Pore opening detection for controlled dissolution of barrier oxide layer and fabrication of nanoporous alumina with through-hole morphology

Pore opening and removal of the oxide barrier layer by wet chemical etching is the last and most critical step in the fabrication of anodic alumina oxide (AAO) with through-hole pore morphology. The time controlled etching method is dependent on many parameters that cannot provide reproducible pore diameters of fabricated AAO. To address the inconsistency of the chemical etching process and achieve fabrication of AAO with desired pore diameters we introduce an electrochemical method for detection of the pore opening point. The method is based on monitoring of current through AAO using a simple electrochemical permeation cell with two electrodes. The dissolution of the oxide barrier film in phosphoric acid and the pore opening point were detected by significant increases of the current between two electrodes during etching. Experimental demonstration and feasibility of this approach for routine fabrication of through-hole AAO membranes using different AAO samples and etching conditions (10–55 °C) combined with scanning electron microscopy study are presented. It was demonstrated that with support of this method, nanometer scale control of the pore opening of AAO can be achieved by chemical etching. The method offers improved reproducibility in fabrication of AAO with desired pore diameters, particularly when AAO is fabricated using different anodization conditions (mild and hard anodization) and different acid solutions (sulfuric, oxalic, and phosphoric).