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GOLD COATING OF SILICA AND ZINC OXIDE
NANOPARTICLES BY THE SURFACE
REDUCTION OF GOLD(I) CHLORIDE
Michael D. English
Submitted in fulfilment of the requirements for the degree of
Master of Science (Research)
Faculty of Science and Technology
Chemistry Discipline
Queensland University of Technology
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride i
Statement of Original Authorship
The work contained in this thesis has not been previously submitted to meet
requirements for an award at Queensland University of Technology or any other
higher education institution. To the best of my knowledge and belief, the thesis
contains no material previously published or written by another person except where
due reference is made.
Signature:______________________________ Date:_________________________
Michael D. English B. App. Sci.
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride iii
Dedication
In honour of my wife Grace English
Who passed away on 19/06/2011
From Glioblastoma Multiforme
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride v
Acknowledgements
A thesis is a study of one very small part of the scientific world with many
limitations such as time and availability of resources. It can never be the definitive
word on a subject, but merely a stepping stone for those who read this work, interpret
and conduct their own research utilising some of the ideas presented within or
generating new ideas aiding in the advancement of science.
There are many people directly or indirectly involved in the production of this thesis
and without their help and guidance over a period of a lifetime the final product
could never have become a reality. If I haven’t thanked anyone by name, please
accept this as my thanks, it is truly appreciated.
First and foremost, I would like to thank my supervisor Associate Professor Eric
Waclawik who saw in me a unique ability when no-one else did and has assisted in
developing that ability. Also, I would like to thank Associate Professor Peter
Fredericks for his input along with QUT in providing funding and a scholarship.
Most of all I would like to thank my recently departed wife Grace who allowed me to
extend my studies in spite of the many obstacles that I had to overcome. Thanks
extends to my children Olivia and David who have expressed an interest in their
father’s academic work and became accustomed to their father completing a thesis
regardless of ongoing obstacles. I also extend thanks to my mother who without her
encouragement to continue with my studies this thesis would never have eventuated.
Finally, I would like to thank all the laboratory staff who assisted me in the use of
unfamiliar instruments, obtaining data and generally being very supportive. Thanks
are also extended to my fellow postgraduate students who without them, the world of
research and interpersonal relationships would be a much poorer experience for all.
Michael D. English 2012
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride vii
Abstract
The possibility of a surface inner sphere electron transfer mechanism leading to
the coating of gold via the surface reduction of gold(I) chloride on metal and semi-
metal oxide nanoparticles was investigated. Silica and zinc oxide nanoparticles are
known to have very different surface chemistry, potentially leading to a new class of
gold coated nanoparticles.
Monodisperse silica nanoparticles were synthesised by the well known Stöber
protocol in conjunction with sonication. The nanoparticle size was regulated solely
by varying the amount of ammonia solution added. The presence of surface hydroxyl
groups was investigated by liquid proton NMR. The resultant nanoparticle size was
directly measured by the use of TEM.
The synthesised silica nanoparticles were dispersed in acetonitrile (MeCN) and
added to a bis acetonitrile gold(I) co-ordination complex [Au(MeCN)2]+
in MeCN.
The silica hydroxyl groups were deprotonated in the presence of MeCN generating a
formal negative charge on the siloxy groups. This allowed the [Au(MeCN)2]+
complex to undergo ligand exchange with the silica nanoparticles, which formed a
surface co-ordination complex with reduction to gold(0), that proceeded by a surface
inner sphere electron transfer mechanism. The residual [Au(MeCN)2]+ complex
was allowed to react with water, disproportionating into gold(0) and gold(III)
respectively, with gold(0) being added to the reduced gold already bound on the
silica surface. The so-formed metallic gold seed surface was found to be suitable for
the conventional reduction of gold(III) to gold(0) by ascorbic acid. This process
generated a thin and uniform gold coating on the silica nanoparticles.
This process was modified to include uniformly gold coated composite zinc oxide
nanoparticles (Au@ZnO NPs) using surface co-ordination chemistry. AuCl dissolved
in acetonitrile (MeCN) supplied chloride ions which were adsorbed onto ZnO NPs.
The co-ordinated gold(I) was reduced on the ZnO surface to gold(0) by the inner
sphere electron transfer mechanism. Addition of water disproportionated the
remaining gold(I) to gold(0) and gold(III). Gold(0) bonded to gold(0) on the NP
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viii Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
surface with gold(III) was reduced to gold(0) by ascorbic acid (ASC), which
completed the gold coating process.
This gold coating process of Au@ZnO NPs was modified to incorporate iodide
instead of chloride. ZnO NPs were synthesised by the use of sodium oxide, zinc
iodide and potassium iodide in refluxing basic ethanol with iodide controlling the
presence of chemisorbed oxygen. These ZnO NPs were treated by the addition of
gold(I) chloride dissolved in acetonitrile leaving chloride anions co-ordinated on the
ZnO NP surface. This allowed acetonitrile ligands in the added [Au(MeCN)2]+
complex to surface exchange with adsorbed chloride from the dissolved AuCl on the
ZnO NP surface. Gold(I) was then reduced by the surface inner sphere electron
transfer mechanism. The presence of the reduced gold on the ZnO NPs allowed
adsorption of iodide to generate a uniform deposition of gold onto the ZnO NP
surface without the use of additional reducing agents or heat.
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride ix
Publications Arising
Proposed Title Status A Novel Method for the Synthesis of
Monodisperse Gold Coated Silica
Nanoparticles
Published online in the Journal of
Nanoparticle Research on
12/01/2012.
ZnO NPs Synthesised Using ZnCl2 and Gold
Coated by use of KCl and AuCl
In preparation.
Gold Coated Zinc Oxide Nanoparticles
Synthesised using ZnI2 and Gold(I) Chloride
In preparation.
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride xi
Table of Contents
Statement of Original Authorship ............................................................................................................i
Dedication ............................................................................................................................................. iii
Acknowledgements ................................................................................................................................. v
Abstract ................................................................................................................................................ vii
Publications Arising ...............................................................................................................................ix
Table of Contents ...................................................................................................................................xi
List of Figures ..................................................................................................................................... xiii
List of Tables ....................................................................................................................................... xiv
Schemes ............................................................................................................................................... xiv
LIST OF ABBREVIATIONS ............................................................................................................... xv
CHAPTER 1: INTRODUCTION ....................................................................................................... 1
1.1 Background .................................................................................................................................. 1
1.2 Project Hypothesis ....................................................................................................................... 2
1.3 Project Aims ................................................................................................................................ 5
1.4 Methodology Used ....................................................................................................................... 6
1.5 Study Outline ............................................................................................................................... 7
1.6 Surface Enhanced Raman Spectroscopy (SERS) ......................................................................... 9
1.7 Synthesis of Bis acetonitrilegold(I) Complex ........................................................................... 11
1.8 Inner Sphere Electron Transfer Mechanism............................................................................... 12
CHAPTER 2: GOLD NANOPARTICLES USING ASCORBIC ACID ....................................... 15
2.1 Introduction ................................................................................................................................ 15
2.2 Experimental .............................................................................................................................. 17
2.3 Results and Discussion .............................................................................................................. 19
2.4 Conclusion ................................................................................................................................. 22
CHAPTER 3: A NOVEL METHOD FOR THE SYNTHESIS OF MONODISPERSE GOLD
COATED SILICA NANOPARTICLES ........................................................................................... 23
3.1 Introduction ................................................................................................................................ 23 3.1.1 Silica nanoparticles ......................................................................................................... 23 3.1.2 Gold Coated Silica Nanoparticles ................................................................................... 24 3.1.3 Uses of Gold Coated Silica Nanoparticles ...................................................................... 24
3.2 Experimental .............................................................................................................................. 25 3.2.1 Materials ......................................................................................................................... 25 3.2.2 Equipment ....................................................................................................................... 25 3.2.3 Synthesis of silica nanoparticles ..................................................................................... 26 3.2.4 Gold coating of silica nanoparticles ................................................................................ 26 3.2.5 Mass spectroscopy of [Au(MeCN)2]
+ ............................................................................. 26
3.2.6 Proton NMR preparation ................................................................................................ 26 3.2.7 Electron microscopy preparation .................................................................................... 27
3.3 Results and Discussions ............................................................................................................. 27 3.3.1 Nanoparticles sizes ......................................................................................................... 27 3.3.2 Morphology .................................................................................................................... 29
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xii Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
3.3.3 Spectroscopy................................................................................................................... 31 3.3.4 1
H NMR .......................................................................................................................... 32 3.3.5 Proposed Mechanism ...................................................................................................... 34
3.4 Conclusion ................................................................................................................................. 35
CHAPTER 4: ZnO NPS SYNTHESISED USING ZnCl2 AND GOLD COATED BY USE OF
KCl AND AuCl .................................................................................................................................... 37
4.1 Introduction................................................................................................................................ 37 4.1.1 Applications of Au@ZnO NPs ....................................................................................... 37 4.1.2 Synthesis of Au@ZnO NPs ............................................................................................ 39
4.2 Experimental .............................................................................................................................. 41
4.3 Results and discussion ............................................................................................................... 43
4.4 Conclusion ................................................................................................................................. 49
CHAPTER 5: GOLD COATED ZINC OXIDE NANOPARTICLES SYNTHESISED USING
ZnI2 AND GOLD(I) CHLORIDE ...................................................................................................... 51
5.1 Introduction................................................................................................................................ 51
5.2 Experimental .............................................................................................................................. 54
5.3 Results and discussion ............................................................................................................... 55
5.4 Conclusion ................................................................................................................................. 62
CHAPTER 6: GENERAL CONCLUSION...................................................................................... 63
6.1 Ascorbic Acid Based Gold Nanoparticles ................................................................................. 63
6.2 A Novel Method for the Synthesis of Monodisperse Gold Coated Silica Nanoparticles .......... 63
6.3 Uniform Gold Coating of Zinc Oxide Nanoparticles Using Gold(I) Chloride and KCl ............ 64
6.4 Gold Coated Zinc Oxide Nanoparticles Synthesised Using ZnI2 and Gold(I) Chloride ............ 65
CHAPTER 7: FUTURE WORK ....................................................................................................... 67
7.1 Ascorbic Acid Based Gold Nanoparticles ................................................................................. 67
7.2 A Novel Method for the Synthesis of Monodisperse Gold Coated Silica Nanoparticles .......... 67
7.3 Gold Coating of Zinc Oxide Nanoparticles ............................................................................... 68
7.4 Surface Inner Sphere Electron Transfer Mechanism ................................................................. 69
REFERENCES .................................................................................................................................... 71
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride xiii
List of Figures
Figure 1. TEM image of gold nanoparticles formed using ascorbic acid and HAuCl4.......... 15
Figure 2(A). Ascorbic acid .................................................................................................... 15
Figure 2(B). Dehydroascorbic acid ....................................................................................... 15
Figure 3. Typical Ascorbic acid gold colloid UV-Vis absorbance peak ..................... 18
Figure 4. UV-Vis spectroscopy results from addition of KOH to HAuCl4 ........................... 21
Figure 5. Plot of pH of HAuCl4 solution vs. gold NP size .................................................... 21
Figure 6. Tetraethyl orthosilicate (TEOS) ............................................................................. 23
Figure 7. (3-aminopropyl)-triethoxysilane (APTES) ............................................................ 24
Figure 8. SEM and TEM images of silica and gold coated silica NPs .................................. 28
Figure 9. Bar graphs of silica NP sizes ................................................................................. 28
Figure 10. TEM images of gold-coated silica NPs ............................................................... 29
Figure 11. Bar graphs of gold coated silica NP sizes ............................................................ 30
Figure 12. Spectroscopy results of silica and gold coated silica NPs ................................... 31
Figure 13. 1H NMR of silica and gold coated silica NPs ...................................................... 33
Figure 14. TEM images of ZnO and Au@ZnO NPs ............................................................. 42
Figure 15. UV-Vis and florescence results of ZnO and Au@ZnO NPs ............................... 44
Figure 16. EDX analysis of ZnO and Au@ZnO NPs ........................................................... 46
Figure 17. XRD images of ZnO and Au@ZnO NPs ............................................................. 47
Figure 18. UV-Vis and florescence spectroscopy of ZnO and Au@ZnO NPs ..................... 56
Figure 19. TEM images of the ZnO and Au@ZnO NPs ....................................................... 57
Figure 20. EDAX results of the ZnO and Au@ZnO NPs ..................................................... 58
Figure 21. XRD spectra of ZnO and Au@ZnO NPs ............................................................. 61
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xiv Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
List of Tables
Table 1. Average hydroxoauric species present a various pH levels .................................... 16
Table 2. pH and maximum absorbance after basifying HAuCl4 ........................................... 19
Table 3. Maximum absorbance after addition of ascorbic acid to HAuCl4 ........................... 20
Table 4. Water bath reaction temperature of HAuCl4 and ASC solutions ............................ 20
Schemes
Scheme 1. Possible surface reactions forming gold coated silica NPs ................................. 34
Scheme 2. ZnO NP synthesis and gold coating scheme ........................................................ 41
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride xv
LIST OF ABBREVIATIONS
APTES ................................................................................... 3-(Aminopropyl)triethoxysilane
ASC .................................................................................................................... Ascorbic acid
AuCl ............................................................................................................... Gold(I) chloride
Au@SiO2 ................................................................................ Gold coated silica nanoparticles
Au@ZnO ....................................................................... Gold coated zinc oxide nanoparticles
CDCl3 ................................................................................................... Deuterated chloroform
ClO4- ............................................................................................................. Perchlorate anion
EDX or EDAX ............................................................ Energy-dispersive X-ray spectroscopy
EtOH ............................................................................................................................ Ethanol
Fe2O3 .................................................................................................................. Iron(III) oxide
FeSO4 ................................................................................................... Iron(II) sulphate
HAuCl4 ................................................................................................... Tetrachloroauric acid
HPLC .............................................................. High-performance liquid chromatography
KCl ............................................................................................................. Potassium chloride
KI ................................................................................................................... Potassium iodide
KOH ....................................................................................................... Potassium hydroxide
MeCN ..................................................................................................................... Acetonitrile
NaBH4 ...................................................................................................... Sodium borohydride
Na2O ................................................................................................................... Sodium oxide
NMR ..................................................................... Nuclear Magnetic Resonance spectroscopy
NOClO4 ........................................................................................... Nitrosyl perchlorate
NPs ...................................................................................................................... Nanoparticles
PATP .................................................................................................... Para Aminothiophenol
PEG ........................................................................................................... Polyethylene glycol
SEM ......................................................................................... Scanning Electron Microscopy
SERS ......................................................................... Surface Enhanced Raman Spectroscopy
SiO2 .................................................................................................................. Silicon Dioxide
TEM .................................................................................. Transmission Electron Microscopy
TEOS .................................................................................................... Tetraethyl orthosilicate
UV-Vis .................................................................. Ultraviolet-visible light spectrophotometry
XRD ............................................................................................................... X-ray diffraction
ZnCl2 .................................................................................................................... Zinc chloride
ZnI2 .......................................................................................................................... Zinc iodide
ZnO ........................................................................................................................ Zinc oxide
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1
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 1
Chapter 1: Introduction
1.1 Background
Gold coated silica nanoparticles are used in Surface Enhanced Raman Spectroscopy
(SERS) and related applications by many research groups of which only a few
significant publications are highlighted here[1-5]. Those groups researching gold
coated zinc oxide for SERS and related applications are limited in terms of the
number of publications available. However, the number of research papers in this
field is growing steadily[6-9].
One over-riding theme in most gold coated silica or zinc oxide nanoparticle
publications is the possibility of tailoring the core of the nanoparticle to a selected
size to increase the wavelength at maximum intensity of the plasmon resonance peak
of core-shell nanoparticles as measured by UltraViolet-Visible Light Spectrophotometry
(UV-Vis spectroscopy). The plasmon resonance peak is the maximum absorbance
peak of the metal coated nanoparticles under examination by UV-Vis spectroscopy
and this peak shifts towards the infrared with increasing size of the underlying
nanoparticle. This has been discussed in a study by Averitt et al[10].
