Synthesis of TiO₂nanotubes: effect of post-treatment on crystallinity and photocatalytic activity

Photocatalytically active TiO 2 nanotubes (TNs) were synthesized by anodizing a Ti foil. The morphology and crystal structure of the TNs were examined by scanning electron microscopy, x-ray diffraction (XRD), and x-ray absorption spectroscopy (XAS). It was revealed that the crystal structure of as-grown TNs was based on the anatase structure. The crystallinity of as-grown TNs was low but increased after annealing below the anatase-rutile transition temperature. The photocatalytic activity of the TNs was evaluated by the photocatalytic degradation of methylene blue aqueous solution. The as-grown TNs showed little photocatalytic activity owing to fluorine residue on the surfaces and their low crystallinity. However, rinsing and annealing post-treatment greatly enhanced TN activity by removing fluorine residue and increasing crystallinity. TNs annealed at 450 °C showed the highest photocatalytic activity due to their increased crystallinity and anatase-rich phase composition.