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Understanding The Role Of Defects In The Radiation Response Of Understanding The Role Of Defects In The Radiation Response Of
Nanoceria Nanoceria
Amit Kumar University of Central Florida
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UNDERSTANDING THE ROLE OF DEFECTS IN THE RADIATION RESPONSE OF
NANOCERIA
by
AMIT KUMAR
Bachelors of Technology (Honours), IIT Kharagpur, India, 2006
Master of Technology, IIT Kharagpur, India, 2006
A dissertation submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
in the Department of Mechanical, Materials and Aerospace Engineering
in the College of Engineering and Computer Science
at the University of Central Florida Orlando, Florida
Summer Term
2012
Major Professor: Sudipta Seal
ii
© 2012 Amit Kumar
iii
ABSTRACT
Nanoscale cerium oxide (nanoceria) have shown to possess redox active property , and
has been widely studied for potential use in catalysis, chemical-mechanical planarization, bio-
medical and solid oxide fuel cell (SOFC), etc. The redox state of nanoceria can be tuned by
controlling the defects within the lattice and thus its physical and chemical properties. Perfect
ceria lattice has fluorite structure and the research in last decade has shown that oxide and mixed
oxide systems with pyrochlore and fluorite have better structural stability under high energy
radiation. However, the current literature shows a limited number of studies on the effect of high
energy radiation on nanoceria. This dissertation aims at understanding the phenomena occurring
on irradiation of nanoceria lattice through experiments and atomistic simulation.
At first, research was conducted to show the ability to control the defects in nanoceria
lattice and understand the effect in tailoring its properties. The defect state of nanoceria was
controlled by lower valence state rare earth dopant europium. Extensive materials
characterization was done using high resolution transmission electron microscopy (HRTEM),
UV-Visible spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and Raman
spectroscopy to understand the effect of dopant chemistry in modifying the chemical state of
nanoceria. The defects originating in the lattice and redox state was quantified with increasing
dopant concentration. The photoluminescence property of the control and doped nanoceria were
evaluated with respect to its defect state. It was observed that defect plays an important role in
modifying the photoluminescence property and that it can be tailored in a wide range to control
the optical properties of nanoceria.
iv
Having seen the importance of defects in controlling the properties of nanoceria, further
experiments were conducted to understand the effect of radiation in cerium oxide thin films of
different crystallinity. The cerium oxide thin films were synthesized using oxygen plasma
assisted molecular beam epitaxy (OPA-MBE) growth. The thin films were exposed to high
energy radiation over a wide range of fluence (1013
to 1017
He+ ions/cm
3). The current literature
does not report the radiation effect in nanoceria in this wide range and upto this high fluence.
The chemical state of the thin film was studied using in-situ XPS for each dose of radiation. It
was found that radiation induced defects within both the ceria thin films and the valence state
deviated further towards non-stoichiometry with radiation.
The experimental results from cerium oxide thin film irradiation were studied in the light
of simulation. Classical molecular dynamics and Monte Carlo simulation were used for
designing the model ceria nanoparticle and studying the interaction of the lattice model with
radiation. Electronic and nuclear stopping at the end of the range were modeled in ceria lattice
using classical molecular dynamics to simulate the effect of radiation. It was seen that
displacement damage was the controlling factor in defect production in ceria lattice. The
simulation results suggested that nanosized cerium oxide has structural stability under radiation
and encounters radiation damage due to the mixed valence states. A portion of the study will
focus on observing the lattice stability of cerium with increasing concentration of the lower
valence (Ce3+
) within the lattice. With this current theoretical understanding of the role of redox
state and defects during irradiation, the surfaces and bulk of nanoceria can be tailored for
radiation stable structural applications.
v
This dissertation is dedicated to my father and mother, who
supported me at each step of my way and have been the source
of motivation and inspiration since the beginning of my studies.
vi
ACKNOWLEDGMENTS
I wish to thank my faculty advisor, Prof. Sudipta Seal, for offering the support and
challenge on various projects during my course of study and research at UCF. With his
motivation and guidance it was impossible to settle with anything less than my best efforts. It
was not easy, but truly cherishable. I am thankful to the dissertation committee members (Dr.
Ram Devanathan, Dr. Helge Heinrich, Dr. Hyoung J. Cho, Dr. Michael Leuenberger and Dr. Lei
Zhai) for providing their valuable time for this dissertation. I would like to extend my sincere
gratitude to Dr. Ram Devanathan for providing valuable insights and mentoring throughout the
research.
I would like thank the scientists at Environmental Molecular Sciences Laboratory
(EMSL) operated by Pacific Northwest National Laboratory for the interaction and opportunity
to work during Summer Research Internship and several short stay through user proposal under
NSF PNNL supplement. I am fortunate to work with excellent mentors like Dr. Theva
Thevuthasan and Shuttha Shuttanandan at EMSL, who imbibed in me excellent work practice
and leadership qualities.
I am thankful to Dr. Ajay S. Karakoti for the conceptual discussions on innumerable
occasions, for instrumentation and practice presentations. It was a pleasure working with Dr.
Suresh Babu, Dr. Satyanarayana Kuchibhatla, Abhilash Vincent, Virendra Singh, David Reid
and Soumen Das. I would like to acknowledge all the colleagues in the Surface Engineering and
Nanotechnology Facility (SNF) for their cooperation and support. I am thankful to the students
with whom I had a chance to work with in collaboration, Janet Dowding, Sanjay Singh and
vii
David Summerlot. I am thankful to David Reid, Priscilla Mendez and Swetha Barkam for proof
reading this dissertation.
I am thankful to Materials Characterization Facility (MCF) at AMPAC, UCF for offering
the characterization facilities required for conducting this research work. I am thankful to the
engineers at MCF, Kirk Scammon and Mikhail Klimov for helping at different stage of handling
the instruments and preparing samples for characterization. I am thankful to Karen, Angie, Kari
and Cindy at UCF for the administrative support during the course of my Ph.D.
I am thankful to the several organizations such as Materials Research Society, American
Vacuum Society, The Mineral, Metals and Materials Society, ICACC for giving me the
opportunity to present my research to the scientific community during their annual technical
meetings. I appreciate the financial support through award, fellowship, travel grants during my
Ph.D. from different agencies (NSF, NIH), institute (UCF), society (AVS, MRS, TMS).
Most of all, I would like to express my greatest gratitude to my parents (Sri Kamala
Prasad and Srimati Hirawati Devi), brother (Akash Deep) and sister (Anjali Anand) for their
undivided support throughout my work. I am grateful to my friends, Raunak Patel, Srikumar Roy
and Rahul Gautam for appreciating me for my work and ongoing advice to this day. I am
thankful to Saroj Karakoti, Deepshikha Singh and Milan Srivastava for being there as a family. I
am thankful to all my roommates during the stay in Orlando, for providing me the comfort of
working and making the days memorable.
At last but not the least, I am truly indebted and thankful to the mentors from my
undergraduate days, Prof. Sanat Kumar Roy, Prof Nirupam Chakraborti and Dr. G. G. Roy, for
motivating me to pursue research work.
viii
TABLE OF CONTENTS
LIST OF FIGURES ....................................................................................................................... xi
LIST OF TABLES ...................................................................................................................... xvii
CHAPTER ONE: INTRODUCTION ............................................................................................. 1
1.1 Introduction to radiation ....................................................................................................... 1
1.1.1 Radiation effect in living organisms .............................................................................. 1
1.1.2 Radiation effect in materials .......................................................................................... 2
1.2 Radiation induced defects ..................................................................................................... 4
1.2.1 Electronic excitation ...................................................................................................... 4
1.2.2 Nuclear interaction ......................................................................................................... 5
1.2.3 Select oxide materials under radiation ........................................................................... 6
1.3 Cerium oxide (ceria, CeO2)................................................................................................... 8
1.3.1 Irradiation study on cerium oxide material .................................................................... 8
1.3.2 Cerium oxide and radiation induced damage in biological medium ........................... 10
1.4 Problem of interest .............................................................................................................. 11
1.5 Approach to the problem .................................................................................................... 12
1.6 Outline of the dissertation ................................................................................................... 13
CHAPTER TWO: DEFECTS IN NANOCERIA ......................................................................... 14
ix
2.1 Introduction ......................................................................................................................... 14
2.1 Experiments ........................................................................................................................ 16
2.1.1 Materials ...................................................................................................................... 16
2.1.2 Characterization methods............................................................................................. 16
2.2 Results and discussion ........................................................................................................ 17
2.2.1 Size and structural properties ....................................................................................... 17
2.2.2 Surface and symmetry modifications........................................................................... 25
2.2.3. Optical and luminescence properties .......................................................................... 36
2.3 Conclusion .......................................................................................................................... 46
CHAPTER THREE: RADIATION EFECT IN NANOCERIA ................................................... 48
3.1 Introduction ......................................................................................................................... 48
3.2. Experimental details........................................................................................................... 49
3.3 Results and discussion ........................................................................................................ 49
3.4 Conclusion .......................................................................................................................... 53
CHAPTER FOUR: CERIA IRRADIATION STUDY-SIMULATION ....................................... 54
4.1 Introduction ......................................................................................................................... 54
4.2 Simulation details................................................................................................................ 55
4.3 Results and discussion ........................................................................................................ 57
4.4 Conclusion .......................................................................................................................... 64
x
CHAPTER FIVE: ROLE OF REDOX STATE IN CERIA IRRADIATION .............................. 65
5.1 Methodology adopted ......................................................................................................... 65
5.2 Simulating nanoceria lattice ................................................................................................ 66
5.3 Radiation in simulated lattices ............................................................................................ 69
5.4 Conclusion .......................................................................................................................... 73
CHAPTER SIX: CONCLUSIONS ............................................................................................... 74
6.1 Intellectual merit ................................................................................................................. 75
APPENDIX A: FLOWCHART FOR MONTE-CARLO IMPLEMENTATION ........................ 77
APPENDIX B: FORTRAN CODE FOR CREATING NANOCERIA LATTICE WITH GRAIN
BOUNDARIES ............................................................................................................................. 79
APPENDIX C: PUBLICATIONS DURING THIS DISSERTATION ...................................... 115
APPENDIX D: COPYRIGHT PERMISSIONS ......................................................................... 119
LIST OF REFERENCES ............................................................................................................ 122
xi
LIST OF FIGURES
Figure 1: XRD pattern measured (a) at RT for ceria sample without doping and ceria samples
doped with Eu at 1, 5, 10, 15, 20 and 30 mol % depicting the broad peaks for (111), (200), (220),
(311) and (b) for 15 mol % Eu doped ceria heat treated at temperature 300°C, 500°C, 700°C and
900°C indicates that the fluorite structure becomes more crystalline at higher annealing
temperature. .................................................................................................................................. 18
Figure 2: Lattice parameter- shown along ordinate, obtained by fitting gaussian curve to the
spectra of i.) ceria doped with Eu at 0, 1, 5, 10, 15, 20 and 30 mol % ii.) 15 mol% Eu doped ceria
heat treated at temperature 300C, 500°C, 700°C and 900°C. Lattice parameter increases linearly
with increasing dopant concentration which indicates that the stress generated due to the larger
ionic size of europium substitution in the lattice position of cerium increases with increase in
dopant concentration. The decrease in lattice parameter value with temperature suggests that
stress in the doped samples is released with higher annealing temperature. ................................ 20
Figure 3: The representative plot of “βcos vs 4sin ” for CE15. The slope and intercept of a
linear fit to the data was used to calculate the stress and strain in the sample.............................. 21
Figure 4: The representative bright field HRTEM images of CE15 (a) as prepared and annealed
at (b) 300°C, (c) 700°C, (d) 900°C and (e) EDX spectra of CE15. The inset of each figure shows
the SAED of the respective HRTEM image ................................................................................. 22
Figure 5: IR spectra of 15 mol% Eu doped nanoceria shows with annealing temperature .......... 24
Figure 6: XPS spectrum of Ce(3d) for 15 mol% Eu doped ceria sample was deconvoluted to give
individual spin-orbit doublet of 3d3/2 and 3d5/2 and the sum of deconvoluted peaks were used to
produce the fit to the actual data ................................................................................................... 26
xii
Figure 7: The high resolution XPS spectra of ceria samples doped with Eu at 1, 5, 10, 15, 20 and
30 mol % ....................................................................................................................................... 27
Figure 8: Oxygen 1s spectra of nanoceria samples doped with 1, 5, 10, 15, 20 and 30 mol % Eu
at room temperature shows the asymmetric behavior changes with dopant concentration and
depicts peak broadening at higher doping concentration (b) Oxygen 1s spectra of 15 mol% Eu
doped ceria at room temperature and heat treated at 300°C, 500°C, 700°C and 900°C shows the
asymmetry in peak decreases with increasing temperature treatment .......................................... 28
Figure 9: (a) A typical asymmetry in high resolution O1s spectrum of 15 mol% Eu doped ceria
was resolved and attributed to the presence of oxygen in different chemical environment as the
deconvolution of the experimental spectra results in peaks corresponding to binding energy of
O2-
ions associated with chemical environment of CeO2, Ce2O3, Eu2O3, OH/O2: Fitted and actual
spectrum are shown (b) Schematic lattice model showing the different chemical environments of
oxygen within the crystal structure of nanoceria .......................................................................... 30
Figure 10: The integrated peak area ratio (IPARi=AOi/AO) obtained from XPS analysis of O1s
spectra of ceria doped samples shows how with doping and heat treatment the oxygen
atmosphere changes (a) at RT for 1, 5, 10, 15, 20 and 30 mol% Eu doping (b) for CE15 heat
treated at 300C, 500°C, 700°C and 900°C. ................................................................................. 31
Figure 11: (a) Raman spectra for room temperature ceria samples doped with 1, 5, 10, 15, 20 and
30 mol % of Eu samples shows a shift towards decreasing wavenumber and increasing
broadening as the amount of dopant is increased (b) Raman spectra for 15 mol % Eu doped ceria
heat treated at 300C, 500C, 700C and 900C shifts slightly towards higher energy with
annealing temperature ................................................................................................................... 33
xiii
Figure 12: (a) Correlation length for the as prepared samples with variation in dopant
concentration and annealing temperature (b) A comparison between ratio of Ce3+
(calculated
from XPS spectra) and defect concentration (calculated from Raman spectra) with dopant
concentration shows both increases proportionately with amount of dopant which indicates a
correlation between the defect concentration increasing with ratio of Ce3+
as more Eu is added. 36
Figure 13: Reflectance spectra of (a) as synthesized samples with dopant concentration (b) CE15
with annealing temperature ........................................................................................................... 37
Figure 14: Excitation spectra with emission at 611 nm for (a) as prepared and (b) annealed
samples. No emission is observed for pure ceria. Among the room temperature samples the PL
first increases upto 15 mol% Eu doping in ceria and then decreases while the PL increases with
increase in annealing temperature of the 15 mol% Eu doped ceria .............................................. 38
Figure 15: Luminescence emission spectra (excitation at 466 nm) for (a) as prepared and (b)
annealed samples, shows maximum PL at RT for 15 mol% Eu doped ceria and increases further
with increase in annealing temperature......................................................................................... 40
Figure 16: Intensity ratio with dopant concentration for as prepared and 900 °C annealed samples
shows the asymmetric position of Eu in the ceria lattice (Ce4+
-O-Eu3+
) is favored on increasing
dopant concentration in both RT and annealed samples ............................................................... 42
Figure 17: Luminescence emission spectra (excitation at 370nm) of (a) as prepared and (b)
annealed samples .......................................................................................................................... 44
Figure 18: PL Peak intensity variation with Eu dopant concentration and annealing temperature
upon (a) emission spectra (excitation at 466nm) and excitation spectra (emission at 612nm) .... 45
xiv
Figure 19: XPS spectrum of Ce (3d) deconvoluted into Ce 3d3/2 and Ce3d5/2 for single-
crystalline ceria thin film (a) and polycrystalline ceria thin film (b). The peaks with hatched
shaded area are characteristic of Ce3+
ions, while those shown by solid continuous lines represent
contribution from Ce4+
ions. ......................................................................................................... 50
Figure 20: The change in XPS spectrum of Ce (3d) for single crystalline ceria (a) and poly
crystalline ceria (b) with cumulative dose of radiation (the spectra are plotted after background
subtraction). The position of upward arrows indicate the increase in contribution of Ce3+
signals
in the spectra and the position of downward arrows indicate the decrease in contribution of Ce4+
signals. .......................................................................................................................................... 51
Figure 21: Variation in Ce3+
concentration with cumulative dose of radiation for single and poly
crystalline ceria thin film. ............................................................................................................. 52
Figure 22: The profile of thermal spike created along z direction within the model ceria lattice
for energy loss of 1 keV/nm.......................................................................................................... 56
Figure 23: Mixing occurring in case of cascade simulation and thermal spike. The results of
thermal spike were plotted on the bottom y axis and that from five cascades, created at random
locations within the lattice and high index directions, on the top y-axis. ..................................... 58
Figure 24: The number of vacancies of Ce (a) and O (b) for different 5 keV Ce cascades. ......... 60
Figure 25: Defects (O vacancy: yellow, Ce vacancy: green, Ce interstitial: blue, O interstitial:
red) towards the end of the cascade simulation of a 5 keV Ce recoil. The area shown is about 3.4
nm x 3.4 nm. ................................................................................................................................. 61
Figure 26: Visualization of the sputtering geometry with an energetic Ce4+
ion incident on CeO2
(111) surface at an angle of 30° to the surface. Ce4+
is shown in cyan and O2-
in red. ................ 63
xv
Figure 27: The configurational energy change in the lattice for 10% Ce3+
prior to carrying out
amorphization and recrystallization. Introduction of Ce3+
in the lattice increases the
configurational energy of lattice and using Monte Carlo with Molecular Dynamics simulation
minimizes the lattice configurational energy. .............................................................................. 67
Figure 28: The evolution of nanoceria lattice with 10% Ce3+
observed during amorphisation step
at T=3750 K at (a) 0.1 ps (b) 10 ps (c) 100 ps and (d) 200 ps. In the starting (t= 0.1 ps) the lattice
is strained and, at T= 3750 K, with time (t=10 ps) the atoms come close to restore the lattice
parameter. Due to the velocity of atoms, the lattice amorphises as seen at t=100 ps. (Ce4+
: red,
Ce3+
: pink, O2-
: cyan) ................................................................................................................... 68
Figure 29: The total energy of nanoceria lattice with 10% Ce3+
on stabilizing at T=3250 K. The
initial decrease in energy of the lattice corresponds to the temperature decrease by 500 K (from
3750K to 3250 K). After ~100 ps, again a drop in energy is observed which can be attributed to
formation of stable lattice nuclei in the system.and this suggests evolution of stable features
within the lattice. ........................................................................................................................... 69
Figure 30: The number of defects in the nanoceria lattices for (a) PKA1, (b) PKA2 and (c)
PKA3. The PKA were initiated at simulation step of 1950. For the three PKAs in the nanoceria
lattices, radiation induces several types of defects (vacancy, interstitials for anions and cations)
and with time most of the defects get annihilated within the lattice leaving only few stable
defects. .......................................................................................................................................... 71
Figure 31: Energy (in eV) of nanoceria lattices with 5, 10, 15 and 20 % Ce3+
before radiation at
T=10K for 10 ps. The energy profiles for each lattice is stable, however, with increasing Ce3+
the
energy of nanoceria lattices increases. .......................................................................................... 72
xvi
xvii
LIST OF TABLES
Table 1: Similarity in properties of UO2 and CeO2 ........................................................................ 9
Table 2: Size, strain, Ce3+
ratio, defect concentration, bandgap for different samples................. 25
Table 3: The number of Ce4+
(Ces) and O2-
(Os) removed and the ratio of O2-
removed to Ce4+
removed (Os/Ces) by 1, 2 and 4 keV Ce4+
ions at an angle of 30° to the CeO2 (111) surface. .... 62
1
CHAPTER ONE: INTRODUCTION
1.1 Introduction to radiation
Radiation is a flow of energy through space or matter in the form of particles (neutrons,
electrons, protons, alpha, heavy ions) or waves (X-rays, Gamma rays). It originates from a wide
variety of sources and is classified in terms of non-ionizing and ionizing radiation. Ionizing
radiation produces ions in matter by knocking off the electron from atoms and modifies the
chemical properties. The most familiar source of radiation (non-ionizing) is the Sun and is a
concern to all human being due to its ultraviolet rays. Some of the sources for high-energy
ionizing radiation are generated by linear accelerators, cyclotrons, synchrotrons, etc. Other
sources of non-ionizing and ionizing radiation are widespread in different occupational settings
and if not properly controlled can be hazardous to the nearby environment. Exposure to radiation
is common to human being through medical applications such as electronic devices, x-rays, CT
scans, radiotherapy and un-desired sources viz. technology enhanced naturally occurring
radioactive materials (TENORM), radioactive waste, nuclear weapons, nuclear power plants, etc.
