The Relationship Between the Growth Rate of Anodic TiO2 Nanotubes, the Fluoride Concentration and the Electronic Current

Anodic TiO2 nanotubes (ATNTs) have received much attention, but the classic field-assisted dissolution (FAD) mechanism involving the participation of fluoride ions has been questioned. Up to now, the relationship between the concentration of fluoride ions in the electrolyte and the growth rate of nanotubes has not been explained. In the same electrolyte with the same fluoride ion concentration, the growth rates of nanotubes obtained at 55 V, 60 V and 65 V are 156 nm/min, 215 nm/min and 402 nm/min. This paper considers why the FAD model involving fluoride ions cannot explain the huge differences in the growth rates. The huge difference is first interpreted reasonably in terms of the electronic current and the oxygen bubble model. The results show that the dissolution of oxide by fluoride ions is weak, and the role of fluoride ions is rather to form an anion-contaminated layer, which promotes the generation of electronic current and oxygen evolution. This theory was then tested by adding boric acid to the electrolyte. Borate can inhibit the penetration of fluoride ions into the contaminated layer, resulting in a significant reduction in electronic current, thus greatly reducing the growth rate of the nanotubes.