Stabilizing and Improving Solar H2 Generation from Zn0.5Cd0.5S

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Stabilizing and improving solar H2 generation from Zn0.5Cd0.5S nanorods@MoS2/RGO hybrids via dual charge transfer pathway Shuai Nan Guo, YuLin Min, JinChen Fan, and QunJie Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06009 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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ACS Applied Materials & Interfaces

Stabilizing and Improving Solar H2 Generation from Zn0.5Cd0.5S nanorods@MoS2/RGO Hybrids via Dual Charge Transfer Pathway ShuaiNan Guo, YuLin Min*, JinChen Fan, QunJie Xu† Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, China

* Corresponding author. Tel./ fax: +86 021 35303242, † Corresponding author. Tel./fax: +86 021 35304734,

E-mail address: [email protected](Y L Min) E-mail address: [email protected](Q J Xu)

ACS Paragon Plus Environment


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Abstract The incorporated Zn0.5Cd0.5S (ZCS) nanorods with MoS2/RGO co-catalysts by a simultaneous reduction reaction was reported. The preparation of RGO and formation of MoS2 with intimate interfacial contact with ZCS were achieved. Through the optimizing of each component proportion, the ZCS@MoS2/RGO hybrid with 1.5 wt.% MoS2 and 3 wt.% RGO showed the highest photocatalytic H2 production activity (2.31 mmol/h) with long time stability (50 h). The relative mechanism has been investigated. It is believed that the stabilizing and improving solar H2 generation is originating from dual charge transfer pathway from excited ZCS to RGO, then to MoS2 due to intimate interfacial structure. KEYWORDS: Zn0.5Cd0.5S nanorods, MoS2/RGO, Photo-reduction, Hydrogen evolution, Charge transfer.


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Introduction Solar H2 generation using semiconductor photocatalysts from aqueous electrolyte is a fascinating, green and sustainable approach to solve current energy crises.1-3 Given the fact that there is ∼43% visible light proportion in the full solar spectrum, which is much more than ∼4% UV light proportion, much attraction for solar-driven photocatalysts have been received in recent several decades.4 As a promising candidate, cadmium sulfide (CdS) photocatalyst has been extensively studied because of its suitable bandgap and band position for H2 evolution, but, the rapid charge recombination and serious photo-corrosion during photocatalysis significantly hinder its effectiveness and durability.5-7 Over the past decade, CdS photocatalyst has been focused on a series of meticulous modification, mainly including bandgap tuning and charge separation. Metal ion implantation 11-13

8-10

and surface modification

have been considered as two efficient routes for driving CdS photocatalysis to a large

extent. In comparison to CdS, although zinc sulfide (ZnS) with a band gap of 3.66 eV fails to utilize visible light, it exhibits better ability in redox and durability. As a results, many efforts have been attempted to combine it with other narrow semiconductors, such as Ag2S, Cu2S et al. to improve their photocatalytic activity.14-16 In particular, Zn doped CdS photocatalysts (Zn1-xCdxS) have been witnessing an increased amount of attention in past years, motivated by well-matched coordination mode between CdS and ZnS, and tunable band gap width and band edge position.17-19 It was found out that the optimal photocatalytic activity was achieved over the Zn0.5Cd0.5S component due to the balanced CB potentials and band gap.20 Despite tremendous efforts, stabilizing and improving solar H2 generation over Zn0.5Cd0.5S photocatalyst is becoming an urgent task. While conventional use of Pt co-catalyst was restricted by cost and abundant source, it is going to exploit highly efficient co-catalysts for facilitating its practicability in low cost.21,22 Composite MoS2/RGO co-catalyst revealed an outstanding performance in charge separation and H2 evolution for various photocatalysts,23-26 which is able to be a suitable candidate instead of Pt. It has a few reports on combined CdS with MoS2/reduced graphene oxide (MoS2/RGO) for H2 generation,27-32 by which it is verified that the configurations among MoS2, RGO and CdS is crucial point to promoting photocatalytic H2 evolution.31, 32 Because the RGO has provided a facile pathway for electron transfer, and MoS2 has a superior hydrogen evolution activity for H2 generation, an intimate


