QUEENSLAND UNIVERSITY OF TECHNOLOGY

Noble metal photocatalysts under visible light and UV light

irradiation

A thesis presented in fulfilment of the requirements for the degree of

Doctor of Philosophy

Xi Chen

(Bachelor of Chemistry, Nankai University, China; PhD of

Chemistry, Nankai University, China)

School of Physical and Chemical Sciences

July 2010

1

ABSTRACT

One of the greatest challenges for the study of photocatalysts is to devise new

catalysts that possess high activity under visible light illumination. This would allow

the use of an abundant and green energy source, sunlight, to drive chemical reactions.

Gold nanoparticles strongly absorb both visible light and UV light. It is therefore

possible to drive chemical reactions utilising a significant fraction of full sunlight

spectrum. Here we prepared gold nanoparticles supported on various oxide powders,

and reported a new finding that gold nanoparticles on oxide supports exhibit

significant activity for the oxidation of formaldehyde and methanol in the air at

ambient temperature, when illuminated with visible light. We suggested that visible

light can greatly enhance local electromagnetic fields and heat gold nanoparticles due

to surface plasmon resonance effect which provides activation energy for the

oxidation of organic molecules. Moreover, the nature of the oxide support has an

important influence on the activity of the gold nanoparticles. The finding reveals the

possibility to drive chemical reactions with sunlight on gold nanoparticles at ambient

temperature, highlighting a new direction for research on visible light photocatalysts.

Gold nanoparticles supported on oxides also exhibit significant dye oxidation

activity under visible light irradiation in aqueous solution at ambient temperature.

Turnover frequencies of the supported gold nanoparticles for the dye degradation are

much higher than titania based photocatalysts under both visible and UV light. These

gold photocatalysts can also catalyse phenol degradation as well as selective oxidation

of benzyl alcohol under UV light. The reaction mechanism for these photocatalytic

oxidations was studied. Gold nanoparticles exhibit photocatalytic activity due to

2

visible light heating gold electrons in 6sp band, while the UV absorption results in

electron holes in gold 5d band to oxidise organic molecules.

Silver nanoparticles also exhibit considerable visible light and UV light absorption

due to surface plasmon resonance effect and the interband transition of 4d electrons to

the 5sp band, respectively. Therefore, silver nanoparticles are potentially

photocatalysts that utilise the solar spectrum effectively. Here we reported that silver

nanoparticles at room temperature can be used to drive chemical reactions when

illuminated with light throughout the solar spectrum. The significant activities for dye

degradation by silver nanoparticles on oxide supports are even better than those by

semiconductor photocatalysts. Moreover, silver photocatalysts also can degrade

phenol and drive the oxidation of benzyl alcohol to benzaldehyde under UV light. We

suggested that surface plasmon resonance effect and interband transition of silver

nanoparticles can activate organic molecule oxidations under light illumination.

3

KEYWORDS

Gold photocatalyst

Silver photocatalyst

Organic degradation

Selective oxidation

Visible light

Ultraviolet light

Surface plasmon resonance

Interband transition

Formaldehyde

Methanol

Dye

Phenol

Alcohol

Aldehyde

4

ABBREVIATIONS

CVD Chemical vapour deposition

DP Deposition-precipitation

EDS Energy dispersive X-ray spectroscopy

FID Flame ionization detector

FT-IR Fourier transform infrared

HRTEM High-resolution transmission electron microscopy

IEP Isoelectric point

IMP Impregnation method

NP Nanoparticle

PVP Poly(N-vinyl-2-pyrrolidone)

SPC Surface photocurrent

SPR Surface plasmon resonance

SRB Sulforhodamine-B

TEM Transmission electron microscopy

TOF Turnover frequency

TPV Transient photovoltage

VOC Volatile organic compounds

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

5

PUBLICATIONS

1. X. Chen, Z. F. Zheng, K. X. Bin, E. Jaatinen, T. F. Xie, D. J. Wang, C. Guo, J. C.

Zhao, H. Y. Zhu, “Supported Silver Nanoparticles as Photocatalysts under

Ultraviolet and Visible Light Irradiation”, Green Chemistry, 2010, 12, 414-419.

2. Z. F. Zheng, J. Teo, X. Chen, H. W. Liu, Y. Yuan, H. Y. Zhu, Z. Y. Zhong,

“Correlate Catalytic Oxidation Activities of Various Titania Nanotubes and Their

Supported Gold Catalysts with Surface OH-Groups Generation on Oxygen

Vacancies of the Titania Surface”, Chem. Eur. J., 2010, 16, 1202-1211.

3. H. Y. Zhu, X. Chen, Z. F. Zheng, K. X. Bin, E. Jaatinen, J. C. Zhao, C. Guo, T. F.

Xie, D. J. Wang, “Mechanism of Supported Gold Nanoparticles as Photocatalysts

under Ultraviolet and Visible Light Irradiation”, Chem. Comm., 2009, 7524-7526.

4. X. Chen, H. Y. Zhu, J. C. Zhao, Z. F. Zheng, X. P. Gao, “Visible Light Driven

Oxidation of Organic Contaminants in Air with Gold Nanoparticle Catalysts On

Oxide Supports”, Angew. Chem. Int. Ed., 2008, 47, 5353-5356.

5. X. Chen, H. Y. Zhu, “Catalysis by supported gold nanoparticles”, Elsevier Science,

in Comprehensive Nanoscience and Technology 2009.

6

TABLE OF CONTENTS

ABSTRACT-------------------------------------------------------------------------------------1

KEYWORDS------------------------------------------------------------------------------------ 3

ABBREVIATIONS----------------------------------------------------------------------------- 4

PUDLICATIONS--------------------------------------------------------------------------------5

TABLE OF CONTENTS-----------------------------------------------------------------------6

STATEMENT OF ORIGINALITY------------------------------------------------------------8

ACKNOWLEDMENTS------------------------------------------------------------------------9

CHAPTER 1------------------------------------------------------------------------10

INTRODUCTION

CHAPTER 2------------------------------------------------------------------------14

LITERATURE REVIEW

2.1. Preparation of gold catalysts --------------------------------------------------------14

2.2. Catalytic CO oxidation---------------------------------------------------------------18

2.2.1. Effect of preparation method-------------------------------------------------18

2.2.2. Effect of particle size and support -------------------------------------------19

2.2.3. Reaction mechanism-----------------------------------------------------------21

2.3. Catalytic oxidation of organic compounds by gold catalysts-------------------23

2.4. Catalytic reduction of organic compounds by silver catalysts------------------26

2.5. Catalytic reduction of organic compounds----------------------------------------27

2.6. Noble metal/semiconductor photocatalysts ---------------------------------------28

2.7. Gold photocatalysts-------------------------------------------------------------------29

2.7.1. Plasmon absorption of noble metal NPs------------------------------------30

2.7.2. Oxidation of organic compounds over gold NPs --------------------------32

2.7.3. Silver photocatalysts-----------------------------------------------------------33

7

2.8. Summary-------------------------------------------------------------------------------35

CHAPTER 3------------------------------------------------------------------------45

VISIBLE LIGHT DRIVEN OXIDATION OF ORGANIC CONTAMINANTS IN AIR

WITH GOLD NANOPARTICLE CATALYSTS ON OXIDE SUPPORTS

CHAPTER 4-----------------------------------------------------------------------58

MECHANISM OF SUPPORTED GOLD NPS AS PHOTOCATALYSTS UNDER

ULTRAVIOLET AND VISIBLE LIGHT IRRADIATION

CHAPTER 5-----------------------------------------------------------------------71

SUPPORTED SILVER NPS AS PHOTOCATALYSTS UNDER ULTRAVIOLET

AND VISIBLE LIGHT IRRADIATION

CHAPTER 6-----------------------------------------------------------------------79

CONCLUSIONS

8

STATEMENT OF ORIGINALITY

The material presented in this thesis has not been previously submitted for a

degree at any other university or institution. To the best of my knowledge, this thesis

contains no material published or written by any other person, except where due

reference is made.

Xi Chen

Apr 2010

9

ACKNOWLEDGEMENTS

I would first and foremost like to thank my principal supervisor, Prof. Huaiyong Zhu

for his guidance, suggestions, support and understanding throughout the past three

years. I sincerely appreciate his help without which I would not have been able to

complete my PhD.

Thanks must go to my associate supervisor, Prof. Ray Frost for his support and for

sharing his scientific knowledge. His wisdom and friendship are much appreciated.

I would also like to thank the various research and technical staffs, Dr Xuebin Ke, Dr

Dongjiang Yang, Dr Hongwei Liu, Dr Chris Carvalho, Dr Llew Rintoul, Dr Thor

Bostrom, Tony Raftery and Dr Loc Doung.

I must thank a number of the chemistry postgraduate students in QUT. The laughter

and friendship throughout the years will not be forgotten.

And finally, I would like to thank my family for their enthusiastic support.

CHAPTER 1

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CHAPTER 1. INTRODUCTION

Environmental problem caused by the utilisation of fossil fuels, which are running

out, is the major driving force for the application development of new clean energy

sources such as sunlight. Photocatalysis which increases the rate of chemical reaction

in the presence of a catalyst under light illumination has become a highly dynamic hot

spot for researchers. In recent years, conventional photocatalysis using semiconductor

materials in particular TiO2 based materials has been the focus of numerous

investigations [1]. However, due to the large band gap structure, usually TiO2 are

only active under UV light irradiation (wavelength below 400 nm), which only

accounts for about 4% of the whole energy of solar spectrum, while visible light

constitutes around 43% of incoming solar energy [2]. Many approaches have been

reported to develop TiO2 based photocatalysts effective under visible light, including

doping TiO2 with metal ions or metal atom clusters [3] and incorporating nitrogen [4]

or carbon [5] into TiO2

Surface plasmon is a charge-density oscillation that can propagate at the interface

between metal and dielectric medium. Noble metal nanoparticles (NPs) have intensive

absorption of visible light due to SPR effect characterised by strong field

. Nonetheless, the search of visible-light-driven photocatalysis

should not be limited to semiconductor materials, but can be extended to other

materials, such as noble metal (gold and silver) materials which exhibit considerable

visible light absorption due to surface plasmon resonance (SPR) effect [6].

CHAPTER 1

11

enhancement at the interface [7, 8]. The enhanced local field strength can be over 500

times larger than the applied field for the structures with sharp edges and concave

curvatures (e.g. nanowires, cubes, triangular plates, and NP junctions) [9]. Moreover,

the plasmon absorption may cause rapid heating of noble metal NPs because of the

large absorption of light energy through SPR effect and the low heat transfer to the

surrounding environment [10, 11]. Under laser irradiation, the temperature of the gold

NPs was measured to be about 2500K by the photon counting system [10].

On the other hand, gold NPs supported on metal oxides are efficient catalysts for

important oxidation process in dark. Supported gold catalysts have been reported to

exhibit extraordinary activity to oxidise carbon monoxide at very low temperatures

(significantly below 273K) [12]. The gold nanomaterials supported on metal oxides

could also catalyse various oxidations of various volatile organic compounds (VOCs)

such as formaldehyde at moderately elevated temperature around 100 oC [13, 14].

Comibining the effects induced by the SPR absorption and the catalytic activity of

gold NPs, we found an important opportunity: if the gold NPs could be heated by SPR

effect under visible light irradiation, VOCs on the NPs could be activated while the

system remains at ambient temperature. This will result in a visible light driven

photooxidation on gold catalysts. Furthermore, it reveals potential to apply the new

gold photocatalysts for organic compound oxidation. To the best of our knowledge,

little attention has been devoted to noble metal photocatalysts, especially gold

photocatalysts, and the precise nature of the photocatalysis mechanisms under visible

light and UV light have not been clarified thoroughly to date. In the thesis, the

CHAPTER 1

12

photocatalytic activities on supported gold NPs for the oxidation of formaldehyde and

methanol by visible light irradiation were investigated first. Then we studied

photocatalytic performances and mechanisms of noble metal NPs in liquid phase

reactions, such as dye degradation, phenol degradation and selective alcohol oxidation

under visible light and UV light illumination, respectively. Our finding that noble

metal NPs on oxide supports can drive a wide range of chemical reactions with light

illumination highlights a new direction for photocatalysis research.

In Chapter 2, recent progress in the preparation and applications of noble metal

materials in catalysis and photocatalysis was reviewed to provide relevant background

to the research of this thesis.

CHAPTER 1

13

REFERENCES

[1] M. I. Litter, “Heterogeneous photocatalysis - Transition metal ions in photocatalytic systems”, Appl. Catal. B 1999, 23, 89-114. [2] N. S. Lews, “Light work with water”, Nature 2001, 414, 589-590. [3] A. L. Linsebigler, G. Q. Lu, J. T. Yates, “Photocatalysis on TiO2

results”, Chem. Rev. 1995, 95, 735-758. surfaces -

principles, mechanisms, and selected[4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, “Visible-light photocatalysis in nitrogen-doped titanium oxides”, [5] S. U. M. Khan, M. Al-Shahry, W. B. Ingler, “Efficient photochemical water splitting by a chemically modified n-TiO

Science 2001, 293, 269-271.

2

[6] S. Eustis, M. A. El-Sayed, “Why gold nanoparticles are more precious than pretty gold: Noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes”, Chem. Soc. Rev. 2006, 35, 209-217.

”, Science 2002, 297, 2243-2245.

[7] X. H. Huang, I. H. El-Sayed, W. Qian, M. A. El-Sayed, “Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods”, J. Am. Chem. Soc. 2006, 128, 2115-2120. [8] H. Yuan, W. H. Ma, C. C. Chen, J. C. Zhao, J. W. Liu, H. Y. Zhu, X. P. Gao, “Shape and SPR evolution of thorny gold nanoparticles promoted by silver ions”, Chem. Mater. 2007, 19, 1592-1600. [9] K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, “The optical properties of metal nanoparticles: The influence of size, shape and dielectric environment”, J. Phys. Chem. B 2003, 107, 668-677. [10] A. Takami, H. Kurita, S. Koda, “Laser-induced size reduction of noble metal particles”, J. Phys. Chem. B 1999, 103, 1226-1232. [11] D. K. Roper, W. Ahn, M. Hoepfner, “Microscale heat transfer transduced by surface plasmon resonant gold nanoparticles”, J. Phys. Chem. C 2007, 111, 3636-3641. [12] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet, B. Delmon, “Low-temperature oxidation of CO over gold supported on TiO2, α-Fe2O3, and Co3O4

[13] M. Jia, Y. Shen, C. Li, Z. Bao, S. Sheng, “Effect of supports on the gold catalyst activity for catalytic combustion of CO and HCHO”, Catal. Lett. 2005, 3, 235-239.

”, J. Catal. 1993, 144, 175-192.

[14] A. Corma, M. E. Domine, “Gold supported on a mesoporous CeO2

matrix as an efficient catalyst in the selective aerobic oxidation of aldehydes in the liquid phase”, Chem. Commun. 2005, 4042-4044.

CHAPTER 2

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CHAPTER 2. LITERATURE REVIEW

Gold was regarded as catalytically inactive for a long time. This changed after the

discovery that gold nanoparticles (NPs) are excellent catalysts for CO oxidation at

low temperature [1]. Now the research on the catalytic activity has become a hot topic.

Scientists have found that the catalytic activity of gold NPs generally depends upon

NP size, support nature and preparation method, and gold catalysts can also catalyse

many reactions other than CO oxidation, including oxidation reactions of organic

compounds. On the other hand, noble metal NPs have intensive absorption of light

energy, which highlights an important opportunity that noble metal NPs could induce

oxidation reactions of organic compounds under light illumination. This chapter

focused on recent progress in these fields, especially in the preparation of gold

catalysts and applications of noble metal materials in catalysis and photocatalysis.

