Effect of electrochemical reduction on the structural and electrical properties of anodic TiO2 nanotubes

The effect of electrochemical reduction on the structural and electrical properties of amorphous as well as annealed TiO2 nanotubes (TNTs) is investigated under ambient conditions. TNTs were prepared by anodizing titanium sheet in ethylene glycol electrolyte containing NH4F and de-ionized water at 40 V for 6 h. Electrochemical reduction is carried out in 1 M aqueous KOH solution for ∼15 s at 3 V. TNTs are characterized by SEM, XRD, XPS and impedance spectrometer. XRD results confirm an increase in d-spacing for (101) and (200) planes, after electrochemical reduction. XPS data reveal that electrochemical reduction produced prominent shifts of ∼0.7-1.0 eV in the binding energies of TNTs. Interestingly, these shifts recover completely (in case of amorphous TNTs) and partially (in case of anatase TNTs) within ∼7 days after reduction process due to oxygen uptake. Partial recovery in the binding energies of anatase TNTs is due to the fact that the oxygen vacancies are thermodynamically more stable as compared to amorphous TNTs. Similarly, the electrochemical reduction process decreases the impedance values of TNTs by more than three orders of magnitudes (from MΩ to kΩ). The impedance values also recover to the similar values before reduction in a span of ∼7days.