Effect of Anodization Temperature on the Morphology and Structure of Porous Alumina Formed in Selenic Acid Electrolyte

The complex study of the influence of the H2SeO4 electrolyte temperature on the porous anodic aluminum oxide (AAO) composition and defects, morphological, luminescent properties is performed. The synthesis temperature increasing leads to decrease of AAO cell diameter from 85-115 nm to 38-58 nm (depending on the electrolyte concentration) and increases AAO walls etching that can even lead to AAO etching to individual fibers at 40°C. Selenium concentration in the samples formed in 0.5-1.5 M H2SeO4 at 5-40°C does not exceed 2 at.% and becomes undetectable at 40°C. It is established that the formation of the nanocrystalline phase Al2O3 in H2SeO4 electrolyte is observed at 40°C. The samples exhibit very weak photoluminescence. It was shown that in AAO formed in H2SeO4 there are 3 types of paramagnetic centers: F+ centers (NₛF=8.2·1015 g-1), newly discovered centers in which the unpaired electron belongs to an oxygen atom (NₛO=1017 g-1), and paramagnetic centers associated with selenate radicals (NₛS=6·1018 g-1). By comparison of the photoluminescence spectra and defect concentrations, it is assumed that the luminescent properties of AAO obtained in selenic acid are exclusively determined by F⁺ centers. The centers associated with other reaction products do not contribute to the AAO luminescent properties.