1.1.1 Radiation effect in living organisms
The effect of radiation on a living system depends on several factors, such as, the type of
radiation being used, the dose of radiation, the specific area of exposure to the body and the
living systems state of health. For humans in general, long term exposure to small amounts of
radiation or short term exposure to large radiation causes radiation sickness (hair loss, reduced
organ function), burns and even induce cancer. Other harmful effects of radiation are premature
2
aging or even death. The effects of radiation exposure can pass through generations via mutation
of genes of the exposed person. The decreasing sensitivity of different cells to radiation follows
the order of: lymphocytes, erythrocytes, granulocytes, epithelial cells, endothelial cells,
connective tissue cells, bone cells, nerve cells, brain cells and muscle cells. Radiation can
directly interact with a molecule to cause damage or it can interact with water in the organism
and produce free radicals, hydronium (H•) and hydroxyls (•OH). These free radicals can damage
the DNA, or produce peroxides which are toxic to the cells. Although the cells have
sophisticated mechanisms (enzyme secretion) to repair such damage, the DNA can either die or
undergo structural damage (ring formation, dicentric formation) from large exposure to radiation.
1.1.2 Radiation effect in materials
Radiation tolerant materials have been a topic of interest for several researchers over the
last decade. The demand for materials which can sustain and prevent radiation damage is ever
increasing. The effect of radiation on materials is a critical aspect for nuclear reactors, nuclear
waste storage, optical applications, solar cell cover glasses, ion exchange and electronic
applications, among others. Several materials have been studied for radiation damage and it is
important to understand the effect of different types of radiation on the physical and mechanical
properties of materials, including lattice structure, lattice defects, ductility, swelling, stress
relaxation and creep behavior. The atomic collision and ionization ability in materials due to the
energy or mass of radiation are affected by temperature, corrosion and other environmental
conditions encountered during their applications. Metals are mostly affected by displacement
damage, while in case of charged particle ionization, the valence electrons moves to the
conduction band. Ceramics are mostly damaged by atomic displacements leading to disruption of
3
the crystal lattice. Polymeric materials undergo crosslinking or, chain scission under radiation.
The effect of radiation is quantified by displacements per atom (dpa) which is measured by the
number of times an atom is displaced through atomic collisions from its lattice site. The key
parameters for the extent of damage from radiation are environment (radiation fluence- ϕ*t,
where, ϕ is the ion flux and t is time) and irradiation temperature. Some of the early reported
radiation effects in materials were motivated from fission reactors by Wigner1, Seitz
2 and
Cawthorne3. These studies showed that irradiation of the fission reactor materials, was
deleterious to the lifetime of the reactor materials. Baffle bolts made of 316CW, 304L, 316L and
347 stainless steels in reactors undergo cracking due to irradiation swelling, which is highly
temperature dependent. The swelling observed in stainless steels in reactors was also found to be
a general behavior of other materials under radiation4. Radiation-induced dimensional changes,
swelling, creep, and microstructural changes have been discussed by Mansur5. Other common
examples of radiation on material properties are listed here. For example, exposing Nd-Fe-B
magnets to ion fluence of 10 x1015
/cm2 leads to a loss of 10% magnetic remanence due to
reversion of domains by collision damage. Optical glasses may undergo darkening, internal
arcing, density change, fracture. Low modulus silicone adhesives have threshold doses of 1 x 108
rads TID.
It has been observed that among organic and inorganic materials, metals and ceramics
withstand radiation better, without losing the structural integrity. Some common oxide materials
for radiation-resistant applications are: tantalum oxide, cerium oxide, titanium oxide and zinc
oxide. Tantalum oxide coatings have proven to be beneficial in solar cell cover glasses.
Radiation resistant glasses are formulated with cerium oxide, e.g. Schott BK7G18, K5G20,
4
LF5G15, SK4G13, SF6G05, others. Titanium oxide and zinc oxide are readily used in
commercial sunscreen products. Various alloys of oxide-doped austentic stainless steels are used
in various reactors components (core barrel, baffle plate, fasteners, weldments). In the following
section we will discuss a brief overview of literature on oxide materials studies for radiation.
1.2 Radiation induced defects
In the following section, the basic concepts of radiation interaction with matter are
discussed, which are presented in great detail in the literature6,7,8,9
. When radiation in the form of
energetic particles, heavy ions, neutrons or electrons, impinges a material; it transfers its energy
to the solid through a series of collisions with the electrons and ion-core of the target atoms.
Thus the loss of kinetic energy is through the electrical forces leading to excitation, ionization or
radiative ways. The collisions can be elastic or inelastic in nature. The elastic collision occurs
with atomic nuclei and the inelastic collision occurs with atomic nuclei or electrons. The
interaction of radiation with matter involves several stages; however the principal consequence
of irradiation is the formation of defects in the material. How the defects evolve in subsequent
damage, decides the materials stability towards radiation. Depending upon the energy and
amount of the radiation, microstructural changes could occur and modify the microscopic and
macroscopic properties of materials. The irreversible structural change occurring in a material is
related to its resistance to amorphization by radiation damage.
1.2.1 Electronic excitation
In a target material for radiation, the number of electrons and their cross-sectional area
are much greater than those of the nuclei, and so the electronic interactions of incoming radiation
5
are numerous. Due to the small mass of electrons, they do not alter the path of incoming
radiation nor result in atomic displacement. However, a significant amount of radiation energy is
lost during the electronic excitation, and ion tracks are formed in the irradiated materials which
can be analyzed using Transmission Electron Microscopy (TEM). The energy loss for radiation
from such inelastic interaction is quantified by the electronic stopping power of the material.
1.2.2 Nuclear interaction
When an energetic particle is incident on a material and results in the displacement of
atom from its lattice site, it creates a primary knock-on atom (PKA). The PKA atom and
recoiling atom interact with other atoms and create a cascade of displaced atoms. For a neutron
with ~2 MeV energy incident on a material with a threshold displacement energy of ~40 eV, the
typical number of displaced atoms in a cascade would be around ~50, 000. The duration of this
size of cascade would be for few picoseconds (ps) and extend to several nanometers within the
lattice. The minimum kinetic energy of a particle required for permanently displacing a lattice
atom to result in stable defect is defined as its threshold displacement energy (Ed). The displaced
atom creates a vacancy, while moving itself to an interstitial lattice site, a substitutional lattice
site, or to another vacancy. Most of initially displaced atoms recombine with the vacancy
position. If the initial radiation energy is more than the displacement energy even after
contribution to electronic excitation, it leaves behind stable Frenkel pairs (vacancy-interstitial
pairs). The defects can interact and form dislocations, voids, etc. The common radiation-induced
defects in materials are vacancies, self-interstitials, interstitial or substitutional impurity,
dislocation loops, voids and clusters. The type of defects and production rate varies with the type
6
of material and the radiation energy10,11
. The microstructural evolution of the defects or changes
in physico-chemical properties under irradiation is dependent on the crystal structure.
1.2.3 Select oxide materials under radiation
In search of host materials for immobilizing toxic radionuclides, reactor applications, etc,
several oxide materials have been studied, either employed in radiative media or conceived to
have resistance to amorphization, during radiation. Radiation effects in such oxide materials of
interest are more complex than in metals and metal alloys due to the presence of cation and anion
sublattices, varying crystal structure and thus the associated defect production and defect
evolution. Cations disorder by cation antisite reactions and anions take part in Frenkel defect
formation. There are numerous studies, which suggest that metal oxides with fluorite crystal
structures have higher resistance towards radiation damage12,13
. Oxides related to fluorite (AO2)
and pyrochlore (A2B2O7) structure have the ability to accommodate radiation-induced defects.
There is a close crystallographic symmetry between pyrochlore and fluorite structures. If
pyrochlore A and B cations were to randomly exchange places with one another, such a
pyrochlore would assume the identical periodicity and structure as fluorite. The larger ionicity of
the fluorite-type structure has been suggested for its radiation tolerance14
. Interstitial-vacancy (I-
V) recombination or the interaction of interstitial and vacancies are some of the other suggested
methods of accommodating the radiation effect in fluorite structures.
Most of the early investigations on radiation effects in oxides were focused on UO215,16
.
Later, several other oxide systems have been studied to understand the radiation resistance
mechanism in oxides. Trachenko et al.17
has explained better radiation resistance of Al2O3, MgO
with respect to TiO2 based on the comparison of short-range covalent and long-range ionic
7
forces. Another well studied system for radiation interaction with materials are zirconate and
titanate pyrochlores (Gd2(ZrxTi1-x)2O7). Wang et al.18
studied the effect of composition on
zirconate and titanate pyrochlores subjected to 1 MeV of Kr+ irradiation and observed a
systematic change in the susceptibility to radiation-induced amorphization with increasing Zr
content. The study suggested Zr- rich pyrochlore Gd2Zr2O7 transformed into a defect-fluorite
structure upon irradiation while the Ti-rich pyrochlore undergoes amorphization at room
temperature. The different behaviors of Ti and Zr rich pyrochlore upon irradiation are related to
the tendencies toward the order-disorder transition.19
Irradiation study on pyrochlore (Gd2Ti2O7)
and zirconalite (CaZrTi2O7) with 1.5 MeV Xe+, 1.0 MeV Kr
+ and 0.6 MeV Ar
+ was pursued by
Wang et.al20
in the temperature range of 20-1073K. They reported that the critical radiation dose
for amorphization increased with temperature and that the heavier ion increased the critical
temperature for amorphization. Before amorphization, the pyrochlore superlattice transformed to
fluorite sublattice, while the zirconolite first transformed to cubic pyrochlore followed by
disordered fluorite structure. Lumpkin et.al21
studied the critical amorphization fluence by
choosing Y2Ti2-xSnxO7 as the system and observed that radiation tolerance increased
significantly with Sn content on the pyrochlore B site. The sample with x=1.6 remains crystalline
on irradiating with 1 MeV Kr+ at 50K. The radiation response of such pyrochlore (A2B2O7)
structure strongly depends on the radius ratio of A to B cations22
, which in turn decides the
energy for defect formation such as cation antisites and anion Frenkel pairs. When the ratio
(rA/rB) is high, the vacancies and interstitials are produced upon irradiation of such pyroclores,
which tend to accumulate instead of recombining. Earlier this behavior was reported by Sickafus
et.al23
in nuclear stopping regime and later it was shown by Patel et.al24
that the same is true in
8
electronic stopping regime as well. Interstitial-vacancy recombination or their interaction leading
to dislocation loops and the radius ratio of cations in complex oxides are some of the structural
features associated with the susceptibility to lattice destabilization under irradiation.
1.3 Cerium oxide (ceria, CeO2)
Cerium oxide or Ceria (CeO2) is a rare-earth oxide material, well known for its wide
spectrum of applications viz. chemical mechanical polishing25
, catalysis26
, ultraviolet shielding27
,
conversion coating, electrolyte in solid oxide fuel cells28,29
etc. Researchers explored that ceria at
the nanosize regime has potential applications in a variety of fields due to its oxygen buffering
capacity30
. Ceria nanoparticles (nanoceria) have also gained considerable attention due to its
recently observed antioxidant property in biological systems31
.
Ceria is a particularly interesting material for understanding radiation-induced
phenomena in nuclear oxide fuels and in designing structural materials for radiation-tolerant
applications. Ceria has close similarities to UO2 (Table 1) and is expected to have properties
similar to UO2. Thus it is an ideal material to study or experiment with radiation in order to
understand the irradiation behavior of oxides and in particular nuclear fuels.
1.3.1 Irradiation study on cerium oxide material
Most of the studies reported on ceria irradiation have been done recently. Sonoda et.al32
and have shown that by irradiating ceria with high-energy ions (210 MeV, Xe14+
), even at an ion
fluence of 1x1015
ions/cm2, the structure remains crystalline. Ohhara et.al
33 showed using Raman
spectra of ceria irradiated with 200 MeV Au ions, that a broad band peak appears at the higher
frequency side of the F2g peak of CeO2 and the intensity of Raman peak increases with
9
increasing ion fluence. In the study, comparison of Raman spectrum from irradiated ceria and
then annealed after irradiation revealed, that the peak was from the creation of oxygen vacancies,
reasoned to
Table 1: Similarity in properties of UO2 and CeO2
Property UO2 CeO2
Lattice Structure Fluorite Fluorite
Lattice Constants 5.471 5.407
Melting Temperature 2870 °C 2700 °C
Radioactive Yes No
be originated upon irradiation. Iwase et.al34
studied swift heavy ion (200 MeV, Xe ions)
irradiated ceria, in the fluence range of 0.6x1013
-6x1013
ions/cm2 and quantified the amount of
Ce3+
increase with ion fluence. Yasunaga et.al35
derived the threshold displacement energies of
the sublattices and migration energy of Ce vacancy in CeO2 by following the growth behavior of
interstitial type dislocation loops caused by different displacement conditions from electron
irradiations (200-3000 keV). The migration energy of Ce ion vacancy was estimated to be
greater than 2.1 eV. Electron irradiations greater than 1500 keV induced Ce ions, below this
energy level only O ions were observed. The threshold displacement energy of the Ce-sublattice
was estimated in the range of 44-58 eV and for O-sublattice to be lower than 33 eV in ceria
lattice. The threshold displacement energy of oxygen for radiation damage in ceria was estimated
theoretically by Walsh et.al36
to be 35.4 eV and compared to that of indium oxide (In2O3) which
is 14.2 eV. They also calculated the energy associated with the formation of first stable anion
10
Frenkel pair to be 5.80 eV for ceria. Atomistic simulation of point’s defects in ceria by
Guglielmetti et.al37
evaluated the oxygen and cerium threshold displacement energies to be 27
eV and 56 eV, respectively. It was shown that oxygen Frenkel pair recombinations are
spontaneous or thermally activated and their lifetimes are shorter than the characteristic time for
oxygen vacancies diffusion. Ceria is a wide-band gap material with energy gap of 3.2 eV which
is still lower than the energy for the anion Frenkel pair formation (5.80 eV). A detailed study by
Lushchik et.al38
suggests that wide-gap oxides materials with band gap less than Frenkel defect
energy are not radiation sensitive, and need high energy or external cation atoms to disrupt the
lattice structure upon irradiation. Thus the literature data suggests that ceria is a potential
candidate for irradiation study aimed at exploring radiation resistance properties.
1.3.2 Cerium oxide and radiation induced damage in biological medium
In addition to exhibiting radiation tolerance behavior from a structural aspect, ceria has
been recently investigated for quenching radiation induced free-radicals in biological systems.
