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Cu2O Decorated with Co-catalyst MoS2 for Solar Hydrogen Production with Enhanced Efficiency under Visible Light Yufei Zhao, Zhi-yu Yang, Yuxia Zhang, Jing Lin, Xin Guo, Zhengtai Ke, Panwei Hu, Guoxiu Wang, Yiming Yan, and Kening Sun J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 10 Jun 2014 Downloaded from http://pubs.acs.org on June 11, 2014

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The Journal of Physical Chemistry

Cu2O Decorated with Co-catalyst MoS2 for Solar Hydrogen Production with Enhanced Efficiency under Visible Light Yu-Fei Zhao a,b, Zhi-Yu Yang a, Yu-Xia Zhang a, Lin Jing a, Xin Guo a, Zhengtai Ke a, Panwei Hu a

a

, Guoxiu Wang b, Yi-Ming Yan a,*, and Ke-Ning Sun a,*

Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemical

Engineering and Environment, Beijing Institute of Technology, Beijing, 100081, China. Fax: +86-10-68918696; Tel: +86-10-68918696; E-mail: [email protected]; [email protected] b

Center for Clean Energy Technology, School of Chemistry and Forensic Science, Faculty of

Science, University of Technology, Sydney, Sydney, NSW 2007, Australia

ACS Paragon Plus Environment

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ABSTRACT: In this work, we have prepared a p-type semiconductor of Cu2O decorated with MoS2 nanosheets as co-catalyst for efficient solar hydrogen production under visible light. Results show that Cu2O decorated with 1.0 wt% MoS2 presents the maximum reduction photocurrent density of 0.17 mA cm-2, which is 7-fold higher than pristine Cu2O. Furthermore, the as-prepared MoS2@Cu2O exhibits remarkable photostability with only 7 % loss of its original photocurrent after 9 hours continuous work. The excellent performance of MoS2@Cu2O is ascribed to the introduction of active sites of MoS2 nanosheets as co-catalyst to the surface of Cu2O nanoparticles, which activates photocatalyst by lowering the electrochemical proton reduction overpotential and also inhibits photo-induced corrosion during the measurement.

KEYWORDS: hydrogen production, p-type semiconductor, cuprous oxide, molybdenum disulfide, photoelectrocatalytic


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1. INTRODUCTION Due to increasing global demand for energy and environmental concerns, much effort has been devoted to exploiting renewable, carbon-free energy sources. Among these, solar energy is the largest source available. One way of utilizing solar energy is to convert it into electricity by employing photovoltaic cells.1-3 The other route is to convert it into chemical energy stored within chemicals, such as hydrogen, methanol and methane, ideally by utilizing photoelectrochemical cells.4-6 Recently, numerous attempts have focused on solar hydrogen production by photo-induced water reduction with available semiconductor photocatalysts. Although solar hydrogen production achieved a breakthrough in 1972,7 the key lies in the use of suitable material to efficiently exploit solar spectrum of visible light, which makes solar hydrogen production applications more feasible. Generally, in terms of solar hydrogen production, p-type semiconductors are advantageous over n-types.8,9 However, of the current known natural crystal structures of semiconductors, p-types are much less known than the ntypes (e.g., TiO2, ZnO, Fe2O3, WO3).10-15 Particularly, as a typical p-type semiconductor, cuprous oxide (Cu2O) is thought to be the most promising candidate for efficient photocatalytic solar hydrogen production from water.16-19 Cu2O has a small band gap of ∼2.0 eV and suitable

conduction band, which gives it efficient visible light absorption and available hydrogen

evolution potential.20-23 Furthermore, copper is naturally abundant, which makes for possible large-scale fabrication of Cu2O photoelectrodes, offering potential competitiveness over other semiconductors. Unfortunately, the practical application of Cu2O in solar hydrogen production is still severely hindered by its low photocatalytic efficiency caused by fast electron-hole recombination and poor stability owing to photo-corrosion. To solve these problems, several attempts have been conducted in the past. For example, Cu2O could be combined with certain n-


