ABSTRACT:

Band structure engineering is an effiffifficient technique to develop desired semiconductor photocatalysts, which was usually carried out through isovalent or aliovalent ionic substitutions. Starting from a UV-activated catalyst ZnGa2S4, we successfully exploited good visible light photocatalysts for H2 evolution by In3+-to-Ga3+ and (Cu+/ Ga3+)-to-Zn2+ substitutions. First, the bandgap of ZnGa2−xInxS4 (0 ≤ x ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn1−2y(CuGa)yGa1.7In0.3S4 (y = 0.1, 0.15, 0.2) provided a further and signifificant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn0.7Cu0.15Ga1.85In0.3S4 possessed the highest H2 evolution rate under pure visible light irradiation using S2− and SO32− as sacrifificial reagents (386 μmol/h/g for the noble-metalfree sample and 629 μmol/h/g for the one loaded with 0.5 wt % Ru), while the binary hosts ZnGa2S4 and ZnIn2S4 (synthesized using the same procedure) show 0 and 27.9 μmol/h/g, respectively. The optimal apparent quantum yield reached to 7.9% at 500 nm by tuning the composition to Zn0.6Cu0.2Ga1.9In0.3S4 (loaded with 0.5 wt % Ru).

INTRODUCTION

Hydrogen generation utilizing solar energy driven photocatalysts has been recognized as a green and promising way to produce clean energy and solve the fossil fuels shortage problem.1 During the past few decades, many photocatalysts have been developed for H2 generation through water splitting. For example, TiO2 2,3 and ZnO4,5 are typical wide bandgap semiconductors and need to be activated by ultraviolet (UV) light, which, however, occupies only 5% of the solar energy; therefore, searching for visible-light-driven photocatalysts is always desired.

Metal sulfides have been widely studied as excellent photocatalysts in the visible light range because the valence bands (VBs) were mainly contributed by the S 3p orbitals instead of O 2p in oxides, resulting in higher VB maxima and narrower bandgaps.6,7 For example, CdS is probably one of the most widely studied metal sulfides as a photochemical water reduction catalyst because of its narrow bandgap (2.4 eV) and appropriate electrochemical potentials.8−10 However, the drawback is also apparent that metal sulfides are not capable of pure water splitting because of the inevitable photocorrosion, which can be solved by using Na2S and Na2SO3 as sacrifificial regents. Though metal sulfides are usually not suitable photocatalysts for pure water splitting, people could use them for photocatalytic H2 production and, in the meantime, consume sulfur compounds from chemical industries or natural resources.

People could either use a visible light responsive metal sulfide directly, or modify a UV-light catalyst by band structure engineering strategy. Very recently, A. Kudo and H. Kaga reported solid solutions between defect chalcopyrite ZnGa2S4 and chalcopyrite CuGaS2, which is in fact a cosubstitution of Cu+ /Ga3+ to Zn2+. 11 The bandgap of ZnGa2S4 can be significantly narrowed (from 3.4 to 2.5 eV) because the incorporation of Cu 3d orbitals largely raised the VB potential. The best performance of H2 evolution was obtained on Zn0.4(CuGa)0.3Ga2S4 loaded with 0.5 wt % Pd, where the apparent quantum yield (AQY) is 15% at 420 nm.As for AQYs, there is an amazing record of 93% at 420 nm for the Pt-PdS/CdS catalyst reported by C. Li et al.12 In addition, ZnIn2S4, although possessing a different structure with ZnGa2S4 (see Figure S1 in the Supporting Information, SI), has been extensively investigated and the highest AQY reaches 34.3% at 420 nm.13,14 The interesting point is that the presumably existed ZnGa2−xInxS4 solid solutions should possess the bandgaps between ZnGa2S4 (∼3.4 eV) and ZnIn2S4 (∼2.5 eV), and the In3+-to-Ga3+ substitution could lower the conduction band (CB) position.15−18 Moreover, according to A. Kudo’s work on ZnGa2S4−CuGaS2 solid solutions, where the modification is started from the host compound ZnGa2S4, we believe that the cosubstitution of Cu+ /Ga3+ to Zn2+ on ZnGa2−xInxS4 would further reduce the bandgap energy by raising the VB position, and allow the absorption edge to redshift. Of course, maintaining a substantial overpotential to H+/ H2 is also necessary during the bandgap modifification.

