Flow-Regulated Growth of Titanium Dioxide (TiO ) Nanotubes in Microfluidics.

Electrochemical anodization of titanium (Ti) in a static, bulk condition is used widely to fabricate self-organized TiO nanotube arrays. Such bulk approaches, however, require extended anodization times to obtain long TiO nanotubes and produce only vertically aligned nanotubes. To date, it remains challenging to develop effective strategies to grow long TiO nanotubes in a short period of time, and to control the nanotube orientation. Here, it is shown that the anodic growth of TiO nanotubes is significantly enhanced (≈16-20 times faster) under flow conditions in microfluidics. Flow not only controls the diameter, length, and crystal orientations of TiO nanotubes, but also regulates the spatial distribution of nanotubes inside microfluidic devices. Strikingly, when a Ti thin film is deposited on silicon substrates and anodized in microfluidics, both vertically and horizontally aligned (relative to the bottom substrate) TiO nanotubes can be produced. The results demonstrate previously unidentified roles of flow in the regulation of growth of TiO nanotubes, and provide powerful approaches to effectively grow long, oriented TiO nanotubes, and construct hierarchical TiO nanotube arrays on silicon-based materials.