In-situ removal of thick barrier layer in nanoporous anodic alumina by constant current Re-anodization

A novel method for thick barrier layer removal of nanoporous anodic alumina (NAA) based on a constant current re-anodization (third anodization step) is reported. The barrier layer consists of an alumina layer that prevents electrical contact between the inner pore volume and the aluminium substrate. Several methods have been reported for the removal of such alumina layer. However, none of them is applicable to NAA produced under high anodization voltages, for example with phosphoric acid (H3PO4) electrolyte. Herein, NAA is obtained by two-step anodization of aluminium at a constant voltage of 195 V in a H3PO4 electrolyte. The novelty of this study consists of the combination of a reduction of the barrier layer thickness with a partial chemical etching in a H3PO4 solution followed by a constant current re-anodization step. This opens branchings in the barrier layer that permit a complete removal with a final chemical etching step in H3PO4 solution. The best condition for removing the barrier layer in the constant current re-anodization step is to fix a constant density current of 17.0 mu A/cm(2). We obtain self-ordered NAA substrates without barrier layer, allowing for the establishment of an electrical contact between the aluminium substrate and the interior of the nanopores. These nanoporous-back metal contact structures can be very important for applications in energy generation, energy storage and optoelectronic devices.