View
1
Download
0
Category
Preview:
Citation preview
Stability and Transformation of
Particulate MgO-based Nanocomposites
Stabilität und Transformation von
MgO-Partikel-basierten Nanokompositen
Der Technischen Fakultät der
Friedrich-Alexander-Universität Erlangen-Nürnberg
zur Erlangung des Doktorgrades
DOKTOR-INGENIEUR
vorgelegt von
Amirreza Gheisi
aus Schiraz
Als Dissertation genehmigt von der Technischen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung : 08.10.2015
Vorsitzende des Promotionsorgans: Prof. Dr. Peter Greil
Gutachter: Prof. Dr. Oliver Diwald
Prof. Dr. Martin Hartmann
Abstract
i
Abstract
The stability of nanostructures and the preservation of their chemical and
physical properties in different chemical environments are critical for their proper
implementation in devices. However, the metastability and high surface and interface
energies specific to nanomaterials are found to impose property changes during
material synthesis or post-synthesis processes. In particular, for metastable metal
oxide nanoparticles, post-synthesis processes, storage and aging can lead to their
transformation and change of their characteristics such as crystal structure, chemical
composition, morphology and size. Transformation through either chemical reactions
or physical processes may also enable the modification of as-synthesized
nanomaterials into derivatives of desired composition and structure. The study of
metastability and transformation in metal oxide nanostructures is challenging and
many issues are still unknown. For such studies it is crucial to have model systems
as starting materials with defined properties. In this work some exemplary types of
vapor phase grown metal oxide nanoparticles are used. These materials constitute a
promising class of model nanostructures because of their narrow particle-size
distribution and defined chemical composition, crystal structure and morphology.
For this work three material systems with different levels of metastability are
produced, namely i) MgO and ZnO nanoparticles, ii) Zn-Mg-O and Fe-Mg-O
nanoparticles and iii) MgO nanocubes with surface adsorbed SixOyClz moieties.
Corresponding characterization work has been performed with focus on the influence
of different thermal treatments and chemical composition of the surrounding
atmosphere. The experiments addressing issues like a) the dependence of the
photoluminescence (PL) properties of vapor phase grown MgO and ZnO
nanoparticles on the surface composition as well as the composition of the
surrounding continuous phase, b) annealing induced changes including phase
separation in metastable ternary nanocomposite particles of Zn-Mg-O and Fe-Mg-O
systems, and c) the metastability of MgO nanocubes that were functionalized with
SixOyClz moieties and their transformation into magnesium oxychloride
Mg3(OH)5Cl∙4H2O fibers in the ambient.
Abstract
ii
a) In the first part of this study, the photoluminescence properties of two
prototypical metal oxide nanoparticle systems, MgO as an ionic insulator
and ZnO as an ionic semiconductor, have been discussed and compared.
Gaseous oxygen suppresses PL emission at MgO nanoparticles due to
deactivation of surface excitons, while it enhances the PL emission of
semiconducting ZnO nanoparticles showing adsorption dependent band
bending. This study shows also that annealing induced changes in surface
composition critically influence the luminescence.
b) Control over the distribution of chemical components and the identification
of synergistic effects inside composites are challenging tasks in the
functionalization of mixed-metal oxide nanoparticles. Annealing protocols in
vacuum or O2 atmosphere are very efficient for the purification of the
particles, i.e. for the removal of carbon based contaminants originating from
precursors. In ternary metal oxide systems, differences in ionic radii/ or
charge can induce the segregation of admixed cations to the particle
surface. In the second part of this work, these effects have been utilized to
produce two types of composite transition metal-magnesium oxide
nanoparticles. Zn-Mg-O nanocomposite particles have been prepared by
flame spray pyrolysis (FSP) in cooperation with Prof. Lutz Mädler and Dr.
Huanjun Zhang from University of Bremen. Based on structural properties
and chemical composition of resulting nanoparticles, optical absorption and
photoluminescence emission properties are discussed as a result of bulk
and surface excitons. For the Zn-Mg-O solid solutions, annealing induced
surface-segregation of Zn2+ ions depletes the MgO-specific PL emission
from particle surfaces. Moreover, it is found that the surface hydroxyls play
a significant role as protecting groups against oxygen as a PL quencher at
the solid-gas interface.
Metal-organic chemical vapor synthesis (MO-CVS) is used for the
generation of nanoparticles of the ternary Fe-Mg-O system. The effect of
annealing on composition, structure and optical properties of particle
powders of different compositions are explored. The types of
nanocomposites discussed range from solid solutions of Fe3+ ions in the
Abstract
iii
periclase structure of MgO exhibiting superparamagnetic properties to phase
separated components of periclase MgO and magnesioferrite MgFe2O4
phases with antiferromagnetic behavior. Moreover, absence of MgO-specific
PL emission on annealed Fe-Mg-O nanoparticles points to the effective
segregation of Fe3+ ions into the surface of the composites.
c) Finally, the phenomenon of spontaneous growth of magnesium oxychloride
Mg3(OH)5Cl∙4H2O needles in air and at room temperature has been
discovered and explored in great detail. It is found that MgO/SixOyClz
material system, formed during surface functionalization of MgO nanocubes
with SiCl4 and O2, is highly metastable upon contact with water vapor. The
results underline the critical impact of parameters such as SiCl4 adsorption
temperature, nature of the surrounding atmosphere, and MgO particle size
and dispersion degree on the aforementioned process of fiber growth.
iv
Kurzfassung
v
Kurzfassung
Die Stabilität von Nanostrukturen und der Erhalt ihrer chemischen und
physikalischen Eigenschaften in chemisch verschiedener Umgebung sind von
entscheidender Bedeutung für ihre Anwendung in Bauteilen. Jedoch führen
Metastabilität und hohe Oberflächen- bzw. Grenzflächenenergie, welche spezifisch
für Nanomaterialien sind, zur Änderung von bestimmten Eigenschaften während der
Synthese und Nachbehandlung. Im Speziellen können Nachbehandlung, Lagerung,
und Alterung für metastabile Metalloxid Nanopartikel zur Umwandlung oder
Änderung ihrer charakteristischen Eigenschaften wie Kristallstruktur, chemische
Zusammensetzung, Morphologie und Größe führen. Transformation durch entweder
chemische Reaktionen oder physikalische Prozesse können aber auch zu
gewünschten Änderungen der Zusammensetzung und Struktur führen. Die
Untersuchung von Metastabilität und Umwandlung in Metalloxid Nanostrukturen stellt
eine große Herausforderung dar und viele Aspekte sind noch unbekannt. Für solche
Untersuchungen ist es entscheidend Modellsysteme mit bekannten Eigenschaften zu
haben. In dieser Arbeit wurden einige dieser modellhaften und in der Gasphase
hergestellten Metalloxid Nanopartikel genutzt. Diese Materialien stellen aufgrund
ihrer engen Partikelgrößenverteilung, ihren definierten chemischen Eigenschaften,
sowie ihrer Kristallstruktur und Morphologie eine vielversprechende Klasse an
modellhaften Nanostrukturen dar.
Für diese Arbeit wurden drei Materialsysteme verschiedener Stufen an
Metastabilität produziert, namentlich i) MgO und ZnO Nanopartikeln, ii) Zn-Mg-O und
Fe-Mg-O Nanopartikeln und iii) MgO Nanowürfel mit auf der Oberfläche adsorbierten
SixOyClz Clustern. Entsprechende Charakterisierungsarbeit wurde mit Fokus auf den
Einfluss verschiedener thermischer Behandlungen und der chemischen
Zusammensetzung der umgebenden Atmosphäre geleistet. Die Experimente
adressieren Themen wie a) Abhängigkeit der Photolumineszenz-Eigenschaften der in
der Gasphase hergestellten MgO- und ZnO-Nanopartikeln, welche die
Oberflächenzusammensetzung, sowie die Zusammensetzung der umgebender
Phase betreffen, b) durch thermische Behandlungen induzierte Änderungen wie
Phasentrennung in ternären Nanokompositpartikeln wie Zn-Mg-O und Fe-Mg-O und
c) die Metastabilität von mit SixOyClz Clustern funktionalisierten MgO Nanowürfeln
Kurzfassung
vi
und deren Transformation in Magnesiumoxychlorid-Fasern (Mg3(OH)5Cl∙4H2O) unter
Umgebungsbedienungen.
a) Im ersten Abschnitt dieser Studie werden die Photolumineszenz-
eigenschaften von zwei prototypischen Metalloxidnanopartikelsystemen,
MgO als ionischer Isolator und ZnO als ionischer Halbleiter, diskutiert und
anschließend miteinander verglichen. Gasförmiger Sauerstoff verringert die
PL Emission der MgO Nanopartikeln aufgrund von Deaktivierung von
Oberflächenexzitonen, wohingegen es die PL Emission der halbleitenden
ZnO Nanopartikeln, die ein adsorptionsabhängiges band bending zeigen
erhöht. Diese Studie zeigt, dass durch thermische Nachbehandlung
induzierte Veränderungen der Oberflächenzusammensetzung kritischen
Einfluss auf die Lumineszenz haben.
b) Kontrolle über die Verteilung von chemischen Bestandteilen und die
Identifizierung von synergistischen Effekten innerhalb der Komposite stellen
für die Funktionalisierung von misch-metalloxidischen Nanopartikeln eine
Herausforderung dar. Thermische Aktivierungsverfahren im Vakuum oder
unter Sauerstoffatmosphäre sind sehr effizient zur Reinigung der Partikeln,
z.B. um Kohlenstoff-basierte Verunreinigungen, die durch Präkursoren
entstanden sind zu entfernen. In ternären Metalloxid Systemen können
unterschiedliche Ionenradien oder -ladungen die Segregation von
zugegebenen Kationen zur Partikeloberfläche hin induzieren. Im zweiten
Teil der Arbeit, wurden diese Effekte verwendet um zwei Typen der
Übergangsmetall-Magnesiumoxid Nanopartikeln zu produzieren. Zn-Mg-O
Nanokompositpartikeln wurden durch Flame Spray Pyrolysis (FSP) in
Kooperation mit Prof. Lutz Mädler und Dr. Huanjun Zhang der Universität
Bremen produziert. Basierend auf strukturellen Eigenschaften und der
chemischen Zusammensetzung der entstandenen Nanopartikeln wurden die
optischen Absorptions- und Photolumineszenzemissionseigenschaften,
welche durch Bulk und Oberflächen Exzitonen hervorgerufen werden,
diskutiert. Oberflächen Segregation von Zn²+ Ionen aufgrund thermischer
Aktivierung von Zn-Mg-O Mischkristallen verringert die MgO spezifischen PL
Emissionen der Partikeloberfläche. Außerdem wurde herausgefunden, dass
Kurzfassung
vii
Hydroxylgruppen auf der Oberfläche eine wichtige Rolle beim Schutz gegen
Sauerstoff als PL-Quencher in der Feststoff-Gas Grenzfläche spielen.
Metall-organische chemische Gasphasensynthese (MO-CVS) wurde für die
Herstellung von Nanopartikeln des ternären Fe-Mg-O Systems verwendet.
Der Einfluss der thermischen Behandlung auf Zusammensetzung, Struktur
und optische Eigenschaften der Nanopartikeln verschiedener
Zusammensetzungen wurde erforscht. Die Typen der diskutierten
Nanokomposite erstreckt sich vom Mischkristall der Fe3+ Ionen in der
Periklasstruktur des MgO, welches superparamagnetische Eigenschaften
zeigt, bis hin zu separierten Bestandteilen von Periklas-MgO und
Magnesioferrit MgFe2O4 Phasen mit antiferromagnetischen Verhalten.
Außerdem weist ein Mangel an MgO-spezifischen PL Emissionen von
thermisch behandelten Fe-Mg-O Nanopartikeln auf eine effektive Fe3+ Ionen-
Segregation zur Oberfläche der Komposite auf.
c) Zuletzt wurden Phänomene von spontan wachsenden
Magensiumoxychlorid-Fasern (Mg3(OH)5Cl∙4H2O) an Luft und bei
Raumtemperatur entdeckt und im genaueren Detail erforscht. Es wurde
erwiesen, dass das MgO/SixOyClz Materialsystem, welches während der
Oberflächenfunktionalisierung von MgO Nanowürfeln mit SiCl4 und O2
geformt wurde, in Kontakt mit Wasserdampf sehr metastabil ist. Die
Ergebnisse betonen den kritischen Einfluss von Parametern wie z.B. die
Adsorptionstemperatur von SiCl4, die Art der umgebenden Atmosphäre, die
MgO Partikelgröße und den Grad der Dispersion auf den oben genannten
Prozess des Nanofaserwachstums.
viii
Publikationen
ix
Publikationen
Aus Gründen der wissenschaftlichen Priorität wurden einige Ergebnisse dieser
Dissertation bereits veröffentlicht:
(1) Gheisi, A. Sternig, A. Redhammer, G. Diwald, O. Thin water films and
magnesium hydroxide fiber growth. RSC Advances 5, 82564-82569, (2015).
(2) Gheisi, A. Neygandhi, C. Sternig, A. Carrasco, E. Marbach, H. Thomele, D.
Diwald, O. O2 Adsorption dependent photoluminescence emission from metal
oxide nanoparticles. Physical Chememistry Chemical Physics 16, 23922-23929
(2014).
(3) Gheisi, A. Sternig, A. Rangus, M. Redhammer, G. Hartmann, M. Diwald, O.
Spontaneous growth of magnesium hydroxide fibers at ambient conditions.
Crystal Growth and Design 14 (9), 4236–4239 (2014).
(4) Zhang, H. Gheisi, A. Sternig, A. Müller, K. Schowalter, M. Rosenauer, A.
Diwald, O. Mädler, L. Bulk and surface excitons in alloyed and phase-separated
ZnO-MgO particulate systems. ACS Applied Materials & Interfaces 4 (5), 2490-
2497 (2012).
x
Acknowledgements
xi
Acknowledgements
The achievements and completion of this thesis would not have been possible
without the support of many people. I would like to acknowledge everyone who has
helped me along the way. First and foremost, I would like to thank Prof. Dr. Oliver
Diwald for giving me the opportunity to undertake my thesis in his group. I am
thankful for all of the hours Prof. Diwald spent giving me feedback and guidance in
my research. Prof. Diwald has been a mentor and he taught me priceless lessons in
both science and life. Furthermore, I want to thank Prof. Diwald for many valuable
advices and discussions regarding the writing part of this work.
It has also been a privilege to have Dr. Andreas Sternig as my colleague. He
has been a great teacher and a good friend. I am very thankful for all of his scientific
and technical advices and supports throughout my research. I also want to thank Dr.
Sternig for the TEM measurements and analysis provided by him in the course of
works presented in chapters 4, 7 and 8.
I would like to thank Prof. Dr. Johannes Bernardi from the University Center for
Transmission Electron Microscopy, Vienna University of Technology for the TEM
measurements and analysis which have been performed for the works presented in
chapters 6 and 10.
I am very grateful to Prof. Dr. Martin Hartmann and Dr. Mojca Rangus from
Erlangen Catalysis Resource Center, University of Erlangen-Nürnberg for solid state
MAS NMR experiments and the data evaluation presented in chapter 7. Especially, I
would like to thank Prof. Hartmann for reviewing this thesis.
I also want to thank Dr. Hubertus Marbach and Dr. Esther Carrasco from the
Chair of Physical Chemistry II, University of Erlangen-Nürnberg for collaborative work
and Auger Electron Spectroscopy (AES) measurements and analysis presented in
chapter 4.
Acknowledgements
xii
I thank Prof. Lutz Mädler and Dr. Huanjun Zhang from Department of
Production Engineering, University of Bremen for providing samples synthesized by
flame spray pyrolysis and collaborative work presented in chapter 5.
It is a pleasure to acknowledge collaborators from Department of Materials
Science and Physics, University of Salzburg. I thank Prof. Dr. Werner Lottermoser
and Mag. Gerold Tippelt for the Mößbauer measurements and the data evaluation
presented in Chapter 6. I also thank Prof. Dr. Günther Redhammer for the analysis of
X-ray diffraction patterns presented in chapters 6, 7 and 8.
I am also grateful for the opportunity to get to know and work with wonderful
group of colleagues from the working group and from the Institute of Particle
Technology. Especially I owe a huge debt of gratitude to colleagues who helped me
in my research and who assisted me in performing experiments; Dr. Stefan
Baumann, Dr. Michael Elser, Dr. Nicolas Siedl, Chris Neygandhi, Daniel Thomele,
Johannes Schneider, Paula Hoppe and Irina Merschmann.
I would also like to express my appreciation to the Austrian Science Fund
(FWF, project I-312) and the German Research Foundation (DFG, project 1613/ 2-1)
for the financial support of this work.
Finally, my sincere gratitude goes to my parents for all their love and
encouragement throughout my life and my studies. And, my deepest gratitude goes
to my loving, supportive and patient wife Mehrnoush whose faithful encouragement
during the last stages of my PhD is so appreciated.
Table of Contents
xiii
Table of Contents
1 Introduction ........................................................................................................ 1
1.1 Stability of Metal Oxide Nanoparticles ............................................................. 1
1.2 MgO Nanoparticles as Probes for Interfacial Changes .................................... 3
1.3 Zn/Fe-Mg-O Metal Oxide Nanocomposites ...................................................... 5
1.4 Scope and Structure of this Work .................................................................... 8
2 Spectroscopic Techniques .............................................................................. 11
2.1 UV-Vis Diffuse Reflectance Spectroscopy ..................................................... 11
2.2 Optical Absorption Spectra of Transition Metal Oxides .................................. 14
2.3 Photoluminescence Spectroscopy ................................................................. 18
3 Experimental Details ........................................................................................ 21
3.1 Sample Preparation ....................................................................................... 21
3.2 UV-Vis Diffuse Reflectance Spectroscopy ..................................................... 28
3.3 Photoluminescence Spectroscopy ................................................................. 30
4 O2 Adsorption Dependent Photoluminescence Emission from Metal Oxide
Nanoparticles ................................................................................................... 31
4.1 Abstract .......................................................................................................... 31
4.2 Introduction .................................................................................................... 31
4.3 Experimental Section ..................................................................................... 33
4.4 Results ........................................................................................................... 36
4.5 Discussion ..................................................................................................... 43
4.6 Conclusions ................................................................................................... 47
4.7 Supporting Information ................................................................................... 49
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO-MgO
Particulate Systems ......................................................................................... 55
5.1 Abstract .......................................................................................................... 55
5.2 Introduction .................................................................................................... 55
5.3 Experimental Section ..................................................................................... 58
5.4 Results and Discussion .................................................................................. 60
5.5 Conclusions ................................................................................................... 73
5.6 Supporting Information ................................................................................... 75
Table of Contents
xiv
6 Fe-Mg-O Nanocomposite Particle Systems: Controlled Synthesis and the
Influence of Annealing on Composition, Structure and Optical Properties 79
6.1 Abstract .......................................................................................................... 79
6.2 Introduction .................................................................................................... 80
6.3 Experimental Section ..................................................................................... 84
6.4 Results ........................................................................................................... 88
6.5 Discussion ................................................................................................... 106
6.6 Conclusions ................................................................................................. 109
6.7 Supporting Information ................................................................................. 111
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient
Conditions ...................................................................................................... 115
7.1 Abstract ........................................................................................................ 115
7.2 Introduction .................................................................................................. 115
7.3 Results and Discussion ................................................................................ 116
7.4 Conclusion ................................................................................................... 120
7.5 Supporting Information ................................................................................. 121
8 Vapor phase based and Water Film mediated Growth of Magnesium
Oxychloride Fibers ......................................................................................... 127
8.1 Abstract ........................................................................................................ 127
8.2 Introduction .................................................................................................. 127
8.3 Experimental Section ................................................................................... 129
8.4 Results and Discussion ................................................................................ 131
8.5 Conclusion ................................................................................................... 144
8.6 Supporting Information ................................................................................. 146
9 Summary ......................................................................................................... 151
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles Size
Distribution and Morphology ........................................................................ 155
10.1 MgO nanoparticle changes influenced by annealing conditions .................. 155
10.2 Mass Spectroscopy during Standard Annealing of MgO ............................. 161
10.3 Annealing Dwell Time Effect ....................................................................... 166
10.4 Summary and Conclusion ........................................................................... 172
Table of Contents
xv
11 Bibliography ................................................................................................... 175
xvi
1 Introduction
1
1 Introduction
1.1 Stability of Metal Oxide Nanoparticles
The study of nanomaterials1 is currently an active area of research in a broad
variety of scientific and technological fields, from physics, chemistry and materials
engineering to medical, electronic and computer sciences. Among different
nanomaterials, pure and mixed metal oxide nanoparticles are the most common
materials in the large number of technical applications, like catalysts, sensors,
constituents of microelectronic devices and optical coatings.2,3 The detailed
understanding of property changes of metal oxide nanoparticles, i.e. their stability, is
not only crucial for improvement of their performances but also for eventual design of
new materials. The fundamental knowledge of their chemical, structural and surface
characteristics is necessary but, on the other hand, is directly linked to the
experimental conditions during the synthesis of the nanoparticles as well as to the
stability and transformations during post-synthesis processing.
Due to the high surface-to-volume ratio, surface structure and composition and
resulting surface reactivity dominate metal oxide nanoparticles properties. Different
intrinsic (ion vacancies and interstitial atoms) and extrinsic (impurities) surface
defects may contribute significantly to the nanoparticles overall properties.4,5 For
instance, surface defects such as oxygen vacancies affect the photoluminescence
properties of nanoparticles like ZnO or surface adsorbed hydroxyl groups influence
the dispersion stability of metal oxide nanoparticles in various liquid media.6,7,8
However, the high dependence of properties on surface and interface implies
challenges with regard to materials control when exposed to different and sometimes
less defined environmental phases. For example, nanoparticles handled in ambient
air are automatically exposed to water and residual gas atmosphere that can result in
significant changes of the optical and structural properties.
1 Nanomaterials are defined as substances with any external dimention in the size range from
approximately 1 nm to 100 nm (nanoscale), or having internal or surface structure in the nanoscale.1
1 Introduction
2
Many characteristics of the metal oxide nanoparticles, such as surface
electronic states or melting temperatures, relate to their high surface-to-volume ratios
as well as to the prevailing existence of non-equilibrium phases, residual stresses,
large portion of defects and interfaces.4,9,10 In the case of metastable nanoparticles,
variation of parameters, such as temperature, pressure, time, radiation and chemical
environment can induce property changes during materials handling, storage and
processing. For instance, thermal activation of the nanoparticles at different
processing steps can lead to increasing of ion diffusion, alloying and phase
separation. Moreover, formation or annihilation of bulk and surface defects and the
associated gain or loss of the related properties can occur at elevated
temperatures.11–13
Thus, stability in terms of physical, chemical and mechanical behavior is of
great importance for effective application of metal oxide nanoparticles. Numerous
stability studies concerned with various aspects of metal oxide nanoparticles such as
chemical composition, shape, size, recrystallization, segregation and phase
separation have been carried out.14–19 In fact, stability is a broad issue that can be
discussed based on the behavior of nanoparticle ensembles in different thermal,
chemical, mechanical and atmospheric conditions.
In this work some exemplary types of pure and composite metal oxide
nanoparticles have been subject to different thermal treatment procedures. For these
nanoparticles annealing induced property changes, i.e. thermal stability during their
processing, play a significant role in the control of their size, shape, composition and
crystal structure. Indeed, many of the metal oxide nanoparticles are solids at
non-equilibrium state, but to understand their thermal stability at different
temperatures it is necessary to know about the thermodynamic phase equilibrium
and related phenomena, such as phase transformation temperatures at specified
pressures, miscibility of components and phase separation. However, it is known that
the thermodynamic properties of nanomaterials differ from those of bulk
materials.20-22 This means that for nanomaterials, the deviation of the phase
transformation processes from bulk materials must be considered. Because of the
wide range of possible metal oxide nanoparticle systems and thermal treatment
1 Introduction
3
conditions, there is no comprehensive study that can describe the kinetic and
thermodynamic properties of such systems. Moreover, thermal stability cannot be
understood and discussed without considering the bulk and surface impurities and
defects, processing time and the nature of the surrounding continuous phase, i.e.
vacuum, gas or liquid.
The well-facetted and highly dispersed MgO nanocubes obtained by chemical
vapor synthesis (CVS) and subsequent thermal annealing, represent an ideal metal
oxide for related studies.23–28 Because of the high thermal stability, MgO
nanoparticles can be treated at high temperatures in order to achieve dehydroxylated
nanoparticle surfaces. This generates a particularly well-suited starting situation to
track surface-related property changes of particulate MgO-based nanoparticles by
spectroscopy.
The aim of the present work is to characterize the influence of annealing and
constituents of surrounding atmosphere on the stability and transformation of the
MgO-based pure and composite nanoparticles. For this purpose, the variations in
physico-chemical properties such as chemical composition, crystal structure, size,
shape and optical properties have been investigated.
1.2 MgO Nanoparticles as Probes for Interfacial Changes
Magnesium oxide as an ionic insulator material (Ebg ≈ 7.6 eV) with rock salt
crystal structure is one of the most representative metal oxides. Particularly, in
nano-size regime the highly dispersed MgO nanoparticles, with cubic morphology,
offer high amount of low coordinated surface active sites such as corners and edges
(Figure 1.1). These surface features make MgO nanoparticles suitable probes for
surface and interface related studies both theoretically23–25,29 and experimentally26–28.
The optical characterization of the surface and interface of MgO nanoparticles
provides an opportunity to establish a correlation between topological surface
features and the corresponding chemical and spectroscopic properties.
1 Introduction
4
Figure 1.1: Schematic representation of topological surface features such as low
coordinated corners (3C), edges (4C). Adapted from reference [30].
Low-coordinated surface anions, which are considered to be chemically reactive
sites, give rise to specific optical transitions in the UV range, which can be assigned
to the formation of surface excitons.29 For surface-dehydroxylated MgO nanocubes
with average particle size of about 6 nm, UV-Vis diffuse reflectance spectra show two
absorption bands at energies far below the bulk absorption threshold of MgO
(7.6 eV). The absorption bands at energies of 5.2 eV (λ = 240 nm) and 4.6 eV
(λ = 240 nm) have been attributed to low-coordinated edges and corners,
respectively.31 Photoluminescence studies have been done to find a connection
between the nature of emission and absorption sites on MgO surfaces. It is shown
that for surface-dehydroxylated MgO nanocubes in vacuum (p < 10-5 mbar), the
photoexcitation of the edge and corner sites results in emission bands at 3.4 eV
(λ = 370 nm) and 3.2 eV (λ = 380 nm), respectively.32 These exclusively surface-
related spectroscopic fingerprints of MgO are the basis for the investigation of the
surface- and structure-property stability of MgO-based nanocomposites in different
chemical and thermal conditions applied in this work.
1 Introduction
5
1.3 Zn/Fe-Mg-O Metal Oxide Nanocomposites
The combination of two metals in an oxide matrix can produce composite
materials with novel structural and chemical properties that rely on synergistic
combination of single-component properties. Nanoparticles of metal oxide composite
materials are increasingly important in many areas of chemistry, physics and material
science.33–36 Among these material systems, nanocomposites containing transition
metals and magnesium oxides have attracted considerable attention because of
particularly their potential for optoelectronic, magnetic and catalytic applications.37–40
Admixture of transition metals (TM) at low concentrations to MgO yields
substitution of magnesium ions with TM cations within the rock-salt structure of
MgO.12,41,42 Annealing of MgO-based nanocomposites with concentrations above the
solubility limit of the admixed TMs in MgO is expected to lead to segregation of the
admixed cations to the particle surface potentially upon phase separation.11,12,43 The
segregation is derived from differences in ionic radii and/or charge of the transition
metal and magnesium ions and occurs to reduce free energy of the system.44–46
Beyond the consideration of ionic radii and charge, the relative electronegativity of
transition metal cations plays a role as well.47 As all transition metals are less
electropositive than Mg, the TM2+-O2- bond is less ionic. This additionally drives the
segregation of TM2+ ions from the bulk into sites at the surface of the nanoparticles,
where the electrostatic potential is weaker. However, the spatially controlled
distribution and arrangement of the transition metal ions in host matrix is highly
challenging and depend on chemical nature of the dopant, chemical environment,
processing temperature and time.12,48
In this work we have generated composite nanoparticles of the Zn-Mg-O and
Fe-Mg-O system to investigate the annealing induced structural and optical property
changes and, consequently, to identify the arrangement of the components in
particular regions of the solid.
1 Introduction
6
1.3.1 Zn-Mg-O Nanocomposites
Magnesium oxide (Ebg ≈ 7.6 eV) and zinc oxide (Ebg ≈ 3.4 eV) belong to
different group of materials with regard to their crystallographic and electronic
structures. The admixture of narrow bandgap semiconductor ZnO to the wide
bandgap insulator MgO provides means to generate tunable bandgap nanomaterials
for use in many fields including optoelectronics, display devices, gas sensors and
photocatalysis to name a few.35,49–52 Most of the related studies on such particle
systems are dedicated to ZnO wurtzite structures with small amounts of MgO
admixed, whereas considerably fewer studies report periclase MgO doped with ZnO.
Due to their defined shape and tunable optical and chemical surface properties such
composites provide novel building blocks for the growth of cubic ZnO
nanostructures.53,54
Different synthesis methods including the sequential implantation of Zn+ and O
-
ions in single crystalline MgO,55 the calcination of polymer/metal salt complexes56 or
reactive electron beam evaporation57 have been employed for the generation of cubic
ZnO structures. However, detailed synthesis and characterization studies on well-
dispersed powders of ZnxMg1−xO nanocrystals are scarce, due to the lack of
appropriate preparation techniques and reliable surface characterization approaches.
In a recent work, that is done in our group, it has been demonstrated that
monocrystalline ZnxMg1−xO nanocomposites with exceptional regular cubic shape
can be obtained using chemical vapor synthesis (CVS) and subsequent vacuum
annealing.12 Such particles are described as insulating cubes which have scaffolds of
a semiconducting component. However, the utilized approach does not allow the
production of nanocomposites with Zn2+ concentrations above 12 at.%.
For the realization of ternary Zn-Mg-O mixed metal oxide with higher Zn2+
concentrations, Zn needs to be added in a more effective and controllable way to the
nanocomposite system. As an alternative approach, in this work, in cooperation with
Prof. Lutz Mädler and Dr. Huanjun Zhang from University of Bremen flame spray
pyrolysis process has been employed to produce nanocomposite samples with
higher Zn2+ concentrations. The composition and surface electronic structure of the
1 Introduction
7
produced samples are explored based on the influence of annealing induced
segregation, phase separation and surface dehydroxylation effects.
1.3.2 Fe-Mg-O Nanocomposites
Nanocomposites comprising iron as transition metal embedded in magnesium
oxide matrix have attracted increasing attention for their excellent catalytic, magnetic
and optical properties.40,58 In technological applications they are used in magnetic
devices39, photovoltaics38, pigments59 and in the fabrication of different types of
catalysts, like catalysts for DeSOx processes60 and carbon nanostructures
synthesis61.
Many methods have been developed in order to produce Fe-Mg-O
nanostructures. Examples are in melt doping of MgO with iron and subsequent high
temperature oxidation62, high temperature annealing of powder mixtures of Fe2O3
and MgO63, combustion of iron and magnesium nitrates in urea solution64, sol-gel
approaches followed by high temperature calcination59, spray pyrolysis38, sputtering65
and hydrothermal procedure66.
The structural, chemical and surface/interface properties of such nanoparticles
critically depend on preparation techniques and their post synthesis treatments. Most
of the wet methods like sol-gel or hydrothermal synthesis have intrinsic
disadvantages with regard to their use for more detailed studies or advanced
applications. Reasons for that are undesirable bulk and surface impurities originating
from synthesis, poor control over cation concentration and broad particle size
distribution.
In this work we employed for the first time a modified metal organic chemical
vapor synthesis (MO-CVS) approach for the growth of Fe-doped magnesium oxide
nanocrystals. This method and subsequent annealing in controlled gas atmosphere
provide condition to produce Fe-Mg-O nanocomposite samples with distinct
morphology, defined distribution of size and desired composition. For the obtained
nanocomposites, structural-, and optical-property changes are explored. These
1 Introduction
8
property changes relate to the different admixed iron concentrations as well as
annealing induced surface segregation and phase separation effects.
1.4 Scope and Structure of this Work
The focus of this work is on the experimental investigation of changes in metal
oxide nanoparticles that are induced by i) annealing and ii) controlled changes of the
surrounding gas phase. In this regard, we extensively explored variations in optical
absorption, photoluminescence emission, bulk and/or surface composition, particle
morphology, size and crystal structure. Different metal oxides and composites
thereof, namely MgO, ZnO, Zn-Mg-O, Fe-Mg-O and MgO/SixClyOz have been
synthesized and investigated in great detail.
The first part is primarily concerned with the effect of surface composition and
oxygen adsorption on the photoluminescence (PL) properties of vapor phase grown
ZnO and MgO nanoparticles (Chapter 4). Annealing induced optical property
changes of these prototypical metal oxides will be discussed in terms of changes in
the surface composition.
The second part deals with the admixture of two types of transition-metals
(Zn2+ and Fe3+) into the MgO matrix using different gas phase synthesis approaches.
The objective of this part was to obtain the information about the surface
functionalization of the MgO by the admixture of a second metal ion component. For
the nanoparticles of the ternary Zn-Mg-O system the correlation between the surface
states and their PL emission properties are discussed (Chapter 5). For Fe-Mg-O
nanocomposites a new gas phase synthesis approach has been established and
used to explore a new class of materials where the effect of annealing on
composition, structure, morphology and optical properties can be studied in detail
(Chapter 6).
In the third part of this work MgO nanoparticles have been exposed to SiCl4 and
O2 in an attempt to cover MgO nanocubes surfaces with SiOx shells. As a result, in
the ambient MgO nanocubes with surface adsorbed SixClyOz moieties transform into
1 Introduction
9
Mg3(OH)5Cl∙4H2O nanofibers (Chapter 7). This part will shed light on the chemical
and physical factors leading to formation of MgO/SixClyOz system and the growth
mechanism of Mg3(OH)5Cl∙4H2O nanofibers (Chapter 8). Moreover, the thermal
stability of the obtained fibers is discussed.
Material’s characterization in terms of particle size distribution, crystal structure,
morphology, and specific surface area is carried out by the help of scanning electron
microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction
(XRD) analysis and nitrogen sorption. The main elemental analysis techniques
employed in this work for compositional analysis of nanomaterials, are inductively
coupled plasma optical emission spectroscopy (ICP-OES) and energy dispersive
X-ray (EDX) spectroscopy. Additionally, UV-Vis diffuse reflectance spectroscopy and
photoluminescence spectroscopy are used to characterize the optical properties of
the nanoparticle ensembles.
10
2 Spectroscopic Techniques
11
2 Spectroscopic Techniques
2.1 UV-Vis Diffuse Reflectance Spectroscopy
2.1.1 Basics
Ultraviolet-Visible (UV-Vis) spectroscopy is used to obtain absorbance spectra
of a wide range of sample types such as liquids, gases and solids. The absorption
behavior of solutions containing low amounts (c < 0.01 M) of dissolved molecules can
be described by Beer-Lambert law (Equation 2.1). This law states that the
concentration of a solute in a dilute solution is directly proportional to the sample
absorbance.
A = ɛ c d Equation 2.1
Where A is absorbance (no unit), ɛ the molar absorptivity (L mol-1 cm-1), c
concentration of the compounds or molecules in solution (mol L-1) and d is the path
length of the sample (cm). The term Absorption refers to the physical process of
absorbing electromagnetic radiation. In absorption, the energy of electromagnetic
radiation is absorbed and excites electrons from the ground state to the excited state
of the compound.
For solid samples the incident beam is not only absorbed or transmitted but
may be also reflected (Figure 2.1). The ratio of the reflected to the incident beam is
defined as the amount of reflectance and can be measured by reflectance
spectroscopy.
Figure 2.1: The interaction of light with solid samples.
2 Spectroscopic Techniques
12
The data provided by this technique is used to describe the optical absorption
behavior and electronic structures of the solid powder samples.
2.1.2 Reflectance Measurement of a Powder Sample
In order to determine the relative amount of radiation reflected from a sample at
any wavelength, reflectance spectroscopy is used. Reflectance consists of two
contributions:
1) Specular reflectance
2) Diffuse reflectance
The mirror-like reflection which is described by the law of reflection is called
specular reflectance. Interaction of electromagnetic radiation with electric field of
particles and molecules can also results in reflection of radiation in all directions. This
is called electromagnetic scattering and refers to diffuse reflectance.
Most samples display a combination of specular and diffuse reflectance.
Depending on the equipment used, it is possible to measure specular reflectance,
diffuse reflectance or total reflectance separately. Diffuse reflectance spectroscopy
concerns the diffusely reflected radiation collected and detected by the detector from
an irradiated sample.
If the wavelength of the incident beam is significantly larger than the surface
variations (roughness) or particle size (d, the diameter of the particle) diffuse
reflectance dominates (i.e. when λ >> d). In nanoparticle powders the radiation
scatters many times within the particle ensemble, which is known as multiple
scattering.67 Several theories have been developed to describe multiple scattering of
light.68 Among these, theory of Kubelka-Munk provides a way to transform
reflectance spectrum into absorbance spectrum and characterize the absorption and
scattering of the irradiated sample layer.
2 Spectroscopic Techniques
13
2.1.3 The Kubelka-Munk Theory
The theory developed by Kubelka and Munk in 1931 describes diffusely
reflected radiation based on absorption (K) and scattering (S) coefficients of a
continuum medium capable of both scattering and absorbing radiation.68 The theory
describes reflectance properties by differential equations for parallel infinitesimal
small layers of thickness dx. For a layer of thickness dx of small-particle ensembles
irradiated in the x direction (Figure 2.2) the downward flux (I) is decreased by
absorption (KIdx) and scattering (SIdx) processes. In turn, the upward flux (J) is
increased by scattering (SJdx) and decreased by absorption (KJdx).
Figure 2.2: Model for the Kubelka-Munk analysis of reflectance data.
