GOLD COATING OF SILICA AND INC XIDE NANOPARTICLES BY THE SURFACE … · 2013-07-04 · Monodisperse silica nanoparticles were synthesised by the well known Stöber protocol in conjunction

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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ii 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 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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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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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

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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


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


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


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


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


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


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


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


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


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


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


(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


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

http://en.wikipedia.org/wiki/Tetraoctylammonium_bromide

16


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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

62


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


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


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


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).

66


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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.

68


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


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.

70


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71


References

1. Ung T, Liz-Marzán L M, Mulvaney P: Optical Properties of Thin Films of

Au@SiO2 Particles. The Journal of Physical Chemistry B 2001, 105:3441-

3452.

2. Li D, Li D-W, Li Y, Fossey J S, Long Y-T: Cyclic electroplating and

stripping of silver on Au@SiO2 core/shell nanoparticles for sensitive and

recyclable substrate of surface-enhanced Raman scattering. Journal of

Materials Chemistry 2010, 20:3688-3693.

3. Kelly K L, Coronado E, Zhao L L, Schatz G C: The Optical Properties of

Metal Nanoparticles: The Influence of Size, Shape, and Dielectric

Environment. The Journal of Physical Chemistry B 2002, 107:668-677.

4. Wang W, Ruan C, Gu B: Development of gold-silica composite nanoparticle

substrates for perchlorate detection by surface-enhanced Raman

spectroscopy. Analytica Chimica Acta 2006, 567:121-126.

5. Liu Y-C, Yu C-C, Hsu T-C: Improved performances on surface-enhanced

Raman scattering based on electrochemically roughened gold substrates

modified with SiO2 nanoparticles. Journal of Raman Spectroscopy 2009,

40:1682-1686.

6. Yang L, Ruan W, Jiang X, Zhao B, Xu W, Lombardi J R: Contribution of

ZnO to Charge-Transfer Induced Surface-Enhanced Raman Scattering in

Au/ZnO/PATP Assembly. The Journal of Physical Chemistry C 2009,

113:117-120.

7. Chen L, Luo L, Chen Z, Zhang M, Zapien J A, Lee C S, Lee S T: ZnO/Au

Composite Nanoarrays As Substrates for Surface-Enhanced Raman

Scattering Detection. The Journal of Physical Chemistry C 2009, 114:93-100.

8. Chen P, Gu L, Xue X, Song Y, Zhu L, Cao X: Facile synthesis of highly

uniform ZnO multipods as the supports of Au and Ag nanoparticles. Materials

Chemistry and Physics 2010, 122:41-48.

9. Luo Y: ZnO microrods photodeposited with Au@Ag nanoparticles:

Synthesis, characterization and application in SERS. Colloid Journal 2009,

71:223-232.

10. Averitt R D, Sarkar D, Halas N J: Plasmon Resonance Shifts of Au-Coated

Au2S Nanoshells: Insight into Multicomponent Nanoparticle Growth.

Physical Review Letters 1997, 78:4217-4220.

11. Hu K-W, Jhang F-Y, Su C-H, Yeh C-S: Fabrication of

Gd2O(CO3)2·H2O/silica/gold hybrid particles as a bifunctional agent for MR

imaging and photothermal destruction of cancer cells. Journal of Materials

Chemistry 2009, 19:2147-2153.

12. Tsai C-L, Chen J-C, Wang W-J: Near-infrared Absorption Property of

Biological Soft Tissue Constituents. Journal of Medical and Biological

Engineering 2001, 21:7-14.

13. Hale G M, Querry M R: Optical Constants of Water in the 200-nm to 200-µm

Wavelength Region. Appl Opt 1973, 12:555-563.

14. Kim J G, Liu H: Variation of haemoglobin extinction coefficients can cause

errors in the determination of haemoglobin concentration measured by near-

infrared spectroscopy. Physics in medicine and Biology 2007, 52:6295-6322.

72


15. Basolo F, Pearson R G: Mechanisms of inorganic reactions: a study of metal

complexes in solution. 2 edn. New York: John Wiley & Sons; 1967.

