Influence of heat treatment on morphological changes of nano-structured titanium oxide formed by anodic oxidation of titanium in acidic fluoride solution

TiO₂ nanotube array (TN) on titanium plate was fabricated by using an electrochemical method. The crystal structure and surface morphology of TN array was examined by X-ray diffraction (XRD) and Field Emission Scanning Electronic Microscopy (FE-SEM), respectively. The stability of the nanotube structure and crystal phase transition was studied at different temperatures in dry oxygen ambient. The as-deposited films were found to be amorphous. The tubes crystallized in the anatase phase at a temperature of 450°C. Anatase crystallites formed inside the tubes walls was transformed completely to rutile at 500°C in dry environment. With the heating temperature increased the intensity of rutile peak increased with decrease in reflection from titanium. Intense rutile peak was observed at 600°C. The average pore diameter as calculated from FE-SEM images was 50-100 nm. At higher temperature tubular structure completely collapsed leaving dense rutile crystallites. A model was proposed to explain the formation mechanism of TN fabricated on titanium plate in HF/H₂SO₄ electrolyte.