ZnIn2S4 as a photocatalyst for photosplitting water into H2 exhibits some fascinating advantages, such as low toxicity, good crystallinity, and considerable chemical stability. Currently, developing ZnIn2S4-based composite photocatalysts with different morphologies has received wide attention in order to improve the photocatalytic activity. In this contribution, a new ZnIn2S4/RGO/MoS2 photocatalytic system has been designed. The presence of the RGO is confirmed by x-ray photoelectron spectroscopy and FT-IR spectra. By optimization of solvothermal reaction temperatures, reaction time, and RGO introduction amount, up to 1.62 mmol/h ⋅ g of hydrogen evolution rate has been achieved.

Photocatalytic water splitting into hydrogen is considered a promising way to solve the energy shortage problem.1 Amongst, developing efficient photocatalysts is the key. A semiconductor photocatalyst for water splitting generally needs a suitable band gap (Eg) and suitable band positions; specifically, the Eg of photocatalyst has to be over 1.23 eV, in the meantime, its conduction band (CB) must be at a more negative potential than the reduction potential of H+/H2 (0 V vs. NHE) while its valence band (VB) at a more positive position than the oxidation potential of O2/H2O (1.23 V vs. NHE).2 Different kinds of water splitting photocatalysts have been investigated so far. Some UV-light-responsive photocatalysts (i.e., TiO2, SrTiO3, La-doped NaTaO3, and K4Nb6O17) exhibit good photocatalytic activity and stability, however, at most 4% of solar energy corresponding to UV light remarkably limits their application.3–5 Therefore, visible light-driven photocatalysts (typically including CdS, ZnxCd1−xS, ZnIn2S4, Ta3N5, and BiV O4) covering a wider solar spectrum exhibit potential attractive prospects.6–12 

As a ternary sulfide semiconductor, ZnIn2S4 exhibits some fascinating advantages, such as low toxicity, good crystallinity, and considerable chemical stability.2 The photocatalytic activity of ZnIn2S4 itself is low; therefore, different ZnIn2S4 morphologies, doped ZnIn2S4 photocatalysts, and ZnIn2S4-based composite photocatalysts have been developed.3–5 Typically, ZnIn2S4 composite photocatalysts have received more attention. For example, Peng et al. reported MWCNTs/ZnIn2S4 composites with 684 μmol/h of H2 production rate.13 Ye et al. synthesized In2S3/ZnIn2S4 bulk composite with 67.8 μmol/h of H2 production rate.14 Guo et al. synthesized a series of Pt-loaded MS/ZnIn2S4 (MS = Ag2S, NiS, CoS, CuS, and MnS) photocatalysts with 201.7 μmol/h of H2 production rate.15 1 wt% La-doped ZnIn2S4 prepared by hydrothermal method exhibits 58.3 μmol/h hydrogen evolution.16 Clearly, modification toward the ZnIn2S4 favors the photocatalytic performance.

Graphene exhibits potential application in photocatalysis due to its good conductivity, large surface, and excellent mechanical strength.17 Currently, some photocatalytic composite systems with the graphene oxide (GO) have been reported,12,18 in which graphene mainly acts as an efficient co-catalyst.19,20 However, the photocatalytic performance of these ZnIn2S4/RGO photocatalysts is still unsatisfied so far. In fact, MoS2 is a good alternative of Pt co-catalyst, which has been used in ZnIn2S4 photocatalyst systems.7,21 In this respect, new ZnIn2S4/RGO/MoS2 photocatalytic systems have been developed in order to further improve the photocatalytic activity. By optimization of solvothermal reaction conditions and RGO deposition amount, 1.62 mmol/h ⋅ g of the hydrogen evolution rate has been achieved, and the co-existence of RGO and MoS2 is found to favor the photocatalytic activity. Meanwhile, the photocatalytic mechanism of ZIS/RGO/MoS2 systems is also suggested.

The ZnIn2S4/RGO composite catalysts were obtained by solvothermal method which are labeled as ZIS/RGO-X-Y, X represents reaction time, Y represents the weight ratio of GO to ZIS. (For detailed experiments, please see the supplementary material.)22 For comparison, ZnIn2S4 sample was also prepared by the same method without using GO. According to the FT-IR spectrum of ZIS, two characteristic peaks at 1611 and 1458 cm−1 are found, which are assigned to the stretching vibration and bending vibration of the adsorbed water.23 To the ZIS/RGO composite photocatalyst, its IR peaks at 1654 and 1458 cm−1 are assigned to skeletal vibration of C=C— and bending vibration of H2O, respectively, in comparison with pure GO and RGO. (For FT-IR spectra, please see Figure S1 in the supplementary material.)22 It is thus suggested that the GO was successfully reduced to RGO during the solvothermal reaction,16 which also proves the formation of ZIS/RGO.

