Transmission Electron Microscopy Study of Gold-Coated Iron Core-Shell and Au/Fe/Au Onion-Like Nanoparticles Synthesized
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crystal structure compounds were selected to make such kind of core-shell nanostructure, where epitaxial growth of the shell structure may occur. Currently, one of most versatile methods to synthesize core-shell structure is wet chemical method such as microemulsion techniques which rely on the self-assembly nature of surfactants to push aqueous reactants into micelle [8]. Due to the dynamic nature of micelles, aqueous components can come together and react to form particles that are constrained to the size of the micelle. By changing the reaction conditions the methods can be even applied sequentially to form core-shell structure. By careful controlling of the synthesis using reserve micelles the nanoparticles can be obtained in a very narrow size distribution and quite uniform. Pure metal iron nanoparticles are unstable in the air. By a coating an iron nanoparticles with a stable noble metal like gold, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming selforganization nanoarrays. In this paper, we present transmission electron microscopy study of these kinds of gold coated iron core-shell and Au/Fe/Au onion-like nanostructures.
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Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
EXPERIMENT The reserve micelle reaction is carried out using cetyltrimethylammonium bromide (CTAB) as the surfactant, octane as the oil phase, and aqueous reactant as the water phase. By varying the water to surfactant ratio (o4=[H2O]/[CTAB]), we are able to carefully control the particle size ranging from 5 to 30 nmometers. A co-surfactant of nbutanol is used to help decrease the fraction of the micellar head group that is neutralized and thereby increase the stability of the micelle. Without the addition of the co-surfactant the amount of free water that is available to carry on reactions is greatly reduced as most of the water is locked in the head group of the CTAB. The metal particles are formed inside the reverse micelle by reduction of a metal salt using sodium borohydride. In our experiment we are utilizing the sequential synthesis offered by the reduction of ferrous
sulfate by sodium borohydride. After the reaction has been allowed to go completion, the micelles within the reaction mixture are expanded to accommodate the shell using a large micelle containing additional sodium borohydride. The shell is formed using an aqueous hydrogen tetracholoroaurate solution. The particles are then washed, collected in a magnetic field and dried under vacuum. The Au/Fe/Au onion-like structure was basically synthesized using same method, but in three steps: In step one, two micelle solutions were prepared using 0. 1M HAuC14 (aq) and the other with 0.6M NaBH4 (aq). These solutions ((o=6) were mixed together using a magnetic stirrer and allowed to react for two hours to ensure that all the gold was reduced. The reaction was carried out in the presence of long wave ultra-vi
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