This red-shift in the plasmon resonance peak is one driving force for the use of
composite nanoparticles in medicine, such as cancer destruction by thermal means as
investigated by Hu et al[11]. A shift to near infrared allows visible light imaging to
take place as well as transfer of laser energy in the well known biological window
from 600nm to 1300nm as mentioned by Tsai et al[12] in a study on absorption of
light by typical fats found in the human body. Tsai et al found there was little
absorption of light by human body fats below about 1300nm. Water is another
significant component of the human body which can absorb visible light. A study by
Hale and Querry[13] found the minimum absorbance for water is approximately
470nm with minimal absorbance across the entire visible light range. The most
significant absorbance component of the human body is haemoglobin, which has
been investigated by Kim and Liu[14]. According to Kim and Liu, the region of
minimum absorbance of visible light by haemoglobin is approximately 700nm for
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2 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
oxygenated blood and approximately 800nm for deoxygenated blood. As such, the
best region for the plasmon peak to be for medical related uses needs to be between
700–800nm to avoid significant visible light absorbance by haemoglobin, fat and
water.
It is possible metallic or semi-metallic core nanoparticles may contribute to the
overall SERS effect via modification of the surface plasmon intensity using a charge
transfer between the inner metal oxide or semi-metal oxide core and surface metal.
This effect has been partially examined in core–shell Au@ZnO nanoparticles by
workers from the Lombardi group[6] which used para-Aminothiophenol (PATP) to
link gold nanoparticles to the ZnO core. This result suggested a transfer of electrons
takes place through the linking compound or ligand, PATP, from the zinc oxide core
to the outer gold shell substantially enhancing the SERS response from the PATP
linker molecule.
This suggestion by the Lombardi group[6] indicated an electron transfer occurs
between the inner nanoparticle core and outer metal shell using a bridging ligand as
the electron transfer mechanism. If such an electron transfer occurs this may enable
reduction of a labile metal cation on the surface of a core nanoparticle. This literature
observation therefore forms a preliminary basis to devise a hypothesis based on a
possible “surface inner sphere electron transfer mechanism” leading to the
reduction of gold(I) on the surface of a nanoparticle.
1.2 Project Hypothesis
Surface Inner Sphere Electron Transfer Mechanism
Co-ordination chemistry has several phenomena of interest, such as associative
ligand exchange processes as discussed by Basolo and Pearson[15] and the inner
sphere electron transfer discovered by Taube[16] that could be applied to the gold
coating of metal and metal oxide nanoparticles. Ruff[17] further expanded the inner
sphere mechanism, which is best represented by the following terminology;
M1n+
-L-M2m+
, where M1n+
is the electron donor,
-L- is the bridging ligand
responsible for the electron transfer and -M2m+
is the electron acceptor. Ugo[18]
suggested that a surface with attached ligands or functional groups can act as an
3
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 3
analogue to conventional co-ordination chemistry, indicating this approach could be
used for gold coating of silica and zinc oxide nanoparticles.
Using this surface co-ordination chemistry analogue indicates anionic ligands
adsorbed onto the surface of nanoparticles may be a form of a surface co-ordination
complex. Applying this surface co-ordination complex analogue to conventional
theory on the inner sphere mechanism indicates it may be possible for the surface co-
ordinated ligand to form a covalent linkage or bridging ligand to a labile metal
species, thus forming the necessary prerequisites for a surface reduction of the labile
metal to occur via a “surface inner sphere electron transfer mechanism”. The metal
cation component of the nanoparticle provides the non-labile metal component of the
“surface inner sphere electron transfer mechanism” with the core nanoparticle
providing the necessary electron reservoir for reduction to occur on the surface.
Using Ruffs’ terminology[17], the metal or metal-like nanoparticle core corresponds
to M1n+
. The bridging ligand or surface coordinated ligand corresponds to -L-
including the possibility of a deprotonated nanoparticle surface or a surface co-
ordinated ligand. The gold ion then corresponds to -M2m+
, which by definition is the
reducible or labile species.
This hypothesised reaction mechanism for the gold coating of zinc oxide
nanoparticles indicates a series of synthetic procedures may need to be followed. The
first potential requirement is the formation of a sufficiently labile metal complex
such as may be synthesised from gold(I) chloride. The next requirement is that a
bridging ligand be present on the surface of the nanoparticle. Suitable bridging
ligands are anionic halogens which in the case of zinc oxide, is simplified by the
synthesis of ZnO NPs from various zinc halogen compounds or by the addition of
halogen salts. A further requirement is that the formed gold(I) complex undergoes a
surface ligand exchange between the surface co-ordinated ligands and the labile
metal cation ligand(s).
Once a surface co-ordination complex is synthesised between the gold(I) and the
nanoparticle core, an electron transfer can take place from the reservoir of free
electrons in the nanoparticle core through the bridging ligand to the co-ordinated
gold(I) reducing gold(I) to gold(0). In order to view the process an additional
4
4 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
reaction may be needed, such as a means to add gold metal to the treated
nanoparticle.
This gold metal could be supplied by gold(I) chloride which has a propensity to
disproportionate into gold(0) and gold(111) as discussed by Bergerhoff[19]
following the addition of water. Since a gold(I) complex will be required for the
initial reaction, a suitable complex could be considered sufficiently stable if minimal
disproportionation occurs within a reasonable time period. This disproportionation
reaction could proceed if a sufficiently weak ligand is used for the formation of the
gold complex so that a ligand exchange with the initial complex can occur through
the addition of water.
Another issue that needs to be considered is the reduction of any aqueous gold(III)
and any unreduced gold(I) adding to the gold coated nanoparticle as gold(0). This
could be adapted from existing gold coated nanoparticle literature (covered in later
chapters) and involves little more than the addition of ascorbic acid or some mild
reductant to the solution, thereby adding gold(0) to the surface of the gold coated
nanoparticle. This gold(0) attraction to other gold(0) atoms is known as aurophilicity
and was examined in a review by Schmidbaur[20]. Aurophilicity is defined as the
intermolecular aggregation of small mononuclear gold complexes via gold-gold
contacts with a bonding energy equivalent to standard hydrogen bonding.
Taking this hypothetical synthesis further indicates it may be possible to deprotonate
siloxy groups in silica nanoparticles and covalently bond gold(I) to the deprotonated
silica nanoparticle surface. The silica nanoparticle may act as an electron reservoir
with the anionic character of surface oxygen acting as the electron transfer pathway,
which will lead to the reduction of the covalently bound gold(I).
This project should provide a novel gold coating technique for both silica and zinc
oxide nanoparticles as well developing a new theory accounting for the hypothesised
gold coating process called the “surface inner sphere electron transfer
mechanism”, which may have relevance to other metal or semi-metallic oxide
nanoparticles. The gold coated silica and zinc oxide nanoparticles may also prove
suitable for use in future SERS applications.
5
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 5
1.3 Project Aims
This project aims to accomplish several interlinked tasks with the overall aim of
providing a synthetic method for the complete and uniform gold coating of both
silica and zinc oxide nanoparticles. In order to achieve these project aims, the project
needs to be broken down into its component parts.
The first phase of the project consisted of gaining an understanding of the synthesis
of gold nanoparticles specifically by the reduction of aqueous HAuCl4 in the
presence of ascorbic acid. This method of gold coating on silica and zinc oxide
nanoparticles was selected because the reaction could be conducted at room
temperature and offered the possibility of controlling parameters such as pH,
concentration, molar ratio (HAuCl4:Ascorbic acid) and temperature.
The second phase of the project was to demonstrate that various sizes of silica
nanoparticles could be synthesised using the alteration of one parameter, namely a
variation in the amount of ammonia used for the hydrolysis of tetraethyl orthosilicate
(TEOS) that is used to synthesise silica nanoparticles by the Stöber[21] method.
The third phase of the project was to synthesise a gold complex that could be formed
in-situ and undergo a ligand exchange with a silica nanoparticle surface and water.
This phase required the solvent to be compatible with water and be reasonably stable
in air and at room temperature. This led to the use of the co-ordinating solvent,
acetonitrile, (MeCN) which is a neutral ligand allowing easy replacement by anionic
ligands or water. Further extension to this process required the gold ion to be easily
reducible to solid gold by a simple electron transfer using a bridging ligand. Only a
gold(I) cation can meet this requirement and this restricts the number of complexes
that can be synthesised to a linear, 2 co-ordinate complex. This new synthetic method
then formed part of the gold coating method for silica and zinc oxide nanoparticles.
This method was modified for use with zinc oxide nanoparticles.
The fourth phase of this project consisted of synthesising zinc oxide by the use of
either chloride or iodide ions, which have very different properties in solution, while
testing the feasibility of the proposed surface electron transfer mechanism and the
6
6 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
necessary oxygen vacancies of zinc oxide essential for adsorbtion of anionic ions.
The gold coating method developed in phase three of the project, being the gold
coating of silica, was modified allowing gold coating of the synthesised zinc oxide to
occur under the determined conditions.
1.4 Methodology Used
Undertaking this study poses significant challenges in developing suitable
techniques, conducting experiments and interpreting the results.
Ascorbic acid based gold nanoparticles are well known and a simple referencing
technique using UV-Vis based on Mie theory[22] is all that is required for reliable
size determination assuming the gold nanoparticles formed are spherical.
In the case of silica nanoparticles the method of synthesis is well known although
most work appears vague in the area of quantities of chemicals required for synthesis
to obtain a certain size nanoparticle. The simplest method of determining the size
results and comparing them to the gold coated silica nanoparticles is by Transmission
Electron Microscopy (TEM). An analysis of the composition can be accomplished by
the use of Energy-dispersive X-ray Spectroscopy (EDX) on a Scanning Electron
Microscopy (SEM) instrument.
Determining the gold complex makeup may be accomplished by the use of mass
spectroscopy by matching the molecular mass of the complex to the theoretical mass.
Proton (1H) Nuclear Magnetic Resonance Spectroscopy (NMR) could also be used as
MeCN contains protons that may provide suitable NMR spectra with gold present.
This technique could also be extended to silica and the gold coating method,
provided all NMR spectra were taken in a suitable deuterated liquid such as
chloroform-d (CDCl3). This technique should also provide information on the
presence of protonated siloxy groups. In the presence of gold, information should
also be provided of covalent attachment to the siloxy functional group with the
absence or reduction in peak size indicating complete coverage of gold on almost all
the nanoparticles in solution.
7
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 7
Additionally, a UV-Vis spectrum with a strong plasmon resonance peak is indicative
of very uniform gold coating of monodisperse core nanoparticles. This use of UV-
Vis spectroscopy is also used in the zinc oxide study where the particles are much
more variable in size and morphology however, a useable plasmon resonance peak
should be obtained in this system. Additionally the zinc oxide and gold coated zinc
oxide nanoparticles were analysed by EDX, X-Ray Diffraction (XRD) and TEM.
When examining synthesised zinc oxide nanoparticles a different approach to testing
the efficacy of gold coating can be taken by monitoring a fluorescence peak that is
known as the “defect peak”, which occurs at roughly 500nm. The absence of this
defect peak is indicative of a successful gold coating on the majority of the zinc
oxide nanoparticles. The same method cannot be used for commercial zinc oxide
because the green light emission peak is not present, so both synthesised and
manufactured zinc oxide nanoparticles (called bulk zinc oxide herein) can act as a
control against each other.
1.5 Study Outline
This thesis contains a significant amount of inter-related work across a number of
areas and as such adopts a linear progression via individual chapters especially in
relation to reviewing the relevant literature. Each chapter is broken down into an
introduction, experimental details, results and discussions and a conclusion. In
combination with the appropriate literature and discussion from chapter one it is
conceivable a published paper could be constructed from each chapter.
Chapter one contains an introduction to the topic, details the hypothesis, the project,
the methodology used, and discusses the literature relevant to the overall project. A
mini review of SERS literature is included from which the “inner sphere electron
transfer mechanism” arises. Literature on the synthesis of the bis acetonitrilegold(I)
complex is used. Relevant literature on the inner sphere mechanism is included along
with some hypothetical reactions leading to the synthesis of gold coated silica and
zinc oxide nanoparticles.
Chapter two details the available literature on a main method for forming gold
nanoparticles which has been substantially modified throughout the thesis for use in
8
8 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
gold coating silica and zinc oxide NPs. This method is based on the reduction of
tetrachloroauric acid by the use of ascorbic acid. Additionally, it was found the size
of gold nanoparticles may be varied using pH control and changes to molar ratios of
the precursor solution. The experiments conducted and the corresponding UV-Vis
spectra obtained will be discussed.
In chapter three, the literature relevant to the synthesis of silica and gold coated silica
nanoparticles is reviewed and discussed with emphasis on reliable synthetic methods
and other possible uses of silica and gold coated silica nanoparticles. The
experiments conducted are listed along with the materials used. The preparation of
the silica nanoparticles, along with the preparation of the gold complex and the
preparation of gold coated silica nanoparticles for analysis is covered. The obtained
results are presented along with a discussion and relevant specific literature relating
to the experimental results. Finally, a conclusion specific to the silica nanoparticles
synthesised and the gold coating of these silica nanoparticles is presented.
In chapter four, the literature relevant to the synthesis of zinc oxide nanoparticles and
the current methods of gold coating zinc oxide nanoparticles is reviewed and
discussed with emphasis on reliable synthetic methods and other possible uses of
zinc oxide nanoparticles gold coated zinc oxide nanoparticles. Fluorescence, a useful
probe specifically for zinc oxide has also been reviewed and discussed. A hypothesis
relating to the synthesis of zinc oxide nanoparticles is also presented with the
relevant literature relating to the suggested method. The experiments conducted are
listed along with the materials used, and the preparation of zinc oxide nanoparticles
for analysis is discussed. The preparation of the gold complex is also covered along
with the preparation of the gold coated zinc oxide nanoparticles for analysis. The
obtained results are presented along with a discussion and specific literature relating
to the experimental results. Finally, a conclusion specific to the synthesis of zinc
oxide nanoparticles and the gold coating of zinc oxide nanoparticles is presented and
is related to the synthesis of the nanoparticles arising from the use of ZnCl2 and KCl.
In chapter five, the synthesis and related literature relevant to the synthesis of ZnO
NPs using ZnI2 will be discussed. Additionally the chemistry of gold interactions
with iodide and iodine will also be discussed. Finally, evidence will be shown that it
9
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 9
is possible to reduce gold(I) chloride in the presence of iodide and iodine by use of
the “surface inner sphere electron transfer mechanism”
The general conclusion in chapter six will include a summary of findings that are
specific to ascorbic acid generated gold nanoparticles, silica and gold coated silica
nanoparticles, zinc oxide and gold coated zinc oxide nanoparticles. It also includes a
conclusion on the proposed “surface inner sphere electron transfer mechanism”
and has been discussed in the context of the overall project aims.
Chapter seven details projected future work that may be conducted concluding the
thesis, including potential significant areas that require further development using
these gold coated composite nanoparticles.
1.6 Surface Enhanced Raman Spectroscopy (SERS)
In conducting this research project the author was taking advantage of a known
charge transfer process, which despite much controversy, may be considered to
contribute to SERS. As such, a review of relevant SERS literature was undertaken to
devise the gold coating methods studied and to devise a hypothesis why the studied
reactions occurred.
The Raman signal amplification now known as SERS was first reported by
Fleischmann et al[23] using pyridine adsorbed on an electrochemically deposited and
roughened silver surface. It was proposed the as yet unidentified SERS effect was
due to increased Raman scattering from the increased number of molecules adsorbed
on the surface.
Later research work by Jeanmaire and Duyne[24] using a similar approach as above,
stated SERS is a result of a charge transfer effect, for example, when pyridine is
chemisorbed onto the silver surface via an anion induced process that leads to an
axial end-on attachment to the surface. Additional work conducted by Albrecht and
Creighton[25] was undertaken to try and understand the SERS effect using a
roughened silver electrode with pyridine, with the authors reporting considerable
enhancement in the order of 105 magnitude over non-SERS Raman.
10
10 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
A more complete explanation of the SERS effect may be found in a recent
publication by Ru and Etchegoin[26], which has detailed information on the subject
with the following definitions provided; SERS is a surface spectroscopy technique
with the molecules of interest having to be in close proximity or in contact with the
metal substrate. The enhancement factor is a result of “plasmon resonances” which
is a shorthand way of stating a family of effects associated with the interaction of
electromagnetic spectrum radiation with the metal substrate. The Raman component
of the term comes from detection of the inelastic scattering of electromagnetic
radiation giving an insight into the molecules chemical makeup. The term “plasmon
resonance” is what is responsible for the SERS effect and relates to metals such as
the noble metals, copper and aluminium that have free conduction electrons. These
free electrons move in a sea of fixed positive metal ions which provides overall
stability to the bulk metal while forming the free electron “plasma” that governs the
optical properties of the metal where the characteristic resonance energies prevail
mainly in the visible light region as used in Raman spectroscopy.