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connection among MoS2, RGO and CdS with numerous junction interfaces would be idea configuration for highly efficient H2 generation. However, in most cases, the CdS/MoS2/RGO was obtained by growth of CdS on MoS2/RGO sheets, resulting in that the formed MoS2 layers obstruct the direct contact between CdS and RGO. In this study, we have reported a different strategy from previous studies to incorporate Zn0.5Cd0.5S nanorods with MoS2/RGO (ZCS@MoS2/RGO) for H2 generation from lactic acid solution. Firstly, the ZCS nanorods were synthesized via solvothermal method. Secondly, the ZCS nanorods were incorporated with MoS2/RGO via dual reduction reaction in presence of GO and (NH4)2MoS4 under UV light irradiation. The total reactions could be expressed as: ZCS + hv → e- + h+

(1)

GO + e- + h+ → RGO

(2)

[MoS4]2- + 2e- → MoS2 + 2S2-

(3)

The resulted ZCS@MoS2/RGO has numerous junction interfaces among ZCS, MoS2 and RGO, which exhibited high H2 production rate (2.31 mmol/h) and stability (up to 50h).

Experimental section Materials. Graphite powder (purity 99.9995%) was obtained from Alpha Aesar. Other chemical reagents were purchased from Aldrich without further purification.

Synthesis of ZCS@MoS2/RGO composite. Here, graphene oxide (GO) was prepared from graphite powder using Hummers method. For the solvothermal synthesis of ZCS nanorods, 50 mL pyridine was dissolved with each 4 mmol CdCl2 and ZnCl2, a 40 mmol thioacetamide as sulphur source were then added with further stirring of 30 min. The mixture was transferred into a Teflon-lined stainless autoclave with a capacity of 80 mL and heated at 180 oC for 20 h. The yellow ZCS powder was rinsed with de-ionized water and dried at 60 oC for 12 h. 0.4 g ZCS was dispersed into 100 mL of 15 vol % triethanolamine (TEOA) solution and different amounts of 1.0 mg/mL−1 (NH4)2MoS4 and GO were added. The mixture was purged with N2 to completely remove oxygen and then irradiated under a UV light for 30 min. The ZCS@MoS2/RGO composites were filtrated, washed with de-ionized water several times, and dried at 60 °C for 12 h. The ZCS@MoS2



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was obtained using same procedure without addition of GO, and the ZCS@RGO was obtained using same procedure without addition of (NH4)2MoS4. ZCS@Pt/RGO was obtained using same procedure only (NH4)2MoS4 instead of H2PtCl6.

Characteristics of ZCS@MoS2/RGO composite. Transmission electron microscopy (TEM) images were obtained using a Cs corrected JEM-2200FS microscope. XRD diffraction measurements were conducted on Rigaku, Japan, RINT 2500 V using Cu Kα radiation. FT-IR spectra were collected using KBr pellets with Bruker FTIR. UV-visible reflection spectra were performed using a spectrophotometer (Shimadzu UV-2401PC), and BaSO4 as the reference was used. Room photoluminescence was measured using a fluorescence spectrometer (Shimadzu, RF-5410PC) in water solution. Scanning electron microscopy (SEM) images were performed using a field emission scanning electron microscope (FESEM, JEOL, FEG-XL 30S). XPS analysis was conducted using an ESCALAB-220I-XL

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photocatalyst/ethanol slurry, and coated on ITO with area of 1.05 × 1.05 cm-2. Then Measurements of the photocurrent were performed in three-electrode system with Pt foil as counter and saturation calomel electrode as reference electrode using an electrochemical workstation (CHI660C). For hydrogen evolution reactions, the electrodes were prepared on a glassy carbon electrode of 3 mm diameter. Briefly, a homogeneous ink was prepared by dispersing 4 mg sample into 1 mL water/ethanol at volume ratio of 3:1 containing 30 µ L Nafion solution (5 wt%) as binder, and sonicated for 1 h. Then 5 µL of the ink (~20 µ g sample) was dropped onto a glassy carbon electrode which was naturally dried. All of electrochemical testing was conducted in N2-saturated 0.5 M H2SO4 using three-electrode system where Ag/AgCl (in 3 M KCl solution) was used as the reference electrode, a graphite rod was used as the counter. Hydrogen electrode (RHE) was applied to unify all potentials in electrochemical characterizations. Photocatalytic test.

