Structure of NiCu Nanoparticles as derived from Monte Carlo and Molecular Dynamics simulations

DMITRII V. GLUKHOV; PAOLA QUAINO; SHOKIRBEK A. SHERMUKHAMEDOV; RENAT NAZMUTDINOV

Vladivostok

Congreso; V.A. Fock Meeting on Quantum and Computational Chemistry; 2015

V.A. Fock Meeting on Quantum and Computational Chemistry

Recently the surface structure of NiCu alloys is in the focus of a number of theoretical studies, while detailed information on segregation phenomena in NiCu nanoparticles (NP) is still lacking. Only a few attempts were made, however, to describe the structure of such NPs with the help of Monte Carlo (MC) and molecular dynamics (MD) method. In this work we employed atomistic MC and MD simulations (NVT ensemble) to gain a deeper insight into the structure of NiCu NPs of different size and shape, as well as in a wide range of their composition. Atomic interactions are described by a set pair Morse potentials and in the framework of Embedded Atom Method. Our computational strategy presumes the MD with annealing technique to model small NPs containing several hundreds atoms. Larger NPs (till 8400 atoms) are simulated using a lattice version of the Monte Carlo method. According to the results of simulations the segregation takes place at any composition of NPs. If the Ni fraction prevails, the Cu atoms tend to assemble in the surface region. In contrast, when the Ni is solute, the Ni atoms prefer to segregate at the particle surface. Our findings agrees qualitatively with the experimental data collected in literature for NiCu alloys. The most interesting prediction is the Ni segregation which supports challenging experimental observations reported in Refs. [1, 2]. The structure of model NPs differs, therefore, from the ?core-shall? and ?Janus-like? type, because the metal atoms are noticeably spread along the radial direction. Most of the metal atoms segregated on the surface area of NPs were found to form an island-like structure, although a minor number of isolated solute atoms are observed as well. This conclusion is supported by angular distributions calculated for the both metal atoms which clearly points to anisotropy. Such a striking feature originates purely from a competition between the interaction energies (enthalpy) and entropic factors and depends on the NP size. The formation of island-like patterns might elucidate the origin of segregation of solute Ni atoms. The Ni-Ni interaction energy which is stronger as compared with Ni-Cu and Cu-Cu might favour, on the one hand, the appearance of island structure. On the other hand, entropy factor is responsible for the extrusion of Ni atoms from core region of NP to its surface. We discuss the effect of size, shape of the NPs and their roughness on the segregation. Finally, emphasis is put on the comparison of the most qualitatively interesting features we observed with available experimental data.