Nanoscale Transformations in Covellite (CuS) Nanocrystals in the Presence of Divalent Metal Cations in a Mild Reducing Environment

We studied the structural and compositional transformations of colloidal covellite (CuS) nanocrystals (and of djurleite (Cu1.94S) nanocrystals as a control) when exposed to divalent cations, as Cd2+ and Hg2+, at room temperature in organic solvents. All the experiments were run in the absence of phosphines, which are a necessary ingredient for cation exchange reactions involving copper chalcogenides, as they strongly bind to the expelled Cu+ ions. Under these experimental conditions, no remarkable reactivity was indeed seen for both CuS and Cu1.94S nanocrystals. On the other hand, in the covellite structure 2/3 of sulfur atoms form covalent S–S bonds. This peculiarity suggests that the combined presence of electron donors and of foreign metal cations can trigger the entry of both electrons and cations in the covellite lattice, causing reorganization of the anion framework due to the rupture of the S–S bonds. In Cu1.94S, which lacks S–S bonds, this mechanism should not be accessible. This hypothesis was pr...