The following differential equations can be derived for fluxes I and J:
dI / dx = - ( K + S ) I + SJ Equation 2.2
dJ / dx = ( K+ S ) J – SI Equation 2.3
In the case of an infinitely thick sample (which is true for the nanoparticle
powder samples investigated in this work) the reflectance is independent of thickness
and is given by R∞ (= Rsample / Rstandard). Under this condition the solution of these
equations is simplified and Kubelka-Munk equation at any wavelength becomes:
F(R∞) ≡ K/S = (1-R∞)2/(2 R∞) Equation 2.4
where F(R∞) is named remission or Kubelka-Munk function. According to this
equation the reflectance at any wavelength is a function of K/S ratio and not their
absolute values. The Kubelka-Munk theory is based on several assumptions and can
be applied only under following experimental conditions :69
i. The sample has high diffusion and low absorption.
ii. The sample thickness is sufficient to avoid loss of radiation by transmission.
2 Spectroscopic Techniques
14
iii. The absorption intensity is very weak (F(R∞) ≤ 1).
iv. Photoluminescence is absent.
Therefore the model holds when the particle sizes are comparable or smaller
than the wavelength of the incident light. By this assumption the scattering coefficient
shows little variations with wavelength over the range of interest and Kubelka-Munk
theory can be successfully used to convert the diffuse reflectance spectrum into a
spectrum that is proportional to an absorbance spectrum.
2.2 Optical Absorption Spectra of Transition Metal Oxides
2.2.1 Basic Concepts
Transition metals both in atomic or ionic form have partially filled d orbitals.70
This definition includes most of the elements in group 3 to 12 in the periodic table.
Table 2.1 shows the elements in the first row of these groups.
Table 2.1: 1st row of elements in group 3 to 12 in the periodic table.
group 3 4 5 6 7 8 9 10 11 12
element Sc Ti V Cr Mn Fe Co Ni Cu Zn
atomic no. 21 22 23 24 25 26 27 28 29 30
valence configuration
d1s2 d2s2 d3s2 d5s1 d5s2 d6s2 d7s2 d8s2 d10s1 d10s2
The oxidation states of the transition metals depend on their state of
complexation and how they exist in corresponding coordination compounds.
Coordination compounds are Lewis acid (electron acceptor) -base (electron donor)
complexes where the transition metal atom or ion acts as a Lewis acid and forms
coordinate bonds with the ligands. Most of the ligands are anions or neutral
molecules (e.g., O2-, OH-, CO, NH3) and the nature of metal-ligand bond can range
from covalent to ionic.71
The arrangement of the d orbitals and their energy differences in the above
mentioned coordination compounds is reflected in their absorption spectra.72 There
are two widely used models to describe the electronic structure of these
complexes:73,74
2 Spectroscopic Techniques
15
1) Crystal-field theory
2) Ligand-field theory
Crystal-field theory is based on the analysis of transition metal ions in solids and
only applies to ions in crystals. In this model the ligands are considered as point
negative charges which repel the d orbital electrons of the metal ion. The crystal-field
theory explains the splitting of the d orbitals into different energies and relates the
optical absorption spectra to this splitting. The splitting energy is called ligand-field
splitting parameter Δ.
In crystal-field theory the overlap of ligands and metal atom orbitals is not
considered. Ligand-field theory takes molecular orbital theory into account and
provides a more complete framework to interpret spectroscopic data and the origin of
d orbitals splitting. Both theories use the ligand-field splitting parameter Δ, to
correlate the spectroscopic properties. Ligand-field theory gives a more complete
description of the electronic structure of the complexes. However, for the transition
metal ions in crystals the crystal-field theory can be used to interpret the energies
and intensities of electronic transitions in a more straightforward manner.72
For transition metal complexes octahedral and tetrahedral coordinations
describe the most important ligand arrangements. According to crystal-field theory
characterizing an octahedral crystal field, ligands are placed as six point negative
charges in an octahedral array (Figure 2.3). These charges interact with the d orbitals
of central metal ion and split them into a lower-energy triply degenerate set (t2g) and
a higher-energy doubly degenerate set (eg).
2 Spectroscopic Techniques
16
Figure 2.3: The orientation of the five d orbitals arising from the ligands of an
octahedral complex: the degenerate eg and t2g orbitals are separated by ligand field
splitting parameter, Δo.72
Another possible arrangement of ligands in a transition metal complex is
tetrahedral coordination. A tetrahedral crystal field splits the d orbitals into two sets of
e and t2 orbitals where e orbitals have lower energy than t2 orbitals (Figure 2.4).
Figure 2.4: The orientation of the five d orbitals arising from the ligands of a
tetrahedral complex: the degenerate e and t2 orbitals are separated by ligand field
splitting parameter, ΔT.72
2 Spectroscopic Techniques
17
An octahedral complex has six metal-ligand bonding interactions. The complex
with this arrangement has lower energy than a tetrahedral complex with just four
metal-ligand bonding interactions. Because of this lower energy the transition metal
complexes present more octahedral than tetrahedral coordination in their
structures.75
2.2.2 Absorption Spectra
The magnitude of ligand field splitting relates to the energy of electronic
transitions and corresponds to absorption of UV-Visible spectrum. However, because
of the presence of electron-electron repulsions within the metal orbitals the
absorption energies are not directly determined by ligand-field splitting. One of the
outcomes of the electron-electron repulsion is that a single transition can lead to
different absorption bands.72
For the electron configuration of 3d5 the electronic ground state of Fe3+ free ion
is described as 6S. 6S is the spectroscopic term symbol describing the degenerate
states in a 3d5 metal. The superscript 6 denotes a sextet state when each electron
occupies an individual d orbital. In a free atom all 5 d orbitals are degenerate but in a
complex not all of them are degenerate and beside electron-electron repulsion the
energy differences between t2g and eg orbitals must be taken into account. When
Fe3+ ion is surrounded by ligands, in octahedral coordination, t2g and eg d orbitals of
Fe3+ ion are separated. The separated orbitals written as t2g3 eg
2 configuration give
rise to
6A1 as the complex ground state. 6A
1 is the spectroscopic term for the d orbital
electrons in octahedral structure of the complex and relates to the spectroscopic term
of Fe3+ free ion in its ground state.
The absorption spectra of transition metal complexes result from electronic
transitions within the d-orbitals (ligand-field transitions) and charge transfer from
metal to ligand or vice-versa. All the ligand field transitions occur between the ground
state of the complex (e.g. 6A1
for Fe3+ in octahedral complex) and states which arise
from different possible electronic configurations of t2g and eg. Charge transfer bands
are related to electronic transitions between ligand and metal orbitals.
2 Spectroscopic Techniques
18
Quantitative interpretation of electronic absorption spectra of transition metal
complexes is possible by extracting the value of ligand field splitting parameter and
its correlation to related electronic transitions. This is done by using energy level
diagrams known as Tanabe-Sugano diagrams (T-S) diagrams76 (Figure 2.5).
Figure 2.5: Schematic of Tanabe-Sugano diagram for high-spin Fe3+ oxides.77
In T-S diagrams energies of the various electronic states E of each electron
configuration and the ligand field splitting parameters Δ are expressed in units of
Racah parameter B. E/B is plotted vs. Δ/B taking the ground state term of free ion as
zero.72 The Racah parameter B measures the electron-electron repulsion among
electrons in d orbitals.73 Using these diagrams it is possible to identify absorption
bands in UV-Vis spectra and calculate ligand field splitting parameter of related
electronic transitions.77
2.3 Photoluminescence Spectroscopy
2.3.1 Basic Principles
Photoluminescence is the spontaneous emission of light from a molecule or a
solid under optical excitation. Photoluminescence spectroscopy is a nondestructive
and extremely sensitive technique to probe electronic states of samples.78 When the
2 Spectroscopic Techniques
19
energy of incident light on a material is sufficient, the photons are absorbed and the
electrons are excited. Excited electrons return to their ground states by releasing the
excess energy following a radiative or nonradiative process (relaxation). In the case
of radiative relaxation the emitted light is called photoluminescence (PL).79
The photoluminescence can be divided into two principle types of transitions:
1) Fluorescence
2) Phosphorescence
Fluorescence and phosphorescence result from internal transitions before
relaxation to a ground state. The difference between them is best explained
considering spin multiplicities. The photon emitted in a transition between states of
the same spin multiplicity (e.g., singlet-singlet or triplet-triplet transitions) is called
fluorescence and the photon emitted in transition between states with different spin
multiplicity (e.g., triplet-singlet transition) is called phosphorescence.80 Fluorescence
has very short lifetimes of about 10-8 s to 10-4 s, whereas phosphorescence usually
has much longer life times in the range of 10-4 s to minutes.
Photoluminescence spectroscopy is widely used in the characterization of
surfaces because of its sensitivity to different electronic states located near surfaces
and interfaces. 78 The PL emission energy and intensity provides information about
the quality of surfaces and interfaces.
2.3.2 Fluorescence Quenching
In radiative transition the part of emission energy which is not dissipated by the
vibrational relaxations may still be reduced by other processes. For solid-gas
interfaces the PL intensity further can be decreased depending on the ability of
surrounding lattice or the surrounding gas-phase molecules to accept electronic
energies. This reduction in intensity is called quenching and can occur by different
mechanisms, such as dynamic and static quenching.
2 Spectroscopic Techniques
20
Dynamic quenching happens by deactivation of excited-state luminophore upon
contact with molecules in surrounding atmosphere. The molecules are called
quencher and are chemically not altered. A wide variety of molecules can act as
dynamic or collisional quenchers. Molecular oxygen is one of the examples with
widely spaced vibrational levels which can accept large quantum of electronic energy
and quench the fluorescence. In static quenching the luminophores and quenchers
form nonluminescent complexes and quenching occurs in the ground state.81
3 Experimental Details
21
3 Experimental Details
3.1 Sample Preparation
The list of samples produced for this work is given in Table 3.1. The Table also
indicates the synthesis methods used for the production of the samples and the
chapters in which the related results are discussed.
Table 3.1: List of metal oxide nanoparticle samples produced for this work, their
synthesis methods and related chapters.
Sample Synthesis Method Chapter
MgO Chemical vapor synthesis 4, 6, 7, 8, 10
Flame spray pyrolysis 5
ZnO Metal organic chemical vapor synthesis 4
Zn-Mg-O Flame spray pyrolysis 5
Fe-Mg-O Metal organic chemical vapor synthesis 6
In this chapter only the synthesis details of the MgO and ZnO samples
produced by chemical vapor synthesis (CVS) and Fe-Mg-O samples produced by
metal organic chemical vapor synthesis (MO-CVS) method are presented. The
experimental details of the samples produced by other methods like flame spray
pyrolysis (FSP) (MgO or Zn-Mg-O in chapter 5) or water-assisted transformation
(chapters 7 and 8) are given in related chapters.
3.1.1 Chemical Vapor Synthesis
Chemical vapor synthesis (CVS) is a so called “bottom-up” method to produce
nanoparticles from precursors in the gas phase. The method is called “MO-CVS”
when a metal organic compound is used as precursor. This technique allows the
production of pure and mixed crystalline metal oxide nanoparticles.
3 Experimental Details
22
3.1.1.1 MgO Synthesis
The CVS apparatus used for the production of MgO nanoparticle samples
consists of two quartz glass tubes which are mounted concentrically inside a
cylindrical furnace (Figure 3.1). The inner glass tube contains two ceramic ships with
Mg metal pieces (1 g of Mg in each ship). Heating of the furnace to a defined
temperature (T = 913 K) allows adjusting the Mg-vapor pressure. An argon gas flow
(QAr = 1000 sccm)1 is led through the inner tube to transport evaporated metal atoms
to the reaction zone where Mg and Ar gas mixture meets oxygen (QO2 = 900 sccm)
coming from the outer quartz glass tube. The exothermic oxidation reaction leads to a
bright stable Mg combustion flame in the reactor and MgO nanoparticles are formed
as a result of homogenous nucleation in the gas phase. A rotary vane pump keeps
the reactor system at a constant low pressure (p = 50 ± 2 mbar). Thanks to
continuous pumping, the residence time of nuclei within the flame remains short
enough to prevent substantial coarsening and coalescence. The MgO nanoparticles
are deposited downstream in a stainless steel net that is kept at the room
temperature.
Figure 3.1: Schematic diagram of the CVS reactor setup that is used for the
production of MgO nanoparticles. (MFC: mass flow controller; T: local operation
temperature; P: pressure gauge)
A by-pass system allows avoiding particle collection during uncontrolled
process conditions, i.e. heating and cooling phase. Therefore the product is only
collected at steady state process condition when the bypass is closed and the main
1 Q = volumetric flow rate
3 Experimental Details
23
way is opened. To finish the particle collection the furnace is turned off and cooled
with pressurized air, which is blown through the space between furnace and outer
glass tube.
3.1.1.2 ZnO Synthesis
ZnO nanoparticles were synthesized by the means of a two-hot-zone MO-CVS
reactor (Figure 3.2). The reactor system employed for this purpose consists of one
quartz glass tube, which is placed inside a heating coil (first heating zone) followed
by a ceramic tube furnace (second heating zone). The first heating zone of the tube
hosts a ceramic ship with 1 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) powder
(≥ 99.0%, Sigma-Aldrich), which is heated to T1 = 523 K to sublimate the precursor.
An oxygen gas flow (QO2 = 650 sccm) is mixed with the precursor in the gas phase
and transports the metal organic vapor to the second heating zone of the tube where
the tube furnace provides a temperature T2 = 1073 K inside the glass tube. At this
position of the tube, zinc precursor is decomposed and ZnO nanoparticles are formed
as a result of oxidation and homogeneous nucleation in the gas phase.
Figure 3.2: Schematic diagram of the MO-CVS reactor setup that is used for the
production of ZnO nanoparticles. (MFC: mass flow controller; T: local operation
temperatures; P: pressure gauge)
Continuous pumping keeps the residence time of nuclei within the second
heating zone of the tube short enough to prevent substantial coarsening and
coalescence. The ZnO nanoparticles are deposited downstream in a stainless steel
net that is kept at the room temperature. The total pressure (p = 15 ± 2 mbar), as well
3 Experimental Details
24
as flow rate and temperatures in the reactor are kept constant during the time of
nanoparticle collection.
3.1.1.3 Fe-Mg-O Nanocomposite Synthesis
For the production of Fe-Mg-O nanoparticles we developed a new MO-CVS
apparatus, which provides control over the concentration of iron in FeMgO
nanocomposite samples. The two-hot-zone reactor system (Figure 3.3) employed for
this purpose consists of two quartz glass tubes, which are mounted concentrically
inside a heating coil (first heating zone) followed by a ceramic tube furnace (second
heating zone).
Figure 3.3: Schematic diagram of the MO-CVS reactor setup that is used for the
production of Fe-Mg-O nanoparticles. (MFC: mass flow controller; T: local operation
temperatures; P: pressure gauge)
In the first heating zone the inner glass tube hosts a ceramic ship with 1 g of
iron (III) acetylacetonate (Fe(C5H7O2)3) powder (≥ 99.9%, Sigma-Aldrich), which is
heated to temperature T1 = 353 K, 363 K or 373 K to sublimate the iron precursor at
adjustable evaporation rates. An argon gas flow (QAr = 1200 sccm) is led through the
inner tube to transport the metal organic vapor to the second heating zone where the
furnace provides a temperature T2 = 913 K. At this position a ceramic ship with 1 g
Mg metal grains is positioned inside the inner tube. Here, the magnesium is
evaporated and mixed with iron precursor vapor. The vapor mixture is then
transported by the argon gas flow to the end of the inner glass tube. There the argon,
magnesium vapor and iron precursor vapor mixture meets oxygen (QO2 = 1200 sccm)
which is flowing through the outer glass tube. At this position of the reactor, the
3 Experimental Details
25
exothermic oxidation reaction leads to a stable Mg combustion flame which
decomposes the iron precursor and Fe-Mg-O nanoparticles are formed as a result of
homogenous nucleation in the gas phase. Because of continuous pumping, the
residence time of nuclei within the flame remains short enough to prevent substantial
coarsening and coalescence. The MgO nanoparticles are deposited downstream in a
stainless steel net that is kept at the room temperature. The total pressure (p = 70 ± 2
mbar), as well as argon and oxygen flow rates and T1 and T2 heating zones
temperatures are kept constant during the time of nanoparticle collection.
3.1.2 Thermal Annealing
After production, the nanoparticle powders were transferred from the reactor
into quartz glass cells (Figure 3.4). This step has to be performed via short time
(~15 min) of contact to air. The cells provide the condition for sample processing and
measurements in defined gas atmosphere and are used for: i) thermal activation of
the sample, ii) optical spectroscopy experiments and iii) sample storage.
Figure 3.4: Quartz glass cells used for thermal activation of the sample, optical
spectroscopic experiments and sample storage.
In the CVS apparatus the samples are produced at reduced pressures and in
controlled gas atmosphere. However, transfer of the samples from the closed reactor
into the quartz glass cell that occurs via short exposure to the ambient and
incomplete precursor decomposition in the case of MO-CVS result in different types
of contaminations. In order to guarantee decontaminated particle surfaces and
3 Experimental Details
26
remove organic residues the activation of the sample by thermal annealing in defined
gas atmosphere is necessary. This post-synthesis treatment can be carried out
before experiments aiming at the exploration of surface/interface properties of the
nanoparticles. The procedure of thermal annealing applied depends on sample
contamination. The here employed thermal annealing procedures are of two types:
1) annealing under vacuum condition and 2) annealing in the presence of oxygen.
Different modifications of these procedures are used by changing the final
temperature or the dwell times at each temperature.
3.1.2.1 Thermal Annealing in Vacuum
In a typical standard annealing program the cell containing as-synthesized
powder sample was first evacuated (p < 10-5 mbar) at room temperature and then
heated to T = 1173 K in 100 K-steps according to the program described in Table 3.2
Just shortly after reaching T = 1173 K sample was cooled down to T = 1123 K and
contacted by 10 mbar of oxygen for 10 minutes to remove organic contaminants.
After evacuation of the oxygen, temperature was again increased to T = 1173 K and
it was kept for 60 minutes at pressures less than 5·10-6 mbar. This thermal treatment
takes 6 to 8 hours (depending on vacuum pumps used and amount of annealed
MgO) and offers perfect conditions to have a clean metal oxide surface.
Table 3.2: Standard thermal annealing program used for the activation of MgO
nanoparticle samples.
temperature (K) rate (K /min) pressure (mbar) dwell time
(min)
373 5 < 1.0 · 10-5 * 0
473 5 < 1.0 · 10-5 * 0
573 5 < 1.0 · 10-5 * 0
673 10 < 1.0 · 10-5 * 0
773 10 < 1.0 · 10-5 * 0
873 10 < 1.0 · 10-5 * 0
973 10 < 1.0 · 10-5 * 0
1073 10 < 1.0 · 10-5 * 0
1173 10 < 1.0 · 10-5 * 0
1123 30 O2 pressure =10 10
1123 30 < 1.0 · 10-5 * 0
1173 30 < 5.0 · 10-6 60
*. Base pressure; before increasing to the next temperature, base pressure must be
reached.
3 Experimental Details
27
3.1.2.2 Thermal Annealing in the Presence of Oxygen
This post synthesis procedure is used for the samples produced with MO-CVS
reactors. High oxygen pressure was applied to remove carbon based contaminants
originating from synthesis. A modification of this procedure is outlined in Figure 3.5.
First the cell containing a nanoparticle powder was evacuated down to a pressure of
p < 10-5 mbar at room temperature.
Figure 3.5: Temperature profile (solid line, left ordinate scale) and applied oxygen
pressure (dashed line, right ordinate scale) during annealing treatment in the
presence of oxygen. (r: heating rate; td: dwell time).
The respective sample was then heated to T = 373 K at a rate = 2.5 K min-1,
held at this temperature for 15 min (dwell time, td) and then again subjected to
evacuation to p < 10-5 mbar. Further annealing steps are carried out at p = 650 mbar
of oxygen. The powder was stepwise heated in oxygen atmosphere to T = 473 K
(r = 5 K min-1, td = 15 min), T = 673 K (r = 10 K min-1, td = 30 min), T = 873 K
(r = 10 K min-1, td = 180 min) and T= 1173 K (r = 10 K min-1, td = 60 min).
3 Experimental Details
28
After each annealing step the sample was cooled to room temperature
(cooling time ≈ 30 min) followed by an evacuation (≈ 15 min) to a base pressure of
p < 10-5 mbar in order to remove water and CO2 as oxidation products.
3.2 UV-Vis Diffuse Reflectance Spectroscopy
UV-Vis diffuse reflectance measurements were carried out with a Perkin Elmer
Lambda 950 spectrometer by using an integrating sphere. In Figure 3.6 the
integrating sphere module of the diffuse reflectance setup is shown.
Figure 3.6: The optical design of the PerkinElmer integrating sphere module.
Adapted from Perkin Elmer Lambda 950 product catalogue.
3.2.1 Integrating Sphere
In this work an integrating sphere is used instead of the standard detection
module for UV/Vis absorption spectroscopy. The sample cuvette is placed behind the
sphere in sample holder. The beam reflected by the sample is reflected onto the
internal reflective surface of the sphere before reaching the detectors inside the
sphere. The sphere's internal surface is made of the polymer Spectralon®, which
offers high degree of diffuse reflectance approaching 100%. The use of Spectralon®
restricts the spectral range to 200 nm - 2500 nm.
The spectrophotometer is equipped with a double-beam sphere made of
Spectralon® with a diameter of 150 mm. The surface area of the ports (entrance or
exit passages for light beams) corresponds to 2.5% of the internal reflective surface
for the specified sphere. The detectors inside the sphere (a photomultiplier for the
3 Experimental Details
29
visible range and a PbS for the NIR) are protected against direct reflectance by
Spectralon® baffles. These baffles are essential for ensuring the accuracy of the
measurements. When measuring reflectance, a specular flap can be used to exclude
specular reflectance so that only diffuse reflectance is measured. For the
measurements done in this work the specular flap was removed to exclude specular
reflectance caused by the quartz cuvette.
3.2.2 Measurement
Before doing measurements the baseline was recorded using a standard
spectralon sample. Spectralon is a fluoropolymer, which has the highest diffuse
reflectance of any known material or coating over the ultraviolet, visible, and near-
infrared regions of the spectrum. This was performed to have a nominal 100%
reflectance collection as baseline. Then the measurement of the samples was carried
out from λ = 200 nm to 2000 nm. The cuvette containing the particles was located in
the sample holder when the entire quartz glass cell (Figure 3.4 left) was in a dark
environment. Figure 3.7 shows that higher reflectivity values are recorded under
vacuum conditions due to luminescence emission from the sample. The figure clearly
reveals how the presence of O2 is decreasing the fluorescence intensity of MgO in
comparison to measurement in vacuum of 5·10-6 mbar. To prevent
photoluminescence emission by the sample, the cell was filled with 10 mbar O2 as a
PL quencher.
Figure 3.7: Reflectance spectra of MgO samples measured in vacuum p < 5·10-6
mbar (a) and in the presence of 10 mbar O2 (b) in order to omit luminescence effects.
The spectra are recorded at T = 298 K.
3 Experimental Details
30
3.3 Photoluminescence Spectroscopy
For photoluminescence (PL) measurements a Fluorolog®-3 Model FL3-22
Luminescence Spectrometer was used. The double-grating excitation and emission
spectrometers of the instrument offer excellent performance in resolution, sensitivity,
and stray-light rejection. This system is perfect for highly scattering samples as
powders investigated in this work. The spectrometer is comprised of the following
components:
1) Excitation source which is a 450 W continuous wave xenon lamp
2) Double-grating monochromator in excitation and emission sides
3) Photomultiplier tube (PMT) as detector
3.3.1 Long-Pass Filters
Long-pass filters attenuate wavelengths shorter than a cut-on wavelength and
transmit longer wavelengths. For PL emission spectra acquisition carried out in this
work long-pass filters are used for two different cut-on wavelengths of λ = 295 nm
and λ = 395 nm on the emission side. The filters eliminate contributions of the first-
and second-order of the excitation light.
4 O2 Adsorption Dependent Photoluminescence Emission
31
4 O2 Adsorption Dependent Photoluminescence Emission
from Metal Oxide Nanoparticles
4.1 Abstract
Optical properties of metal oxide nanoparticles are subject to synthesis related
defects and impurities. Using photoluminescence spectroscopy and UV diffuse
reflectance in conjunction with Auger electron spectroscopic surface analysis we
investigated the effect of surface composition and oxygen adsorption on the
photoluminescence properties of vapor phase grown ZnO and MgO nanoparticles.
On hydroxylated MgO nanoparticles as a reference system, intense
photoluminescence features exclusively originate from surface excitons, the radiative
deactivation of which results in collisional quenching in an O2 atmosphere.
Conversely, on as-prepared ZnO nanoparticles a broad yellow emission feature
centered at hvEm = 2.1 eV exhibits an O2 induced intensity increase. Attributed to
oxygen interstitials as recombination centers this enhancement effect originates from
adsorbate-induced band bending, which is pertinent to the photoluminescence active
region of the nanoparticles. Annealing induced trends in the optical properties of the
two prototypical metal oxide nanoparticle systems, ZnO and MgO, are explained by
changes in the surface composition and underline that particle surface and interface
changes that result from handling and processing of nanoparticles critically affect
luminescence.
4.2 Introduction
Defect engineering belongs to the important challenges in the development of
functional particle systems. Moreover, control over defect populations in a particle
powder is required to endow it with new and desirable properties. The corresponding
approaches are particularly demanding since the generation, stabilization or
annihilation of defects with functional or unwanted properties needs to be monitored
along the entire process chain. This spans a wide range beginning with the
production of particle powders using a variety of different synthesis routes, to particle
processing and integration into the device and, ultimately, to device operation which
4 O2 Adsorption Dependent Photoluminescence Emission
32
corresponds to the exposure of materials to electric current, heat, radiation or
mechanical stress.
Motivated by the photoelectronic properties of ZnO nanomaterials and the rich
spectrum of related applications, there is a continuously growing number of
publications related to the research topic ‘‘ZnO nanoparticles and photoluminescence
properties’’. Recent research activities address the relationship between different
defect types and photoluminescence emission features.82–93 Although it is well
established that surfaces and interfaces dominate and control the properties of
nanomaterials,94 the chemical and physical nature of ZnO nanoparticles remains
unspecified in most cases. Considering the fact that minor changes in synthesis,
handling and processing can alter the surface properties of the particles, a
particularly unsatisfactory situation is created where an increasing number of
publications report discrepant results for the same nanoparticulate material.
In this paper we demonstrate that vapor phase grown ZnO nanoparticles, which
were grown under oxygen rich conditions, show bright photoluminescence emission
that is linked to oxygen interstitials in the near surface region. Characteristic
adsorbate-induced changes in band bending of semiconducting metal oxides
substantially enhance photoluminescence emission in O2 atmosphere. A powder of
ionic MgO nanoparticles with purely surface dependent optical properties, on the
other hand, represents a well-suited reference system in order to assess the impact
of the nanoparticle surfaces on photoluminescence intensity changes. This study
involves a semiconducting and an insulating oxide and underlines the determining
influence of sample history and, concomitantly, the nature of the nanoparticle
surfaces with regard to their photoluminescence properties. Moreover, it clearly
shows that differences in the nature and composition of the surrounding continuous
phase (vacuum, gas or solution) may lead to substantial variations in observed
photoluminescence intensities.
Bare and well-facetted nanoparticle surfaces that are free from any type of
surface adsorbate certainly represent an ideal starting point for related studies.
However, required sample activation approaches typically involve high temperature
treatment under vacuum. Under such conditions, metal oxide nanomaterials typically
4 O2 Adsorption Dependent Photoluminescence Emission
33
undergo substantial particle coarsening and coalescence due to their limited thermal
stability. As a result, most nanoparticles deteriorate and transform into ill-shaped
microcrystalline materials displaying very complex surface and interface
characteristics.5 The aim of the present study is to document the influence of oxygen
in the surrounding continuous phase on the properties of as-synthesized metal oxide
particles in comparison to those which were subjected to moderate annealing.
4.3 Experimental Section
4.3.1 Material Synthesis
For the production of MgO and ZnO nanoparticles we used chemical vapor
synthesis (CVS). The details of the MgO production technique are given
elsewhere.95,96 ZnO nanoparticles were synthesized by means of a two-hot-zone
CVS reactor (see Figure S4.1 in Supporting Information) The reactor system
employed for this purpose consists of one quartz glass tube, which is placed inside a
heating coil (first zone) followed by a ceramic tube furnace (second zone). The first
zone of the tube hosts a ceramic ship with zinc acetate dihydrate powder (≥ 99.0%,
Sigma-Aldrich), which is heated to T = 523 K to sublimate the precursor. An oxygen
stream (650 sccm) is mixed with the precursor in the gas phase and transports the
metal organic vapor to the second zone of the tube where the furnace provides
T = 1073 K inside the glass tube. At this zone, the precursor is decomposed and ZnO
nanoparticles form as a result of oxidation and homogeneous nucleation in the gas
phase. The total pressure (15 ± 2 mbar) as well as the flow rate and temperatures in
the reactor are kept constant during the time of nanoparticle collection.
4.3.2 Annealing
After production and short contact time with air, the nanoparticle powders were
transferred into quartz glass cells, within which thermal sample activation was
performed. Annealing treatment is used according to a defined procedure
(Supporting Information, Figure S4.2) for dehydration, dehydroxylation and removal
of carbon based surface contaminants. At the beginning, the cell containing
nanoparticle powder was evacuated to p < 10-5 mbar at room temperature. The
4 O2 Adsorption Dependent Photoluminescence Emission
34
respective sample was then heated to T = 373 K at a rate (r) of 2.5 K min-1, held at
this temperature for 15 min (dwell time, td) and then again evacuated to p < 10-5
mbar. Further annealing steps were carried out at p = 650 mbar of oxygen. The
powder was stepwise heated in an oxygen atmosphere to T = 473 K (r = 2.5 K min-1,
td = 15min) and T = 673K (r = 5 K min-1, td = 60 min). Before each new annealing step
fresh oxygen was added. After each annealing step the sample was cooled to room
temperature (cooling time ≈ 30 min) followed by an evacuation to a base pressure of
p < 10-5 mbar in order to remove water and CO2 as oxidation products (Supporting
Information, Figure S4.2).
4.3.3 Structure and Morphology
X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8
Advance diffractometer using Cu Ka radiation (λ = 154 pm). For transmission
electron microscopic measurements with a Phillips CM300 UT TEM operated at
300 kV, small amounts of the metal oxide powders were cast on a carbon grid just by
immersing the sample grid into the dry powder.
4.3.4 Spectroscopy
The Auger electron spectroscopy (AES) measurements have been performed in
a UHV chamber equipped with a Leo Gemini electron column and a hemispherical
electron energy analyzer (Omicron Nanotechnology/NanoSAM). During the
measurements the base pressure is below 3∙10-10 mbar. Auger spectra were acquired
at primary electron beam energy of 5 kV and a beam current of 3 nA in the constant
retardation ratio mode. The AES are acquired in the direct form, i.e. as N vs. E. Three
or four surveys are acquired in different areas of each sample. Each spectrum has
been taken in a specific area of the surface only once and the beam is blanked
afterwards before moving the sample stage to a different location. A scanned area
mode is used for the AES acquisition in order to spread the electron dose (a
maximum value of 1.2 x 10-2 C/cm2 for Mg) over a whole area of 117 x 88 μm2 and
thus to effectively reduce the local electron dose. This procedure guarantees
minimization of unwanted electron beam induced effects and to achieve spatially
averaged chemical information of the provided samples.97,98 Apart from the
4 O2 Adsorption Dependent Photoluminescence Emission
35
corresponding metal and oxygen elements, carbon has been detected in the samples
exposed to ambient air. The relative concentrations of each element cA can be
obtained by using the following formula:
where IA, IB and IC correspond to the peak areas for the three elements of interest
(O, C, Zn or Mg) or to the peak-to-peak intensities in the derivative spectrum. The
selected peaks for the oxides correspond to the main transitions KL2,3L2,3 for O and
Mg and L3M4,5M4,5 for ZnO.99,100 SA, SB and SC refer to the corresponding sensitivity
factors which were taken from reference 101. For proof of consistency the
concentration estimates have been derived from both the original spectra (via area
analysis) as well as from the correspondingly derived ones (via peak heights). The
peak areas were determined via linear background subtraction. The intensities (areas
or peak-to-peak heights) of the different elements in each survey were estimated and
the corresponding averaged values were determined and used to calculate the
concentrations according to the formula given above.
UV diffuse reflectance spectra were acquired at room temperature using quartz
glass cells with a Perkin-Elmer Lambda 950 spectrophotometer, equipped with an
integrating sphere. The reflectance spectra were converted into absorption spectra
using the Kubelka Munk transform procedure. For photoluminescence (PL)
measurements, a Fluorolog®-3 Model FL3-22 spectrometer with a continuous wave
450 watt Xenon arc lamp was used for excitation. The double-grating excitation and
emission spectrometers of the instrument offer excellent performance in resolution,
sensitivity, and stray-light rejection. This system is well-suited for strongly scattering
samples such as the nanoparticle powders investigated in this work. PL spectroscopy
was performed at room temperature using a quartz glass cell that guarantees
vacuum conditions better than 5∙10-6 mbar.
4 O2 Adsorption Dependent Photoluminescence Emission
36
4.4 Results
4.4.1 Structure and Particle Size
ZnO and MgO nanoparticle powders were characterized both directly after
synthesis and after controlled annealing at T = 673 K (SI, Figure S4.3). Figure 4.1
shows representative X-ray diffraction patterns (XRD) for ZnO and reveals that all
diffraction peaks are consistent with those of the wurtzite phase. No additional
crystalline phases were observed. Annealing of ZnO nanoparticles results in a
narrowing of the diffraction features widths, which points to a volume increase of the
coherently scattering solid and, therefore, to particle coarsening.
Figure 4.1: X-ray diffraction patterns of ZnO nanoparticles (a) after synthesis; (b)
after annealing in oxygen (T = 673 K, p = 650 mbar O2). Vertical lines correspond to
the standard XRD pattern of wurtzite ZnO (JCPDS No. 36-1451).
From the full width at the half maximum (FWHM) of the diffraction peaks102,103
the average crystallite sizes were calculated to be 6 ± 1 nm and 10 ± 1 nm for ZnO
after synthesis and after annealing treatment, respectively (details of the procedure
are found in the Supporting Information). Figure 4.2 shows TEM images of the metal
oxide nanoparticles after synthesis (Figure 4.2 a and c) and after annealing (Figure
4.2 a and d). All samples are relatively homogeneous in terms of particle size and
shape (see also Figure 4.3).
4 O2 Adsorption Dependent Photoluminescence Emission
37
The powders of vapor phase grown nanoparticles can be characterized as
ensembles of loosely agglomerated, more (MgO) or less (ZnO) facetted
nanoparticles. TEM images also show comparable particle sizes for both types of
samples.
Figure 4.2: TEM images of (a) ZnO nanoparticles after synthesis; (b) ZnO
nanoparticles after annealing in oxygen (T = 673 K, p = 650 mbar O2); (c) MgO
nanoparticles after synthesis and (d) MgO nanoparticles after annealing in oxygen
(T = 673 K, p = 650 mbar O2).
After vacuum annealing at T = 673 K the particle size distribution in the ZnO
nanoparticle powder remains narrow and peaks at an average particle size of 10 nm.
This value is perfectly consistent with the average crystallite domain size (see
above). MgO nanoparticles, on the other hand, exhibit a higher thermal stability. Their
average crystallite domain size of d = 6 ± 1 nm does not change upon annealing (SI,
Figure S4.4). While MgO is known for its low sinterability even at high temperatures,
ZnO exhibits a high sinterability with high grain growth even at lower temperatures.
These well-established phenomena arise from differences in the interface energetics
4 O2 Adsorption Dependent Photoluminescence Emission
38
between the two metal oxides.104 Supply of thermal energy to the metal oxide
nanoparticle ensemble initiates ion diffusion and allows the grains to coarsen and to
reorganize towards thermodynamic equilibrium. As observed for vapor phase grown
ZnO nanoparticles, annealing temperatures higher than T = 673 K can induce
significant particle size disproportionation effects (Figure 4.3).5
Figure 4.3: Cumulative particle size distribution plots for ZnO nanoparticle after gas
phase synthesis and after annealing to T = 673 and T = 873 K under vacuum and a
dry oxygen atmosphere.
4.4.2 Surface Composition
After vacuum annealing to T = 673 K and at p < 10-5 mbar metal oxide
nanoparticles remain partially hydroxylated and may also retain residual surface
carbonates or adsorbed CO2. Previous photoelectron spectroscopy measurements
on MgO nanoparticles revealed that – irrespective of vacuum annealing procedures,
which were applied ex situ and prior to the measurement – MgO nanoparticles
instantaneously adsorb carbon dioxide and other carbonaceous species from the
air.105 On the highly dispersed MgO with enhanced surface basicity as compared to
ZnO this process occurs within a few minutes of exposure to air and the surface
contamination level is typically in the range between 20 to 30%. Systematic FT-IR
studies on highly dispersed MgO samples reveal that only after vacuum annealing
induced desorption at temperatures higher than T = 873 K, the concentration of
hydrogen bonded neighboring surface OH groups becomes negligibly small.106–109
4 O2 Adsorption Dependent Photoluminescence Emission
39
Since the present study starts with as-synthesized nanoparticles we expect the
coexistence of non-crystalline surface hydroxides, physisorbed water as well as
surface hydroxyls. After annealing to T = 673 K, i.e. above temperature of Mg(OH)2
conversion into MgO, only chemisorbed hydroxyls remain.106
The contaminants on ZnO nanoparticle powders mostly originate from the
synthesis process, i.e. the thermal decomposition of zinc acetate dihydrate
[Zn(CH3COO)2•2H2O].110 Macroscopically, the lightly ochre color of the as-
synthesized ZnO nanoparticle powder (SI, Figure S4.3) changes upon annealing to
T = 673 K to white color. This change in color indicates that the organic remnants
from the precursor material became decomposed and eliminated during annealing.
Attenuated total reflection (ATR)-IR spectroscopic measurements of ZnO
nanoparticle powders in air (spectra shown in Figure S4.5) reveal weak absorption
related to surface hydroxyls and carboxylates and substantiate this point. A recent
FT-IR study on ZnO nanoparticles111 which were derived from similar precursor
materials revealed that IR active and surface adsorbed CO2 112,113 exhibit thermal
stabilities up to T = 1073 K.
