Nanopore architectures in anodic aluminum oxide: effects of anodization voltage and time on planar and non-planar aluminum substrates

This research investigates the effects of anodization voltage and duration on the nanopore architecture of anodic aluminum oxide (AAO) formed on various aluminum (Al) substrates, encompassing both planar and non-planar geometries. Following a two-step anodization process, planar Al substrates developed a porous layer confined to the flat surface. In contrast, Al wire, owing to their curved geometry, experienced an enhanced local electric field, resulting in thicker oxide layers distributed around the circumference. The most pronounced 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 substrates studied. Field-emission scanning electron microscopy was employed to characterize the AAO morphology. The results revealed a direct correlation between the applied voltage and AAO pore diameter, 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. The 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. Additionally, the surface geometry of the Al substrates influenced the distribution of AAO pore diameters. The dual-layer AAO on Al tubes exhibited larger pores and greater interpore distances, which are attributed to differences in oxide growth direction and electrochemical field distribution. These findings provide valuable insights for the design and engineering of non-planar AAO materials for a wide range of applications.