The effect of pH and composition of sulfuric-oxalic acid mixture on the self-ordering configuration of high porosity alumina nanohole arrays

High-density structures with well-ordered nanohole arrays have been obtained by the self-ordering growth of nanopores using 25-40V anodization voltages in a sulfuric-oxalic acid mixture. In each anodization voltage, two types of electrolyte mixture were used with low and high sulfuric concentrations at a constant pH value. The ordering and regularity of a nanohole array was quantified by analysis of fast Fourier transformations of scanning electron and atomic force microscope images. The results show that the long range ordering of nanohole array is superior for high sulfuric concentration, while regularity is more pronounced for low sulfuric concentration. The interpore distance in the oxalic-sulfuric mixture is higher in the case of low sulfuric concentration. It is almost proportional to 2.6 nm V-1 and 2.5 nm V-1 for the electrolyte mixture with low and high sulfuric concentrations, respectively. While the porosity is nearly 10% for samples made in standard sulfuric and oxalic acid using 25V and 40V anodization voltage, respectively, it increases up to 50% for the sample made with 37.5V in an oxalic-sulfuric mixture.