We characterized the surface composition and stoichiometry of the ZnO
nanoparticles with Auger electron spectroscopy in more detail. The estimated
concentrations of as-synthesized ZnO nanoparticles as well as annealed ZnO
nanoparticle powders are provided in Table 4.1.
Table 4.1: Surface composition of ZnO nanoparticles as determined by Auger
electron spectroscopy
Sample Relative concentration
Zn O C
ZnO as-synthesized 0.38 0.57 0.05
ZnO annealed 0.43 0.51 0.05
A relative carbon concentration of 0.05 was found in both cases. The results in
Table 4.1 suggest that the oxygen content exceeds that of Zn in both samples and is
higher in the as-synthesized sample. Comparison of the Auger spectra (not shown)
with those in the literature reveals a good agreement in energy and position of the
4 O2 Adsorption Dependent Photoluminescence Emission
40
respective features. For a reliable determination of the ZnO
nanoparticles’ stoichiometry we used the atomic ratio of O to Zn (i.e. the ratio of O
intensity to Zn intensity corrected by their respective sensitivity factors,
(IO/SO)/(IZn/SZn)) and compared these values with data reported for different faces of
atomically clean ZnO single crystals114 (SI, Table S4.1). As a conclusion, the
annealed ZnO nanoparticle samples were found to correspond to a stoichiometric
compound, while the as-synthesized ZnO contains excess oxygen above the
stoichiometric composition.
4.4.3 Optical Properties
4.4.3.1 UV-Vis Diffuse Reflectance Spectroscopy
Before as well as after annealing the MgO nanoparticle powders adopt a white
slightly bluish opalescent color. The related UV-Vis diffuse reflectance spectra show
absorption features at energies above hv = 5.0 eV and hv = 5.4 eV for vacuum
annealed and as-synthesized MgO nanoparticle powders, respectively. The shift in
the absorption threshold is attributed to the annealing induced transformation of
photoluminescence inactive surface hydroxides – present on the as-synthesized
nanoparticle sample – into the oxide as well as to the desorption of surface adsorbed
water from the particle surface. A fraction of low coordinated surface elements
become uncovered and addressable by light excitation with sub band gap energy.26
In comparison to MgO as an insulator – the band gap E = 7.8 eV corresponds
to a wavelength below λ = 200 nm – the band gap of ZnO (E = 3.4 eV) corresponds
to the absorption threshold at λ = 370 nm and is clearly observable in the spectra of
ZnO (Figure 4.4 a after synthesis and Figure 4.4 c after annealing).
4 O2 Adsorption Dependent Photoluminescence Emission
41
Figure 4.4: UV-Vis diffuse reflectance spectra of (a) ZnO after synthesis; (b) MgO
after synthesis; (c) ZnO after annealing in oxygen (T = 673 K, p = 650 mbar O2);
(d) MgO after annealing in oxygen (T = 673 K, p = 650 mbar O2). The spectra were
acquired at T = 298 K and in the presence of 10 mbar O2 in order to omit
luminescence effects. The dashed vertical lines indicate the excitation energies
selected for the photoluminescence emission measurements.
4.4.3.2 Photoluminescence Spectroscopy
We chose the excitation energies for the two metal oxides on the basis of the
most intense PL emission intensities and used hvExc = 4.6 eV (λExc = 270 nm) as an
excitation energy for ZnO and hvExc = 5.2 eV (λExc = 240 nm) as an excitation energy
for MgO (Figure 4.4). Whereas the respective excitation energy for ZnO exceeds the
optical band gap, the 5.2 eV excitation light used for MgO exclusively addresses
localized defect states related to coordinatively unsaturated surface ions or
interfaces.30,115–118 Figure 4.5 compares the PL emission spectra obtained on ZnO
4 O2 Adsorption Dependent Photoluminescence Emission
42
under vacuum (p < 10-5) and in an oxygen atmosphere (p = 10 mbar O2). Figure 4.5 a
corresponds to a ZnO powder sample after synthesis. Spectrum acquisition under
vacuum reveals a weak UV luminescence band centered at hvEm = 3.3 eV
(λEm = 376 nm).
Figure 4.5: Photoluminescence (PL) spectra of (a) ZnO after synthesis; (b) ZnO after
annealing in oxygen (T = 673 K, p = 650 mbar O2); upon excitation a hvExc = 4.6 eV,
the respective PL emission spectra are recorded either under vacuum (p < 10-5 mbar)
or in an O2 atmosphere (10 mbar) at T = 298 K.
In addition, there is a broad emission feature with a maximum at hvEm = 2.1 eV
(λEm = 590 nm) and of only small intensity. In an oxygen atmosphere the intensity is
enhanced, while the band in the UV region becomes completely extinguished. Figure
4.5 b shows the spectra for annealed ZnO nanoparticles. Both spectra –irrespective
of whether they were acquired under vacuum or in an oxygen atmosphere – do not
show any UV emission band. In comparison to the as-synthesized sample, they show
a red-shifted visible band centered at hvEm = 2 eV (λEm = 630 nm) of by a factor of 3-4
reduced intensity.
As the MgO nanoparticles have been subjected to the same annealing
procedure, their surfaces remain hydroxylated. Despite the fact that after synthesis
most coordinatively unsaturated surface elements remain covered by adsorbates, the
respective particle system already shows substantial photoluminescence (Figure
4.6 a), such as the band centered at hvEm = 3.2 eV (λEm = 390 nm). Vacuum
annealing at T = 673 K leads to an intensity enhancement by a factor of about five. In
4 O2 Adsorption Dependent Photoluminescence Emission
43
contrast to the PL emission effects observed for ZnO, the MgO related bands are
perfectly quenched in the presence of gaseous oxygen (see blue in Figure 4.6).
Figure 4.6: Photoluminescence (PL) spectra of (a) MgO nanoparticles after
synthesis; (b) MgO nanoparticles after annealing in oxygen (T = 673 K, p = 650 mbar
O2). Samples are excited at hvExc = 5.2 eV and acquired at T = 298 K either under
vacuum (black line, p < 10-5 mbar) or in an O2 atmosphere (blue line, 10 mbar).
4.5 Discussion
The effect of O2 adsorption on the photoluminescence (PL) emission properties
of two prototypical metal oxides with high and comparable surface-to-volume ratios is
in the focus of this study. As a first important conclusion, the nature of the
surrounding gas phase has a substantial effect on the photo-excited states formed
inside a nanoparticle powder. Moreover, oxygen adsorption effects can disclose
valuable information about the location of underlying defects.119,120 Table 4.2
summarizes the two opposite trends observed for O2 adsorption on the PL emission
intensity of insulating MgO nanoparticles, on the one hand, and semiconducting ZnO
nanoparticles, on the other hand.
Table 4.2: O2 pressure dependent photoluminescence emission trends in metal
oxide nanoparticle systems
hvExc / eV hvEm/ eV Intensity change with O2 partial pressure Ref.
ZnO 4.6 2.1 ↑ This study
MgO 5.2 3.2 ↓ 115
4 O2 Adsorption Dependent Photoluminescence Emission
44
The ZnO photoluminescence emission at hvEm = 2.1 eV (λEm = 600 nm)
corresponds to yellow light and – when observed on ZnO nanostructures – is
attributed to deep trap states related to oxygen interstitials.84,92,121,122 DFT stability
diagrams indicate that oxygen interstitials represent stable defects in ZnO structures
under oxygen rich conditions.85,122 A recent STM luminescence spectroscopy study
on atomically clean ZnO thin films92 has revealed that an emission feature at
hvEm = 2.1 eV (λEm = 595 nm) is subject to the growth conditions in the presence of
excess of atomic oxygen.
We observe a positive PL intensity dependence on the presence of gaseous
oxygen (Figure 4.5 a). The O2 adsorption effect proves the underlying defect’s
location in the near surface region, i.e. in the range of the depletion layer of the
semiconductor particle, where adsorbate induced surface potential changes and
band bending become active.120 While molecular oxygen acts for surface excited
states of MgO as a photoluminescence quencher (see below), the absence of PL
quenching in the case of ZnO nanoparticles rules out that the underlying excitonic
transition is localized at distinct defects direct at the nanoparticle surface.
Materials specific issues need to be included into the discussion of the optical
ZnO nanoparticle properties: the PL emission effect is strongest on the
as-synthesized sample derived from the thermal decomposition of Zn acetate
dihydrate.123 On such samples, AES analysis points to a surface contamination with
carbon that roughly corresponds to 5% (Table 4.1). As the nanoparticles emerge
from the oxidative decomposition of an oxygen rich Zn precursor, we assume that
they exhibit a surplus in oxygen. Taking into account the measurement uncertainty of
the quantitative Auger analysis, the as-synthesized ZnO particles are in fact enriched
in oxygen above the stoichiometric composition (SI, Table S4.1), whereas those
which – prior to the AES analysis – had been annealed to T = 673 K correspond to a
stoichiometric compound. The oxygen excess may also stem from oxygen containing
adsorbates such as surface carbonates or carboxylates. The photoluminescence
fingerprint, i.e. the emission band in the yellow light range, however, points to oxygen
interstitials inside the lattice of the vapor phase grown nanoparticles. Annealing to
T = 673 K initiates particle coarsening and, concomitantly, leads to crystallite domain
size increase from 6 ± 1 nm to 10 ± 1 nm (Figure 4.2 and Figure 4.3). This reveals
4 O2 Adsorption Dependent Photoluminescence Emission
45
that ion mobility and mass transfer are significant at these temperatures. We
therefore attribute the PL emission intensity decrease in Figure 4.5 b to the thermally
induced annihilation of growth related lattice interstitials. MgO nanoparticles are
completely different in this respect. As an ionic insulator and a nonreducible metal
oxide we can expect a stoichiometric compound exhibiting a substantially higher
thermal stability with regards to sintering and in comparison to ZnO (Figure 4.1,
Figure 4.2, Figure 4.3 and Figure S4.2).
In ZnO photogenerated electrons and holes either recombine radiatively by a
direct band-to-band recombination mechanism producing the UV emission band
(Figure 4.5 a) or via a trap assisted mechanism upon emission of photons having
less energy than the optical band gap.
The effect of oxygen on the intensity of ZnO nanoparticles can be consistently
explained by band bending on ionic semiconductors.120 Adsorption of respective
acceptor molecules (Figure 4.7) enhances surface band bending and drives
photogenerated holes into the surface near region.121 We suggest that subsequent
hole trapping at oxygen interstitials as deep trap states enforces their recombination
with photogenerated electrons yielding yellow photoluminescence emission.
For powders of entirely dehydroxylated MgO nanocubes, it is well-established
that two absorption bands – far below the bulk absorption threshold of MgO
(7.8 eV) – are associated with corner (4.6 eV) and edge sites (5.2 eV).118 In addition,
two closely spaced photoemission bands at 3.2 eV and 3.4 eV are linked to the
photoexcitation of corners and edges, respectively.124,125
4 O2 Adsorption Dependent Photoluminescence Emission
46
Figure 4.7: Schematics of the energy band diagram (not to scale) for ZnO
nanoparticles after synthesis containing oxygen interstitials. The vertical arrows in
downward direction depict the radiative recombination process that produces the PL
emission band at 2.1 eV (λ = 600 nm, Figure 4.5). O2 adsorption (right panel)
enhances surface band bending and drives photogenerated holes into the surface
near region. Subsequent hole trapping by deep trap states such as oxygen
interstitials Oi enforces trap assisted recombination and yields yellow
photoluminescence emission.
As an entirely new insight from this study, hydroxylated MgO particle systems,
which were investigated right after synthesis and prior to any activation treatment
under vacuum, show appreciable surface dependent photoluminescence. Consistent
with the effect of hydroxyls on the electronic structure of MgO surface elements,126
the respective emission feature with a maximum at hvEm = 3.2 eV is red-shifted in
comparison to those observed on adsorbate free particle surfaces. While in the case
of ZnO where the PL emission originating from surface near region is only indirectly
affected by the adsorption of electron acceptors or donors, the excitonic properties of
MgO nanoparticles involve localized excited states and are exclusively surface
related.
The ZnO precursor used for this study, i.e. Zn(CH3COO)2·2H2O, is widely
employed for ZnO nanoparticle synthesis in the gas phase as well as in
solution.127-133 The majority of these studies lack information on synthesis related
remnants and their potential influence on the optical material properties. This study
also involves a detailed AES analysis and shows that at least a surface fraction of 5%
4 O2 Adsorption Dependent Photoluminescence Emission
47
related adsorbates survives high vacuum treatment at T = 673 K. While these
contaminants do not seem to affect the properties of the photoluminescing
nanoparticles, molecular oxygen in the gas phase does. It must be concluded that
every change in its sticking properties as well as in its concentration in the
surrounding continuous phase critically affects the photoluminescence emission
yield, irrespective of whether the surface excited state is directly located at the
surface (MgO) or in the depletion layer underneath the surface of the semiconductor
(ZnO).
Photoluminescence is a well-suited spectroscopic technique to perform
adsorption studies on semiconducting metal oxide particle systems.134–136 Thus, a
systematic investigation of synthesis related additives and contaminants belongs to
the characterization challenges associated with the identification of defects and other
material specific factors that determine the photoelectronic properties of ZnO
nanostructures. Irrespective of whether one deals with semiconducting or insulating
metal oxide nanoparticles, a reliable discussion of their photoluminescence
properties always requires us to address the nature and composition of the particle
interfaces as well as the composition of the surrounding continuous phase. This is
particularly important for defect engineering and for the stabilization of derived
functional properties in nanomaterials where synthesis and processing matters.
4.6 Conclusions
This work compares the photoluminescence of two prototypical metal oxide
nanoparticle systems with identical surface-to volume ratios, ZnO as an ionic
semiconductor and MgO as an ionic insulator. As inferred from Auger electron
analysis and photoluminescence emission, the growth of ZnO nanoparticles under
oxygen rich conditions generates oxygen interstitials which act as deep traps and
assist in radiative charge carrier recombination upon emission of yellow light
(hvEm = 2.1 eV). The presence of oxygen in the surrounding continuous phase was
found to have a critical and – for the two metal oxides – opposite influence on the
measured photoluminescence intensities. While it quenches photoemission from
surface excited states on MgO nanoparticles, it substantially enhances the emission
intensity on ZnO because of adsorbate-induced band bending across the
4 O2 Adsorption Dependent Photoluminescence Emission
48
semiconductor interface. This study underlines that resolving controversies in the
reported optical properties of technically relevant metal oxide nanoparticles requires
a more complete documentation of structural and compositional properties of bulk
and interfaces and – in particular – the address of their photoluminescence
determining surface property changes.
4 O2 Adsorption Dependent Photoluminescence Emission
49
4.7 Supporting Information
4.7.1 Schematic of the Reactor
Figure S4.1: Schematic of the reactor used for production of ZnO nanoparticles.
4.7.2 Annealing Treatment Procedure
Figure S4.2: Temperature profile (solid line, left ordinate scale) and applied oxygen
pressure (dashed line, right ordinate scale) during annealing treatment.
4 O2 Adsorption Dependent Photoluminescence Emission
50
4.7.3 Photos of ZnO and MgO Powders
Figure S4.3: Photos taken from powder samples after synthesis: a) ZnO, b) MgO;
and after annealing in oxygen (T = 673 K, p = 650 mbar O2): c) ZnO, d) MgO.
4.7.4 Analysis of the XRD Reflexes Widths
Pseudo-Voigt functions were used to determine the full-width at half maximum
(FWHM) of the main reflexes and the average crystallite sizes were calculated with
the Scherrer equation102:
D = Kλ / [W cos(θ)]
K is a constant which depends on the particle morphology and varies from 0.89 to
1.39 rad. Here K = 1 was used, which corresponds to an average volume of the
apparent size D independently of a particular morphology.103 λ is the wavelength of
CuKα radiation (in nm), W is the full width at half-maximum (FWHM in radian), and θ
is the diffraction angle (deg.).
4 O2 Adsorption Dependent Photoluminescence Emission
51
4.7.5 Powder XRD of MgO Nanoparticles
Figure S4.4: Powder XRD data of MgO nanoparticles after synthesis and after
annealing to T = 673 K.
4.7.6 FT-IR Spectroscopy
The infrared spectra of the samples were measured by the attenuation total
reflection (ATR) technique using a Varian FTS-3100 spectrometer. The FT-IR
experiments were performed under ambient conditions. A small amount of sample
powder was casted on the ATR crystal. A total of 64 scans were accumulated for
each spectrum to obtain a reasonable signal to noise ratio with a spectral resolution
of 4 cm-1.
Figure S4.5: Infrared spectra of a) Zinc acetate dihydrate (precursor used to
synthesize ZnO); b) ZnO nanoparticles after synthesis; c) ZnO nanoparticles after
subsequent annealing in oxygen (T = 673 K, p = 650 mbar O2). All the spectra were
recorded in air and at room temperature.
4 O2 Adsorption Dependent Photoluminescence Emission
52
ATR-Infrared spectroscopy measurements were performed to investigate the
potential presence of synthesis related organic remnants or surface adsorbed
species which originate from oxidation of the precursor during annealing. An IR
spectrum of the precursor clearly shows absorption peaks between 1110 cm-1 and
1730 cm-1 that are attributed to the fingerprint of the stretching modes of the acetate
groups (COOH).137 In addition to the dihydrate of the precursor salt, also water from
ambient adsorbs on the sample powder and results in a broad absorption feature
between 2600 cm-1 and 3550 cm-1. It is noticeable that for the sample after synthesis
all the absorption band intensities decrease upon decomposition and oxidation
reaction of the precursor at 1073 K which corresponds to the temperature of the flow
reactor during ZnO synthesis. All features are completely removed from ZnO sample
in the course of annealing. Since the ATR-IR studies were carried out in the ambient
but shortly after synthesis and annealing treatment, the above results indicate that
the density of the acetate and hydroxyl groups remarkably decreases by synthesis
and post-treatment conditions applied.
4.7.7 Surface Composition Analysis by Auger Electron Spectroscopy
Before quantification and consistent with reference 101, all the spectra in this
study have been divided by the kinetic energy E. Since the intensity/ energy
response function (IERF) for AES instruments operated in the constant retardation
ratio is proportional to E∙Δ(E) ,i.e. proportional to the product of the kinetic energy by
the detector efficiency Δ(E) [see for example 101], an additional correction to E-1 is
necessary. This has been checked by using it with AES of a MgO sample previously
annealed at around T = 1073 K in the UHV system (to avoid the presence of
contaminations) and following the evaluation procedure with this stoichiometric
compound. A minor correction over the factor E-1 was necessary in the O region of
E-0.985 and none in the Mg, validating essentially the procedure. These small
corrections were included in the evaluations for ZnO in Table S4.1 though.
4 O2 Adsorption Dependent Photoluminescence Emission
53
Table S4.1 compares the atomic O/ Zn ratios for the ZnO nanoparticle characterized
in this study with results obtained on different atomically clean ZnO single crystals
faces reported in reference.114,138
4.7.8 Photoluminescence spectroscopy
Figure S4.6: Photoluminescence (PL) spectra of a) ZnO after synthesis; b) ZnO after
annealing in oxygen (T = 673 K, p = 650 mbar O2); upon excitation a hvExc = 5.2 eV,
the respective PL emission spectra are recorded either under vacuum (p < 10-5 mbar)
or in an O2 atmosphere (10 mbar) at T = 298 K.
Table S4.1
sample atomic ratio
O/Zn
single crystal ZnO (from Ref.114)
Prism Face 1.15
Zinc Face 1.05
Oxygen Face 1.25
annealed ZnO Derivative analysis 1.29 ± 0.06
Area analysis 1.19 ± 0.14
as-synthesized ZnO Derivative analysis 1.41 ± 0.04
Area analysis 1.50 ± 0.12
4 O2 Adsorption Dependent Photoluminescence Emission
54
Figure S4.6 shows the PL emission spectra of ZnO sample obtained upon
excitation with hvExc = 5.2 eV (λExc = 240 nm). Figure S4.6 a and b indicate for ZnO
that excitation at hvExc = 5.2 eV produces qualitatively the same emission feature as
excitation at hvExc = 4.6 eV (λExc = 270 nm).
Figure S4.7: Photoluminescence (PL) spectra of a) MgO after synthesis; b) MgO
after annealing in oxygen (T = 673 K, p = 650 mbar O2); upon excitation a hvExc = 4.6
eV, the respective PL emission spectra are recorded either under vacuum (p < 10-5
mbar) or in an O2 atmosphere (10 mbar) at T = 298 K.
Figure S4.7 a reveals the emission spectrum for well dispersed MgO
nanoparticles just after synthesis. Whereas fully dehydroxylated MgO nanocubes
excitation at hvExc = 4.6 eV (λExc=270 nm) leads to corner excitation and produces
one PL emission band centered at hvEm= 3.3 eV (λEm = 380 nm), here one excitation
process contributes to the photoluminescence emission. Subsequent annealing
treatment (T = 673 K and p = 650 mbar O2) on MgO changes the shape of spectra
due to partial dehydroxylation of the sample surface and a band centered at
λEm = 400 nm is present (Figure S4.7 b). Figure S4.7 a and b both prove that unlike
ZnO, PL emission of MgO is quenched in O2 atmosphere.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
55
5 Bulk and Surface Excitons in Alloyed and Phase-
Separated ZnO-MgO Particulate Systems
5.1 Abstract
The rational design of composite nanoparticles with desired optical and
electronic properties requires the detailed analysis of surface and bulk contributions
to the respective overall function. We use flame spray pyrolysis (FSP) to generate
nanoparticles of the ternary Zn-Mg-O system the compositions of which range from
solid solutions of Zn2+ ions in periclase MgO to phase separated particle mixtures
which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The
structure and composition of the composite ZnxMg1−xO (0 ≤ x ≤ 0.3) particles are
investigated using X-ray diffraction and high-resolution transmission electron
microscopy, whereas UV diffuse reflectance and photoluminescence (PL)
spectroscopy are used for the investigation of their optical properties. Vacuum
annealing has been carried out to track the effects of stepwise elimination of surface
adsorbates on the photoexcitation and PL emission properties. We demonstrate that
for Zn0.1Mg0.9O particles, the admixed ZnO suppresses the MgO specific surface
excitons and produces a PL emission band at λEm = 470 nm. Although gaseous
oxygen partially reduces the emission intensity of hydroxylated particles, it leads to
entire quenching in completely dehydroxylated samples after vacuum annealing at
T = 1173 K. Consequently, surface hydroxyls at the solid-gas interface play a
significant role as protecting groups against the PL-quenching effects of O2. The
obtained results are relevant for the characterization of ZnO-based devices as well as
for other metal oxide materials where the impact of the surface composition on the
photoelectronic properties is usually neglected.
5.2 Introduction
Composite nanoparticle systems are increasingly important for applications that
rely on the controlled synergistic combination of single-component properties. To
understand their property changes during synthesis, processing and under operation
conditions in a device, the rational development of functional nanomaterials requires
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
56
careful evaluation of the physical properties in the bulk and at the interfaces.
Magnesium oxide and zinc oxide are very distinct materials with regard to their
crystallographic and electronic structures. The photoexcitation of highly dispersed
MgO and other alkaline earth oxides, for instance, leads to the generation of surface
excitons that depend on the chemical composition of the particle surface, and
therefore, can be employed as surface probes.26,32 On the other hand, the
semiconductor ZnO (bandgap Ebg ≈ 3.4 eV) with its bulk luminescence properties has
attracted great attention because of its potential for UV light-emitting diodes (LEDs),
lasers and various other optoelectronic applications.84,139,140 Moreover, admixing ZnO
into MgO (Ebg ≈ 7.6 eV) provides means for engineering the bulk electronic
structure.35,49,50 Most of related studies on such particle systems are dedicated to
ZnO wurtzite structures with small amounts of MgO admixed, whereas considerably
fewer studies report periclase MgO doped with ZnO. A promising aspect of such
composites is related to the metastable cubic ZnO phase53 that is expected to be
compatible for p-type doping.141 Different synthesis methods including the sequential
implantation of Zn+ and O+ ions in single-crystalline MgO,55 the calcination of
polymer/metal salt complexes56 or reactive electron beam evaporation142 have been
employed for the generation of cubic ZnO structures. But detailed synthesis and
characterization studies on powders of well−dispersed ZnxMg1−xO nanocrystals are
scarce, because of the lack of appropriate preparation techniques and reliable
surface characterization approaches, which are crucial to understand the material
properties related to the gas−solid interface.
In a previous work we have shown that monocrystalline ZnxMg1−xO (x denotes
the molar ratio between Zn and (Zn+Mg)) nanocubes of exceptional regular cubic
shape and edge lengths below 25 nm can be produced by chemical vapor synthesis
(CVS).12 In line with ab initio calculations, the annealing induced Zn2+ segregation
into low coordinated surface sites of MgO nanocubes was tracked with UV diffuse
reflectance, FT−IR and PL spectroscopy. We observed completely new PL emission
features which were perfectly quenched upon exposure to gaseous oxygen,
demonstrating that corresponding excitation and subsequent radiative deactivation
processes are directly linked to excitons formed at the surface of the composite
nanocubes. Higher ZnO loadings with concentrations above the solubility limit of Zn2+
in MgO are expected to lead to particle systems where annealing induced phase
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
57
separation143 would generate highly dispersed ZnO deposits in contact with thermally
stable MgO based support particles. Such an approach would provide particulate
model systems where a systematic comparison between photoexcited surface states
on MgO as well as electronic transitions induced in ZnO can provide substantial
insights into stability and surface electronic structure of pure and composite ZnO
nanomaterials. However, as a major shortcoming of the direct combustion of Zn and
Mg vapors (CVS), an upper concentration limit of approximately 12 at % ZnO results
from the fact that during the combustion process Zn vapor cools down the flame and
in this way prevents the reproducible metal combustion over longer periods of time.
For the realization of composite ZnO/MgO systems with higher Zn2+ concentrations,
longer production times and higher yields, Zn vapor needs to be added in a more
effective and controllable way to the combustion zone. As an alternative approach
the versatile flame spray pyrolysis process can be scaled up to production
rates of kg h−1.144–146 ZnO and MgO nanoparticles were successfully produced with
flame spray pyrolysis in the past. However, there exists only one report on the flame
spray synthesis of ZnxMg1−xO nanoparticles which focuses on Mg−doping of ZnO and
is limited to the description of basic powder properties.147
This paper has three major objectives: first, we want to explore the potential of
flame spray pyrolysis (FSP) with respect to the generation of particles of the ternary
Zn-Mg-O system the compositions of which range from solid solutions of Zn2+ ions in
periclase MgO12 to mixtures where phase separation into a MgO rich periclase and a
ZnO rich wurtzite phase occurs. Second, we want to study the influence of annealing
induced surface dehydroxylation and removal of surface contaminants on the surface
electronic structure of ZnO based particle systems. As shown in previous papers32,148
the in depth characterization of interfacial effects requires treatment and
measurements of nanoparticle powders in defined gas atmospheres such as high
vacuum (p < 1∙10−5 mbar) or oxygen atmosphere. The third objective of this study
aims at the identification of potential synergistic effects between ZnO and MgO that
originate from surface and bulk doping, as well as from segregation and phase
separation effects.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
58
5.3 Experimental Section
5.3.1 Chemicals
Anhydrous magnesium(II) acetylacetonate (Strem Chemicals, 98%) and zinc(II)
naphthenate (Strem Chemicals, 65% in mineral spirits in naphthenic acid) were used
as received as the doped MgO. A mixture of xylene (BDH, ≥ 98.5%) and ethanol
absolute (RDH, ≥ 99.8%) was used as solvents for preparing the precursor solutions.
To increase the Mg(II) concentration in the precursor solutions, 2-ethylhexanoic acid
(RDH, ≥ 99%) was used to dissolve the Mg(II) salt.
5.3.2 FSP Synthesis
Zinc-doped MgO (ZnxMg1−xO) powders were synthesized in a flame spray
reactor. Detailed description of the setup can be found elsewhere.149 For the
preparation of the precursor solutions, 56.8 g of Mg(II) acetylacetonate was dissolved
in a mixture of 2-ethylhexanoic acid (167.2 mL), ethanol absolute (100.0 mL) and
certain amount of xylene, to make a final volume of 500.0 mL, corresponding to an
Mg(II) concentration of 0.5 mol/L. The Zn(II) solution (0.5 mol/L) was made by diluting
81.7 g of Zn(II) naphthenate (in naphthenic acid as received) with xylene to a final
volume of 250.0 mL. The Mg(II) and Zn(II) solutions were mixed at chosen volume
ratio to achieve the desired Zn(II) doping levels while keeping the total metal
concentration of 0.5 mol/L in the precursor solutions.
During flame spray pyrolysis experiments, the mixed precursor solution was fed
by a syringe pump (KD Scientific) at a fixed flow rate of 5.0 mL/min. The liquid was
dispersed into fine droplets by O2 gas (5.0 L/min, 1.5 bar) at the spray nozzle exit.
The spray was ignited by a CH4/O2 (1.5 and 3.2 L/min, respectively) supporting flame
to form a self−sustained flame. The flow rates of all gases were controlled by
calibrated mass flow controllers (Bronkhorst High-Tech). The generated ZnxMg1−xO
particles were directed by a vacuum pump and collected on a water-cooled glass
fiber filter (Whatman GF−6) placed 450 mm above the nozzle. After being collected
from the filter surface, the powders were sieved to remove the glass fibers.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
59
5.3.3 XRD, N2 Physisorption, HRTEM, EDS, and HAADF−STEM
Characterizations
Powder X-ray diffraction analysis was carried out on a PANalytical X’pert
diffractometer with Cu Kα radiation source. Silicon zero−background plates were
used as the sample holder. The scanning program covers a 2θ range between 15°
and 140° at a step size of 0.03°. Rietveld refinements of the XRD patterns were
conducted using commercial refinement software (Bruker AXS Topas 4.2) to analyze
the crystal structure and quantify the phase concentration of the products. The
Brunauer−Emmett−Teller specific surface area (BET SSA) of the as−synthesized
ZnxMg1−xO powder was measured through N2 physisorption at liquid nitrogen
temperature using a Quantachrome Nova analyzer. The samples were degassed at
T = 473 K for 4 h before dosing with N2.
High-resolution transmission electron microscopy (HRTEM) was conducted on
an FEI Titan 80/300 electron microscope operated at 300 kV to investigate the
morphology and the microscopic elemental composition of the as−synthesized
powders. In particular, energy dispersive X-ray spectra (EDS) were collected from the
samples in the scanning transmission electron microscopy (STEM) mode, in which
Z-contrast imaging was performed with a high-angle annular dark-field (HAADF)
detector. The EDS spectra were used to analyze the spatial distribution of Mg, Zn,
and O elements on the length scale of a few hundred nanometers. Electron
diffraction was also performed to study the crystal structure of the samples. The TEM
samples were prepared by dispersing the as-synthesized particles in isopropanol
using an ultrasonic bath. Several drops of the suspension were dropped onto carbon-
coated copper grids, which were then subjected to vacuum storage before TEM
experiments.
5.3.4 Vacuum Annealing and Optical Investigations
To investigate the dependence of their optical properties on the surrounding
atmosphere, we transferred the powder samples into quartz glass cells, within which
thermal activation of the powders and spectroscopic measurements were performed
in high vacuum (at base pressure < 5∙10−6 mbar unless otherwise specified). A
typical procedure for dehydration, dehydroxylation and the removal of carbon−based
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
60
surface contaminants is as follows: the as-synthesized powders are heated to
T = 1123 K in high vacuum at a rate of 5 K min−1 and are then brought into contact
with 10 mbar O2 at this temperature. Subsequently, the sample−cell temperature is
raised to T = 1173 K and is kept at this temperature for 30 min before being cooled
down to room temperature.
UV diffuse reflectance spectra were acquired in the presence of 10 mbar of O2
using a Perkin-Elmer Lambda 950 spectrophotometer equipped with an integrating
sphere and then converted to absorption spectra using the Kubelka−Munk
transformation. PL spectra were measured on a Horiba Jobin Yvon Fluorolog−3
system (FL3−22) using a continuous wave 450 W xenon arc lamp for excitation. The
spectrometer is equipped with a double monochromator in emission and excitation to
guarantee optimal stray-light rejection. After vacuum annealing, the processed
powders were further characterized by N2 physisorption and TEM.
5.4 Results and Discussion
5.4.1 Characterization Results
5.4.1.1 Powder XRD
The powder XRD patterns of the as-synthesized ZnXMg1−XO samples are
shown in Figure 5.1. The XRD pattern of the as-synthesized MgO shows a periclase
structure with the most intense diffraction peak (002) at 42.96 . The relative positions
of other weaker peaks of this sample also match the periclase space group (fm3 m),
although with some small shifts in 2θ. Rietveld refinement results in an average
crystallite size of 9.9 nm (Table 5.1). With Zn doping at a loading of 10% (x = 0.1),
the XRD pattern is rather similar to that of pure MgO. No signature of other crystalline
phases including wurtzite ZnO can be identified, indicating a solid solution of
Mg-Zn-O with Zn being well blended into the lattice structure of periclase MgO during
the flame spray pyrolysis process. Rietveld refinement results in an average
crystallite size of 10.5 nm, slightly larger than that of the pure MgO sample.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
61
Figure 5.1: Powder XRD patterns and Rietveld refinement results from FSP-made
ZnXMg1-XO samples. The raw XRD data, the refined patterns and their differences
are presented in black, red and blue colors, respectively. Diffraction peaks for cubic
MgO and hexagonal ZnO are also shown as bars to indicate their 2θ positions.
When the Zn loading is further increased (x = 0.3), diffraction peaks start to
emerge at 31.93°, 34.14°, 47.44°, 56.89°, and 68.04°, corresponding to (010), (002),
(012), (110), and (112) diffractions of wurtzite ZnO. The peak at 36.66° should be
contributed by both ZnO (011) and MgO (111) diffractions, if we compare the
intensity here with those at x = 1 and x = 0.9. These results show that at a high Zn
loading (30%), a certain amount of ZnO is phase-separated from periclase MgO to
form wurtzite structures. Rietveld refinement gives an average periclase crystallite
size of 10.4 nm and wurtzite crystallite size of 9.8 nm. Quantitatively, a wurtzite
concentration of 5.9% in weight percentage was estimated. Therefore, the majority of
Zn is blended into the periclase MgO lattices. The nominal formula for this solid
solution is calculated as Zn0.27Mg0.73O, corresponding to a wurtzite-to-periclase molar
ratio of 1:25 (calculation details in the Supporting Information).
It is also observed that as the Zn loading increases, the diffraction peaks of the
periclase phase slightly shift to the lower angle. Based on Bragg’s law (nλ = 2d sin θ),
the d-spacing and hence the lattice constant are expected to increase. As shown in
Table 5.1, the a value increases from 4.22 Å for pure MgO to 4.24 Å for Zn0.27Mg0.73O
accordingly. This lattice expansion can be explained by the slightly larger ionic radius
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
62
of Zn2+ (0.74 Å) than that of Mg2+ (0.72 Å) in the periclase phase.150 Such a trend
further supports that Zn2+ has been incorporated into the Mg2+ sites in the cubic
lattice as the Zn loading increases, leading to a monotonous, although small,
expansion of the unit cell. On the other hand, the lattice constants of the segregated
wurtzite phase at x = 0.3 were also found to differ slightly from those of pure
FSP−synthesized ZnO (XRD and Rietveld refinement data not shown here). The
lattice constants in the a and c-axis of the separated wurtzite phase are 3.24 Å and
5.22 Å, respectively, indicating a slight compression in the a-axis and expansion in
the c−axis, as compared to the refined 3.25 Å (a) and 5.21 Å (c) of the pure ZnO
sample. Such a change is consistent with the reported trend for Mg-doped ZnO thin
films.151,152
Table 5.1: Summary of the results from powder XRD and N2 physisorption analyses
on ZnXMg1-XO.
a Calculated from Rietveld refinement of the powder XRD patterns. b Calculated using
the formula dBET = 6/(ρSSA), where ρ is the molar-average density of the samples
based on the densities of cubic ZnXMg1-XO and hexagonal ZnO, g m−3. c The ZnO
crystallite sizes are not calculated because no diffraction peaks corresponding to the
wurtzite phase were observed from the powder XRD profiles.
Although the wurtzite-structured Mg-Zn-O solid solutions (mostly epitaxially
grown thin films) have been actively investigated during the past decade,151,153–155
their counterparts, i.e., the cubic-structured solid solutions, have been much less
addressed, especially in the powder or particle form. The solubility limit of ZnO in the
FSP-made samples measured here, 27% (molar percentage), closely matches that
predicted by MgO-ZnO phase diagram as reported by Segnit and Holland.141 In their
samples
after synthesis after vacuum annealing
dXRD (nm)a SSA
(m2/g)
dBETb
(nm)
aMgOa
(Å)
dXRD (nm)a SSA
(m2/g) dBET
b (nm) MgO
phase ZnO
phase MgO
phase ZnO
phase
MgO 9.9 N/Ac 190 8.9 4.22 10.6 N/Ac 150 11.2
Zn0.1Mg0.9O 10.5 N/Ac 172 8.9 4.22 11.9 N/Ac 110 13.9
Zn0.3Mg0.7O 10.4 9.8 143 9.1 4.24 49.1 54.0 17 76.1
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
63
work, a solubility limit of 28.8% for ZnO at 1000 °C in the cubic Mg-Zn-O solid
solution was estimated.
5.4.1.2 N2 Physisorption
The results of N2 physisorption (7-point BET) analysis on the FSP−made
ZnXMg1−XO powders are also shown in Table 5.1. As the Zn loading increases from 0
to 30% (molar), the specific surface area decreases from 190 m2/g to 143 m2/g. On
the basis of the formula dBET = 6/(ρSSA), the average particle size is estimated to be
8.9 nm, 8.9 nm, and 9.1 nm for 0%, 10%, and 30% Zn-doped MgO samples,
respectively. These results indicate that introducing Zn into MgO does not
significantly change the average particle size. The particle sizes estimated by N2
physisorption here are close to those calculated from Rietveld refinement, indicating
single−crystalline particles formed.