16. Taube H, Myers H, Rich R L: Observations on the Mechanism of Electron

Transfer in Solution. Journal of the American Chemical Society 1953,

75:4118-4119.

17. Ruff I: Extension of the "band model" to the inner-sphere mechanism of

electron-transfer reactions. The Journal of Physical Chemistry 1968,

72:1792-1797.

18. Ugo R: The Contribution of Organometallic Chemistry and Homogeneous

Catalysis to the Understanding of Surface Reactions Catalysis Reviews

1975, 11:225-297.

19. Bergerhoff G: Preparation of copper(I) and gold(I) compounds in

acetonitrile. Zeitschrift fuer Anorganische und Allgemeine Chemie 1964,

327:139-142.

20. Schmidbaur H: Ludwig Mond Lecture. High-carat gold compounds.

Chemical Society Reviews 1995, 24:391-400.

21. Stöber W, Fink A, Bohn E: Controlled growth of monodisperse silica spheres

in the micron size range. Journal of Colloid and Interface Science 1968,

26:62-69.

22. Haiss W, Thanh N T K, Aveyard J, Fernig D G: Determination of Size and

Concentration of Gold Nanoparticles from UV−Vis Spectra. Analytical

Chemistry 2007, 79:4215-4221.

23. Fleischmann M, Hendra P J, McQuillan A J: Raman Spectra of Pyridine

Adsorbed at a Silver Electrode. Chemical Physics Letters 1974, 26:163-166.

24. Jeanmaire D L, Van Duyne R P: Surface Raman Electrochemistry Part I.

Heterocyclic, Aromatic and Aliphatic Amines Adsorbed on the Anodized

Silver Electrode. Journal of Electroanalytical Chemistry 1977, 84:1-20.

25. Albrecht M G, Creighton J A: Anomalously Intense Raman Spectra of

Pyridine at a Silver Electrode. Journal of the American Chemical Society

1977, 99:5215-5217.

26. Ru E L, Etchegoin P: Principles of Surface-Enhanced Raman Spectroscopy

and related plasmonic effects, Elsevier; 2009.

27. Lombardi J R, Birke R L: A Unified Approach to Surface-Enhanced Raman

Spectroscopy. The Journal of Physical Chemistry C 2008, 112:5605-5617.

28. Moskovits M, Suh J S: Surface selection rules for surface-enhanced Raman

spectroscopy: calculations and application to the surface-enhanced Raman

spectrum of phthalazine on silver. The Journal of Physical Chemistry 1984,

88:5526-5530.

29. Goolsby A D, Sawyer D T: Electrochemistry of gold(I) and its complexes in

acetonitrile. Analytical Chemistry 1968, 40:1978-1983.

30. Johnson P R, Pratt J M, Tilley R I: Experimental Determination of the

Standard Reduction Potential of the Gold(I) Ion. JCS Chemical

Communications 1978, 14:606-607.

31. Faraday M: The Bakerian Lecture: Experimental Relations of Gold (and

Other Metals) to Light. Philosophical Transactions of the Royal Society of

London 1857, 147:145-181.

32. Turkevich J, Stevenson P C, Hillier J: The Formation of Colloidal Gold.

Journal of Physical Chemistry 1953, 57:670-673.

33. Frens G: Particle size and sol stability in metal colloids. Colloid & Polymer

Science 1972, 250:736-741.

73


34. Brust M, Walker M, Bethell D, Schiffrin D J, Whyman R: Synthesis of Thiol-

derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System.

Journal of the Chemical Society, Chemical Communications 1994:801-802.

35. Stathis E C, Fabrikanos A: Preparation of Colloidal Gold. Chemistry and

Industry 1958, July 5:860-861.

36. Andreescu D, Sau T K, Goia D V: Stabilizer-free nanosized gold sols Journal

of Colloid and Interface Science 2006, 298:742-751.

37. Wang S, Qian K, Bi X, Huang W: Influence of Speciation of Aqueous HAuCl4

on the Synthesis, Structure, and Property of Au Colloids. The Journal of

Physical Chemistry C 2009, 113:6505-6510.

