Synchronously achieved surface Bi0 metallisation and incorporation of Bi3+/5+ ions into a W6+, N3− doped TiO2 lattice by the hydrothermal-reduction method: surface plasmonic resonance effect for efficient photocatalysis

Bi3+/5+ ions were incorporated into W6+ and N3− doped TiO2 by the sol–gel and hydrothermal reduction methods. The products obtained by these methods were different and were designated as BiWNT and Bi0-BiWNT, respectively. Bi was incorporated into the TiO2 lattice by the sol–gel method and some of Bi was reduced to the Bi0 state on the surface of the catalyst in the hydrothermal method. The PXRD results confirmed the anatase phase for the BiWNT sample and mixed anatase and rutile phases for the Bi0-BiWNT sample. Incorporation of higher or lower valence metal/non-metal ions into the TiO2 lattice is usually accompanied by the formation of neutral/ionized oxygen and titanium vacancies. An effective strategy is to monitor the inherently created defects/imperfections which can influence the properties such as the electronic structure, optical absorption, charge transport mechanism and surface acidity/basicity. Estimation of the Bi5+/Bi3+/Bi0 content in the BiWNT and Bi0-BiWNT samples was carried out by XPS analysis of Bi 4f7/2 and 4f5/2 spin states. The charge transfer process in the bicrystalline framework of Bi0-BiWNT depends on the positions of the energy levels within the band gap and it can be either the Type II heterojunction mechanism or the direct Z-scheme mechanism or it can be both of these mechanisms. Higher photocatalytic activity of the Bi0-metallised sample could be ascribed to various factors such as the bicrystalline framework of the lattice, presence of substitutional N, and surface plasmonic effect of Bi0 deposits, and to the unique defect chemistry of the sample.