Plasmonic Photocatalyst for H2 Evolution in Photocatalytic Water Splitting

Jiun-Jen Chen,† Jeffrey C. S. Wu,*,† Pin Chieh Wu,‡ and Din Ping Tsai‡

Department of Chemical Engineering and Department of Physics, National Taiwan UniVersity,Taipei, Taiwan 10617

ReceiVed: August 6, 2010; ReVised Manuscript ReceiVed: NoVember 27, 2010

The effect of surface plasmon resonance (SPR) on the photocatalytic water splitting was studied by employingthe photocatalyst, Au/TiO2, to produce renewable solar hydrogen. It is well-known that metal particles onTiO2 can behave as electron traps, retarding the recombination of electron-hole pairs, thereby improvingreaction activity. However, the electron trap is not the only mechanism responsible for the photoreactionenhancement. Our experiment on methylene blue photodegradation over Au particles proved that the SPRphenomenon was also involved in the photoreaction enhancement. Furthermore, the photocatalytic watersplitting was performed on Au/TiO2 prepared by the photodeposition method. The production of hydrogenwas significantly increased because Au particles not only acted as electron traps as well as active sites butalso played an important role in the SPR enhancement. The intensified electric field at the interface betweenthe Au particle and the subdomain on TiO2 was illustrated by finite element method (FEM) electromagneticsimulation.

1. Introduction

Due to global environmental problems and energy issues,scientists have paid a great deal of attention to the utilizationof solar energy for the production of hydrogen from water usingphotocatalysts. Water splitting has been studied for a long timesince the discovery of the Honda-Fujishima effect,1 whichinvolves a TiO2 semiconductor electrode. Water splitting onsemiconductors is initiated by the absorption of a photon withenergy equal to, or greater than, the semiconductor bandgap.This promotes electrons from the valence band (VB) to theconduction band (CB), with the consequent formation ofelectron-hole pairs. The produced electrons and holes, respec-tively, induce the reduction of the H+ ion and the oxidation ofH2O, both absorbing on the semiconductor surface. The overallphotocatalytic water splitting reaction is thus formulated as ineq 1.

One of the major problems of photoreaction is its low activitydue to the high recombination rate of photogenerated electron-hole pairs, setting a limit to the efficiency of light energyconversion. In recent years, several research groups have madeefforts to increase the photoactivity of semiconductor metaloxides, for example, adding sacrificial agents to efficientlyconsume either e- or h+2-4 or modifying photocatalysts by noblemetal loading to favor the separation of charge carriers.5-9

Methanol and other organic species are commonly used assacrificial agents in photocatalytic water splitting. They cancapture photogenerated valence-band holes more efficiently thanwater, making conduction band electrons readily available forhydrogen production from water. Although the quantum ef-

ficiency of such a method may be increased with the expenseof sacrificial agents, it is not pure water splitting consideredfrom the viewpoint of solar energy conversion.

The loading of noble metal is usually used to enhance theactivity of photoreaction. The presence of noble metal particleson the surface of the photocatalyst increases the electron-holepair separation because photogenerated electrons can be capturedby the noble metal. Noble metal particles can serve as electrontraps. Under light irradiation, the electrons are transferred fromthe TiO2 conduction band to the metal, and the holes ac-cumulated in the TiO2 valence band. Hence, photogeneratedelectrons and holes are efficiently separated. However, the activesites are blocked, resulting in an activity decrease when toomuch noble metal is loaded.8 In addition, the noble metal clustersat a higher concentration may work as a recombination center.The recombination rate between electrons and holes increasesexponentially with the increase in loading concentration becausethe average distance between trapping sites decreases byincreasing the number of the clusters confined within a particle.5

Noble metal particles, such as gold and silver, are interestingcatalytic nanomaterials because the peculiar activities arestrongly related to their size, shape, and surface charge. Theoptical properties of gold nanoparticles are dominated by theirsurface plasmon resonance (SPR), defined as the collectivemotionsof theconductionelectrons inducedbylight irradiation.10,11

This is associated with a considerable enhancement of theelectric near-field. The resonance wavelength strongly dependson the size and shape of the nanoparticles, the interparticledistance, and the dielectric property of the surroundingmedium.12,13 As shown in Figure 1, electrons from the valenceband are excited to the conduction band in TiO2 by UV lightirradiation. The electrons then migrate to the gold particle onTiO2. The SPR effect induced by appropriate visible lightirradiation can boost the energy intensity of trapped electronsresulting in the photocatalytic activity enhancement.14

To investigate the sole SPR phenomenon of nanogoldparticles, the photodegradation of methylene blue (MB) aqueoussolution was conducted over Au particles deposited on a quartz

* Corresponding author. Phone: 886-223631994. E-mail: cswu@ntu.edu.tw.† Department of Chemical Engineering.‡ Department of Physics.

