In situ thermo-Raman study of titanium oxide nanotubes

Titania nanotubes were obtained by alkaline hydrothermal treatment of anatase powder at 100 °C followed by washing with an HCl aqueous solution. The resulting nanotubes were composed by one or two structural layers with an interlayer space ca. 0.826 nm, showing inner and outer diameters of around 5 and 10 nm, respectively. The walls of the nanotubes were composed of layers of an orthorhombic structure with lattice parameters (in nm) a = 0.301, b = 0.382 and c = 0.826. The thermal stability of the structure depended on the annealing conditions. Under inert atmosphere, the structure transformed into anatase at 250 °C, while under oxidizing conditions the structure was more stable, where the transition started at 400 °C. During annealing, the nanotubular structure releases hydroxyls and Ti4+ cations are self-reduced into Ti3+generating a non-stoichiometric anatase phase, which collapse the initial nanotubular morphology. The self-reduction is enhanced when annealing is carried out in inert atmosphere in comparison with that in air, producing highly oxygen-deficient anatase.