Enhanced photocatalytic oxidation of propylene over V-doped TiO2 photocatalyst: Reaction mechanism between V5+ and single-electron-trapped oxygen vacancy

The novel V-doped TiO2 (V-TiO2[NTA]) samples with different (0-10%) concentration have been successfully prepared by a facile solid state sintering method using the nanotubular titanic acid (NTA) as titanium precursor for the first time, which shows remarkable photocatalytic activity for degradation of propylene under visible-light irradiation. Based on the density function theory (DFT) calculations in conjunction with a series of experimental characterization techniques including Raman spectra, TEM, XPS, ESR, and UV-vis DRS, a new V4+ ions formation mechanism by means of the electron trapped at single-electron-trapped oxygen vacancy (SETOV, V-o(center dot)) not Ti3+ has been proposed, and the origin of the observed remarkable improved photocatalytic activity of the V-TiO2[NTA] samples has also been investigated. At low concentration, the enhancement of photocatalytic activity for the V-TiO2[NTA] samples firstly come from the synergistic effect of V and V-o(center dot) co-doping: a part of V5+ is reduced to V4+ by V-o(center dot) into TiO2 lattice, others exist on the surface in the form of V2O5. The incorporation of V4+ in TiO2 lattice induces some new states (around the top of the valence band, due to O 2p and V 3d orbitals; around the bottom of the conduction band, due to Ti 3d, O 2p, and V 3d orbitals) near the edge of the valence and conduction bands, respectively, causing an effective narrowing of the band gap. The narrowing of gap is responsible for the red-shifted and increased light-absorption. The presence of V5+/V4+ redox couple facilitates the efficient separation and migration of photo-induced e(-)/h(+) pairs to generate active species. Secondly, the enhancement of photocatalytic activity with low doping concentration may also be ascribed to the increased surface area. At high doping concentration, the reasons for the decreased photocatalytic activity is that the same trapping sites may act as the recombination centers and excess V dopant may occupy the active sites of surface. (C) 2015 Elsevier B.V. All rights reserved.