A facile method to crystallize amorphous anodized TiO₂ nanotubes at low temperature

Anodic growth of TiO₂ nanotubes has attracted intensive interests recently. However, the as-prepared TiO₂ nanotubes are usually amorphous and they generally need to be crystallized by sintering above 450 °C. Here, we report on a facile method to crystallize amorphous anodized TiO₂ nanotubes at a low temperature. We find that, simply by immersing them into hot water, the anodized TiO₂ nanotubes can be transformed from amorphous to crystalline state at a temperature as low as 92 °C. Results indicate that the hot water treatment might be a versatile strategy to crystallize amorphous anodized TiO₂ nanotubes at low temperature. Field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, UV-vis spectroscopy, and Brunauer-Emmett-Teller (BET) analysis via N₂ adsorption are used to characterize the resulting samples. In addition, the TiO₂ nanotubes in powder form are taken as photocatalysts to explore their potential applications. Results indicate that the sample after 35 h of hot water treatment shows the highest photoactivity, which is as efficient as the commercial photocatalyst Degussa P25. The photocatalytic testing results demonstrate that the hot water treatment reported in this study can be an alternative approach to the conventional methods.