High Thermoelectric Performance in 2D Sb₂Te₃ and Bi₂Te₃ Nanoplate Composites Enabled by Energy Carrier Filtering and Low Thermal Conductivity

Thermoelectrics arean important class of materials with greatpotential in alternative energy applications. In this study, two-dimensional(2D) nanoplates of the layered chalcogenides, Sb2Te3 and Bi2Te3, are synthesized and compositesof the two are investigated for their thermoelectric properties. Thetwo materials, Sb2Te3 and Bi2Te3, were synthesized as hexagonal, 2D nanoplates via a colloidalpolyol route. The as-synthesized Sb2Te3 andBi(2)Te(3) vary drastically from one another intheir lateral and vertical dimensions as revealed by scanning electronmicroscopy and atomic force microscopy. The single crystalline nanoplatenature is deduced by high-resolution transmission electron microscopyand selected area electron diffraction. Nanoplates have well-definedhexagonal facets as seen in the scanning and transmission electronmicroscopy images. The nanoplates were consolidated as an anisotropicnanostructured pellet via spark plasma sintering. Preferred orientationobserved in the powder X-ray diffraction pattern and scanning electronmicroscopy images of the fractured pellets confirm the anisotropicstructure of the nanoplates. Thermoelectric properties in the paralleland perpendicular directions were measured, revealing strong anisotropywith a significant reduction to thermal conductivity in the perpendiculardirection due to increased phonon scattering at nanoplate interfaces.All compositions, except that of the 25% Bi2Te3 nanoplate composite, behave as degenerate semiconductors with increasingelectrical resistivity as the temperature increases. The Seebeck coefficientis also increased dramatically in the nanocomposites, the highestreaching 210 mu V/K for 15% Bi2Te3. Theincrease in Seebeck is attributed to energy carrier filtering at thenanoplate interfaces. Overall, these enhanced thermoelectric propertieslead to a drastic increase in the thermoelectric performance in theperpendicular direction, with zT similar to 1.26, forthe 15% Bi2Te3 nanoplate composite at 450 K.