Sangkyu Lee

Hanyang University, Sŏul, Seoul, South Korea

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Publications (30)115.35 Total impact

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    ABSTRACT: The inherently low electrical conductivity of TiO2-based electrodes as well as the high electrical resistance between an electrode and a current collector represents a major obstacle to their use as an anode for lithium ion batteries. In this study, we report on high-density TiO2 nanotubes (NTs) branched onto a carbon nanofiber (CNF) "tree" that provide a low resistance current path between the current collector and the TiO2 NTs. Compared to a TiO2 NT array grown directly on the current collector, the branched TiO2 NTs tree, coupled with the CNF electrode, exhibited similar to 10 times higher areal energy density and excellent rate capability (discharge capacity of similar to 150 mA.h.g(-1) at a current density of 1,000 mA.g(-1)). Based on the detailed experimental results and associated theoretical analysis, we demonstrate that the introduction of CNFs with direct electric contact with the current collector enables a significant increase in areal capacity (mA.h.cm(-2)) as well as excellent rate capability.
    Nano Research 04/2015; 7(4):491-501. DOI:10.1007/s12274-014-0415-1 · 6.96 Impact Factor
  • Seungki Hong, Sangkyu Lee, Ungyu Paik
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    ABSTRACT: Here we propose a core-shell tubular nanostructure consisted of hollow carbon nanofiber and manganese oxide (MnO2) for the application of high capacitance electrochemical capacitors. Hollow nanostructured carbon nanofibers are prepared using an electrospinning technique with a dual nozzle. The hollow channel of carbon nanofibers enables the uptake of MnO2 precursor solution inside the hollow carbon nanofiber, leading to the formation of MnO2 layer on both the inner and outer surfaces of hollow carbon nanofiber. The utilization of both surfaces of hollow carbon nanofiber increases the effective reaction sites of electrode materials contacted with an electrolyte as well as maximizes the loading mass of MnO2 on the surface of hollow carbon nanofiber (94% compared to carbon contents), consequently enabling the fabrication of electrochemical capacitors with the increased specific capacitance of 237 F/g.
    Electrochimica Acta 09/2014; 141:39–44. DOI:10.1016/j.electacta.2014.07.047 · 4.09 Impact Factor
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    ABSTRACT: Nitridated Si–Ti–Ni (STN) ternary alloy as an anode for lithium ion batteries is prepared by the NH3 nitridation of STN alloy. The nitridated STN electrode shows 14% increased reversible capacity and 4 times higher rate capability at 3 C compared to those of pristine STN electrode. These improvements are attributed to the formation of nitride compounds (such as SiNx/SiNxOy, TiNx/TiNxOy and NiNx/NiNxOy) on STN alloy, which benefits both mechanics and kinetics.
    Journal of Power Sources 05/2014; 253:282–286. DOI:10.1016/j.jpowsour.2013.12.041 · 5.21 Impact Factor
  • Electrochimica Acta 04/2014; 125:536-542. DOI:10.1016/j.electacta.2014.01.142 · 4.09 Impact Factor
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    ABSTRACT: Si-Ge composites have recently been explored as an anode material for lithium-ion batteries due to their stable cycle performance and excellent rate capability. Although previous reports show the benefits of Si-Ge composites on electrochemical performance, the specific mechanism and structural effects have been overlooked. Here, the structural effect of Si-Ge heterogeneous nanostructures on both mechanics and kinetics is systematically studied through theoretical analysis and detailed experimental results. Si-Ge and Ge-Si core–shell nanowires are employed for this study. The Si-Ge core–shell nanowires show a much improved electrochemical performance, especially cycle performance and rate capability, when compared to those of the Ge-Si core–shell nanowires electrode. On the basis of the detailed experimental results and associated theoretical analysis, its is demonstrated that the strain distribution and Li diffusivity and/or diffusion path are significantly affected by the Si-Ge heterostructure, which induce different mechanics and kinetics associated with lithium.
    Advanced Functional Materials 03/2014; 24(10). DOI:10.1002/adfm.201302122 · 10.44 Impact Factor
