Preparation and Photocatalytic Behavior of MoS2 and WS2

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Preparation and Photocatalytic Behavior of MoS2 and WS2 Nanocluster Sensitized TiO2 Wingkei Ho, Jimmy C. Yu,* Jun Lin, Jiaguo Yu, and Puishan Li Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China Received January 19, 2004. In Final Form: March 27, 2004 A new approach has been developed for the fabrication of visible light photocatalysts. Nanoclusters of MoS2 and WS2 are coupled to TiO2 by an in situ photoreduction deposition method taking advantage of the reducing power of the photogenerated electrons from TiO2 particles. The photocatalytic degradation of methylene blue and 4-chlorophenol in aqueous suspension has been employed to evaluate the visible light photocatalytic activity of the powders. The blue shift in the absorption onset confirms the size quantization of MS2 nanoclusters, which act as effective and stable sensitizers, making it possible to utilize visible light in photocatalysis. Quantum size effects alter the energy levels of the conduction and valence band edges in the coupled semiconductor systems, which favors the interparticle electron transfer. In addition, the coupled systems are believed to act in a cooperative manner by increasing the degree of charge carrier separation, which effectively reduces recombination.

Introduction Since the discovery of photoinduced water splitting on TiO2 electrodes in 1972, TiO2 has been widely studied due to its potential applications in air purification, water disinfection, and hazardous waste remediation.1-7 Besides single-component photocatalysis, a lot of research has focused on heterogeneous semiconductor systems in order to improve the photocatalytic efficiency and functionality of TiO2. An efficient charge separation can be obtained by coupling two semiconductor particles with different energy levels, e.g., WO3/TiO2 ,8 SnO2/TiO2, 9-14 ZnO/ TiO2.15,16 The improvement of efficiency of photocatalytic reactions is explained as the result of a vectorial transfer of photogenerated electrons and holes from a semiconductor to another. Apart from environmental purification, designing heterogeneous semiconductor systems for other new applica* Corresponding author: e-mail [email protected]; Tel (852) 2609-6268; Fax (852) 2603-5057. (1) Hoffmann, M. S.; Martin, T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (2) Fox, M. A.; Duby, M. T. Chem. Rev. 1993, 93, 341. (3) Kamat, P. V. Chem. Rev. 1993, 93, 267. (4) Linsebigler, A. L.; Lu, G.; Yates, Jr. J. T. Chem. Rev. 1995, 95, 735. (5) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. Rev. 2000, 1, 1. (6) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier Science: New York, 1993. (7) Tada, H.; Yamamoto, M.; Ito, S. Langmuir 1999, 15, 3699. (8) Kwon, Y. T.; Song, K. Y.; Lee, W. I.; Choi, G. J.; Do, Y. R. J. Catal. 2000, 191, 192. (9) Cao, Y.; Zhang, X.; Yang, W.; Du, H.; Bai, Y.; Li, T.; Yao, J. Chem. Mater. 2000, 12, 3445. (10) Tada, H.; Hattori, A.; Tokihisa, Y.; Imai, K.; Tohge, N.; Ito, S. J. Phys. Chem. B 2000, 104, 4585. (11) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Chem. Mater. 1996, 8, 2180. (12) Bedja, I.; Kamat, P. V. J. Phys. Chem. 1995, 99, 9182. (13) Vinodgopal, K.; Kamat, P. V. Environ. Sci. Technol. 1995, 29, 841. (14) Hattori, A.; Tokihisa, Y.; Tada, H.; Tohge, N.; Ito, S.; Hongo; K.; Shiratsuchi, R.; Nogami, G. J. Sol-Gel Sci. Technol. 2001, 22, 53. (15) Marci, G.; Augugliaro, V.; Lopez-Munoz, M. J.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. B 2001, 105, 1026. (16) Marci, G.; Augugliaro, V.; Lopez-Munoz, M. J.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. B 2001, 105, 1033.

