2D XANES–XEOL Spectroscopy Studies of Morphology-Dependent Phase Transformation and Corresponding Luminescence from Hierarchical TiO2 Nanostructures

Hierarchical TiO2 nanostructures with the nanograss at the top and the close-ended nanotube at the bottom were synthesized by an extended electrochemical anodization process. Upon thermal annealing, the hierarchical TiO2 nanostructures exhibit a morphology-dependent phase transformation behavior: the amorphous as-prepared nanotube layer undergoes an amorphous-anatase-rutile phase transformation upon annealing at 550 °C and 850 °C, respectively; the nanograss, on the other hand, remains in the anatase phase with an annealing temperature up to 850 °C and becomes anatase-dominant anatase–rutile-multiphase at 900 °C. In addition, the nanotube and nanograss exhibit intriguing light-emitting properties. A two-dimensional X-ray absorption near-edge structure – X-ray excited optical luminescence (2D XANES–XEOL) spectroscopy technique is used to investigate such phenomenon, which correlates the origin of the luminescence with the local chemical entrainment of the nanostructured TiO2. Implications of these observations are thoroughly discussed. These properties are desirable for the engineering of novel TiO2 nanoheterostructures for photocatalysis as well as optical and photonic devices, especially the fabrication of a stable high temperature anatase phase.