Morphological effects of alternating voltage anodizing in the production of Tio₂ nanostructures

Titania nanostructures were grown by anodizing using titanium foils as both cathode and anode in an ethylene glycol solution containing 0.1% ammonium fluoride without any additional water. A square-pulse alternating voltage profile was applied during the anodizing: 20 V for 5 min followed by 80 V for 1 min successively during 90 min. A loose, opaque layer composed by titania nanotube array, 3 μm in length, 130 – 200 nm in diameter and wall thickness of 40 – 80 nm, was formed. The titania nanotube array grew perpendicular to the substrate and had evenly spaced rings, 200 nm, along the outside walls. After the removal of the opaque, loose layer, we found another layer composed by nanobowls corresponding to the footprints of the titania nanotubes in a barrier layer between them and the metallic substrate.