Growth and energetic stability of AgNi core–shell clusters F. Baletto a , C. Mottet b , A. Rapallo c , G. Rossi a , R. Ferrando a, * a INFM and IMEM/CNR, Dipartimento di Fisica dell’Universit a di Genova, via Dodecaneso 33, Genova 16146, Italy b CRMC2/CNRS, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France c Istituto per lo studio delle macromolecole del CNR––sezione di Genova, Via De Marini 6, Genova I-16149, Italy Available online 4 June 2004 Abstract The growth of AgNi clusters is studied by means of Molecular Dynamics simulations at sizes between 200 and 300 atoms. It is found that core–shell structure with an Ni core and an Ag external shell are the most favorable in good agreement with experimental data. Core–shell structures energetics is studied also by a global optimization genetic algorithm, which predicts that the global minimum is an Ni core covered by an Ag shell already at small sizes. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Clusters; Growth; Molecular dynamics; Nickel; Silver; Alloys Core–shell structures can be very important for application in catalysis [1], since the strained shells may present unusual electronic properties, and these nanoparticles have a very favorable surface/ volume ratio. For example, the increase of cata- lytic activity in core–shell nanoclusters has been experimentally demonstrated [2] in the case of Pd shells on Au cores. Recently developed experi- mental and theoretical studies [3–6] have shown that clusters of several bimetallic systems, such as AuAg, AgNi, AgPd, AgCu, present preferentially core–shell structures. In experiments, core–shell clusters are prepared by different methods: for example, AgPd particles have been prepared by chemical decomposition of organometallic com- pounds on oxides powders (MgO, Al 2 O 3 ) [7], obtaining particles with a 6 nm diameter and an apparent Ag shell 1 nm thick. The Ag shell is well accommodated on the Pd core. Also AgNi core– shell clusters with average 3 nm diameter have been recently produced [3,4] by metal laser vaporization and subsequent cooling in inert gas atmosphere. The reason why bimetallic systems such as AgCu, AgPd, AgNi could prefer to form core–shell structure is that these Ag compounds show a strong tendency to phase separation in the bulk, with Ag segregating at the surface [8,9]. These tendencies are especially strong in AgCu and AgNi. In fact, bulk AgNi and AgCu present a large miscibility gap in a wide range of T , while AgPd bulk alloys present a phase diagram with a continuous series of solid solutions over the whole range of concentrations at high T . In general, we expect AgCu clusters structure to be similar to the one of AgNi clusters because of the strong ten- dency of Ag to segregate on Cu too. Our aim is to theoretically study whether the formation of core–shell AgNi structure is favor- able both from the kinetic and the energetic point * Corresponding author. Tel.: +39010-3536214; fax: +39010- 311066. E-mail address: ferrando@fisica.unige.it (R. Ferrando). 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.05.044 Surface Science 566–568 (2004) 192–196 www.elsevier.com/locate/susc