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

We report a comprehensive study on the effect of H2SeO4 electrolyte temperature on the composition, defect, morphological, and luminescent properties of porous anodic aluminum oxide (AAO). An increase in the synthesis temperature led to a decrease in the AAO cell diameter from 85–115 nm to 38–58 nm (depending on the electrolyte concentration) and enhanced the etching of the AAO walls, which even resulted in the disintegration of the AAO into individual fibers at 40 °C. The selenium concentration in the samples formed in 0.5–1.5 M H2SeO4 in the temperature range of 5–40 °C did not exceed 2 at.% and fell below the detection limit at 40 °C. The formation of a nanocrystalline Al2O3 phase was observed in the H2SeO4 electrolyte at 40 °C. The samples exhibited weak photoluminescence. We identified three types of paramagnetic centers in AAO formed in H2SeO4: F+ centers (NsF = 8.2 × 1015 g−1), newly discovered centers with an unpaired electron localized on an oxygen atom (NsO = 1017 g−1), and centers associated with selenate radicals (NsS = 6 × 1018 g−1). By comparing the photoluminescence spectra and defect concentrations, we conclude that the luminescence of AAO formed in selenic acid is exclusively due to F+ centers, while other paramagnetic centers do not contribute.