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    ABSTRACT: To elucidate the influence of electrode geometry on the photocatalytic performance of TiO2, herein, we report the synthesis of three-dimensional in situ Gd-doped TiO2 nanofibers (TiO2-NFs) using a simple electrospinning technique. The as-spun pristine TiO2-NFs show a higher photocatalytic (PC) activity (k = 0.013 m−1) than the TiO2 nanoparticles (TiO2-NPs) (k = 0.006 m−1) electrode, which could be attributed to the fast electron transport in the 1D NFs. In addition, Gd-doped TiO2-NFs show nearly five-fold enhancement in the PC degradation rate due to synergistically higher electron transport and production of HO˙ due to the effects of morphology and doping, respectively. In striking contrast, Gd-doping has no influence on the PC activity of TiO2-NPs due to increased grain boundaries, signifying the vital role of the electrode architecture. The mechanism of Gd doping in pure anatase TiO2 is investigated using density functional theory (DFT) calculations. The influence of Gd-doping and the electrode architecture on the charge recombination and flat-band potential variation in TiO2 are discussed elaborately using ultraviolet photoelectron spectroscopy (UPS) and Mott-Schottky analysis, and the implications of these findings for designing doped 3D fibrous photoelectrodes are discussed.
    RSC Advances 01/2014; 4(23). DOI:10.1039/c3ra46851h · 3.71 Impact Factor
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    ABSTRACT: The relationship between surface coverage of the shell layer in core‐shell nanostructured electrodes and the resulting electrochemical performance of the electrodes is described. Systematic studies and theoretical modeling reveal that partially covered core‐shell nanostructured electrodes are beneficial to release the stress induced by lithiation and delithiation, leading to the excellent cycle performance without any capacity fading up to 50 cycles.
    Advanced Energy Materials 01/2014; 4(1). DOI:10.1002/aenm.201300472 · 14.39 Impact Factor
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    ABSTRACT: The nanoweb structure of TiO2 anode, cross-linked between electrospun nanofibers, is directly fabricated on the current collector by utilizing the fluidity of low glass transition temperature polymer at room temperature. This characteristic enables us to fabricate the nanoweb structure by direct electrosspinning on the current collector, followed by uniaxial pressing. This proposed structure facilitates electron transport through the direct conducting pathways between TiO2 active materials and current collector as well as provides strong adhesion strength to the current collector without polymeric binders. Consequently, we could achieve stable cycle performance up to 100 cycles and the excellent rate capability of ~ 60 % at high rate charge/discharge condition of 10 C.
    ACS Applied Materials & Interfaces 11/2013; 5(22). DOI:10.1021/am404082h · 5.90 Impact Factor
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    ABSTRACT: The development of various technologies has led to the advent of a variety of deformable devices. Despite significant technological advancement in this area, it is still challenging to integrate different devices due to limitations such as substrate issues and differences among growth and deposition conditions. Creating an interconnection between two different devices currently requires the use of metallic wires/lines to build electrical connections. Here, we demonstrate a LEGO-like assembly of the free-standing film of individually operable components encapsulated in a polymer overcoat layer, leading to the production of an integrated architecture without additional electrical connections. The free-standing components are produced by the peeling-off process. The sticky nature of the polymer layer enables the construction of supercapacitor arrays and simple RLC circuits by interlocking the individual components. We expect that this approach will enable the fabrication of a variety of custom-built devices using a LEGO-like assembly method.
    10/2013; 5(10). DOI:10.1038/am.2013.51
  • 223th ECS Meeting; 05/2013
  • 223th ECS Meeting; 05/2013
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    ABSTRACT: A TiO2 nanofiber anode is glued using a sol-gel reaction to improve cycle performance and Coulombic efficiencies. The hydrolysis-condensation process produces TiO2 nanoglues consisting of TiO2 nanoparticles, providing strong adhesion between nanofibers and the current collector as well as between nanofibers, rendering favourable transporting properties of electrons and Li ions in the TiO2 nanofiber electrode, and eventually leading to the excellent performance of Li ion batteries exhibiting an excellent initial Coulombic efficiency (∼85%), cycle performance up to 100 cycles, and high rate capability (60% at 5 C).
    Nanoscale 03/2013; DOI:10.1039/c3nr00157a · 6.74 Impact Factor