tions has received much attention. Watanabe and coworkers demonstrated enhanced photoinduced hydrophilicity on WO3/TiO2 thin films.17,18 They revealed that the surface modification of TiO2 by WO3 led to the charge transfer between the TiO2 and WO3 layers. The photogenerated holes accumulate at the surface of TiO2, whereas electrons accumulate within the WO3 layer, hence enhancing the photoinduced hydrophilic conversion. In addition, Tatsuma et al. showed that the WO3/TiO2 photocatalysis system exhibited an energy storage ability, in which TiO2 gives photoexcited electrons under UV irradiation and WO3 stores those electrons and releases them in the dark to carry on the reaction.19-21 Recently, attempts have been made to use small band gap semiconductors such as CdS to photosensitize TiO2 in visible light.22-26 By coupling two semiconductor systems, electrons can be injected from an excited small band gap semiconductor into TiO2 under visible light. However, there are two prerequisites for the small band gap semiconductor: (i) the band gap value of the semiconductor should be near that for optimum utilization of solar radiant energy, and (ii) its conduction band minimum should be higher than that of TiO2. Unfortunately, only a few semiconductors (CdS and ZnO) can fulfill the above conditions. Most of the energy levels of the conduction band minimum of small band gap semiconductors are less negative than those of TiO2. This means that electrons (17) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 4714. (18) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 2000, 12, 1923. (19) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Chem. Mater. 2001, 13, 2838. (20) Tatsuma, T.; Saitoh, S.; Ngaotrakanwiwat, P.; Ohko, Y.; Fujishima, A. Langmuir 2002, 18, 7777. (21) Ngaotrakanwiwat, P.; Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Phys. Chem. Chem. Phys. 2003, 5, 3234. (22) Sant, P. A.; Kamat, P. V. Phys. Chem. Chem. Phys. 2002, 4, 198. (23) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435. (24) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol. A: Chem. 1995, 85, 247. (25) Yin, H.; Wada, Y.; Kitamura, T.; Sakata, T.; Mori, H.; Yanagida, S. Chem. Lett. 2001, 3, 334. (26) Yu, J. C.; Wu, L.; Lin, J.; Li, P.; Li, Q. Chem. Commun. 2003, 1552.

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would not be transferred from these potential sensitizers to titanium dioxide. During the past 20 years molybdenum and tungsten dichalcogenides have been extensively studied because of their potential use as electrodes for electrochemical solarenergy conversion.27-37 The interest in these semiconductors stems from their band gaps (1.1-1.7 eV) that closely match the solar spectrum and from their stability against photocorrosion.38,39 Unfortunately, the conduction band energy levels of bulk Mo and W dichalcogenides are also less negative than that of TiO2. This means that electrons would not be transferred from these potential sensitizers to titanium dioxide. Accordingly, it has been reported that MoS2 and WS2 nanoclusters exhibited quantum confinement effects.40-43 Because of this confinement, the band gaps of MoS2 and WS2 nanoclusters can be increased significantly, leading to a change in their redox potentials as well. Such appropriate alternation in the energy levels of the conduction and valence band edges would allow MoS2 and WS2 nanoclusters to act as photosensitizers for visible light in the heterogeneous semiconductor systems. Conventionally, the synthesis of molybdenum and tungsten disulfides generally involves thermal decomposition of ammonium thiosalts44,45 and sulfidation of molybdenum and tungsten oxides.46,47 Afanasiev et al. reported an alternative preparation method for MoS2 from the reduction of (NH4)2MoS4 by hydrazine.48 This method, however, requires a reflux of 6 h and a thermal treatment at 623-673 K under nitrogen flow to provoke the redox reactions. Herein, a novel and simple in situ photoinduced deposition method for the fabrication of MS2/TiO2 (M ) Mo or W) photocatalyst is demonstrated. Our new method is more effective as it takes advantage of the reducing power of the photogenerated electrons to convert M6+ to

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Figure 1. Schematic diagram of the experimental setup of in situ photoinduced fabrication of MS2 sensitized TiO2.

M4+ directly on TiO2 particles. The most attractive feature of this in situ reduction and deposition procedure is that the quantum-sized MS2 nanoclusters, which can act as photosensitizers, are inherently bound to TiO2. Moreover, this report reveals for the first time that the coupling of TiO2 photocatalyst with quantum-sized MS2 nanoclusters exhibits visible light activity in the degradation of methylene blue and 4-chlorophenol. Experimental Section