38. Haiss W, Thanh N T K, Aveyard J, Fernig D G: Determination of Size and

Concentration of Gold Nanoparticles from UV-Vis Spectra. Analytical

Chemistry 2007, 79:4215-4221.

39. Sim L, Fielding A, English M, Waclawik E, Rockstroh A, Soekmadji C,

Vasireddy R, Russell P, Nelson C: Enhancement of Biological Effectiveness

of Radiotherapy Treatments of Prostrate Cancer Cells in vitro using Gold

Nanoparticles. Poster Presentation, 2011 International Nanomedicine

Conference; 14-16th July 2011; Coogee Beach, Sydney. 2011

40. English M D, Waclawik E R: A Novel Method for the Synthesis of

Monodisperse Gold Coated Silica Nanoparticles. Journal of Nanoparticle

Research 2012, 14:1-10.

41. Rao K S, El-Hami K, Kodaki T, Matsushige K, Makino K: A novel method

for synthesis of silica nanoparticles. Journal of Colloid and Interface Science

2005, 289:125-131.

42. Hiramatsu H, Osterloh F E: pH-Controlled Assembly and Disassembly of

Electrostatically Linked CdSe−SiO2 and Au−SiO2 Nanoparticle Clusters.

Langmuir 2003, 19:7003-7011.

43. Qu Q, Peng S, Mangelings D, Hu X, Yan C: Silica spheres coated with C18-

modified gold nanoparticles for capillary LC and pressurized CEC

separations. ELECTROPHORESIS 2010, 31:556-562.

44. Salgueiriño-Maceira V, Correa-Duarte M A, Farle M, López-Quintela A,

Sieradzki K, Diaz R: Bifunctional Gold-Coated Magnetic Silica Spheres.

Chemistry of Materials 2006, 18:2701-2706.

45. White I M, Oveys H, Fan X: Increasing the Enhancement of SERS with

Dielectric Microsphere Resonators. Spectroscopy 2006, 21:36, 38-42.

46. Turner M, Golovko V B, Vaughan O P H, Abdulkin P, Berenguer-Murcia A,

Tikhov M S, Johnson B F G, Lambert R M: Selective oxidation with dioxygen

by gold nanoparticle catalysts derived from 55-atom clusters. Nature 2008,

454:981-983.

47. Ueda A, Haruta M: Nitric Oxide Reduction with Hydrogen, Carbon

Monoxide, and Hydrocarbons over Gold Catalysts. Gold Bulletin 1999, 32:3-

11.

48. Shi Y-L, Asefa T: Tailored Core−Shell−Shell Nanostructures: Sandwiching

Gold Nanoparticles between Silica Cores and Tunable Silica Shells.

Langmuir 2007, 23:9455-9462.

49. Kim J-H, Chung H-W, Lee T R: Preparation and Characterization of

Palladium Shells with Gold and Silica Cores. Chemistry of Materials 2006,

18:4115-4120.

50. Hasan M, Bethell D, Brust M: The Fate of Sulfur-Bound Hydrogen on

Formation of Self-Assembled Thiol Monolayers on Gold: 1H NMR

74


Spectroscopic Evidence from Solutions of Gold Clusters. Journal of the

American Chemical Society 2002, 124:1132-1133.

51. Williams D H, Fleming I: Spectroscopic Methods in Organic Chemistry. 5th

edn. Berkshire: McGraw-Hill; 1995.

52. Haldar K K, Sen T, Patra A: Au@ZnO Core−Shell Nanoparticles Are

Efficient Energy Acceptors with Organic Dye Donors. The Journal of

Physical Chemistry C 2008, 112:11650-11656.

53. Terakawa M, Tanaka Y, Obara G, Sakano T, Obara M: Randomly-grown

high-dielectric-constant ZnO nanorods for near-field enhanced Raman

scattering. Applied Physics A: Materials Science & Processing 2011,

102:661-665.

54. Strunk J, Kähler K, Xia X, Comotti M, Schüth F, Reinecke T, Muhler M:

Au/ZnO as catalyst for methanol synthesis: The role of oxygen vacancies.

Applied Catalysis A: General 2009, 359:121-128.