5.4.1.3 TEM/Electron Diffraction
The morphology of the as-synthesized ZnXMg1−XO particles was studied using
transmission electron microscopy, as shown in Figure 5.2. The low− resolution TEM
images (Figure 5.2 a, c and e) show comparable particle sizes at various Zn
loadings. Individual particles are interconnected to form aggregates, which is a typical
feature for flame-synthesized materials. HRTEM imaging (Figure 5.2 b, d and f)
reveals the highly crystalline nature of the as-synthesized particles, regardless of the
Zn loading. Sharp edges of the single particles are visible with no evidence of an
amorphous covering layer. The crystallite size in each sample is about 10 nm, which
closely matches the results from Rietveld refinement of the XRD patterns and the
BET analysis. The observation again suggests that these nanoparticles are
individually single− crystalline. Electron diffraction probing (insets in Figure 5.2 a, c
and e) shows for a Zn loading of 10% a similar set of rings (Figure 5.2 g and h),
whereas for the 30% Zn loading, new rings occur belonging to the diffraction patterns
of wurtzite ZnO. The electron diffraction patterns corroborate the powder XRD
results.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
64
Figure 5.2: Results of TEM characterization on the ZnXMg1−XO samples. The insets
in a, c, and e show the electron diffraction patterns of each sample.
5.4.1.4 EDS and HAADF−STEM
Energy-dispersive X-ray spectroscopy in TEM mode was used to analyze the
chemical compositions of the ZnXMg1−XO particle aggregates. The Zn loadings
detected by conventional EDS probing are 9.5% and 25.6% for the nominal loadings
of 10% and 30%, respectively, suggesting a good fidelity of the product composition
to the precursor solution design. Furthermore, by conducting EDS analysis under the
HAADF−STEM mode, we expect to investigate the distribution of the Mg, Zn and O
elements on the submicrometer length scale while retaining a resolution of a few
nanometers, in order to identify the events of ZnO phase separation. Using this
mode, as the electron beam scans the sample, the EDS spectra were collected at a
series of locations and the total intensity of the collected X-ray intensity is
decomposed according to the Mg, Zn and O energy windows to give element-specific
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
65
photon intensities along the scanned path on the length scale of hundreds of
nanometers. We combine EDS with HAADF−STEM imaging because it is difficult, on
this length scale, to precisely reveal the spatial distribution of different elements by
Z-contrast imaging alone, considering the powder nature of the samples and their
uneven thickness. The results are shown in Figure 5.3.
Figure 5.3: Results of EDS analysis under the STEM–HAADF mode. a, c and e)
STEM–HAADF images of MgO, Zn0.1Mg0.9O and Zn0.3Mg0.7O, respectively; b, d and
f) Spatial variation of the EDS signal intensities of different elements in MgO,
Zn0.1Mg0.9O and Zn0.3Mg0.7O, respectively. For each sample, a typical EDS spectrum
was recorded first and used to define the energy windows for different elements,
which were then used to decompose the total counts of photons to element–specific
intensities.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
66
For the pure MgO sample (Figure 5.3 a and b), the signal intensity of Mg goes
in phase with that of O on a length scale of 80 nm. At 10% Zn loading (Figure 5.3 c
and d), the change of the intensity in the Mg signal is still in phase with that of the O
signal; in addition, the measured Zn signal is also in phase with those of Mg and O,
suggesting a homogeneous blending of Zn into the MgO lattice. Such results of
microscopic compositional analysis are consistent with the XRD analysis results,
which show no noticeable phase separation of ZnO at 10% of Zn. When the Zn
loading increases to 30%, the patterns of the EDS signals bear more structures, as
shown in Figure 5.3 e and f. The scanning distance is about 500 nm. In contrast to
the synchronized trends in Figure 5.3 b and d, in certain regions in Figure 5.3 f the
Mg and Zn signals show out−of−phase trends of variation. For instance, there are
regions where the intensity of Mg drops significantly while the Zn intensity keeps
almost constant; in certain other regions, the intensities of the Mg and Zn signals vary
in opposite directions. Such information indicates that at 30% Zn loading, the
distribution of Zn within the MgO matrix is inhomogeneous. The results are in good
agreement with the powder XRD data from which ZnO phase separation is identified.
Furthermore, it should be within the relatively Zn-abundant regions (e.g., the
rightmost part of the highlighted “Zn-rich” region in Figure 5.3 f) that the segregation
of wurtzite ZnO is expected to occur. Note that the trends found in Figure 5.3 are
typical and the elemental distribution results are representative of all samples.
5.4.1.5 Vacuum Annealing, Structural Changes, and Optical Properties
In comparison to the UV diffuse reflectance spectra of the MgO sample (Figure
5.4 a), the spectra of the as-synthesized Zn0.1Mg0.9O and Zn0.3Mg0.7O samples show
Zn2+- induced changes in the absorption properties (Figure 5.4 b and c, respectively).
These changes are either characterized by a red-shifted absorption band
(Zn0.1Mg0.9O, Figure 5.4 b) or an absorption threshold at 3.4 eV which is consistent
with the band gap of ZnO (Figure 5.4 c).140 This latter observation of the Zn0.3Mg0.7O
sample is supported by the XRD data (Figure 5.1, Table 5.1) and EDS analysis under
the STEM mode (Figure 5.3 e and f). Phase separation into an MgO−rich periclase
phase and a ZnO−rich wurtzite phase should have occurred during the flame
synthesis where the particles are exposed to flame temperatures of above 2000 K.
While vacuum annealing at T = 1173 K does not affect the absorption edge of
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
67
Zn0.3Mg0.7O (gray and black curves in Figure 5.4 c), the absorption−edge shift
observed for the Zn0.1Mg0.9O sample (Figure 5.4 b) points to a modification of the
electronic structure that originates from the annealing-induced Zn2+ segregation into
the particle surface.12
Figure 5.4: UV diffuse reflectance spectra of (a) MgO, (b) Zn0.1Mg0.9O and (c)
Zn0.3Mg0.7O before (black) and after (grey) vacuum annealing at T = 1173 K for 30
minutes. The spectra were acquired at room temperature and in the presence of
10 mbar O2.
For a clearer comparison, the UV diffuse reflectance spectra of the annealed
powders are plotted in Figure 5.5 a in conjunction with their TEM images shown in
Figure 5.5 b−d. Vacuum annealing of MgO leads to the optical absorption behavior
(Figure 5.5 a) and the cubic shape (Figure 5.5 b) that are characteristic of MgO
nanocubes with an average edge length of 10 nm. Both observations are consistent
with the properties of CVS−grown MgO powders.25,31 The sharpness of the cubic
morphology is not preserved upon admixing of Zn2+ (Figure 5.5 c and d).
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
68
Figure 5.5: (a) UV diffuse reflectance spectra and (b–d) TEM images of MgO,
Zn0.1Mg0.9O and Zn0.3Mg0.7O nanoparticles after vacuum annealing at T = 1173 K for
30 minutes (the spectra were acquired at room temperature in the presence of
10 mbar O2).
Powder XRD analysis and Rietveld refinement results show that vacuum
annealing causes different degrees of crystallite-size increase in periclase and
wurtzite domains (Table 5.1). The average crystallite sizes of MgO and Zn0.1Mg0.9O
(both possessing only the periclase phase) increase by 7% and 14%, respectively. In
the case of Zn0.3Mg0.7O, the crystallite size of the periclase domain grows by a factor
of 4.7, whereas the crystallite size of the wurtzite phase increases by a factor of 5.5.
TEM imaging results, as shown in Figure 5.5 d and Figure S5.1 (Supporting
Information), are in good agreement with the crystallite sizes estimated by XRD
analysis. Through high resolution TEM studies on the vacuum annealed Zn0.3Mg0.7O
sample, we identified the phase-separated wurtzite domains with characteristic d-
spacing of 0.247 nm of the (110) planes (see Figure S5.1 b in the Supporting
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
69
Information), which are in contrast to the periclase domains with the d-spacing of
0.212 nm of the (002) planes (see Figure S5.1 d in the Supporting Information).
These observations are consistent with the powder XRD patterns of the vacuum
annealed Zn0.3Mg0.7O sample (see Figure S5.1 in the Supporting Information). Such
trends appear to indicate that Zn2+ ions inside the periclase lattice facilitate the
crystallite growth, possibly by enhancing the mobility of the ions and/or ion vacancies.
Meanwhile, such effects should favor the preferential segregation of Zn2+ ions into
low coordinated surface sites.12 Additional EDS analysis under the STEM−HAADF
mode rules out the possibility of annealing−induced compositional changes such as
the depletion of ZnO into the gas phase.
Based on the UV diffuse reflectance data (Figure 5.5 a), we chose 270 nm and
340 nm as the excitation wavelengths for probing the PL emission properties of these
powders.31 For dehydroxylated MgO nanocubes, λExc = 270 nm (hvExc = 4.6 eV)
selectively excites the corner sites, whereas a wavelength of λExc = 340 nm
(hvExc = 3.6 eV) should not produce photoexcited states in MgO. The photon energy
of 3.6 eV; however, exceeds the band gap of ZnO and can be used to probe
ZnO-specific electronic transitions. The as-synthesized MgO sample shows a broad
PL emission feature with a maximum at λEm = 470 nm (Figure 5.6, black lines); the
shape of the spectrum and the position of the maximum intensity are independent of
the excitation wavelength. Admixing of 10% (atomic) Zn leads to a broadening of the
emission feature and a blue shift of its maximum to λEm = 440 nm; again, the position
of the maximum emission is independent of the excitation wavelength (Figure 5.6,
gray lines). The PL emission intensity of both MgO and Zn0.1Mg0.9O decreases upon
changing the excitation wavelength from λExc = 270 to λExc = 340 nm. A narrow and
so far unexplained emission feature at λEm = 680 nm (hvEm = 1.8 eV) is observed as
well.
For Zn0.3Mg0.7O, both the shape of the PL emission spectrum and the position
of its maximum depend on the excitation energy: with λExc = 270 nm, an emission
band at λEm = 560 nm with a shoulder at 430 nm suggests that the overall spectrum
is composed of at least two contributions (Figure 5.6 a, dashed line). An excitation
wavelength of λExc = 340 nm produces an emission band at λEm = 590 nm (Figure
5.6 b, dashed line), the intensity of which is comparable to the one being subjected to
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
70
excitation with λExc =270 nm. All spectra in Figure 5.6 are insensitive to the presence
of O2 in the gas phase, i.e., no evidence is found for the energy transfer between the
photoexcited particles and O2 (previously established as a PL quencher31,140) from
the gas phase.
Figure 5.6: PL emission spectra of as-synthesized ZnXMg1−XO nanoparticles
acquired at p < 10–5 mbar and T = 298 K using excitation wavelengths of (a) 270 and
(b) 340 nm, respectively. The emission spectra are not affected by the presence of
10 mbar O2.
To investigate the optical properties of the partially hydroxylated samples, the
samples were subjected to annealing at T = 873 K prior to spectroscopic
measurements. Previous studies on MgO nanoparticles have shown that vacuum
annealing in combination with O2 treatment is effective for the elimination of carbon
based surface contaminants;26,32 meanwhile, the concentration of hydrogen−bonded
neighboring surface OH groups becomes negligibly low and only the bands
pertaining to isolated surface hydroxyl groups remain.25,107 As a result, the emission
with a maximum at λEm = 560 nm which was previously observed on as−synthesized
Zn0.3Mg0.7O (Figure 5.6 a) vanishes after being annealed at T = 873 K (Figure 5.7 a
and Figure S5.2 a in the Supporting Information). Pure MgO powders show a strong
surface dependent PL emission feature with a maximum at 470 nm (Figure 5.7). This
band is quenched by O2 from the gas phase, which is attributed to the protonated
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
71
corner sites of partially hydroxylated MgO nanocubes.25 This observation is in good
agreement with the findings on CVS−grown MgO powders. Interestingly, Zn2+
admixed into MgO decreases the intensity of the PL emission. This effect is attributed
to the annealing-induced segregation of Zn2+ ions into the edge and corner sites in
the periclase domain, thus inducing the depletion of MgO-specific surface excitons.12
As outlined in the Supporting Information (Figure S5.2 b) molecular oxygen does not
entirely quench the PL emission band at λEm = 440 nm (hvEm = 2.8 eV). This is
different from dehydroxylated particle surfaces and indicates that surface hydroxyls
counteract energy transfer between photoexcited surface states and molecular
oxygen.156
Figure 5.7: PL emission spectra of ZnXMg1−XO nanoparticles after vacuum annealing
at T = 873 (a) and T = 1173 K (b) (The dwell time at each temperature is 30 minutes;
the spectra were acquired at < 10–5 mbar and T = 298 K using λExc=270 nm). The PL
emission of MgO is effectively quenched upon the addition of 10 mbar O2 irrespective
of the annealing temperature. The PL emission of the Zn0.1Mg0.9O sample is
quenched by 10 mbar O2 only after annealing at T = 1173 K.
The intensity loss of the green emission from the partially hydroxylated samples
may have its origin in the annealing−induced annihilation of intrinsic bulk− or surface
defects including the surface hydroxyls.12,82,157,159 To address the latter possibility, we
performed a control experiment by exposing the partially dehydroxylated Zn0.3Mg0.7O
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
72
sample to air for 24 h (see Figure S5.2 a in the Supporting Information).
Subsequently, after being pumped down to <1∙10−6 mbar, the sample did not present
a restoration of the PL emission observed before annealing. Apparently, simple
surface hydroxylation via contact with moisture in the air is insufficient to restore the
ZnO specific PL behavior. In case of the MgO samples, a PL emission at hvEm = 2.7
eV (Figure 5.7 a, black line) grows with the increase in annealing temperature while
the band maximum shifts to 3.2 eV (Figure 5.7 b, black line). This trend is consistent
with the stepwise dehydroxylation of MgO nanocube surfaces upon generation of
bare metal oxide particle surfaces where surface excitons can selectively form at
corners and edges.31
Figure 5.8: Schematic summary of the correlation between the surface states of the
ZnXMg1−XO particles and their PL emission properties. High vacuum annealing leads
to dehydroxylated MgO and Zn-Mg-O surfaces and morphological transformation of
the particles. The size of the flashes qualitatively denotes the relative emission
intensity that changes upon annealing.
Figure 5.8 schematically summarizes the observed trends in the photoelectronic
properties of pure and Zn2+ admixed MgO cleaning. Starting with particles the
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
73
surfaces of which are covered with adsorbates that are generated from the synthesis
process (Figure 5.8, left), a PL emission at λEm = 590 nm is observed only for the
Zn0.3Mg0.7O sample with a separated wurtzite phase (bottom of Figure 5.8). The
corresponding excitation energy is below the threshold energy required to address
low−coordinated surface sites on MgO particles. The independence of the PL
emission on the surrounding gas atmosphere, in particular from molecular oxygen as
a PL quencher, indicates that the underlying process corresponds to the radiative
deactivation of bulk excitons.
Annealing at T = 873 K irreversibly extinguishes this ZnO specific PL feature
and facilitates the formation of surface excitons on MgO particles (Figure 5.7).
Moreover, such treatment not only removes carbon−based surface contaminants and
eliminates the surface hydroxyls, but also favors ion mobility inside the lattice,
therefore allowing for partial reorganization of the particles (Figure 5.8, right). While
the MgO particles transform into cubes and essentially retain their high dispersion
(Table 5.1), the admixing of Zn2+ ions reduces the thermal stability of the host
component and leads to coarsened and morphologically less sharply defined
particles. PL emission measurements clearly demonstrate that MgO specific surface
excitons are strongly depleted by annealing-induced Zn2+ segregation into low
coordinated surface sites where surface excitons are formed and/or can radiatively
deactivate. A new PL emission band with its maximum at λEm = 520 nm is observed,
which should originate from the Zn2+-decorated MgO particle surface and is related to
the chemical composition at the interface.
5.5 Conclusions
It has been demonstrated that flame spray pyrolysis is a valuable synthesis
approach for the generation of ternary Zn-Mg-O nanoparticles. Powder XRD, Rietveld
refinement and HRTEM−EDS collaboratively show that admixing 10% (atomic) of Zn
into MgO leads to Zn0.1Mg0.9O solid solutions, whereas admixing 30% of Zn gives
rise to the phase separation into an MgO-rich periclase phase and a ZnO-rich
wurtzite phase. Such structural differences critically affect the optical properties of the
particles, as revealed by UV diffuse reflectance and PL emission measurements:
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
74
(1) The as-synthesized MgO nanoparticles show PL characteristics that are
related to surface excitons associated with the corner sites of MgO
nanocubes.31,140
(2) Phase-separated wurtzite ZnO inside the as-synthesized Zn0.3Mg0.7O
particles gives rise to a green PL emission at λEm = 590 nm. This process is
not quenched by molecular oxygen in the gas phase. Removal of adsorbed
water, hydroxyls, and possible carbonaceous contaminants by vacuum
annealing and oxidation at T = 873 K leads to the extinction of this emission
process. Subsequent surface hydroxylation via contact with air moisture
does not restore this feature.
(3) Upon vacuum annealing, surface-segregation of Zn2+ ions causes depletion
of MgO-specific PL emission. While exposure to gaseous O2 partially
reduces the PL emission intensity of hydroxylated Zn0.1Mg0.9O particles, it
quenches entirely the emission in dehydroxylated particles. Consequently,
hydroxyls can be regarded as protecting groups against gaseous oxygen as
a PL quencher at the solid-gas interface.
The present study clearly underlines the importance of sorting out the bulk and
interface contributions to the overall optical performances of composite oxide
nanomaterials that can be further engineered as components for optoelectronic
applications.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
75
5.6 Supporting Information
5.6.1 Calculation of the Chemical Formula of the Periclase–Phase Solid
Solution
From Rietveld refinement, a weight percentage of the wurtzite phase of 5.9% is
estimated. We assume the wurtzite phase consists of only ZnO, while the periclase
phase contains both Zn and Mg. Starting with 0.3 mol of ZnO and 0.7 mol of MgO as
determined by the precursor solution design, the mass of wurtzite ZnO is:
(0.3 × 81.41 + 0.7 × 40.30) × 5.9% = 3.11(g)
The rest of the total mass in the periclase phase:
(0.3 × 81.41 + 0.7 × 40.30) × 94.1% = 49.53 (g)
The molar ratio between Zn and Mg in the periclase phase is:
((0.3 × 81.41 – 3.11) / 81.41) / 0.7 = 0.26/0.7
The formula of the periclase phase compound is:
Zn0.26Mg0.7O0.96 or Zn0.27Mg0.73O
The formula weight of the periclase phase compound is then 51.39 g mol–1.
Therefore, the molar ratio between the wurtzite and the periclase phase is:
((3.11/81.41) / (49.53/51.39)) = 1/25
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
76
5.6.2 TEM and XRD of the Vacuum Annealed Zn0.3Mg0.7O Sample
Figure S5.1: TEM images and XRD patterns showing different crystalline domains in
the vacuum annealed Zn0.3Mg0.7O sample. The measured distances of 0.247 nm (b)
and 0.212 nm (d) correspond to the (011) and (002) d–spacing in the hexagonal
(a and b) and cubic (c and d) domains, respectively. The (011) and (002) planes
correspond to the strongest reflections in the XRD patterns of the wurtzite (solid
circles) and the periclase (empty triangles) phases.
5 Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO−MgO
77
5.6.3 Photoluminescence Spectra
Figure S5.2: a) Effects of annealing and exposure to air/ O2 atmosphere on the PL
emission properties of the Zn0.3Mg0.7O sample. The annealing time is 30 minutes.
b) Room temperature photoluminescence emission spectra (λExc = 340 nm) of
hydroxylated Zn0.1Mg0.9O nanoparticle powders after vacuum annealing and
oxidation at T = 873 K. Different to samples with entirely dehydroxylated particle
surfaces molecular oxygen does not entirely quench the PL emission band at
hvEm = 2.8 eV which indicates that surface hydroxyls counteract energy transfer
between photoexcited surface states and molecular oxygen.
78
6 Fe-Mg-O Nanocomposite Particle Systems
79
6 Fe-Mg-O Nanocomposite Particle Systems: Controlled
Synthesis and the Influence of Annealing on
Composition, Structure and Optical Properties
6.1 Abstract
Fe-Mg-O nanocomposite particles have been prepared by chemical vapor
synthesis (CVS) and subsequent annealing in controlled gas atmosphere. The
composition of the composite Fe-Mg-O (Fe/(Fe+Mg) = 1, 6 or 9 at.%) particles is
investigated by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-
OES) and EnergyDispersive X-ray (EDX) spectroscopy. The structure of the obtained
nanoparticles has been characterized by X-ray diffraction and transmission electron
microscopy, whereas Mößbauer spectroscopy is used to investigate the valence
state of Fe ions and magnetic properties of the samples. The results demonstrate
that the Fe-Mg-O composite can reveal superparamagnetic properties in case of
particle systems to be characterized as solid solutions of Fe3+ ions in periclase MgO
or antiferromagnetic behavior in case of phase separated particle mixtures containing
periclase MgO and magnesioferrite MgFe2O4 phases.
Fe-Mg-O nanoparticle powders are further explored by UV-Vis diffuse
reflectance and photoluminescence (PL) spectroscopy to investigate their optical
properties. The absorption spectra of samples consist of several absorption bands in
the energy range 2.6 eV ≤ hv ≤ 5.6 eV. These are attributed to different ligand field
and ligand-to-metal charge transfer electronic transitions. The absence of MgO
specific PL emission originating from surface excitons suggests annealing induced
surface segregation of Fe3+ ions for Fe-Mg-O nanocomposite particles. The results
show that the trends in the optical properties are subject to different iron
concentrations and annealing induced changes.
6 Fe-Mg-O Nanocomposite Particle Systems
80
6.2 Introduction
The Fe-Mg-O system as one of the prevailing mineral systems in Earth’s lower
mantle has gained research interest over a long time because of its importance in
understanding physical and chemical properties of the Earth.159,160 Moreover, certain
phases of the Fe-Mg-O system like Magnesioferrite (MgFe2O4) exhibit optical,
chemical and magnetic properties with potential for applications as photovoltaic
devices38, gas sensors162, magnetic devices39, catalysis40 and pigments59. Many
studies discuss the behavior of natural Fe-Mg-O systems under different conditions
of temperature and pressure. Different preparative techniques have been used and
proposed to yield desired Fe-Mg-O solid phases with specific structural, electronic
and magnetic properties.
The methods that have been employed for the fabrication of Fe-Mg-O
nanostructures are very diverse. Examples are in melt doping of MgO with iron and
subsequent high temperature oxidation62, high temperature annealing of powder
mixtures of Fe2O3 and MgO63, combustion of iron and magnesium nitrates in urea
solution64, sol-gel approaches followed by high temperature calcination59, spray
pyrolysis38, sputtering162 and hydrothermal procedures66.
Structural studies on Fe-Mg-O systems revealed that the admixture of iron to
magnesium oxide can give rise to ferric (Fe3+) or ferrous (Fe2+) states of iron in the
mixture. The valence state of the iron ions in the mixture are subject to heat
treatment.162 Investigations reveal for both valence states, that the iron ions replace
magnesium ions in the cationic sublattice of the magnesium oxide structure. In
octahedral sites inside the cubic lattice the ionic radius of Fe2+ (r = 0.078 nm) is
different from that of Fe3+ (r = 0.064 nm).150 Compared with the ionic radius of Mg2+
ion (r = 0.072 nm) the divalent iron ion is slightly larger and the trivalent iron ion is
slightly smaller when substituting magnesium.
6 Fe-Mg-O Nanocomposite Particle Systems
81
Figure 6.1: Schematic diagram of Fe2+ and Fe3+ substituting Mg2+ in MgO crystal
lattice.
Upon substitution of Mg2+ ions in the cationic sub lattice, Fe2+ ion donates two
valence electrons to its neighboring oxygen atoms and induces strain in the
surrounding lattice (Figure 6.1 left).163 Several studies reveal that Fe2+ ions tend to
cluster in order to decrease the strain they exert on the lattice.164–166
Upon replacement of Mg2+ by Fe3+ ions, the positive excess charge of Fe3+ can
be compensated by formation of Mg2+ vacancies.62 For every two Fe3+ ions there is
one Mg2+ vacancy which guarantees charge compensation (Figure 6.1 right). The
Fe3+ ions are reported to be either isolated in sites of octahedral symmetry or
clustered in tetrahedral sites.64,168
For Fe-Mg-O composite systems heat treatment can induce phase changes and
phase separation. This depends on total iron concentration, oxygen partial pressure
applied during annealing and annealing temperature. Phase diagrams of the ternary
Fe-Mg-O system have been reported for different atmospheres and at high
temperatures, typically for temperatures above T = 1273 K.168,169 These phase
diagrams reveal coexisting phases for Fe concentrations above Fe3+ solubility limit in
MgO lattice at XFe = 0.3 at.% and at T = 1273 K.41,42 A typical phase diagram of the
Fe-Mg-O system is shown in Figure 6.2. It can be seen that at T = 1273 K in the
samples with 0 at.% < XFe < 64 at.% both phases of magnesiowustite ((Mg,Fe)O) and
magnesioferrite (MgFe2O4) coexist. At the same temperature the samples with an
iron concentration of 64 at.% < XFe < 70 at.% only consist of magnesioferrite phase.
As another important point it is important to note the coexistence between the
magnesioferrite and hematite (Fe2O3) phase for samples with iron concentrations XFe
6 Fe-Mg-O Nanocomposite Particle Systems
82
> 70 at.% and at T = 1273 K. It should be mentioned that phase diagram in Figure
6.2 does not specify the relative distributions of Fe2+/Fe3+ ions at a given composition
and temperature.
Figure 6.2: Phase diagram in the system Fe-Mg-O in air. After Phillips et al.42,168
XFe = nFe/(nFe+nMg). X: molar ratio; n: number of moles.
The magnesioferrite phase with the chemical formula of MgFe2O4 has a cubic
spinel structure. The spinel structure is based on fcc packing of O2- ions and has the
general formula of AB2O4 where A is a divalent and B a trivalent cation. In unit cell of
the normal spinel (Figure 6.3 left) having 64 tetrahedral and 32 octahedral interstices
the A ions occupy 8 of the tetrahedral and the B ions 16 of the octahedral interstices.
In the inverse spinel structure (Figure 6.3 right) A and B ions are equally distributed
over these 16 octahedral interstices while the remaining number of B ions occupy the
8 tetrahedral interstices. The deviation from these two types of cation distributions in
octahedral and tetrahedral sites is called inversion. In MgFe2O4 the distribution of A
and B cations corresponds to an intermediate state between normal and inverse
spinel structure. In any case the degree of inversion sensitively depends on the
thermal history of the sample.170
6 Fe-Mg-O Nanocomposite Particle Systems
83
Figure 6.3: Schematic diagram of the normal (left) and inverse (right) spinel structure
with the general formula of A(II)B2(III)O4. In the normal spinel 8 of tetrahedral and 16
of octahedral sites are occupied by divalent (A) and trivalent (B) ions, respectively. In
inverse spinel divalent (A) ions swap with half of the trivalent (B) ions. Adapted from
reference [171].
While the above mentioned information which stems from equilibrium
thermodynamics is very valuable for a first orientation, the situation may be different
for nanomaterials which were synthesized under non equilibrium conditions.
Recently, nanoparticles of ternary Fe-Mg-O system like MgFe2O4 have received a lot
of attention because they can exhibit novel magnetic properties that are different from
their bulk counterparts.58 The structural, chemical and surface/interface properties of
such nanoparticles critically depend on preparation techniques and their post
synthesis treatments. Most of the wet methods like sol-gel or hydrothermal synthesis
have intrinsic disadvantages with regard to their use for more detailed studies or
advanced applications. Reasons for that are undesirable bulk and surface impurities
originating from synthesis, poor control of cation concentration and broad particle
size distribution.
This work aims to study the potential of metal organic chemical vapor synthesis
(MO-CVS) as a method to synthesize Fe-Mg-O nanoparticle composites using a Fe
(III) organic precursor. In the first part we explored the production of Fe-Mg-O
nanoparticles and established a robust process which provides control over iron
concentration and compositional homogeneity. In addition the effect of post-synthesis
thermal treatment on the as synthesized samples with different iron concentrations
was investigated in order to track the impact of thermal processing on structure and
6 Fe-Mg-O Nanocomposite Particle Systems
84
compositional changes. In the third part we explored the role of total iron
concentration and annealing treatment on the optical properties of Fe-Mg-O mixed
oxides.
6.3 Experimental Section
6.3.1 Material Synthesis
Fe-Mg-O nanoparticles were produced via metal organic chemical vapor
synthesis (MO-CVS). The details of the MgO preparation, which is based on
controlled combustion of metal vapor with oxygen under reduced pressure is given
elsewhere.95,115 Here we used for the first time a modified MO-CVS procedure, which
provides control over the concentration of iron in Fe-Mg-O nanocomposite samples.
The two-hot-zone reactor system (Figure 6.4) employed for this purpose consists of
two quartz glass tubes, which are mounted concentrically inside a heating coil (first
heating zone with T1 as operation temperature) followed by a ceramic tube furnace
(second heating zone with T2 as operation temperature).
Figure 6.4: Schematic diagram of the reactor setup used for production of Fe-Mg-O
nanoparticles. (MFC: mass flow controller; T: local operation temperatures;
P: pressure gauge)
In the first heating zone the inner glass tube hosts a ceramic ship with
iron (III) acetylacetonate (Fe(C5H7O2)3) powder (≥ 99.9%, Sigma-Aldrich) (1 g), which
is heated to temperature T1 = 353 K or 363 K or 373 K to sublimate the iron
6 Fe-Mg-O Nanocomposite Particle Systems
85
precursor and to adjust the evaporation rates. An argon gas flow (QAr = 1200 sccm)1
is led through the inner tube to transport the metal organic vapor to the second
heating zone where the furnace provides a temperature T2 = 913 K. At this position a
ceramic ship with Mg metal grains (1 g) is positioned inside the inner tube. Here, the
magnesium is evaporated and the resulting metal vapor becomes mixed with the
gaseous iron precursor. The vapor mixture is then transported by the argon gas flow
to the end of the inner glass tube. There the argon, magnesium vapor and iron
precursor vapor mixture meets oxygen (QO2 = 1200 sccm) which is flowing through
the outer glass tube. At this reactor position, the exothermic oxidation reaction
(2Mg + O2 → 2MgO) leads to a stable Mg combustion flame which decomposes the
iron precursor. As a result of homogenous nucleation and particle growth Fe-Mg-O
nanoparticles are formed. Because of continuous pumping, the residence time of
nuclei within the flame is so short that it prevents substantial particle coarsening and
coalescence. The MgO nanoparticles are deposited in a stainless steel net that is
kept downstream at the room temperature. The total pressure (p = 70 ± 2 mbar), as
well as argon and oxygen flow rates and T1 and T2 heating zones temperatures are
kept constant during nanoparticle collection.
6.3.2 Annealing
Annealing treatment was used for dehydration, dehydroxylation of the particle
surfaces and for removal of carbon based contaminants originating from either
synthesis or short contact of the nanoparticle powders with air. This post-synthesis
treatment is performed according to an optimized procedure2 which is outlined in
Figure 6.5.
1 Q = volumetric flow rate
2 This optimized procedure is selected on the basis of previous exploratory annealing procedure
evaluations which also included the compositional analysis done on Fe-Mg-O samples. For this
purpose measurement of local concentration at different positions of each sample is carried out by
EDX. The investigations revealed that i) removal of remnant carbon species (from incomplete
precursor decomposition in the reactor) requires annealing at oxygen pressures of p = 650 mbar and
ii) samples with homogeneous distribution of iron and magnesium can be retained when the samples
are kept at T= 873 K for more than 2h.
6 Fe-Mg-O Nanocomposite Particle Systems
86
Figure 6.5: Temperature profile (solid line, left ordinate scale) and applied oxygen
pressure (dashed line, right ordinate scale) during annealing treatment.
At the beginning, the cell containing a nanoparticle powder was evacuated
down to a pressure of p < 10-5 mbar at room temperature. The respective sample
was then heated to T = 373 K at a rate = 2.5 K min-1, held at this temperature for
15 min (dwell time, td) and then again subjected to evacuation to p < 10-5 mbar.
Further annealing steps are carried out at p = 650 mbar of oxygen. The powder was
stepwise heated in oxygen atmosphere to T = 473 K (r = 5 K min-1, td = 15 min),
T = 673 K (r = 10 K min-1, td = 30 min), T = 873 K (r = 10 K min-1, td = 180 min) and
T = 1173 K (r = 10 K min-1, td = 60 min).
After each annealing step the sample was cooled to room temperature
(cooling time ≈ 30 min) followed by an evacuation (≈ 15 min) to a base pressure of
p < 10-5 mbar in order to remove water and CO2 as oxidation products.
6.3.1 Structure and Morphology
X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8
Advance diffractometer using Cu Kα radiation (λ = 154 pm). For electron microscopic
measurements, small amounts of the metal oxide powders were cast on a carbon
6 Fe-Mg-O Nanocomposite Particle Systems
87
grid for investigation. Scanning electron microscopy (SEM) measurements were
performed on a Zeiss Gemini Ultra 55 microscope operating at 20 kV equipped with
an EDX (energy-dispersive X-ray emission) detector which allows for local
composition analysis. The transmission electron microscopy (TEM) investigations
were performed on TECNAI F20 analytical microscope equipped with a field emission
gun and an S-twin objective lens.
6.3.2 Spectroscopy
The metal composition of the Fe-Mg-O nanoparticles was analyzed by
inductively coupled plasma - optical emission spectroscopy (ICP-OES), in a
PerkinElmer OptimaTM 8300. For ICP-OES measurements the powder samples were
dissolved in 5 vol.% HNO3. EDX spectroscopy was employed to characterize the
local composition at different sample positions and to investigate the compositional
homogeneity in the Fe-Mg-O nanoparticle agglomerates. The local resolution of
chemical information for EDX analysis is limited to the penetration depth and
scattering of the primary electrons and, therefore, to the volume where characteristic
X-rays are emitted of the sample. With acceleration voltages of 20 kV the sampled
volume in EDX studies are in the order of few μm3 for bulk Fe-Mg-O samples.
Therefore the change in the ratio between Fe and Mg of each sample is tracked by
EDX over more than 50 positions1 with spatial resolution of approximately 10 μm.
For Mössbauer spectroscopy the powder sample was embedded in a copper
ring and mounted in a conventional Mössbauer apparatus (Halder Elektronik GmbH)
with constant acceleration (time mode procedure). The absorber was exposed to a
nominal 50 mCi (1.8 GBq) 57Co source in Rh (Wissel Ltd). The transmitted intensities
were stored in a multichannel analyzer with 1024 channels (Halder Elektronik
GmbH). In order to improve statistics, the obtained spectra were folded to 510
channels. For a good resolution and a good peak / background ratio high counting
rates (generally half a million) were chosen. Before and after a sequence with
identical velocity adjustments a -iron spectrum was recorded in order to determine
1 Figure S6.1 in Supporting Information provides an example of how the local distributions of Fe
and Mg ratios were measured for many local spots of the samples.
6 Fe-Mg-O Nanocomposite Particle Systems
88
the calibration factor - the velocity scale was subsequently recalculated from
channels to mm/s.
UV-Vis diffuse reflectance spectra were acquired at room temperature using
quartz glass cells with a Perkin-Elmer Lambda 950 spectrophotometer, equipped with
an integrating sphere. The reflectance spectra were converted into absorption
spectra using the Kubelka Munk transform procedure. For photoluminescence (PL)
measurements, a Fluorolog®-3 Model FL3-22 spectrometer with a continuous wave
450 watt Xenon arc lamp was used. PL spectroscopy was performed at room
temperature using quartz glass cell that guarantee vacuum condition better than
5·10-6 mbar.
6.4 Results
Part I: Composition, Structure and Morphology
6.4.1 Nanoparticle Composition
ICP-OES and EDX analysis are used to investigate the integral Fe
concentration as well as the compositional homogeneity (local concentration) of Fe in
the nanocomposite samples, respectively. The integral concentration of a given
element is its total amount in a volume of sample which is used for elemental
analysis. The local concentration is the amount of element in a certain position on the
length scale of tens of µm of the sample. The concentration values of each element
in atomic percent can be determined from ICP-OES and EDX results. Based on
these values the relative iron concentration, i.e. Fe/(Fe+Mg), is calculated and plotted
as XFe. Table 6.1 shows the integral XFe values for Fe-Mg-O samples produced by
using different operation temperatures T1 in the first heating zone of the
reactor (Figure 6.4). The concentrations are measured after controlled annealing of
each sample at T = 873 K and T = 1173 K (Figure 6.5).
6 Fe-Mg-O Nanocomposite Particle Systems
89
Table 6.1: Relative iron concentrations determined by ICP-OES. The results are
shown for Fe-Mg-O samples produced by using different operation temperatures T1
in the first heating zone of the reactor and after controlled annealing of each sample
to T = 873 K and T = 1173 K. XFe = nFe/(nFe+nMg). X: molar ratio; n: number of moles.
The analysis of the Fe-Mg-O samples by ICP-OES (Table 6.1) clearly shows
that setting the temperature T1 and, thus, the sublimation rate of the iron precursor
determines the integral Fe concentration. Table 6.1 shows how by increasing T1 from
T = 353 K to T = 373 K the integral Fe concentration increases from XFe = 1 at.% to
XFe = 10 at.%. Variations in concentration between samples annealed at T = 873 K
and T = 1173 K are attributed to preparation and sampling errors.
The Fe-Mg-O samples produced with T1 = 353 K, 363 K and 373 K will be
called (1%) Fe-Mg-O, (6%) Fe-Mg-O and (9%) Fe-Mg-O based on their integral
concentrations.The integral concentrations obtained from ICP-OES give no
information about the compositional homogeneity of Fe and Mg in nanoparticle
samples; therefore the local concentrations were investigated by EDX. The
compositional homogeneity is defined as the probability of having uniform
composition (in terms of XFe) at different specified sample positions throughout the
nanocomposite powder. This is described by composition distribution function which
is obtained from cumulative XFe values (Figure 6.6).
