Effective Activation of Melamine for Synchronous Synthesis of Catalytically Active Nanosheets and Fluorescence-Responsive Quantum Dots

Because of the low reactivity of cyclic nitrides, liquid-phase synthesis of carbon nitride introduces challenges despite its favorable potential for energy-efficient preparation and superior applications. In this study, we demonstrate a strong interaction between citric acid and melamine through experimental observation and theoretical simulation, which effectively activates melamine-condensation activity and produces carbon-rich carbon nitride nanosheets (CCN NSs) during hydrothermal reaction. Under a large specific surface area and increased light absorption, these CCN NSs demonstrate significantly enhanced photocatalytic activity in CO 2 reduction, increasing the CO production rate by approximately tenfold compared with hexagonal melamine (h-Me). Moreover, the product selectivity of CCN NSs reaches up to 93.5% to generate CO from CO 2 . Furthermore, the annealed CCN NSs exhibit a CO conversion rate of up to 95.30 μmol/(g h), which indicates an 18-fold increase compared with traditional carbon nitride. During the CCN NS synthesis, nitrogen-doped carbon quantum dots (NDC QDs) are simultaneously produced and remain suspended in the supernatant after centrifugation. These QDs disperse well in water and exhibit excellent luminescent properties (QY = 67.2%), allowing their application in the design of selective and sensitive sensors to detect pollutants such as pesticide 2,4-dichlorophenol with a detection limit of as low as 0.04 µmol/L. Notably, the simultaneous synthesis of CCN NSs and NDC QDs provides a cost-effective and highly efficient process, yielding products with superior capabilities for catalytic conversion of CO 2 and pollutant detection, respectively.