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 the 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 the exchange energy where the partial bond energy can be empirically calculated from the experimental values of dimer energy, the pressure-dependent surface energy and the bulk cohesive energy of the constituent metals. The advantage of this procedure is the ability to estimate the pressure-dependent surface composition of the nanoparticles. The method has been applied to oxygen-adsorbed Pd–Ag systems; and it is found that while Ag segregate in clean 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-pressure 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.