Preparation of self-ordered nanoporous anodic aluminum oxide membranes by combination of hard anodization and mild anodization

Anodic aluminum oxide (AAO) is one of the most important templates for fabrication of nano-materials. In this paper, we present a fast two-step anodization method to prepare self-ordered AAO films. The first step is hard anodization (HA) in H2SO4 and the second step is mild anodization (MA) in H2C2O4. HA in H2SO4 at a wide range of anodization voltage provided a fast formation of ordered arrays of pits on Al sheet, and in the second step, arrays of pits on Al substrate affected nucleation initially, and the anodization voltage of H2C2O4 MA determined the degree of AAO later. Then the best anodization voltage of H2C2O4 MA was validated to be around 40 V. The improved combination of HA and MA not only shortens the time of the first anodization step compared to conventional two-step anodization which was reported by Masuda and Fukuda (1995), but also keeps a highly ordered AAO within large area. Moreover, the relationship between the size of arrays of pits formed in H2SO4 HA and degree of order of AAO formed in H2C2O4 MA was investigated in detail.