Junghyun An
Seoul National University
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Publication
Featured researches published by Junghyun An.
Energy and Environmental Science | 2013
Uk Sim; Tae-Youl Yang; Joonhee Moon; Junghyun An; Jinyeon Hwang; Jung-Hye Seo; Jouhahn Lee; Kye Yeop Kim; Joohee Lee; Seungwu Han; Byung Hee Hong; Ki Tae Nam
Carbon-based catalysts have been attracting attention in renewable energy technologies due to the low cost and high stability, but their insufficient activity is still a challenging issue. Here, we suggest that monolayer graphene can be used as a catalyst for solar-driven hydrogen evolution reaction on Si-photocathodes, and its catalytic activity is boosted by plasma treatment in N2-ambient. The plasma treatment induces abundant defects and the incorporation of nitrogen atoms in the graphene structure, which can act as catalytic sites on graphene. The monolayer graphene containing nitrogen impurities exhibits a remarkable increase in the exchange current density and leads to a significant anodic shift of the onset of photocurrent from the Si-photocathode. Additionally, monolayer graphene shows the passivation effect that suppresses the surface oxidation of Si, thus enabling the operation of the Si-photocathode in neutral water. This study shows that graphene itself can be applied to a photoelectrochemical system as a catalyst with high activity and chemical stability.
Advanced Materials | 2014
Joonhee Moon; Junghyun An; Uk Sim; Sung-Pyo Cho; Jin Hyoun Kang; Chul Chung; Jung-Hye Seo; Jouhahn Lee; Ki Tae Nam; Byung Hee Hong
High-quality N-doped graphene quantum sheets are successfully fabricated from as-grown monolayer graphene on Cu using nitrogen plasma, which can be transferred as a film-like layer or easily dispersed in an organic solvent for further optoelectronic or photoelectrochemical applications.
Energy and Environmental Science | 2015
Uk Sim; Joonhee Moon; Junghyun An; Jin Hyoun Kang; Sung Eun Jerng; Junsang Moon; Sung-Pyo Cho; Byung Hee Hong; Ki Tae Nam
Photoelectrochemical hydrogen production from solar energy has been attracting much attention in the field of renewable energy technology. The realization of cost-effective hydrogen production by water splitting requires electrolysis or photoelectrochemical cells decorated with highly efficient co-catalysts. A critical requirement for catalysts in photoelectrochemical cells is not only the ability to boost the kinetics of a chemical reaction but also to exhibit durability against electrochemical and photoinduced degradation. In the race to replace previous noble-metal catalysts, the design of carbon-based catalysts represents an important research direction in the search for non-precious, environmentally benign, and corrosion-resistant catalysts. Herein, we suggest graphene quantum sheets as a catalyst for the solar-driven hydrogen evolution reaction on Si nanowire photocathodes. The optimum nanostructures of the Si photocathodes exhibit an enhanced photocurrent and a lower overpotential compared to those of a planar Si surface. This significant enhancement demonstrates how graphene quantum sheet catalysts can be used to produce Si nanowire photocathodes as hydrogen evolution reaction catalysts with high activity.
Small | 2017
Vinayak S. Kale; Uk Sim; Jiwoong Yang; Kyoungsuk Jin; Sue In Chae; Woo Je Chang; Arun Kumar Sinha; Heonjin Ha; Chan-Cuk Hwang; Junghyun An; Hyo-Ki Hong; Zonghoon Lee; Ki Tae Nam; Taeghwan Hyeon
There is an urgent need to develop metal-free, low cost, durable, and highly efficient catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using hydrothermal process. Sulfur-modified graphitic carbon nitride (S-modified g-CN x ) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. The sulfur modification in the g-CN x structure leads to excellent oxygen evolution reaction activity by lowering the overpotential. Compared with the previously reported nonmetallic systems and well-established metallic catalysts, the S-modified g-CN x nanostructures show superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm-2 and a Tafel slope of 120 mV dec-1 with long-term durability of 91.2% retention for 18 h. These inexpensive, environmentally friendly, and easy-to-synthesize catalysts with extraordinary performance will have a high impact in the field of oxygen evolution reaction electrocatalysis.
