International Journal of Photoenergy

Volume 2013 (2013), Article ID 247516, 5 pages

http://dx.doi.org/10.1155/2013/247516

Research Article

Green Synthesis of Feather-Shaped MoS₂/CdS Photocatalyst for Effective Hydrogen Production

Yang Liu, Hongtao Yu, Xie Quan, and Shuo Chen

Key Laboratory of Industrial Ecology and Environmental Engineering of Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Received 31 May 2013; Revised 4 September 2013; Accepted 4 September 2013

Academic Editor: Pierre Pichat

Copyright © 2013 Yang Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

MoS₂/CdS photocatalyst was fabricated by a hydrothermal method for H₂ production under visible light. This method used low toxic thiourea as a sulfur source and was carried out at 200°C. Thus, it was better than the traditional methods, which are based on an annealing process at relatively high temperature (above 400°C) using toxic H₂S as reducing agent. Scanning electron microscopy and transmission electron microscopy images showed that the morphologies of MoS₂/CdS samples were feather shaped and MoS₂ layer was on the surface of CdS. The X-ray photoelectron spectroscopy testified that the sample was composed of stoichiometric MoS₂ and CdS. The UV-vis diffuse reflectance spectra displayed that the loading of MoS₂ can enhance the optical absorption of MoS₂/CdS. The photocatalytic activity of MoS₂/CdS was evaluated by producing hydrogen. The hydrogen production rate on MoS₂/CdS reached 192 μmol·h⁻¹. This performance was stable during three repeated photocatalytic processes.

1. Introduction

Solar hydrogen production from water can provide a clean and renewable energy. It has been considered to be the most promising approach for solving energy and environmental issues at a global level. In this context the fabrication of effective photocatalysts is an important area of research. Many semiconductors such as TiO₂ [1], ZnO [2], Nd₂O₅ [3], and CdS [4] have been reported as useful photocatalysts for hydrogen production. Among these photocatalysts, CdS has received the most attentions, due to its superior light absorption and appropriate conduction-band level [5–8]. However, bare CdS photocatalyst usually suffers from photocorrosion [8, 9], which can be improved by loading a cocatalyst such as noble metal (Pt [4, 10], Au [11], and Rh [12, 13]), WC [14], and WS₂ [15] on the surface of CdS. From the resources and environmental point of view, noble metal and tungsten are limited by their rare availability and high price. Therefore, there is an emerging urge for exploring alternative cocatalysts.

Recently, MoS₂ was reported to be a good cocatalyst, and it has been experimentally confirmed that hydrogen production on CdS with MoS₂ loading is even more efficient than that of CdS with noble metal loading [16–19]. However, for the fabrication of MoS₂/CdS photocatalyst, the poisonous H₂S gas has to be employed as sulfur source, and the calcinations temperature is relatively high (above 400°C [17, 19]). These disadvantages limited the development of this promising photocatalyst. Therefore, it is worthy to find a green method at relatively low temperature with nontoxic sulfur source for the preparation of MoS₂/CdS photocatalyst.

Herein, we developed a hydrothermal method for synthesizing MoS₂/CdS photocatalyst. This method was carried out at only 200°C, and its sulfur source was less toxic thiourea. Their photocatalytic performances were evaluated by producing hydrogen under visible light irradiation.

2. Materials and Methods

2.1. Fabrication of MoS₂/CdS Photocatalyst

According to the pioneer work, thiourea has been chosen as sulfur source to synthesize sulfide [16]. CdCl₂·2H₂O and Na₂MoO₄·2H₂O worked as precursors of Cd and Mo, respectively. Briefly, CdCl₂·2H₂O and thiourea with the molar ratio of 1 : 3 were dissolved in 80 mL deionized water; then various amounts of Na₂MoO₄·2H₂O were added into the above solution. The solution was mixed homogeneously in a Teflon-lined stainless steel autoclave (100 mL) followed by sonication for 1 h. Then the Teflon-lined stainless steel autoclave was heated in an air blowing thermostatic oven at 200°C for 24 h. The obtained precipitate was washed with ethanol and water and dried in a vacuum chamber overnight at room temperature.

2.2. Characterization

The morphology of samples was observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G² F30). The X-ray photoelectron spectroscopy (XPS) was performed with a VG ESCALAB250 surface analysis system using a monochromatized Al Kα X-ray source (300 W, 20 mA, and 15 kV). The crystal structures of the samples were investigated by an X-ray diffractometer (XRD, Shimadzu LabX XRD-6000) employing Cu Kα radiation accelerating voltage of 40 kV and current of 30 mA over the 2θ range of 20–80°. The optical absorption property of the samples was measured by a Shimadzu UV-2450 spectrophotometer with the scanning range from 200 to 800 nm.

2.3. Hydrogen Production Experiments

Hydrogen production experiments were carried out in a Pyrex top-irradiation glass reactor connected to a closed gas-circulation system. The photocatalyst powder (50 mg) was introduced into a 100 mL aqueous solution containing 0.5 M Na₂S and 0.5 M Na₂SO₃ as the sacrificial agent. After stirring, the suspension was irradiated from the top of the reactor by a 300 W Xe lamp with a cut-off filter ( nm). The temperature of reactant solution was maintained constantly at 10°C by a flow of cooling water during the reaction. The H₂ gas was quantified by an online gas chromatograph (Shimadzu, GC-14C, TCD, molecular sieve 5 Å).

