TiO2 nanotube arrays grown in ethylene glycol-based media containing fluoride: Understanding the effect of early anodization stages on the morphology

Abstract

In this work, we develop a phenomenological model describing the growth of TiO2 nanotube arrays (NTA-TiO2) by titanium anodization in ethylene glycol-based electrolytes containing fluoride. NTA-TiO2 formation is described in terms of the well-known High-Field mechanism (HFM) considering film thinning by dissolution, and the electronic current contribution according to the Oxygen Bubble Mold (OBM) model, as it has been previously reported. For the first time, the inclusion of the nucleation of oxygen bubbles as the rate determining step at early anodization times is considered. By simultaneous fitting of the potentiostatic j/t profiles and the time evolution of morphological features such as pore radius, layer thickness and density of pores, the behavior observed for several experimental conditions is accurately described, in contrast to previously reported Field Assisted Dissolution (FAD) and OBM models. Proposed model allows obtaining physical and phenomenological information concerning the formation and growth of nanoporous oxide films as well as their morphological properties. Results also indicate that fluoride species play a key role at the early anodization stages, determining the subsequent film morphology evolution. Also, conclusive evidence for a chemical-type instead of field-assisted film dissolution process is obtained.