Hae Kyung Jeong

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnels equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.

Valence band of graphite oxide

We investigated the valence band structure of graphite oxide by photoelectron spectroscopy at the Pohang Accelerator Laboratory, Korea. The typical sp2 hybridization states found in graphite were also seen in graphite oxide. However, the π state disappeared near the Fermi level because of bonding between the π and oxygen-related states originating from graphite oxide, indicating electron transfer from graphite to oxygen and resulting in a downward shift of the highest occupied molecular orbital (HOMO) state to higher binding energies. The band gap opening increased to about 1.8 eV, and additional oxygen-related peaks were observed at 8.5 and 27 eV. The electronic states of graphite were also found in graphite oxide. Thus, graphite oxide has an electronic structure similar to that of pristine graphite except for the states near the Fermi level and oxygen-related states.

Hydrolysis-induced immobilization of Pt(acac)2 on polyimide-based carbon nanofiber mat and formation of Pt nanoparticles

Electrospun polyimide (PI)-based carbon nanofibers have recently garnered much interest due to their high conductivity and high mechanical strength. Promising applications include electrodes for supercapacitors, filters, sensors, and fuel cells. Here, we demonstrate that Pt nanoparticles can be loaded on the surface of PI nanofibers via an immobilization process induced by hydrolysis. The uniform distribution and sizes of Pt nanoparticles were controlled further by carbonization. Pt(acac)2 dissolved in acetone was impregnated on the hydrolyzed PI nanofibers. Pt ions were localized exclusively on the surface of PI nanofibers by precise control of the hydrolysis process. Our X-ray photoelectron spectroscopy results show that Pt ions in Pt(acac)2 molecules (40%) are immobilized on the hydrolyzed PI surface while some of them (60%) bind to O in the carboxylic group to form a PtO structure, and then are fully decomposed into Pt nanoparticles during carbonization. Using density functional calculations, we show that the binding of Pt(acac)2 on hydrolyzed PI is strong with a binding energy of −4.3 eV, which originates mostly from Pt–O binding and π-stacking between (acac) and PAA, confirming experimental observations of robust formation of Pt nanoparticles on hydrolyzed PI. The cyclic voltammetric test demonstrates that our robust carbon nanofiber mat can be utilized for fuel cell electrodes.

Nonspecific cleavage of proteins using graphene oxide

In this article, we report the intrinsic catalytic activity of graphene oxide (GO) for the nonspecific cleavage of proteins. We used bovine serum albumin (BSA) and a recombinant esterase (rEstKp) from the cold-adapted bacterium Pseudomonas mandelii as test proteins. Cleavage of BSA and rEstKp was nonspecific regarding amino acid sequence, but it exhibited dependence on temperature, time, and the amount of GO. However, cleavage of the proteins did not result in complete hydrolysis into their constituent amino acids. GO also invoked hydrolysis of p-nitrophenyl esters at moderate temperatures lower than those required for peptide hydrolysis regardless of chain length of the fatty acyl esters. Based on the results, the functional groups of GO, including alcohols, phenols, and carboxylates, can be considered as crucial roles in the GO-mediated hydrolysis of peptides and esters via general acid-base catalysis. Our findings provide novel insights into the role of GO as a carbocatalyst with nonspecific endopeptidase activity in biochemical reactions.

Improved dispersion of graphite derivatives by solution plasma

Solution plasma is applied to graphite and thermally reduced graphite oxide in ambient conditions in order to improve their dispersion. Changes in morphology, oxygen functional groups, defects, decomposition temperatures, oxygen to carbon atomic ratio, conductivity, stability, and degree of dispersion are systematically investigated by using scanning electron microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, Thermogravimetric analysis, X-ray photoelectron spectroscopy, Zeta potential analyzers, and ultraviolet–visible spectrophotometry. Dramatic enhancement of the dispersion after the simple plasma treatment is introduced without evident change of conductivities. This simple, easy, economical, and eco-friendly plasma method could functionalize and reform material efficiently in many application fields.

Electronic structure of cyclodextrin–carbon nanotube composite films

The electronic structures of two kinds of cyclodextrin–carbon nanotube (αCD–CNT and γCD–CNT) composite films are investigated by using (angular dependent) photoelectron spectroscopy to gain insight as to why the αCD–CNT and γCD–CNT composite films show different performances in biosensor applications. The γCD–CNT composite film is likely to have the CD localized on the surface rather than in the bulk of the film, while αCD–CNT has CD relatively more concentrated within the bulk of selvedge region of the film, rather than the surface. The results indicate that the CD, of the γCD–CNT composite, may be more bioactive, and possibly a better sensor of biomolecules due to the favorable surface position compared with that of αCD–CNT. The valence band of αCD–CNT and γCD–CNT show little difference from the CNT film except for a density of states, originating from CD, evident at a binding energy near 27 eV below Fermi level, meaning that there are few or no redox interactions between the CD and the CNT. The absence of a redox interaction between the CD and the CNT permits a clear electrochemical response to occur when guest biomolecules are captured on the composites, providing a route to biosensor applications.

Effective Reduction of Graphene Oxide for Energy-storage Devices

Graphene is a two-dimensional allotrope of carbon with the honeycomb structure, and it has the precious properties such as high thermal conductivity (∼5000 Wm−1K−1) and skyscraping electron mobility (200,000 cm2V−1s−1) [1]. Moreover, graphene can be made from graphite which is cheap and has a large reserve in nature so that it has become the “rising star” for high technology research work [2]. Among many routes to produce graphene, the chemical method through graphene oxide (GO) is considered as an easy and economical way [3]. GO, however, is an insulator unfortunately so it is needed to reduce oxygen functional groups for recovering the graphene structure of high conductivity. Chemical or thermal reduction processes of GO have been investigated [4–6]. Each pathway has certain advantages and disadvantages, resulting in different degrees of regeneration of the graphene structure. Our recent research has been investigated the grain size effect on the electrochemical performance of the chemically reduced GO [7], finding that the smallest grain size, 5 μm, provided the best electrochemical performance due to better reduction compared to the other chemically reduced GO

Plane wave density functional theory studies of the structural and the electronic properties of amino acids attached to graphene oxide via peptide bonding

We studied via plane wave pseudopotential total-energy calculations within the local spin density approximation (LSDA) the electronic and the structural properties of amino acids (alanine, glycine, and histidine) attached to graphene oxide (GO) by peptide bonding. The HOMO-LUMO gap, the Hirshfeld charges, and the equilibrium geometrical structures exhibit distinctive variations that depend on the species of the attached amino acid. The GO-amino acid system appears to be a good candidate for a biosensor.

ELECTROCHEMICAL PROPERTIES OF POLY SODIUM 4-STYRENESULFONATE INTERCALATED GRAPHITE OXIDE ELECTRODE IN AN AQUEOUS ELECTROLYTE

The electrochemical properties of poly sodium 4-styrenesulfonate intercalated graphite oxide (PSSGO) have been investigated in a 1 M H2SO4 electrolyte. We observed capacitor behavior at scan rate of 1–25 mV/s in a cyclic voltammetry. Specific capacitance obtained from galvanostatic charge and discharge measurements were 6 F/g to 102 F/g at 1 A/g to 0.1 A/g, respectively. The specific capacitance of PSSGO is relatively high compared to that of the precursor graphite oxide in which the specific capacitance was 11–20 F/g at 0.03 A/g. Capacitance retention was 73% after 3000 cycles, proving reliable cyclic stability up to 3000 cycles.