Unfolding the Anatase-to-Rutile Phase Transition in TiO2 Nanotubes Using X-ray Spectroscopy and Spectromicroscopy

Abstract

This work reports a study of the anatase-to-rutile phase transition (ART) in a highly ordered TiO2 nanotube (NT) specimen fabricated using an electrochemical process followed by thermal annealing at 750 °C (NT750). Two-dimensional X-ray absorption near-edge structure–X-ray excited optical luminescence spectroscopy reveals the hierarchically two-layered structure of NT750 by resolving the surface anatase luminescence and bulk rutile optical emission. Scanning transmission X-ray microscopy analysis of a sliced NT750 lamella spatially differentiates the top nanotubular anatase structure from the denser rutile bottom layer with a gradual ART interface layer. On the basis of these results together with the known behavior of size and anisotropy dependence of ART in TiO2 nanocrystal, we propose the “bottom-up” mechanism for ART in anodic TiO2 NTs. This result is particularly relevant to the fundamental understanding of phase transition in nanostructures as well as the fabrication of desired TiO2 NT mixed-phase c...