Thermally Induced Domain Migration and Interfacial Restructuring in Cation Exchanged ZnS–Cu_1.8S Heterostructured Nanorods

Partial cation exchange reactions can be used to rationally design and synthesize heterostructured nanoparticles that are useful targets for applications in photocatalysis, nanophotonics, thermoelectrics, and medicine. Such reactions introduce intraparticle frameworks that define the spatial arrangements of different materials within a heterostructured nanoparticle, as well as the orientations and locations of their interfaces. Here, we show that upon heating to temperatures relevant to their synthesis and applications, the ZnS regions and Cu_1.8S/ZnS interfaces of heterostructured ZnS–Cu_1.8S nanorods migrate and restructure. We first use partial cation exchange reactions to synthesize a library of seven distinct samples containing various patches, bands, and tips of ZnS embedded within Cu_1.8S nanorods. Upon annealing in solution or in air, ex situ TEM analysis shows evidence that the ZnS domains migrate in different ways, depending upon their sizes and locations. Using differential scanning calorimetry, we correlate the threshold temperature for ZnS migration to the superionic transition temperature of Cu_1.8S, which facilitates rapid diffusion throughout the nanorods. We then use in situ thermal TEM to study the evolution of individual ZnS–Cu_1.8S nanorods upon heating. We find that ZnS domain migration occurs through a ripening process that minimizes small patches with higher-energy interfaces in favor of larger bands and tips having lower-energy interfaces, as well as through restructuring of higher-energy Cu_1.8S/ZnS interfaces. Notably, Cu_1.8S nanorods containing multiple patches of ZnS thermally transform into ZnS–Cu_1.8S heterostructured nanorods having ZnS tips and/or central bands, which provides mechanistic insights into how these commonly observed products form during synthesis.