Woo-Jae Kim

Gachon University, 성남시, Gyeonggi-do, South Korea

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Publications (55)266.28 Total impact

  • Quynh Bui Vu Ngoc · Myung-Soo Choi · Woo-Jae Kim
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    ABSTRACT: We develop a quantitative analytical method to estimate the number of functional groups on single-walled carbon nanotube (SWNT) surfaces by simply measuring the Raman features of functionalized SWNTs. We use diazonium chemistry to attach functional groups to the SWNT surfaces and calculate the number of functional groups on the SWNTs by measuring their densities and using the analytical method developed in this study. We successfully establish a linear relationship between the independently estimated number of functional groups per carbon atom and the area ratio of the disorder mode (D peak, 1289 cm−1) to the tangential mode (G peak, 1582 cm−1) from the Raman measurements. From this study, we find that the injected reagents do not completely react with SWNT after 0.0263 functional groups per carbon atom are already attached to SWNT surfaces, e.g., 2.354 functional groups per nm (6, 5) of SWNT, therefore above this range, there is no linear relationship between the number of functional groups on SWNTs estimated from injected reagent concentration and the D to G area ratio (AD/G) of the Raman features. However, the number of functional groups per carbon atom (nf) estimated using the analytical method developed in this study linearly depends on the area ratio of D to G peaks of the Raman spectrum even above this range (nf = 0.02AD/G), therefore this simple and quick estimation scheme can provide an analytical basis for a variety of SWNT applications. We take into account the decrease of the number of surfactants by the coverage of functional groups on SWNTs in this scheme, and also verified that this relation is valid irrespective of SWNT diameter distributions.
    No preview · Article · Jan 2016 · RSC Advances
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    ABSTRACT: Varied morphologies and compositions of bismuth tungstate nanocomposites have been investigated as promising materials for photocatalytic applications. Among these nanocomposites, hierarchically structured bismuth sulfide (Bi 2 S 3)/bismuth tungstate (Bi 2 WO 6) hybrids have significant photocatalytic efficiency toward heavy metal ions. To simplify the synthetic procedure for this desirable composite, we developed a robust single-step hydrothermal synthesis for the formation of hierarchically structured heterocatalysts of Bi 2 S 3 / Bi 2 WO 6 with a high yield (>95%). The synthesized heterostructures were characterized by various spectroscopic, microscopic, and surface area analysis techniques, which confirmed the successful incorporation of Bi 2 S 3 into the Bi 2 WO 6 matrix and were used to optimize pore size for enhanced catalytic activity. The resulting Bi 2 S 3 /Bi 2 WO 6 heterocatalysts were used to remove toxic Cr(VI) ions via reduction to water insoluble Cr(III) utilizing visible-light irradiation. We also investigated the role of citric acid as a hole scavenger in the reduction of Cr(VI) with minimizing the rate of electron−hole recombination during photocatalysis. Likewise, the observed catalytic activity was significantly enhanced under a condition of an appropriate balance between hierarchical structure of catalysts and the amount of hole scavenger.
    Full-text · Article · Oct 2015 · ACS Sustainable Chemistry & Engineering
  • Chang Yong An · Kai Zhuo · Woo-Jae Kim · Chan-Hwa Chung
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    ABSTRACT: A dendritic Pd–Cu electrode has been prepared by co-electrodeposition of palladium and copper with high cathodic over-potentials, in which hydrogen evolution occurred. The dendritic Pd–Cu was electrochemically de-alloyed to selectively remove copper. This de-alloyed Pd–(Cu) electrode has a dramatically increased surface-area that is appropriate for electrochemical sensor applications. Electrode characteristics for use as a non-enzymatic sensor were monitored by cyclic voltammetry and amperometry, in which detection signals for hydrogen peroxide (H2O2) were precisely measured. The prepared dendritic Pd–(Cu) electrode demonstrated outstanding electrocatalytic activity for H2O2 reduction with a detection limit of 1.78 × 10−8 M, a wide dynamic range of 50 μM to 10 mM, and high sensitivity of 915.25 μA cm−2 mM−1.
