Microstructural Models of Alumina Nanotubes and Anodic Porous Alumina Film Formed in Sulfuric Acid

Electrochemical stepwise-anodization of aluminum in dilute sulfuric acid results in the formation of alumina nanotubes (ANTs) which is due to hexagonal split of anodic porous alumina (APA) film along the cell boundaries containing many voids, that is, the ANTs are the completely detached cell of the APA film. The inner diameters of the ANTs are in the range of 10-20nm, and the aspect ratio (inner diameter/length) of the ANTs can be about 80. The relations found for pore diameter, cell diameter and barrier layer thickness are around 1, 2.7 and 0.85 nm/V, respectively. Transmission electron microscope (TEM) reveals that the ANT wall has a three-shell structure: an outer shell (metal/oxide interface) consisting of pure alumina oxide, a middle shell of the hydrated oxide or/and hydroxide and an inner shell (oxide/electrolyte interface) of anion incorporated oxide with the thickness ratio of 1:1:2. The structural change of ANTs induced by e-beam irradiation in TEM indicates that the thermal instability of the hydrated oxide or/and hydroxide within the cell wall might be an alternative origin contributing to the self-organization of the cells, leading to a densely packed triangular cell lattice of the APA film.