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type semiconductors of the more positive conduction bands such as TiO2, ZnO, rGO, etc., forming the n-p junction to allow the efficient transport of photogenerated electrons from Cu2O to the n-type semiconductor conduction band that results in the improved photostability of Cu2O.24-27 Another practical approach is to coat Cu2O with a thin protective carbon layer, which could combat the photo-corrosion problem of Cu2O, as well as improve the water splitting performance.28 In addition, CuO and NiOx have been combined with Cu2O to improve its performance both in photocatalytic activity for hydrogen production and stability during the long term operation.29,30 Despite the progress, a facile, scalable, cheap and environmentally friendly strategy is desired for synthesizing Cu2O-based composite with enhanced performance as photoelectrode for efficient solar hydrogen generation. In past years, the co-catalyst’s loading on the transition metal oxide semiconductor photocatalysts have received great attention because co-catalysts could provide reaction active sites and decrease the activation energy for gas evolution. Recently, molybdenum disulfide (MoS2), possessing a layered structure similar to graphene, has been extensively investigated as a promising electrocatalyst for H2 evolution.31-34 It is believed that MoS2 is a potential substitute for noble metals (such as Pt), which can work as an efficient co-catalyst to enhance the H2 production efficiency in solar hydrogen generation.35,36 Therefore, we expect that MoS2 could not only promote the separation of photo-excited electrons and holes, but also offer more active sites for H2 generation.37 As far as we know, there are only a few studies reporting the use of MoS2 in solar H2 production. For instance, Yu et.al. reported a composite material consisting of TiO2 grown in the presence of a layered MoS2/graphene hybrid as photocatalyst for H2 evolution with high activity and Zong et al. reported the enhancement of photocatalytic H2 production activity


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of CdS by loading MoS2 as co-catalyst,35,38 which have confirmed that MoS2 could act as a cocatalyst for improving the photocatalytic activities of the semiconductors. In this work, we develop a simple method for the synthesis of Cu2O nanoparticles decorated with layered MoS2 used for solar H2 production by photocatalytic water reduction. The results demonstrate that the activity of the Cu2O is significantly enhanced by the presence of the layered MoS2 co-catalyst. Moreover, the resulting MoS2@Cu2O composite exhibits more remarkable photostability than the unmodified Cu2O. We demonstrate that chemical deposition synthesis of MoS2 decorated Cu2O may offer a feasible route in the preparation of an effective semiconductor used as photoelectrode for solar H2 production. 2. EXPERIMENTAL SECTION Preparation of MoS2. The MoS2 nanoparticle was synthesized by a one-step reaction referring to Yu’s method.35 In brief, 0.242 g of Na2MoO4·2H2O (1.0 mmol) and 0.381g of thiourea (5.0 mmol) were dissolved in 60 mL distilled water. The homogeneous solution was transferred to a 100 mL Teflon-lined autoclave and held at 210 oC for 24 h. After that, the black precipitate was collected by centrifugation, washed three times with distilled water and ethanol, and then dried in an oven at 80 ℃ for 12 h.

Preparation of MoS2@Cu2O Composite. The MoS2@Cu2O composite were prepared by a typical procedure.39 1.250 g of CuSO4·5H2O (5 mmol) was dissolved in 0.729 g of CTAB (2.0 mmol) aqueous solution (500 mL), and followed by the addition of 3.6 mg of MoS2 (0.023 mmol). Then 1.4 mL N2H4·H2O solution was added into the above solution with continuous stirring. After 10 mins, the precipitate was centrifuged, washed three times with deionized water


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and ethanol, and then dried in a vacuum oven at 60 ℃ for 8 h. Other samples with a different mass (0 wt%, 0.1 wt%, 3 wt%) percentage from MoS2 were prepared with the same procedure. Material Characterization. The surface morphology and composition of the prepared Cu2O and MoS2@Cu2O were examined by X-ray diffraction (XRD, X’ Pert PRO MPD), scanning electron microscopy (SEM, FEI QUANTA-250) and X-ray photoelectron spectroscopy (XPS, PHIQUANTERA-2). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken with a Tecnai-G2-F30 field emission transmission electron microscope operating at an accelerating voltage of 300 kV. UV-vis absorption spectra were acquired by a spectrophotometer (UV-2450, Shimadzu, Japan). Electrochemical

and

Photoelectrochemical

Measurements.

