Super-fast fabrication of self-ordered nanoporous anodic alumina membranes by ultra-hard anodization

Abstract Although the fast fabrication of long-range ordered porous anodic alumina (PAA) membranes has already been realized by hard anodization, achieving super-fast oxide growth rates has been still very challenging. Herein, we report a new generation of the anodization process, called ultra-hard anodization (UHA), in which the mean growth rate of self-ordered PAA reached 2900 µm/h in the first 80 s of the process, resulting in a membrane thickness of 58 µm. By comparison, the same thickness can be obtained using the conventional hard anodization and mild anodization processes in 2400 s and 10 h, respectively. While the current density (J) reaches a high value of 2400 mA/cm2, burning and dielectric breakdown phenomena are avoided by controlling the barrier layer temperature and the diffusion length. In addition, the mechanism of oxide growth is explained on the basis of the high field theory and the diffusion-limited path of ionic species through the nanopores, taking into account the effect of temperature and sprinkling rate of electrolyte on J. By blending 0.3 M oxalic acid with phosphoric or sulfuric acid separately, the UHA process is also performed at target voltages ranging from 85 to 150 V, thereby allowing for a wide range of interpore distances. From a practical point of view, we synthesize high aspect ratio polymer nanowires (up to 320 µm) in the resulting membranes. Our findings open a new approach for the super-fast fabrication of the self-ordered PAA, facilitating the development of nanodevices and their commercial applications.