Origin of self-organisation in porous anodic alumina films derived from analogy with Rayleigh–Bénard convection cells

Abstract

Porous anodic aluminium oxide (PAOX) has different practical applications (e.g. filters with uniform pore sizes, adsorbers, porous templates for functional nanomaterials), but the formation mechanism is still poorly understood. Equal-sized hexagonally ordered pores are formed during anodic oxidation of aluminium in water solutions of some acids at certain concentrations and temperatures, and comparatively high electrode potentials. Today, a limited range of pore diameters and the degree of hexagonal ordering are reached with empirically found conditions. Here, a theoretical model explaining the appearance of honeycomb structure in porous anodic alumina is presented. The proposed mechanism is based on a dissipative self-organization process, but not on the earlier accepted field-assisted dissolution of pre-formed dense alumina. Our analysis rests on the concept that electrolyte currents near aluminium anode are organized in the same way as well-known Rayleigh–Bénard convection currents. A simple yet effective way to predict pore formation in unexplored electrolytes is suggested. The validity of theoretical considerations is experimentally confirmed by the growth of hexagonally arranged porous alumina in a new electrolyte–aqueous formic acid solution.