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