Controllable current oscillation and pore morphology evolution in the anodic growth of TiO₂ nanotubes

The spatial heterogeneities and temporal instabilities in the anodic growth of TiO₂ tubes are very important for nanostructure fabrication, but few ordered cases have been reported. In this work, we represent a new current oscillation with pore morphology evolution in the formation of anodic TiO₂ tubes. Small (less than 8.0% of the minimum value) and fast (period ~10⁰ s) current oscillation was formed under static conditions in a wide range, while significant morphological change such as periodical narrowing, abruption and small pits appeared in the pore with characteristic length scale of 10¹-10² nm. Surprisingly, the roughness in the pore would be totally eliminated instead of being enhanced by high speed stirring or periodically modulated voltage with the current oscillation still being enhanced, which indicates an important involvement of the ion transport process. It has also been found that the growth rate could be significantly accelerated by tuning the stirring rate or the periodical modulation of the voltage. The mechanism has been described with consideration of the local reactions and the ion transport with a key involvement of the convection process, which can be strongly influenced by the mechanical stirring and the modulated voltage.