Nonmetal-metal-semiconductor-promoted P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst with superior photocatalytic activity and stability

Recently, modifying TiO₂ to develop visible-light-driven photocatalysts via doping with nonmetal elements, metals and semiconductors has attracted much attention. In this study, a facile and reproducible sol-gel method was utilized for the first time to codope nonmetal (P), metal (Ag) and narrow band gap semiconductors (Ag₂O and Ag₃PO₄) in TiO₂ to prepare a P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite photocatalyst for developing a novel visible-light-driven photocatalyst. The crystal structure, morphology and optical properties of P/Ag/Ag₂O/Ag₃PO₄/TiO₂ were systemically characterized, and the photocatalytic abilities were evaluated under simulated solar light, in comparison with pure TiO₂ and Ag/Ag₂O/TiO₂ photocatalysts. The results indicated that the P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite photocatalyst exhibited a strong absorption band in the visible light region, a small band gap (about 2.2 eV) and extremely high separation efficiency of photo-excited electron-hole pairs, which could account for its enhanced photocatalytic performance. The degradation ratio of rhodamine B (Rh B) by P/Ag/Ag₂O/Ag₃PO₄/TiO₂ reached 97.1% after 60 min under simulated solar light with only 13 mW cm^-2 light intensity, while just 26.6% and 66.7% of Rh B were degraded by the as-prepared TiO₂ and Ag/Ag₂O/TiO₂ photocatalysts, respectively. Furthermore, the radical trapping experiments implied that photo-generated holes and O₂^- radicals in the P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite photocatalyst were the major active species for Rh B degradation. Metallic Ag could effectively trap the photo-generated electrons from Ag₃PO₄ and Ag₂O to reduce the possibility of decomposing Ag₃PO₄ and Ag₂O, resulting in an improved stability of the P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite with high photocatalytic ability.