The cyclic nature of porosity in anodic TiO2 nanotube arrays

Abstract

Precise shape engineering and physico-chemical properties of TiO₂ nanotubes (NTs) obtained by electrochemical anodization are key-issues that enable their practical applications. This work describes the general features that govern the porosity and geometrical parameters (pore diameter, inter-pore distance, wall and barrier layer thickness) of self-ordered TiO₂ NTs. We observed a complex cyclic porosity trend during constant voltage anodizations. We propose that these porosity changes are the outcome of the microscopic mechanisms behind NT formation and growth, aligned with non-equilibrium oxidation–dissolution processes at their bottoms. The self-ordered regime of TiO₂ NTs was found for porosity values in the range of 5.0% to 5.6%. Finally, a detailed analysis of the anodization curves enabled us to extract significant and comprehensive information on the nanotubes' organization mechanisms during the growth process.