Surface segregation and relaxation calculated by the embedded-atom method: Application to face-related segregation on platinum-nickel alloys

Abstract

The embedded-atom method (EAM) [M.S. Daw and M. I. Baskes, Phys. Rev. Lett. 50, 1285(1983; Phys. Rev. B 29, 6443 (1984)] is applied to surface segregation and surface relaxation of binary alloys. This method needs no ad hoc size-mismatch strain energy and can handle arbitrary interlayer distances at the surface. Three low-index faces of platinum-nickel alloys are studied.Previously, low-energy electron diffraction investigations have established a face-related segregation on platinum-nickel alloys, with platinum enrichment on the (111) surface and nickel enrichment on the (110) surface. This work shows that EAM is capable of reproducing the experimentally determined segregation and relaxation with a good accuracy. In addition, EAM predicts the existence of a metastable concentration profile on the Pto &Nio 5(110) surface. I. ImRuDUCTIOXIn the present paper the embedded-atom method' (EAM) is used to describe the energetics of atoms in substitutionally disordered binary alloys, where both multilayer segregation and multilayer relaxation are found to occur at the surface. The energy expression in the EAM is a function of atomic distances. This makes it possible to find both the composition and the geometry near the surface. A statistical approach is used to simulate the equilibrium state of the alloys.In this work the EAM is applied to platinum-nickel alloys which show a face-related segregation ' with platinum enrichment on the (111)surface and nickel enrichment on the (110) surface. The (110) surface is particularly interesting because of the large contraction of the spacing between the first and second layers. It is well known that platinum-nickel alloys are exceptions from simple segregation criteria, which are successfully applied to a large number of other alloys. The EAM gives a quantitatively correct description of the (111) surface of platinum-nickel alloys with respect to both concentration profile and surface relaxation. For the (110) surface the EAM indicates the existence of two equilibrium states of segregation. One state is in good agreement with experiment, and the other can be interpreted as a metastable state which may be obtained by a special treatment of the crystal. It is concluded that a good description of the surface relaxation is crucial for the calculation of segregation. The EAM is a semiempirical model, proposed by Daw and Baskes, ' for the energy in a pure metal or in an alloy. This model is used in a number of different applications, ' ' ' where various bulk and surface properties are studied. In a previous work Foiles' used Monte Carlo simulations with energies from the EAM to predict both concentrations and atomic positions for copper-nickel alloys. Surface energies and surface relaxations for pure metals are obtained, and the surface reconstructions on Pt(110) (Ref. 11) and Au(100) (Ref. 14) are predicted. These results make the EAM a promising model for energies at surfaces of pure metals and alloys.Surface segregation is of great technological importance. The properties of alloy su...

Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles

“…Using previous EAM potentials [26], the segregation energies for Ni in Pt (100) surfaces are determined to be: -0.01 eV in the first layer and -0.20 eV in the second layer; the segregation energies for Pt in Ni (100) surfaces are: -0.32 eV in the first layer and 0.00 eV in the second layer. These EAM potentials give an accurate description of segregation profile in (100) surfaces of Pt-Ni alloys [19,21]. Our MEAM results of segregation energy in Table V agree well with the EAM calculations.…”

“…IV) simulations. In this work, we chose to simulate segregation in extended surfaces of disordered Pt-Ni alloys at T=1200 K for a direct comparison with previous experiments [12][13][14][15][16][17][18] and simulations [19][20][21][22][23][24]. T=600 K is a typical reduction temperature for bimetallic nanoparticle catalysts [5] and is used here to investigate segregation in Pt-Ni nanoparticles.…”

“…To do that would require a much larger simulation cell and an MC algorithm that allows changes in the number of atoms from the fcc-crystal terminated surface configurations. It is also worth pointing out that none of previous theoretical work [19][20][21][22][23][24] explicitly takes the correct surface reconstruction into account when determining surface segregation profiles of Pt-Ni alloys. …”

“…It has been reported that the Pt-Ni alloy catalysts may even have enhanced activity compared to pure platinum catalysts, depending on how the surfaces are prepared [11]. Surface segregation phenomena in the Pt-Ni system have been extensively studied both in experiment [12][13][14][15][16][17][18] and theory [19][20][21][22][23][24]. Some very interesting phenomena in the Pt-Ni single crystal alloy surfaces (such as anisotropic surface segregation, oscillatory segregation profiles, and face-related segregation reversal) have been found.…”

Synthesis, electrochemical characterization and molecular dynamics studies of surface segregation of platinum nano-alloy electrocatalysts

“…8, which is in agreement with the behavior observed experimentally for different Pt x Ni y single crystal surfaces. [57][58][59][60] Finally, we have analyzed the atom coordination in the bulk and at the surface to obtain a qualitative understanding of the bulk morphology and the surface faces of the nanoparticles. Fig.…”