Rapid growth of titanium oxide nanotubes under the critical breakdown voltage: Evidence against the dissolution reaction of fluoride ions

Rapid fabrication of anodic titanium oxide nanotubes is limited to its growth mechanism, because the growth mechanism is still dominated by the dissolution reaction of fluoride ions. Here, the growth rates of the nanotubes anodized at 50 V in four electrolytes of NH4F (0.2, 0.3, 0.4 and 0.5 wt%) increased slowly with the increasing concentration of NH4F. This phenomenon seems to be consistent with the dissolution reaction of fluoride ions. However, when titanium was anodized at the critical breakdown voltages, the growth rates of nanotubes obtained in 0.2, 0.3, 0.4 and 0.5 wt% NH4F electrolytes were 2.191, 0.869, 0.538 and 0.497 μm min−1, respectively. Therefore, the growth rates of nanotubes decreased with the increasing concentration of NH4F. Compared with the growth rate (0.140 μm min−1) under non-breakdown conditions (50 V), the growth rate (2.191 μm min−1) of nanotubes under the critical breakdown voltage was greatly increased. Obviously, the phenomenon of rapid growth of nanotubes in low concentration of NH4F electrolyte is contrary evidence of the dissolution reaction of fluoride ions and the dissolution equilibrium theory. •Rapid growth rate of anodic TiO2 nanotubes can reach 2.19 μm min−1.•Equilibrium state appears in the process of anodization at low voltage.•Non-equilibrium state appears in anodization at critical breakdown voltage.•Growth rate is related to anodizing current rather than NH4F concentration.•Evidences against dissolution reaction of fluoride ion and equilibrium are proposed.