Growth of anodic TiO2 nanotubes in mixed electrolytes and novel method to extend nanotube diameter

• Correlation between compact anodic TiO2 and porous anodic TiO2 films is elucidated.
• Ionic current is used to form oxide and increases the oxide forming efficiency.
• Electronic current gives rise to O2 gas and decreases the oxide forming efficiency.
• A novel approach to enlarging the nanotube diameter is presented.
• Nanotube diameters increase with increasing H3PO4 amount in a given NH4F solution.

ABSTRACTIt is well known anodic TiO2 nanotubes (ATNTs) can be obtained by the anodization of Ti foils in fluoride-containing solutions, and the nanotube diameter is proportional to the applied voltages. However, the growth kinetics of ATNTs and the relationship between structural features and anodizing parameters still remain unclear. Challenges always remain in the fabrication of ATNTs with large diameters due to the undesired breakdown event under the high voltage in NH4F solutions. Here, an interesting approach is first proposed to overcome these particular challenges. A series of constant current anodizing processes in fluoride-free H3PO4 solutions, NH4F solutions of different concentrations (0.7 wt%, 0.5 wt% and 0.2 wt%), and different mixed electrolytes containing both NH4F and H3PO4, have been compared in detail. And we mainly focused on the influence of the different ratios of NH4F and H3PO4 on the outer diameters of ATNTs and the correlation between two types of films. The interesting results show that the nanotube diameter greatly increases with H3PO4 amount in the solutions with a given concentration of NH4F. In contrast, the nanotube length decreases with the increase of H3PO4 amount in the solutions with a given concentration of NH4F. The experimental findings and the undesired breakdown phenomenon can be elucidated by the theory of the electronic current and ionic current rather than by the field-assisted dissolution.

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