Thermal Conductivity of TiO2 Nanotubes

Thermal conductivity of TiO2 nanotubes prepared from a NaOH treatment of TiO2 particles with subsequent acid washing and annealing has been investigated. The obtained TiO2 nanotubes have a tetragonal anatase structure, and have a typical inner diameter of about 4-5 nm, wall thickness of about 2-3 nm, and length up to several hundred nanometers. TiO2 nanotubes show a significantly reduced thermal conductivity of about 0.40-0.84 W/(m.K) (average 0.62 W/(m.K)) at room temperature, as compared to about 8.5 W/ (m.K) for the bulk TiO2 materials. The great suppression in thermal conductivity can be understood by means of increased phonon-boundary scattering and enhanced phonon confinement in TiO2 nanotubes with unique nanotubular morphology, small featured sizes, and large surface area (similar to 258 m(2)/g). A theoretical analysis including the surface scattering and size confinement effects of phonon transport in TiO2 nanotubes is also reported, which results in an intrinsic thermal conductivity of 0.30-0.77 W/(m.K) (average 0.54 W/(m.K)) for individual TiO2 nanotubes with wall thickness of 2-3 nm, in harmony with the experimental values.