2.1. Preparation of gold catalysts

Many gold catalysts were typically prepared by impregnation (IMP) method. In

the IMP method, a metal oxide support is immersed in an aqueous solution of HAuCl4

and then the solution is heated to disperse HAuCl4 crystallites over the support

surfaces. The dried precursor is calcined in air or reduced in a hydrogen stream to

obtain Au NPs [2]. However, during the heating process the interaction between

HAuCl4 and the metal oxide support is weak, and the size of Au particles obtained by

IMP method is larger than 20 nm as shown in Figure 1 [2]. In order to produce gold

with diameters below 10 nm on a variety of metal oxide supports, several new

techniques have been developed as briefly described below.

CHAPTER 2

15

Figure 1. Transmission electron microscopy (TEM) micrograph of Au/SiO2

catalyst

prepared by IMP method [2].

1. Co-precipitation [3]: An aqueous solution of HAuC14 and a metal nitrate are

added into an aqueous solution of Na2CO3

2. Co-sputtering [4]: Gold NPs are simultaneously sputter-deposited on a

substrate to produce thin film in an oxygen atmosphere, and then the film is annealed

in air.

. The obtained precipitation is then washed,

dried and finally calcined in air to obtain catalyst samples with strong interaction

between gold NPs and supports.

3. Chemical vapour deposition (CVD) [5]: The vapour of an organic gold

compound is introduced onto an evacuated metal oxide support. The adsorbed gold

compound is pyrolysed in air to prepare small gold particles. This method can be

applied to a wide variety of metal oxides.

4. Deposition-precipitation (DP) [6]: Scheme 1 shows the detailed procedure

proposed by Haruta [7]. The pH of an aqueous solution of HAuC14 is adjusted to a

fixed value in the range of 6-10 with dilute NaOH solution due to the amphoteric

properties of Au(OH)3. During the process the concentration (around 10-3 M) and

temperature (323-363 K) of the solution need to be controlled carefully so that the

CHAPTER 2

16

partially hydrolysed species can react on the support surface, resulting in the

deposition of Au(OH)3

on the surfaces of support metal oxides.

HAuCl4 (PH = 2-3) HAuCl4 (PH = 6-10) Au(OH)3/Support Au/Support

Scheme 1. Procedure in the deposition-precipitation method [7].

The remarkable influence of the pH on the particle size of Au prepared by DP is

shown in Figure 2 when TiO2 is the support [8]. Above pH 6 the main Au species in

solution is transformed from AuC14- to Au(OH)nCl4-n (n = 1-3) and the mean

diameter of Au particles in the calcined catalysts becomes smaller than 4 nm [9]. The

advantage is that the metal oxide support immersed in the solution can be in any form,

such as a powder, bead, honeycomb and thin film. However, one of the constraints of

DP is that its application can be only to metal oxides whose isoelectric points (IEPs)

are above 5. Gold hydroxide cannot be deposited on oxides with low IEPs, such as

SiO2 and WO3

5. NaBH

.

4 reduction: Zhong et al. [10] reported an efficient method for the

preparation of highly dispersed supported Au catalysts. First, support is dispersed in

10 ml deionised water. Then HAuCl4 and lysine are added. The pH of the suspension

is adjusted to 5-6 with a dilute NaOH solution. Next, the suspension is subjected to

sonication in order to facilitate dispersion and deposition of the Au colloids onto the

catalyst support, and during the sonication, freshly prepared NaBH4

is injected

quickly. The suspension turns dark in colour immediately, and the precipitation is

separated using a centrifuge and washed with deionised water. Most of the Au

particles in the obtained samples are below 5 nm and highly dispersed.

CHAPTER 2

17

4 5 6 7 8 9 10

5

10

15

20

Au N

P siz

e (nm

)

PH

Figure 2. Influence of the HAuC14 solution pH on the mean diameter of Au particles

for the Au/TiO2 catalysts prepared by DP method. Gold content in the HAuC14

solution corresponded to 13 wt% with respect to TiO2

. Calcination was conducted in

air at 400°C [8].

The five methods above can be classified into two categories. The first is based on

the preparation of well-mixed precursors — hydroxide and oxide of Au by DP,

coprecipitation, co-sputtering and NaBH4 reduction. These precursor mixtures can be

transformed into metallic gold particles attached to the crystalline metal oxides, such

as α-Fe2O3, Co3O4 and ZrO2

during the calcination process. The second technique is

to utilise the deposition or adsorption of gold compounds, such as organogold

complex by CVD. Gold particles that are strongly attached to oxide supports and are

stable at relatively high temperature can be produced by all these methods. The strong

affinity with the oxide supports results in surprisingly high catalytic activity for CO

and organic compound oxidation [7].

CHAPTER 2

18

2.2. Catalytic CO oxidation

The oxidation of CO to CO2 by gold catalyst received a great deal of initial

publicity [11]. In the 1980s Haruta et al. [1, 2, 12] reported that supported gold

catalysts could exhibit the activity to oxidise CO at very low temperatures,

significantly below 273K. This property has not been observed for other metals. They

found that α-Fe2O3

was an excellent support and suggested that the preparation

method was crucial to obtain high catalytic activity [1]. By electron microscopy

investigation they found that the active sites on the catalysts were small gold NPs with

diameter of about 2-4 nm [2]. This discovery started the rapid growth in studies

relating to heterogeneous catalysis with gold NPs. Many research groups prepared

gold catalysts in various ways and studied their catalytic activity under different

conditions.

2.2.1. Effect of preparation method

The preparation method of gold NPs generally plays a crucial role in the catalytic

activity. The IMP method was considered as a disadvantage due that poor CO

oxidation catalysts were obtained [12]. However, sequential reduction-oxidation-

reduction treatment could considerably enhance the CO catalytic oxidation activity,

clearly illustrating the significance of the pre-treatment procedure [13]. Bamwenda et

al. [14] observed greater activity for gold catalysts synthesised by DP method than

those of the catalysts synthesised by CVD and IMP methods. They explained that the

DP method yielded hemispherical gold particles that had strong interaction with the

support while CVD and IMP methods simply loaded spherical particles on the support.

The coprecipitation method also produced highly dispersed gold particles with a mean

size below 5nm as active catalysts for CO oxidation. The close contact and high

CHAPTER 2

19

dispersion of Au particles to the support was indispensable for the catalysis [14].

According to Yuan and co-workers [15], highly dispersed active gold catalysts could

also be synthesised by reacting Au-phosphine complex with as-precipitated wet metal

hydroxide supports. However, conventional metal oxide and hydroxide supports have

been found to be unsuitable [16]. By modifying the synthesis procedure using

different solvents, or treating catalysts with a temperature-programmed reduction-

oxidation procedure, highly dispersed active gold catalysts could be obtained, even on

a conventional support such as TiO2

Furthermore, the activity of Au/TiO

[17].

2 catalysts with reduction treatment at 773K

was found to be relatively high [18]. It could be speculated that the decomposition

during the Au precursor reduction-calcination treatment increased the interaction

between Au particles and TiO2

support [17]. Dominguez et al. [19] suggested that

high pre-treatment temperature could lead to the generation of structural vacancies by

the elimination of carbonates, which could modify the gold oxidation state.

2.2.2. Effect of particle size and support

As mentioned above, significant catalytic activity was observed for small Au

particles [13, 20, 21]. Lopez et al. [20] reported that CO oxidation rate for 2 to 4 nm

particles was more than two orders of magnitude larger than for 20 to 30 nm particles.

They proposed that the main effect of decreasing the gold particle size was to increase

the concentration of low-coordinated Au atoms. Further evidences for the particle size

effect were obtained via CO oxidation studies on different catalysts [21, 22]. In these

studies Au clusters supported on TiO2 thin films were prepared under ultra-high

vacuum conditions with average metal cluster sizes that varied from 2.5 to 6.0 nm,

while the catalytic activity measurements were performed in a reactor contiguous to

CHAPTER 2

20

the surface analysis chamber. The specific rates of reaction were dependent on the Au

cluster size with a maximum occurring at 3.2 nm [22].

The contribution of the support is also important in determining the CO oxidation

activity [13, 14, 23]. Ribeiro et al. [24] compared catalytic activity of gold particles

supported on Al2O3, ZrO2 and 10% ZrO2/Al2O3 in CO oxidation. In this case

Au/ZrO2 samples exhibited the best performance. However, Grunwaldt et al. [25]

reported that the Au/TiO2 catalyst showed significantly higher activity than the

Au/ZrO2 catalyst, when the particle size on both supports was comparable. The

uncalcined Au/TiO2 catalysts also exhibited high activity, whereas the uncalcined

Au/ZrO2 catalysts were inactive under the same conditions. They suggested that the

different number of the low-coordinated gold sites and different interactions between

gold NPs and oxide supports could lead to different activities on the two supports [23].

Gold clusters on CeO2 were found to be catalytically active at 353 K for CO oxidation

[26]. Moreover, Au/Mn2O3 materials were observed to exhibit high activities for low-

temperature CO oxidation. The reaction rates for CO oxidation were comparable with

the highly active Au catalysts supported on other oxides [27]. Schubert et al. [28]

made a thorough comparison of gold catalysts on different support materials, and

suggested that metal oxide-supported Au catalysts could be grouped into two

categories with respect to CO oxidation. (1) Gold catalysts with inert support

materials, such as SiO2, Al2O3, or MgO, were intrinsically less active. Catalysts with

a relatively high activity could be prepared on these supports as well, but only if gold

existed in a highly dispersed state. These catalysts showed a strong dependence on the

metal particle size and lost their activity rapidly with increasing size of the gold

particles. For these systems oxygen adsorption occurred directly on the gold particles,

either at defect sites (steps, edges, and kinks) or facilitated by variations in the

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21

electronic structure of small metal particles. (2) Au catalysts supported on reducible

transition metal oxides such as Fe2O3

Gold catalysts supported on metal hydroxides could also show high catalytic

activities in low-temperature CO oxidation. Among the obtained Au catalysts, those

on Mn(OH)

exhibited a significantly enhanced activity for

CO oxidation, which was attributed to their ability to provide reactive oxygen [28].

The existence of an oxygen reservoir on the support could reduce the turnover

frequency (TOF) dependence on the gold particle diameter, since oxygen dissociation

was no longer rate-limiting. As a consequence, the TOF was not governed by particle

size effects as suggested for inert support materials. However, this made the

performance probably sensitive toward the microcrystalline structure of the metal-

support interface, so that the activity of such systems often depended crucially upon

the pre-treatment method. It was postulated that the independence of the TOF from

the Au particle size could apply only to low metal loadings, where the metal particles

were sufficiently distant from each other and the oxygen supply was not rate-limiting.

2 and Co(OH)2 were most highly active even at 203 K [13]. Those on

Fe(OH)3 and Ti(OH)4

were also able to catalyse CO oxidation at low temperatures

(203-273 K). The catalysts on metal hydroxides exhibited much greater CO oxidation

activity than those on the corresponding metal oxides [13, 29].

2.2.3. Reaction mechanism

There has been much discussion about the active site and reaction mechanism of

gold catalysts. In 2000 Bond and Thompson [30] suggested that Au0, Aux+ and the

metal oxide support contributed to activity. They proposed a model where Au atoms

at the interface between the gold and the support were the active oxidation centres.

These peripheral gold atoms were responsible for the oxygen activation in the CO

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22

oxidation. Hao and Gates [31] have shown that the activated Au/MgO catalyst

contained gold clusters and these were principally Au0. On the basis of IR

characterisation of the catalyst under CO oxidation conditions, it has been suggested

that CO2 oxidised Au0 in supported clusters to form Aux+ sites. They inferred that one

or both of these species might be involved in the catalytic reaction, perhaps with the

charged species being at the metal oxide support interface. A similar conclusion was

made in other studies [23, 26, 32], which suggested that gold could be oxidised to Au+

in the presence of CO and Aux+/Au0

Catalytic activities of well-ordered monolayers and bilayers of gold atoms that

completely covered the TiO

redox couples were active in low-temperature CO

oxidation.

2 support have been studied by Chen and Goodman [33].

They found that gold bilayers were more active than a monolayer, and the

combination of the first- and second-layer Au sites was necessary to promote reaction

between CO and O2. It was likely that the interaction of the first-layer Au with Ti3+ of

the support, yielding Aux–, was crucial for oxygen activation. However, it has been

shown that CO could strongly adsorb on the Au bilayer structure. By studying

Au/TiO2

On the other hand, gold clusters exhibited better activity for CO oxidation than the

mononuclear gold species [35]. Based on the analysis of scanning transmission

electron microscopy, Herzing et al. [36] proposed that active sites in gold NPs with

high catalytic activity for CO oxidation were the bilayer clusters that were about 0.5

nm in diameter and contained only 10 gold atoms.

catalyst activity Choudhary and Goodman [34] have shown the evidence to

support the model systems proposed in the study of Chen and Goodman.

Electronic structures of gold NPs supported on TiO2 have been investigated by

Okazaki et al. [37] utilising electron holography methods, scanning tunnelling

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23

microscopy and first-principles calculations. The dependence of the mean inner

potential of the gold NPs on TiO2

on the size of gold particles was observed. Mean

inner potential was given as the zero-order Fourier coefficient of the crystal potential.

Inner potentials of gold NPs with size 5 nm became larger than the bulk value. These

authors suggested that this tendency for the potential of the gold nanocrystals to

increase their size correlated well with the catalytic behaviour of these particles for

CO oxidation.

2.3. Catalytic oxidation of organic compounds by gold catalysts

Supported Au catalysts could catalyse many reactions involving various organic

compounds. Turner et al. [38] showed that small 55-atom gold clusters (1.4 nm)

supported on inert materials (BN, SiO2 and C) were efficient catalysts for the

selective oxidation of styrene to benzaldehyde. They found a significant size threshold

in catalytic activity because catalytic activity decreased once the Au particle was

above 2 nm in diameter. The binding energy of Au 4f7/2 for the NP was 1.1 eV higher

than the value of bulk Au, indicating that catalytic activity of gold nanoclusters arose

from the altered electronic structure. The oxidation results appeared to coincide with

the report by Hughes et al. for Au/C catalyst [39]. Hayashi and Haruta have reported

that gold supported on TiO2

Supported gold NPs are active and selective for the oxidation of alcohols [41-56].

High pH of the reaction system could play a crucial role in enhancing the catalytic

activity. The effect of basicity was to provide an OH

exhibited excellent selectivity in the partial oxidation of

propylene, propane and iso-butane to propylene oxide, acetone and tert-butanol,

respectively [40].

- anion and form Au-OH- site,

which was essential for hydrogen abstraction from alcohol [56]. Conte et al. [57]

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24

showed that alcohol oxidation proceeded through formation of Au-H intermediate and

the rate determining step was C-H bond cleavage. The oxygen’s role in this reaction

was to remove the hydrogen from the gold surface, maintaining the catalytic cycle,

not to oxidise the alcohol directly. With gold alone the reaction process was difficult,

though Biella and Rossi reported that gold catalysts were also active for oxidation of

gas-phase compounds without base addition [58]. Gold on carbon was observed to be

active for oxidation of various alcohols, such as propenol, butanol and ethylene glycol

[59]. Su et al. [60] demonstrated that the combination of gold NPs on Ga2O3 support

was effective for the solvent-free oxidation of benzyl alcohol with molecular oxygen.

Benzaldehyde could be obtained with high selectivity under 403 K. The Au/Ga2O3

catalyst was stable and the alcohol conversion still remained better than 98% after

recycling four times. With base addition, glycerol was oxidised to glyceric acid

entirely using 1% Au/charcoal and 1% Au/graphite catalyst under mild reaction

conditions (60 °C, water as solvent) [61-63]. Under the same conditions glyceric acid

was also obtained with supported Pd and Pt catalysts, but the main products were

undesired by-products, such as CO2, HCHO and HCOOH. Moreover, various gold

catalysts (supported on TiO2, MgO, Al2O3 and Fe2O3

The support materials could be a crucial factor to control gold catalyst activity.

