Long-wavelength photoresponsive gallium zinc oxynitride for efficient oxygen evolution and Z-scheme water splitting reactions

Abstract

Long-wavelength photoresponsive GaN:ZnO, a solid solution of GaN and ZnO, can be obtained by reacting Ga₂O₃ and Zn₃N₂ in the presence of ZnX₂ (X = halogen) in a sealed evacuated tube. However, the activity of GaN:ZnO for overall water splitting via one-step excitation under visible light remains low due to the small particle size, which leads to uncontrollable aggregation and lack of suitable cocatalysts. In this study, well-dispersed particulate GaN:ZnO with a ZnO concentration of 66 mol%, optical absorption up to about 600 nm, and anisotropic crystalline facets was obtained using δ-Ga₂O₃ as a raw material and adding Zn to the starting material. The resulting GaN:ZnO exhibited an apparent quantum yield of 11.9% at 420 nm in the oxygen evolution reaction when loaded with an IrO_x cocatalyst. It also served as an oxygen evolution photocatalyst in a photocatalyst sheet for Z-scheme water splitting, in combination with La₅Ti₂Cu_0.9Ag_0.1O₇S₅, whose absorption edge wavelength was about 700 nm, as a hydrogen evolution photocatalyst. This work, which demonstrates the activation of long-wavelength photoresponsive GaN:ZnO, expands the possibilities of constructing photocatalytic systems that effectively utilize long-wavelength visible light for water splitting.