Transition from core–shell to Janus chemical configuration for bimetallic nanoparticles

Abstract

In order to determine the possibilities to control the chemical configuration of bimetallic nanoparticles, we have considered CuAg nanoparticles synthesized by a physical route as a model in this study. The synthesis was made by pulsed laser deposition under ultra-high vacuum conditions, via a sequential deposition procedure. We show that the temperature of the substrate and the absolute quantity of Ag in a particle are the main parameters that drive the chemical configuration. To explain the transition from a core-shell configuration to a Janus configuration as a function of Ag quantity, we have conducted density-functional theory calculations and atomistic molecular dynamics simulations to investigate the stability of this system. The results are presented together with the experimental observations.

Morphological instability of core-shell metallic nanoparticles

“…The asymmetric core however does not extend to the 8-th shell because C atoms gain in occupying subsurface positions, as we have seen for fcc clusters 7,9 , so that none or very few C atoms appear at the cluster surface. The displacement of the core from the symmetric position is quantified in asymmetric core and the metastable centered symmetric core increases, becoming very large for k = 5 especially for AgNi and AgCo (see Table II).…”

“…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.…”

“…Chemical ordering patterns can be intermixed (randomly or phase ordered), core-shell and multishell. Also phase separated patterns have been found, such as the so-called as Janus particles 3,4 , and the ball-and-cup configurations [5][6][7] . This variety of geometries and patterns reflects the complexity of nanoalloy energy landscapes 8 , which renders the theoretical prediction of most stable structures quite challenging.…”

“…In particular, the cases of poorly miscible pairs, such as AgCu, AgCo, AgNi, and AuCo, have been investigated, both by experiments and simulations, by several groups that have explored a variety of properties and potential applications 2,[5][6][7] . AgCo, AgNi, and AuCo nanoalloys are studied for their interesting magneto-optical properties which stem from the fact that they are composed by a ferro-(Co or Ni) and a non-ferromagnetic metal (Ag or Au), the latter presenting a sharp surface plasmon resonance (SPR).…”

Interfacial Structures and Bonding in Metal-Coated Gold Nanorods

“…Factors influencing chemical ordering include metal-metal bond strengths, surface energies, atomic size mismatch, charge transfer, substrate and surfactant binding and specific electronic or magnetic effects. The chemical ordering observed for a particular NA depends critically on the balance of these factors, as well as on the synthetic method and experimental conditions [7,9].…”