Fabrication and formation mechanism of gradient TiO2 nanotubes via bipolar anodization

Bipolar electrochemistry is a promising technology especially in the fields of micro- and nanoscience, but it does not receive enough attention at present. In this paper, bipolar electrochemistry was successfully employed to fabricate TiO2 nanotubes with the diameter, length and composition gradients on titanium foils (as bipolar electrodes, BPE) within an applied voltage range of 50–120 V. The formation mechanism of the nanotube gradients and the role of the interface potential difference between BPE and the electrolyte on the forming driving force were thoroughly discussed. The results showed that the TiO2 nanotubes geometries could readily be controlled by adjusting the location on BPE and the applied voltages. The largest diameter and tube length occurred near the edge of BPE anode and then decreased with the distance away, but increased with the applied voltages. However, at certain locations, 35 mm away from the edge of BPE anode at 120 V, 30 mm at 100 V, 25 mm at 75 V and 15 mm at 50 V, massive little cylindrical oxides, rather than hollow nanotubes began to appear. The formation mechanism of the gradient nanotubes was generalized into the filed-assisted oxidation, chemical and field-assisted dissolution, and the synergy of two processes, which corresponded to three stages of the current density-time curves, the initial exponential drop, then rapid rise, and subsequent increase at different rates at 50–120 V, respectively. The average current density was also increased with an increase of the applied voltage. The interface potential difference E between the electrolyte and BPE titanium, instead of the applied voltages Etotal, essentially determined the driving force of anodic actions, leading to the formation of TiO2 nanotube gradients. Since E was less than Etotal, the largest diameter and length of nanotubes were less than those in conventional anodization at the same applied voltages. However, at 30 V, the insufficient interface potential difference E cannot bring about hollow tubular structure on the whole BPE titanium. This study on the fabrication and formation mechanism of the gradient TiO2 nanotubes will provide important scientific values and engineering application prospects, such as in screening the performance of catalytic, sensing and biomedical materials, or steering the molecular or macroscopic motion.