Pt-Enhanced Mesoporous Ti³⁺/TiO₂ with Rapid Bulk to Surface Electron Transfer for Photocatalytic Hydrogen Evolution

Pt-doped mesoporous Ti³⁺ self-doped TiO₂ (Pt-Ti³⁺/TiO₂) is in situ synthesized via an ionothermal route, by treating metallic Ti in an ionic liquid containing LiOAc, HOAc, and a H₂PtCl₆ aqueous solution under mild ionothermal conditions. Such Ti³⁺-enriched environment, as well as oxygen vacancies, is proven to be effective for allowing the in situ reduction of Pt⁴⁺ ions uniformly located in the framework of the TiO₂ bulk. The photocatalytic H₂ evolution of Pt-Ti³⁺/TiO₂ is significantly higher than that of the photoreduced Pt loaded on the original TiO₂ and commercial P25. Such greatly enhanced activity is due to the various valence states of Pt (Ptⁿ⁺, n = 0, 2, or 3), forming Pt-O bonds embedded in the framework of TiO₂ and ultrafine Pt metal nanoparticles on the surface of TiO₂. Such Ptⁿ⁺-O bonds could act as the bridges for facilitating the photogenerated electron transfer from the bulk to the surface of TiO₂ with a higher electron carrier density (3.11 × 10²⁰ cm^-3), about 2.5 times that (1.25 × 10²⁰ cm^-3) of the photoreduced Pt-Ti³⁺/TiO₂ sample. Thus, more photogenerated electrons could reach the Pt metal for reducing protons to H₂.