Figure 6.6 shows composition distribution function of (1%) Fe-Mg-O, (6%)
Fe-Mg-O and (9%) Fe-Mg-O samples. For all samples after annealing at T = 873 K
narrow distribution functions with median values consistent with the integral values
were observed. Figure 6.6 b reveals how annealing to T = 1173 K affects the
composition distribution function. For the samples with higher Fe content, i.e. (6%)
Fe-Mg-O and (9%) Fe-Mg-O the determined functions are clearly broadened.
sample name (1%) Fe-Mg-O (6%) Fe-Mg-O (9%) Fe-Mg-O
temperature T1 in reactor first zone 353 K 363 K 373 K
annealing temperature 873 K 1173 K 873 K 1173 K 873 K 1173 K
XFe / at.% ± 10 % 1.6 1.4 6.2 5.5 9.4 9.7
6 Fe-Mg-O Nanocomposite Particle Systems
90
However, for the (1%) Fe-Mg-O sample the composition distribution function remains
as narrow as after annealing to T = 873 K.
Figure 6.6: Composition distribution functions of Fe-Mg-O samples produced with
different iron precursor temperatures (T1) obtained by local EDX measurements; a)
samples annealed to T = 873 K; b) samples annealed to T = 1173 K. The minimum,
maximum and median values of each distribution curve are indicated.
XFe = nFe/(nFe+nMg). X: molar ratio; n: number of moles.
6.4.2 Mößbauer Spectroscopy
Mössbauer spectroscopy has been used as an effective method to determine
the valence state of the Fe ions involved, symmetry / distortion of the surrounding
charge distribution and presence or absence of magnetic ordering in the Fe-Mg-O
samples described above. As a first generally important result which applies for all
samples (see Figure S6.2 and Table S6.4 in Supporting Information) the Mössbauer
spectra reveal the presence of Fe3+ ions in a distorted octahedral environment. No
Fe2+ related phase has been detected. Since Fe3+ was employed as a precursor and
synthesis and post-synthesis treatment was performed in oxidizing environment this
result is not surprising.
The Mössbauer spectrum of the (1%) Fe-Mg-O sample annealed at T = 873 K
reveals no distinct absorptions (Figure S6.2). In contrast, the Mössbauer spectra of
the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples annealed at T = 873 K reveal
6 Fe-Mg-O Nanocomposite Particle Systems
91
doublets which are indicative of superparamagnetic phases of Fe3+. Each of these
spectra is fitted with two doublets whose isomer shifts () and quadrupole splittings
(QS) are consistent with those of Fe3+ in 6 fold coordination.41,64,172,173 However,
different QS values of the two fitted doublets of each spectrum suggest variations in
the degree of distortion in the octahedral sites.
The samples with different concentrations were further annealed at T = 1173 K
and after cooling to room temperature subjected to Mössbauer spectroscopic
measurements. The spectrum of the (1%) Fe-Mg-O sample annealed at T = 1173 K
reveals two doublets (Figure S6.2 and Table S6.4). Similar to the spectra of
(6%) Fe-Mg-O and (9%) Fe-Mg-O samples annealed at T = 873 K, the spectrum of
the (1%) Fe-Mg-O sample annealed at T = 1173 K indicates the existence of Fe3+ in
a 6 fold coordination. However, the site distribution of the two sorts of Fe3+ ions
reveals a ratio of approximately 2/3 to 1 which means random occupation.
The Mössbauer spectra of the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples
annealed at T = 1173 K display complicated patterns and can be deconvoluted into
three sextets and one doublet (Figure S6.2 and Table S6.4). The distribution of the
hyperfine fields (Table S6.4) and the absence of Fe2+ are indicative of the spinel type
compound magnesioferrite MgFe2O4 with antiferromagnetic behavior.41,172 The
doublet is ascribed to a superparamagnetic phase which coexists with the
magnetically ordered phase. Both spectra clearly indicate the separation of samples
into a magnetically ordered phase with antiferromagnetic behavior and a
superparamagnetic phase.
6.4.3 Structure and Morphology
6.4.3.1 SEM
SEM images of the undoped MgO nanocubes and the Fe-Mg-O nanocomposite
which were annealed to temperatures of T= 873 K and T = 1173 K are shown in
Figure 6.7. It can be seen that the samples annealed to temperature of T= 873 K
correspond to fluffy and loosely bound powders (Figure 6.7 a, c, e and g). Annealing
of the samples to T = 1173 K leads to more compact powders (Figure
6 Fe-Mg-O Nanocomposite Particle Systems
92
6.7 b, d, f and h). These compact powders consist of particulate objects in the size
range of tens of nanometers (insets in Figure 6.7 f and h).
Figure 6.7: SEM images of the MgO and Fe-Mg-O samples after annealing to
T = 873 K (left) and T = 1173 K (right): a and b) MgO; c and d) (1%) Fe-Mg-O and;
e and f) (6%) Fe-Mg-O; g and h) (9%) Fe-Mg-O.
The SEM images of the samples annealed to T = 1173 K show Fe-Mg-O
nanocomposite powders are more coarse grained compared to the undoped MgO
nanoparticle powders. With increasing iron content of the nanocomposites, the trend
6 Fe-Mg-O Nanocomposite Particle Systems
93
towards a compact and coarse grained powder increases. This is observed in the
SEM images of the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples (Figure 6.7 f and h)
annealed to T = 1173 K.
6.4.3.2 TEM
TEM images of the undoped MgO sample annealed to temperatures of
T = 873 K (Figure 6.8 a) and T = 1173 K (Figure 6.8 b) show agglomerates consisting
of nanoparticles with cubic morphology. For the undoped MgO annealed at T = 873 K
rounded cubes as a result of multiple stepped edges can be observed in some
regions of the sample (Figure 6.8 a, inset). Increasing the annealing temperature of
the MgO samples to T = 1173 K does not change the particle size distribution of the
particle ensemble (Figure 6.9). However, a small increase in median value (~5 nm)
can be seen.
Figure 6.8: TEM images of the MgO samples after annealing to T = 873 K (a) and
T = 1173 K (b).
6 Fe-Mg-O Nanocomposite Particle Systems
94
Figure 6.9: Cumulative particle size distribution plots for undoped MgO (blue) and
Fe-Mg-O samples after annealing to T = 873 K (a) and T = 1173 K (b). The
distribution was obtained by measuring the size of 300 to 400 particles at different
sample regions from TEM images.
Figure 6.10 a and b reveal a variety of morphological features of the primary
particles for the (1%) Fe-Mg-O sample annealed to temperature of T = 873 K. The
images indicate that particles with less regular shapes coexist with more cubic
crystallites having multiple stepped surfaces.
Figure 6.10 c and d indicate that annealing of the (1%) Fe-Mg-O sample at
T = 1173 K lead to growth of some nanocubes to larger dimensions (~50 nm to
80 nm) however the majority of the particles remain smaller in size. TEM, moreover,
reveals that by annealing at T = 1173 K, the particles adopt cubic morphology. This
development is similar to that of undoped MgO particles; however, the particle size
distributions of (1%) Fe-Mg-O and undoped MgO samples are different.
6 Fe-Mg-O Nanocomposite Particle Systems
95
Figure 6.10: TEM images of the (1%) Fe-Mg-O samples after annealing to T = 873 K
(a and b) and T = 1173 K (c and d).
From the detailed analysis of many TEM images taken from different sample
regions it can be concluded that (1%) Fe-Mg-O sample retains its narrow size
distribution with an average particle size of about 7 nm (= median) after annealing to
T = 873 K (Figure 6.9 a). Compared to undoped MgO the lower average particle size
must be attributed to the presence of iron in the lattice of MgO which seems to limit
crystal growth in the temperature range of 300 K ≤ T ≤ 873 K. However, annealing of
the sample at T = 1173 K significantly increases particle growth and leads to broad
particle size distribution (Figure 6.9 b).
6 Fe-Mg-O Nanocomposite Particle Systems
96
Figure 6.11 and Figure 6.12 represent the TEM images of (6%) Fe-Mg-O and
(9%) Fe-Mg-O samples after oxidative heat treatment. For both samples which were
annealed to T = 873 K the uniformity in particle size is similar. This is more clearly
reflected in the cumulative particle size distribution curves of the samples with the
average particle size of about 11 nm (Figure 6.9 a). However, particles exhibit
several morphological features such as cubic (or cuboid), semi-hexagonal or rounded
shapes (insets in Figure 6.11 a and Figure 6.12 a).
Figure 6.11: TEM images of the (6%) Fe-Mg-O samples after annealing to T = 873 K
(a) and T = 1173 K (b).
Annealing of the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples at T = 1173 K
leads to significant particle growth (Figure 6.11 b and Figure 6.12 b). Large crystal
domains are surrounded by much smaller particles. Unlike the samples which were
annealed to T = 873, the samples annealed to T = 1173 K exhibit significantly
broadened particle size distributions (Figure 6.9 b).
6 Fe-Mg-O Nanocomposite Particle Systems
97
Figure 6.12: TEM images of the (9%) Fe-Mg-O samples after annealing to T = 873 K
(a) and T = 1173 K (b).
From the Fe and Mg composition distributions (Figure 6.6) and particle size
distributions (Figure 6.9) of the samples we can conclude at this point:
For the (1%) Fe-Mg-O, the compositional homogeneity i.e. the composition
distribution function is retained as a result of increasing the annealing temperature
from T = 873 K to T = 1173 K. However annealing of this sample to temperature of
T = 1173 K broadens the particle size distribution. From this we conclude that
annealing to temperature of T = 1173 K drives the crystal growth at some sample
regions. Different from particle size distribution, the cubic shape of the particles is
retained when the (1%) Fe-Mg-O sample is annealed at T = 1173 K.
In contrast to the thermal stability of the (1%) Fe-Mg-O sample, annealing of the
(6%) Fe-Mg-O and (9%) Fe-Mg-O samples to T = 1173 K significantly changes their
composition distribution function. The (6%) Fe-Mg-O as well as (9%) Fe-Mg-O
samples behave in the same way. Annealing to T = 1173 K broadens particle size
distribution of (6%) Fe-Mg-O and (9%) Fe-Mg-O samples. This crystal growth at
some regions of these samples is exemplified by TEM observations. Particles of
these samples beside cubic shape exhibit some other morphological features like
hexagonal or rounded shapes (insets in Figure 6.11 and Figure 6.12).
6 Fe-Mg-O Nanocomposite Particle Systems
98
6.4.3.3 XRD
The XRD patterns of undoped MgO and Fe-Mg-O samples are presented in
Figure 6.13 and reveal the effect of annealing treatment. For all samples annealed to
T = 873 K the peak positions match the diffraction lines of MgO periclase phase
(JCPDS card # 45-0946) (Figure 6.13 a). No additional diffraction features which
would point to iron segregation and phase separation of ferric or ferrous phase are
observed. This proves phase homogeneity throughout all powder samples which are
annealed up to temperature of T = 873 K.
Figure 6.13: X-ray diffraction (XRD) patterns of MgO and Fe-Mg-O powder samples
after annealing to T = 873 K (a) and T = 1173 K (b). 1) MgO; 2) (1%) Fe-Mg-O; 3)
(6%) Fe-Mg-O; 4) (9%) Fe-Mg-O. Vertical lines correspond to the standard XRD
patterns of periclase MgO (solid lines, JCPDS card # 45-0946) and spinel
magnesioferrite (MgFe2O4) (dashed lines, JCPDS card # 36-0398).
The diffraction patterns of the samples annealed to T = 1173 K reveal that MgO
(Figure 6.13 b-1) and (1%) Fe-Mg-O (Figure 6.13 b-2) retain their periclase phase
without emergence of a second crystalline phase. After annealing to T = 1173 K the
XRD patterns further reveal additional diffraction lines for (6%) Fe-Mg-O and
(9%) Fe-Mg-O samples. The most intense peaks are attributed to (220), (311), (422)
and (511) planes of the magnesioferrite spinel phase (JCPDS card # 36-0398).
6 Fe-Mg-O Nanocomposite Particle Systems
99
The XRD patterns in Figure 6.13 b reveal narrowing of the diffraction features
widths of the periclase phase. This points to volume increase of the coherently
scattering solid and, therefore, annealing induced particle coarsening which is in
good agreement with particle growth deduced from TEM images. The average
crystalline domain sizes of periclase and magnesioferrite phases of the samples were
measured by Scherrer equation and from the reflex broadening (Table 6.2). For the
samples annealed to temperature of T = 873 K, the calculated average crystalline
domain sizes of periclase phase adopt similar values. The (1%) Fe-Mg-O sample,
however, shows slightly smaller domain size. The average crystalline domain size
increases by annealing of the samples from T = 873 K to T = 1173 K. For both
periclase and magnesioferrite phases this effect is smaller for the (9%) Fe-Mg-O
sample than (1%) Fe-Mg-O and the (6%) Fe-Mg-O samples.
Table 6.2: Average crystalline domain sizes of periclase phase and magnesioferrite
phase determined by Scherrer equation and from full width at half maximum (FWHM)
of the diffraction peaks.
sample annealed at T = 873 K annealed at T = 1173 K
periclase phase, average domain size / nm
MgO 11 ± 1 17 ± 1
(1%) Fe-Mg-O 9 ± 1 34 ± 1
(6%) Fe-Mg-O 12 ± 1 35 ± 1
(9%) Fe-Mg-O 12 ± 1 25 ± 1
magnesioferrite phase, average domain size / nm
(1%) Fe-Mg-O - -
(6%) Fe-Mg-O - 33 ± 1
(9%) Fe-Mg-O - 27 ± 1
Part II: Optical Spectroscopy
In the following discussion we will concentrate on optical absorption and
photoluminescence spectra of (1%) Fe-Mg-O and (9%) Fe-Mg-O samples.
6 Fe-Mg-O Nanocomposite Particle Systems
100
6.4.4 UV-Vis Diffuse Reflectance Spectroscopy
The UV-Vis diffuse reflectance spectra were acquired at room temperature in
the presence of 10 mbar O2 to avoid possible photoluminescence. After data
acquisition they are converted to absorption spectra via the Kubelka-Munk transform
procedure (Figure 6.14).
Figure 6.14: Room temperature UV-Vis diffuse reflectance spectra of MgO and
Fe-Mg-O powder samples after oxidative heat treatment: a) (1%) Fe-Mg-O sample
annealed to T = 873 K; b) (9%) Fe-Mg-O sample annealed to T = 873 K; c)
(1%) Fe-Mg-O sample annealed to T = 1173 K; d) (9%) Fe-Mg-O sample annealed to
T = 1173 K. The sum curve and the individual resolved absorption bands are shown
as colored lines. The spectra are measured at T = 298 K and in the presence of
10 mbar O2.
Undoped MgO nanoparticle powders show absorption band with maxima at
hv = 5.7 eV (λ = 220 nm) for samples annealed up to T = 873 K and T = 1173 K.
This absorption band is attributed to the electronic excitation of 4-coordinated anions
located in edges of MgO nanocubes.26 Unlike MgO the optical absorption spectra of
Fe-Mg-O samples reveal additional overlapping absorption bands in the UV-Vis
6 Fe-Mg-O Nanocomposite Particle Systems
101
range. The absorption spectra can be characterized as a superposition of absorption
bands with different band maxima. The absorption spectra can be deconvoluted into
several bands via Gaussian curve fitting using OriginPro 9.0 peak analyzer. The
results of the curve-fitting are shown in Figure 6.14 and Table 6.3.
Table 6.3: Curve fitting results related to spectra shown in Figure 6.14. FWHM = full
width at half maximum height of the fitted band.
sample annealed to
T = band
center max / eV
FWHM integral area /
%
(1%) Fe-Mg-O 873 K A 2.6 0.38 1.0
B 3.1 0.33 1.7
C 3.3 0.40 2.7
D 4.3 0.90 47.4
E 5.0 0.60 8.5
F 5.6 0.90 38.7
(1%) Fe-Mg-O 1173 K A 2.6 0.38 3.7
B 3.1 0.33 4.0
C 3.3 0.40 6.3
D 4.3 0.90 49.2
E 5.0 0.60 18.0
F 5.6 0.90 18.9
(9%) Fe-Mg-O 873 K A 2.6 0.42 7.6
B 3.1 0.50 9.9
C 3.3 0.75 23.7
D 4.3 1.00 34.9
E 5.0 0.70 11.8
F 5.6 0.90 12.2
(9%) Fe-Mg-O 1173 K A 2.6 0.38 6.0
B 3.1 0.33 6.8
C 3.3 0.40 12.8
D 4.3 0.90 38.3
E 5.0 0.60 14.0
F 5.6 0.90 22.2
Absorption spectrum of (1%) Fe-Mg-O sample annealed at T = 873 K is shown
in Figure 6.14 a. The spectrum consists of 6 absorption bands in the UV-Vis range.
The spectrum shows a well-defined band of low intensity centered on hv = 2.6 eV
(λ = 480 nm) (A) and two slope changes which suggest the presence of two bands
centered near hv = 3.1 eV (λ = 400 nm) (B) and hv = 3.3 eV (λ = 370 nm) (C). Two
intense and well-defined bands at hv = 4.3 eV (λ = 290 nm) (D) and hv = 5.6 eV
(λ = 222 nm) (F) are also present.
6 Fe-Mg-O Nanocomposite Particle Systems
102
Figure 6.14 b shows the spectrum of (9%) Fe-Mg-O sample annealed to
T = 873 K. The fitted spectrum in Figure 6.14 b is composed of 6 bands around
hv = 2.6 eV (λ = 480 nm) (A), hv = 3.1 eV (λ = 400 nm) (B), hv = 3.3 eV (λ = 370 nm)
(C), hv = 4.3 eV (λ = 290 nm) (D), hv = 5.0 eV (λ = 250 nm) (E) and hv = 5.6 eV
(λ = 222 nm) (F) (Table 6.3).
Figure 6.14 c and d show the absorption spectra of Fe-Mg-O samples after
annealing to T = 1173 K. Comparison between Figure 6.14 a and c reveals a
decrease in relative intensity of the absorption bands D and C centered at
hv = 4.3 eV (λ = 290 nm) and hv = 3.3 eV (λ = 370 nm), respectively, while the
relative intensities of bands A, B and C remain essentially constant. Increase of the
annealing temperature from T = 873 K to T = 1173 K, additionally, leads to a change
in relative intensity of the absorption bands D and F centered at hv = 4.3 eV
(λ = 290 nm) and hv = 5.6 eV (λ = 222 nm), respectively.
In comparison to (1%) Fe-Mg-O sample the absorption spectrum of
(9%) Fe-Mg-O sample (Figure 6.14 d) undergoes more significant changes upon
annealing to T = 1173 K.
Figure 6.14 further shows absorption spectra of MgO samples annealed to
T = 873 K and T = 1173 K and reveals one intense band at hv = 5.7 eV (λ = 220 nm).
Finally, it is important to note that all spectra shown along with Figure 6.14 can be
consistently deconvoluted by one set of bands, A to F, with constant band-maxima
and widths (Table 6.3).
6.4.4.1 Electronic Absorption Spectra of Fe3+ Oxides
To clarify the origin of the spectral bands it is worthwhile to first discuss Fe
related optical absorption spectra in minerals. Absorption bands in the UV-Vis range
correspond to electronic transitions within the 3d5 atomic orbitals of Fe3+ cations.
There are essentially three types of electronic transitions: 1) Fe3+ ligand field
transitions, 2) transitions due to excitation of magnetically coupled adjacent Fe3+
cations, and 3) ligand-to-metal charge transfer (LMCT) transitions.77,174–176
6 Fe-Mg-O Nanocomposite Particle Systems
103
Based on the ligand field theory for an ion with the electron configuration of 3 d5
the electronic ground state of Fe3+ free ion is described by the term 6S. When Fe3+
ion is surrounded by ligands in octahedral coordination, the d atomic orbitals of Fe3+
ion are split into two groups of orbitals expressed as t2g and eg which are separated
by ligand field splitting energy. The separated orbitals written as (t2g)3(eg)
2
configuration characterize the 6A1 state as the ground state.
The absorption bands in the spectra of Fe3+ containing minerals which are
associated with Fe3+ ligand field transitions basically originate from excitations of the
6A1 (6S) ground state to higher energy states (4E,
4A1 (4G), 4E (4D), 4T1 (4P),…).
Simultaneous excitation of two adjacent Fe3+ cations gives rise to additional bands in
the visible region. LMCT transitions occur at energies higher than ligand field
transitions and produce absorption bands below near UV region.77 The following
band assignment is based on work by Sherman and Waite77 about electronic spectra
of Fe3+ oxides.
6.4.4.2 Assignment of the absorption bands in Fe-Mg-O spectra
The absorption band A centered around hv = 2.6 eV (λ = 480 nm) (Figure 6.14
and Table 6.3) is assigned to the 6A1 +6A1
→ 4T1 (4G) +4T1 (
4G) excitation of Fe3+-Fe3+
pair. Absorption bands B centered at hv = 3.1 eV (λ = 400 nm), C at hv = 3.3 eV
(λ = 370 nm) and D at hv = 4.3 eV (λ = 290 nm) are attributed to the
6A1 → 4E, 6A1 (
4G), 6A1 → 4E (4D) and 6A1 → 4T1 (4P) ligand field transitions of Fe3+,
respectively. The additional absorption bands of E centered at hv = 5.0 eV
(λ = 250 nm) and F at hv = 5.6 eV (λ = 222 nm) are attributed to ligand-to-metal
charge transfer transitions.
6.4.5 Photoluminescence Spectroscopy
The photoluminescence spectra of undoped MgO and Fe-Mg-O powder
samples annealed to T = 873 K and T = 1173 K are given in Figure 6.15, Figure 6.16
and in supporting information Figure S6.3. The spectra recorded in vacuum show that
6 Fe-Mg-O Nanocomposite Particle Systems
104
both emission band position and intensity change with Fe ion concentration,
annealing temperature and excitation energy.
Figure 6.15: Room temperature photoluminescence (PL) spectra of MgO and
Fe-Mg-O powder samples after annealing to T = 873 K (a) and T = 1173 K (b). PL
emission spectra are recorded in vacuum (p < 10-5 mbar) and at T = 298 K using an
excitation energy hvExc = 5.2 eV (λExc = 240 nm).
Figure 6.15 a shows the PL emission spectra of samples annealed to T = 873 K
for excitation energy hvExc = 5.2 eV (λExc = 240 nm). The spectra reveal for MgO a
strong emission band centered at hvEm = 3.2 eV (λEm = 390 nm), for (1%) Fe-Mg-O a
weak and broad emission feature with maxima around hvEm = 3 eV (λEm = 410 nm)
and in the case of the (9%) Fe-Mg-O sample the absence of any emission. Sample
annealing to T = 1173 K enhances the MgO specific emission. The PL emission
features of Fe-Mg-O samples however are perfectly quenched after annealing to
T = 1173 K.
6 Fe-Mg-O Nanocomposite Particle Systems
105
Figure 6.16: Room temperature photoluminescence (PL) spectra of MgO and
Fe-Mg-O powder samples after annealing to T = 873 K (a) and T = 1173 K (b). PL
emission spectra are recorded in vacuum (p < 10-5 mbar) and at T= 298 K using an
excitation energy hvExc = 4.6 eV (λExc = 270 nm).
The PL emission intensity of MgO and Fe-Mg-O samples decreases upon
changing the excitation energy from hvExc = 5.2 eV (λExc = 240 nm) to hvExc = 4.6 eV
(λExc = 270 nm). MgO samples annealed to T = 873 K and T = 1173 K show broad PL
emission feature with maximum at hvEm = 2.9 eV (λEm = 430 nm) (Figure 6.16); the
position of the maximum intensity is independent of the annealing temperature. The
PL spectrum of the (1%) Fe-Mg-O sample annealed to T = 873 K reveals a broad
emission feature with maximum at hvEm = 3.2 eV (λEm = 380 nm). Annealing to
T = 1173 K leads to blue shift of the PL emission maximum to hvEm = 4 eV
(λEm = 310 nm) (Figure 6.16 b). For the (9%) Fe-Mg-O samples the excitation energy
of 4.6 eV (λExc = 270 nm) produces no measurable PL emission (Figure
6.16 a and b).
PL measurements in oxygen atmosphere (p = 10 mbar O2) (spectra not shown
here) results in PL emission quenching of all bands observed for MgO and for
(1%) Fe-Mg-O samples.
6 Fe-Mg-O Nanocomposite Particle Systems
106
6.5 Discussion
In the present study the Fe-Mg-O nanocomposite particles of adjustable
composition and synthesized by the MO-CVS approach are characterized. The effect
of oxidative post-synthesis heat treatment on the compositional homogeneity of the
samples, the particle size distribution and the optical absorption properties is
investigated.
The Fe/(Fe+Mg) concentrations as determined by ICP-OES clearly shows
increasing iron concentrations with increasing evaporation temperatures of the iron
precursor in the MO-CVS reactor (Table 6.1). We considered samples with three
different iron concentrations for local elemental characterization by EDX. Although
the EDX results are not representative for the entire powder sample they illustrate
how uniformly iron is distributed throughout the particle ensemble. As a result,
compositional homogeneity changes with the annealing temperature. The
composition distribution function of the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples
changes above an annealing temperature of T = 873 K. However, for the
(1%) Fe-Mg-O sample increase of the annealing temperature from T = 873 K to
T = 1173 K does not change the composition distribution function.
Mößbauer spectroscopy points to the exclusive existence of Fe3+ ions in the
Fe-Mg-O nanocomposite samples, irrespective from the total iron concentration and
annealing temperature. Addition of iron to the magnesium oxide lattice leads to
substitution of Mg2+ ions by Fe3+ ions. For all sample types which were annealed at
T = 873 K Mößbauer spectroscopy reveals the presence of Fe3+ ions in the 6 fold
coordination state. This is also true for the (1%) Fe-Mg-O sample annealed to
T = 1173 K. For the (6%) Fe-Mg-O and (9%) Fe-Mg-O samples annealed to
T = 1173 K, however, Mößbauer spectroscopy points to the presence of a
magnesioferrite phase which adopts the spinel structure with Fe3+ ions which are
distributed between the octahedral and tetrahedral interstices.
From the particle size distributions as determined by TEM analysis it is found
that the Fe-Mg-O particle ensembles annealed at T = 873 K exhibit average particle
sizes smaller than that of undoped MgO. We assume that up to this temperature the
6 Fe-Mg-O Nanocomposite Particle Systems
107
incorporation of Fe3+ ions in the magnesium oxide lattice has a stabilizing effect. The
origin for stabilizing effect observed at T = 873 K is presently unresolved. By
annealing of the samples to higher temperature this stabilization effect is lost and
broadened particle size distributions with much larger particles are observed after
annealing to T = 1173 K (Figure 6.9). Except for (1%) Fe-Mg-O nanoparticles which
retain the cubic shape, most of the large particles formed at T = 1173 K adopt less
regular morphologies.
The Fe3+ ions in the lattice of MgO produce cation vacancies in order to
maintain the charge balance.62 Admixture of a larger amount of Fe3+ ions to the
lattice favors vacancy concentration enhancement and facilitates ion diffusion and
thus, sintering.
The structural characterization done by XRD in parallel to TEM analysis
confirms crystallite domain growth for all samples upon annealing temperature
increase. For (6%) Fe-Mg-O and (9%) Fe-Mg-O samples which were annealed to
T = 1173 K we observe phase separation in addition to particle growth. In line with
Mößbauer spectroscopy, the XRD patterns of the (6%) Fe-Mg-O and (9%) Fe-Mg-O
samples indicate the presence of magnesioferrite and periclase phases.
The influence of annealing on crystal growth and phase separation of Fe-Mg-O
samples is sketched in Figure 6.17. The magnesium flame in reactor (T ≈ 2000 K)
during oxidation reaction generates a non-equilibrium solid with high concentration of
iron (> 1 at.%) in the as-synthesized samples. This leads to formation of metastable
Fe-Mg-O nanocomposites with Fe3+ ions accommodated in different sites of the MgO
lattice. Consistent with the low solubility limit of Fe3+ in MgO lattice (XFe = 0.3 at.% at
T = 1273 K), annealing of the metastable particles to T = 873 K results in the surface
segregation of Fe3+ ions. Ion diffusion enhanced by the Mg2+ vacancies clearly favors
the crystal growth. Annealing of (1%) Fe-Mg-O sample to T = 1173 K further
enhances the ion mobility which would favor the thermodynamically driven
transformation into cubic particles. For samples with higher total iron concentrations
this leads to the phase separation into magnesioferrite and periclase phases.
.
6 Fe-Mg-O Nanocomposite Particle Systems
108
Figure 6.17: Schematic illustrating the process of Fe3+ concentration dependent
crystal growth and phase separation upon annealing.
During phase separation several intermediate Fe-Mg-O structures are expected
to form. An exact identification of intermediate phases, however, is rather difficult at
this stage and would require more experiments and characterization work that is out
of scope of current work.
Optical properties of Fe-Mg-O nanocomposite samples are investigated using
UV-Vis diffuse reflectance and photoluminescence spectroscopy. The optical
absorption spectra of the Fe-Mg-O samples were deconvoluted into different bands
(Table 6.3). On the basis of band maxima they are assigned to electronic transitions
in different ligand fields, to the simultaneous excitation of magnetically coupled
adjacent Fe3+ cations or to ligand-to-metal charge transfer (LMCT) transitions. In the
light of the ligand field theory variations in absorption band intensities can be
explained by essentially two factors: 1) Fe3+ concentration and 2) annealing
temperature.
6 Fe-Mg-O Nanocomposite Particle Systems
109
Different to the absorption properties of (1%) Fe-Mg-O samples, absorption
spectra of annealed (9%) Fe-Mg-O samples show increased absorption band
intensities in the energy range of UV-Visible spectra. Ligand field transitions are in
principle spin forbidden but magnetic coupling of neighbor Fe ions can break this
condition.77 In particular, the intensity increase of band A in spectra of samples with
higher iron concentration (Figure 6.14 b & d) is attributed to an enhanced degree of
Fe3+- Fe3+ coupling as compared to samples with lower iron concentration.
As inferred from compositional and structural analysis, supply of higher thermal
energy to the Fe-Mg-O samples during annealing increases Fe3+ions diffusion in the
crystal lattice and affects the ion distribution in the structure. This produces phase
separation in samples with iron concentrations above 1 at.% and affects the average
Fe-to-Fe distance as well. Moreover, annealing to T = 1173 K expands metal-oxygen
(M-O) interatomic distance in oxide materials containing Fe and Mg ions.177 Changes
in the Fe-to-Fe distance and M-O bond length leave a critical influence on magnetic
coupling of the neighboring Fe ions.175 A combination of all these effects may explain
the Fe3+ ion concentration dependent variation of absorption band intensities
observed (Figure 6.14).
In comparison with MgO nanoparticles which exhibit intense PL emission
effects which exclusively originate from the excitation of surface low coordinated
sites, admixture of Fe3+ to MgO quenches PL emission. Related trends in PL
emission with increase of Fe3+ concentration in the nanocomposite samples (Figure
6.15 and Figure 6.16) are linked to surface segregated Fe3+ ions as static PL
quencher species. On this basis we conclude that annealing induces surface
segregation of Fe3+ ions for the Fe-Mg-O nanocomposites.
6.6 Conclusions
Fe-Mg-O nanocomposite particles were produced by metal organic chemical
vapor synthesis using iron (III) acetylacetonate as metal precursor. This gives a very
good control over the iron concentration and provides the opportunity to study
composition and stability of metastable ternary nanocomposites of Fe-Mg-O systems.
6 Fe-Mg-O Nanocomposite Particle Systems
110
Mößbauer spectroscopy proves the existence of iron ions in their trivalent state. This
technique also reveals the formation of superparamagnetic nanoparticles in the case
of (1%) Fe-Mg-O samples annealed at T = 873 K and annealed at T = 1173 K. For
(6%) Fe-Mg-O and (9%) Fe-Mg-O samples it could have been found that annealing
to T = 1173 K leads to the separation into a periclase phase and a magnesioferrite
with antiferromagnetic property. Mößbauer results are in perfect agreement with
results from X-ray diffraction and electron microscopy and point to annealing induced
particle growth. Crystal growth and phase separation is mediated by cation vacancies
which emerge from the charge imbalance of Fe3+ ions in the lattice. This enhances
ion mobility and ultimately leads to phase separation and spinel formation. An
explanation for this process is based on the non-equilibrium nature of the here
described nanoparticles: chemical vapor synthesis generates Fe-Mg-O
nanocomposites with incorporated Fe3+ ions above solubility limit in MgO lattice
(XFe = 0.3 at.% at T = 1273 K). Annealing to T = 873 K and T = 1173 K induces
crystal growth and surface segregation of Fe3+ ions. This is driven by
thermodynamics in combination with Fe3+ induced increase of the ion vacancy
concentration in MgO lattice which facilitates ion diffusion. By annealing of the
samples to temperatures above T = 873 K, excess Fe3+ ions aggregate and some of
them move into the tetrahedral interstices. This leads to phase separation and to
precipitation of magnesioferrite spinel.
This study underlines for Fe-Mg-O nanocomposites that differences in ionic
charge drives the annealing induced particle growth and segregation of the admixed
Fe3+ cations. Knowledge about synthesis and controlled processing of such material
systems enables the production of nanocomposites with defined compositional,
structural and optical properties. These nanomaterials may find applications in areas
such as catalysis, magnetic devices and optoelectronics.
6 Fe-Mg-O Nanocomposite Particle Systems
111
6.7 Supporting Information
6.7.1 Example of Local Concentration Measurement
Figure S6.1: Local concentration determination for a Fe-Mg-O sample as measured
by energy dispersive X-ray (EDX) spectroscopy. The figure reveals data of
composition analysis made of 5 different sample positions (A, B, C, …) and spots
(colored) to measure the relative Fe and Mg concentrations and their spatial
distribution over a larger area of the casted nanoparticles.
6 Fe-Mg-O Nanocomposite Particle Systems
112
6.7.2 Mößbauer Spectroscopy
Figure S6.2: The Mößbauer spectra of the Fe-Mg-O samples measured at room
temperature after annealing to T = 873 K and T = 1173 K. a & d) (1%) Fe-Mg-O;
b & e) (6%) Fe-Mg-O; c & f) (9%) Fe-Mg-O
6 Fe-Mg-O Nanocomposite Particle Systems
113
Table S6.4: The high fine parameters of the fitted curves to the Mößbauer spectra of
the Fe-Mg-O samples.
sample annealed at
T = Ssp
/ mm s
-1
QS / mm s
-1
H(0) / T
/ mm s
-1
A / %
2
(1%) Fe-Mg-O 873 K - - - - - -
(6%) Fe-Mg-O 873 K 1 0.3(3) 1.0(x) - 0.5(6) 49.7 0.35
2 0.3(3) 0.5(x) - 0.4(1) 50.3
(9%) Fe-Mg-O 873 K 1 0.3(x) 1.0(x) - 0.3(x) 46.0 0.41
2 0.3(x) 0.5(1) - 0.3(x) 54.0
(1%) Fe-Mg-O 1173 K 1 0.302(5) 0.95(3) - 0.54(4) 63.0 0.35
2 0.34(2) 0.6(1) - 0.358(2) 37.0
(6%) Fe-Mg-O 1173 K 1 0.30(4) 0.9(8) - 0.630(2) 19.2 1.31
2 0.27(7) 0.02(2) 46.04(5) 0.506(6) 35.2
3 0.25(1) -0.02(5) 43.8(1) 0.446(0) 23.1
4 0.28(1) -0.02(5) 40.82(9) 0.716(0) 20.6
(9%) Fe-Mg-O 1173 K 1 0.28(3) 0.8(7) - 0.694(2) 15.4 3.14
2 0.27(5) 0.01(2) 45.45(3) 0.538(4) 37.5
3 0.263(9) -0.01(4) 42.77(7) 0.486(0) 25.1
4 0.273(8) -0.03(4) 39.04(7) 0.800(0) 20.6
In most cases, we obtained quite well resolved spectra which were fitted with a
conventional self-made refinement routine using Lorentzian line shapes and different
sets of Mössbauer parameters corresponding to the non-equivalent crystallographic
sites involved. The parameters displayed in Table S6.4 have the following meaning:
Isomer shift relative to α-iron [mm/s]; Half width of the lines [mm/s]; Quadrupole
splitting QS = 1/2 eQVzz1+ɳ2/3 [mm/s] where Vzz = z-component of the electric field
gradient efg, Q = nuclear quadrupole moment, ɳ = asymmetry parameter (Vxx-Vyy)/Vzz
with |Vzz| ≥ |Vyy|≥|Vxx|; Internal magnetic field H(0) [T]; Amount of area [%] of the
relevant subspectrum – a measure for the site occupation of the corresponding Fe
ion.
The convergence of the iteration and the 2-value of the final fit served as an
indication of the quality of the refinement. The rather low values of 2 can be
considered as being rather good. The values are given with errors in round brackets;
where the latter exceeds the value, an “x” is given instead. The samples are listed in
the same order as the corresponding spectra of Figure S6.2, the underlying
subspectra are denoted as Ssp.; contributions lower than 2% (amount of area) are
left off.
6 Fe-Mg-O Nanocomposite Particle Systems
114
6.7.3 PL Spectra
Figure S6.3: Room temperature photoluminescence (PL) spectra of MgO and
Fe-Mg-O powder samples after annealing to T = 873 K (a) and T = 1173 K (b). PL
emission spectra are recorded in vacuum (p < 10-5 mbar) and at T= 298 K using an
excitation energy hvExc = 3.2 eV (λExc = 380 nm).
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
115
7 Spontaneous Growth of Magnesium Hydroxide Fibers at
Ambient Conditions
7.1 Abstract
Spontaneous transformation paths of nanomaterials point to guiding principles
for synthesis. We describe the room temperature transformation of MgO nanocubes
into Mg3(OH)5Cl·4H2O nanofibers in air and investigated the underlying formation
mechanism using electron microscopy, X-ray diffraction, and solid-state NMR
spectroscopy. Upon contact with water vapor, the magnesium hydroxide needles
were found to grow out of agglomerates of highly dispersed MgO nanocubes with
preadsorbed SiCl4. Corresponding one-dimensional nanostructures do not form on
low surface area materials. The presented growth approach is potentially extendable
to other hydrolyzable metal oxides at ultrafine dispersion.
