Study of the Thermoelectric Properties of Bi₂Te₃/Sb₂Te₃ Core-Shell Heterojunction Nanostructures

Thermoelectric materials convert heat energy into electricity, hold promising capabilities for energy waste harvesting, and may be the future of sustainable energy utilization. In this work, we successfully synthesized core-shell Bi₂Te₃/Sb₂Te₃ (BTST) nanostructured heterojunctions via a two-step solution route. Samples with different Bi₂Te₃ core to Sb₂Te₃ shell ratios could be synthesized by controlling the reaction precursors. Scanning electron microscopy images show well-defined hexagonal nanoplates and the distinct interfaces between Bi₂Te₃ and Sb₂Te₃. The similarity of the area ratios with the precursor ratios indicates that the growth of the Sb₂Te₃ shell mostly took place on the lateral direction rather than the vertical. Transmission electron microscopy revealed the crystalline nature of the as-synthesized Bi₂Te₃ core and Sb₂Te₃ shell. Energy-dispersive X-ray spectroscopy verified the lateral growth of a Sb₂Te₃ shell on the Bi₂Te₃ core. Thermoelectric properties were measured on pellets obtained from powders via spark plasma sintering with two different directions, in-plane and out-of-plane, showing anisotropic properties due to the nanostructure alignment in the pellets. All samples showed a degenerate semiconducting character with the electrical resistivity increasing with the temperature. Starting from Sb₂Te₃, the electrical resistivity increases with the increase in amounts of Bi₂Te₃. Thermal conductivity is lowered due to the increase in interfaces and additional phonon scattering. We show that the out-of-plane direction of the BTST 1-3 sample (where 1-3 indicates the ratio of BT to ST) demonstrates a high Seebeck value of 145 μV/K at 500 K which may be attributed to an energy filtering effect across the heterojunction interfaces. The highest overall zT is observed for the BTST 1-3 sample in the out-of-plane direction at 500 K. The zT values increase continuously over the measured temperature range, indicating a probable higher value at increased temperatures.