An Amorphous-to-Anatase Phase Transition Study of TiO₂ Nanotubes by Temperature-Dependent Synchrotron-Based In Situ X-ray Diffraction

TiO₂ nanotubes (NTs) with amorphous and crystalline structures have attracted interest due to their wide range of applications, particularly black TiO₂, which addresses the limitations of conventional TiO₂ (a wide band gap of 3.0-3.2 eV). Understanding the amorphous-to-anatase phase transition is crucial for phase control of crystalline TiO₂. This study investigates the size-dependent phase transition behavior of TiO₂ and black TiO₂ NTs using temperature-dependent synchrotron-based X-ray diffraction. Thermal treatments reveal that phase transitions occur in the ranges of 210-240 °C for normal NTs and 270-300 °C for black NTs. The onset temperature and crystallization growth are dependent on size, especially NT length, at least in the system under investigation. We observe anisotropic crystallization in quantum confinement, with the unit cell undergoing compression in the <i>a</i>-axis and expansion in the <i>c</i>-axis during crystallization. The longest black NTs (BV50T4) show a significant difference in the <i>c</i>/<i>a</i> ratio. Defects, such as oxygen vacancies, localize on specific NT planes.