Surface-segregation-induced phase separation in epitaxial Au/Co nanoparticles: Formation and stability of core-shell structures

We have studied formation and stability of core-shell structures in epitaxial Au/Co nanoparticles (NPs) by using atomic-resolution scanning transmission electron microscopy. As the particle size reduces, number of NPs having Au-shell increases and their frequency of occurrence reached 65%. Au segregation proceeds during particle growth at 520 K. The core-shell structure formation is particle size-dependent; the critical diameter dividing the Au-shell and the Co-shell structures is about 11 nm, below which the Au-shell is stable. After annealing at 800 K for 3.6 ks, Au-shell NPs were conserved while the Co-shell NPs changed to two-phase structures with a planar interface separating Au and Co. There is a local energy minimum where the Co-shell NP is metastable in the as-deposited state. A simple model based on surface and interfacial energies suggests stability of Au-shell structures. Surface-segregation-induced phase separation in small NPs, due to low surface free energy of Au, will be responsible for the Au-shell formation.