Flow-Regulated Growth of Titanium Dioxide (TiO2 ) Nanotubes in Microfluidics.

Electrochemical anodization of titanium (Ti) in a static, bulk condition is used widely to fabricate self-organized TiO2 nanotube arrays. Such bulk approaches, however, require extended anodization times to obtain long TiO2 nanotubes and produce only vertically aligned nanotubes. To date, it remains challenging to develop effective strategies to grow long TiO2 nanotubes in a short period of time, and to control the nanotube orientation. Here, it is shown that the anodic growth of TiO2 nanotubes is significantly enhanced (≈16-20 times faster) under flow conditions in microfluidics. Flow not only controls the diameter, length, and crystal orientations of TiO2 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) TiO2 nanotubes can be produced. The results demonstrate previously unidentified roles of flow in the regulation of growth of TiO2 nanotubes, and provide powerful approaches to effectively grow long, oriented TiO2 nanotubes, and construct hierarchical TiO2 nanotube arrays on silicon-based materials.