(27) Tributsch, H. J. Electrochem. Soc. 1978, 125, 1086. (28) Lewerenz, H. J.; Heller, A.; Di Salvo, F. J. J. Am. Chem. Soc. 1980, 102, 1877. (29) Lewerenz, H. J.; Gerischer, H.; Lubke, M. J. Electrochem. Soc. 1984, 131, 100. (30) Calabrese, G. S.; Wrighton, M. S. J. Am. Chem. Soc. 1981, 103, 6273. (31) Kam, K. K.; Parkinson, B. A. J. Phys. Chem. 1982, 86, 463. (32) Calabrese, G. S.; Buchanan, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 5786. (33) Baglio, J. A.; Calabrese, G. S.; Kamieniecki, E.; Kershaw, R.; Kubiak, C. P.; Ricco, A. J.; Wold, A.; Wrighton, M. S.; Zoski, G. D. J. Electrochem. Soc. 1983, 129, 1461. (34) Baglio, J. A.; Calabrese, G. S.; Harrison, D. J.; Kamieniecki, E.; Ricco, A. J.; Wrighton, M. S.; Zoski, G. D. J. Am. Chem. Soc. 1983, 105, 2246. (35) Simon, R. A.; Ricco, A. J.; Harrison, D. J.; Wrighton, M. S. J. Phys. Chem. 1983, 87, 4446. (36) Calabrese, G. S.; Buchanan, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 5594. (37) Cabrera, C. R.; Abruna, H. D. J. Electrochem. Soc. 1988, 135, 1436. (38) Coehoorn, R.; Haas, C.; Dijkstra, J.; Flipse, C. J. F.; de Groot, R. A.; Wold, A. Phys. Rev. B 1987, 35, 6203. (39) Coehoorn, R.; Haas, C.; de Groot, R. A. Phys. Rev. B 1987, 35, 6203. (40) Huang, J. M.; Laitinen, R. A.; Kelley, D. F. Phys. Rev. B 2000, 62, 10995. (41) Wilcoxon, J. P.; Newcomer, P. P.; Samara, G. A. J. Appl. Phys. 1997, 81, 7934. (42) Thurston, T. R.; Wilcoxon, J. P. J. Phys. Chem. B 1999, 103, 11. (43) Wilcoxon, J. P. J. Phys. Chem. B 2000, 104, 7334. (44) Afanasiev, P.; Geantet, C.; Thomazeau, C.; Jouget, B. Chem. Commun. 2000, 1001. (45) Alonso, G.; Berhault, G.; Aguilar, A.; Collins, V.; Ornelas, C.; Fuentes, S.; Chianelli, R. R. J. Catal. 2002, 208, 359. (46) DiPaola, A.; Palmisano, L.; Derrigo, M.; Augugliaro, V. J. Phys. Chem. B 1997, 101, 876. (47) Van der Vlies, A. J.; Prins, R.; Weber, Th. J. Phys. Chem. 2002, 106, 9277. (48) Afanasiev, P.; Xia, G. F.; Berhault, G.; Jouguet, B.; Lacroix, M. Chem. Mater. 1999, 11, 3216.

Preparation of MoS2 and WS2 Nanocluster Sensitized TiO2. A 300 mL Pyrex photochemical reactor was used, in which 0.3 g of TiO2 (Degussa P25) was suspended in a 300 mL aqueous solution of (NH4)2MoS4 or (NH4)2WS4 (0.25 M) as shown in Figure 1. Hydrazine (0.1 M) was added to react with the positively charged holes, thus inhibiting the h+/e- recombination during the preparation process. A 300 W high-pressure mercury lamp was used as a UV light source. N2 was continuously bubbled through the sample solution before and during the irradiation for the removal of oxygen. After irradiation, the powder that was separated from the aqueous solution by centrifuge was calcined in N2 at different temperatures for the crystallization of the deposited MS2. It should be emphasized that the as-prepared powders were washed with deionized water and 1 M HCl aqueous solution several times to remove the precursors and unsupported MS2 nanoclusters. The concentrations of WS2 and MS2 were found to be 0.3 and 0.6 mol %, respectively, as determined by XPS. Characterization. XPS measurements were performed on a Phi Quantum 2000 system with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to C 1s peak at 284.8 eV of the surface adventitious carbon. UV/vis spectra were achieved using a UV-vis spectrophotometer (Cary 100 scan spectrophotometers, Varian). ESR spectra were recorded by JEOL ESR spectrometer (TE100) equipped with a UV source at 77 K. Photocatalytic Activity Measurements. The photocatalytic activities of the samples for methylene blue degradation were measured. A 300 W tungsten lamp was positioned inside a cylindrical Pyrex vessel surrounded by a circulating cooling water jacket and a cutoff filter solution jacket (HCl aqueous solution of CuSO4 (0.5 M) and K2CrO4 (0.002 M)) which cuts off wavelengths shorter than 400 nm and longer than 660 nm. O2 was continuously bubbled throughout the reaction. A typical reaction mixture for the irradiation contained the following initial concentrations: methylene blue (8 mg/dm3) and MS2 nanocluster sensitized TiO2 (350 mg/dm3). Deionized water was used through-