“Plasmon” can be defined as a “quantum quasi-particle representing the elementary
excitations, or modes, of the charge density oscillations in a plasma”. As such, a
plasmon is simply to plasma charge density as photons are to an electromagnetic
field.
Lombardi and Birke[27] generated a universal theory for SERS that incorporates the
magnetic field enhancement as well a chemical enhancement and charge transfer
enhancement. However it does point out that SERS enhancement predominates from
the magnetic field enhancement with the chemical and charge transfer factors
contributing to the overall enhancement. This is at odds with Moskovits and Suh[28]
who argue that all SERS enhancement occurs from the intense electromagnetic field
enhancement by excitation of the plasmon field. As covered by Lombardi and
workers[6], it is possible the charge transfer process from an inner core of zinc oxide
to an outer shell of gold via the linking molecule PATP contributes to additional
excitement of the plasmon field on the surface of the nanoparticle, thereby enhancing
the intense magnetic field and generating a more intense SERS enhancement. This
seems a more reasonable explanation for the additional SERS enhancement observed
than enhancement by a charge transfer or chemical mechanism alone.
11
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 11
1.7 Synthesis of Bis acetonitrilegold(I) Complex
A significant problem in this project relates to the formation of a suitable gold(I)
complex that is easy to synthesise and relatively stable in organic solutions but may
be manipulated in aqueous solutions. It is also well known that many organic
solvents such as acetonitrile possess a free pair of electrons that may form a co-
ordinate bond with transition metals amongst others. This leads to the possibility of
acetonitrile being used as a solvent and a co-ordinating ligand for gold(I).
The synthesis of a bis acetonitrilegold(I) complex [Au(MeCN)2]ClO4, was achieved
by Bergoff[19] by the reduction of gold(III) ions from HAuCl4 using FeSO4 in the
presence of NOClO4 suspended in MeCN. Goolsby and Sawyer[29] stated that
gold(I) can only exist in water as a stable complex. They prepared a bis acetonitrile
gold(I) chloride complex by the partial electrochemical reduction of HAuCl4 in the
presence of tetraethyl ammonium perchlorate in MeCN. This complex was found to
form a stable 2 co-ordinate complex being highly soluble in acetonitrile with a
stability constant of 1.4 x 1012
.
Some additional work on the stability constant of [Au(MeCN)2]ClO4 was reported
by Johnson et al[30] by preparing the bis acetonitrilegold(I) complex by anodically
dissolving gold into MeCN with added tetraethyl ammonium perchlorate. It was
found that the complex remains stable for about 5 minutes in the presence of aqueous
HClO4 before disproportionation occurs. Apart from these potentiometric works on
bis acetonitrile gold(I), little appears to have been published.
Worth noting is the fact that gold(I) chloride may be successfully dissolved in
acetonitrile forming a relatively stable bis acetonitrile gold(I) complex until the
addition of water begins disproportionation to gold(0) and gold(III) within 5 minutes.
This simple bis acetonitrilegold(I) complex could be an ideal solution to the
synthesis of a gold(I) complex. This gold(I) complex is relatively stable, easy to
make and can be successfully ligand exchanged with water, disproportionating and
adding additional gold(0) to the initially surface reduced gold. Also, additional
gold(III) that is produced can also be reduced to gold(0) thus adding to the gold shell.
12
12 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
1.8 Inner Sphere Electron Transfer Mechanism
The inner sphere electron transfer mechanism was discovered by Taube[16] who
was eventually awarded the Nobel prize for his efforts. The simple explanation of
this electron transfer mechanism is that a covalent linkage forms between a bridging
ligand and the metal cations present via an anionic atom or molecule. Taubes’
experiment validated this mechanism by synthesising the compound [CoCl(NH3)5]2+
and then reducing it with Cr2+
in HClO4. When the medium for the reaction
contained radioactive Cl, the mixing between the Cl- and Cr
3+ was found to be less
than 0.5% indicating the transfer of Cl-
from the reducing agent to the oxidizing
agent is direct. The Cl- that was bonded to the cobalt(III)
now becomes bonded to the
chromium(II) forming the bi-metallic intermediate compound; [Co(NH3)5(μ-
Cl)Cr(H2O)5]4+
where "μ-Cl" indicates the chloride bridging ligand between
cobalt(III) and the chromium(II). Chloride then serves as an electron flow “bridge”
between the cobalt(III) and chromium(II) which causes the cobalt(III) to reduce to
cobalt (II) and chromium(II) to be oxidised to chromium(III). Since the radioactive
Cl- doesn’t show up in the new chromium complex formed; [CrCl(H2O)5]
2+ it must
be concluded that the normal chloride ion transferred to the new [CrCl(H2O)5]2+
complex. Therefore, chloride must serve as an electron transfer mechanism between
cobalt(III) and the chromium(II) as per the following reaction:
[CoCl(NH3)5]2+
+ [Cr(H2O)6]2+
→ [Co(NH3)5(H2O)]2+
+ [CrCl(H2O)5]2+ ....................
1
This was further elaborated on and extended by Ruff[17] indicating that an electron
donor and electron acceptor needs to be present. The formula given by Ruff for an
inner sphere mechanism is M1n+
-L-M2m+
where M1n+
is a stable metal ion, -L- is the
anionic bridging ligand and M2m+
is a labile metal ion. This is analogous to the
hypothesised surface electron transfer mechanism where the nanoparticle is a metal
or semi-metal oxide that can act as the electron donor, the adsorbed anion atoms or
molecules that act as the bridging ligand. While the proposed bis acetonitrilegold(I)
complex can potentially exchange a acetonitrile ligand to bond with, almost
covalently, with the adsorbed anion. This is likely to be the case for zinc oxide
nanoparticles as the halogen ion is adsorbed to the zinc ion itself which forms the
non labile metal and the zinc oxide nanoparticle serves as the electron donor through
the surface zinc ions. The suggested hypothetical reactions are shown below:
13
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 13
ZnOn + mKX → [(ZnO)nXm]m-
+ mK+
.....................................................................................................
2
[(ZnO)nXm]m-
+ m.[Au(MeCN)2]+
→ [(ZnO)n(X Au(MeCN))m]+ m.MeCN ............
3
ZnOn refers to zinc oxide nanoparticles.
KX refers to the use of a potassium salt with halogen.
[(ZnO)nXm]m-
refers to the zinc oxide nanoparticle with adsorbed or
surface co-ordinated anionic halogen ligands.
[Au(MeCN)2]+
refers to the co-ordinated gold(I) complex in acetonitrile.
[(ZnO)n(XAu(MeCN))m] refers to the formed surface co-ordinated
complex between bis acetonitrilegold(I) and the anionic halogenated zinc
oxide nanoparticle.
MeCN refers to the surface exchanged acetonitrile ligand.
In the case of silica the situation is more complex in forming the bridging ligand. In
this case, it is misnomer to call the required surface group a ligand, but the same
process can occur. This bridging process requires the deprotonation of the siloxy
group which could easily be accomplished by the use of MeCN. The deprotonation
by acetonitrile allows a bonded oxygen with anionic character to be available to
ligand exchange MeCN, from the bis acetonitrilegold(I) complex, to form a surface
co-ordination complex with the bonded negative character oxygen. The silica
nanoparticle itself will act as the electron donor with the negative character oxygen
forming the electron bridge allowing the transfer of an electron from the nanoparticle
to the gold(I) surface complex, and thus reducing it to gold(0). The hypothetical
reaction for this mechanism is shown below:
(SiO2)(n-m)(OH)m + m.MeCN→ ((SiO2)(n-m)(O-)m + m.MeCNH
+ ........................................... 4
m.[Au(MeCN)2]+
→ [(SiO2)(n-m)OAu(MeCN)] + m.MeCN ......................................................
5
(SiO2)(n-m) is the core of the nanoparticle.
((SiO2)(n-m)(OH)m) indicates the surface protonated silica nanoparticle.
14
14 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
(OH)m indicates the protonated siloxy functional groups on the surface of
the nanoparticle.
(SiO2)(n-m)(O-)m indicates the deprotonated siloxy functional groups on the
silica nanoparticle surface.
[(SiO2)(n-m)OAu(MeCN)m] refers to the nanoparticle forming a surface co-
ordination complex from the added bis acetonitrilegold(I) complex after
ligand exchanging with an acetonitrile ligand for the deprotonated siloxy
“surface ligand”.
OAu is the central component of the surface complex and is able to
transfer charge from the nanoparticle through the deprotonated siloxy
group to gold(I).
15
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 15
Chapter 2: Gold Nanoparticles using Ascorbic Acid
Figure 1. TEM image of gold nanoparticles formed using ascorbic acid and HAuCl4.
2.1 Introduction
Gold nanoparticles have a long history in scientific literature beginning with Faraday[31] who
reduced gold(III) chloride using phosphorus in an aqueous solution. A significant synthesis method
was developed by Turkevich et al[32], whereby aqueous HAuCl4 was brought to the boil and
aqueous sodium citrate was added turning the solution a ruby red colour. This was refined by
Frens[33], which focused on concentration parameters. A non aqueous method was developed by
Brust and Schiffrin[34], whereby aqueous HAuCl4 was transferred from the aqueous phase to a
toluene organic phase by the use of tetraoctylammonium bromide (TOAB), which acts as a
stabilising agent after the addition of Sodium Borohydride (NaBH4) which causes the reduction of
the gold.
The method of most relevance to this project is the reduction of aqueous HAuCl4 by the addition of
ascorbic acid as originally conducted by Stathis and Fabrikanos[35].
Figure 2(A). Ascorbic acid (ASC). Figure 2(B). Dehydroascorbic acid.
Andreescu[36], added additional refinements such as pH control. In this method, the monodispersity
of the resultant gold nanoparticles is very high with significant stability at basic pH. The size of the
16
16 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
gold nanoparticles can also be modified by pH adjustment. Additionally, the reaction is conducted at
room temperature and pressure using ascorbic acid (ASC, vitamin C), an easily metabolised
compound. Gold(III) is reduced by the ascorbic acid via the oxidation of ascorbic acid to the radical
semidehydroascorbate, with further oxidation to stable dehydroascorbate, which then is available to
cap the resultant gold nanoparticles. The overall reaction is best described as:
2HAuCl4 + 3C6H8O6 → 2Au0
+ 3C6H6O6 + 8HCl ................................................................................................................
1
Gold(III) can be reduced in solution using pH adjustment through the gradual replacement of
chloride ions by added hydroxide. This gradual addition has the effect of slowing down the reaction
kinetics so larger particles can form[36]. These oxoauric acid compounds are formed by stepwise
substitution of the chloride ligands of the original aqueous tetrachloroauric acid. With the relative
reduction potential reducing to 0.00v at a pH of 10.35, this effect should increase the particle size
and decrease the monodispersity under the premise that a fast reaction controls the monodispersity
but at the expense of size.
Table 1. Average hydroxoauric species present at various pH levels and the reduction potential of
these species at particular pH readings when using ascorbic acid as the reductant[37]. Not all redox
potentials have been quantified in this paper.
pH Average Formula Relative Redox Potential
2.91 [AuCl2.91(OH)1.09]-
+0.66v
3.39 [AuCl2.56(OH)1.44]- -
4.01 [AuCl2.46(OH)1.54]- -
5.01 [AuCl2.43(OH)1.57]- -
6.16 [AuCl1.09(OH)2.91]- +0.59v
7.52 [AuCl0.83(OH)3.17]- -
8.01 [AuCl0.67(OH)3.33]- +0.53v
10.35 [AuCl0.10(OH)3.90]- 0.00v
Sizes and concentrations of gold nanoparticles can be calculated using UV-Vis spectroscopy in co-
junction with formulas and a table developed by Haiss et al[38]. For gold hydrosols with particle
17
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 17
diameters larger than 35nm, theoretical and experimental results for the surface plasmon peak in the
extinction spectrum can be precisely fitted by the following equation[38]:
λspr = λ0 + L1exp(L2d) ................................................................................................................................................................................
2
Particle diameters (d) can also be directly calculated without reference to fitting software from the
peak position using the following fitted parameters (λ0 = 512, L1 = 6.53, L2 = 0.0216), where the
average of the absolute error in calculating experimentally observed particle diameters has been
shown to be only 3% (Haiss et al[38] using UV-Vis spectroscopy).
2.2 Experimental
Hydrogen tetrachloroauric acid (99.9%), ascorbic acid (reagent grade), pelletised potassium
hydroxide (Univar, 85%) were all purchased from Sigma Aldrich Australia and used as received.
Equipment consisted of standard pH colour test strips, a wide (20cm) flat glass container, a hot water
bath (ice cream container), a series of 100ml beakers, a stirring bar and a magnetic stirring plate.
The original experiment consisted of dissolving 0.059mmol (0.02g) of HAuCl4 into 100ml of
deionised water containing 0.18mmol (0.01g) of KOH. The solution pH was approximately 11,
roughly measured by pH strips. Then 0.114mmol (0.02g) of ascorbic acid was dissolved into
100.0ml of deionised water. The treated beakers were then poured slowly into opposite ends of the
flat bottomed glass container (20cm) with magnetic stirring bar. Within 10 seconds a clear, blue
colour resulted giving a maximum absorbance at 601nm.
The amount of KOH added to the deionised water was reduced by half for each successive
experiment that gave an initial approximate starting pH of 10, 6 and 5. These gave maximum
absorbance at 563, 533 and 529nm, with details of each experiment given in Table 2.
This experiment was modified by dissolving 0.02g of HAuCl4 in 50.0ml of deionised water plus 2
drops of 5% HCl aqueous solution. The amount of ascorbic acid used each time was varied from
0.015g, 0.017g, 0.022g, 0.025g and 0.028g. The maximum absorbance wavelengths were 618nm,
601nm, 619nm, 608nm and 683nm respectively. A higher amount of ASC (0.03g) caused
coagulation. The conclusion reached was 0.02g provided the least dispersion against all other
amounts. All subsequent testing and all additional experiments used this 1:1, w:w ratio generally
18
18 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
being: 0.02g HAuCl4 dissolved in 50.0ml deionised water vs. 0.02g ASC dissolved in 50.0ml
deionised water as seen in Table 3.
After this test, the temperature was varied by immersing both beakers in a hot water bath with a
thermometer in the ascorbic acid beaker. The reaction was tested at 25, 30, 35 and 40°C generating
maximum absorbances at 602, 583, 549 and 552nm as detailed in Table 4.
The procedure for routine synthesis depended on changing the initial pH either by dropwise addition
of 5% HCl or 5% KOH to obtain the desired size of the resultant gold NPs is detailed below:
HAuCl4 (0.02g) was accurately weighed into a beaker and dissolved in 50.0ml of deionised water
The pH was adjusted, usually by addition of 2-3 drops of 5% aqueous KOH or 5% aqueous HCl
solution. Ascorbic acid (0.02g) was accurately weighed into a beaker and dissolved in 50.0ml of
deionised water was added to dissolve it.
A thermometer was placed into the ascorbic acid solution and both beakers were held in a hot water
bath until the temperature was about 35°C and then both beakers were removed and slowly poured
into opposite ends of a flat bottomed glass container (20cm) with stirring. This procedure was used
subsequently in further biological related experiments by another researcher who was conducting
prostate cancer research using standard radiation treatments[39].
The entire experiment was subsequently modified and used for an undergraduate laboratory teaching
experiment.
An aqueous KOH solution (≈0.05M, 100.0ml) was prepared by adding solid KOH (≈0.02g, 2-3
pellets) to a volumetric flask to which deionised water was added. An aqueous solution (≈ 0.6 mM,
200.0ml) of HAuCl4 was prepared by adding HAuCl4 (0.05g) to a volumetric flask and filling with
deionised water. The HAuCl4 solution (20mL) was then added to 6 beakers. Aqueous KOH (≈0.05
M) solution was added to each of these beakers by serial addition starting from 0.00ml, in 1ml
increments to 6mL in total then made up to 25mL using deionised water.
A solution of aqueous ascorbic acid (1.4mM) was made up by adding powdered ascorbic acid
(0.05g) to a volumetric flask to which deionised water was added. In turn, the ascorbic acid solution
(25mL) was pipetted into 6, 50mL beakers.
