Tracking the Local Effect of Fluorine Self-Doping in Anodic TiO2 Nanotubes

We report herein a study in which we reveal the role of F- incorporated in the very anodic TiO2 nanotubes prepared electrochemically from a Ti foil using a fluoride based electrolyte. X-ray absorption near edge structure (XANES), resonant X-ray emission spectroscopy (RXES), and X-ray photoelectron spectroscopy (XPS) have been used to examine the as-prepared and the annealed TiO2 nanotubes. It is found that the additional electron resulting from the substitution of O-2(-) by self-doped F- in the TiO2 lattice is localized in the t(2g) state. Consequently, a localized Ti3+ state can be tracked by a d-d energy loss peak with a constant energy of 1.6 eV in the RXES, in contrast to TiO2 nanostructures where this peak is hardly noticeable when F- is driven out of the lattice upon annealing.