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    ABSTRACT: Tin oxide (SnO2) is a promising candidate as an anode for lithium ion batteries because of its high theoretical capacity. However, poor capacity retention caused by large volume changes during cycling, large initial irreversible capacity, and low rate capability frustrate its practical application. We have developed a ternary nanocomposite based on tin indium oxide (SnO2–In2O3) and graphene nanosheet (GNS) synthesized via a facile solvothermal method. The incorporation of In2O3 into SnO2 can improve the electrochemical property of SnO2 and reduce the charge transfer resistance of electrode leading to the enhanced reversible capacity and rate capability. The graphene nanosheet in the composite electrode can accommodate high volume expansion/contraction during cycling resulting in excellent capacity retention. As an anode for lithium ion batteries, the SnO2–In2O3/GNS nanocomposite exhibits a remarkably improved electrochemical performance in terms of lithium storage capacity (962 mAh g−1 at 60 mA g−1 rate), initial coulombic efficiency (57.2%), cycling stability (60.8% capacity retention after 50 cycles), and rate capability (393.25 mAh g−1 at 600 mA g−1 rate after 25 cycles) compared to SnO2/GNS and pure SnO2–In2O3 electrode.
    Electrochimica Acta 02/2013; 91:275–281. DOI:10.1016/j.electacta.2012.12.070 · 4.09 Impact Factor
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    ABSTRACT: This paper explores transport in transparent thin film transistors formed using a liquid precursor to indium zinc oxide, delivered to target substrates by electrohydrodynamic jet (e-jet) printing. Under optimized conditions, we observe field effect mobilities as high as 32 cm2V−1s−1, with on/off current ratios of 103 and threshold voltages of 2 V. These results provide evidence that material manipulated in fine-jet, electric field induced liquid flows can yield semiconductor devices without any adverse effects of residual charge or unintentional doping. E-jet printing methods provide levels of resolution (∼1.5 μm) that provide a path to printed transistors with small critical dimensions.
    Applied Physics Letters 03/2012; 100(10). DOI:10.1063/1.3691177 · 3.52 Impact Factor
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    ABSTRACT: Nearly all research in micro- and nanofabrication focuses on the formation of solid structures of materials that perform some mechanical, electrical, optical, or related function. Fabricating patterns of charges, by contrast, is a much less well explored area that is of separate and growing interesting because the associated electric fields can be exploited to control the behavior of nanoscale electronic and mechanical devices, guide the assembly of nanomaterials, or modulate the properties of biological systems. This paper describes a versatile technique that uses fine, electrified liquid jets formed by electrohydrodynamics at micro- and nanoscale nozzles to print complex patterns of both positive and negative charges, with resolution that can extend into the submicrometer and nanometer regime. The reported results establish the basic aspects of this process and demonstrate the capabilities through printed patterns with diverse geometries and charge configurations in a variety of liquid inks, including suspensions of nanoparticles and nanowires. The use of printed charge to control the properties of silicon nanomembrane transistors provides an application example.
    Nano Letters 02/2010; 10(2):584-91. DOI:10.1021/nl903495f · 13.03 Impact Factor
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    Ungyu Paik, Sangkyu Lee, Jea-Gun Park
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    ABSTRACT: The effect of physicochemical properties of solvents on the microstructure of polyvinyl carbazole (PVK) film for non-volatile polymer memory was investigated. For the solubilization of PVK molecules and the preparation of PVK films, four solvents with different physicochemical properties of the Hildebrand solubility parameter and vapor pressure were considered: chloroform, tetrahydrofuran (THF), 1,1,2,2-tetrachloroethane (TCE), and N,N- dimehtylformamide (DMF). The solubility of PVK molecules in the solvents was observed by ultraviolet- visible spectroscopy. PVK molecules were observed to be more soluble in chloroform, with a low Hildebrand solubility parameter, than solvents with higher values. The aggregated size and micro-/nano-topographical properties of PVK films were characterized using optical and atomic force microscopes. The PVK film cast from chloroform exhibited enhanced surface roughness compared to that from TCE and DMF. It was also confirmed that the microstructure of PVK film has an effect on the performance of non-volatile polymer memory.
