Role of crystallinity on the nanomechanical and electrochemical properties of TiO2 nanotubes

• TiO2 nanotubes were annealed at different temperatures to induce crystallization.
• Mechanical response of the nanotubes was correlated with their crystallinity.
• Evolution of rutile phase reduced the donor concentration drastically.
• Preferential growth of rutile phase enhanced the passivity.

In order to study the effect of annealing on the nanomechanical and electrochemical properties, TiO2 nanotubes synthesized via anodic oxidation were subjected to thermal treatment at various temperatures. Structural studies revealed that as-anodized nanotubes are amorphous, but they become crystalline when subjected to thermal treatment in air. TiO2 nanotubes annealed up to 450 °C crystallizes in pure anatase phase, whereas a mixture of anatase and rutile phase was observed for sample annealed at 600 °C. Interestingly, the evolution of rutile phase at 600 °C leads to decrease in crystallite size of the anatase phase. Nanoindentation studies showed that crystallite size plays a strong role in hardness, whereas porosity has significant influence on the elastic modulus of the nanotubes. Presence of two time constants in electrochemical impedance spectroscopy measurement presumably implies the formation of bilayer passive film. Mott Schottky analysis showed n-type semiconducting behavior with decrease in donor concentration (1019 cm− 3) upon increasing the annealing temperature. However, the evolution of rutile phase decreases the donor concentration drastically by an order of a magnitude (1018 cm− 3). Potentiodynamic polarization studies revealed that rutile phase favors enhanced passivation behavior exhibiting lower passive current density (10− 7 A cm− 2) compared to the pure anatase nanotubes (10− 6 A cm− 2). The enhanced passivation behavior of rutile phase was confirmed by Raman spectroscopy.