Adsorption-introduced MC simulation technique for segregation studies in Pd-Ag nanoparticles

We report here a new approach to introduce the effect of chemisorption in t he Monte-Carlo simulation procedure for studying the segregation behaviour in bimetallic nanoparticles at low pressures. A coordination-dependent pair interaction energy is constructed in terms of partial bond energies and th e exchange energy where the partial bond energy can be empirically calculat ed from the experimental values of dimer energy, the pressure-dependent sur face energy and the bulk cohesive energy of the constituent metals. The adv antage of this procedure is the ability to estimate the pressure-dependent surface composition of the nanoparticles. The method has been applied to ox ygen-adsorbed Pd-Ag systems; and it is found that while Ag segregate in cle an particles, at higher oxygen pressures Pd atoms segregate to the surface. The present technique has been argued to be suitable for higher pressures ( > 10(-5) Torr) when coverage cannot be accurately determined. At low-pres sure regime(< 10(-5) Torr) the results compare qualitatively well with the previous MC results where the effects of adsorbates were considered through a coverage-dependent additional term in the configuration energy. (C) 2000 Elsevier Science B.V. All rights reserved.