Oxygen could be activated by the support, leading to fast activity recovery of the

catalysts [57]. In the study of benzyl alcohol oxidation over the Au/Ga

) exhibited activities for the

selective oxidation of aminoalcohols to aminoacids [64]. Only a few percents of

amine oxide by-product could be detected.

2O3 catalyst,

the significantly enhanced activity was attributed to a strong interaction between the

gold NPs and the Ga2O3 support as well as the enhanced alcohol dehydrogenation

capabilities [60]. Corma and co-workers showed that Au/CeO2 catalysts were active

CHAPTER 2

25

for the solvent-free selective oxidation of alcohols to aldehydes [65] and aldehydes to

acids [66]. The catalyst TOF was 12500 h-1 for the conversion of 1-phenylethanol into

acetophenone at 160 °C, which was higher than the catalytic activities with supported

Pd catalysts under same reaction conditions [67]. The interaction between gold and

oxide support led to positively charged gold. Then the alcohol reacted with the Lewis

acid sites of Au/CeO2

Alloying is also used to enhance the catalytic activity of gold catalysts. Enache et

al. [69, 70] showed that alloying Pd with the Au on a TiO

to form a metal alkoxide, and subsequently underwent a rapid

hydride transfer from C-H to produce the ketone. Tsunoyama et al. [68] reported that

gold nanoclusters supported on poly(N-vinyl-2-pyrrolidone) (PVP) could catalyse

benzyl alcohol oxidation in water at ambient temperature. The reactants could access

the gold particle surface, due to the interaction through multiple coordination of PVP.

2 support (Au-rich core with

Pd-rich shell) achieved very high TOFs for the oxidation of alcohols and improved the

selectivity to aldehydes. In this case Au acted as an electronic promoter for Pd, which

led to electronical influences of the Pd catalytic properties significantly. Then hydride

abstraction reaction from alcohols could be activated. Au-V and Au-Nb catalysts were

also found to be very active in the oxidation of methanol to CO2

These studies have been extended to the sugar oxidation. High catalytic activity

about the oxidation of glucose was observed [72, 73], and gold particle size seemed to

be the major factor influencing the catalytic reaction activity. The addition of Pd or Pt

to Au/C catalysts has been found to enhance reaction rates for the selective oxidation

of d-sorbitol to gluconic and gulonic acids [74]. Basheer et al. [75] have shown that a

simple capillary reactor could be used for the selective oxidation of glucose, thus

oxidation in a flow reactor was feasible using a supported gold catalyst. Ishida et al.

[76] found that gold catalytic mechanisms for glucose oxidation were different

[71].

CHAPTER 2

26

between gas-phase and liquid-phase reactions. The support effect was the most

important for the gas-phase reactions, while Au particle size was critical in the liquid

phase. The rate determining step in the liquid phase reaction was suggested to be the

oxidation of glucose, because glucose could adsorb on gold surface easily [75]. Such a

reaction was found to be first order with respect to oxygen concentration.

Gold catalysts could also oxidise aldehydes. Jia et al. [77] investigated that the

catalytic activities of gold catalysts based on different supports prepared by the DP

method could be observed for the oxidation of HCHO. All these catalysts exhibited

good activity and the Au/CeO2 catalyst showed the highest activity among them with

the 100% conversion at 353 K. Similar to the case of CO oxidation, the gold catalysts

on the as-precipitated hydroxides exhibited higher activity than those on the

corresponding oxide supports. In the catalysts on hydroxide precipitate the gold NPs

dispersed homogenously on the support surface and had more active sites. Gold

supported on a carbon support could oxidise various aldehydes to their corresponding

carboxylic acids in water solution under mild conditions without activity loss on

recycling [78]. Corma and Domine [79] reported that gold supported on CeO2

catalysed the selective aerobic oxidation of aliphatic and aromatic aldehydes better

than other catalysts such as Pt/C/Bi materials. Marsden et al. [80] described an

attractive route to produce esters from aldehydes in which gold NPs exhibited a highly

catalytic ability. The most economic oxidant, air, was used with the oxidation even

took place at 200K. Fristrup et al. [81] found in isotopic labelling experiments, that C-

H bond activation was the rate-determining step to oxidise aldehydes over gold

catalysts.

2.4. Catalytic oxidation of organic compounds by silver catalysts

CHAPTER 2

27

Silver catalysts have also been observed to be active in selective alcohol oxidation.

The partial oxidation mechanism of gas-phase benzyl alcohol over SiO2-supported

silver catalysts prepared by NaBH4 reduction has been studied. Gaseous oxygen

molecule could interact with the silver surface through electron transfer from the

electron-rich metallic silver bulk to the oxygen, followed by the formation of atomic

oxygen species. The surface silver species could be positive-charged silver species,

although the bulk was metallic silver. The adsorbed atomic oxygen species could

activate the C-H bond of benzyl alcohol to produce corresponding aldehyde [82].

Pestryakov et al. [83] reported that silver catalysts supported on foam ceramics were

promising for the partial oxidation of methanol to formaldehyde due to their high

catalytic properties. Modifying additives of ZrO2 and CeO2 raised activity and

selectivity of supported silver catalysts. A novel catalyst of silver NPs over a zeolite

film-coated copper grid was reported by Shen et al. [84]. It exhibited high catalytic

activity and selectivity at a relatively low temperature for the partial oxidation of 1,2-

propylene glycol to methyl glyoxal. Moreover, 150 nm Ag2

O particles supported on

CuO could also catalyse the oxidation of aromatic aldehydes to corresponding

carboxylic acids in high yields [85].

2.5. Catalytic reduction of organic compounds

Supported gold catalysts could exhibit high activity and selectivity for the

reduction of organic compounds. Guzman et al. [86] studied the ethene hydrogenation

with mononuclear gold supported on MgO powder at 353K. Experimental data

indicated that Au3+ was the predominant surface gold species during the catalysis, and

ethyl-gold species was the reactive intermediates. Other alkenes, alkynes and α,β-

unsaturated aldehydes were also investigated in heterogeneous gold-catalysed

CHAPTER 2

28

reduction [87-90].

Functionalised anilines have been important industrial intermediates [91]. Corma

et al. [92, 93] found that they could be synthesised through selective reduction of nitro

groups with catalysts of gold NPs supported on TiO2 or Fe2O3

. These two gold

catalysts could reduce over 98% 3-nitrostyrene with the selectivity that 96% of the

product was 3-vinylaniline. Neither Pt nor Pd catalysts were selective for the

reduction reactions. They proposed that the fast step was the reduction reaction from

nitrobenzene to phenylhydroxylamine, followed by slow reduction of

phenylhydroxylamine to aniline [94].

2.6. Noble metal/semiconductor photocatalysts

The working mechanism of semiconductor photocatalysts is well known:

semiconductors generate electron-hole pairs when light irradiation energy is enough

to overcome band gap, and the photogenerated electron-hole subsequently can induce

the degradation of the organic compounds [95]. Studies indicated that one of the

methods proposed to enhance the photocatalytic activity of semiconductor materials

was the surface modification with noble metal NPs [96-100]. Once noble metal

particles contact with semiconductor surface, the Fermi level of noble metal shifts to

the semiconductor Fermi level. Then the photogenerated electrons from

semiconductor are transferred to noble metal NPs resulting in effective charge

separation. Moreover, oxygen can trap electrons from gold NPs readily and enhance

the photocatalytic activity

This mechanism could be supported by the report of Sonawane and Dongare [96].

By studying thin films of Au/TiO2 prepared by simple sol-gel dip coating method,

they showed that the photocatalytic activity of phenol decomposition by Au/TiO2

CHAPTER 2

29

photocatalyst could be improved by 2-2.3 times that of TiO2. Similar experimental

results on the photocatalytic activity of Au/TiO2

Moreover, oxygen could trap electrons from gold NPs readily and enhance the

photocatalytic activity. Capturing and transferring photoelectrons could result in the

decrease of recombination of electrons and holes. Tian et al. [98] reported that

Au/TiO

thin films were reported for the

reaction of methylene blue degradation [97].

2 samples prepared by washing treatment showed higher photocatalytic

activity for methyl orange photodegradation than those prepared by rotary evaporation.

Such a mechanism was also supported by the study of Wu et al. [99] for the

mechanism of methanol reformation on Au/TiO2 photocatalyst. Four basic steps were

involved in the reformation reaction: (1) photogeneration of excited electrons to

semiconductor conduction band; (2) the electrons transferred to gold particles and

reduced the protons to produce hydrogen; (3) the holes oxidised H2O and CH3OH,

and its reaction intermediate products adsorbed on TiO2; (4) the final intermediate

HCOOH was oxidised to CO2

. Moreover, Zheng et al. [100] reported that the

photocatalytic activity of Ag/ZnO photocatalysts depended on the dispersion of Ag

particles in the photocatalyst and photocatalyst particles in the dye solution. The

higher the dispersions of metallic Ag in Ag/ZnO photocatalyst and Ag/ZnO catalyst in

the dye solution were, the higher the photocatalytic activity of Ag/ZnO photocatalyst

should be.

2.7. Noble metal photocatalysts

Semiconductor photocatalysts such as TiO2 and ZnO have a serious drawback:

they cannot efficiently utilise visible light due to the band gap. For example, the band

gap of TiO2 semiconductor is 3.0-3.2 eV, electron-hole pairs and degradation of

CHAPTER 2

30

organic compounds only can occur in the UV-illuminated process. However, UV

radiation accounts for less that 4% energy of the incoming sunlight, while the visible

light (wavelength above 400 nm) constitutes around 43% of solar energy [101]. Hence,

one of the greatest challenges for photocatalyst study is to devise new catalysts that

exhibit high activity when illuminated by visible light. This will allow us to efficiently

use sunlight, the abundant and clean energy source with low cost, to drive chemical

reactions.

Various methods have been developed to produce visible light photocatalysts [96-

100]. Researchers have concentrated on the modification of semiconductor materials.

However, the search for these photocatalysts should not be limited to semiconductors,

as other materials, such as noble metal NPs may offer superior photocatalytic

properties via alternative novel mechanisms.

2.7.1. Plasmon absorption of noble metal NPs

An important feature of the noble metal NPs is that they have intensive absorption

of visible light due to surface plasmon resonance (SPR) effect [102, 103]. Plasmon is

charge-density oscillations propagating in a plasma, and surface plasmon is plasmon

oscillations that can propagate at the interface of metal and dielectric medium. As

shown in Figure 3 [104], the incoming irradiation, which is an oscillating

electromagnetic field, induces surface plasmon oscillation of the metal electrons. As

the wave in front of the light passes through metal NP, the metal electron density is

polarised to one side and oscillates in resonance with the light frequency. According

to quasistatic approach, the electromagnetic field outside the particle, Eout, has a direct

radio to (εin − εout)/(εin + 2εout) [105]. Here εin is defined as the dielectric constant of

the metal NP, and εout is the dielectric constant of the external environment. It is

CHAPTER 2

31

suggested that the maximum enhancement occurs when (εin + 2εout) approaches zero

(εin ≈ −2εout). Noble metals such as gold and silver can fit the equation that εin ≈

−2εout, and they are the most suitable metals for the generation of a surface plasmon.

The resonance condition depends on the shape, size and dielectric constants of both

the metal and the surrounding material. As the shape or size of the NP changes, the

surface geometry changes and a shift of the electric field density appears on the

surface. This causes a change in the oscillation frequency of the electrons and strong

field enhancement of the local electromagnetic fields near the rough surface of noble

metal NPs [105]. The enhanced local field strength can be over 500 times larger than

the applied field for the noble metal nanomaterials with thorny structures, edges and

concave curvatures, such as nanowires, cubes, triangular plates and NP junctions

[106].

Incoming light +++

+

---- +

+++

--- -

Figure 3. Origin of surface plasmon resonance due to coherent interaction of the

electrons in the conduction band with light [104].

The SPR absorption may also cause rapid heating of the NPs [107, 108].

Noble metal NPs also exhibit considerable UV light absorption due to the

interband transition (the transition of 5d electrons to the 6sp band for gold, and from

Under

irradiation of a pulsed Nd:YAG laser at 532 nm, the temperature of the gold particles

measured by the photon counting system was about 2500K. The high temperature was

caused by the large absorption of pulsed laser energy by the gold particles through

SPR effect and the low heat transfer to the surrounding environment [107].

CHAPTER 2

32

4d to the 5sp band for silver) [109-112]. The optical absorption due to interband

transitions has been observed to dominate the plasmon absorption on decreasing the

particle size [109], and the interband absorption was found to be very sensitive to the

thermal character of the distribution [110].

2.7.2. Gold photocatalysts

The combination of these two properties, the SPR absorption and the catalytic

activity of noble metal NPs, offers an interesting hypothesis: as the light absorption by

the noble metal NPs heats the particles, this may be sufficient to activate molecules on

the NPs to induce the reaction of the molecules. This means we can drive reactions on

noble metal NPs by visible light at ambient temperature. Recently, we reported a new

finding [2]: when illuminated with visible light, gold NPs on oxide supports (ZrO2,

CeO2 and Fe2O3) could exhibit significant activity for oxidation of formaldehyde and

methanol in air at room temperature (25 °C). The TOF, being about 1.2 × 10-3

molecules of HCHO (Au atom)-1 s-1, was comparable to the frequencies for the CO

oxidation on the gold catalysts, by heating the reaction system to 80 °C or above. The

catalytic activity was found to be dependent on the intensity of light irradiation, which

indicated undoubtedly that the reaction was driven by visible light. The band gaps of

oxides in this study were much larger than the energies of visible light photons. It was

also impossible that charge separation could be conducted by the transfer of

photoexcited electrons from gold NPs to oxides [107]. We tentatively suggested that

the irradiation of incident light with wavelength in the range of the SPR band might

result in two consequences: First, the light absorption by the gold NPs could heat

these NPs up quickly [108]. Second, the interaction between the oscillating local

electromagnetic fields and polar molecules could also assist in activating the

CHAPTER 2

33

molecules. The proposed reaction mechanism was distinctly different from that

occurring in the reaction catalysed by semiconductor photocatalysts. Our finding that

supported gold NPs could absorb visible light revealed that the photocatalytic activity

and mechanism of the gold NPs in the photocatalysts of Au/oxide had not been

comprehensively recognised. The finding highlighted a new direction of catalysis and

heralded significant changes in the economics and environmental impact of the

chemical production.

2.7.3. Silver photocatalysts

Silver photocatalysts could exhibit high activities for the photocatalytic

decomposition of NOx into N2 and O2 under UV irradiation [113-116]. It has been

reported that UV irradiation of Ag+/ZSM-5 photocatalyst in the presence of 1 Torr of

N2O at 298 K led to the efficient formation of N2 and O2 [113, 114]. The yield of N2

molecules increased with a good linearity versus the irradiation time, indicating that

the reaction proceeded photocatalytically [113, 114]. On the other hand, only a small

amount of N2 was observed on Ag0/ZSM-5 photocatalyst. These results clearly

indicated that Ag+ ions played a crucial role in the photocatalytic decomposition of

NOx. It was observed that the effective wavelength range for the photcatalysis was

from 200 nm to 250 nm, where the absorption (excitation) of the supported Ag+ ions

occurred [115, 116]. The silver electron transferring from the 5s orbital of photo-

excited Ag+ into the π anti-bonding molecular orbital of NOx could weak the N-O

bond. At the same time, an electron transfer from a π-bonding molecular orbital of

another NO to the vacant 4d orbital of Ag+ could result in further weakening of the N-

O bond, leading to the decomposition of NOx [115, 116].

