Showing papers by "Kwang S. Kim published in 2019"

Superb water splitting activity of the electrocatalyst Fe3Co(PO4)4 designed with computation aid.

Abstract: For efficient water splitting, it is essential to develop inexpensive and super-efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, we report a phosphate-based electrocatalyst [Fe3Co(PO4)4@reduced-graphene-oxide(rGO)] showing outstanding OER performance (much higher than state-of-the-art Ir/C catalysts), the design of which was aided by first-principles calculations. This electrocatalyst displays low overpotential (237 mV at high current density 100 mA cm−2 in 1 M KOH), high turnover frequency (TOF: 0.54 s−1), high Faradaic efficiency (98%), and long-term durability. Its remarkable performance is ascribed to the optimal free energy for OER at Fe sites and efficient mass/charge transfer. When a Fe3Co(PO4)4@rGO anodic electrode is integrated with a Pt/C cathodic electrode, the electrolyzer requires only 1.45 V to achieve 10 mA cm−2 for whole water splitting in 1 M KOH (1.39 V in 6 M KOH), which is much smaller than commercial Ir-C//Pt-C electrocatalysts. This cost-effective powerful oxygen production material with carbon-supporting substrates offers great promise for water splitting. The sluggish kinetics of the oxygen evolution reaction (OER) is the main obstacle in water splitting which is generally catalyzed by precious metals. Here, authors report a DFT predicted non-precious bimetallic phosphate electrocatalyst that displays high OER activity and stability.

Dual Emission of Water-Stable 2D Organic–Inorganic Halide Perovskites with Mn(II) Dopant

Abstract: Moisture-delicate and water-unstable organic–inorganic halide perovskites (OI-HPs) create huge challenges for the synthesis of highly efficient waterstable light-emitting materials for optoelectronic devices. Herein, a simple acid solution–assisted method to synthesize quantum confined 2D lead perovskites through Mn doping is reported. The efficient energy transfer between host and dopant ions in orange light-emitting Mn2+-doped OI-HPs leads to the most efficient integrated luminescence with a photoluminescence quantum yield over 45%. The Mn2+ substitution of Pb2+ and passivation with low dielectric constant molecules such as phenethylamine, benzylamine, and butylamine enhance water resistivity, leading to water stability. The dual emission process of this water-stable 2D Mn-doped perovskite will help in developing highly efficient 2D water-stable perovskites for practical applications.

Non-adiabatic dynamics of ring opening in cyclohexa-1,3-diene described by an ensemble density-functional theory method

Abstract: The dynamics of the ring opening in the S1 state of cyclohexa-1,3-diene (CHD) is studied by a new direct mixed quantum-classical non-adiabatic dynamics approach which employs the decoherence-induce

Pt-like hydrogen evolution on a V2O5/Ni(OH)2 electrocatalyst

Abstract: We report a highly efficient and cost-effective binder-free catalyst for the hydrogen evolution reaction (HER) using V2O5 particles on nickel foam (NF) (V2O5/Ni(OH)2@NF). This low-cost catalyst exhibits Pt-like activity with a low overpotential of 39 mV at 10 mA cm−2 (lowest among V-based materials which are known to be generally non-explosive and safe) and long-term stability in a 1 M KOH solution. The overall performance is highly comparable to that of a commercial 20% Pt/C catalyst on NF. Furthermore, the V2O5/Ni(OH)2@NF outperforms the Pt/C catalyst at a higher current density (100 mA cm−2) which is more preferable for industrial applications. First principles calculations show that the remarkable HER activity is ascribed to the near-zero adsorption free energy (ΔGH*) on the Ni-site of Ni(OH)2@NF and the Ni- and O-sites of in situ generated V2O5@NF, due to the charge transfer arising from adsorbed O atoms on Ni(111), along with high conductivity of NF. O-adsorption on the Ni transition metal surface downshifts the d-band center of the transition metal, which helps in quick hydrogen desorption by weakening the hydrogen binding strength. As a result, most Ni fcc sites of V2O5/Ni(OH)2@NF are more active than pristine Ni fcc sites. The V2O5/Ni(OH)2@NF catalyst initiates overall water splitting at 1.53 V in a 6 M KOH solution for solar-to-hydrogen generation in a two-electrode set-up using a solar panel.

