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.