CHAPTER 2

34

The effect of O2 addition on the photocatalytic decomposition of NOx over

Ag+/ZSM-5 catalyst was also investigated. It was found that the addition at high

pressures (above 1 Torr) could not lead to the oxidation of Ag+ to Ag2+ in the

Ag+/ZSM-5 photocatalyst, in contrast to the easy oxidation of Cu+ to Cu2+ in the

Cu+/ZSM-5 photocatalyst [115, 116]. The excellent chemical stability of Ag+,

especially in an oxidative atmosphere, is one of the advantages in the utilisation of the

Ag+/ZSM-5 materials as photocatalysts for NOx elimination in the atmosphere.

Moreover, evacuation treatment at high temperature is not necessary to produce

Ag+/ZSM-5 photocatalyst, unlike the preparation of Cu+/ZSM-5 which requires the

treatment above 973 K to produce Cu+

Jacobs et al. [117] reported that photochemical/thermal cleavage reaction of water

into H

as active species [115, 116].

2 and O2 could be catalysed on Ag+/Y zeolite. Under sunlight irradiation Ag+/Y

zeolite photocatalysts exhibited good activities for the O2 evolution, accompanied by

the reduction of Ag+ ions into Ag clusters. Then the thermal treatment of the reduced

Ag+/Y at 873 K led to the evolution of H2. Thus, photochemical/thermal cleavage of

water proceeded on Ag+

Fourier

transform

/Y zeolite photocatalysts, and this process could be repeated

several times with almost same efficiency. The structural changes of the silver species

supported on Y zeolite during the photocatalytic process were investigated by

infrared (FT-IR) spectroscopy. After sunlight irradiation, the intensity of

the FT-IR band due to the Ag+-CO species decreased, while the FT-IR band due to the

surface OH group appeared [118]. These results indicated the reduction of Ag+ ions.

Moreover, Ozin et al. [119, 120] reported that UV irradiation of Ag/Y zeolite at

ambient temperature led to the selective dimerisation of the hydrocarbons, such as

methane to ethane, ethane to n-butane, and propane to hexane.

CHAPTER 2

35

It has been known for a long time that silver halides are photosensitive materials

that have been extensively used as source materials in photographic films. Kakuta et

al. [121] reported that AgBr/SiO2 photocatalyst was used for hydrogen generation

from CH3OH/H2O solution under UV illumination. H2 was continuously evolved

even after UV illumination for 200 h. As suggested by Kakuta et al., Ag0 species

could be formed on AgBr during the early stage of the light irradiation. Then electron-

hole separation might occur smoothly in the presence of Ag0 species. The latter could

catalyse H2

The visible light photocatalytic properties of Ag/AgCl due to SPR effect were

reported only recently. Wang et al. [122] found that AgCl photocatalyst was highly

efficient and stable under visible light illumination for dye photodegradation. The rate

of the dye decomposition over AgCl photocatalyst was found to be faster than that

over N-doped TiO

production from alcohol radicals formed by photogenerated holes.

2 by a factor of eight. Under visible light irradiation, AgCl

photogenerated electrons were expected to be trapped by O2 to form superoxide ions

(O2-

) and other reactive oxygen species.

2.8. Summary

The noble metal NPs, with proper control of the particle size and the suitable

selection of oxide support materials, have been found to be very active catalysts for

oxidation and selective oxidation reactions. Fundamental research aiming at

interpreting good catalytic activity and selectivity of noble metal materials at an

atomic scale is also invaluable for understanding the photocatalytic properties of

noble metal NPs, as the combination of the catalytic activity and the light absorption

property of noble metal NPs creates great opportunity for a new class of

photocatalysts. The noble metal photocatalysts have moderate redox ability, and the

CHAPTER 2

36

processes with the photocatalysts generally work under moderate conditions.

Moreover, such light absorption is a local effect, limited to the noble metal particles

so that the light only heats the noble metal NPs which generally account for a few

percent of the overall catalyst mass to elevated temperature, while the reaction system

remains at temperatures close to the ambient temperature. Therefore, such a process

will require much lesser energy input to catalyse reactions. These properties make the

photocatalysts attractive for applications over a wide range from final chemical

synthesis and environmental remediation, and present future opportunities for

industrial and environmental applications, especially for the processes using sunlight,

the most abundant energy in the world to drive reactions and degrade pollutants.

CHAPTER 2

37

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CHAPTER 3

45

CHAPTER 3. VISIBLE LIGHT DRIVEN

OXIDATION OF ORGANIC CONTAMINANTS IN

AIR WITH GOLD NP CATALYSTS ON OXIDE

SUPPORTS

Introductory Remarks

In this chapter, we reported that gold NPs could catalyse the oxidation of the

organic compounds under visible light illumination at ambient temperature. The

article describing our finding was published in Angewandte Chemie International

Edition in 2008 [1]. To the best of our knowledge, it was the first report on

photocatalytic activity of gold NPs, though it has been discovered that gold NPs can

enhance semiconductor photocatalytic activity once gold particles contact with its

surface [2]. In our experiment, the band gaps of supports were above 5.0 eV. Visible

light could not excite electrons from the valence band to the conduction band. Hence,

the mechanism of photocatalytic activity over gold NPs was not similar to that in

semiconductor photocatalysts, such as TiO2

The size of Au particles produced by conventional IMP method is relatively large.

In order to produce small gold NPs with good photocatalytic activities, trisodium

citrate can be added into the reaction solution to prevent the agglomeration of the

obtained gold NPs. The support contribution is also very important in determining the

oxidation activity of gold materials. Oxide supports in gold catalysts have been

classified into “inert” and “active” supports [3]. The active supports, such as ZrO

materials.

2,

can adsorb oxygen molecules. Silica is an inert support which could not adsorb

CHAPTER 3

46

oxygen. Usually active supports result in good catalytic activities. In this chapter we

investigated ZrO2, SiO2, CeO2 and Fe2O3

According to our suggested mechanism of gold-catalysed reactions, light energy

could heat gold NPs up at a rate of 3-5 °C per second. On account of the heat transfer

from gold to support or atmosphere, probably the temperature of gold NPs reaches to

several tens degrees. Then we suggested that the gold photocatalysts in our

experiment could degrade organic compounds which could be oxidised over gold

materials at moderately elevated temperature in dark. It was the reason why we chose

HCHO as the reactant in the gold-catalysed reactions.

as potential gold supports. The support’s

effect was studied through the difference of photocatalytic activities over various gold

photocatalysts.

In our experiment gaseous specimens were analysed by gas chromatography.

Specimens passed through a mechanize where CO2 was converted to methane. Then

the CO2

concentration could be measured by flame ionization detector (FID).

[1] X. Chen, H. Y. Zhu,

[2] R. S. Sonawane, M. K. Dongare, J. Mol. Catal. A. 2006, 243, 68-76.

J. C. Zhao, Z. F. Zheng, X. P. Gao, Angew. Chem. Int. Ed.

2008, 47, 5353-5356.

[3] M. M. Schubert, S. Hackenberg, A. C. Veen, M. Muhler, V. Plzak, R. J. Behm, J.

Catal. 2001, 197, 113-122.

STATEMENT OF CONTRIBUTION

The authors listed below have certified that: 1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in their field of expertise;

2. They have public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria; 4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication on the Australian Digital Thesis database consistent with any limitations set by publisher requirements. In the case of this chapter:

Visible-Light-Driven Oxidation of Organic Contaminants in Air with Gold Nanoparticle Catalysts on Oxide Supports

Xi Chen, Huaiyong Zhu, Jincai Zhao, Zhanfeng Zheng, and Xueping Gao Published in the journal: Angew. Chem. Int. Ed. 2008, 47, 5353-5356.

Contributor Statement of contribution Xi Chen Developing the experimental methods, conducted most of

experimental and characterisation work, problem resolving, data interpretation, manuscript revising.

Huaiyong Zhu Proposing the concept of noble metal nanoparticles as photocatalysts, designing the experiment and set-up, organising the research, interpreting data and writing manuscript.

Jincai Zhao Conceptual design on surface plasmon, assisted with data interpretation of photocatalysis and manuscript revising.

Zhanfeng Zheng Reaction system design, assisted with data discussion and manuscript revising.

Xueping Gao Conducted TEM measurements and explain the obtained data, manuscript revising.

halla

Due to copyright restrictions, this article is not available here. Please consult the hardcopy thesis available from QUT Library or view the published version online at: http://dx.doi.org/10.1002/anie.200800602

CHAPTER 3

51

Supporting information

TOF is defined as the number of reactant moles that the catalyst can convert per

catalytic site per unit time. In this chapter, it can be given by

TOF = (Mreact * C) / (MAu

Here M

* t)

react is defined as the initial mole number of organic reactant. C is

photocatalytic conversion after light illumination. MAu

SPR effect of gold NPs was observed to be only applicable to the oxidation of polar

species, which was also verified by conducting the acetylene oxidation under blue

light illumination at ambient temperature. No acetylene conversion was detected using

Au/ZrO

is the gold mole number in the

photocatalyst, and t is the illumination time.

2

When the Au/ZrO

photocatalyst.

2 photocatalysts was heated at 500 oC, the HCHO conversion

decreased to 8% because the gold NP size increased from about 20–30 nm to over 100

nm. A blank experiment under the otherwise identical conditions but without Au-NPs

(calcined ZrO2

When illuminated with visible light, gold NPs dispersed on oxide supports exhibited

significant activity for oxidation of formaldehyde and methanol in air at room

temperature. It was believed that the visible light was absorbed by gold NPs due to

surface plasmon resonance effect, and the particles were heated up quickly to the

temperature at which the organic molecules were activated to react with oxygen.

Because the light heated the gold NPs only, it required much lesser energy input to

activate the reaction, compared to conventional catalytic oxidation under directly

supports only) was conducted. No conversion was observed.

CHAPTER 3

52

heating. This finding revealed the possibilities to drive other reactions with abundant

sunlight on gold NPs at ambient temperature.

The size distributions of the gold NPs on four supports were calculated from the

TEM images and given in Supporting information (SI Figure 1). The distributions on

ZrO2 and SiO2 were broad, and with peak values at 27 and 53 nm, respectively. Most

of the gold NPs on CeO2 were below 10 nm, and most of the particles on Fe2O3

supports were between 10 and 30 nm. The mean sizes of gold particles were in an

order of Au/CeO2 < Au/Fe2O3 < Au/ZrO2< Au/SiO

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

30

Au/ZrO2

Freq

uenc

y / %

Size distribution / nm

2.

0 2 4 6 8 100

5

10

15

20

25

30

Freq

uenc

y / %

Size distribution / nm

Au/CeO2

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

30

Freq

uenc

y / %

Size distribution / nm

Au/Fe2O3

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

30

Freq

uenc

y / %

Size distribution / nm

Au/SiO2

SI Figure 1. Size distribution of the gold NPs on four supports. In each sample, 60

gold NPs were counted.

CHAPTER 3

53

We could obtain the absorption spectra by the gold NPs in a catalyst from the

difference between the spectra of the catalysts and oxide supports (SI Figure 2). The

absorbed irradiation energy by the gold NPs was then derived from the overlap area of

the absorption spectrum of the gold NPs and the spectra of the irradiation tube sources

(SI Figure 3) as well as the irradiation energy (for instance 0.17W/cm2 for blue light).

The TOF per unit of the irradiation energy absorbed by gold NPs of the catalysts,

normalized TOF, was calculated with the data of gold content and HCHO conversion

of the catalyst. The absorbed energy by the gold NPs on SiO2 was lower than those

particles on other oxides. This could be an important reason for the relative low

activity of Au/SiO2

sample. The activities of the catalysts on various oxides in the

normalized turnover frequency were given in supporting information (SI Figure 4).

300 400 500 600 700 8000.0

0.5

1.0

1.5

Abso

rban

ce

Au/ZrO2

ZrO2

Wave length / nm300 400 500 600 700 800

0.0

0.5

1.0

1.5

Wave length / nm

CeO2

Au/CeO2Abso

rban

ce

300 400 500 600 700 8000.0

0.5

1.0

1.5

Wave length / nm

Fe2O3

Abso

rban

ce Au/Fe2O3

300 400 500 600 700 8000.0

0.2

0.4

Wave length / nm

Abso

rban

ce

SiO2

Au/SiO2

SI Figure 2. UV-Visible spectra of Au/ZrO2, Au/CeO2, Au/Fe2O3, Au/ZrO2 and

Au/SiO2, the spectra are also compared with those of the corresponding oxide support.

Absorption spectra by the gold NPs (red curves) are obtained from the difference

between the spectra of the catalyst and oxide support.

CHAPTER 3

54

300 400 500 600 700 8000

20

40

60

80

100

120

4

3

2

Abs

orpt

ion

inte

nsity

/ %

Wave length / nm

1

SI Figure 3. Absorption intensity of gold NPs on different supports. 1) Au/Fe2O3; 2)

Au/CeO2; 3) Au/ZrO2 and 4) Au/SiO2. Blue dot line shows irradiation intensity of six

blue light lamps, while the red dot line shows irradiation intensity of six red light

lamps.

CHAPTER 3

55

0

2

4

6

8

10

12

14

Au/SiO2Au/CeO2Au/Fe2O3

Tur

nove

r fre

quen

cy

/ 10-4

Au-

atom

-1 s

-1

Au/ZrO2

SI Figure 4. The influence of the oxide supports on the TOF of HCHO oxidation

reaction. The bars in blue is the HCHO turnover frequency (in 10-4 Au-atom-1 s-1)

under illumination of blue light (with wavelength between 400 and 500 nm) and the

bars in red is the conversion under red light (with wave length between 600 and 700

nm).

CHAPTER 3

56

SI Figure 5. Gaseous photocatalytic system. The glass vessel was put in a wood

chamber with light tubes (18 W/tube, Philips) as the light source. Air conditioning

was applied in the chamber to maintain the temperature at 25 oC.

CHAPTER 3

57

Erratum page

There were some errors in this chapter. The light intensities under the irradiation

of six, four and two blue light tubes should be 0.011, 0.008 and 0.006 W cm-2,

respectively. The intensity by six red light tubes should be 0.010 W cm-2. Sunlight

intensity was measured to be 0.001 W cm-2

. The absorbed energy and normalized

TOF data listed in Table 1 should be replaced.

Table 1. Gold content, absorption of irradiation energy and turnover frequency of the

gold NPs on various oxides

Au/ZrO Au/CeO2 Au/Fe2 2O Au/SiO3 2 Gold content (wt%) 2.44 2.62 3.10 2.24 Absorbed energy by gold NPs under blue light (W cm-2 0.011

) 0.010 0.008 0.007

Turnover frequency under blue light (Au-atom-1 s-1 1.2 × 10

) 6.5 × 10-3 4.6 × 10-4 0 -4

Normalized turnover frequency under blue light (cm2 J-1Au-atom-1

1.1 × 10

)

6.5 × 10-1 5.7 × 10-2 0 -2

Absorbed energy by gold NPs under red light (W cm-2 0.009

) 0.010 0.010 0.005

Turnover frequency under red light (Au-atom-1 s-1 9.4 × 10

) 5.6 × 10-4 3.4 × 10-4 2.7 × 10-4 -4

Normalized turnover frequency under red light (cm2 J-1Au-atom-1 1.0 × 10

) 5.6 × 10-1 3.4 × 10-2 5.4 × 10-2 -2

CHAPTER 4

58

CHAPTER 4. MECHANISM OF SUPPORTED

GOLD NPS AS PHOTOCATALYSTS UNDER

ULTRAVIOLET AND VISIBLE LIGHT

IRRADIATION

Introductory Remarks

In this chapter, it was reported that gold NPs could exhibit photocatalytic activity

for organic compound degradation in aqueous solution. Our article demonstrating the

finding was published in Chemical Communications recently [1]. After it was

discovered that gold NPs could induce the oxidation of HCHO gas, gold

photocatalytic activity in some liquid-phase reactions was studied. Our finding in the

project could lead to further use of noble metal photocatalyst in applications, such as

wastewaters treatment to eliminate environmental organic contamination.