Energy and Environmental Science | 2015
Uk Sim; Joonhee Moon; Junghyun An; Jin Hyoun Kang; Sung Eun Jerng; Junsang Moon; Sung-Pyo Cho; Byung Hee Hong; Ki Tae Nam
Correction for ‘N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production’ by Uk Sim et al., Energy Environ. Sci., 2015, DOI: 10.1039/c4ee03607g.
ACS Applied Materials & Interfaces | 2017
Uk Sim; Joonhee Moon; Joohee Lee; Junghyun An; Hyo-Yong Ahn; Dong Jin Kim; Insu Jo; Cheolho Jeon; Seungwu Han; Byung Hee Hong; Ki Tae Nam
Photoelectrochemical cells are used to split hydrogen and oxygen from water molecules to generate chemical fuels to satisfy our ever-increasing energy demands. However, it is a major challenge to design efficient catalysts to use in the photoelectochemical process. Recently, research has focused on carbon-based catalysts, as they are nonprecious and environmentally benign. Interesting advances have also been made in controlling nanostructure interfaces and in introducing new materials as catalysts in the photoelectrochemical cell. However, these catalysts have as yet unresolved issues involving kinetics and light-transmittance. In this work, we introduce high-transmittance graphene onto a planar p-Si photocathode to produce a hydrogen evolution reaction to dramatically enhance photon-to-current efficiency. Interestingly, double-layer graphene/Si exhibits noticeably improved photon-to-current efficiency and modifies the band structure of the graphene/Si photocathode. On the basis of in-depth electrochemical and electrical analyses, the band structure of graphene/Si was shown to result in a much lower work function than Si, accelerating the electron-to-hydrogen production potential. Specifically, plasma-treated double-layer graphene exhibited the best performance and the lowest work function. We electrochemically analyzed the mechanism at work in the graphene-assisted photoelectrode. Atomistic calculations based on the density functional theory were also carried out to more fully understand our experimental observations. We believe that investigation of the underlying mechanism in this high-performance electrode is an important contribution to efforts to develop high-efficiency metal-free carbon-based catalysts for photoelectrochemical cell hydrogen production.
ACS central science | 2018
Jimin Park; Ki Dong Yang; Na Young Kim; Kang-Won Jung; Viet-Duc Le; Hee-Jin Lim; Junghyun An; Kyoungsuk Jin; Yong-Hyun Kim; Ki Tae Nam; Dae Won Moon
One of the remaining challenges in material chemistry is to unveil the quantitative compositional/structural information and thermodynamic nature of inorganic materials especially in the initial nucleation and growth step. In this report, we adopted newly developed time-of-flight medium-energy-ion-scattering (TOF-MEIS) spectroscopy to address this challenge and explored heterogeneously grown nanometer-sized calcium phosphate as a model system. With TOF-MEIS, we discovered the existence of calcium-rich nanoclusters (Ca/P ∼ 3) in the presence of the non-collagenous-protein-mimicking passivating ligands. Over the reaction, these clusters progressively changed their compositional ratio toward that of a bulk phase (Ca/P ∼ 1.67) with a concurrent increase in their size to ∼2 nm. First-principles studies suggested that the calcium-rich nanoclusters can be stabilized through specific interactions between the ligands and clusters, emphasizing the important role of template on guiding the chemical and thermodynamic nature of inorganic materials at the nanoscale.
Advanced Functional Materials | 2016
Ki Dong Yang; Yoonhoo Ha; Uk Sim; Junghyun An; Chan Woo Lee; Kyoungsuk Jin; Younghye Kim; Jimin Park; Jung Sug Hong; Jun Ho Lee; Hye-Eun Lee; Hui-Yun Jeong; Hyungjun Kim; Ki Tae Nam
Nanoscale | 2018
Joonhee Moon; Uk Sim; Dong Jin Kim; Hyo-Yong Ahn; Junghyun An; Heonjin Ha; Kyoung Soon Choi; Cheolho Jeon; Jouhahn Lee; Ki Tae Nam; Byung Hee Hong
Bulletin of The Korean Chemical Society | 2018
Junghyun An; Kyoungsuk Jin; Uk Sim; Ki Tae Nam