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.