A New Benchmark for TiO2 Nanotube Array Growth by Anodization

We report on the anodic formation of a self-standing 720 μm thick TiO2 nanotubular membrane by complete consumption of a 250 μm thick titanium foil sample. By employing double sided electrochemical oxidation of titanium in an electrolyte comprised of water, NH4F, and ethylene glycol, we obtain two highly ordered, hexagonal close-packed titania nanotube arrays 360 μm in length that are separated by a thin compact oxide layer; the individual nanotubes in each array have an aspect ratio of ∼2200. The potentiostatic anodization of titanium in an ethylene glycol, NH4F, and water electrolyte dramatically increases the rate of nanotube array growth to approximately 15 μm/h, representing a growth rate ∼750−6000% greater than that seen, respectively, in other polar organic or aqueous based electrolytes previously used to form TiO2 nanotube arrays. We consider the effects of electrolyte composition, applied potential, and anodization duration on the length and diameter of the resulting nanotubes in terms of a growth rate model, with results suggesting that reduced hydroxyl ion injection from the electrolyte, which enables faster high field ionic conduction through the barrier layer, is responsible for the high nanotube growth rates achieved. Furthermore, as reported herein for the first time, we are able to make self-standing TiO2 nanotube array films ranging in thickness from 50 to 360 μm.