Photocatalytic H2 Evolution on CdS Loaded with WS2 as Cocatalyst

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Photocatalytic H2 Evolution on CdS Loaded with WS2 as Cocatalyst under Visible Light Irradiation Xu Zong, Jingfeng Han, Guijun Ma, Hongjian Yan, Guopeng Wu, and Can Li*

J. Phys. Chem. C 2011.115:12202-12208. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/26/19. For personal use only.

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China ABSTRACT: WS2/CdS photocatalysts with different amounts of WS2 cocatalyst are prepared by loading WS2 on CdS with an impregnation
sulfidation approach. The transformation process of the tungsten species loaded on CdS together with the junctions formed between WS2 and CdS are clearly demonstrated with X-ray photoelectron spectroscopy and transmission electron microscopy. Photocatalytic H2 production on WS2/CdS catalysts under visible light (λ > 420 nm) shows that WS2 cocatalyst plays a crucial role in H2 production. Under optimum conditions, the rate of H2 evolution can be increased by up to 28 times when CdS was loaded with only 1.0 wt % WS2, and WS2 demonstrates a catalytic performance comparable or even superior to those of noble metals. Photocatalytic reaction results and electrochemical measurements indicate that the significantly enhanced H2 evolution of WS2/ CdS catalyst mainly owns to the junctions formed between WS2 and CdS and the excellent performance of WS2 as a cocatalyst in catalyzing H2 evolution.

1. INTRODUCTION Photocatalytic H2 production from water splitting using semiconductor photocatalysts has drawn considerable attention as a promising way of resolving global energy and environmental problems.1
4 The development of visible-light-driven photocatalysts is indispensable for the utilization of the main part of the solar spectrum, which is of great importance for the practical application of the semiconductor photocatalytic system.5,6 Up to now, a lot of photocatalysts including (oxy)nitrides, (oxy)sulfides, and silicide have been demonstrated to be able to produce H2 under visible light.7
15 Among the photocatalysts under investigation, CdS is a good candidate semiconductor for its excellent light absorption property in the visible region.16
19 Moreover, the component of CdS is less expensive and the preparation method of CdS is relatively simple, which makes the CdS-based photocatalyst a more suitable choice compared with other photocatalysts for fundamental understanding. CdS alone demonstrated extremely low photocatalytic activity for the production of H2 under visible light. After surface modification with cocatalysts, the photocatalytic activity of CdS can be greatly improved.20,21 Therefore, the study on cocatalyst which could promote the separation of photoexcited electrons and holes and decrease the activation potentials for H2 evolution is absolutely paramount for developing the CdS-based photocatalysts as well as other photocatalysts.22
25 However, for most cases, noble metals or their oxides which are very expensive and in limited sources are used as the cocatalysts.26
29 Therefore, it is highly desirable to find less expensive alternative cocatalysts.30,31 In our recent work, it was found that MoS2 can act as an excellent cocatalyst for CdS, and the photocatalytic activity of CdS for H2 evolution was greatly enhanced by loading MoS2 on CdS.32 As members of the transition metal dichalcogenide compounds, WS2 and MoS2 possess extremely similar crystal structure and chemical property. Moreover, MoS2 and WS2 are active r 2011 American Chemical Society

components in many H2 involved reactions such as hydrodesulfurization and hydro-denitrogenation.33
38 We anticipate that WS2 could also act as a cocatalyst similar to MoS2 for CdS in photocatalytic H2 production. In the present study, WS2/CdS photocatalyst was fabricated and WS2 was investigated as a cocatalyst for photocatalytic H2 production under visible light. To understand the functions of the cocatalyst, factors influencing the activities of WS2/CdS catalyst for H2 evolution are investigated. We found that WS2 demonstrates similar property to MoS2 as a cocatalyst, which can also significantly enhance the photocatalytic activity of CdS in H2 production.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Commercial CdS powder was obtained from Shenyang Reagent Wuchang (cubic phase, 98%). (NH4)2WS4 was synthesized according to the literature and used as precursor for WS2 to be loaded on CdS.39 Different amounts of tungsten species were loaded on CdS by an impregnation method from ammonia solution containing different amounts of (NH4)2WS4. After vacuum drying at 333 K for 6 h, the (NH4)2WS4/CdS precursor was heated at temperatures from 423 to 773 K for 2 h in H2S to obtain the WS2/CdS catalyst. In most cases, CdS commercially obtained was directly used as the precursor to be loaded with WS2. The as-prepared sample was denoted as WS2/CdS. In some cases, CdS was first treated at 773 K for 5 h in H2S and then loaded with WS2. The resulting sample was denoted as WS2/CdS (HT). The amount of the tungsten species loaded on CdS was calculated by supposing that the (NH4)2WS4 was completely converted to WS2. Received: January 22, 2011 Revised: April 24, 2011 Published: May 26, 2011 12202

