M-Dependent activity of MCo2O4 spinels for water splitting and H2 production on Zn0.5Cd0.5S under visible light

Production of H2 from water splitting is challenging. Herein, we report an increased production of H2 on Zn0.5Cd0.5S (ZnCdS) in aqueous solution under a 420 nm LED lamp, via 4 wt % MCo2O4 (M = Mg, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Without addition of sacrifices, the rate of H2 evolution (RH, in a unit of mmol g-1 h-1) was 0.0154 for ZnCdS, and 0.057-0.551 for MCo2O4/ZnCdS. On addition of Na2S/Na2SO3, the corresponding RH increased to 18.5 and 31.5-78.2, respectively. In both cases, no H2 was observed for MCo2O4. However, the RH obtained in presence of Na2S/Na2SO3 had a negative correlation with Ms and R2, respectively, where Ms was the magnetization degree of MCo2O4, and R2 was the charge transfer resistance for MCo2O4/ZnCdS at solid-liquid interface. Electrochemical measurement showed that MCo2O4/ZnCdS was more active than ZnCdS for proton reduction, water oxidation, and water(sulfide/sulfite) photo-oxidation, respectively. According to the solid photoluminescence, band parameters, and open circuit potentials, formation of a p-n junction is proposed, which facilitates the photoelectron transfer to ZnCdS for proton reduction, and the photohole transfer to MCo2O4 for water (sulfide/sulfite) oxidation. It is also proposed that the magnetic field of MCo2O4 inhibits the charge transfer between MCo2O4 and ZnCdS. This work suggests that MCo2O4 spinels are good co-catalysts, but their magnetization need to be minimized.