Self-Ordering Regimes of Porous Anodic Alumina Layers Formed in Highly Diluted Sulfuric Acid Electrolytes

Nanoporous anodic alumina films with long-range hexagonal order have been obtained from a series of highly diluted sulfuric acid electrolytes. A simple linear relationship was established between the self-ordering voltages and acid concentrations (28, 29, and 30 V for 0.2, 0.1, and 0.05 M electrolytes, respectively). Besides establishing new self-ordering regimes, our experimental work sheds new light on some fundamental principles of honeycomb anodic alumina formation. It suggests that the spontaneous self-organization of a stable nanoscale structure originates from the electrohydrodynamic (EHD) convection rather than from Marangoni-type instability at the anode surface. Theoretical analysis displays a decreasing exponential functional relationship between electrolyte concentration and the critical values of the earlier found electrochemical analogue of Rayleigh number, which can be used for prediction of hexagonal cell pattern in currently unexplored anodizing electrolytes.