55. Carabineiro S A C, Machado B F, Bacsa R R, Serp P, Drazic G, Faria J L,

Figueiredo J L: Catalytic performance of Au/ZnO nanocatalysts for CO

oxidation. Journal of Catalysis 2010, 273:191-198.

56. Dhas V, Muduli S, Lee W, Han, S-H,., Ogale S: Enhanced conversion

efficiency in dye-sensitized solar cells based on ZnO bifunctional nanoflowers

loaded with gold nanoparticles. Applied Physics Letters 2008, 93:243108

57. Imran Jafri R, Sujatha N, Rajalakshmi N, Ramaprabhu S: Au-MnO2/MWNT

and Au-ZnO/MWNT as oxygen reduction reaction electrocatalyst for polymer

electrolyte membrane fuel cell. International Journal of Hydrogen Energy

2009, 34:6371-6376.

58. Chang, C-C,., Chiu, N-Fu,., Lin D S, Chu-Su Y, Liang, Y-H,., Lin, C-W,.

High-Sensitivity Detection of Carbohydrate Antigen 15-3 Using a Gold/Zinc

Oxide Thin Film Surface Plasmon Resonance-Based Biosensor. Analytical

Chemistry 2010, 82:1207-1212.

59. Wei Y, Li Y, Liu X, Xian Y, Shi G, Jin L: ZnO nanorods/Au hybrid

nanocomposites for glucose biosensor. Biosensors and Bioelectronics 2010,

26:275-278.

60. Liu C, Mi C C, Li B Q: Transient Temperature Response of Pulsed-Laser-

Induced Heating for Nanoshell-Based Hyperthermia Treatment. IEEE

Transactions on Nanotechnology 2009, 8:697-706.

61. Wang, X-H,., Shi J, Dai S, Yang Y: A sol-gel method to prepare pure and

gold colloid doped ZnO films. Thin Solid Films 2003, 429:102-107.

62. Joshi R K, Hu Q, Alvi F, Joshi N, Kumar A: Au Decorated Zinc Oxide

Nanowires for CO Sensing. The Journal of Physical Chemistry C 2009,

113:16199-16202.

63. Nicoll F H: Temperature Dependence of the Emission Bands of Zinc Oxide

Phosphors. Journal of the Optical Society of America 1948, 38:817.

64. Wang Z G, Zu X T, Zhu S, Wang L M: Green luminescence originates from

surface defects in ZnO nanoparticles. Physica E: Low-dimensional Systems

and Nanostructures 2006, 35:199-202.

65. Bahnemann D W, Kormann C, Hoffmann M R: Preparation and

Characterization of Quantum Size Zinc Oxide: A Detailed Spectroscopic

Study. Journal of Physical Chemistry 1987, 91:3789-3798.

66. Chen L, Xu J, Holmes J D, Morris M A: A Facile Route to ZnO Nanoparticle

Superlattices: Synthesis, Functionalization, and Self-Assembly. The Journal

of Physical Chemistry C 2010, 114:2003-2011.

75


67. Li K, Lai Y, Zhang W, Jin L: Fe2O3@Au core/shell nanoparticle-based

electrochemical DNA biosensor for Escherichia coli detection. Talanta 2011,

84:607-613.

68. Wang X, Kong X, Yu Y, Zhang H: Synthesis and Characterization of Water-

Soluble and Bifunctional ZnO-Au Nanocomposites. Journal of Physical

Chemistry C 2007, 111:3836-3841.

69. Link S, El-Sayed M A: Shape and size dependence of radiative, non-radiative

and photothermal properties of gold nanocrystals. International Reviews in

Physical Chemistry 2000, 19:409-453.

70. Mulvaney P: Surface Plasmon Spectroscopy of Nanosized Metal Particles.

Langmuir 1996, 12:788-800.

71. Wang X, Kong X, Yu Y, Zhang H: Synthesis and Characterization of Water-

Soluble and Bifunctional ZnO−Au Nanocomposites. The Journal of Physical

Chemistry C 2007, 111:3836-3841.

