Size- and Composition-Dependent Structural Stability of Core-Shell and Alloy Pd-Pt and Au-Ag Nanoparticles

The structural stability and thermodynamic properties of core-shell nanoparticles (NPs) are very important for their applications. In this work, the bond energy (BE) model, originally for cohesive energy of pure NPs, has been generalized to core-shell nanosolids. The core-shell NPs can be divided into three regions: the core, the interface, and the shell. By summation of the atomic contributions of the three regions, the relation for the cohesive energy of core-shell NPs can be derived. Using this model, we discuss the cohesive energy and the phase stability of Pd-Pt and Au-Ag core-shell and alloy NPs. For 6.2 nm PtPd bimetallic NPs, the cohesive energy exhibits a linear relationship with the composition of Pd. Compared with Pt@Pd (core @shell) and PdPt nanoalloy NPs, the Pd@Pt NPs are most unstable. There exists critical composition for the stability of Pt@Pd NPs and PdPt nanoalloy, i.e., 6.2-nm PdPt nanoalloy is more stable than Pt@Pd NPs when the compositing of Pd exceeds 62%. The critical size increases with the increase of the composition of Pt. For Au-Ag bimetallic nanoparticles, the Ag@Au NPs are most unstable compared with Au@Ag and AuAg nanoalloy NPs. There exists critical composition for the stability of Au@Ag and AuAg nanoalloy NPs. The critical size increases with increasing of the composition of Au. We also use the generalized BE model to predict the melting of core shell NPs. For Pt@Pd NPs, the melting temperature of the Pt core increases with the increase of core size. Furthermore, the predicted surface melting temperature of Pt@Pd NPs is 1246 K, while the melting temperature of the Pd shell in Pt@Pd NPs lies in the region between the surface melting temperature and the particle melting temperature of Pd NPs.