Fabrication of alumina with ordered tapered-nanopore nested in micro-bowl hierarchical structure by a combined anodization

We report that porous anodic alumina (PAA) with ordered tapered-nanopore nested in micro-bowl hierarchical structure can be facilely and controllably fabricated by a combined anodization process. It includes three main steps: I, stable anodization in citric acid to fabricate micro-bowl; II, mild anodization and chemically dissolving PAA to fabricate nanopit pattern; III, multi-step mild anodization and etching pore-widening to fabricate tapered-nanopore. Although the used anodization technology in each step is well established, the smart combination for realizing hierarchical structure is unreported but very effective. It is found that the original interpore distance of the micro-bowl obtained by stable anodization in citric acid can be enlarged by the followed mild anodization, which can be tuned from 0.88 μm to 1.3 μm. The structural parameters of tapered-nanopore can be tailored by changing the mild anodization conditions. Because of the curvature of the micro-bowl, the average number of the tapered-nanopore can be as high as 1.68 × 1015/cm2, which is about 1.5 times more than that (1.16 × 1015/cm2) on the flat surface. This method is simple and effective, which opens a door for boosting investigations into the physical and chemical properties of micro- and nanostructure made of metal or polymer materials as combining with nanoimprinting or electrochemical deposition.