The ability of cerium to switch valence state in redox environments motivated several
researchers to investigate the quenching of free-radicals produced due to radiation exposure in
living organisms39,40
. The early investigation of ceria for therapeutic application show that
nanoceria prevented retinal degeneration induced by reactive oxygen species (ROS)41
. It was
shown by Seal et al., that nanoceria has SOD mimetic/antioxidant properties. This property was
shown beneficial in preventing radiation induced cell damage.42
Normal and Tumor cells were
irradiated with 10 Gy and the cell viability was measured by MTT assay at 24 and 48 hours of
post radiation. It was observed that normal cells pretreated with 10nm of ceria nanoparticles
prior to radiation were protected. Radioprotection from nanoceria was also evidenced by in-vitro
11
(with CCL135-normal lung fibroblast) and in-vivo (athymic nude mice) studies.43
The normal
lung fibroblast cells were exposed to increasing doses of radiation (0-30 Gy) and the cell
viability decreased with increasing radiation. When nanoceria was administered to cells before
radiation, these nanoparticles significantly protected the normal lung fibroblast cells. Another
system studied for nanoceria effectiveness in preventing radiation-induced damage was
gastrointestinal epithelium, which conferred radioprotection to the good cells.44
The cells (CRL
1541) were pretreated with ceria nanoparticles before radiation exposure, and dose-dependent
reduction in expression for reactive oxygen species was observed. In the same report, an in-vivo
study was carried on mice pretreated with intraperitoneal injections of ceria before exposing the
abdominal area to radiation. The results of cell apoptosis by TUNEL and caspase 3 stains assay
indicate significant protection from ionizing radiation. Thus recent investigations on nanoceria
by in-vitro and in-vivo assays for preventing radiation induced damage to the healthy cells,
reveal the therapeutic potential of nanoceria.
1.4 Problem of interest
The theoretical and experimental approaches carried out thus far on irradiated ceria have
found that ceria exhibits radiation tolerant properties. Despite great effort, a detailed
understanding of the radiation interaction with nanoceria at the microstructural level requires
more research. Nanoceria displays dual valence state of Ce3+
and Ce4+
and energetic ions from
radiation lead to structural modification, such as changes in grain size and volume, which can
alter further interaction. It is known that crystal structures with lower ionicity show stronger
resistance to radiation cascade damage, while crystal structures with larger ionicity show less
sensitivity towards disordering by thermal spikes. Nanoceria with different redox states will have
12
different ionic characteristics and hence are assumed to perform differently under radiation. The
ionicity can also originate from the structural modification of nanoceria lattice by radiation. The
properties of nanoceria depend on grain size, oxygen environment, temperature, and synthesis
method. Some of the important factors to consider while accessing the ceria interaction with
radiation are effect of interfaces, grain boundary, dopants and temperature among others.
Thus, to utilize the potential of nanoceria for radiation resistant applications, it is
imperative to study in detail the performance of ceria under radiation. The objective of the work
presented here is to show the variations in the properties of nanoceria with varying the redox
state and thus the defect state. The behavior of nanoceria with different crystallinity under high
energy ionizing radiation is analyzed.
1.5 Approach to the problem
This dissertation presents systematic experiments to control the defect state of nanoceria
lattice by modifying the valence state through doping. Such experiments require suitable choice
of synthesis method and dopant. The doped nanoparticle samples will be characterized and
analyzed in detail to access the defect state of nanoceria lattice.
Radiation effects in nanoceria will be studied by synthesis of high quality crystalline
ceria samples and exposing them to high energy ionizing radiation. The defect state of the
irradiate sample will be characterized using spectroscopic methods at various radiation doses.
Such experiment provides detail about the residual state of nanoceria after irradiation.
The results on ceria irradiation will be analyzed in the light of atomistic simulation. The
simulation will capture the evolution of cascade damage over picosecond (ps) time scales.
13
The focus of the work in this dissertation is to carry out fundamental studies on nanoceria
upon radiation through experiment and simulation. This will contribute to the present
understanding of the irradiation behavior of nanoceria. However, as we have seen in section
1.3.2, that nanoceria imparts protection from radiation induced damage in biological system; the
fundamental study conducted here may benefit the therapeutic applications of nanoceria.
1.6 Outline of the dissertation
The following chapters of this dissertation are organized as explained below. Chapter 2
demonstrates one of the methods for controlling the defects in the synthesis of oxide nanoparticle
systems and how these defects can be used to modify the property of synthesized nanoparticle.
Chapter 3 provides experimental results from irradiation of cerium oxide thin films. Chapter 4
shows how atomistic simulation can be used as an effective tool to understand the events
occurring on cerium oxide thin film irradiation. Chapter 5 shows the effect of stoichiometry on
the irradiation of nanoceria lattice studied using atomistic simulation. Chapter 6 summarizes the
thesis and predicts the future scope of research on similar areas.
14
The work presented in this chapter has been published before
Reprinted with permission from Langmuir, 2009, 25, 10998-11007. Copyright
2009 American Chemical Society.
CHAPTER TWO: DEFECTS IN NANOCERIA
2.1 Introduction
In chapter one (1.2), we reviewed that defects in materials and effect of irradiation are
interdependent. While understanding the role of defects in determining materials property is
important, it is equally important to be able to change the defect state experimentally. In the
present work, a suitable property of nanoceria was selected to study with respect to change in the
defect state. As mentioned in section 1.3.2 recent efforts have shown a new frontier for
application of nanoceria in biomedical applications for inhibiting radiation induced damage43
. It
has been shown that ceria nanoparticles can mimic the radical scavenging properties analogous
to naturally existing antioxidants31
. Due to the non-toxic nature and biocompatibility, nanoceria
have the potential to be used in many biomedical applications. However, ceria shows weak
emission characteristics which limit its identification in biological and cellular studies. So the
photoluminescent property of nanoceria was selected to optimize by modifying the defects and
thereby the associated electronic state of nanoceria.
The approach used here is to modify the nanoceria lattice through doping the parent
matrix with elements which can emit by itself upon excitation in the UV-visible range of
spectrum.45
Europium (Eu) belongs to rare earth metal group and has a strong red emission when
doped in different matrices. Eu is considered a suitable choice as dopant to meet the afore-
15
mentioned purpose of enhancing emission in ceria for three reasons i) it can be excited from
ultraviolet to visible light,46
ii) the ionic radius47
of Eu3+
(0.1066 nm) being close to that of
cerium (Ce) (Ce3+
: 0.1143nm, Ce4+
: 0.097 nm) favors extensive solubility with ceria lattice and
iii) it modifies the trivalent valence state of Ce in the doped lattice and thus altering the defect
state. Metal oxides such as alkaline, transition and lanthanide oxides and some sulfides that
exhibit luminescence on doping with Eu, only few studies are reported on doping nanoceria with
Eu. Earlier Wang et al.48
have shown that doping Eu is not efficient in improving the
photoluminescence (PL) properties of ceria. On the contrary, few studies49
have shown
prominent emission upon doping Eu in ceria. Li et al.50
, using citrate and polyethylene glycol,
synthesized 0.1 to 10% Eu doped ceria and attributed the broad band observed in the excitation
spectrum to the charge transfer (CT) transition from O2-
to Ce4+
. The role of defect concentration
such as oxygen-ion vacancies is a topic of debate in open literature; for example, in systems like
zinc oxide (ZnO) it has been shown that the defects act as the emitting centre while in other
system such as ceria it can act as the PL quenching center. In an earlier report51
it has been
shown that the emission of 5 wt% Eu doped ceria can be influenced by the defects. Here we
quantitatively demonstrate the role of oxygen-ion vacancies as PL quenching centers in Eu
doped ceria system. The PL properties of Eu-doped ceria system are investigated with respect to
dopant concentration and annealing temperature. The mechanism of efficient energy transfer has
been correlated with the surface and defect structure. Ce3+
aids in the PL properties of Eu, while
oxygen vacancy concentration adversely affects the PL of Eu by seizing the radiative route of
emission. The effects of Eu ion concentration on the emission and symmetry are vital as it can
tune the oxygen-ion vacancy concentration.
16
2.1 Experiments
2.1.1 Materials
Cerium nitrate (99%, Aldrich), europium nitrate (99.9%, Aldrich), ammonium hydroxide
(30 wt%, Alfa Aesar) were used as received. Deionized water (18.0 MΩ) purified in Barnstead
system was used in all the experiments. Eu doped ceria nanoparticles were synthesized via co-
precipitation technique. Cerium nitrate and stoichiometric amount of europium nitrate were
dissolved in 500 ml deionized water and hydrolyzed with ammonium hydroxide in order to
maintain a pH of 9. The resultant solution was stirred and allowed to settle overnight. The
precipitates were washed with water multiple times to remove any weakly adhered ions on the
surface and dried at 120 °C. Using the above procedure, samples with varying mole fractions of
Eu were prepared and the samples were coded as CEX where X = 1, 5, 10, 15, 20, 25 and 30
mol%. A similar procedure was adopted to prepare nanoceria without any dopant (CE0). In order
to evaluate the structure, symmetry, luminescence modifications, CE15 sample (selected based
on the maximum emission among the various dopant concentration, as discussed later in this
chapter) was annealed at different temperatures. The annealed samples were designated as CE15-
T, where T is the annealing temperature (300, 500, 700 and 900°C).
2.1.2 Characterization methods
To determine the size, lattice parameter and strain in the samples, the as prepared
nanopowders were characterized by X-ray diffraction (XRD) and high resolution transmission
electron microscope (HRTEM). The X-ray diffraction patterns were collected using Rigaku
D/MAX XRD with CuKα source (λ=0.1541 nm) at a scan rate of 0.01°/step over 20-70°(2)
range. The HRTEM images were taken on FEI Tecnai F30 operated at 300 kV. X-ray
17
photoelectron spectroscopy (XPS) experiments were carried out using Physical Electronics
(PHI5400 ESCA) spectrometer with a monochromatic Al Kα X-ray source operated at 300 W
and a base pressure of 5×10-8
torr. The acquisition time of the sample was kept low to minimize
any changes in the surface oxidation state under X-ray irradiation. Raman spectra were collected
using a Horiba JobinYvon LabRam infrared (IR) micro-Raman system using a 633 nm helium-
neon laser with a spatial resolution of 200 nm to obtain the electronic structure. Ultraviolet-
visible (UV-Vis) spectra were recorded using Lambda 650 UV/Vis spectrophotometer to
evaluate the optical properties of the samples. Fourier Transform Infrared (FTIR) spectra was
collected using Perkin-Elmer Spectrum One to study the variation in surface features with
temperature and the PL properties were investigated using Hitachi-7000 spectrophotometer.
2.2 Results and discussion
2.2.1 Size and structural properties
Samples were analyzed by XRD to understand the structural and size modifications. For
the as prepared particles (Figure 1a), the diffraction peaks correspond to (111), (200), (220) and
(311) planes of cubic fluorite ceria (CeO2) structure (JCPDS no. 88-2326) having Ce in 4+
oxidation state. Diffraction peaks corresponding to either europium oxide or hydroxide were not
observed indicating the formation of solid solution in the entire composition range. Upon doping,
the peak position (2θ) shifts towards lower angles in comparison to pure nanoceria (CE0). Ce ion
can exist in two different oxidation states of 3+ and 4+ having an ionic radii of 0.1143 and 0.097
nm, respectively while Eu can exist in 3+ (0.1066 nm) and 2+ (0.125 nm). Since XRD pattern
indicates the presence of Ce predominantly in 4+ oxidation states, doping higher ionic radii Eu
ions can induce a corresponding increase in lattice parameter introducing internal tensile stress in
18
the ceria matrix. With annealing temperature, the peaks become sharper and increase in intensity
due to the growth of nanoparticles (Figure 1b).
Figure 1: XRD pattern measured (a) at RT for ceria sample without doping and ceria samples doped with
Eu at 1, 5, 10, 15, 20 and 30 mol % depicting the broad peaks for (111), (200), (220), (311) and (b) for 15
mol % Eu doped ceria heat treated at temperature 300°C, 500°C, 700°C and 900°C indicates that the
fluorite structure becomes more crystalline at higher annealing temperature.
19
Even upon high temperature annealing, the resultant peaks could be indexed to pure
ceria, without any Eu phase separation suggesting that the Eu ions are doped in the lattice of
ceria. Lattice parameter (a) was calculated by fitting the (311) peaks with Gaussian and the full
width at half maximum (FWHM) was obtained from the fit. To compensate for the broadening in
peaks due to instrument, the correction in FWHM was done using equation ( 1 ).
222 insthklhkl FWHMFWHM ( 1 )
where, FWHMhkl is the full width at half maximum intensity of the reflection (h k l) plane and
FWHMinst is the instrumental broadening. The instrumental broadening in the pattern of the
sample was corrected using a standard quartz sample. A FWHM curve (FWHM peak as a
function of 2) was obtained using the scan pattern from the standard reference material. The
standard sample was run under identical instrumental conditions as those of the Eu doped and the
undoped ceria samples, so that the broadening of the standard is exactly the instrumental
broadening in the pattern of the sample. The lattice parameter for (h k l) plane was calculated
using the equation ( 2 ):
222 lkhda ( 2 )
The calculated lattice parameters are shown in Figure 2 as a function of dopant concentration and
annealing temperature for CE15. The lattice parameter of CE0 was found to be higher than that
of the bulk ceria (0.541 nm). With the increase in dopant concentration there is an increase in
lattice parameter,52
which can be attributed to the presence of Eu ions in the ceria lattice. The
lattice parameter increases linearly from 0.5412 nm in CE0 to 0.546 nm in CE30. The width of
20
Figure 2: Lattice parameter- shown along ordinate, obtained by fitting gaussian curve to the spectra of i.)
ceria doped with Eu at 0, 1, 5, 10, 15, 20 and 30 mol % ii.) 15 mol% Eu doped ceria heat treated at
temperature 300C, 500°C, 700°C and 900°C. Lattice parameter increases linearly with increasing dopant
concentration which indicates that the stress generated due to the larger ionic size of europium
substitution in the lattice position of cerium increases with increase in dopant concentration. The decrease
in lattice parameter value with temperature suggests that stress in the doped samples is released with
higher annealing temperature.
the peaks are influenced by size and strain of the particles and the two factors were separated by
using standard Williamson-Hall plot53
given by equation ( 3 ).
cos
sin4
cos
9.0
d
d
tstrainsize
( 3 )
where, is the FWHM of the diffraction peaks after correcting for instrumental broadening, is
the wavelength of the incident x ray, is the diffraction angle, t is the average crystal size, and
d is the difference of the d spacing corresponding to a typical peak. The size and strain in the
21
sample were obtained from the y-intercept and the slope of the straight line fitted from the plot
between cos and sin as shown in Figure 3. Size and strain obtained for different dopant
concentration and annealing temperatures are reported in Table 2. The as prepared nanoparticles
were in the size range of 5-7 nm but on annealing at 900 oC the size increases up to 35 nm for
CE15-900.
Figure 3: The representative plot of “βcos vs 4sin ” for CE15. The slope and intercept of a linear fit to
the data was used to calculate the stress and strain in the sample
To evaluate the size and morphology, HRTEM characterization was carried out. Multiple
TEM and HRTEM images were taken over different locations of the grid in order to calculate the
average nanoparticle size. Representative bright field HRTEM images along with selected area
electron diffraction (SAED) patterns for CE15, annealed at different temperatures, are shown in
Figure 4a-e. The average crystallite size of CE15 in the as synthesized condition was 5 nm as
22
Figure 4: The representative bright field HRTEM images of CE15 (a) as prepared and annealed at (b)
300°C, (c) 700°C, (d) 900°C and (e) EDX spectra of CE15. The inset of each figure shows the SAED of
the respective HRTEM image
23
shown in Figure 4a. As evident from TEM and XRD results annealing the samples at higher
temperature lead to increase in particle size due to growth in order to reduce the surface energy.
The inset of Figure 4a shows the ring patterns corresponding to (111), (200), (220) and (311) in
SAED representing the fluorite structure. Figure 4b-d shows the HRTEM images of CE15
annealed at 300, 700 and 900°C, respectively. From the HRTEM image it is observed that the
average crystallite size increases with annealing temperature from 5 nm as prepared to 15 nm at
300°C. At higher annealing temperature the particle size increases further from 30 nm at 700°C
to 35-45 nm at 900°C. As evident from the XRD results annealing the samples at higher
temperature leads to the increase in particle size due to its growth in order to reduce the surface
energy. Figure 4e shows the representative energy dispersive X-ray (EDX) spectra for CE15
indicating the presence of Ce, Eu and O. From EDX quantification, the calculated Eu
concentration was found to be within ±5 at% from the anticipated value. Presence of Cu can be
attributed to the carbon coated copper grids used for TEM analysis.From EDX quantification
(Figure 4e), the calculated Eu concentration was found to be within ±5 % from the anticipated
value. In order to understand the associated structural modification with heat-treatment, FT-IR
spectra of the powders were recorded (Figure 5). In the as prepared condition, FTIR spectra
showed the presence of adsorbed hydroxyl species on the surface of the samples and these could
be responsible for quenching the emission through non radiative routes. The concentration of
adsorbed hydroxyl groups decrease with increase in annealing temperature as the hydroxyl
groups detach from the surface.54
FT-IR spectra (Figure 5) were recorded for the annealed
samples to identify the effect of heat-treatment causing any structural changes. The observed
absorption peaks are located at 3420(L1), 1632(L2), 1480(L3) and 1320(L4) cm-1
. The former
24
two peaks (L1, L2) are attributed to the hydroxyl (OH) bond stretching and bending mode,
respectively. The peak at 1480 and 1320 cm-1
corresponds to nitrate group, adhering to the
surface of nanoparticles. From the FTIR spectra, it can be concluded that with the increase in
annealing temperature the concentration of surface hydroxyl group decreases.
