AuO_x Surface Oxide Layer as a Hole-Transferring Cocatalyst for Water Oxidation over Au Nanoparticle-Decorated TiO₂ Photocatalysts

The development of a photocatalyst having high activity for four-electron oxidation of water is important because water oxidation is the rate-determining step of water splitting. Titanium(IV) oxide (TiO2) modified with gold (Au) nanoparticles (Au/TiO2) was prepared by the deposition precipitation (DP) method and then thermally treated under a flow of oxygen (O2) or hydrogen (H2) at various temperatures. These Au/TiO2 samples were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). When the Au/TiO2 sample was treated in H2, all of the Au nanoparticles were in a metallic state. When the Au/TiO2 sample was treated in O2, Au atoms at the outer surface of Au nanoparticles were partially oxidized, resulting in the formation of an Au core–AuOx shell-like structure in Au/TiO2. In water oxidation under visible-light irradiation, Au/TiO2 plasmonic photocatalysts having the Au core–AuOx shell-like structure were more active than metallic Au/TiO2 samples. Model reactions in which oxidation and reduction products were deposited on the Au/TiO2 photocatalyst indicated that the reduction and oxidation sites were the TiO2 surface and the AuOx layer, respectively. The role of an AuOx layer and the reaction mechanism of water oxidation over Au/TiO2 plasmonic photocatalyst are discussed on the basis of the results.