Crystallization and high-temperature structural stability of titanium oxide nanotube arrays

The stability of titanium oxide nanotube arrays at elevated temperatures was studied in dry oxygen as well as dry and humid argon environments. The tubes crystallized in the anatase phase at a temperature of about 280 degreesC irrespective of the ambient. Anatase crystallites formed inside the tube walls and transformed completely to rutile at about 620 degreesC in dry environments and 570 degreesC in humid argon. No discernible changes in the dimensions of the tubes were found when the heat treatment was performed in oxygen. However, variations of 10% and 20% in average inner diameter and wall thickness, respectively, were observed when annealing in a dry argon atmosphere at 580 degreesC for 3 h. Pore shrinkage was even more pronounced in humid argon environments. In all cases the nanotube architecture was found to be stable up to approximately 580 degreesC, above which oxidation and grain growth in the titanium support disrupted the overlying nanotube array.