Showing papers by "Franklin Kim published in 2010"

Graphene Oxide Sheets at Interfaces

Abstract: Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge −COOH groups is affected by pH, GO’s amphiphilicity can be tuned by pH. In addition, size-dependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule−colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble mat...

Graphene Oxide: Surface Activity and Two‐Dimensional Assembly

Abstract: Graphene oxide (GO) is a promising precursor for preparing graphene-based composites and electronics applications. Like graphene, GO is essentially one-atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two-dimensional assemblies over large areas is discussed.

Visualizing Graphene Based Sheets by Fluorescence Quenching Microscopy

Abstract: Graphene based sheets have stimulated great interest due to their superior mechanical, electrical, and thermal properties. A general visualization method that allows quick observation of these single atomic layers would be highly desirable as it can greatly facilitate sample evaluation and manipulation, and provide immediate feedback to improve synthesis and processing strategies. Here we report that graphene based sheets can be made highly visible under a fluorescence microscope by quenching the emission from a dye coating, which can be conveniently removed afterward by rinsing without disrupting the sheets. Current imaging techniques for graphene based sheets rely on the use of special substrates. In contrast, the fluorescence quenching mechanism is no longer limited by the type of substrate. Graphene, reduced graphene oxide, or even graphene oxide sheets deposited on arbitrary substrates can now be readily visualized with good contrast for layer counting. Direct observation of suspended sheets in solution was also demonstrated. The fluorescence quenching microscopy offers unprecedented imaging flexibility and could become a general tool for characterizing graphene based materials.

Graphene oxide as surfactant sheets

Abstract: Graphite oxide sheet, now referred to as graphene oxide (GO), is the product of chemical oxidation and exfoliation of graphite powders that was first synthesized over a cen- tury ago. Interest in this old material has resurged in recent years, especially after the dis- covery of graphene, as GO is considered a promising precursor for the bulk production of graphene-based materials. GO sheets are single atomic layers that can readily extend up to tens of microns in lateral dimension. Therefore, their structure bridges the typical length scales of both chemistry and materials science. GO can be viewed as an unconventional type of soft material as it carries the characteristics of polymers, colloids, membranes, and as highlighted in this review, amphiphiles. GO has long been considered hydrophilic due to its excellent water dispersity, however, our recent work revealed that GO sheets are actually amphiphilic with an edge-to-center distribution of hydrophilic and hydrophobic domains. Thus, GO can adhere to interfaces and lower interfacial energy, acting as surfactant. This new property insight helps to better understand GO's solution properties which can inspire novel material assembly and processing methods such as for fabricating thin films with controllable microstructures and separating GO sheets of different sizes. In addition, GO can be used as a surfactant sheet to emulsify organic solvents with water and disperse insoluble materials such as graphite and carbon nanotubes (CNTs) in water, which opens up opportunities for creating functional hybrid materials of graphene and other π-conjugated systems.

Self-propagating domino-like reactions in oxidized graphite

Abstract: Graphite oxide (GO) has received extensive interest as a precursor for the bulk production of graphene-based materials. Here, the highly energetic nature of GO, noted from the self-propagating thermal deoxygenating reaction observed in solid state, is explored. Although the resulting graphene product is quite stable against combustion even in a natural gas flame, its thermal stability is significantly reduced when contaminated with potassium salt by-products left from GO synthesis. In particular, the contaminated GO becomes highly flammable. A gentle touch with a hot soldering iron can trigger violent, catastrophic, total combustion of such GO films, which poses a serious fire hazard. This highlights the need for efficient sample purification methods. Typically, purification of GO is hindered by its tendency to gelate as the pH value increases during rinsing. A two-step, acid–acetone washing procedure is found to be effective for suppressing gelation and thus facilitating purification. Salt-induced flammability is alarming for the fire safety of large-scale manufacturing, processing, and storage of GO materials. However, the energy released from the deoxygenation of GO can also be harnessed to drive new reactions for creating graphene-based hybrid materials. Through such domino-like reactions, graphene sheets decorated with metal and metal oxide particles are synthesized using GO as the in situ power source. Enhanced electrochemical capacitance is observed for graphene sheets loaded with RuO2 nanoparticles.

Method of forming a film of graphite oxide single layers, and applications of same

Abstract: A method for forming a film of graphite oxide single layers. In one embodiment, the method includes the steps of preparing a solution of graphite oxide to allow a plurality of graphite oxide single layers to be formed and dispersed in the solution; and applying the solution of graphite oxide onto an air-water interface of water to form a film at the air-water interface, wherein the film comprises a plurality of graphite oxide single layers and is characterized by a packing density, wherein the film of graphite oxide single layers is formed with no presence of a surfactant or stabilizing agent.

Co-Assembly of Nanoparticles in Evaporating Aerosol Droplets: Preparation of Nanoporous Pt/TiO2 Composite Particles

Abstract: Nanoporous Pt/TiO2 micro-particles were synthesized via an aerosol assisted co-assembly (AACA) route. Aerosol droplets were produced from a colloidal mixture of 5 nm Pt and 20 nm TiO2 nanoparticles, which formed spherical micro-aggregates of Pt and TiO2 with average diameter of around 1.2 μm. The resulting composite micro-particles have very open structure with pore sizes ranging from 20 to 200 nm. Pt nanoparticles were found to be well dispersed on the surface of the supporting TiO2 particles. Electrocatalytic application of the nanoporous Pt/TiO2 composites was examined through methanol oxidation reaction. The performance of 20 wt% Pt/TiO2 particles was found to be comparable to that of commercial 20 wt% Pt/carbon black catalyst.

High-Throughput Imaging of Graphene Based Sheets by Fluorescence Quenching Microscopy and Applications of Same

Abstract: A method for imaging a graphene-based film. In one embodiment, the method includes the steps of providing a graphene-based film on a surface of a medium; forming a fluorescent coating over the graphene-based film to form a sample; illuminating the sample with light of a specific wavelength or wavelengths, which is absorbed by the fluorescent coating to cause the fluorescent coating to emit light of wavelengths longer than that of the absorbed light, which is quenched by the graphene-based film such that a visibility contrast is formed between the graphene-based film and the fluorescent coating; and imaging the graphene-based film from the visibility contrast.