Effect of pH on the barrier layer of TiO₂ nanoporous films potentiostatically grown in aqueous media containing fluoride ions

The effect of pH on the potentiostatic anodization growth of TiO ₂ film in 0.1MHClO₄/0.05MNH₄F solution was electrochemically characterized by EIS measurements. The increase in electrolyte pH modified considerably the voltammetric behavior of Ti electrode by diminishing the active dissolution region of the material and giving rise to two current plateaus that indicate modification of properties of the formed oxide. Five different formation potentials (E_F) were selected from this study to potentiostatically grow oxide films, and to evaluate their morphology, the modification of their properties, and their interaction with the electrolyte. SEM images of the formed films showed that the E_F increase resulted in a larger diameter of the pores formed, but an increase in pH led to a decrease in this parameter. In-situ characterization by EIS indicated that pH increase in the electrolyte used in anodization causes lower interaction between the formed oxide and F^- ions in the solution due to the change in the surface charge excess of Ti oxide, resulting in an increase in the compact layer resistance and a decrease in both the time constant associated with F^- ion adsorption and F^- ion diffusion coefficient within the compact oxide layer.