Formation, evolution and characteristics of copper sulfide nanoparticles in the reactions of aqueous cupric and sulfide ions

Colloidal copper sulfides produced in reactions of aqueous copper and sulfide ions are important for many materials applications, environment and mineral processing. Here, CuxS nanoparticles formed and aged at varying copper sulfate to sodium sulfide ratios were studied using in situ UV–vis–NIR spectroscopy, dynamic light scattering, X-ray absorption spectroscopy, ex situ TEM, X-ray photoelectron spectroscopy and Raman scattering, and DFT + U calculations. It was established that the ratio of aqueous Cu2+ to S2− ions of 1:2 is critical for the reaction, which yields disordered covellite-like 4–6 nm Cu0.7S nanoparticles comprised polysulfide species at this and higher sulfide concentrations; upon aging, the particles release sulfur and transform to chalcocite-like structure (Cu2-xS, x < 1). Conversely, at the “excess” of copper ions, Cu2-xS-type particles grew into 12–14 nm “covellitic” nanoparticles. The optical absorbance at 1100–1200 nm commonly attributed to localized surface plasmon resonance increased with time and was lowest for Cu2+/S2− = 1:2. DFT + U calculations found that polysulfide stabilizes copper-deficit covellite, while Cu vacancies in chalcocite are more favorable and destabilized by S–S bonding; the Fermi level energy increases and the hole density decreases with polysulfide formation. We believe that CuS2 clusters form initially, and following conversion of disulfide to polysulfide and then monosulfide ions rather than release of Cu determines the character of CuxS nanoparticles.