Au-Rh bimetallic nanoparticles spontaneously form subnanometer Au overlayers whose thickness is limited by anisotropic strain
Synopsis
This work reports that bimetallic nanoparticles can form a thermodynamically controlled "shell-dimer" architecture; using Au-Rh as a model system, atomic-resolution imaging and molecular dynamics show that an ultrathin Au overlayer forms on Rh, stabilized by competition among surface, interfacial, and strain energies, with anisotropic strain limiting its growth to the subnanometer scale and changes in surface chemistry able to destabilize it altogether; across a range of bimetallic nanoparticles, overlayer formation is associated with elemental immiscibility and lattice mismatch.
Interpretation
Bimetallic nanoparticles can form a thermodynamically controlled "shell-dimer" architecture in which one metal forms an overlayer only a few atomic layers thick on another. Atomic-scale surface structures of nanoparticles were previously challenging to predict; this work proposes and verifies a predictable overlayer configuration. Using Au-Rh as a model system, combining atomic-resolution imaging and molecular dynamics simulations.
The stability of the ultrathin Au overlayer on Rh is governed by competition among surface, interfacial, and strain energies, and anisotropic strain limits its growth to the subnanometer scale. Attributes the thickness limitation of the overlayer to anisotropic strain, rather than solely to thermodynamic equilibrium. Supported jointly by atomic-resolution imaging and molecular dynamics simulations.
Changes in surface chemistry can destabilize the overlayer altogether. Shows that the overlayer is not fixed but depends on the surface chemical environment. The text reports results showing surface chemistry changes destabilize the overlayer.
Across a range of bimetallic nanoparticles, overlayer formation is associated with elemental immiscibility and lattice mismatch. Extends the Au-Rh finding to a broader set of bimetallic systems, proposing a generalizable association. Observations across a range of bimetallic nanoparticles.
Perspective
The results apply to predicting and controlling surface structures of bimetallic nanoparticles, especially systems with elemental immiscibility and lattice mismatch; atomic-scale evidence is given for the Au-Rh model system, and the association is extended to a range of bimetallic nanoparticles. This provides a basis for designing multimetallic nanoparticles in surface-property-dependent contexts such as catalysis and energy.
The association between overlayer formation and elemental immiscibility and lattice mismatch was observed across a range of bimetallic nanoparticles; its generality and quantitative criteria still require further testing. The specific conditions and reversibility of overlayer destabilization by surface chemistry changes, and the microscopic mechanism by which anisotropic strain limits growth, are open questions worth watching.