Enhanced thermoelectric properties of p-type polycrystalline SnSe by regulating the anisotropic crystal growth and Sn vacancy*

Thermoelectric selenides have attracted more and more attentions recently. Herein, p-type SnSe polycrystalline bulk materials with good thermoelectric properties are presented. By using the SnSe2 nanostructures synthesized via a wet-chemistry route as the precursor, polycrystalline SnSe bulk materials were successfully obtained by a combined heat-treating process under reducing atmosphere and following spark plasma sintering procedure. As a reference, the SnSe nanostructures synthesized via a wet-chemistry route were also fabricated into polycrystalline bulk materials through the same process. The thermoelectric properties of the SnSe polycrystalline transformed from SnSe2 nanostructures indicate that the increasing of heattreating temperature could effectively decrease the electrical resistivity, whereas the decrease in Seebeck coefficient is nearly invisible. As a result, the maximum power factor is enhanced from 5.06\times {10}^{-4}\,{\rm{W}}/{\rm{m}}\cdot {{\rm{K}}}^{2} to 8.08\times {10}^{-4}\,{\rm{W}}/{\rm{m}}\cdot {{\rm{K}}}^{2} at 612 °C. On the other hand, the reference sample, which was obtained by using SnSe nanostructures as the precursor, displays very poor power factor of only 1.30\times {10}^{-4}\,{\rm{W}}/{\rm{m}}\cdot {{\rm{K}}}^{2} at 537 °C. The x-ray diffraction (XRD), scanning electron microscope (SEM), x-ray fluorescence (XRF), and Hall effect characterizations suggest that the anisotropic crystal growth and existing Sn vacancy might be responsible for the enhanced electrical transport in the polycrystalline SnSe prepared by using SnSe2 precursor. On the other hand, the impact of heat-treating temperature on thermal conductivity is not obvious. Owing to the boosting of power factor, a high zT value of 1.07 at 612 °C is achieved. This study provides a new method to synthesize polycrystalline SnSe and pave a way to improve the thermoelectric properties of polycrystalline bulk materials with similar layered structure.