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

Production of H 2 from water splitting is challenging. Herein, we report an increased production of H 2 on Zn 0.5 Cd 0.5 S (ZnCdS) in aqueous solution under a 420 nm LED lamp, via 4 wt % MCo 2 O 4 (M = Mg, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Without addition of sacrifices, the rate of H 2 evolution ( R H , in a unit of mmol g ―1 h ―1 ) was 0.0154 for ZnCdS, and 0.057―0.551 for MCo 2 O 4 /ZnCdS. On addition of Na 2 S/Na 2 SO 3 , the corresponding R H increased to 18.5 and 31.5―78.2, respectively. In both cases, no H 2 was observed for MCo 2 O 4 . However, the R H obtained in presence of Na 2 S/Na 2 SO 3 had a negative correlation with M s and R 2 , respectively, where M s was the magnetization degree of MCo 2 O 4 , and R 2 was the charge transfer resistance for MCo 2 O 4 /ZnCdS at solid-liquid interface. Electrochemical measurement showed that MCo 2 O 4 /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 MCo 2 O 4 for water (sulfide/sulfite) oxidation. It is also proposed that the magnetic field of MCo 2 O 4 inhibits the charge transfer between MCo 2 O 4 and ZnCdS. This work suggests that MCo 2 O 4 spinels are good co-catalysts, but their magnetization need to be minimized.