INTRODUCTION
On basis of two-dimensional nature, single atom thickness, and
unprecedented properties, graphene gains the honor “wonder material”
with great potential for various applications from composites to
electronics. Although prominent progress has been made in the last two
decades, graphene still faces some challenges for advancing its
practical applications. A major one is to massively acquire high-quality
but low-cost graphene in a green and efficient manner. For that matter,
the bottom-up strategy that resorts to chemical synthesis to produce
graphene is not under consideration for its production-scale limitation
and high-cost at the current stage of technique
development.1,2 By contrast, the top-down one turns to
mechanical exfoliation of graphite to get graphene and/or FLG,
exhibiting a high feasibility for cost-efficient massive
production.3 Of the developed methods under top-down
strategy, LPE arouses a particular interest.4,5Besides possessing the general advantages of top-down strategy, LPE
renders as-exfoliated FLG directly forming a stable uniform dispersion,
greatly facilitating integration of FLG into various practical systems
through diverse dispersion-based processes. LPE has accordingly become
one of the most extensively explored methods for producing graphene.
Any energy source capable of providing an energy density high enough to
overcome the cleavage energy of graphite (35−61 meV/atom)6 is competent for FLG exfoliation. Under this
guideline, the probe/bath sonicators that can afford thousands of W
L-1 of energy densities have been extensively employed
for exfoliation of graphite.5,7 It is a pity, however,
that the sonication-driven exfoliation method is not practicable for
scale production. To achieve >1.0 mg mL-1of as-exfoliated FLG dispersion, one has to significantly extend the
sonication time (as long as hundreds of hours), which induces not only a
very low production rate (down to 3.1 mg h-1) but a
remarkable amount of defects built upon the graphene basal
plane.3-5,8-10 What’s more, the exfoliation process is
strongly influenced by many other factors, such as vessel geometry,
vessel position in bath, water level of bath, and energy output of
sonicator that usually deviates from the rated value. This makes poor
reproducibility become another shortcoming for scale-up of
sonication-driven LPE. Against this background, a fluid dynamics-based
LPE performed by a vortex fluidic device, jet cavitation device,
rotor-stator mixer, or even kitchen blender emerges
recently.7,11-13 In such a process, graphite moves
fast with liquid and can be repeatedly exfoliated at different positions
of the vessel. Relative to sonication exfoliation, this exfoliation way
has a higher possibility to get basal defect-free FLG because the energy
density deposited on graphene basal plane is much lower than that
exerted by sonicator,7 thus making it a promisingly
efficient process for mass production of high-quality FLG. Of all the
hired devices for driving fluid dynamics, the rotor-stator mixer is of
particular concern.7,14-16 As an easily available
high-shear mixer, it has been industrially employed for many
applications, such as dispersing nanoparticles in liquids. When the
shear rate () is higher than 104s-1, it was shown to be also applicable for
exfoliation of graphite into FLG.7 With the optimized
processing parameters, the FLG production rate reaches 5.3 g
h-1, far higher than any reported work, and may be
further improved to 100 g h-1 by scaling up the liquid
volume to 10 m3. Accordingly, the rotor-stator
mixer-mediated shear exfoliation is regarded as, even if not exclusive,
one of the most industrially feasible methods for mass production of
defect-free FLG.7
Considering that water is a low-cost and eco-friendly solvent and
aqueous FLG dispersion has many important applications, such as
water-soluble polymer composites and conductive
coatings,17,18 it is highly desirable for FLG
exfoliation to be performed in aqueous liquid. However, the big surface
energy mismatch is a huge hurdle to exfoliate and disperse FLG in water.
A forthright solution is to utilize a water-soluble stabilizer. By far,
various surfactants, 19-21 aromatic
compounds,22-24 polymers,25-36 and
even inorganic nanoparticles 37-39 have been explored
as stabilizers. Among them, polymer stabilizers attract specific
attention. Compared with small molecules, polymers having longer chains
may provide more active sites to solvate nano substance and higher
steric volume to push nano substance away from each other for
stabilization. In fact, many research efforts have revealed that
polymers are assuredly more efficient than small molecules in achieving
higher FLG concentrations.21-23 Additionally, it is
different from most of small molecules, which often has adverse effects
on the performances of FLG-derived products, that polymers act
concurrently as a useful component in subsequent applications of aqueous
FLG dispersion, contributing some functionalities, such as chemical
sensing,25 enzyme immobilization,30and DNA recognition.35 These merits promote polymers
to come into the most prevalent stabilizers for FLG exfoliation.
As an effort to advance the practical applications of graphene, a
polymer stabilizer-assisted, rotor-stator mixer-mediated shear
exfoliation method was developed for efficient production of
high-quality FLG in aqueous liquid. For this,
VIB-co -VI-co -Py (Figure 1), a water-soluble
vinylimidazole-based polymer,25 was selected as
stabilizer. It has been previously proven a successful stabilizer in
producing FLG by sonication-driven aqueous phase exfoliation of
graphite.40 Another reason for selection of
VIB-co -VI-co -Py is due to its imidazole rings, which have
bioactivity and pharmaceutical activity and exist in many biomolecules
and drugs. 41 It is reasonably presumed that the
VIB-co -VI-co -Py exfoliated FLG is applicable to the
biomedical uses. In this work, VIB-co -VI-co -Py was
demonstrated also successful in FLG production by shear exfoliation of
graphite. The rotor-stator mixer-mediated FLG exfoliation was executed
in aqueous liquid and discussed from aspects of FLG concentration,
production yield, and dispersion stability. Then the dispersed flakes
were carefully characterized. It was found that the as-exfoliated FLG
forms a stable colloidal dispersion with the production rate up to 0.86
g h-1 by 5.0 L of aqueous liquid. To our knowledge,
this is the highest production rate per volume (0.17 g
L-1 h-1) hitherto for LPE with the
liquid volume higher than 5.0 L. In addition, the shear-exfoliated FLG
shows a high exfoliation level, high quality, and excellent
redispersibility in water. Finally, the as-obtained FLG was demonstrated
to perform well in two typical applications, i.e. flexible
conductive films and polymer composite hydrogels.