Electrochemical

and

photoelectrochemical measurements were performed on an electrochemical analyzer (CHI660C Instruments) in a standard three-electrode system using the prepared samples as the working electrodes, a Platinum wire as the counter electrode, and SCE as a reference electrode. Potentials reported in this study were all quoted against normal hydrogen electrode (NHE) by ENHE = ESCE + 0.23 + 0.059 × pH. A Xe lamp (71PX5002 , Saifan) was used as the light source, connecting with monochromator to yield monochromatic irradiation at the electrode. The supporting electrolyte was 25% (v/v) methanol (scavenger) mixed with Na2SO4 (0.1 mol L-1, pH=7) aqueous solution. The working electrodes were prepared as follows: the catalyst ink was first prepared by dispersing material (4.0 mg) in H2O (0.5 mL) by ultrasonication, and then catalyst ink dispersions (200 μL) was drop directly onto a cleaning indium tin oxide (ITO) glass surface by pipette and placed in a vacuum oven to speed dry. The scan rate of the Transient photocurrent−time and Linear-sweep voltammograms (LSV) is 2 mV s-1. The photocurrent


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densities produced at bias -0.1 V vs. SCE under light irradiation of 480 nm (light on/off cycles: 10s) and the stability of the materials was conducted at the same condition (light on). The impedance spectroscopy of the materials was recorded under the following condition: amplitude 50 mV, a potential window from 0.6V to -0.5V (vs. Ag/AgCl), and without illumination. Photocatalytic Measurements. The photocatalytic H2 evolution experiments were performed in a sealed quartz flask (250 mL) using the recirculation cooling water to control the temperature. A 350 W Xe arc lamp was used as light source with an optical filter (≥ 420 nm) to remove the light in the ultraviolet region (10 cm away from the photocatalytic reactor) (Fig. S1, ESI). Typically, the prepared material (80 mg) was suspended in 25% (v/v) methanol/water (150 mL) by ultrasonication for 15 min, and then bubbled with nitrogen through the reactor for 30 min to remove the oxygen before irradiation. A continuous magnetic stirrer was applied at the bottom of the reactor in order to keep the photocatalyst particles in suspension status during the whole experiment. Hydrogen was analyzed by gas chromatograph (GC-2014A series, Shimadzu, Japan, TCD, nitrogen as a carrier gas and 5Å molecular sieve column). 3. RESULTS AND DISCUSSION The layered MoS2 decorated p-type semiconductor Cu2O for efficient solar hydrogen production was synthesized by two steps. At first, the layered MoS2 (Figure. S2, EIS) was obtained through hydrothermal reaction of Na2MoO4·2H2O and thiourea at 210 oC for 24 h. Subsequent a facile and simple chemical deposition synthesis method was applied for the MoS2@Cu2O using the Hexadecyl trimethyl ammonium Bromide (CTAB) as the surfactant to control the morphology.


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Figure 1. Characterization of MoS2@Cu2O composites: (a) SEM images of the MoS2@Cu2O composites, (b, c) TEM and HRTEM images for MoS2@Cu2O composites. (d) X-ray diffraction (XRD) of pristine Cu2O and MoS2@Cu2O composites, respectively.