7.2 Introduction
Spontaneous transformations of nanostructures are important key to their
chemical synthesis and application for two reasons: first, the underlying mechanisms
may provide guiding principles for the synthesis and controlled spatial arrangement of
anisotropic nanostructures.178–181 Second, knowledge about the transformation
behavior of nanomaterials in the environment is needed in order to reliably assess
the potential risk to biological systems. Depending on the nature of particle surfaces
and their interactions with gases in the atmosphere, disperse systems can undergo
completely changed interfacial chemistries with altered reaction networks as
compared to bulk materials.182
MgO nanocubes are excellent model systems for surface chemistry studies on
unsupported nanomaterials.25,118 At the same time, they are well-suited building
blocks for pure and composite nanostructures.143,183 We investigated different
approaches for SiCl4 adsorption on related particle ensembles (Supporting
Information and Chapter 8, Section 8.4.1) and discovered an unprecedented
transformation process at ambient conditions.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
116
7.3 Results and Discussion
The following combined electron microscopy and diffraction study was
performed to explore the underlying mechanism. A dry powder of MgO nanoparticles
which was casted on the SEM grid can be characterized as agglomerates exhibiting
a fine-grained particulate structure.184 The high dispersion results from the MgO
nanocrystals of cubic morphology and an average edge length of approximately
6 nm. MgO nanocubes exhibit a periclase structure and are phase pure (Figure 7.1
d1). The relatively broad diffraction features are related to the limited crystalline
domain sizes (d ≈ 6 nm).
Figure 7.1: (a) SEM image of a MgO nanocubes agglomerate after room
temperature storage in humid air (p(H2O) = 32 mbar); (b) SEM and TEM images of
MgO nanocubes after contact with SiCl4 and subsequent room temperature storage
in vacuum (p < 10−5 mbar). Additional TEM images are shown in the Supporting
Information. (c) EM images of SiCl4 contacted MgO nanocubes after room
temperature storage in humid air; (d) X-ray diffraction patterns of (d1) MgO
nanocubes, (d2) MgO nanocubes after storage in humid air; (d3) MgO after SiCl4/O2
contact and storage in a vacuum (p < 10−5 mbar), and (d4) SiCl4 contacted MgO
nanocubes kept in humid air. In all experiments, the room temperature storage time
was 14 days.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
117
After 3 days of sample contact with water-saturated air (p(H2O) = 32 mbar) at
room temperature, MgO nanocubes were quantitatively converted in to the brucite
phase (Figure 7.1 d2) according to
MgO + H2O MgO(OH)2 Equation 7.1
From the widths of the diffraction features, we determined for the crystallite
dimensions x001 and x110 coherence lengths of 5 and 10 nm, respectively, using the
Scherrer equation. This is in good agreement with the plate like morphology reported
of brucite nanocrystals.185,186 In the case of MgO which was previously contacted with
SiCl4 and O2 and exposed to water vapor thereafter, the material was converted into
a solid that is characterized by an entirely different X-ray diffraction (XRD) pattern
(Figure 7.1 d4). Phase analysis on the basis of pattern indexing and lattice parameter
refinement led to the compound Mg3(OH)5Cl·4H2O, thereby closely matching the
PDF database card #7-420.13 The refined cell parameters and atomic coordinates
were determined by Rietveld refinement of the powder XRD and are summarized in
Table S7.1, Table S7.2 and Table S7.3 (Supporting Information).
The transformation of MgO into Mg(OH)2 and Mg3(OH)5Cl·4H2O needles can
also be tracked by 25Mg NMR spectroscopy. A narrow resonance at 26.0 ppm as
observed for MgO nanocubes reveals a well-ordered Mg environment without defects
and impurities (Figure 7.2 a). Exposure to water vapor triggers the reaction described
in Equation 7.1, and the 25Mg NMR spectrum shows broad quadrupolar resonance
line (δ = 13 ppm, CQ = 3.0 MHz, ηQ = 0) characteristic for Mg(OH)2 (Figure 7.2
b).187,188 The spectrum also indicates a residual small fraction of MgO. The single
magnesium resonance observed for the chlorinated MgO powder which was stored in
a vacuum (Figure 7.2 c) indicates that the MgO nanocubes remain essentially
unaltered at this stage of sample treatment.
Exposure of chlorinated MgO nanocubes to water vapor initiates growth of
Mg3(OH)5Cl·4H2O needles. The 25Mg NMR spectrum of this new phase (Figure 7.2 d)
shows a signal at 0 ppm. Since 25Mg is a quadrupolar nucleus, its line shape also
gives some information about the orientation of the nucleus in the local electric field.
Narrow and symmetrical NMR resonance line of the Mg3(OH)5Cl·4H2O reflects a
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
118
symmetric environment of Mg in the MgO6 octahedra. In addition, the spectrum
confirms that Mg has been quantitatively incorporated into Mg3(OH)5Cl·4H2O.
Figure 7.2: Solid state single-pulse 25Mg MAS NMR spectra of MgO nanocubes
(a) before and (b) after exposure to H2O vapor (p(H2O) = 32 mbar) in a closed
sample chamber. The spectrum of the MgO powder sample which was contacted
with SiCl4/O2 and stored in a vacuum thereafter is shown as trace (c). Trace (d)
corresponds to the spectrum of Mg3(OH)5Cl·4H2O.
We carried out control experiments on commercially available low surface area
MgO powders (Aldrich #529699) with average domain sizes of dXRD = 35 nm and
found that - irrespective from whether the oxide was contacted with SiCl4/ O2 prior to
H2O vapor exposure or not - the powders became only superficially transformed into
the hydroxide of the brucite modification (with crystallite dimensions x001 and x110 of
28 and 25 nm, respectively. Further details in Chapter 8, Section 8.6.5). The absence
of needle growth as concluded from the corresponding XRD and transmission
electron microscopy (TEM) data clearly underlines the critical importance of high
surface-to-volume ratios for the here reported transformation process.
The associated increase in surface reactivity makes chlorinated MgO
nanocubes to physicochemically dynamic materials in environmental media.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
119
Humidity, i.e., the presence of water molecules in the gas phase, is key to the growth
of Mg3(OH)5Cl·4H2O needles. In the presence of water vapor, the hydrolyzable Mg-O
surface elements break. In a way similar to the hydrolysis and polycondensation
reactions of typical sol−gel reactions, the broken Mg-O bond leads to the formation of
Mg2+ and OH− ions to form magnesium hydroxide, which spreads and forms thin
ribbon-like films on top of the particle agglomerates. The transformation is driven by
the high energy of highly dispersed chlorinated MgO/ Clx particles and SixClyOz
moieties (related experimental evidence see Supporting Information) as educts and
the thermodynamic stability of the newly formed compound Mg3(OH)5Cl·4H2O. Figure
7.3 illustrates the growth mechanism.
Figure 7.3: Schematic illustrating the growth mechanism of magnesium oxychloride
fibers from MgO nanocubes kept in H2O vapor environment. (1) MgO nanocubes; (2)
MgO nanocubes covered with SixClyOz layers (experimental NMR evidence in
Supporting Information); (3) Mg3(OH)5Cl·4H2O fibers start to grow out from the grains
of chlorinated and agglomerated nanocubes (see corresponding TEM image in the
inset and in the Supporting Information) as a result of contact with water vapor.
(4) After 14 days, the formation of hedgehog like structures is completed.
We assume that a thin liquid layer forms on the surface of the chlorinated
nanocube agglomerates and provides a new reaction medium upon contact with
water vapor. The local supersaturation of dissolved ions leads to the precipitation of
Mg3(OH)5Cl·4H2O seeds that anisotropically grow out of the reaction pool (Figure
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
120
7.3) with [010] as the growth direction and the (101) plane being subject to fast
material addition and growth. Further needle growth is sustained by ion transport
inside the aqueous film and across the concentration gradient.
7.4 Conclusion
Studies on the interplay between hydration state, microstructure, and
mechanical properties of oxyhydrates revealed the growth of Mg3(OH)5Cl·4H2O
needles in aqueous pastes of MgO/ MgCl2 mixtures.189–191 More recently material
chemists reported the solution synthesis of oxychloride nanorods upon contact of
nanocrystalline MgO with concentrated aqueous magnesium chloride solution.192 As
a completely novel effect, the here reported transformation process does occur in air
and leads to 1-D hydroxides with interfacial regions that are susceptible to ion
exchange. The overall process points to a simple and direct way to spatially control
needle growth via arrangement of MgO nanocube ensembles (Figure 7.1 a) as
seeding regions. Moreover, constitutional water at the surface as well as inside the
bulk structure provides additional functionality for the deposition of catalyst
particles,193 for ion insertion and exchange reactions that are critical for materials
design in energy conversion technology194–196 as well as for the development of
structural materials.191,196 From a different perspective, these results underline the
substantial metastability of nanomaterials and that many unintended transformations
may easily occur in both environmental and biological systems.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
121
7.5 Supporting Information
7.5.1 MgO Exposure to SiCl4
SiCl4 was cleaned employing the freeze-pump-thaw method prior to the
adsorption on MgO nanocubes. The base pressure of the cell containing MgO
nanocube powder, which was kept at room temperature, was less than 10-5 mbar.
The cell was immersed into liquid nitrogen in order to keep a constant temperature of
T = 77 K (low temperature adsorption = LTA). Typically, during one cycle, 200 mg of
MgO nanocube powder was exposed to SiCl4 gas with a pressure of 300 mbar for 5
minutes and subjected to subsequent oxidation step in O2 atmosphere (p = 700
mbar) for another period of 5 minutes. The numbers of SiCl4 and O2 molecules
provided in each reaction step correspond to 3x1021 for both gases (T= 298 K).
During subsequent evacuation to a base pressure of p < 10-4 mbar gaseous
reactants that did not adsorb on the particle surface were removed and the powder
sample was then subjected to a new cycle with fresh reactants. At the end of 6th cycle
evacuation to the base pressure is done at T = 298 K. The results presented in this
study were obtained on MgO nanocube powders which were subjected to 6 cycles.
7.5.2 Water assisted Needle Growth
The growth of needle-like structures results from the contact of chlorinated MgO
nanocubes (see part 7.5.1) with water vapor at room temperature. To keep this
process under controlled conditions, the chlorinated MgO powder samples were kept
under constant water vapor pressure (32 mbar H2O) inside a closed chamber (Figure
S7.1) at T = 298 K.
Figure S7.1: Experimental set up for controlled exposure of chlorinated MgO
nanocube powders to water vapor.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
122
7.5.3 Analysis of the X-ray Diffraction Pattern
Indexing of the powder diffraction pattern of the sample contacted with SiCl4
and exposed to water vapour thereafter was successfully done using EXPO2013198
leading to a primitive monoclinic unit cell, space group P2/m. The determined lattice
parameters closely match those of Mg3(OH)5Cl·4H2O.198 Taking the atomic model of
Sugimoto et al. the crystal structure of Mg3(OH)5Cl*4H2O was refined using the
Rietveld method with FULLPROF.199 Thereby the atomic parameters of Mg, O and Cl
were allowed to vary freely, while the H-positions where fixed to the values given in
reference [198]. The isotropic atomic displacement parameters were grouped
together for the oxygen atoms and the Mg-atoms, but allowed the refine freely. It was
noted by Sugimoto et al. (2007) that the free Cl atom shows a mixed occupation with
0.5 OH2 + 0.5 Cl- This was confirmed in our refinement however with a slightly
different occupation of this site with 0.62(1) O + 0.38(1) Cl-.
Figure S7.2: Rietveld refinement of the XRD pattern shown in Figure 7.1 d4. The
upper tick marks correspond to Mg3(OH)5Cl·4H2O (F5-Phase) while the lower ones
are from the 2.0 wt.% impurity of tridymite SiO2. The difference plot between
observed and calculated intensities is shown in the lowest part of the Figure,
mismatch around 12°, 21° and 37° 2Theta is due to distinct internal micro-strain of
the sample which could not be modeled perfectly.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
123
Table S7.1: Refined structural parameters for Mg3(OH)5Cl·4H2O (F-Phase) as
extracted from Rietveld refinements.
Space group P2/m Temperature 298 K a (Å) 9.6553(6) b (Å) 3.15207(19) c (Å) 8.3087(6)
(°) 90.00000
(°) 114.022(4)
(°) 90.00000
Unit cell volume (Å3) 230.97(3) Radiation Cu K1,2
Table S7.2: Refined atomic coordinates
ID x/a y/b z/c Uiso
Mg1 0.8045(7) 0 0.6981(7) 0.0234(18) Mg2 ½ ½ ½ 0.0234(18) O1 0.7258(9) ½ 0.5171(13) 0.0087(16) O2 0.5833(9) ½ 0.6813(9) 0.0087(16) O3 1.0218(8) ½ 0.6879(9) 0.0087(16) O4 0.8714(7) ½ 0.8821(10) 0.0087(16) Cl1a 0.6792(6) ½ 0.1104(7) 0.0087(16)
a the occupation of this site is 0.62(1) O + 0.38(1) Cl
Uiso = isotropic atomic displacement parameter
Table S7.3: Fixed atomic coordinates as taken from Sugimoto et al. 198
H1 0.73240 ½ 0.41200 0.02533 H2 0.58620 ½ 0.79360 0.02533 H3 1.10570 -0.18750 0.75870 0.02533 H4 1.04960 0.18750 0.60780 0.02533 H5 0.80000 ½ 0.95340 0.02533 H6 0.96590 ½ 0.96880 0.02533 H7 0.76790 ½ 0.08650 0.02533 H8 0.58200 ½ 0.01860 0.02533
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
124
Figure S7.3: Graphical representation of the crystal structure of Mg3(OH)5Cl·4H2O
(F5-Phase) in a projection onto the a-c plane.
7.5.4 Solid State NMR Experiments
Solid-state 25Mg and 29Si MAS (magic-angle spinning) NMR (nuclear
magnetic resonance) spectra were recorded on a 500 MHz Agilent NMR system
using Agilent 3.2 mm and 6 mm T3 HXY MAS Solids Probes. The 29Si and 25Mg
Larmor frequencies amounted to 99.30 MHz and 30.60 MHz, respectively. Single
pulse NMR spectra repetition delays were 5 s and 20 s, the number of scans
accumulated was in the order of magnitude of 10,000 and 1000. All the samples
were spun at 5 kHz during measurement.
20 0 -20 -40 -60 -80 -100 -120
(b)
/ ppm
(a)
29Si sp MAS NMR
Figure S7.4: 29Si MAS NMR spectra of the MgO nanocubes after exposure to SiCl4
and O2 (a) and storage in vacuum (b) which indicates the presence of short
polychlorosiloxane chains.
7 Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions
125
29Si single pulse MAS NMR spectrum (Figure S7.4) of MgO immediately after
chlorination shows only the presence of SiCl4 species (-21 ppm). However, after
storing the chlorinated sample in vacuum for two weeks, also peaks attributed to
SixClyOz compounds are observed. The much higher relative intensities of the -SiCl3
(-45 ppm) resonance compared to the -SiCl2- (-71 ppm) peak indicate the presence
of short chlorosiloxane chains, predominantly Cl3Si-O-SiCl3 and Cl3Si-O-SiCl2-O-
SiCl3.
7.5.5 SiCl4/ O2 Contacted MgO Nanocubes: Additional TEM Images
Figure S7.5: TEM images of MgO nanocubes after exposure to SiCl4/ O2 and
storage in vacuum. The structures which are in focus show clear-cut edges specific
to MgO nanocubes as the starting material.
126
8 Growth of Magnesium Oxychloride Fibers
127
8 Vapor phase based and Water Film mediated Growth of
Magnesium Oxychloride Fibers
8.1 Abstract
Highly dispersed metal oxides that are covered with thin films of water can give
rise to the spontaneous and spatially controllable growth of oxide and hydroxide
fibers in the ambient. Understanding the underlying formation mechanism is needed
for the exploration of related microstructure evolution and represents a critical
requirement for the rational development of industrial binders as well as for ceramic
precursors. This work examines the parameters of MgO nanocube functionalization
with oxychlorides, using SiCl4 as a water free chlorine ion source, and explores the
subsequent transformation of obtained composites into magnesium oxychloride
Mg3(OH)5Cl*4H2O fibers, which correspond to the main component of the oxychloride
cement phase. Specifically we show how the temperature of the functionalization
process as well as the materials’ level of dispersion determine the reaction pathway
to either obtain MgCl2·6 H2O and Mg(OH)2 or magnesium oxychloride fibers. Lessons
to be learned from this unique route to synthesize Mg3(OH)5Cl*4H2O nanofibers upon
water vapor contact can be applied to a variety microstructural evolution processes
that involve metal oxide nanoparticles in combination with superficial water which
acts both as a reactant as well as reaction medium for the hydration process.
8.2 Introduction
The implementation of nanostructured metal oxides and hydroxides into
functional and structural materials requires knowledge about their stability in
changing chemical environments. These will never be fully understood without the
examination of specific aspects of that complexity. Tailored particle systems with
interface properties that are accessible to experimental methods are indispensable
model systems for this purpose.13 This is also true for ceramics or cement-bonded
materials, where structure, texture and reactivity of the different constituent mineral
and mineral-like phases determine the functionality and stability of the composite. As
a nano- and microporous multicomponent system, cement is chemically highly
8 Growth of Magnesium Oxychloride Fibers
128
reactive. Various aspects or properties related to chemical interfaces are, however,
still not understood.200,201,202,203
Oxychloride (Sorel) cement corresponds to a fast curing cement for patching
the surface of freeways as well as other applications such as stucco, flame retardant
coatings and molded cement objects. Besides its good binding ability with inorganic
and organic compounds, the superior thermal, chemical and mechanical stability
makes it a high performance material which draws continuously further
attention.204,205 Sorel cement is typically produced by mixing an aqueous MgCl2 brine
solution with MgO powder.206,198 There is general agreement in the literature that this
cement emerges on the basis of a complex reaction network, which involves water,
MgO and MgCl2 admixed in specific proportions as starting materials189,191 but
considerable controversy as to what chemical reaction is responsible for the setting
reaction.
Relevant to Sorel cement formation, we recently discovered a spontaneous
formation process of magnesium oxychloride (Mg3(OH)5Cl*4H2O) fibers which grow
out of MgO-based nanoparticle agglomerates.207 This process occurs under ambient
conditions and starts from MgO nanocubes that are covered with SixOyClz moieties.
With a presumably important effect on the entire transformation process the impact of
different materials parameters, such as the required degree of dispersion, but also
process parameters, like the temperature for SiCl4 and O2 adsorption have remained
unexplored. In this study, we investigated their influence in detail to gain insights into
the underlying growth mechanism, to identify parameters for fiber growth optimization
and, ultimately, to control microstructure development. Moreover, an important
aspect related to the spontaneously forming needle-like shaped magnesium
oxychlorides relates to their stability towards annealing induced dehydration and their
potential to restore the parental oxide inside novel microstructures.
This Chapter is structured as follows: in the first part we provide a detailed
description of the MgO functionalization process which provides the reactive
precursor material leading to spontaneous growth of magnesium oxychloride fibers.
In particular, the challenge of achieving compositional homogeneity over the entire
8 Growth of Magnesium Oxychloride Fibers
129
nanoparticle ensemble is at the focus of this part. In the second part, insights about
the SiCl4 adsorption temperature, the effect of powder dispersion and surface area
enabled us to delineate the growth mechanism in the ambient, which involves thin
liquid water films as reaction medium. In the third part of this paper we addressed the
decomposition behavior of the magnesium oxychloride (Mg3(OH)5Cl*4H2O) fibers in
order to provide enabling knowledge for applications that operate at elevated
temperatures such as high temperature coatings, architectural materials or abrasive
tools.208,209,210,211
8.3 Experimental Section
8.3.1 Chemical Vapor Synthesis of MgO
MgO nanoparticles were produced via a chemical vapor synthesis (CVS)
technique which is based on controlled combustion of metal vapor within a flow
reactor system.212,143 For further materials processing the samples are transferred
through ambient air into a quartz glass tube. In order to guarantee bare particle
surfaces as well as to achieve cubically shaped nanocrystals, thermal sample
activation via vacuum annealing is employed. This procedure leads full
dehydroxylation of sample surfaces as proved by FT-IR spectroscopy.107 A typical
procedure utilized for dehydration, dehydroxylation and removal of carbon−based
surface contaminants is as follows: as-synthesized powders are heated to T = 1123
K in high vacuum with a rate of 5 K min−1 and then are brought into contact with 10
mbar O2 at this temperature. Subsequently, the temperature is raised to T = 1173 K
and, at pressures p < 5∙10-6 mbar, kept at this temperature for 1 h before being
cooled down to room temperature.213 Beside CVS MgO we also employed
commercial nanocrystalline MgO (529699 Aldrich) for reference experiments (Figure
S8.1 and Figure S8.2).
As revealed by Transmission Electron Microscopy CVS MgO consists of highly
dispersed monocrystalline nanocubes with a high portion of edge and corner features
(Supporting Information, Figure S8.1 a). In contrast to MgO nanocubes, commercial
MgO can be characterized as an assembly of less regular shaped particles
(Supporting Information, Figure S8.1 b). Whereas the Scanning Electron Microscopy
8 Growth of Magnesium Oxychloride Fibers
130
(SEM) images of CVS MgO nanoparticles reveal a fine-grained material with an open
and porous secondary structure (Supporting Information, Figure S8.1 c), micrographs
related to commercial MgO particles reveal their more coarse grained nature.
Respective particles do not exhibit any characteristic and prevailing shape.
XRD confirms for both types of MgO powders the exclusive presence of
crystalline particles with Periclase structure (Supporting Information, Figure S8.2).
The average crystallite sizes, as determined from reflex broadening using the
Scherrer equation102,103, correspond to 6 ± 1 nm and 35 ± 1 nm for CVS MgO and
commercial MgO, respectively. While the measured specific surface area of CVS
MgO, which corresponds to SBET = 300 ± 20 m2·g-1, is perfectly consistent with the
XRD derived crystallite domain size, the situation is different for commercially
available MgO: in comparison to a XRD derived value of SXRD = 50 m2·g-1 we
measured with N2 sorption analysis a value of SBET = 12 ± 1 m2·g-1 which is lower by
a factor of five. This discrepancy points to polycrystalline grains exhibiting a large
fraction of intergranular solid interface area that is not susceptible to N2 adsorption.
Both CVS and commercial MgO were processed equally prior to functionalization and
growth studies described in this work.
8.3.2 MgO Surface Functionalization via SiCl4/ O2 Exposure
MgO is exposed to SiCl4 and O2 in a cyclic process. The schematic of the setup
used for this process is shown in Figure 8.1. Prior to adsorption SiCl4 is cleaned
employing the freeze-pump-thaw method. The base pressure of the sample tube at
room temperature (T = 298 K) is less than p = 10-5 mbar. For each cycle, the lower
part of the glass tube with the MgO nanocubes powder (adsorbent) was immersed
into a liquid nitrogen bath in order to keep T = 77 K as a constant temperature of
adsorption. Samples which were functionalized in this way will be designated in the
following as low temperature adsorption (LTA) samples.
8 Growth of Magnesium Oxychloride Fibers
131
Figure 8.1: Schematic illustrating the setup used for the MgO exposure to SiCl4/O2.
8.4 Results and Discussion
8.4.1 Surface Functionalization of MgO Particles and Spontaneous
Fiber Growth
Critical for functionalization as well as for the below described transformation
process is a - in terms of composition - homogeneous surface coverage of the MgO
particle ensembles with silicon tetrachloride (SiCl4) which acts here as a carbon free
source for chlorine.214 We performed respective gas phase functionalization by
SiCl4/ O2 admission at T = 298 K (Room temperature adsorption, RTA) or T = 77 K
(Low temperature adsorption, LTA). On this basis we identified the following
sequence of reaction steps as a particularly robust approach (Figure 8.2): during the
first cycle, approximately 200 mg of MgO nanocube powder is exposed to SiCl4 vapor
- with a vapor pressure of 300 mbar at room temperature - for 5 minutes and after
pumping subjected to O2 atmosphere (p = 700 mbar) for another period of 5 minutes.
8 Growth of Magnesium Oxychloride Fibers
132
Figure 8.2: Time-dependent development of SiCl4 and O2 partial pressures
describing the functionalization cycles employed for MgO nanocubes at T = 298 K or
at T = 77 K.
Gaseous reactants that did not adsorb on the particle surface are removed
upon subsequent pumping to a base pressure of p < 10-4 mbar and new reactant
molecules are supplied to the powder sample in the course of each subsequent
cycle. The results presented in this study were obtained on MgO nanocube powders
which were subjected to 6-cycles of room (RTA) or low temperature (LTA)
adsorption. After completion of the 6th cycle evacuation to the base pressure of
p < 10-4 mbar was completed at T = 298 K. Estimated surface coverage for CVS and
commercial MgO are listed in Table S8.1 of the Supporting Information.
For comparison, we performed experiments where SiCl4 was added to
dehydroxylated MgO nanocube powders (Figure 8.3 a) either at T = 77 K (LTA,
Figure 8.3 b) or at T = 298 K (RTA, Figure 8.3 c) and found that entirely different
reaction paths do occur dependent on the temperature of adsorption. At room
temperature, a strongly exothermic and poorly controllable process gives rise to a
sample heterogeneity which is visible to the eye (see Figure 8.3 c). Different powder
regions with particle agglomerates of varying quality and optical appearance are
formed as a result of an exothermic reaction. Parts of the originally fluffy MgO
nanocube powder (sample shown in Figure 8.3 a) becomes transformed into
compact white flakes in some regions of the sample (c2 in Figure 8.3). Both TEM
analysis (lower right panel in Figure 8.3) and powder X-ray diffraction (lower left
panel in Figure 8.3) reveal that only a fraction of the starting material becomes
converted into the substantially larger grains. These correspond to a so far unknown
8 Growth of Magnesium Oxychloride Fibers
133
intermediate which, within the time of the XRD measurement (< 5 hours), transforms
into a crystalline MgCl2·6H2O phase (Bischofite, c2 in Figure 8.3). The retained
second fraction inside the inhomogeneous powder agglomerates consists of MgO
nanocube with periclase structure (c1 in Figure 8.3).
Figure 8.3: Digital micrograph in the middle of the upper panel (a) shows a white and
opalescent MgO nanocube powder. The image on the left hand (b) shows a
homogeneous white and compact powder which results from MgO exposure to
SiCl4/O2 at T = 77 K (Figure 8.2). The inhomogeneous appearance of sample in the
right (c) originating from the same sample treatment at T = 298 K results from the
coexistence of two main fractions: c1) agglomerates of unreacted MgO nanocubes
agglomerates and c2) more compact and deeply white powder parts with a XRD
pattern specific to MgCl2·6H2O grains. The energy profile in the middle panel
underlines that adsorption at T = 77 K (b) inhibits the strongly exothermic reaction
that transforms MgO into Forsterite and further intermediates.
8 Growth of Magnesium Oxychloride Fibers
134
For specimen isolated from the more compact regions of the powder sample
(Figure 8.3 c2), phase analysis revealed magnesium dichlorhydrate MgCl2·6H2O
(Bischofite) as the predominant phase (69%) together with Forsterite (28%) and
periclase (3%) as minor fractions (Supporting Information, Figure S8.3). The sample
heterogeneity develops directly after SiCl4/ O2 adsorption and, thus, before the
reaction cell’s (Figure 8.1) vacuum was broken for sample transfer for materials
characterization. Water from the ambient, which is unavoidable in the present XRD
measurement set up, leads to the transformation of a so far unknown precursor
phase into magnesium dichlorhydrate (Figure 8.3 c2).
TEM images and XRD powder patterns related to fractions c1 and c2 (Figure
8.3) clearly reveal the difference between the starting material (MgO) and
MgCl2·6H2O originating from the strongly exothermic reaction between SiCl4 and
MgO, respectively.
On the other hand, earlier powder XRD and solid state 29Si MAS NMR studies
revealed that the LTA sample consists of MgO nanocubes which are covered with
surface adsorbed chlorosiloxane species SixClyOz.207 Despite their metastability and
high reactivity in the ambient (see below), the chemical composition as well the
composites’s nano- / microstructure can be retained over weeks by storage in
vacuum. Thus, for the here described functionalization approach, the temperature
during adsorption is the determining factor. In case of the room temperature
adsorption (RTA) experiment the thermal energy in conjunction with the heat of
reaction released allows the system to overcome the activation barrier towards
formation of forsterite and another, so far unidentified intermediate phase which in air
quickly transforms into MgCl2·6H2O (middle panel in Figure 8.3). Because of the
limited sample homogeneity of room temperature contacted MgO powder (RTA
Figure 8.3 c) we only continued with LTA samples on the investigation of reactivity
under ambient conditions.
We compared the reactivity of MgO nanocubes that were functionalized with
SixClyOz (LTA) towards water vapor with that of nanocubes with bare surfaces. A
sketch of the performed experiments together with the characteristic microstructures
obtained in the course of these experiments is provided in Figure 8.4. A closed
8 Growth of Magnesium Oxychloride Fibers
135
reaction system was used for keeping the different samples in water saturated air at
T = 298 K and for a specified time.
Figure 8.4: Schematic illustrating the H2O vapor phase induced transformation of
CVS MgO (a) into different microstructures, such as Mg(OH)2 nanosheet
agglomerates (b) or magnesium oxychloride fiber ensembles (c) (out of SixClyOz
functionalized MgO).
The MgO particles are arranged in particle agglomerates (Figure 8.4 a).
Imaging with Scanning Electron Microscopy (SEM) reveals on these agglomerates no
characteristic morphological features in the µm size range. Room temperature
exposure of bare MgO particle surfaces to water vapor produces thin lamellar
features (e.g. see arrow in the upper right panel of Figure 8.4) which, as will be
demonstrated below, correspond to Mg(OH)2 nanosheets. A completely new
microstructural situation, however, originates from water contact of chlorosiloxane
8 Growth of Magnesium Oxychloride Fibers
136
covered MgO nanocube agglomerates for exposure times t ≥ 3 days1 (lower panel in
Figure 8.4): assemblies of needle-like shaped crystals, which have grown radially out
of the individual agglomerates are observed. We also checked for related
microstructural transformation behavior on commercially available nanocrystalline low
surface area MgO material (SBET = 12 m²·g-1) which - prior to storage in water vapor
saturated air - had been subjected to functionalization cycles identical to those
applied to CVS MgO nanocubes (Supporting Information, Table S8.1). Analysis of the
XRD pattern (Supporting Information, Figure S8.4 a) and the SEM images
(Supporting Information, Figure S8.4 b) points to the exclusive formation of crystalline
Mg(OH)2 and the absence of fiber growth or additional crystallographic phases.
The microstructural changes observed for both pure and surface functionalized
MgO nanocubes (bottom of Figure 8.4) are in line with crystallographic structure
changes (Figure 8.5).
Figure 8.5: Left panel: (a) powder XRD pattern of a MgO powder after storage in
water-saturated air (p(H2O) = 32 mbar), (b) a chlorinated CVS MgO powder after
storage in vacuum (p < 10-4 mbar) and (c) a chlorinated CVS MgO powder after
storage in water-saturated air (p(H2O) = 32 mbar). The diffraction patterns are
consistent with a) the brucite phase Mg(OH)2; b) Periclase MgO and c) with the F5
1 Analysis of the XRD patterns reveal that chlorinated MgO nanocubes completely convert into
Mg3(OH)5·Cl∙4H2O fibers after 3 days storage in water-saturated air. TEM images show that water-
contacted samples consist of non-transformed MgO-related agglomerates in addition to fibers.
Absence of any XRD pattern that can be related to the observed non-transformed parts suggests their
low amount or their amorphous nature. However, TEM images reveal minor amount of non-
transformed agglomerates in samples stored for 14 days in water-saturated air compared to those
stored for 3 days.
8 Growth of Magnesium Oxychloride Fibers
137
phase of magnesium oxychloride (Mg3(OH)5·Cl∙4H2O). The right panel contains a
graphical representation of the corresponding crystal structure adapted from
reference [198].
Water vapor converts MgO nanocubes into Mg(OH)2 (a in left panel of Figure
8.5). However, the interaction between chlorosiloxane covered MgO nanocubes and
H2O vapor gives rise to the F5 phase Mg3(OH)5·Cl*4 H2O as concluded from the
XRD powder pattern ( Figure 8.5 c).207,215 A graphical representation of this structure
is provided in the right panel of Figure 8.5. It is important to note that that in the
course of all the experiments performed we have never obtained any evidence for
magnesium oxychloride carbonatation which would result from CO2 uptake from the
gas phase.215
For complementation of materials characterization we analyzed the LTA
samples at the different stages of their transformation with Transmission Electron
Microscopy (Figure 8.6).
Figure 8.6: Representative TEM images of a) chlorinated MgO nanocubes; b)
chlorinated MgO nanocubes after storage in vacuum (p < 10-4 mbar, 14 days) and c)
after 14 days of contact of chlorinated MgO nanocubes with water vapor (p(H2O)=32
mbar).
8 Growth of Magnesium Oxychloride Fibers
138
The granular contrast in Figure 8.6 a and b indicates a high degree of
crystallinity of the particles after low temperature contact with SiCl4/ O2 (Figure 8.2).
Displaying sharp edges and corners their cubic habitus is essentially retained.
Furthermore, consistent with the crystallite domain size, the particle sizes, as
determined by TEM analysis, remain in the range below 10 nm. Irrespective from the
retained primary particle properties, the agglomerates have become more compact
(upper micrographs of Figure 8.6 a and b) in comparison to MgO nanocube powders
before functionalization (Supporting Information, Figure S8.1).216,213
From microscopy and XRD data in conjunction with the NMR evidence for
siloxane formation on MgO surfaces 207 we infer that SiCl4/ O2 adsorption (Figure 8.2)
does not affect the primary particle properties but decreases their average distance
and by that increases the powder density. This explains the changes in the scattering
properties and, thus, the altered macroscopic optical appearance as revealed by the
digital images in Figure 8.3 b. The larger Mg3(OH)5·Cl*4H2O fibers (Figure 8.6) are
arranged as bundles (inset in Figure 8.6 c) with diameters in the range between 100
and 300 nm. After 14 days of H2O exposure we did not evidence residues of MgO
nanocubes which previously had served as Mg2+ ion source for magnesium
oxychloride fiber growth.
8.4.2 A Gas Phase Transformation Process with Thin Water Films as
Liquid Reaction Medium
Magnesium oxychloride fibers grow in aqueous MgO/ MgCl2 mixtures and give
rise to characteristic microstructures which are found in Sorel cement
phases.190,189,217,192 In the course of this study we identified a transformation process
that is based on the interaction between a water vapor saturated gas phase and a
highly dispersed metal oxide nanoparticle powder which was previously processed in
a water free environment. It is important to note at this point, that thin-film water is
ubiquitous in humid environments.218 Under such conditions nanomaterials become
instantaneously covered by water with film thicknesses ranging between one
molecular layer to few nanometers.219 These films provide a reaction medium220,221
with an essentially unexplored surface chemistry.219,218 The here described
transformation process as well as the unexpected reactivity of the chlorinated MgO
8 Growth of Magnesium Oxychloride Fibers
139
nanocube ensemble is attributed to the presence of water multilayers as a confined
liquid solvent medium for ions (Figure 8.7). Concentration gradients related to the
Mg2+, OH- and Cl- ions inside these water films enable ion transport along the
anisotropically growing structures. This sustains the growth process via diffusion and
material precipitation at the top part of the growing fibers.
Figure 8.7: Scheme illustrating the mechanism of fiber growth with [010] as the
growth direction and the (101) plane being subject to fast material addition and
growth. Contact with water vapor leads to the instantaneous formation of condensed
water films which sustain ion transport necessary to add new Mg3(OH)5Cl.4H2O units
to the top of the growing crystalline fibers.
We used SiCl4 as a water free chlorine ion source 214 and coadsorbed O2 at low
temperatures. Upon subsequent sample warming to room temperature under
dynamic vacuum conditions, this surface mixture transforms a fraction of activated
SiCl4/ O2 molecules into chlorosiloxane species that remain on the MgO nanocube
surfaces. Corresponding materials’ state is metastable and undergoes the
spontaneous transformation towards Mg3(OH)5Cl*4H2O fibers at room temperature
(Figure 8.4 and Figure 8.7). Comparison of the two types of nanocrystalline MgO
samples, the CVS MgO nanocube powder (Supporting Information, Figure S8.1 a
and b) and commercially available MgO (Supporting Information, Figure S8.1 c and
d), revealed the critical impact of the degree of dispersion on the transformation
process (Figure 8.4 and Figure 8.7). On CVS MgO, where reactive SiCl4/O2
adsorption (Figure 8.2) can occur, the estimated number of monolayer equivalents
8 Growth of Magnesium Oxychloride Fibers
140
corresponds to approximately 1 and is about 28 times higher for commercially
available nanocrystalline MgO (Supporting Information, Table S8.1). For the latter
type of material the dispersion is too low to enable effective intermixture between
SiCl4/ O2 derived adsorbates, on the one hand, and activating MgO surface sites, on
the other. As a consequence, SixClyOz production is negligible. The evacuation step
performed during warm up and in the last step of each cycle (Figure 8.2) removes all
physisorbed and weakly bound SiCl4/ O2 molecules. In the subsequent step,
exposure of the low surface area material towards H2O (Figure 8.4) leads to the
quantitative transformation of MgO into brucite (Supporting Information, Figure S8.4).
Thus, the concentration of surface activated chlorsiloxanes SixClyOz is insufficient to
engage the reaction path towards magnesium oxychloride needle formation. An
additional explanation relates to the complex reaction network which also requires
Mg-O dissolution to enrich the liquid water film with Mg2+ ions. Size dependent
dissolution effects 213 as well as microstructural parameters which hamper mass
transfer and eliminate the chance for magnesium oxychloride growth in case of the
commercially available starting material may play an additional role.
Silicon originating from SiCl4 is neither part of the crystalline product nor does it
contribute to any other crystalline phase as investigated by XRD measurements. We
expect that after fiber growth amorphous SiOx remains in the base region of the fiber
assemblies.
8.4.3 Vacuum Annealing Induced Decomposition of Oxychloride Fibers
The emergence of anisotropic magnesium oxychloride prompts the question
whether their thermally induced dehydration can be used to obtain nanocrystalline
MgO in more organized microstructures. For this reason, the annealing induced
decomposition of the Mg3(OH)5Cl*4H2O fibers was studied by powder X-ray
diffraction and electron microscopy. Vacuum annealing was applied to the
magnesium oxychloride fibers to temperatures above that of Mg(OH)2 decomposition,
i.e. at T = 673 K, T = 873 K and T = 1173 K.
