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Supercritical Fluid Processing of Graphene and Graphene Oxide 47 edges. This not only promises to deliver handles for exploiting graphene’s intrinsic properties but also should lead to new properties altogether. Interaction of graphene with electron-donor and electron acceptor molecules causes marked changes in the electronic structure and properties of graphene [24, 25]. Thus, electron donor molecules, such as aniline and tetrathiafulvalene (TTF), soften (i.e. shift to lower frequency) the Raman G band of few-layer graphene while electron-acceptor molecules, such as nitrobenzene and tetracyanoethylene (TCNE), stiffen (i.e. shift to higher frequency) the G band. As demonstrated by Subrahmanyam et al. the graphene can be functionalized through noncovalent modification without affecting its electronic structure by wrapping with surfactants or through π-π interaction with a pyrene derivative such as 1-pyrenebutanoic acid succinimidyl ester (PYBS). Recently, we have reported the SCF exfoliation and surface modification of graphene using 1-PSA molecules [30]. The π-π interaction between the graphene layer and aromatic ring of modifier molecules resulted in the modifier-graphene composite without any damages to the graphene surface and its electronic conductivity under SCF conditions. In the present work, we have found that methanol, which is cheaper solvent than that were used in our previous report can also be effective in exfoliation of graphite. This can also yield a high quality and large scale graphene solution, which are stable for several days without any surfactant. In this chapter, we highlight the structural and molecular engineering of graphene and GO by exfoliation, chemical functionalization, and reduction under SCFs. Such structural and molecular engineering of graphene under SCFs plays a crucial role in retaining and restoring the original properties of the resulting graphene sheets. We present the detailed study of the role of SCFs conditions on the exfoliation, covalent and non-covalent functionalization and GO reduction in one pot process. The solvent and molecular engineering under SCFs is of great interest because it enables the attachment of suitable organic molecules through π-π interaction. In this chapter, we are presenting a comparative study on SCF process over other methods that have been used for the exfoliation, surface modification and GO reduction. The different reaction conditions such as type of SCF, reaction time, temperature, solvents and organic molecules ratio had the significant effect on the exfoliation and surface modification of the graphene. We have also noticed the influence of some reaction conditions on the structure and electronic properties of the graphene or reduced GO. 2. Experimental section 2.1 Preparation of graphene by supercritical methanol The preparation of graphene by supercritical methanol was similar to that reported in our previous study. First we pretreated graphite flakes with dilute sulphuric acid and nitric acid. The exfoliations were performed in a stainless steel reactor having a maximum volume of 10 ml. In a typical experiment, 30 mg of pre-treated graphite crystals were taken in a stainless steel reactor vessel and dispersed in 5 ml methanol by low power sonication(AS ONE US cleaner, US-4R , 40kHz, 160 W) for 10-minutes. Then, the sealed reactor vessel was heated up to 300-400 oC for 30-60 minutes in a special designed tube furnace (AKICO, Japan). The reactor reaches optimum temperature within the 3 minutes, therefore, the reaction time mentioned above includes ramp up time. The pressure was maintained at 38- 40 MPa by adjusting the volume and temperature of the reactor vessel. The reaction was terminated by submerging the hot reactor in an ice-cold water bath. The exfoliated GS were collected by repeatedly washing and centrifuging with fresh solvents and vacuum dried over night at 100 oC.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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