Jae Ha Shim

Korea Advanced Institute of Science and Technology , Seoul, Seoul, South Korea

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Publications (6)50.54 Total impact

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    ABSTRACT: A selective preparation and the formation mechanism of hexagonal and cubic CoO nanoparticles from the reaction of [Co(acac)(2)] (acac=acetylacetonate) and amine have been investigated. CoO nanoparticles with a hexagonal pyramidal shape were yielded under decomposition conditions with amine. Importantly, the addition of water altered the final phase to cubic and comprehensively changed the reaction mechanism. The average sizes of the hexagonal and cubic CoO nanoparticles could be controlled either by changing the amine concentration or by using different reaction temperatures. Detailed formation mechanisms are proposed on the basis of gas chromatography-mass spectrometry data and color changes of the reaction mixture. The hexagonal CoO phase is obtained through two distinct pathways: solvolysis with C-C bond cleavage and direct condensation by amine. On the other hand, the cubic CoO nanoparticles were synthesized by strong nucleophilic attack of hydroxide ions from water and subsequent C-C bond breaking. The resulting caboxylate ligand can stabilize a cobalt hydroxide intermediate, leading to the generation of a thermodynamically stable CoO phase.
    Chemistry - An Asian Journal 06/2011; 6(6):1575-81. DOI:10.1002/asia.201100031 · 3.94 Impact Factor
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    ABSTRACT: Pure rocksalt (c-CoO) and wurtzite (h-CoO) phases of cobaltous oxide (CoO) have been selectively prepared through thermodynamically and kinetically controlled reactions of a single molecular precursor Co(acac)3 (acac: acetylacetonate), respectively. Changing thermal decomposition conditions of the precursor produces different phases and distinct morphologies of the cobaltous oxides. Hexagonal pyramidal shaped h-CoO nanocrystals have been formed by a flash heating of the reaction mixture (185 °C for 2 h, kinetic control condition), whereas cube shaped c-CoO nanocrystals have been produced by a prolonged heating at a relatively low temperature (130 °C for 12 h, thermodynamic control condition). Addition of o-Dichlorobenzene (o-DCB) to the reaction mixture alters the reaction condition to the thermodynamic control regime by slowing down the decomposition rate of the precursor. Further increase of the concentration of o-DCB in the reaction mixture changes the morphology of product from h-CoO hexagonal pyramids to h-CoO nanorods with various aspect ratios and finally to c-CoO nanocrystals. Air oxidation at 240 °C for 5 h of either h-CoO or c-CoO nanocrystals yields spinel Co3O4 nanocrystals with retention of the original crystal morphology. During the oxidation process, the h-CoO phase has been converted into Co3O4 via formation of the c-CoO phase, but the c-CoO phase has been directly oxidized to Co3O4. The electrochemical properties of the h-CoO, c-CoO, and spinel Co3O4 nanocrystals toward lithium exhibit characteristic features reflecting their Gibbs free energies. This work allows understanding of the detailed mechanism and energetics of selective formation, phase transformation, morphology control, and electrochemical properties in the closely related nanostructured cobalt oxides.
    Chemistry of Materials 07/2010; 22(15). DOI:10.1021/cm101138h · 8.54 Impact Factor
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    ABSTRACT: Monodisperse Pt and PtRu/C(60) hybrid nanoparticles with controlled diameters were synthesized by the reduction of metal precursors in the presence of C(60); the resulting metal/C(60) hybrid nanocatalyst exhibited a remarkable enhancement of methanol oxidation activity over commercial E-TEK catalysts.
    Chemical Communications 10/2009; DOI:10.1039/b911068b · 6.83 Impact Factor
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    ABSTRACT: (Figure Presented) Hollow nan': Hollow face-centered cubic (fcc) Co nanoparallelepipeds were prepared by thermolysis of solid fcc CoO nanoparallelepipeds in oleylamine (see TEM image). The solid fcc CoO nanoparallelepipeds are reduced by the oleylamine surfactant to form hollow fcc Co nanoparallelepipeds. Voids and fcc Co are generated on the surface of the solid fcc CoO nanoparallelepipeds by the removal of oxide as carbon monoxide.
    Angewandte Chemie International Edition 11/2008; 47(49):9504-8. DOI:10.1002/anie.200803048 · 11.26 Impact Factor
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    ABSTRACT: Monodisperse, phase-pure PtRu colloidal nanoalloy have been synthesized by the coreduction of Pt(acac)2platinum and Ru(Acac)3 (acac-acetylacetonate) by 1,2-hexadecanediol as a reducing agent in octyl ether in the presence of oleylamine and oleic acid as surfactants. The PtRu/Vc catalyst was generated by impregnation on the Vulvan carbon support, and the surfactant was removed by acetic acid treatment. These carbon-supported Pt-Ru catalyst exhibits a excellent catalytic activity for methanol oxidation. The high-quality of these catalyst is due to nanosize and monodispersity of the catalyst particles and excellent dispersion of the catalysts on the carbon support by colloidal impregnation and mild activation of the catalyst. These catalysts are expected to be promising materials for the anode, where methanol is catalytically oxidized in the direct methanol fuel cell (DMFC) to optimize DMFC performance.
    Chemistry of Materials 08/2006; 18(18). DOI:10.1021/cm052881k · 8.54 Impact Factor
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    ABSTRACT: Highly crystalline, phase- and size-controlled CoO nanocrystals of hexagonal and cubic phases have been prepared by thermal decomposition of Co(acac)3 in oleylamine under an inert atmosphere. Kinetic and thermodynamic control for the precursor formation leads to two different seeds of hexagonal and cubic phases at higher temperatures. The crystal size of both CoO phases can be easily manipulated by changing the precursor concentration and reaction temperature.
    Journal of the American Chemical Society 06/2005; 127(17):6188-9. DOI:10.1021/ja050359t · 11.44 Impact Factor