Unexpected Multi-Step Transformation of AgCuS to AgAuS During Nanoparticle Cation Exchange

Cation exchange reactions can modify the compositions of colloidal nanoparticles, providing easy access to compounds or nanoparticles that may not be accessible directly. The most common nanoparticle cation exchange reactions replace monovalent cations with divalent cations or vice versa, but some monovalent-to-monovalent exchanges have been reported. Here, we dissect the reaction of as-synthesized AgCuS nanocrystals with Au⁺ to form AgAuS, initially hypothesizing that Au⁺ could be selective for Cu⁺ (rather than for Ag⁺) based on a known Au⁺-for-Cu⁺ exchange and the stability of the targeted AgAuS product. Unexpectedly, we found this system and the putative cation exchange reaction to be much more complex than anticipated. First, the starting AgCuS nanoparticles, which match literature reports, are more accurately described as a hybrid of Ag and a variant of AgCuS that is structurally related to mckinstryite Ag₅Cu₃S₄. Second, the initial reaction of Ag-AgCuS with Au⁺ results in a galvanic replacement to transform the Ag component to a AuyAg_1-y alloy. Third, continued reaction with Au⁺ initiates cation exchange with Cu⁺ in Au_yAg_1-y-AgCuS to form Au_yAg_1-y-Ag₃Cu_xAu_1-xS₂ and then Au_yAg_1-y-AgAuS, which is the final product. Crystal structure relationships among mckinstryite-type AgCuS, Ag₃Cu_xAu_1-xS₂, and AgAuS help to rationalize the transformation pathway. These insights into the reaction of AgCuS with Au⁺ reveal the potential complexity of seemingly simple nanoparticle reactions and highlight the importance of thorough compositional, structural, and morphological characterization before, during, and after such reactions.