Core-Shell Nanoparticles Driven by Surface Energy Differences in the Co-Ag, W-Fe, and Mo-Co Systems

Core–shell nanoparticles are known to form in binary systems using a one-step gas-condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in surface energy between component atoms creates a preferential surface phase leading to a core–shell structure. Here, core–shell nanoparticles have been observed in systems with enthalpy as low as −5 kJ mol−1 and a surface energy difference of 0.5 J m−2 (Mo–Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The compositions and size dependence of the core–shell structures are also compared and no core–shell structures are observed below a critical size of 8 nm.