    No preview · Article · Jul 2015 · Sensors and Actuators B Chemical
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    ABSTRACT: In this work, we demonstrate that Ar gas flow can be optically detected using mechanical vibration of a semiconducting single-walled carbon nanotube (SWCNT) bundle as a platform. A change in the photoluminescence (PL) intensity was induced by out-of-focusing of the SWCNT bundle of interest due to vibration caused by the introduced gas stream, for which a gas flow control system was installed in an optical microscope. The PL intensity was found to change systemically with the Ar flow rates in a range of relatively large flow rate intervals [0.70 to 3.0 standard cubic liters per minute (SLM) with 0.1–0.5 SLM intervals] with a noticeable hysteresis. It was, however, difficult to obtain a detectable PL change in a range of very small flow rate intervals (0.67 to 0.70 SLM with a 0.01 SLM interval). The detailed results and underlying mechanism are discussed in detail.
    No preview · Article · Dec 2014 · Journal of Nanoscience and Nanotechnology
  • Hyerim Choi · Won Jung Yoon · Heather Yang · Woo-Jae Kim
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    ABSTRACT: We demonstrate a simple and efficient method for separating metallic from semiconducting single-walled carbon nanotubes (SWNTs) using density-gradient ultracentrifugation. Density differences between metallic and semiconducting SWNTs, which enable SWNT separation by electronic type, are created using a single surfactant, i.e., sodium dodecyl sulfate (SDS), rather than a complex mixtures of surfactant, as is used in current separation schemes. SDS strongly adsorbs onto the surface of metallic SWNTs over semiconducting SWNTs by the mirror-charge phenomenon. Therefore, metallic SWNT-SDS assemblies have relatively smaller buoyant densities than semiconducting SWNT-SDS assemblies; thus, the metallic assemblies are easily collected at the most buoyant top fractions, whereas the semiconducting assemblies are collected at the bottom fractions. We also demonstrate that this protocol is valid regardless of the SWNT production method; that is, SWNTs grown by high-pressure carbon monoxide conversion (HiPco) and the arc discharge method. Optical absorption shows that the heavy bottom fractions consist of highly pure semiconducting nanotubes, whereas the buoyant top fractions consist of highly pure metallic nanotubes. In addition, films made of the separated metallic SWNTs exhibit lower sheet resistances than unsorted SWNTs by 53% for arc discharged and 64% for HiPco SWNTs, as expected.
    No preview · Article · Dec 2014 · Journal of Nanoscience and Nanotechnology
  • Jo-Eun Um · Won Jung Yoon · Hyung-Wook Choi · Woo-Jae Kim
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    ABSTRACT: In this paper, we describe a novel method for selectively attaching TiO2 promoters on Pd surfaces in Pd/SiO2 catalysts using selective chemical vapor deposition (CVD). Ti was selectively deposited on Pd active sites over a SiO2 support under a hydrogen atmosphere when titanium tetraisopropoxide was used as a Ti precursor. The Pd-Ti/SiO2 catalyst modified by CVD exhibits approximately 1.8 times higher ethylene selectivity than the un-modified Pd/SiO2 catalyst in the selective hydrogenation of acetylene due to the effective geometric modification of the Pd surface, which is beneficial to ethylene selectivity, by the selectively deposited Ti species.
