Nanocrystallinity Enhancement of TiO₂ Nanotubes by Post-Hydrothermal Treatment

In the present research, a modified post-hydrothermal treatment with different temperature of 80-150^oC has been applied on as-annealed TiO₂ nanotubes derived from conventional hydrothermal process. The treatment has two-fold objectives, i.e. enhancing the nanocrystallinity of anatase TiO₂, and at the same time maintaining the integrity of nanotube structures. The resulting TiO₂ nanotubes were characterized by using XRD and UV-Vis spectroscopy. The XRD analysis revealed that the as dried TiO₂ nanotubes contain combined crystalline structures of sodium-titanate and anatase. It was also found that by increasing the post-hydrothermal temperature from 80 to 150^oC, the nanocrystallinity of nanotubes enhances as indicated by increasing the crystallite size of anatase TiO₂ from 6.93 to 7.81 nm. The anatase crystallite growth affected the optical characteristic of nanotubes, as represented with the reduction of the band gap energy, E_g from 3.75 to 3.67 eV by using Kubelka-Munk analysis for the obtained UV-Vis reflectance spectra. TEM observation confirms that the integrity of nanotubes structure can be well-maintained upon post-hydrothermal treatment.