Solar light-driven photocatalytic hydrogen evolution over ZnIn2S4 loaded with transition-metal sulfides
A series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn2S4, while Ag2S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn2S4, the activity for H2 evolution was increased by up to 1.6 times, compared to the ZnIn2S4 only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion.
KeywordsPhotocatalytic Activity Hydrogen Evolution Ag2S IrO2 SnS2
Since the discovery of photo-induced water splitting on TiO2 electrodes , 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, 5, 6, 7, 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, 10, 11, 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, 14, 15, 16, 17]. In our previous work [18, 19, 20, 21, 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)  or transition-metal oxide (e.g., NiO , Rh2-yCryO3 , RuO2 , IrO2), loaded on the surface of the base photocatalyst can be beneficial to photocatalytic H2 and/or O2 evolution for water splitting . 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, 29, 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.
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.
Average rates of H2 evolution over Pt-loaded MS/ZnIn2S4.
Content of MS
Rate of hydrogen evolution
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.
The authors acknowledge the support by the National Basic Research Program of China (No. 2009CB220000), Natural Science Foundation of China (No. 50821064 and No. 90610022), and the U.S. Department of Energy. One of the authors (SS) was also supported by China Scholarship Council and the Fundamental Research Funds for the Central Universities (No. 08142004 and No. 08143019).
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