Geometries and segregation properties of platinum-palladium nanoalloy clusters

A detailed study is made of Pt, Pd and Pt-Pd bimetallic clusters, (PtPd)(M), with up to 56 atoms, modelled by the many-body Gupta potential. A Genetic Algorithm is used to find the lowest energy structures for each nuclearity and composition. A variety of structure types (icosahedral, decahedral, fcc close-packed and disordered) are observed for Pt clusters. The Pd clusters have similar geometries to those of Pt, though more icosahedral clusters and fewer disordered structures are found than for Pt. Global minima are generally more difficult to find for the bimetallic Pt-Pd clusters, due to the presence of homotops (structures with identical geometries but with different arrangements of the Pt and Pd atoms) as well as geometrical isomers. The structures found for the bimetallic clusters are different to those of either of the pure element clusters, with more decahedral structures and fewer icosahedra. Segregation is observed in the Pt-Pd clusters, with most having Pt-rich cores and Pd-rich surfaces. This is explained in terms of the lower surface energy of Pd and the higher cohesive energy of Pt. Doping of Pt atoms into Pd clusters (and vice versa) is found to lead to significant changes in cluster geometry. The effect of varying the Pt-Pd parameters of the Gupta potential on the geometrical structures and atomic segregation in Pt-Pd clusters is investigated and the parameters obtained by averaging the Pt-Pt and Pd-Pd parameters are found to give best agreement with experiment. Our results are generally in good agreement with previous experimental and theoretical studies of Pt, Pd and Pt-Pd clusters and related alloy systems.