Surface segregation in bimetallic nanoclusters: Geometric and thermodynamic effects

Nanocluster morphology (size, crystallographic faces, surface defects), cou pled to thermodynamic driving forces, influence surface segregation phenome na. These geometric and thermodynamic effects are studied at several temper atures and overall compositions for Cu-Ni, Ag-Cu, and Ag-Rh. The bond order metal simulation model [Zhu, L.; DePristo, A. E. J Catal 1997,167,400] is used in Monte Carlo simulations to describe interatomic interactions. Nanoc lusters from 110 to 729 atoms are arranged on stacks of layers correspondin g to face-centered cubic crystals. The exposed surfaces are (111), (110), a nd (331) faces, and the overall composition of the clusters defined in term s of the segregating atoms ranges from 5 to 95%. The effect of surface step s on surface segregation is also investigated. It is found that as overall concentration increases, the effect of surface defects on segregation pheno mena becomes more pronounced. The increase in the number of low coordinatio n sites due to the presence of surface steps modifies the layer-by-layer co mposition profile, causing surface and inner sites in the proximity of the step to become occupied by the segregating atoms. The population on other s urfaces becomes depleted, resulting in a lowered total surface segregation. These effects are more noticeable not only at high overall composition but also at high temperatures, especially for the less segregating system, Ag- Cu. (C) 2001 John Wiley & Sons, Inc.