FIGURE 8 a) TEM image of FLG-P-80 hybrid (scale bar − 500 nm), b) SAED patterns of (i) circled and (ii) framed flakes in a), and c) intensity analyses along the dashed lines in b).
frequently seen in our FLG sample. It seems that the adsorbed pyrenyl-containing polymers promote the folding as a result of π-π interaction with the other graphene surface. The selected area electron diffraction (SAED) patterns exhibit a diagnostic six-fold symmetry expected for graphene (Figure 8b), meaning that shear mixing induces few lattice defects in the graphene framework.55 Further, the SAED pattern can be used for determination of the layer number of flake. As previously proposed, the flake with stronger {1100} diffraction spot than {2110} one is single-layer graphene, while one with weaker {1100} spot is FLG.8 According to this proposition, the circled flake (Figure 8a) that has an intensity ratio of {1100} to {2110} (I{1100}/I{2110} ) of 1.6 (Figure 8c-i) is recognized as a single-layer graphene; the framed flake (Figure 8a) having anI{1100}/I{2110}value of 0.5 (Figure 8c-ii) is identified as a FLG.
The quality (defect content) of FLG-P-80 hybrid was assessed by Raman spectroscopy. For comparison, graphite was also evaluated in parallel. Each spectrum given in Figure 9 is averaged from five spectra collected from different locations on a 1.0 cm diameter sample. The defect content is defined as the D-to-G band intensity ratio, denoted generally asID/IG . It is noticed thatID/IG ofFLG-P-80 increases to 0.30 from 0.05 for graphite, suggesting some new defects having been introduced. Even so, this value accords with that of stabilizer- assisted sonication- and other shear-exfoliated FLG (ID/IG = 0.10−0.80) 8,12,15,25,43 and is much lower than that of rGO reduced by hydrazine (ID/IG = 1.44)56 or sodium borohydride (ID/IG = 1.08).57 It is well-known that rGO includes a high content of basal and edge defects resulting from severe oxidation treatment. A small increase ofID/IG ofFLG-P-80 thus predicts that the shear-exfoliation process adopted here produces very few basal defects and merely moderate quantity of edge ones. Besides being confirmed by AFM and TEM (Figures 7a and 8), this prediction is endorsed by the Raman study onFLG-P-20 hybrid that was prepared from the dispersion produced at tM = 20 min. Compared withFLG-P-80 hybrid, a lower defect content of 0.14 is noticed in the FLG-P-20 hybrid, exactly corresponding with the larger lateral size of FLG in its master dispersion (Figure 4b) and thus less edges per unit mass. The 2D-band is another Raman spectral feature, which exhibits the different shapes between graphite and FLG-Phybrids. A sharp peak (2680 cm-1) followed by a shoulder one (2625 cm-1) is observed on the graphite sample, but only a relatively broad single peak (2670 cm-1) appears in the spectra of FLG-Phybrids. The observations manifest once again that the exfoliated graphitic materials are few-layer graphene.58