Model for Formation of Self-Ordered Anodic Oxide Nanomaterials

Abstract

Porous anodic films are self-organized porous oxides nanomaterials formed by electrochemical oxidation. Pores in porous alumina and nanotubes in TiO2nanotube layers initiate after an initial stage of barrier oxide growth. The porous film eventually evolves into a hexagonally-ordered array of parallel cylindrical pores or nanotubes of order 10-100 nm diameter with a characteristic spacing that scales directly with the cell voltage. We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide (1-3). The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of metal and oxygen ions (1). Stress at the film interfaces is generated by ion-transfer processes and relaxed by viscous flow, resulting in layers with elevated compressive or tensile stress within a few nm of the interfaces (2,3). Experimental evidence for these layers has been found from in situ stress measurements (2-4). We present linear stability analysis incorporating the surface stress boundary conditions in the coupled oxide flow-electrical migration model of Houser and Hebert (1). The calculations reveal self-organized pattern formation with steady-state pore spacing-voltage scaling ratios that compare favorably to those found experimentally, for both porous alumina and TiO2nanotube layers. The scaling ratios are independent of oxide viscosity or uncertainties in the values of any other model parameters. In the case of Al anodization, oxide flow resulting in pore patterns is driven by elevated stress near the oxide-solution interface, through a mechanism that has common features with the Marangoni instability in thin liquid films. Stress near the oxide surface is produced because strong electrolyte anion adsorption suppresses oxide growth on the surface. Such near-surface stress is not present in TiO2nanotube layers (5). Instead, compressive stress driving flow arises at the metal-oxide interface from the volume change upon conversion of metal to oxide. REFERENCES J. E. Houser and K. R. Hebert, Nature Mater., 8, 415 (2009).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E737 (2018).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E744 (2018).Ö. Ö. Çapraz, P. Shrotriya, P. Skeldon, G. E. Thompson, K. R. Hebert, Electrochim. Acta, 167, 404 (2015). Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 295, 418 (2019).