Tubular V-free and V-doped TiO2 derived from H2Ti12O25 hollow spheres: Vapour-thermal synthesis, morphology evolution, and photocatalytic performance

Vanadium (V)-free and V-doped H 2 Ti 12 O 25 hollow spheres (HTOHSs) were first synthesised via vapour-thermal method at 290 °C using common chemicals and solvents, over super-critical temperature (243 °C) of ethanol (vapour source). Then, they were annealed at 600 °C under different processing conditions to obtain titanium dioxide (TiO 2 ) photocatalysts . All catalysts were characterised by means of XRD , SEM, TEM, XPS , UV–vis DRS, FT-IR, N 2 adsorbtion-desorbtion and fluorescence lifetime. Results indicated that after the treatment in air, V-free HTOHSs were transformed into open-ended hollow tubes with uniform length of ~5 µm and diameter of ~1 µm, and walls of hollow tubes consisted of aggregated nanosheets . Furthermore, HTOHS crystallised into anatase TiO 2 (white) phase. The treatment in N 2 atmosphere led to the breaking of longer tubes into shorter ones. In contrast, after the treatment in N 2 atmosphere or by reduction using NaBH 4 as a reductant, V-doped HTOHS resulted in the formation of anatase TiO 2 (black) samples and consisted of fewer tubes and more deformed spheres. In this study, fluorescence lifetime ( τ ) of photo-generated carriers corresponded well with the ratio of oxygen vacancy , indicating that oxygen vacancy dominates the lifetime even though it is very sensitive to many factors. The evolution of structure and morphology and photocatalytic mechanism were analysed and discussed.