Effect of heat treatment on morphology, crystalline structure and photocatalysis properties of TiO2 nanotubes on Ti substrate and freestanding membrane

► TiO2 nanotubes are fabricated in a glycol solution by anodization. ► We examine morphology and crystalline changes of TiO2 nanotubes during annealing. ► The TiO2 nanotubes with or without the Ti substrate have different properties on morphology and crystalline changes. ► Models to explain the changes of TiO2 nanotubes are presented. ► Photocatalytic activity is examined for the TiO2 nanotubes on the Ti substrate.