A Direct Observation of the Fine Aromatic Clusters and Molecular Structures of Biochars

Biochar is a promising carbon material to improve soil quality and relieve global warming by adding it into soil. Though biochar has been used for millennia as a soil ingredient, fundamental studies on the structure of biochar have only recently attracted great interest, and molecular-level structures are being pursued. In this study, by applying solvent exfoliation, high-resolution transmission electron microscopy with Cs-correction and quadrupole time-of-flight mass spectroscopy, we report, for the first time, an atomic-level observation of the fine aromatic cluster and molecular structure of the exfoliated biochars. The observed aromatic clusters are generally smaller than 5 nm and decorate the graphene-like aggregates at different depths. The Stone-Wales transformation in the dispersed biochars was observed, which is critical to form carbon nanotubes, fullerenes, and other carbon nanostructures in the natural environment. In addition, the C-C bond length decreases as the pyrolysis temperature increases from 500 to 700 degrees C, indicating that aliphatic carbon transforms into aromatic carbon. The quaternary structure of biochar, including heterogeneous phases, graphene-like aggregates, nanosized aromatic clusters, and atomic arrangements, is concepfualized to enhance our knowledge of the biochar structure, which greatly aids in understanding the novel functions of biochar and offers a molecular guide for potential applications of biochar.