Alumina nanostructures prepared by two-step anodization process

Alumina nanostructures were obtained by two-step anodization process. The porous anodic aluminum oxide（AAO） membranes were anodized in oxalic acid, which was subsequently treated with chemical etching process with 1.0 mol/L sodium hydroxide solution, or mixed solution of phosphoric acid （6.0%） and chromic acid （1.8%）, respectively. Field emission scanning electron microscopy（FE-SEM） and transmission electron microscopy（TEM） were employed to character the morphology and structure of the obtained alumina nanostructure. It is found that alumina nanowires are generated in the acidic chemical etching solution, while nanotube structures are formed in the alkaline solution. The influence of acid and alkaline solution on alumina morphologies in the chemical etching process was discussed.