The formation of highly ordered TiO2 nanotubes array by hard anodization

Abstract

The regularly ordered valve metal oxide nanotubes can be formed by our well-established electrochemical anodization processes. Among them, the regular arrays of TiO2 nanotubes are promising candidates for solar energy conversion as a photoelectrochemical cell. Numerous researchers and technicians have made efforts to increase the growth rate of titania nanotubes by anodization in terms of the control of various parameters. A high applied voltage favors fast nanoporous TiO2 films but it produces less regular TiO2 nanotubes. It is not clearly understood how the applied voltage and the electrolyte components have influences on the formation and growth of TiO2 nanotubes. In this study, we investigated the relation between an applied voltage and electrolyte condition. The ratio of the molar concentration of ammonium fluoride to water was systematically varied with an applied voltage in order to prevent burning or breakdown and thus to achieve the fabrication of regular nanotubes array even at an high voltage such as 300 V. The high electric field strength results in the fast growth (1-1.6μm/min) of the array of nanotubes on the surface of the titanium. With an increasing anodization temperature, the thickness of the nanotube walls gets increased due to the acceleration of the oxidation of Ti in the oxide/metal interface. Under the hot electrolyte (56℃) condition, a hexagonal closed packed array of nanotubes were revealed partially on the top of the surface region. Another effort is currently underway to form such a regular and uniform array of zirconium oxide nanotubes.