Influence of acid radical ions upon photoluminescence properties of anodic aluminum oxide

Photoluminescence (PL) properties of anodic aluminum oxide (AAO) membranes with an ordered nanopore array formed in a mixture of sulfuric acid and oxalic acid solution with various volume ratios by two-step anodizing process were investigated under an excitation at 250 nm. Measurements reveal that the PL band in the wavelength range of 350-450 nm originates from optical transitions from oxygen vacancies, the 288 nm and 328 nm peaks attribute to the impurities of SO₄^2- and C₂O₄^2- in AAO, respectively. With the increasing ratio of SO₄^2- vs. C₂O₄^2-, the intensity of 328 nm peak reaches a maximum value and then decreases, but the peak located at 288 nm vanishes nearly, then appears gradually. Analyse suggests that there is likely to be nonradiative energy transfer between the two kinds of PL centers originated from SO₄^2- and C₂O₄^2-. An energy transfer mechanism was proposed to explain the behaviors of the emission at 328 nm, and it was tentatively suggested by the PL properties of AAO membranes with dual-layered structure fabricated from sulfuric acid and oxalic acid.