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Kwak, In Hye,Kwon, Ik Seon,Abbas, Hafiz Ghulam,Seo, Jaemin,Jung, Gabin,Lee, Yeron,Kim, Doyeon,Ahn, Jae-Pyoung,Park, Jeunghee,Kang, Hong Seok The Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.5
<P>Two-dimensional layered MoS2 has recently been considered as an excellent catalyst for the water-splitting hydrogen evolution reaction (HER). Herein, we synthesize 1T′ phase MoS2 that was intercalated with a series of alkylated <I>p</I>-phenylenediamines (PDs). The substituted N atoms produced S vacancies, leading to a composition of MoS2−2xNx (<I>x</I> = 0.1). The more abundant methyl groups induce a larger charge transfer, resulting in excellent HER performance: for tetramethyl PD, the overpotential is 0.15 V at 10 mA cm<SUP>−2</SUP> with a Tafel slope of 35 mV dec<SUP>−1</SUP>. The catalytic activity of the complexes depends on the concentration of the intercalated molecules, showing an optimum at a concentration of 8 mol%. First-principles calculations showed that the intercalated complexes (1T′ phase) having N atom-S vacancy (N-VS) pairs are stabilized by a large charge transfer from the PD molecules that is enhanced by the methyl groups (<I>i.e</I>., 0.40e-0.84e per molecule at 6.25 mol% intercalation). The charge transfer increases the density of states at and just above the Fermi level, thereby increasing the electron concentration at low cathodic bias. The active sites for the Volmer reaction are found to be N atoms in the proximal N-VS pairs. The activation barrier for the Heyrovsky reaction becomes higher at higher concentrations of the intercalants, suggesting that the experimental HER performance is also kinetically controlled.</P>
Kwak, In Hye,Kwon, Ik Seon,Debela, Tekalign Terfa,Seo, Jaemin,Ahn, Jae-Pyoung,Yoo, Seung Jo,Kim, Jin-Gyu,Park, Jeunghee,Kang, Hong Seok The Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.39
<P>Two-dimensional (2D) MoS2 nanostructures have attracted much attention in recent years because of their excellent electrocatalytic activity toward the hydrogen evolution reaction (HER). Herein, we report unique 2D hybrid nanostructures of MoS2 and melamine synthesized <I>via</I> a one-step solvothermal process. Remarkably, few-layer metallic 1T′ phase MoS2 nanoflakes and orthorhombic phase melamine aggregate to form nanoplates. At a controlled concentration, the melamine molecules intercalated into the 1T′ phase MoS2 by forming charge-transfer complexes. The hybrid complexes with 7% intercalated melamine exhibited excellent performance for the catalytic HER, with a current of 10 mA cm<SUP>−2</SUP> at 0.136 V (<I>vs.</I> RHE) and a Tafel slope of 37 mV dec<SUP>−1</SUP>. First-principles calculations showed that the intercalation of hydrogen-bonded melamine clusters could stabilize the 1T′ phase MoS2<I>via</I> substantial charge transfer. The activation barrier was calculated for the Volmer-Heyrovsky reactions, by identifying the active sites of the Volmer reaction as the basal S atoms above the hydrogen-bonded amine group of melamine. This rationalizes the dependence of the catalytic activity on the concentration of intercalated melamine. The present study highlights the opportunities for producing unique 2D hybrid complexes to enhance the HER catalytic activity by controlling the intercalating organic molecules.</P>
Kwak, Sung Chul,Lee, Cheol,Kim, Ju-Young,Oh, Hyun Mee,So, Hong-Seob,Lee, Myeung Su,Rho, Mun Chual,Oh, Jaemin Pharmaceutical Society of Japan 2013 Biological & pharmaceutical bulletin Vol.36 No.11
<P>Excessive osteoclastic bone resorption plays a critical role in inflammation-induced bone loss such as rheumatoid arthritis and periodontal bone erosion. Therefore, identification of osteoclast targeted-agents may be a therapeutic approach to the treatment of pathological bone loss. In this study, we isolated chlorogenic acid (CGA) from fructus of Gardenia jasminoides to discover anti-bone resorptive agents. CGA is a polyphenol with anti-inflammatory and anti-oxidant activities, however, its effects on osteoclast differentiation is unknown. Thus, we investigated the effect of CGA in receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL)-induced osteoclast differentiation and RANKL signaling. CGA dose-dependently inhibited RANKL-mediated osteoclast differentiation in bone marrow macrophages (BMMs) without any evidence of cytotoxicity. CGA inhibited the phosphorylation of p38, Akt, extracellular signal-regulated kinase (ERK), and inhibitor of nuclear factor-kappa B (IκB), and IκB degradation by RANKL treatment. CGA suppressed the mRNA expression of nuclear factor of activated T cells c1 (NFATc1), TRAP and OSCAR in RANKL-treated bone marrow macrophages (BMMs). Also, overexpression of NFATc1 in BMMs blocked the inhibitory effect of CGA on RANKL-mediated osteoclast differentiation. Furthermore, to evaluate the effects of CGA in vivo, lipopolysaccharide (LPS)-induced bone erosion study was carried out. CGA remarkably attenuated LPS-induced bone loss based on micro-computed tomography and histologic analysis of femurs. Taken together, our findings suggest that CGA may be a potential treatment option for osteoclast-related diseases with inflammatory bone destruction.</P>
Kwak, In Hye,Abbas, Hafiz Ghulam,Kwon, Ik Seon,Park, Yun Chang,Seo, Jaemin,Cho, Min Kyung,Ahn, Jae-Pyoung,Seo, Hee Won,Park, Jeunghee,Kang, Hong Seok The Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.14
<P>We synthesized cobaltocene (7-20%)-intercalated WS2 nanosheets using a solvothermal process. The intercalation of cobaltocene between the expanded 1T′ phase WS2 layers was confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy. The intercalated complexes exhibited excellent performance for the catalytic hydrogen evolution reaction, with a Tafel slope of 40 mV dec<SUP>−1</SUP> and a current density of 10 mA cm<SUP>−2</SUP> at 0.17 V (<I>vs.</I> RHE). Spin-polarized density functional theory calculations showed that cobaltocene is intercalated with a fivefold symmetry parallel to the WS2 plane, driven by substantial charge transfer. Reaction pathway calculations suggest that the basal S atoms just above the Co atom are the active sites, and the activation barrier of the Heyrovsky reaction determines the catalytic activity.</P>