Controllable wet etching of porous anodic alumina toward highly ordered hierarchical interfaces

Anisotropic etching is a powerful technique for fabrication of micro/nanostructures. However, methods used to perform anisotropic etching, are very limited and costly. Wet chemical etching is a cost-effective and convenient method but it generally forms isotropic features. Here, the anisotropic wet etching of porous anodic alumina (PAA) using phosphoric acid to controllably form various structures is presented. Using photo-patterned SU-8 as protective layers and 5 wt% phosphoric acid as etchants at 60 °C, the exposed areas of PAA are vertically etched along the direction of inherent nanopores, while the areas coated with SU-8 are protected from etching. Arbitrary alumina micropillar arrays are achieved and the length of micropillar is determined by the thickness of the PAA films. In addition to cylindrical pillar arrays, sharp and concentric cone-shaped pillar arrays are obtained upon fine-tuning the etching conditions. The as-prepared super-hydrophilic alumina substrates with considerable surface roughness can be transformed into super-hydrophobic surfaces of 150° water contact angle. Hole arrays have also been fabricated using a reversed pattern protective layer before etching. These substrates are further used as templates to replicate various structures of polymers and thus provide a facile approach to fabricate hierarchical structures for a range of applications.