Highly ordered titanium oxide (TiO2) nanotubes were prepared by electrolytic anodization of titanium electrodes. Morphological evolution and phase transformations of TiO2 nanotubes on a Ti substrate and that of freestanding TiO2 membranes during the calcinations process were studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction microscopy. The detailed results and mechanisms on the morphology and crystalline structure were presented. Our results show that a compact layer exists between the tubular layer and Ti substrate at 600 degrees C, and the length of the nanotubes shortens dramatically at 750 degrees C. The freestanding membranes have many particles on their tubes during calcinations from 450 to 900 degrees C. The TiO2 nanotubes on the Ti substrate transform to rutile crystals at 600 degrees C, while the freestanding TiO2 membranes retain an anatase crystal with increasing temperature to 800 degrees C. The photocatalytic activity of TiO2 nanotubes on a Ti substrate annealed at different temperatures was investigated by the degradation of methyl orange in aqueous solution under UV light irradiation. Due to the anatase crystals in the tubular layer and rutile crystals in the compact layer, TiO2 nanotubes annealed at 450 degrees C with pure anatase crystals have a better photocatalytic activity than those annealed at 600 degrees C or 750 degrees C. (C) 2011 Elsevier B.V. All rights reserved.