Photocatalytic hydrogen evolution was performed in a Pyrex quartz reactor and light source is located at side face. In detail, 30mg of catalyst powders was dispersed in 200 mL electrolyte containing 40 mL lactic acid of sacrificial agent. 1 mL gas was collected with constant time


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interval using a glass injection syringe, and the total amounts of H2 were analyzed using a gas chromatograph.

Results and discussion Figure 1 a and b exhibit the TEM images of typical ZCS@MoS2/RGO (3 wt.% RGO, 1.5 wt.% MoS2), where the RGO sheets are uniformly decorated with the ZCS nanorods. The SEM image of ZCS shows that the average diameter was smaller than 100 nm with length up to several micrometers (Figure S1a). TEM images and corresponding selected area electron diffraction (SAED) pattern of the ZCS nanorods confirm single crystal characteristics with growth direction along [001] (Figure S1b and c). In addition, it is clear that the MoS2 sheets have an obvious tendency to grow at the interface between RGO and ZCS, which may be correlated with the dominant electron transfer route from ZCS to RGO. As shown in Figure 1b, the TEM observation indicates a MoS2 location at interfacial contact formed between the ZCS and RGO. The element mapping and EDX spectra demonstrate the homogeneous distribution of Zn, Cd, Mo, S and C elements in the entire range (Figure S2). Therefore, such a good interfacial contact for the ZCS@MoS2/RGO could be expected to promote the charge transport as well as succedent H2 generation. All diffraction peaks in the XRD patterns of the ZCS nanorods and typical ZCS@MoS2/RGO (Figure 1c) are assigned to hexagonal wurtzite phase (JCPDS number 49-1032). Notes that the XRD pattern of ZCS nanorods is different from that of either ZnS (JCPDS number 65-0309) or CdS (JCPDS number 41-1049). Due to high dispersibility and low content of MoS2/RGO, no obvious peaks could be assigned to them in the typical ZCS@MoS2/RGO. However, the diffraction peak of MoS2 at around 14o of two-theta degree in the ZCS@MoS2/RGO with ~2.5 wt.% MoS2 and 5 wt.% RGO could be clearly observed (Figure S3). The influence of MoS2/RGO on the BET surface area of the typical ZCS@MoS2/RGO was surveyed using N2 adsorption-desorption isotherms. As shown in Figure 1d, the special BET surface area of ZCS nanorods is 32.8 m2 g-1, which is significantly increased to 77.6 m2 g-1 due to introduction of the two dimensional MoS2/RGO. It is well-known that a high surface area could increase surface active sites and charge transfer of photocatalyst, resulting in the photocatalytic activity improvement. Their pore size



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distribution is shown in Figure S4, it can be observed that the ZCS only has nanopores, while the ZCS@MoS2/RGO has broader size distribution to mesopores and macropores.

Figure 1. (a) TEM and (b) HR-TEM image of the typical ZCS@MoS2/RGO. (c) XRD patterns of the ZCS nanorods and typical ZCS@MoS2/RGO. (d) Nitrogen adsorption– desorption isotherms the ZCS nanorods and typical ZCS@MoS2/RGO.


