A study on the structure/phase transformation of titanate nanotubes synthesized at various hydrothermal temperatures

A conversion from commercial titania (TiO₂) nanoparticles to nanotubes was achieved by hydrothermal method with 10 M NaOH solution at various reaction temperatures ranging from 70 to 150 °C over 48 h. Most of intercalated sodium in as-synthesized titanate nanotubes was removed by washing with 0.1 M HCl solution for 1 h. The samples were then dried at room temperature and annealed at 300, 400, 500, and 600 °C in air for 1 h. With increasing reaction temperature, the morphology varied from spherical particles to two-dimensional nano-sheets to one-dimensional nanotubes. At 110 °C, nanosheets transformed to tube-like structure. The reaction temperature is a key factor in determining the overall aspect ratio of the tubular material. X-ray diffraction supports the structural transformation indicating the gradual changes in the phase and crystallinity of the synthesized powder. Tubular structure collapsed when annealed at 600 °C and converted to anatase phase totally. O 1s peak is found built-up of sub-peaks of H₂O, -OH, Ti-O. Annealing at 600 °C reduces the peak intensity of H₂O (531.01 eV) and -OH, while that of Ti-O increases. It is found that annealing removes the chemical bonds such as H₂O, -OH from the titanate are also converts the bonded states of titanate to that of titania.