Figure 5: IR spectra of 15 mol% Eu doped nanoceria shows with annealing temperature
25
Table 2: Size, strain, Ce3+
ratio, defect concentration, bandgap for different samples
Sample
Dopant
Concentration
(mol%)
Annealing
temperature
(oC)
Size
(nm)
Strain
(x10-3
)
Band
gap
(eV)
%Ce3+
Defect
concentration
(x1020
cm-3
)
CE0 0 - 5-7 0.87 3.30 - 1.32
CE1 1 - 5-7 1.2 3.24 18.3 1.61
CE5 5 - 5-7 1.8 3.24 18.9 1.9
CE10 10 - 5-7 1.8 3.23 19.6 2.25
CE15 15 - 5-7 4.6 3.29 21.2 2.97
CE20 20 - 5-7 4.8 3.24 22.0 3.39
CE30 30 - 5-7 6.0 3.23 23.5 4.03
CE15-300 15 300 6.9 4.0 3.27 19.0 2.32
CE15-500 15 500 7.7 2.0 3.27 18.8 2.06
CE15-700 15 700 12.6 0.4 3.32 16.5 1.53
CE15-900 15 900 34.7 0.1 3.36 14.0 1.17
2.2.2 Surface and symmetry modifications
Since addition of Eu ion in the ceria lattice can influence the dynamics of Ce3+
to Ce4+
, to
understand the changes in the valence chemistry and binding energy of constituent elements XPS
analysis55
was carried out. The recorded XPS spectra were charge corrected with respect to the
C(1s) peak at 284.6 eV. Figure 6 shows the XPS spectra for CE15, the fitted curve and the
26
corresponding deconvoluted peaks. The peaks denoted by v0, v’, u0, u’ are characteristic peaks
of Ce3+
ions, while those marked by v, v’’, v’’’, u, u’’, u’’’ are of Ce4+
ions.
Figure 6: XPS spectrum of Ce(3d) for 15 mol% Eu doped ceria sample was deconvoluted to give
individual spin-orbit doublet of 3d3/2 and 3d5/2 and the sum of deconvoluted peaks were used to produce
the fit to the actual data
The deconvoluted Ce(3d) spectrum is relatively complex due to the presence of Ce in 3+
and 4+ oxidation states as well as multiple d-splitting. The peaks in the spectrum of Ce were
deconvoluted using the PeakFit (4.0) software. The spin-orbit doublet 3d3/2 (880.31 and 898.25
eV) and 3d5/2 (898.81 and 916.7 eV) is clearly observed for both valence states of Ce. XPS
results indicate that Ce is in mixed valence states of 3+ (880.40, 885.5, 898.81, 903.7 ±0.7 eV)
and 4+ (882.7, 888.96, 898.2, 901.3, 907, 916.7 ±0.7 eV). The integrated area under the curve of
each deconvoluted peak was used to calculate the concentration of Ce3+
ions56
as in equation ( 4 )
27
i
uuvv
A
AAAACe
'0
'03
( 4 )
where Ai is the integrated area for peak “i”.
From the analysis of the XPS spectra, it was found that the concentration of Ce3+
increases from
18.3% for CE1 to 23.5% for CE30 and decreased with annealing temperature. Although Eu ion
can exist in 2+ or 3+ oxidation states, in the present case Eu predominantly exist in trivalent state
as indicated by the presence of characteristic peaks at 1134±0.7 eV and 1163.5±0.7 eV.57
The
peak positions of Eu in 3+ and 2+ state differ by large binding energy difference of about 9 eV
and no peak was observed in the region of 1124±1 eV (Eu2+
3d5/2) and 1153±1 eV (Eu2+
3d3/2)
corresponding to 2+ oxidation state of Eu. Both peaks of Eu3+
showed a shift by 0.7 eV towards
lower binding energy with increasing dopant concentration as shown in Figure 7.
Figure 7: The high resolution XPS spectra of ceria samples doped with Eu at 1, 5, 10, 15, 20 and 30 mol
%
28
The decrease in binding energy can be attributed to increase in valence electron density.58
The
FWHM of the Eu3d5/2 decreases from 5.23 to 4.99 eV, respectively, as the dopant concentration
increases from 1 to 30 mol%. For CE15, the Ce3+
concentration was found to be 21.2 % which
decreases to 14.0 % upon annealing at 900°C. Although XPS identified the oxidation states of Ce
and Eu, the modifications in crystal symmetry upon doping with Eu can be understood by
observing the O(1s) spectra.
Figure 8: Oxygen 1s spectra of nanoceria samples doped with 1, 5, 10, 15, 20 and 30 mol % Eu at room
temperature shows the asymmetric behavior changes with dopant concentration and depicts peak
broadening at higher doping concentration (b) Oxygen 1s spectra of 15 mol% Eu doped ceria at room
temperature and heat treated at 300°C, 500°C, 700°C and 900°C shows the asymmetry in peak decreases
with increasing temperature treatment
29
Analyzing the spectra of O(1s) peak of Eu doped ceria samples at room temperature shows an
increase in asymmetry with dopant concentration as shown in Figure 8a which indicates the
distortion of the crystal structure.
In XPS the energy provided by the absorbed photon causes the excitation of electronic
and atomic structure of the specimen surface. Hence, the ejected electron from the sample
appears with kinetic energy dependent on atomic oscillations associated with the type of bonds
within the lattice. Therefore, the observed asymmetry in O(1s) peak is due to the O2-
ions located
in different chemical environment such as oxygen attached to Ce3+
, Ce4+
, Eu3+
, H+, within the
sample leading to alternative energy transfer mechanisms. The effect of heat-treatment on O1s
spectra is depicted in Figure 8b. With increasing annealing temperature the asymmetry in oxygen
peak decreases due to the reordering or removal of chemical species such as H2O and OH.
In order to understand the associated local environmental modifications around oxygen,
the asymmetric O(1s) peak of CE15 sample was fitted into four Gaussian-Lorentzian peaks
namely Oa, Ob, Oc, Od centered at 529.0±0.2 eV, 529.6±0.2 eV, 531.4±0.2 eV & 533.5±0.2 eV
respectively as shown in Figure 9a. Binding energy of O(1s) for Ce3+
-O2-
is reported to be higher
than that of Ce4+
-O2-
. For example, Praline et.al have reported the binding energy of Ce2O3
(530.3 eV) to be 0.7 eV higher than CeO2 (529.6 eV).59
Europium oxide (Eu2O3) has higher
binding energy of 531.40 eV compared to Ce-O bonds,60
thus Eu3+
-O2-
will have a higher
binding energy as compared to Ce3+
-O2-
based on its higher electronegativity and oxygen affinity
relative to those of Ce3+
ions. Figure 9b shows the schematic lattice model of ceria with different
chemical environment around lattice oxygen-ion.
30
Figure 9: (a) A typical asymmetry in high resolution O1s spectrum of 15 mol% Eu doped ceria was
resolved and attributed to the presence of oxygen in different chemical environment as the deconvolution
of the experimental spectra results in peaks corresponding to binding energy of O2-
ions associated with
chemical environment of CeO2, Ce2O3, Eu2O3, OH/O2: Fitted and actual spectrum are shown (b)
Schematic lattice model showing the different chemical environments of oxygen within the crystal
structure of nanoceria
The Oa peak on lower binding energy side of the O(1s) spectrum is attributed to O2-
ions
surrounded by Ce4+
ions, which corresponds to Ce-O bond in CeO2. The deconvoluted peak Ob
can be ascribed to to the O2-
ions in Ce-O bond where Ce is present in the 3+ state. The peak Oc
is associated with the binding energy of O2-
ions that are specific to Eu3+
-O2-
configuration.61
The
deconvoluted peak Od is related to either the O2-
ions that are in oxygen-deficient regions within
the Eu doped ceria lattice or the chemisorbed OH species on the surface as suggested by the IR
spectra (Figure 5) of the heat-treated samples. Similar analysis of O1s peak was carried out for
other doped samples. The Integrated Peak Area Ratio (IPARi=AOi/AO) for Oa, Ob, Oc, Od is
representative of the relative amount of different species Oa (Ce4+
-O2-
), Ob (Ce3+
-O2-
), Oc (Eu3+
-
O2-
) and Od (O2-
-H+). For the room temperature samples it was observed that with increasing
dopant concentration, the IPAR of Oc and Ob increases while that of Oa decreases as shown in
31
Figure 10a. This implies that increase in Eu3+
is accompanied by an increase in Ce3+
concentration and simultaneous decrease in the concentration of Ce4+
. Thus it is clear that
addition of Eu in ceria lattice replaces the Ce4+
ion in lattice and creates oxygen vacancies. The
IPAR for Od changes marginally from 0.045 for CE1 to 0.056 for CE30. The CE15 sample
annealed at temperatures from 300°C to 900°C revealed that the IPAR for Oa increases from
0.56 to 0.72 and the IPAR for Ob decreases from 0.13 to 0.09 with increasing annealing
temperature (Figure 10b). The IPAR for Oc peak in 15 mol% Eu doped ceria sample decreases
rapidly from 0.24 at room temperature to 0.147 at 500°C and then decreased marginally to a
value of 0.142 at 900°C.
Figure 10: The integrated peak area ratio (IPARi=AOi/AO) obtained from XPS analysis of O1s spectra of
ceria doped samples shows how with doping and heat treatment the oxygen atmosphere changes (a) at RT
for 1, 5, 10, 15, 20 and 30 mol% Eu doping (b) for CE15 heat treated at 300C, 500°C, 700°C and 900°C.
The IPAR of Od for 15 mol% doped sample decreases from 0.05 at room temperature to
0.03 at 900°C, which suggests that with heat treatment, the chemisorbed -OH species (as
detected in FTIR spectra, Figure 5) on the surface is released and this is expected to enhance the
luminescence by seizing the non-radiative routes of energy transfer. It must be noted that the
32
peak separation in Figure 7b is actually due to the reduction in hydroxide content. At room
temperature the contribution from OH at higher binding energy is enough to merge with the
cumulative peak of Eu3+
-O2-
, Ce3+
-O2-
and Ce4+
- O2-
. As the sample is heat treated the OH
content in the sample reduces and the peak intensity diminishes which causes a dip in spectra
between the binding energy corresponding to positions of O2-
-H+ and Eu
3+-O
2-. Thus the OH
binding energy peak which was initially merged in the overall spectra due to higher contribution
from OH, separates out in case of heat-treated sample as the intensity of the peak decreases with
higher annealing temperatures. Also from the analysis of O1s peak of the doped samples, it was
found that at room temperature the binding energy corresponding to Oa and Oc peak decreased
by 0.8 eV and 0.49 eV respectively as the dopant concentration was increased from 1 mol% to
30 mol%. The XPS results indicate that the chemistry of the doped sample and thus the
luminescence can be controlled by appropriate amount of doping followed by annealing at
suitable temperature.
Further investigation of electronic structure of the doped samples was carried out using
Raman spectroscopy. Figure 11a-b shows the Raman spectra for the as prepared and annealed
samples. On doping ceria with Eu the Ce4+
ions are partly replaced by Eu3+
ions, which create
oxygen-ion vacancies to compensate for the reduced positive charge of the cations. These
oxygen-ion vacancies change the valency of Ce from 4+ to 3+ thereby increasing the Ce3+
concentration in the lattice as supported by the XPS results. The introduction of O2-
vacancies in
Eu doped ceria samples distorts the fluorite lattice. In presence of impurities or lattice disorder,
the phonon function is spatially confined which results in wavenumber shifts and band
broadening.
33
Figure 11: (a) Raman spectra for room temperature ceria samples doped with 1, 5, 10, 15, 20 and 30 mol
% of Eu samples shows a shift towards decreasing wavenumber and increasing broadening as the amount
of dopant is increased (b) Raman spectra for 15 mol % Eu doped ceria heat treated at 300C, 500C,
700C and 900C shifts slightly towards higher energy with annealing temperature
34
Figure 11a shows the Raman shift of 5.84 cm-1
towards lower wavenumber for CE5, CE10 and
CE15 with respect to CE1 sample. With respect to CE1, a shift of 0.17 cm-1
for CE20 and a shift
of 0.11 cm-1
for is observed. In comparison to CE15, a shift of 8.55 cm-1
, 12.31 cm-1
, 14.18 cm-1
,
15.12 cm-1
towards higher wavenumber is observed on annealing CE15 at 300°C, 500°C, 700°C
and 900°C respectively. In a perfect crystal of cerium oxide, each cerium atom is surrounded by
8 oxygen-ion and for symmetrical stretching vibrations of CeO8 vibrational unit results in a first
order allowed Raman active triply degenerate F2g mode at 465 cm-1
.62
As reported in earlier
studies on Raman spectra of ceria nanoparticles the F2g mode shifts towards lower energies with
increasing line-width while the line shape becomes asymmetric with decreasing particle size.63
Similar shift has been observed in case of Eu doped ceria samples with respect to bulk ceria and
on increasing the dopant concentration above 15 mol%, the degree of shift is reduced. With
increasing annealing temperature this peak shifts towards higher energy. The peak shift can be
directly correlated to oxygen-ion vacancies in doped ceria samples. The concentration of
oxygen-ion vacancies increases with the amount of dopant64
and with annealing these vacancies
tend to migrate and get annihilated at the surface leading to improved phonon lifetime which
would in turn broaden and increase (for Eu doped ceria) the intensity of the peaks in PL
measurement. The evolution of Raman line can be attributed to the combined effect of phonon
confinement, and residual strain in the sample.65
As seen from the XRD results (Table 2) the
strain in the sample increases with dopant concentration and so does the broadening in Raman
line (Figure 11a). The shape change in Raman line and the lattice disorder induced by dopant is
explored by interpreting the data using Spatial Correlation Model.66
According to this model, the
Raman line intensity I(w) at frequency w is given by equation ( 5 ).
35
20
2
31
0
22
2/)(4exp
qww
qdLqI ( 5)
where, q is the wave vector expressed in units of 2/a (a is lattice constant). The term
exp(-q2L
2/4) represents the Gaussian spatial correlation function, where L is the coherence length
representing average extension of the material homogeneity region. 0 is the FWHM of Raman
line for perfect crystal (9.2 cm-1
). The intensity from the model was calculated by taking the sum
over three equally weighted branches of phonon dispersion, as done previously,67
and the defect
concentration (N) was calculated using the equation ( 6 ).
3
4
3
LN
( 6)
The value of correlation length obtained from the model as shown in Figure 12a varies from 11.4
to 8.4 at RT. Figure 12b compares the amount of Ce3+
obtained from XPS data and the defect
concentration calculated using the model for Raman data. With increasing dopant concentration
from CE1 to CE30, the Ce3+
concentration increases from 0.183 to 0.235, but the increase in
defect concentration is more steeper (from 1.61x1020
cm-3
to 4.03 x1020
cm-3
) (Figure 12).
Analyzing the effect of the two phenomena (increase in Ce3+
concentration and increase in defect
concentration) on luminescence, it is expected that the former will enhance the luminescence
while the latter will quench the luminescence properties. Therefore it is predicted that increase in
luminescence by Eu doping in ceria lattice will be quenched after reaching an optimal point
where the negative effect of dopant concentration will overcome the luminescence intensification
by increasing Ce3+
concentration.
36
Figure 12: (a) Correlation length for the as prepared samples with variation in dopant concentration and
annealing temperature (b) A comparison between ratio of Ce3+
(calculated from XPS spectra) and defect
concentration (calculated from Raman spectra) with dopant concentration shows both increases
proportionately with amount of dopant which indicates a correlation between the defect concentration
increasing with ratio of Ce3+
as more Eu is added.
2.2.3. Optical and luminescence properties
In order to understand the modification of optical properties upon doping; absorption and
luminescence spectra were recorded. Figure 13a-b shows the normalized optical reflectance
spectra of as-prepared Eu doped ceria samples at different dopant concentrations and 15 mol%
Eu doped ceria annealed at various temperatures. In these spectra, low reflectance means high
absorption in the corresponding wavelength region. The band-gap was calculated by extending
the slope to 0% R. The band gap was found to be about 370nm (~3.3eV±0.1) but upon annealing
CE15, the bandgap shifted marginally towards higher energy. On annealing, the particle size
increases with a reduction in Ce3+
concentration.68
The optical transmittance spectra (Figure 13)
indicated a blue shift of band gap due to the valence transition of Ce3+
to Ce4+
induced by
thermal annealing. The decrease of Ce3+
concentration will eliminate some localized states
37
within the band gap due to the corresponding decrease of defects (vacancies). Thus the blue shift
of Eg among the Eu doped ceria samples is due to the decrease in Ce3+
content, with increasing
grain size induced by heat treatment and not due to the quantum size confinement. Similar trend
in blue shift has been observed for CeO2 films on sapphire68
and Eu doped ceria prepared by sol-
gel50
, when Eu concentration increases to 5% and 10%.
Figure 13: Reflectance spectra of (a) as synthesized samples with dopant concentration (b) CE15 with
annealing temperature
Figure 14a depicts the excitation spectra for emission at 611 nm in the as prepared condition
while Figure 14b depicts the spectra of 15 mol% Eu doped ceria with annealing temperature.
Although pure ceria (CE0) does not show any peak on following the emission at 611 nm, a sharp
peak at 466 nm characteristic to Eu 4f-4f transitions69
was observed in Eu doped ceria samples.
Since the O2-
-Eu3+
CT lie in much shorter wavelength region and CeO2 has a band gap around
3.3 eV (~370 nm), the broad peak is related to mainly Ce4+
-O2-
CT70
with some overlapping with
the intra-configurational Eu 4f-4f transitions.
38
Figure 14: Excitation spectra with emission at 611 nm for (a) as prepared and (b) annealed samples. No
emission is observed for pure ceria. Among the room temperature samples the PL first increases upto 15
mol% Eu doping in ceria and then decreases while the PL increases with increase in annealing
temperature of the 15 mol% Eu doped ceria
39
As evident from Figure 14a with increasing amount of Eu doping, the intra-4f6 transition bands
of Eu3+
become stronger. The intensity was found to increase with increase in dopant
concentration but beyond 15 mol% intensity decreases due to concentration quenching effect.
