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

Abstract

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.