Size and Composition Effects in the Structure and Properties of Polymer-Protected Bimetallic Particles
- PDF / 199,960 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 168 Views
Size and Composition Effects in the Structure and Properties of Polymer-Protected Bimetallic Particles 1
2
Marie-José Casanove, Pierre Lecante, Marie-Claire Fromen, Marc Respaud , David Zitoun , 2 2 2 2 3 Cheng Pan , Karine Philippot , Catherine Amiens , Bruno Chaudret and Fabrice Dassenoy Centre d’Elaboration de Matériaux et d’Etudes Structurales, CNRS, B.P. 4347, F-31055 Toulouse cedex 4, France 1 Laboratoire de Physique de la Matière Condensée, INSA, F-31077 Toulouse cedex 4, France 2 Laboratoire de Chimie de Coordination, CNRS, F-31077, Toulouse cedex 4, France 3 Fritz Haber Institüt, Berlin, Germany ABSTRACT We investigate the structural evolution of PtRu and CoRh ultra fine bimetallic particles as a function of their composition in the whole range of stoichiometry. Isolated particles embedded in an organic matrix are synthesized using chemical techniques in mild conditions. Their size, dispersion and structure are analyzed by HRTEM, WAXS and EXAFS techniques. The magnetic behavior of cobalt-based alloys is investigated by SQUID magnetometry. In the PtRu alloys, we demonstrate the role of composition in the structural transition from the face-centered cubic to the hexagonal close packed structure in nanoparticles and detail the mechanism of the transition. We point out the effect of size reduction and core-shell atomic distribution in the structure and the enhanced magnetization in CoRh bimetallic particles. INTRODUCTION Nanometer sized metal particles, well known to exhibit unique physical and chemical properties, arouse an increasing interest in different fields as catalysis, micro-electronics and magnetism [1-3]. For instance, a non magnetic metal as Rh becomes magnetic when the particle size decreases to the nanometer scale [4]. Many of their properties are even more spectacular in bimetallic particles: improvement of the activity and/or selectivity of metallic catalysts, occurrence of a higher spontaneous spin polarization in 4d metals alloyed with 3d elements, etc. Besides, bimetallic particles offer the opportunity to study combined size, composition and segregation effects in binary alloys and possibly to investigate new phases. In this paper, we focus on the structural behavior of bimetallic isolated particles of nanometer size synthesized by decomposition of organometallic precursors in the presence of a polymer. The systems studied, namely, PtRu and CoRh, all present a continuous series of solid solutions in the structural phase diagram of the bulk alloys. However, in bulk PtRu, the solubility of Ru in the Pt face-centered cubic (fcc) phase does not exceed 70 at% while the solubility of Rh in the fcc phase of Co reaches 100 at%. Besides, the various elements present in these alloys behave differently in nanometer-sized particles synthesized in the same way as the bimetallic ones. Pt, Rh and Ru crystallize in their bulk structure, namely fcc for Pt and Rh and hcp for Ru, although size effects induce a contraction of their metal-metal bond length [5]. An original polytetrahedral structure was evidenc
Data Loading...