Figure 15a-b shows the emission spectra of various doped and annealed samples respectively on
exciting at 466 nm. With 466 nm as the excitation wavelength the observed effect of variation in
emission can be correlated to the dopant concentration. Earlier it has been shown that through
charge transfer the energy will not be transferred to the Eu efficiently.51
With increasing Eu3+
concentration there is an increase in intensity upto 15 mol%, further increase in dopant
concentration decreases the Eu3+
-Eu3+
distance leading to an effective energy transfer between
neighboring ions. Hence, the excited state moves to the quenching sites dissipating the energy
non-radiatively. As a result of the concentration quenching, the emission intensity decreases.
Critical concentration for observing the concentration quenching depends on the solubility of
dopants in the matrix and varies with the type of dopants and the matrix. In case of phosphors
very low solid solubility of dopants results in the segregation of the dopant material leading to
concentration quenching even at lower concentrations. For example, in Y2O3 matrix the
concentration quenching from Eu was observed at 3.5% and 5% for Gd2O3 matrix.71,72
Both Eu
and Ce belong to the lanthanide series and demonstrate higher solid solubility and thus Eu can be
dispersed evenly throughout the ceria matrix even at relatively higher Eu concentration. Due to
higher solid solubility the concentration quenching was observed at a higher Eu dopant
concentration (15 mol %) in the present experiments. In order to understand the structural
modification and emission characteristics, CE15 which showed maximum intensity in the as
prepared conditions, was annealed at different temperatures.
40
Figure 15: Luminescence emission spectra (excitation at 466 nm) for (a) as prepared and (b) annealed
samples, shows maximum PL at RT for 15 mol% Eu doped ceria and increases further with increase in
annealing temperature
41
The emission intensity was found to increase with increase in annealing temperature. PL shows
the characteristic Eu3+
emission, that can be assigned to various transitions 5D0-
7FJ (J=0, 1, 2,
etc)51
and these emissions can reveal much about the local environment of the Eu3+
ion. The
multiple peaks in the spectra are due to the splitting of Eu3+
4f shell. The Judd-Ofelt (J-O) ,
theory73,74
is the most useful and popular method in the analysis of spectroscopic studies of rare
earth ions in different hosts. As per J-O theory, the emission lines are a cumulative effect of the
Magnetic Dipole (MD) transition and the Electric Dipole (ED) transition depending on the
specific environment of Eu3+
in any matrix. According to the J-O theory, ED transition (5Do-7F2)
centered at about 611 and 629 nm, is only allowed in absence of inversion symmetry and is
hypersensitive to the local electric field. On the other hand, the MD transition (5D0-
7F1) with
emission at 591 nm is a magnetic dipole allowed transition, which is insensitive to the crystal
environment. Ceria has fluorite structure with every Ce-ion surrounded by eight equatorial
oxygen-ions in Oh symmetry. The emission intensity of Eu3+
is very critical to its location in the
lattice i.e. type of environment around Eu3+
ions.75
Upon replacing Ce4+
with Eu3+
, the symmetry
can be either lowered or improved depending on the site occupied by the dopant. In the present
case, ED transition intensity was found to be higher, indicating that Eu3+
mainly occupies the
lattice sites which reduce the Oh inversion symmetry. The intensity ratio of ED allowed
transition (5D0-
7F2) and the MD allowed transition (
5D0-
7F1) known as intensity or asymmetry
ratio is a measure of the degree of distortion from the inversion symmetry of the local
environment of the Eu3+
ion in the lattice. The influence of doping concentration on the
asymmetry ratio was also observed for Eu doped SnO2.76
Figure 16 shows that the intensity ratio
42
increases for as prepared samples with respect to dopant concentration and samples annealed at
900°C.
Figure 16: Intensity ratio with dopant concentration for as prepared and 900 °C annealed samples shows
the asymmetric position of Eu in the ceria lattice (Ce4+
-O-Eu3+
) is favored on increasing dopant
concentration in both RT and annealed samples
This suggests that an asymmetric position of Eu in the CeO2 lattice is favored with increasing
dopant concentration; not only at annealed temperature but also at RT. On excitation at 466 nm,
the intensity ratio of the as prepared samples increased from 0.5 to 1.35 indicating more disorder
in the crystal lattice with dopant concentration. Since Ce can exist in 3+ and 4+ oxidation states,
coexistence of (Ce3+
-O-Eu3+
) and (Ce4+
-O-Eu3+
) arrangements in the crystalline environment is
possible. As mentioned in section 2.2.1 the size of Eu3+
ion is close to Ce3+
than to the size of
43
Ce4+
which implies (Ce3+
-O-Eu3+
) configuration has more symmetry as compared to (Ce4+
-O-
Eu3+
) configuration. Higher mol% of doping generates lattice distortion of local environment
around Eu3+
ions as supported by the XRD results and thus favors the electric dipole induced
emission (5D0-
7F2). On annealing, due to diffusion kinetics Ce
4+ concentration increases as also
supported from the XPS results and thus the asymmetric (Ce4+
-O-Eu3+
) configuration in the
lattice increases which suggests that ED transition will be more favorable than MD transition.
In order to understand the host lattice influence on emission, ceria matrix was excited at 370 nm
and has been shown in Figure 17. It is seen that luminescence properties of the doped sample
strongly depend on the composition of samples. A blue shift is observed in the luminescent
spectra of Eu substituted ceria, which is probably due to the influence of 5d electron states of
Eu3+
in the crystal field because of ionic size variation causing crystal defects. Figure 18 shows
how the variation in the peak intensity at 610 nm for the doped samples excited at 466 nm
changes as a function of dopant concentration and annealing temperature. It is observed from the
above two plots that the PL is highest for 15 mol% doping of Eu in ceria nanoparticles. On
annealing the samples the luminescence intensity first decreases and then becomes very efficient
with increasing temperature. The increase in PL intensity upon annealing the samples can be
understood by comparing the concentration of defects and trivalent cerium in the system.
With annealing the defect concentration in the lattice, calculated using the spatial correlation
model on Raman data, is suppressed from 2.969x1020
cm-3
for CE15 to 1.16x1020
cm-3
for CE15-
900. The XPS results indicate that annealing the CE15 decreases the Ce3+
/Ce4+
ratio from 0.21 at
room temperature to 0.14 at 900°C. On heat treatment the decrease in concentration of Ce3+,
decreases the luminescence intensity of the 15 mol% Eu doped ceria.
44
Figure 17: Luminescence emission spectra (excitation at 370nm) of (a) as prepared and (b) annealed
samples
45
Figure 18: PL Peak intensity variation with Eu dopant concentration and annealing temperature upon (a)
emission spectra (excitation at 466nm) and excitation spectra (emission at 612nm)
46
Also with increasing annealing temperature the defect concentration which act as the undesired
sites offering non-radiative transfer route to electrons in Eu doped ceria decreases and thus
favors PL. This increase in PL of Eu doped ceria due to decrease in defect concentration is unlike
that observed in Zinc oxide (ZnO) or Titania(TiO2).77
In case of ZnO or TiO2 nanoparticles the
observed emissions are not due to band edge emission but attributed to the band edge free
excitons and bound excitons. On nanoscale regime and at low temperature annealing, these
exciton formation leads to more enhanced luminescence of ZnO or TiO2. The observed emission
in the Eu doped ceria is due to the defect concentration and changes with annealing temperature.
The present data from PL suggests that at low temperature annealing the effect of decreasing
Ce3+
concentration on lowering the luminescence intensity transcends the increase in
luminescence by decreasing the defect concentration. With further heat treatment the increase in
luminescence intensity due to the reduction in the oxygen-ion vacancy concentration is much
more prominent than the effect of decreasing Ce3+
/Ce4+
ratio. The explanation is based on data
from 15 mol% Eu doped ceria however, the same effect must hold true for all other doped
samples as depicted in Figure 18. Therefore due to the cumulative effect of decreasing defect
concentration and decreasing Ce3+
/Ce4+
ratio with increasing annealing temperature reveals a
minimum in emission intensity when plotted with respect to temperature and then increases
sharply with temperature.
2.3 Conclusion
Nanocrystalline ceria with various dopant concentrations were prepared by a simple room
temperature chemical precipitation technique. The size, structure, surface, defect and optical
properties were characterized. The PL intensity of nanoceria increases with dopant concentration
47
and saturates at 15 mol% dopant. On further increase in the dopant amount, quenching of PL was
observed due to concentration effect. The defect concentration and Ce3+
/Ce4+
ratio primarily
determines the Eu3+
emission. Contrary roles of trivalent cerium and oxygen ion vacancies in
affecting the PL response of Eu doped ceria were identified. Reducing the concentration of
oxygen ion vacancies by annealing at suitable temperature resulted in an increase in the PL
intensity. The work presented shows quantitatively the effect of concentration of defects and
trivalent cerium on the emission properties of Eu doped ceria.
Thus it is observed that the properties of nanoceria which are affected by defect state can
be modified through doping. The defect state can be quenched by annealing and as shown in this
work, it modifies the property further.
48
The work presented in this chapter has been published before
Reprinted with permission from J. Phys. Chem. C 2012, 116 (1), 361-366 Copyright
2012 American Chemical Society.
CHAPTER THREE: RADIATION EFECT IN NANOCERIA
3.1 Introduction
In the earlier chapter we observed that defects in nanoceria lattice can be controlled and
associated properties. The redox chemistry of nanoceria is associated with defects within the
lattice and can be modified to suit a relevant application. As mentioned in section 1.3.1, CeO2 is
considered to be surrogate of nuclear fuel, UO2.78
Thus, knowledge gained about the redox
chemistry of ceria will be beneficial to transformative research on ceramic materials for fission
reactor applications and matrices for immobilizing toxic radionuclides. The objective of this
work is to investigate the influence of ion irradiation on the cerium (Ce) charge state in cerium
oxide.
There exist several methods to synthesize nanoceria in different shape and morphology.
In order to examine fundamental behavior under radiation, a well-defined ceria lattice has to be
synthesized. Experiments with ceria samples without specific shape or morphology will benefit
the learning from ceria irradiation experiments to other similar oxide materials with minimum
reservation. It would be ideal to study irradiation in thin films of ceria to avoid the effect from
shape and morphology of studied ceria system. Thus, in the present study, the influence of
radiation dose on the Ce oxidation state changes in high quality single and poly crystalline ceria
thin films were examined using in situ XPS analysis.
49
3.2. Experimental details
Highly oriented single and poly crystalline ceria thin films (~100 nm thick) were grown
by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The optimum set of parameters
used to grow single79
and poly80
crystalline ceria thin films were investigated earlier and used in
the current work. The single crystal films were grown with (111) orientation on (111) plane of 10
mole % yttria stabilized zirconia (YSZ) and the polycrystalline films were grown on the (0001)
plane of sapphire (Al2O3) substrate. High purity Ce rods were used as the source material in an e-
beam evaporator under 2 × 10-5
Torr oxygen plasma environment. After deposition, an area of 2
mm × 5 mm of both ceria thin films were irradiated with 2 MeV He+ ions using the National
Electrostatics Corporation (NEC) model 9SDH-2, 3.0-MV tandem electrostatic accelerator in the
EMSL, Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory.
The fluence extends over several orders of magnitude from 1013
to 1017
ions/cm2. The change in
the oxidation state of Ce after irradiation was measured in-situ by x-ray photoelectron
spectroscopy (XPS) attached to an ultrahigh vacuum (UHV) end station maintaining a pressure
of 8×10-10
Torr. The ceria samples were plasma cleaned before each dose of radiation and
subsequent XPS measurements.
3.3 Results and discussion
Irradiation of a material can result in electron cascades, heat, point defects, and defect
clusters. Subsequent damage evolution can give rise to dislocation loops, gas bubbles, voids, ion
tracks or even amorphization. In the case of ceria, formation of such defects can influence the Ce
oxidation state. Previous studies based on scanning tunnelling microscopy and density functional
theory have shown that oxygen vacancies in ceria are associated with two Ce3+
ions in the next
50
nearest neighbour position.81,82
Thus, it was important to follow the dynamics of Ce4+
to Ce3+
on
the sample irradiated using He+ ions. In-situ XPS analysis was carried out to understand the
changes in the valence chemistry of the cerium. The recorded XPS spectra were charge corrected
with respect to the C(1s) peak at 284.6 eV.83
Figure 19 (a, b) shows the XPS spectra of single
and polycrystalline ceria thin films before radiation exposure. The Ce(3d) spectrum has
contributions from Ce in 4+ and 3+ state each with multiple d-splitting and hence was
deconvoluted using the PeakFit (4.0) software.
Figure 19: XPS spectrum of Ce (3d) deconvoluted into Ce 3d3/2 and Ce3d5/2 for single-crystalline ceria
thin film (a) and polycrystalline ceria thin film (b). The peaks with hatched shaded area are characteristic
of Ce3+
ions, while those shown by solid continuous lines represent contribution from Ce4+
ions.
The integrated area under the curve of each deconvoluted peak was used to calculate the
concentration of Ce3+
ions as explained elsewhere. The Ce3+
concentration was found to be 1.2 %
and 4.4 % for unirradiated single and poly crystalline ceria thin films, respectively. The low
concentration of Ce3+
appearing in the single crystalline ceria thin film is due to surface defects,
while the higher concentration in the polycrystalline sample might be due to an additional grain
boundary contribution.
51
The two ceria thin film samples were bombarded with 2 MeV He+ ions for fluence range
extending from 1013
to 1017
ions/cm2 at room temperature and the XPS spectra were recorded
after each exposure. Figure 20 (a, b) shows the variation in XPS spectrum (after background
correction) with cumulative doses of radiation for the single and poly crystalline samples. From
the relative change in shape of the Ce 3d spectra it is apparent that with increasing radiation dose
the concentration of Ce3+
increases for both single and poly-crystalline ceria thin film.
Figure 20: The change in XPS spectrum of Ce (3d) for single crystalline ceria (a) and poly crystalline
ceria (b) with cumulative dose of radiation (the spectra are plotted after background subtraction). The
position of upward arrows indicate the increase in contribution of Ce3+
signals in the spectra and the
position of downward arrows indicate the decrease in contribution of Ce4+
signals.
A few studies have reported reduction of ceria under prolonged exposure to the X-ray
source that is used for recording the spectrum.84
The X-ray beam damage on Ce 3d core level has
been studied in detail with respect to X-ray power density, exposure time to the beam, and
sample charge effect.85
The results show that with no charge effect, Ce3d spectra do not change
noticeably up to 500 min. In view of the possibility of beam damage on ceria samples, we
grounded the sample and kept the acquisition time short enough to prevent any changes in the
spectra. For ceria samples reported in the literature with significant amount of carbon, e.g.
52
carbon from the organic precursor solvent in spray pyrolysis of doped ceria,86
carbon can aid the
reduction of ceria. The sample surface was cleaned by oxygen plasma to avoid such reduction
from the presence of carbon. However, it is not feasible to completely avoid Ce reduction in the
unirradiated sample while recording the spectrum. One can take the Ce3+
concentration in
unirradiated CeO2 as a baseline from which the relative changes in Ce3+
concentration can be
determined as a function of radiation exposure.
The change in concentration of Ce3+
with 2 MeV He+ irradiation fluence for single and
poly crystalline ceria films is shown in Figure 21.
Figure 21: Variation in Ce3+
concentration with cumulative dose of radiation for single and poly
crystalline ceria thin film.
53
It appears that the Ce3+
concentration for the single crystalline ceria approaches that of
the polycrystalline sample with increasing radiation fluence. There is no distinct difference
between the change in proportion of Ce3+
in the single crystal and polycrystal given that the
polycrystalline ceria had a higher Ce3+
proportion in the unirradiated state.
3.4 Conclusion
Irradiation with 2 MeV He+ leads to an increase in the proportion of Ce
3+ by 10 to 15% in
both single and polycrystalline ceria. These results suggest that radiation can change the valence
state of Ce on the surface of its nanoparticles which indicates a possible mechanism for radiation
protection imparted to healthy cells by ceria nanoparticles documented in our previous studies.
54
The work presented in this chapter has been published before
Reprinted with permission from J. Phys. Chem. C 2012, 116 (1), 361-366 Copyright
2012 American Chemical Society.
CHAPTER FOUR: CERIA IRRADIATION STUDY-SIMULATION
4.1 Introduction
Radiation effects in materials have been investigated experimentally and theoretically by
a number of researchers in recent years. Interaction of energetic particles and beams with
materials results in energy transfer by electronic and nuclear stopping processes. A shower of
energetic electrons, local heating, and displacement of atoms from their lattice sites are produced
in crystals. The evolution of these excited electrons, energy and charge localisation, formation
of atomic-level defects, and the interaction of radiation-induced defects with impurities, pre-
existing defects and interfaces will determine the radiation response of the material. Thus in
order to understand the performance of a material under radiation it is imperative to know the
atomic-level processes that take place during irradiation.
In the present study, atomistic simulation of ceria irradiation is carried out. The
simulation will be done for radiation doses similar to that used in experiments carried out on
ceria irradiation in chapter three. The influence of radiation dose on the Ce oxidation state
observed, in high quality single and poly crystalline ceria thin films, using in situ XPS analysis
are interpreted using the Molecular dynamics simulation results.
55
6
expij
ij
ij
ij
ijijr
CrArV
4.2 Simulation details
Classical molecular dynamics (MD) was used to separately model the electronic stopping
and nuclear stopping at the end of range of the ion in a model ceria lattice to identify the
individual effects. To carry out the simulation of electronic and nuclear stopping corresponding
to the radiation damage, the molecular dynamics code DL_POLY_4.01 was used.87
The
simulation cell for ceria in the cubic fluorite structure consisted of 32000 Ce4+
ions and 64000
O2-
ions with periodic boundaries. All production runs were performed in the microcanonical
ensemble (NVE) with dynamically varying time steps at a temperature of 300 K following
equilibration in the isothermal, isobaric ensemble at 300 K and zero external pressure for 20 ps.
