Role of Electrode Reaction Kinetics in Self-Ordering of Porous Anodic Alumina

Porous anodic aluminium oxide with long-range ordered structure is of high practical importance in modern materials science and nanotechnology owing to its wide applications as membranes or templates for various nanostructures. However, the fundamental reason of self-organized growth of honeycomb-like porous anodic alumina films solely within a narrow region of processing conditions remains unknown. Here we show that the formation of the long-range ordered porous structure occurs only when anodization rate is limited by migration in barrier layer, which separates metal and electrolyte, or by diffusion in pores, whereas the mixed control of anodic oxidation process leads to disordered porous structures. The universality of this conclusion is experimentally confirmed for anodization in oxalic and sulfuric acid solutions, which are the most versatile anodizing electrolytes. The predictive power of the suggested model is demonstrated for anodic oxidation of aluminium in 2 M sulfuric acid electrolyte, in which mild and hard anodization regimes leading to the formation of highly ordered porous structures are developed. It allows one to intentionally choose proper anodization conditions instead of simple empirical search that has been used so far. (C) 2017 Elsevier Ltd. All rights reserved.