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Figure 1. XPS spectra for Cd 3d, S 2p, and W 4f regions of the 1.0 wt % WS2/CdS catalysts prepared at different temperatures.

2.2. Characterization of Catalysts. The as-prepared WS2/ CdS samples were characterized by X-ray photoelectron spectra (XPS) on an Amicus X-ray photoelectron spectrometer with Mg KR radiation. TEM and HRTEM micrographs were taken using a Tecnai G2 Spirit (FEI Co.) transmission electron microscope. The Brunauer
Emmett
Teller (BET) surface areas were measured on a Micromeritics ASAP 2000 system at liquid nitrogen temperature. 2.3. Photocatalytic Reactions. The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g of catalyst was suspended in 200 mL of aqueous solution containing 20 mL of lactic acid (LA) as a sacrificing agent. Different noble metals, including Pt, Ru, Rh, and Au, were loaded on CdS in situ by photoreduction method using H2PtCl6, RuCl3, RhCl3, and HAuCl4 aqueous solutions, respectively. The suspension was then thoroughly degassed and irradiated by a Xe lamp (300 W) equipped with an optical cutoff filter (λ > 420 nm) to eliminate ultraviolet light and a water filter to remove infrared light. The temperature of the reactant solution was maintained at 283 ( 5 K by a flow of cooling water during the reaction. The amount of H2 produced was analyzed using an online gas chromatography. The activities

of different catalysts were compared by the average rate of H2 evolution in the first 5 h. The apparent quantum efficiency (Φ) was estimated by the method reported in the literature and calculated using the following equation: Φ (%) = (2  R/I)  100, where R and I represent the number of evolved H2 molecules and the number of incident photons, respectively.40 Here Φ is quantum efficiency, where it is assumed that all incident photons are absorbed by the photocatalyst. The number of incident photons was measured using a calibrated Si photodiode. 2.4. Electrochemical Measurements. The electrochemical properties of WS2 were investigated using electrodes fabricated as follows. The CdS/FTO electrode was first prepared by chemical bath deposition of CdS layer on glass/FTO substrate that was cleaned in advance by ultrasonication in acetone, ethanol, and water.41 The as-prepared CdS/FTO electrode was then heated at 573 in H2S K for 2 h to promote the crystallinity of CdS. The WS2/CdS/FTO electrodes were prepared by loading WS2 on CdS/FTO electrode by impregnation from a (NH4)2WS4 methanol solution. Impregnation was performed by dropping a small amount of (NH4)2WS4
methanol solution onto the as-prepared CdS/FTO electrode using a pipet. The sample was dried and then heated at 573 K in H2S for 2 h to convert 12203

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Figure 2. TEM images of 1 wt % WS2/CdS catalysts prepared at (A) 573, (B) 673, (C) 723, and (D) 773 K and (E, F) HRTEM images of as-prepared 1 wt % WS2/CdS catalysts at 773 K and (G, H) HRTEM images of 1 wt % WS2/CdS catalysts recovered after photocatalytic reaction. The white circles indicate the presence of WS2 slabs.