The interactions between the ions were described by the potential model and parameters
from the work of Sayle et al.88
and modified at close separations by the Ziegler-Biersack-
Littmark potential89
to avoid unphysical attraction between the ions. Briefly, the potential is
based on Born model of the ionic solid where ions interact via attractive long-range Coulombic
terms balanced by short-range repulsive interactions that have the form as in equation ( 7 ).
( 7 )
Here rij refers to the distance between the interacting ions i and j. The values for the potential
parameters Aij, ρij, Cij can be found in the work of Sayle et al.88
The Ewald sum was used to
calculate the electrostatic part of the potential.
Given the energy of the process studied and the large system size needed, the use of
density functional theory to model these radiation damage events is not feasible. At the same
time, MD simulations cannot model electronic stopping effects directly. The effects of electronic
56
stopping were modelled indirectly in the form of a thermal spike resulting from the energy
transfer from the electron cascades to the lattice through electron-phonon coupling.90
The
thermal spike ran along the z-axis ([001] direction) of the simulation box with energy deposition
of 0.44 keV/nm corresponding to the peak electronic stopping of 2 MeV He+ ions. A thermal
spike with a higher energy of 1.0 keV/nm was also simulated. A Gaussian profile in the X-Y
plan with a standard deviation of 1 nm was chosen for the spikes as shown in Figure 22.
Figure 22: The profile of thermal spike created along z direction within the model ceria lattice for energy
loss of 1 keV/nm.
57
The X and Y boundary layers with a thickness of 0.5 nm were coupled to a heat bath at
300 K. The simulation was run for 40 ps in the microcanonical ensemble (NVE). Nuclear
stopping at the end of range of the He ion was simulated by creating primary knock-on atoms
(PKAs) at random locations within the ceria lattice. PKAs along five high-index
crystallographic directions were simulated to obtain good defect production statistics. The five
different PKAs were initiated in the following five high angle directions namely, (871) for
PKA1, (345) for PKA2, (932) for PKA3, (552) for PKA4 and (356) for PKA5. Direct simulation
of this irradiation is not computationally feasible due to the large mean free path of 2 MeV He+
ions in the ceria lattice. Instead a Ce4+
PKA was chosen at random within the lattice model (away
from the boundary) and given a kinetic energy of 5 keV typical to the energy of a recoil atom
resulting from energy transfer from the He+ ion to the ceria lattice near the end of range.
Finally, to model sputtering processes at the surface, a surface model of ceria was created
as follows. The top half of the simulation cell did not contain atoms, while the bottom half
contained 8640 Ce4+
and 17280 O2-
ions in the fluorite structure with the (111) surface exposed.
A Ce4+
PKA was initiated from the vacuum region on top towards the surface at an angle of 30º
to the surface. The sputtering was accessed for PKA atom with energy of 1, 2, or 4 keV chosen
in the range of ion energies for sputtering. The simulation time for observing the sputtering
process in the constant NVE ensemble at 300 K was 30 ps. The number of Ce4+
and O2-
removed
from the surface was counted at the end of the simulation.
4.3 Results and discussion
One of the main effects of radiation in materials is the displacement of atoms from lattice
sites. The extent of atomic displacement in thermal spikes and displacement cascades can be
58
quantified in terms of the mixing parameter given by <R2>/6E, where <R
2> is the mean square
displacement of atoms and E is the energy deposited per unit volume.91
Figure 23: Mixing occurring in case of cascade simulation and thermal spike. The results of thermal spike
were plotted on the bottom y axis and that from five cascades, created at random locations within the
lattice and high index directions, on the top y-axis.
The mixing parameter is presented in Figure 23 for five PKAs and the thermal spike
corresponding to 0.44 keV/nm energy deposition. The mixing is much less in the case of the
thermal spike which suggests that the 5 keV PKAs is more effective in producing displacements.
However, displaced atoms do not always create defects because of the possibility of dynamic
59
defect recombination or replacement collision events. In the case of thermal spikes
corresponding to energy loss of 0.44 keV/nm, no stable defects were created. The perfect crystal
was restored within a period of 1 ps. This result was verified by performing two thermal spikes
with a heat bath at the boundary and two thermal spikes with no heat bath (constant NVE
simulation). The thermal spikes arising from the passage of 2 MeV He+ are too weak to create
defects in the ceria lattice. Even though atoms are displaced and atomic mixing occurs, the
displaced ions create a chain of replacements on their sublattice, which restores crystalline order.
When the thermal spike energy was increased to 1 keV/nm, two oxygen Frenkel pairs were
produced. The energy loss of 1 keV/nm is more than twice the energy used in the experiments.
These simulations reveal that 1 keV/nm is close to the energy deposition threshold for defect
creation in ceria and that the thermal spike mechanism cannot explain the observed reduction of
Ce4+
.
Having ruled out the influence of thermal spikes, we studied defect creation by 5 keV
Ce4+
PKAs. Figure 24a shows the number of cation vacancies and Figure 24b shows the number
of anion vacancies as a function of time. For PKAs along all five crystallographic directions, the
peak number of vacancies occurs around 0.2 ps and is three to four times more on the anion
sublattice than on the cation sublattice. The Ce and O vacancies recombine with the
corresponding interstitials leaving behind a total of about 17 Frenkel pairs, of which most of
them were found on the anion sublattice.
60
Figure 24: The number of vacancies of Ce (a) and O (b) for different 5 keV Ce cascades.
The calculated number of Frenkel pairs, averaged over the five cascades, at cation (Ce4+
)
sublattice was 4.6 and that at anion (O) sublattice was 12.2. Relative to the stoichiometry, a
disproportionately higher number of O Frenkel pairs are produced in CeO2. However, the
material was not amorphized. Such radiation tolerance of the fluorite structure is well
documented in the literature.92
Such ability to resist defect creation is also exhibited by the
related pyrochlore structures as indicated in section 1.2.3. Figure 25 shows the final defect state
of the PKA1 cascade after 30 ps of simulation. It is evident from the defect configuration that,
vacancies are well separated from interstitials. The final number of O vacancies created is almost
three times as many as the number of Ce vacancies. This again highlights the preferential defect
production on the anion sublattice. The preferential production of anion vacancies that are
separated from interstitials can lead to the reduction of Ce4+
. In our simulation, the ion charges
were fixed, while in the experiment Ce4+
ions may switch to the 3+ charge state to ensure local
charge neutrality. We know from earlier simulation studies that Ce3+
does not disrupt the fluorite
lattice of ceria.93
61
Figure 25: Defects (O vacancy: yellow, Ce vacancy: green, Ce interstitial: blue, O interstitial: red)
towards the end of the cascade simulation of a 5 keV Ce recoil. The area shown is about 3.4 nm x 3.4
nm.
This mechanism is likely to have a similar effect in single and polycrystalline ceria thin
films. While cascade simulations point to oxygen depletion as a mechanism governing the
valence state change of Ce from 4+ to 3+, it is also possible to bring about this change locally by
the radiation induced creation of oxygen Frenkel pairs as indicated by some studies.94
62
Table 3: The number of Ce4+
(Ces) and O2-
(Os) removed and the ratio of O2-
removed to Ce4+
removed
(Os/Ces) by 1, 2 and 4 keV Ce4+
ions at an angle of 30° to the CeO2 (111) surface.
PKA energy 1 keV Ce 2 keV Ce 4 keV Ce
Run Ces Os Os/Ces Ces Os Os/Ces Ces Os Os/Ces
1 2 7 3.50 3 9 3.00 7 23 3.29
2 2 6 3.00 4 14 3.50 7 74 10.57
3 5 17 3.40 4 16 4.00 7 99 14.14
4 1 3 3.00 0 3 NA 9 104 11.56
5 2 6 3.00 1 5 5.00 1 3 3.00
Average
3.18
3.88
8.51
Standard
deviation
0.25
0.85
5.07
Irradiation with 2 MeV He+ is likely to create defects in the bulk of the sample and
sputtering effects are not especially relevant to this irradiation condition. However, for the sake
of completeness and to shed light on previous experimental studies of sputtering effects in
ceria,95
the number of O2-
and Ce4+
ions removed by Ce4+
ion sputtering is shown in Table 3 for
five simulation runs at three different ion energies. Figure 26 shows the sputtering geometry
visualized using the VMD package.96
The ratio of the number of O2-
removed to Ce4+
removed is
greater than 2.0 for all cases. This average increases from about 3.2 to about 8.5 as the incident
Ce4+
ion energy increases from 1 keV to 4 keV, and the scatter in the data also increases.
63
Figure 26: Visualization of the sputtering geometry with an energetic Ce4+
ion incident on CeO2 (111)
surface at an angle of 30° to the surface. Ce4+
is shown in cyan and O2-
in red.
The increase in the standard deviation may be an indication that we need to go to a larger
simulation cell at higher energies. Even though a disproportionately higher number of O2-
was
removed, charge neutrality is maintained in the simulation, because the removed ions move into
the ‘vacuum region’ at the top and are still a part of the simulation cell. In experiments, some of
the Ce4+
at the surface may switch to Ce3+
to compensate for the oxygen loss from preferential
sputtering. Such effects can be significant in irradiated ceria nanoparticles and nanograins.
64
Finally, it is pertinent to point out that ionizing radiation can cause reduction of Ce4+
by
charge localization processes at the end of the electron cascade as outlined by Itoh and
Stoneham.97
Ab initio simulation of these highly energetic processes is beyond current
computational capabilities.
4.4 Conclusion
The present study has integrated simulation and experiment to shed light on the reduction
of Ce4+
in irradiated ceria. Irradiation with 2 MeV He+ leads to an increase in the proportion of
Ce3+
by 10 to 15% in both single and polycrystalline ceria. The simulation study suggests that
thermal spikes arising from electronic stopping due to 2 MeV He+ are not responsible for the
observed reduction. Nuclear stopping appears to be the governing process for the change in
valence state upon irradiation for both ceria thin films. Direct reduction as a result of charge
localization at the end of electron cascades and reduction due to preferential production of
oxygen Frenkel pairs by nuclear stopping are possible causes of the reduction of Ce4+
. The
present study also shows that in experiments where sputtering is dominant, oxygen will be
sputtered preferentially leading to Ce4+
reduction at the surface. These results suggest that
radiation can change the valence state of Ce on the surface of nanoparticles of ceria and indicate
a possible mechanism for radiation protection imparted to healthy cells by ceria nanoparticles
documented in our previous studies. Hence, future efforts will be directed towards in situ studies
of radiation damage in nanoceria.
65
CHAPTER FIVE: ROLE OF REDOX STATE IN CERIA IRRADIATION
In the earlier chapters we have studied the effect of radiation on ceria lattices using
experiment and simulation. We have demonstrated that atomistic simulation provides great detail
on defect chemistry in the ceria lattice with radiation and supported the experimental
observations. The experimental analysis of material irradiation on the events occurring during
the radiation is based on characterization after the radiation event. The detailed characterization
of irradiated material with established techniques such as TEM, XPS and Rutherford Back
Scattering (RBS) can provide great detail on the overall performance of material. In the case of
nanosize ceria lattice, the defect states of the lattice are of prime interest influencing irradiation
behavior. The objective of the work presented in this section is to simulate realistic nanoceria
lattice models with different redox states and carry out irradiation on the model lattices.
5.1 Methodology adopted
Modeling the irradiation effect within a nanoceria lattice requires an accurate
representation of the lattice with close similarity to the experimental observations. It takes a
considerable effort to create impartial dislocations or grain boundaries within a lattice of
nanometer size and the results from study utilizing such lattice models may be biased due to the
artificial placement of the defects within the lattice.98,99
The recent approach of structural evolution simulates lattice models with features, such
as defects, dislocations, grain-boundaries, etc., without using crystal symmetry
operations.100,88,101
In this approach the experimentally observed complex structures evolve over
time, and it is based on the accurate description of the interatomic potentials of the component
ions. The strategy suggested in Sayle’s work88,102,103
is to simulate the crystallization process by
66
implementing amorphisation and re-crsytallisation of the lattice model. A perfect lattice of the
desired nanocrystal is chosen and strained. Then the lattice is raised to very high temperatures
close to the melting point of the material causing the structure to amorphise. Following the
amorphisation step, the nanocrystal is cooled down slowly to lower temperatures in steps. In the
work presented here, a similar procedure was adopted with appropriate modifications (explained
later) in the method of choosing the starting lattice prior to amorphisation.
5.2 Simulating nanoceria lattice
Four different nanoceria crystals were to be generated with 5, 10, 15 and 20 % Ce3+
for
the radiation study. The molecular dynamics software DL_POLY 4.03 was used for carrying out
the structural evolution of the lattice structure. The potential model used is the same as in chapter
four (section 4.2). Four perfect ceria lattices were generated with 11 unit cells in each x, y and z
direction comprising a total of 5324 Ce4+
and 10648 O2-
ions. Depending on the desired Ce3+
percentage (5,10,15, and 20) in the nanoceria model; (266, 532, 798 and 1064) Ce4+
in the lattice
were substituted randomly by Ce3+
, and accordingly, oxygen vacancies were created to maintain
charge balance. Learning from the Sayle’s work, considerable tension (38%) was introduced into
the nanoceria by scaling the lattice positions of the ions. The strained lattices will try to restore
its natural lattice parameter and at higher temperature, the collision between ions leads to
amorphisation. However, following the strain created in the lattice and prior to amorphisation,
the Ce3+
substituted lattices were modified using a method implementing Monte Carlo coupled
with Molecular Dynamics. This step is critical in determining the final lattice structure. The
flowchart of Monte-Carlo procedure on lattice is shown in Appendix A. Creating a lattice with
experimentally resolvable features involves diffusion of atoms over large atomic distances and
67
MD cannot lead to diffusion of ions within lattice over extended distance. The use of Monte
Carlo (MC) simulation to obtain a low-energy configuration lattice followed by MD relaxation at
very low temperature (10 K) is an important step to model the real lattice. This process of MC
with MD was repeated in the subsequent steps of strained lattices (5, 10, 15 and 20 % Ce3+
).
Figure 27 shows the configurational energy change while carrying out MC-coupled MD steps in a
representative strained ceria lattice with 10% Ce3+
. The MD relaxation following several MC
steps was done at 10 K only for one ps. Relaxation for longer times using MD may hinder
amorphisation in the later steps and make the lattice stable at a local minimum energy
configuration instead of producing the desired amorphisation.
Figure 27: The configurational energy change in the lattice for 10% Ce3+
prior to carrying out
amorphization and recrystallization. Introduction of Ce3+
in the lattice increases the configurational
energy of lattice and using Monte Carlo with Molecular Dynamics simulation minimizes the lattice
configurational energy.
68
From Figure 27, it is observed that carrying MC steps coupled with MD results in a lattice with a
lower energy configuration than carrying out MC in isolation. When no favorable change in the
lattice was observed during MC steps, the lattices (5, 10, 15 and 25 % Ce3+
) were allowed to
undergo amorphisation at elevated temperatures (~3750 K) for 250 picoseconds. Figure 28 shows
the ceria lattice at different times of amorphisation.
Figure 28: The evolution of nanoceria lattice with 10% Ce3+
observed during amorphisation step at
T=3750 K at (a) 0.1 ps (b) 10 ps (c) 100 ps and (d) 200 ps. In the starting (t= 0.1 ps) the lattice is strained
and, at T= 3750 K, with time (t=10 ps) the atoms come close to restore the lattice parameter. Due to the
velocity of atoms, the lattice amorphises as seen at t=100 ps. (Ce4+
: red, Ce3+
: pink, O2-
: cyan)
Having obtained an amorphous system with stable energy, the lattices were cooled down
in steps. As the temperature of the system decreases, the ions nucleate a fluorite structured
crystal, and with decreasing temperature, Ce and O ions keep condensing onto such nuclei. For
each drop in the temperature, the simulation is run for sufficient time so that the energy of the
69
configuration becomes stable. The cooling procedures were adopted for lattices to stabilize at
3250 K (for 250 ps), 2750 K (for 200 ps), 2000 K (for 200 ps), 1000 K (for 50 ps) and at 10 K
(for 20 ps). Figure 29 shows the change in energy during cooling at 3250K for 10% Ce3+
lattice.
The decrease in energy of the lattice suggests the formation of stable features within the lattice.
Figure 29: The total energy of nanoceria lattice with 10% Ce3+
on stabilizing at T=3250 K. The initial
decrease in energy of the lattice corresponds to the temperature decrease by 500 K (from 3750K to 3250
K). After ~100 ps, again a drop in energy is observed which can be attributed to formation of stable lattice
nuclei in the system.and this suggests evolution of stable features within the lattice.
5.3 Radiation in simulated lattices
The nanoceria lattices obtained at the end of the structural evolution technique were used
for irradiation study. In chapter four we studied radiation in ceria lattices by simulating the
nuclear stopping by displacement cascade and electronic stopping by thermal cascade. In section
70
4.3 we compared the effect of both types of radiation events corresponding to the irradiation
energy (2 MeV) of He+ particles used in experiments carried out in chapter 3. From the results it
was observed that thermal cascades were not able to produce any stable defects and only
displacement cascades were responsible for the stable defects observed in the ceria lattice. Thus
for the current study only displacement cascades were performed on the simulated nanoceria
lattices. The displacement cascades in the ceria lattice were performed by generating PKA in
(111), (321) and (345) directions with respect to the lattice frame. The PKA atoms were created
by providing 0.5 keV of kinetic energy to a randomly chosen atom in the specified direction.
