Role of the solvent medium in the wet-chemical synthesis of CuSbS₂, Cu₃SbS₃, and bismuth substituted Cu₃SbS₃

Co-thermal decompositions of equimolar concentrations of thiourea complexes ([Cu(tu)₃]Cl and [Sb(tu)₂]Cl₃) in ethylene glycol and ethanolamine were attempted. Based on the results from powder X-ray diffraction, Raman spectroscopy, and energy dispersive spectral analysis of the products, Cu₃SbS₃ in cubic symmetry emerged from ethylene glycol. In contrast, orthorhombic CuSbS₂ resulted from the ethanolamine medium. The generation of copper-rich and copper-poor sulfides was found to be the major reason behind the formation of these stoichiometries of the Cu-Sb-S system as verified by reacting the preformed Cu-S and Sb-S species in ethanolamine and ethylene glycol. Attempts to include bismuth (up to 50 mol%) for antimony were successful only in ethylene glycol medium. The inclusion of bismuth stabilized the orthorhombic form of Cu₃SbS₃, possibly due to the non-bonded lone pairs present on Bi³⁺ and Sb³⁺-ions. The inclusion of bismuth confirmed from the successful refinement of powder X-ray diffraction pattern by the Rietveld method, Raman spectroscopy, and energy dispersive spectroscopy analysis. CuSbS₂, Cu₃SbS₃ (cubic) and Cu₃Sb_0.50Bi_0.50S₃ showed broad absorption extending up to visible region in their UV-visible spectra. The bandgap values of 1.31, 1.40 and 0.94 eV were estimated by Tauc plots for Cu₃SbS₃, CuSbS₂, and Cu₃Sb_0.50Bi_0.50S₃, respectively.

Graphic abstract

CuSbS₂, Cu₃SbS₃ (cubic) and Cu₃Sb_0.50Bi_0.50S₃ showed broad absorption extending up to visible region in their UV-visible spectra. The bandgap values of 1.31, 1.40 and 0.94 eV were estimated by Tauc plots for Cu₃SbS₃, CuSbS₂, and Cu₃Sb_0.50Bi_0.50S₃, respectively.