72. Yu H, Chen M, Rice P M, Wang S X, White R L, Sun S: Dumbbell-like

Bifunctional Au−Fe3O4 Nanoparticles. Nano Letters 2005, 5:379-382.

73. Daniel M-C, Astruc D: Gold Nanoparticles: Assembly, Supramolecular

Chemistry, Quantum-Size-Related Properties, and Applications toward

Biology, Catalysis, and Nanotechnology. Chemical Reviews 2003, 104:293-

346.

74. Statham P J: Pile-up Correction for Improved Accuracy and Speed of X-Ray

Analysis. Microchimica Acta 2006, 155.

75. Leff D V, Ohara P C, Heath J R, Gelbart W M: Thermodynamic Control of

Gold Nanocrystal Size: Experiment and Theory. Journal of Physical

Chemistry 1995, 99:7036-7041.

76. Brust M, Fink J, Bethell D, Schiffrin D J, Kiely C: Synthesis and reactions of

functionalised gold nanoparticles. Journal of the Chemical Society, Chemical

Communications 1995:1655-1656.

77. Teirlinck P A M, Petersen F W: The nature of gold-iodide adsorption onto

coconut-shell carbon. Minerals Engineering 1996, 9:923-930.

78. Kelsall G H, Welham N J, Diaz M A: Thermodynamics of Cl-H2O, Br-H2O,

I-H2O, Au-Cl-H2O, Au-Br-H2O and Au-I-H2O systems at 298 K. Journal of

Electroanalytical Chemistry 1993, 361:13-24.

79. Wang Q, Geng B, Wang S: ZnO/Au Hybrid Nanoarchitectures: Wet-

Chemical Synthesis and Structurally Enhanced Photocatalytic Performance.

Environmental Science & Technology 2009, 43:8968-8973.

80. Qi J, Dang X, Hammond P T, Belcher A M: Highly Efficient Plasmon-

Enhanced Dye-Sensitized Solar Cells through Metal@Oxide Core–Shell

Nanostructure. ACS Nano 2011, 5:7108-7116.

81. Zheng Y-Z, Tao X, Hou Q, Wang D-T, Zhou W-L, Chen J-F: Iodine-Doped

ZnO Nanocrystalline Aggregates for Improved Dye-Sensitized Solar Cells.

Chemistry of Materials 2010, 23:3-5.

82. Hoffmann M R, Martin S T, Choi W, Bahnemann D W: Environmental

Applications of Semiconductor Photocatalysis. Chemical Reviews 1995,

95:69-96.

83. Norman V J: Oxygen chemisorbed on zinc oxide: The determination of

reactive oxygen. Australian Journal of Chemistry 1966, 19: 1133-1141

84. Morgan K J: Some reactions of inorganic iodine compounds. Quarterly

Reviews, Chemical Society 1954, 8:123-146.

76


85. Wong G T F, Zhang L-S: The kinetics of the reactions between iodide and

hydrogen peroxide in seawater. Marine Chemistry 2008, 111:22-29.

86. Haynes W M (Ed.). CRC Handbook of Chemistry and Physics, Section 5:

Thermochemistry, Electrochemistry, and Kinetics: Electrochemical Series, 92

edition. Boca Raton CRC Press/Taylor and Francis; 2012.

87. Shannon R: Revised effective ionic radii and systematic studies of interatomic

distances in halides and chalcogenides. Acta Crystallographica Section A

1976, 32:751-767.

88. Wang C, Li Q, Mao B, Wang E, Tian C: A different chemical route to

synthesise ZnO nanoparticles. Materials Letters 2008, 62:1339-1341.

89. Schmidbaur H: The aurophilicity phenomenon: A decade of experimental

findings, theoretical concepts and emerging applications. Gold Bulletin 2000,

33:3-10.

90. Dobson G, Grossweiner L I: Primary Processes in the Photo-Oxidation of

Iodide Ion in Ethanol. Radiation Research 1964, 23:290-299.

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GOLD COATING OF SILICA AND INC XIDE NANOPARTICLES BY THE SURFACE … · 2013-07-04 · Monodisperse silica nanoparticles were synthesised by the well known Stöber protocol in conjunction