Liquid-Phase Exfoliation of Biochars in Green Solvents and Correlation with Solvent Parameters

Liquid-phase exfoliation (LPE) is a process frequently used to yield small sheets of layered materials. These materials are prepared via direct or indirect sonication in an ideal solvent, and the sheets produced often present remarkable chemical and physical properties. Unfortunately, the preferred solvents for exfoliation processes are frequently toxic and possess several health risks. In this work, we show the use of LPE in green solvents to access nanostructures of biochar. Biochar is a material produced after thermochemical treatment of biomass residues and it is an important tool for the sequestration of greenhouse gases. Herein, hardwood and softwood biomass residues (e.g. sludge, bark, and sawdust) are used to prepare pristine and oxidized biochars which are then exfoliated in a range of solvents. Stable dispersions containing up to 75% by weight of exfoliated biochar could be obtained. A range of solvents were screened for LPE of biochars to identify ‘green' options that could afford highly concentrated dispersions. The properties of the biochar before and after exfoliation were evaluated using Raman spectroscopy and Transmission Electron Microscopy. Correlations between effective LPE of biochar in a solvent and different solvent parameters were established. For example, LPE of oxidized biochars is more efficient in hydrogen-bond accepting solvents due to the increased concentration of carboxylic acid and alcohol functional groups within this material, when compared with pristine biochars.