Graphene Sheets from Graphitized Anthracite Coal: Preparation ...

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Energy & Fuels Article Figure 4. XPS spectra C1s peaks of coal-based graphite oxides and graphene sheets: TX-C-GO (a), TX-C-GS (b), TX-NC-GO (c), and TX-NC-GS (d). place during the oxidation process; therefore, the driving force resulting from gases (H 2O and CO 2) formed during DBD is not strong enough to overcome interlayer van der Waals force for complete expansion. Typical morphologies of TX-NC-GS and TX-C-GS are shown in Figure 2. It is revealed that layer structures were formed after expansion via DBD plasma (Figure 2c,f). Highmagnification SEM images (Figure 2d,g) of GS show a wrinkled, thin, sheetlike structure similar to that of graphene prepared from natural flake graphite by thermal expansion 22 and sonication-assisted chemical reduction. 9 TEM images (Figure 2e,h) show that these sheet structures have plenty of ripples and folded regions in their basal planes and seem to be transparent, possibly owing to their being ultrathin or even single sheets. It is obvious that the layer of TX-NC-GS appears to be thicker than that of TX-C-GS due to the incomplete exfoliation of the former. This is consistent with the volume change observation during plasma-assisted exfoliation, as shown in Figure 2a,b. There are a variety of oxygen-containing groups on the surface of TX-NC-GO and TX-C-GO (FTIR spectra; see Figure 3a), which mainly include the C�O, C−O, and O−H. After H 2 plasma reduction, the peaks for oxygen functional groups were significantly reduced with fewer C�O groups left in TX-NC-GS and TX-C-GS compared with their corresponding precursors. Generally, the wide peak at 1100−1420 cm −1 could mainly be assigned to the �C−H stretch, and the peak at 1583 cm −1 could be assigned to the aromatic C�C stretch. According to the FTIR spectra of coal-based graphite oxides, the C�C bond almost cannot be detected, while there is a visible C�C peak located at 1583 cm −1 in their GS. The typical peak of C�O located at 1731 cm −1 of TX-NC-GS is 5189 stronger than that of TX-C-GS, which may indicate that the former has more oxygen-containing groups. The coal-based graphene sheets were further characterized via N 2 adsorption and desorption at 77 K for deeply understanding their pore structure. According to the results, the BET surface area values of TX-C-GS and TX-NC-GS are 306 and 135 m 2 /g, respectively. As shown in Figure 3b, the isotherms have the characteristics of type IV with a type H3 hysteresis loop at relatively high pressure, revealing that these materials are composed of aggregated sheets (loose assemblages) possessing typical mesopore structures, corresponding with distributions of the most probable pore sizes of 3.69 nm (TX-NC-GS) and 4.02 nm (TX-C-GS), as shown in the inset of Figure 3b and the microscopy observations mentioned above. The significant structural changes occurring during the transformation from coal-based graphite to the GS are reflected in their Raman spectra (Figure 3c). As known, the G band corresponds to the first-order scattering of the E 2g mode observed for sp 2 carbon domains, and the pronounced D band is the disordered band associated with structural defects, amorphous carbon, or edges that can break the symmetry and selection rule. 23 The Raman spectra of TX-NC-G and TX-C-G display a prominent G peak as the feature at 1581 cm −1 . The strong 2D peak at 2663 cm −1 indicates that the Taixi coal has been transformed into graphite-like carbon after graphitization treatment. As expected, the I D/I G ratio of TX-C-G is smaller than that of TX-NC-G, which indicates that TX-C-G possesses higher graphitization degree than TX-NC-G, consistent with the results obtained from XRD. From the Raman spectra of coal-based graphite oxides (TX-NC-GO and TX-C-GO), it can be seen that the G bands seem to be broadened compared with dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192
Energy & Fuels Article Figure 5. Electrochemical performance of coal-derived graphene sheets. (a) Cyclic voltammetry curves of GS with the scanning rates of 5 mV/s. (b) Specific capacitance against different discharge current density with 6 M KOH solution as an electrolyte. (c) Galvanostatic charge/discharge curves with the current density of 100 mA/g. (d) The cycling performance at a charge/discharge current of 100 mA/g. their precursors, meanwhile, the D bands at 1337 cm −1 become much more prominent, indicating the reduction in size of the in-plane sp 2 domains, possibly due to the extensive oxidation. 10 The Raman spectra of the TX-NC-GS and TX-C-GS also contain both G and D bands (at 1590 and 1350 cm −1 , respectively), however, with a decreased I D/I G intensity ratio compared to that in TX-NC-GO and TX-C-GO, which suggests an increase in the average size of the sp 2 domains and restoration of sp 2 network during reduction of GO in H 2 discharge plasma process. 24,25 It should be noted that no obvious 2D bands can be observed in the Raman spectra of TX- NC-GS and TX-C-GS due to the disordered structure (defects, vacancies, or distortions of the sp 2 domains) caused by the use of strong acid (H 2SO 4) and strong oxidant (KMnO 4) during the oxidation stage. We can reasonably believe that it is very difficult for the graphene sheets to retrieve the graphitic structures through DBD plasma treatment. Compared with TX- NC-GS, the D band of TX-C-GS is weaker and wider, which suggests fewer structural defects in the latter than the former. As mentioned above, it can be concluded that the crystallinity of coal-derived carbon remarkably affects the structures and the properties of the graphene obtained from chemical exfoliation. 26 The oxygen-containing functional groups of TX-NC-GO, TX-NC-GS, as well as TX-C-GO and TX-C-GS were characterized by X-ray photoelectron spectroscopy (XPS). The high-resolution C1s spectrum of the TX-C-GO (Figure 4a) and TX-NC-GO (Figure 4c) reveal that there are three main components arising from C�C (aromatic rings), C−O (alkoxy and epoxy), and C�O (carboxyl) groups. After DBD plasma treatment, carbon with different chemical valences remains for TX-NC-GS and TX-C-GS in XPS spectra; however, the peak intensities of them are much smaller than those in TX- 5190 NC-GO and TX-C-GO. As shown in Figure 4b,d, there are no O−C�O groups left in TX-NC-GS and TX-C-GS and the C�C bonds become dominant, while the peak intensities of C�O and C−O are obviously lower than that of TX-NC-GO and TX-C-GO, which indicates that most of the oxygencontaining groups in TX-NC-GO and TX-C-GO have been removed and the majority of the conjugated graphene networks are restored 27 by DBD plasma treatment for only 5 min. The residual oxygen-containing groups on the graphene sheets will benefit the wettability of the electrode in supercapacitor and metal or metal oxide catalyst grafting for heterogeneous catalysis. However, compared with TX-C-GS, it is worth noting that there are more significant C�O and C−O peaks left in the XPS spectrum of TX-NC-GS, which is consistent with the FTIR results. The electrochemical properties of these coal-derived graphene were measured in a 6 M KOH aqueous electrolyte with a three-electrode supercapacitor cell at room temperature by a CHI 660D electrochemical workstation. Figure 5a shows the CV measurement results of TX-NC-GS and TX-C-GS at the scanning rate of 5 mV/s. The CV curve of TX-C-GS exhibits a more typical rectangular shape than that of TX-NC- GS, indicating that the TX-C-GS has a better capacitive behavior and charge propagation at the surface of electrode following the electric double layer charging mechanism compared with TX-NC-GS. The specific capacitances of TX- NC-GS and TX-C-GS against the discharge current density are shown in Figure 5b, which was calculated from the galvanostatic charge and discharge curves at different current density, using the following equation 28 C = IΔt/ mΔV dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192
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