Joonhee Moon

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

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.

The effects of 100 nm-diameter Au nanoparticles on dye-sensitized solar cells

Gold nanoparticles of ∼100 nm in diameter were incorporated into TiO2 nanoparticles for dye-sensitized solar cells (DSSCs). At the optimum Au/TiO2 mass ratio of 0.05, the power-conversion efficiency of the DSSC improved to 3.3% from a value of 2.7% without Au, and this improvement was mainly attributed to the photocurrent density. The Au nanoparticles embedded in the nanoparticulate-TiO2 film strongly absorbed light due to the localized surface-plasmon resonance, and thereby promoted light absorption of the dye. In the DSSCs, the 100 nm-diameter Au nanoparticles generate field enhancement by surface-plasmon resonance rather than prolonged optical paths by light scattering.

One‐Step Synthesis of N‐doped Graphene Quantum Sheets from Monolayer Graphene by Nitrogen Plasma

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.

N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production

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.

Smart Contact Lenses with Graphene Coating for Electromagnetic Interference Shielding and Dehydration Protection

Recently, smart contact lenses with electronic circuits have been proposed for various sensor and display applications where the use of flexible and biologically stable electrode materials is essential. Graphene is an atomically thin carbon material with a two-dimensional hexagonal lattice that shows outstanding electrical and mechanical properties as well as excellent biocompatibility. In addition, graphene is capable of protecting eyes from electromagnectic (EM) waves that may cause eye diseases such as cataracts. Here, we report a graphene-based highly conducting contact lens platform that reduces the exposure to EM waves and dehydration. The sheet resistance of the graphene on the contact lens is as low as 593 Ω/sq (±9.3%), which persists in an wet environment. The EM wave shielding function of the graphene-coated contact lens was tested on egg whites exposed to strong EM waves inside a microwave oven. The results show that the EM energy is absorbed by graphene and dissipated in the form of thermal radiation so that the damage on the egg whites can be minimized. We also demonstrated the enhanced dehydration protection effect of the graphene-coated lens by monitoring the change in water evaporation rate from the vial capped with the contact lens. Thus, we believe that the graphene-coated contact lens would provide a healthcare and bionic platform for wearable technologies in the future.

Strain Relaxation of Graphene Layers by Cu Surface Roughening

The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene.

Correction: N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen production

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.

Effects of Poly(N‐isopropylacrylamide) (PNIPAAm) on the Conformational Change of Poly(N 5‐hydroxyalkyl glutamine) (PHAG) in PHAG‐PNIPAAm Block Copolymer with Temperature

Diblock copolymers consisting of poly(N 5 -hydroxyalkylglutamine) (PHAG) and poly(N-isopropylacrylamide) (PNIPAAm) were prepared by aminolysis with aminoalkanols of the side-chain ester of poly(γ-benzyl L-glutamate) (PLBG) as a part of PBLG-PNIPAAm block copolymers. The molecular weight ratio of the initial PBLG to the resulting PHAG was nearly 0.35. The effect of PNIPAAm on the conformational change of PHAG in PHAG-PNIPAAm block copolymers with temperature was investigated by circular dichroism. Poly[N 5 -(2-hydroxyethyl)-L-glutamine] (PHEG) and the PHEG-PNIPAAm copolymer (GNE) stayed in a randomly coiled conformation whereas poly(N 5 -(3-hydroxypropyl)-L-glutamine] (PHPG), poly(N 5 -(4-hydroxybutyl)-L-glutamine) (PHBG), PHPG-PNIPAAm copolymer (GNP), and PHBG-PNIIPAm copolymer (GNB) underwent conformational transitions with temperature.The conformational change of the PHPG block in GNP copolymer occurred from an α-helix to a random coil after the incorporatoin of PNIPAAm into the copolymer. The thermodynamic parameters of the thermally induced helix-coil transition for PHBG and PHBG-PNIIPAm in aqueous solution were calculated.

Double-Layer Graphene Outperforming Monolayer as Catalyst on Silicon Photocathode for Hydrogen Production.

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.

A highly efficient and stable organic additive for the positive electrolyte in vanadium redox flow batteries: taurine biomolecules containing –NH2 and –SO3H functional groups

The vanadium redox flow battery (VRFB) is one of the most promising energy storage systems for large-scale applications. However, its commercialization has been hampered due to the low thermal stability and slow kinetics of the positive electrolyte. To solve these problems, we applied taurine and its derivatives containing both amine (–NH2) and sulfonic acid (–SO3H) groups as organic additives. Among these candidates, taurine showed the lowest overpotential and the fastest reaction kinetics, which resulted in high energy efficiency (87.9% at 50 mA cm−2) and capacity retention (87.6% after 100 cycles). Taurine was found to improve the thermal stability of the positive electrolyte by forming a vanadium–additive complex, suppressing irreversible precipitation at high temperature. Additionally, –NH2 and –SO3H groups in taurine induced nitrogen/sulfur bifunctional doping on the carbon electrode, which increased the wettability and reversibility by enhancing charge transfer and mass transport rates. Through this functional group-oriented approach that introduces taurine into the positive electrolyte, we believe that an important breakthrough has been made in VRFB technologies, which could significantly improve the VRFB performance and direct a lot of attention towards new organic additives for various redox flow batteries.