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.