19
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 19
A magnetic stirring bar in a large 1L beaker was set rotating on a magnetic stirring plate. A treated
gold and one of the ascorbic acid beakers were brought up to 30°C by simply immersing the beakers
in a hot water bath and monitored using a thermometer. Both beakers were immediately removed
from the hot water bath at 30°C and poured into, from opposite ends of the 1L beaker
simultaneously. UV-Vis spectroscopy was conducted on the resultant reaction components which
were used undiluted in UV-Vis spectroscopy examination as shown in Figure 4.
2.3 Results and Discussion
Figure 3. Typical ascorbic acid gold colloid UV-Vis absorbance peak
Table 2. pH and maximum absorbance after basifying HAuCl4 aqueous solution with approximate
adjusted pH and the resultant maximum absorbance of the nanoparticles.
KOH added (grams) Approximate pH (test strips) Maximum Absorbance, nm
0.01 11 601
0.005 10 563
0.0025 6 533
NIL 5 529
20
20 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
The results shown in Table 2 indicate the possibility that gold nanoparticles may be tailored in size
by adjusting the HAuCl4 solutions’ pH. Adjusting the amount of added ascorbic acid as shown in
Table 3 and the temperature as shown in Table 4 may also influence the NP size. However, what is
not shown is that collected nanoparticle solutions below pH 7 tend to coagulate over several days,
therefore, for long term storage, a pH >7 is very desirable.
Table 3. Maximum absorbance obtained after the addition of ascorbic acid to HAuCl4 solution
Amount Ascorbic Acid Used, grams Maximum absorbance, nm
0.015 618
0.017 601
0.022 619
0.025 608
0.028 683
0.03 COAGULATED
Table 4. Water bath reaction temperature of the about to be reacted HAuCl4 and ASC solutions and
the wavelengths obtained at maximum absorbance.
Reaction Temperature 0C Maximum Absorbance, nm
25 602
30 583
35 549
40 552
Observing Figure 4 it is seen the more acidic solution where nil KOH was added expressed the
highest monodispersity, which can be judged by the peak width at half height which is narrowest.
Aggregation is present after addition of a small amount of KOH giving a second plasmon resonance
peak at about 680nm. This is in line with observations based on storage of the gold hydrosols which
indicated that when acidic (i.e. pH < 7.00) they coagulated. The strongest plasmon resonance is seen
after 3ml KOH was added with an initial pH of 9.59 giving a calculated particle size using equation 2
21
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 21
and shown in Figure 5. This resulted in a 101nm average size gold nanoparticle with a reasonable
monodispersity judging by the lack of the second coalescence peak at 680nm. The resonance
wavelength peaks became confused after this point indicating a possible reversal of the relative
reduction potential sign. This observation is consistent with a probable charge reversal noting that
from Table 1 a zero relative reduction potential occurs at a pH of 10.35. Clearly there are many
unanswered questions in this experiment. However, across the majority of the pH range a red shift
progression is evident.
Figure 4. UV-Vis spectroscopy results from addition of KOH to HAuCl4
Figure 5. Plot of pH of HAuCl4 solution vs. gold NP size using KOH.
Gold NP size results as shown in Figure 5 were calculated from Figure 4 using size tables from Haiss
et al[38]. On examination of Figure 5 a very quick increase in size (67nm and 92nm) is evident
60
70
80
90
100
110
2 3 4 5 6 7 8 9 10 11
nm
pH
22
22 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
between pH 2.43 - 2.72. This quickly tapers off from 97nm to 101nm at 3.90 to 9.59 pH. An
aberration was noted at pH 10.28 reducing the size to 82nm which then returned to 101nm at pH
10.53. This reversal in size effect requires further investigation in the 10 to 11 pH range but it is
suspected this effect is closely connected to a sign reversal of the relative reduction potential
occurring from a pH of 10.35 onwards. Additionally, the increase in gold NP size as the pH
increases is thought to be due to substitution of chloride ligands by hydroxide ligands[37] forming
more stable hydroxide substituted chloro-hydroxo-gold complexes in which the gold(III) central
metal is slower to reduce. This slowing in reaction time therefore allows more time for larger gold
NPs to form before the solution is depleted of reduceable gold(III) complexes.
2.4 Conclusion
The size of gold nanoparticles can be controlled by varying the pH of the hydrogen tetrachloroauric
acid solution by addition of aqueous KOH as in Figure 4 and HCl in Figure 5. This result suggests
this simple method can be further developed for the specific size production of spherical,
monodisperse gold nanoparticles. These experiments show the ability to reduce various hydroxo
substituted chloroauric species using ascorbic acid at 30°C.
23
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 23
Chapter 3: A Novel Method for the Synthesis of
Monodisperse Gold Coated Silica
Nanoparticles
3.1 Introduction
The following is an excerpt of a research paper with additional comments recently published in the
Journal of Nanoparticle Research[40]. All information regarding the motivation for this work can be
referred back to chapter one with the main novel areas covered being the deprotonation of silica
nanoparticles, the formation of a surface co-ordination complex, the formation of a gold(I)
acetonitrile complex and the surface inner sphere electron transfer mechanism.
Additional comments are included in this chapter that were not present in the original accepted
journal version, which may help to clarify some points. However this series of experiments were
based on the development of the hypothesis regarding the surface inner sphere electron transfer
mechanism described in Chapter 1.
3.1.1 Silica nanoparticles
Silica nanoparticles were synthesised that were relatively monodisperse, spherical and easily
prepared by the simple hydrolysis reaction of tetraethyl orthosilicate (TEOS) with ammonia in
ethanol and water by Stöber[21]. This method has been subsequently modified by Rao et al[41] by
incorporating ultrasonics to produce very monodisperse and uniform silica nanoparticles. These
silica nanoparticles can be varied in size by changing parameters such as the initial amount of TEOS,
the amount of water added and the amount of concentrated ammonia solution used for the hydrolysis.
Figure 6. Tetraethyl orthosilicate (TEOS)
24
24 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
3.1.2 Gold Coated Silica Nanoparticles
The classic method used to synthesise gold coated silica nanoparticles consists of synthesising gold
colloids based on the work of Turkevich et al[32]. Aqueous tetrachloroauric acid was brought to the
boil and aqueous sodium citrate was added to reduce Au3+
to Au0. A significant variation on
synthesising gold nanoparticles was used by Stathis and Fabrikanos[35] who added aqueous ascorbic
acid to HAuCl4 at room temperature to obtain gold nanoparticles. This method was studied in some
detail in Chapter 2. To date there has been no indication the gold colloid method (developed by
Stathis and Fabrikanos) has been used to directly synthesise composite Au@SiO2 NPs.
Gold-coated silica nanoparticles have been prepared by Hiramatsu and Osterloh[42]. They
functionalised the silica nanoparticles with (3-aminopropyl)-triethoxysilane (APTES) that enabled
gold colloids (prepared by the Turkevich method) to be electrostatically attached via the APTES
linker molecule.
Figure 7. (3-aminopropyl)-triethoxysilane (APTES).
This method resulted in rough, gold seed surfaced nanoparticles. To form a complete gold shell on
the silica surface, HAuCl4 and ascorbic acid solutions were added. The unfortunate drawback of this
process is the very great variability in morphology and general reluctance of the gold colloid to stick
to the linker molecules. This current method depends on the creation of a seed surface, which will
allow reduction of HAuCl4 by ascorbic acid to proceed. There has been no known study on why this
method works. However, it is alluded to in Chapter 1 that APTES may act as an electron transfer
ligand or bridge to enable reduction to occur.
3.1.3 Uses of Gold Coated Silica Nanoparticles
The deposition of metal nanoparticles such as gold onto silica nanoparticles has attracted
considerable attention, because these systems can be used in a very diverse range of applications.
When combined with a suitable capping ligand, gold-shell silica-core (Au@SiO2) nanoparticles have
25
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 25
been used in HPLC as a stationary phase[43]. Au@SiO2 nanoparticles have also been used as a
means for providing a thermal effect for the destruction of cancer[11]. When Au@SiO2 nanoparticles
are combined with an iron core, magnetic directional control in biomedical applications is
possible[44]. In Raman spectroscopy, Au@SiO2 nanoparticles can be used for the SERS detection of
analytes such as perchlorate in groundwater[4], and for single molecule detection of analytes by
taking advantage of the dielectric core to generate additional SERS enhancement[45]. Gold colloids
supported on silica have also been used for the selective oxidation of styrenes by di-oxygen in order
to generate epoxides, which are a key component of much chemical synthesis[46]. Catalytic
reduction of nitrogen oxides by hydrogen, carbon monoxide and hydrocarbons in the presence of
excess oxygen has also been achieved using supported gold colloids on silica[47].
In the case of the novel Au@SiO2 NPs reported in this chapter, the main application expected is the
ability to form a surface coating on suitable substrates for SERS applications involving gaseous
phase compounds. In other words, a SERS based gas sensor.
3.2 Experimental
3.2.1 Materials
Gold(I) Chloride, 99.9% metals basis, tetraethyl orthosilicate, (TEOS) 98% reagent grade, ascorbic
acid, reagent grade and CDCl3 , 99.8% deuterated and polyethylene glycol (PEG), molecular weight
approximately 2000g mol-1
, were purchased from Sigma Aldrich Australia. Acetonitrile (MeCN),
HPLC grade, 99.9%, was purchased from Labscan, concentrated ammonia, 28%, was purchased
from Univar with absolute ethanol purchased from Merck.
3.2.2 Equipment
The sonicator used was a Branson Model 1510 in combination with an Eppendorf model 5424
centrifuge and standard 2.0 ml plastic vials. A Bruker 400MHz spectrometer was used for liquid
solution 1H NMR using CDCl3. Positive ion mass spectrometry data was collected on a Fisons
Quattro triple quadropole mass spectrometer equipped with electrospray interface. Transmission
electron microscopy was conducted on a JEOL 1200 operating at 100kV using a standard tungsten
filament. A Philips CM200 TEM operating at 200kV was used to calculate nanoparticle sizes using a
manual method. A FEI Quanta environmental SEM operating at high vacuum was used for scanning
electron microscopy along with EDX analysis.
26
26 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
3.2.3 Synthesis of silica nanoparticles
Silica nanoparticles were synthesised using the well known Stöber method combined with the
improved method of Rao, with only a few modifications. Concentrated ammonia solution, (28%) was
combined with deionised water (9.0ml) and absolute ethanol (27.0ml) and sonicated for 15 minutes.
To this mixture, TEOS (2.4ml) was added to absolute ethanol (50.0ml) and sonicated for a further 2
hours. The mixture was then centrifuged @ 10,000rpm for 15 minutes after which the supernatant
was removed. The recovered solid pellet was redispersed by sonication in MeCN, while
concentrating the volume down to 24.0ml in total through a further 2 further cycles. The amount of
concentrated ammonia added for hydrolysis was varied from 2.0 to 14.0ml without additional
changes.
3.2.4 Gold coating of silica nanoparticles
Gold(I) chloride was dissolved in MeCN (20.0ml) with the assistance of several minutes of
sonication, giving a very pale, clear, yellow solution. To this solution, 2.0ml of the concentrated
silica nanoparticles in MeCN was added, with a variation in molar ratio between AuCl and TEOS
from 1 to 2.5. After stirring the solution for several hours, the colour changed to a clear, golden -
light brown colour. Deionised water (10.0ml) was added and allowed to stir for 15 minutes. Aqueous
ASC (3mM, 20.0ml) was added dropwise, which turned the solution a near opaque, mid brown
colour, which when held up to white light was clear purple in colour.
3.2.5 Mass spectroscopy of [Au(MeCN)2]+
Gold chloride (0.1g) was dissolved in MeCN, and then the solvent was removed by rotary
evaporation. Pale yellow crystals remained and were redissolved in MeCN and injected into the mass
spectrometer with the addition of pure water as the eluent. The positive ion spectra were taken. No
collision cell analysis was attempted.
3.2.6 Proton NMR preparation
For proton NMR, [Au(MeCN)2]+ was prepared using the same protocol as for the mass spectroscopy
experiments, except that the final solvent used was CDCl3 and the resultant solution was directly
added to a glass NMR tube from the preparation flask. A blank of MeCN in CDCl3 was also prepared
for comparison purposes.
Previously synthesised silica nanoparticles were placed in two, 2ml Eppendorf vials, and were
topped up with either [Au(MeCN)2]+
in MeCN for the gold coating of the silica nanoparticles step, or
in MeCN only, followed by sonication to disperse the mixture.
27
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 27
These MeCN-silica-[Au(MeCN)2]+
and MeCN-silica mixtures were then centrifuged @ 10,000rpm
for 5 minutes with the supernatant being decanted. CDCl3 was directly added to the remaining solid
pellets and redispersed using sonication for several minutes. This process was repeated two
additional times and then the respective mixtures, along with dilution using CDCl3, were added to
glass NMR tubes for immediate 1H NMR analysis.
It was noted that over several hours all samples with added [Au(MeCN)2]+
gradually turned the silica
nanoparticles brown or where there was no silica particles, the glass NMR tube became partially
coated with gold.
3.2.7 Electron microscopy preparation
Samples of silica and Au@SiO2 for SEM analysis were prepared by centrifuging the relevant 2ml
Eppendorf vials for 5 minutes @ 10,000rpm and decanting the supernatant, leaving behind solid
pellets. These vials were then topped up with ethanol, with the nanoparticles being redispersed by
sonication for several minutes. The mixture was then dropped onto carbon tape, which was stuck to
an aluminium stub, allowed to dry and coated (3 times) with a conductive coating of carbon using a
Cressington 208 turbo carbon coater. Preparation of similar samples for TEM examination consisted
of drop casting the same nanoparticle mixtures (except for the gold shell nanoparticles) onto gold
sputter coated formvar copper grids and dried in an oven (65°C) prior to use. In the case of the gold
shell nanoparticles, polyethylene glycol (PEG) was also added to the vial.
3.3 Results and Discussions
3.3.1 Nanoparticles sizes
Monodisperse silica nanoparticles were successfully synthesised with a modified Stöber process
combined with the use of an ultrasonic bath method[41] to demonstrate how the size of the formed
nanoparticles varied by alteration of the volume of added concentrated ammonia solution, as seen in
Figure 8.
28
28 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Figure 8. SEM (top) and TEM (bottom) micrographs of silica NPs made from TEOS with varying
amounts of NH3 (28% v/v) (A) 2ml NH3, (B) 5ml NH3, (C) 10ml NH3 (D) 14ml NH3.
From examination of the TEM micrographs it can be seen in Figures 8A and 8B, the silica
nanoparticles were different sizes but spherical. In Figures 8C and 8D the silica nanoparticles were
very monodisperse and essentially spherical. In all cases it was noted that twinned nanoparticles
occasionally formed.
Nanoparticle size distributions obtained by direct measurement were extracted from the micrographs
as shown in Figure 8 and collated in Figure 9. These distributions strongly indicated that the molar
ratio range of ammonia to TEOS strongly influenced the silica NP monodispersity and the NPs size.
Figure 9. Bar graphs of silica NP diameter. (A) 65nm, S.D. 8nm, (B) 170nm, S.D. 12nm (C) 430nm,
S. D. 15nm (D) 430nm, S.D. 17nm.
29
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 29
As in Figure 8, Figures 9A, 9B, 9C and 9D confirm there is evidence for monodispersity being
achieved in all cases for the different added amounts of ammonia compared to TEOS. It is worth
noting in Figures 8C and 8D and Figures 9C and 9D that the silica nanoparticles are of a very similar
size. It therefore appears that there is a maximum amount of ammonia required for the formation of
the large size of nanoparticles for the specific amount of TEOS used.
3.3.2 Morphology
From Figure 10 it is readily seen how the silica nanoparticles were uniformly coated with a layer of
gold. Direct physical measurement extracted from the TEM micrographs as seen in Figure 10, gave
the gold coating thickness on the silica NPs as shown in Figure 11.
Figure 10. TEM images of gold-coated silica NPs, where 5ml NH3 was added but with varying
molar ratios of AuCl:TEOS. (A) 1.0:1, (B) 1.5:1, (C), 2:1 (2.5:1 coalesced so not shown).
