Atomistic Simulations of fcc Ft 75 Ni 25 and Ft 75 Re 25 Cubo-octahedral Nanoparticles
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Atomistic Simulations of fcc Pt75Ni25 and Pt75Re25 Cubo-octahedral Nanoparticles Guofeng Wang 1, M.A. Van Hove 1,2,3, P.N. Ross 1, and M.I. Baskes 4 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 2 Advanced Light Source, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 3 Department of Physics, University of California, Davis, CA 95616 4 MST-8 Structure and Property Relations Group, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT We have developed interatomic potentials for Pt-Ni and Pt-Re alloys within the modified embedded atom method (MEAM). Furthermore, we applied these potentials to study the equilibrium structures of Pt75Ni25 and Pt75Re25 nanoparticles at T=600 K using the Monte Carlo method. In this work, the nanoparticles are assumed to have disordered fcc cubo-octahedral shapes (terminated by {111} and {100} facets) and contain from 586 to 4033 atoms (corresponding to a diameter from 2.5 to 5 nm). It was found that, due to surface segregation, (1) the Pt75Ni25 nanoparticles form a surface-sandwich structure: the Pt atoms are enriched in the outermost and third atomic shells, while the Ni atoms are enriched in the second atomic shell; (2) the equilibrium Pt75Re25 nanoparticles adopt a core-shell structure: a Pt-enriched shell surrounding a Pt-deficient core. INTRODUCTION Developing low-cost yet high-performance electrodes is a major thrust in fuel cell research. In the past years, considerable progress has been made in characterizing and understanding the catalytic properties of Pt and Pt-bimetallic bulk alloy surfaces [1]. Due to high surface-volume ratio, Pt and Pt-bimetallic nanoparticles can perform better than thin films as electro-catalysts in lower temperature polymer electrolyte fuel cells. Since Pt is a precious active catalyst with a wide range of applications, it is highly desirable to design catalyst nanoparticles in which the Pt atoms are arranged predominantly at the outer surfaces. Complementary to experimental techniques, atomistic simulations can provide much insight into the equilibrium structures of nanoparticles in the nanoscale level and is the approach we take in this work. We chose to investigate Pt-Ni and Pt-Re nanoparticles, because Pt-Ni and Pt-Re alloys are currently under experimental scrutiny and also represent two different types of surface segregation in bimetallic alloys. In the (111) and (100) surfaces of Pt-Ni alloys, the Pt atoms will be enriched in the first and third atomic layers while the Ni atoms are enriched in the second atomic layer. In contrast, the concentrations of Pt atoms will be highest in the outermost layer of Pt-Re surfaces and decrease gradually deeper inside the bulk. Moreover, Pt-Ni is an fcc-fcc bimetallic alloy, while Pt-Re is an fcc-hcp bimetallic alloy. It is quite challenging to develop a generic model to describe both systems. Baskes [2,3] solved this problem by adding directional bonding to the original embedded-atom method (
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