29,488 research outputs found
Utilization of multiple graphene layers in fuel cells. 1. An improved technique for the exfoliation of graphene-based nanosheets from graphite
An improved, safer and mild method was proposed for the exfoliation of graphene like sheets from graphite to be used in fuel cells. The major aim in the proposed method is to reduce the number of layers in the graphite material and to produce large quantities of graphene bundles to be used as catalyst support in polymer electrolyte membrane fuel cells. Graphite oxide was prepared using potassium dichromate/sulfuric acid as oxidant and acetic anhydride as intercalating agent. The oxidation process seemed to create expanded and leafy structures of graphite oxide layers. Heat treatment of samples led to the thermal decomposition of acetic anhydride into carbondioxide and water vapor which further swelled the layered graphitic structure. Sonication of graphite oxide samples created more separated structures. Morphology of the sonicated graphite oxide samples exhibited expanded the layer structures and formed some tullelike translucent and crumpled graphite oxide sheets. The mild procedure applied was capable of reducing the average number of graphene sheets from 86 in the raw graphite to nine in graphene-based nanosheets. Raman spectroscopy analysis showed the significant reduction in size of the in-plane sp2 domains
of graphene nanosheets obtained after the reduction of graphite oxide
Ceramic Composite Thin Films
A ceramic composite thin film or layer includes individual graphene oxide and/or electrically conductive graphene sheets dispersed in a ceramic (e.g. silica) matrix. The thin film or layer can be electrically conductive film or layer depending the amount of graphene sheets present. The composite films or layers are transparent, chemically inert and compatible with both glass and hydrophilic SiOx/silicon substrates. The composite film or layer can be produced by making a suspension of graphene oxide sheet fragments, introducing a silica-precursor or silica to the suspension to form a sol, depositing the sol on a substrate as thin film or layer, at least partially reducing the graphene oxide sheets to conductive graphene sheets, and thermally consolidating the thin film or layer to form a silica matrix in which the graphene oxide and/or graphene sheets are dispersed
Ultra-strong Adhesion of Graphene Membranes
As mechanical structures enter the nanoscale regime, the influence of van der
Waals forces increases. Graphene is attractive for nanomechanical systems
because its Young's modulus and strength are both intrinsically high, but the
mechanical behavior of graphene is also strongly influenced by the van der
Waals force. For example, this force clamps graphene samples to substrates, and
also holds together the individual graphene sheets in multilayer samples. Here
we use a pressurized blister test to directly measure the adhesion energy of
graphene sheets with a silicon oxide substrate. We find an adhesion energy of
0.45 \pm 0.02 J/m2 for monolayer graphene and 0.31 \pm 0.03 J/m2 for samples
containing 2-5 graphene sheets. These values are larger than the adhesion
energies measured in typical micromechanical structures and are comparable to
solid/liquid adhesion energies. We attribute this to the extreme flexibility of
graphene, which allows it to conform to the topography of even the smoothest
substrates, thus making its interaction with the substrate more liquid-like
than solid-like.Comment: to appear in Nature Nanotechnolog
Layer-by-layer polypyrrole coated graphite oxide and graphene nanosheets as catalyst support materials for fuel cells
For the production of advanced types of catalyst support materials, the distinguished properties of graphene nanosheets were combined with the structural properties of conducting polypyrrole by the incorporation of graphene nanosheets into a polymer matrix by the proposed simple and low-cost fabrication technique. A precise tuning of electrical conductivity and thermal stability was achieved by controlling the polymer thickness of randomly dispersed graphene nanosheets. Initially, graphene nanosheets were fabricated in large quantities via a mild chemical synthetic route involving graphite oxidation, ultrasonic treatment, and chemical reduction. Then, polypyrrole/graphene nanosheet composites with improved conductivity, thermal stability, and high surface area were synthesized by in situ polymerization with the different pyrrole feed ratios. Although graphite oxide sheets have electrically insulating property, partially oxidized graphite oxide was also utilized as conductive fillers in polymer matrix. However, polypyrrole/graphene nanosheet composites have better electrical conductivity than polypyrrole/graphite oxide composites
Layer-by-layer polypyrrole coated graphite oxide and graphene nanosheets as catalyst support materials for fuel cells
For the production of advanced type of catalyst support materials, the distinguished properties of graphene nanosheets were combined with the structural properties of conducting polypyrrole by the incorporation of graphene nanosheets into a polymer matrix by the proposed simple and low-cost fabrication technique. A precise tuning of electrical conductivity and thermal stability was also achieved by controlling the thickness of randomly dispersed graphene nanosheets by a layer-by-layer polymer coating. Initially, graphene nanosheets were fabricated in large quantities via a mild chemical synthetic route involving graphite oxidation, ultrasonic treatment and chemical reduction. Then, polypyrrole/graphene nanosheet composites with improved