(NH4)2WS4 to WS2. For comparison, the Pt/CdS/FTO electrode was prepared by dropping a small amount of H2PtCl6 aqueous solution on CdS/FTO electrode followed by heating at 573 K for 2 h in H2S. Electrochemical experiments were performed in a threeelectrode cell made of quartz. A Pt plate and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, and the CdS/FTO, Pt/CdS/FTO, and WS2/CdS/ FTO electrodes were used as working electrodes. 0.5 M of Na2SO4 solution was used as the electrolyte, and the electrolyte was saturated with argon prior to electrochemical measurements. Current
voltage curves were taken on a Princeton Applied Research potentiostat (2273).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterizations with XPS, TEM, HRTEM, and BET. The chemical states of Cd, S, and W species in the as-

prepared WS2/CdS catalysts were investigated by the XPS technique. Figure 1 shows the XPS spectra for Cd 3d, S 2p, and W 4f of the 1 wt % WS2/CdS catalysts prepared at temperatures from 473 to 773 K. As shown in Figure 1, the Cd 3d peaks at 411.9 and 405.1 eV and the S 2p peaks at 161.7 and 162.7 eV confirm S2
and Cd2þ in CdS,42 and the positions of the Cd 3d and S 2p peaks remain almost unchanged for the 1 wt % WS2/CdS catalysts prepared at different temperatures. Four peaks at 32.7, 34.8, 35.8, and 37.9 eV are observed for W 4f. The peaks at 32.7 and 34.8 eV can be assigned to the tungsten species in WS2 while the peaks at higher binding energies of 35.8 and 37.9 eV can be assigned to the tungsten species in WO3.43 With the increase of the heating temperature, the intensities of the peaks at 35.8 and 37.9 eV are gradually decreased while those of the peaks at 32.7 and 34.8 eV are drastically increased, indicating the general transformation of tungsten species from WO3 to WS2 with increased sulfidation temperature. It should be noted that because (NH4)2WS4 is not stable during the impregnation process, a large amount of (NH4)2WS4 undergos reaction to form ammonium tungstate. Therefore, tungsten species in the form of WO3 was observed on the surface of WS2/CdS prepared at high temperatures. TEM and HRTEM characterizations were then done on 1 wt % WS2/CdS catalysts prepared at different temperatures to investigate

the morphology of WS2 and the interfacial structures between WS2 and CdS. As shown in Figure 2, WS2 particles with typical layered structure were not observed on the surface of the 1 wt % WS2/CdS catalyst prepared at a temperature below 573 K, while WS2 slabs with characteristic layered structure were found on the surface of the 1 wt % WS2/CdS catalyst prepared at a temperature higher than 673 K. In fact, no crystalline WS2 was observed on the surface of CdS when the preparation temperature is lower than 623 K, while crystalline WS2 slabs appeared on the surface of CdS when the preparation temperature is higher than 673 K, indicating the gradual transformation of amorphous WS2 to crystalline WS2 at elevated temperatures. Moreover, it is easy to find that the (002) plane of WS2 tend to grow along the surface of CdS to form intimate junctions with CdS. It has been pointed out that undesirable phase boundary would impair the efficiency of charge separation and collection, and the detrimental influence of phase boundaries can be reduced by lattice matched heterointerfaces. For the layered materials the specific properties of the van der Waals plane may offer advantages in this aspect.44 Therefore, it is supposed that the characteristic stack mode of WS2 would make it form a more desirable junction with CdS, which will lead to more efficient electron transfer between adjacent components. Moreover, it is worth noting that singlelayer WS2 is widely distributed on the surface of CdS, which could afford much higher efficiency than that of multilayer WS2 by avoiding the electron tunneling through the WS2 layers. The chemical compositions of the as-prepared catalysts were then analyzed employing the STEM mode of HRTEM characterization. As shown in Figure 3, the signals assigned to Cd and S in CdS can be easily detected. Moreover, tungsten element is observed in four independent regions of WS2/CdS catalysts, indicating the homogeneous distribution of tungsten species on the CdS catalyst. The BET surface area of the commercial CdS and 1 wt % WS2/CdS samples prepared at 473, 573, 673, and 773 K is 15.1, 13.2, 11.6, 6.9, and 3.8 m2 g
1, respectively. 3.2. Photocatalytic H2 Evolution on WS2/CdS with Different Loading Amount of WS2 as Cocatalyst. In the above XPS analysis, WS2 and WO3 were found to coexist on the surface of CdS. Because the conduction band of WO3 locates at a more positive position than the potential of H2 evolution, WO3 is supposed to be inactive for photocatalytic H2 evolution. Therefore, 12204

