Siwei Zhao

Formation mechanism of multilayer TiO2 nanotubes in HBF4 electrolyte

Multilayer anodic TiO2 nanotubes are first fabricated in HBF4-containing electrolyte by a one-step galvanostatic anodization. These nanotubes demonstrate unique A-shaped sidewalls, which are different from the traditional V-shaped nanotubes formed in NH4F-containing electrolyte. Further, the formation mechanism of the multilayer TiO2 nanotubes is proposed. During the anodizing process, the total anodizing current could be separated into ionic current and electronic current. The oxygen bubbles, induced by the electronic current, play a significant role in shaping the nanotube architectures. The bottoms of TiO2 nanotubes could be broadened under the pressure of oxygen bubbles. Thus the wall at the bottom of the nanotube becomes thinner. When the pressure of oxygen bubbles reaches a certain value, it will break the sidewalls of the nanotubes, resulting in the formation of the A-shaped sidewalls of TiO2 nanotubes. However, in NH4F-containing electrolyte, the oxygen bubbles escape from the top of nanotubes, and then the V-shaped sidewall thickness profiles of TiO2 nanotubes are formed. Owing to the inflating effect of oxygen bubbles on the nanotube walls, the whole TiO2 nanotube layer is finally divided into nanotube multilayers in HBF4-containing electrolyte.