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Figure 4. UV-vis diffuse reflectance spectra of pure TiO2, 0.3 mol % WS2/TiO2, and 0.5 mol % MoS2/TiO2. Figure 2. Proposed model of in situ photoinduced fabrication of MS2 sensitized TiO2.

Figure 5. Energy levels of the conduction and valence band edges vs normal hydrogen electrode (NHE) for pure TiO2, WS2, and MoS2 with various sizes at pH 7. (Asterisk indicates band gap increase due to the quantum-sized effect.)

Figure 3. High-resolution XPS spectra of the Mo 3d and W 4d regions for the MS2 sensitized TiO2. The raw spectra after a Shirley-type background subtraction were fitted using a nonlinear least-squares fitting program with GaussianLorentzian peak shapes. out this study. At the given irradiation time intervals, a sample (3 mL) was taken out and analyzed by UV/vis spectroscopy. The photocatalytic activities of the MS2 sensitized TiO2 samples were also measured by the degradation of 4-chlorophenol in an aqueous solution. O2 was bubbled into the solution throughout the experiment. A 300 W tungsten halogen lamp with a 400 nm cutoff filter was used as visible light source. 0.2 g of photocatalyst was suspended in a 200 mL aqueous solution of 2.2 × 10-4 M 4-chlorophenol. The concentrations of 4-chlorophenol and its degradation products were measured by a HPLC system (Waters Baseline 810 with a Waters 486 tunable UV absorbance detector, and a Supelco LC-18-DB column). The mobile phase was a 40:60 methanol:water mixture, and the flow rate was 1 mL/min. The organic compounds were detected at 220 nm. Millipore disks were used to separate the catalysts before analysis of the solution.

Results and Discussion The formation of MS2 nanocluster sensitized TiO2 mainly involves the elimination of ammonium sulfide, deposition of MS2 nanoclusters on TiO2 particles, and crystallization of MS2 nanoclusters. The proposed model of in-situ photoinduced fabrication of MS2 sensitized TiO2 is illustrated in Figure 2. The ratios of both Mo4+ and W4+ to S2- are about 1:2 according to the XPS analysis results. In addition, the high-resolution XPS spectra in Figure 3 also show doublet peaks for Mo 3d (228.8 and 231.8 eV) and W 4d (245.3 and 257.9 eV). These are assigned to the

Mo4+ ion in MoS2 and the W4+ ion in WS2, respectively.49 No features corresponding to MoS2 and WS2 can be observed in the samples without UV irradiation. Evidently, this confirms the efficient formation of MoS2 and WS2 by TiO2 photoreduction. UV-vis diffuse reflectance spectra of the MoS2 and WS2 nanocluster sensitized TiO2 show extended absorption down to 700 and 620 nm, respectively (Figure 4). It should be noted that bulk MoS2 and WS2 have absorption edges at 1040 nm (1.23 eV band gap) and 920 nm (1.35 eV band gap), respectively.50 The large blue shifts indicate the presence of a strong quantum confinement effect in the new photocatalysts.40-43 As a result of this confinement, the band gaps of MS2 nanoclusters are increased, and their redox potentials change accordingly. This issue is illustrated in Figure 5, which shows the redox potentials of TiO2 and MS2 (based upon the lowest energy indirect transition in both materials) with bulk and quantum sizes vs the normal hydrogen electrode potential (NHE). The appropriate alternation in the energy levels of the conduction and valence band edges allow MoS2 and WS2 nanocluster to act as sensitizers for visible light TiO2 photocatalysis. Photocatalytic activities of the samples prepared under different conditions were evaluated by the degradation of methylene blue. It is not surprising that the visible light photocatalytic activity is strongly dependent on both the UV irradiation time for photoinduced deposition and the calcination temperature (Table 1). These factors greatly influence the formation and crystallization of MS2 nanoclusters on TiO2 particles. The optimums in photocatalytic activity were obtained for the samples. Obviously, both MoS2 and WS2 nanocluster sensitized TiO2 exhibit high (49) Wagner, C.; Muilnberg, G. Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (50) Kam, K. K.; Parkinson, B. A. J. Phys. Chem. 1982, 86, 463.

