Control of Cobalt Nanoparticle Size by the Germ-growth Method in Inverse Micelle System: Size-dependent Magnetic Propert
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Control of cobalt nanoparticle size by the germ-growth method in inverse micelle system: Size-dependent magnetic properties X. M. Lin and C. M. Sorensen Condensed Matter Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506
K. J. Klabunde Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
G. C. Hajipanayis Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716 (Received 1 May 1998; accepted 22 October 1998)
Control of Co particle size was archived by a germ-growth method during inverse micelle synthesis. Magnetic coercivity and blocking were both a function of the particle ˚ Interparticle dipolar interaction was proven to be size, which ranged from 38 to 88 A. important in order to interpret the magnetic properties for large-size particles. I. INTRODUCTION
Research during the last decade has demonstrated that nanosize semiconductor and metal particles have unique properties compared to the bulk materials, largely due to their small size and high surface to volume ratio.1 Fundamentally, as we decrease the size, the electronic structure changes from continuous bands of the bulk material to discrete energy levels of atoms.2 It is therefore no surprise to find that nanoparticles, which are a bridge between the bulk and the atomic state, have a significant size dependence for electrical, optical, magnetic, and chemical properties. Examples include the red shift of the photoluminescence peak in semiconductor nanoparticles is strongly dependent on the particle size,3 magnetic nanoparticles prepared in gas phase have a size-dependent enhanced magnetic moment,4 and dissociate chemisorption of D2 was found to occur with great sensitivity to the size of iron, cobalt, and niobium nanoparticle catalyst.5 Therefore, control of nanoparticle size is crucial in studying their properties. The inverse micelle technique has been established as a simple method to obtain bulk quantities of nanoparticles.6 The size of particles synthesized inside the inverse micelle can be varied by changing the [water]/ [surfactant] ratio. An increase of the ratio increases the size of the water pool inside the inverse micelle, and therefore allows bigger particles to form. Pileni et al.7 have used this method to synthesize different sizes of metal and semiconductor nanoparticles. We have synthesized cobalt nanoparticles in the inverse micelle system using NaBH4 reduction of Co21 ions and studied the morphology and magnetic properties of particles formed at different temperatures.8 Unfortunately, the size of the cobalt particles prepared in this manner cannot be adjusted by changing the [water]/[surfactant] ratio, since it has been already proven that the final product of BH42 1542
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4, Apr 1999
Downloaded: 11 Mar 2015
reduction is very sensitive to the [water]/[surfactant] ratio.9 A high concentration of water would form Co2 B rather than Co metal inside the micel
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