    No preview · Article · Nov 2014 · Journal of Industrial and Engineering Chemistry
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    ABSTRACT: Although silver bromide has recently drawn considerable attention due to its high photocatalytic activity, it tends to form agglomerated metallic silver under the irradiation of visible light. Therefore, photocatalytic activity decreases with time and cannot be applied for repeated uses. To overcome this limitation, in the present work, we complexed AgBr with nitrogen doped (N-doped) and amine functionalized reduced graphene oxide (GN). N-doped and/or amine functionalized graphene shows intrinsically good catalytic activity. Besides, amine groups can undergo complexation with silver ions to suppress its reduction to metallic Ag. As a result, this complexed catalysts show excellent photocatalytic activity for the degradation of methylene blue (MB) dye under the irradiation of visible light. Photocatalytic degradation of MB shows that the catalytic activity is optimized at a condition of 0.5 wt% GN, under which ~ 99 % of MB was degraded only after 50 min of visible light irradiation. Notably, the complexed catalyst is quiet stable and retained almost full of its catalytic activity even after greater than ten repeated cycles. Moreover, the catalyst can also efficiently decompose 2-chlorophenol, a colorless organic contaminant, under visible light exposure. Detailed experimental investigation reveals that hydroxyl (·OH) radicals play an important role for dye degradation reactions. A relevant mechanism for dye degradation has also been proposed.
    Full-text · Article · Nov 2014 · ACS Applied Materials & Interfaces
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    ABSTRACT: Thermopower waves are a recently developed energy conversion concept utilizing dynamic temperature and chemical potential gradients to harvest electrical energy while the combustion wave propagates along the hybrid layers of nanomaterials and chemical fuels. The intrinsic properties of the core nanomaterials and chemical fuels in the hybrid composites can broadly affect the energy generation, as well as the combustion process, of thermopower waves. So far, most research has focused on the application of new core nanomaterials to enhance energy generation. In this study, we demonstrate that the alignment of core nanomaterials can significantly influence a number of aspects of the thermopower waves, while the nanomaterials involved are identical carbon nanotubes (CNTs). Diversely structured, large-area CNT/fuel composites of one-dimensional aligned CNT arrays (1D CNT arrays), randomly oriented CNT films (2D CNT films), and randomly aggregated bulk CNT clusters (3D CNT clusters) were fabricated to evaluate the energy generation, as well as the propagation of the thermal wave, from thermopower waves. The more the core nanostructures were aligned, the less inversion of temperature gradients and the less cross-propagation of multiple thermopower waves occurred. These characteristics of the aligned structures prevented the cancellation of charge carrier movements among the core nanomaterials and produced the relative enhancement of the energy generation and the specific power with a single-polarity voltage signal. Understanding this effect of structure on energy generation from thermopower waves can help in the design of optimized hybrid composites of nanomaterials and fuels, especially designs based on the internal alignment of the materials. More generally, we believe that this work provides clues to the process of chemical to thermal to electrical energy conversion inside/outside hybrid nanostructured materials.
    Full-text · Article · Sep 2014 · Nanoscale Research Letters
  • Jo-Eun Um · Chan-Hwa Chung · Doh C. Lee · Pil J. Yoo · Woo-Jae Kim
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    ABSTRACT: Sidewall functionalization of single-walled carbon nanotubes (SWNTs) has previously been used to effectively attach a variety of functional groups onto SWNTs; however, there has been little investigation into the reversibility of these reactions, which is critical for the restoration of the original optical and electrical properties of SWNTs. In this study, we investigated the complete removal of functional groups attached to the sidewalls of SWNTs by a thermal annealing process at high temperature to restore the genuine optical and electrical properties of SWNTs. Hydroxyphenyl groups were covalently attached to SWNTs for this purpose. Functionalized SWNTs typically show a significantly larger sheet resistance and completely lose their typical characteristic optical properties due to the covalent attachment of the functional groups onto the SWNT surfaces. However, we found that all of the functional groups could be completely removed by an annealing temperature above 400oC in a nitrogen environment, as confirmed by FT-IR, Raman and UV-vis-nIR absorption analyses. Accordingly, the original optical and electrical properties were also recovered upon annealing at above 400oC, as confirmed by the recovery of the original sheet resistance value. To demonstrate the effectiveness of annealing, we evaluated transparent films, which were prepared predominantly with metallic SWNTs, using the separation technique, which is based on selective covalent functionalization. A functionalized metallic SWNT film recovers its original high conductivity upon annealing at 400oC by the removal of the functional groups, which demonstrates the effectiveness of this annealing process.