conductivity, thermal stability and high surface area were synthesized by in situ polymerization with the different pyrrole feed ratios. Although graphite oxide sheets have electrically insulating property, partially oxidized graphite oxide was also utilized as conductive fillers in polymer matrix. However, polypyrrole/graphene nanosheet composites have better electrical conductivity than polypyrrole/graphite oxide composites
Position dependent photodetector from large area reduced graphene oxide thin films
We fabricated large area infrared photodetector devices from thin film of
chemically reduced graphene oxide (RGO) sheets and studied their photoresponse
as a function of laser position. We found that the photocurrent either
increases, decreases or remain almost zero depending upon the position of the
laser spot with respect to the electrodes. The position sensitive photoresponse
is explained by Schottky barrier modulation at the RGO film-electrode
interface. The time response of the photocurrent is dramatically slower than
single sheet of graphene possibly due to disorder from the chemically synthesis
and interconnecting sheets
SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS FOR DYE-SENSITIZED SOLAR CELLS
Graphene, which is a two-dimensional carbon material, exhibits unique properties that promise its potential applications in photovoltaic devices. Dye-sensitized solar cell (DSSC) is a representative of the third generation photovoltaic devices. Therefore, it is important to synthesize graphene with special structures, which possess excellent properties for dye-sensitized solar cells.
This dissertation research was focused on (1) the effect of oxygen content on the structure of graphite oxide, (2) the stability of graphene oxide solution, (3) the application of graphene precipitate from graphene oxide solution as counter electrode for DSSCs, (4) the development of a novel synthesis method for the three-dimensional graphene with honeycomb-like structure, and (5) the exploration of honeycomb structured graphene (HSG) as counter electrodes for DSSCs.
Graphite oxide is a crucial precursor to synthesize graphene sheets via chemical exfoliation method. The relationship between the oxygen content and the structures of graphite oxides was still not explored. In this research, the oxygen content of graphite oxide is tuned by changing the oxidation time and the effect of oxygen content on the structure of graphite oxide was evaluated. It has been found that the saturated ratio of oxygen to carbon is 0.47. The types of functional groups in graphite oxides, which are epoxy, hydroxyl, and carboxylgroups, are independent of oxygen content. However, the interplanar space and BET surface area of graphite oxide linearly increases with increasing O/C ratio.
Graphene oxide (GO) can easily dissolve in water to form a stable homogeneous solution, which can be used to fabricate graphene films and graphene based composites. This work is the first research to evaluate the stability of graphene oxide solution. It has been found that the introduction of strong electrolytes (HCl, LiOH, LiCl) into GO solution can cause GO precipitation. This indicates that the electrostatic repulsion plays a critical role in stabilizing aqueous GO solution. Furthermore, the HCl-induced GO precipitation is a feasible approach to deposit GO sheets on a substrate as a Pt-free counter electrode for a dye-sensitized solar cell (DSSC), which exhibited 1.65% of power conversion efficiency.
To explore broad and practical applications, large-scale synthesis with controllable integration of individual graphene sheets is essential. A novel strategy for the synthesis of graphene sheets with three-dimensional (3D) Honeycomb-like structure has been invented in this project based on a simple and novel chemical reaction (Li2O and CO to graphene and Li2CO3). The simultaneous formation of Li2CO3 with graphene not only can isolate graphene sheets from each other to prevent graphite formation during the process, but also determine the locally curved shape of graphene sheets. After removing Li2CO3, 3D graphene sheets with a honeycomb-like structure were obtained. This would be the first approach to synthesize 3D graphene sheets with a controllable shape. Furthermore, it has been demonstrated that the 3D Honeycomb-Structured Graphene (HSG) possesses excellent electrical conductivity and high catalytic activity. As a result, DSSCs with HSG counter electrodes exhibit energy conversion efficiency as high as 7.8%, which is comparable to that of an expensive noble Pt electrode
Visualizing Graphene Based Sheets by Fluorescence Quenching Microscopy
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 afterwards 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 types of substrates. Graphene, reduced graphene oxide, or even
graphene oxide sheets deposited on arbitrary substrates can now be readily
visualized by eye 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.Comment: J. Am. Chem. Soc., Article ASA
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