Thermally Driven Solid-Phase Ion Exchange for In Situ Transition from Heterocore–Shell to Alloy Nanostructures

Rational design and precise control of ion exchange reactions constitute fundamental prerequisites for fabricating multielement nanostructures with tailored functionalities in emerging applications spanning nanophotonics, semiconductor devices, photocatalysis, and batteries. A variety of nanostructures have been successfully synthesized by regulating thermodynamic parameters, including temperature, ionic species, and ligands, during ion exchange processes. However, critical gaps persist in directly resolving the atomic-scale spatial rearrangement mechanism, which necessitates systematic investigation of dynamic ion exchange behavior and the correlated microstructure evolution. Herein, we employ in situ electron microscopy to decipher solid-state ion migration pathways in heterocore-shell nanowire systems, quantitatively analyzing formation processes and reaction kinetics under a uniform thermal field. Our findings demonstrate that thermal activation promotes Ag ion diffusion along Te_XSe_<i>y</i>@Se core-shell NWs and the bulk diffusion of Te, enabling the formation of homogeneous Ag₂Se_<i>x</i>Te_1-<i>x</i> alloys through intracrystalline reorganization, differing from electron beam irradiation, which induces surface-dominated Ag ion diffusion, yielding dual-core-shell architectures. Phase-field simulations unambiguously reconstruct these divergent transport pathways and structural transformations. Besides, with a sufficient Ag supply, up to 5.5 mm migration distance can be achieved, resulting in a centimeter-scale nanofilm. This mechanistic understanding of solid-state ion kinetics and phase transition dynamics establishes a paradigm for intentional interconversion between multiphase heterostructures and monophasic systems, providing a design framework for next-generation functional nanomaterials.