Band energy levels and compositions of CdS-based solid solution and their relation with photocatalytic activities

CdS-based solid solution (AgIn)_xCd_2(1-x)S₂ (x = 0, 0.02, 0.05, 0.1, 0.2, 1.0) was prepared through hydrothermal reaction. The physicochemical and optical properties of the as-prepared solid solution samples were characterized by X-ray diffraction (XRD), diffuse reflectance UV-visible absorption spectroscopy (DRS) and N₂ adsorption-desorption isotherms. Through controlling the composition of CdS solid solution, its band-gap energy (E_g) can be easily tuned from ca. 2.4 to 1.8 eV. The band edge energy levels of all samples were determined through the measurement of the open circuit potential (OCP) and onset potential of photocurrent (E _onset) for their respective electrodes. The decrease in E_g of the solid solutions is attributed to the downward and upward shift of conduction and valence band potentials, respectively. It was found that In 5s5p and Cd 5s5p states mainly contribute to the bottom of conduction band, and Ag 4d and S 3p states mainly contribute to the top of valence band as confirmed by theoretical density of states (DOS) calculation. To test the photocatalytic activity of the prepared solid solution materials, the evolution of H ₂ and the reduction of polyoxometalate (POM: PMo₁₂O ₄₀^3-) under visible light irradiation were used as probe reactions. The photocatalysis test results were well correlated with the band structures determined from the photoelectrochemical measurements and DOS calculation. The relative positioning between the band-edge levels and the substrate conversion potentials was the most critical and this can be controlled with the solid solution-based photocatalysts.