TiO₂ nanotubes: Efficient suppression of top etching during anodic growth: Key to improved high aspect ratio geometries

We report on an approach to obtain anodic self-organized TiO₂ nanotube layers with a significantly improved tube morphology. Before the anodic tube growth, a rutile layer is formed on the Ti substrate by high temperature oxidation in air. This layer protects the tube tops during anodization against chemical etching in the fluoride-containing electrolyte. As a result, much longer (higher aspect ratio) nanotubes can be grown that still have an intact tube top (i.e., the formation of undesired etching-induced "nanograss" and "nanospikes" is suppressed). This technique is particularly significant for applications that require open tubes and coherent tube walls.