Dye Sulforhodamine-B (SRB) was found to be degraded by titania materials

under visible light irradiation [2]. During the photosensitisation process the excitation

of the dye molecules occurs. The excited electrons are then injected into the titania

conduction band and reduce molecular oxygen to produce the oxidising species which

are responsible for the dye degradation. SRB in this chapter was the reactant degraded

over a gold photocatalyst. Because gold particles were considered as catalytically

active sites, TOFs of gold photocatalysts and titania materials were calculated and

compared. Experiment results showed that the activities of supported gold NPs for

SRB degradation were much higher than titania materials under both visible and UV

light. On account that phenol has been present in waste effluents of pulp and paper

industry, we also collected experimental results for phenol degradation over various

CHAPTER 4

59

gold photocatalysts. Moreover, the study of selective alcohol oxidation was conducted

in this chapter.

[1] H. Y. Zhu, X. Chen, Z. F. Zheng, X. B. Ke, E. Jaatinen, J. C. Zhao,

[2] J. C. Zhao, C. C. Chen, W. H. Ma, Top. Catal. 2005, 35, 269-278.

C. Guo, T. F.

Xie, D. J. Wang, Chem. Commun. 2009, 7524-7526.

STATEMENT OF CONTRIBUTION

The authors listed below have certified that: 1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in their field of expertise;

2. They have public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria; 4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication on the Australian Digital Thesis database consistent with any limitations set by publisher requirements. In the case of this chapter:

Mechanism of Supported Gold Nanoparticles as Photocatalysts under Ultraviolet and Visible Light Irradiation

Huaiyong Zhu, Xi Chen, Zhanfeng Zheng, Xuebin Ke, Esa Jaatinen, Jincai Zhao, Cheng Guo, Tengfeng Xie,

Published in the journal: Chem. Commun., 2009, 7524–7526. and Dejun Wang

Contributor Statement of contribution Huaiyong Zhu Developing the mechanism for gold photocatalysts, organising

the research, data interpretation and writing manuscript. Xi Chen Conducting most of experimental and analysis work, developing

experimental methods, assisting in developing the mechanism, data interpretation, revising manuscript.

Zhanfeng Zheng Developing the mechanism, conducting experiment of semiconductor, assisted with conceptual design and data analysis and revising manuscript.

Xuebin Ke Developing the mechanism, assisted with experimental design and synthesis, data analysis and revising manuscript.

Esa Jaatinen Developing the mechanism and interpretation of important concepts and mechanism, revising manuscript.

Jincai Zhao Interpretation of important concepts and mechanism on surface plasmon resonance and photocatalysis, and revising manuscript.

Cheng Guo Interpreting reaction mechanism and revising manuscript. Tengfeng Xie Conducted SPR and TPV measurements and explain the

obtained data. Dejun Wang Conducted SPR and TPV measurements and explain the

obtained data.

Mechanism of supported gold nanoparticles as photocatalysts under

ultraviolet and visible light irradiationw

Huaiyong Zhu,*a Xi Chen,a Zhanfeng Zheng,a Xuebin Ke,a Esa Jaatinen,a Jincai Zhao,b

Cheng Guo,cTengfeng Xie

dand Dejun Wang

d

Received (in Cambridge, UK) 18th August 2009, Accepted 7th October 2009

First published as an Advance Article on the web 4th November 2009

DOI: 10.1039/b917052a

Gold nanoparticles strongly absorb both visible light and

ultraviolet light to drive an oxidation reaction for a synthetic

dye, as well as phenol degradation and selective oxidation of

benzyl alcohol under UV light.

A key step in improving the photocatalysis process is the

development of new catalysts that allow us to use sunlight,

the abundant and green energy source, to drive chemical

reactions.1 It is well known that gold nanoparticles

(Au-NPs) strongly absorb visible light due to the so-called

surface plasmon resonance (SPR) effect.2 The SPR effect is the

collective oscillation of conduction electrons in the nano-

particles, which resonate with the electromagnetic field of the

incident light. SPR absorption may cause rapid heating of the

nanoparticles,3 which can induce oxidation of formaldehyde

in the air at ambient temperatures.4 Au-NPs also exhibit

considerable ultraviolet (UV) light absorption, causing the

transition of 5d electrons to the 6sp band (interband transition).5

Due to the higher photon energy, it is logical to expect that UV

light is also able to drive chemical reactions on Au-NPs. This

implies the full solar spectrum can be used for driving

reactions with the new photocatalysts of Au-NPs. However,

the nature of UV light absorption by the Au-NPs is different

from that of visible light absorption,5 and the precise nature of

the reaction mechanism for the catalysis under visible light has

not been clarified to date. In this study, we prepared a series of

samples of Au-NPs supported on zeolite Y, ZrO2 and SiO2,

and investigated their photocatalytic performance and photo-

electrical properties. Here we verify that Au-NPs at room

temperature can be used to drive chemical reactions under

light illumination throughout the solar spectrum, and in the

process gain some understanding of the gold photocatalysis

mechanism which is different from that for conventional

semiconductor photocatalysts.

About 8% of gold (metal state) was loaded on the supports

by the impregnation method. Energy dispersive X-ray spectro-

scopy (EDS) results and X-ray photoelectron spectroscopy

(XPS) spectra is shown in the Electronic Supplementary

Informationw (ESI) Table S1 and Fig. S1, respectively. Trans-

mission electron microscopy (TEM) images (Fig. S2 in ESIw)indicate that gold exists in these samples as nanoparticles. As

can be seen in Table 1, the gold photocatalysts exhibited better

catalytic performance to degrade dye sulforhodamine-B (SRB)

under UV irradiation than under blue light (wavelength in the

range 400–500 nm with the maximum intensity at 450 nm).

They not only decompose dye molecules under UV light faster

than under visible light, but are also able to oxidize phenol in

aqueous solution, which they cannot catalyze under visible

light. After 120 h of UV irradiation, the Au-NPs on zeolite Y,

SiO2 and ZrO2 converted 21%, 28% and 45% of phenol,

respectively. In the dark, SRB was not decomposed with any

one of the three gold photocatalysts. A blank experiment

under the otherwise identical conditions but without Au-NPs

(aqueous solutions with these oxide supports—ZrO2, zeolite Y

and SiO2 only) was conducted. No conversion above 3% was

observed. ZrO2 has a band gap of about 5 eV4 and according

to the UV-Vis absorption measurements (Fig. 1a), the band

gaps of zeolite Y and SiO2 are slightly larger than that for

ZrO2. These supports alone (in the absence of the Au-NPs),

exhibit little light absorption and no charges are generated

from them under irradiation with light with wavelengths

longer than 300 nm. Therefore, under irradiation of light with

wavelengths above 300 nm, almost all photogenerated charges

that lead to catalytic activity must arise from the Au-NPs.

Furthermore, the SRB degradation activity of the Au-NPs

on zeolite Y and ZrO2 is higher than that of nitrogen-doped

TiO2 under blue light. Given that the Au-NPs are the active

component for the photocatalysis and gold accounts for 8% of

the catalyst mass, the efficiency of supported Au-NPs for the

Fig. 1 Sample characterisation: (a) UV-Visible spectra; (b) surface

photocurrent spectra of the Au-NPs supported on zeolite Y and zeolite

Y (trace in black); (c) transient photovoltage spectra of the Au-NPs

supported on zeolite Y and zeolite Y (trace in black).

a School of Physical and Chemical Sciences, Queensland University ofTechnology, Brisbane, Qld 4001, Australia.E-mail: hy.zhu@qut.edu.au; Fax: +61 7 3138 1804;Tel: +61 7 3138 1581

b Institute of Chemistry, The Chinese Academy of Science,Beijing 100080, China

c College of Science, Nanjing University of Technology,Nanjing 210009, China

dCollege of Chemistry, Jilin University, Changchun 130012, Chinaw Electronic supplementary information (ESI) available: Experimentaldetails and characterisation data. See DOI: 10.1039/b917052a

7524 | Chem. Commun., 2009, 7524–7526 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

SRB degradation is much higher than titania materials under

both visible and UV light as indicated by the turnover

frequencies4 (Table 1).

Photocatalytic decomposition is complete oxidation of the

organic compound,6 which involves electron transfer from the

organic molecules to the oxidant, such as oxygen. In principle,

selective oxidation of an organic compound could be achieved

if its complete oxidation requires multiple-electron transfer

and we are able to regulate the electron transfer process by

tuning the experimental conditions. Indeed, we found that

under UV light irradiation the Au-NPs on zeolite Y (50 mg)

were able to oxidize benzyl alcohol in toluene (as solvent) to

benzaldehyde with oxygen as the oxidation agent in the

presence of 30 mg sodium hydroxide in order to increase

photocatalytic conversion and selectivity.7 23% conversion

was achieved in 48 h with high selectivity toward the product

benzaldehyde (100%). No conversion was observed when the

control experiments were conducted either without Au-NPs or

in the dark. Selective oxidation for producing aldehydes from

corresponding alcohols is a very important process for the

fine chemical industry.8 This finding reveals the potential

application of the new Au-NPs photocatalysts beyond

environmental remediation.

In the photocatalytic redox reactions the supported Au-NPs

seem to function as initializors and mediators of the electron

transfer for the oxidation reactions. The light absorption by

the samples in the UV and visible light range, the surface

photocurrent (SPC) and, the transient photovoltage (TPV)

spectra of the samples were measured to determine whether

light absorption by Au-NPs can induce electron transfer from

the particle to oxygen or not (Fig. 1). A surface current and a

transient photovoltage will arise whenever excess light-induced

charge carriers are separated in space, with the intensity of the

spectra signal being proportional to the number of the

photogenerated charges.9 These spectra also explicitly give

the dependence of the electron transfer on the wavelength of

the incident light allowing the spectral regions under which

electron transfer occurs to be identified.

The absorption peak at 520 nm in the UV-Vis spectra of the

gold supported on zeolite Y (Fig. 1a) is attributed to the SPR

absorption of the Au-NPs, which originates from the intraband

excitation of 6sp electrons (see Scheme 1).2,10 Considerable

absorption in the UV region is also observed; it results from

the interband excitation of electrons from 5d to 6sp.5,10

From the SPC spectra (Fig. 1b) it is apparent that UV

absorption produces a much larger surface photocurrent than

that induced by the SPR absorption under visible light

irradiation. Nonetheless, a large initial photovoltage is observed

in the TPV spectrum (Fig. 1c) which was measured with a

532 nm laser that has a much higher intensity than the light

used for the SPC measurement. This indicates that visible light

irradiation does generate electrical surface charges when

sufficiently intense. The SPC spectrum shown in Fig. 1b also

indicates that the interband absorption (UV) results in a much

larger proportion of electron transfer from the Au-NPs to

the oxygen molecule (Scheme 1) than the intraband SPR

absorption (visible). Consequently, more positive charges are

left in lower energy levels (in 5d band) of the Au-NPs when

they are exposed to UV light. Given the relatively high

electronegativity of gold, the Au-NPs can capture electrons

from the organic molecules adsorbed on them to neutralize the

positive charges, oxidizing the organic compound. The

reaction rate, at which the photocatalytic oxidation occurs,

increases with increasing positive charge number. This offers

an explanation for the observation that the reaction under UV

light is faster than under visible light. Furthermore, the ability

of the Au-NPs to capture electrons appears to be determined

by the position of the positive charges in the electron band

Table 1 Absorption of irradiation energy and catalytic activity of the gold photocatalysts to degrade SRB

Photocatalyst Au–zeolite Y Au–ZrO2 Au–SiO2 N-doped TiO2 TiO2 (P25)

Conversion under blue light (%)a 37 46 27 34 14Absorbed energy by Au-NPs or TiO2 under blue light (W cm�2) 0.165 0.152 0.095 0.021 0.008Turnover frequency under blue light(Au-atom�1 s�1 or Ti-atom�1 s�1)b

5.1 � 10�6 6.1 � 10�6 3.6 � 10�6 1.5 � 10�7 8.8 � 10�9

Normalized turnover frequency under blue light(cm2 J�1 Au-atom�1 or cm2 J�1 Ti-atom�1)b

3.1 � 10�5 4.0 � 10�5 3.8 � 10�5 7.1 � 10�6 1.1 � 10�6

Conversion under UV light (%)a 51 64 44 49 75Absorbed energy by Au-NPs or TiO2 under UV light (W cm�2) 0.123 0.138 0.098 0.075 0.086Turnover frequency under UV light(Au-atom�1 s�1 or Ti-atom�1 s�1)b

5.4 � 10�6 6.8 � 10�6 4.7 � 10�6 2.2 � 10�7 3.3 � 10�7

Normalized turnover frequency under UV light(cm2 J�1 Au-atom�1 or cm2 J�1 Ti-atom�1)b

4.4 � 10�5 4.9 � 10�5 4.8 � 10�5 2.9 � 10�6 3.8 � 10�6

a SRB conversions from replicate runs agree to within �4%. b Turnover frequency data in the table were calculated from the conversion after

1 h of irradiation.

Scheme 1 The diagram of band structures of supported Au-NPs and

the proposed mechanism for photocatalysis using supported Au-NPs.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 7524–7526 | 7525

structure of the supported Au-NPs. The visible light absorption

results in positive charges in gold’s 6sp band, which can

capture electrons from the molecules that are easier to be

oxidized such as dye, HCHO and methanol.4 While the UV

absorption by the Au-NPs yields positive charges in the lower

5d band of gold, which are able to oxidize the molecules that

are more difficult to be oxidized such as phenol. The transfer

mechanism we propose to explain these photocatalytic

observations with the Au-NPs is illustrated in Scheme 1.

Molecular oxygen was determined to be the oxidant of the

photocatalytic reactions, since the reduction product of

oxygen, H2O2, was detected in the reaction solutions with

Potassium Iodide Indicator paper. When the reaction was

conducted in argon atmosphere, instead of air, with other

experimental conditions remaining the same, the SRB degra-

dation was only a few percents of that in air, possibly caused

by oxygen adsorbed on the catalysts.

Oxygen molecules on the Au-NPs or the interface between

the support and the Au-NPs seize the energetic electrons in

high excited energy levels of gold’s 6sp band, forming O2�

species. Then O2� reacts with H+ to yield other active species

such as HO2� or OH� radicals.11 When electron transfer occurs

in large numbers, the SPC and TPV measurements should be

able to detect it. As the Au-NPs absorb visible light through

the SPR effect, the 6sp electrons gain energy and migrate to

higher intraband energy levels.5,10 Soon after light irradiation

the plasmon heats electron gas to an elevated temperature

(about 400–2000 K) within a time scale of the order of 100 fs

or less through electron–electron collision.6 During the process

gold electron gas obeys the Fermi–Dirac distribution at an

elevated temperature. Then electron–phonon interactions,

which share the electron energy with the nanoparticle lattice,

take place over a time scale from 500 fs to 10 ps. Therefore, it

is conceivable that the electron gas remains in an excited ‘hot’

state for up to 0.5–1 ps.12 Very recently, Furube et al. found

that the electron transfer from gold nanoparticles to titanium

oxide is fast, taking less than 240 fs.13 It is possible in our case

that a small number of excited electrons in gold gain sufficient

energy (above the green line in Scheme 1) to be captured by

oxygen molecules adsorbed on the Au-NPs under visible light

irradiation with moderate intensity. The weak SPC signal

indicates that most of these excited electrons are not captured

by oxygen molecules. On the other hand, it has been suggested

that the supported Au-NPs can attract electrons from the

organic molecules on the nanoparticles.14 Also SRB dye

molecules are excited under light irradiation. The excited

SRB molecules (SRB*) are able to inject their electrons to

the substrate.11 This additional ‘‘dye sensitization’’ effect of

the excited SRB molecules on the Au-NPs faciliate the formation

of O2� species. It may combine with the SPR effect of the

Au-NPs, producing a high rate of dye degradation. At higher

visible light intensities, such as under laser illumination, the

quantity of the positive charges increases and more electrons

can gain enough energy by multiple absorptions and are

captured by oxygen so that we observe an enhanced photo-

voltage signal, with the rate of the photo-catalytic oxidation

increasing as a function of light intensity (see Fig. S2 in ESIw).When the Au-NPs absorb UV irradiation, gold’s 5d elec-

trons are excited to the 6sp band, and many of the excited

electrons are at high energy levels (the green line or above in

Scheme 1) where oxygen molecules can seize them. Thus we

observe a large surface photovoltage signal under UV light.

