Stability of aqueous suspensions of titanate nanotubes

The long-term stability of titanate nanotubes, which were produced by alkaline hydrothermal treatment of TiO2, was studied at room temperature in acidic, pure water, and basic aqueous suspensions. In pure water and basic (0.1 mol dm(-3) NaOH) solutions, the nanotubes were stable and minimal morphological changes occurred. In 0.1 mol dm(-3) H2SO4, suspended titanate nanotubes slowly transformed to rutile nanoparticles of ca. 3 nm size, which were agglomerated into ellipsoidal particles. The porosity, crystal structure, and morphology of protonated titanates and TiO2 have been studied for intermediate states during the transformation by nitrogen adsorption, XRD, Raman spectroscopy, SEM, and HRTEM. The rate of conversion of nanotubes to nanoparticles has been related to the concentration of soluble titanium(IV) in solution, which depends on the nature of the acid. Thermodynamic and kinetic aspects of the acid transformation are discussed.