Optimum self-ordered nanopore arrays with 130-270nm interpore distances formed by hard anodization in sulfuric/oxalic acid mixtures

A hard anodization (HA) technique was employed using different mixtures of sulfuric/oxalic acid for fabrication of highly self-ordered alumina nanopore arrays. The sulfuric acid concentration was changed from 0.005 to 0.05 M and the anodization voltage was changed from 60 to 140 V. An optimum self-ordering was achieved in a wide range of interpore distances from 130 to 270 nm. For each mixture, there was a region of anodization voltage in which the self- ordering took place. The width of this region decreases with increasing sulfuric acid concentration. In these regions, the optimum voltages with the best self-ordering versus the sulfuric acid concentration follow a second order exponential decay function. Our investigations show that current-time characteristics at optimum anodization voltages were identical for all mixtures. For each electrolyte mixture, the interpore distance was dependent upon the anodization voltages with proportionality constants of 2.1 nm V-1 and 2.2 nm V-1 for high and low voltages, respectively. The porosity of the samples (about 3.5%) follows the porosity rule of HA.