Growth Mechanism of Self-Assembled TixWyO Nanotubes Fabricated by TiW Alloy Anodization

Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process with an aid of anodic aluminum oxide (AAO) for several decades. However, tubular nanostructures have not been reported by using an AAO-assistant anodization technology until now. In this study, we successfully synthesized self-organized TixWyO nanotubes with cap structures for the first time by using a two-step anodization process from Al-coated TiW alloy layers. The TixWyO nanotubes were observed only in the shaped pores of AAO with large enough pore size, and otherwise the nanostructures would become solid rods due to the aggregation of TixWyO. The wall of nanotubes was similar to 30% inside the pores and the other similar to 70% penetrated into the AAO template. Furthermore, no tubular nanostructures were found unless the percentage of W element in TiW alloys is larger than 50%, attributing to the formation of a void by oxygen bubbles arising from the oxidation reaction of tungsten metal. Besides, the height of the nanotubes is limited by an electric field of 0.82 GV/m due to the dielectric breakdown effect. Finally, a bottom-up growth mechanism at the oxide/metal interface is proposed, based on the observation of the cap structure on the top of the tubular architecture. (C) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License.