Highly concentrated suspensions of graphene stabilized with surfactant were prepared using ultrasonic exfoliation. Concentrations of up to 1.5% w/w (15 mg/mL) were achieved through the continuous addition of the surfactant during the exfoliation process. Previous methods typically add the surfactant only once, prior to the commencement of sonication. The vast increase in the available solid-liquid interfacial area through delamination results in the rapid depletion of the surfactant from solution through adsorption. This leads to a change in the liquid-vapor surface tension outside of the optimum range for the efficient production of graphene sheets. By continuously replacing the surfactant to lower the surface tension during sonication and the production of the graphene surface area, the concentration of particles was significantly increased. Cationic, anionic, and nonionic surfactants were studied and all showed significant increases in the concentration of graphene produced using this continuous addition method.
Vol. 28, no. 40 (Oct 2012), pp. 14110-14113