Particle Size Control, Structure and Magnetic Characterization of Cobalt Nanoparticles
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0998-J01-04
Particle Size Control, Structure and Magnetic Characterization of Cobalt Nanoparticles Abhishek Singh1, Nirav Parekh1, Gregory Young1, Kiumars Parvin2, Maninder Kaur2, David Bruck3, Spencer Wong3, and Mehdi Varasteh4 1 Chemical and Materials Engineering, San Jose State University, One Washington Square, San Jose, CA, 95192 2 Physics, San Jose State University, One Washignton Square, San Jose, CA, 95192 3 Biology, San Jose State University, One Washington Square, San Jose, CA, 95192 4 Alza Corporation, Mountain View, CA, 94039
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ABSTRACT Cobalt nanoparticles were synthesized by means of a hot metal reduction reaction with cobalt chloride as precursor material. The size of cobalt nanoparticles was controlled by the choice of surfactant and the molar ratio of surfactant-to-reagent. Surfactants with larger alkane side chains yielded a smaller average nanoparticle size (diameter) and tighter size distribution, as these chains provided steric hindrance to the growth of the nanoparticles after initial nucleation. For each alkane side chain, a high molar ratio of surfactant-to-reagent (HSR) rendered nanoparticles with smaller particle size, while a low molar ratio of surfactant-to-reagent (LSR) produced larger nanoparticles. Measurements on transmission electron microscope images of cobalt particles synthesized with tri-octylphosphine revealed an average particle size of 6.9 nm (HSR) and 9.1 nm (LSR), while particles synthesized with tri-butylphosphine had a mean diameter of 12.5 nm (HSR) and 14.9 nm (LSR). X-ray diffraction profiles indicated that particles had metastable ε-cobalt structure. Room temperature magnetization measurements showed ferromagnetic behavior with highly square M-H loops indicative of single domain particles with coercive fields in the range of 400-500 Oe.
INTRODUCTION A solution-based, high temperature synthesis as a result of oxidation-reduction of a metallic salt was used for synthesizing mono-dispersive cobalt nanoparticles [1,2,3,4]. This technique involved a mixture of reagents in dioctyl ether at elevated temperatures, providing discrete nucleation sites and allowing particle size control [3,4]. The resultant nanoparticles consisted of an inorganic crystalline core surrounded by an organic monolayer, which prevented oxidation and aggregation of the nanoparticles [2,4]. We investigated the impact of two surfactants and the surfactant-to-reagent molar ratio on cobalt nanoparticle size, structure, and magnetic properties utilizing transmission electron microscopy, x-ray diffraction, and magnetization measurements. EXPERIMENTS The synthesis was carried out in an inert environment due to the oxygen-sensitive nature of the reagents. Initially, 0.52 g CoCl2 (anhydrous) was added to 80 mL dioctyl ether and 1.3 mL oleic acid. The CoCl2 was allowed to dissolve during heating to 100∞C. An organophosphine surfactant was added at 100∞C, after which the solution was heated to 205∞C. Lithium triethylborohydride in tetrahydrofuran (6 mL of 1.0 M) was then added to reduce the Co ions (Co2+) t
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