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.