A simplified method for synthesis of band-structure-controlled (CuIn)_xZn_2(1-x)S₂ solid solution photocatalysts with high activity of photocatalytic H₂ evolution under visible-light irradiation

(CuIn)_xZn_2(1-x)S₂ (x = 0.01-0.5) microspheres were prepared by a simple hydrothermal method. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Visible diffuse reflectance spectra (UV-Vis), Raman scattering spectra and Brunauer-Emmett-Teller (BET) surface area measurement. It was found that the (CuIn)_xZn_2(1-x)S₂ samples formed solid solution only in the presence of surfactant cetyltrimethylammonium bromide (CTAB). CTAB enabled the increase of the surface area for the (CuIn)_xZn_2(1-x)S₂ solid solutions. Diffuse reflection spectra of the solid solutions shifted monotonically to long wavelength side as the value of x increased. The photocatalytic H₂ evolution activity under visible-light irradiation of the solid solutions was evaluated. The result showed that the activity depended on their corresponding compositions closely. Ru (1.5 wt%)-loaded (CuIn)_0.2Zn_1.6S₂ showed the highest photocatalytic activity of 198.09 μmol h^-1 under visible-light irradiation, and the apparent quantum yield amounted to 15.45% at 420 nm. Furthermore, the density functional theory (DFT) calculations showed that the solid solution with the ration of ZnS and CuInS₂, 6:1, was a direct band gap semiconductor. The valence band consisted of the hybrid orbital of S 3p and Cu 3d and the conduction band consisted of In 5s5p orbital mixed with Zn 4s4p. The energy band structure resulted in the visible-light response of the solid solution, and affected its photocatalytic performance.