Quantitative relationship between nanotube length and anodizing current during constant current anodization

The growth kinetics of anodic TiO2 nanotubes (ATNTs) still remains unclear. ATNTs are generally fabricated under potentiostatic conditions rather than galvanostatic ones. The quantitative relationship between nanotube length and anodizing current (Jtotal) is difficult to determine, because the variable Jtotal includes ionic current (Jion) (also called oxide growth current Jgrow=Jion) and electronic current (Je), which cannot be separated from each other. One successful approach to achieve this objective is to use constant current anodization rather than constant voltage anodization, that is, through quantitative comparison between the nanotube length and the known Jtotal during constant current anodization, we can estimate the relative magnitudes of Jgrow and Je. The nanotubes with lengths of 1.24, 2.23, 3.51 and 4.70 μm, were formed under constant currents (Jtotal) of 15, 20, 25 and 30 mA, respectively. The relationship between nanotube length (y) and anodizing current (x =Jtotal=Jgrow+Je) can be expressed by a fitting equation: y=0.23(x-10.13), from which Jgrow (Jgrow = x -10.13) and Je (∼10.13 mA) could be inferred under the present conditions. Meanwhile, the same conclusion could also be deduced from the oxide volume data. These results indicate that the nanotube growth is attributed to the oxide growth current rather than the dissolution current.