The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidation

We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose, N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 mu m, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures >= 280 degrees C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 degrees C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 degrees C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.