We
studied the structural and compositional transformations of
colloidal covellite (CuS) nanocrystals (and of djurleite (Cu<sub>1.94</sub>S) nanocrystals as a control) when exposed to divalent cations, as
Cd<sup>2+</sup> and Hg<sup>2+</sup>, 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<sup>+</sup> ions. Under these experimental conditions, no remarkable
reactivity was indeed seen for both CuS and Cu<sub>1.94</sub>S 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 Cu<sub>1.94</sub>S, 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 covellite
nanocrystals, while it had no noticeable effect on the Cu<sub>1.94</sub>S ones. Once inside the covellite particles, Cd<sup>2+</sup> and
Hg<sup>2+</sup> cations engaged in exchange reactions, pushing the
expelled Cu<sup>+</sup> ions toward the not-yet exchanged regions
in the same particles, or out to the solution, from where they could
be recaptured by other covellite nanoparticles/domains. Because no
good solvating agent for Cu ions was present in solution, they essentially
remained in the nanocrystals