Fig. 1
Representative surface morphology images of TiO2 nanotubes under different voltage: a 20 V, b 30 V, c 40 V and d 50 V. The diameters of the nanotubes fabricated under 30 V, 40 V and 50 V were, respectively (98.4 ± 11.4) nm, (136 ± 21.9) nm and (238 ± 33.5) nm, respectively. e, f Cross-sectional view of mechanically fractured sample showing that the length of the tubes was approximately 1.34 ± 0.26 µm