Introduction

Since the discovery of photo-induced water splitting on TiO2 electrodes [1], solar-driven photocatalytic hydrogen production from water using a semiconductor catalyst has attracted a tremendous amount of interest [2, 3]. To efficiently utilize solar energy, numerous attempts have been made in recent years to realize different visible light-active photocatalysts [4–8]. Among them, sulfides, especially CdS-based photocatalysts with narrow band gaps, proved to be good candidates for photocatalytic hydrogen evolution from water under visible light irradiation [9–12]. However, CdS itself is not stable for water splitting, and Cd2+ is hazardous to environment and human health. A number of nontoxic multicomponent sulfides without Cd2+ ions have been developed to show comparable photocatalytic efficiency for hydrogen evolution [13–17]. In our previous work [18–22], hydrothermally synthesized ZnIn2S4 was found to have photocatalytic and photoelectrochemical properties that made it a good candidate for hydrogen production from water under visible light irradiation. On the other hand, a solid co-catalyst, typically noble metal (e.g., Pt, Ru, Rh) [23] or transition-metal oxide (e.g., NiO [24], Rh2-yCryO3 [25], RuO2 [26], IrO2[27]), loaded on the surface of the base photocatalyst can be beneficial to photocatalytic H2 and/or O2 evolution for water splitting [25]. Nevertheless, there have been only a limited number of studies in which metal sulfides acted as co-catalysts to enhance the activity of a semiconducting photocatalyst [28–30]. For instance, Li and co-workers observed that dual co-catalysts consisting of noble metals (Pt, Pd) and noble-metal sulfides (PdS, Ru2S3, Rh2S3) played a crucial role in achieving very high efficiency for hydrogen evolution over CdS photocatalyst [29, 30]. In this study, a series of transition-metal sulfides (MS: Ag2S, SnS, CoS, CuS, NiS, and MnS) were deposited on hydrothermally synthesized ZnIn2S4 by a simple precipitation process. The photocatalytic activities for hydrogen evolution over these MS/ZnIn2S4 products were investigated. We demonstrated that transition-metal sulfides, such as CuS, combined with Pt could act as dual co-catalysts for improving photocatalytic activity for hydrogen evolution from a Na2SO3/Na2S aqueous solution under simulated sunlight.

Experimental section

All chemicals are of analytical grade and used as received without further purification. ZnIn2S4 products were prepared by a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthetic method as described in our previous studies [18, 19]. For the synthesis of MS/ZnIn2S4 (MS = Ag2S, SnS, CoS, CuS, NiS, and MnS), 0.1 g of prepared ZnIn2S4 was dispersed in 20 mL of distilled water and ultrasonicated for 20 min. Under stirring, a desired amount of 0.1 M AgNO3 (J.T.Baker Chemical Co., Phillipsburg, NJ, USA), SnCl2 (Sigma-Aldrich, Milwaukee, WI, USA), Co(NO3)2 (Aldrich), Cu(NO3)2 (Fluka Chemical Company, Buchs, Switzerland), Ni(NO3)2 (Fluka), or Mn(CH3COO)2 (Alfa-Aesar, Ward Hill, MA, USA) aqueous solution was dropped into the above suspension, followed by a drop-wise addition of 0.1 M Na2S·9H2O (Sigma-Aldrich) aqueous solution, containing double excess of S2- relative to the amount of metal ions. The resulting suspension was stirred for another 20 min, then the MS/ZnIn2S4 precipitate was collected by centrifugation and washed with distilled water for several times, and dried overnight at 65°C. The weight contents of transition-metal sulfides (MS) in these MS/ZnIn2S4 products were controlled at 0.5% to approximately 2.0%.

X-ray diffraction patterns were obtained from a PANalytical X'pert diffractometer (PANalytical, Almelo, The Netherlands) using Ni-filtered Cu Kα irradiation (wavelength 1.5406 Å). UV-visible absorption spectra were determined with a Varian Cary 50 UV spectrophotometer (Varian Inc, Cary, NC, USA) with MgO as reference. Morphology inspection was performed with a high-resolution scanning electron microscope (SEM, Hitachi S-4300, Tokyo, Japan). Transmission electron microscopy (TEM) study was carried out on a JEOL JEM 2010 instrument (JEOL Ltd., Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD, Shimadzu/Kratos Analytical, Hadano, Kanagawa, Japan) with monochromatic Al Kα radiation (hν = 1,486.69 eV) and with a concentric hemispherical analyzer. Elemental Analysis was conducted on the Bruker S4 PIONEER X-ray fluorescence spectrum (XRF, Bruker AXS GmbH, Karlsruhe, Germany) using Rh target and 4-kW-maximum power.

