Zinc–cobalt oxides as efficient water oxidation catalysts: the promotion effect of ZnO

Abstract

Herein, we report the promotion effect of ZnO in water oxidation catalyzed by Co. Zinc–cobalt oxides—ZnCo_xO_y were prepared via the calcination of Zn–Co_x-coordination polymers. The results of XRD, Co K-edge XANES and EXAFS show that the Co/Zn ratio greatly affected the oxidation state of Co and local structure of the ZnCo_xO_y oxides. With a Co/Zn ratio higher than 3.0, Zn(II) prefers to substitute in the lattice of Co₃O₄. The integrated ZnO and Co₃O₄ composites were formed at a Co/Zn ratio less than 2.0. The HR-TEM images show that ZnO and Co₃O₄ compactly contact to form the interfaces in the composites. In both the chemical water oxidation and the visible-light-driven photocatalytic water oxidation ([Ru(bpy)₃]²⁺–persulfate system), Zn substituted in the spinel structured Co₃O₄ oxide cannot significantly improve the water oxidation activity and only the integrated ZnO and Co₃O₄ composites afford much higher TOFs than Co₃O₄. This suggests the existence of the cooperation effect between ZnO (water adsorption site) and Co₃O₄ (water oxidation site). Our results provide a facile approach to design composite catalysts for the water oxidation reaction.