8 Growth of Magnesium Oxychloride Fibers
141
Figure 8.8: Powder XRD patterns of a) Mg3(OH)5Cl*4H2O as starting material and of
the same material after 30 minutes of vacuum annealing up to the temperatures
specified b) T = 673 K, c) T = 873 K and d) T = 1173 K. The XRD pattern in b
contains diffraction peaks attributed to low hydrates of MgOHCl, and MgCl2*xH2O as
well as the signature which arises from the MgO periclase phase.222 Patterns c and d
reveal the predominant abundance of MgO periclase in addition to low intensity
peaks that are consistent with the Forsterite phase (magnesium silicate).
In the course of annealing to T = 673 K the F5 phase Mg3(OH)5Cl*4H2O
decomposes into periclase (MgO, 73 %) admixed to components containing lower
hydrates of MgOHCl or MgCl2*xH2O223 (Figure 8.8). The conversion into MgO seems
to be nearly completed after annealing to T = 873 K (Figure 8.8 c). From the width of
the MgO specific diffraction peaks we derived an average crystallite domain size of
dXRD = 35 nm which remains in the regime of nanocrystalline materials. Our results
are consistent with previous reports on the thermal decomposition of MgOHCl into
MgO and HCl gas at temperature above T = 750 K.223
MgOHCl MgO + HCl Equation 8.1
Figure 8.8 c reveals the additional presence of low intensity peaks related to
bischofite (10%) and to the Forsterite phase of magnesium silicate (Mg2SiO4, 17%).
The latter contribution indicates that residual silicon has become partially
8 Growth of Magnesium Oxychloride Fibers
142
reincorporated into the crystalline solid. The retransformation to MgO seems to be
completed after annealing at T = 1173 K i.e. at a temperature at which MgO particle
surfaces become dehydroxylated under high vacuum conditions.107 Representative
SEM images in Figure 8.9 shows the related microstructural changes.
Figure 8.9: SEM images of Mg3(OH)5Cl*4H2O samples isolated after vacuum
annealing at a) T = 673 K, b) T = 873 K, and c) T = 1173 K for 30 minutes in each
case.
Figure 8.9 a reveals the morphology changes of the Mg3(OH)5Cl*4H2O sample
after annealing at T = 673 K. The fiber bundles become more compact and at various
places we observed the detaching of thin fiber components.
8 Growth of Magnesium Oxychloride Fibers
143
Although the chemical constituents of the original fibers are partially
decomposed, these images show particles with needle- and ribbon-like morphology
which are different from those typically observed for MgO or Forsterite.213 From the
SEM images taken from the sample that was annealed at T = 873 K it becomes
apparent that the decomposition product is characterized by particulate segregates or
deposits around the original fiber (Figure 8.9 b). Moreover, the overall product
morphology of the sample which was previously annealed to T = 1173 K (Figure 8.9
c) strongly resembles the materials situation related to the sample after annealing to
T = 873 K. Some larger crystallites (> 100 nm) with hexagonal or less regular shapes
and with sharp edges can be identified as well (Figure 8.10).
Figure 8.10: TEM images of particulate products after thermal decomposition of
Mg3(OH)5Cl*4H2O via vacuum annealing to T =1173 K for 30 minutes.
The cartoon of Figure 8.11 summarizes the chemical transformation of MgO
nanocube powders into magnesium oxychloride fiber assemblies (1-3) followed by
their annealing induced pseudomorphic transformation into MgO nanocrystal based
fibers (3-4).
8 Growth of Magnesium Oxychloride Fibers
144
Figure 8.11: Schematic illustrating the organization of MgO nanoparticles into fibers
via the different stages of magnesium oxychloride fiber growth (1 → 3) and
subsequent vacuum annealing (3 → 4).
The structural and in particular the microstructural development represents an
entirely new way to generate MgO based nanostructures which are organized in
microstructures and exhibit a substantially enhanced level of spatial organization.
This opens an interesting promising route to generate nanocrystalline model systems
for surface chemistry studies that will contribute to our understanding of mechanical
property evolution in mineral binders. Moreover, we believe that such model systems
will be extremely useful in completely different fields such as the materials
development for adsorption/ desorption cycles and catalysis.
8.5 Conclusion
Agglomerated MgO nanoparticles can be functionalized with surface
oxychlorides via the gas phase using SiCl4 as chlorine source. This produces
metastable high energy materials which, as a result of H2O vapor exposure, undergo
spontaneous room temperature conversion into magnesium oxychloride fibers. Via
the detailed exploration of the process parameters that are critical for the formation of
magnesium oxychloride fibers we also demonstrate the great potential of CVS MgO
particles as a model system for the elucidation of growth of ceramic anisotropic
nanostructures, the associated heterogeneous chemistry and, ultimately, the
evolution of resulting microstructures. Links between materials’ dispersion and
functionalization temperature, on the one hand, and resulting hydroxide and
oxychloride structures, on the other, were established. An important aspect relates to
the localization of the microstructure evolution observed, i.e. the base of
8 Growth of Magnesium Oxychloride Fibers
145
Mg3(OH)5Cl*4H2O fiber growth. Spatially confined reaction media both in the µm
range (agglomerates of functionalized MgO nanocubes) as well as in the range of
few nanometers (thin liquid films as media for ion transport and reaction) were shown
to determine the transformation and microstructure evolution. Finally we addressed
the annealing induced re-transformation of Mg3(OH)5Cl*4H2O needles into spatially
organized metal oxide nano- and microstructures and demonstrate a novel way of
assembling metal oxide nanoparticles well-organized microstructures such as fibers.
8 Growth of Magnesium Oxychloride Fibers
146
8.6 Supporting Information
8.6.1 Electron Microscopy
Figure S8.1: TEM images of CVS MgO (a) and commercial MgO (b) nanoparticles
as reference material. SEM images of CVS MgO (c) and commercial MgO (d)
nanoparticles. All images were taken after oxidation (T = 1123 K, p = 10 mbar O2)
and vacuum annealing (T = 1173 K, p < 5∙10-6 mbar).
Transmission Electron Microscopy clearly shows that CVS MgO is
characterized by agglomerates of highly dispersed monocrystalline nanocubes with a
high portion of edge and corner features (Figure S8.1 a). In contrast to MgO
nanocubes, commercial MgO can be characterized as an assembly of less regular
shaped particles (Figure S8.1 b).
8 Growth of Magnesium Oxychloride Fibers
147
8.6.2 Powder X-ray Diffraction
Figure S8.2: Powder XRD patterns of CVS MgO and commercial MgO nanoparticle
powders after oxidation (T = 1123 K, p = 10 mbar O2) and vacuum annealing
(T = 1173 K, p < 5 ·10-6 mbar). The diffractograms clearly reveal that both particle
types adopt crystal structures of the cubic phase. (Diffraction peaks indicating 2θ
positions for the periclase structure are indicated as bars.)
8.6.3 Estimated Surface Coverages of SiCl4/ O2 on Nanocrystalline MgO
Samples
Table S8.1: MgO sample weight, surface area and active sites available during the
exposure experiment. Pressure, number of molecules, monolayer equivalent (MLE)
and incident molecular flux of SiCl4 and O2 provided during one cycle.
*. According to a previous study the estimated number of active sites (corners and
edges) corresponds to roughly 3% of the total surface area available.
SiCl4/ cycle O2 / cycle
weight (mg)
surface area (m
2)
Est. number of active
sites* (m
-2)
pressure (mbar) at
RT
number of provided
molecules
MLE (%)
incident molec.
flux ( m
-2 s
-1)
pressure (mbar) at
RT
incident molecular
flux (m
-2 s
-1)
CVS MgO 200 60 1.8 300 3 × 1021
118 1·1027
700 6·1027
commercial MgO
200 2.5 << 1 300 3 × 1021
2832 1·1027
700 6·1027
8 Growth of Magnesium Oxychloride Fibers
148
Table S8.1 lists surface area of MgO powder samples, the estimated number of
active surface sites, number of molecules supplied at a given pressure, monolayer
equivalents (MLE, i.e. molecules provided to achieve a monolayer coverage at a
sticking coefficient of S = 1) and incident molecular flux of SiCl4 and O2 per cycle.
The gas properties are measured at T = 298 K considering the volume and other
conditions of gas reservoir before exposure.
8.6.4 XRD Phase Analysis of Reaction Products after contact of CVS
MgO with SiCl4 at RT
The quantitative phase analysis – with a good quality of refinement (e.s.d. of
wt% ~1 %) - is demonstrated in Figure S8.3.
Figure S8.3: XRD pattern and results of the phase analysis for the same powder
indicated in Figure 8.3 as fraction c2 and obtained after CVS MgO exposure to
SiCl4/O2 at room temperature.
8 Growth of Magnesium Oxychloride Fibers
149
8.6.5 Control Experiment with Commercial MgO
Figure S8.4: Commercial MgO after exposure to SiCl4/O2 and subsequent exposure
to water-saturated air (p(H2O) = 32 mbar): a) Powder XRD pattern revealing the
exclusive existence of brucite Mg(OH)2; b) SEM image.
150
9 Summary
151
9 Summary
The impact of different processing parameters in conjunction with nature and
composition of surrounding atmosphere on the metal oxide nanoparticle properties
optical absorption, photoluminescence emission, chemical composition, crystal
structure, morphology and size have been investigated. Several vapor phase grown
metal oxide nanoparticles and composite thereof have been synthesized and
explored using different experimental methods.
MgO and ZnO were synthesized by chemical vapor synthesis (CVS) and metal
organic-chemical vapor synthesis (MO-CVS) methods, respectively. An opposite
effect of O2 adsorption on the photoluminescence (PL) emission properties of these
two prototypical metal oxides has been observed. While molecular oxygen acts for
surface excited states of MgO as a PL quencher, it enhances the PL emission at
hvEm = 2.1 eV for ZnO nanoparticles. Stronger enhancement in PL emission was
found for as-synthesized ZnO nanoparticles as compared to the annealed ZnO
nanoparticles. Auger electron spectroscopic analysis reveals the existence of oxygen
interstitials in the surface near region of as-synthesized ZnO and the annihilation of
these defects by annealing. The enhancement in PL emission of as-synthesized
ZnO is explained by band bending effects on ionic semiconductors. For ZnO
nanoparticles gaseous oxygen affects the surface band bending by producing an
electric field near the surface that separates the electron-hole pair and promotes hole
transport to the surface. Subsequent hole trapping by oxygen interstitials as deep
trap states enforces their recombination with photogenerated electrons yielding
yellow PL light.
Particles of the ternary Zn-Mg-O system were prepared by the means of flame
spray pyrolysis (FSP) and subsequent vacuum annealing. Solid solution of Zn2+ in
periclase MgO is obtained for the nanoparticles with Zn2+ concentration of 10 at.%.
Higher Zn2+ concentrations of 30 at.% leads to mixtures where phase separation into
an MgO rich periclase and a ZnO rich wurtzite phase results from thermal treatment.
While gaseous oxygen partially reduces the PL emission of the hydroxylated
(as-synthesized) nanoparticles containing 10 at.% Zn2+, it entirely quenches the
emission in dehydroxylated particles after vacuum annealing. It is found that the PL
9 Summary
152
emission observed at hvEm = 2.1 eV for the as-synthesized Zn-Mg-O particles with
separated wurtzite phase is not quenched by gaseous oxygen but it is depleted after
vacuum annealing. Moreover, hydroxyls were found to act as protecting groups
against gaseous oxygen as a PL quencher at the solid−gas interface.
Using MO-CVS and subsequent annealing procedures in oxygen atmosphere,
Fe-Mg-O nanocomposite particles of different compositions (Fe/(Fe+Mg) = 1, 6 or
9 at.%) are synthesized. Fe-Mg-O nanoparticles that are annealed up to T = 873 K
are characterized as solid solutions of Fe3+ in periclase MgO which can reveal
superparamagnetic properties. Annealing of (1%) Fe-Mg-O samples up to T = 1173 K
leads to coarsening of the cubic particles that show superparamagnetic properties.
Annealing of (6% and 9%) Fe-Mg-O up to T = 1173 K leads to phase separated
particle mixtures containing periclase MgO and magnesioferrite MgFe2O4 phases that
are characterized as particles with antiferromagnetic behavior. The results point to
annealing induced crystal growth and phase separation which is facilitated by cation
vacancies emerging from the charge imbalance of Fe3+ ions in the Mg2+ lattice. In
addition, the optical absorption and PL emission properties of Fe-Mg-O particles are
investigated using UV-Vis diffuse reflectance and PL spectroscopy. The absorption
bands in the energy range 2.6 eV ≤ hv ≤ 5.6 eV are observed and attributed to
different ligand field and ligand-to-metal charge transfer electronic transitions. It is
found that the admixture of Fe3+ to MgO quenches the MgO specific PL emission
originating from surface excitons. This points to annealing induced surface
segregation of Fe3+ ions in Fe-Mg-O nanocomposites.
In the last part of this work, MgO is exposed to SiCl4 and O2 via the vapor phase
in a cyclic process. It is found that different reaction paths can occur depending on
adsorption temperature. While at room temperature adsorption (T = 298 K) of SiCl4 a
strong exothermic process leads to sample heterogeneity, at low temperature
(T = 77 K) adsorption homogeneous surface coverage of the MgO particle
ensembles with SiCl4 occurs. The low temperature adsorption leads to MgO
nanocubes which are covered with surface adsorbed chlorosiloxane SixClyOz species
as revealed by microscopy and XRD data in conjunction with the NMR studies. The
obtained MgO-SixClyOz material system is highly metastable and transforms
spontaneously into magnesium oxychloride Mg3(OH)5Cl∙4H2O needle-like structures
9 Summary
153
upon contact with water vapor. The absence of needle formation in the case of low
surface area MgO reveals the importance of high materials dispersion for the here
reported spontaneous phase transformation. This study also suggests that
condensed water layer on the growing fibers provides an ion transport and reaction
medium for their directional growth. Moreover, it is demonstrated that annealing
retransforms magnesium oxychlorides into the parental magnesium oxide particles
assembled in well-ordered fiber-like microstructures.
The here presented results have shown several annealing induced property
changes in metastable metal oxide nanoparticles. On the other hand it also became
clear that the chemical composition of the surrounding continuous phase strongly
influences the properties of the vapor phase grown metal oxide nanoparticles during
post-synthesis processing, aging and, ultimately, measurement. Moreover, aside
from the progress in the functionalization of MgO nanocubes with oxychlorides, the
growth of magnesium oxychloride Mg3(OH)5Cl∙4H2O fibers via water vapor contact
introduces a new approach for the fabrication of oxide and hydroxide fibers in the
ambient. In general, the results underline the necessity of considering sample history
and nature of the surrounding atmosphere in characterization of the metal oxide
nanoparticles. Related insights advance the understanding of the process-induced
property changes that give rise to differences in the functional performance of
identical materials that are treated differently or to formation of completely new type
of nanomaterials.
154
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
155
10 Appendix I: Impact of Annealing Processes on MgO
Nanoparticles Size Distribution and Morphology
This chapter will discuss the impact of post-synthesis annealing processes on
particle morphology and size distribution of MgO nanoparticle samples produced by
chemical vapor synthesis (CVS). The synthesis approach is already described in
detail in section 3.1.1.1. The motivation for these studies comes from the unexpected
crystal growth which has been observed for MgO nanoparticles annealed in the
presence of residual gas atmosphere and high pressure of oxygen (pO2 = 650 mbar),
compared to those annealed under vacuum condition. This chapter is divided into
three parts. First, the structural and morphological characterization results which
show the differences in MgO nanoparticle samples annealed by two different
annealing programs will be discussed. Second, mass spectroscopic analysis done on
MgO samples annealed under vacuum condition will be shown at different evacuation
and annealing steps. Third, the crystal growth trend will be presented which is
investigated by considering different dwell times used in annealing programs.
10.1 MgO nanoparticle changes influenced by annealing conditions
Throughout this work it has been seen that vapor phase grown MgO
nanoparticle properties like particle size and morphology are influenced by annealing
conditions. In this context annealing condition refers to the presence or absence of
residual gases namely H2O, O2 and COx. Two employed annealing programs are:
i) annealing in vacuum condition named standard program and ii) annealing in the
presence of oxygen and residual gases named program number 1 (P1).
10.1.1 Standard Annealing
In this typical standard annealing program the cell containing as-synthesized
powder sample was first evacuated (p < 10-5 mbar) at room temperature and then
heated to T = 1173 K in 100 K-steps according to the program described in Table
10.1 and Figure 10.1.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
156
Table 10.1: Standard vacuum annealing program used for the activation of
CVS-grown MgO.
temperature (K) rate (K /min) pressure (mbar) dwell time† (min)
373 5 < 1.0 · 10-5 * 0
473 5 < 1.0 · 10-5 * 0
573 5 < 1.0 · 10-5 * 0
673 10 < 1.0 · 10-5 * 0
773 10 < 1.0 · 10-5 * 0
873 10 < 1.0 · 10-5 * 0
973 10 < 1.0 · 10-5 * 0
1073 10 < 1.0 · 10-5 * 0
1173 10 < 1.0 · 10-5 * 0
1123 30 O2 pressure = 10 10
1123 30 < 1.0 · 10-5 * 0
1173 30 < 5.0 · 10-6 60
*. Base pressure; before increasing to the next temperature, base pressure must be
reached. †. Dwell time is the programed time that system remains at each
temperature after reaching base pressure.
Just shortly after reaching T = 1173 K sample was cooled down to T = 1123 K
and contacted by 10 mbar of oxygen for 10 minutes to remove organic contaminants.
After evacuation of the oxygen, temperature was again increased to T = 1173 K and
it was kept for 60 minutes at pressures less than 5·10-6 mbar. This thermal treatment
takes 6 to 8 hours (depending on vacuum pumps used and amount of annealed
MgO) and offers perfect conditions to have a clean metal oxide surface as proved by
FT-IR spectroscopy.107
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
157
Figure 10.1: Temperature profile (black line, left ordinate scale) and pressure profile
(blue line, right ordinate scale) during standard annealing. At T = 1123 K the samples
were oxidized with 10 mbar O2 for 10 minutes.
10.1.2 P1 Annealing
In this annealing method, outlined in Figure 10.2, at the beginning the cell
containing MgO nanoparticles powder was evacuated (p < 10-5 mbar) at room
temperature. The respective sample was then heated to T= 373 K at a rate of
2.5 K min-1, held at this temperature for 15 min (dwell time, td) and then was again
evacuated to p < 10-5 mbar. Further annealing steps are carried out at p = 650 mbar
of oxygen. The powder was stepwise heated in oxygen atmosphere to T = 473 K
(r = 5 K min-1, td = 15 min), T = 673 K (r = 10 K min-1, td = 30 min), T = 873 K (r = 10 K
min-1, td = 180 min) and T = 1173 K (r = 10 K min-1, td = 60 min).
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
158
Figure 10.2: Temperature profile (solid line, left ordinate scale) and applied oxygen
pressure (dashed line, right ordinate scale) during P1 annealing procedure.
(r: heating rate; td: dwell time). Cooling times are not shown.
After each annealing step the sample was cooled down to room temperature
(cooling time ≈ 30 min) followed by an evacuation (≈ 15 min) to base pressure of
p < 10-5 mbar in order to remove water and CO2 as oxidation products.
10.1.3 Results
10.1.3.1 Scanning Electron Microscopy
SEM images of the MgO nanoparticle powders after standard (Figure 10.3 a)
and P1 (Figure 10.3 b) annealing programs show visible differences in agglomerate
appearance. The SEM images of standard annealed sample correspond to fluffy and
loosely bond powders. The SEM images of P1 annealed sample reveal more
compact powders consisting of particulate objects in the size range of tens of
nanometers.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
159
Figure 10.3: SEM images of MgO nanoparticle powder samples a) after standard
annealing; b) after P1 annealing. The SEM images show powders of sample after P1
annealing are coarser grained compared to samples after standard annealing.
10.1.3.2 Transmission Electron Microscopy
The TEM images of the MgO nanoparticle powder samples annealed by
standard method show high dispersion (Figure 10.4 a and b) with narrow particle size
distribution and average particle size of about 4 nm (Figure 10.5 a). The small
particle sizes as well as absence of sintering and solid-solid interfaces in this sample
lead to strong charging effects during transmission electron analysis. Movement of
particle agglomerates under electron beam made high resolution imaging very
difficult. Although the shape of the particles at this low resolution is not completely
clear but mostly particles with cubic morphology can be seen. Inspection of TEM
images of P1 annealed MgO sample (Figure 10.4 c and d) reveals that in spite of
significant increase in average particle size to about 18 nm, the particle size
distribution remains narrow (Figure 10.5 b). The P1 annealed MgO samples reveal
more compact powders with interconnected cubic particles compared to standard
annealed samples.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
160
Figure 10.4: TEM images of MgO nanoparticle powder samples a and b) after
standard annealing; c and d) after P1 annealing. The TEM images show powders of
samples after P1 annealing have interconnected cubic particles and are more
compact compared to samples after standard annealing.
Figure 10.5: Cumulative particle size distribution plots for MgO nanoparticle powder
samples a) standard annealed; b) P1 annealed. The distribution is obtained from the
edge length of particles at different regions of each sample based on TEM images
(about 300 particles measured for each distribution).
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
161
10.1.3.3 X-ray Diffraction
X-ray diffraction (XRD) patterns of MgO nanocubes confirm that all samples
possess periclase cubic structure (JCPDS card # 45-0946) irrespective of annealing
method applied (Figure 10.6). The XRD pattern of the P1 annealed sample reveal
narrowing of the periclase phase diffraction features widths, which points to volume
increase of the coherently scattering solid and, therefore, to particle coarsening by
annealing in oxygen. This is in good agreement with particle growth deduced from
TEM images. Using Scherrer equation and from the full width at half maximum
(FWHM) of the MgO diffraction peaks the average crystalline domain sizes can be
calculated as 5 ± 1 nm and 16 ± 1 nm for standard and P1 annealed samples,
respectively.
Figure 10.6: X-ray diffraction patterns of MgO nanoparticle powder samples
a) standard annealed; b) P1 annealed. Vertical lines correspond to the standard XRD
pattern of periclase MgO (JCPDS card # 45-0946).
10.2 Mass Spectroscopy during Standard Annealing of MgO
Mass spectrometer was used to qualitatively track the composition of the
residual gas atmosphere and desorbed species (especially H2O) during cell
evacuation and annealing of MgO. For this purpose 0.2 g of MgO nanoparticle
powder produced by chemical vapor synthesis was evacuated and standard
annealed using the setup shown in Figure 10.7.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
162
Figure 10.7: Schematic diagram of the vacuum annealing setup equipped with mass
spectrometer.
The experimental steps are as below:
1) Prior to evacuation, the as synthesized MgO powder sample was kept in air
for 1 hour to allow complete water adsorption from humidity
2) The sample was slowly evacuated by gradually opening the valve (sample
valve) between sample glass cell and vacuum chamber while only the rotary
vacuum pump was connected. The opening of the valve took almost 50
minutes. During this time mass spectrometer was working and the metering
valve was half opened at a fixed value of 500. This amount of opening in
metering valve guarantees the pressure less than 10-4 mbar which is needed
for the safe operation of Faraday detector in mass spectrometer analyzer.
3) After completely opening of the sample valve, turbo molecular pump was
connected.
4) The sample was annealed stepwise following the standard annealing
program until reaching to p < 5·10-6 mbar at T = 873 K. The sample was
then cooled down overnight.
5) The day after the sample was directly annealed to T = 973 K, T = 1073 K
and T = 1173 K until reaching to p < 5·10-6 at T = 1173 K when it was cooled
down afterwards.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
163
10.2.1 Mass Spectroscopy during Initial Evacuation
Mass spectroscopy during sample valve opening at room temperature
(T = 297 K) shows the trend view of the relative pressure (or relative abundance)
changes of desorbed species (Figure 10.8). Figure 10.8 also contains the changes of
the total pressure in mass spectrometer chamber measured by cold cathode gauge.
The trend graphs as well as mass spectra at different times reveal that at the
beginning the majority of detected species consist of N2 and O2 (Figure 10.9 a)
whereas at the last minutes mainly OH and H2O are detected (Figure 10.9 b). It
should be mentioned that very small amounts (i.e. low partial pressure as recorded
by MS) of other masses which may are the fragment ions (e.g. CO2 fragments) are
also recorded by mass spectrometer.
Figure 10.8: Trend graph of relative pressure changes over time of the sample valve
opening. Partial pressures (link ordinate scale) are recorded by mass spectrometer
and total pressure (right ordinate scale) is measured by cold cathode pressure gauge
installed in the MS chamber. The fluctuation (peaks) of the curves arises from
stepwise opening of the sample valve. The related mass spectra of the time slots
indicated by vertical dashed lines are shown in Figure 10.9.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
164
Figure 10.9: Mass spectra related to time slots indicated in Figure 10.8 by vertical
dashed lines. a) at t = 42.18 min; b) at t = 50.62 min.
10.2.2 Mass Spectroscopy during Vacuum Annealing
The temperature and vacuum chamber pressure profiles during vacuum
annealing are illustrated in Figure 10.10. The figure indicates how the total pressure
in the vacuum chamber increases due to heating induced desorption of molecules as
well as the time necessary to reach again a pressure less then p<10-5 mbar. It is
observed that at around T = 573 K the total pressure reaches the maximum value.
This temperature relates to maximum water and hydroxyl desorption as revealed
from mass spectra. It is further found out that the dwell times are highly dependent on
the vacuum capacity of the pumps used.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
165
Figure 10.10: Temperature (black line, left ordinate scale) and vacuum chamber
pressure (blue line, right ordinate scale) profiles during standard annealing. Unlike
standard annealing there is no oxidation step at T= 1123 K. The related mass spectra
of the time slots indicated by vertical dashed lines are shown in Figure 10.11.
To qualitatively compare the main desorbed species during the standard
annealing, for some selected time slots, (four vertical dashed lines indicated in Figure
10.10) the mass spectra recorded by mass spectrometer are shown in Figure 10.11.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
166
Figure 10.11: Mass spectra of the desorbed species during standard annealing
related to the time slots indicated in the Figure 10.10.
10.3 Annealing Dwell Time Effect
In this part the influence of different annealing dwell times on MgO crystal
growth and particle morphology is described. The effect of different dwell times at
T = 1173 K is discussed for annealing in vacuum condition as well as annealing in
the presence of oxygen and residual gases. Changes in crystal size are investigated
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
167
by analysis of XRD pattern peak broadening and particle size distributions derived
from TEM micrographs. Changes in morphology are also detected with TEM
micrographs.
10.3.1 Description of applied annealing programs
The vacuum annealing employed corresponds to standard annealing program
which is described in section10.1.1 but extended to variable dwell times at
T = 1173 K (Figure 10.12). 3 dwell times used for annealing of the sample at T =
1173 K are 1h, 7h and 10h. Due to experimental reasons the sample is cooled down
to room temperature over night after reaching T = 1173 K for the first time.
Figure 10.12: Temperature profile (black line, left ordinate scale) and pressure profile
(blue line, right ordinate scale) during vacuum annealing. At T = 1123 K the samples
were oxidized with 10 mbar O2 for 10 minutes. At T = 1173 K the MgO samples were
annealed with different dwell times of 1h, 7h and 10h to investigate crystal shape
and domain size changes. td = dwell time.
An annealing program named P2 has been employed to study the influence of
variable dwell times in the presence of oxygen and residual gases. 3 dwell times
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
168
used for P2 annealing of the sample at T = 1173 K are 1h, 7h and 10h. P2 annealing
program is described in Figure 10.13 and carried out as follows: First at room
temperature the sample is evacuated to a pressure of p < 10-5 mbar to remove
surrounding gas atmosphere. Mass spectroscopy results (Figure 10.9 and Figure
10.11) suggest that at this stage the sample`s surface remains covered with
adsorbed water molecules. After initial evacuation a constant oxygen pressure (p =
650 mbar) is applied and sample is heated to T = 873 K (r = 10 K∙min-1) and kept at
this temperature for 30 minutes. Subsequently, the sample is cooled down to room
temperature and the supernatant gases are removed by evacuating to p < 10-5 mbar.
Prior to the next heating step to T = 1173 K (r = 10 K∙min-1) replacement of residual
gas atmosphere with fresh oxygen (p = 650 mbar) is performed. At T = 1173 K
identical dwell times as vacuum annealing are applied. Finally the sample is cooled
down to room temperature and annealing program is finished.
Figure 10.13: Temperature profile (black line, left ordinate scale) and applied oxygen
pressure (blue line, right ordinate scale) during P2 annealing. At T = 1173 K the MgO
samples were annealed with different dwell times of 1h, 7h and 10h. td = dwell time.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
169
10.3.2 Results
The effect of P2 annealing in the presence of oxygen and residual gases on
MgO particle size and morphology in comparison to standard annealing in vacuum
condition was studied. The analysis was done by means of TEM micrographs and x-
ray diffraction and can be summarized as follows:
Figure 10.14: TEM images of the MgO powder samples after a) 1h, b) 7h and c) 10h
standard annealing in vacuum at T = 1173 K.
TEM micrographs reveal that long dwell times up to 10h at T = 1173 K does not
influence the cubic shape of MgO nanoparticles by standard annealing (Figure
10.14). Compared to standard annealing, P2 annealing in the presence of oxygen
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
170
and residual gases lead to dramatic change in the shape of particles for all dwell
times used (Figure 10.15).
Figure 10.15: TEM images of the MgO powder samples after a) 1h, b) 7h and c) 10h
P2 annealing at T = 1173 K. The annealing is performed in the presence of oxygen
and residual gases.
In the case of P2 annealing all particles are more or less rounded in shape and
almost no cubes were observed (Figure 10.15). Moreover, high amount of sintering
can be seen between single crystals. The TEM images reveal steps and terraces on
the particle surfaces.
On the basis of the TEM micrographs, particle size distributions (PSD) were
derived (Figure 10.16 left). For MgO nanoparticles annealed with dwell times of 1h,
7h and 10h, standard annealing leads to almost identical PSDs having median
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
171
values of around 4 to 5 nm. In the case of P2 annealed nanoparticles, the PSDs are
broadened and shifted to bigger sizes with median values in the range of 20 nm
(td = 1h) and 25 nm (td = 7h and 10h). For P2 annealing, increase of dwell time from
1h to 7h leads to more particle growth, compared to standard annealing (Figure
10.16).
Figure 10.16: Left panel: Cumulative particle size distributions (PSD) of the annealed
MgO samples showing particle growth in the case of P2 annealing. Right panel:
Median values of the cumulative PSDs in dependence of dwell time at T = 1173 K.
Colored areas represent the borders of the respective distributions, showing a
significant broadening in case of P2 annealing.
XRD patterns for the respective samples confirm the trend in crystal growth
which has been seen from the results of particles size distributions. A significant
narrowing of the reflexes in case of P2 annealing is observed, implying an increase in
crystallite domain size. The XRD results are depicted in Figure 10.17. Using Scherrer
equation and from the full width at half maximum of the most pronounced reflexes
(related to 200 and 220 planes), an increase in crystallite domain size for the P2
annealed samples can be observed (Figure 10.18). Furthermore, with increasing the
dwell time at T = 1173 K, the crystallite sizes of P2 annealed samples seem to grow,
whereas no significant crystal growth can be seen for the standard annealed
samples.
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
172
Figure 10.17: X-ray diffraction (XRD) patterns of MgO powder samples a) standard
annealed; b) P2 annealed. The patterns attribute to the samples annealed with
different dwell times of 1h, 7h and 10h at T = 1173 K. Vertical lines correspond to the
standard XRD pattern of periclase MgO (JCPDS card # 45-0946).
Figure 10.18: Crystallite dimensions x200 and x220 of standard vacuum annealed and
P2 annealed samples with different dwell times of 1h, 7h and 10h at T = 1173 K.
10.4 Summary and Conclusion
From the results discussed in this appendix, it became clear that annealing in
the presence of oxygen and residual gases has a big influence on the particle size
and morphology of the vapor phase grown MgO samples. The MgO particles can
grow to sizes in the range of 20 nm by using P1 (Figure 10.2) or P2 (Figure 10.13)
annealing programs. Unlike P1 and standard annealing methods which lead to
10 Appendix I: Impact of Annealing Processes on MgO Nanoparticles
173
particles with cubic shape (Figure 10.4 and Figure 10.14), P2 annealing gives rise to
particles with non-cubic shape (Figure 10.15).
The reason for different effects of these annealing programs can be found in
differences in process conditions. As described in Figure 10.1 standard annealing is
carried out under dynamic vacuum condition which means that all desorbing species,
especially water, are pumped away instantly. Influences of the residual gases are
therefore negligible. In P1 annealing the first heating step is performed under the
same condition as standard annealing. This means that P1 annealing up to T = 373 K
is done under dynamic vacuum followed by evacuation to p < 10-5 mbar. Moreover,
before each heating step in P1 annealing, residual gases are replaced by fresh
oxygen. This leads to an efficient removal of adsorbed water species from the particle
surfaces, the same as standard annealing. In contrast, P2 annealing procedure
provides the condition for only one residual gas replacement step which is after
T = 873 K. Therefore in P1 annealing up to T = 873 K the presence of residual water
must be considered.
The results suggest that annealing in the absence of water allows the (100)
surfaces become the most stable MgO surfaces whereas annealing in the presence
of adsorbed water destabilizes the (100) planes and allows other surfaces like (111)
become stabilized. As a result, MgO particles with cubic shape are obtained by
annealing in the water free condition and particles with non-cubic shape are obtained
by annealing in the presence of residual water. More detailed investigations are
required in order to gain in-depth understanding of the residual water induced
changes of MgO nanocubes during annealing. Such insight will improve the control
over particle size and morphology which determine the chemical and physical
properties of MgO nanoparticles like their surface activity and optical properties.
174
11 Bibliography
175
11 Bibliography
[1] International organization for standardization. Terms describing categories of nanostructured material. ISO/TS 80004-4:2011(en). Available at www.iso.org (Last seen: October 2015).
[2] Oksam, G. Metal oxide nanoparticles: synthesis, characterization and application, Journal of Sol-Gel Science and Technology 37, 161-164 (2006).
[3] Hahn, Y.-B. & Umar, A. Metal oxide nanostructures and their applications, American Scientific Publishers (2010).
[4] Pacchioni, G. & Freund, H. Electron transfer at oxide surfaces. The MgO paradigm: From defects to ultrathin films, Chemical Reviews 113, 4035–4072 (2012).
[5] Jupille, J. & Thornton, G. Defects at oxide surfaces, Springer International Publishing (2015).
[6] Wang, Z. G. Zu, X. T. Zhu, S. & Wang, L. M. Green luminescence originates from surface defects in ZnO nanoparticles, Physica E: Low-dimensional systems and nanostructures 35, 199–202 (2006).
[7] Faure, B. et al. Dispersion and surface functionalization of oxide nanoparticles for transparent photocatalytic and UV-protecting coatings and sunscreens, Science and Technology of Advanced Materials 14, 23001 (2013).
[8] Kango, S. et al. Surface modification of inorganic nanoparticles for development of organic–inorganic nanocomposites, Topical Issue on Polymer Hybrids 38, 1232–1261 (2013).
[9] Reed, J. & Ceder, G. Role of electronic structure in the susceptibility of metastable transition-metal oxide structures to transformation, Chemical Reviews 104, 4513–4534 (2004).
[10] Scaramuzza, S. Agnoli, S. & Amendola, V. Metastable alloy nanoparticles, metal-oxide nanocrescents and nanoshells generated by laser ablation in liquid solution: influence of the chemical environment on structure and composition, Physical Chemistry Chemical Physics (2015).
[11] Berger, T. Schuh, J. Sterrer, M. Diwald, O. & Knözinger, E. Lithium ion induced surface reactivity changes on MgO nanoparticles, Journal of Catalysis 247, 61–67 (2007).
[12] Stankic, S. Sternig, A. Finocchi, F. Bernardi, J. & Diwald, O. Zinc oxide scaffolds on MgO nanocubes, Nanotechnology 21 (2010).
[13] Berger, T. & Diwald, O. Defects in metal oxide nanoparticle powders, in defects at oxide surfaces by J. Jupille and G. Thornton, Springer International Publishing (2015).
[14] Mowbray, D. J. et al. Trends in metal oxide stability for nanorods, nanotubes, and surfaces, Journal of Physical Chemistry C 115, 2244–2252 (2011).
11 Bibliography
176
[15] Leite, E. R. et al. Development of metal oxide nanoparticles with high stability against particle growth using a metastable solid solution, Advanced Materials 14, 905 (2002).
[16] Riss, A. Elser, M. J. Bernardi, J. & Diwald, O. Stability and photoelectronic properties of layered titanate nanostructures, Journal of the American Chemical Society 131, 6198–6206 (2009).
[17] Vayssieres, L. On the thermodynamic stability of metal oxide nanoparticles in aqueous solutions, International Journal of Nanotechnology 2, 411–439 (2005).
[18] Zhu, J. et al. Two-step synthesis of Fe2O3 and Co3O4 nanoparticles. Towards a general method for synthesizing nanocrystalline metal oxides with high surface area and thermal stability, RSC Advances 2, 121–124 (2012).
[19] Korotcenkov, G. & Cho, B. K. Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement, Sensors and Actuators B: Chemical 156, 527–538 (2011).
[20] Hornyak, G. L. Introduction to nanoscience & nanotechnology, CRC Press (2009).
[21] Jiang, Q. & Yang, C. C. Size effect on the phase stability of nanostructures, Current Nanoscience 4, 179-200 (2008).
[22] Wilde, G. Nanostructured Materials. 1st ed., Elsevier (2009).
[23] McKenna, K. P. Sushko, P. V. & Shluger, A. L. Inside powders: A theoretical model of interfaces between MgO nanocrystallites, Journal of the American Chememical Society 129, 8600–8608 (2007).
[24] Sushko, P. V. Gavartin, J. L. & Shluger, A. L. Electronic properties of structural defects at the MgO(001) surface, Journal of Physical Chemistry B 106, 2269-2276 (2002).
[25] Mueller, M. et al. Effect of protons on the optical properties of oxide nanostructures, Journal of the American Chememical Society 129, 12491–12496 (2007).
[26] Spoto, G. et al. Carbon monoxide MgO from dispersed solids to single crystals: A review and new advances, Progress in Surface Science 76, 71–146 (2004).