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Figure 3. STEM image and EDX analysis of (A) as-prepared 1 wt % WS2/CdS catalyst and (B) 1 wt % WS2/CdS catalyst recovered after photocatalytic reaction.

it is reasonable to conclude that WS2 rather than WO3 is the active component for catalytic H2 evolution. For the purpose of elaboration, in the following sections we suppose that WS2 is the only tungsten species loaded on CdS. Figure 4 shows the rate of H2 evolution on WS2/CdS catalysts with different amounts of WS2 loadings, together with those on WS2, CdS, and Pt/CdS catalysts for a comparison. As shown in Figure 4A, no H2 was detected when WS2 alone was used as the catalyst, suggesting that WS2 is not active for photocatalytic H2 evolution. CdS alone shows activity for photocatalytic H2 evolution, but the activity is extremely low (ca. 15 μmol h
1), indicating that CdS lacks the catalytic sites for H2 evolution. After loading only 0.1 wt % of WS2 on CdS, the rate of H2 evolution on WS2/CdS is increased to 198.3 μmol h
1. With the increase of the loading of WS2 on CdS, the rate of H2 evolution on WS2/CdS is further increased initially and achieves a maximum when the loading amount of WS2 on CdS is about 1 wt %. The rate of H2 evolution on WS2/CdS at the optimum loading is 28 times higher than that on CdS alone, and the quantum efficiency for the photocatalytic H2 evolution was determined to be 5.0% at 420 nm under the reaction conditions. Further loading of WS2 on CdS led to a decrease in the photocatalytic H2 evolution. As is shown in Figure 4A, excess loading of WS2 will considerably block the absorption of the incident light by CdS. Moreover, recombination centers may be introduced at high loadings, which is another factor leads to the decreased activity. A similar volcano-like relationship between the loading amount of cocatalyst and photocatalytic activity was also observed on the CdS catalysts loaded with Pt (Figure 4B). However, the optimum amount of Pt loaded on CdS was 0.2 wt % in the present reaction condition. The rate of H2 evolution on CdS was increased by about 30 and 24 times by loading Pt with amounts of 0.2 and 1 wt %, respectively. For comparison, different noble metals such as Pt, Ru, Rh, and Au were also tested as the H2 evolution cocatalysts. 1 wt % of these cocatalysts was loaded on CdS with photoreduction method from the corresponding metal complexes. As shown in Figure 5A, the rates of H2 evolution were 355, 293, 207, and 45.5 μmol h
1 when employing Pt, Ru, Rh, and Au as cocatalysts

Figure 4. Rate of H2 evolution on CdS photocatalysts under visible light (λ > 420 nm) loaded with different amounts of (A) WS2 and (B) Pt as cocatalysts. Reaction conditions: catalyst, 0.1 g; 10 vol % lactic solution, 200 mL; light source, xenon lamp (300 W) with cutoff filter. The inset in (A) is the UV
vis diffuse reflectance spectra for CdS and WS2/CdS with different amounts of WS2.

on CdS, corresponding to apparent quantum efficiencies of 4.3%, 3.6%, 2.5%, and 0.5%, respectively. Among all the noble metals tested, Pt is demonstrated to be the best promoter for H2 evolution, while the rate of H2 evolution for 1 wt % Pt/CdS is still lower than that for 1 wt % WS2/CdS. Although at the optimum loading, 0.2 wt % Pt/CdS shows higher activity than 12205

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Figure 6. Rate of H2 evolution from lactic acid solution on 1 wt % WS2/CdS and CdS photocatalysts prepared at different temperatures under visible light (λ > 420 nm). Reaction conditions: catalyst, 0.1 g; 10 vol % lactic solution, 200 mL; light source, xenon lamp (300 W) with cutoff filter.

