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
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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
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
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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
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2
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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
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from CH3OH/H2O solution under UV illumination. H2 was continuously evolved
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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
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efficient and stable under visible light illumination for dye photodegradation. The rate
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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
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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.
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37
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[100] Y. H. Zheng, C. Q. Chen, Y. Y. Zhan, X. Y. Lin, Q. Zheng, K. M. Wei, J. F. Zhu, “Photocatalytic activity of Ag/ZnO heterostructure nanocatalyst: Correlation between structure and property”, J. Phys. Chem. C 2008, 112, 10773-10777.
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[101] N. S. Lews, “Light work with water”, Nature 2001, 414, 589-590. [102] 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. [103] 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. [104] 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. [105] L. M. Liz-Marzan, “Tailoring surface plasmons through the morphology and assembly of metal nanoparticles”, Langmuir 2006, 22, 32-41. [106] 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. [107] Y, Tian, T. Tatsuma, “Mechanisms and applications of plasmon-induced charge separation at TiO2
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[113] M. Matsuoka, W. S. Ju, H. Yamashita, M. Anpo, “Investigations on the local structure of Ag
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CHAPTER 2
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reactivities for the elimination of N2O into N2 and O2
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O Solution”, J. Phys. Chem. B 1999, 103, 5917-5919.
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
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: [email protected]; 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
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
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
CHAPTER 4
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
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: [email protected];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
(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
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
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).
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.
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