Nanoporous Anodic Aluminum Oxide or AAO is a self-organized material with honeycomb-like structure formed by high density arrays of uniform and parallel nanopores. The diameter of the nanopores can be as low as 5 nanometers and as high as several hundred nanometers, and length can be controlled from few tens of nanometers to few hundred micrometers. Nanoporous AAO is formed by electrochemical oxidation (anodization) of aluminum in liquid electrolytes in the conditions that balance the growth and the localized dissolution of aluminum oxide to form arrays of nanopores. In the absence of such dissolution, dense anodic aluminum oxide films are formed with limited thickness.
Anodizing aluminum has been widely used since early last century for corrosion protection and as decorative coatings. The porous nature of anodic alumina films has been discovered in the 1930s and further elaborated in the 1950s-1970s. Processes for producing anodic aluminum oxide membranes using chromic acid, sulfuric acid, oxalic acid, or phosphoric acid appear in a patent attributed to Alan W. Smith of the Boeing Company in 1974.
Starting in the late 1980s, due to its uniform nanostructure, AAO began to attract interest in the area of nanotechnology, in particular as a template for deposition of the uniform arrays of nanowires. Since several key publications on using AAO for bottom-up templated nanofabrication appeared by the mid-1990s, AAO became widely recognized and very popular platform for design and synthesis of high density arrays of nanostructures (nanowires, nanotubes) and functional nanocomposites.
AAO-based nanomaterials have a broad range of applications, from nanoelectronics and magnetic storage media to photonics and energy conversion to nanoporous substrates and nanotags for bioanalysis. The number of AAO-related publications in this area increased exponentially from 1990 to 2005, with over 75% of the papers focused on use of AAO in nanotechnology, and continues to grow rapidly.
The significance of AAO in science and technology is underpinned by that fact its structure and chemistry could be controllably engineered at the nanoscale over very large areas and in practical formats, enabling development of new materials and products with desired properties and functionality.