[27] Cornu, D. et al. Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis, Physical Chemistry Chemical Physics 15, 19870–19878 (2013).
[28] Stankic, S. et al. Novel optical surface properties of Ca2+-doped MgO nanocrystals, Nano Letters 5, 1889–1893 (2005).
[29] A L Shluger et al. Self-trapping holes and excitons in the bulk and on the (100) surfaces of MgO, Journal of Physics: Condensed Matter 3, 8027 (1991).
[30] Shluger, A. L. Sushko, P. V. Kantorovich, L. N. Spectroscopy of low-coordinated surface sites: Theoretical study of MgO, Physical Review B 59, 2417 (1999).
11 Bibliography
177
[31] Stankic, S. et al. Size-dependent optical properties of MgO nanocubes, Angewandte Chemie International Edition 44, 4917–4920 (2005).
[32] Sternig, A. et al. Photoluminescent nanoparticle surfaces: The potential of alkaline earth oxides for optical applications, Advanced Materials 20, 4840–4844 (2008).
[33] Heifets, E. Eglitis, R. Kotomin, E. Maier, J. & Borstel, G. Ab initio modeling of surface structure for SrTiO3 perovskite crystals, Physical Review B 64, 235417 (2001).
[34] Rodriguez, J. A. Electronic and chemical properties of mixed-metal oxides: basic principles for the design of DeNOx and DeSOx catalysts, Metallic Oxides: filling the gap between catalysis and surface science 85, 177–192 (2003).
[35] Lin, S. S. et al. Phosphorus doped Zn1-xMgxO nanowire arrays, Nano Letters 9, 3877–3882 (2009).
[36] Gaskov, A. & Rumyantseva, M. In sensors for environment, health and security, edited by M.-I. Baraton, Springer, 3-30 (2009).
[37] Sun, M. Liu, H. Liu, Y. Qu, J. & Li, J. Graphene-based transition metal oxide nanocomposites for the oxygen reduction reaction, Nanoscale (2014).
[38] Ingler Jr, W. B. & Khan, S. U. M. Photoresponse of spray pyrolytically synthesized magnesium-doped iron (III) oxide (p-Fe2O3) thin films under solar simulated light illumination, Thin Solid Films 461, 301–308 (2004).
[39] Orna, J. et al. FeO/MgO/Fe heteroepitaxial structures for magnetic tunnel junctions, IEEE Transactions 44, 2862–2864 (2008).
[40] Rakmak, N. Wiyaratn, W. Bunyakan, C. & Chungsiriporn, J. Synthesis of Fe/MgO nano-crystal catalysts by sol–gel method for hydrogen sulfide removal, Chemical Engineering Journal 162, 84–90 (2010).
[41] Bhide, V. G. & Tambe, B. R. Investigation of the MgO∶Fe system using the mössbauer effect, Journal of Materials Science 4, 955-961 (1969).
[42] Groves, G. W. & FINE, M. E. Solid solution and precipitation hardening in
Mg‐Fe‐O alloys, Journal of Applied Physics 35, 3587–3593 (1964).
[43] Myrach, P. et al. Temperature-dependent morphology, magnetic and optical properties of Li-doped MgO, ChemCatChem 2, 854–862 (2010).
[44] Lee, W. Han, J. W. Chen, Y. Cai, Z. & Yildiz, B. Cation size mismatch and charge interactions drive dopant segregation at the surfaces of manganite perovskites, Journal of the American Chemical Society 135, 7909–7925 (2013).
[45] Battaile, C. C. Najafabadi, R. & Srolovitz, D. J. Simulation of segregation to free surfaces in cubic oxides, Journal of the American Ceramic Society 78, 3195–3200 (1995).
[46] Wynblatt, P. Rohrer, G. S. & Papillon, F. Grain boundary segregation in oxide ceramics, Ceramic and Metal Interfaces 23, 2841–2848 (2003).
[47] Noguera, C. Physics and chemistry at oxide surfaces, Cambridge University Press (1996).
11 Bibliography
178
[48] Skvortsova V. Trinkler, L. Transition metal ions luminescence in neutron irradiated magnesium oxide, IOP Conference Series: Materials Science and Engineering 15, 012055 (2010).
[49] Özgür, Ü. et al. A comprehensive review of ZnO materials and devices, Journal of Applied Physics 98, 41301 (2005).
[50] Takeuchi, I. et al. Monolithic multichannel ultraviolet detector arrays and continuous phase evolution in MgxZn1−xO composition spreads, Journal of Applied Physics 94, 7336–7340 (2003).
[51] Diler, E. Rioual, S. Lescop, B. Thierry, D. & Rouvellou, B. Stability of ZnMgO oxide in a weak alkaline solution, Thin Solid Films 520, 2819–2823 (2012).
[52] Maemoto, T. and Ichiba, N . and Ishii, H. and Sasa, S. and Inoue, M. Structural and optical properties of ZnMgO thin films grown by pulsed laser deposition using ZnO-MgO multiple targets, Journal of Physics: Conference Series 59, 670 (2007).
[53] Bates, C. H. White, W. B. & Roy, R. New high-pressure polymorph of zinc oxide, Science 137, 993 (1962).
[54] Pan, X. et al. Fabrication of p-type ZnMgO films via pulsed laser deposition method by using Li as dopant source, Applied Surface Science 253, 6060–6062 (2007).
[55] Eijt, S. W. H. Roode, J. de, Schut, H. Kooi, B. J. & Hosson, J. T. M. de. Formation and stability of rocksalt ZnO nanocrystals in MgO, Applied Physics Letters 91 (2007).
[56] Lu, G. Lieberwirth, I. & Wegner, G. A General polymer-based process to prepare mixed metal oxides: The Case of Zn1-xMgxO nanoparticles, Journal of the American Chemical Society 128, 15445–15450 (2006).
[57] Chen, J. Shen, W. Z. Chen, N. B. Qiu, D. J. and Wu, H. Z. The study of composition non-uniformity in ternary MgxZn1− xO thin films, Journal of Physics: Condensed Matter 15, L475 (2003).
[58] Biasi, R. S. de, Cardoso, L. H. G. Campos, J. B. de, Sanchez, D. R. & Cunha, J. B. M. d. Influence of annealing on magnesioferrite nanoparticles synthesized by a sol-gel/combustion method, Materials Research 12, 225–227 (2009).
[59] Candeia, R. A. et al. MgFe2O4 pigment obtained at low temperature, Materials Research Bulletin 41, 183–190 (2006).
[60] Rodriguez, J. A. et al. Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S–O bonds, The Journal of Chemical Physics 115, 10914–10926 (2001).
[61] Palizdar, M. Ahgababazadeh, R. Mirhabibi, A. Brydson, R. & Pilehvari, S. Investigation of Fe/MgO catalyst support precursors for the chemical vapour deposition growth of carbon nanotubes, Journal of Nanoscience and Nanotechnology 11, 5345–5351 (2011).
[62] Yager, T. A. & Klngery, W. D. The kinetics of clustering reactions in iron-doped MgO, Journal of Materials Science 16, 483-488 (1981).
11 Bibliography
179
[63] Hansson, R. Hayes, P. C. & Jak, E. Phase equilibria at sub-solidus conditions in the Fe-Mg-Zn-O system in air, Scandinavian Journal of Metallurgy 33, 355–361 (2004).
[64] Coquay, P. Grave, E. de, Peigney, A. Vandenberghe, R. E. & Laurent, C. Carbon nanotubes by a CVD method. Part I: Synthesis and characterization of the (Mg, Fe)O catalysts, Journal of Physical Chemistry B 106, 13186–13198 (2002).
[65] Kotnala, R. K. et al. Influence of annealing on humidity response of RF sputtered nanocrystalline MgFe2O4 thin films, Thin Solid Films 519, 6135–6139 (2011).
[66] Mahmoud, H. R. El-Molla, S. A. & Saif, M. Improvement of physicochemical properties of Fe2O3/MgO nanomaterials by hydrothermal treatment for dye removal from industrial wastewater, Powder Technology 249, 225–233 (2013).
[67] Bohren, C. F. & Huffman, D. R. Absorption and scattering of light by small particles, Wiley (1998).
[68] Christy, A. A. Kvalheim, O. M. & Velapoldi, R. A. Quantitative analysis in diffuse reflectance spectrometry: A modified Kubelka-Munk equation. Infrared and raman spectroscopy, Vibrational Spectroscopy 9, 19–27 (1995).
[69] Jackson, S. D. Metal oxide catalysis, Wiley (2009).
[70] Nič, M. Jirát, J. Košata, B. Jenkins, A. & McNaught, A. IUPAC compendium of chemical terminology (2009).
[71] Nakamura, D. Kurita, Y. Ito, K. & Kubo, M. Covalency of metal-ligand bonds in potassium hexahaloplatinates (IV) studied by the pure quadrupole resonance of halogens, Journal of the American Chemical Society 82, 5783–5787 (1960).
[72] Atkins, P. W. Inorganic chemistry. 5th edition, Oxford University Press (2010).
[73] Morin, F. J. Oxides of the 3d transition metals, Bell System Technical Journal
37, 1047‐1084 (1958).
[74] Förster, H. in Characterization I, edited by H. Karge & J. Weitkamp, Springer, pp. 337-426 (2004).
[75] Lakshmi, S. Endo, T. & Siva, G. in Advanced aspects of spectroscopy, edited by M. Akhyar Farrukh (2012).
[76] Tanabe, Y. & Sugano, S. On the absorption spectra of complex ions. I, Journal of Physical Society 9, 753–766 (1954).
[77] Sherman, D. & Waite T David. Electronic spectra of Fe3+ oxides and oxide hydroxides in the near IR to near UV. American Mineralogist Journal, 1262–1269 (1985).
[78] Gfroerer, T. H. in Encyclopedia of analytical chemistry, John Wiley & Sons Ltd. (2000).
[79] Andrew R. Barron (ed.). Physical methods in chemistry and nano science (2014).
[80] Anpo, M. & Che, M. in Advances in catalysis, edited by Werner, B. C. G. a. H. K. and Haag, O. 119–257, Academic Press (1999).
11 Bibliography
180
[81] Lakowicz, J. R. Principles of fluorescence spectroscopy. 3rd edition, Springer (2006).
[82] van Dijken, A. Meulenkamp, E. A. Vanmaekelbergh, D. & Meijerink, A. Identification of the transition responsible for the visible emission in ZnO using quantum size effects, Journal of Luminescence 90, 123–128 (2000).
[83] Ischenko, V. et al. Zinc oxide nanoparticles with defects, Advanced Functional Materials 15, 1945–1954 (2005).
[84] Djurisić, A. B. & Leung, Y. H. Optical properties of ZnO nanostructures, Small 2, 944–961 (2006).
[85] Janotti, A. & Van de Walle, Chris G. Native point defects in ZnO, Physical Review B 76, 165202 (2007).
[86] Brauer, G. et al. Characterization of ZnO nanostructures. A challenge to positron annihilation spectroscopy and other methods, Physica Status Solidi (c) 6, 2556–2560 (2009).
[87] Wang, D. et al. Positron annihilation study of the interfacial defects in ZnO nanocrystals. Correlation with ferromagnetism, Journal of Applied Physics 107, 23524 (2010).
[88] Klingshirn, C. et al. 65 years of ZnO research - old and very recent results, Physica Status Solidi (b) 247, 1424–1447 (2010).
[89] Knutsen, K. E. et al. Zinc vacancy and oxygen interstitial in ZnO revealed by sequential annealing and electron irradiation, Physical Review B 86, 121203 (2012).
[90] Zhang, H. et al. Bulk and surface excitons in alloyed and phase-separated ZnO–MgO particulate systems, ACS Applied Materials & Interfaces 4, 2490–2497 (2012).
[91] Drouilly, C. et al. Role of oxygen vacancies in the basicity of ZnO. From the model methylbutynol conversion to the ethanol transformation application, Applied Catalysis A: General 453, 121–129 (2013).
[92] Stavale, F. Nilius, N. & Freund, H.-J. STM luminescence spectroscopy of intrinsic defects in ZnO(0001) thin films, Journal of Physical Chemistry Letters 4, 3972–3976 (2013).
[93] Stavale, F. Pascua, L. Nilius, N. & Freund, H.-J. Luminescence properties of nitrogen-doped ZnO, Journal of Physical Chemistry C 118, 13693–13696 (2014).
[94] Baer, D. R. et al. Surface characterization of nanomaterials and nanoparticles: Important needs and challenging opportunities, Journal of Vacuum Science & Technology A 31, 50820 (2013).
[95] Knoezinger, E. Diwald, O. & Sterrer, M. Chemical vapour deposition - a new approach to reactive surface defects of uniform geometry on high surface area magnesium oxide, Journal of Molecular Catalysis A Chemical 162, 83–95 (2000).
[96] Siedl, N. Koller, D. Sternig, A. K. Thomele, D. & diwald, O. Photoluminescence quenching in compressed MgO nanoparticle systems, Physical Chemistry Chemical Physics 16, 8339–8345 (2014).
11 Bibliography
181
[97] Walz, M.-M. Vollnhals, F. Schirmer, M. Steinrück, H.-P. & Marbach, H. Generation of clean iron nanocrystals on an ultra-thin SiO(x) film on Si(001), Physical Chemistry Chemical Physics 13, 17333–17338 (2011).
[98] Vittadini, A. et al. Defects in oxygen-depleted titanate nanostructures, Langmuir 28, 7851–7858 (2012).
[99] Bassett, P. J. Gallon, T. E. Prutton, M. & Matthew, J. High resolution auger electron spectrum of MgO(100), Surface Science 33, 213–218 (1972).
[100] Onsgaard, J. Barlow, S. M. & Gallon, T. E. Clean ZnO surfaces and oxygen adsorption on the (0001) surface studied by electron spectroscopy, Journal of Physics C: Solid State Phys. 12, 925–942 (1979).
[101] Mroczkowski, S. Calculated Auger yields and sensitivity factors for KLL–NOO transitions with 1–10 kV primary beams, Journal of Vacuum Science & Technology A 3, 1860 (1985).
[102] Weidenthaler, C. Pitfalls in the characterization of nanoporous and nanosized materials, Nanoscale 3, 792–810 (2011).
[103] Weibel, A. Bouchet, R. Boulc, F. & Knauth, P. The big problem of small particles. A comparison of methods for determination of particle size in nanocrystalline anatase powders, Chemistry of Materials 17, 2378–2385 (2005).
[104] Castro, R. H. R. Tôrres, R. B. Pereira, G. J. & Gouvêa, D. Interface energy measurement of MgO and ZnO. Understanding the thermodynamic stability of nanoparticles, Chemistry of Materials 22, 2502–2509 (2010).
[105] Sternig, A. diwald, O. Gross, S. & Sushko, P. V. Surface decoration of MgO nanocubes with Sulfur oxides: Experiment and theory, Journal of Physical Chemistry C 117, 7727–7735 (2013).
[106] Knözinger, E. Jacob, K.-H. Singh, S. & Hofmann, P. Hydroxyl groups as IR active surface probes on MgO crystallites, Surface Science 290, 388–402 (1993).
[107] Diwald, O. Sterrer, M. & Knözinger, E. Site selective hydroxylation of the MgO surface, Physical Chemistry Chemical Physics 4, 2811–2817 (2002).
[108] Chizallet, C. et al. Identification of the OH groups responsible for kinetic basicity on MgO surfaces by 1H MAS NMR, Journal of Catalysis 268, 175–179 (2009).
[109] Chizallet, C. et al. Role of hydroxyl groups in the basic reactivity of MgO. A theoretical and experimental study, Oil & Gas Science and Technology - Rev. IFP 61, 479–488 (2006).
[110] This is reasoned by the fact, that in the relevant temperature range of the CVS reactor system, i.e. in the range between 700 and 1100 K, Zn exhibits a substantially smaller vapor pressure than Mg in order to be directly combusted at reduced oxygen pressures.
[111] Hlaing Oo, W. M. McCluskey, M. D. Lalonde, A. D. & Norton, M. G. Infrared spectroscopy of ZnO nanoparticles containing CO2 impurities, Applied Physics Letters 86, 73111 (2005).
11 Bibliography
182
[112] Noei, H. Wöll, C. Muhler, M. & Wang, Y. Activation of carbon dioxide on ZnO nanoparticles studied by vibrational spectroscopy, Journal of Physical Chemistry C 115, 908–914 (2011).
[113] Buchholz, M. Weidler, P. G. Bebensee, F. Nefedov, A. & Wöll, C. Carbon dioxide adsorption on a ZnO(1010) substrate studied by infrared reflection absorption spectroscopy, Physical Chemistry Chemical Physics 16, 1672–1678 (2014).
[114] Weisz, S. Z. Calibration of oxygen Auger signal from single-crystal ZnO surfaces, Journal of Vacuum Science & Technology A 5, 302 (1987).
[115] Siedl, N. Koller, D. Sternig, A. K. Thomele, D. & diwald, O. Photoluminescence quenching in compressed MgO nanoparticle systems, Physical Chemistry Chemical Physics 16, 8339–8345 (2014).
[116] Shluger, A. L. Sushko, P. V. & Kantorovich, L. N. Spectroscopy of low-coordinated surface sites: Theoretical study of MgO, Physical Review B 59, 2417–2430 (1999).
[117] Beck, K. M. et al. Energy and site selectivity in O-atom photodesorption from nanostructured MgO, Surface Science 602, 1968–1973 (2008).
[118] Sternig, A. Stankic, S. Müller, M. Siedl, N. & diwald, O. Surface exciton separation in photoexcited MgO nanocube powders, Nanoscale 4, 7494–7500 (2012).
[119] Stevanovic, A. & Yates, J. T. Probe of NH3 and CO adsorption on the very outermost surface of a porous TiO2 adsorbent using photoluminescence spectroscopy, Langmuir 28, 5652–5659 (2012).
[120] Zhang, Z. & Yates, J. T. Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces, Chemical Reviews 112, 5520–5551 (2012).
[121] Sun, W.-C. Yeh, Y.-C. Ko, C.-T. He, H. & Chen, M.-J. Improved characteristics of near-band-edge and deep-level emissions from ZnO nanorod arrays by atomic-layer-deposited Al2O3 and ZnO shell layers, Nanoscale Research Letters 6, 556 (2011).
[122] Janotti, A. & Van de Walle, Chris G. LDA + U and hybrid functional calculations for defects in ZnO, SnO2, and TiO2, physica status solidi (b) 248, 799–804 (2011).
[123] Hlaing Oo, W. M. McCluskey, M. D. Lalonde, A. D. & Norton, M. G. Infrared spectroscopy of ZnO nanoparticles containing CO2 impurities, Applied Physics Letters 86, 73111 (2005).
[124] Stankic, S. Bernardi, J. diwald, O. & Knözinger, E. Optical surface properties and morphology of MgO and CaO nanocrystals, Journal of Physical Chemistry B 110, 13866–13871 (2006).
[125] Sternig, A. Mueller, M. McCallum, M. Bernardi, J. & Diwald, O. BaO clusters on MgO nanocubes: A quantitative analysis of optical-powder properties, Small 6, 582–588 (2010).
11 Bibliography
183
[126] Müller, M. et al. Effect of protons on the optical properties of oxide nanostructures, Journal of the American Chemical Society 129, 12491–12496 (2007).
[127] Mar, G. L. Timbrell, P. Y. & Lamb, R. N. Factors influencing the chemical vapor deposition of oriented ZnO films using zinc acetate, Chemistry of Materials 7, 1890–1896 (1995).
[128] Spanhel, L. Colloidal ZnO nanostructures and functional coatings. A survey, Journal of Sol-Gel Science & Technology 39, 7–24 (2006).
[129] Li, Z. Luan, Y. Mu, T. & Chen, G. Unusual nanostructured ZnO particles from an ionic liquid precursor, Chemical Communications, 1258–1260 (2009).
[130] Ghosh, M. et al. Role of ambient air on photoluminescence and electrical conductivity of assembly of ZnO nanoparticles, Journal of Applied Physics 110, 54309 (2011).
[131] Hu, X. Masuda, Y. Ohji, T. Saito, N. & Kato, K. Low-temperature fabrication of bunch-shaped ZnO nanowires using a sodium hydroxide aqueous solution, Journal of Nanoscience & Nanotechnology 11, 10935–10939 (2011).
[132] Yue, S. Yan, Z. Shi, Y. & Ran, G. Synthesis of zinc oxide nanotubes within ultrathin anodic aluminum oxide membrane by sol–gel method, Materials Letters 98, 246–249 (2013).
[133] Kołodziejczak-Radzimska, A. & Jesionowski, T. Zinc oxide—From synthesis to application. A review, Materials 7, 2833–2881 (2014).
[134] Idriss, H. & Barteau, M. A. Photoluminescence from zinc oxide powder to probe absorption and reaction of oxygen, carbon monoxide, hydrogen, formic acid, and methanol, Journal of Physical Chemistry 96, 3382–3388 (1992).
[135] Idriss, H. Application of luminescence techniques to probe surface–adsorbate interactions on oxide single crystals, Journal of Vaccum Science & Technology A 11, 209 (1993).
[136] Idriss, H. & Barteau, M. A. in Impact of Surface Science on Catalysis, 261–331, Elsevier (2000).
[137] Xia, Y. Xiong, Y. Lim, B. & Skrabalak, S. E. Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?, Angewandte Chemie International Edition English 48, 60–103 (2009).
[138] Briggs, D. & Grant, J. T. Surface analysis by Auger and x-ray photoelectron spectroscopy, IM Publications (2003).
[139] Klingshirn, C. ZnO: Material, Physics and Applications, ChemPhysChem 8, 782–803 (2007).
[140] Woll, C. The chemistry and physics of zinc oxide surfaces, Progress in Surface Science 82, 55–120 (2007).
[141] Segnit, E. R. & Holland, A. E. The system MgO-ZnO-SiO2, Journal of the American Ceramic Society 48, 409–413 (1965).
[142] Chen, J. Shen, W. Z. Chen, N. B. Qiu, D. J. & Wu, H. Z. The study of composition non-uniformity in ternary MgxZn1-x O thin films, Journal of Physics: Condensed Matter 15, L475-L482 (2003).
11 Bibliography
184
[143] Sternig, A. et al. Phase Separation at the Nanoscale. Structural properties of BaO segregates on MgO-based nanoparticles, Journal of Physical Chemistry C 115, 15853–15861 (2011).
[144] Mueller, R. Mädler, L. & Pratsinis, S. E. Nanoparticle synthesis at high production rates by flame spray pyrolysis, Chemical Engineering Science 58, 1969–1976 (2003).
[145] Tani, T. Watanabe, N. Takatori, K. & Pratsinis, S. E. Morphology of oxide particles made by the emulsion combustion method, Journal of the American Ceramic Society 86, 898–904 (2003).
[146] Tani, T. Mädler, L. & Pratsinis, S. E. Journal of Nanoparticle Research 4, 337–343 (2002).
[147] Hendy, S. C. & Brown, I. W. M. Advanced materials and nanotechnology. Proceedings of the international conference (AMN-4), Dunedin, New Zealand, 8-12 February 2009 (American Institute of Physics, Melville, N.Y. 2009).
[148] Spoto, G. et al. Carbon monoxide MgO from dispersed solids to single crystals: a review and new advances, Progress in Surface Science 76, 71–146 (2004).
[149] Mädler, L. et al. Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flame-made Pt/SnO2 nanoparticles, Sensors and Actuators B: Chemical 114, 283–295 (2006).
[150] Stokłosa, A. & Laskowska, B. Ionic radii in mixed-valence and nonstoichiometric metal oxides and their polymorphic forms, Journal of Chemical Crystallography 38, 913–925 (2008).
[151] Ohtomo, A. et al. MgxZn1-xO as a II–VI widegap semiconductor alloy, Applied Physics Letters 72, 2466 (1998).
[152] Kim, Y.-I. Page, K. & Seshadri, R. Synchrotron x-ray study of polycrystalline wurtzite Zn1-xMgxO (0 ≤ x ≤ 0.15). Evolution of crystal structure and polarization, Applied Physics Letters 90, 101904 (2007).
[153] Choopun, S. et al. Realization of band gap above 5.0 eV in metastable cubic-phase MgxZn1-xO alloy films, Applied Physics Letters 80, 1529 (2002).
[154] Park, W. I. Yi, G.-C. & Jang, H. M. Metalorganic vapor-phase epitaxial growth and photoluminescent properties of Zn1-xMgxO (0 ≤ x ≤ 0.49) thin films, Applied Physics Letters 79, 2022 (2001).
[155] Wang, Y. S. Thomas, P. J. & O'Brien, P. Optical properties of ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions, Journal of Physical Chemistry B 110, 21412–21415 (2006).
[156] Stankic, S. Bernardi, J. Diwald, O. & Knoezinger, E. Optical surface properties and morphology of MgO and CaO nanocrystals, Journal of Physical Chemistry B 110, 13866–13871 (2006).
[157] Vanmaekelbergh, D. & van Vugt, L. K. ZnO nanowire lasers, Nanoscale 3, 2783–2800 (2011).
[158] John, T. T. Priolkar, K. R. Bessière, A. Sarode, P. R. & Viana, B. Effect of [OH-] linkages on luminescent properties of ZnO nanoparticles, Journal of Physical Chemistry C 115, 18070–18075 (2011).
11 Bibliography
185
[159] Madi, K. Forest, S. Cordier, P. & Boussuge, M. Numerical study of creep in two-phase aggregates with a large rheology contrast: Implications for the lower mantle, Earth and Planetary Science Letters 237, 223–238 (2005).
[160] Larico, R. Justo, J. F. & Assali, L. V. C. Iron in magnesium oxide at high pressures: a first principles theoretical investigation, Physica Status Solidi B 250, 750–754 (2013).
[161] Kotnala, R. K. et al. Influence of annealing on humidity response of RF sputtered nanocrystalline MgFe2O4 thin films, Thin Solid Films 519, 6135–6139 (2011).
[162] Sonder, E. Stratton, T. G. & Weeks, R. A. Kinetics of Fe2+ oxidation and Fe3+ reduction in MgO single crystals, Journal of Chemical Physics 70, 4603–4608 (1979).
[163] Larico, R. Justo, J. F. & Assali, L. V. C. Iron in magnesium oxide at high pressures: a first principles theoretical investigation, Physica Status Solidi B 250, 750–754 (2013).
[164] Carles, V. & Rousset, A. Elaboration and characterization of submicronic magnesio-wüstite (Mg1−xFex)O powders, Solid State Ionics 83, 309–321 (1996).
[165] Jing, J. & Campbell, S. J. Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O, Hyperfine Interact 68, 283-286 (1992).
[166] Woods, K. N. & Fine, M. E. Nucleation and growth of magnesioferrite in MgO containing 0.9% Fe3+, Journal of the American Ceramic Society 52, 186–188 (1969).
[167] Skinner, A. R. PhD thesis: Electron paramagnetic resonance of some 3d ions in magnesium oxide (1986).
[168] PHILLIPS, B. SÕMIYA, S. & MUAN, A. Melting relations of magnesium oxide-Iron oxide mixtures in air, Journal of the American Ceramic Society 44, 167–169 (1961).
[169] Jung, I.-H. Decterov, S. A. & Pelton, A. D. Critical thermodynamic evaluation and optimization of the Fe–Mg–O system, Journal of Physics and Chemistry of Solids 65, 1683–1695 (2004).
[170] Inglis, A. D. Clustering in iron-doped magnesium oxide (1981).
[171] Lee, D. K. et al. A facile synthesis of multi metal-doped rectangular ZnO nanocrystals using a nanocrystalline metal-organic framework template, Nanoscale 6, 10995–11001 (2014).
[172] Stoch, P. Mössbauer spectroscopy study of the crystallization of vitrified waste inceneration ashes, Physics of Non-Crystalline Solids 10 345–346, 153–156 (2004).
[173] Dyar, M. D. Agresti, D. G. Schaefer, M. W. Grant, C. A. & Sklute, E. C. Mössbauer spectroscopy of earth and planetary materials, Annual Review Of Earth And Planetary Sciences 34, 83–125 (2006).
[174] Smith, G. Evidence for absorption by exchange-coupled Fe2+-Fe3+ pairs in the near infra-red spectra of minerals, Physics & Chemistry of Minerals 3, 375-383 (1978).
11 Bibliography
186
[175] Scheinost, A. C. Chavernas, A. Barron, V. & Torrent, J. Use and limitations of second-derivative diffuse reflectance spectroscopy in the visible to near-infrared range to identify and quantify Fe oxide minerals in soils, Clays and Clay Minerals 46, 528–536 (1998).
[176] Taran, M. Koch-Müller, M. & Langer, K. Electronic absorption spectroscopy of natural (Fe2+, Fe3+)-bearing spinels of spinel s.s.-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures, Physics & Chemistry of Minerals 32, 175-188 (2005).
[177] Hazen, R. M. & Prewitt, C. T. Effects of temperature and pressure on interatomic distances in oxygen-based minerals, American Mineralogist 62, 309–315 (1977).
[178] Meldrum, F. C. & Cölfen, H. Controlling mineral morphologies and structures in biological and synthetic systems, Chemical Reviews 108, 4332–4432 (2008).
[179] Patzke, G. R. Zhou, Y. Kontic, R. & Conrad, F. Oxide nanomaterials: synthetic developments, mechanistic studies, and technological innovations, Angewandte Chemie International Edition English 50, 826–859 (2011).
[180] Chang, S.-Y. et al. Spontaneous growth of one-dimensional nanostructures from films in ambient atmosphere at room temperature. ZnO and TiO2, Journal of Materials Chemistry 21, 4264 (2011).
[181] Peng, X. Jin, J. Ericsson, E. M. & Ichinose, I. General method for ultrathin free-standing films of nanofibrous composite materials, Journal of the American Chemical Society 129, 8625–8633 (2007).
[182] Finlayson-Pitts, B. J. Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols, Physical Chemistry Chemical Physics 11, 7760–7779 (2009).
[183] Sternig, A. Müller, M. McCallum, M. Bernardi, J. & Diwald, O. BaO clusters on MgO nanocubes: a quantitative analysis of optical-powder properties, Small 6, 582–588 (2010).
[184] McKenna, K. P. Koler, D. Sternig, A. Siedl, N. Govind, N. Sushko, P. V. Diwald, O. Optical properties of nanocrystal interfaces in compressed MgO nanopowders, ACS Nano 5, 3003-3009 (2011).
[185] Pang, H. Ning, G. Gong, W. Ye, J. & Lin, Y. Direct synthesis of hexagonal Mg(OH)2 nanoplates from natural brucite without dissolution procedure, Chemical Communications (Camb.) 47, 6317–6319 (2011).
[186] Ding, Y. et al. Nanoscale magnesium hydroxide and magnesium oxide powders. Control over size, shape, and structure via hydrothermal synthesis, Chemistry of Materials 13, 435–440 (2001).
[187] MacKenzie, K. J. D. & Meinhold, R. H. Thermal decomposition of brucite, Mg(OH)2: a 25Mg MAS NMR study, Thermochimica Acta 230, 339–343 (1993).
[188] Pallister, P. J. Moudrakovski, I. L. & Ripmeester, J. A. Mg ultra-high field solid state NMR spectroscopy and first principles calculations of magnesium compounds, Physical Chemistry Chemical Physics 11, 11487–11500 (2009).
11 Bibliography
187
[189] Matkovic, B. Popovic, S. Rogic, V. Zunic, T. & Y, J. F. Reaction products in magnesium oxychloride cement pastes. System MgO-MgCl2-H2O, Journal of the American Ceramic Society 60, 504–507 (1977).
[190] Montle, J. F. & Mayhan, K. G. Magnesium oxychloride as a fire retardant material, Journal of Fire and Flammability/Fire Retardant Chemistry Supplement, 243–254 (1974).
[191] Chau, C. K. & Li, Z. Microstructures of magnesium oxychloride Sorel cement, Advances in Cement Research 20, 85–92 (2008).
[192] Jeevanandam, P. Mulukutla, R. S. Yang, Z. Kwen, H. & Klabunde, K. J. Nanocrystals to nanorods. A precursor approach for the synthesis of magnesium hydroxide nanorods from magnesium oxychloride nanorods starting from nanocrystalline magnesium oxide, Chemistry of Materials 19, 5395–5403 (2007).
[193] Brown, M. A. Carrasco, E. Sterrer, M. & Freund, H.-J. Enhanced stability of gold clusters supported on hydroxylated MgO(001) surfaces, Journal of the American Chemical Society 132, 4064–4065 (2010).
[194] Goodenough, J. B. Evolution of strategies for modern rechargeable batteries, Accounts of Chemical Research 46, 1053–1061 (2012).
[195] Fabbri, E. Pergolesi, D. & Traversa, E. Materials challenges toward proton-conducting oxide fuel cells: a critical review, Chemical Society Reviews 39, 4355–4369 (2010).
[196] Matkovic, B. & Young, J. F. Microstructure of magnesium oxychloride cements, Nature-Physical Science 246, 79–80 (1973).
[197] Altomare, A. et al. A kit of tools for phasing crystal structures from powder data, Journal of Applied Crystallography 46, 1231–1235 (2013).
[198] Sugimoto, K. Dinnebier, R. E. & Schlecht, T. Structure determination of Mg3(OH)5Cl.4H2O (F5 phase) from laboratory powder diffraction data and its impact on the analysis of problematic magnesia floors, Acta Crystallographica B 63, 805–811 (2007).
[199] Rodríguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction, Physica B: Condensed Matter 192, 55–69 (1993).
[200] Artioli, G. & Bullard, J. W. Cement hydration. The role of adsorption and crystal growth, Crystal Research and Technology 48, 903–918 (2013).
[201] Weber, J. Bayer, K. & Pintér, F. in Historic Mortars, edited by J. Válek, J. J. Hughes & C. J. W. P. Groot, Springer, 89–103 (2012).
[202] Krauss Juillerat, F. Gonzenbach, U. T. Elser, P. Studart, A. R. & Gauckler, L. J. Microstructural control of self-setting particle-stabilized ceramic foams, Journal of the American Ceramic Society 94, 77–83 (2011).
[203] Holzer, L. Muench, B. Wegmann, M. Gasser, P. & Flatt, R. J. FIB-nanotomography of particulate systems—part I. particle shape and topology of interfaces, Journal of the American Ceramic Society 89, 2577–2585 (2006).
[204] Misra, A. K. & Mathur, R. Magnesium oxychloride cement concrete, Bulletin of Materials Science 30, 239–246 (2007).
11 Bibliography
188
[205] Tan, Y. Liu, Y. Zhao, Z. Paxton, J. Z. & Grover, L. M. Synthesis and in vitro degradation of a novel magnesium oxychloride cement, Journal of Biomedical Materials Research A 103, 194–202 (2015).
[206] Sorel, S. T. On a new magnesium cement, Comptes Rendus hebdomadires des Seances de Academie des sciences 65, 102-4 (1867).
[207] Gheisi, A. et al. Spontaneous growth of magnesium hydroxide fibers at ambient conditions, Crystal Growth & Design 14, 4236–4239 (2014).
[208] Castellar, M. D. de, Lorente, J. C. Traveria, A. & Tura, J. M. Cracks in sorel's cement polishing bricks as a result of magnesium oxychloride carbonatation, Cement and Concrete Research 26, 1199–1202 (1996).
[209] Alonso, C. & Fernandez, L. Dehydration and rehydration processes of cement paste exposed to high temperature environments, Journal of Materials Science 39, 3015–3024 (2004).
[210] Dinnebier, R. E. Freyer, D. Bette, S. & Oestreich, M. 9Mg(OH)2·MgCl2·4H2O, a high temperature phase of the magnesia binder system, Inorganic Chemistry 49, 9770–9776 (2010).
[211] Li, G. Yu, Y. Li, J. Wang, Y. & Liu, H. Experimental study on urban refuse/magnesium oxychloride cement compound floor tile, Cement and Concrete Research 33, 1663–1668 (2003).
[212] Sternig, A. Koller, D. Siedl, N. diwald, O. & McKenna, K. Exciton formation at Solid–Solid interfaces. A systematic sxperimental and ab Initio study on compressed MgO nanopowders, Journal of Physical Chemistry C 116, 10103–10112 (2012).
[213] Baumann, S. O. et al. Size effects in MgO cube dissolution, Langmuir 31, 2770–2776 (2015).
[214] Different to HCl, SiCl4 can be applied in water free form via the gas phase and - also important for the highly sensitive surface reaction reported here - exhibits a lower reactivity than TiCl4.
[215] Runčevski, T. Dinnebier, R. E. & Freyer, D. Dehydration of the sorel cement phase 3Mg(OH)2·MgCl 2 ·8H2O studied by in situ synchrotron X-ray powder diffraction and thermal analyses, Zeitschrift für anorganische und allgemeine chemie 640, 100–105 (2014).
[216] Sutter, E. A. & Sutter, P. W. Size-dependent phase diagram of nanoscale alloy drops used in vapor−liquid−solid growth of semiconductor nanowires, ACS Nano 4, 4943–4947 (2010).
[217] Chau, C. K. & Li, Z. Microstructures of magnesium oxychloride sorel cement, Advances in Cement Research 20, 85–92 (2008).
[218] Newberg, J. T. McIntire, T. M. & Hemminger, J. C. Reaction of bromide with bromate in thin-film water, Journal of Physical Chemistry A 114, 9480–9485 (2010).
[219] Garde, S. & Schlossman, M. L. Water at functional interfaces, MRS Bulletin 39, 1051–1053 (2014).
11 Bibliography
189
[220] Zhang, H. et al. A new vapor-phase hydrothermal method to concurrently grow ZnO nanotube and nanorod array films on different sides of a zinc foil substrate, Chemistry 18, 5165–5169 (2012).
[221] Liu, P. et al. A facile vapor-phase hydrothermal method for direct growth of titanate nanotubes on a titanium substrate via a distinctive nanosheet roll-up mechanism, Journal of the American Chemical Society 133, 19032–19035 (2011).
[222] Up to now we did not succeed in an unambiguous identification of the intermediate phases such as 3Mg(OH)2*MgCl2*5.4 and 4.6 H2O. See also reference [215].
[223] Kashani-Nejad, S. et al. MgOHCl thermal decomposition kinetics, Metallurgical and Materials Transactions B 36, 153-157 (2005).
190
Recommended