Cation-π-Induced Exfoliation of Graphite by a Zwitterionic Polymeric Dispersant for Congo Red Adsorption

Water dispersion remains a technical challenge for effective applications of graphene. In this paper, graphene nanosheets were successfully produced by directly exfoliating expanded graphite in water in the presence of zwitterionic polymeric dispersant, succinic anhydride-functionalized polyethylenimine (PEI-SA). The effects of various parameters, including SA percentage in PEI-SA, dispersant concentration, pH, centrifugation speed, and initial graphite concentration on the exfoliation and stabilization of graphene were studied. Cation-π interaction between the protonated amines in PEI-SA and polyaromatic structure in graphite, together with the zwitterionic property and the steric hindrance of PEI-SA, facilitated the exfoliation and stabilization of graphene. Graphene nanosheets with negligible defects were obtained by dispersant-assisted liquid-phase exfoliation. The concentration of exfoliated graphene suspension under the optimal condition was 0.56 mg/mL with the yield as high as 11.2% and the suspension remained stable for more than 6 months. The stability of exfoliated graphene suspension was a function of pH as determined by UV–vis absorbance, hydrodynamic size, and zeta potential. The exfoliated graphene showed greater potential as an adsorbent for Congo red (CR) removal from water compared to that of pristine graphene.