Preparation of freestanding tubular alumina through-hole membranes by two-layer anodization

Tubular ordered alumina through-hole membranes were prepared from a tubular Al substrate by two-layer anodization using concentrated H2SO4. Alumina membranes that formed on the inner surface of the tubular Al substrate in oxalic acid were detached by selective dissolution of the bottom part of the anodic alumina formed in concentrated H2SO4. The most important feature of this process is that tubular alumina membranes with an ordered hole arrangement can be prepared repeatedly from a single Al substrate by repeating this process. The geometrical structures including the hole interval, hole size, and thickness can be controlled by adjusting the anodizing conditions.