title: Nanoscale Transformations in Covellite (CuS) Nanocrystals in the Presence of Divalent Metal Cations in a Mild Reducing Environment.
abstract: 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 covellite nanocrystals, while it had no noticeable effect on the Cu1.94S ones. Once inside the covellite particles, Cd2+and Hg2+cations engaged in exchange reactions, pushing the expelled Cu+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.
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Yi
authors.initials: Y
authors.lastname: Xie
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy ; IMEM-CNR , Parco Area delle Scienze 37/A, 43124 Parma, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Bertoni
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy ; Optical Materials Engineering Laboratory, ETH Zurich , 8092 Zurich, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Riedinger
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Ayyappan
authors.initials: A
authors.lastname: Sathya
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Mirko
authors.initials: M
authors.lastname: Prato
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Sergio
authors.initials: S
authors.lastname: Marras
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Renyong
authors.initials: R
authors.lastname: Tu
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Teresa
authors.initials: T
authors.lastname: Pellegrino
authors.affiliation: Istituto Italiano di Tecnologia (IIT) , via Morego, 30, 16163 Genova, Italy.
authors.forename: Liberato
authors.initials: L
authors.lastname: Manna