Atomic-Scale Structure and Stress Release Mechanism in Core–Shell Nanoparticles

Abstract

Core−shell nanoparticles find applications in catalysts, sensors, and theranostics. The full internal 3D atomic structure, however, cannot be resolved by current imaging and diffraction techniques. We analyzed the atomic positions and stress-release mechanism in a cubic Au−Pd core−shell nanoparticle in approximately 1000 times higher resolution than current experimental techniques using large-scale molecular dynamics simulation to overcome these limitations. The core− shell nanocube of 73 nm size was modeled similarly to solution synthesis by random epitaxial deposition of a 4 nm thick shell of Pd atoms onto a Au core of 65 nm side length using reliable interatomic potentials. The internal structure reveals specific deformations and stress relaxation mechanisms that are caused by the +4.8% lattice mismatch of gold relative to palladium and differential confinement of extended particle facets, edges, and corners by one, two, or three Au−Pd interfaces, respectively. The three-dimensional lattice strain causes long-range, arc-like bending of atomic rows along the faces and edges of the particle, especially near the Au−Pd interface, a bulging deformation of the Pd shell, and stacking faults in the Pd shell at the corners of the particle. The strain pattern and mechanism of stress release were further characterized by profiles of the atomic layer spacing in the principal crystallographic directions. Accordingly, strain in the Pd shell is several times larger in the extended facets than near the edges and corners of the nanoparticle, which likely affects adsorption, optical, and electrochemical properties. The findings are consistent with available experimental data, including 3D reconstructions of the same cubic nanoparticle by coherent diffractive imaging (CDI) and may be verified by more powerful experimental techniques in the future. The stress release mechanisms are representative for cubic core−shell nanoparticles with fcc structure and can be explored for different shapes by the same methods.

Accurate simulation of surfaces and interfaces of ten FCC metals and steel using Lennard–Jones potentials

“…The reliability of computed structural, thermal, and mechanical properties can be excellent with 0.1 to 5% error relative to experiments, clearly better than with DFT methods 34 . Surface energies are computed too low by up to 50%, i.e., up to ten times larger error compared to LJ potentials and experimental data 30,34,52 . Mechanical properties can be excellent with~5% deviation and better than LJ potentials 34 , while deviations up to 40% from experiments are also found depending on the EAM 30,52 .…”

“…S1i) 29 , strain fields and defects in core-shell nanoparticles (Supplementary Fig. 1j) 30 , and properties of alloys (Supplementary Fig. 1k) 31 .…”

Interface-mediated Kirkendall effect and nanoscale void migration in bimetallic nanoparticles during interdiffusion

“…Hence, the center of the NP becomes saturated with vacancies. The lack of structural defects in the core further means that the accumulated vacancies can only condense at interfacial defects that were originally present in the NPs, such as stacking faults at the bimetallic interface 32–35 . For Pd–Au NPs, the vacancy flux is directed towards the Au shell.…”

On Compton scattering as a source of background in coherent diffraction imaging experiments

“…One of the ultimate aims of such experiments is to characterize the internal structure and quantify the stress relaxation mechanisms. Large-scale molecular dynamics calculations have been employed to simulate the core-shell Au/Pd particles (Nathanson et al, 2018). It is predicted that strain in the palladium shell is larger on the faces than at the edges and corners.…”