RESULTS AND DISCUSSION

FLG exfoliation was performed by high-shear mixing of graphite in an aqueous solution of VIB-co -VI-co -Py. In this process, the mixer rotor speed was optimized and fixed at 4,500 rpm (Supporting Information (SI): Section I), corresponding to = 5.6 × 104 s-1 that is safely beyond the minimum required for FLG exfoliation (min ≈ 104 s-1).7 As to centrifugation, an important procedure for collection of FLG, it was fixed at 2,000 rpm/90 min. Previous studies have shown that 500−2,000 rpm/90 min is enough for removing most of unwanted thick particles, and further increasing centrifugation speed induces basal defect-rich FLG sheets concentrated in the supernatant.21,43 Hence, this work on FLG exfoliation centers only on the parameters of VIB-co -VI-co -Py concentration (CP ), initial graphite concentration (CG,i ), and mixing time (tM ).
Using FLG concentration (CG ) as an evaluation index, optimization of CP was performed at the constant CG,i of 3.0 mg mL-1 and tM of 20 min. After the shear mixing and centrifugation separation, a homogeneous colloidal dispersion is obtained regardless of variation ofCP (inset of Figure 2a). In the absence of VIB-co -VI-co -Py, however, the graphitic particles completely settle in water. For fast determination ofCG , a well-developed spectroscopic method 8 was adopted with an absorption coefficient ofα G,660 = 2,751 mL mg-1m-1 (SI: Section II). Figure 2a depicts theCG dependence onCP . It is shown thatCG increases with increasingCP and reaches the maximum of 6.3 μg mL-1 at CP = 2.0 mg mL-1. Further increase ofCP causes the leveling off ofCG . Similar dependence also takes place at CG,i = 25, 50, and 80 mg mL-1, where CG reach the maxima of 0.082, 0.181, and 0.239 mg mL-1respectively at CP = 1.75, 2.0, and 2.0 mg mL-1 (Figure 2b). The criticalCP of ~2.0 mg mL-1 is obviously higher than 0.6 mg mL-1 that was optimized for VIB-co -VI-co -Py in the sonication-driven FLG exfoliation.25 Besides, in sonication exfoliation,CG begins to decrease whenCP exceeds 0.6 mg mL-1. The reason behind these disparities may be due to the different morphology and dimension of VIB-co -VI-co -Py against sonication and shear mixing. As was previously noticed, micelles are formed in aqueous solutions of VIB-co -VI-co -Py and their average sizes (26.3−465.2 nm)