An atomistic view of the interfacial structures of AuRh and AuPd nanorods

Abstract

In this work we address the challenge of furthering our understanding of the driving forces responsible for the metal-metal interactions in industrially relevant bimetallic nanocatalysts, by taking a comparative approach to the atomic scale characterization of two core-shell nanorod systems (AuPd and AuRh). Using aberration-corrected scanning transmission electron microscopy, we show the existence of a randomly mixed alloy layer some 4-5 atomic layers thick between completely bulk immiscible Au and Rh, which facilitates fully epitaxial overgrowth for the first few atomic layers. In marked contrast in AuPd nanorods, we find atomically sharp segregation resulting in a quasi-epitaxial, strained interface between bulk miscible metals. By comparing the two systems, including molecular dynamics simulations, we are able to gain insights into the factors that may have influenced their structure and chemical ordering, which cannot be explained by the key structural and energetic parameters of either system in isolation, thus demonstrating the advantage of taking a comparative approach to the characterization of complex binary systems. This work highlights the importance of achieving a fundamental understanding of reaction kinetics in realizing the atomically controlled synthesis of bimetallic nanocatalysts.

Monitoring in situ the colloidal synthesis of AuRh/TiO2 selective-hydrogenation nanocatalysts

“…[4][5][6][7] Using either chemical or physical preparation techniques, they showed that Au tends to cover the Rh surface. Using scanning transmission electron microscopy (STEM), Chantry et al 8,9 highlighted the tendency of Rh either to deposit in an uneven way on Au nanorods and/or to form mixed alloy layers at the topmost surface depending on kinetic issues during the formation of such nanostructures. Similarly, Sneed et al 10 observed the formation of island overgrowth of Rh onto Au nanocubes or octahedra.…”

Interfacial Structures and Bonding in Metal-Coated Gold Nanorods

“…4, to AuRh nanorods synthesised using a seed-mediated sequential growth method via a wet chemical route. The main results have been reported in two recent publications [51,80]. Figure 5 shows EDX maps of Au and Rh overlaid on the same image.…”

“…Where the sample is of uniform thickness, HAADF image contrast variation can be directly related to sample elemental variation. Figure 3 shows a typical HAADF image of segregated AuPd nanorods [51], demonstrating a clear difference in image intensity between the Au core (Z ¼ 79) and the Pd shell (Z ¼ 46).…”

“…If low-coordinated Au atoms are present on the surface, they would aggregate together at step or kink sites of the growing Rh island and could be buried by newly deposited Rh atoms. This is a possible scenario to explain the observed mixing at the interface of Rh-coated Au nanorods [51].…”

Structural studies of Au–Pd bimetallic nanoparticles by a genetic algorithm method

“…Especially, bimetallic nanoparticles, composed of two kinds of metal elements, have attracted persistently substantial attention not only because of their combining properties of each constituent metal but also because of their additional peculiar properties not observed in their monometallic counterparts [4,10,11].…”