As shown in Figure 1(a), the samples derived from solvothermal reactions at 200, 210, and 220 °C can present similar XRD patterns while fixing the RGO amount is 0.5 wt%. With increasing the hydrothermal temperature, the crystallinity of ZnIn2S4 improves slightly. However, further increasing reaction temperatures was not carried out due to the temperature limitation of the Teflon reactors. Therefore, the highest reaction temperature is 220 °C in this work. Figure 1(b) presents XRD patterns of samples with and without RGO. Similar XRD profiles are observed for all the samples, which characteristic diffraction peaks 2θ at 21.8°, 30.4°, and 52.9° are ascribed to the (009), (110), and (107), respectively, crystal planes of the hexagonal ZnIn2S4 (JCPDS: 49-1562). No other impurities such as ZnS, In2S3, and other unreacted reactants are found, indicating the phase purity of ZIS. In comparison with the ZIS samples prepared under hydrothermal condition, however, some ZIS peaks are broadened or even invisible,24 indicating unsatisfied crystallinity of the ZIS prepared in dimethylformamide (DMF)/ethylene glycol (EG) mixed solvent.7 No obvious diffraction peak shift and the variation in crystallinity of ZIS and ZIS/RGO are observed. Even increasing the reaction time while fixing the reaction temperature at 220 °C, no obvious difference is observed from the XRD results of the ZIS/RGO samples, as shown in Figure 1(c). It is thus suggested that the introduction of a small quantity of GO does not change the structure of ZIS even extending the reaction time.

FIG. 1.

XRD patterns of (a) ZIS/RGO-0.5 samples at different reaction temperatures; (b) ZIS/RGO-0.5 samples at different reaction times; and (c) ZIS-6h, ZIS/RGO-6h-0.5, ZIS-36h, and ZIS/RGO-36h-0.5.

FIG. 1.

XRD patterns of (a) ZIS/RGO-0.5 samples at different reaction temperatures; (b) ZIS/RGO-0.5 samples at different reaction times; and (c) ZIS-6h, ZIS/RGO-6h-0.5, ZIS-36h, and ZIS/RGO-36h-0.5.

Close modal

Representative SEM images of ZIS and ZIS/RGO samples are shown in Figure 2. As can be seen, the ZnIn2S4 and ZnIn2S4/RGO composite samples have similar morphology, which are independent of the reaction time. Besides, the ZIS crystallites self-assemble into floriated microspheres with diameters ranging from 1 to 5 μm, and each microsphere consists of numerous petals formed zigzag structure, similar with the ZIS derived from hydrothermal method7 and other sulfide photocatalysts.25,26 When the reaction temperature is fixed at 220 °C, no obvious variation on the morphologies between the ZIS and ZIS/RGO is found for the reaction time at 6 and 36 h, respectively. And ZIS petals are surrounded by some RGO wrinkles, basically in accordance with the TEM image, as shown in Figure 2(e). Further HRTEM image shows the lattice spacing is about 0.28 nm fitting with (107) crystal plane of the hexagonal ZIS decorated with RGO with characteristic diffraction peaks, as shown in Figure 2(f).22 

FIG. 2.

SEM images of (a) as-prepared ZIS-6h sample, (b) ZIS-36h sample, (c) ZIS/RGO-6h-0.5 sample, (d) ZIS/RGO-36h-0.5 sample, TEM image of (e) ZIS/RGO-36h-0.5 sample, and HRTEM image of (f) ZIS/RGO-36h-0.5 sample.

FIG. 2.

SEM images of (a) as-prepared ZIS-6h sample, (b) ZIS-36h sample, (c) ZIS/RGO-6h-0.5 sample, (d) ZIS/RGO-36h-0.5 sample, TEM image of (e) ZIS/RGO-36h-0.5 sample, and HRTEM image of (f) ZIS/RGO-36h-0.5 sample.