Figure 5. (A) Time courses of photocatalytic H2 evolution on CdS loaded with 1 wt % of different cocatalysts under visible light (λ > 420 nm). (B) Repated time courses of photocatalytic H2 evolution on 1 wt % WS2/CdS. Reaction conditions: catalyst, 0.1 g; 10 vol % lactic solution, 200 mL; light source, xenon lamp (300 W) with cutoff filter.

that of 1 wt % WS2/CdS and the optimum loadings of Rh, Ru, and Au were not tested in the studies, the present results clearly indicated that WS2 could function as an efficient cocatalyst with capability comparable with those of noble metals. To investigate the stability of WS2/CdS photocatalysts, we carried out photocatalytic reaction for repeated three sycles. The reaction system was evacuated every 5 h. As is shown in Figure 5B, the rate of H2 evolution on WS2/CdS decreased with every reaction cycle. From the HRTEM, STEM, and EDX characterization results shown in Figure 2G,H and Figure 3B, it was found that the WS2/ CdS photocatalyst preserved its original integrity well. Moreover, it is worth noting that similar phenomena were also observed on CdS loaded with different noble metals (Figure 5A). Therefore, the photocorrosion of CdS during the photocatalytic reactions is supposed to be the main reason for the decreased activity with reaction time, which has been pointed to be a major problem for metal sulfide photocatalysts. 3.3. Effect of the Preparation Temperature of WS2/CdS on H2 Evolution. The preparation temperature of the WS2/CdS catalysts could influence the surface area, crystallinity of WS2 and CdS, and the junctions formed between CdS and WS2, thereby influencing the photocatalytic properties. Therefore, WS2/CdS catalysts prepared at different temperatures were tested to investigate the effect of the heating temperature on the photocatalytic activities of the WS2/CdS catalysts. Figure 6 shows the rate of H2 evolution on the CdS and 1 wt % WS2/CdS catalysts prepared by heating the commercial CdS and (NH4)2WS4/CdS precursor at temperatures from 423 to 773 K in H2S. Photocatalytic H2 evolution was found to occur over the untreated CdS catalyst with an average rate of 5.5 μmol h
1, and the activities of the CdS catalysts are increased from 12.5 to 31.4 μmol h
1 with

Figure 7. Time courses of photocatalytic H2 evolution from lactic acid solution on 1 wt % WS2/CdS (HT) catalysts prepared at different temperatures under visible light (λ > 420 nm). Reaction conditions: catalyst, 0.1 g; 10 vol % lactic solution, 200 mL; light source, xenon lamp (300 W) with cutoff filter.

the increase of the heating temperatures from 473 to 773 K. But for all the CdS catalysts preapared at high temperatures, their activities remained extremely low. After loading 1 wt % of WS2 on CdS at different temperatures, the activity of CdS was dramatically enhanced. The rate of H2 evolution on 1 wt % WS2/CdS prepared at 473 K is 184 μmol h
1, which is about 15 times higher than that on CdS catalyst prepared at the same temperature. With the increase of the heating temperature, the overall activities of 1 wt % WS2/CdS increased first and attained the highest when the temperature was up to 573 K. Further increase of the heating temperature will decrease the rate of H2 evolution for 1 wt % WS2/CdS catalyst, which should be ascribed to the drastic decrease of the surface area of 1 wt % WS2/CdS catalyst heated at elevated temperatures. To eliminate the influence of the surface area on the photocatalytic activity, CdS was first pretreated at 773 K for 5 h in H2S and then WS2 was loaded on the as-treated CdS to prepare the 1 wt % WS2/CdS (HT) catalysts. We found that this pretreatment can prepare WS2/CdS (HT) catalysts with similar surface area. Figure 7 shows the rate of H2 evolution on the 1 wt % WS2/CdS (HT) catalysts prepared at different temperatures. As shown in Figure 7, with the increase of the heating temperature the activities of the 1 wt % WS2/CdS (HT) catalysts increase first and 12206

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Table 1. Rate of H2 Evolution under Visible Light (λ > 420 nm) on WS2, CdS, WS2/CdS, and the Mechanical Mixtures of WS2 and CdSa rate of H2 evolution entry

catalyst

(μmol h
1)

1

WS2 (0.01 g)

2

CdS (0.1 g)

