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

摘要

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 proven by the experimental evidence that adding ascorbic acid increased the fraction of metal ions incorporated in the covellitenanocrystals, while it had no noticeable effect on the Cu1.94S ones. Once inside the covellite particles, Cd²⁺ andHg²⁺ cations engaged in exchange reactions, pushing theexpelled Cu⁺ ions toward the not-yet exchanged regionsin the same particles, or out to the solution, from where they couldbe recaptured by other covellite nanoparticles/domains. Because nogood solvating agent for Cu ions was present in solution, they essentiallyremained in the nanocrystals.