Hydrothermal processing of hydrogen titanate/anatase-titania nanotubes and their application as strong dye-adsorbents.

The nanotubes of pure hydrogen titanate and anatase-titania have been synthesized via hydrothermal treatment of as-received anatase-titania particles. The formation mechanism of anatase-titania nanotubes via hydrothermal has been discussed in detail in view of the finger-prints produced by characterizing the intermediate and end products using various microscopic and spectroscopic techniques such as scanning electron microscope, high-resolution transmission electron microscope, X-ray diffraction, Brunauer, Emmett, and Teller specific surface-area measurement, Fourier transform infrared spectroscope, diffuse reflectance, photoluminescence, thermal gravimetric and differential thermal analyses. The obtained results strongly support the rollup mechanism, involving multiple nanosheets, for the formation of anatase-titania nanotubes with the formation of different intermediate hydrothermal products having various morphologies such as sodium titanate having aggregated rectangular block-like structures, hydrogen sodium titanate and pure hydrogen titanate having highly aggregated unresolved fine-structures containing nanotubes, and finally, the pure anatase-TiO2 nanotubes. It is demonstrated that, during the hydrothermal treatment, the nanotubes of pure hydrogen titanate are formed first coinciding with the stable solution-pH during washing, indicating the completion of ion-exchange process, and a drastic increase in the specific surface-area of the hydrothermal product. The anatase-titania nanotubes are then derived from the pure hydrogen titanate nanotubes via thermal treatment. The use of pure hydrogen titanate and anatase-titania nanotubes for an organic textile dye-removal, from an aqueous solution under the dark condition, via surface-adsorption mechanism has been demonstrated. It is shown that, the specific surface-area and the surface-charge govern the maximum dye-absorption capacity of the anatase-TiO2 nanotubes under the dark condition.