Mapping the morphology and thermal stability of AgCo nanoalloys: Composition-driven phase segregation and functional implications

Bimetallic silver-cobalt (AgCo) nanoparticles (NPs) form an immiscible nanoalloy, where functional behavior is dictated by phase segregation and morphology. Here we combine molecular dynamics (MD) annealing simulations with XRD, SAED, TEM, EDS, and EELS characterizations to establish a composition-morphology-thermal stability framework for AgCo NPs. The simulations map the energetic competition between Co@Ag Core-Shell and Janus-like architectures, with Ag preferentially segregating to the surface and similar stability of Core–Shell and Janus configurations at intermediate compositions. Experimentally, the synthesized AgCo NPs remain oxide-free and exhibit coexisting metallic Ag and Co phases, confirming chemical stability. The particles are predominantly spherical; Ag-rich systems favor isotropic growth, whereas Co-rich compositions show a stronger tendency toward aggregation. Thermal analyses demonstrate that AgCo nanoparticles undergo a two-step transformation pathway rather than a single melting point: Ag-rich regions destabilize first, and Co-rich domains melt at higher temperature, while the Ag-transition temperature and peak shape are morphology dependent. These results define a thermal switching window in which the Ag surface structure can be reconfigured while the Co-rich framework remains comparatively stable, providing design rules for magnetically recoverable, thermally responsive AgCo nanoalloys.