Fig.1 Schematic diagrams of the AAO membrane (a) and cross-sectional structure (b)

   Porous AAO membranes are made up of numerous hexagonal unit cells (Fig. 1a). Each unit cell contains three different distinct parts: 

    (1) A hexagonal inner layer, also called the “skeleton”, which is made up of the common internal walls between the unit cells.

    (2) An outer layer, between the central pore and the inner layer.

    (3) An interstitial rod inside the inner layer, at the triple cell junction. 

    Interpore distance (D_int), pore diameter (D_p), barrier layer thickness (t_b), pore wall thickness (t_w), pore density (ρ_p), and porosity (P) are important structural parameters (Fig. 1b). For porous AAO membranes with highly ordered pore arrangement, the relationship among these parameters can be expressed as follows:

    These parameters are mainly dependent on the electrolyte type, anodization voltage (U), anodization current density (J), and temperature (T). e.g., Dint is linearly dependent on the U with a proportionality constant ζ, D_int = ζ U at a given temperature. For porous AAO membranes formed by mild anodization (MA) and hard anodization (HA) conditions, ζ_MA ≈ 2.5 nm/V and ζ_HA ≈ 2.0 nm/V at 1 °C in 0.3 M oxalic acid electrolyte, respectively.