Competitive Cation Exchange and Etching Processes in Metal Sulfide Nanoparticles from Anion/Ligand Cooperativity and Exogenous Cations

Cation exchange reactions enable the construction of sophisticated multimaterial architectures within nanoparticles, with certain regions capable of being etched to create void spaces and sculpted morphologies. However, the chemical reactivity that leads to etching often remains unknown and therefore uncontrollable, making it an undesirable side reaction rather than a reaction that can be harnessed for nanostructure design. Here, we report the discovery that spectator anions and exogenous cations that are assumed to be innocent during nanoparticle cation exchange reactions instead drive an etching process that is competitive with exchange. Attempts to exchange some of the Cu⁺ cations in roxbyite Cu_1.8S nanorods with Ga³⁺ using GaCl₃ proceed as expected, but when the same reaction is applied to heterostructured ZnS-Cu_1.8S, CdS-Cu_1.8S, and CuInS₂-Cu_1.8S nanorods, Ga³⁺ exchange does not occur, but instead the Cu_1.8S regions are etched. Through a comprehensive series of experiments, we identified three reaction components that work cooperatively to favor etching over exchange: Cl^- anions from GaCl₃ or another chloride salt, trioctylphosphine that is present to help drive the cation exchange reaction, and the presence of exchangeable metal cations in solution that can originate from the non-Cu_1.8S regions of the heterostructured nanorods. These insights reveal that nanoparticle cation exchange and etching exist on a continuum and that these competitive processes can be harnessed to control and tune nanostructural features.