Controlled Interconversion of Nanoarray of Ta Dimples and High Aspect Ratio Ta Oxide Nanotubes

We report the controlled formation of either high-aspect-ratio Ta2O5 nanotubes; or an organized nanoarray of Ta dimples by Ta anodization in a single H2SO4 + HF solution. Dimpled Ta is the stable surface morphology in the first few seconds, followed by the growth of dense and fully vertically aligned Ta2O5 nanotubes (up to 2.5 mu m long). After 2 min, the dimpled surface morphology reappears, related to the build-up of a resistive Ta fluoride surface layer.