In Figures 10A, 10B and 10C a lighter region can be distinguished between the darker inner, silica
sphere and the outer dark layer. Since the TEM micrographs were obtained on a gold coated formvar
copper grid, it was necessary to add PEG to enable a usuable contrast to be obtained. Analysis of the
light region gives shell thicknesses, respectively in Figures 10A, 10B and 10C, of approximately
2.4nm, 4.7nm and 7.4nm. This increase in shell thickness observed was consistent with the increased
amount of AuCl added, although a direct linear increase was not evident.
30
30 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Figure 11. Bar graph of size measurements of the gold coated silica nanoparticles obtained by
analysis of TEM images. (A) 200nm, S.D. 25nm, (B) 200nm, S.D. 13nm, (C) 200nm, S.D. 33nm.
The pure silica nanoparticles used for the core of the gold shell nanoparticles and the Au@SiO2
nanoparticle samples could not be directly compared to each other in the same measurement to
ascertain the difference in sizes, and hence shell thicknesses. However, the Au@SiO2 nanoparticles
in Figures 10A, 10B and 10C had a significantly larger diameter than the silica nanoparticle
precursor shown in Figure 8B. Diameter distributions are given in Figure 11.
Depending on the molar ratio of AuCl to TEOS we were able to readily generate gold layers of
approximate thicknesses of 2.4nm, 4.7nm and 7.4nm on the exterior of 170nm silica nanoparticles as
shown in Figures 10A, 10B and 10C respectively. It should be noted that when a ratio of
AuCl:TEOS greater than 2.5:1 was used in the synthesis, there was a strong tendency for the
Au@SiO2 NPs to coagulate and precipitate out of solution. This coagulation phenomenon requires
further investigation to determine the process responsible. It should also be noted that since these
Au@SiO2 NPs were not stabilized by a capping agent, this tendency to coalesce was exacerbated
compared to core-shell systems formed using organic linker molecules. However, within the confines
of the molar ratios of AuCl to TEOS described above, for 170nm diameter silica nanoparticle cores,
the Au@SiO2 core-shell nanoparticles could be maintained as a stable colloid at room temperature
for up to one day after which, they settled out of solution. A quick shake of the vial would disperse
the colloid back into suspension, although for a few hours only.
The TEM results in Figure 10 are similar to results obtained in attempts to synthesise very uniform
Au@SiO2 NPs using the traditional linker molecule and gold colloid approach[48]. Our results
demonstrate that there might be few benefits to be gained by using organic linker molecules, since
their Au@SiO2 NP synthetic strategy also yielded NPs that readily agglomerated.
31
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 31
3.3.3 Spectroscopy
A typical silica nanoparticle sample’s UV-Vis spectrum is given in Figure 12A
which was similar to spectra observed in a previous study where both palladium
shells around a silica core and palladium around a gold core were synthesised[49].
The UV-Vis absorbance spectra obtained was the same for all types of silica
nanoparticles prepared.
Figure 12. (A) UV-Vis of 430nm silica NPs, (B) SEM of 430nm silica NPs, scale
bar 1.0µm, (C) UV-Vis of gold coated 430nm silica NPs with ratio of TEOS:AuCl of
1:2.5, (D) SEM of gold coated 430nm silica NPs, scale bar 1.0µm, (E) EDX of the
gold coated 430nm silica NPs, (F) Diffraction image of gold coated 430nm silica
NPs. The 430nm silica NPs were made using 14ml added ammonia.
Upon formation of the gold shell, the NPs UV-Vis spectra incorporated a classic gold
plasmon resonance peak at 560nm, which was considerably higher than the
Au@SiO2 NP plasmon peak reported previously using the alternative linker molecule
and gold colloid method[42]. It is noteworthy that the silica cores used in our
32
32 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
experiment are more than double the diameter of the silica cores used in the earlier
gold colloid method, and the as-formed Au@SiO2 nanoparticles possessed
considerably improved monodispersity. This is also reflected in the narrower and
very distinctive gold plasmon resonance peak observed in the UV-Vis spectrum in
Figure 12C. Unfortunately these very large Au@SiO2 nanoparticles tended to
agglomerate (Figure 6D), which might be expected for such a massive composite
nanoparticle.
In the EDX spectrum, taken on the same batch and area as seen in Figure 6D, it can
be seen that silicon, oxygen and gold were all present. The structure of the particles
was reinforced by electron diffraction results from the Au@SiO2 nanoparticles which
demonstrated a characteristic polycrystalline pattern and confirmed the presence of a
gold crystal structure that possessed no formal orientation. This result is expected for
a complete, spherical gold metallic shell.
3.3.4 1H NMR
The gold complex was readily made by simply dissolving AuCl directly into MeCN.
This generated a positively charged co-ordination complex as can be seen by the
positive mass ion in the mass spectra results of Figure 13B where the mass to charge
ratio matched the expected positive metal co-ordination complex [Au(MeCN)2]+.
33
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 33
Figure 13. (A) 1H NMR of the gold complex in CDCl3, (B) Mass spectra of the gold
complex, M+ = 279.0184 m/z, (C)
1H NMR spectra of silica NPs in CDCl3, (D)
1H
NMR spectra of gold silica complex NPs. (* is residual water)
A 1H NMR was taken of the complex, since it is known that gold nanoparticles can
generate significant upfield shifts in adsorbed molecules depending on the proximity
of the protons to the gold[50]. Very significant upfield shifts in the 1H NMR of the α
CH2 of dodecanethiol adsorbed onto the gold surface were observed in this previous
study, along with line broadening of the spectra.
The expected 1H NMR upfield shift occurred in Figure 13A, where the MeCN peak
was also present, appearing to emanate from a weak ligand exchange with the excess
CDCl3. This indicated that the uncharged acetonitrile ligand was easily displaced.
The chemical shift of un-coordinated MeCN occurred at the expected 2.01ppm
chemical shift, whereas the attached MeCN ligand experienced a significant upfield
shift to 1.57ppm. Other peaks observed in Figure 13A were related to the original
AuCl compound. Some water was also present at 2.67 ppm.
In Figure 13C the presence of the siloxy group signal occurred at 6.45ppm along
with residual TEOS peaks at 3.72ppm and 1.25ppm. In Figure 13D, where the gold
complex was added, the siloxy peak was no longer present, and was replaced by a
peak at 1.60ppm, which shifted slightly downfield from the peak observed in Figure
7A (1.57ppm). It is suspected this slight downfield shift is a result of coordination by
34
34 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
an inner sphere mechanism. This is consistent with the formation of a charged siloxy
group via the protonation of the excess MeCN that was originally added to the silica
NPs. This close proximity of Au+ and SiO
- is allowed for in this mechanism. This is
consistent with some deshielding[51] being provided to the attached MeCN ligand
via the covalently attached anionic siloxy group and gold(I).
3.3.5 Proposed Mechanism
The proposed Au@SiO2 NP formation mechanism is depicted schematically in
Scheme 1. It begins with the addition of MeCN to the silica nanoparticles, thereby
initiating deprotonation of the siloxy groups on the silica nanoparticle surface. The
charged anion is then able to displace one of the MeCN ligands from the added bis
acetonitrile gold(I) complex.
Scheme 1. Schematic of the likely sequence of surface reactions occurring at the
surface of the silica nanoparticles, in solution and in the electrical double layer.
Scheme 1 covers the initial addition of MeCN to the silica NPs. The addition of
[Au(MeCN)2]+
in MeCN and finally, the addition of ascorbic acid in order to form
the gold shell using in-situ Au(III).
While the solution is non-aqueous, the transfer of an electron from the surface anion
occurs, allowing the inner sphere mechanism to proceed, reducing the Au+ to Au
0
while leaving it coordinated to MeCN.
It is expected that the addition of water will result in the positive charged MeCN
being deprotonated, forming a trace amount of acid. The water can also displace co-
ordinated MeCN on the surface of the reduced gold and with the gold complex in
solution forming a possible intermediate, being bis aqua gold(I).
35
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 35
The bis aqua gold(I) complex is now free to disproportionate into Au0
and Au3+
. The
formed Au0
binds to the surface co-ordinated Au0 forming a thin shell of metallic
gold which acts as a seed surface. This seed surface allows for the well known
deposition of Au0
resulting from the reduction of Au3+
by ASC.
3.4 Conclusion
A new synthetic method for production of size-monodisperse silica and gold-coated
silica nanoparticles were comprehensively analysed using both TEM and SEM to
determine the structure, dimensions and the layer thickness of the Au@SiO2 NPs. All
indications are that a thin, uniform gold shell formed on the exterior of the silica
NPs. This was confirmed by EDAX results which indicated the gold shell was
present, along with the expected silicon and oxygen peaks. Electron diffraction
results indicated the gold shell was polycrystalline and non-directional in orientation.
The bare silica nanoparticles and the gold-shell silica nanoparticles were also
examined by UV-Vis spectroscopy, with the bare silica exhibiting a spectrum that
decreased in intensity with an increase in the incident irradiation wavelength. The
gold-shell silica nanoparticles produced a UV-Vis spectrum that is normally
associated with small size, monodisperse gold nanoparticles such as those
traditionally made by the Turkevich method. This demonstrated the utility of the
method as an alternative to the prior linker-molecule gold-colloid seed-shell growth
method.
1H NMR spectroscopy of the silica nanoparticles indicated that a large number of
siloxy functional surface groups initially present were removed by the addition of the
bis acetonitrile gold(I) complex. This result is consistent with a surface co-ordination
complex being synthesised. The bis acetonitrile gold(I) complex was identified from
the positive ion mass spectra and 1H NMR spectra, where the latter exhibited a
distinctive upfield shift typical of nearby deshielding by gold(I). Upon addition of the
bis acetonitrile gold(I) complex to the silica NPs, this shift moved downfield. This
indicated that gold(I) was covalently linked, rather than co-ordinated to the anionic
siloxy functional group on the silica nanoparticle.
36
36 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
A plausible mechanism for the formation of the stable, uncapped Au@SiO2 core-
shell nanoparticles was proposed. This was consistent with the 1H NMR and mass
spectroscopy studies, revealing that a bis acetonitrile gold(I) complex was produced
by the simple addition of solid AuCl to MeCN. It was also shown that upon the
addition of the bis acetonitrile gold(I) complex to silica NPs in MeCN, the complex
could successfully ligand exchange with a silica nanoparticle surface as evidenced by
the disappearance of the hydroxyl groups (Figure 6D). The slight downfield shift
from the bis acetonitrile gold(I) complex, when attached to the silica surface, can be
attributed to the formation of a covalent coupling between gold(I) and the silica
surface siloxy anion. This observation suggests the mechanism of attachment
proceeds by a surface inner sphere electron transfer mechanism.
With further attention to the development of the synthetic protocol, along with the
additional determination of the mechanism of nanoparticle formation, a useful
method has been developed to produce a core-shell silica-gold nanoparticle that is
potentially useful for a wide range of applications.
The concept of treating the surface of a silica nanoparticle as part of a surface
coordination complex, while using an inner sphere mechanism to directly deposit
gold metal has been partially validated by this study.
37
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 37
Chapter 4: ZnO NPs Synthesised Using
ZnCl2 and Gold Coated by use of
KCl and AuCl
4.1 Introduction
This chapter is devoted to an as yet to be submitted paper, which consists of a
method of synthesising zinc nanoparticles and also a method for gold coating them.
The method is based on the “surface inner sphere mechanism” and corresponds
heavily on the previous chapter where silica was used as the core material. The
results show that the proposed surface inner sphere electron transfer mechanism’s
versatility cab be applied to gold coating of zinc oxide nanoparticles.
4.1.1 Applications of Au@ZnO NPs
The potential applications for Au@ZnO NPs, whether these ZnO NPs are fully
coated with gold or decorated with gold colloids are considerable. For example,
gold-shell (i.e. coated) ZnO NPs have been used to investigate the charge transfer
process from a ZnO core to the gold shell using Rhodamine-6G[52, 53]. Thiophenol
SERS spectra have also been examined using gold shell coated ZnO NPs[8]. One
potential catalysis application is methanol synthesis from CO, CO2 and H2 using ZnO
NPs decorated with gold colloids[54]. Another potential catalysis application is
oxidation of CO in the presence of oxygen[55] using gold colloid decorated ZnO
NPs. Dye-sensitised ZnO nanoflowers[56] decorated with gold colloids have even
been examined in a photovoltaic device configuration, where results indicate
considerable efficiency gains arise in such heterostructured materials. In hydrogen
fuel cells, ZnO NPs decorated with gold colloids and multi walled carbon
nanotubes[57] are possible alternatives to platinum catalysts. Au@ZnO NPs have
potential uses in nano-medicine where they have been employed in the identification
of a carbohydrate antigen tumor marker found in breast cancer[58] and also as a
glucose sensor[59]. An analogue of Au@ZnO, being Au@SiO2, was recently
investigated for the thermal destruction of cancer[60].
38
38 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Current chemical methods of gold decoration or shell formation on ZnO NPs occur
through the erratic decoration of gold colloids of various morphological ZnO NPs
types often followed by uneven gold shell formation. These methods use HAuCl4
either directly, or via the formation of gold colloids. These procedures can be
separated into 3 classes such as: 1) the reduction of gold(III) onto ZnO NPs, 2) direct
deposition of preformed gold colloids onto ZnO NPs or 3) the use of a linker
molecule to electrostatically attach gold colloids.
Chemical synthesis of gold NPs onto ZnO NPs[61] has been achieved by initially
placing a ZnO thin film coated glass slide into a mixture of HAuCl4, isopropanol and
hydrochloric acid. The treated slide was then calcinated, forming gold colloids
interspersed with the ZnO NP film. Gold colloids directly deposited onto ZnO
NPs[62] was achieved through the immersion of ZnO nanorods into a solution of
gold colloids prepared by the Turkevich method[32]. Another variation of gold
colloid decoration on ZnO NPs was prepared by the addition of ZnO NPs to gold
colloids formed by the addition of NaBH4 to a solution of HAuCl4 and polyvinyl
alcohol[54], which resulted in irregularly scattered gold NPs across the ZnO surface.
A SERS study by Yang et al[6] recently used gold colloids electrostatically bonded
to a glass slide that had been pre-treated by placing it in an aqueous
poly(diallyldimethylammonium) chloride. The bound gold colloid film was then
placed in an aqueous solution of zinc nitrate and hexamethylenetetramine at 90°C.
This generated a Au/ZnO film, which was then immersed into an aqueous mixture of
p-aminothiophenol (PATP) with the gold colloids giving an enhanced SERS
response from the PATP. It was suggested that a charge transfer mechanism[24]
between the ZnO and final gold layer was responsible for the enhanced SERS
response[6]. Since SERS response relies on plasmon resonance generating an intense
electromagnetic field on the surface of a gold NP[28], as long as the gold shell is able
to accept electrons from the ZnO NPs’ core, it was proposed this charge transfer
process resulted in a more intense plasmon resonance peak in Au@ZnO NPs and
hence an increased SERS response.
39
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 39
4.1.2 Synthesis of Au@ZnO NPs
The charge transfer effect from the ZnO core to the gold outer shell[6] could be
exploited by taking advantage of a well known but under-utilised co-ordination
chemistry effect known as the inner sphere electron transfer mechanism described
by Taube[16]. This mechanism involved the reduction of [CoCl(NH3)5]2+
using Cr2+
in HClO4. The Cl-
that was co-ordinately bonded to the Co3+
also formed a
coordinate bond to the Cr2+
that generated the intermediate co-ordinate complex
[Co(NH3)5(μ-Cl)Cr(H2O)5]4+
in the process. The chloride anion served as an electron
bridge between the Co3+
and Cr2+
, reducing Co3+
to Co2+
while oxidising Cr2+
to Cr3+
.
This mechanism was later generalised and extended by Ruff [17], indicating an
electron donor and electron acceptor is required for this reaction to occur.
Gold cation reduction by an inner sphere electron transfer mechanism was recently
applied to silica nanoparticle surfaces by English and Waclawik[40], whereby gold
was directly deposited on silica nanoparticles surfaces. To achieve this reduction, the
silica NPs’ surface was first deprotonated in acetonitrile (MeCN). Gold(I) dissolved
in MeCN was then added, which formed an initial gold seed shell. This shell then
allowed the further deposition of gold via the reduction of disproportionated gold(I)
in aqueous solution, which results in a complete and uniform gold shell.
