Synthesis of Narrow Band Gap Gallium Zinc Nitride Oxide Solid Solutions for Photocatalytic Water Splitting under Visible Light

(Ga1–xZnx)(N1–xOx), a solid solution of GaN and ZnO, is known as a photocatalyst capable of water splitting under visible light irradiation. However, it has been difficult to obtain well-crystallized (Ga1–xZnx)(N1–xOx) with a long absorption edge wavelength at a gram scale due to the loss of the ZnO component that is essential for band gap narrowing or the production of an explosive hydrogen byproduct during the synthesis. In this work, we demonstrate the gram-scale synthesis of ZnO-rich (Ga1–xZnx)(N1–xOx) with high crystallinity and a long-wavelength photoresponse by reacting hydrogen-free solid nitrogen sources in sealed evacuated tubes. The obtained (Ga1–xZnx)(N1–xOx) was almost stoichiometric and absorbed visible light up to a wavelength of 580 nm. Furthermore, the material could be activated for overall water splitting under visible light irradiation by the use of a mild nitridation treatment and by loading with cocatalysts to give an apparent quantum yield of 2 × 10–2% at 430 nm. This opens new possibilities for obtaining oxynitride photocatalysts with a long-wavelength photoresponse for water splitting through the use of hydrogen-free solid nitrogen sources in sealed evacuated tubes.