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70 Graphene – Synthesis, Characterization, Properties and Applications Also Nolan et al [26] have deposited graphene on metal catalysts by this method. Carbon formed from CO in CO2 at around 500°C deposited as nanotubes and encapsulating carbons on a supported Ni catalyst without H2 or as filaments if H2 was present. By a thermodynamic model, they explained how hydrogen in low concentrations controls filament morphology and why equilibrium is shifted from that for graphite during carbon deposition. Carbon deposition reaction rates at low carbon activity in the absence of hydrogen were reported. When hydrogen was present, a series of hydrocarbons formed, as in fuel synthesis (Fischer-Tropsch) chemistry. Surface vinyl species that have been recently shown to be intermediates in Fischer-Tropsch chemistry also polymerized to form graphene. The formation of vinyls from CO and H via surface alkyls occurred at a greater rate than methane formation when the supply of hydrogen was limited. Hydrogen from the bulk catalyst metal (not surface adsorbed) hydrogenates the surface alkyls, indicating that hydrogen solubility may control the metal-catalyzed formation of various hydrocarbons and eventually solid graphitic carbon. 2.4 High Temperature High Pressure technique Graphene has been produced by a high pressure-high temperature (HPHT) growth process from the natural graphitic source material by utilising the molten Fe-Ni catalysts for dissolution of carbon [27]. The method may lead to a more reliable graphene synthesis and facilitate its purification and chemical doping. 2.5 Sputtering During the co-sputtering of C and Cu into a carbon matrix [28], a demixing (segregation) occurs of the carbon and copper species due to their very low solubilities that leads to the formation of nanometric copper precipitates homogeneously distributed in a more or less graphitic matrix. These precipitates have an elongated shape in the direction of the thin film growth. When the deposition was performed at 273 K for copper atomic concentrations CCu > 55%, as well as for all thin films synthesized at 573 K whatever the CCu value, the formation of graphene layers parallel to the surface of the copper precipitates was observed so that an encapsulation of the Cu aggregates in carbon cages occurs. 2.6 Wet chemistry and sonication Graphene-like carbon sheets can be synthesized, for instance, from adamantane in the solution phase at ambient temperature. Adamantane, C10H16, has a tetracyclic ring structure with four cyclohexanes in chair conformation. The two dimensional carbon structures have been obtained by introducing ferrocene as the catalyst precursor and adamantane as the carbon source under sonication [29], proving that cyclohexane structures in adamantane can serve as a building block for graphene formation. The synthesized carbon sheets were characterized and confirmed by X-ray diffraction, high-resolution electron microscopy and atomic force microscopy. Also, nanostructured CNx thin films were prepared by supersonic cluster beam deposition (SCBD) [30]. Films containing bundles of well-ordered graphene multilayers, onions and nanotubes embedded in an amorphous matrix were grown alongside purely amorphous films by changing the deposition parameters. Graphitic nanostructures were synthesized without using metallic catalysts.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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