Overall water splitting using (oxy)nitride photocatalysts

Abstract

Oxynitride photocatalysts with d¹⁰ electronic configuration are presented as effective non-oxide catalysts for overall water splitting. Germanium nitride (β-Ge₃N₄) having a band gap of 3.8-3.9 eV modified with RuO₂ nanoparticles as a cocatalyst is shown to achieve stoichiometric decomposition of H₂O into H₂ and O₂ under UV irradiation (λ > 200 nm). A novel solid solution of GaN and ZnO, (Ga_1-xZn_x)(N_1-xO_x), with a band gap of 2.4-2.8 eV (depending on composition) achieves overall water splitting under visible light (λ > 400 nm) when loaded with an appropriate cocatalyst. The narrower band gap of the solid solution is attributed to the bonding between Zn and N atoms at the top of the valence band. The photocatalytic activity of (Ga_1-xZn_x)(N_1-xO_x) for overall water splitting is strongly dependent on both the cocatalyst and the crystallinity and composition of the material. The quantum efficiency of (Ga_1-xZn_x)(N_1-xO_x) with Rh and Cr mixed-oxide nanoparticles is 2-3 % at 420-440 nm, which is the highest reported efficiency for overall water splitting in the visible-light region.