Applying this information to the problem of gold coating on ZnO NPs, suggests a
chloride bridging ligand and a reduceable form of gold is required. Since electron
flow from ZnO to gold has been shown possible by charge transfer[6], it may be
possible to reduce gold ions, such as Au+, directly onto ZnO NP surfaces via chloride
bridging ligands. A problem in designing such an approach is that Au+
disproportionates in water[19] forming gold(0) and gold(III),
precluding the initial
use of aqueous conditions. Fortunately, Goolsby and Sawyer[29] prepared a highly
MeCN soluble, linear, two co-ordinate gold(I) complex in the presence of acetonitrile
with added tetraethyl ammonium perchlorate by the partial electrochemical reduction
of HAuCl4. The bis acetonitrilegold(I) co-ordination complex formed, could be
duplicated by dissolving AuCl in MeCN, which functions as a co-ordinating ligand
to Au+
ions[40]. It is then possible to surface exchange an acetonitrile ligand from the
formed bis acetonitrilegold(I) co-ordinated complex with an adsorbed or co-
40
40 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
ordinated anionic chloride ligand on the ZnO surface. This forms the basis of a
complete gold shell around the ZnO NPs after sufficient gold build-up has been
achieved by additional synthetic steps. This gold shell may then provide a gold
plasmon resonance peak easily observed by UV-Vis spectroscopy. The possible
ligand exchange reaction between a ZnO NP with co-ordinated chloride ligands and
a bis acetonitrilegold(I) co-ordination complex, which leads to the initial reduction of
Au+
to gold(0) on the ZnO NP is shown below.
[(ZnO)nCl-m] + m[Au(MeCN)2]
+ → [(ZnO)n(Cl
-Au
+(MeCN))m] + mMeCN ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 1
Where [(ZnO)nCl-m] refers to the ZnO NP with adsorbed or surface co-ordinated
chloride ligands, [Au(MeCN)2]+
refers to the co-ordinated gold(I) complex in
acetonitrile, [(ZnO)n(Cl-Au
+(MeCN))m] refers to the formed surface co-ordinated
complex between bis acetonitrilegold(I) and the halogenated ZnO NP, and MeCN
refers to the surface exchanged acetonitrile ligand.
Another issue relating to ZnO NP surfaces is the presence of a green emission
peak[63] in the photoluminescence spectrum, which has been attributed to the
presence of oxygen vacancies or crystal defects[64]. The presence of a plasmon
resonance peak upon complete gold coating of ZnO NP surfaces should also be
expected to be accompanied by a decrease or even complete absence of the ZnO
surface oxygen-defect emission. Equally, the presence of adsorbed chloride ions in
combination with the presence of a fully formed gold shell on ZnO NP surfaces
should lead to elimination of the green emission peak. This is due to the electronic
transition responsible for green light emission[65] being unavailable if the electrons
responsible are conducted away via the chloride bridging ligand to the formed gold
shell.
The disproportionation of Au+
when co-ordinated with MeCN to form gold(0) and
gold(III), which occurs after the addition of water[19], can also be taken advantage
of. The aurophilic attraction[20] between atoms of gold(0) can assist in bonding the
gold(0) produced in solution to already-reduced gold(0) on the ZnO NP surface. This
may form a “seed surface” for further gold deposition. Any remaining gold(III) in
solution can readily be reduced by using aqueous ascorbic acid (ASC)[35]. This
41
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 41
method, modified by Hiramatsu and Osterloh[42] was used to decorate silica
nanoparticles with gold colloids prepared by the Turkevich method[32], which were
linked to the silica core by (3-aminopropyl)-triethoxysilane.
The scheme used to gold coat ZnO NPs (shown below) is an adaptation of a method
we devised in order to prepare Au@SiO2 core-shell NPs[40], making use of a
modified ZnO NP synthesis method[66]. The chemically synthesised ZnO NPs were
added to ethanolic KCl. AuCl was dissolved in MeCN was then added followed by
H2O and aqueous ASC synthesising the Au@ZnO NPs.
Scheme 2. ZnO NP synthesis and gold coating scheme.
4.2 Experimental
All chemicals were purchased from Sigma-Aldrich Australia unless otherwise
stated. AuCl (99.9%), KCl (99%), Na2O (80%), ascorbic acid, reagent grade, Chem
Supply undenatured EtOH, 95%, Univar KOH pellets (85%) and Merck MeCN
(HPLC grade, 99.9%). Commercial ZnO NPs (99.8% 10 – 30nm) were purchased
from SkySpring Nanomaterials.
Na2O (0.04g) was added to absolute EtOH (20.0ml) in an aqua regia cleaned flask
and sonicated till dissolved. ZnCl2 was dissolved in absolute EtOH (20.0ml) by
sonication and added to the flask. KOH (≈ 0.1g, 1 pellet) was added to absolute
EtOH (20.0ml), ground with a glass rod and sonicated till dissolved. This ethanolic
KOH mixture was added to the flask and allowed to reflux for about 4 hours with
stirring.
42
42 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
The flask was removed from the heat source, allowed to cool and settle for about 30
minutes after which the supernatant was removed via a pipette. KCl (0.04g) was
partially dissolved in absolute EtOH (80.0ml) by sonication for about 15 minutes and
then was added to the flask with any adhering KCl washed in by absolute EtOH.
Sonication was carried out for approximately 15 minutes prior to further treatments.
Commercial ZnO NPs (0.04g) were added to flask, followed by the addition of
absolute EtOH (40.0ml). Sonication was required for about 15 minutes to disperse
the nanoparticles. Potassium chloride (0.04g) was partially dissolved in absolute
ethanol (40.0ml) by sonication for about 15 minutes and was then added to the flask,
with any adhering KCl washed in by additional absolute EtOH with sonication for
approximately 15 minutes prior to further treatments.
AuCl (0.04g) was added to MeCN (40.0ml) and sonicated till dissolved. The mixture
was quickly added to the sonicating nanoparticles whether synthesised or
commercial ZnO. This mixture was sonicated for about 15 minutes prior to the
addition of deionised water (10.0ml). The mixture was sonicated for an another 15
minutes before transference to a stirring plate.
Ascorbic acid (0.04g) was added to deionised water and stirred till dissolved. This
mixture was added slowly dropwise to the flask till complete and left to settle
overnight for XRD and EDX characterisation or withdrawn immediately and diluted
as required for UV-Vis and florescence spectroscopy characterisation processes.
The sonicator used was a Branson Model 1510 in combination with an Eppendorf
model 5424 centrifuge and standard 2.0 ml plastic vials as required for washing the
prepared nanoparticles. Typical centrifuge setting was 2 minutes @ 14,000rpm.
UV-Vis spectroscopy was conducted on a dual beam Cary 100 using absolute EtOH
as a blank. Florescence experiments were conducted using a 325nm excitation laser
on a Cary Eclipse florescence spectrometer. TEM imaging was conducted using a
Philips CM200 TEM set at 200kV using photographic film subsequently processed
to a positive image using an Epson scanner. EDAX elemental analysis was
conducted using FEI Quanta 3D Focused Ion Beam SEM. A PANanalytical XPert
43
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 43
Pro Multi Purpose Diffractometer was used for obtaining powder XRD crystallinity
patterns.
4.3 Results and discussion
Figure 14. TEM images of ZnO and Au@ZnO NPs, (A) Commercial ZnO NPs with
added KCl. (B) Commercial gold treated ZnO NPs. (C) Chemically synthesised ZnO
NPs with KCl. (D) Chemically synthesised gold treated ZnO NPs.
The TEM results presented in Figure 14 indicate that the quality of the gold coating
of ZnO nanoparticles achieved through chloride salt treatment, followed by Au(I)
complex seeding and subsequent reactions is strongly influenced by the surface
chemistry of the ZnO NP cores. The same gold-shell coating procedure was applied
to a commercially sourced ZnO sample (Figure 14A), as to ZnO NPs synthesised
from the ZnCl2 starting material described by Reaction Scheme 1 (Figure 14C). The
gold coating of the commercial ZnO NP cores (Figure 14B) appeared incomplete.
Large differences in contrast occurred between ZnO crystals and the solid Au in
TEM, so an effective coating with gold was expected to lead to far darker-contrast
and possibly slightly larger nanoparticles in Figure 14B compared to Figure 14A. In
fact, only minimal gold coverage of the commercial ZnO NPs was evident in Figure
14B which would be consistent with a lack of chloride adsorption (and hence less
44
44 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
gold coating). In both the commercial ZnO and synthesised ZnO NPs, it was
observed that after addition of ethanolic KCl, their ease of dispersal via sonication
was significantly enhanced indicating that adsorption of chloride could assist
generation of a stable sol. However, the commercial ZnO NPs after addition of
ethanolic KCl and sonication took considerably longer to disperse than those
synthesised in Scheme 2.
In contrast to the commercially sourced ZnO NPs, complete and uniform gold
coverage of the synthesised ZnO NPs was evident, which can be seen in Figure 14D.
The nanoparticles in this figure appear to be faceted, hexagonal shaped nano-crystals,
typical of nano-crystalline gold. There also appeared to be no evidence of any
unreacted or even partially-coated ZnO NPs in this sample based on the consistent,
dark contrast that the nanoparticle samples possessed. This result is similar to that
seen by Li et al[67] who used a Fe2O3 NP core and synthesised very similar shaped
gold nano-crystals while still preserving the underlying Fe2O3 NP core. The
differences noticed between the samples at a microscopic level in TEM were obvious
at a macroscopic level during solution preparation and gold coating. During the
dropwise addition of aqueous ASC solution, the colour of the synthesised Au@ZnO
NPs in Figure 14D changed from a cloudy mid-yellow colour to a clear, very deep
purple and then finally to a clear, dark purple-brown colour. These overall colour
changes are very familiar and characteristic of gold colloid and sol formation.
The synthesised Au@ZnO NPs were allowed to stand overnight, during which they
coalesced, and enabled the collection of the opaque, dark purple to black product.
Redispersion was easily achieved in EtOH by sonication for a few minutes, which
regenerated the original clear, dark purple-brown colour. During the attempted gold
coating of the commercial ZnO (as seen in Figure 14B), the solution underwent
similar colour changes to that seen with the synthesised ZnO NPs (shown in Figure
14D), with one notable and significant difference. The final colour of the solution
was a cloudy light purple. After centrifugation of the partially gold coated
commercial ZnO NPs, a considerable amount of white ZnO was evident in the pellet
that was extracted and only a small portion of opaque light to dark purple powder
was evident. This mixture of uncoated ZnO NPs and partially coated ZnO NPs
undoubtedly gave rise to the cloudy purple colour. Centrifugation of the Au@ZnO
45
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 45
NPs in Figure 14D resulted in an opaque purple to black coloured pellet with no
evidence of any uncoated ZnO.
Figure 15. UV-Vis and fluorescence intensity spectroscopic results of ZnO and
Au@ZnO NPs, (A) UV-Vis spectra of commercial and chemically synthesised ZnO
NPs (B) UV-Vis spectra of gold treated commercial ZnO NPs and gold coated
chemically synthesised ZnO NPs (C) Fluorescence intensity spectra of commercial
ZnO NPs and chemically synthesised ZnO NPs. (D) Fluorescence intensity spectra of
commercial gold treated ZnO NPs and chemically synthesised gold coated ZnO NPs.
(E) UV-Vis spectra at each stage of gold coating chemically synthesised ZnO NPs.
(F) Fluorescence intensity spectra at each stage of gold coating chemically
synthesised ZnO NPs.
Consistent with the direct TEM observations, when the samples were examined by
UV-Vis spectroscopy, no readily discernable gold plasmon peak appeared above the
ZnO absorption in the optical absorption spectrum for the gold-treated commercial
ZnO NPs (Figure 15B). This observation is similar to the results obtained by Wang et
al[68], who produced ZnO-Au composite NPs by a conventional reduction of Au(III)
on ZnO NP surfaces, without the use of amino-silane type linkers. Between 400 –
800nm the optical absorption spectra of their ZnO-Au composites possessed a weak
and broadened gold plasmon resonance peak. When our results for the attempted
gold coating of commercial ZnO in Figure 15B are compared to the optical
46
46 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
absorption spectrum of gold coated ZnO NPs synthesised from ZnCl2 in Figure 15B,
the distinctive gold plasmon resonance peak, centred at 570nm, is notable for its
high-intensity of absorbance. It is well known that the surface plasmon resonance
peak (λspr) of gold nanoparticles in water red-shift with increased NP size[22]. The
position of the plasmon band of these gold-coated ZnO NPs were red-shifted by
approximately 40nm further than might be expected for similar sized (≈ 50nm) gold
NPs. Although solvent dielectric and other factors can have a strong influence on
λspr[69, 70], Wang et al[68], rationalised a similar observation in their Au@ZnO
nano-composite materials, in terms of peak position, as being due to the electron
charge transfer[71] from gold to ZnO. The transferred electrons accumulate at the
ZnO side of the Au-ZnO interface causing an induced electron deficiency on the Au
surface, which is thought to be the cause of broadening and red-shifting of the
surface plasmon band. This is consistent with several previous reports[69, 72, 73].
There is no significant green emission from the commercial ZnO NPs in figure 15C.
This could be related to the absence of crystal defects that prevent significant
adsorbtion of oxygen[64]. In comparison, the chemically synthesised ZnO NPs in
Figure 15D, exhibits significant green emission related to the presence of oxygen
vacancies and hence the presence of crystal defects[64]. Since this green emission is
related to the number of oxygen vacancies, it indicates significant chloride
adsorption is possible onto chemically synthesised ZnO NPs surfaces with limited
chloride adsorption on the commercial ZnO NPs. The presence of chloride is
required for the surface reduction of Au+,
as in the surface inner sphere
mechanism[40], and is consistent with the UV-Vis spectra in Figures 15B and 15E.
The fluorescence intensity in Figure 15F decreased significantly after the addition of
AuCl dissolved in MeCN. This is similar to observations by Wang et al[71], where
the addition of gold colloids to the ZnO surface decreased the green emission
intensity. Following addition of aqueous ASC, significant suppression of green
emission was effected, which indicated complete gold shells were formed over the
ZnO NPs’ cores. This observation corresponds to TEM images of the fully gold
coated, chemically synthesised, ZnO NPs from Figure 14D. This reduction of the
ZnO green PL-emission indicates the oxygen vacancies, which formerly gave rise to
the green emission intensity, were occupied by chloride ions which acted as an
47
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 47
electron flow bridge to the gold shell and thus prevented the electronic transition
pathway[65] that gives rise to the green PL-emission.
Figure 16. EDX elemental analysis images of ZnO and Au@ZnO NPs,
(A) Commercial ZnO NPs. (B) Chemically synthesised ZnO NPs. (C) Commercial
gold treated ZnO NPs (D) Chemically synthesised Au@ZnO NPs.
Note: In (D) there is a Zn Kα sum peak normally corresponding to a P Kα peak[74].
Heavy gold deposition on the chemically synthesised ZnO NPs is evident in Figure
16D, which is consistent with the intense gold plasmon resonance peak observed in
Figures 15B and 15E.
In all ZnO and Au@ZnO samples, both zinc and oxygen were identified by EDX
elemental analysis (Figures 16A to 16D). Peaks due to gold were also obvious in
Figure 16C and 16D, except the amount was considerably less in the commercial
ZnO NPs. Worth noting, is the absence of chloride in the commercial ZnO NPs in
Figure 16A and the limited presence in Figure 16C. In all cases, KCl was added in
excess to the commercial and chemically synthesised ZnO NPs. In Figure 16B,
48
48 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
chloride is readily apparent in the chemically synthesised ZnO. This indicates
chloride adsorption or surface co-ordination to a ZnO NP surface occurs when the
surface possesses oxygen vacancies. This is are signified by the presence of a green
emission band emission band seen in Figures 15D and 15F in the chemically
synthesised ZnO NPs.
Figure 17. XRD images of ZnO and Au@ZnO NPs, (A) Commercial ZnO NPs.
(B) Chemically synthesised ZnO NPs. (C) Commercial gold treated ZnO NPs (D)
Chemically synthesised Au@ZnO NPs. In all cases the ZnO NPs and Au@ZnO NPs
were examined as a thin film on silicon slides.
