Split TiO2 nanotubes − Evidence of oxygen evolution during Ti anodization

The fabrication and growth mechanism of TiO2 nanotubes have attracted widespread interest for decades. Conventional Ti anodization generally produces only non-split TiO2 nanotubes. In this work, completely split TiO2 nanotubes, whose morphology is similar to that of porous anodic alumina, were produced under breakdown conditions. The appearance of so many split TiO2 nanotubes in such a short period of time (900 s) cannot be explained by the classical field-assisted dissolution theory, viscous flow model or dissolution equilibrium theory. Here, we use the oxygen bubble mould theory, separating the electronic current and ionic current from the total current. The results show that both ionic and electronic currents reach a steady value during conventional anodization, while the electronic current keeps increasing during anodization under breakdown conditions. We suggest that the split nanotubes are formed by intense oxygen evolution due to the increasing electronic current – the nanotube walls are cleaved by the pressure of oxygen bubbles. We believe that the split nanotube array provides solid evidence of oxygen evolution due to the electronic current. •Conventional Ti anodization only produces non-split nanotube arrays.•For the first time, split nanotube arrays are produced under breakdown conditions.•A non-equilibrium state appears in anodization under breakdown conditions.•Electronic current keeps rising under breakdown conditions, which accelerates oxygen evolution.•Intense oxygen evolution contributes to the splitting of the TiO2 nanotubes.