Unraveling the Role of Sodium Thiosulfate in Copper Sulfide Synthesis

Our study reveals that the green copper sulfide synthesis from sodium thiosulfate and copper salts is a two-step process, where an intermediate product acts as a sacrificial template. The effect of various synthesis parameters, including the temperature applied in both reaction steps, solvent composition (water and ethylene glycol in ratios of 1:0, 3:1, 1:1, and 1:3), and copper precursor (CuSO4, Cu(NO3)2, CuCl2), on the morphology of the intermediate and final products was investigated in detail. By adjusting the ethylene glycol content, it was possible to obtain a hexagonal rod-like intermediate product with varying dimensions, which decomposed into tubular copper sulfide at room temperature. In aqueous solvents with a low precursor concentration (<0.10 M), the solubility limit of the rod-like sacrificial template was not exceeded, leading to the direct precipitation of CuS from the solution attaining eventually the spherical shape. Comprehensive structural analysis unveiled that the intermediate product was composed of sodium, copper(I), thiosulfate anion, and hydroxyl group, suggesting a Na2[Cu3(S2O3)2(OH)] formula. Furthermore, X-ray diffraction (XRD) ex situ time-resolved studies indicated that this compound decomposed to chalcocite (Cu2S) and subsequently to covellite (CuS) in the course of the reaction.