The positive charges left in the 5d band have a lower energy

and therefore a greater affinity for capturing electrons from the

adsorbed organic molecules than those in the 6sp band. This

property can be utilized for two reaction regimes. First, under

UV illumination, the photocatalysts can oxidize the compounds

that they cannot oxidize under visible light, such as phenol.

Second, the greater ability to capture electrons under UV light

can be used for the oxidation of a compound to an intermediate

that is a useful chemical; meanwhile we manipulate the experi-

mental conditions to prevent further oxidation, achieving

selective oxidation with high selectivity. The selective oxidation

of benzyl alcohol to benzaldehyde is an example of this regime,

which was observed only under UV irradiation.

The major experimental observations in this study, the band

structures of the gold nanoparticles and the tentative mecha-

nism we proposed for the photocatalysis using the supported

Au-NP are summarized in Scheme 1. Given that the 6sp band

overlaps with the 5d band in terms of energy scale, the

suggested mechanism also offers the potential to switch

the specific reactions on or off by tuning the wavelength of

the irradiating light.

The finding in this study reveals a new class of photocata-

lysts and a possible pathway by which various chemical

reactions on the photocatalysts can be driven with sunlight

at ambient temperatures for environmental remediation and

fine chemical production.

Notes and references

1 N. S. Lewis, Nature, 2001, 414, 589; Z. Zou, J. Ye, K. Sayama andH. Arakawa, Nature, 2001, 414, 625.

2 P. Mulvaney, Langmuir, 1996, 12, 788; P. V. Kamat, J. Phys.Chem. B, 2002, 106, 7729; K. L. Kelly, E. Coronado, L. L. Zhaoand G. C. Schatz, J. Phys. Chem. B, 2003, 107, 668; H. Yuan,W. H. Ma, C. C. Chen, J. C. Zhao, J. W. Liu, H. Y. Zhu andX. P. Gao, Chem. Mater., 2007, 19, 1592; R. Wilson, Chem. Soc.Rev., 2008, 37, 2028; S. Eustis and M. A. El-Sayed, Chem. Soc. Rev.,2006, 35, 209; H. Y. Guo, F. X. Ruan, L. H. Lu, J. W. Hu, J. A. Pan,Z. L. Yang and B. Ren, J. Phys. Chem. C, 2009, 113, 10459.

3 A. Takami, H. Kurita and S. Koda, J. Phys. Chem. B, 1999, 103,1226; D. K. Roper, W. Ahn and M. Hoepfner, J. Phys. Chem. C,2007, 111, 3636.

4 X. Chen, H. Y. Zhu, J. C. Zhao, Z. F. Zheng and X. P. Gao,Angew. Chem., Int. Ed., 2008, 47, 5353.

5 S. Link, C. Burda, Z. L. Wang and M. A. El-Sayed, J. Chem.Phys., 1999, 111, 1255; C. Voisin, N. Del Fatti, D. Christofilos andF. Vallee, J. Phys. Chem. B, 2001, 105, 2264; B. Balamurugan andT. Maruyama, Appl. Phys. Lett., 2005, 87, 143105.

6 M. I. Litter, Appl. Catal., B, 1999, 23, 89; A. L. Linsebigler,G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735.

7 J. J. Zhu, J. L. Figueiredo and J. L. Faria, Catal. Commun., 2008,9, 2395.

8 M. Hudlicky, Oxidation in Organic Chemistry, American ChemicalSociety, Washington, DC, 1990.

9 Y. Du, M. Yang, J. Yu, Q. Pan and R. Xu, Angew. Chem., Int. Ed.,2005, 44, 7988; Q. Fang, G. S. Zhu, Z. Jin, M. Xue, X. Wei,D. J. Wang and S. L. Qiu, Angew. Chem., Int. Ed., 2006, 45, 6126.

10 K. Yamada, K. Miyajima and F. Mafune, J. Phys. Chem. C, 2007,111, 11246.

11 J. C. Zhao, C. C. Chen and W. H. Ma, Top. Catal., 2005, 35, 269.12 S. Link and M. A. El-Sayed, Int. Rev. Phys. Chem., 2000, 19, 409.13 A. Furube, L. C. Du, K. Hara, R. Katoh and M. Tachiya, J. Am.

Chem. Soc., 2007, 129, 14852.14 A. Grirrane, A. Corma and H. Garcıa, Science, 2008, 322, 1661.

7526 | Chem. Commun., 2009, 7524–7526 This journal is �c The Royal Society of Chemistry 2009

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63

Supplementary Information Experimental Details

Preparation and characterization of catalysts. Gold nanoparticles (about 8% of the

overall catalyst mass) were loaded to the supports by the deposition-precipitation

method. A HAuCl4 aqueous solution of 50 ml H2O and 200 mg HAuCl4 was

prepared and pH was adjusted to 8-10 with 0.1M NaOH solution. Then, 2 g of support

powder was dispersed into the HAuCl4

Surface photocurrent (SPC) and transient photovoltage (TPV). The SPC

measurements were performed on the system constituted of a source of

monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540),

and a photovoltaic cell. A 500 W xenon lamp (CHFXQ500W, Global xenon lamp

power) and a double-prism monochromator (Hilger and Watts, D300) provided

monochromatic light. A comb-like ITO electrode with an external bias (10.0 V) on its

two sides was used. The sample chamber for TPV measurements consisted of ITO

electrode, a 10 μm thick mica spacer as electron isolator, and a platinum wire gauze

electrode (with a transparency of about 50%). The construction was a sandwich-like

solution, the resulting suspension was stirred

for 2 hours at 80°C. The solid in the suspension was separated, washed extensively

with deionized water and dried overnight at 80°C and heated at 300°C for 4 hours.

Transmission electron microscopy (TEM) images were taken with a JEOL 2010

microscope employing an accelerating voltage of 200 kV. The UV-visible spectra

were examined by Cary5000, Stheno. Energy dispersive X-ray spectroscopy (EDS)

experiment was attached on FEI Quanta 200 Environmental SEM. X-ray

photoelectron spectroscopy (XPS) test was attached on Kratos Analytical Axis Ultra

X-ray photoelectron spectrometer.

CHAPTER 4

64

structure of ITO electrode-sample-mica-gauze platinum electrode. During the

measurement, the gauze platinum electrode was connected to the core of a BNC cable

which input signals to the oscilloscope. The samples were excited from platinum wire

gauze electrode with a laser radiation pulse (wavelength of 532 nm and pulse width of

5 ns) from a third-harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.).

The intensity of the pulse was regulated with a neutral gray filter and determined with

an EM500 single-channel joulemeter (Molectron, Inc.). The TPV signals were

registered with a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix).

Photocatalytic tests. The photocatalytic activity of the catalysts for decomposing

sulforhodamine-B (SRB) and phenol in aqueous solution at 25 °C was tested in

similar procedure. For example, 50 mg of photocatalyst powder was dispersed in

aqueous suspension of SRB (50 mL, 2×10-5 M). The catalyst concentration was 1.0

g/L. pH of the suspension was adjusted to 3 by 0.1M HNO3 and then the suspension

was placed in a chamber with 6 light tubes (18 W/tube, from Philips, light intensity

was 0.011 W/cm2) as the light source, as shown in SI Figure 5, Chapter 3. The vessel

was in a chamber with 6 blue light tubes (18 W/tube, from Philips) or 6 UV light

tubes (20 W/tube, light intensity was 0.014 W/cm2, wavelength around 365 nm) as the

light source. In order to study the effect of light intensity to SRB photodegradation,

the same experiments were conducted except turning off 2 or 4 blue light tubes (the

light intensity was reduced to 0.008 and 0.006 W/cm2, respectively). An air condition

was installed in the chamber to maintain the temperature at 25 °C as the light

illumination could cause an increase in temperature of the vessel. The suspension was

magnetically stirred in the dark for 30 min prior to irradiation to establish

adsorption/desorption equilibrium between the dye and the surface of the catalyst

under ambient air-equilibrated conditions. At given irradiation time intervals, 4 mL

CHAPTER 4

65

aliquots were collected, centrifuged and filtered through a Millipore filter (pore size

0.45 μm) to remove the catalyst particulates. The filtrates were analysed at the

wavelength of maximal absorption (565 nm) in the UV-vis spectra of SRB using

Varian 50. The data shown in Table 1 were obtained after 1 hour of irradiation. For

comparison we also prepared nitrogen-doped TiO2 by annealing TiO2 at 550°C in N2

The selective oxidation of benzyl alcohol to benzaldehyde was conducted in a

toluene solution under UV irradiation. 50 mg of gold photocatalyst powder was

dispersed in toluene solution of benzyl alcohol (10%). Then the suspension was

placed in a chamber in which 6 UV light tubes as the light source and the temperature

was maintained at 25 °C with an air condition. 30 mg NaOH was added into the

suspension and the air in the container of the suspension was replaced with pure

oxygen. Prior to light irradiation the suspension was magnetically stirred in the dark

for 30 min. At given irradiation time intervals, 2 mL aliquots were collected,

centrifuged and filtered through a Millipore filter (pore size 0.45 μm) to remove the

catalyst particulates. The filtrates were analysed in a Gas Chromatography (HP6890

Prometheus) to measure concentration change of benzyl alcohol.

gas for 4 hours, and the activity of this sample for SRB photodegradation was tested

in the same procedures used for the Au-NPs photocatalysts. In the photocatalytic

phenol degradation experiments, aqueous suspensions of organic compounds (100

mL, 1 mM) and 100 mg of gold photocatalyst powders were placed in the vessel.

Then the vessel was in a chamber with 6 UV light tubes as the light source. The

filtrates were analysed at 270nm in the UV-vis spectra using the Varian 50.

CHAPTER 4

66

90 89 88 87 86 85 84 83

3

2

1

4f7/2

4f5/2

Binding energy (eV)

Inte

nsity

(a.u

.)

Figure S1 Binding energy of Au 4f5/2 and Au 4f7/2 for gold photocatalysts. (1)

Au/Zeolite Y; (2) Au/ZrO2; (3) Au/SiO2.

CHAPTER 4

67

Figure S2 The transmission electron microscope images of the photocatalysts of gold

nanoparticles on supports. (a) gold on zeolite Y, Au/zeolite Y, (b) gold on zirconia,

Au/ZrO2 and (c) gold on silica, Au/SiO2

.

CHAPTER 4

68

0.00 0.05 0.10 0.150

10

20

30

40

50

Conv

ersio

n (%

)

Light intensity (W/cm2)

Figure S3 The relationship between SRB conversion and the light intensity. The

symbols ■, ● and ▲ represent the results obtained over Au/ZrO2, Au/zeolite Y and

Au/SiO2, respectively.

CHAPTER 4

69

0 10 20 30 40 500

5

10

15

20

25

30

Freq

uenc

y / %

Size distribution / nm

Au/Y

0 10 20 30 40 500

5

10

15

20

25

30

Au/ZrO2

Freq

uenc

y / %

Size distribution / nm0 10 20 30 40 50

0

5

10

15

20

25

30

Freq

uenc

y / %

Size distribution / nm

Au/SiO2

Figure S4. Size distribution of the gold NPs on three supports. In each sample, 60

gold NPs were counted.

Figure S5. Irradiation intensity of blue light lamps (blue curve) and UV lamps (red

curve).

Table S1 Gold content of the gold nanoparticles on various oxides analysed by EDS

Au-Zeolite Y Au-ZrO Au-SiO2 2 Gold content (wt%) 8.06 8.21 8.22

CHAPTER 4

70

Erratum page

In this chapter, the light intensities were measured to calculate the normalized

TOFs. However, there were some errors about the light intensity values found during

thesis revising. The correct light intensities under blue and UV irradiation should

change to 0.011 and 0.014 W cm-2

, respectively. The turnover frequency data listed in

Table 1 should also be replaced.

Table 1. Absorption of irradiation energy and catalytic activity of the gold

photocatalysts to degrade SRB.

[a] SRB conversions from replicate runs agree to within ±4%. [b] Turnover frequency data in the table were calculated from the conversion after 1 hour irradiation.

Photocatalyst Au-zeolite

Y

Au-ZrO

Au-SiO2

N-doped TiO2

TiO2

2 (P25)

Conversion under blue light (%) 37 a 46 27 34 14

Absorbed energy by Au-NPs or TiO2 under blue light (W cm-2

0.011 )

0.010 0.006 0.001 0.001

Turnover frequency under blue light (Au-atom-1 s-1 or Ti-atom-1 s-1)

5.1·10b

6.1·10-6 3.6·10-6 1.5·10-6 8.8·10-7

Normalized turnover frequency under blue light (cm

-9

2 J-1 Au-atom-1 or cm2 J-1 Ti-atom-1)

4.6·10b

6.1·10-4 6.0·10-4 1.5·10-4 8.8·10-4

Conversion under UV light (%)

-6

51 a 64 44 49 75

Absorbed energy by Au-NPs or TiO2 under UV light (W cm-2

0.008 )

0.009 0.007 0.005 0.006

Turnover frequency under UV light (Au-atom-1 s-1 or Ti-atom-1 s-1)

5.4·10b

6.8·10-6 4.7·10-6 2.2·10-6 3.3·10-7

Normalized turnover frequency under UV light (cm

-7

2 J-1 Au-atom-1 or cm2 J-1 Ti-atom-1)

6.8·10b

7.6·10-4 6.7·10-4 4.4·10-4 5.5·10-5 -5

CHAPTER 5

71

CHAPTER 5. SUPPORTED SILVER NPS AS

PHOTOCATALYSTS UNDER ULTRAVIOLET

AND VISIBLE LIGHT IRRADIATION

Introductory Remarks

Gold NPs could exhibit photocatalytic activities to degrade various organic

compounds in aqueous solution, as shown in chapter 4. On account that silver NPs

were also found to exhibit strong absorption of visible light [1] and UV light [2], an

important opportunity is presented: we could drive the oxidation reactions on silver

catalysts under light irradiation at ambient temperature. In this chapter, we reported

that silver NPs were observed to be good photocatalysts in aqueous solution for

various photodegradation reactions of organic compounds. The significant activities

for dye degradation by silver NPs on oxide supports were even better than those on

semiconductor photocatalysts. We suggested that SPR effect could activate organic

molecules adsorbed on the silver NPs for the oxidation. Our article describing the

finding was published in Green Chemistry this year [3].

[1] H. Yuan, Chem. Mater. 2007, 19, 1592-1600.

[2] P. V. Kamat, J. Phys. Chem. B 2002, 106, 7729-7744.

[3] X. Chen, Z. F. Zheng, X. B. Ke, E. Jaatinen, T. F. Xie, D. J. Wang, C. Guo, J. C.

Zhao, H. Y. Zhu, Green Chemistry 2010, 12, 414-419.

