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CARBON 47 (2009) 493–499 495 the HRTEM to connect to the graphene to measure its elec- Fig. 3 – The C1s XPS spectra of GO, TEGO, reduced TEGO, and pristine HOPG. The C1s XPS spectrum of GO clearly indicates a considerable degree of oxidation with four components that correspond to non-oxygenated ring C (284.3 eV), C in C–O bonds (286.1 eV), carbonyl C (287.4 eV), and carboxylate carbon (O–C@O, 288.1 eV). tronic conductivity. 3. Results and discussion Fig. 1 shows the typical SEM images of the different starting graphites used in our experiment. HOPG, consisting of a highly ordered stack of graphene layers along the c-axis, has a large lateral size (referring to the particle size along the plane direction perpendicular to the c-axis) of more than 1000 lm. NFG has a plate-like morphology with a size of >600 lm and thickness of several micrometers, and FGP has a mean lateral size of <30 lm. Note that these two types of flake graphite are single crystal graphite with high purity. KG (>200 lm) was obtained by the crystallinization of carbon in molten steel during the manufacturing process. It has a high crystallinity but lower than HOPG and NFG. As to AG, it has an average lateral size of less than 30 lm, and lower crys- tallinity than HOPG and flake graphite but higher than KG. AFM was used to examine the detailed structure of the synthesized samples after spin-coating the supernatant onto a 300 nm SiO2/Si substrate. It was found that most of the graphene shows a relative smooth planar structure (Fig. 2a), similar to that obtained by mircomechanical cleavage [2]. This was further verified by SEM (Fig. 2b) and TEM (Fig. 2c) observa- tions, indicating the high-quality of these graphenes. In addi- tion, XPS measurements revealed that the C/O ratio (10.8– 14.9) of the TEGO reduced by H2 is evidently more than that of GO (1.7–2.5) and TEGO (5.6–7.9), and the corresponding C1s peak structure of the reduced TEGO is similar to that of pristine HOPG, without significant signals corresponding to the C–O species of GO (Fig. 3). These results suggest that the final graphene is reduced by the thermal expansion and reduction processes, without excessive covalent chemical functionalization as in the case of GO. It is worth noting that no significant holes were found in the platelets although most of the functional groups were removed during thermal expan- sion and H2 reduction processes (Fig. 2c). A great number of AFM measurements were performed to determine the number of graphene layers prepared from dif- ferent starting graphite materials. Fig. 4a–c shows the typical AFM images of the graphene derived from KG, FGP, and AG, respectively. In order to determine the number of layers of the obtained graphenes according to the AFM measurements, it is prerequisite to know the thickness and interlayer spacing of the graphenes with different number of layers. It has been theoretically predicted that the interlayer spacing of GO is 0.7 nm, due to a 0.44 nm increase in graphene thickness Fig. 1 – Typical SEM images of (a) HOPG, (b) NFG, (c) KG, (d) FGP, and (e) AG. Fig. 2 – Typical morphology of the graphene derived from AG. (a) AFM image of the graphene on a SiO2/Si wafer, displaying a relative smooth planar structure. (b) SEM image of the graphene hanging over a holey carbon film, showing a flat and transparent structure. (c) TEM image of the graphene.PDF Image | graphene with a pre-determined number of layers
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