Published: March 03, 2011 r2011 American Chemical Society 6360 dx.doi.org/10.1021/jp200288y | J. Phys. Chem. C 2011, 115, 6360–6366 ARTICLE pubs.acs.org/JPCC Structural Characterization of Surfactant-Coated Bimetallic Cobalt/Nickel Nanoclusters by XPS, EXAFS, WAXS, and SAXS Luisa Sciortino, † Francesco Giannici, † Antonino Martorana, † Angela Monia Ruggirello, † Vincenzo Turco Liveri, † Giuseppe Portale, ‡ Maria Pia Casaletto, § and Alessandro Longo* , § † Dipartimento di Chimica “Stanislao Cannizzaro”, Universit  a di Palermo, Parco D’Orleans II, I-90128 Palermo, Italy ‡ Netherlands Organization for Scientific Research (NWO), 6 rue Jules Horowitz, BP220, 38043 Grenoble Cedex, France § Istituto per lo Studio dei Materiali Nanostrutturati, CNR, Via Ugo La Malfa 153, I-90146 Palermo, Italy 1. INTRODUCTION The preservation of nanoparticle identity from agglomeration and chemical decomposition, allowing size-control and long- term storage without losing unique properties, is one of the most stringent requirements of an effective nanoparticle synthetic strategy. This can be achieved by the use of a suitable coating agent, which could also involve an easy separation of nanopar- ticles from the reaction medium. 1-5 A peculiar class of coating agents is composed of surfactants which, owing to their amphiphilic nature, are able to physically or chemically adsorb at the nanoparticle surface through their hydrophilic heads and to build up a hydrophobic layer formed by the oriented alkyl chains, thus hindering the nanoparticle 3growing/coalescence processes and, at the same time, allow- ing a protection of their chemical composition from most external reactants. Further advantages of surfactants arise from their functionalization as carriers of nanoparticle precursors and their ability of self-assembly as pure components, to form a supramo- lecular structure that directs the nanoparticle 2D and 3D spatial arrangement. 5-8 Other requirements arise when one aims at the production of nanoparticles composed of two or more components, in which a well-defined structural arrangement is crucial to control their properties. In such a case, the control of the composition and the internal structure becomes of the utmost importance to modulate the physicochemical features of the nanoparticles. In principle, this can be achieved by varying the synthetic protocol. 9-12 The core-shell nanoparticles can be effectively formed using separate reduction reactions in two stages with different reaction temperatures through the heterogeneous nucleation process. For example, Cu-Ni core-shell nanoparticles have been prepared using this technique. Recently, Yamauchi et al. showed that the shapes of the Ni-Co nanoparticles can be easily controlled using different nickel precursors. It was also shown that the shape of the Ni core plays a key role in determining the final shape of a Ni- Co (core-shell) nanocrystal, because Ni nanoparticles act as catalysts for the redox reaction of the Co 2þ ions. 13 However, this mechanism has been investigated for metal clusters with dia- meters in the range of 40-50 nm, and the possibility of ex- changing the two metals in the core of the nanoparticles was not investigated. In order to finely control the structure, chemical composition, and growth of nanosized Co-Ni particles of a few nanometers in diameter, we investigated the effect of changing the se- quence of the chemical reduction of Co 2þ and Ni 2þ ions in the reverse micelle solutions of cobalt bis(2-ethylhexyl)phosphate, Co(DEHP) 2 , and nickel bis(2-ethylhexyl)phosphate, Ni(DEHP) 2 . 14 In particular, the reduction reactions were carried out by mix- ing, sequentially or contemporaneously, fixed amounts of 0.3 M Received: January 11, 2011 Revised: February 3, 2011 ABSTRACT: Cobalt-nickel bimetallic nanoparticles were synthesized by changing the sequence of the chemical reduction of Co(II) and Ni(II) ions confined in the core of bis(2- ethylhexyl)phosphate 2 , and Ni(DEHP) 2 . The reduction was carried out by mixing, sequentially or contemporaneously, fixed amounts of n-heptane solution of Co(DEHP) 2 and Ni(DEHP) 2 micelles with a solution of sodium borohydride in ethanol at a fixed (reductant)/(total metal) molar ratio. This procedure involves the rapid formation of surfactant-coated nanoparticles, indicated as Co/Ni (Co after Ni), Ni/Co (Ni after Co), and Co þ Ni (simultaneous), followed by their slow separation as nanostructures embedded in a sodium bis(2-ethylhexyl)phosphate matrix. The resulting composites, together with those obtained by reducing the n-heptane solutions of pure Co(DEHP) 2 or Ni(DEHP) 2 , were characterized by XPS, EXAFS, WAXS, and SAXS. The data analysis confirms the presence of nanometer-sized surfactant-coated cobalt, nickel, and cobalt/nickel particles. As expected, the composition and internal structure of cobalt/nickel bimetallic nanoparticles are influenced by the preparation sequence as well as by the “chemical affinity” between the surfactant and the metal. However, some atomic-scale physicochemical processes play a subtle role in determining the structural features of bimetallic nanoparticles. Further effects due to the competition between nanoparticle growing process and surfactant adsorption at the nanoparticle surface were observed.