Showing papers by "Kwang S. Kim published in 2017"
TL;DR: In this paper, the development of technology for clean-energy production has become the major research priority worldwide, and the globalization of advanced energy conversion technologies like rechargeable metal-air batteries, regenerated fuel cells, and water splitting devices has been majorly benefitted by the developing of apposite catalytic materials that can proficiently carry out the pertinent electrochemical processes like oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction(HER), and water hydrolysis.
Abstract: The persistently increasing energy consumption and the low abundance of conventional fuels have raised serious concerns all over the world. Thus, the development of technology for clean-energy production has become the major research priority worldwide. The globalization of advanced energy conversion technologies like rechargeable metal–air batteries, regenerated fuel cells, and water-splitting devices has been majorly benefitted by the development of apposite catalytic materials that can proficiently carry out the pertinent electrochemical processes like oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and water hydrolysis. Despite a handful of superbly performing commercial catalysts, the high cost and low electrochemical stability of precursors have consistently discouraged their long-term viability. As a promising substitute of conventional platinum-, palladium-, iridium-, gold-, silver-, and ruthenium-based catalysts, various transition-metal (TM) i...
774 citations
TL;DR: This work demonstrates the synthesis of mesoporous silicon hollow nanocubes (m-Si HCs) derived from a metal-organic framework (MOF) as an anode material with outstanding electrochemical properties and exhibits outstanding cycle stability.
Abstract: Controlling the morphology of nanostructured silicon is critical to improving the structural stability and electrochemical performance in lithium-ion batteries. The use of removable or sacrificial templates is an effective and easy route to synthesize hollow materials. Herein, we demonstrate the synthesis of mesoporous silicon hollow nanocubes (m-Si HCs) derived from a metal–organic framework (MOF) as an anode material with outstanding electrochemical properties. The m-Si HC architecture with the mesoporous external shell (∼15 nm) and internal void (∼60 nm) can effectively accommodate volume variations and relieve diffusion-induced stress/strain during repeated cycling. In addition, this cube architecture provides a high electrolyte contact area because of the exposed active site, which can promote the transportation of Li ions. The well-designed m-Si HC with carbon coating delivers a high reversible capacity of 1728 mAhg–1 with an initial Coulombic efficiency of 80.1% after the first cycle and an excelle...
167 citations
TL;DR: A conductivity-enhanced bifunctional electrocatalyst of nanoscale-coated Pt-Pd alloys on both tin-doped indium (TDI) and reduced graphene oxide (rGO), denoted as Pt- Pd@TDI/rGO, which has remarkable catalytic performances attributed to the role of TDI in enhancing the catalytic activity by protecting Pt from oxidation.
Abstract: A key challenge in developing fuel cells is the fabrication of low-cost electrocatalysts with high activity and long durability for the two half-reactions, i.e., the methanol/ethanol oxidation reaction (MOR/EOR) and the oxygen reduction reaction (ORR). Herein, we report a conductivity-enhanced bifunctional electrocatalyst of nanoscale-coated Pt–Pd alloys on both tin-doped indium (TDI) and reduced graphene oxide (rGO), denoted as Pt–Pd@TDI/rGO. The mass activities of Pt in the Pt–Pd@TDI/rGO hybrid toward MOR, EOR, and ORR are 2590, 1500, and 2690 mA/mg, respectively. The ORR Pt specific activity and mass activity of the electrocatalyst are 17 and 28 times larger, respectively, than commercial Pt/C catalysts. All these remarkable catalytic performances are attributed to the role of TDI in enhancing the catalytic activity by protecting Pt from oxidation as well as rapid mass/charge transfer due to the synergistic effect between surface Pt–Pd alloys and TDI/rGO.
96 citations
TL;DR: Red-light absorbing carbon nitride sheets lead to remarkable proliferation and osteogenic differentiation by runt-related transcription factor 2 (Runx2) activation, a key transcription factor associated with osteoblast differentiation in human bone marrow-derived mesenchymal stem cells.
Abstract: Human bone marrow-derived mesenchymal stem cells (hBMSCs) present promising opportunities for therapeutic medicine. Carbon derivatives showed only marginal enhancement in stem cell differentiation toward bone formation. Here we report that red-light absorbing carbon nitride (C3N4) sheets lead to remarkable proliferation and osteogenic differentiation by runt-related transcription factor 2 (Runx2) activation, a key transcription factor associated with osteoblast differentiation. Accordingly, highly effective hBMSCs-driven mice bone regeneration under red light is achieved (91% recovery after 4 weeks compared to 36% recovery in the standard control group in phosphate-buffered saline without red light). This fast bone regeneration is attributed to the deep penetration strength of red light into cellular membranes via tissue and the resulting efficient cell stimulation by enhanced photocurrent upon two-photon excitation of C3N4 sheets near cells. Given that the photoinduced charge transfer can increase cytoso...