A thermally stable, barium-stabilized α-CsPbI3 perovskite for optoelectronic devices

Abstract: The all-inorganic perovskite CsPbI3 has emerged as an alternative photovoltaic material to organic–inorganic hybrid perovskites due to its non-volatile composition and comparable photovoltaic performance. However, its spontaneous deformation from the light-active black phase to a light-inactive yellow phase under ambient conditions, poor air stability, low thermal stability as well as high-temperature processing are challenging issues in the fabrication of CsPbI3-based solar cells. Herein, we introduce a new surface passivation strategy using camphor sulfonic acid (CSA) to improve the surface morphology and air stability of Ba-stabilized α-CsPbI3 perovskites at low temperature. The surface passivated, Ba-doped α-CsPbI3 was thermally stable upon annealing and highly photo-stable over a year, and it also exhibited a band gap of ∼1.72 eV, which is suitable for optoelectronic applications. The all-inorganic solar cell based on the Ba-doped α-CsPbI3 retained 98% of its initial PCE value even after 700 h, and red light-emitting diodes (LED) exhibited light emission at 700 nm with a bandwidth of 39 nm. To date, this is the first study on surface passivated, Ba-stabilized α-CsPbI3, which provides opportunities for the development of highly efficient tandem solar cells and other optoelectronic devices.

Multiphotoluminescence from a Triphenylamine Derivative and Its Application in White Organic Light‐Emitting Diodes Based on a Single Emissive Layer

Abstract: White organic light-emitting diode (WOLED) technology has attracted considerable attention because of its potential use as a next-generation solid-state lighting source. However, most of the reported WOLEDs that employ the combination of multi-emissive materials to generate white emission may suffer from color instability, high material cost, and a complex fabrication procedure which can be diminished by the single-emitter-based WOLED. Herein, a color-tunable material, tris(4-(phenylethynyl)phenyl)amine (TPEPA), is reported, whose photoluminescence (PL) spectrum is altered by adjusting the thermal annealing temperature nearly encompassing the entire visible spectra. Density functional theory calculations and transmission electron microscopy results offer mechanistic understanding of the PL redshift resulting from thermally activated rotation of benzene rings and rotation of 4-(phenylethynyl) phenyl)amine connected to the central nitrogen atom that lead to formation of ordered molecular packing which improves the π-π stacking degree and increases electronic coupling. Further, by precisely controlling the annealing time and temperature, a white-light OLED is fabricated with the maximum external quantum efficiency of 3.4% with TPEPA as the only emissive molecule. As far as it is known, thus far, this is the best performance achieved for single small organic molecule based WOLED devices.

Compositional and Dimensional Control of 2D and Quasi-2D Lead Halide Perovskites in Water

Abstract: Blue light emitting two dimensional (2D) and quasi-2D layered halide perovskites (LHPs) are gaining attention in solid-state lighting applications but their fragile stability in humid condition is one of the most pressing issues for their practical applications. Though water is much greener and cost effective, organic solvents must be used during synthesis as well as the device fabrication process for these LHPs due to their water-sensitivity/instability and consequently, water-stable blue-light emitting 2D and quasi-2D LHPs have not been documented yet. Here, water-mediated facile and cost-effective syntheses, characterizations, and optical properties of 16 organic–inorganic hybrid compounds are reported including 2D (A′)2PbX4 (A′ = butylammonium, X = Cl/Br/I) (8 compounds), 3D perovskites (4), and quasi-2D (A′)pAx−1BxX3x+1 LHPs (A = methylammonium) (4) in water. Here, both composition and dimension of LHPs are tuned in water, which has never been explored yet. Furthermore, the dual emissive nature is observed in quasi-2D perovskites, where the intensity of two photoluminescence (PL) peaks are governed by 2D and 3D inorganic layers. The Pb(OH)2-coated 2D and quasi-2D perovskites are highly stable in water even after several months. In addition, single particle imaging is performed to correlate structural–optical property of these LHPs.

