Thermal conductivity and secondary porosity of single anatase TiO₂ nanowire.

Single anatase TiO₂ nanowire is synthesized using the electrospinning technique with the sol-gel method and is suspended over a pre-processed 100 µm-wide TEM grid for further characterization. The diameters of the nanowires fall in the range of 250-400 nm. The transient electrothermal (TET) method is adopted to acquire the voltage-time (U-t) profile of the Ir-coated nanowire under step Joule heating. The intrinsic thermal diffusivity of single anatase TiO₂ nanowires varies from 1.3 to 4.6 × 10⁻⁶ m² s⁻¹, and the thermal conductivity changes distinctly from 1.3 to 5.6 W m⁻¹ K⁻¹, much lower than the value of the bulk counterpart: 8.5 W m⁻¹ K⁻¹. The density and thermal conductivity increase significantly with the diameter, largely because at larger diameters less secondary porosity is left by decomposition of organic composites and their escape from the wire during calcination. The density of TiO₂ nanowires is found to be much lower than that of the bulk counterpart. This is supported by the SEM image of the secondary porous surface. High secondary porosity is observed for TiO₂ nanowires, ranging from 18% to 63%. This very high secondary porosity confirms that the decomposition of PVP content may distort the fibrous matrix and leave vacancies. In addition, the transition from amorphous to anatase phase could also create a porous state due to crystal particle aggregation.