General Trends in Core-shell Preferences for Bimetallic Nanoparticles

Core-shell nanoparticles have gathered much attention of the scientific community owing to their potential applications in various fields including biomedical imaging and catalysis. Predicting core-shell preference1-3 is, however, still largely based on a few experimental observations and limited theorical studies, and hence development of new core-shell nanoparticles is normally built on a trial-and-error approach. Here we present general trends of core-shell preferences for 45 bimetallic nanoparticle systems studied by molecular dynamics (MD) and Monte Carlo (MC) simulations. Simulations were performed using LAMMPS code and the embedded-atom method (EAM) potentials were employed for simulating the interactions between atoms in the bimetallic nanoparticle systems composed of 10 metals; Ag, Cu, Au, Pd, Fe, Co, Ni, Pt, Al, and Mo. In order to quantify the core-shell preference, the MD/MC results were analysed to identify surface atoms using the alpha-shapes method. The core and shell compositions of the preferred equilibrium structures of bimetallic combinations were then used to categorize each combination into one of four different types depending on the level of core- shell tendency: mixed, core-shell, highly segregated core-shell, Janus-like. The categorized MD/MC results were also analysed using principal component analysis (PCA) and linear discriminant analysis (LDA) to determine the primary factors that dictate core-shell tendency. Eight possible factors were considered, and cohesive energy and atomic radius are found to be the two primary factors that have an ‘additive’ effect on the segregation level and core-shell preference in the bimetallic nanoparticles studied. In the majority of the investigated combinations, the element with higher cohesive energy has smaller atomic radius and tend to occupy the core. Highly segregated structures (highly segregated core-shell or Janus-like) are expected to form when both the relative cohesive energy difference is greater than ~ 20 % and the relative atomic radius difference is greater than ~ 4 %. However, when the element with higher cohesive energy has larger atomic radius, the core-shell tendency decreases. The general trend observed in the current study can be used as a guide in nanoparticle synthesis methods in which heat-induced surface segregation phenomena play an essential role, and in predicting the equilibrium structures of bimetallic nanoparticles.