    No preview · Article · Sep 2014 · RSC Advances
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    ABSTRACT: Composite materials of Sn-doped TiO2 nanoparticles (4–6 nm) uniformly anchored onto reduced graphene oxide (RGO) sheets were fabricated by a hydrothermal process. When compared with an undoped TiO2/RGO composite material, the partial substitution of Sn4+ for Ti4+ in TiO2/RGO in different doping concentrations not only increased the specific surface area, but also narrowed the band gap energy of the material. It also resulted in a shift in the photoresponse of the material into the visible light region, an increase in the relative intensity of its Raman (D/G) band, and changes in its X-ray photoelectron spectra; these results implied strong interactions between the Sn species and nanosized TiO2 as well as between the TiO2 and RGO layers. The presence of RGO led to enhanced adsorptivity of methyl orange onto the material compared with that of either pure TiO2 or Sn-doped TiO2. Increasing the Sn dopant content to optimal value facilitated photoactivity under both UV and visible light irradiation. The incorporation of both RGO and Sn dopants significantly modified the electronic structure of TiO2, altering the electron transfer direction and efficiently prohibiting the recombination of photo-induced charge carriers.
    No preview · Article · Mar 2014 · Applied Catalysis A General
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    ABSTRACT: La oxide is known to be the best promoter among reducible metal oxides for acetylene hydrogenation. However, it requires high-temperature reduction, which is not feasible in commercial processes. To maintain the enhanced catalytic performance by La oxide addition while lowering the reduction temperature for application in commercial process, we added Ti oxide as a second promoter, which has a higher reducibility than La oxide. The Ti oxide is added to the Pd surface, which has been partially covered by La oxide, and maintains the modified geometric and electronic structures of the Pd catalyst induced by the high-temperature-reduced La oxide even after low-temperature reduction, as confirmed by H2 chemisorption and X-ray photoelectron spectroscopy. Surprisingly, Ti oxide further modifies the electronic structure of Pd, even for low-temperature reduction, due to its high reducibility, leading to higher ethylene selectivity than when La oxide is used exclusively. We also confirmed that a similar additive effect also applies to other metal oxides, i.e., Nb2O5.
    No preview · Article · Feb 2014 · Applied Catalysis A General
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    ABSTRACT: We report colloidal growth of PbSe nanosheets and finely size-tuned PbSe nanocrystals (NCs) via simple control of reaction parameters. The approach involves slow injection of precursors with excess amounts of oleic acid. Retarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, causes attachment of PbSe NCs through the (110) planes, which are more reactive than the (100) facet, into a two-dimensional geometry. In contrast, such attachment processes can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs dispersed in chloroform. For instance, the slow injection of Pb and Se precursors into a reaction solution containing Cd chalcogenide NCs results in the growth of spherical PbSe NCs, as the Cd chalcogenide NCs hinder the PbSe nuclei from merging via (110) planes. Compared to conventional rapid-injection methods, PbSe NCs grow slowly, which enables fine control of NC size. Ab initio calculations suggest that Cd precursors strongly bound on the surface of PbSe NCs may impede nanosheet formation and may slow PbSe NC growth.
    No preview · Article · Jan 2014 · RSC Advances
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    ABSTRACT: A porous hydrous ruthenium oxide for supercapacitors is fabricated by electro-deposition accompanied with hydrogen-evolution reaction. This high-surface-area electrode (207.5 m2 g−1) is developed by electrochemical de-alloying process of copper from as-deposited Ru–(Cu) foam (23.5 m2 g−1). The three-dimensional structures with open pores and numerous dendritic morphologies produce good electrochemical performances by providing the pathway for facile penetration of electrolytes into active electrode surface. The three-dimensional porous hydrous ruthenium oxide exhibits good capacitance performance of 809 F g−1 at 1.5 A g−1, high energy density (112 Wh kg−1), and excellent stability retention (∼98%) even after 3000 cycles.
