Fabrication of free‐standing TiO2 nanotube membranes with through‐hole morphology

In the present work, we show a simple and robust ex‐situ method to fabricate free‐standing membranes consisting of vertically oriented, both‐side‐open TiO₂ nanotube arrays. In this method, self‐organized TiO₂ nanotube membranes with different thickness ranging from seven to tens of micrometers could be easily separated from the metallic Ti substrate by applying a reverse‐bias voltage at the end of anodization. The stress developing at the TiO₂ nanotubes/Ti interface during H₂ gas generation facilitates the separation of the TiO₂ membranes. This procedure leads to an intact, free‐standing TiO₂ nanotube membrane with closed bottoms. After exposing the TiO₂ membrane to HF vapor, the barrier layer at the closed bottoms was etched away, and then a free‐standing TiO₂ membrane with through‐hole morphology was obtained. The bottom‐opening process is a slow barrier layer thinning process. Meanwhile, it is found that the mean diameter of tube bottoms monotonously decreases with the increasing the etching time.