Ki Min Nam

Single‐Crystalline Hollow Face‐Centered‐Cubic Cobalt Nanoparticles from Solid Face‐Centered‐Cubic Cobalt Oxide Nanoparticles

Hollow nanoparticles are of great interest because of their applications in catalysis, nanoelectronics, photonics, drug delivery system, nanoreactors, lubrication, and chemical storage. Various known hollow spheres include those composed of carbon, polymers, metals, and inorganic materials. Diverse synthetic methods have been developed to prepare these hollow nanoparticles, such as removal of the templating core, galvanic replacement, and through the Kirkendall effect. Cobalt exhibits hexagonal closed-packed (hcp Co) and face-centered cubic (fcc Co) structures in the bulk, and a metastable cubic structure labeled e-Co in the nanometer range. Cobalt nanostructures have been widely studied because of their potential applications, mainly in ultrahighdensity magnetic storage. However, reports on hollow cobalt nanoparticles are very limited thus far, although they are interesting materials in terms of their unusual magnetic domains and quantum properties. We herein report that fcc Co hollow nanoparallelepipeds have been prepared by thermolysis of fcc CoO solid nanoparallelepipeds in oleylamine (C18H35NH2). The fcc CoO solid nanoparallelepipeds, surprisingly, are reduced by the oleylamine surfactant to form fcc Co hollow nanoparallelepipeds. This new phenomenon could signify an important methodology to produce constituent metal (M) hollow nanoparticles from metal oxide (MO) solid nanoparticles. In our previous work, we reported phaseand sizecontrolled syntheses of hexagonal and cubic CoO nanocrystals (hcp CoO and fcc CoO). The fcc CoO solid nanoparallelepipeds in oleylamine undergoes reduction at high temperatures (270–290 8C) to transform into hollow nanoparallelepipeds composed of cubic metallic Co (fcc Co). Figure 1 shows the evolution of the morphology of fcc Co

Designed Synthesis of Well-Defined Pd@Pt Core-Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Phosphidation of Li4Ti5O12 nanoparticles and their electrochemical and biocompatible superiority for lithium rechargeable batteries

Phosphidated-Li(4)Ti(5)O(12) shows high capacity with a significantly enhanced kinetics opening new possibilities for ultra-fast charge/discharge of lithium rechargeable batteries. The in vitro cytotoxicity test proves its fabulous cell viability, indicating that the toxicity problem of nanoparticles can be also solved by phosphidation.

Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction

Modification of the electronic structure and lattice contraction of Pt alloy nanocatalysts through control over their morphology and composition has been a crucial issue for improving their electrocatalytic oxygen reduction reaction (ORR) activity. In the present work, we synthesized PtCo alloy nanocubes with controlled compositions (Pt(x)Co NCs, x = 2, 3, 5, 7, and 9) by regulating the ratio of surfactants and the amount of Co precursor to elucidate the effect of the composition of nanocatalysts on their ORR activity. Pt(x)Co NCs had a Pt-skin structure after electrochemical treatment. The electrocatalysis experiments revealed a strong correlation between ORR activity and Co composition. Pt₃Co NCs exhibited the best ORR performance among the various Pt(x)Co NCs. From density functional theory calculations, a typical volcano-type relationship was established between ORR activity and oxygen binding energy (E(OB)) on NC surfaces, which showed that Pt₃Co NCs had the optimal E(OB) to achieve the maximum ORR activity. X-ray photoelectron spectroscopy and X-ray diffraction measurements demonstrated that the electronic structure and lattice contraction of the Pt(x)Co NCs could be tuned by controlling the composition of NCs, which are highly correlated with the trends of E(OB) change.

Comprehensive design of carbon-encapsulated Fe3O4 nanocrystals and their lithium storage properties.

Controlling the bulk and surface structure of metal oxide nanostructures is crucial to obtain superior electronic and electrochemical properties. However, the synthetic or post-treatment techniques for preparing such structures, especially those with complex configurations, still remains a challenge. Herein, we report a completely novel approach-an amorphous carbon coating on the surface coupled with a controlled metal oxidation state in the bulk-via a simple glucose treatment. The bulk and surface structures of iron oxides are controlled by the carbothermal reaction associated with the decomposition of glucose. These novel configurations of iron oxides possess an amorphous carbon layer and ferrous state with high electronic conductivity, which definitely enhances their electrochemical properties compared to pristine iron oxides. Our findings provide an effective solution for the synthesis of complex metal oxide nanostructures, which can pave the way to further expand the electronic or electrochemical applications of metal oxides.

Hollow Sn–SnO2 Nanocrystal/Graphite Composites and Their Lithium Storage Properties

Hollow spheres have been constructed by applying the Kirkendall effect to Sn nanocrystals. This not only accommodates the detrimental volume expansion but also reduces the Li(+) transport distance enabling homogeneous Li-Sn alloying. Hollow Sn-SnO2 nanocrystals show a significantly enhanced cyclic performance compared to Sn nanocrystal alone due to its typical structure with hollow core. Sn-SnO2/graphite nanocomposites obtained by the chemical reduction and oxidation of Sn nanocrystals onto graphite displayed very stable cyclic performance thanks to the role of graphite as an aggregation preventer as well as an electronic conductor.

Improved Photoelectrochemical Water Oxidation by the WO3/CuWO4 Composite with a Manganese Phosphate Electrocatalyst

We describe a composite of the n-type semiconductors for the photoelectrochemical oxygen evolution reaction (OER). A simple drop-casting technique of mixed precursors and a one-step annealing process were used in the synthesis of the WO3/CuWO4 composite. The composite showed improved photocurrent for water oxidation compared to either of the two compounds individually. We discuss possible electron-hole separation mechanisms in two semiconductors comprising a primary photon-absorbing semiconductor of CuWO4 with a secondary semiconductor of WO3. When the WO3/CuWO4 composite is simultaneously irradiated, the photogenerated hole from the WO3 valence band transfers to CuWO4, which results in an enhanced charge separation of CuWO4. Furthermore, the OER catalytic activity of manganese phosphate (MnPO) was compared to manganese oxide nanoparticles (Mn2O3) by electrochemical measurements, showing that the manganese phosphate was more efficient for the OER reaction. To investigate the effect of catalysts on semiconductors, manganese phosphate was deposited on the WO3/CuWO4 composite. The result demonstrates the promise of manganese phosphate for improving the photocurrent as well as the stability of the WO3/CuWO4 composite.

Monodisperse Pt and PtRu/C60 hybrid nanoparticles for fuel cell anode catalysts

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.

New Crystal Structure: Synthesis and Characterization of Hexagonal Wurtzite MnO

Most transition metal oxides have a cubic rocksalt crystal structure, but ZnO and CoO are the only stable transition metal oxides known to possess a hexagonal structure. Unprecedented hexagonal wurtzite MnO has been prepared by thermal decomposition of Mn(acac)(2) on a carbon template. Structural characterization has been carried out by TEM, SAED, and a Rietveld analysis using XRD. The experimental and theoretical magnetic results indicate magnetic ordering of the hexagonal wurtzite MnO. Density functional calculations have been performed in order to understand the electronic and piezoelectric properties of the newly synthesized hexagonal wurtzite MnO.

The Role of Water for the Phase-Selective Preparation of Hexagonal and Cubic Cobalt Oxide Nanoparticles

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.