Multistep Regioselectivity and Non-Kirkendall Anion Exchange of Copper Chalcogenide Nanorods

Chemical reactions that modify the compositions of nanoparticles are important for optoelectronic and catalytic applications. Understanding how they occur, and the unique features that can be produced as a result, is an important prerequisite to designing intricate nanostructures with complex morphologies. Here, we report the conversion of alpha-chalcocite copper sulfide nanorods into weissite copper telluride nanorods through anion exchange. By examining the elemental composition, morphology, and crystallinity post exchange, it was found that the tellurium ions replaced sulfur ions to generate weissite in a way that maintained the (pseudo-)hexagonally close-packed sublattice as well as the morphology and crystallinity. Unusually, the anion exchange proceeded without inducing voids in the product nanoparticles. Such voids, produced through the Kirkendall effect, are commonly observed during nanocrystal anion exchange reactions, yet can be important to avoid minimizing defects. The lack of void formation was explained by the balancing of inward and outward anion diffusion offered by nanoscopic pathways that formed within the nanorods at the early stages of the anion exchange reaction. The presence of these exchange-facilitating faults results in an unusual multistep process altering the locations at which copper telluride regions emerged during partial exchange. Three different anion movement regimes resulted in three distinct geometries. Initially, a near-isotropic exchange produced a copper sulfide/copper telluride core-shell structure. As the exchange progressed, the defect-mediated movement resulted in irregularly shaped copper sulfide domains within copper telluride. Phase segregation then led to a unique double-core copper sulfide/copper telluride heterostructure. This work offers insights into the mechanism behind anion exchange, highlights the design capabilities emergent from defective materials, and creates new opportunities for rational synthesis of complex nanoheterostructures.