High Thermoelectric Performance in SnTe Nanocomposites with All-Scale Hierarchical Structures

SnTe has attracted considerable attention as an environmentally friendly thermoelectric material. The thermoelectric figure of merit ZT value is related to low thermal conductivity that can be successfully realized using fabrication of nanostructures. However, the practical realization of SnTe nanostructured composites is often limited by long reaction time, low yield, and aggregation of nanoparticles. Herein, a simple substitution reaction between Cu₂Se and SnTe was adopted to realize Cu_1.75Te-SnTe nanocomposites with unique all-scale hierarchical structures. On the atomic level, the substitution Se_Te is introduced into the lattice via the reaction between Cu₂Se and SnTe; on the nanoscopic level, Cu_1.75Te nanoinclusions with 10 nm size are evenly distributed at the grain boundaries of SnTe with average grain size less than 1 μm; on the mesoscopic level, these SnTe grains stack up to larger particles (10-20 μm), which are further surrounded by Cu_1.75Te grains with a predominant size of 1-2 μm. These hierarchical structures, together with additional SnTe stacking faults, can effectively scatter phonons with different wavelengths to reduce the lattice thermal conductivity. At 873 K, a thermal conductivity value of 0.49 W·m^-1·K^-1 was obtained in the SnTe nanocomposite sample with 0.057 Cu_1.75Te molar content, which is 40% lower than that of the pristine SnTe. By using the same approach for scattering phonons across integrated length scales, a ZT value of 1.02 (∼80% enhancement, compared with that of the pristine SnTe) was achieved at 873 K for the sample of the SnTe nanocomposite with 0.034 Cu_1.75Te molar content. This large increase in ZT values highlights the role of multiscale hierarchical architecture in controlling phonon scattering, offering a viable alternative to realize higher performance thermoelectric bulk materials.