Alkaline earth metal as a novel dopant for chalcogenide solid solution: Improvement of photocatalytic efficiency of Cd_1-xZn_xS by barium surface doping

Novel Ba(x)-Cd_0.8Zn_0.2S photocatalysts, which has not been reported yet, was synthesized for the first time by thermal sulfuration method. These samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, N₂ absorption-desorption isotherm, ultraviolet visible diffuse reflectance spectroscopy, density functional theory calculation and photoluminescence spectroscopy. Results showed that Ba mainly existed on the surface of Cd _1-xZn_xS particles, because of the largest atom radius compared to Cd and Zn atoms, in the chemical form of Ba-S-Cd/Zn bonding. The successful surface doping finally induces the element gradient from bulk phase to surface, which efficiently promotes the photogenerated charges' transition and separation. On the basis of various characterization results, the mechanism of this promotion effect is proposed. The element gradient efficiently enhances the transition of electrons, while the noticeable doping influence on valence band significantly promotes the holes' migration. Thus, the bulk recombination of photoelectrons and holes was efficiently suppressed, and the hydrogen production was improved efficiently.