Direct emission from quartet excited states triggered by upconversion phenomena in solid-phase synthesized fluorescent lead-free organic–inorganic hybrid compounds

Abstract: We report, for the first time, the solid-phase gram-scale synthesis of two lead-free, zero-dimensional (0D) fluorescent organic–inorganic hybrid compounds, [Bu4N]2[MnBr4] (1) and [Ph4P]2[MnBr4] (2), achieved by grinding the organic and inorganic precursor salts. The solid-phase synthetic route has several advantages for modulating molecular dimensionalities. During grinding, organic cations and Mn2+ cations are co-crystallized together in the solid-state, forming a 0D assembly at the molecular level where each individual metal center is surrounded by organic cations. Both compounds exhibit an emission peak at 520 nm and a photoluminescence (PL) quantum yield (QY) of 47%. Here, we also report, for the first time, the upconversion phenomena which trigger different emission energies occurring in different quartet states of Mn, 4T1(4G), 4T2(4G), 4A1(4G), 4E(4G), 4T2(4D), 4E(4D), and 4T1(4P). These optical properties are unusual phenomena which break Kasha's rule of emission. Single particle imaging and low-temperature PL measurement are performed to obtain a deeper insight into these ground products. These results pave a new path to develop highly fluorescent non-toxic hybrid compounds with remarkable optical properties.

First-order and continuous melting transitions in two-dimensional Lennard-Jones systems and repulsive disks.

Abstract: In two-dimensional Lennard-Jones (LJ) systems, a small interval of melting-mode switching occurs below which the melting occurs by first-order phase transitions in lieu of the melting scenario proposed by Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY). The extrapolated upper bound for phase coexistence is at density ρ∼0.893 and temperature T∼1.1, both in reduced LJ units. The two-stage KTHNY scenario is restored at higher temperatures, and the isothermal melting scenario is universal. The solid-hexatic and hexatic-liquid transitions in KTHNY theory, even so continuous, are distinct from typical continuous phase transitions in that instead of scale-free fluctuations, they are characterized by unbinding of topological defects, resulting in a special form of divergence of the correlation length: ξ≈exp(b|T-T_{c}|^{-ν}). Here such a divergence is firmly established for a two-dimensional melting phenomenon, providing a conclusive proof of the KTHNY melting. We explicitly confirm that this high-temperature melting behavior of the LJ system is consistent with the melting behavior of the r^{-12} potential and that melting of the r^{-n} potential is KTHNY-like for n≤12 but melting of the r^{-64} potential is first order; similar to hard disks. Therefore we suggest that the melting scenario of these repulsive potentials becomes hard-disk-like for an exponent in the range 12

Selective separation of Xe/Kr and adsorption of water in a microporous hydrogen-bonded organic framework

Abstract: We have studied the adsorption properties of Xe and Kr in a highly microporous hydrogen-bonded organic framework based on 1,3,5-tris(4-carboxyphenyl)benzene, named HOF-BTB. HOF-BTB can reversibly adsorb both noble gases, and it shows a higher affinity for Xe than Kr. At 1 bar, the adsorption amounts of Xe were 3.37 mmol g−1 and 2.01 mmol g−1 at 273 K and 295 K, respectively. Ideal adsorbed solution theory (IAST) calculation predicts selective separation of Xe over Kr from an equimolar binary Xe/Kr mixture, and breakthrough experiments demonstrate the efficient separation of Xe from the Xe/Kr mixture under a dynamic flow condition. Consecutive breakthrough experiments with simple regeneration treatment at 298 K reveal that HOF-BTB would be an energy-saving adsorbent in an adsorptive separation process, which could be attributed to the relatively low isosteric heat (Qst) of adsorption of Xe. The activated HOF-BTB is very stable in both water and aqueous acidic solutions for more than one month, and it also shows a well-preserved crystallinity and porosity upon water/acid treatment. Besides, HOF-BTB adsorbs about 30.5 wt%, the highest value for HOF materials, of water vapor during the adsorption–desorption cycles, with a 19% decrease in adsorption amounts of water vapor after five cycles.