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Figure 2. (a) Full XPS spectra of the typical ZCS and ZCS@MoS2/RGO. (b) Zn 2p XPS spectra. (c) Cd 3d XPS spectra. (d) Mo 3d XPS spectra. (e) C1s XPS spectrum of GO. (f) C1s XPS spectrum of the typical ZCS@MoS2/RGO. In most of the proposed mechanisms of photo-assisted synthesis of semiconductor/RGO composites, the interaction between surface hydroxyl or amine functional groups of photocatalyst and carboxyl functional groups acts as main force to couple each other.33-36 The strongly coupled semiconductor/RGO using photo-assisted synthesis method would present an efficient interface for charge transfer from ZCS to RGO.22,37,38 In order to investigate the interfacial interaction of the ZCS@MoS2/RGO, the chemical states of the ZCS nanorods and the typical ZCS@MoS2/RGO were compared by the X-ray photoelectron spectroscopy (XPS). The full XPS survey spectra of the ZCS nanorods and the typical ZCS@MoS2/RGO are shown in Figure 2a, demonstrating Zn, Cd and S as major elements in both samples, and Mo and C as minor elements in the typical ZCS@MoS2/RGO. The high-resolution Zn 2p XPS spectra are shown in Figure 2b, where two peaks at binding energies of 1022.3 eV and 1045.3 eV in Zn 2p XPS spectra could be assigned to Zn 2p3/2 and Zn 2p1/2, respectivily, which are consistent with the previous work.24 XPS spectra of Cd 3d in Figure 2c reveal two binding energies at 405.3 eV (Cd 3d5/2) and 412.0 eV (Cd 3d3/2), which is similar to Cd2+ in CdS.39 In addition, XPS signals of Mo 3d in Figure 2d are observed at binding energies around 229.4 eV and 232.4 eV in the typical ZCS@MoS2/RGO which ascribed to doublet Mo3d5/2 and Mo3d3/2, which are consistent with the data of MoS2.40 Compared to the ZCS nanorods, the typical ZCS@MoS2/RGO shows both binding energy of Zn 2p and Cd 3d towards high value, which can be a strong evidence for presence of interactions between ZCS and RGO sheets.40,41 To confirm the deoxygenation of GO using the photo-assisted reduction method, XPS spectrum of C1s of ZCS@MoS2/RGO is compared with that of initial GO. As shown in Figure 2e, four functional groups associated with C-C at 284.6 eV, C-O at 286.9 eV, C=O at 287.8 eV and carboxyl at 288.8 eV are presented in initial GO, indicating the sufficient oxidation of graphite. In stark contrast, the peaks for all oxygen containing functional groups are decreased or disappeared in the XPS spectrum of C1s from ZCS@MoS2/RGO (Figure 2f), indicating a considerable reduction of GO by the photo-assisted reduction method. The results agree with their FT-IR spectra (Figure S5). As mentioned above, ZCS nanorod can



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coordinated or bonded with functional groups of GO by electrostatic adsorption to form an interfacial structures. Under light irradiation, the generated electrons/holes by the excited ZCS nanorod could be injected to GO, the electron could readily reacted with those oxygen containing functional groups, leading to an efficient deoxygenation.42,43

Figure 3. (a) UV–vis absorption spectra of the ZCS nanorods and typical ZCS@MoS2/RGO. Inset is the bandgaps converted from UV–vis absorption spectra. (b) Photocurrent response of the ZCS nanorods and typical ZCS@MoS2/RGO measured in NaSO3/Na2S electrolyte at 0.3 V vs. Ag/AgCl. Figure 3a shows the UV-visible absorption spectra of the ZCS nanorods and the typical ZCS@MoS2/RGO. The UV-visible absorption spectra of ZCS nanorods shows an efficient absorption edge to 530 nm, corresponding to bandgap of 2.48 eV. According to Xing et al. results, the Cd1-xZnxS photocatalyst with the bandgap between 2.4 and 2.5 eV could be better for H2 generation.44 Compared to ZCS nanorods, the typical ZCS@MoS2/RGO only shows a raising absorption intensity in the wavelength longer than 530 nm, indicating the unchanged bandgap, while the raising absorption intensity in longer than 530 nm region is due to the reintroduction of black MoS2/RGO.23-32 Figure 3b shows the photocurrent responses of the ZCS nanorods and the typical ZCS@MoS2/RGO, it is clear that the ZCS@MoS2/RGO has much higher photocurrent density than that of ZCS nanorods, suggesting much easily charge transfer in ZCS@MoS2/RGO.