The morphology and chemical composition of the as-prepared 1.0 wt% MoS2@Cu2O composite were first investigated by SEM, TEM, XRD. Fig. 1a shows the SEM image of the MoS2@Cu2O sample, which reveals well-defined nanospheres with the diameter of 200-400 nm. As comparison, the SEM image of pristine Cu2O (Fig. S3, EIS) shows similar morphology and is almost the same size as MoS2@Cu2O. The results suggest that introducing 1.0 wt% layered MoS2 to the Cu2O does not change the evolution procedure of Cu2O nanoparticles or affect the morphology and structure of the resulting material. TEM image of the MoS2@Cu2O composite is shown in Fig. 1b. It shows that the surface of the Cu2O nanospheres was clearly covered with a layer-structured material, which is reasonably supposed to be MoS2 nanosheets. The highresolution TEM (HRTEM) image (Figure 1c) verified that the MoS2 sheets were successfully


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bound to the Cu2O surface, where the dark lattice fringes spaced by 0.62 nm is in good agreement with the (002) van der Waals planes of MoS2 (6.1 Å ). The presence of MoS2 in the obtained MoS2@Cu2O was further confirmed by XRD. As shown in Fig. 1d, the peaks at 36.5°, 42.4° and 61.5° observed with MoS2@Cu2O were clearly assigned to the characteristic peaks of Cu2O, which is exactly the same for that of the pristine Cu2O. A very slight peak at 14.1° was found, corresponding to the typically characteristic peak of MoS2 (002). However, the peak is so weak that we cannot directly confirmed MoS2 is introduced into the composite by XRD. The weak XRD signal of the MoS2 observed here is mainly due to its low content in the MoS2@Cu2O materials.

Figure 2. (a, b, c) XPS spectrum of Cu (2p), Mo (3d) and S (2p) in the MoS2@Cu2O composites. (d) UV-vis absorption spectra of the MoS2@Cu2O composites compared with those of pristine Cu2O.


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In order to further investigate the composition, MoS2@Cu2O composite were characterized by X-ray photoelectron spectroscopy (XPS, Figure 2a, b, c), which could verify the valence states of the elements in the materials. It can be seen that the typical XPS peaks of the Cu (2p) at 952.9 and 932.7 eV for MoS2@Cu2O were observed, demonstrating the existence of a cuprous state. Again, the typical peaks for Mo (3d) at 231.9 and 228.8 eV, and S (2p) at 162.0 and 164.1 eV clearly demonstrate the successful decoration of MoS2 at the surface of Cu2O. The optical properties of the MoS2@Cu2O and pristine Cu2O were measured using UV-vis absorption spectra, as shown in Fig. 2d. Compared with the Cu2O, the as-prepared MoS2@Cu2O showed an enhanced absorption in the long wavelength region ranging from 500 to 700 nm (red solid line in Fig. 2a), which might be due to the strong absorption of the MoS2 layer in this region. We noted that the peak of the adsorption spectra MoS2@Cu2O sample shifted to long wavelength for ca. 15 nm, compared with that of the pristine Cu2O. To estimate the bandgap energy of semiconductor samples, the UV-vis absorption spectra of the samples were transformed using the Kubelka-Munk function and plotted against the energy of light, which is shown as the inset in Fig. 2a.40,41 We found that there was no significant change in bandgap value of the pristine Cu2O nanoparticles and MoS2@Cu2O nanocomposite. From the intercept of the plot, we can calculate that the bandgap value of both Cu2O and MoS2@Cu2O is 1.97 eV, which agrees with the theoretical bandgap value of Cu2O (~2.0 eV). It suggests that the decorated MoS2 was not incorporated into the Cu2O lattice; therefore, it has no significant effect on the bandgap of the ptype semiconductor.


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Figure 3. Photoelectrocatalytic and photocatalytic activities characterization of MoS2@Cu2O composites and pristine Cu2O nanoparticles (a) Transient photocurrent−time profiles at bias -0.1 V vs. SCE for MoS2@Cu2O composites and pristine Cu2O nanoparticles coated on ITO glass under chopped light irradiation of 480 nm. The scan rate is 2 mV s-1. (b) H2 evolution from the pristine Cu2O and 1.0 wt%MoS2@Cu2O composites under visible light irradiation.