Formation of a photoactive quasi-2D formamidinium lead iodide perovskite in water

Abstract: There has been no report on the synthesis of fluorescent α-phase stabilized quasi-2D FAPbI3 [FA = CH(NH2)2] perovskite nanocrystals (NCs) in water. We report the top–down synthesis of fluorescent quasi-two dimensional (2D) α-FAPbI3 NCs in aqueous media by controlling the electronic states of lead and the NCs size of FAPbI3. The product was stable in ambient conditions for more than six months. We explored a detailed synthetic study, their emissive properties and phase stabilization mechanism corroborated by density functional theory (DFT) calculations. Single particle imaging and photoluminescence (PL) study of α-FAPbI3 NCs bear the signature of dual emission, which indicates the formation of self-trapped excited states. Our findings will pave the way to develop phase-stabilized hybrid perovskite NCs in water, beneficial for optoelectronic devices.

Effect of Organic–Cation Exchange Reaction of Perovskites in Water: H-Bond Assisted Self-Assembly, Black Phase Stabilization, and Single-Particle Imaging

Abstract: Despite outstanding performance of organic–inorganic lead halide perovskite (OILHP)-based solar cells and light-emitting devices, they are still unusable for practical applications due to instabili...

Author Correction: Multicomponent electrocatalyst with ultralow Pt loading and high hydrogen evolution activity

Abstract: An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Band Gap Narrowing of Zinc Orthogermanate by Dimensional and Defect Modification

Abstract: Dimensional reduction and defect control are powerful techniques for enhancing the physical and chemical properties of raw materials. Herein, we report the new type of sheetlike or two-dimensional-...

Quantum Monte Carlo Study of the Water Dimer Binding Energy and Halogen-π Interactions.

Abstract: Halogen-π systems are involved with competition between halogen bonding and π-interaction. Using the diffusion quantum Monte Carlo (DMC) method, we compare the interaction of benzene with diatomic halogens (X2: Cl2/Br2) with the typical hydrogen bonding in the water dimer, taking into account explicit correlations of up to three bodies. The benzene-Cl2/Br2 binding energies (13.07 ± 0.42/16.62 ± 0.02 kJ/mol) attributed to both halogen bonding and dispersion are smaller than but comparable to the typical hydrogen bonding in the water dimer binding energy (20.88 ± 0.27 kJ/mol). All of the above values are in good agreement with those from the coupled-cluster with single, double, and noniterative triple excitations (CCSD(T)) results at the complete basis set limit (benzene-Cl2/Br2: 12.78/16.17 kJ/mol; water dimer: 21.0 kJ/mol).

Signature of multilayer growth of 2D layered Bi2Se3 through heteroatom-assisted step-edge barrier reduction

Abstract: During growth of two-dimensional (2D) materials, abrupt growth of multilayers is practically unavoidable even in the case of well-controlled growth. In epitaxial growth of a quintuple-layered $${{\rm{Bi}}}_{2}{{\rm{Se}}}_{3}$$ film, we observe that the multilayer growth pattern deduced from in situ x-ray diffraction implies nontrivial interlayer diffusion process. Here we find that an intriguing diffusion process occurs at step edges where a slowly downward-diffusing Se adatom having a high step-edge barrier interacts with a Bi adatom pre-existing at step edges. The Se–Bi interaction lowers the high step-edge barrier of Se adatoms. This drastic reduction of the overall step-edge barrier and hence increased interlayer diffusion modifies the overall growth significantly. Thus, a step-edge barrier reduction mechanism assisted by hetero adatom–adatom interaction could be fairly general in multilayer growth of 2D heteroatomic materials.

