Realizing High Thermoelectric Properties in Bi₂S₃ Bulk via Band Engineering and Nanorods Compositing

Bismuth sulfide is a promising thermoelectric material because of its low cost and toxicity; however, its low electrical conductivity limits its thermoelectric properties. In this study, Bi₂S₃+x wt% HfCl₄ (x = 0, 0.25, 0.5, 0.75, and 1.0) bulk samples are fabricated using a combination of melting and spark plasma sintering. The microstructures, electronic structures, and thermoelectric properties of the composites are characterized. The results of electronic structure calculations show that doping with HfCl₄ produces an impurity energy level that narrows the bandgap and allows the Fermi energy level to enter the conduction band, leading to a favorable increase in carrier concentration. By regulating the HfCl₄ doping concentration, the electrical conductivity of the 0.75 wt% doped sample reaches 253 Scm^-1 at 423 K and its maximum ZT value is 0.47 at 673 K. Moreover, the sample is compounded with Bi₂S₃ nanorods prepared by the hydrothermal method, reducing thermal conductivity by 30% due to the introduction of additional interfaces and pores. This resulted in a final ZT value of 0.61 at 673 K, which is approximately eight times higher than that of pure Bi₂S₃. This step-by-step optimization approach provides a valuable methodology for enhancing the performance of other thermoelectric material systems.