Abstract
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.
Highlights
Metallic nanoparticles (NPs) have been actively studied due to their excellent properties including catalytic activities, magnetic, and magnetotransport properties, which are useful for industrial, environmental, and biomedical applications
NPs can be characterized by their structural properties,[1] where the chemical composition is an important factor determining structure and properties of NPs via alloying, ordering, or phase separation
Core-shell structure is one of the well-known examples from the viewpoints of phase separation, and it has been intensively studied for catalytic applications.[2,3,4]
Summary
Metallic nanoparticles (NPs) have been actively studied due to their excellent properties including catalytic activities, magnetic, and magnetotransport properties, which are useful for industrial, environmental, and biomedical applications. Surface-segregation-induced phase separation in epitaxial Au/Co nanoparticles: Formation and stability of core-shell structures Konno1 1Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan 2Department of Materials Science, Tohoku University, 6-6 Aramaki, Sendai 980-8579, Japan (Received 14 April 2017; accepted 6 June 2017; published online 14 June 2017)
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