Deep Eutectic Solvent-Assisted Microwave Synthesis of Thermoelectric AgBiS₂ and Cu₃BiS₃

AgBiS2 and Cu3BiS3 were synthesized via a microwave-assisted solution route using a deep eutectic solvent (DES). The DES of choice consists of choline chloride and thiourea, with thiourea acting as a sulfur source. The DES synthesis route provides a fast, environmentally friendly, and low-temperature alternative to high-temperature (HT) synthesis. We compared DES synthesis to the synthesis from an aqueous solution and determined that the DES is preferred for the synthesis of Cu3BiS3, while deionized water is the better solvent for the microwave-assisted synthesis of AgBiS2. The syntheses of AgBiS2 and Cu3BiS3 proceed via Bi2S3 intermediate that reacts with the +1 cations in solution when synthesis is carried out in DES or water. The DES synthesis yields ∼0.5 μm particles with a rod-like (AgBiS2) or rock-like (Cu3BiS3) morphology. The measured optical indirect band gap is 0.9 eV for AgBiS2 and 1.21 eV for Cu3BiS3. AgBiS2 powder is thermally stable in vacuum up to 700 °C as opposed to Cu3BiS3 powder. In situ HT powder X-ray diffraction revealed that Cu3BiS3 shows structural transitions upon heating. The transformation of Cu3BiS3 from the room-temperature P212121 polymorph to the HT polymorph occurs between 100 and 122 °C. The HT polymorph is stable until 476 °C and completely decomposes to crystalline Cu2S and “Bi2S3” melt at 527 °C. The Cu3BiS3 polymorphic transition at 100 °C is evident from the measured Seebeck coefficient, resistivity, and thermal conductivity. Cu3BiS3 is a p-type semiconductor. The thermoelectric figure of merit, zT, at 230 °C of the DES-synthesized Cu3BiS3 is comparable to the previously reported thermoelectric properties of the Cu3BiS3 synthesized via the HT route from elements.