72 citations
TL;DR: 2D features, defects, edges, and substrate effects of Gr are discussed first, and key insights into the functionalized Gr hybrid materials lead to the applications for not only energy storage and electrochemical catalysis, green chemistry, and electronic/spintronic devices but also biosensing and medical applications.
Abstract: This spotlight discusses intriguing properties and diverse applications of graphene (Gr) and Gr analogs. Gr has brought us two-dimensional (2D) chemistry with its exotic 2D features of density of states. Yet, some of the 2D or 2D-like features can be seen on surfaces and at interfaces of bulk materials. The substrate on Gr and functionalization of Gr (including metal decoration, intercalation, doping, and hybridization) modify the unique 2D features of Gr. Despite abundant literature on physical properties and well-known applications of Gr, spotlight works based on the conceptual understanding of the 2D physical and chemical nature of Gr toward vast-ranging applications are hardly found. Here we focus on applications of Gr, based on conceptual understanding of 2D phenomena toward 2D chemistry. Thus, 2D features, defects, edges, and substrate effects of Gr are discussed first. Then, to pattern Gr electronic circuits, insight into differentiating conducting and nonconducting regions is introduced. By utiliz...
54 citations
TL;DR: In this paper, the role of A-site cation in carrier transport of perovskite solar cells (PSCs) has been uncovered, i.e., tuning the Frohlich electron-phonon (e-ph) coupling of longitudinal optical phonon.
Abstract: As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frohlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frohlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.
42 citations
TL;DR: The new method, the state-interaction state-averaged REKS(4,4), is capable of describing several excited states of a molecule involving double bond cleavage, polyradical character, or multiple chromophoric units and can describe a wide range of multireference phenomena.
Abstract: An extended variant of the spin-restricted ensemble-referenced Kohn-Sham (REKS) method, the REKS(4,4) method, designed to describe the ground electronic states of strongly multireference systems is modified to enable calculation of excited states within the time-independent variational formalism. The new method, the state-interaction state-averaged REKS(4,4), i.e., SI-SA-REKS(4,4), is capable of describing several excited states of a molecule involving double bond cleavage, polyradical character, or multiple chromophoric units. We demonstrate that the new method correctly describes the ground and the lowest singlet excited states of a molecule (ethylene) undergoing double bond cleavage. The applicability of the new method for excitonic states is illustrated with π stacked ethylene and tetracene dimers. We conclude that the new method can describe a wide range of multireference phenomena.
30 citations
TL;DR: Green synthesis of ∼10 nm sized CuO nanoparticles (NPs) aggregated on ∼400 nm sized 50-facet Cu2O polyhedral nanocrystals opens up new possibilities of replacing the usage of expensive CO oxidation materials.
Abstract: As carbon monoxide oxidation is widely used for various chemical processes (such as methanol synthesis and water-gas shift reactions H2O + CO ⇄ CO2 + H2) as well as in industry, it is essential to develop highly energy efficient, inexpensive, and eco-friendly catalysts for CO oxidation. Here we report green synthesis of ∼10 nm sized CuO nanoparticles (NPs) aggregated on ∼400 nm sized 50-facet Cu2O polyhedral nanocrystals. This CuO-NPs/50-facet Cu2O shows remarkable CO oxidation reactivity with very high specific CO oxidation activity (4.5 μmolCO m–2 s–1 at 130 °C) and near-complete 99.5% CO conversion efficiency at ∼175 °C. This outstanding catalytic performance by CuO NPs over the pristine multifaceted Cu2O nanocrystals is attributed to the surface oxygen defects present in CuO NPs which facilitate binding of CO and O2 on their surfaces. This new material opens up new possibilities of replacing the usage of expensive CO oxidation materials.
29 citations
TL;DR: Detailed biochemical, biophysical, and computational studies were able to provide a foundation of how to originate molecular formulas to devise chemical tools capable of controlling the reactivities of various pathological components through distinct mechanisms.
Abstract: To elucidate the involvement of individual and inter-related pathological factors [i.e., amyloid-β (Aβ), metals, and oxidative stress] in the pathogenesis of Alzheimer's disease (AD), chemical tools have been developed. Characteristics required for such tool construction, however, have not been clearly identified; thus, the optimization of available tools or new design has been limited. Here, key structural properties and mechanisms that can determine tools' regulatory reactivities with multiple pathogenic features found in AD are reported. A series of small molecules was built up through rational structural selection and variations onto the framework of a tool useful for in vitro and in vivo metal-Aβ investigation. Variations include: (i) location and number of an Aβ interacting moiety; (ii) metal binding site; and (iii) denticity and structural flexibility. Detailed biochemical, biophysical, and computational studies were able to provide a foundation of how to originate molecular formulas to devise chemical tools capable of controlling the reactivities of various pathological components through distinct mechanisms. Overall, this multidisciplinary investigation illustrates a structure-mechanism-based strategy of tool invention for such a complicated brain disease.
28 citations
TL;DR: A nanoparticulate material whose PL is tunable across the entire visible range and is achieved without adjusting particle size, any postsynthetic doping, or surface modification is described.