STATEMENT OF CONTRIBUTION

The authors listed below have certified that: 1. They meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the publication in their field of expertise;

2. They have public responsibility for their part of the publication, except for the responsible author who accepts overall responsibility for the publication;

3. There are no other authors of the publication according to these criteria; 4. Potential conflicts of interest have been disclosed to (a) granting bodies, (b) the

editor or publisher of journals or other publications, and (c) the head of the responsible academic unit, and

5. They agree to the use of the publication in the student’s thesis and its publication on the Australian Digital Thesis database consistent with any limitations set by publisher requirements. In the case of this chapter:

Supported Silver Nanoparticles as Photocatalysts under Ultraviolet and Visible Light Irradiation

Xi Chen, Zhanfeng Zheng, Xuebin Ke, Esa Jaatinen, Tengfeng Xie, Dejun Wang, Cheng Guo, Jincai Zhao and Huaiyong Zhu

Accepted in the journal: Green Chemistry, 2009.

Contributor Statement of contribution Xi Chen Conceptual design on experimental idea and scientific method,

conducted most of experimental and characterisation work, data interpretation, paper writing.

Zhanfeng Zheng Developed photoreaction mechanism, part of sample preparation, assisted with data analysis and paper revising.

Xuebin Ke Developing mechanism of silver photocatalysts, part of sample preparation, assisted with data discussion and paper revising.

Esa Jaatinen Developing photoreaction mechanism, clarifying important concepts and data interpretation, revising paper.

Tengfeng Xie Conducted SPR and TPV measurements and explain the obtained data.

Dejun Wang Conducted SPR and TPV measurements and explain the obtained data.

Cheng Guo Data interpretation of reaction mechanism, revising paper Jincai Zhao Conceptual design on surface plasmon, revising paper. Huaiyong Zhu Organising the research, developing mechanism for silver

photocatalysts, data interpretation and revising paper.

PAPER www.rsc.org/greenchem | Green Chemistry

Supported silver nanoparticles as photocatalysts under ultraviolet andvisible light irradiation

Xi Chen,a Zhanfeng Zheng,a Xuebin Ke,a Esa Jaatinen,a Tengfeng Xie,b Dejun Wang,b Cheng Guo,c

Jincai Zhaod and Huaiyong Zhu*a

Received 16th October 2009, Accepted 2nd December 2009First published as an Advance Article on the web 26th January 2010DOI: 10.1039/b921696k

The significant activity for dye degradation by silver nanoparticles (NPs) on oxide supports wasbetter than popular semiconductor photocatalysts. Moreover, silver photocatalysts can degradephenol and drive oxidation of benzyl alcohol to benzaldehyde under ultraviolet light. We suggestthat surface plasmon resonance (SPR) effect and interband transition of silver NPs can activateorganic molecules for oxidation under ultraviolet and visible light irradiation.

Introduction

Photocatalysts show great potential as drivers of chemical reac-tions when illuminated by sunlight at ambient temperatures.1-3

One of the great challenges in this field is devising newcatalysts that consist of nanoparticles (NPs) usually below10 nm size regime and possess high activity when illuminatedwith either visible light or ultraviolet (UV) light.1 The newphotocatalysts will enable us to use sunlight, the abundantand green energy source, to drive useful chemical reactions.Once sunlight is utilized as a substitute of fossil fuel to drivereactions for production of important chemicals and environ-mental remediation, this will alleviate our reliance on fossilfuel energy and reduce energy consumption and CO2 emissions.Conventional semiconductor photocatalysts, in particular TiO2

based materials have been extensively investigated.2 As thesephotocatalysts have a large band gap, photocatalysis can onlyoccur when UV light is absorbed. Since more than 43% ofthe solar energy is in the visible part of the spectrum,3 manyapproaches have been proposed to develop photocatalysts thatcan perform under visible light.1,3-4 It is well known that gold,silver and copper NPs strongly absorb visible-light due to the so-called surface plasmon resonance (SPR) effect.5 The SPR effectis the collective oscillation of conduction electrons in the NPs,which resonate with the electromagnetic field of the incidentlight. Also these excited electrons will return to their thermalequilibrium states and release heat to the lattice and surroundingmedium.6 This heating effect may also induce reactions of themolecules adsorbed on the particles. Indeed, we found thatwhen illuminated with visible light, gold NPs dispersed onoxide supports exhibited significant activity for oxidation offormaldehyde and methanol in air at room temperature due to

aSchool of Physical and Chemical Sciences, Queensland University ofTechnology, Queensland, 4001, Australia. E-mail: hy.zhu@qut.edu.au;Fax: +61 731381804; Tel: +61 731381581bCollege of Chemistry, Jilin University, Changchun, 130012, ChinacCollege of Science, Nanjing University of Technology, Nanjing, 210009,ChinadInstitute of Chemistry, Chinese Academy of Science, Beijing, 100080,China

the SPR effect.7 Since visible light absorption heats the electronsand excites them from ground state to higher energy levels, theprobability that a conduction electron participates in chemicalreactions involving electron transfer is greater.

Silver NPs also exhibit considerable UV light absorptiondue to the interband transition (the transition of 4d electronsto the 5sp band).8 Therefore, silver NPs are potentially pho-tocatalysts that utilize the full solar spectrum. Silver NPs onthe surface of semiconductors and electron-donor substancescause charge separation of photogenerated electron-hole pairs,thus enhancing the overall photocatalytic activity.9 However, thephotocatalytic activity of the silver NPs themselves has not beenrecognized. While silver ions were reported to be photoactivefor certain reactions, such as nitric oxide decomposition andcarbon-hydrogen bond activation,10 to date even the precisenature of the reaction mechanism for the catalysis involvingplasmonic silver materials has not been clarified. Here we verifythat silver NPs at room temperature can be used to drivechemical reactions when illuminated with light throughout thesolar spectrum, and in the process gain some understandinginto the mechanism behind the photocatalytic process (whichis different from that for the conventional semiconductorphotocatalysts).

Experimental

Silver NPs preparation

Solution-phase reduction methods11 were used to prepare thesilver NPs supported on different oxides. ZrO2, Zeolite Y andamorphous SiO2 powders were chosen as supports because oftheir band gaps (above ~5.0 eV), which are much larger than theenergies of the photons of visible light (below 3.0 eV). Hence,the light cannot excite electrons from the valence band to theconduction band of the support. It is also impossible for thesilver NPs on the support to reduce its band gap enough forvisible light photons to be absorbed. Thus, the observed visiblelight absorption and catalytic activity by the photocatalysts isdue to the supported silver NPs. The AgNO3 solution (3 ¥10-3 M) containing suspended 0.5 g ZrO2, SiO2 or zeolite Y

414 | Green Chem., 2010, 12, 414–419 This journal is © The Royal Society of Chemistry 2010

(surface areas are 34, 47, 653 m2 g-1, respectively) was irradiatedwith six UV lamps (20 W/tube, from Philips). The irradiatedmixture was then centrifuged for 2 h, and the obtained Ag/oxideprecipitate was washed with deionized water, dried at 80 ◦C andheated at 450 ◦C for 6 h.

Sample characterization

Transmission electron microscopy (TEM) studies of supportedsilver NPs were carried out on Philips CM200 TEM withan accelerating voltage of 200 kV. The silver content wasdetermined by energy dispersive X-ray spectroscopy (EDS)using FEI Quanta 200 Environmental SEM. X-ray photoelec-tron spectroscopy (XPS) analysis was performed on a KratosAnalytical Axis Ultra X-ray photoelectron spectrometer. X-raydiffraction (XRD) was carried out using a PANalytical with Cu-Ka radiation. The surface photocurrent (SPC) measurementswere performed on the system constituted of a source ofmonochromatic light, a lock-in amplifier (SR830-DSP) witha light chopper (SR540), and a photovoltaic cell. A 500 Wxenon lamp (CHFXQ500W, Global xenon lamp power) and adouble-prism monochromator (Hilger and Watts, D300) providemonochromatic light. A comb-like ITO electrode with an exter-nal bias (10.0 V) on its two sides was used. The sample chamberfor transient photovoltage (TPV) measurements consists of anITO electrode, a 10 mm thick mica spacer as electron isolator,and a platinum wire gauze electrode (with a transparency ofabout 50%). The construction is a sandwich-like structure of ITOelectrode-sample-mica-gauze platinum electrode. During themeasurement, the gauze platinum electrode was connected to thecore of a BNC cable which input signals to the oscilloscope. Thesamples were excited from platinum wire gauze electrode witha laser radiation pulse (wavelength of 532 nm and pulse widthof 5 ns) from a third-harmonic Nd:YAG laser (Polaris II, NewWave Research, Inc.). The intensity of the pulse was regulatedwith a neutral gray filter and determined with an EM500 single-channel joulemeter (Molectron, Inc.). The TPV signals wereregistered with a 500 MHz digital phosphor oscilloscope (TDS5054, Tektronix).

Photocatalytic tests

In the photocatalytic dye sulforhodamine-B (SRB) degradationexperiment under air atmosphere, aqueous suspensions of SRB(50 mL, 2 ¥ 10-5 M) and 50 mg of photocatalyst were placedin a glass vessel, which was in a chamber with 6 light tubes(18 W/tube, Philips, light intensity 0.011 W cm-2, wavelengtharound 450 nm) as the light source. The pH of the solutionswas adjusted to 2.5 with 0.1 M HNO3. An air conditionerwas installed in the chamber to maintain the temperature at25 ◦C as the light illumination could cause an increase intemperature of the vessel. Before irradiation the suspensionswere magnetically stirred in the dark for 30 min to establishadsorption/desorption equilibrium between the dye and thecatalyst. At given irradiation time intervals, 4 mL aliquots werecollected, centrifuged, and then filtered through a Millipore filter(pore size 0.45 mm) to remove the catalyst particulates. Thefiltrates were analyzed at the wavelength of maximal absorption(565 nm) in the UV-vis spectra of SRB using a Varian 5000. Forcomparison we also product nitrogen-doped TiO2 by annealing

TiO2 at 550 ◦C in N2 gas for 4 h to test SRB photodegradation.In the photocatalytic phenol degradation experiment, aqueoussuspensions of organic compounds (100 mL, 1 mM) and 100 mgof silver photocatalyst powders were placed in the vessel. Thenthe vessel was in a chamber with 6 UV light tubes as the lightsource (20 W/tube, NEC, light intensity was 0.014 W cm-2,wavelength around 365 nm). The filtrates were analysed at270 nm in the UV-vis spectra using the Varian 50. In the benzylalcohol degradation experiment, 50 ml toluene suspensions ofbenzyl alcohol (10%) and 50 mg of silver photocatalyst powderswere placed in the glass vessel. Then the vessel was in a chamberwith 6 UV light tubes as the light source. In order to increasephotocatalytic activity, 30 mg NaOH was added into the benzylalcohol solution and the vessel was filled with pure oxygenas the reaction atmosphere. The filtrates were analysed in GCHP6890 Prometheus to measure the concentration change ofbenzyl alcohol.

Results and discussion

In this study, we loaded silver NPs onto various typical oxidesupports12 and used these photocatalysts for degrading a rangeof organic compounds in aqueous solution under either visiblelight or UV irradiation at room temperature. TEM images ofsilver NPs supported on ZrO2 (Ag/ZrO2), amorphous SiO2

(Ag/SiO2) and Zeolite Y (Ag/Zeolite Y) are shown in Fig. 1.These images indicate that silver exists in these samples as NPs.Most of the silver particles (the dark-colour substance) on thesesupports were found to have dimensions below 10 nm, which canlead to changes in surface and electronic structure providingan opportunity to control catalytic activity and selectivity.13

The silver contents in Ag/Zeolite Y, Ag/ZrO2 and Ag/SiO2

samples were found by EDS to be 7.39, 7.48 and 7.56 wt% ofthe overall photocatalyst mass, respectively. XPS analysis shownin Fig. 2 indicated that the silver exists in metal state. However,no silver peaks can be identified through XRD pattern of ourphotocatalysts (shown in Fig. 3), probably due that the loadedsilver did not form large particles, but was dispersed in thesupport structure.

Fig. 1 Transmission electron microscopy images of the photocatalystsof silver NPs on supports. (a) Silver on zeolite Y, Ag-zeolite Y. (b) Silveron zirconia, Ag-ZrO2. (c) Silver on silica, Ag-SiO2.

Dyes are of special interest as their use in the textileand industrial industries is becoming a significant source ofenvironmental contamination.14 Under visible light irradiationsilver NPs dispersed on oxide supports exhibited significantactivity for SRB degradation at 25 ◦C (An air conditioner wasinstalled to maintain the temperature as the light illuminationcould cause an increase in the vessel temperature), which is evenbetter than can be achieved with the widely reported nitrogen

This journal is © The Royal Society of Chemistry 2010 Green Chem., 2010, 12, 414–419 | 415

Fig. 2 Binding energy of Ag 3d5/2 and Ag 3d3/2 for silver photocatalysts.(1) Ag/Zeolite Y; (2) Ag/ZrO2; (3) Ag/SiO2.

Fig. 3 XRD analysis of silver photocatalysts. (1) Ag/Zeolite Y; (2)Ag/ZrO2; (3) Ag/SiO2.

doped TiO2 photocatalysts. The comparison of degradationcurves of SRB in aqueous solutions using silver photocatalystsafter 180 min of blue light irradiation are shown in Fig. 4.Silver supported on zeolite exhibited the highest degradationability among these materials. SRB content decreased by 74%in 3 h under blue light irradiation with intensity of 0.011W cm-2. The photocatalysts of silver supported on zirconiaand silica (Ag/ZrO2 and Ag/SiO2) are also very active for SRBdegradation (shown in Fig. 4). After 3 h of blue light irradiation,71% and 66% of the dye were degraded with Ag/SiO2 andAg/ZrO2, respectively, slightly lower than the degradation ratewith Ag/Zeolite Y. We also found a trend that the ability of thephotocatalysts to degrade SRB increased with the increasingsilver content. The Ag/Zeolite Y samples containing 4.5 wt%and 1.5 wt% of silver were also prepared. The SRB contentdecreased to 56% and 40% under blue light irradiation in 3 h,respectively. In the dark, SRB was not decomposed with any oneof the three silver photocatalysts. A blank experiment under theotherwise identical conditions but without silver NPs (solutionswith these oxide supports - Zeolite Y, ZrO2 or SiO2 powderonly) was also conducted, and no SRB conversion above 3%was observed. Moreover, silver photocatalysts were stable underrepeated application. About 82% SRB conversion catalyzed by

Fig. 4 Degradation curves of SRB under blue light using differentphotocatalysts. (1) N-doped TiO2. (2) Ag/Zeolite Y. (3) Ag/SiO2. (4)Ag/ZrO2.

Ag/Zeolite Y can be maintained within 5 photodegradationrecycles.

We also studied the effect that the intensity of the lightirradiation had on the SRB degradation reaction. No concen-tration changes were detected if the experiment was conductedwithout light irradiation. When the light intensity was reducedto 0.008 and 0.006 W cm-2 (by turning off 2 or 4 blue lighttubes, respectively), the SRB conversion by Ag/SiO2 decreasedfrom 71% to 50% and 27%, respectively. The wavelength ofthe irradiation also affects the photocatalytic activity. Underred light irradiation (of 6 light tubes from Philips, with overalllight intensity 0.010 W cm-2 and wavelength around 650 nm)about 56% and 39% of SRB was converted by using Ag/SiO2

and Ag/ZrO2 as catalysts in 3 h, which is substantially lowerthan that under blue light (71% and 66%). These observationsshow that the SRB degradation is undoubtedly driven by visiblelight. Next, as the calcination temperature of silver samples was300 ◦C, only 49% SRB was degraded over Ag/Zeolite Y underblue light for 3 h due to weak interaction between silver andsupport. While high calcination temperature (600 ◦C) can leadto big silver paricles. 53% SRB was converted under the sameexperimental condition except the calcination temperature.