    No preview · Article · Dec 2013 · Journal of Power Sources
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    ABSTRACT: Highly efficient exfoliation of individual single-walled carbon nanotubes (SWNTs) was successfully demonstrated by utilizing biocompatible phenoxylated dextran, a kind of polysaccharide, as a SWNT dispersion agent. Phenoxylated dextran shows greater ability in producing individual SWNTs from raw materials than any other dispersing agent, including anionic surfactants and another polysaccharide. Furthermore, with this novel polymer, SWNT bundles or impurities present in raw materials are removed under much milder processing conditions compared to those of ultra-centrifugation procedures. There exists an optimal composition of phenoxy groups (∼13.6 wt%) that leads to the production of high-quality SWNT suspensions, as confirmed by UV-vis-nIR absorption and nIR fluorescence spectroscopy. Furthermore, phenoxylated dextran strongly adsorbs onto SWNTs, enabling SWNT fluorescence even in solid-state films in which metallic SWNTs co-exist. By bypassing ultra-centrifugation, this low-energy dispersion scheme can potentially be scaled up to industrial production levels.
    No preview · Article · May 2013 · Nanoscale
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    ABSTRACT: Electron beam deposition technique was used for formation of gadolinium Ca doped GDC electrolyte thin films. Ca_GDC electrolyte thin films were grown by evaporating Gd0.1Ce0.9O1.95 ceramic powder. The evaporating Ca_GDC electrolyte powder was synthesized by solid-state reaction method. The GDC thin films were deposited on porous Ni-GDC anode pellet. Operating technical parameters that influence thin film microstructure and crystallite size were studied. It was found that electron gun power (changed in the range of 0.60 to 1.05 kW) has the influence on the crystallite size of GDC thin films and decreased linearly increasing electron beam gun power. The influence of electron gun power and annealing temperature on thin film structure and surface morphology were investigated by X-ray diffraction (XRD) using CuKα-radiation in the range of 2 = 20∼80°C. The XRD peaks was showed formation of single-phase monoclinic structure was confirmed by X-ray diffraction (XRD) for the doped Ca GDC electrolyte. The patterns indicated the intensity of well-cubic fluorite structure phase.The morphology and the size of the prepared particles were investigated with a field-emission scanning electron microscope (FE-SEM). The performance of the cells was evaluated over 500∼800°C using humidified hydrogen as fuel and air as oxidant.
    No preview · Article · Dec 2012 · Molecular Crystals and Liquid Crystals
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    ABSTRACT: Nanomaterials have recently received significant attention as photothermal agents and fluorescence contrast agents for molecular therapeutics due to their unique optical properties (e.g. light absorption). In particular, single-walled carbon nanotubes (SWNTs) have been recently utilized as photothermal agents (for photothermal therapy) because of the solubility of SWNTs as well as their light absorbing capability at the near-infrared region. In this work, we have developed the SWNT-based photothermal agents using pyrene-based PEGylation of SWNTs. FT-IR and/or H-NMR spectroscopies have validated the PEGylation of SWNTs, and it is shown that water-soluble SWNTs are able to generate heat under near-infrared (NIR) irradiation of 5W/cm2. Moreover, it is found that our pyrene-based PEGylated SWNTs exhibit the fluorescence characteristic under the excitation wavelength of 340 nm. Our study sheds light on the pyrenyl PEGylated SWNTs as photothermal agents and/or fluorescence contrast agents for the future applications in molecular therapeutics.
    No preview · Article · Dec 2012 · BioChip journal
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    ABSTRACT: Diazonium salts preferentially react with metallic single-walled carbon nanotubes (SWNT) over semiconducting SWNT, enabling the separation of SWNT by electronic type. Therefore, the reaction selectivity of diazonium salts for metallic SWNT is crucial for high purity separation of both metallic and semiconducting SWNT. Herein, we developed an efficient method of increasing the reaction selectivity by manipulating the redox potential of diazonium salts. The electron affinity of diazonium salts is effectively lowered when the para-substituent of the diazonium salts is an electron-donating group, (i.e., 4-hydroxy and 4-propargyloxy) rather than an electron-withdrawing group (i.e., 4-nitro, 4-carboxy, and 4-cholro). The reduction potential of 4-hydroxyphenyl and 4-propargyloxyphenyl diazonium salt was greater than the oxidation potential of semiconducting SWNT; therefore, the electron transfer reaction between these two reagents was effectively suppressed, leading to a highly selective reaction for metallic SWNT. We confirmed that this highly selective reaction scheme can be used to separate SWNT, and high purity semiconducting SWNT can be obtained via density-induced separation.
