A phosphoric anion layer inhibits electronic current generation and nanotube growth during anodization of titanium

Nowadays the formation mechanism of anodic TiO₂ nanotubes has attracted extensive attention. Field-assisted dissolution (TiO₂ + 6F^- + 4H⁺ → [TiF₆]^2- + 2H₂O) has been considered as the causal link to the formation and growth of nanotubes. But it is hard for this theory to explain three stages of the current-time curve. Here, the anodization of titanium was studied by adding different concentrations of H₃PO₄ (0%, 4 wt%, 6 wt%, 8 wt%, and 10 wt%) in ethylene glycol containing the same concentration of NH₄F (0.5 wt%). The results prove that under the action of the same concentration of NH₄F, the growth rate of nanotubes decreases obviously with the increase of H₃PO₄ concentration, and the second stage of the current-time curve is also prolonged simultaneously. These experimental facts cannot be interpreted by field-assisted dissolution theory and the viscous flow model. Here, an anion layer formed by H₃PO₄ and the electronic current theory are ably used to explain these facts reasonably for the first time.