15

3

WS2 (0.002 g) þ CdS (0.1 g)b

47

4 5

WS2 (0.01 g) þ CdS (0.1 g)b WS2 (0.001 g)/CdS (0.1 g)c

184 420

not detected

a

CdS treated at 573 K was used in the reaction. b Catalysts were simply mixed in the reaction solution. c WS2 was loaded on CdS by heating the (NH4)2WS4/CdS precursor at 573 K in H2S; reaction conditions: 10 vol % lactic solution (200 mL); light source, xenon lamp (300 W) with cutoff filter.

achieve a maximum at a heating temperature of 673 K. Because the surface area and the crystallinity of CdS are almost the same for the WS2/CdS (HT) catalyst, the changes of the catalytic efficiency of WS2 and the junctions formed between WS2 and CdS with the heating temperature are supposed to be the main factors affecting the activities of the catalysts. 3.4. Effects of the Junction Formed between CdS and WS2 on H2 Evolution. Table 1 shows the rate of H2 evolution on WS2, CdS, WS2/CdS, and mechanical mixture of WS2 and CdS. WS2 is not active, and CdS demonstrates very low activity in photocatalytic H2 evolution. The rate of H2 evolution on a mechanical mixture of CdS (0.1 g) and WS2 (0.002 g) was increased by only 3 times compared with CdS (0.1 g), and the activity can be further increased by adding more WS2 in the mixture. The fact that the activity of the mixture is higher than that of CdS alone indicates that there is some contact between WS2 and CdS in the mixture during the stirring of the reaction solution, and the contact is favorable for the charge separation between the two components. When the amount of WS2 used in the mixture is increased, the contacts between WS2 and CdS are improved, which leads to much higher activity. It is very interesting to note that the activity of WS2 (0.001 g)/CdS (0.1 g) is 9 times higher than that of the mixture of CdS (0.1 g) and WS2 (0.002 g) even though smaller amount of WS2 was used. Our early work demonstrates that the heterogeneous junctions formed between CdS and WS2 is crucial for the interelectron transfer between the two components. Heterogeneous junction between WS2 and CdS can be derived when the WS2/CdS catalyst is pretreated at high temperatures, and the junction can facilitate the interelectron transfer between CdS and WS2 more efficiently. Therefore, the activity of WS2/CdS is much higher than the mechanical mixture of CdS and WS2. 3.5. Electrochemical Characterization. The reduction of protons to H2 is a very important step in the photocatalytic H2 production reactions. It has been reported that the catalyzing property of MoS2 for H2 evolution is supposed to be a very important reason for the enhanced activity of MoS2/CdS.32 WS2 has previously been proposed as a catalyst for the hydrogen evolution reaction.45 In the present work, electrochemical measurements were conducted to investigate the role of WS2. The current
voltage curve of the WS2/CdS/FTO electrode is shown in Figure 8 together with those of the CdS/FTO and Pt/CdS/ FTO electrodes for a comparison. Cathodic current that was attributed to the reduction of water to H2 was observed for the

Figure 8. Current
voltage curves of the as-prepared CdS/FTO, Pt/ CdS/FTO, and WS2/CdS/FTO electrodes in 0.5 M Na2SO4 solution.

Scheme 1. Proposed Reaction Mechanism for Photocatalytic H2 Production on WS2/CdS Catalyst

CdS/FTI electrode in the potential range
0.9 to
1.3 V vs SCE, while the current is extremely low. There was a great increase in the cathodic current for WS2/CdS/FTO and Pt/CdS electrode/FTO. Therefore, WS2 was clearly demonstrated to be a good catalyst similar to Pt that can efficiently catalyze the evolution of H2 and, as a result, enhance the rate of H2 evolution on WS2/CdS catalysts. Since the preparation methods of WS2 for the WS2/FTO and WS2/CdS/FTO electrodes are almost the same as that of WS2/CdS catalyst, it is reasonable to think that WS2 loaded on CdS can act as a H2 evolution cocatalyst for CdS. 3.6. Proposed Reaction Mechanism. Based on the above results, the proposed mechanism for photocatalytic H2 production on WS2/CdS catalyst is shown in Scheme 1. Upon photoexcitation, electron and hole pairs are produced in the conduction band (CB) and valence band (VB) of CdS, respectively. The holes are consumed by the lactic acid. The photogenerated electrons will transfer to the surface of CdS and reduce protons to H2. Because CdS lacks the active sites for H2 evolution, the rate of H2 evolution on bare CdS is extremely low. However, when the photogenerated electrons transfer from CdS to the WS2 particles, protons can be efficiently reduced to produce H2 because WS2 is a good catalyst for the reduction of protons. Moreover, intimate junctions can be formed between WS2 and CdS, which facilitates the electron transfer from CdS to WS2. The above two factors are supposed to the main reasons that lead to the enhanced activity of the WS2/CdS photocatalyst.