We have then investigated the photoelectrochemical properties of MoS2@Cu2O composite using it as photocathode for photo-induced water reduction to generate H2. In order to get adjustable measurements and make more accurate evaluation on the photoelectrochemical


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performance of the electrode, we utilized the monochromator to yield monochromatic irradiation on the electrode. As shown in Fig. 3a, the transient photocurrent responses of Cu2O or MoS2@Cu2O nanohybrid materials modified ITO electrodes were recorded for several on-off cycles under visible light irradiation (480 nm) at bias -0.1 V vs. SCE (suppress photogenerated holes and drive photogenerated electrons to reduce protons to H2). Four samples of MoS2@Cu2O were prepared with different mass loading of MoS2 (0.0 wt%, 0.1 wt%, 1.0 wt%, and 3.0 wt%) and then examined for their photoelectrochemical performance. Apparently, all MoS2@Cu2O samples exhibited noticeable improvement of the photocurrent density. We found that 1.0 wt% MoS2@Cu2O showed the best performance, which achieved a 7-fold increase in photocurrent density when compared with the pristine Cu2O material (The following tests of MoS2@Cu2O all adopt 1.0 wt% percentage). However, a decrease of photocurrent density was also obeserved with 3.0 wt% MoS2@Cu2O compared with that of 1.0 wt% MoS2@Cu2O. Thus, it seems that there exists an optimal mass loading of MoS2 which would give maximum photoelectrochemical performance for such a MoS2 decorating Cu2O photocatalyst. This might be explained by the fact that much thicker layered MoS2 at the surface can lead to lower optical absorption for the MoS2@Cu2O hybrid, resulting in the poor utilization of solar energy and it can also cause light absorption competition between the MoS2 and Cu2O.36 What’s more, the light absorbed by the MoS2 can’t be utilized for hydrogen evolution because of the conduction band bottom-edge.35 For a contrast experiment, mechanically mixed pristine Cu2O nanoparticles and MoS2 (with a mass ratio 100:1) was prepared. The mixture yielded a photocurrent density of ca. 0.05 mA cm-2, which is higher than that of the pristine Cu2O, but significantly lower than that achieved by the MoS2@Cu2O composite with the same percentage (Fig. S4, EIS). It indicates that physical mixing of Cu2O nanospheres and MoS2 does not give efficient charge transport and high


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photoelectrochemical activity. This is probably due to the poor interaction between the Cu2O nanospheres and MoS2, which should be overcome by chemical deposition that results in uniform distribution of MoS2 on the surface of Cu2O nanoparticles. Thus, this photocurrent density enhancement observed with MoS2@Cu2O strongly indicated that MoS2 could act as favourable co-catalyst to effectively accelerate the photoelectrochemical courses of p-type semiconductor Cu2O. Photocatalytic H2 evolution over the MoS2@Cu2O composite was conducted in 150 mL of aqueous methanol solution (25 vol%) as sacrificial reagents under visible light irradiation (≥ 420 nm). The gas was collected with a gas tight syringe and measured with gas chromatography. As depicted in Fig. 3b, there was a linear evolution of H2 up to 5 h for both MoS2@Cu2O and Cu2O photocatalysts. There was much more efficient H2 production that achieved with MoS2@Cu2O compared to Cu2O, which can be also ascribed to the active sites of MoS2 for hydrogen evolution and also the good interaction of the materials. The photocatalytic H2 evolution results are consistent with those of the photocurrent measurements and further confirmed the existence of the MoS2 in the hybrid could result in high activities.


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Figure 4. (a, b) Motte - Schottky plots for pristine Cu2O nanoparticles and MoS2@Cu2O composites coated onto ITO glass in 0.1 M Na2SO4 electrolyte (pH=7). (c) Schematic energy level diagrams of MoS2@Cu2O composites and the pristine Cu2O compared with the potentials for water reduction in 0.1 M Na2SO4 electrolyte (pH=7).


