ABSTRACT
Although anodic alumina membranes (AAMs) were first reported 50 years ago [1], the material has attracted increasing attention from scientists in the fields of materials science, electrochemistry, nanomaterials, and nanotechnology in recent years [2]. The hexagonally arranged nanoscale pores are often used as a template for fabrication of many of the nanostructured materials such as nanowires, nanotubes, and nanodots fabricated by deposition of various substrates, including metals, semiconductors, oxides, and polymers, into the pores of alumina membranes [3]. In general, two different methods of fabrication of AAMs can be distinguished, (1) prepatterned guided anodization [4] and (2) selforganized anodization [5]. The anodization of imprinted aluminum
leads to an ideal hexagonal, square, and triangle arrangement of pores in the final structure, and the size of the structure is determined by dimensions of the master mold/stamp used for aluminum patterning [4]. On the other hand, a self-organized process of aluminum anodization is usually performed in a sulfuric, oxalic, or phosphoric acid solution [5]. And the two-step self-organized anodization of aluminum results in the formation of hexagonal arrangement of pores. The optimized anodizing conditions result in a layer of selfordered honeycomb arrays of uniformly sized parallel channels with a cylindrical shape closed at the pore bottoms. The anodization carried out in electrolytes containing phosphoric acid leads to AAMs with the highest pore diameters and interpore distances. Depending on the used electrolyte and applied potential, anodizing results in AAMs with a pore diameter ranging from about 10 nm to over 200 nm [5]. A lattice constant of the AAMs, being an interpore distance of the ordered structure, can vary from about 35 nm to 500 nm. The depth of nanoporous channels of AAMs can be easily adjusted between a few and hundreds of microns by varying anodizing time. This part reviews current research progresses that center on the formation of one-dimensional (1D) nanostructures of AAMs. The main contents of this article are organized into three sections. The formation mechanism of AAMs is briefly introduced in the first section. The second section demonstrates the new results in regard of the different morphologies of AAMs. The final section draws the conclusions and presents the authors’ personal views of the future research directions. 4.2 Formation Mechanism of AAMs
The most popular mechanism for the self-adjustment of pores in AAMs is believed to be based on mechanical stress. By varying the applied voltage, thus varying the current efficiency for oxide formation and the volume expansion, Jessensky et al. found that optimal conditions for the growth of ordered pore structures were accompanied by a moderate expansion of the aluminum under the optimal applied voltage [6]. They suggested that mechanical stress associated with the expansion of aluminum during oxide formation was the cause of repulsive forces between neighboring pores during the oxidation process, and this was the driving force for the self-
ordering process. Neither too large nor too small an expansion would result in long-range ordering of the pores. Patermarakis and coworkers presented results from investigations of the growth kinetics of porous anodic alumina films formed using sulfuric acid as the anodizing electrolyte [7]. In these studies, different kinetic models based on Faraday’s law were developed, which describe the growth of these films under galvanostatic conditions. Treatment of the experimental data using these models provided information about the thickness, pore density, and porosity of the oxide films and the dependence of these parameters on anodization process variables (electrolyte type and concentration, current density, temperature, and time of anodization). Recently, Su et al. proposed an equifield strength model to explain pore generation, self-adjustment of pore size, and ordering of AAMs [8]. They also found that the relative dissolution rate of water during the anodization is very important in the determination of the porosity [8]. In their work, they applied the equifield strength model to explain all the experimental observations during the formation of the porous AAMs, including pore merging and splitting, and elucidated that the relationship of the relative dissolution rate of water during the anodization and the porosity can be used not only for the ordered pore arrays but also for disordered pores in AAM.
