Yeoheung Yoon

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.

Highly Air‐Stable Phosphorus‐Doped n‐Type Graphene Field‐Effect Transistors

Phosphorus-doped double-layered graphene field-effect transistors (PDGFETs) show much stronger air-stable n-type behavior than nitrogen-doped double-layered graphene FETs (NDGFETs), even under an oxygen atmosphere, due to strong nucleophilicity, which may lead to real applications for air-stable n-type graphene channels.

Highly Sensitive and Selective Gas Sensor Using Hydrophilic and Hydrophobic Graphenes

New hydrophilic 2D graphene oxide (GO) nanosheets with various oxygen functional groups were employed to maintain high sensitivity in highly unfavorable environments (extremely high humidity, strong acidic or basic). Novel one-headed polymer optical fiber sensor arrays using hydrophilic GO and hydrophobic reduced graphene oxide (rGO) were carefully designed, leading to the selective sensing of volatile organic gases for the first time. The two physically different surfaces of GO and rGO could provide the sensing ability to distinguish between tetrahydrofuran (THF) and dichloromethane (MC), respectively, which is the most challenging issue in the area of gas sensors. The eco-friendly physical properties of GO allowed for faster sensing and higher sensitivity when compared to previous results for rGO even under extreme environments of over 90% humidity, making it the best choice for an environmentally friendly gas sensor.

Nitrogen-doped partially reduced graphene oxide rewritable nonvolatile memory.

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGODMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7-2.1 eV and contained a sp2 C═C framework with 45-50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGODMF film lost memory behavior. Polarization of charged PrGODMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGODMF memory effects that followed voltage-driven rewrite-read-erase-read processes.

Cancer Therapy Using Ultrahigh Hydrophobic Drug-Loaded Graphene Derivatives

This study aimed to demonstrate that curcumin (Cur)-containing graphene composites have high anticancer activity. Specifically, graphene-derivatives were used as nanovectors for the delivery of the hydrophobic anticancer drug Cur based on pH dependence. Different Cur-graphene composites were prepared based on polar interactions between Cur and the number of oxygen-containing functional groups of respective starting materials. The degree of drug-loading was found to be increased by increasing the number of oxygen-containing functional groups in graphene-derivatives. We demonstrated a synergistic effect of Cur-graphene composites on cancer cell death (HCT 116) both in vitro and in vivo. As-prepared graphene quantum dot (GQD)-Cur composites contained the highest amount of Cur nano-particles and exhibited the best anticancer activity compared to the other composites including Cur alone at the same dose. This is the first example of synergistic chemotherapy using GQD-Cur composites simultaneous with superficial bioprobes for tumor imaging.

Can Commonly Used Hydrazine Produce n‐Type Graphene?

A simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (rGO) sheets (see figure) as an n-type semiconductor through the treatment of as-prepared GO sheets with the commonly used reducing reagent hydrazine, followed by rapid thermal annealing (RTA) is described.

Synthesis of Highly n‐Type Graphene by Using an Ionic Liquid

The development of an n-type, graphene-based semiconductor is currently a significant research interest. Graphene is easily p-doped by adsorbates such as oxygen and moisture and thus a p-type semiconductor can be easily prepared. However, the development of an n-type semiconductor is required to fabricate a complementary circuit, and nitrogendoped (N-doped) materials would be useful in real device applications. Chemical doping is an important method used to modulate the electrical properties of graphene. Both theoretical calculations and detailed experiments have proved that chemical doping with foreign atoms, such as nitrogen, is an effective approach to achieve n-type semiconductors. [1] nType semiconductors can be obtained by replacing carbon atoms with nitrogen atoms in the graphene framework. The lone electron pairs of nitrogen atoms play an important role in producing a delocalized conjugated system with sp 2 hybridized carbon frameworks [2–4] that can enhance the reactivity and electrocatalytic properties of graphene. Substitutional N-doped multilayer graphene sheets were synthesized by adding NH3 gas during the chemical vapor deposition (CVD) growth of graphene, [1] and the monolayer growth of N-doped graphene sheets by using poly(methyl methacrylate) (PMMA) and pyridine was recently reported. [5, 6] The nitrogen-doping behavior of graphene and reduced graphene oxide (rGO) through electrical joule heating and thermal annealing in NH3, respectively, has been reported to produce an n-type semiconductor. [7, 8] However, the systematic investigation of graphene doping is required to achieve a large N-doping effect in real device applications. A high atomic percentage of dopant nitrogen is important in the fabrication of n-type rGO with a large shift of the Dirac point (DP). The catalyst-free synthesis of N-doped graphene by thermal annealing of graphene oxide (GO) with melamine through a bulk reaction was reported to provide a high atomic percentage of nitrogen on the rGO surface. [9] How

Highly hydrophilic and insulating fluorinated reduced graphene oxide

A facile method for the synthesis of highly fluorinated reduced graphene oxide from graphene oxide using BF3-OEt2 solution and alkylthiol/alkylamine on the Gram scale has been described using a detailed mechanism. The maximum fluorination was as high as 38 wt% and the fluorinated reduced graphene oxide produced has great wettability and high insulating properties.