Investigations on current transients in porous alumina films during re-anodizing using the electrochemical quartz crystal microbalance

Current transients and mass variations in as-prepared and heat-treated anodic alumina films were measured during re-anodizing by means of voltammetry and electrochemical quartz crystal microbalance (EQCM), respectively. Aluminum electrodes (100 nm) on quartz crystals were prepared by thermal evaporation. Anodic alumina films were formed on the surface of Al electrodes in aqueous solutions of oxalic (0.3 M) and phosphoric (0.6 M) acid in the potentiostatic regime. The EQCM experiments did not detect an overshoot in the mass variation of the Al electrode during re-anodizing of heat-treated anodic alumina films. The observed current overshoot in transients proved the presence of electrons and electron holes injected from the contacts in the bulk of the oxide. This can be explained by the emergence of excess electrons in the barrier layer of the alumina films due to a change in the mobility of the electrons.