Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides.

Anodic TiO2 nanotubes have been studied widely for two decades because of their regular tubular structures and extensive applications. However, the formation mechanism of anodic TiO2 nanotubes remains unclear, because it is difficult to find convincing evidence for popular field-assisted dissolution or field-assisted injection theories and the oxygen bubble model. Here, in a bid to find direct evidence that oxygen bubbles form nanotube embryos, a new method is applied to handle this challenge. Before nanotube formation, a dense cover layer was formed to make nanotubes grow more slowly. Many completely enclosed nanotube embryos formed by oxygen bubbles were found beneath the dense cover layer for the first time. The formation of these enclosed and hollow gourd-shaped embryos is convincing enough to prove that the nanotubes are formed by the oxygen bubble mold, similar to inflating a football, rather than by field-assisted dissolution. Based on the 'oxygen bubble model' and ionic current and electronic current theories, the formation and growth process of nanotube embryos is explained clearly for the first time. These interesting findings indicate that the 'oxygen bubble model' and ionic current and electronic current theories also apply to anodization of other metals.