AAO-assisted synthesis of highly ordered, large-scale TiO₂ nanowire arrays via sputtering and atomic layer deposition

Highly ordered nanoporous anodic aluminum oxide (AAO) thin films were fabricated in oxalic acid under a constant voltage via a two-step anodization process. To investigate the high-aspect-ratio (7.5:1) filling process, both sputtering and atomic layer deposition (ALD) were used to form TiO₂ nanowires. Field emission scanning electron microscopy and high-resolution transmission electron microscopy images indicated that mushroom-like TiO₂ structures were sputtered onto the AAO template surface, and the ALD-coated TiO₂ exhibited fine filling results and clear crystal grain boundaries. Large-scale and free-standing TiO₂ nanowire arrays were liberated by selectively removing the aluminum substrate and AAO template via a wet etching process with no collapsing or agglomeration after the drying process. ALD-deposited TiO₂ nanowire arrays that were 67 nm in diameter and 400 nm high were transferred from the AAO template. The ALD process enabled the rapid, simple synthesis of highly ordered TiO₂ nanowire arrays with desired parameters such as diameter, density, and thickness determined using diverse AAO templates.