Study of the Thermoelectric Properties of BiTe/SbTe 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 BiTe/SbTe (BTST) nanostructured heterojunctions via a two-step solution route. Samples with different BiTe core to SbTe shell ratios could be synthesized by controlling the reaction precursors. Scanning electron microscopy images show well-defined hexagonal nanoplates and the distinct interfaces between BiTe and SbTe. The similarity of the area ratios with the precursor ratios indicates that the growth of the SbTe shell mostly took place on the lateral direction rather than the vertical. Transmission electron microscopy revealed the crystalline nature of the as-synthesized BiTe core and SbTe shell. Energy-dispersive X-ray spectroscopy verified the lateral growth of a SbTe shell on the BiTe 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 SbTe, the electrical resistivity increases with the increase in amounts of BiTe. 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 is observed for the BTST 1-3 sample in the out-of-plane direction at 500 K. The values increase continuously over the measured temperature range, indicating a probable higher value at increased temperatures.