Signature of a quantum dimensional transition in the spin- 1 2 antiferromagnetic Heisenberg model on a square lattice and space reduction in the matrix product state

Abstract: We study the spin-$\frac{1}{2}$ antiferromagnetic Heisenberg model on an $\ensuremath{\infty}\ifmmode\times\else\texttimes\fi{}N$ square lattice for even $N$'s up to 14. Previously, the nonlinear sigma model perturbatively predicted that its spin-rotational symmetry breaks asymptotically with $N\ensuremath{\rightarrow}\ensuremath{\infty}$, i.e., when it becomes two dimensional (2D). However, we identify a critical width ${N}_{c}=10$ for which this symmetry breaks spontaneously. It shows the signature of a dimensional transition from one dimensional (1D) including quasi-1D to 2D. The finite-size effect differs from that of the $N\ifmmode\times\else\texttimes\fi{}N$ lattice. The ground-state (GS) energy per site approaches the thermodynamic limit value, in agreement with the previously accepted value, by one order of $1/N$ faster than when using $N\ifmmode\times\else\texttimes\fi{}N$ lattices in the literature. Methodwise, we build and variationally solve a matrix product state (MPS) on a chain, converting the $N$ sites in each rung into an effective site. We show that the area law of entanglement entropy does not apply when $N$ increases in our method and the reduced density matrix of each effective site has a saturating number of dominant diagonal elements with increasing $N$. These two characteristics make the MPS rank needed to obtain a desired energy accuracy quickly saturate when $N$ is large, making our algorithm efficient for large $N$'s. Furthermore, the latter enables space reduction in MPS. Within the framework of MPS, we prove a theorem that the spin-spin correlation at infinite separation is the square of staggered magnetization and demonstrate that the eigenvalue structure of a building MPS unit of $\ensuremath{\langle}g|g\ensuremath{\rangle},|g\ensuremath{\rangle}$ being the GS is responsible for order, disorder, and quasi-long-range order.

Band Gap Narrowing of Zinc Orthogermanate by Dimensional and Defect Modification

Abstract: Dimensional reduction and defect control are powerful techniques for enhancing the physical and chemical properties of raw materials. Herein, we report the new type of sheetlike or two-dimensional-like zinc orthogermanate (Zn₂GeO₄, denoted as S-ZGO) with a one-step hydrothermal reaction and its application to photocatalytic water splitting. S-ZGO is directly grown along the surface of the Zn foil, since the growth rate of a crystal facet can be modified through the restricted reaction of dissolved precursor (GeO₂) with a solid precursor (Zn foil) during the hydrothermal reaction. For further modification, the oxygen vacancies are introduced on the surface of S-ZGO using thermal hydrogen treatment and photodeposition-driven low amount of Pt/RuO₂ co-catalysts loading. Notably, this reduced dimension decreases the band gap to 4.09 eV for S-ZGO (from 4.5 eV for the bulk), and the hydrogenation of S-ZGO further decreases the band gap to 3.88 eV. The origin of band gap narrowing is demonstrated with the density functional theory showing increased density of states at the edge of the conduction band (CB) and valence band (VB), and a new defect level between the CB minimum and VB maximum. As a possible application, we demonstrate that S-ZGO/H₂ loaded with Pt/RuO₂ exhibits the H₂ rate of 167.0 μmol h–¹ g–¹ and the O₂ rate of 83.0 μmol h–¹ g–¹, ∼8 times those of the rodlike ZGO photocatalysts.

Supra-Binary Ferroelectricity in a Nanowire

Abstract: We report the prediction and observation of supra-binary ferroelectricity in a ferroelectric nanowire (FNW) covered with a semi-cylindrical gate that provides an anisotropic electric field in the FNW. There are gate-voltage-driven transitions between four polarization phases in FNW's cross section, dubbed axial-up, axial-down, radial-in and radial-out. They are determined by the interplay between the topological depolarization energy and the free energy induced by an anisotropic external electric field, in clear distinction from the conventional film-based binary ferroelectricity. When the FNW is mounted on a biased graphene nanoribbon (GNR), these transitions induce exotic current-voltage hysteresis in the FNW-GNR transistor. Our discovery suggests new operating mechanisms of ferroelectric devices. In particular, it enables intrinsic multi-bit information manipulation in parallel to the binary manipulation employed in data storage devices.