Heejoun Yoo

High-Quality Reduced Graphene Oxide by a Dual-Function Chemical Reduction and Healing Process

A new chemical dual-functional reducing agent, thiophene, was used to produce high-quality reduced graphene oxide (rGO) as a result of a chemical reduction of graphene oxide (GO) and the healing of rGO. Thiophene reduced GO by donation of electrons with acceptance of oxygen while it was converted into an intermediate oxidised polymerised thiophene that was eventually transformed into polyhydrocarbon by loss of sulphur atoms. Surprisingly, the polyhydrocarbon template helped to produce good-quality rGOC (chemically reduced) and high-quality rGOCT after thermal treatment. The resulting rGOCT nanosheets did not contain any nitrogen or sulphur impurities, were highly deoxygenated and showed a healing effect. Thus the electrical properties of the as-prepared rGOCT were superior to those of conventional hydrazine-produced rGO that require harsh reaction conditions. Our novel dual reduction and healing method with thiophene could potentially save energy and facilitate the commercial mass production of high-quality graphene.

Vertical alignments of graphene sheets spatially and densely piled for fast ion diffusion in compact supercapacitors.

Supercapacitors with porous carbon structures have high energy storage capacity. However, the porous nature of the carbon electrode, composed mainly of carbon nanotubes (CNTs) and graphene oxide (GO) derivatives, negatively impacts the volumetric electrochemical characteristics of the supercapacitors because of poor packing density (<0.5 g cm(-3)). Herein, we report a simple method to fabricate highly dense and vertically aligned reduced graphene oxide (VArGO) electrodes involving simple hand-rolling and cutting processes. Because of their vertically aligned and opened-edge graphene structure, VArGO electrodes displayed high packing density and highly efficient volumetric and areal electrochemical characteristics, very fast electrolyte ion diffusion with rectangular CV curves even at a high scan rate (20 V/s), and the highest volumetric capacitance among known rGO electrodes. Surprisingly, even when the film thickness of the VArGO electrode was increased, its volumetric and areal capacitances were maintained.

Anti‐Solvent Derived Non‐Stacked Reduced Graphene Oxide for High Performance Supercapacitors

An anti-solvent for graphene oxide (GO), hexane, is introduced to increase the surface area and the pore volume of the non-stacked GO/reduced GO 3D structure and allows the formation of a highly crumpled non-stacked GO powder, which clearly shows ideal supercapacitor behavior.

Dual Functions of Highly Potent Graphene Derivative–Poly-l-Lysine Composites To Inhibit Bacteria and Support Human Cells

Dual-function poly(L-lysine) (PLL) composites that function as antibacterial agents and promote the growth of human cell culture have been sought by researchers for a long period. In this paper, we report the preparation of new graphene derivative-PLL composites via electrostatic interactions and covalent bonding between graphene derivatives and PLL. The resulting composites were characterized by infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. The novel dual function of PLL composites, specifically antibacterial activity and biocompatibility with human cells [human adipose-derived stem cells and non-small-cell lung carcinoma cells (A549)], was carefully investigated. Graphene-DS-PLL composites composed of 4-carboxylic acid benzene diazonium salt (DS) generated more anionic carboxylic acid groups to bind to cationic PLLs, forming the most potent antibacterial agent among PLL and PLL composites with high biocompatibility with human cell culture. This dual functionality can be used to inhibit bacterial growth while enhancing human cell growth.

A low ion-transfer resistance and high volumetric supercapacitor using hydrophilic surface modified carbon electrodes

A hydrophilic surface modified carbon electrode shows a good electrolyte affinity with homogeneous dispersibility in water, resulting in low ion-transfer resistance and a uniform and dense electrode to give a high volumetric capacitor. The hydrophilic carbon electrode exhibits a superior capacitance (58 F cm−3, 99.3 mF cm−2) and is stable up to 5000 cycles.

n‐Type Reduced Graphene Oxide Field‐Effect Transistors (FETs) from Photoactive Metal Oxides

Graphene is of considerable interest as a next-generation semiconductor material to serve as a possible substitute for silicon. For real device applications with complete circuits, effective n-type graphene field effect transistors (FETs) capable of operating even under atmospheric conditions are necessary. In this study, we investigated n-type reduced graphene oxide (rGO) FETs of photoactive metal oxides, such as TiO(2) and ZnO. These metal oxide doped FETs showed slight n-type electric properties without irradiation. Under UV light these photoactive materials readily generated electrons and holes, and the generated electrons easily transferred to graphene channels. As a result, the graphene FET showed strong n-type electric behavior and its drain current was increased. These n-doping effects showed saturation curves and slowly returned back to their original state in darkness. Finally, the n-type rGO FET was also highly stable in air due to the use of highly resistant metal oxides and robust graphene as a channel.