Joonwon Lim

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Three‐Dimensional Shape Engineered, Interfacial Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors

DOI: 10.1002/adma.201303503 Assembly of graphene into functional macroscopic objects, such as fi lms, [ 1 ] sheets, [ 2 ] fi bers, [ 3 ] foams, [ 4,5 ] and other complex architectures, [ 6 ] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modifi cation, two-dimensional graphene may straightforwardly exploit strong interlayer π – π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area. [ 7,8 ]

N-doped graphitic self-encapsulation for high performance silicon anodes in lithium-ion batteries

N-doped sites at CNT and graphene trigger spontaneous encapsulation of Si particles by simple pH control at room temperature. Significantly, N-doped CNT encapsulated Si composite electrode materials show remarkable cycle life and rate performance in battery operations. Superior capacity retention of 79.4% is obtained after 200 cycles and excellent rate capability of 914 mA h g−1 is observed at a 10 C rate.

Liquid crystal size selection of large-size graphene oxide for size-dependent N-doping and oxygen reduction catalysis.

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.

Synergistic Concurrent Enhancement of Charge Generation, Dissociation, and Transport in Organic Solar Cells with Plasmonic Metal–Carbon Nanotube Hybrids

Plasmonic nanostructures are synthesized by decorating B- or N-doped carbon nanotubes (CNTs) with Au nanoparticles. While the plasmonic nanoparticles promote exciton generation and dissociation, the B- and N-doped CNTs enable charge-selective transport enhancement in the organic active layer. Such concurrent enhancements of all the principal energy-harvesting steps improve the device efficiency up to 9.98% for organic single-junction solar cells.

Selective and Regenerative Carbon Dioxide Capture by Highly Polarizing Porous Carbon Nitride

Energy-efficient CO2 capture is a stringent demand for green and sustainable energy supply. Strong adsorption is desirable for high capacity and selective capture at ambient conditions but unfavorable for regeneration of adsorbents by a simple pressure control process. Here we present highly regenerative and selective CO2 capture by carbon nitride functionalized porous reduced graphene oxide aerogel surface. The resultant structure demonstrates large CO2 adsorption capacity at ambient conditions (0.43 mmol·g(-1)) and high CO2 selectivity against N2 yet retains regenerability to desorb 98% CO2 by simple pressure swing. First-principles thermodynamics calculations revealed that microporous edges of graphitic carbon nitride offer the optimal CO2 adsorption by induced dipole interaction and allows excellent CO2 selectivity as well as facile regenerability. This work identifies a customized route to reversible gas capture using metal-free, two-dimensional carbonaceous materials, which can be extended to other useful applications.

Dopant-specific unzipping of carbon nanotubes for intact crystalline graphene nanostructures.

Atomic level engineering of graphene-based materials is in high demand to enable customize structures and properties for different applications. Unzipping of the graphene plane is a potential means to this end, but uncontrollable damage of the two-dimensional crystalline framework during harsh unzipping reaction has remained a key challenge. Here we present heteroatom dopant-specific unzipping of carbon nanotubes as a reliable and controllable route to customized intact crystalline graphene-based nanostructures. Substitutional pyridinic nitrogen dopant sites at carbon nanotubes can selectively initiate the unzipping of graphene side walls at a relatively low electrochemical potential (0.6 V). The resultant nanostructures consisting of unzipped graphene nanoribbons wrapping around carbon nanotube cores maintain the intact two-dimensional crystallinity with well-defined atomic configuration at the unzipped edges. Large surface area and robust electrical connectivity of the synergistic nanostructure demonstrate ultrahigh-power supercapacitor performance, which can serve for AC filtering with the record high rate capability of −85° of phase angle at 120 Hz.

Highly tunable refractive index visible-light metasurface from block copolymer self-assembly

The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification.

Direct Growth of Polyaniline Chains from N‐Doped Sites of Carbon Nanotubes

Polymer grafting from graphitic carbon materials has been pursued for several decades. Unfortunately, currently available methods mostly rely on the harsh chemical treatment of graphitic carbons which causes severe degradation of chemical structure and material properties. A straightforward growth of polyaniline chain from the nitrogen (N)-doped sites of carbon nanotubes (CNTs) is presented. N-doping sites along the CNT wall nucleate the polymerization of aniline, which generates seamless hybrids consisting of polyaniline directly grafted onto the CNT walls. The resultant materials exhibit excellent synergistic electrochemical performance, and can be employed for charge collectors of supercapacitors. This approach introduces an efficient route to hybrid systems consisting of conducting polymers directly grafted from graphitic dopant sites.

3D Tailored Crumpling of Block-Copolymer Lithography on Chemically Modified Graphene

Novel 3D self-assembled nanopatterning is presented via tailored crumpling of chemically modified graphene. Block-copolymer self-assembly formed on a layer of chemically modified graphene provides highly dense and uniform 2D nanopatterns, and the controlled crumpling of the chemically modified graphene by mechanical instabilities realizes the controlled 3D transformation of the self-assembled nanopatterns.