Structure of alumina nanowires synthesized by chemical etching of anodic alumina membrane

We report the structure of alumina nanowires (ANWs) synthesized by the chemical etching of an anodic alumina membrane (AAM). The AAM was prepared by the two-step anodization method followed by the lift-off process. The field emission transmission electron microscopy analyses showed that the AAM consists of a gel-like structure, an outer layer, and an inner layer, which are distinguished by their anion impurity concentration. The fabricated ANWs appeared to have a two-oxide-layer structure, similar to the core-shell structure in a coaxial cable. The inner oxide layer may be composed of relatively pure alumina and the outer oxide layer of the byproducts of the reaction between alumina and the anodization or etching solution. The fabricated ANWs have a flexible nature, with some of them being sufficiently malleable to form L-shaped ANWs. One possible formation process of the two-oxide-layer structured ANW is discussed.