Hydrothermal formation mechanism of lignocellulose-derived carbon dots: From sawdust to carbon nanomaterials with blue fluorescence

The conversion of lignocellulose into fluorescent lignocellulose-derived carbon dots (LCDs) via hydrothermal carbonization (HTC) has attracted significant attention. However, the degradation pathways of lignocellulose components, their contributions to LCDs formation, and the temporal evolution of LCDs' formation remain unclear. Given that HTC conditions influence LCDs' structure and fluorescence properties, an orthogonal design was applied to investigate these effects. Results show that HTC temperature and feedstock type are two key factors due to the diverse hydrolysis behaviors of components. A temperature of 190 °C effectively balances fluorescence clusters' generation and consumption, and the softwood, Pinus sylvestris var. mongolic, with its high hemicellulose and G-type lignin content, is an ideal feedstock. During HTC, LCDs are formed with a carbon core of conjugated cyclic structures from cross-linked, polymerized, and carbonized carbocyclic compounds, and a shell of polymer clusters from aliphatic hydrolysates. Here, hemicellulose and amorphous cellulose contribute to aliphatic and carbocyclic compounds, imparting hydrophilicity and fluorescence, while lignin supplies carbocyclic compounds essential for fluorescent carbon cores. As conjugation increased, the maximum fluorescence quantum yield (FQY) of LCDs reached 3.4 %. This work offers a theoretical basis for feedstock selection and explains the temporal formation process of LCDs, which is of great significance for the high-value utilization of lignocellulosic biomass and the regulation of LCDs' structures and optical properties.