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.