Prediction of distinct surface segregation effects due to coordination-dependent bond-energy variations in alloy nanoclusters

Abstract

The first implementation of a recently introduced method based on the extraction of the coordination dependence of surface bond-energy variations ͑CBEV͒ from density-functional theory ͑DFT͒ computed puremetal surface-energy anisotropy is reported. In particular, polynomial functions fitted to DFT data computed previously for Pt, Pd, and Rh are used as input energetics for statistical-mechanical computations of Pt-Pd 923-atom cuboctahedron-cluster compositional structures ͑and Pt-Rh͑111͒ as a test case͒ using the free-energy concentration expansion method ͑FCEM͒. The major findings concern the roles of preferential strengthening of intrasurface and surface-subsurface interlayer bonds leading to quite unique segregation characteristics: ͑i͒ strong Pt segregation at certain ͑111͒ surface sites of the Pt-Pd clusters, accompanied, at relatively high overall Pt composition, by weaker Pt segregation forming Pt-Pd ordered ͑100͒ structure, whereas Pd segregates mainly at the edge and vertex sites; ͑ii͒ dominant Pd subsurface segregation. The high computation efficiency of the CBEV/FCEM approach allows the determination of the complete temperature dependence of atomic-exchange processes among surface sites, as well as between subsurface and deeper sites, reflected in the corresponding configurational heat-capacity curves. Compared to other approaches, the high transparency of this method helps to elucidate the origin of the distinct bond-energy-variation effects on site-specific segregation in alloy nanoclusters.

Comparative modelling of chemical ordering in palladium-iridium nanoalloys

“…For surface and subsurface sites coordination-dependent bond energy variations (CBEV, δw pq ) were extracted from DFT computed surface energies via 21,22…”

“…The energetics required by FCEM can be provided by the extraction of coordination dependent bond energy variations (CBEV) from DFT computed surface energies. 21,22 The extension of FCEM/CBEV from central-symmetric to more complex chemical orderings has recently been reported for the study of Pt-Ir clusters. 23 The model was able to characterise "quasi-Janus" behaviour in the system, as has recently been investigated by Bochicchio and Ferrando.…”

Molecular dynamics simulations of the internal temperature dependent diffusing and epitaxial behaviors of Pd–Ag cluster beam deposition

“…13 The dynamical properties of alloy clusters, e.g., the surface segregations, 14,15 are different from that of monometallic clusters and dependent on the internal temperature 16 and the composition of the clusters. 13 The dynamical properties of alloy clusters, e.g., the surface segregations, 14,15 are different from that of monometallic clusters and dependent on the internal temperature 16 and the composition of the clusters.…”

Structural studies of Au–Pd bimetallic nanoparticles by a genetic algorithm method

“…The superior properties of metallic nanoparticles depend on surface atoms and their distributions [1,16,17], which are directly associated with the sizes and shapes of the particles [12].…”