Enhanced Thermoelectric Performance of Nanostructured Nickel-Doped Ag₂Te

We report on the thermoelectric (TE) properties of nickel-doped Ag2–xNixTe (x = 0, 0.015, 0.025 & 0.055, 0.115, 0.155) nanostructures (NSs) in the temperature (T) range of 5–575 K. The electrical resistivity (ρ) of Ag2Te nanostructure shows metallic behavior in 5–300 K initially that evolves into two metal to insulator transitions at low- and mid-temperature regimes with increasing x due to Mott-variable range hopping (VRH) and Arrhenius transports, respectively. Their Seebeck coefficient varies nearly in a linear fashion in this temperature range, showing metallic or doped degenerate semiconducting behavior. Notably, this behavior of Seebeck coefficient (S ∝ T) is in contrast to Mott-VRH conduction (S ∝ T1/2) as observed in ρ. The steady increase in ρ and S with the sharp decrease in thermal conductivity between 410 and 425 K associated with the structural phase transition accomplishes a maximum TE figure of merit (ZT) of 0.86 ± 0.1 near 480 K in x = 0.155. This is ∼83% more compared to that of bulk Ag2Te and shows a significant improvement over the best value reported for Ag2Te NSs thus far. This study, therefore, shows that simultaneous nanocomposite formation, doping, and nanostructuring could be an effective strategy for tuning the electron and phonon transports to improve the TE properties of a material.