Nanograined thermoelectric Bi₂Te_2.7Se_0.3 with ultralow phonon transport prepared from chemically exfoliated nanoplatelets

Herein, we report on a scalable synthesis of surfactant-free Bi ₂Te_2.7Se_0.3 nanocrystals by chemical exfoliation and subsequent spark plasma sintering to fabricate nanostructured thermoelectric bulk materials. The exfoliated n-type Bi₂Te _2.7Se_0.3 nanoplatelets were shown to transform into nanoscroll-type crystals (∼5 nm in diameter, ∼50 nm in length) by ultrasonication. The thermoelectric performance of the Bi₂Te _2.7Se_0.3 nanocrystals was found to be recoverable by minimizing surface oxides by chemical reduction of the exfoliated suspensions. Nanostructured bulk materials, composed of plate-like grains with ∼50 nm thickness, were prepared by sintering of the ultrasonicated sample using a spark plasma sintering technique. The resulting compound showed drastic reduction of lattice thermal conductivity (0.31 W m^-1 K^-1 @ 400 K) due to enhanced phonon scattering at highly dense grain boundaries without deterioration of the power factor (21.0 × 10^-4 W m^-1 K^-2 @ 400 K). The peak ZT value of the present compound (∼0.8 @400 K) is comparable to that of n-type single crystalline Bi ₂(Te,Se)₃, which is one of the highest among the reported values for n-type materials synthesized by a soft chemical route.