Near-band-gap luminescence from TiO2 nanograss–nanotube hierarchical membranes

Abstract

Freestanding TiO2 nanograss–nanotube hierarchical membranes are synthesized from a Ti metal foil by electrochemical anodization. It is found that the two nanostructures exhibit different luminescence properties, which are also affected by the crystal phases upon phase transformation. An unusual near-UV luminescence is observed from the amorphous nanograss, which is found to be excitation element specific. The amorphous nanotube shows no luminescence. Upon calcination, both nanograss and nanotubes are crystalized into the anatase phase with some rutile phase present, and both structures emit visible green luminescence at slightly different energies. The luminescence mechanism is explored using UV–vis spectroscopy, X-ray absorption near-edge structures (XANES), and X-ray excited optical luminescence (XEOL), and its implications are presented.