Phase Transformation and Electrochemical Behavior of Hexagonal TiO2 Nanotubes Under Different Annealing Temperatures and Heating Rates

In this study, hexagonal titanium dioxide nanotubes (hTNTs) fabricated by sonoelectrochemical anodization were thermally modified in air to investigate the influence of annealing temperature and heating/cooling rate on phase evolution, structural stability and electrochemical behavior. The samples were annealed at 450 °C, 550 °C, and 650 °C for 2 h using heating/cooling rates of 6 °C/min, 10 °C/min, and 20 °C/min. The hexagonal nanotubular morphology remained preserved after thermal treatment. However, increasing annealing temperature and heating/cooling rate promoted crack formation due to the thermally induced stress relaxation and phase transformation. The anatase content increased with increasing heating/cooling rate, indicating kinetically limited anatase-to-rutile transformation, whereas annealing at 650 °C promoted partial rutile formation. Electrochemical studies demonstrated that annealing temperature and heating/cooling rate affected the electrochemical behavior of hTNTs through different mechanisms. Increasing annealing temperature promoted structural ordering and partial anatase-to-rutile transformation, leading to reduced current response and enhanced electrochemical stability. In contrast, heating/cooling rate significantly affected impedance behavior and diffusion-related processes, indicating changes in charge transfer kinetics and ion transport within the nanotubular oxide layer. The results demonstrate that thermal treatment kinetics play an important role in controlling the phase composition and electrochemical behavior of hTNTs, providing insight into the thermal optimization of hexagonal TiO2 nanotubes for advanced functional applications.