Aqueous exfoliated graphene by amphiphilic nanocellulose and its application in moisture-responsive foldable actuators

Graphene is a promising material for diverse applications, such as in composites, optoelectronics, photovoltaic cells, and energy storage devices. However, high-yielding liquid exfoliation of good-quality graphene in high concentrations remains a challenge. In this study, amphiphilic 2,2,6,6-tetramethylpiperidin-1-yl-oxyl (TEMPO)-mediated cellulose nanofibrils (CNFs) were demonstrated in robust aqueous exfoliation of graphite into high quality graphene in high yields and stable dispersions with graphene concentration up to 1 mg mL-1. Over 50% of graphene flakes exfoliated were 3 layers or less, of which ca. 5% were monolayer, and another 47% were multilayers, leaving only 3% as un-exfoliated graphitic platelets. Outstanding yields up to 84.2% were achieved at an optimized 0.2 g g-1 graphite : CNF feed ratio. The dispersed graphitic flakes are stabilized by Coulomb repulsion from the surface bound charged CNFs. Aqueous graphene suspensions stabilized by CNFs were easily vacuum filtered into nanopapers that exhibited rapid moisture triggered motion and spontaneous recovery in the absence of moisture, resembling actions of biological motor cells in "shame plant" leaves. Such unique moisture responsive behavior is attributed to the highly accessible, charged CNF surfaces and the recovery is due to the inherently hydrophobic graphene. This facile aqueous exfoliating approach using amphiphilic CNFs as multi-functional exfoliating, dispersing and structural-forming agents for moisture-responsive graphene nanopaper opens up a large-area of potential applications toward biologically inspired sensors and actuators.