Three different PKAs were studied to have a good statistical representation of the irradiation on
the nanoceria lattices containing different valence states. The defects created in the nanoceria
lattices due to radiation were analyzed over time. Figure 30 shows the number of defects in the
nanoceria lattices for the three different PKAs. The number of defects shown in Figure 30 is the
sum of interstitial and vacancy type defects for both cations and anions. For all the three PKA’s,
we can see that with radiation of the nanoceria lattice, many defects are generated, and with time
most of the defects are quenched in the lattice with only few stable defects remaining. In the
beginning of irradiation Ce and O ions get displaced from their lattice positions following the
corresponding interstitial leaving behind the corresponding vacancy. From the earlier defect
study during ceria irradiation reported in chapter four, we observed that Ce and O interstitials
recombine with near-by corresponding vacancies. Only the well-separated cationic and anionic
interstitials and their corresponding vacancies were stable. The similarity between the profiles
for the number of defects in nanoceria lattices in the current study and thoseobserved for cationic
71
and anionic defects in the chapter 4, suggests that defect recombination leads to quenching of the
defects with time.
Figure 30: The number of defects in the nanoceria lattices for (a) PKA1, (b) PKA2 and (c) PKA3. The
PKA were initiated at simulation step of 1950. For the three PKAs in the nanoceria lattices, radiation
induces several types of defects (vacancy, interstitials for anions and cations) and with time most of the
defects get annihilated within the lattice leaving only few stable defects.
As seen in Figure 30, the average # of defects from the three PKAs after a few ps of
radiation is same for nanoceria lattices with 5, 10 and 15% Ce3+
. The nanoceria lattice with 20 %
Ce3+
shows more stable defects than the lower percentage of Ce3+
. To understand this, the lattice
72
energy of the different nanoceria lattices was evaluated before radiation and is shown in Figure
31. It is noticed that the energy of simulated nanoceria lattices using the crystallization of
amorphous lattice increases with increase in concentration of Ce3+
. The size of the nanoceria
lattice simulated in the current work is ~10 nm. In this size of nanoceria lattice, 5% Ce3+
is found
most stable. We have not studied concentration of Ce3+
lower than 5% in the current study.
Figure 31: Energy (in eV) of nanoceria lattices with 5, 10, 15 and 20 % Ce3+
before radiation at T=10K
for 10 ps. The energy profiles for each lattice is stable, however, with increasing Ce3+
the energy of
nanoceria lattices increases.
Using the Kroger-Vink notation104
it is represented by the chemical equation (8), that
nanoceria lattices with increasing Ce3+
concentration will lead to anion vacancy in the lattice.
( )
( ) ( ) (8)
This increasing vacancy in the lattice or the strain from Ce3+
at Ce4+
lattice sites may be the
reason for the increasing lattice energy with increasing Ce3+
.
73
We have observed in chapter four that radiation in the ceria lattice results in preferentially
anion vacancies rather than cation vacancies. In our current simulation of irradiation in a
nanoceria lattice we cannot dynamically switch the charge states in cerium. However, we expect
similar phenomena to occur in the case of the nanoceria lattice as seen in the ceria lattice, that
anion vacancies are more preferentially formed than cation vacancies. And this may result in
charge localization on Ce sites, leading to a reduction in Ce4+
at random sites in the lattice, as
represented in equation (9).
→
(9)
Thus with increasing Ce3+
concentration in the nanoceria lattice, the ability to accommodate
defects by the above equation decreases due to the presence of anion vacancies that are already
in the lattice. Thus nanoceria with 20% Ce3+
shows higher number of defects for all PKAs upon
irradiation.
5.4 Conclusion
A modified approach utilizing Monte-Carlo coupled to molecular dynamics method has
been utilized to model nanoceria lattices of different stoichiometries. Four different nanoceria
lattices with 5, 10, 15 and 20% Ce3+
were created and used for radiation study.
It was observed that nanoceria lattices with 5 and 10% Ce3+
showed similar responses to
irradiation in terms of the number of stable defects generated. The lattice with 20% Ce3+
had a
higher number of stable defects. The observed difference in radiation behavior with changing
Ce3+
concentration was explained by the stability of Ce3+
in the lattice, which results in anion
vacancies that might lead to less stable defects from radiation.
74
CHAPTER SIX: CONCLUSIONS
The work presented in this dissertation was focused on understanding the role of defects
in the ceria lattice. Nanoceria was doped with another rare earth element, Europium, adopting a
room temperature wet chemical synthesis procedure. The changes in the redox state and defect
concentration were quantified and the effect of defect state of the lattice was studied on a
therapeutically relevant property (developing nontoxic bio-imaging), photoluminescence (PL).
The defect concentration and Ce3+
/Ce4+
ratio primarily determines the Eu3+
emission. It was
observed that the defect concentration and the redox state play contrary role in modifying the PL.
Both the redox state and defect concentration were controlled using dopant concentration and
annealing temperature. An optimum concentration (~15% in our study) of dopant is required to
get maximum PL intensity. Reducing the defect concentration by annealing at suitable
temperature resulted in an increase in the PL intensity. The work shows quantitatively the
emission properties of Eu doped ceria can be tailored by modifying the concentration of defects
and trivalent cerium on. It is evident from this part of dissertation that property of nanoceria is
dependent on the defect state of the lattice.
Experiments were carried to synthesize ceria thin film of different crystallinity and
irradiate them with high-energy ionizing radiation in a wide range of ion fluence using a reactor
located at Pacific Northwest National Lab, WA. The irradiation was carried out in steps, so that
the state of the irradiated material can serve as the reference for further irradiation. An interesting
observation was made, that radiation affects the valence sate of ceria thin film which is related to
the defect state. The results obtained from the experiments on irradiated ceria were studied using
classical molecular dynamics simulation. Cascade and thermal displacement in nanoceria lattice
75
were modeled to simulate the irradiation of nanoceria and the defects in the simulated lattice
were analyzed to bring change in the redox state of cerium. Also, the effect of sputtering as a
possibility during ceria irradiation has been considered and studied using molecular dynamics
simulation. The work deciphers the role of lattice defects, dopants on ceria irradiation response
and provides insights into the microstructural process.
After gaining insights into the radiation and defect chemistry in ceria thin film through
experiment and simulation, ceria nanoparticles with different redox state were studied under
irradiation using atomistic simulation. A modified approach utilizing molecular dynamics
assisted Monte-Carlo method to structural evolution of crystals was used to simulate nanoceria
with variable stoichiometry. Using the modified approach, nanoceria with four different
concentration of Ce3+
were modeled and used for irradiation study. Displacement cascades were
carried out to determine the role of valence states in nanoceria exposed to radiation. It was
observed that for higher concentration of Ce3+
, the number of stable defects were more in the
lattice.
6.1 Intellectual merit
The interdependence of redox state of nanoceria and defect concentration within the
lattice was elucidated quantitatively. It was shown that the defect concentration in nanoceria
lattice can be tuned in a wide range with appropriate selection of dopant, synthesis method and
temperature treatment.
The change in valence state of ceria thin film due to the defects produced upon irradiation
was evidenced using in-situ XPS in a wide range of radiation dose. Using atomistic simulation,
76
the radiation events in ceria lattice were studied and the in-depth analysis of defect states within
model lattice supported the results from ceria irradiation experiments.
A modified approach, utilizing Monte-Carlo assisted Molecular Dynamics, is suggested
to simulate crystallization of nanocrystals using structural evolution strategy. Using the
suggested method, nanoceria lattices with 5, 10, 15 and 20% Ce3+
were simulated. Displacement
cascades study was carried out to understand the role of valence states in nanoceria irradiation.
77
APPENDIX A: FLOWCHART FOR MONTE-CARLO
IMPLEMENTATION
78
79
APPENDIX B: FORTRAN CODE FOR CREATING NANOCERIA
LATTICE WITH GRAIN BOUNDARIES
80
program nclatt
implicit none
!
! Program to generate Ceria simulation cell of desired size
! Written by Ram Devanathan at PNNL, Richland, WA
! Modified by Amit to create nanoceria lattice with grain boundaries of desired no., size and
! orientation while maintaining the stoichiometry
!
real::xfr(24), yfr(24), zfr(24)
real::alatt, clatt, tstep, cell1, cell5, cell9,zero
real::xstart, ystart, zstart, tottime
real::tmpx(2),tmpy(2),tmpz(2)
real::hexlatt,sqrt3 ! Hexlatt-edge length of the hexagon
real:: space1,space2 ! For deciding intergranular spacing
real::pi,ang1,ang2,ang3 ! Lattice orientations
integer::natoms,ncer
integer::id, ncellx, ncelly, ncellz, levcfg, imcon, nstep
integer::iatom(4), indx, ix, iy, iz
character*8::sym(8),nullc
character*72::header
logical::atstat,chk(3) ! Contstraints for hexagonal unit cell
real,allocatable::latt1(:,:),latt2(:,:),latt3(:,:),latt4(:,:)
81
integer::i,j,ce(4),ox(4),diff(4),addce(4),rmoxy(4)
integer,allocatable::label1(:),label2(:),label3(:),label4(:) ! 1:Ce4+; 2:Ce3+; 3:O2-
! Four label created, assuming diff stoic for each grain
real::tmp1,tmp2
integer::ranum,iseed !For random no generation
real*8::ran1 !For random no generation
!---------------------Read the unit cell------------------------
open(1,file='ceria.in',status='old')
read(1,'(a72)') header
read(1,*) natoms, ncer
do indx = 1, natoms
read (1,*) id, xfr(indx), yfr(indx), zfr(indx)
end do
read(1,*) ncellx, ncelly, ncellz, levcfg, imcon, nstep, tottime
read(1,*) alatt, clatt, tstep
close(1)
!---------------------------------------------------------------
cell1 = real(ncellx)*alatt
cell5 = real(ncelly)*alatt
cell9 = real(ncellz)*clatt
zero = 0.0
82
xstart = -0.5*cell1
ystart = -0.5*cell5
zstart = -0.5*cell9
nstep=ncellx*ncelly*ncellz*12
sqrt3=sqrt(3.)
hexlatt=2*alatt ! Decides the size of individual lattice
pi=3.14159265
ang1=0
ang2=0
ang3=0
open(2,file='latt1.xyz',status='unknown')
!write(2,"(i8)") iatom(1)
write(2,'(a72)') header
open(3,file='latt2.xyz',status='unknown')
!write(3,"(i8)") iatom(2)
write(3,'(a72)') header
open(8,file='latt3.xyz',status='unknown')
!write(8,"(i8)") iatom(3)
83
write(8,'(a72)') header
open(9,file='latt4.xyz',status='unknown')
!write(9,"(i8)") iatom(4)
write(9,'(a72)') header
sym(1) = 'Ce4+ '
sym(2) = ' Ce4+ '
sym(3) = ' Ce4+ '
sym(4) = ' Ce4+ '
sym(5) = ' Ce4+ '
sym(6) = ' Ce4+ '
sym(7) = ' Ce4+ '
sym(8) = ' Ce4+ '
iatom = 0
! ----------------------------------------------------------------
! Write all the Ce atoms
do ix = 0, ncellx - 1
do iy = 0, ncelly - 1
do iz = 0, ncellz - 1
do indx = 1, ncer
tmpx(1)=xstart + real(ix)*alatt + xfr(indx)*alatt
84
tmpy(1)=ystart + real(iy)*alatt + yfr(indx)*alatt
tmpz(1)=zstart + real(iz)*clatt + zfr(indx)*clatt
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(1)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(1)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(1)+sqrt3*(tmpy(1)+hexlatt))).AND.(zero.GT.(tmpx(1)+sqrt3*(tmpy(1)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(1)+sqrt3*(-tmpy(1)+hexlatt))).AND.(zero.GT.(tmpx(1)-
sqrt3*(tmpy(1)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3)) ! To check the atom is within
the hexagonal lattice
if(atstat.EQV. .TRUE.)then
iatom(1) = iatom(1) + 1
write(2,"(a8,3(1x,g20.10))") sym(1),tmpx(1),tmpy(1),tmpz(1)
end if
tmpx(2)=tmpx(1)*cos(ang1)-tmpy(1)*sin(ang1)
tmpy(2)=tmpx(1)*sin(ang1)+tmpy(1)*cos(ang1)
85
tmpz(2)=tmpz(1)
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-
sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(2) = iatom(2) + 1
write(3,"(a8,3(1x,g20.10))")sym(2),tmpx(2),tmpy(2),tmpz(2)
end if
tmpx(2)=tmpx(1)*cos(ang2)-tmpy(1)*sin(ang2)
tmpy(2)=tmpx(1)*sin(ang2)+tmpy(1)*cos(ang2)
tmpz(2)=tmpz(1)
86
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-
sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(3) = iatom(3) + 1
write(8,"(a8,3(1x,g20.10))")sym(3),tmpx(2),tmpy(2),tmpz(2)
end if
tmpx(2)=tmpx(1)*cos(ang3)-tmpy(1)*sin(ang3)
tmpy(2)=tmpx(1)*sin(ang3)+tmpy(1)*cos(ang3)
tmpz(2)=tmpz(1)
87
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-
sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(4) = iatom(4) + 1
write(9,"(a8,3(1x,g20.10))")sym(4),tmpx(2),tmpy(2),tmpz(2)
end if
end do
end do
end do
end do
ce=iatom
88
write(*,*)'Ceria Atom in Latt1, 2, 3, 4: ',ce
! Write all the O atoms
do ix = 0, ncellx - 1
do iy = 0, ncelly - 1
do iz = 0, ncellz - 1
do indx = ncer+1, natoms
tmpx(1)=xstart + real(ix)*alatt + xfr(indx)*alatt
tmpy(1)=ystart + real(iy)*alatt + yfr(indx)*alatt
tmpz(1)=zstart + real(iz)*clatt + zfr(indx)*clatt
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(1)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(1)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(1)+sqrt3*(tmpy(1)+hexlatt))).AND.(zero.GT.(tmpx(1)+sqrt3*(tmpy(1)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(1)+sqrt3*(-
tmpy(1)+hexlatt))).AND.(zero.GT.(tmpx(1)-sqrt3*(tmpy(1)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
89
iatom(1) = iatom(1) + 1
write(2,"(a8,3(1x,g20.10))")
sym(5),tmpx(1),tmpy(1),tmpz(1)
end if
tmpx(2)=tmpx(1)*cos(ang1)-tmpy(1)*sin(ang1)
tmpy(2)=tmpx(1)*sin(ang1)+tmpy(1)*cos(ang1)
tmpz(2)=tmpz(1)
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-
tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(2) = iatom(2) + 1
90
write(3,"(a8,3(1x,g20.10))")sym(6),tmpx(2),tmpy(2),tmpz(2)
end if
tmpx(2)=tmpx(1)*cos(ang2)-tmpy(1)*sin(ang2)
tmpy(2)=tmpx(1)*sin(ang2)+tmpy(1)*cos(ang2)
tmpz(2)=tmpz(1)
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-
tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(3) = iatom(3) + 1
91
write(8,"(a8,3(1x,g20.10))")sym(7),tmpx(2),tmpy(2),tmpz(2)
end if
tmpx(2)=tmpx(1)*cos(ang3)-tmpy(1)*sin(ang3)
tmpy(2)=tmpx(1)*sin(ang3)+tmpy(1)*cos(ang3)
tmpz(2)=tmpz(1)
chk(1)=.FALSE.
chk(2)=.FALSE.
chk(3)=.FALSE.
atstat=.FALSE.
if((zero.LT.(tmpx(2)+(sqrt3/2)*hexlatt)).AND.(zero.GT.(tmpx(2)-
(sqrt3/2)*hexlatt)))chk(1)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)+sqrt3*(tmpy(2)
-hexlatt))))chk(2)=.TRUE.
if((zero.LT.(tmpx(2)+sqrt3*(-
tmpy(2)+hexlatt))).AND.(zero.GT.(tmpx(2)-sqrt3*(tmpy(2)+hexlatt))))chk(3)=.TRUE.