The chemically synthesised Au@ZnO NPs (Figure 17D) formed from the
synthesised ZnO NPs exhibited a classic gold nano-crystalline pattern [75]. Leff et
al[75] when investigating the XRD patterns of dodecanethiol capped gold
nanoparticles using the well known Brust method[76] reported a similar result. The
nano-crystalline pattern obtained here was very similar to Leff et al [75]giving the
same 2θ crystal parameters being, (111), (200), (220) and (311) respectively. Li et
al[67] coated Fe2O3 NPs and found a uniform and complete layer of gold that formed
very similar hexagonal shaped nano-composite NPs similar to those in this
49
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 49
investigation. In both studies, there is little evidence when using XRD of the
underlying NP core after the gold coating. The XRD crystal patterns (Figures 17A
and 17B) of the commercial and synthesised ZnO NPs were exhibited prior to the
gold treatment. After gold treatment, the commercial gold coated ZnO NPs still
exhibited a typical crystalline ZnO pattern (Figure 17C), along with nano-crystalline
gold peaks at (111), (200), (220) and (311)[75]. The ZnO NPs have a typical
Wurtzite ZnO type crystal similar to those reported by Wang et al [68].
The full coating of the ZnO NPs was only able to be obtained for the chemically
synthesised ZnO NPs, which corresponds to the previous TEM results (Figure 14D).
Further evidence of a full coating was observed in the plasmon resonance peaks in
Figures 15B and 15E, along with the absence of green emission in Figures 15D and
15E. The presence of chloride along with gold in Figure 16C, also indicates the need
for the presence of chloride in the synthesis process to give a complete gold coating.
The presence of ZnO nano-crystalline XRD peak in the commercial gold treated ZnO
samples indicates that not all of the ZnO NPs were gold coated. This is in line with
TEM observations in Figure 14B and the absence of a plasmon resonance peak in
Figures 14B and 14D. The initial absence of a green emission peak as seen in Figure
15C which indicates few oxygen vacancies are present, appears to coincide with a
minimal amount of chloride adsorption on the ZnO surfaces (Figures 16A and 16C).
The presence of the underlying ZnO core is definitely observed in the XRD results
given in Figure 17C. This indicates that commercial ZnO NPs were unable to adsorb
sufficient chloride to be satisfactorily gold coated. Since ZnO and gold peaks are
both exhibited it must be concluded these commercial NPs were not fully gold
coated.
4.4 Conclusion
Uniform gold coated ZnO NPs were synthesised from chemically synthesised ZnO
NPs, which exhibited a green emission peak indicative of oxygen vacancies that were
subsequently occupied by absorbed chloride ions. On addition of dissolved AuCl in
acetonitrile the chloride ligand was responsible for electron flow from the ZnO core
through to the surface co-ordinated Au+ reducing it to gold(0). Addition of water
50
50 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
disproportionated the remaining bis acetonitrilegold(I) in solution to gold(0), which
added to the previously reduced Au+. The resultant gold(III)
remaining in solution
was reduced to gold(0) by the addition of ASC adding to the previously formed gold
seed surface.
Through application of an analogue of co-ordination chemistry,
specifically a surface inner sphere electron transfer mechanism, this investigation has
proved it possible to synthesise a new class of Au@ZnO NPs that may find use in a
wide range of applications.
51
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 51
Chapter 5: Gold Coated Zinc Oxide
Nanoparticles Synthesised using
ZnI2 and Gold(I) Chloride
5.1 Introduction
A new variation on the synthesis of ZnO NPs was tried using zinc iodide (anhydrous)
and sodium oxide. This does present a dilemma since iodide and iodine in aqueous
solution is rather excellent at dissolving gold[77]. However, some chloride could
remain from the addition of AuCl and should be able to act as a bridging ligand
allowing gold(I) to be surface-reduced.
The successful gold coating of zinc oxide nanoparticles using iodide, will provide
evidence that the proposed surface inner sphere electron transfer mechanism is
stronger than the dissolution of gold in iodide solutions. This dissolution reaction has
been studied by the gold mining industry[78], however, the results are still pertinent
here.
As discussed in Chapter 4, it was discovered that chloride could act as the bridging
ligand for the reduction of gold(I). Using iodide and gold(I) allowed the reduction of
gold(I) onto the ZnO NP core without the use of any additional reductants or water.
This was a most unexpected and encouraging result. It was expected iodide would be
adsorbed or incorporated into the ZnO NP matrix as a side result of the synthesis
method. As seen in the EDAX results obtained in the following discussion, no iodine
was present.
The ability to enhance the optoelectronic properties of a metal oxide semiconductor,
such as ZnO, with plasmon enhancement via the addition of noble metals such as
silver, palladium and gold currently lies at the core of an increasing number of quite
disparate projects ranging from photo-catalysis of environmental contaminants[79]
52
52 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
through to plasmon-enhanced dye-sensitised solar cells (DSSCs)[80]. It is well
known that plasmon enhancement is brought about by a charge transfer effect in
these types of devices[24], typically via an attached molecule from the underlying
substrate to the surface of a NP. Since charge transfer effects are of particular
relevance to further development of DSSCs, catalysis and other related plasmonic
processes, a synthetic development that could engender an enhanced plasmon effect
in a composite by using direct gold deposition on ZnO NPs via an electron transfer
process would be a singularly useful advance.
In a chemical synthesis study, gold NPs interspersed with ZnO NPs[80] was
achieved by immersing a ZnO thin film into a mixture of HAuCl4, isopropanol and
HCl followed by calcination. Gold colloids synthesised by the Turkevich method[61]
can also be directly deposited onto ZnO nanorods[32] were directly deposited onto
ZnO nanorods[62]. Strunk et al[54] has shown that ZnO NPs decorated with gold
colloids may be prepared by adding ZnO NPs to gold NPs that have been generated
by addition of aqueous NaBH4 to a solution of HAuCl4 and polyvinyl alcohol. To
date, no suitable method combines surface reduction of gold onto suitably treated
ZnO NPs which yield plasmon-enhanced NP devices that relies on electron transfer
via a ligand bridge from an electronegative central nanoparticle core to a surface
metal. However, a promising new route was recently devised whereby gold was
directly reduced onto silica nanoparticles by surface reduction of gold(I) chloride[40]
This used the deprotonated silica nanoparticle surface to generate the surface electron
transfer mechanism, which reduced acetonitrile co-ordinated gold(I). To prove that
the gold coating of ZnO NPs can be achieved, the optical properties of the
component materials, Au and ZnO, were used to characterise the reaction. A well
known property of zinc oxide which could be used for this purpose is the green
oxygen vacancy defect-based photoluminescence emission peak of ZnO[63]. The
intensity of the green emission peak is sensitive to the presence of adsorbed oxygen
on a ZnO NP surface, also known as a ZnO defect peak[64].
Beside plasmonic-based enhancement, other pathways to improve dye-sensitised
solar cell efficiencies have used doping of the semiconductor metal oxides, primarily,
TiO2 and ZnO. For instance, studies of thin film DSCs constructed from iodine-
doped ZnO[81] (synthesised using zinc acetate and iodic acid), indicate that such
ZnO:I nano-crystalline aggregates may also provide a very effective photo-electrode
53
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 53
material. Surface bound oxygen on ZnO NP surfaces should allow iodide to react
with it as demonstrated by Hoffman et al[82]. A review published by Hoffman et
al[82], indicated that the presence of heat (or visible light) generates a reactive
oxygen species via the surface reduction of di-oxygen, which in turn produces a
super oxide that self reacts giving hydrogen peroxide. The presence of hydrogen
peroxide is then free to react with iodide and was investigated by Norman[83] who
indicated that only the O- form of chemisorbed oxygen reacts with iodide in solution.
Additionally, Morgan indicated in a review[84] that iodide and hydrogen peroxide
may form free oxygen, water, hydroxide and iodine. Iodine, being present in a basic
solution is then regenerated as iodide. This reaction has been studied in simulated
seawater (pH ≈ 8.0) by Wong and Zhang[85] who determined that the rate of H2O2
decomposition is directly proportional to the concentration of I-. Clearly,
regenerating I-
from formed I2 under basic conditions will keep the rate of H2O2
decomposition constant and hence prevent adsorption of oxygen into the ZnO NP
thereby partially eliminating the oxygen based green florescence emission or defect
peak. This should leave only the available unreacted oxygen to generate significant
change in the green emission or defect peak of ZnO. One additional factor to take
into account is the redox potential of I-
and gold(I), being +1.692v overall under
standard SHE conditions[86]. In a basic solution, any iodine formed from the
reduction of Au+
will be oxidised back to iodide. This iodide (I-) cannot be absorbed
easily into the ZnO matrix due to the well known considerable size differential
between itself and oxygen (O2-
)[87], (206pm vs. 126pm respectively), compared to
Cl- and Zn
2+, (167pm and 88pm respectively). Therefore, Cl
- is easily absorbed by
the ZnO surface simply due to the smaller size of Cl-, which is more comparable to
the size of O
2- rather than the much larger I
-.
Once gold(I) is reduced onto a suitably treated ZnO NP surface the green
photoluminescence emission peak should be eliminated. The absence of the green
emission peak is related to an electronic transition[65], whereby electrons are
transferred via a bridging ligand from the central electronegative NP core to the
surface reduced gold. This theoretical prediction is in line with the surface inner
sphere electron transfer mechanism[40]. The adsorbed chloride now acts as an
electron transfer bridge or bridging ligand to the gold shell. This electronic transition
54
54 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
may provide additional plasmonic interaction with incident visible light, which is
essential for DSSC, related catalysis or other plasmonic based applications.
5.2 Experimental
Na2O (80%), ZnI2 (anhydrous, 10 mesh, 99.999%) and AuCl (99.9%) were
purchased from Sigma-Aldrich Australia. Absolute EtOH (99.5%) was purchased
from Chem Supply. KOH (pellets, analytical reagent) were purchased from Selby-
Biolab and HPLC grade acetonitrile (MeCN 99.9%) was obtained from Lab Scan.
All chemicals were used as received, without further purification or modification.
Na2O (0.04g) was dissolved in absolute EtOH (20.0ml) by sonication. Anhydrous
ZnI2 was dissolved in absolute EtOH (20.0ml) using sonication. Both mixtures were
combined and stirred. Solid KOH (≈ 0.1g, one pellet) was dissolved in absolute
EtOH (20.0ml) by sonication and then added to the stirring mixture. Anhydrous KI
was dissolved in 20.0ml absolute EtOH by sonication and added. The mixture was
left to reflux for 4 hours.
After removing the heat, the white precipitate was left to settle out over several
hours. The clear, colourless supernatant was pipetted off. Absolute EtOH (50.0ml)
was added to the white precipitate and left to stir (≈ 5 minutes). AuCl (0.04g) was
then dissolved in spectroscopic grade MeCN (40.0ml) with the help of sonication.
This gold solution was slowly added dropwise to the KI treated mixture to give a
clear, dark purple solution which agglomerated after approximately 15 minutes. All
spectroscopic readings were taken within 5 minutes of addition of the gold(I)
solution.
UV-Vis was conducted on a dual beam Varian-Cary 100 instrument using absolute
EtOH as a blank. Florescence experiments were conducted for 325nm light
excitation with a Cary-Eclipse florescence spectrometer. TEM imaging was
conducted using a Philips CM200 TEM set at 200kV. Images were obtained using
photographic film subsequently processed to a positive image by an Epson scanner.
EDAX elemental analysis was conducted using FEI Quanta 3D Focused Ion Beam
55
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 55
SEM. A PANanalytical XPert Pro Multi Purpose Diffractometer was used for
obtaining powder XRD crystallinity patterns.
5.3 Results and discussion
In Figure 18A and Figure 18B the expected gold plasmon peak of the core-shell
composite appears as a broad absorption band centred at 540nm. There is also
evidence of aggregation of the plasmonic-nanoparticle systems at higher wavelengths
due to a very broad absorption at wavelengths greater than 650nm (Figure 18B).
Nanoparticle aggregation was noted throughout the experiments by a broad peak
above 650nm. The ZnO UV-Vis spectrum in 18B is typical of ZnO NPs. The
presence of ZnO is readily appreciated in this figure by the presence of an absorption
peak with a steep cut-off just below 380nm (the bulk ZnO optical band-gap
absorption cut-off), as might be expected for ZnO quantum dots. In addition, the
fluorescence spectrum of ZnO in Figure 18C possesses a green florescence peak at
561nm. It is worth noting since iodine was used in the synthesis of these ZnO NPs
this oxygen vacancy defect-related peak is considerably reduced from that which
might normally be expected in a ZnO NP chemical synthesis[64], since the iodine
acted as a physisorbed oxygen scavenger[82]. This dramatic reduction of the green
emission peak is seen in the enlarged Figure 18D. Another reason why this defect-
peak is of such low intensity could be due to the product possessing considerable
crystallinity[64]. This was confirmed by the sharp FWHM that was obtained by
powder XRD for these product fractions. Confirmation that the gold coating method
was effective for these Au@ZnO NPs was observed as a considerable reduction in
fluorescence intensity of the ZnO defect peak, followed by the complete elimination
of the fluorescence intensity at 561nm ( Figures 18C and 18D).
56
56 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Figure 18. (A) Expanded UV-Vis spectra of ZnO and Au@ZnO NPs in gold
plasmon region (B) UV-Vis spectra of ZnO and Au@ZnO NPs (C) Expanded
fluorescence intensity of ZnO and Au@ZnO NPs in the ZnO oxygen vacancy defect
region (D) Fluorescence intensity of ZnO and Au@ZnO NPs.
A series of experiments were performed in order to determine how the approximate
morphology of the synthesised ZnO nano-materials changed at each synthetic step.
Figure 19A shows the SEM image of ZnO NPs prepared under reflux without the
addition of KI. It is seen that these NPs are approximately spherical and of similar
size. The procedure was then repeated again, except a small molar equivalent amount
of KI was added, under the premise that this could be surface adsorbed onto the ZnO
NPs and subsequently act as a bridging surface ligand for the surface reduction of
gold(I)[40]. It is seen in Figure 19B, that the presence of the added KI generated a
much more crystalline ZnO NP product than in Figure 19A. The result is similar to
that of Wang et al[88] where ZnO NPs were synthesised from zinc metal and iodine
crystals in EtOH. This observation is also consistent with Figures 18C and 18D
where the presence of only a very small green emission peak indicated increased
nano-crystallinity.
57
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 57
Following incorporation of the gold (I) chloride step into the procedure, (Figure
19C), confirmed the presence of larger solid, black NPs attributed to the formation of
Au@ZnO. This result is consistent with the spectroscopic observations given in
Figures 18A and 18B, which showed large gold-coated nano-crystallites giving rise
to the plasmon peak at approximately 540nm. Nanoparticle aggregation is also
clearly seen in Figure 19C, which is also consistent with the spectroscopic results in
Figures 18A and 18B. It is well worth noting that no added capping agent was used.
Only the presence of surface-adsorbed iodine/iodide could be considered as a
“capping agent”. In Figure 19C, the surface of the ZnO NPs was coated with gold as
shown by the uniform black colour in the TEM micrographs. Worth noting is the
relatively uniform morphology of the gold surface as seen in the micrograph in
Figure 18C. Close inspection of the SEM micrograph of Figure 18A indicated that
the ZnO NPs appeared to undergo a secondary agglomeration process prior to the
gold coating of ZnO NPs. In Figure 19B, this secondary agglomeration process could
be discerned. Secondary agglomeration prior to the formation of the Au@ZnO NPs
by this method may warrant further investigation.
Figure 19. (A) As made ZnO NPs using ZnI2 and Na2O in EtOH without added KI
(B) As made ZnO NPs using ZnI2 and Na2O in EtOH with added KI (C) As made
Au@ZnO NPs using ZnI2 and Na2O in EtOH with added KI and AuCl dissolved in
MeCN
58
58 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Figure 20. EDAX results of the nanoparticles synthesised (A) ZnO as made using
ZnI2 without added KI. (B) ZnO as made using ZnI2 plus added KI (C) Au@ZnO
using KI as the reductant and AuCl as the gold source.
In Figure 20, an elemental analysis was conducted by EDAX. Upon incorporation of
iodine into the synthesis, it was expected that some of the iodine present should have
become trapped in the. However, this did not appear to occur (Figures 20A, 20B and
20C). The NPs were sonicated for one minute to disperse in EtOH and then allowed
to dry (≈ 50°C) on a piece of carbon tape on an aluminium stub prior to SEM and
EDAX analysis. In Figure 20A, ZnO NPs were made without added KI and resulted
in EDAX peaks assigned to Zn and O as expected. In Figure 20B, samples where KI
was added, gave a similar result to Figure 20A. What appeared to be missing was any
peak which could be ascribed to iodine. It would appear on the basis of this result
59
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 59
that iodine did not adsorb onto the ZnO NPs’ surfaces. In Figure 20C, the full
sequence of gold coating was carried out. The presence of Au, Zn and O peaks were
expected, however, a signature peak due to the presence of chloride was detected.
