An atomistic view of the interfacial structures of AuRh and AuPd nanorods† Ruth L. Chantry, a Ivailo Atanasov, b Wilai Siriwatcharapiboon, b Bishnu P. Khanal, c Eugene R. Zubarev, c Sarah L. Horswell, b Roy L. Johnston b and Z. Y. Li * a 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. Introduction Interest in bimetallic nanoparticles has grown considerably in recent years, in particular because of the signicant additional potential they offer over monometallic counterparts for use as industrial catalysts. 1–5 This potential derives not only from the possibility to combine the properties of their constituent metals, and gain enhancements from synergistic interactions between them, but also from the ability to control those prop- erties through altering structure and chemical ordering. 6–10 The resulting tunability of bimetallic nanoparticles offers signi- cant advantages over monometallic equivalents that makes them highly desirable systems for the development of rationally designed catalysts tailored to specic reactions. 9–13 However, a full realization of the potential that bimetallic nanoparticles offer requires a fundamental understanding of the interactions between their constituent metals at the atomic scale. The atomic structure of nanoparticles is determined by complex interactions between a range of structural and energetic parameters, including lattice spacing, symmetry, and surface and cohesive energies. In binary systems this complexity is made even greater by the need to accommodate two sets of parameters and the differences between them. As such the driving forces behind metal–metal interactions in bimetallic nanocatalysts are not easy to identify. Here we present a comparative study of the interfacial structures of two catalytically relevant systems, Au core Rh shell and Au core Pd shell nanorods. Conducting a direct comparison between these two systems provides the opportunity to gain insight into key factors that inuenced their formation, which cannot be identied easily by considering the parameters of either system alone. The systems form ideal comparators as they utilize the same Au-core template with over-grown shell metals of adjacent atomic numbers (45 and 46, respectively). A key difference in their properties is their bulk miscibility: Au and Pd are miscible in the bulk, whereas Au and Rh are completely immiscible, 14 yet despite this, the synthesis of both alloyed and segregated nanoparticles have been reported for both systems. 15–31 While AuPd nanoparticles have been extensively studied, using a wide range of techniques, in both segregated and alloyed forms, 15–27 this is not the case for AuRh nanoparticles, where the work to date has been carried out primarily using surface science techniques on systems formed via thermal evaporation. 28,29 The successful over-growth of Rh on Au cubes 32 and nanorods 30 using wet chemical methods has recently been reported, with the latter study revealing a complex deposition and growth pattern driven by the reaction kinetics, which calls for detailed structural characterization of this system. a Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. E-mail: z.li@bham.ac.uk b School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK c Department of Chemistry, Rice University, Houston, TX 77005, USA † Electronic supplementary information (ESI) available: Additional STEM images, details of molecular dynamics simulation, and descriptions of sample synthesis. See DOI: 10.1039/c3nr02560h Cite this: Nanoscale, 2013, 5, 7452 Received 17th May 2013 Accepted 11th June 2013 DOI: 10.1039/c3nr02560h www.rsc.org/nanoscale 7452 | Nanoscale, 2013, 5, 7452–7457 This journal is ª The Royal Society of Chemistry 2013 Nanoscale PAPER Published on 14 June 2013. Downloaded by Rice University on 06/01/2014 20:25:39. View Article Online View Journal | View Issue