Photocatalytic hydrogen evolution was performed in a side-window reaction cell. A 300-W solar simulator (AM 1.5; Newport Corporation, Irvine, CA, USA) was used as the light source. The amount of hydrogen evolved was determined using a gas chromatograph (CP-4900 Micro-GC, thermal conductivity detector, Ar carrier; Varian Inc., Palo Alto, CA, USA). In all experiments, 100 mL of deionized water containing 0.05 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell. Here, sacrificial agent was used to scavenge photo-generated holes. Argon gas was purged through the reaction cell for 30 min before reaction to remove air. Pt (1.0 wt.%) as a co-catalyst for the promotion of hydrogen evolution was deposited in situ on the photocatalyst from the precursor of H2PtCl6·xH2O (Aldrich; 99.9%). In all cases, the reproducibility of the measurements was within ± 10%. Control experiments showed no appreciable H2 evolution without solar light irradiation or photocatalyst.

Results and discussion

The ZnIn2S4 products prepared by the CTAB-assisted hydrothermal method possessed a hexagonal structure and morphology of microspheres comprising of numerous petals, and showed an absorption edge at about 510 nm (Additional file 1, Figure S1-3). Compared to pure ZnIn2S4, the obtained MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) displayed different absorption profiles (Additional file 1, Figure S4), with enhanced absorption in the visible light region from 550 to 800 nm. Such additional broad band (λ > 550 nm) can be assigned to the absorption of transition-metal sulfides.

We investigated the photocatalytic activity for hydrogen evolution over MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS). Photocatalytic activities for hydrogen evolution over MS/ZnIn2S4 were evaluated by loading 1 wt.% Pt as co-catalyst. Figure 1 shows the average rates of H2 evolution over Pt-loaded MS/ZnIn2S4 photocatalysts under simulated solar irradiation in the initial 20-h period. The Pt-ZnIn2S4 showed a photocatalytic activity for H2 production at the rate of 126.7 μmol·h-1, which is comparable to reported values in previous literatures [18–20]. The hydrogen production rates of Pt-MS/ZnIn2S4 photocatalysts varied with different kinds of loaded transition-metal sulfides. The Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) photocatalysts displayed enhanced activities for hydrogen evolution under solar irradiation. In particular, the H2 evolution rate greatly increased to 200 μmol·h-1 after loading 1.0 wt.% of CuS on ZnIn2S4. In this CuS/ZnIn2S4 sample, the formation of CuS (copper monosulfide) could be evidenced by XPS analysis results shown in Figure S5 (Additional file 1). The survey scan spectrum (Figure S5A of Additional file 1) indicated the presence of Cu, Zn, In, and S in the sample [21, 31]. The binding energies shown in Figure S5E (Additional file 1) for Cu 2p3/2 and Cu 2p1/2 were 952.5 and 932.5 eV, respectively, which are close to the reported value of Cu2+[31]. The actual molar ratio of Cu:Zn:In:S was 0.011:0.2:0.39:1.01 as confirmed by XRF analysis result, with weight content of CuS calculated to be 1.15 wt.%, which is quite close to the proposed stoichiometric ratio. The photocatalytic activities for hydrogen evolution over Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) in the initial 20-h period were measured to increase in the order of SnS <Ag2S <CuS. Generally, these transition-metal sulfides (SnS, Ag2S, and CuS) alone are not photocatalytically active for H2 evolution, as no H2 was detected when they were used as the catalysts. Thus, the improvement of photocatalytic performances of Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) can be related to the enhanced separation of photo-generated electrons and holes induced by the hybridization of MS with ZnIn2S4. In this photocatalysis system, transition-metal sulfides (MS = Ag2S, SnS, and CuS) combined with noble-metal Pt acted as dual co-catalysts for photocatalytic hydrogen evolution. However, when transition-metal sulfides (MS = CoS, NiS, and MnS) were loaded on ZnIn2S4, the rates of H2 evolution over Pt-MS/ZnIn2S4 (MS = CoS, NiS, and MnS) were sharply decreased. Instead of the role as effective co-catalysts, these transition-metal sulfides (i.e., CoS, NiS, and MnS) may work as the recombination center of photo-generated electron-hole pairs, which lowered the photocatalytic activity for hydrogen evolution over Pt-MS/ZnIn2S4 (MS = CoS, NiS, and MnS). Further investigation is needed and also under way to provide enough supporting information to evidence the negative effects of CoS, NiS, and MnS, although main attention has focused on the more effective co-catalysts such as Ag2S, SnS, and CuS in the following discussion.

Figure 1
figure 1

Average rates of H 2 evolution. The average rates of H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) under solar light irradiation in the initial 20-h period.

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Figure 2 shows the reaction time depended H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) under solar irradiation. Pt-SnS/ZnIn2S4 and Pt-CuS/ZnIn2S4 exhibited stable activity in the period of 34-h experiment. However, the rate of H2 production over Pt-Ag2S/ZnIn2S4 had a significant drop after irradiation for approximately 20 h. This deactivation may result from gradual reduction of Ag2S particles loaded on the surface of ZnIn2S4 to metallic Ag by photo-generated electrons during the reaction. Similar deactivation of photocatalyst was previously observed for CdS modified with Ag2S [32]. However, this result is quite different from our previous report on Pt-Ag2S/CdS, in which the high dispersion of Ag2S in the nanostructure of CdS contributed to stable photocatalytic activity for hydrogen evolution [33]. Taking into account the reduction potential (vs. normal hydrogen electrode (NHE)) of Ag+/Ag (0.80 V), Cu2+/Cu (0.34 V), and Sn2+/Sn (-0.14 V), reduction of Ag2S by photo-generated electrons is easier than photoreduction of CuS and SnS. Therefore, Pt-MS/ZnIn2S4 (MS = SnS and CuS) turned to be more stable than Pt-Ag2S/ZnIn2S4 during the photocatalytic reaction for hydrogen evolution.