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Figure 4. (a) Effect of MoS2 content on the photocatalytic H2 evolution rate of the ZCS@MoS2/RGO composites with constant 3 wt.% RGO content. (b) Effect of RGO content on the photocatalytic H2 evolution rate of the ZCS@MoS2/RGO composites with constant 1.5 wt.% MoS2 content. (c) Comparison of the H2 generation rate of the ZCS nanorods, ZCS@MoS2/RGO (1.5 wt.% MoS2 and 3 wt.% RGO), 1.5 wt.% ZCS@MoS2 and 3 wt.% ZCS@RGO. (d) Comparison of the H2 generation rate of the ZCS@MoS2/RGO (1.5 wt.% MoS2 and 3 wt.% RGO), and ZCS@Pt/RGO (1.5 wt.% Pt and 3 wt.% RGO) in lactic acid and Na2S-Na2SO3 containing electrolyte. The photocatalytic H2 production activities over ZCS@MoS2/RGO were measured by using lactic acid as the sacrificial agent under solar light. Figure 4a reveals the activities of H2 evolution of ZCS incorporated with MoS2/RGO at different MoS2 content (0.5~2.5 wt.%). It can be observed that the photocatalytic activities of ZCS@MoS2/RGO increases with increasing the MoS2 content to 1.5 wt.%, and producing highest H2 generation rate of 2.31 mmol/h. The effect of the content of RGO on the photocatalytic H2 production activity of ZCS@MoS2/RGO was also investigated by fixing the 1.5 wt.% MoS2 as constant. As shown in Figure 4b, the H2 generation of ZCS@MoS2/RGO at 3 wt.% RGO content present a highest



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photocatalytic activity. Therefore, the optimizing MoS2 and RGO content for photocatalytic activity of ZCS should be at 1.5 wt.% and 3 wt.%, respectively. To clearly elucidate the role of MoS2/RGO, the photocatalytic activities of H2 generation of ZCS, 1.5 wt.% ZCS@MoS2, 3 wt.% ZCS@RGO and ZCS/MoS2@RGO (1.5 wt.% MoS2 and 3 wt.% RGO) were compared. As shown in Figure 4c, the ZCS/MoS2@RGO reveals a constant H2 generation rate with linear increase until 25 h, after then, the decrease in H2 generation rate is ascribed to the consumption of lactic acid. With further adding the sacrificial agent of lactic acid, the H2 generation rate could be returned back. The stable H2 generation could be maintained at least 50 h. In addition, the great stability of ZCS/MoS2@RGO could be also revealed under visible light (λ > 420 nm), as shown in Figure S6. However, the ZCS alone presents a poor photocatalytic H2 activity, which is 82.5 time lower than the ZCS/MoS2@RGO. For 1.5 wt.% ZCS@MoS2 and 3 wt.% ZCS@RGO, the photocatalytic H2 activities have remarkable improvement, but they still are 1~2 times lower than the ZCS/MoS2@RGO. Moreover, the H2 generation rates over ZCS@MoS2 and ZCS@RGO exhibit serious hysteresis after irradiation of 10 h, although lactic acid was added, which might be due to the photocorrosion of ZCS. On the basis of above results, the ZCS decorated on MoS2@RGO significantly improves and stabilize its photocatalytic activities for H2 generation compared to ZCS in either MoS2 or RGO. It was well-known that surface-bound proton is the main active species, which is to be responsible for H2 generation.23-32 To understanding the effect on lactic acid electrolyte, we have compared the photocatalytic H2 production activity of ZCS@MoS2/RGO with ZCS@Pt/RGO in both lactic acid and Na2S-Na2SO3, as shown in Figure 4d. The ZCS@MoS2/RGO exhibits the much higher H2 activities than the ZCS@Pt/RGO in lactic acid solution. When used Na2S–Na2SO3 instead of lactic acid as the hole scavenger, the ZCS@Pt/RGO has obvious advantage than the ZCS@MoS2/RGO, the phenomenon is agreed with Chang’s results.45 It was demonstrated that the unsaturated S atoms on exposed edges of MoS2 more easily capture H+ than is the case in Na2S-Na2SO3 solution due to the abundance of H+ in the lactic acid electrolyte, facilitating H2 generation. As a result, the H2 generation rate of ZCS@MoS2/RGO hybrid is much higher than ZCS@Pt/RGO in lactic acid electrolyte. Therefore,

