Lignin-assisted direct exfoliation of graphite to graphene in aqueous media and its application in polymer composites
Introduction
Graphene has inspired ever-increasing enthusiasm in the past few years owing to its exceptional chemical and physical properties. It is a promising material for many fields such as nanoelectronics, energy storage and composites [1], [2], [3], while for most of its potential applications it is necessary to mass produce high-quality graphene economically. Micromechanical cleavage of graphite crystals [4] or chemical vapor deposition [5] can yield very high-quality monolayer and few-layer graphene. These methods have, however, disadvantages of low throughput or high cost. A popular low-cost method to prepare graphene in large quantities is chemical oxidation of graphite to graphene oxide (GO) in liquid media followed by thermal or chemical reduction, whereas this process inevitably leaves numerous defects, giving poorer transport properties to the reduced GO (rGO) compared with that given by the physical preparation methods [6], [7]. Moreover, the oxidation/reduction process involves the usage of hazardous and/or corrosive chemicals [8]. Alternatively, direct exfoliation of graphite via sonication in certain organic solvents [9], [10], organic solvent with layered silicate [11], water-surfactant solutions [12], [13], [14], [15] or ionic liquids [16] is considered as a promising low-cost route for the production of high-quality graphene. Besides less destruction to the carbon basal plane and hence without the need for the reduction step, the direct exfoliation is also easily scalable and allows for more precise chemical modification of the graphene [17], [18]. Compared with the organic solvents, exfoliation of graphite in aqueous media is more economical and eco-friendly. Therefore, exploration of a low-cost method to produce high-concentration aqueous dispersions of high-quality graphene is of particular importance.
Lignin, the second most abundant natural polymer after cellulose, accounts for up to 35 wt% of wood [19]. As a low-cost byproduct from pulp and paper industry, currently lignin goes mostly to energy recovery, while utilization of lignin in high-value products is receiving increasing attention. Lignin has been used either directly or after chemical modification as binders, heavy metal sequestrants, photostabilizers or component for composites and copolymers [20], [21]. Recently, it has been reported that lignin peroxidase can efficiently degrade oxidized and reduced graphene oxide nanoribbons [22]. Lignin consists of three types of phenylpropanoid subunits connected in complex ways [19], [21]. This makes it potentially useful for dispersing graphene in aqueous media owing to its amphiphilic nature and the possibility of π–π interaction between lignin and graphene. Indeed it has been reported that sodium lignosulfonate (SLS) is an effective stabilizing agent for reduction of GO in water [23]. This method, however, still includes the oxidation and reduction steps, and direct exfoliation of graphite using lignin-based surfactants has not been realized. Moreover, to tackle environmental concerns, the output of SLS is declining year to year because sulfite pulping process produces more pollutants than kraft pulping process that gives alkali lignin (AL) [20].
In this article, for the first time we report a simple, low-cost and environmentally benign method for preparing high-concentration aqueous dispersions of graphene via AL-assisted direct exfoliation of graphite. The graphene stabilization mechanism is elucidated and good quality of the graphene produced is demonstrated with solid experimental evidence. The results also show that the free-standing graphene thin films prepared from the graphene aqueous dispersions exhibit good electrical conductivity. Furthermore, using epoxy as a model polymer matrix, the graphene prepared using this low-cost approach is proven to be more efficient reinforcing and toughening filler than commercial nano-carbon additives, including single-walled carbon nanotubes (SWCNT) and carbon nanofibers (CNF).
Section snippets
Materials
Natural graphite flake (43319, −10 mesh, 99.9%) was obtained from Alfa Aesar (USA). AL was purchased from TCI America (USA, TCI product number: L0082, softwood lignin) and used without further purification. SWNT and CNF were purchased from Chengdu Institute of Organic Chemistry (China) and Grupo Antolin Ingenieria (Spain). The Epolam 5015 and hardener 5015 used as the epoxy matrix were supplied by Axson (USA).
Preparation of the graphene dispersion and graphene film
A desired amount of AL was added to 500 mL Millipore water in a glass beaker. When AL
Influence of dispersing parameters on graphite exfoliation
Fig. 1a and b shows the photographs of sonicated AL and AL–graphite aqueous dispersions with different AL feed concentrations. Without AL, obviously most graphite particles settle down. By contrast, homogeneous black dispersions are obtained when AL is added even at the AL feed concentration of only 0.01 mg mL−1. The Tyndall effect is observed from all the dispersions, showing that both AL and sonicated AL–graphite dispersions are colloid systems. The effects of AL feed concentration (CAL),
Conclusions
In conclusion, a low-cost, environmentally friendly and efficient method has been developed to prepare graphene through direct exfoliation of graphite in aqueous media using AL as surfactant. Using optimized dispersion parameters, the concentration of graphene dispersion can reach 0.65 ± 0.3 mg mL−1, and the dispersion contains a considerable amount of monolayer graphene. The exfoliation and stabilization mechanism lies in the adsorption of AL on graphene surface via π–π interaction during the
Acknowledgment
This work was supported by Science and Engineering Research Council of the Agency for Science, Technology and Research (A∗STAR) Singapore under Grant No. 11 2300 4027.
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These authors contributed equally to this work.