atstat=(chk(1).AND.chk(2).AND.chk(3))
if(atstat.EQV. .TRUE.)then
iatom(4) = iatom(4) + 1
92
write(9,"(a8,3(1x,g20.10))")sym(8),tmpx(2),tmpy(2),tmpz(2)
end if
end do
end do
end do
end do
write(2,"(i8)")iatom(1)
write(3,"(i8)")iatom(2)
write(8,"(i8)")iatom(3)
write(9,"(i8)")iatom(4)
close(2)
close(3)
close(8)
close(9)
!----------- Wrote the grains in file -----------
ox=iatom-ce
write(*,*)'Ce Atom in Latt1, 2, 3, 4: ',ce
write(*,*)'Oxy Atom in Latt1, 2, 3, 4: ',ox
93
allocate(label1(iatom(1)),label2(iatom(2)),label3(iatom(3)),label4(iatom(4)))
label1=3 ! 1:Ce4+, 2:Ce3+, 3:O2-, 4:Vacancy
label2=3
label3=3
label4=3
label1(1:ce(1))=1
label2(1:ce(2))=1
label3(1:ce(3))=1
label4(1:ce(4))=1
!----------- Now check for charge balance and -----------
!----------- find whether to substitute Ce4+ -----------
!----------- with Ce3+ or remove O2- ions. -----------
diff=ox-2*ce
do i=1,4
if(diff(i).ge.0)then
rmoxy(i)=ox(i)-2*ce(i)
addce(i)=0
j=0
94
do while (j.lt.rmoxy(i))
ranum=int(ran1(iseed)*ox(i))+ce(i)
SELECT CASE (i)
CASE (1)
if(label1(ranum).eq.3)then
label1(ranum)=4
else
j=j-1
end if
CASE (2)
if(label2(ranum).eq.3)then
label2(ranum)=4
else
j=j-1
end if
CASE (3)
if(label3(ranum).eq.3)then
label3(ranum)=4
else
j=j-1
end if
CASE (4)
95
if(label4(ranum).eq.3)then
label4(ranum)=4
else
j=j-1
end if
CASE DEFAULT
write(*,*)'None of the cases selected'
END SELECT
j=j+1
end do
end if
end do
do i=1,4
if(diff(i).lt.0)then
rmoxy(i)=0
addce(i)=4*ce(i)-2*ox(i)
j=0
do while (j.lt.addce(i))
ranum=int(ran1(iseed)*ce(i))+1
SELECT CASE (i)
CASE (1)
96
if(label1(ranum).eq.1)then
label1(ranum)=2
else
j=j-1
end if
CASE (2)
if(label2(ranum).eq.1)then
label2(ranum)=2
else
j=j-1
end if
CASE (3)
if(label3(ranum).eq.1)then
label3(ranum)=2
else
j=j-1
end if
CASE (4)
if(label4(ranum).eq.1)then
label4(ranum)=2
else
j=j-1
97
end if
CASE DEFAULT
write(*,*)'None of the cases selected'
END SELECT
j=j+1
end do
end if
end do
write(*,*)'# of Ce3+ ion to introduce',addce
write(*,*)'# of O2- vacancy to create',rmoxy
!----------- Cross-check the label for -----------
!----------- stoichiometry and correct -----------
!----------- replacement -----------
ix=0
iy=0
iz=0
indx=0
do i=1,iatom(1)
if(label1(i).eq.1)ix=ix+1
if(label1(i).eq.2)iy=iy+1
98
if(label1(i).eq.3)iz=iz+1
if(label1(i).eq.4)indx=indx+1
end do
write(*,*)"Lattice1: Ce4+:",ix," Ce3+:",iy," O2-:",iz," Vacancy:",indx
ix=0
iy=0
iz=0
indx=0
do i=1,iatom(2)
if(label2(i).eq.1)ix=ix+1
if(label2(i).eq.2)iy=iy+1
if(label2(i).eq.3)iz=iz+1
if(label2(i).eq.4)indx=indx+1
end do
write(*,*)"Lattice2: Ce4+:",ix," Ce3+:",iy," O2-:",iz," Vacancy:",indx
ix=0
iy=0
iz=0
indx=0
do i=1,iatom(3)
99
if(label3(i).eq.1)ix=ix+1
if(label3(i).eq.2)iy=iy+1
if(label3(i).eq.3)iz=iz+1
if(label3(i).eq.4)indx=indx+1
end do
write(*,*)"Lattice3: Ce4+:",ix," Ce3+:",iy," O2-:",iz," Vacancy:",indx
ix=0
iy=0
iz=0
indx=0
do i=1,iatom(4)
if(label4(i).eq.1)ix=ix+1
if(label4(i).eq.2)iy=iy+1
if(label4(i).eq.3)iz=iz+1
if(label4(i).eq.4)indx=indx+1
end do
write(*,*)"Lattice4: Ce4+:",ix," Ce3+:",iy," O2-:",iz," Vacancy:",indx
!----------- Now reading the grains for creating lattice -----------
!-------------------------------------------------------------------
100
iatom=iatom-rmoxy ! To introduce vacancy
allocate(latt1(iatom(1),3),latt2(iatom(2),3),latt3(iatom(3),3),latt4(iatom(4),3))
! Vacancy will be incorporated while reading the files only
! Substitution of Ce3+ will occur while writing the files
open(2,file='latt1.xyz',status='old')
read(2,*)
do ix=1,iatom(1)
if(label1(ix).ne.4)read(2,"(a8,3(1x,g20.10))")nullc,latt1(ix,1),latt1(ix,2),latt1(ix,3)
end do
close(2)
open(3,file='latt2.xyz',status='old')
read(3,*)
do ix=1,iatom(2)
if(label2(ix).ne.4)read(3,"(a8,3(1x,g20.10))")nullc,latt2(ix,1),latt2(ix,2),latt2(ix,3)
end do
close(3)
open(8,file='latt3.xyz',status='old')
read(8,*)
do ix=1,iatom(3)
101
if(label3(ix).ne.4)read(8,"(a8,3(1x,g20.10))")nullc,latt3(ix,1),latt3(ix,2),latt3(ix,3)
end do
close(8)
open(9,file='latt4.xyz',status='old')
read(9,*)
do ix=1,iatom(4)
if(label4(ix).ne.4)read(9,"(a8,3(1x,g20.10))")nullc,latt4(ix,1),latt4(ix,2),latt4(ix,3)
end do
close(9)
! -----------Writing Nanoceria Lattice-------------------------
nstep=3
open(11,file='CONFIG.xyz',status='unknown')
open(12,file='CONFIG',status='unknown')
ncellx=2 !Determines the x size of lattice
ncelly=2 !Determines the y size of lattice
! The z size was determined by z in ceria.in file
space1=alatt/8 ! Spacing between grains in x direction
space2=alatt/8 ! Spacing between grains in y direction
102
cell1 = real(ncellx)*(2*sqrt3*hexlatt+space1+space2)
cell5 = real(ncelly)*(3*hexlatt+space1+space2)
cell9 = real(ncellz)*clatt
nstep=ncellx*ncelly*sum(iatom)
write(12,'(a72)') header
write(12,'(3i10,2g20.10)') levcfg, imcon, nstep, tstep, tottime
if(imcon.gt.0) write(12,'(3f20.10)')cell1,zero,zero,zero,cell5,zero,zero,zero,cell9
write(11,"(i10)")nstep
write(11,'(a72)') header
!-----Write all the cerium4 ions
id=0
do iy=1,ncelly
do ix=1,ncellx
j=1
do indx=1,ce(1)
tmp1=latt1(indx,1)+(ix-1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt1(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
103
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
if(label1(j).eq.1)then
write(11,"(a8,3(1x,g20.10))")sym(1),tmp1-cell1/2,tmp2-
cell5/2,latt1(indx,3)
id=id+1
write(12,'(a8,i10)') sym(1), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt1(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(2)
tmp1=latt2(indx,1)+sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt2(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+sqrt3*hexlatt+space1
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
if(label2(j).eq.1)then
104
write(11,"(a8,3(1x,g20.10))")sym(2),tmp1-cell1/2,tmp2-
cell5/2,latt2(indx,3)
id=id+1
write(12,'(a8,i10)') sym(2), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt2(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(3)
tmp1=latt3(indx,1)+1.5*sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt3(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+1.5*sqrt3*hexlatt+spa
ce1
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)+1.5*hexlatt+space1
if(label3(j).eq.1)then
105
write(11,"(a8,3(1x,g20.10))")sym(3),tmp1-cell1/2,tmp2-
cell5/2,latt3(indx,3)
id=id+1
write(12,'(a8,i10)') sym(3), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt3(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(4)
tmp1=latt4(indx,1)+0.5*sqrt3*hexlatt+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt4(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+0.5*sqrt3*hexlatt
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)+1.5*hexlatt+space1
if(label4(j).eq.1)then
write(11,"(a8,3(1x,g20.10))")sym(4),tmp1-cell1/2,tmp2-
cell5/2,latt4(indx,3)
106
id=id+1
write(12,'(a8,i10)') sym(4), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt4(indx,3)
endif
j=j+1
end do
end do
end do
!-----Write all the cerium3 ions
sym(1) = ' Ce3+ '
sym(2) = ' Ce3+ '
sym(3) = ' Ce3+ '
sym(4) = ' Ce3+ '
do iy=1,ncelly
do ix=1,ncellx
j=1
do indx=1,ce(1)
tmp1=latt1(indx,1)+(ix-1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt1(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
107
if(label1(j).eq.2)then
write(11,"(a8,3(1x,g20.10))")sym(1),tmp1-cell1/2,tmp2-
cell5/2,latt1(indx,3)
id=id+1
write(12,'(a8,i10)') sym(1), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt1(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(2)
tmp1=latt2(indx,1)+sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt2(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+sqrt3*hexlatt+space1
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
if(label2(j).eq.2)then
write(11,"(a8,3(1x,g20.10))")sym(2),tmp1-cell1/2,tmp2-
cell5/2,latt2(indx,3)
id=id+1
108
write(12,'(a8,i10)') sym(2), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt2(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(3)
tmp1=latt3(indx,1)+1.5*sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt3(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+1.5*sqrt3*hexlatt+spa
ce1
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)+1.5*hexlatt+space1
if(label3(j).eq.2)then
write(11,"(a8,3(1x,g20.10))")sym(3),tmp1-cell1/2,tmp2-
cell5/2,latt3(indx,3)
id=id+1
write(12,'(a8,i10)') sym(3), id
109
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt3(indx,3)
endif
j=j+1
end do
j=1
do indx=1,ce(4)
tmp1=latt4(indx,1)+0.5*sqrt3*hexlatt+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt4(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)+0.5*sqrt3*hexlatt
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)+1.5*hexlatt+space1
if(label4(j).eq.2)then
write(11,"(a8,3(1x,g20.10))")sym(4),tmp1-cell1/2,tmp2-
cell5/2,latt4(indx,3)
id=id+1
write(12,'(a8,i10)') sym(4), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt4(indx,3)
endif
110
j=j+1
end do
end do
end do
!-----Write all the oxygen ions
do iy=1,ncelly
do ix=1,ncellx
do indx=ce(1)+1,iatom(1)
tmp1=latt1(indx,1)+(ix-1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt1(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
write(11,"(a8,3(1x,g20.10))")sym(5),tmp1-cell1/2,tmp2-
cell5/2,latt1(indx,3)
id=id+1
write(12,'(a8,i10)') sym(5), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt1(indx,3)
end do
do indx=ce(2)+1,iatom(2)
111
tmp1=latt2(indx,1)+sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt2(indx,2)+(iy-1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
write(11,"(a8,3(1x,g20.10))")sym(6),tmp1-cell1/2,tmp2-
cell5/2,latt2(indx,3)
id=id+1
write(12,'(a8,i10)') sym(6), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt2(indx,3)
end do
do indx=ce(3)+1,iatom(3)
tmp1=latt3(indx,1)+1.5*sqrt3*hexlatt+space1+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt3(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
write(11,"(a8,3(1x,g20.10))")sym(7),tmp1-cell1/2,tmp2-
cell5/2,latt3(indx,3)
id=id+1
112
write(12,'(a8,i10)') sym(7), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt3(indx,3)
end do
do indx=ce(4)+1,iatom(4)
tmp1=latt4(indx,1)+0.5*sqrt3*hexlatt+(ix-
1)*(2*sqrt3*hexlatt+space1+space2)
tmp2=latt4(indx,2)+1.5*hexlatt+space1+(iy-
1)*(3*hexlatt+space1+space2)
if(tmp1.lt.0)tmp1=tmp1+ncellx*(2*sqrt3*hexlatt+space1+space2)
if(tmp2.lt.0)tmp2=tmp2+ncelly*(3*hexlatt+space1+space2)
write(11,"(a8,3(1x,g20.10))")sym(8),tmp1-cell1/2,tmp2-
cell5/2,latt4(indx,3)
id=id+1
write(12,'(a8,i10)') sym(8), id
write(12,'(3g20.10)') tmp1-cell1/2,tmp2-cell5/2,latt4(indx,3)
end do
end do
end do
deallocate(latt1,latt2,latt3,latt4)
close(11)
113
close(12)
stop
end
!********************* FUNCTION *********************
function ran1(idum)
implicit none
integer,PARAMETER::ia=16807,im=2147483647,iq=127773,ir=2836,ntab=32
real*8,PARAMETER::am=1./im,eps=1.2e-7,rnmx=1.-eps,ndiv=1+(im-1)/ntab
integer::idum,j,k,iv(ntab),iy
real*8::ran1
save iv,iy
data iv /ntab*0/,iy/0/
if((idum.le.0).or.(iy.eq.0))then
idum=max(-idum,1)
do j=ntab+8,1,-1
k=idum/iq
idum=ia*(idum-k*iq)-ir*k
if(idum.lt.0)idum=idum+im
114
if(j.le.ntab)iv(j)=idum
end do
iy=iv(1)
endif
k=idum/iq
idum=ia*(idum-k*iq)-ir*k
if(idum.lt.0)idum=idum+im
j=1+iy/ndiv
iy=iv(j)
iv(j)=idum
ran1=min(am*iy,rnmx)
return
end
!-------------------- Function end ------------------
115
APPENDIX C: PUBLICATIONS DURING THIS DISSERTATION
116
1. Kumar, R. Devanathan, V. Shutthanandan, S. Kuchibhata, A. S. Karakoti, Y. Yong, S.
Thevuthasan, and S. Seal, "Radiation-Induced Reduction of Ceria in Single and
Polycrystalline Thin Films," Journal of Physical Chemistry C, vol. 116, pp. 361-366, Jan
2012.
2. Kumar, P. Zhang, A. Vincent, R. McCormack, R. Kalyanaraman, H. J. Cho, and S. Seal,
"Hydrogen selective gas sensor in humid environment based on polymer coated
nanostructured-doped tin oxide," Sensors and Actuators B-Chemical, vol. 155, pp. 884-
892, Jul 2011.
3. Kumar, S. Babu, A. S. Karakoti, A. Schulte, and S. Seal, "Luminescence Properties of
Europium-Doped Cerium Oxide Nanoparticles: Role of Vacancy and Oxidation States,"
Langmuir, vol. 25, pp. 10998-11007, Sep 2009.
4. V. Singh, S. Singh, S. Das, A. Kumar, W. T. Self, and S. Seal, "A facile synthesis of
PLGA encapsulated cerium oxide nanoparticles: release kinetics and biological activity,"
Nanoscale, vol. 4, pp. 2597-2605, 2012.
5. J. M. Dowding, T. Dosani, A. Kumar, S. Seal, and W. T. Self, "Cerium oxide
nanoparticles scavenge nitric oxide radical ((NO)-N-center dot)," Chemical
Communications, vol. 48, pp. 4896-4898, 2012.
6. S. Das, S. Singh, S. Chigurupati, M. R. Mughal, A. Kumar, C. R. Patra, S. Oommen, N.
E. Vlahakis, D. Mukhopadhyay, W. T. Self, M. P. Mattson, and S. Seal, "Cerium oxide
nanoparticles can induce pro-angiogenesis and facilitate wound healing," Wound Repair
and Regeneration, vol. 20, pp. A20-A20, Mar-Apr 2012.
117
7. U. M. Bhatta, I. M. Ross, T. X. T. Sayle, D. C. Sayle, S. C. Parker, D. Reid, S. Seal, A.
Kumar, and G. Mobus, "Cationic Surface Reconstructions on Cerium Oxide
Nanocrystals: An Aberration-Corrected HRTEM Study," Acs Nano, vol. 6, pp. 421-430,
Jan 2012.
8. S. Singh, T. Dosani, A. S. Karakoti, A. Kumar, S. Seal, and W. T. Self, "A phosphate-
dependent shift in redox state of cerium oxide nanoparticles and its effects on catalytic
properties," Biomaterials, vol. 32, pp. 6745-6753, Oct 2011.
9. N. S. N. Shirato, J. Strader, A. Kumar, A. Vincent, P. Zhang, A. Karakoti, P.
Nacchimuthu, H. J. Cho, S. Seal, and R. Kalyanaraman, "Thickness dependent self
limiting 1-D tin oxide nanowire arrays by nanosecond pulsed laser irradiation,"
Nanoscale, vol. 3, pp. 1090-1101, 2011.
10. T. X. T. Sayle, B. J. Inkson, A. Karakoti, A. Kumar, M. Molinari, G. Mobus, S. C.
Parker, S. Seal, and D. C. Sayle, "Mechanical properties of ceria nanorods and
nanochains; the effect of dislocations, grain-boundaries and oriented attachment,"
Nanoscale, vol. 3, pp. 1823-1837, 2011.
11. J. H. Zou, J. H. Liu, A. S. Karakoti, A. Kumar, D. Joung, Q. A. Li, S. I. Khondaker, S.
Seal, and L. Zhai, "Ultralight Multiwalled Carbon Nanotube Aerogel," Acs Nano, vol. 4,
pp. 7293-7302, Dec 2010.
12. P. Zhang, A. Vincent, A. Kumar, S. Seal, and H. J. Cho, "A Low-Energy Room-
Temperature Hydrogen Nanosensor: Utilizing the Schottky Barriers at the
Electrode/Sensing-Material Interfaces," Ieee Electron Device Letters, vol. 31, pp. 770-
772, Jul 2010.
118
13. V. Singh, A. Karakoti, A. Kumar, A. Saha, S. Basu, and S. Seal, "Precursor Dependent
Microstructure Evolution and Nonstoichiometry in Nanostructured Cerium Oxide
Coatings Using the Solution Precursor Plasma Spray Technique," Journal of the
American Ceramic Society, vol. 93, pp. 3700-3708, Nov 2010.
14. S. Singh, A. Kumar, A. Karakoti, S. Seal, and W. T. Self, "Unveiling the mechanism of
uptake and sub-cellular distribution of cerium oxide nanoparticles," Molecular
Biosystems, vol. 6, pp. 1813-1820, 2010.
15. V. Shah, P. Dobiasova, P. Baldrian, F. Nerud, A. Kumar, and S. Seal, "Influence of iron
and copper nanoparticle powder on the production of lignocellulose degrading enzymes
in the fungus Trametes versicolor," Journal of Hazardous Materials, vol. 178, pp. 1141-
1145, Jun 2010.
16. F. Rispoli, A. Angelov, D. Badia, A. Kumar, S. Seal, and V. Shah, "Understanding the
toxicity of aggregated zero valent copper nanoparticles against Escherichia coli," Journal
of Hazardous Materials, vol. 180, pp. 212-216, Aug 2010.
17. Rotavera, A. Kumar, S. Seal, and E. L. Petersen, "Effect of ceria nanoparticles on soot
inception and growth in toluene-oxygen-argon mixtures," Proceedings of the Combustion
Institute, vol. 32, pp. 811-819, 2009.
18. A. S. Karakoti, S. Singh, A. Kumar, M. Malinska, S. Kuchibhatla, K. Wozniak, W. T.
Self, and S. Seal, "PEGylated Nanoceria as Radical Scavenger with Tunable Redox
Chemistry," Journal of the American Chemical Society, vol. 131, pp. 14144-+, Oct 2009.
119
APPENDIX D: COPYRIGHT PERMISSIONS
120
121
122
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