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)