Scanning electron microscopy of heat treated TiO2 nanotubes arrays obtained by anodic oxidation

Scanning electron microscopy was used to investigate the anatase-rutile transformation of self-organized TiO2 nanotubes obtained on titanium foil by anodizing and subsequent heat treatment. The anodizing was carried out at 20V in an 1% v/v HF acid and ethylene glycol: water (50: 50) electrolyte at room temperature. The anodized samples were initially pre-heat treated at 450 degrees C for 4 hours to modify the amorphous structure of TiO2 nanotubes into anatase structure. Then, the samples were heated between 600 to 800 degrees C for different times, in order to promote the transformation to rutile structure. The formation of TiO2 nanotubes is evident by SEM images. Notably, when the samples are treated at high temperature, the formation of rutile crystals starts to become evident at the nanotubes located on the originally grain boundaries of the titanium. Thus, the anatase - rutile transformation has a close relationship with the microstructure of the titanium, more exactly with grain boundaries.