Rapid and tunable growth of a well-ordered hexagonal nanoporous anodic aluminum oxide (AAO) structure by two-step high-temperature anodization

In this study, we have developed a swift and well-ordered growth of a nanoporous anodic aluminum oxide (AAO) structure using two-step high temperature anodization of the pure aluminum substrate. The pre-anodization surface treatment of the aluminum substrate assisted in the formation of well-organized nanoporous structures. The two-step anodization process was performed in 0.3 mol/L of oxalic acid at 20 °C with 40 and 45 V to obtain tunable pore diameters. The high temperature of the electrolyte solution facilitated the rapid growth of the nanoporous AAO structure. The top surface image of AAO shows a well-ordered nanoporous structure with an average pore diameter of 70 nm at 40 V and 100 nm at 45 V. The SEM cross-sectional view also illustrates the well-ordered nano-channel and the elemental mapping elaborates the presence of aluminum and oxygen. The thickness of the nanoporous AAO structure was determined using SEM for three anodization time spans (20, 24, and 28 h), in which an increasing trend was observed. The fabricated AAO has a higher thickness and a well-ordered nanoporous structure, which shows that it can be used as a template for fabricating nanostructured materials.