3. Results and Discussion

The morphology of the samples was observed by SEM and TEM. The SEM image showed that the samples looked like feather cluster, and the length and width of a feather were about 5 μm and 1 μm, respectively (Figure 1(a)). The TEM image displayed that the feather was composed of fusiform structures (Figure 1(b)). To further magnify, the lattice spacings can be distinguished in Figure 1(c). The magnified HRTEM image in Figure 1(c) exhibits the interlayer spacing of 0.32 nm and 0.62 nm, which correspond to the (101) plane of hexagonal CdS and the (002) plane of hexagonal MoS₂, respectively. It indicated that both MoS₂ and CdS have a good crystallization, and MoS₂ layer was less than 3 nm coated on the surface of CdS.

(a)

(b)

(c)

Figure 1: SEM and TEM images of MoS₂/CdS samples.

The chemical composition of samples was investigated by XPS. Figure 2(a) showed the XPS spectrum for Mo . The and peaks located at 231.7 and 225.9 eV indicated the presence of Mo⁴⁺ cations. The S spectrum can be found in Figure 2(b). The split peaks of S were at 162.94 and 161.25 eV corresponding to a doublet composed of and . As Figure 2(c) shows, the doublet peaks at 412 and 405.1 eV were ascribed to Cd and . These binding energies are all consistent with the reported values for the MoS₂ and CdS. Together with the results of TEM and XRD, the above results of XPS confirmed that the sample was composed of MoS₂ and CdS.

(a)

(b)

(c)

Figure 2: XPS spectra of MoS₂/CdS: (a) Mo , (b) S and (c) Cd .

The crystal structures of MoS₂, CdS, and MoS₂/CdS are investigated by an X-ray diffraction (XRD). As shown in Figure 3, for CdS, the main characteristics peaks correspond, respectively, to the reflection (100), (002), (101), (102), (110), (103), (112), (202), (203), and (105) crystal faces of hexagonal wurtzite structure CdS (JCPDS 41-1049). Compared with MoS₂, no XRD peaks belonging to MoS₂ were detected in MoS₂/CdS, indicating the low amount and fine distribution of MoS₂ on the CdS.

Figure 3: XRD of MoS₂, CdS, and MoS₂/CdS.

UV-vis diffuse reflectance spectra of MoS₂, CdS, and MoS₂/CdS were shown in Figure 4. It could be seen that the loading of MoS₂ enhanced the light absorption of the MoS₂/CdS composite, which would result in higher light energy utilization.

Figure 4: DRS of MoS₂, CdS, and MoS₂/CdS.

To observe the effect of MoS₂ ratio to the photocatalytic capability of MoS₂/CdS photocatalysts, MoS₂/CdS samples with various MoS₂ ratios (0 wt%, 5.8 wt%, 6.9 wt%, 10.6 wt%, 16.4 wt%, and 100 wt%) were synthesized, and their respective hydrogen production rates were measured (Figure 5). The hydrogen production rate corresponding to CdS was 11.5  μmol·h⁻¹. The value enhanced obviously, once MoS₂ was coated on the surface of CdS, and reached its maximum on MoS₂ (6.9 wt%)/CdS. Further increase in MoS₂ ratio, the H₂ production rate began to reduce, which could be explained by overloading of MoS₂.

Figure 5: The time courses of photocatalytic H₂ production on MoS₂/CdS with various MoS₂ ratios under visible light irradiation.

To evaluate the stability of MoS₂ (6.9 wt%)/CdS, three repeated photocatalytic processes were performed. After the third cycle, the H₂ production was 168  μmol·h⁻¹, which reduced only by 7% compared to that of the first one (Figure 6). This insignificant reduction suggested the stability of the MoS₂ (6.9 wt%)/CdS photocatalyst. In other words, the photocorrosion, the inherent drawback of CdS, had been inhibited effectively by loading of MoS₂.

Figure 6: Three consecutive cycling experiments using the same MoS₂ (6.9 wt %)/CdS.

Due to the quantum confinement effect, CB potential of nanoscale MoS₂ has been reported to be about −0.2 eV versus NHE [16], which is sufficiently negative to reduce H⁺ to H₂ but more positive than that of CdS (−0.52 eV versus NHE) [20, 21]. Therefore, the photogenerated electrons may transfer from the CB of CdS to the CB of MoS₂ and reduce water to H₂ on the surface of MoS₂. This mechanism agrees with the literature [17, 19] and is shown in Figure 7.

Figure 7: Schematic diagram of H₂ production mechanisms on MoS₂/CdS.

4. Conclusions

A MoS₂/CdS photocatalyst has been successfully synthesized by a green hydrothermal method, which can avoid the disadvantages (such as high energy consumption and toxic sulfur source) of the conventional methods. By controlling the ratio of MoS₂, the H₂ evolution capability of MoS₂/CdS is 17 times greater than that of CdS. It is believed that this green synthesis method can be used to prepare competitive sulfide photocatalysts for efficient solar hydrogen production.

Acknowledgments

The work was supported by the National Basic Research Program of China (2011CB936002). Thanks are due to Shahzad Afzal for the English corrections.

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