Superficial segregation, wetting, and dynamical equilibrium in bimetallic clusters: A Monte Carlo study

Abstract

Using Monte Carlo simulations on a lattice-gas model within the pseudo-grand-canonical ensemble, we study the competition between superficial segregation, wetting and a core dynamical equilibrium for nanoparticles made of thousands of atoms in a system that tends to phase separate, e.g., Cu-Ag. Increasing the chemical potential difference ⌬ between Ag and Cu ͑or the nominal Ag concentration͒ at a temperature lower than the critical temperature for the phase separation in the infinite crystal, we show that the cluster goes through different stages: ͑i͒ Ag-superficial segregation that involves the vertices first, then the edges, and finally the ͑111͒ and ͑001͒ facets; ͑ii͒ prewetting that leads to Ag enrichment on the shells close to the cluster surface; ͑iii͒ a dynamical equilibrium that affects all the internal shells jointly, similar to the first-order phase transition due to the miscibility gap in an infinite crystal; and ͑iv͒ again standard segregation. Moreover, we show that a similar behavior occurs for the cluster facets if the temperature is lower than the critical temperatures of the first-order phase transition of the corresponding surfaces of semi-infinite crystals. A remarkable consequence of those dynamical equilibria is that very different concentrations of the facets on one hand and of the whole cluster on the other hand can be observed at a given ⌬.

Morphological instability of core-shell metallic nanoparticles

“…Besides being poorly miscible in bulk phases 42 , these systems have the element of larger atomic radius (Ag or Au) presenting smaller cohesive and surface energies than the other element. These features are all in favour of Ag or Au surface segregation in these nanoparticles, a behaviour that has been confirmed, both experimentally and computationally, by several studies 2,[5][6][7][9][10][11][13][14][15][16][17][19][20][21][22][23][24][25][29][30][31][32]35 . However, the fact that the cluster surface is expected to contain mostly Ag or Au does not determine completely chemical ordering.…”

Enhanced photocatalytic, electrochemical and photoelectrochemical properties of TiO2 nanotubes arrays modified with Cu, AgCu and Bi nanoparticles obtained via radiolytic reduction

“…According to some theoretical and experimental studies, AgCu clusters tend to form core-shell structures in which copper is located in the core of nanoparticles while silver atoms segregate on the surface and create shell [74] , [75] , [76] . This is related to such properties of Cu and Ag atoms as relative strength of bonds, surface energies, size and electronegativity [34] , [77] .…”

How to determine accurate chemical ordering in several nanometer large bimetallic crystallites from electronic structure calculations

“…Furthermore, lattice models depend on a set of energetic parameters that are often fitted on bulk or surface quantities obtained by experiments or ab initio calculations by ad hoc procedures. 23 – 25 …”