ZnGa_2–<i>x</i>In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) and Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>Ga_1.7In_0.3S₄ (0.1 ≤ <i>y</i> ≤ 0.2): Optimize Visible Light Photocatalytic H₂ Evolution by Fine Modulation of Band Structures

Abstract

Band structure engineering is an efficient 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 ZnGa_2–<i>x</i>­In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>­Ga_1.7In_0.3S₄ (<i>y</i> = 0.1, 0.15, 0.2) provided a further and significant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn_0.7Cu_0.15­Ga_1.85In_0.3S₄ possessed the highest H₂ evolution rate under pure visible light irradiation using S^2– and SO₃^2– as sacrificial reagents (386 μmol/h/g for the noble-metal-free 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 Zn_0.6Cu_0.2­Ga_1.9In_0.3S₄ (loaded with 0.5 wt % Ru)