On the growth sequence of highly ordered nanoporous anodic aluminium oxide

Anodic aluminium oxide films were fabricated by well known two-step anodizing process in oxalic acid electrolyte. The ordering characteristics (ordered pore domains, average pore diameter size and through-pore arrangement) of anodic aluminium oxide films, obtained in different growth sequences, were identified by microscopic analysis such as ex situ contact-mode atomic force microcopy and scanning electron microscopy. Flattened areas in which some pits are seen mostly cover the electropolished surface of aluminium. Single anodizing of aluminium produces a broad distribution of nanopore size, whereas induces a highly ordered hemispherical pattern, which plays the ordered nucleation sites for the second anodizing step. Moreover, a quasi-linear growth behavior exists for the ordered domain growth versus the duration of first step anodizing. The through-pore arrangement of ideally grown membranes is not influenced by increasing the duration of second step anodizing.