2H2O98Catalyst/hν

2H2 + O2 (1)

J. Phys. Chem. C 2011, 115, 210–216210

10.1021/jp1074048 2011 American Chemical SocietyPublished on Web 12/14/2010

plate. The combination effect of SPR and photocatalysis wasstudied using nanogold particles on TiO2. A numerical simula-tion was used to discern the SPR phenomenon of the Au/TiO2

system in aqueous solution. The simulation used a frequency-domain three-dimensional finite element method to solveMaxwell’s equations of electric field distributions. In the end,the photocatalytic water splitting was performed to illustratethe SPR-enhanced photoactivity.

2. Experimental Section

2.1. Preparation of Au on a Quartz Plate and Au/TiO2.A gold dispersion was prepared according to the sodium citratereduced method.15 Distilled water was added to 2 mL of HAuCl4

solution containing 50 mg of gold and made up to 500 mL.When the solution was boiling, 50 mL of 1% sodium citratesolution was added under vigorous stirring. After 30 min ofcontinuous boiling, the solution was allowed to cool. Thismethod produces a stable, deep-red suspension of gold particles.A pipet was used to take 8 mL of nanogold suspension solution.Then the solution was spread on a clean quartz plate. To increasethe adhesion of nanogold particles on the quartz plate, theresulting sample was then heated from room temperature to 100°C. Next, the heating temperature was increased from 100 to500 °C within 30 min and kept at 500 °C for 60 min.16,17

The photodeposition of nanogold particles on TiO2 wascarried out by the method suggested in the literature.18,19 DegussaP25, i.e., TiO2, was precalcined in air at 773 K for 4 h. Thecalcined TiO2 powder was then added into a beaker containingthe appropriate amount of 0.002 M HAuCl4 solution, and thesolution pH was adjusted to 5.5 by dropwise addition of 0.2 NNaCO3. The suspension solution was irradiated with the 100W high-pressure mercury lamp operated at 8 W cm-2 for 1 hwith vigorous stirring. The color of solution became clear,indicating the completion of Au photodeposition. Then thesuspension was filtrated and washed several times with distilledwater, until no Cl- was detected. The solid was dried undervacuum at room temperature for 16 h.

2.2. Characterization. The Au loading on TiO2 by photo-deposition was estimated from the residual Au concentrationin the precursor solution measured by atomic absorptionspectroscopy (GBC 906AA). The light absorption of photo-catalysts was characterized by reflective diffusive UV-visspectroscopy (Varian, Cary 100). Field-emission scanningelectron microscopy (FE-SEM) was carried out on a Hitachimodel S-800 instrument. The X-ray photoelectron spectroscopy(XPS) was carried out to determine the chemical compositionof the as-prepared Au/TiO2 particles and the chemical status of

various species. The XPS was carried out on a Thermo ThetaProbe instrument. The photocatalyst was pressed into a pelletand stuck to the sample holder using a carbon tape. Carbon(1s, 284.5 eV) was used as an internal standard for bindingenergy calibration. Transmission electron microscopy (TEM)of the photocatalysts was carried out on a Hitachi model H-7100instrument. The particle size distribution (PSD) of gold nano-particles was measured by the Particle Size and Zeta PotentialAnalyzer (Malvern, Nano-ZS).

2.3. Photocatalytic Activity Measurement. 2.3.1. Photo-degradation of Methylene Blue. The methylene blue (MB)aqueous solution of 2.4 × 10-5 M was photodegraded in a glassreactor at 25 °C. The Au-deposited quartz plate (5 cm × 5 cm× 1 mm) immersed in the solution was irradiated by the Xelamp (λ > 400 nm). The Varian, Cary 100, reflective diffusiveUV-vis spectroscope was used to measure the concentrationsof the MB aqueous solution based on the absorption peak of664.3 nm during the photodegradation.