the spectacular photocatalytic H2 evolution of ZCS@MoS2/RGO could be ascribed to


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simultaneous efficient H2 evolution activity and charge transfer process. The reasons underlying the improved photocatalytic H2 generation are further investigated by above mentioned two key factors. Figure 5a shows the electrochemical measurements of Pt/C, MoS2, RGO and MoS2/RGO in 0.5 m H2SO4 solution. The MoS2/RGO exhibits a small onset overpotential of 150 mV for HER, beyond which the cathodic current increases rapidly. Howevere, the onset overpotential of the MoS2 and RGO is higher than 250 mV, indicating their poor catalytic activity for H2 evolution in acid electrolyte.46,47 A photoluminescence (PL) spectra was recorded to investigate the efficiency of separation of photogenerated charge in the ZCS, ZCS@MoS2, ZCS@ RGO and ZCS@MoS2/RGO. As shown in Fig. 5b, the PL signal of all samples is located at around 530 nm, which is originated from the band-to-band emission of ZCS. Compared to ZCS, the decreased PL intensities of the ZCS@MoS2 and ZCS@ RGO indicate that the both MoS2 and graphene could facilitate the charge transfer and separation. Notably, the ZCS@MoS2/RGO exhibits an almost disappearance in PL intensity, suggesting a more efficient charge separation.

Figure 5. (a) Polarization curves ofPt/C, MoS2, RGO and MoS2/RGO measured in 0.5 m H2SO4 solution. (b) Photoluminescence spectra of the ZCS nanorods, ZCS@MoS2/RGO (1.5 wt.% MoS2 and 3 wt.% RGO), 1.5 wt.% ZCS@MoS2 and 3 wt.% ZCS@RGO under 360 nm excitation wavelength. Here, we hypothesize that the interfacial structures of ZCS@MoS2/RGO is able to be responsible for its stabilized and improved activity, the reactions might be as follows: 1) the triphasic interface facilitates a fast electron transfer from excited ZCS to RGO, then to MoS2. Compared to ZCS@MoS2, the great electron transport efficiently reduces possibility of



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electron and hole recombination. (2) MoS2 as co-catalyst promotes H2 generation in lactic acid containing electrolyte compared to RGO. (3) The obtained ZCS@MoS2/RGO by dual reduction reaction ensures formation efficiencies of triphasic interface, which shortens electron transport distance reaching MoS2. The schematic diagram is shown in Figure 6a. Figure 6b shows schematic energy level diagrams of ZCS nanorods, RGO, and MoS2 in comparison with the potentials associated with H2 and O2 evolution. The photogenerated electron from ZCS nanorods could be rapidly injected into RGO, then further transferred to MoS2 to induce the hydrogen reduction by their intimate interfacial structure. Theoretically, the lower overpotential and efficient charge transfer of HER are two crucial factors to promote H2 reduction, therefore, in either ZCS@MoS2 or ZCS@RGO, the photogenerated electrons from ZCS nanorods could not be maximized for H2 generation due to according low electron transfer or high overpotential.

Figure 6. The schematic diagram (a) and energy diagram (b) of ZCS@MoS2/RGO under solar light irradiation. Conclusion


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We demonstrate a dual reduction reaction for the synthesis of ZCS@MoS2/RGO composites for H2 generation. The photocatalytic H2 activities of the as-prepared ZCS@MoS2/RGO could be significantly enhanced by the different ratios of MoS2 and RGO, and an optimizing condition was found at 1.5 wt.% MoS2 and 3 wt.% RGO, which reveals H2 production rate of 2.31 mmol/h with long term stability at least 50 h. The outstanding photocatalytic activity of ZCS@MoS2/RGO for the H2 generation under solar light irradiation could be contributed by following reasons. First, RGO facilitates electron transport; second, MoS2 enhances hydrogen evolution activity; finally, the formed ZCS@MoS2/RGO hybrids have an efficient triphasic interface to shorten electron transport distance.

ASSOCIATED CONTENT Supporting Information. Additional TEM images, Element mapping, XRD patterns, Pore size distribution, FT-IR spectra and Photocatalytic H2 evolution under visible light. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION * Corresponding author: [email protected] (Y. L. Min)

Acknowledgement: This work was supported by the National Science Foundation of China (NSFC) (Grants nos.21271010) and Shanghai Municipal Education Commission (No:15ZZ088)

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Stabilizing and Improving Solar H2 Generation from Zn0.5Cd0.5S

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