Effect of annealing temperature and atmosphere on the structural, morphological and luminescent properties of TiO₂ nanotubes

Titanium dioxide (TiO₂) nanotubes prepared on titanium metal substrate are thermally annealed at various temperatures in nitrogen, air, oxygen and vacuum. In all annealing atmospheres, except in vacuum, the anatase phase becomes less prominent than the rutile phase for annealing temperatures above 600 °C. The anatase phase is stable above 600 °C in vacuum but does not evolve significantly up to 900 °C. The morphologies of the nanotubes tend to deteriorate with increased annealing temperature, particularly in nitrogen, air and oxygen atmospheres. However, the integrity of the nanotubes is maintained up to 900 °C in vacuum. The photoluminescence emissions suggest mainly the presence of oxygen vacancies and self-trapped excitons, with respective emission bands around 2.5 eV and 2.3 eV. The results show that the annealing temperature and atmosphere strongly influence the crystalline and optical properties of the TiO₂ nanotubes.