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To better understand and explain the enhanced performance observed with the MoS2@Cu2O composites, the Mott−Schottky equation was further applied for studying the properties of the pristine Cu2O and MoS2@Cu2O composite. Fig. 4a, b showed the Mott-Schottky plots for the pristine Cu2O and MoS2@Cu2O in 0.1 M Na2SO4 electrolyte. The negative slope in the linear region (Figure 4a, b) verified the p-type conductivity of the both the pristine Cu2O and MoS2@Cu2O. Flatband potentials were obtained by the calculation of the intersection of the line fit to the 1/C2 measurements with the x-axis, which is 0.93 V for pristine Cu2O and 1.19 V for [email protected],43 The flatband potentials could be equated to the valence band (VB) edges (≤ 0.1 V) for the p-type conductors.44-46 As a consequence, one can calculate the conduction band (CB) as ECB by using the optical bandgaps, and thus depict the energy scheme of the catalysts, which is shown in Fig. 4c (the potential of H+/H2 is calculated by ENHE = ESCE + 0.23 + 0.059 × pH ). It can be seen that the ECB of MoS2@Cu2O is by 0.26 V more oxidizing than the ECB of pristine Cu2O. However, such a shift could not be utilized to explain the enhanced photocurrent observed with MoS2@Cu2O, as the proton is more difficult to reduce at more positive Fermi levels.


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Figure 5. LSVs for MoS2@Cu2O composites and pristine Cu2O without the light irradiation in a faradic box with a scan rate of 2 mV s-1.

Therefore, in order to obtain more insight into the mechanism, linear scan voltammograms (LSVs) were recorded for Cu2O and MoS2@Cu2O in 0.1 M Na2SO4 solution without the light irradiation (Fig. 5). The plots reveal cathodic currents for the reduction of protons at the modified electrodes. Remarkably, we found that the potential for reducing protons at the Cu2O electrode is -1.21 V (vs. SCE), while the reduction potential raised to -0.72 V (vs. SCE) for the MoS2@Cu2O electrode. The observed reduction potential with MoS2@Cu2O is consistent with that reported by Bonde et. al at the molybdenum disulfide nanoparticles modified electrode.47 The significant decrease of the overpotential of H+ reduction for MoS2@Cu2O can be attributed to the active site provided by the MoS2 nanosheets on the surface and also the good interaction between the two materials, therefore improving the electron transfer rate, facilitating holes/electrons separation at the surface, and thus enhancing kinetics of water reduction. As a


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result, we can deduce that the increased activity of the MoS2@Cu2O photocatalyst can be mostly attributed to the lowering of the H+ reduction overpotential caused by the presence of co-catalyst MoS2 nanosheets at the surface of Cu2O. Typically, overpotentials are usually needed to be considered for realizing the reaction in photoelectrocatalytic water splitting. The presence of electrocatalysts on photocathode as co-catalysts can enhance the photocurrent density and also result in the positive shift of onset potential in the electrocatalytic test.

Figure 6. Time dependent photocurrent density of bare Cu2O and MoS2@Cu2O composites on ITO glass under chopped light irradiation of 480 nm, at bias -0.1 V vs. SCE.

The photostability of a photoelectrode is also a crucial factor in determining the performance of hydrogen production. The photostability of the MoS2@Cu2O and pristine Cu2O photocathodes was evaluated by continuously monitoring the photocurrent under illumination (480 nm) over an irradiation course of 9 hours and the results are presented in Fig. 6. It was found that


