The conductivity of graphite oxide films is modulated using reducing agents. It is found that the sheet resistance of graphite oxide film reduced using sodium borohydride (NaBH4) is much lower than that of films reduced using hydrazine (N2H4). This is attributed to the formation of CN groups in the N2H4 case, which may act as donors compensating the hole carriers in reduced graphite oxide. In the case of NaBH4 reduction, the interlayer distance is first slightly expanded by the formation of intermediate boron oxide complexes and then contracted by the gradual removal of carbonyl and hydroxyl groups along with the boron oxide complexes. The fabricated conducting film comprising a NaBH4-reduced graphite oxide reveals a sheet resistance comparable to that of dispersed graphene.

Efficient Reduction of Graphite Oxide by Sodium Borohydride and Its Effect on Electrical Conductance

Percolation and Bruggeman's effective media theories, as they apply to the electrical conductivity of composites, are reviewed, and a general effective media (GEM) equation, which combines most aspects of both percolation and effective media theories, is introduced. It is then shown that the GEM equation quantitatively fits electrical resistivity (conductivity) as a function of the volume fraction data for binary composites. The parameters used to fit the experimental data are the electrical resistivities of the two phases, the percolation threshold for the lower resistivity phase (oc), and an exponent t. Preliminary work, showing how the GEM equation can be used to model the piezoresistivity of composites by postulating that oc is a function of the independent variable, is also presented.

Electrical Resistivity of Composites

Carbon nanostructures have emerged as likely candidates for a wide range of applications, driving research into novel synthetic techniques to produce nanotubes, graphene and other carbon-based materials. Single sheets of pristine graphene have been isolated from bulk graphite in small amounts by micromechanical cleavage1, and larger amounts of chemically modified graphene sheets have been produced by a number of approaches2,3,4,5,6,7. Both of these techniques make use of highly oriented pyrolitic graphite as a starting material and involve labour-intensive preparations. Here, we report the direct chemical synthesis of carbon nanosheets in gram-scale quantities in a bottom-up approach based on the common laboratory reagents ethanol and sodium, which are reacted to give an intermediate solid that is then pyrolized, yielding a fused array of graphene sheets that are dispersed by mild sonication. The ability to produce bulk graphene samples from non-graphitic precursors with a scalable, low-cost approach should take us a step closer to real-world applications of graphene. Most techniques for producing graphene use graphite as a starting material and are labour-intensive. The direct chemical synthesis of carbon nanosheets in gram-scale quantities from the common laboratory reagents ethanol and sodium has now been demonstrated. The ability to produce bulk graphene samples from non-graphitic precursors with a scalable, low-cost approach should take us a step closer to real-world applications of graphene.

Gram-scale production of graphene based on solvothermal synthesis and sonication

Low-cost, high-yield production of graphene nanosheets (GNs) is essential for practical applications. We have achieved high yield of edge-selectively carboxylated graphite (ECG) by a simple ball milling of pristine graphite in the presence of dry ice. The resultant ECG is highly dispersable in various solvents to self-exfoliate into single- and few-layer (≤ 5 layers) GNs. These stable ECG (or GN) dispersions have been used for solution processing, coupled with thermal decarboxylation, to produce large-area GN films for many potential applications ranging from electronic materials to chemical catalysts. The electrical conductivity of a thermally decarboxylated ECG film was found to be as high as 1214 S/cm, which is superior to its GO counterparts. Ball milling can thus provide simple, but efficient and versatile, and eco-friendly (CO2-capturing) approaches to low-cost mass production of high-quality GNs for applications where GOs have been exploited and beyond.

/pdf/edge-carboxylated-graphene-nanosheets-via-ball-milling-3e9sldaxvt.pdf

Edge-carboxylated graphene nanosheets via ball milling

Graphene oxide (GO) has attracted intense interest for its use as a precursor material for the mass production of graphene-based materials, which hold great potential in various applications. Insights into the structure of GO and reduced GO (RGO) are of significant interest, as their properties are dependent on the type and distribution of functional groups, defects, and holes from missing carbons in the GO carbon lattice. Modeling the structural motifs of GO can predict the structural evolution in its reduction and presents promising directions to tailor the properties of RGO. Two general reduction approaches, chemical and thermal, are proposed to achieve highly reduced GO materials. This review introduces typical chemical oxidation methods to produce GO from pure graphite, then summarizes the modeling progress on the GO structure and its oxidation and reduction dynamics, and lastly, presents the recent progress of RGO preparation through chemical and thermal reduction approaches. By summarizing recent studies on GO structural modeling and its reduction, this review leads to a deeper understanding of GO morphology and reduction path, and suggests future directions for the scalable production of graphene-based materials through atomic engineering.

/pdf/graphene-oxide-and-its-reduction-modeling-and-experimental-29kq1c0ost.pdf

Graphene oxide and its reduction: modeling and experimental progress

The conductivity of four graphite powders was measured as a function of the volume fraction of graphite (or air) along (axial) and across (transverse) the direction of compression. The conductivity varies by between two and four orders of magnitude during these measurements. The results are shown to fit an equation which expresses the conductivity of the mixture as a function of the conductivity of the graphite and two morphology parameters. One parameters is the critical volume fraction, at which the graphite-air mixture would become insulating (percolation threshold) and the other an exponent is a combination of the effective demagnetisation coefficients of the grains and the critical volume fraction. Electron micrographs of the powder grains are shown and volume orientation percentage measurements of the graphite using X-rays were made in order to try to understand the results better.

http://iopscience.iop.org/article/10.1088/0022-3727/21/1/015/pdf

N Deprez

Papers

The analysis of the electrical conductivity of graphite conductivity of graphite powders during compaction