Tailoring the Structural Motif of AgCo Nanoalloys: Core/Shell versus Janus-like I. Parsina and F. Baletto* Physics Department, King’s College London, WC2R 2LS, London, U.K. ReceiVed: October 12, 2009; ReVised Manuscript ReceiVed: December 7, 2009 This paper deals with the computational study of the growth of small silver-cobalt clusters, modeled by a semiempirical potential. Both atom-by-atom growth molecular dynamics and simulations of freezing are used to identify formation patterns of silver-cobalt nanoalloys. Different pathways leading to core/shell and bicompartmentalized configurations, reminiscent of Janus geometry, are observed in cobalt-rich and in silver- rich alloys, respectively. The physical origin of the formation of asymmetric structures and their evolution toward core/shell motifs, as well as the opposite process, are discussed. Introduction Metallic nanoalloys 1 are one of the most exciting research objects in materials science because their optical, catalytic, and magnetic properties, different from their bulk values, may have numerous immediate technological applications. Because the physical and chemical properties of nanoparticles are a direct consequence of their electronic structure and, thus, of their geometrical configuration, one of the goals of nanoscience is to be able to design clusters of optimal geometry for a given application simply by tuning their size and chemical composi- tion. 2 Generally speaking, a bimetallic nanoparticle AB can present various chemical orderings, ranging from a mixed to a completely segregated morphology. 1,3 Mixed alloys can be either random or ordered, as is the case for the L1 0 geometry of AuCu and AuAg alloys, 4-6 whereas segregated motifs often manifest themselves as a core/shell order where a core of metal A is surrounded by an external B shell, 7,8 or multishell alloys, where the A and B metallic layers are alternating as A-B-A 9 or A-B- A-B. 10 In addition, layered or bicompartmentalized patterns are a possibility. The latter has been recently referred to as “Janus” particles, paying patronage to the Roman god with two faces, Janus, known as the custodian of the Universe. 11,12 Nanomixtures of metals that are miscible only at the nanoscale are attracting a lot of attention. 13-19 This is the case for the silver (Ag) and cobalt (Co) system, which belongs to the class of immiscible materials with silver segregation at cobalt interfaces, composed by a ferro- and a nonferromagnetic metal. These nanoalloys are of interest due to an observed giant magneto- resistance effect, which peaks when a low concentration of cobalt cluster is embedded in a silver film. 20 Although Co core and Ag shell are appealing due their catalytic applications, as demonstrated in ref 21, Janus-like AgCo clusters have potential due to their magneto-optical features that could be used for many applications, ranging from the switching on/off of their catalytic properties depending on the orientation of the nanoparticle itself to plasmonics use for cancer treatment and integrated circuits. 22,23 Today, the basic science behind the preparation, as well as the structural characteristic of bicompartmentalized nanoclusters, is poorly understood. Therefore, proposing the circumstances under which we can design asymmetric particles to occur is undoubtedly of interest. In this paper, our aim is to identify, through molecular dynamics (MD) simulations, the role of size and chemical composition during the formation of AgCo nanoparticles, in order to answer what is the predominant chemical order as a function of cobalt concentration in a size range from 55 up to 600 atoms. Because of the strong tendency of silver to surface segregate on cobalt, a mixed alloy seems to be very unlikely. Moreover, global optimization (GO) results at selected sizes between 34 and 147 have shown clearly the formation of a core/shell motif with a Co core . 24 For the purposes of this work, we distinguish two families of segregated structures: the core/shell, including a multionion core/shell structure, found for other silver nanoalloys, 9 and the Janus motifs. The Janus and Janus-like patterns individuate nanopar- ticles that exhibit two well-defined subunits, as recently found during melting simulations of AgNi 25 and in experiments for AgCu. 26 We have considered and compared two possible formation methods: the atom-by-atom deposition of cobalt over a silver seed and the freezing of AgCo nanodroplets of various sizes and composition. We refer to the former method as inVerse deposition growth because the seed metal (Ag) has a strong tendency to surface segregate on the deposited metal (Co). The inverse deposition could lead to qualitatively different results from the direct deposition. 9 Recently, it has been shown that the direct deposition of silver over cobalt results in the formation of a core/shell shape and, in particular, the anti-Mackay icosahedron at Ag 92 Co 55 , which is the most favorable structure also from an energetic and thermodynamic point of view. 24 We find that the inverse deposition of cobalt easily produces asymmetric structures, namely, up to 15% at small sizes and up to 40-45% at medium-large sizes. This asymmetric structural motif consists of two subunits: one is a pure Ag cluster, often delimited only by (111) facets, and the other is an icosahedral Co core covered by a single monolayer of silver. In the following, these are referred to as Janus-like structures because they clearly consist of two distinct “faces”. At any concentration, cobalt prefers to bunch together and grow around subsurface sites. The cobalt unit tries to adopt an icosahedral arrangement, in agreement with the strong tendency of pure cobalt to form Ih at small and intermediate sizes. Anyway, the determination of the best structural motif is far from our analysis: focus on the dynamical formation of a core/shell against Janus-like chemical ordering as a function of cobalt concentration. Moreover, we address the solid-solid transformation path of a motif into the other. In fact, further deposition of cobalt over a * To whom correspondence should be addressed. E-mail: francesca.baletto@kcl.ac.uk. J. Phys. Chem. C 2010, 114, 1504–1511 1504 10.1021/jp909773x  2010 American Chemical Society Published on Web 01/05/2010