Chemical and Structural Segregation in Ternary Pt–Pd–Cu Nanoparticles: Molecular Dynamics Simulation

Abstract

Isothermal molecular dynamics, the LAMMPS program, and embedded atom method, have been employed to simulate the uniform heating of ternary Pt₁₇₀₀Pd₁₇₀₀Cu₅₁₀₀ nanoparticles (consisting of 8500 atoms, about 6.4 nm in size) and subsequent cooling (quenching) of nanodroplets to a final temperature of 300 K. The results obtained for the nanoparticles of this size have been compared with the data on smaller particles (4.6 nm) with the same percentage of the components (Pt_20%Pd_20%Cu_60%). The object of the research has been chosen due to the potential application of ternary Pt−Pd−Cu nanoparticles as nanocatalysts. To analyze the structural changes occurring in the nanoparticles during heating and cooling, the temperature dependences of the potential (cohesive) component of the specific internal energy and the gyration radii have been found and analyzed for all atomic subsystems (Pt, Pd, and Cu). Furthermore, the local structure of the nanoparticles has been investigated by the common neighbor analysis (CNA) implemented using the OVITO program. The composition of the two outer monolayers of solidified nanodroplets has been analyzed. The performed studies have revealed that the heating of nanoparticles with an initial uniform component distribution results in surface segregation of Pd atoms. The surface segregation of Pd is also characteristic of final configurations corresponding to solidified nanodroplets. In turn, Pt atoms are observed in the first (outer) atomic monolayer before nanoparticle melting and in the second and lower monolayers at all stages of heating and quenching. Not only the chemical segregation, i.e., the spatial separation of the particle components has been observed and described, but also the structural segregation, i.e., the separation into areas (grains) with specific crystalline structures. One of the most interesting results is the discovery of a multi-twinned structure and the icosahedral morphology in the final configurations (temperature of 300 K) formed after nanodroplet quenching.