Scalable synthesis of Cu-Sb-S phases from reactive melts of metal xanthates and effect of cationic manipulation on structural and optical properties.

We report a simple, economical and low temperature route for phase-pure synthesis of two distinct phases of Cu-Sb-S, chalcostibite (CuSbS2) and tetrahedrite (Cu12Sb4S13) nanostructures. Both compounds were prepared by the decomposition of a mixture of bis(O-ethylxanthato)copper(II) and tris(O-ethylxanthato)antimony(III), without the use of solvent or capping ligands. By tuning the molar ratio of copper and antimony xanthates, single-phases of either chalcostibite or tetrahedrite were obtained. The tetrahedrite phase exists in a cubic structure, where the Cu and Sb atoms are present in different coordination environments, and tuning of band gap  energy was investigated by the incorporation of multivalent cationic dopants, i.e. by the formation of Zn-doped tetrahedrites Cu12-xZnxSb4S13 (x = 0.25, 0.5, 0.75, 1, 1.2 and 1.5) and the Bi-doped tetrahedrites Cu12Sb4-xBixS13 (x = 0.08, 0.15, 0.25, 0.32, 0.4 and 0.5). Powder  X-ray diffraction (p-XRD) confirms single-phase of cubic tetrahedrite structures for both of the doped series. The only exception was for Cu12Sb4-xBixS13 with x = 0.5, which showed a secondary phase, implying that this value is above the solubility limit of Bi in Cu12Sb4S13 (12%). A linear increase in the lattice parameter a in both Zn- and Bi-doped tetrahedrite samples was observed with increasing dopant concentration. The estimated elemental compositions from EDX data are in line with the stoichiometric ratio expected for the compounds formed. The morphologies of samples were investigated using SEM and TEM, revealing the formation of smaller particle sizes upon  incorporation of  Zn. Incorporation of Zn or Bi into Cu12Sb4S13 led to an increase in band gap energy. The estimated band gap energies of Cu12-xZnxSb4S13 films ranges from 1.49 to 1.6 eV, while the band gaps of Cu12Sb4-xBixS13 films increases from 1.49 to 1.72 eV with increasing x.