Atomic structure of three-layer Au/Pd nanoparticles revealed by aberration-corrected scanning transmission electron microscopy

The study of nanomaterials can be greatly improved with the use of aberration-corrected transmission electron microscopy (TEM), which provides image resolutions at the level of I A and lower. Sub-Angstrom image resolution can yield a new level of understanding of the behavior of matter at the nanoscale. For example, bimetallic nanoparticles are extremely important in catalysis applications; the addition of a second metal in many cases produces much-improved catalysts. In this paper, we study the structure and morphology of Au/Pd bimetallic particles using primarily the high-angle annular dark-field (HAADF) imaging mode in an aberration-corrected STEM/TEM. It is well established that, when recorded under appropriate illumination and collection geometries, incoherent HAADF-STEM images are compositionally sensitive and provide direct information on atomic positions. We matched the experimental intensities of atomic columns with theoretical models of three-layer Au/Pd nanoparticles, in different orientations. Our findings indicate that the surface layer of the nanoparticle contains kinks, terraces and steps at the nanoscale. The effect of adding a second metal induces the formation of such defects, which might very likely promote the well-known improved catalytic activity of this system.