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

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 (N_sF = 8.2 × 10¹⁵ g^-1), newly discovered centers with an unpaired electron localized on an oxygen atom (N_sO = 10¹⁷ g^-1), and centers associated with selenate radicals (N_sS = 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.