4. CONCLUSIONS The rate of H2 evolution on CdS under visible light (λ > 420 nm) is significantly enhanced by loading WS2 as a cocatalyst. The activity of CdS is increased by up to 28 times when loaded 12207

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The Journal of Physical Chemistry C with only 1 wt % of WS2, and WS2 as a cocatalyst demonstrates comparable catalytic performance with those of noble metals. The excellent H2 activation property of WS2 together with the junctions formed between WS2 and CdS is supposed to be mainly responsible for the enhanced photocatalytic activity of WS2/CdS. This work also presents a possibility for the use of WS2 as a substitute for noble metals as cocatalyst in the photocatalytic H2 production.

’ AUTHOR INFORMATION Corresponding Author

*Tel þ86-411-84379070; Fax þ86-411-84694447; e-mail canli@ dicp.ac.cn.

’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grants 20503034 and 20673112), the National Key Basic Research and Development Program (Grant 2009CB220010), and Programme Strategic Scientific Alliances between China and The Netherlands (Grant 2008DFB50130). ’ REFERENCES (1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (2) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253. (3) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (4) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141. (5) Liu, G.; Yang, H. G.; Wang, X. W.; Cheng, L. N.; Pan, J.; Lu, G. Q.; Cheng, H. M. J. Am. Chem. Soc. 2009, 131, 12868. (6) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (7) Wang, X. W.; Liu, G.; Chen, Z. G.; Li, F.; Wang, L. Z.; Lu, G. Q.; Ming, C. H. Chem. Commun. 2009, 3452. (8) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (9) Sato, J.; Saito, N.; Yamada, Y.; Maeda, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K.; Inoue, Y. J. Am. Chem. Soc. 2005, 127, 4150. (10) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286. (11) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (12) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2002, 124, 13547. (13) Lei, Z. B.; You, W. S.; Liu, M. Y.; Zhou, G. H.; Takata, T.; Hara, M.; Domen, K.; Li, C. Chem. Commun. 2003, 2142. (14) Ritterskamp, P.; Kuklya, A.; Wustkamp, M. A.; Kerpen, K.; Weidenthaler, C.; Demuth, M. Angew. Chem., Int. Ed. 2007, 46, 7770. (15) Liu, H.; Yuan, J.; Shangguan, W.; Teraoka, Y. J. Phys. Chem. C 2008, 112, 8521. (16) Bao, N. Z.; Shen, L. M.; Takata, T.; Domen, K. Chem. Mater. 2008, 20, 110. (17) Ma, G. J.; Yan, H. J.; Shi, J. Y.; Zong, X.; Lei, Z. B.; Li, C. J. Catal. 2008, 260, 134. (18) Buhler, N.; Meier, K.; Reber, J. F. J. Phys. Chem. 1984, 88, 3261. (19) Yan, H. J.; Yang, J. H.; Ma, G. J.; Wu, G. P.; Zong, X.; Lei, Z. B.; Shi, J. Y.; Li, C. J. Catal. 2009, 266, 165. (20) Matsumura, M.; Furukawa, S.; Saho, Y.; Tsubomura, H. J. Phys. Chem. 1985, 89, 1327. (21) Reber, J. F.; Rusek, M. J. Phys. Chem. 1986, 90, 824. (22) Trasatti, S. J. Electroanal. Chem. 1972, 39, 163. (23) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735. (24) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83.

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