Core/shell formation and surface segregation of multi shell icosahedral silver-palladium bimetallic nanostructures: A dynamic and thermodynamic study

• Nanostructures of PdmAgn clusters for m + n = 55 and 147 have been studied.
• Structures favor the formation of palladium-core surrounded by silver shell.
• Calculated thermodynamic parameters confirm the energetic results.
• Core/shell formation is favored at concentration of silver.
• Silver segregation on surface while palladium penetration to core is observed.

Core/shell formation and surface segregation of multi shell icosahedral bimetallic silver-palladium nanostructures with the size of 55 and 147 atoms were studied by using the Molecular Dynamics simulations, and calculating Helmholtz free energy changes in the penetration of palladium atoms from shell to core, core to shell transition of silver and melting temperatures by using statistical mechanical densities of states. In 55 atoms icosahedra, two core–shell motifs, Ag13Pd42 and Pd13Ag42 with their isomers Pd13(Pd29Ag13) and Ag13(Ag29Pd13) were considered. Similarly in 147 atoms icosahedra, all mutations corresponding to the occupations of either silver atoms or palladium atoms in the core, inner shell or outer shell and their isomers generated by interchanging thirteen core atoms with thirteen atoms of the other type in the inner and outer shells were considered. It is found that the palladium-core clusters are more stable than the silver-core clusters and cohesive energy increases with the palladium composition. Phase transition of each cluster was studied by means of constant volume heat capacity. The trend in variation of melting temperature is accordance with the energy trend. Helmholtz free energy changes in palladium penetration, core to shell transition of silver and in surface mixing and segregation revealed the thermodynamic stability of the formation of PdcoreAgshell structures especially at silver rich environment and the surface segregation of silver.

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