Photocatalytic H₂ evolution under visible-light irradiation over band-structure-controlled (CuIn)_xZn_2(1-x)S₂ solid solutions

(CuIn)_xZn_2(1-x)S₂ solid solutions between a ZnS photocatalyst with a wide band gap and CuInS₂ with a narrow band gap showed photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents SO₃^2- and S^2- under visible-light irradiation (λ ≥ 420 nm). Pt (0.5 wt %)-loaded (CuIn)_0.09Zn_1.82S₂ with a 2.3-eV band gap showed the highest activity for H₂ evolution, and the apparent quantum yield at 420 nm amounted to 12.5%. H₂ evolved at a rate of 1.5 L h^-1 m^-2 under irradiation with a solar simulator (AM 1.5). Diffuse reflection and photoluminescence spectra of the solid solutions shifted monotonically to a long wavelength side, as the ratio of CuInS₂ to ZnS increased in the solid solutions. The photocatalytic H₂ evolution depended on the composition as well as the photophysical properties. DFT calculations suggested that the visible-light response should be derived from the contribution of Cu 3d and S 3p orbitals to the valence band and that of In 5s5p and Zn 4s4p orbitals to the conduction band, respectively. The contribution of these orbitals to the energy bands affected the photophysical and photocatalytic properties.