Close modal

The presence of RGO in the composites can also be confirmed by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3, all of the Zn, In, S, and C elements were detected. The peaks at 1021.0 and 1045.5 eV correspond to the binding energies of Zn2p3/2 and Zn2p1/2, respectively, while the peaks at 444.9 and 452.5 eV correspond to the binding energies of In3d5/2 and In3d3/2, respectively. Besides, the peaks at 161.96 and 162.89 eV correspond to S2p3/2 and S2p1/2, respectively. A broad asymmetric curve can be deconvoluted into two peaks at 284.8 and 286.4 eV corresponding to the sp2-hybridized carbon (C—C) and ether bond (C—O), respectively.16,27,28 In the meantime, a small peak at 288.8 eV may be assigned to the residue organic solvents (DMF or EG). It is thus deduced that our solvothermal reaction indeed leads to the formation of ZIS/RGO nanocomposites with a highly reductive degree.7,16

FIG. 3.

XPS spectra of ZIS/RGO-36h-0.5 photocatalyst: (a) Zn2p core-level spectra, (b) In3d core-level spectra, (c) S2p core-level spectra, (d) C1s core-level spectra.

FIG. 3.

XPS spectra of ZIS/RGO-36h-0.5 photocatalyst: (a) Zn2p core-level spectra, (b) In3d core-level spectra, (c) S2p core-level spectra, (d) C1s core-level spectra.

Close modal

According to the UV-vis spectra and the relationship between (αhυ)1/2 and photon energy, the band gaps of ZIS-6h and ZIS-36h are estimated to be 2.76 and 2.69 eV, respectively, while the band gaps of ZIS/RGO composites are 2.73 and 2.58 eV. (For UV-vis spectra, please see Figure S2 in the supplementary material.)22 It is thus suggested that the ZIS/RGO composite samples have relatively narrower band gaps than the corresponding ZIS samples; in the meantime, with the reaction time increasing, the band gaps of ZIS samples slightly decrease.

MoS2 as a co-catalyst is photo-assisted deposited on the ZnIn2S4/RGO surface before the H2 evolution activity measurement by following our previous work.7 Preliminary investigation reveals that 0.425 wt% MoS2 deposited on ZIS/RGO can present the highest H2 evolution rate, and the following experiments are all based on 0.425 wt% MoS2 co-catalyst.7 (For detailed relationship between the H2 evolution rates and different MoS2 loading amount, please see Table S1 in the supplementary material.)22 Figure 4(a) presents the H2 evolution rates of ZnIn2S4/RGO/MoS2 photocatalysts with different loading amount of RGO while the solvothermal reaction time is fixed at 6 h. We can see that the ZnIn2S4/MoS2 alone presents a lower photocatalytic activity with the H2 evolution rate of 0.72 mmol/h ⋅ g, indicating unsatisfactory photo-generated charge separation and transportation abilities of ZnIn2S4/MoS2 for H2 reduction reaction. When a small amount of RGO is introduced into the ZnIn2S4 system, the H2 evolution rate gradually enhances. And the 0.51 wt% of RGO is involved, the H2 evolution rate is enhanced to 1.1 mmol/h ⋅ g; however, further increasing the loading amount of RGO will lead to a decrease in the photocatalytic H2 evolution. Such a decrease in the photocatalytic activity with a higher loading of RGO may be due to the shading effect, that is, the extra RGO can block the light absorption of ZnIn2S4 or the active sites, thus leading to a negative effect of hydrogen evolution.21 For comparison, Pt co-catalyst is also loaded on the ZnIn2S4/RGO surface by in-situ photo-deposition method. For 0.425 wt% Pt-ZnIn2S4-36h/RGO sample, 0.97 mmol/h ⋅ g of the H2 evolution rate is obtained, indicating the MoS2 as a co-catalyst can exhibit better catalytic activity than the conventional Pt co-catalyst to ZnIn2S4/RGO systems, in good agreement with our previous work.7 

FIG. 4.

(a) Hydrogen evolution rates based on 220 °C reaction temperature and 6 h reaction time with different loading amounts of RGO; (b) hydrogen evolution rates of ZIS/RGO-6h-0.5 samples at different reaction temperatures while fixing 6 h reaction time; (c) hydrogen evolution rates of ZIS/RGO-X-0.5 samples based on 220 °C reaction temperature with different reaction time; (d) comparison of hydrogen evolution rates of samples based on optimal reaction temperature and reaction time with and without RGO or MoS2 co-catalysts; (e) proposed mechanism for photocatalytic H2 evolution on the ZIS/RGO/MoS2 photocatalyst.

FIG. 4.

