Thermally Induced Domain Migration and Interfacial Restructuring in Cation Exchanged ZnS-CuS 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 CuS/ZnS interfaces of heterostructured ZnS-CuS 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 CuS nanorods. Upon annealing in solution or in air, 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 CuS, which facilitates rapid diffusion throughout the nanorods. We then use thermal TEM to study the evolution of individual ZnS-CuS 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 CuS/ZnS interfaces. Notably, CuS nanorods containing multiple patches of ZnS thermally transform into ZnS-CuS heterostructured nanorods having ZnS tips and/or central bands, which provides mechanistic insights into how these commonly observed products form during synthesis.