Effect of counter-ion and solvent on the morphology and barrier layer properties of nanoporous/nanotubular TiO₂ films grown by anodization in fluoride containing media

Nanoporous TiO₂ films were formed by anodization in three different electrolytes fluoride-containing, at different formation potentials for 2.5 hours. Film topography and morphology were characterized by AFM and SEM images, respectively. Aditionally, an in-situ study of the anodic film growth was carried out by electrochemical impedance spectroscopy. The growth ratio for the pore size obtained from SEM images was not directly proportional to the barrier layer growth ratio derived from EIS. The main role of the counter-ion seems to be insertion of fluoride ions altering the properties of the formed oxide, decreasing the film resistance and causing a more aggressive chemical attack, observed in the increase in pore size and film roughness. The solvent tunes the chemical attack of the anodic layer, and could also be establishing the titanium oxide stoichiometry, thus altering oxygen vacancies in the film and the migration of fluoride ions through the oxide lattice.