Tailoring microstructure and transport properties of bismuth sulfide via microwave-assisted synthesis for enhanced thermoelectric performance

This study presents a systematic optimization of microwave-assisted hydrothermal synthesis and thermal annealing of bismuth sulfide (Bi₂S₃) for thermoelectric applications. Using a design of experiments (DOE) approach, the optimal synthesis parameters were identified as a sulfur-to-bismuth (S:Bi) precursor ratio of 2.06, microwave temperature of 160 °C, and reaction duration of 16 min. These conditions yielded monoclinic cannonite (Bi₂SO₇) with a predicted dimensionless figure of merit (ZT) of 1.61 × 10⁻⁷. The experimental validation produced a ZT of 1.73 × 10⁻⁷, differing by only 7.2 % from the predicted value. A comparison with a Bi₂S₃ reference sample (BS9; S:Bi = 2, 80 °C, 30 min) revealed that although Bi₂SO₇ exhibited a higher as-synthesized ZT, thermal annealing increased the ZT of Bi₂S₃ but decreased that of Bi₂SO₇. In Bi₂S₃, annealing enhanced the crystallinity, electrical conductivity, and Seebeck coefficient, yielding a power factor of 23.36 μW/m·K² and a ZT of 0.0133. However, excessive annealing leads to sulfur volatilization and performance loss. Structural and thermal stability analyses confirmed phase retention up to 600 °C, supporting the feasibility of Bi₂S₃-based materials for high-temperature thermoelectric applications. These findings establish a framework for optimizing Bi₂S₃ synthesis and post-treatment toward superior performance through doping and advanced annealing strategies.