Junhyung Kim

Carbon Nanotube/Manganese Oxide Ultrathin Film Electrodes for Electrochemical Capacitors

Multiwall carbon nanotube (MWNT)/manganese oxide (MnO2) nanocomposite ultrathin film electrodes have been created via redox deposition of MnO2 on layer-by-layer (LbL)-assembled MWNT films. We demonstrate that these LbL-assembled MWNT (LbL-MWNT)/MnO2 thin films consist of a uniform coating of nanosized MnO2 on the MWNT network structure using SEM and TEM, which is a promising structure for electrochemical capacitor applications. LbL-MWNT/MnO2 electrodes yield a significantly higher volumetric capacitance of 246 F/cm3 with good capacity retention up to 1000 mV/s due to rapid transport of electrons and ions within the electrodes. The electrodes are generated with two simple aqueous deposition processes: the layer-by-layer assembly of MWNTs followed by redox deposition of MnO2 at ambient conditions, thus providing a straightforward approach to the fabrication of high-power and -energy electrochemical capacitors with precise control of electrode thickness at nanometer scales.

Quantum-sized gold clusters as efficient two-photon absorbers

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.

Surface Composition Tuning of Au–Pt Bimetallic Nanoparticles for Enhanced Carbon Monoxide and Methanol Electro-oxidation

The ability to direct bimetallic nanoparticles to express desirable surface composition is a crucial step toward effective heterogeneous catalysis, sensing, and bionanotechnology applications. Here we report surface composition tuning of bimetallic Au-Pt electrocatalysts for carbon monoxide and methanol oxidation reactions. We establish a direct correlation between the surface composition of Au-Pt nanoparticles and their catalytic activities. We find that the intrinsic activities of Au-Pt nanoparticles with the same bulk composition of Au0.5Pt0.5 can be enhanced by orders of magnitude by simply controlling the surface composition. We attribute this enhancement to the weakened CO binding on Pt in discrete Pt or Pt-rich clusters surrounded by surface Au atoms. Our finding demonstrates the importance of surface composition control at the nanoscale in harnessing the true electrocatalytic potential of bimetallic nanoparticles and opens up strategies for the development of highly active bimetallic nanoparticles for electrochemical energy conversion.

Critical Size for the Observation of Quantum Confinement in Optically Excited Gold Clusters

We present a systematic study of optical properties of a series of hexanethiolate-capped Au clusters of varying sizes using femtosecond transient absorption, time-resolved fluorescence, and two-photon absorption cross-sectional measurements. An abrupt change in optical properties and their trends has been found at the 2.2 nm size. Displacively excited vibrations with a period of 450 fs have been detected in the transient absorption signal for smaller clusters < or = 2.2 nm. These results strongly suggest an emerging optical gap between the highest occupied and lowest unoccupied orbitals in the narrow size range at 2.2 nm.

Virus-templated Au and Au–Pt core–shell nanowires and their electrocatalytic activities for fuel cell applications

A facile synthetic route was developed to make Au nanowires (NWs) from surfactant-mediated bio-mineralization of a genetically engineered M13 phage with specific Au binding peptides. From the selective interaction between Au binding M13 phage and Au ions in aqueous solution, Au NWs with uniform diameter were synthesized at room temperature with yields greater than 98 % without the need for size selection. The diameters of Au NWs were controlled from 10 nm to 50 nm. The Au NWs were found to be active for electrocatalytic oxidation of CO molecules for all sizes, where the activity was highly dependent on the surface facets of Au NWs. This low-temperature high yield method of preparing Au NWs was further extended to the synthesis of Au/Pt core/shell NWs with controlled coverage of Pt shell layers. Electro-catalytic studies of ethanol oxidation with different Pt loading showed enhanced activity relative to a commercial supported Pt catalyst, indicative of the dual functionality of Pt for the ethanol oxidation and Au for the anti-poisoning component of Pt. These new one-dimensional noble metal NWs with controlled compositions could facilitate the design of new alloy materials with tunable properties.

Optically Excited Acoustic Vibrations in Quantum-Sized Monolayer-Protected Gold Clusters

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

Pt-Covered Multiwall Carbon Nanotubes for Oxygen Reduction in Fuel Cell Applications

Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm(2) Pt at 0.9 ViR-corrected versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst -46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO4 at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt at 0.9 ViR-corrected vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs.

Oxygen Reduction Activity of PtxNi1-x Alloy Nanoparticles on Multiwall Carbon Nanotubes

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Nonlinear optical properties of quantum sized gold clusters

Gold clusters with the sizes close to the Fermi wavelength of electron shows interesting quantum size effects. Linear and nonlinear optical properties show dramatic trends when the sizes of clusters are in the range of quantum confinement. We have investigated the size dependence of the non-linear optical property of two-photon absorption (TPA) cross-sections of the gold clusters. Absolute TPA cross-sections are measured by a combination of one and two-photon excited fluorescence upconversion measurements. Large cross-sections and abrupt changes in the trend of cross-sections are observed in the size-dependence when size is reduced from nanoparticles to cluster. The results can be attributed to the appearance of quantum confinement in these monolayer protected gold clusters.