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.