Tuning the geometry of porous alumina layers via anodization in mixtures of different acids

Porous anodic aluminum oxide (AAO) layers have been obtained by two-step anodization of high-purity Al in two types of acid mixtures, i.e., in H₂C₂O₄–H₃PO₄ and, for the first time, in H₂SO₄–H₃PO₄ systems. The kinetics of oxide formation was examined by monitoring the current vs. time curves while the morphology of the resulting layers was carefully verified by scanning electron microscopy (SEM). A special emphasis was put on establishing correlations between electrolyte composition, the kinetics and effectiveness of oxide growth, and the morphological features of AAO layers (pore and cell diameter, porosity), as well as pore arrangement. It was confirmed that the addition of H₃PO₄ to both H₂C₂O₄ and H₂SO₄ electrolytes results in a significant decrease in oxide growth rate, and worsening of pore arrangement, while the values of pore diameter and interpore distance are much less affected. Moreover, the presence of a small amount of phosphoric acid in the reaction mixture allowed for a noticeable increase in pore ordering if anodization was carried out beyond the self-ordering regime, or performing controlled anodization even at voltages at which the burning phenomenon is typically observed. It is strongly believed that manipulating the electrolyte composition by adding another acid may provide another degree of freedom to control the morphology of the resulting nanostructured alumina layers.