Nanostructured Anodic Oxides : fabrication , application & future trends 2015

Numerous metals are subjected to the anodic anodic oxidation. As a results one can obtain amorphous barrier-type oxide, crystalline barrier-type oxide or amorphous nanoporous oxide. Two decades ago Masuda and Fukuda published results on self-organized two step anodization allowing to form highly-ordered nanoporous oxides. It was a milestone. Currently highly-ordered nanoporous anodic aluminum oxide is obtained with various electrolytes to form nanostructures with a wide range of geometrical features. It allows applications for this material as a template for nanofabrication of variety of nanowires, nanotubes and nanodots. Also other anodic oxides attract attention of the researchers. For example, anodic titania is currently applied as a key material in Dye Sensitized Solar Cells (DSSC). Anodic titania was found to be also an efficient catalyst for water splitting and carbon dioxide removal. Additional, anodic zirconia nanotubes serve as efficient support for Surface Enhanced Raman Spectroscopy. Introduction Two decades ago Masuda and Fukuda published in Science their milestone paper about twostep self-organized anodization [1]. Generally, anodic oxidation allows the formation of hard, compact films on the surface on valve metals. Before 1995 it was also well-known that within a certain pH range, anodizing in certain electrolytes enables the formation of porous layers on aluminum. However, publication of Masuda’s and Fukuda’s results allowed nanotechnology to bloom: their findings allowed to form hexagonally arranged, well-ordered, nanoporous anodic alumina with a high degree of ordering and uniformity of pore diameter. Since then anodic aluminum oxide has become one of the most frequently applied templates for nanofabrication. Contribution to many fields of science and technology is acknowledged to the nanowires, nanotubes and nanodots made of metals, oxides, salts and polymers, formed from nanoporous anodic aluminum oxide. Fabrication of the nanoporous anodic aluminum oxide The general principle of two-step self-organized anodization is as follows: at the first step of anodization, a poorly ordered oxide is formed; next, the poorly formed oxide is removed by chemical etching – on the aluminum surface a highly-ordered concave hexagonal array is obtained; reanodization conducted at the same set of operating conditions enables the formation of highly-ordered anodic aluminum oxide (AAO) [1]. Typically, for fabrication of well-ordered AAO, three electrolytes are most frequently used: sulfuric acid, oxalic acid and phosphoric acid, with voltages ranging from 15-25, 20-100 and 120-195 V, respectively (Fig. 1). Much fundamental research was conducted to investigate the influence of operating conditions like applied voltage, temperature of the electrolyte and duration of each anodizing step on the morphology of the anodic oxides (pore diameter, interpore distance, porosity, oxide layer thickness) and their arrangement, which is mainly investigated with the use of advanced software employing fast Fourier transform. The major findings of the fundamental research are [2-6]:  Pore diameter of AAO increases linearly with anodizing voltage from 12 nm up to microns  Interpore distance increases linearly with the anodizing voltage from 35 nm up to microns  The greater temperature of the electrolyte, the greater the pore diameter; there is no temperature effect on the interpore distance  The longer the second step of anodization, the greater the pore diameter at fixed interpore distance  The greater anodizing voltage the greater current density and as a consequence, the greater the oxide growth rate; current density and the oxide growth rate increase exponentially with the