Formation process of dielectric alumina nanowires: Three critical steps depending on etching rates of oxide layers in anodic aluminum oxide membrane

Various diameters of core-shell structured alumina nanowires (ANWs) are produced by a dilute NaOH droplet etching method from highly ordered anodic aluminum oxide (AAO) membrane. Field emission scanning electron microscope observation reveals that ANWs formed from AAO membrane, regardless of their diameter, follow three critical steps depending on the etching rates of oxide layers in AAO membrane. The highly ordered AAO membrane is prepared by the two-step anodization method, followed by the subsequent lift-off process. Field-emission transmission electron microscope study shows that the oxide layers in the membrane are composed of gel-like structured layer, outer layer, and inner layer, which are categorized by the concentration of anion impurities that entered during the anodization process. At the first step, the gel-like structured layer is removed as soon as a dilute NaOH solution is introduced. The second step is to manifest an image of triangular triple points, surrounded by three adjacent pores. Lastly, the triangular-cross-section ANWs become circular-cross-section ones as further etching occurs, with relatively pure alumina as a core and the byproducts of reaction between alumina and aqueous NaOH solution as a shell.