Junghoon Oh

Generation of B-doped graphene nanoplatelets using a solution process and their supercapacitor applications.

Chemically modified graphene (CMG) nanoplatelets have shown great promise in various applications due to their electrical properties and high surface area. Chemical doping is one of the most effective methods to tune the electronic properties of graphene materials. In this work, novel B-doped nanoplatelets (borane-reduced graphene oxide, B-rG-O) were produced on a large scale via the reduction of graphene oxide by a borane-tetrahydrofuran adduct under reflux, and their use for supercapacitor electrodes was studied. This is the first report on the production of B-doped graphene nanoplatelets from a solution process and on the use of B-doped graphene materials in supercapacitors. The B-rG-O had a high specific surface area of 466 m(2)/g and showed excellent supercapacitor performance including a high specific capacitance of 200 F/g in aqueous electrolyte as well as superior surface area-normalized capacitance to typical carbon-based supercapacitor materials and good stability after 4500 cycles. Two- and three-electrode cell measurements showed that energy storage in the B-rG-O supercapacitors was contributed by ion adsorption on the surface of the nanoplatelets in addition to electrochemical redox reactions.

Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping

Chemically modified graphene platelets, produced via graphene oxide, show great promise in a variety of applications due to their electrical, thermal, barrier and mechanical properties. Understanding the chemical structures of chemically modified graphene platelets will aid in the understanding of their physical properties and facilitate development of chemically modified graphene platelet chemistry. Here we use (13)C and (15)N solid-state nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy to study the chemical structure of (15)N-labelled hydrazine-treated (13)C-labelled graphite oxide and unlabelled hydrazine-treated graphene oxide, respectively. These experiments suggest that hydrazine treatment of graphene oxide causes insertion of an aromatic N(2) moiety in a five-membered ring at the platelet edges and also restores graphitic networks on the basal planes. Furthermore, density-functional theory calculations support the formation of such N(2) structures at the edges and help to elucidate the influence of the aromatic N(2) moieties on the electronic structure of chemically modified graphene platelets.

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Oxidized carbon nitrides: water-dispersible, atomically thin carbon nitride-based nanodots and their performances as bioimaging probes.

Three-dimensional (3D) carbon nitride (C3 N4 )-based materials show excellent performance in a wide range of applications because of their suitable band structures. To realize the great promise of two-dimensional (2D) allotropes of various 3D materials, it is highly important to develop routes for the production of 2D C3 N4 materials, which are one-atom thick, in order to understand their intrinsic properties and identify their possible applications. In this work, water-dispersible, atomically thin, and small carbon nitride nanodots were produced using the chemical oxidation of graphitic C3 N4 . Various analyses, including X-ray diffraction, X-ray photoelectron, Fourier-transform infrared spectroscopy, and combustion-based elemental analysis, and thermogravimetric analysis, confirmed the production of 3D oxidized C3 N4 materials. The 2D C3 N4 nanodots were successfully exfoliated as individual single layers; their lateral dimension was several tens of nanometers. They showed strong photoluminescence in the visible region as well as excellent performances as cell-imaging probes in an in vitro study using confocal fluorescence microscopy.

Generation of ultra-high-molecular-weight polyethylene from metallocenes immobilized onto N-doped graphene nanoplatelets.

Catalytic natures of organometallic catalysts are modulated by coordinating organic ligands with proper steric and electronic properties to metal centers. Carbon-based nanomaterials such as graphene nanoplatelets are used with and without N-doping and multiwalled carbon nanotube as a ligand for ethylene polymerizations. Zirconocenes or titanocenes are immobilized on such nanomaterials. Polyethylenes (PEs) produced by such hybrids show a great increase in molecular weight relative to those produced by free catalysts. Specially, ultra-high-molecular-weight PEs are produced from the polymerizations at low temperature using the hybrid with N-doped graphene nanoplatelets. This result shows that such nanomaterials act a crucial role to tune the catalytic natures of metallocenes.

Finely tuning oxygen functional groups of graphene materials and optimizing oxygen levels for capacitors

Oxygen-containing, chemically modified graphene (CMG) systems have been intensively investigated for various applications. The development of methods that allow fine control of the oxygen functionality would help better understand the mechanisms associated with CMGs, facilitate optimization of the material properties, and provide standards for chemical characterization purposes. Here, the authors report a new method for finely controlling the levels of oxygen in CMG materials based on the refluxing of aqueous colloidal suspensions of graphene oxide for specific reflux times, which does not require additional reducing agents. Chemical analysis confirmed that the oxygen levels can be finely controlled and they can provide spectroscopic tools to monitor the oxygen levels of CMG-based systems. This system was applied to help provide a fundamental foundation for the correlation between the oxygen groups and capacitive features.

Effect of degree of reduction on the anode performance of reduced graphene oxide in Li-ion batteries

Graphene-based materials are considered to be promising anodes for achieving high energy and power densities in Li-ion batteries (LIBs). Despite intensive research on reduced graphene oxide (rG-O) on this aspect, few have addressed the relationship between LIB performance and the degree of reduction of graphene oxide. In this work, we study the performance of LIB anodes using rG-O materials with different oxygen levels. A series of rG-O samples is produced by refluxing an aqueous graphene oxide suspension for different durations. The C/O ratios of the materials are found to increase gradually from approximately 1 to 6, with no heteroatoms other than oxygen atoms. The rG-O sample with the lowest degree of reduction delivers the highest anodic capacity and good durability.

Electrocatalytic performances of N-doped graphene with anchored iridium species in oxygen reduction reaction

Development of new systems with high catalytic performances in the oxygen reduction reaction (ORR) for practical applications in fuel cells and metal–air batteries is a challenge. We develop a one-pot solution method for producing a novel hybrid material consisting of Ir species anchored on N-doped graphene. The hybrid is synthesized by reacting graphene oxide with IrCl3 xH2O in dimethylformamide under reflux. Chemical and structural analyses confirm the attachment of Ir atoms to the N and O atoms of the N-doped graphene-based materials. The hybrid shows a good electrocatalytic performance for the ORR in alkaline media, with an onset potential of 0.88 V (versus the reversible hydrogen electrode), high long-term durability, and good tolerance for methanol poisoning.

Production of Metal-Free Composites Composed of Graphite Oxide and Oxidized Carbon Nitride Nanodots and Their Enhanced Photocatalytic Performances.

A novel metal-free composite (GN) composed of two types of carbon-based nanomaterials, graphite oxide (GO) and 2D oxidized carbon nitride (OCN) nanodots was produced. Chemical and morphological characterizations reveal that GN contains a main component of GO with well-dispersed 2D OCN nanodots. GN shows enhanced photocatalytic performance for degrading an organic pollutant, Rhodamine B, under visible light.

Electrochemistry of Layered Graphitic Carbon Nitride Synthesised from Various Precursors: Searching for Catalytic Effects

Graphitic carbon nitride (g-C3 N4 ), synthesised by pyrolysis of different precursors (dicyandiamide, melamine and urea) under varying reaction conditions (air and nitrogen gas) is subjected to electrochemical studies for the elucidation of the inherent catalytic efficiency of the pristine material. Contrary to popular belief, pristine g-C3 N4 shows negligible, if any, enhancement in its electrochemical behaviour in this comprehensive study. Voltammetric analysis reveals g-C3 N4 to display similar catalytic efficiency to the unmodified glassy carbon electrode surface on which the bulk material was deposited. This highlights the non-catalytic nature of the pristine material and challenges the feasibility of using g-C3 N4 as a heterogeneous catalyst to deliver numerous promised applications.