TiO2 Nanotubes: Morphology, Size, Crystallinity, and Phase-Dependent Properties from Synchrotron-Spectroscopy Studies

This work presents a study of two series (anodization-time-dependent and anodization-voltage-dependent) of TiO2 nanotubes (NTs) prepared via control anodization and annealing methods. The desired phases of amorphous, anatase, mixed-phase (anatase and rutile), and rutile have been obtained, and their size-dependent phase transition behavior is investigated using synchrotron-based X-ray techniques. The local symmetry and electronic properties and the origin of luminescence from the TiO2 NTs are tracked by X-ray absorption near-edge structures (XANES) and X-ray excited optical luminescence (XEOL) techniques, respectively. It is found that the anatase-to-rutile (A-R) transition at a given annealing temperature highly depends on the size of NTs. That is, the longer the NTs, the slower the A-R phase transformation that would take place, while shorter NTs have a lower anatase fraction in the mixed-phase structure at the same annealing temperature. In addition, surface nucleation dominates the A-R phase transition in the temperature range of 700–800 °C. The power of XANES and XEOL in phase recognition is illustrated.