Voltage oscillations and morphology during the galvanostatic formation of self-organized TiO2 nanotubes

The galvanostatic formation of self-organized nanotubular TiO2 layers on titanium was investigated in 1 M (NH4)(2)SO4+0.5 wt % NH4F electrolytes. Under specific electrochemical conditions (anodization above a critical current density) large potential oscillations were observed. The electrochemical and morphological investigations show that these oscillatory phenomena are coupled with sequential growth of the TiO2 nanotube layer and a subsequent lift-off of these layers. The origin of the detachment process is related to the formation of disordered porous film underneath the first tubular layer at higher voltages. Under galvanostatic conditions that do not lead to lift-off, a nanotubular TiO2 film with high aspect ratio tubes of medium pore diameter 60 nm and thickness up to 1 mu m can be obtained.