    No preview · Article · Oct 2012 · Chemistry of Materials
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    ABSTRACT: Pd/SiO2 for acetylene hydrogenation was doubly promoted with La oxide and Si, the latter being selectively deposited on Pd instead of on SiO2. La oxide promoted the ethylene selectivity of the catalyst due to its strong interaction with Pd after the catalyst had been reduced at high temperature, e.g., 500 °C, and Si prevented the migration of La oxide in the catalyst when the reduced catalyst was exposed to air. Consequently, the doubly promoted catalyst showed high ethylene selectivity comparable to that of 500 °C-reduced Pd–La/SiO2 even when the catalyst had been oxidized and subsequently reduced at low temperature, e.g., 300 °C.
    No preview · Article · Jul 2012 · Catalysis Communications
  • Woo-Jae Kim · Sang Heup Moon
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    ABSTRACT: This article reviews our previous studies to develop high performance catalysts for acetylene hydrogenation. Ethylene selectivity of Pd catalysts in acetylene hydrogenation was improved by adding Si, metal oxides showing SMSI behavior, and either Ag or Cu as promoters. The promoter effect was further enhanced by maximizing the interactions between Pd and added promoters. Chemical vapor deposition was used for the addition of Si, high-temperature reduction for the SMSI metal oxides, and a surface redox (SR) for Ag and Cu. Si modified the Pd surface geometrically, SMSI metal oxides modified Pd both electronically and geometrically, and Ag modified Pd largely electronically. Cu added by SR modified Pd electronically to a small extent, but preferentially decorated the low-coordination sites of Pd such that the ethylene selectivity of the optimum catalyst was significantly promoted from that of Pd, while its activity remained comparable to that of Pd. The sensitivity of ethylene selectivity to the surface structure of Pd was demonstrated using model catalysts containing uniform-sized Pd particles in either cubic or spherical shapes. The three stage deactivation of the Pd catalyst and the self-regenerative behavior in early stages of deactivation were also investigated.
    No preview · Article · May 2012 · Catalysis Today
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    ABSTRACT: Amorphous indium–gallium–zinc-oxide (a-IGZO) thin films were prepared using the facing targets sputtering (FTS) method as a function of input power at room temperature. The a-IGZO films were used a channel layer for thin film transistors (TFTs). The electrical, optical, and structural properties of a-IGZO thin films were measured by Hall Effect measurement, UV/vis spectrometer and X-ray diffractometer. The performance and device characteristics of the a-IGZO TFTs were measured by using a semiconductor parameter analyzer. The transfer characteristics of a-IGZO TFTs exhibited saturation mobility of 10.83cm2/Vs and threshold voltage of 5.13V.
    No preview · Article · Jan 2012 · Ceramics International

Publication Stats

1k Citations
266.28 Total Impact Points


  • 2010-2016
    • Gachon University
      • • Department of Chemical and Biological Engineering
      • • Department of Environmental and Energy Engineering
      • • Department of Electrical Engineering
      성남시, Gyeonggi-do, South Korea
  • 2011
    • Stanford University
      • Department of Chemical Engineering
      Palo Alto, California, United States
  • 2008-2011
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, Massachusetts, United States
  • 2006-2007
    • University of Illinois, Urbana-Champaign
      • Department of Chemical and Biomolecular Engineering
      Urbana, Illinois, United States
    • Sungkyunkwan University
      • Department of Chemical Engineering
      Sŏul, Seoul, South Korea