Chemical tuning of TiO₂ nanoparticles and sintered compacts for enhanced thermoelectric properties

A novel, fast combustion method for synthesizing anatase TiO₂ nanoparticles (average diameter ∼14 nm) codoped with N and Nb in a single step is reported. XRD, STEM-EDX, and XPS measurements confirm that Nb ions are incorporated into the tetragonal lattice on Ti sites, while N ions occupy O sites, and likely also interstitial sites. Sintering of pellets of codoped powders under reducing conditions produced polycrystalline samples with the rutile structure. Chemically tuned samples have power factors up to 9.87 × 10^-4 W m^-1 K^-2, 7 times higher than that of pure TiO₂ sintered under the same conditions. In addition, the thermal conductivity is considerably lower at 2.6-4.0 W m^-1 K ^-1 as a result of greater grain-boundary and point-defect scattering. The figure of merit, ZT, is improved to 0.35 at 700 C, which is the highest value reported for a TiO₂ material to date, and is comparable to the highest values of any n-type thermoelectric oxide. Our material also exhibits good thermal stability in a pure N₂ atmosphere and is an excellent candidate for thermoelectric power generators. Consequently, the combustion technique represents a promising new strategy for preparing foreign-atom-doped metal oxides; the chemical tuning approach, a combination of foreign-atom-doped nanoparticle synthesis and optimized sintering process, can be applied to prepare superior thermoelectric materials.