FIGURE
2Dependence of CG onCP withCG,i keeping constant at a) 3.0 mg
mL-1 and b) 25, 50, and 80 mg mL-1.
Inset: photograph of an aqueous FLG dispersion produced withCP = 2.0 mg mL-1 andCG,i = 3.0 mg mL-1(therein the Tyndall effect is shown).
increase with increasing CP (0.1−2.0 mg
mL-1).25 It was well documented that
micelles, especially the large-sized ones, cannot fit into adjacent
nanoparticles.44 As a consequence, the osmotic
pressure of micelles around nanoparticles generates a strong depletion
attraction, causing aggregation of nanoparticles and thus decrease of
the dispersion concentration. In the sonication process, the cavitation
effect makes it possible that no micelles or only small ones are formed
in aqueous solutions until CP is higher
than 0.6 mg mL-1. Under the shear-mixing condition,
however, the critical CP for formation
of micelles with sizes large enough to create effective depletion
attraction seems to be improved to higher than 3.0 mg
mL-1 (maximal CPexamined in this work), owing to serious micelle breakage brought by the
combined high shear, cavitation, and collision
effects.3 Higher CPbut absence of large-sized micelles is helpful to facilitate
exfoliation, dispersion, and stabilization of FLG, particularly in the
cases of higher CG,i . Almost constantCG observed within 2.0−3.0 mg
mL-1 of CP is
attributed to adsorption of polymers on the graphitic particle surface
already reaching saturation and more polymers having little influence on
dispersing FLG.
The aqueous solution of CP = 2.0 mg
mL-1 was then selected as liquid medium to study the
influence of CG,i onCG . As shown in Figure 3a,CG increases monotonously withCG,i in the detected range of 3.0−80 mg
mL-1. Such an increase trend is different from that of
sonication exfoliation, where CG usually
tends to decrease beyond a certain CG,i(10−60 mg mL-1) whose specific value depends on
selection of sonication mode and water-soluble
stabilizer.25,30,45 It is well established that sound
cavitation is the primary driving force for sonication-assisted FLG
exfoliation.3 Too highCG,i may block the sound propagation and
thus weaken the exfoliation efficiency. In the case of shear
exfoliation, however, shearing force dominantly responsible for FLG
exfoliation is long-range relative to sound wave, and its transmission
is little affected by CG,i (at least
within the currently examined CG,irange). Figure 3a also reveals that CGrelates with CG,i in an empirical form
of CG ∝CG,i 1.17. This
power-law correlation was as well observed in sonication exfoliation
with the exponents falling in 0.5−0.8.30,46,47 Much
higher value achieved here (1.17) reflects an obviously faster increase
of CG withCG,i , indicating high-shear mixing more
efficient in FLG exfoliation than sonication. What should be pointed out
is that the exponent of 1.17 is realized by merely 20 min oftM and, meanwhile,CG is up to 0.24 mg
mL-1. In stark contrast, it usually requires several
or even tens of hours to get such a comparableCG by sonication
exfoliation.21,48 Despite monotonous increase ofCG withCG,i , a parabolic-like dependence of
production yield
(CG/CG,i ) onCG,i is noted withCG/CG,i reaching
the maximum of 0.38% at CG,i = 50 mg
mL-1 (Figure 3b), compared with a quasi-linear decline
of CG/CG,i with
increasing CG,i in sonication
exfoliation.25,30,46 This again demonstrates that the
shear exfoliation is superior to sonication exfoliation from the
viewpoint of capacity for improvingCG/CG,i .