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

Abstract

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.

Cubic AgBiS2 Powder Prepared Using a Facile Reflux Method for Photocatalytic Degradation of Dyes

“…Here, the estimated value of bandgap is around 0.98 eV by assuming direct bandgap via plotting (αhν) 2 vs. (hν), as shown in Figure 7b. This result is in a good agreement with the reported values of AgBiS 2 grown via other methods [6,23]. While the AgBiS 2 powders provide a bandgap of 0.92 eV via the indirect bandgap Tauc plot, their linear fitting relationship is worse than that of the direct bandgap.…”

“…As shown in Figure 6, the black thermogravimetric (TG) curve represents the weight of the powders, the derivative thermogravimetric (DTG) curve represents the rate of weight change of the powders, and the blue differential thermal analysis (DTA) curve corresponds to the heat absorption and exothermic change of the powder sample as the temperature rises. It can be seen from the figure that when the temperature rises from room temperature to 700 • C, the weight loss of the powder sample does not exceed 5%; thus, it can be considered that AgBiS 2 has thermal stability below 700 • C. These results are comparable to previously reported values [23]. The observed weight gain or loss, as well as the absorptive or exothermic processes of the system observed in the curves, occur due to the small number of heterogeneous phases in the powder beyond the accuracy of XRD testing, including the desorption of adsorbed water, the heat absorption process of impurity phase oxidation, and the heat release process of decomposition.…”

Three-Fold Coordination of Copper in Sulfides: A Blockade for Hole Carrier Delocalization but a Driving Force for Ultralow Thermal Conductivity

“…The ρ(T) curve follows a nondegenerate semiconducting behavior with values decreasing from 130 Ω cm at 300 K to 0.36 Ω cm at 563 K. In the previous studies on Cu 3 BiS 3 , Wei et al reported a comparable value of 140 Ω cm at RT, while Adeyemi et al measured an electrical resistivity value of 17.2 Ω cm at 318 K, i.e., one order of magnitude lower than our value. 29,30 This is mainly explained by the larger grain sizes (1 order of magnitude larger) in the compounds synthesized by Adeyemi et al The high electrical resistivity in our sample is consistent with its high Seebeck coefficient S of 800 μV K −1 at RT (Figure S8b) and relatively low hole concentration of 1.7 × 10 16 cm −3 . Those values attest to the relatively high purity and stoichiometry of our sample thanks to the synthesis approach which combines mechanical-alloying followed by SPS, preventing heavy sulfur loss and stoichiometric deviation.…”

“…The pristine phase Cu 3 BiS 3 has an extremely low lattice thermal conductivity (κ ≈ κ L due to the negligible electronic contribution) of 0.43 W m –1 K –1 at 300 K, which decreases as the temperature increases to reach 0.32 W m –1 K –1 at 563 K (Figure a). These values are slightly higher than those reported by Jia et al (0.32 W m –1 K –1 at 300 K), Adeyemi et al (0.29 W m –1 K –1 at 313 K), and Wei et al (0.17 W m –1 K –1 at 300 K). These differences could be the result of additional impurity phases and/or lower bulk sample density in these prior studies.…”

“…Among all the thermoelectric copper-rich sulfides, the tetrahedrite family Cu 12– x T x Sb 4 S 13 is, to the best of our knowledge, the only one that involves 3-fold coordinated copper. In this respect, the recent synthesis of the copper-rich sulfide wittichenite Cu 3 BiS 3 with low lattice thermal conductivity, − which was previously studied as a thin film for photovoltaic absorbers, − appears as a very interesting route for the investigation of structural features (coordinations, cationic networks) governing the electronic and thermal conductivity of these materials.…”