Abstract
Surface segregation phenomena dictate core–shell preference of bimetallic nanoparticles and thus play a crucial role in the nanoparticle synthesis and applications. Although it is generally agreed that surface segregation depends on the constituent materials’ physical properties, a comprehensive picture of the phenomena on the nanoscale is not yet complete. Here we use a combination of molecular dynamics (MD) and Monte Carlo (MC) simulations on 45 bimetallic combinations to determine the general trend on the core–shell preference and the effects of size and composition. From the extensive studies over sizes and compositions, we find that the surface segregation and degree of the core–shell tendency of the bimetallic combinations depend on the sufficiency or scarcity of the surface-preferring material. Principal component analysis (PCA) and linear discriminant analysis (LDA) on the molecular dynamics simulations results reveal that cohesive energy and Wigner–Seitz radius are the two primary factors that have an “additive” effect on the segregation level and core–shell preference in the bimetallic nanoparticles studied. When the element with the higher cohesive energy also has the larger Wigner–Seitz radius, its core preference decreases, and thus this combination forms less segregated structures than what one would expect from the cohesive energy difference alone. 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 Wigner–Seitz radius difference is greater than ∼4%. Practical guides for predicting core–shell preference and degree of segregation level are presented.
Highlights
Bimetallic nanoparticles (NPs) with core−shell structures have shown improved performances in magnetics,[1−3] catalysis,[4−6] and optics[7,8] for a variety of applications
The bimetallic NPs with 0.5:0.5 composition are solidified as the temperature is decreased to room temperature, and their equilibrium structures are found after Monte Carlo simulations are performed
A combination of molecular dynamics (MD) and Monte Carlo (MC) simulations were carried out to predict the core−shell preferences of 45 bimetallic nanoparticle combinations consisting of 10 different metals
Summary
Bimetallic nanoparticles (NPs) with core−shell structures have shown improved performances in magnetics,[1−3] catalysis,[4−6] and optics[7,8] for a variety of applications. They have been successfully created using colloidal synthesis[9,10] or gas-phase condensation,[11,12] with flexible control over materials, particle size, and shell thickness. To design core−shell NPs with desired properties, it is crucial to understand the mechanisms that govern core−shell preference when two materials are mixed, that is, whether a core−shell structure will form and, if so, which material occupies the core and which forms the shell. We present a general trend in core−shell preference for bimetallic NPs that has been found by extensive molecular dynamics (MD) and Monte Carlo (MC) simulation studies carried out for 45 bimetallic NPs.
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