Exfoliation of graphite oxide was achieved by ultrasonication (Heilscher) of the dispersion (0.1%) at 500 W and 50% amplitude for 30 min. The obtained brown dispersion was then subjected to 30 min of centrifugation at 3,000 rpm to remove any unexfoliated graphite oxide.
Reduction of graphene oxide (GO) was done according to the procedure reported by Stankovich et al. . In a typical procedure for chemical conversion of GO to reduced graphene, the resulting homogeneous dispersion (0.5 g in 500 mL) was mixed with 5.0 ml of hydrazine hydrate solution. After being vigorously shaken or stirred for a few min, the flask was put into a water bath (~100 C) for 24 h. Filtration of the dispersion was achieved through an anodisc membrane filter (47 mm in diameter, 0.2 micrometer pore size, Whatman). Synthetic or natural reduced graphene powder was washed with plenty of deionized water and freeze-dried for 2 days.
TGA of natural and synthetic graphite oxide show ()that major mass loss about 60% occurs at ~200oC which is due to decomposition of oxygen-containing functional groups in the graphite oxide . On the other hand, both chemically reduced natural and synthetic graphene show much increased thermal stability than GO.
Further confirmation of natural and synthetic graphite oxides and graphenes were checked by XPS analysis ().The photoelectron peaks of natural and synthetic graphite oxide were curve fitted with two peaks at 284.7 eV,286.7 eV, assigned to graphitic carbon (C-C) and carbon singly bonded to oxygen (C-O-C and C-O-), respectively. In reduced natural and synthetic graphene case, only one predominant peak is observed that can be attributed to graphitic carbon. The narrow scan C 1s XPS spectra of natural and synthetic graphene are quite similar to that of
Graphite oxide and reduced graphene were characterized by thermogravimetric analysis (TGA) in nitrogen at a heating rate of 1℃ / min from room temperature to 800℃ using TA instruments TGA Q50. Images of reduced graphene were taken at various magnifications using a scanning electron microscope(SEM). Spectra of X-ray Photon Spectroscopy(XPS) were recorded on a physical electronics quantum 2000 scanning ESCA microprobe with AlK α excitation at 15 kV acceleration voltages and 50 W for a probing size of 200 ㎛. The chamber pressure was maintained at 10-8Torr.
Review of the most important methods of graphite chemical oxidation was reported. In all graphite oxides comparatively small amount of carboxyl groups and high amount of hydroxyl groups were observed. Prolonged drying over phosphorus pentoxide at ambient temperature allowed to preserve epoxy groups that easily desorbed at slightly higher temperature. It can be concluded that:
Relatively small amount of the edge carboxyl groups were identified in all spectra. Hydroxyl/epoxy group content is comparable in all graphite oxides. XPS technique confirmed higher amount of the sp2 carbon atoms in GO-H. Though Hummers method is the fastest to obtain graphite oxide the final material possesses unoxidized domains. Atomic composition of graphite oxides was presented in table 2. It should be pointed out that in GO-B very small amount of potassium and chlorine was additionally detected but generally this method results in highly-oxidized graphite with the highest oxygen content and the lowest C/O ratio. Higher amount of nitrogen in form of NO2 and NO3 is compensated by lack of the sulfur contaminations. The lower C/O ratio in GO-H with respect to the GO-S is caused by the 2 % higher content of oxygen in GO-H that was assigned on O 1s spectrum to some oxides and probably is a consequence of humidity.
We perform complete feasability assessments, work with your team to formulate a development proposal and aid in the execution of the plan from pilot production through scale-up to commercial production." Also supplies nanotubes and related graphitic nanostructures."Eikos"develops unique Carbon Nanotube Formulations for Coatings.
Signals broadening and decrease in intensities are caused by crystallites size diminishing. Using Scherrer formula it was calculated that the crystallites height (Lc) decreased after oxidation from 47 nm in graphite to 8.7, 9.3 and 4.6 nm for GO-B, GO-S and GO-H respectively. In all graphite oxides signal at 26 ° disappeared and newer one located close to 10 ° appeared. The interlayer distances for GO-B, GO-S and GO-H were 0.74, 0.75 and 0.73 nm respectively. The qualitative analysis of the oxygen-containing groups was performed by FT-IR ATR. Figure 4 presents infrared spectra that confirmed a lot of oxygen within graphite.
Carbon materials such as graphite and graphene exhibit high electrical conductivity. We examined the electrical conductivity of synthetic and natural graphene powders after the chemical reduction of synthetic and natural graphite oxide from synthetic and natural graphite. The trend of electrical conductivity of both graphene (synthetic and natural) was compared with different graphite materials (synthetic, natural, and expanded) and carbon nanotubes (CNTs) under compression from 0.3 to 60 MPa. We found that synthetic graphene showed a marked increment in electrical conductivity compared to natural graphene. Interestingly, the total increment in electrical conductivity was greater for denser graphite; however, an opposite behavior was observed in nanocarbon materials such as graphene and CNTs, probably due to the differing layer arrangement of nanocarbon materials.
Turbostratic structure of graphite oxide stems from breaking of p-p bonds that can be observed by color change during oxidation. Matuyama  observed that graphite treated with oxidizing mixture of 2:1 sulfuric to nitric acid had a blue color while addition of chlorate changed color to brown or even green. In our experiments very dark blue mixture of GO-S was obtained that turned into dark green after dilution with 10 l of deionized water, however, finally filter cake was brown after drying. Graphite oxide slurry prepared by Brodie method was intensively yellow and darkened to yellowishbrownish after drying. In case of Hummers graphite oxide mixture was at first brown and turned yellow after manganese reduction. Solid product was the darkest from the all graphite oxides. XRD analysis is probably the most convenient method to study oxidation progress. Oxygen embedded between graphene layers causes an increase in the interlayer distance from 0.34 nm in graphite to about 0.7 nm in graphite oxide. Consequently characteristic graphitic signal shift from 26 ° to 10-15 ° is observed. Fig 3 presents XRD patterns for GO-B, GO-S and GO-H.
Carbon materials such as graphite and graphene are versatile, environmentally friendly, and exhibit high electrical conductivity . Graphite is comprised of about three millions layers in one millimeter thickness and an individual layer of graphite is considered as graphene. Although, graphene layers are held together by weak bonds that allow the layers to slide over each other, the large numbers of bonds hold the material together as a solid . Graphene is important for fundamental studies and technological applications due to its unique structure and wide range of unusual properties -. The electronic properties of graphene are strongly dependent upon their structures in such a way that the variation in the number of graphene layer may result in a striking change in their electronic properties . Accordingly, it is important to explore the large scale production of graphene with varying number of layers for their fundamental and extensive applications in many demanding sectors.