Zinc Germanium Oxynitride as a Photocatalyst for Overall Water Splitting under Visible Light

Abstract

A solid solution of zinc oxide and germanium nitride (Zn1+xGe)(N2Ox) (x = 0.44) is demonstrated to be an effective photocatalyst for overall water splitting under ultraviolet and visible light. The catalyst is prepared by reaction of GeO2 and ZnO under ammonia flow (20 mL·min-1) at 1123 K for 15 h. The crystal structure of the material is investigated by a combination of Rietveld analysis and the maximum-entropy method using neutron powder diffraction data. The (Zn1.44Ge)(N2.08O0.38) catalyst is confirmed to have a wurtzite-type structure (space group P63mc) and to be the solid solution where the oxygen atoms are substituted for nitrogen atoms. The (Zn1.44Ge)(N2.08O0.38) catalysts thus prepared exhibit a band gap of ca. 2.7 eV and corresponding activity at visible wavelengths. The decrease in band gap compared to the starting materials is attributed to larger valance band dispersion resulting from the energy difference between O2p and N2p orbitals and from the p−d repulsion between Zn3d and N2p+O2p electrons in the upper valance band, which raises the top of the valance band. (Zn1.44Ge)(N2.08O0.38) powder modified by surface loading with RuO2 nanoparticles at 5 wt % achieves overall water splitting under both ultraviolet and visible irradiation.