Freestanding ZrO2 nanotube membranes made by anodic oxidation and effect of heat treatment on their morphology and crystalline structure

An integrated freestanding zirconia (ZrO2) membrane was fabricated via a two-step electrochemical anodization with a zirconium (Zr) foil, followed by a simple and safe detachment of the formed ZrO2 membrane. The resulting membrane consists of highly ordered, vertically aligned, and one-side open ZrO2 nanotube arrays. The effect of heat treatment on the morphology and crystalline structure of the integrated freestanding ZrO2 membranes was studied using scanning electron microscopy, transmission electron microscopy and X-ray diffraction. These results showed that the nanotubes were stable to annealing at 500 °C and then gradually transformed to wires along their long axis from 600 to 900 °C. The as-prepared samples possessed amorphous structures, while both monoclinic and tetragonal phases were found after annealing at a temperature between 400 °C and 900 °C. The present study not only demonstrates a facile approach to produce high quality ZrO2 membranes in a simple manner, but also provides valuable insights into the temperature dependent morphology and crystalline transformations in anodic ZrO2 nanotube arrays.