Enhanced asymmetrical transport of carriers induced by local structural distortion in chemically tuned titania: A possible mechanism for enhancing thermoelectric properties

Optimizing all three thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) simultaneously is a nontrivial task, especially for the Seebeck coefficient and electrical conductivity. Here, we demonstrate that chemically tuned rutile TiO₂ sintered from N- and Nb-codoped nanoparticles possesses not only an enhanced power factor but also a decreased thermal conductivity. In samples produced under strongly reducing conditions, the electrical resistivity is decreased markedly to 3.96×10^-5 Ωm while maintaining a high Seebeck coefficient. In this manner, the power factor can be greatly improved to 9.87×10^-4 W m^-11 K^-2 at 785 C with a significantly enhanced figure of merit ZT of 0.35 at 700 C. Theoretical modeling based on density functional theory suggests that enhanced asymmetrical carrier transport (i.e., with the minority carrier relatively localized on one side of the Fermi level but the majority carrier delocalized on the other) can be induced by locally distorting the crystal structure, since this imposes local perturbations on the intrinsic periodic potential field, which may be responsible for the improved power factor. The greatly improved thermoelectric behavior of chemically tuned rutile TiO₂ provides a promising route to producing practical thermoelectric materials without the use of toxic heavy elements. Our findings also provide a promising new strategy for enhancing the thermoelectric properties of other thermoelectric materials.