Fabrication of tapered, conical-shaped titania nanotubes

Using anodic oxidation with a time-dependent linearly varying anodization voltage, we have made films of tapered, conical-shaped titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10-23 V at rates varying from 2.0-0.43 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the titanium substrate during the initial stages of the anodization process enabled us to propose a mechanism of nanotube formation.