EXPERIMENTAL SECTION

Photocatalytic activities were tested on a gas-closed circulation system equipped with a vacuum line (CEL-SPH2N system), a reaction vessel, and a gas sampling port that is directly connected to a gas chromatograph (Shanghai Techcomp-GC7900, TCD detector, molecular sieve 5A, N2 gas carrier). In a typical run, 50 mg of a catalyst was dispersed by a magnetic stirrer in 50 mL of an aqueous solution containing 0.1 mol L−1 Na2S and 0.5 mol L−1 Na2SO3 in a 150 mL Pyrex glass reactor with a quartz cover. The solution was kept stirring, and a 10 °C recycling water bath was applied to keep the reaction vessel at a constant temperature. The light irradiation source was generated by an external 300W Xe-lamp (CEL-HXF300, Beijing AuLight) with or without a (cutoffff or bandpass) fifilter laid on the top of the reaction vessel.

RESULTS AND DISCUSSION

Synthesis and Phase Identifification. The syntheses of ZnGa2−xInxS4 were successful when 0 ≤ x ≤ 0.4. Powder XRD patterns of as-prepared ZnGa2−xInxS4 are basically the same. All match well with the reference pattern of the defect chalcopyrite ZnGa2S4 (see Figure 1a). A slight left-shift of the refection peaks can be observed, especially for the (220) and (204) peaks at ∼57.5°/2θ, indicating the unit cell expansion induced by the In3+-substitution. More accurate unit cell determination was performed by Le Bail refifinements on powder XRD patterns using TOPAS,22 and the refifined cell parameters are shown in Figure 1b. The linear increase of the length for a-, c-axes as well as the cell volume (see Figure 1a) along with the increase of x is solid evidence for the successful syntheses of pure ZnGa2−xInxS4 (0 ≤ x ≤ 0.4). Further increasing the In3+-concentration in the starting material would not give a larger cell lattice; instead, an impurity phase (Ga,In)2S3 appears. Then it is conclusive that the upper limit of the In3+-doping is around x = 0.4, which is 20 atom %. ZnIn2S4 was also prepared at the same condition, whose powder XRD pattern is consistent with the reference (see Figure S2, Supporting Information).

At the beginning we had assumed there existed a wide range of solid solutions between ZnGa2S4 and ZnIn2S4. Apparently, it is not the case experimentally. At a reaction temperature below 650 °C, In2S3 would not react with other reagents. At 650 °C, the reaction between ZnS, Ga2S3, and In2S3 could proceed but in a very slow speed and incomplete even after a long period of time. So the fifinal reaction temperature was selected to be 700 °C, at which the reaction was completed in just 2 h. At 700 °C, ZnGa1.6In0.4S4 is thermodynamically stable, while “ZnGa1.5- In0.5S4” is not. This is the major reason why there was the impurity (Ga,In)2S3 when x = 0.5. Hypothetically, a higher reaction temperature, i.e., 750 °C, might raise the In3+-doping upper limit. However, In2S3 would volatilize at the heating zone of the vacuumed tube furnace, and transfer to and solidify at the cold end of the tube. The volatilization of In2S3 may partially be solved if performing the syntheses in a small and sealed quartz tube, like that in the syntheses of Zn1−2x(CuGa)xGa2S4. 11 It will be shown later that the ZnGa1.7In0.3S4 possesses the best photocatalytic performance of water reduction. As a consequence, we incorporated CuGaS2 into this specifific composition, forming Zn1−2y(CuGa)yGa1.7In0.3S4 (y = 0.1, 0.15, 0.2). The corresponding powder XRD patterns are shown in Figure 2. The cell volume decreases when y increases, which is as expected due to the replacement of Zn2+ by smaller cations (i.e., Zn2+, 0.60 Å; Cu+ , 0.60 Å; Ga3+, 0.47 Å). Finally, all the used catalysts show the same powder XRD patterns with those of as-synthesized samples, indicating the stability in our photocatalytic conditions (see Figure S3, Supporting Information).