Morphological and chemical characterization of highly ordered conical-pore anodic alumina prepared by multistep citric acid anodizing and chemical etching process

Highly ordered, conical-pore anodic alumina (AAO) membranes with interpore distance (D c ) between ca. 530 and 620 nm and thickness ranging between 2.4 and 7.8 μm, were produced. In the fabrication process aluminum surface was first pre-patterned by the anodization in etidronic acid solution. Then, the regular arrays of Al concaves were used as nucleation sites to grow AAO during the second anodization, which was carried out in highly concentrated citric acid solution (20 wt%) and at relatively high temperature (33–35 °C). The conical pore shape was engineered by a multistep process combining anodization in the citric acid electrolyte and the subsequent chemical pore broadening in phosphoric acid solution. The morphological analyses has revealed that the geometrical parameters of the Al concaves were successfully transferred to the AAO membranes. Furthermore, FTIR spectra analysis confirmed that the electrolyte species, such as phosphonate and citric ions, are being embedded into the AAO framework during the anodization. The graded-index structure formed in AAO can be used for a production of antireflective coatings operating in a broad spectral range.