When More Is Less: Plastic Weakening of Single Crystalline Ag Nanoparticles by the Polycrystalline Au Shell.

It is well-known that in the case of bulk polycrystalline metals, a reduction in the grain size leads to material hardening, since the grain boundaries represent efficient barriers for slip transfer between the adjacent crystalline grains. Here, we show that coating single crystalline Ag nanoparticles with a thin polycrystalline Au layer leads to a weakening of the particles. Moreover, while the single crystalline Ag nanoparticles yield in a single large displacement burst when loaded in compression, their Ag-Au core-shell counterparts demonstrate a more homogeneous deformation with signs of strain hardening. Our molecular dynamics simulations demonstrate that particle weakening at low strains is attributed to the plasticity confinement in the polycrystalline shell, in which the grain boundaries play a dual role of dislocations sources and sinks. At higher strains, the plasticity within the Ag core is initiated by the dislocations nucleating at the Ag-Au interphase boundary. The widespread of energy barriers for dislocations nucleation at the interphase boundaries and their lower value as compared to the barriers for surface nucleation ensure particle weakening and more homogeneous deformation. The results of this study show that adding imperfect material to superstrong single crystalline metal nanoparticles makes them weaker. At the same time, thin nanocrystalline coatings can be employed to improve the formability of metals at the nanoscale.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation.

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge. We report here an unusual twin boundary migration process in a single platinum nanoparticle during CO oxidation using Bragg coherent diffraction imaging as the characterisation tool. Density functional theory calculations show that twin migration can be correlated with the relative change in the interfacial energies of the free surfaces exposed to CO. The x-ray technique also reveals particle reshaping during the reaction. In situ and non-invasive structural characterisation of defects during reaction opens new avenues for understanding defect behaviour in confined crystals and paves the way for strain and defect engineering.