(a) Hydrogen evolution rates based on 220 °C reaction temperature and 6 h reaction time with different loading amounts of RGO; (b) hydrogen evolution rates of ZIS/RGO-6h-0.5 samples at different reaction temperatures while fixing 6 h reaction time; (c) hydrogen evolution rates of ZIS/RGO-X-0.5 samples based on 220 °C reaction temperature with different reaction time; (d) comparison of hydrogen evolution rates of samples based on optimal reaction temperature and reaction time with and without RGO or MoS2 co-catalysts; (e) proposed mechanism for photocatalytic H2 evolution on the ZIS/RGO/MoS2 photocatalyst.

Close modal

In terms of 0.51 wt% RGO/ZIS/MoS2 photocatalyst, the effect of different solvothermal temperatures on the H2 evolution rates is also investigated, as shown in Figure 4(b). As can be seen, with the hydrothermal temperatures increasing, the H2 evolution rate gradually increases; however, the ZnIn2S4/RGO photocatalyst at 210 °C exhibits almost the same photocatalytic activity as that at 220 °C. Therefore, all the following investigation on the photocatalytic performance is based on the ZnIn2S4/RGO photocatalysts derived from the solvothermal reaction at 220 °C. Generally, there are several possible factors related to the improved photocatalytic H2 evolution rates, such as the crystallinity and specific surface areas. Here, it is supposed that the crystallinity of the ZnIn2S4 samples slightly improves as the solvothermal temperature increasing; however, this influence is little probably due to the solvothermal reaction itself.

To further optimize the photocatalytic performance, the effect of the solvothermal reaction time on the H2 evolution rate has been investigated. As shown in Figure 4(c), with the increase of the reaction time, the photocatalytic H2 evolution rate increases first, then decreases. As high as 1.62 mmol/h ⋅ g has been achieved with the apparent quantum efficiency of 0.4% at 420 nm, which is based on the ZIS/0.5 wt% RGO/MoS2 photocatalyst derived from 36 h solvothermal reaction. This enhancement of photocatalytic activity is suggested mainly due to the light absorption is slightly red-shifted for the samples derived from longer reaction time, as in Figure S2.22 Furthermore, preliminary stability test reveals that the H2 evolution amount is proportional to the reaction time in 5 h and no obvious degradation of the ZIS/RGO/MoS2 composite photocatalyst is found. (For UV-visible spectra and time course of H2 evolution based on ZIS/RGO-36h-0.5/MoS2, please see Figures S2 and S3 in the supplementary material.)22 

In terms of our previous work and the literatures, a proposed mechanism for the photocatalytic process of ZnIn2S4/RGO/MoS2 system under visible light irradiation is shown in Figure 4(e). First, GO is reduced to RGO in the typical solvothermal reaction, and it is supposed that the ZIS may grow on highly conductive RGO.20 Under visible light irradiation, the photogenerated electrons are excited from the VB to CB of ZIS, leaving the positive holes in the VB of ZIS. As usual, the photo-generated holes will oxidize the lactic acid as the sacrificial reagent, and the photo-generated electrons in the CB of ZIS can be quickly transport through the highly conductive graphene to MoS2, where H+ is reduced to hydrogen atom, then releases H2. It is supposed that the recombination between the photo-generated electrons and holes could be suppressed effectively.7,13 This is further supported by the photocatalytic performance of the ZIS-based photocatalysts with different co-catalysts.29 As shown in Figure 4(d), the ZIS without any co-catalyst gives 0.32 mmol/h ⋅ g of H2 evolution rate. When the RGO is introduced into the ZIS system, the H2 evolution rate is enhanced to 0.46 mmol/h ⋅ g, suggesting the RGO acts as a co-catalyst, in agreement with Li’s work.20 In comparison with the H2 evolution rate of 1.46 mmol/h ⋅ g for ZIS/MoS2 photocatalytic system, the performance of ZIS/RGO/MoS2 can exhibit 1.62 mmol/h ⋅ g. Obviously, the co-existence of RGO and MoS2 is effective to enhance the hydrogen evolution, thus suggesting that the RGO and MoS2 act as dual co-catalysts to improve the photocatalytic activity of the ZIS.

In summary, RGO and MoS2 have been introduced into ZIS photocatalyst systems simultaneously for the first time. Up to 1.62 mmol/h ⋅ g of H2 evolution rate has been achieved by optimization toward the amount of RGO, solvothermal reaction temperature, and reaction time. A mechanism is suggested that the RGO and MoS2 as dual co-catalysts to enhancement of the ZIS photocatalyst system.

This work is financially supported from the MOST (No. 2012CB932904) and NSFC (Nos. 51421002 and 91233202).

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Supplementary Material