It has been reported that self-organized nanotubular oxide layers are formed on various metals and alloys by anodization in fluoride containing electrolytes. In particular, the nanotubular structure of TiO₂ has been extensively studied due to a wide range of application of TiO₂. To tune the properties of the layers, the optimization of anodization condition has been carried out. However, in such trials, the chemical aspects of anodization condition have been mainly focused, that is, only few works has reported the metallurgical aspects of the condition. In the present work, we examined the effects of heat-treatment on anodization behavior and resulting morphology of the TiO₂ architecture.

Specimens were cut from pure titanium sheets (purity; 99.5 %). The specimens were encapsulated into a silica quartz tube where the vacuum level was controlled less than approximately 2x10^-5 torr. The specimens in the silica tube were heat-treated under several conditions. After the heat-treatments, the surface of the specimens was ground with SiC abrasive papers and then mirror-finished with diamond pates and colloidal silica suspension. The specimens were anodized at 50 V in a mixture of ethylene glycol, water and ammonium fluoride. After the anodization, the surface and cross-sectional morphology of resulting oxide layers was observed.

Under an optimized heat-treatment condition examined, the specimen consisting of large grains in millimeter range was obtained. The anodization of the specimen revealed that its current behavior was similar to that obtained on as-received specimen. In addition, the obtained structure of anodic oxide layer on the heat-treated specimen was also similar to that on as-received specimen, that is, nanotubular structure was obtained. However, the thickness of the nanotubular oxide layer and the ordering of nanotubes were clearly different. Higher thickness of nanotubular oxide layer was obtained and improved ordering of nanotubes was found on the heat-treated specimen. These results indicate that the substrate property could affect not only the growth but also arrangement of nanotubes.