W. Lee and K. Nielsch
When an aluminum substrate is electrochemically oxidized under constant potential, the surface of aluminum will be covered by a nanoporous oxide layer. This process has been known as anodization and has intensively been utilized for various industrial applications, including the formation of electrically insulating layers, anti-corrosion coatings, and decorative coloration of metal surfaces. The (aluminum or surface finishing) industry employs two types of anodization processes for the formation of thick aluminum oxide (alumina) coatings: mild and hard. The latter, which is carried out at high voltage by using sulphuric acid, results in the rapid growth of thick porous oxide layer. These porous oxides film are normally even more disordered than those produced by mild anodization. This aspect is an important concern for the current nanotechnology research. However, so far in industry with fabrication facilities for bulk material this aspect has been completely ignored.
Our group is developing a new anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called “hard anodization (HA)” process that has widely been employed in industry since the 1960s for various industrial applications (e.g., surface finishing of aluminum cooking ware, automotive engineering, machinery, medical implantation, etc.) for high-speed fabrication of mechanically robust, very thick and low-porosity anodic alumina. In academic research, however, the HA process has been out of focus in the past four decades and was not applied for the development of nanostructured materials so far due to difficulties in controlling important structural parameters, such as pore size, interpore distance, and the aspect ratio of nanopores of the resulting alumina membranes.
Fig. 1: (a) SEM micrographs of the AAO specimens formed by mild anodization for 2 h (left column) and hard anodization for 2 h (right column of (a)). The arrangements of the pores for the respective samples are shown in the upper SEM micrographs. The thicknesses of the respective samples are indicated in the cross-sectional SEM micrographs (lower part of the left and right column). (b) Oblique view in cross-section of the anodic alumina membrane with modulated pore diameter. The inset shows a magnified view of the area marked by the white rectangle.
Recently we have found that by pre-anodizing aluminum surfaces, carefully controlling the surface heat evolution and appropriately selecting the anodizing voltage that a new self-ordering regime can be achieved that produces highly ordered porous templates that are thicker, produced 25-35 times faster and provide a new interpore size domain unavailable previously (see Fig. 1). Based on findings on the self-ordering behavior, we have realized perfectly ordered alumina membranes with ratios of pore depth to pore diameter larger than 1,000 of uniform nanopores with constant and periodically modulated diameters (see Fig. 1). These well-defined three-dimensional (3D) nanoarchitectures have the potential for a broad range of nanotechnology applications like 3D photonic crystals, meta-materials, microfluidics and for the template-based synthesis of multifunctional nanowires and nanotubes.