2.3.2. Photocatalytic Splitting of Water. The photocatalyticsplitting of water to generate hydrogen and oxygen was carriedout using the system shown in Figure 2. TiO2 with 3 wt % ofAu was prepared for the water splitting experiment. In a typicalreaction, 0.2 g of photocatalyst was added to 140 mL ofdeionized water in the Pyrex reactor. Before photocatalyticreaction, the reactor was heated at 50 °C and evacuated for 30min with continuous stirring to remove dissolved air in the water.Next, the reactor was purged with high-purity argon gas andevacuated again. This Ar purge/evacuation process was repeatedfive times, and then the residual air content was checked byGC. The reactor was irradiated using a Xe lamp (λ > 400 nm)after the residual air in the reactor was confirmed to benegligible. The light intensity in front of the reactor wasmeasured using a Lumen meter (Goldilux, GRP-1 70234). Theintensity of incident visible light (λ > 400 nm) was 1.68 W/cm2,which was projected onto the reactor-side surface of 96cm2. The UV source was a 254 nm UV lamp and was insertedinto the center of the reactor. Cooling water was circulated insidethe reactor to maintain the reaction temperature at 25 °C. Thereaction was carried out for 7 h, and the reaction products wereanalyzed by GC using an online sampling loop (1 mL) atintervals of 1 h. The GC (China Chromatography 2000 GC)system was equipped with a 3.5 m Molecular Sieve 5A columnand a thermal conductivity detector, with Ar flowing at 20 mL/min as the carrier gas. Blank reactions were performed withoutphotocatalyst in the presence of light and with photocatalyst inthe dark. In both cases, no production of hydrogen was observed.

2.4. Simulation of Surface Plasmon Resonance. Three-dimensional FEM electromagnetic simulation (COMSOL Mul-tiphysics) was used to simulate the electromagnetic fielddistribution of a 3 nm nanogold particle on a 100 nm × 100nm × 50 nm TiO2. A normal incident light with linearpolarization transverse to the plane of incidence (i.e., TMpolarization) radiated from the top of the gold nanoparticle. TM(short for transverse magnetic) polarized light means theelectromagnetic wave in which the magnetic field vector iseverywhere perpendicular to the plane of incidence (the planeof incidence which includes the normal to the surface and theincident wave vector). In our numerical computation andsimulation, refractive indexes of 1.335, 2.592, and 3.300 wereused for water, gold, and TiO2, respectively, and the lightwavelength of 562.8 nm was employed. The size of the goldnanoparticle is quite small as compared with that of TiO2. Wechoose the periodic boundary condition for the simulation ofmany gold nanoparticles attached to the TiO2.

Figure 1. Schematic illustration of Au-loaded TiO2 for water splittingby the SPR effect.

Plasmonic Photocatalyst for H2 Evolution J. Phys. Chem. C, Vol. 115, No. 1, 2011 211

3. Results and Discussion

3.1. Photocatalyst Characterization. Figure 3(a) shows theTEM of gold nanoparticles prepared by the sodium citratereduced method. The gold nanoparticles dispersed well in liquidsolution. From the TEM micrograph, the average size of gold

nanoparticles was approximately 20 nm, which is consistent withthe PSD result shown in Figure 3(b).

Figure 4 shows the TEM micrographs of pure TiO2 (P25)and Au/TiO2 prepared by the photodeposition method. The nearspherical Au particles were observed as dark spots havingobvious contrast with the TiO2 support as shown in Figure 4(b).The mean size of Au was near 3 nm, which is smaller than thatof the particles produced by the sodium citrate reduced method.The morphology and size of Au particles on the quartz plateare shown in Figure 5. The Au particles aggregated slightlyafter calcination, leading to an increased size to about 50 nm.

The plasmon absorption arises from the collective oscillationsof the free conduction band electrons that are induced by theincident electromagnetic radiation in Au0 particles. Figure 6shows the UV-vis spectrum of gold particles on the quartzplate, indicating an absorption band between the wavelengthof 500 and 600 nm. The maximum absorption is at 533.4 nm.

Figure 2. Apparatus for photocatalytic water splitting. A, Ar for purging; B, ultrahigh-purity Ar; C, on/off valve; D, sampling loop; E, six-wayvalve; F, on/off valve; G, GC; H, magnetic stirrer; I, UV; J, water lock; K, Xe lamp; PI, pressure gauge; and R, Pyrex reactor.

Figure 3. (a) TEM micrograph of nanogold dispersion and (b) particlesize distribution.

Figure 4. TEM micrograph of (a) TiO2 (P25) and (b) 3.0 wt % Au on TiO2.

212 J. Phys. Chem. C, Vol. 115, No. 1, 2011 Chen et al.

According to the relevant references,12,13 the absorption bandis ascribed to the plasmonic resonance of metallic Au particles.The size of the nanogold particles would effect the position ofthe maximum absorption peak of SPR.