Figure 2
figure 2

Time courses of H 2 evolution. The time courses of H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) under solar light irradiation.

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Table 1 shows the dependence of photocatalytic activity for H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = SnS and CuS) on the loading amount of transition-metal sulfides. With the increase of SnS-loading from 0 to 2.0 wt.%, the rate of H2 evolution over Pt-SnS/ZnIn2S4 does not show significant changes. In contrast, the photocatalytic performance of Pt-CuS/ZnIn2S4 depends strongly on the amount of CuS-loading, and the optimum loading amount of CuS is approximately1.0 wt.%. The progressive regression of photocatalytic activity observed with the amount of CuS increasing from 1.0 to 2.0 wt.% may be due to the fact that excess CuS particles loaded on the surface of ZnIn2S4 could act as the optical filter or charge recombination center instead of co-catalyst for charge separation [19, 32].

Table 1 Average rates of H2 evolution over Pt-loaded MS/ZnIn2S4.
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To visualize hybridization of CuS with ZnIn2S4, ZnIn2S4, and CuS/ZnIn2S4 photocatalysts were investigated by TEM. A representative TEM image of ZnIn2S4 is shown in Figure 3A, which shows the formation of microspheres, 1-2 μm in diameter and comprised of a circle of micro-petals. The ED pattern (inset of Figure 3A) substantiates that the ZnIn2S4 microsphere is of a hexagonal phase. The TEM image in Figure 3B shows that some nanoparticles are loaded on the surface of ZnIn2S4 microspheres. Such nanoparticles were confirmed by the ED pattern (inset in Figure 3B) to be CuS with typical orthorhombic structure. Thus, nanosized CuS particles dispersed on the ZnIn2S4 surface would act as the charge-transfer co-catalyst, together with photodeposited Pt particles. The Pt-CuS dual co-catalysts improved the charge separation and therefore increased the photocatalytic activity.

Figure 3
figure 3

TEM images (A) ZnIn 2 S 4 and (B) CuS/ZnIn 2 S 4 .

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Figure 4 illustrates the process of photo-generated charge transfer for photocatalytic hydrogen evolution over Pt-CuS/ZnIn2S4 in an aqueous solution containing Na2SO3/Na2S under simulated sunlight. Band gap excitation produces electron-hole pairs in ZnIn2S4 particles. The excited electrons are subsequently channeled to Pt sites, which reduce protons to generate hydrogen. On the other hand, the valence band potential of ZnIn2S4, deduced from the conduction band potential (0.29 V vs. NHE) [22] and the band gap energy (2.43 eV), is about 2.72 V vs. NHE, which is more positive than the OH-/O2 redox potential [4]. The valence band potential of CuS is less positive than the OH-/O2 redox potential [34]. Such a difference of valence band potentials makes it possible for the excited holes to transfer from ZnIn2S4 to CuS to react with Na2S/Na2SO3 electron donor in the solution. Therefore, Pt and CuS are supposed to act as the reduction and oxidation co-catalyst, respectively, which leads to more efficient charge separation, thus improves photocatalytic activity of Pt-CuS/ZnIn2S4. Similar benefits of dual co-catalysts on photocatalytic activity have been observed for CdS loaded with noble metals as reduction catalysts and noble-metal sulfides as oxidation catalysts [29, 30]. It is noteworthy that replacing noble-metal sulfides (such as PdS) by transition-metal sulfides (such as CuS) as the co-catalysts would help lower the cost of photocatalysts for solar-hydrogen production. Moreover, seeking effective co-catalyst candidates could be expanded to other transition-metal sulfides such as FeS and SnS2, etc. Detailed research on this subject is still an ongoing progress in our group.

Figure 4
figure 4

Schematic illustration of photo-generated charge-transfer process for photocatalytic hydrogen evolution over Pt-CuS/ZnIn 2 S 4 . From an aqueous solution containing Na2SO3/Na2S under simulated solar light.

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Conclusions

In summary, a series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfides: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts were developed. It is found that Ag2S, SnS, and CuS could enhance the photocatalytic activity of hydrogen evolution over ZnIn2S4 to varying degrees, while SnS, CoS, and NiS would reduce the activity. Among them, the Pt-CuS/ZnIn2S4 photocatalyst exhibited the most efficient and stable activity for hydrogen evolution. This can be attributed to the fact that the dual co-catalysts of Pt and CuS entrapped photo-induced electrons and holes for reduction and oxidation reaction, respectively, improving charge separation and hence the photocatalytic activity. Application of transition-metal sulfides as co-catalysts opens up an opportunity toward realizing high-efficiency, low-cost photocatalysts for solar-hydrogen conversion.