Effects of Heat Treatment on the Changes in Morphology, Crystal Structure and Fluoride Concentration of TiO2 Nanotubes

The changes of morphology, crystal structure and fluoride concentration of TiO2 nanotubes were investigated systematically as a function of heat-treatment temperature and time using SEM, XRD and EDS. The TiO2 nanotubes were grown electrochemically on the Ti surface by anodizing in ethylene glycol containing 0.1 M NH4F + 2% H2O. Decomposition of the TiO2 nanotubes was found to begin first inside of the nanotubes into fine particles with sizes of 50 ~ 150 nm and then it occurred in the surface region. The particles were randomly distributed on the surface region while they were aligned into straight lines within the interior region of the oxide after heat-treatment for 3 h at 450 ~ 700 °C. The TiO2 nanotubes were transformed from amorphous into anatase crystal without destruction of the nanotublar structure after 10 h at 250 °C in air and anatase crystallization was also observed to occur with the formation of fine particles after 1 h at 450 °C. The fluorine concentration in TiO2 nanotubes was found to decrease linearly with log t, which was attributed to the removal of fluorine from the nanotubes by diffusion process during the heat-treatment in air.