Lignin-assisted direct exfoliation of graphite to graphene in aqueous media and its application in polymer composites

Abstract

In this work, graphene is facilely prepared by direct exfoliation of graphite in aqueous media with the assistance of alkali lignin (AL). The graphite is successfully exfoliated into single- and multi-layer graphene, with about 20% monolayer ones, and the concentration of graphene in the aqueous dispersions can reach 0.65 ± 0.03 mg mL⁻¹. The nanosheets are stabilized in the dispersions owing to π–π interaction between the graphene and AL, and the electrostatic repulsion between negative charged AL, as verified by fluorescence and zeta-potential measurements. The results also show that there are very few defects in the basil-plane of the graphene so that the free-standing graphene films prepared from the dispersions exhibit good electrical conductivity. Finally, the addition of only 0.1 wt% such graphene sheets to an epoxy resin gives 15%, 49%, and 97% enhancement in tensile strength, critical stress intensity factor and critical strain energy release rate, respectively.

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).