Figure 7 shows the UV-vis spectrum of TiO2 (P25) andphotodeposited Au/TiO2. For the TiO2 (P25), the absorption edgewas around 400 nm, which is between the absorption edge ofanatase (387 nm) and rutile (418 nm). This suggests that theTiO2 consists of both anatase and rutile phases.20 Since the sizeof Au particles produced by the photodeposition method wassmaller than that of the sodium citrate reduced method, the SPRabsorption peaks were found at somewhat different positions.In the case of Au/TiO2, the absorption band was shifted to near562.8 nm.

Figure 8 shows the XPS spectra of 3 wt % Au/TiO2. As shownin Figure 8(a), the spin-orbit components (2p3/2 and 2p1/2) of Ti2p are well deconvoluted by two peaks at approximately 459.1and 464.7 eV, corresponding to Ti4+ in a tetragonal structure.Meanwhile, the O 1s XPS spectrum (Figure 8(b)) shows anarrow peak with slight asymmetric distribution and a bindingenergy of 530.4 eV. This peak was attributed to the Ti-O inTiO2. The double peaks for nanogold particles were centeredat 83.5 and 87.2 eV as shown in Figure 8(c). According to the

relevant literature,21-23 the doublet peaks located at 83.3 and87.2 eV for nanogold particles can be assigned to the charac-teristic doublets of Au0 loaded on TiO2, suggesting that onlyelemental Au is formed on the TiO2 surface.

3.2. MB Degradation. Figure 9 shows the result of thedegradation of MB solution. The factors affecting MB removalcan be categorized into three kinds.24-27 The first is theadsorption of MB on gold particles, and the second is the lighteffect alone, under which MB would be photodegraded. Thethird is the photocatalytic degradation of MB in the solution.Prior to the photocatalytic reaction, blank experiments wereconducted. To verify the adsorption effect and the light effectalone on the degradation of MB, respectively, reaction with theliquid solution containing nanogold particles in the dark andthe reaction with liquid solution under light irradiation withoutany nanogold particles were performed. Figure 9 shows thatafter 6 h the adsorption effect contributed 4% in MB reduction,while 14% was observed for the light effect alone. However,when a Xe lamp containing visible light was used as the lightsource, 31.5% in MB degradation was found for nanogoldparticles after 6 h of irradiation. Since the Au particle is not aphotocatalyst, this degradation is not caused by photoinducedelectrons generated under light irradiation. The electromagneticfield of incident light couples with the oscillations of conductionelectrons in gold particles, resulting in strong-field enhancementof the local electromagnetic fields near the surface of goldnanoparticles. Such enhanced local field strength can be muchhigher than the applied electromagnetic field (i.e., incident light).Thus, it is suggested that when the wavelength of incident light(533.4 nm) matches with the SPR band of Au the resonance ofthe electrons was induced, as if forming an extra electronmagnetic field to enhance the light effect, thus increasing thedegradation rate of MB. In another experiment, a 250-450 nmfilter was attached to the light source so that only light withinthis wavelength range may pass through. The SPR effect wasfaded out by blocking away the SPR absorption band of Au(i.e., 533.4 nm). Only 19% in MB degradation resulted after6 h of irradiation, which is nearly equal to the summation ofMB degradation efficiency in both blank tests (i.e., 4% adsorp-tion + 14% light effect alone). Therefore, the extra ∼13%increase in MB degradation was attributed to the SPR on Auparticles.

3.3. Photocatalytic Splitting of Water. The yields of H2

and O2 evolution from the photocatalytic water-splitting reaction

Figure 5. SEM micrograph of Au particles on a quartz plate.

Figure 6. UV-vis spectrum of Au particles dispersed on the quartzplate.

Figure 7. UV-vis spectrum of TiO2 (P25) and photodeposited Au/TiO2.

Plasmonic Photocatalyst for H2 Evolution J. Phys. Chem. C, Vol. 115, No. 1, 2011 213

by Au/TiO2 are listed in Table 1. The product ratio of H2 andO2 was about 2:1, which fitted the stoichiometric ratio of watersplitting. The H2 evolution from pure water using TiO2 and Au/TiO2 photocatalysts prepared by the photodeposition method isshown in Figure 10 (O2 evolution not shown). It can be seenthat the activity of water splitting was improved by the loadingof Au on TiO2. After 7 h of UV irradiation on TiO2, only asmall amount of H2 was produced, which was about 0.79 µmol/g-cat. Under the same conditions, Au/TiO2 greatly enhancedthe production of H2, which was about 35.04 µmol/g-cat. Thisimprovement was due to the presence of nanogold particles thatplayed the role of the electron sinks, retarding the recombination

