Nanopore Architectures in Anodic Aluminum Oxide: Effects of Anodization Voltage and Time on Planar and Non-Planar Aluminum Substrates

This research investigates the effect of voltage changes and anodization duration on the nanopore architecture of anodic aluminum oxide (AAO) across various aluminum (Al) substrates, including both planar and non-planar forms. After a two-step anodization, planar Al substrates developed a porous layer limited to their flat surface. In contrast, the Al wire, due to its curved shape, experienced an intensified local electric field, resulting in thicker oxide layers around its circumference. The most significant effect was observed in hollow Al tubes, where nanoporous layers formed simultaneously on both the inner and outer surfaces. This dual-surface anodization significantly increased the effective surface area and produced the thickest oxide layers among all tested substrates. Field-emission scanning electron microscopy was used to analyze the morphology of the AAO. The findings indicated a direct relationship between the applied voltage and the diameter of AAO pores, with pore sizes increasing from 30.0 to 150.0 nm for planar substrates and from 30.0 to 220.0 nm for non-planar substrates as the voltage increased from 40 V to 100 V. AAO thickness ranged from 12.7 to 47.0 µm for planar substrates and from 14.0 to 60.0 µm for non-planar substrates. The surface structure of the Al substrates also influenced the distribution of AAO pore diameters. The dual-layer AAO on Al tubes exhibited larger pores and greater interpore distances, which can be attributed to differences in oxide growth direction and electrochemical field distribution. These findings offer valuable guidance for the engineering of non-planar AAO materials for diverse applications.