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Table 1. Photocatalytic Activities of MoS2 and WS2 Nanocluster Sensitized TiO2 Samples under Different Preparation Conditions for Methylene Blue Degradation samples WS2/TiO2 WS2/TiO2 WS2/TiO2 WS2/TiO2 WS2/TiO2 WS2/TiO2 MoS2/TiO2 MoS2/TiO2 MoS2/TiO2 MoS2/TiO2 MoS2/TiO2

UV irradiation calcination photocatalytic activity tempb (K) (A/A0, t ) 4 h)c timea (h) 2.5 2.5 2.5 2.5 1 4.5 1.5 1.5 1.5 0.5 1

473 523 573 623 523 523 523 573 623 573 573

0.79 0.60 0.86 0.91 0.69 0.68 0.85 0.68 0.78 0.88 0.92

a

UV irradiation time of the samples in photoinduced deposition. b Calcination in N for 1 h. c A ) initial absorbance of methylene 2 0 blue (λmax at 664 nm), and A ) absorbance of methylene blue after 4 h visible light irradiation.

Figure 6. Photodegradation of methylene blue for the MoS2 and WS2 nanocluster sensitized TiO2 and pure TiO2 under visible light irradiation (400-660 nm). A represents the absorbance of methylene blue (λmax at 664 nm).

Figure 8. ESR spectrum of WS2 nanocluster sensitized TiO2 powder recorded at 77 K in the presence of oxygen under visible light (λ > 425 nm) irradiation.

tion. These results are consistent with those in the degradation of methylene blue. No photoactivity was observed for the pure TiO2 sample. The high photocatalytic activity of the new photocatalysts under visible light can be attributed to the electron transfer from the quantum-sized MS2 nanoclusters. The conduction band edges of MS2 nanoclusters are deemed to be higher than that of TiO2 due to the quantum confinement effect. This facilitates the interfacial electron transfer from MS2 nanoclusters to TiO2. Under visible light irradiation, only MS2 is activated. Photogenerated electrons transfer from the conduction band of MS2 into TiO2 and accumulate at the lower-lying conduction band of TiO2, while holes accumulate at the valence band of WS2. Consequently, the photogenerated electron is then scavenged by the oxygen in water, finally forming hydroxyl radicals to degrade the methylene blue and 4-chlorophenol. The major reactions that occur can be summarized as follows:51-53

MS2/TiO2 + hv (>400 nm) f MS2 (h+ + e-)/TiO2 MS2 (h+ + e-)/TiO2 f MS2 (h+)/TiO2 (e-) TiO2 (e-) + O2 f O2O2- + H2O f HO2• + OHHO2• + H2O f H2O2 + OH• H2O2 f 2OH• Figure 7. Photodegradation of 4-chlorophenol for the MoS2 and WS2 nanocluster sensitized TiO2 and pure TiO2 under visible light irradiation (λ > 400 nm).

photocatalytic activity under visible light in Figure 6. The reactions follow a first-order rate law. No photodegradation of methylene blue occurs when pure TiO2 is irradiated with visible light, since it is inactive under visible light irradiation (λ > 400 nm). 4-Chlorophenol, whose photocatalytic degradation mechanism is quite different from that of methylene blue, was selected as another target substrate in this study. The most efficient photocatalysts on methylene blue degradation were used. Figure 7 illustrates the photocatalyzed disappearance of 4-chlorophenol in the presence of different samples. The results indicate that both MoS2 and WS2 sensitized TiO2 powders are efficient in the photodegradation of 4-chlorophenol under visible light irradia-

OH• + organic compounds f CO2 + H2O A signal identified to Ti3+ radical (g ) 1.992) is detected in the ESR spectrum (Figure 8), which indicates the transfer of photogenerated electrons from the conduction band of MS2 to that of TiO2 under visible light.54,55 This further confirms the interfacial electron transfer from MS2 to TiO2. Another possible reason for hydroxyl radical production could be when the semiconductor valence band potential is larger than +1.20 V vs NHE. Photogenerated holes transfer from quantum-sized MS2 to bound H2O and (51) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (52) Matthews, R. W. J. Catal. 1988, 111, 264. (53) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Toxicol. Environ. Chem. 1988, 16, 89. (54) Howe, R. F.; Gratzel, M. J. Phys. Chem. 1987, 91, 3906. (55) Gratzel, M.; Howe, R. F. J. Phys. Chem. 1990, 94, 2566.

