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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 O2- by self-doped F- in the TiO2 lattice is localized in the t2g 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.

The electronic structure and optical properties of Pd-modified TiO2 nanotubes (NTs) with a vertically aligned nanotubular structure grown by a two-step electrochemical anodization method have been studied using X-ray spectroscopy. X-ray absorption near-edge structure (XANES) at the Ti L3,2- and O K-edges was used to investigate the TiO2 NTs before and after Pd modification. It was found that Pd nanoparticles (NPs) are uniformly coated on the NT surface. The Pd L3-edge of the deposited Pd NPs shows a more intense whiteline and a blue shift for the Pd L3-edge absorption threshold relative to Pd metal, indicating charge depletion from the Pd 4d orbital as a result NP formation. The lattice of Pd is slightly contracted upon NP formation, although it remains fcc as revealed by extended X-ray absorption fine structure (EXAFS) analysis at the Pd K-edge. X-ray-excited optical luminescence (XEOL) together with XANES with element and site specificity was used to study the optical luminescence from TiO2 NTs. It was found that the defect-originated XEOL intensity dropped noticeably in the Pd NP-coated NTs, suggesting a Pd NP-TiO2-interaction-mediated reduction in the radiative recombination of electrons and holes. Further evidence is provided by Ti 2p resonant inelastic X-ray scattering (RIXS), in which no low-energy loss features (d-d transitions) were observed. The implications of these results are discussed.