Facile in situ synthesis of visible-light plasmonic photocatalysts M@TiO2 (M = Au, Pt, Ag) and evaluation of their photocatalytic oxidation of benzene to phenol

We developed a facile in situ method of preparing noble-metal plasmonic photocatalysts M@TiO2 (M = Au, Pt, Ag). In this method, the UV irradiation of TiO2 powder dispersed in absolute ethanol generates some Ti3+ ions on the surface of TiO2 particles and these Ti3+ ions, upon addition of a noble-metal salt in the dark, reduce the metal cations to deposit metal nanoparticles on the TiO2 surface. This Ti3+-ion-assisted synthesis leads to a homogeneous loading of noble-metal nanoparticles on the surface of TiO2 particles, which allows photocatalytic reactions to take place under visible-light on the whole TiO2 surface. Among the three photocatalysts M@TiO2 (M = Au, Pt, Ag), Au@TiO2 exhibits a high yield (63%) and selectivity (91%) for the oxidation of benzene to phenol in aqueous phenol. For this photocatalytic reaction, our study suggests a mechanism in which the visible-light absorption by the Au nanoparticles causes electron transfer from the Au nanoparticles to the TiO2 particle, and the electron-depleted Au oxidizes phenoxy anions to form phenoxy radicals that oxidize benzene to phenol.