Self-organized TiO2 nanotubes prepared in ammonium fluoride containing acetic acid electrolytes

In the present work we report that TiO2 nanotube layers can be formed by electrochemical anodization of titanium in a non-aqueous electrolyte (CH3COOH/NH4F). The morphology of resulting layers is strongly affected by the applied potential. At low potentials the layers consist of ordered nanotubes that have a diameter of approx. 20 nm and a length of a few 100 nm. In this case the individual tubes are connected with each other via bridging rings on the sidewall of the tubes. At higher potentials, assemblies of nanotubes in morphology resembling coral reefs are obtained - these arrangements of the tubes can be ascribed to local breakdown events on the layers. The individual tubes in the reef structure have a diameter of approx. 20 nm and a spacing of approx. 50 nm and are clearly separated from each other-the connecting rings on the sidewall have disappeared.