of electron-hole pairs. Moreover, nanogold particles alsooffered active sites resulting in the substantial increase of H2

yield.Figure 10 also shows the result of H2 yield for Au/TiO2 under

both UV and visible light irradiation after 7 h. As expected,the additional visible light further boosted the H2 yield of Au/TiO2 to 53.75 µmol/g-cat. Such a result was due to the SPReffect generated by nanogold particles under the irradiation ofappropriate visible light. In summary, under the irradiation ofboth UV and visible light, nanogold particles not only playedthe role of electron sinks but also offered active sites as well asthe SPR enhancement to significantly increase the productionof H2. Since TiO2 is a UV responsive photocatalyst, sufficientelectron-hole pairs will be generated under UV irradiation to

Figure 8. XPS spectra of 3.0 wt % Au/TiO2. (a) Ti 2p, (b) O 1s, and(c) Au 4f.

Figure 9. Results of MB photodegradation.

TABLE 1: Photocatalytic Water Splitting to Form H2 andO2 over 3.0 wt % Au/TiO2 after 7 h of Irradiationa

yield (µmol/g-cat)

catalysts and conditions H2 O2

Au/TiO2 irradiated by UV and visible light 53.75 25.87Au/TiO2 irradiated by UV 35.04 17.52Au/TiO2 irradiated by visible light ND NDTiO2 irradiated by UV 0.79 0.38

a ND: not detected. UV: wavelength ) 254 nm, intensity ) 30mW cm-2. Visible: wavelength > 400 nm, intensity ) 1.68 Wcm-2.

Figure 10. Photocatalytic activity for water splitting (photocatalyst0.2 g of 3.0 wt % Au/TiO2). O2 evolution is not shown.

214 J. Phys. Chem. C, Vol. 115, No. 1, 2011 Chen et al.

give H2. On the other hand, under the irradiation of visible lightalone, no H2 production was observed on Au/TiO2. This impliesthat water splitting with only the SPR effect from nanogoldparticles cannot be successfully performed. Therefore, for thewater splitting over TiO2 loaded with nanogold particles, theability to offer electron sinks as well as active sites should bethe major merit of Au, while the effect of SPR is less significantbut important.

Although the real phenomenon of SPR on Au/TiO2 iscomplicated, it can be elucidated by simulation. The numericalcalculation of electromagnetic field intensity of the Au particleon TiO2 could be illustrated by the three-dimensional FEMsimulation. As shown in Figure 11, there are three cases in ourcomputations, namely, TiO2, with a water interface, a 3 nm fullspherical gold nanoparticle on the TiO2, and a 3 nm halfspherical gold nanoparticle on the TiO2. Simulation resultsshown in Figure 12 clearly demonstrate the occurrence ofelectromagnetic field enhancement in the vicinity of goldnanoparticle in close proximity to TiO2 under the normalillumination of a linear TM polarized light. The maximumintensity of the electric field in the second and third case is4.19 and 1.88 times higher than that in the first case, which hasa water-TiO2 interface only. This indicates that near-fieldintensity enhancement of surface plasmon is present to ef-fectively promote the photocatalytic process.

4. Conclusion

The surface plasmon resonance on the metal-loaded photo-catalysts can be an important way to utilize the full spectrum

in sunlight for solar energy harvest, especially for wide bandgapmaterials, such as TiO2. Thus, both photon energies of UV andvisible light can be absorbed and converted to chemical energy,i.e., hydrogen via water splitting. From the result of methyleneblue degradation, the influence of the excitation wavelengthrelated to the plasmon band absorption was observed inaccordance with electromagnetic theory. The degradation ef-ficiency increased when the incident wavelength matched withthe surface plasmon resonance absorption band of nanogoldparticles. The photocatalytic water splitting is one of the bestdirect routes to generate renewable hydrogen from sunlight. Ourresults in water splitting clearly indicated that the role of electronsinks is not the only mechanism responsible for the activityenhancement by incorporating Au particles. The SPR phenom-enon of Au that functions to provide extra electromagnetic fieldwas found to be important as well for the enhancement of H2

production in photocatalytic water splitting.

Acknowledgment. The authors would like to acknowledgethe National Science Council of Taiwan for financial supportof this research under project no. 98-2120-M-002-004.

References and Notes

(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38.

