Gwan Yeong Jung

Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in the Hydrogen Evolution Reaction

Metal sulfide-based nanostructured materials have emerged as promising catalysts for hydrogen evolution reaction (HER), and significant progress has been achieved in enhancing their activity and durability for the HER. The understanding of nanoscale size-dependent catalytic activities can suggest critical information regarding catalytic reactivity, providing the scientific basis for the design of advanced catalysts. However, nanoscale size effects in metal sulfide-based HER catalysts have not yet been established fully, due to the synthetic difficulty in precisely size-controlled metal sulfide nanoparticles. Here we report the preparation of molybdenum sulfide (MoS2) nanoparticles with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm, and their size-dependent catalytic activity in the HER. Using density functional theory (DFT) calculations, we identified the most favorable single-, double-, and triple-layer MoS2 model structures for the HER, and calculated elementary step energetics of the HER over these three model structures. Combining HER activity measurements and the DFT calculation results, we establish that the turnover frequency of MoS2 nanoparticles in the HER increases in a quasi-linear manner with decreased layer numbers. Cobalt-promoted MoS2 nanoparticles also exhibited similar HER activity trend. We attribute the higher HER activity of smaller metal sulfide nanoparticles to the higher degree of oxidation, higher Mo-S coordination number, formation of the 1T phase, and lower activation energy required to overcome transition state. This insight into the nanoscale size-dependent HER activity trend will facilitate the design of advanced HER catalysts as well as other hydrotreating catalysts.

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Highly sensitive and selective liquid-phase sensors based on a solvent-resistant organic-transistor platform.

Liquid-phase sensing of various organic solvents is performed for the first time, using a solvent-resistant organic-transistor platform. Sensors composed of a cross-linked poly(3-hexylthiophene) (P3HT)-azide co-polymer and a calixarene derivative exhibit highly sensitive and selective sensing behavior, owing to the selective binding effects of the liquid analytes with the calixarene-functionalized P3HT-azide, extending the range of their use in practical applications.

Exploration of Gate-Opening and Breathing Phenomena in a Tailored Flexible Metal–Organic Framework

Flexible metal-organic frameworks (MOFs) show the structural transition phenomena, gate opening and breathing, upon the input of external stimuli. These phenomena have significant implications in their adsorptive applications. In this work, we demonstrate the direct capture of these gate-opening and breathing phenomena, triggered by CO2 molecules, in a well-designed flexible MOF composed of rotational sites and molecular gates. Combining X-ray single crystallographic data of a flexible MOF during gate opening/closing and breathing with in situ X-ray powder diffraction results uncovered the origin of this flexibility. Furthermore, computational studies revealed the specific sites required to open these gates by interaction with CO2 molecules.

Edge-Terminated MoS2 Nanoassembled Electrocatalyst via In Situ Hybridization with 3D Carbon Network

Transition metal dichalcogenides, especially MoS2 , are considered as promising electrocatalysts for hydrogen evolution reaction (HER). Since the physicochemical properties of MoS2 and electrode morphology are highly sensitive factor for HER performance, designed synthesis is highly pursued. Here, an in situ method to prepare a 3D carbon/MoS2 hybrid catalyst, motivated by the graphene ribbon synthesis process, is reported. By rational design strategies, the hybrid electrocatalysts with cross-connected porous structure are obtained, and they show a high HER activity even comparable to the state-of-the-art MoS2 catalyst without appreciable activity loss in long-term operations. Based on various physicochemical techniques, it is demonstrated that the synthetic procedure can effectively guide the formation of active site and 3D structure with a distinctive feature; increased exposure of active sites by decreased domain size and intrinsically high activity through controlling the number of stacking layers. Moreover, the importance of structural properties of the MoS2 -based catalysts is verified by controlled experiments, validating the effectiveness of the designed synthesis approach.

Chiral self-sorted multifunctional supramolecular biocoordination polymers and their applications in sensors

Chiral supramolecules have great potential for use in chiral recognition, sensing, and catalysis. Particularly, chiral supramolecular biocoordination polymers (SBCPs) provide a versatile platform for characterizing biorelated processes such as chirality transcription. Here, we selectively synthesize homochiral and heterochiral SBCPs, composed of chiral naphthalene diimide ligands and Zn ions, from enantiomeric and mixed R-ligands and S-ligands, respectively. Notably, we find that the chiral self-sorted SBCPs exhibit multifunctional properties, including photochromic, photoluminescent, photoconductive, and chemiresistive characteristics, thus can be used for various sensors. Specifically, these materials can be used for detecting hazardous amine materials due to the electron transfer from the amine to the SBCP surface and for enantioselectively sensing a chiral species naproxen due to the different binding energies with regard to their chirality. These results provide guidelines for the synthesis of chiral SBCPs and demonstrate their versatility and feasibility for use in various sensors covering photoactive, chemiresistive, and chiral sensors.Chiral supramolecules may be used in chiral recognition, sensing and catalysis. Here the authors selectively synthesized homochiral and heterochiral supramolecular biocoordination polymers of naphthalene diimide ligands with alanine termini and Zn ions.

Monolithic heterojunction quasi-solid-state battery electrolytes based on thermodynamically immiscible dual phases

Traditional single-phase electrolytes, which are widely used in current state-of-the-art rechargeable batteries, have difficulties simultaneously fulfilling different chemical/electrochemical requirements of anodes and cathodes. Here, we demonstrate a new class of monolithic heterojunction quasi-solid-state electrolytes (MH-QEs) based on thermodynamically immiscible dual phases. As a proof-of-concept of the MH-QEs, their application to lithium–sulfur batteries is explored. Driven by combined effects of structural uniqueness and thermodynamic immiscibility, the electrode-customized MH-QEs provide exceptional electrochemical performance that lies far beyond those accessible with conventional battery electrolytes.

Highly Enantioselective Graphene-Based Chemical Sensors Prepared by Chiral Noncovalent Functionalization

As a basic characteristic of the natural environment and living matter, chirality has been used in various scientific and technological fields. Chiral discrimination is of particular interest owing to its importance in catalysis, organic synthesis, biomedicine, and pharmaceutics. However, it is still very challenging to effectively and selectively sense and separate different enantiomers. Here, enantio-differentiating chemosensor systems have been developed through spontaneous chiral functionalization of the surface of graphene field-effect transistors (GFETs). GFET sensors functionalized using noncovalent interactions between graphene and a newly synthesized chiral-functionalized pyrene material, Boc-l-Phe-Pyrene, exhibit highly enantioselective detection of natural acryclic monoterpenoid enantiomers, that is, ( R)-(+)- and ( S)-(-)-β-citronellol. On the basis of a computational study, the origin of enantio-differentiation is assigned to the discriminable charge transfer from ( R)-(+)- or ( S)-(-)-β-citronellol into graphene with a significant difference in binding strength depending on surface morphology. The chemosensor system developed herein has great potential to be applied in miniaturized and rapid enantioselective sensing with high sensitivity and selectivity.