    Journal of Semiconductor Technology and Science 03/2008; 8(1). DOI:10.5573/JSTS.2008.8.1.046 · 0.62 Impact Factor
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    ABSTRACT: We report a novel one-step method for the preparation of hierarchically patterned Au nanoparticles in a conducting polymer matrix by controlling the interface properties between Au nanoparticles and the conducting polymer matrix. The terminal group of capping molecules for the Au nanoparticles was modified to change the interface properties, not to change the size of the Au nanoparticles which affects their intrinsic properties. By modulating the interface properties, it is possible to construct Au nanoparticle-conducting polymer composites with two different structures: one presents a triple layer in which the conducting polymer layer is sandwiched between Au nanoparticle layers at the top and bottom; the other exhibits a form like a raisin cake in which Au nanoparticles are homogeneously organized in the conducting polymer matrix. High-resolution transmission electron microscopy was used to study the morphology and patterning of Au nanoparticles in the conducting polymer matrix.
    Nanotechnology 02/2008; 19(7):075606. DOI:10.1088/0957-4484/19/7/075606 · 3.67 Impact Factor
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    ABSTRACT: Transparent ZrO2–polydimethylsiloxane (PDMS) nanocomposites with high refractive index were prepared by dispersing ZrO2 nanoparticles in a PDMS matrix via ligand molecule engineering. The ligand molecule consists of a diamine head group that adsorbs strongly onto the ZrO2 nanoparticles, and a siloxane tail group with a double-tailed structure that can be easily stretched within the PDMS matrix and yields a greater steric hindrance compared to single-tailed structure. The transmission electron micrograph shows that the designed Si-based ligand molecule has a role to play in keeping ZrO2 nanoparticles fully-dispersed even after the ZrO2 nanoparticles were transferred into the PDMS matrix. UV-vis spectra indicate that ZrO2–PDMS nanocomposite had a high transparency of 93.3% in the whole visible range. The refractive index of the ZrO2–PDMS nanocomposite could be varied from 1.39 to 1.65 simply by increasing the ZrO2 content from 0 to 20.8% v/v.
    Journal of Materials Chemistry 01/2008; 18(15). DOI:10.1039/b715338d · 6.63 Impact Factor
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    ABSTRACT: The preparation of BaMgAl10O17:Eu2+ (BAM) blue phosphor layer for a plasma display panel by the addition of a newly designed energetic dispersant (hereafter referred to as SAIT7) and its resulting photoluminescence (PL) efficiency is investigated. The addition of SAIT7 increases the maximum solids loading of BAM phosphor paste from 27.91 to 35.87 vol% and yields a highly packed microstructure of the phosphor layer. The PL intensity is increased by 7.57% compared to the BAM blue phosphor layer prepared without SAIT7. In conclusion, the addition of SAIT7 increased the packing density of the phosphor layer and resulted in improved luminescent properties of the phosphor layer.
    11/2007; 22(12):3309 - 3315. DOI:10.1557/JMR.2007.0426
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    ABSTRACT: A dispersion technology for Ni particles suspended in a non-aqueous medium based on the quantitative evaluation of surface acid-base properties of Ni particles is described. A quantitative analysis of surface acid-base properties of Ni particles was performed using non-aqueous titration. Dimethylamino ethanol and acetic acid were used as probe molecules to detect surface acid-base amounts of Ni particles. The dispersion system was designed on the basis of the amounts of surface acid-base sites on the Ni particle surface. Rheological behavior and agglomerate particle size data demonstrate that the dispersion stability of the designed Ni suspension is markedly improved, as expected. Therefore, the design strategy to improve the dispersion stability of Ni particles was successful. This strategy is expected to be applicable to dispersion systems of other particles suspended in a non-aqueous medium.
    Journal of Colloid and Interface Science 09/2007; 312(2):265-71. DOI:10.1016/j.jcis.2007.03.061 · 3.55 Impact Factor

Publication Stats

223 Citations
115.35 Total Impact Points

Institutions

  • 2003–2015
    • Hanyang University
      • Division of Materials Science and Engineering (MSE)
      Sŏul, Seoul, South Korea
  • 2010
    • University of Illinois, Urbana-Champaign
      • Department of Mechanical Science and Engineering
      Urbana, Illinois, United States