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MoS2@Cu2O exhibited promising stability with almost 93% of the initial photocurrent remaining after 9 hours illumination. By contrast, the Cu2O modified electrode had only kept 51% of its initial photocurrent after the test. We noted that the observed photostability is also prominent in comparison with the previously reported results of NiOx@Cu2O or C-x-/Cu2O material.27,28 The inactivation of Cu2O is mainly caused by the self-photo-corrosion occurred at the surface during the photoelectrochemical test, as the potentials for the oxidation and reduction of Cu2O lie exactly within the band gap of Cu2O. The introduction of MoS2 as co-catalyst can efficiently suppress such a self-photo-corrosion, which is due to the surface phase junction between the Cu2O and MoS2 could lead to an efficient interfacial electron transfer from the surface of Cu2O to MoS2. What’s more, the XPS of the pristine Cu2O and MoS2@Cu2O after the photostability test are presented in Fig. S5, which imply that the photocorrosion occurred during the photostability process for the pristine Cu2O and MoS2@Cu2O composite. However, there are much less metallic copper and cupric states found in the MoS2@Cu2O composite than those of the pristine Cu2O, which suggests that the MoS2@Cu2O catalyst owns much better photostability compared with the pristine Cu2O. Again, it confirms that introducing MoS2 into the Cu2O can highly improve the photocatalytic stability. 4. CONCLUSION In conclusion, we have synthesized MoS2 decorated p-type semiconductor Cu2O for efficient solar hydrogen production by a facile and simple chemical deposition synthesis method. MoS2 nanosheets were utilized as co-catalyst of Cu2O particles to enhance the photoeletrocatalytic and photocatalytic performance. The edges of the nanosized MoS2 crystallites can accept electrons and act as active sites for H+ reduction, therefore promoting the dissociation of water, photocurrent density and the production of H2. Electrochemical measurements reveal that MoS2


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activates the photocatalyst with high performance by lowering the electrochemical proton reduction overpotential. Meanwhile, the presence of MoS2 as co-catalyst at the surface of Cu2O also greatly prohibits the photo-corrosion of the resulting MoS2@Cu2O catalyst. Our study shows that the development of noble-metal-free Cu2O composites such as the present ones containing an inexpensive and environmentally benign MoS2 like co-catalyst, is feasible and has great potential for solar H2 production. ASSOCIATED CONTENT Supporting Information The home-made measurement system for the photocatalytic water splitting. SEM image and XRD pattern of layered MoS2, The SEM of pure Cu2O, the transient photocurrent−time profile of the MoS2 mixed with Cu2O composite. The XPS of the pristine Cu2O and MoS2@Cu2O composites after the photostability test. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; Fax: +86-10-68918696; Tel: +86-10-68918696; Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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Financial support from the National Natural Science Foundation of China (Grant nos. 21175012, 21006015 and 21070623), Ministry of Science and Technology (2012DFR40240) and the Chinese Ministry of Education (Project of New Century Excellent Talents in University) is gratefully acknowledged. The authors would like to thank Dawei Su from University of Technology, Sydney for the TEM test. REFERENCES (1) Khan, M. A. M.; Kumar, S.; Alhoshan, M.; Al Dwayyan, A. S. Spray pyrolysed Cu2ZnSnS4 absorbing layer: A potential candidate for photovoltaic applications. Opt. Laser Technol., 2013, 49, 196-201. (2) Seminovski, Y.; Palacios, P.; Wahnon, P. Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe. Sol. Energ. Mater. Sol. C., 2013, 114, 99-103. (3) Wang, Y.; Chen, E.; Lai, H.; Lu, B.; Hu, Z.; Qin, X.; Shi, W.; Du, G. Enhanced light scattering and photovoltaic performance for dye-sensitized solar cells by embedding submicron SiO2/TiO2 core/shell particles in photoanode. Ceram. Int., 2013, 39, 5407-5413. (4) Joo, J. B.; Zhang, Q.; Dahl, M.; Lee, I.; Goebl, J.; Zaera, F.; Yin, Y. Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity. Energy Environ. Sci., 2012, 5, 6321-6327. (5) Zhao, Y. F.; Zhang, Y. X.; Yang, Z. Y.; Yan, Y. M.; Sun, K. N. Synthesis of MoS2 and MoO2 for their applications in H2 generation and lithium ion batteries: a review. Sci. Technol. Adv. Mater. 2013, 14 (6)Du, P. W.; Eisenberg, R. Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges. Energy Environ. Sci., 2012, 5, 6012-6021.


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