High-concentration aqueous dispersions of graphene produced by exfoliation of graphite using cellulose nanocrystals Pedro M. Carrasco a , Sarah Montes a , Ignacio Garcı ´a a , Maryam Borghei b , Hua Jiang b , Ibon Odriozola a , Germa ´ n Caban ˜ ero a , Virginia Ruiz a, * a New Materials Department, IK4-CIDETEC, Paseo Miramo ´n 196, E-20009 San Sebastia ´ n, Spain b Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland ARTICLE INFO Article history: Received 18 September 2013 Accepted 27 December 2013 Available online 7 January 2014 ABSTRACT Stable high-concentration aqueous dispersions (>1 mg ml 1 ) of single and few-layer graphene flakes were produced by direct exfoliation of graphite using cellulose nanocrys- tals (CNC). Biodegradable and widely available from renewable sources, CNC have proven to be very efficient graphene stabilizers even at low concentrations (0.2 mg ml 1 ), thus enabling remarkably high graphene/CNC ratios (up to 3.8). Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Producing processable graphene flakes in large quantities remains an ongoing challenge for large-scale applications. To that end, top-down colloidal approaches such as the exfo- liation of powdered graphite in the liquid phase is a very promising route due to its simplicity, utilization of low-cost and readily available graphite, high-throughput potential, possibility of integration with other processes such as blend- ing or casting and no need of transferring processes from the growth substrate [1,2]. The most common route for liquid- phase exfoliation (LPE) involving graphite functionalization is graphite oxidation followed by exfoliation to yield graphene oxide, which is subsequently reduced to obtain reduced graphene oxide whose properties differ from pristine graph- ene. On the other hand, direct exfoliation of unfunctionalized graphite by sonication and good dispersion of resulting graphene sheets require the use of stabilizers that interact noncovalently with graphene and prevent their stacking. Var- ious stabilizers have been proposed both in organic and aque- ous media such as ionic and non-ionic surfactants [3–6], polymers [7,8], organic salts [9] and aromatic molecules [10–13]. Direct exfoliation and stabilization in aromatic solvents [14] or ionic liquids [15] has also been demonstrated yet it suffers from low throughput and poor reproducibility. Despite its capability for large-scale processing, in many cases the resulting dispersions of graphene from LPE are at low concentrations (in the order of 0.1 mg ml 1 ) or require long-lasting sonication times (up to several days). Therefore, considerable effort has been recently focused on developing more efficient exfoliation routes capable of yielding stable high concentration graphene dispersions with minimal amount of stabilizer to preserve graphene properties [16,17]. In this regard, attaining high exfoliation degrees by means of exfoliating agents and stabilizers that are widely available, low-cost, environmentally-friendly and biodegradable would definitely place LPE one step closer towards industrial viability. To this end, we demonstrate here that cellulosic nanoma- terials, specifically cellulose nanocrystals (CNC) are very effi- cient for graphite exfoliation by tip sonication and stabilization of resulting graphene flakes in aqueous disper- sions at high concentrations. CNCs can be regarded as very promising graphene stabilizers due to their interesting fea- tures such as low density, high surface area, good mechanical properties, biodegradability and availability from renewable 0008-6223/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.12.086 * Corresponding author. E-mail address: vruiz@cidetec.es (V. Ruiz). CARBON 70 (2014) 157 – 163 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon