Effect of water and fluoride content on morphology and barrier layer properties of TiO₂ nanotubes grown in ethylene glycol-based electrolytes

This paper studies the effect of H₂O and NH₄F content on morphology and barrier layer properties of TiO₂ nanotubes grown by potentiostatic anodization in ethylene glycol-based electrolytes. The increase in these two variables leads to an increase in the chemical attack of the formed oxide. However, each of these variables plays a different role in the formation of TiO₂ nanotubes. On the one hand, a higher percentage of H₂O in the electrolyte leads to a transition from a nanoporous to a nanotubular structure, as well as to a greater diameter of the tubes and a decrease in their length and barrier layer thickness. In contrast, a higher NH₄F concentration decreases nanotube diameter and increases their length modifying barrier layer properties due to insertion of F^- ions into the lattice. This diminishes the barrier layer resistance, but increases both the adsorption and the diffusion coefficient of F^- ions. The different roles of H₂O and NH₄F in film formation are also associated with the presence of sub-oxides detected by XPS.