Green Solvents for the Liquid-Phase Exfoliation of Biochars
Juliana L. Vidal, Stephanie M. V. Gallant, Evan P. Connors, D. Douglas Richards, Stephanie MacQuarrie, Francesca M. Kerton
Abstract
Exfoliation can be used to weaken and break van der Waals interactions within layered materials and produce small monolayered counterparts with remarkable properties. In the current study, liquid-phase exfoliation (LPE) using ultrasound and organic solvents is explored as a method to break down layered structures in biochars. Unfortunately, preferred solvents that can effectively disperse and stabilize the sheets produced during exfoliation often possess several health risks. In this work, we show that LPE in greener solvents can be used to access nanostructures of biochars to further improve the applications of this biobased material. Herein, pristine and oxidized biochars are exfoliated in a range of solvents to allow the identification of benign alternatives, which have been classified as nonhazardous or less hazardous by various solvent guides. The majority of biochar nanostructures produced consists of stacked nanosheets containing between two and eight layers with 15 nm thickness in average. Correlations between the LPE of biochars and different solvent parameters are established, and surface modification of biochars has potential to increase their exfoliation in more benign solvents. The LPE of oxidized biochars is more efficient in hydrogen-bond-accepting solvents due to the increased concentration of functional groups on their surface. Dispersions containing 0.20–0.75 mg/mL exfoliated oxidized biochars were obtained in solvents such as polyethylene glycols and ε-caprolactone. The LPE of pristine biochars in dimethyl carbonate and ethyl acetate gives similar yields to the most commonly used solvent for this process, N-methyl-2-pyrrolidone.