N-type nanostructured thermoelectric materials prepared from chemically synthesized ultrathin Bi ₂Te ₃ nanoplates

We herein report on the large-scale synthesis of ultrathin Bi ₂Te ₃ nanoplates and subsequent spark plasma sintering to fabricate n-type nanostructured bulk thermoelectric materials. Bi ₂Te ₃ nanoplates were synthesized by the reaction between bismuth thiolate and tri-n-octylphosphine telluride in oleylamine. The thickness of the nanoplates was ∼1 nm, which corresponds to a single layer in Bi ₂Te ₃ crystals. Bi ₂Te ₃ nanostructured bulk materials were prepared by sintering of surfactant-removed Bi ₂Te ₃ nanoplates using spark plasma sintering. We found that the grain size and density were strongly dependent on the sintering temperature, and we investigated the effect of the sintering temperature on the thermoelectric properties of the Bi ₂Te ₃ nanostructured bulk materials. The electrical conductivities increased with an increase in the sintering temperature, owing to the decreased interface density arising from the grain growth and densification. The Seebeck coefficients roughly decreased with an increase in the sintering temperature. Interestingly, the electron concentrations and mobilities strongly depended on the sintering temperature, suggesting the potential barrier scattering at interfaces and the doping effect of defects and organic residues. The thermal conductivities also increased with an increase in the sintering temperature because of grain growth and densification. The maximum thermoelectric figure-of-merit, ZT, is 0.62 at 400 K, which is one of the highest among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. This increase in ZT shows the possibility of the preparation of highly efficient thermoelectric materials by chemical synthesis.