Figure 11. Schemes of Au on TiO2 simulation. (a) TiO2 only, (b) afull spherical gold particle on TiO2, and (c) a half spherical gold particleon TiO2. Figure 12. FEM electromagnetic intensity simulation of (a) TiO2 only,

(b) a full spherical gold particle on TiO2, and (c) a half spherical goldparticle on TiO2. The diameter of the gold nanoparticle is 3 nm, andthe size of TiO2 is 100 nm × 100 nm × 50 nm. A wavelength of562.8 nm and TM polarized light is incident from the top. The definitionof electromagnetic intensity is |E| ) (|Ex|2 + |Ey|2 + |Ez|2)(1/2), and thesubscripts denote the component of total electric field E.

Plasmonic Photocatalyst for H2 Evolution J. Phys. Chem. C, Vol. 115, No. 1, 2011 215

(2) Yang, H. H.; Guo, L. J.; Yan, W.; Liu, H. T. J. Power Sources2006, 159, 1305–1309.

(3) Xing, C.; Zhang, Y.; WeiYan; Guo, L. Int. J. Hydrogen Energy2006, 31, 2018–2024.

(4) Luan, J. F.; Pan, B. C.; Paz, Y.; Li, Y. M.; Wu, X. S.; Zou, Z. G.Phys. Chem. Chem. Phys. 2006, 11, 6289–6298.

(5) Sreethawong, T.; Yoshikawa, S. Catal. Commun. 2005, 6, 661–668.

(6) Kudo, A.; Miseki, Y. Chem. Soc. ReV. 2009, 38, 253–278.(7) Kwak, B. S.; Chae, J.; Kim, J.; Kang, M. Bull. Korean Chem. Soc.

2009, 30, 1047–1053.(8) Yu, Z.; Chuang, S. Appl. Catal. B, EnViron. 2008, 83, 277.(9) Sasaki, Y.; Nemoto, H.; Saito, K.; Kudo, A. J. Phys. Chem. C 2009,

113, 17536–17542.(10) Merlen, A.; Gadenne, V.; Romann, J.; Chevallier, V.; Patrone, L.;

Valmalette, J. C. Nanotechnology 2009, 20, 1–7.(11) Wu, D. J.; Xu, X. D.; Liu, X. J. Solid State Commun. 2008, 148,

163–167.(12) Noguez, C. J. Phys. Chem. C 2007, 111, 3806–3819.(13) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426.(14) Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami,

H.; Ohki, Y.; Yoshida, N.; Watanabe, T. J. Am. Chem. Soc. 2008, 130,1676–1680.

(15) Enustun, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317–3328.

(16) Yogi, C.; Kojima, K.; Wada, N.; Tokumoto, H.; Takai, T.;Mizoguchi, T.; Tamiaki, H. Thin Solid Films 2008, 516, 5881–5884.

(17) Yogi, C.; Kojima, K.; Takai, T.; Wada, N. J. Mater. Sci. 2009, 44,821–827.

(18) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. Catal.Lett. 1997, 44, 83–87.

(19) He, P. K.; Zhang, M.; Yang, D. M.; Yang, J. J. Surf. ReV. Lett.2006, 13, 51–55.

(20) Chiarello, G. L.; Selli, E.; Forni, L. Appl. Catal. B: EnViron. 2008,84, 332–339.

(21) Epling, W. S.; Hoflund, G. B.; Weaver, J. F.; Tsubota, S.; Haruta,M. J. Phys. Chem. 1996, 100, 9929–9934.

(22) Tada, H.; Suzuki, F.; Ito, S.; Akita, T.; Tanaka, K.; Kawahara, T.;Kobayashi, H. J. Phys. Chem. B 2002, 106, 8714–8720.

(23) Iliev, V.; Tomova, D.; Bilyarska, L.; Tyuliev, G. J. Mol. Catal. A:Chem. 2007, 263, 32–38.

(24) Otsuka-Yao-Matsuo, S.; Ueda, M. J. Photochem. Photobiol., A2004, 168, 1–6.

(25) Wang, J. H.; Nonami, T. J. Mater. Sci. 2004, 39, 6367–6370.(26) Ramaswamy, V.; Jagtap, N. B.; Vijayanand, S.; Bhange, D. S.;

Awati, P. S. Mater. Res. Bull. 2008, 43, 1145–1152.(27) Mohamed, S. H. J. Phys. D: Appl. Phys. 2010, 43, 1–8.

JP1074048

216 J. Phys. Chem. C, Vol. 115, No. 1, 2011 Chen et al.