Bipolar Electrochemical Anodization Route for the Fabrication of Porous Anodic Alumina with Nanopore Gradients

Porous anodic alumina (PAA) films with homogeneous nanopores are achieved by traditional anodization. Here, we present a unique anodization technique based on bipolar electrochemistry to fabricate PAA films with nanopore gradients. In an oxalic acid solution dissolved in ethylene glycol, a stable bipolar anodization process is realized. The PAA film prepared at 280 V exhibits a continuous change in interpore distance from ∼171 to ∼83 nm over a range of only 5 mm on the aluminum sheet. Higher driving voltages lead to larger interpore distances and steeper nanopore gradients. Further, no direct electrical connection is required for this bipolar anodization.