Moreover, these silver photocatalysts also can degrade organiccompounds under UV light irradiation at room temperature.The UV light absorption by silver NPs can excite interband tran-sition and be utilized to drive photoreactions. TiO2 is the mostwidely studied photocatalyst under UV light irradiation, andnitrogen-doped TiO2 can exhibit high activity when illuminatedby visible light.4 We compare the SRB photodegradation activityof the supported silver particles with that of TiO2 material inTable 1. SRB conversions from replicate runs agree to eachother within ±3%. The results confirm that silver NPs supportedon oxides are superior to TiO2 based photocatalysts for SRBdegradation under both blue and UV light irradiation, giventhat the silver particles are the active photocatalysis componentand that silver accounts for about 7.5 wt% of the catalyst mass.

The silver photocatalysts exhibited better catalytic activityunder UV irradiation than under blue light. They not onlydecompose dye molecules under UV light faster than undervisible light, but also are able to oxidize phenol in aqueoussolution, which they cannot catalyze under visible light. After120 h of UV light irradiation 41% of phenol was degradedby Ag/ZrO2. The photocatalytic conversion of phenol byAg/Zeolite and Au/SiO2 was 37% and 38%, respectively. Theconversion of blank experiment (without catalyst under UV irra-diation) was below 1%. These experimental results indicate thatsilver particles supported on oxides can catalyze the degradationof organic compounds without involving photosensitizationprocess like dyes in aqueous solution at ambient temperature.15

The photocatalytic decomposition of organic compoundsdiscussed above involves transfer of multiple electrons fromthe organic molecules to oxygen—the oxidant. In principle,selective (or partial) oxidation of an organic compound canbe achieved with these photocatalysts if we regulate the electrontransfer process by tuning the experimental conditions. Indeed,supported silver NPs were found to be effective catalysts foroxidation of benzyl alcohol in toluene to benzaldehyde. 11%benzyl alcohol conversion was achieved in 48 h under UV lightirradiation with 62% of the product being benzaldehyde, when

416 | Green Chem., 2010, 12, 414–419 This journal is © The Royal Society of Chemistry 2010

Table 1 Absorption of irradiation energy and catalytic activity of the photocatalysts to degrade SRB

Under blue light Under UV light

Conversiona Absorb energy TOFb Normalized TOFb Conversiona Absorb energy TOFb Normalized TOFb

Photocatalyst % W cm-2 atom-1 s-1cm2 J-1 (Ag orTiO2)-atom-1 s-1 % W cm-2 atom-1 s-1

cm2 J-1 (Ag orTiO2)-atom-1 s-1

Ag-zeolite Y 68 0.007 1.6 ¥ 10-5 2.3 ¥ 10-3 75 0.007 1.8 ¥ 10-5 2.6 ¥ 10-3

Ag-ZrO2 48 0.007 1.1 ¥ 10-5 1.6 ¥ 10-3 55 0.006 1.3 ¥ 10-5 2.2 ¥ 10-3

Ag-SiO2 50 0.004 1.2 ¥ 10-5 3.0 ¥ 10-3 64 0.004 1.5 ¥ 10-5 3.8 ¥ 10-3

N-doped TiO2 34 0.001 1.5 ¥ 10-7 1.5 ¥ 10-4 49 0.005 2.2 ¥ 10-7 4.4 ¥ 10-5

TiO2 (P25) 14 0.001 8.8 ¥ 10-9 8.8 ¥ 10-6 75 0.006 3.3 ¥ 10-7 5.5 ¥ 10-5

a SRB conversions from replicate runs agree to within ±3%. b Turnover frequency data in the table were calculated from the conversion after 1 h ofirradiation.

Ag/zeolite Y was used as the photocatalyst. Partial oxidationfor producing aldehydes from corresponding alcohols is a veryimportant process for the fine chemical industry.16 We alsoincreased the selectivity for producing benzaldehyde to 100% byadjusting the pH with NaOH solution17 and using pure oxygeninstead of air, though the overall conversion fell to 4% in 48 h.

Based on these facts, we conclude that the photocatalyticprocess with supported silver NPs does not occur via the samemechanism as found for semiconductor photocatalysts, such asTiO2.2 In our study the silver NPs were supported on zeolite,ZrO2 and SiO2. ZrO2 has a band gap of about 5 eV.7 The bandgaps of zeolite Y and SiO2 are slightly larger than that for ZrO2,according to the UV-vis absorption measurements shown inFig. 5a by Cary5000 UV-Vis spectrometer. These supports alone(in the absence of the NPs) exhibit little light absorption. Whenlight wavelengths are above 330 nm, the illumination cannotexcite electrons of the supports from the valence band to theconduction band. Thus, all the photogenerated charges that leadto catalytic activity originate from the silver NPs. The silver in thephotocatalysts remains in a metal state as indicated by the XPSanalysis (Fig. 2), which is dissimilar to AgCl which can donateelectrons and exhibit photocatalytic activity by the oxidation ofCl- ions to Cl0 atoms under light irradiation.9

The photocatalytic reactions (degradation and selectiveoxidation) involve electron transfer from the molecules of theoxidized reactant to those of the reduced reactant. We believethat the silver NPs initialize and mediate the electron transferfor the photooxidation reactions. It has been reported that the

silver doped on TiO2 surface can interact strongly with theoxygen atoms and give rise to an electron transfer to the Ti3d states.18 In order to determine whether light irradiation caninduce electron transfer from the silver particle to the oxygenmolecules (or oxygen adsorbed on the support), SPC and TPVspectra of the samples were also analysed and shown in Fig. 5. Asurface current and a transient photovoltage will arise wheneverexcess light-induced charge carriers are separated in space, withthe signal intensity being proportional to the number of thephotogenerated charges.19

These spectra also explicitly exhibit the dependence of theelectron transfer on the illumination wavelength allowing thespectral regions under which electron transfer occurs to beidentified. For typical isolated spherical silver NPs the SPRabsorption is generally around 380 nm, but the absorptionband of the supported NPs is significantly red-shifted to above410 nm.20 Aggregation of the silver NPs and non-sphericalshaped particles, which we observed in the samples in thepresent study, broaden the absorption resonance at low lightintensity.21 The silver NPs exhibit UV absorption, due to theinterband excitation of electrons from 4d to 5sp. From the SPCspectra (Fig. 5b) it is apparent that UV absorption producesa much larger surface photocurrent than that induced by theSPR absorption under visible light irradiation. Also, a largeinitial photovoltage is observed in the TPV spectrum (Fig. 5c)which was measured with a 532 nm laser that has a much higherintensity than the light used for the SPC measurement. Thisindicates that visible light irradiation does generate electrical

Fig. 5 (a) Light absorption of silver photocatalysts, zeolite Y and oxides in UV and visible light range. (b) Surface photocurrent spectra of silverNPs supported on zeolite Y (trace in blue) and zeolite Y (trace in black). (c) Transient photovoltage spectra of silver NPs supported on zeolite Y(trace in blue) and zeolite Y (trace in black).

This journal is © The Royal Society of Chemistry 2010 Green Chem., 2010, 12, 414–419 | 417

surface charges when sufficiently intense. The SPC spectrumalso indicates that the interband absorption (UV) results in amuch larger proportion of electron transfer from silver NPsto the oxygen molecules than the SPR absorption (visible).Consequently, more positive charges (holes) are left in the silverNPs under UV illumination.

To explain these photocatalytic observations, we propose atentative transfer mechanism as illustrated in Scheme 1. Beforelight illumination the silver electron occupancy obeys the Fermi–Dirac distribution. Blue light irradiation will excite the SPR andis strongly absorbed. Silver electrons are excited from withinthe outermost sp band to higher energy states.5 Soon after lightabsorption the plasmon loses energy causing rapid heating ofelectron gas to an elevated temperature (about 400–2000 K)within a time scale of the order of 100 fs or less throughelectron–electron collision.22 Then the electrons share the heatenergy from the ‘hot’ electron gas with the NP lattice throughelectron–phonon collisions. The time scale varies from 500 fsto 10 ps.23 Therefore, it is possible that the electrons withenough energy may be captured by a body once the captureprocess takes within 1 ps, according to Furube’s finding thatthe electron transfer from gold NPs to titanium oxide takes lessthan 240 fs.24 We suggested that under visible light irradiationat moderate intensity, a very small number of silver electronswith high temperature gain sufficient energy (above the greenline in Scheme 1) to be captured by oxygen molecules. However,most of the electrons are excited to lower energy levels (below

Scheme 1 The diagram of the band structures of the supported silverNPs and the proposed photocatalysis mechanism.

the green line in Scheme 1), which cannot be captured. Thus, aweak SPC signal and slow dye degradation are observed undermoderate visible light illumination. The holes left in the 5sp bandcan capture electrons from excited SRB molecules (SRB*), duethat the photosensitization process under light irradiation whichinvolves initial excitation of the dye molecules can be helpful forinjecting dye electrons.15

When silver particles absorb UV irradiation, electrons ofthe 4d band can be excited to the 5sp tates band,8 and manyphotogenerated electrons are in high energy (the green line orabove in Scheme 1) where oxygen molecules can seize them,yielding large surface photovoltage signals under UV light. Theholes left in the inner d band have a greater affinity for capturingelectrons from the adsorbed organic molecules than those inthe outermost sp band. Thus the presence of holes in the dband allows any attached phenol to be degraded by acceptingits electrons. Since different mechanisms are responsible fordegrading organic compounds under visible and UV light, thephotocatalysts ability to oxidize specific compounds will dependon the illumination wavelength. This property can be utilizedfor two reaction regimes. First, under UV illumination, thephotocatalysts can oxidize the compounds that they cannotoxidize under visible light, such as phenol. However, furtherresearch can be done with other reduction substances whosereduction potential is lower than that of O2/O2

-. Probablythis will provide visible light driven catalytic activity of phenoldegradation to silver photocatalysts. Second, the illuminationwavelength can be used as a control parameter to determinewhether a specific reaction will take place or not. The abilityfor the photocatalysts to capture electrons under UV light is auseful feature that can be used for producing desired chemicalsunder the experimental conditions which can prevent furtheroxidation, such as selective oxidation of benzyl alcohol tobenzaldehyde by silver photocatalysts.

Conclusions

In summary, silver NPs are good photocatalysts under ambienttemperature for degrading organic compounds. Silver NPssupported on oxides can exhibit significant oxidation activityfor a synthetic dye under visible light illumination. Thesephotocatalysts can also catalyze phenol degradation as wellas selective oxidation of benzyl alcohol under UV light. Thefindings indicate conceptually that it is possible to drive variouschemical reactions with visible light. Therefore, as in the caseof photochemistry driven by surface plasmon, it is a distinctpossibility that environmental remediation and fine chemicalproduction can be performed using the most efficient light sourceavailable—visible light. This will alleviate our reliance on fossilfuel energy and concerns in regards to global warming. Ourfindings also show the potential to switch on or off specificreactions by tuning the light wavelength. This development willlead to a new direction in photocatalysis research.

Acknowledgements

Financial Supports from the Australian Research Coun-cil (ARC), and 973 program (2007CB613306) and NSFC(20537010) of China are gratefully acknowledged.

418 | Green Chem., 2010, 12, 414–419 This journal is © The Royal Society of Chemistry 2010

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Supporting information

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Figure S1. Size distribution of the silver nanoparticles on three supports. In each

sample, 60 silver nanoparticles were counted.

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CHAPTER 6. CONCLUSIONS

A critical research direction for the study of photocatalysis is to devise new

catalysts that utilise sunlight, the abundant and green energy source, to drive chemical

reactions. Gold NPs can strongly absorb both visible light and UV light because of

SPR effect and interband transition, respectively. The SPR absorption leads to rapid

heating of gold NPs. Researchers have discovered that gold NPs could catalyse

various oxidations of volatile organic compounds (VOCs) such as HCHO at

moderately elevated temperature. Here we reported that gold NPs on ZrO2 and SiO2

supports could catalyse the VOC oxidations under visible light irradiation at ambient

temperature. When Au/ZrO2 was used as the photocatalyst, HCHO content decreased

by 64% in two hours under blue light irradiation, meanwhile the CO2 content in the

vessel increased. The photocatalytic activity was found to be dependent on the

intensity of light irradiation. According to the UV-vis absorption measurements, oxide

supports alone exhibited little light absorption and no charges could be generated

from them under visible light irradiation. We suggested that the catalytic activities of

gold materials were not caused by the mechanism in semiconductor photocatalysts,

but were due to the SPR effect of gold NPs. The SPR absorption could heat these NPs

up quickly. The interaction between the oscillating local electromagnetic fields and

polar molecules might also assist in activating the molecules. On account that the gold

content in this study was 2-4 wt% of the supports, it required much lower energy

input to activate VOC oxidation, compared to the conventional catalytic oxidation.

Our finding would alleviate our reliance on fossil fuel and concerns in regards to

global warming. Moreover, it revealed further possibilities to catalyse other chemical

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reactions under sunlight at ambient temperature, and highlighted a new catalysis

direction.

We also prepared a series of gold NP samples supported on zeolite Y, ZrO2 and

SiO2, and investigated their photocatalytic performances and photoelectrical

properties. We found that gold NPs at ambient temperature could be used to drive

chemical reactions in the liquid phase, such as dye degradation, under light

illumination throughout the solar spectrum. The dye SRB conversion depended on the

light intensity. We found that these gold photocatalysts could also oxidise phenol.

After 120 h of UV irradiation, the Au-NPs on zeolite Y, SiO2 and ZrO2 converted

21%, 28% and 45% of phenol, respectively. Moreover, under UV light irradiation the

Au-NPs on zeolite Y could oxidise benzyl alcohol in toluene to benzaldehyde with

oxygen as the oxidation agent in the presence of sodium hydroxide. The gold

photocatalysis mechanism was different from that in the conventional semiconductor

photocatalysts. On the basis of surface photocurrent (SPC) and transient photovoltage

(TPV) spectra, we suggested that the visible light absorption resulted in positive

charges in gold’s 6sp band. The positive charges had an affinity for electrons and

could capture them from excited SRB molecules adsorbed on the Au-NPs. While

under UV illumination, gold’s 5d electrons were excited to the 6sp band, and many of

the excited electrons were at high energy levels where oxygen molecules could seize

them. The positive charges left in the 5d band had a lower energy and therefore a

greater affinity for capturing electrons from the adsorbed organic molecules than

those in 6sp band, resulting that the gold photocatalysts could oxidise the compounds

that they could not oxidise under visible light, such as phenol and benzyl alcohol. Our

suggested mechanism offered the potential to switch the specific reactions on or off

by tuning the wavelength of the irradiating light. This study also revealed possibilities

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to drive various chemical reactions under sunlight at ambient temperature for

environmental remediation and fine chemical production.

It is well known that both gold and silver NPs strongly absorb the energy of

visible light due to the SPR effect. Moreover, silver NPs also exhibit considerable UV

light absorption due to the interband transition. Therefore, silver NPs could be

potentially photocatalysts that utilise the full solar spectrum. Here we prepared the

silver NPs supported on different oxides through solution-phase reduction methods,

and reported that silver NPs were found to be good photocatalysts to degrade dyes

under visible light irradiation at ambient temperature. Silver NPs supported on zeolite

exhibited the highest degradation ability among our prepared silver materials. In 3

hours under blue light irradiation SRB content decreased by 74%. Meanwhile 71%

and 66% of the dye were photodegraded with Ag/SiO2 and Ag/ZrO2, respectively.

The TOF results confirmed that silver NPs supported on oxides were superior to TiO2

based photocatalysts for dye photodegradation. Silver photocatalysts could also

eliminate phenol and catalyse selective alcohol oxidation in aqueous solution under

UV light, while they could not catalyse these reactions under visible light. The SPR

effect and interband transition of silver NPs could catalyse organic molecule

oxidations under light illumination. Since these reactions were driven by light

adsorption, it provided a distinct opportunity that environmental remediation and fine

chemical production could be performed under the light, especially the clean and

efficient energy source - visible light.