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produce hydrogen peroxide in the presence of O2, finally yielding hydroxyl radicals.56

2MS2 (h+) + 2H2O f H2O2 + 2H+ H2O2 f 2OH• OH• + organic compounds f CO2 + H2O Obviously, the quantum size effects increase the band gap in nanoscale MS2 and also shift the valence band enough to permit hydroxyl radical production. It should be noted that the oxidation potential of bulk MS2 is not sufficiently large to produce hydroxyl radicals. Although bulk MS2 is activated by the visible light, Serpone et al.24 found that tungsten disulfide on exposure to visible light irradiation was inactive toward phenol oxidation. Also, no photoresponse for hydrogen evolution was observed by Sobczynski et al.57 for silica-supported tungsten disulfide powders under visible light (λ > 435 nm). These observations suggest that the absence of activity of the bulk MS2 systems under visible irradiation could be explained by the unsuitable position of the bands of bulk MS2. In fact, both the conduction band and valence band of bulk MS2 may not be cathodic and anodic enough respectively to allow direct or indirect redox reactions of the substrate by means of the photogenerated charges. It must be noted, however, that Wilcoxon et al. showed very strong size dependence of MoS2 on the photocatalytic oxidation of pentachlorophenol and phenol due to the quantum size effects.42,43 Quantum size effect also shifts the reduction potentials of MS2 to more positive values, thus allowing the transfer of holes from MS2 to the water molecules to produce OH radicals. This prevents the accumulation of holes in valence band of MS2 and decreases the possibility of anodic corrosion. Additionally, detailed band-structure calculations by Coehoorn et al.38,39 have shown that the optical transitions responsible for the creation of hole-electron pairs are between the metal dz2 and nonmetal pz orbitals (56) Pelizzetti, E.; Visca, M. In Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: NewYork, 1983; p 261. (57) Sobczynski, A.; Yildiz, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T.; Webber, S. E.; White, J. M. J. Phys. Chem. 1988, 92, 2311.

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for these materials. The strong covalent interaction between the antibonding orbitals leads to the remarkable stability of Mo and W dichalcogenides against photocorrosion. This explains why MS2 nanocluster sensitized TiO2 powders are quite stable toward photocorrosion and show photocatalytic activity even after several hours of visible light irradiation. Another significant advantage from the alternation of the energy levels in the coupled semiconductor systems is that it allows interparticle electron transfer, which enhances charge separation and improves the efficiency of the interfacial charge-transfer processes compared to that of single-component photocatalysis. The surface trapping of electrons and holes before recombination may also be more efficient in small particles. The photogenerated electron-hole pairs have a much shorter distance to travel to reach the surface in a small cluster. Once the electrons and holes have been trapped at the interface, they can then participate in redox reactions, hence enhancing the photocatalytic activity. Conclusion This study demonstrates a novel approach for the fabrication of MoS2 and WS2 nanocluster sensitized TiO2. The new photocatalysts make possible the efficient utilization of visible light in photocatalysis. An explanation for the visible light photoactivity of the coupled systems is provided where the presence of two semiconductors with different energy levels for the corresponding conduction and valence bands probably leads to the photooxidation of the methylene blue and 4-chlorophenol. The Ti3+ signal observed in ESR measurement confirms the vectorial displacement of electrons from quantum-sized MS2 nanoclusters to TiO2. By the coupling of different semiconductor systems with appropriate alternation in the energy levels of the conduction and valence band edges, it should be possible to tailor the properties of a photocatalyst. Acknowledgment. This work was supported by the Innovation and Technology Fund (ITS/118/01) and by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project CUHK4027/ 02P). We acknowledge Dr. Daniel Kwong and Ms. Anna Chan of Hong Kong Baptist University for their assistance in ESR measurements. LA049838G