Synthesis by Self-Assembly of Iron-Cobalt Nanoalloys
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Synthesis by Self-Assembly of Iron-Cobalt Nanoalloys Melissa A. Zubris and Rina Tannenbaum School of Materials Science and Engineering, Georgia Institute of Technology Atlanta, GA 30332-0245 ABSTRACT In this paper we are proposing the synthesis of iron and cobalt nanoalloys via the codecomposition of iron and cobalt carbonyl precursors in the presence of polystyrene as the surface stabilizing agent. In order to form iron-cobalt nanoalloys with no preferential aggregation of metal atoms resulting in phase segregation, the decomposition kinetics of the iron pentacarbonyl and dicobalt octacarbonyl precursors had to be firmly established. The kinetics of cobalt cluster formation has been thoroughly investigated, but data for iron pentacarbonyl decomposition is relatively scarce. To fully understand the formation of the iron nanoclusters, a kinetic study was performed by varying carbonyl concentrations and reaction media in order to establish reaction order and rate constants. Our results suggest this decomposition to be a higher order process (not first order as previously assumed), with a complicated intermediate mechanism, which has been postulated and experimentally verified. By using this kinetic data, we will be able to predict the necessary conditions for the creation of new in-situ iron-cobalt nanoalloys using carbonyl precursors. INTRODUCTION Nanoalloys are an exciting new class of materials in the growing field of nanotechnology. Nanoalloys consist of the nanoscale co-aggregation of two or more metals with a potential to form compositionally-ordered phases or superstructures that have properties unlike those of the individual types of clusters or of bulk alloys of the constituent metals.1-4 A convenient route to the formation of metal nanoalloy systems is the co-transformation and co-aggregation of their respective organometallic precursors.5-8 If a stable nanoalloy is to be created, there must be no preferential aggregation of metal atoms into mono-metallic clusters in the system, i.e., there should be no kinetically-induced phase separation of the metal atoms within the nanoalloy cluster during its formation. This can be achieved by the concurrent transformation reactions of the organometallic precursors, and hence, these transformations should display similar kinetic features, i.e. similar reaction rates. The control of the individual reaction rates of each of the species in the system (by varying component concentrations, solvents or temperature) may be used to modulate the aggregation and initial phase separation of the different metallic atoms in the nanoalloy clusters. A successful method for the preparation of zero-valent metal nanoclusters is the decomposition of metal carbonyls into metal clusters under an inert atmosphere to reduce the probability of oxidation. In addition, by decomposing these carbonyls in a polymer matrix, the size of the metal nanocluster may be controlled. The polymer adsorbs onto the surface of the cluster, restricting the extent of metal-metal interactions, thereby lim
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