The only chloride used in the synthetic procedure was AuCl, dissolved in MeCN.
The absence of iodine in the EDAX spectra was surprising as it was expected to be
present. However, the lack of an I-peak in the EDAX spectrum could be rationalised
by comparing empirical size calculations of atomic radii in crystals as per
Shannon[87], whereby it would be considered unlikely for iodine/iodide to be
incorporated into the ZnO NPs due to size differential between O and I. Clearly the
reduction mechanism of gold(I) must have proceeded via the presence of chloride in
the last step of the synthesis and not via adsorption of iodide on the ZnO surface in
the second step of the synthesis. The observation of the presence of chloride and the
absence of iodide suggests a possible reaction sequence after the formation of the
surface oxygen free ZnO NPs and addition of the [Au(MeCN)2]+
mixture, which
contained the chloride ions:
(ZnO)n + mCl- → [(ZnO)nClm]
m- (1)
[(ZnO)nClm]m-
+ m[Au(MeCN)2]+
→ [(ZnO)n(ClAu(MeCN))m]+ mMeCN (2)
The question as to whether the reduction of surface co-ordinated Au0
occurs through
the surface inner sphere electron transfer mechanism[40] is not straightforward.
Chloride is clearly present in the gold shell coated ZnO as seen in the Au@ZnO NP
EDAX in Figure 20C. This chloride could only arise from added AuCl as it is not
present prior to addition of AuCl, as seen in Figures 20A and 20B. Additionally,
metallic gold is present as seen in the UV-Vis shown in Figures 18A and 18B. Nano-
crystalline, metallic gold is also shown to be present in Figure 21B completely
encapsulating the ZnO NPs. Also, gold is present in the EDAX spectra in Figure
20C. It can therefore be concluded that an intermediate gold(I) surface co-ordination
complex forms, which then allows gold(I) to be reduced to metallic gold. The close
proximity of gold atoms on the ZnO NPs surface allows complete encapsulation of
the ZnO NPs to occur via aurophilic effects of nano-gold metal[20]. This complete
encapsulation by gold then allows the previously formed surface co-ordination
complex to remain intact with the reduced gold, rather than separating as would be
60
60 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
expected if this reaction occurred between individual molecules without reference to
the ZnO NP surface.
After the gold “seed surface” is formed the iodide in solution would then be free to
adsorb onto the formed gold shell via the following proposed reaction:
[(ZnO)n(ClAu(MeCN))m] + mI- → [(ZnO)n(ClAuI )m]
m- + mMeCN (3)
This adsorbed I-
could be oxidised to elemental I by excess Au+
in the acetonitrile
solvent, forming Au0
and thereby producing more Au0
at the nanoparticle surface.
This would free both the elemental gold and iodine, allowing the elemental gold so-
formed to be attracted to the gold shell by aurophilicity[20, 89]. The free I could
then combine with available I-
to form I2-
and then disproportionate into I3-
and I-,
regenerating the I- catalyst as per Dobson and Grossweiner[90]. Their investigation
looked at the photolysis of iodide in ethanol which proceeds in visible light by a fast
second order decay process.
[(ZnO)n(ClAuI )m]m-
+m[Au(MeCN)2]m+
→ [(ZnO)n(AuClAuI(MeCN))m ]0
+mMeCN (4)
[(ZnO)n(Au2ClI(MeCN))m ]0 → [(ZnO)n(Au Cl)m ]
0 + mAu
0 + mI + mMeCN (5)
[(ZnO)n(Au Cl)m ]0 + mAu
0 → [(ZnO)n(Au2 Cl)m ]
0 (6)
I + I- → I2
- (7)
2I2- → I3
- + I
-
This process is then free to continue as iodide is regenerated until the reactants are
exhausted as per the continuing reaction below:
[(ZnO)n(Au2 Cl)m ]0+ mI
- → [(ZnO)n(Au2 ClI)m ]
m- (8)
[(ZnO)n(Au2 ClI)m ]m-
+m[Au(MeCN)2]+→[(ZnO)n((Au2ClAu I(MeCN))m]
0 +mMeCN (9)
[(ZnO)n((Au2ClAu I(MeCN))m ]0 → [(ZnO)n(Au2 Cl)m ]
0 + mAu
0 + mI + m(MeCN) (10)
[(ZnO)n(Au2 Cl)m ]0 + mAu
0 → [(ZnO)n(Au3 Cl)m ]
0 (11)
61
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 61
Considerably larger Au@ZnO NPs than the ZnO precursor NPs can form via a
second order conglomeration of the ZnO NPs as seen in Figure 19A. This secondary
gold coating process involves the formation of the Au@ZnO NPs by another process,
which is best described as a “surface outer sphere electron transfer mechanism”
that involves a redox reaction between gold(I) and adsorbed I-
without providing
access to the underlying chloride bridging ligand and ZnO NPs’ free electron
reservoir. The reduced gold is then able to absorb iodide from solution allowing
reduction of the gold and reformation to the surface by aurophilicity after
disintegration of the surface formed co-ordination complex.
Worth noting is that this reaction is conducted in a non-aqueous environment and so
the formation of an aqueous, almost insoluble AuI precipitate is avoided due to
MeCN being available as an uncharged ligand for the co-ordination of AuI giving a
probable [AuI(MeCN)]0
species. However, due to the large excess of MeCN, the
formed complex is shifted towards [Au(MeCN)2]0
which would account for the
presence of the iodide/iodide cycle. Iodine can be generated in the presence of the
proposed [Au(MeCN)2]0, species which is free to bind with existing reduced gold on
the Au@ZnO surface.
Figure 21. XRD spectra of (A) ZnO as made using ZnI2 and added KI (B) Au@ZnO
using KI as the reductant and AuCl as the gold source.
In Figure 21, synthesised ZnO NPs with added KI was compared with the Au@ZnO
NPs. It is immediately obvious from the XRD pattern in Figure 21A that nano-
crystalline ZnO was synthesised. The experimental method results in Wurzite peaks
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62 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
that are sharp and well defined. In Figure 21B the ZnO peaks all but disappeared, to
be replaced by the distinctive gold nano-crystalline peaks, which are also sharp and
well defined. This indicates the core-shell NPs were synthesised with a high level of
crystallinity in both the core and shell. Both these observations are consistent with
the minimal presence of the defect peak seen in Figures 18C and 18D. Finally these
observations of the XRD images of the ZnO and Au@ZnO nano-crystal structures
are consistent with the micrographs in Figures 19B and 19C.
5.4 Conclusion
A new method for the synthesis of monodisperse ZnO NPs with a high degree of
nano-crystallinity has been developed. This is observed by the disappearence of a
defect peak seen in the ZnO UV-Vis and photoluminescence spectra. High
crystallinity was confirmed using powder X-ray diffraction, where sharp, well
defined XRD peaks were prominent.
Additionally, a new synthetic method has been described for the complete gold
coating of ZnO NPs without the use of a reducing agent, heat or light by a simple
one pot process. The presence of this gold coating is evident by the growth of a
prominent plasmon peak in the absorption spectrum and the observation of complete
coverage of NPs with gold and uniform morphology using TEM. The signature
crystal structure of elemental gold was also observed in the case of the Au@ZnO
core-shell NPs.
Finally for both the ZnO NPs and Au@ZnO NPs the presence of the required
elements was confirmed by EDAX. The absence of iodine and presence of chloride
could be explained by the surface inner sphere electron transfer mechanism and the
surface outer sphere electron transfer mechanism operating in tandem. The outer
sphere surface electron transfer mechanism might best be described as being the
initial formation of a gold-iodide surface complex, whereby the redox couple so-
formed in the absence of water, can generate iodine and gold(0). This allowed the
reduced gold to be attracted to the reduced surface gold by aurophilicity and thereby
allowed the formation of a gold shell on the ZnO NP surface.
63
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 63
Chapter 6: General Conclusion
6.1 Ascorbic Acid Based Gold Nanoparticles
Ascorbic acid generated gold nanoparticles have not been as widely studied as citrate
derived gold nanoparticles. However, the fact is this type of gold nanoparticle is very
simple to make and uses a very non hazardous material (ascorbic acid) in their
synthesis and capping. This capping agent should be easily metabolised by living
cells making this method an excellent way to derive gold nanoparticles for biological
applications.
Since the size of gold nanoparticles can be controlled by varying the pH of the
precursor HAuCl4 solution, these NPs can then be used in biological applications
where the biological window is of paramount importance.
Additionally, the results although preliminary, suggest this simple method of
synthesising gold nanoparticles can be further developed and modified for the
specific size and type production of spherical, monodisperse gold nanoparticles that
may be required in future applications, such as SERS.
6.2 A Novel Method for the Synthesis of Monodisperse
Gold Coated Silica Nanoparticles
Application of the SERS “charge transfer” theory allowed the derivation of a new
synthetic procedure for uniformly gold coated silica nanoparticles. This experiment
was expressly designed to take advantage of this phenomenon.
A modified synthesis method was developed for stable solutions of silica NPs. It was
found these NPs were remarkably similar in size, with a morphology that was almost
entirely spherical.
The surface chemistry of the silica NPs was investigated using 1H NMR, which
demonstrated that a large number of siloxy groups were present on the silica NP
64
64 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
surface. Possibly this may be one of the first investigations, albeit limited in scope, of
the surface chemistry of silica NPs using liquid NMR.
Additionally a gold 2 co-ordinate complex was shown to exist when gold(I) chloride
is dissolved in acetonitrile by the use of mass spectroscopy.
1H NMR also enabled the gold coating mechanism of formation to be observed
directly. Possibly this has not been achieved before using NPs in a liquid medium.
The amount of literature on NPs undergoing 1H NMR experiments was found to be
very limited as well.
As a result of this successful experiment a close investigation of the literature
relating to the inner sphere mechanism allowed a hypothesis to be derived
culminating in the publication (in press) of a new general theory termed “surface
inner sphere electron transfer mechanism”, which could then be translated to
different classes of metal based NPs.
6.3 Uniform Gold Coating of Zinc Oxide Nanoparticles
Using Gold(I) Chloride and KCl
To test the proposed general theory of the “surface inner sphere electron transfer
mechanism” an experiment was proposed that combined a new class of ZnO NPs
with the potential bridging ligand chloride in comparison to the deprotonated siloxy
group in the Au@SiO2 experiments.
To conduct this experiment, a literature search was conducted to find a suitable
synthesis method to form ZnO NPs based on chloride. This method was subsequently
modified to incorporate additional chloride.
It was found that gold(I) in acetonitrile could be successfully reduced on the ZnO NP
surface and was easily determined by the reduction of the florescence peak. The
complete gold coating, as used in the prior synthesis of Au@SiO2 NPs eliminated the
oxygen deficient peak.
65
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 65
This experiment has provided a new class of previously unknown composite
Au@ZnO NPs and further validated the hypothesis of the “surface inner sphere
electron transfer mechanism” as being a possible general theory of NP synthesis.
6.4 Gold Coated Zinc Oxide Nanoparticles Synthesised
Using ZnI2 and Gold(I) Chloride
To further test the hypothesis of formation it was decided to use a ZnO synthesis
method involving iodide. This is simply due to the size differential between iodine,
which is considerably larger in empirical diameter than oxygen, zinc or chloride. As
such it was not expected to be on the surface.
However, it was expected using the developed process of ZnO NP synthesis using
ZnI2 some iodine would be present within the NP generating a very electronegative
core assisting the gold(I) reduction. However, subsequent analysis revealed the
absence of iodine but a significant presence of chloride that could only come from
added AuCl dissolved in MeCN.
Very similar spectroscopy results were obtained from this experiment as the one
using ZnCl2. A crucial difference to the prior experiment was the iodine/iodide
couple resulting from the reductive potential of iodide in basic organic solution along
with AuCl. This allowed complete elimination of added water and ASC reductant
obtaining a relatively simple one pot process.
This additional Au@ZnO experiment provided additional proof for the hypothesis of
the “surface inner sphere electron transfer mechanism”, which may be a general
theory relating to the synthesis of gold coated NPs based on an inner metal or
metalloid core which can act as non labile electron source (SiO2) and bridging ligand
or absorb a suitable bridging ligand instead (ZnO).
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66 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 67
Chapter 7: Future Work
7.1 Ascorbic Acid Based Gold Nanoparticles
Ascorbic acid based gold nanoparticles certainly need further investigation. The
exact mechanism of formation is unknown to any great extent. This mechanism
could be investigated by the way of 1H NMR as used in the gold coating of silica
nanoparticles, which is also a new technique to be applied to nanoparticles.
The stability of these NPs has yet to be investigated. Measurements such as Zeta
potential and long term changes have yet to be investigated fully. Changes due to
storage have yet to be investigated such as pH, temperature, oxygen content and
related solvents.
These NPs have yet to be studied in a biological context to the extent they warrant.
The use of these NPs in cancer research, catalysis and related applications deserves
study to a significant degree. This also applies to various potential capping agents
that may be used and that needs to be developed.
7.2 A Novel Method for the Synthesis of Monodisperse
Gold Coated Silica Nanoparticles
The synthesised Au@SiO2 NPs is a new class of nano-composites and as such not a
lot is known about them. Further physical investigation such as HRTEM, X-Ray
crystallography and related techniques are required to fully determine their structural
makeup. This could also include additional NMR experiments.
Further experimentation to determine the synthesis parameters is also required as this
area remains unexplored. Additionally, variations in the reducing agents and solvents
remain as yet unexplored.
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68 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
Further experiments may derive a hollow type of gold nanoparticle after dissolving
the inner core. This raises the potential of inserting within the gold shell a cancer
drug that could be released within a cancer cell upon application of light or radiation,
thereby targeting this most insidious disease. As such, further understanding of these
Au@SiO2 and the related stability issues need further study.
Additionally, gold NPs have a very strong SERS effect. Synthesis of Janus type NP
surfaces using silica NPs could provide a rough gold surface, for this application,
may also be possible. It could also be possible to tailor the surface to suit various
chemicals (eg BTEX), which can then be detected at lower levels than thought
possible.
A further use extending from these is also the potential development of a breath
analysis sensor, which combined with appropriate computer software renders easy
detection of diseases or illegal substances.
7.3 Gold Coating of Zinc Oxide Nanoparticles
The development of this new class of fully gold coated ZnO NPs whether using
ZnCl2 or ZnI2, is so new almost every area of their synthesis needs investigation. This
extends from analysis of these NP structures through to determination of synthesis
parameters. None of this has been determined as yet.
No form of NMR has as yet been applied to these unique NPs. No HRTEM or
detailed structural studies have been conducted. Examining these NPs by all
available means is therefore open.
The use of these unique Au@ZnO NPs is one area which is totally unexplored. The
very heavy gold loading may lend itself suitable for say, as a catalyst for the
synthesis of methanol. Perhaps they could be used as a SERS substrate for similar
applications such as gas sensing as Au@SiO2 could be put to.
69
Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 69
Again, cancer applications are a possibility. ZnO is considered to be toxic to cancer
cells, however, gold coated ZnO may not be. It may be possible to directly unload
the ZnO at the site of a cancer cell thereby destroying it after the application of light
or radiation.
7.4 Surface Inner Sphere Electron Transfer Mechanism
This mechanism is far from exploited in the field of surface organometallic
chemistry. As it appears to be a general theory relating to the synthesis of gold
coated NPs, additional types of metal oxides or metalloid oxide based NPs can be
tried. The proviso on the use of this mechanism is the ability of the NP either by
itself or in co-junction with a bridging ligand to form the non labile component of the
mechanism.
This developed theory should also prove useful in unexpected ways as yet to be
discovered. A possible example could be related to the gold coating of glass after the
glass undergoes suitable surface modification. Other applications remain as yet
undetermined. However, a new tool has been made available for those who wish to
pursue a course of scientific discovery.
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70 Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride
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Gold Coating of Silica and Zinc Oxide Nanoparticles by the Surface Reduction of Gold(I) Chloride 71
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