Synthesis of Co@Au core-shell nanoparticles in non-aqueous solution and their characterization
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Synthesis of Co@Au core-shell nanoparticles in non-aqueous solution and their characterization Zhihui Ban, C. J. O’Connor Advanced Materials Research Institute University of New Orleans, New Orleans, LA, USA, [email protected] ABSTRACT A homogeneous non-aqueous solution reactions method has been developed to prepare gold-coated cobalt (Co@Au) nanoparticles. After the sample was washed with 8% HCl, XRD (X-Ray Diffraction), TEM (transmission electron microscopy), and magnetic measurements SQUID (Superconducting Quantum Interference Device) are utilized to characterize the nanocomposites. XRD shows the pattern of sample, which is responding to gold and cobalt, no cobalt oxide was found. TEM results show that the average size of Co@Au nanoparticles is about 10 nm and we can find core-shell structure of the sample. SQUID results show that the particles are ferromagnetic materials at 300K. So the goldcoated cobalt nanoparticles (Co@Au) can be successfully prepared by the homogeneous nonaqueous approach. This kind of core-shell materials is stable in acid condition, which would give many opportunities for bio- application. 1 INTRODUCTION A combination of nanotechnologies and biosciences will be one of the leading areas of research and development in the 21st century; magnetic nanoparticles will certainly play an extremely important role. However, as the particle size decreases, the reactivity of the [1] particle increases, and the magnetic properties are influenced more by surface effects . So far the passivation of magnetic nanoparticles (γ - Fe2O3, Co, Fe, et. al.) by another [ 2-7] inert layer (SiO 2 , gold, polymer et. al.) has been developed . However, the nonmetallic layer potentially reduces the magnetic properties of the magnetic nanoparticles. Gold has become a favored coating material because of a simple synthetic [8] procedure and its chemical functionality . The presence of the diamagnetic coating passivates the magnetic core thus protecting the magnetic properties without having a pronounced effect on the magnetic properties. The presence of the diamagnetic coating allows for a surface that can be derivatized to allow for greater versatility while not reducing the magnetic properties. It is expected that iron nanoparticles can avoid being oxidized and maintain their magnetic properties (such as coercivity or blocking temperature) by gold coating. The reverse micelles method for synthesis of Fe@Au [9] nanoparticles had been developed by our group . Here, in this paper, we will present another novel strategy for synthesis the gold-coated cobalt nanoparticles by partially replacement reaction. The idea of the method is using reducing agents (Na, K, Li) to reduce MLn (M = Fe, Co, Ni; L = Cl, Br; n = 2, 3) to form metallic core nanoparticles,
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and then using the surface of metallic core particles to reduce Au3+ to form gold coated mettallic nanoparticles, as shown in scheme 1. Xia has used galvanic replacement reaction for generating metal nanostructureswith hollow interiors[10].
Metal salt solution
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