Non-thermal effect of microwave on the chemical structure and luminescence properties of biomass-derived carbon dots via hydrothermal method

Highlights: • Microwave non-thermal effect was confirmed in the preparation of carbon dots. • Microwave non-thermal effect promoted the formation of more luminescent structures. • Microwave non-thermal effect led to stronger luminescence and red shifts of the absorption and emission. • Microwave non-thermal action would not disappear with the change of reaction condition. Carbon dots (CDs) is a burgeoning nanomaterial; in particular, those derived from biomass are more sustainable, economical and eco-friendly. The microwave-assisted hydrothermal (MHTC) method was increasingly used to prepare biomass-based CDs as a new green technology. However, the non-thermal effect of microwave remains controversial. In this work, the microwave non-thermal effect in MHTC preparation of CDs from soymeal (S-CDs) was confirmed through comparative experiments between microwave-assisted and conventional hydrothermal (HTC) method. S-CDs prepared by the MHTC method obtained higher relative proportions of self-doping nitrogen, pyridine N, amine N, quaternary N and conjugate sp² C than that by the HTC method for the microwave non-thermal effect, which led to stronger luminescence and red shifts of the absorption and emission. The optimization effect of microwave non-thermal action on S-CDs structure and luminescence performance would not disappear with reaction temperature and residence time. The luminescence intensities of S-CDs obtained by the MHTC method were all stronger than that by the HTC method among the different reaction conditions. The performance optimization caused by microwave non-thermal effect could be briefly explained as macroscopic luminescence enhancement due to more fluorescent carbon nano-particles and microscopic luminescence enhancement due to the formation of key luminescence chemical structures.