Formation of ordered arrays of nanoporous or nanotubular oxides of Al, Ti, and other valve metals during anodization has been reported extensively. These nanostructured oxides could find applications in solar energy conversion, energy storage, photocatalysts for environmental remediation, sensors, templates for nanowire growth, and supports for various catalysts because of their unique geometry and resultant physical properties. Various theories on the formation mechanism of the ordered nanostructures have been proposed in the literature including the field assisted dissolution mechanism[1], localized dielectric breakdown model[2], comparison of the Gibb's free energies of the compact oxide formation and chemical dissolution[3], and morphological instability of the oxide due to ion migration and oxide dissolution[4].

In this presentation, formation of ordered nanoporous anodic oxides of different substrate materials such as pure Ti, Ti-Mn alloy, Zr-W alloy, and bismuth will be analyzed based on the I-t plots and surface perturbation model proposed by Asaro and Tiller[5]. Compressive stresses are developed due to thickening of oxide layer and electrostriction. This compressive stress undulates the barrier film which leads to the surface perturbation. The perturbation releases strain energy of the oxide layer which is balanced by the increase in the surface area. These surface undulations affect the local chemical potentials because of the curvature effect. The presence of surface active species in the anodization electrolyte such as [MeF₆]^2- helps enhance the surface perturbation. The formation of ordered nanostructure could be explained based on the interaction between electrostatic strain energy and surface energy during anodization in specific environments.

[1] Z. Su and W. Zhou, Adv. Mater., 2008, 20, 3663–3667.

[2] Z. Su, W. Zhou, F. Jiang and M. Hong, J. Mater. Chem., 2011, 22, 535–544.

[3] M. Wang, Y. Liu and H. Yang, Electrochim. Acta, 2012, 62, 424–432

[4] K. R. Hebert, S. P. Albu, I. Paramasivam and P. Schmuki, Nat. Mater., 2012, 11, 162–166.

[5] R. J. Asaro and W. A. Tiller, Metall. Trans., 1972, 3, 1789 – 1796.