Bottom-Up Synthesis of SnTe-Based Thermoelectric Composites

There is a need for the development of lead-free thermoelectric materials for medium-/high-temperature applications. Here, we report a thiol-free tin telluride (SnTe) precursor that can be thermally decomposed to produce SnTe crystals with sizes ranging from tens to several hundreds of nanometers. We further engineer SnTe–Cu2SnTe3 nanocomposites with a homogeneous phase distribution by decomposing the liquid SnTe precursor containing a dispersion of Cu1.5Te colloidal nanoparticles. The presence of Cu within the SnTe and the segregated semimetallic Cu2SnTe3 phase effectively improves the electrical conductivity of SnTe while simultaneously reducing the lattice thermal conductivity without compromising the Seebeck coefficient. Overall, power factors up to 3.63 mW m–1 K–2 and thermoelectric figures of merit up to 1.04 are obtained at 823 K, which represent a 167% enhancement compared with pristine SnTe.

Upon TOPTe injection, the color of the solution gradually changed from dark yellow to dark brown within several minutes. The SnCl2/TOPTe-OAm solution was then degassed at room temperature for 20 min and further vacuumed at 120 ºC for 20 min with magnetic stirring.
Subsequently, the mixture was heated to 280 ºC at a rate of ~10 ºC /min and reacted at this temperature for 60 min under Ar atmosphere. When the reaction was completed, the solution was cooled down to room temperature in a water bath. The harvested crude solution containing chloroform was centrifuged at 9000 rpm for 5 min. The re-dispersed precipitate using a mixture solvent of chloroform and ethanol (Vchloroform: Vethanol=1:2) was washed and centrifuged more than one time at 8000 rpm for 3 min. The fine product was dried in a vacuum and stored in a glovebox for further use.
Synthesis of copper telluride nanocrystals (Cu 1.5 Te NCs). Cu1.5Te NCs were synthesized by S-3 a colloidal method reported from our group with minor modifications. 1 In a typical synthesis, 10 mmol of Cu (AC)2 was mixed with 50 mL of OAm in a 100 mL three-neck flask. The mixture was kept under vacuum for 20 min at room temperature, then heated to 120 ºC and maintained at this temperature for 30 min under vacuum to remove low boiling point impurities. Then the temperature was increased to 220 ºC under Ar. After ~10 min, the initial bright yellow solution became clear light brown. At this point, 10 mL of 1 M TOPTe was injected, and the light brown solution immediately changed color to deep green. Upon injection, the temperature of the reaction mixture dropped to ~210 •C. The mixture was allowed to recover the 220 ºC and maintained at this point for 30 min. Afterwards, the colloidal solution was rapidly cooled to room temperature with a water bath at an initial approximate rate of ~80 •C/min. Finally, 25 mL of chloroform were added to the crude solution and the mixture was sonicated for several minutes. The final deep green product was precipitated by centrifugation at 8000 rpm for 5 min.
Then it was redispersed in chloroform and precipitated one more time by centrifugation in the presence of ethanol. Finally, Cu1.5Te NCs were redispersed in chloroform and kept in an Arfilled glovebox for further use.
SnTe-Cu 1.5 Te precursor. Briefly, to prepare a SnTe-Cu1.5Te precursor solution, a certain amount mass fraction of Cu1.5Te nanocrystals dispersed in OAm (10 mg·mL -1 ) were added into the as-prepared SnTe precursor ink. After a period of intense sonication and stirring, SnTe-Cu1.5Te precursor solutions were formed. This precursor was thermally decomposed following the same procedure as described above for the SnTe NCs.
Annealing and hot-press. A certain amount of the dried nanopowders was heated to 580 °C at a rate of 10 °C/min and maintained for 120 min under Ar atmosphere. The annealed powders were loaded into a graphite die within the glovebox containing Ar gas, and then compressed and sintered into pellets by using home-made hot-pressing machine under the axial compressive stress of 40 MPa at 500 °C for 5 min. Finally, dense disk-shaped pellets with diameter of approximate Ф10 mm were obtained, which were then stored in a glovebox and polished before further measurements.
Material characterizations. SEM analysis was done in a Zeiss. Size and shape of the NCs were analyzed by transmission electron microscopy (TEM) using a ZEISS LIBRA 120 S-4 instrument, operating at 120 kV. X-ray powder diffraction (XRD) were performed on a Bruker AXS D8 Advance X-ray diffractometer with Ni-filtered (2 mm thickness) Cu Kα radiation (=1.5406 Å) operating at 40 kV and 40mA to identify phase and structure of samples. X-ray photoelectron spectroscopy (XPS) Analysis was performed on a SPECS system. Thermogravimetric analysis (TGA) was carried out on a Netzsch DSC at a heating rate of 10 K min -1 with argon gas atmosphere.
TE performance measurement. The hot-pressed sintered disk samples were polished with abrasive paper. The pellets are approximately10 mm in diameter and 1-1.5 mm in thickness.
Seebeck coefficient and electrical resistivity were simultaneously measured on LSR-3 Linseis system under helium atmosphere. Electrical properties were obtained by measuring the samples resistivity by the standard four-point probe method. The thermal diffusivity (λ) was measured on LFA 1000 Laser Flash (Netzsch, Germanny), and the total thermal conductivity ( tot ) was calculated by the equation tot = λ . is the specific heat capacity, which was determined by the Dulong-Petit law. 2 is the density of samples, and it was calculated via the Archimedes drainage method. For evaluation of carrier concentration (nH) and mobility (μH) at room temperature, Hall coefficient measurement system were performed on a Hall Effect Analyzer by Linseis Company with a magnetic field of 0.6 T (ezHEMS, NanoMagnetics) to obtain these two parameters via nH= 1/eRH and μH= σRH, respectively, where e is the electronic charge, RH is Hall coefficient.
The κe is proportional to the electrical conductivity σ according to the Wiedemann-Franz relation (equation 2).
Here, is the Boltzmann constant. e is the electron charge. ℎ is the Planck constant. The scattering factor is assumed to be- The reduced Fermi level η is a dimensionless parameter that corresponds one-to-one to the Seebeck coefficient S. 5 Thus, the calculation of η can be derived from the measured S by using the following relationship