Abstract: Materials exhibiting excitation-wavelength-dependent photoluminescence, PL, are useful in a range of biomedical and optoelectronic applications. This paper describes a nanoparticulate material whose PL is tunable across the entire visible range and is achieved without adjusting particle size, any post-synthetic doping, or surface modification. A straightforward thermal decomposition of rhenium (VII) oxide precursor yields nanoparticles that comprise Re atoms at different oxidation states. Studies of time-resolved emission spectra and DFT calculations both indicate that tunable PL of such mixed-valence particles originates from the presence of multiple emissive states that become “active” at different excitation wavelengths. In addition, the nanoparticles exhibit photocatalytic activity that, under visible-light irradiation, is superior to that of TiO2 nanomaterials.
27 citations
TL;DR: Micro-photoluminescence measurements are performed and it is observed that the PL peak redshifts nonlinearly in mono- and bi-layer MoS2 as the excitation power is increased.
Abstract: Due to its unique layer-number dependent electronic band structure and strong excitonic features, atomically thin MoS2 is an ideal 2D system where intriguing photoexcited-carrier-induced phenomena can be detected in excitonic luminescence. We perform micro-photoluminescence (PL) measurements and observe that the PL peak redshifts nonlinearly in mono- and bi-layer MoS2 as the excitation power is increased. The excited carrier-induced optical bandgap shrinkage is found to be proportional to n4/3, where n is the optically-induced free carrier density. The large exponent value of 4/3 is explicitly distinguished from a typical value of 1/3 in various semiconductor quantum well systems. The peculiar n4/3 dependent optical bandgap redshift may be due to the interplay between bandgap renormalization and reduced exciton binding energy.
TL;DR: Diatomic halogen molecules X2 (X = Cl/Br) favor the edge-to-face conformation on benzene with significant electrostatic interaction via halogen bonding whereas on coronene, the two conformations are compatible.
TL;DR: Two-dimensional excitonic photoluminescence from graphene grown on a Cu(111) surface is reported, which shows an unexpected and remarkably sharp strong emission near 3.16 eV and as temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene.
Abstract: Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on a Cu(111) surface, which shows an unexpected and remarkably sharp strong emission near 3.16 eV (full width at half-maximum ≤3 meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene. The observed PL originates from the significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene π and Cu d orbitals of the first and second Cu layers at a shifted saddle point 0.525(M+K) of the Brillouin zone. This finding provides a pathway to engineering optoelectronic graphene devices, while maintaining the outstanding electrical properties of graphene.
TL;DR: In this paper, the dispersion corrected density functional theory (DFT) with hybrid functionals was used to analyze the properties of CX4 adsorbed on benzene, coronene, and graphene.
Abstract: Since carbon tetrahalides CX4 (X = Cl/Br) are adsorbed on carbon nanotubes and graphene sheets, we have studied the structures, adsorption energies, and electronic properties of CX4 adsorbed on benzene, coronene, and graphene using dispersion corrected density functional theory (DFT) with hybrid functionals. As compared with the benzene–CX4 complexes (with binding energy of ∼14/15 kJ/mol) where electrostatic energy is significant due to the halogen bonding effect, the graphene–CX4 complexes show about three times the benzene–CX4 binding energy (∼40/45 kJ/mol) where the dispersion interaction is overwhelming with insignificant electrostatic energy. Since the X atoms in CX4 are slightly positively charged and the X atom’s ends are particularly more positively charged due to the σ-hole effect, CX4 behaves as an electron acceptor. This results in electron transfer from locally negatively charged C sites of benzene/coronene to CX4. In contrast, no electron transfer occurs from graphene to CX4 because of the la...
TL;DR: From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional "standing-up" crystals.
Abstract: Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a “lying-down” pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional “standing-up” crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complem...
TL;DR: In this paper, the role of band splitting and band narrowing in the magnetic and electronic properties of low dimensional itinerant strongly correlated electronic systems was analyzed, particularly in 3D transition elements.
Abstract: We have analyzed the role of band narrowing/broadening in the magnetic and electronic properties of low dimensional itinerant strongly correlated electronic systems, which is particularly important in 3d transition elements. Density functional theory and mean field results have been coupled to explain the magnetic phenomenon in a one-dimensional monatomic chain of 3d transition elements Sc, Ti, V, Cr, Mn, Fe, Co, and Ni. We found that in the ferromagnetic ground state not only the band splitting but also the relative band narrowing/broadening play a crucial role, and the bandwidths of both spins become different as a manifestation of correlated spin hopping interaction. For less than the half-filled band (i.e., for Sc, Ti, V, and Cr), the up spin band broadens, and the down spin band narrows. For the (half and) more than half filled band (i.e., for Mn, Fe, Co, and Ni), the down spin band broadens, and the up spin band narrows. As a result, (in most of the cases) only one spin channel is present for conduc...