Photocatalytic hydrogen evolution from CdS–ZnO–CdO systems under visible light irradiation: Effect of thermal treatment and presence of Pt and Ru cocatalysts

A CdS–CdO–ZnO mixture annealed at different temperatures and loaded with Ru or Pt cocatalysts has been investigated in the production of hydrogen from aqueous solutions containing SO32− + S2− as sacrificial reagents under visible light. The physicochemical characterization of the CdS–CdO–ZnO catalyst revealed significant changes in the crystalline structure and visible light absorption capacity as a result of thermal treatments. Catalytic activity was found to be strongly dependent on physicochemical changes associated with thermal annealing. Hydrogen evolution over the CdS–CdO–ZnO catalyst was enhanced in the sample annealed at 773 K by the better contact between the CdS and CdO–ZnO phases, which improved physical charge separation. CdS–CdO–ZnO catalyst activity was significantly improved by the addition of Pt or Ru cocatalysts. Among the noble metals studied, activity promotion was higher for the sample loaded with Ru. The enhancement of activity associated with Ru loading is linked to a good interaction between Ru oxide particles and CdS, which reduces the possibility of electron–hole recombination, thus resulting in more efficient water splitting.