Study of the effect of nitric acid on the structure and optical properties of alkali lignin-based carbon dots

Hydrothermal reactions can convert lignin into carbon dots, and the process often uses acids as additives, but the mechanism of action is not clear. In this study, lignin-based carbon dots were successfully prepared by HNO₃-assisted one-pot hydrothermal method. The mechanism of the influence of the acidic environment on the structure and optical properties of lignin-based carbon dots was also investigated by changing the addition amount of HNO₃. It was found that the particle size distribution of carbon dots collected was 1-5 nm, and they could emit bright blue fluorescence under violet light irradiation with the highest fluorescence quantum yield of 10.17%. HNO₃ acts on the branched chains and ether bonds of alkali lignin, prompting the depolymerization of lignin and re-cross-linking and condensation to form lignin-based carbon dots. With the increase of HNO₃ addition, the carbon core of lignin-based carbon dots gradually transformed from amorphous structure to complete graphene-like structure, and the emission wavelength of lignin-based carbon dots shifted from 517 nm to 499 nm, and the fluorescence quantum yield was increased from 2.61% to 10.17% by the effect of integrated N doping, which is of great significance for the analysis of the conformational relationship of lignin-based carbon dots, and for the guidance of the high-efficiency synthesis of lignin-based carbon dots.