Red, yellow, green, and blue light-emitting highly crystallized graphene quantum dots derived from lignin: controllable syntheses and light-emitting diode applications

The synthesis of low-cost and high-efficiency multicolour emitting graphene quantum dots (GQDs) is of utmost importance for their prospective applications. Lignin, a renewable biomass, contains aromatic compounds that are well-suited for the preparation of polycyclic aromatic GQDs. In this study, we successfully synthesized red, yellow, green, and blue light-emitting single-layered GQDs with high crystallization using hydrothermal methods, thereby covering the full spectrum of visible light. High-resolution transmission electron microscopy (HRTEM) images revealed the presence of a two-dimensional crystal lattice with the 100 (0.21 nm) and 002 (0.24 nm) facets of graphite, indicating the high crystallization of the synthesized GQDs. Additionally, the average height of all GQDs was found to be below 1 nm, confirming their single-layered structure. This observation supports the notion that mono-benzene compounds tend to form fused polycyclic arenes under high pressure and temperature conditions. Moreover, by analyzing X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared (FT-IR) spectra, we investigated the formation mechanism of GQDs derived from lignin at the molecular level. The cleavage of ether bonds, aldol condensation, benzylic acid rearrangement, and dehydration were identified as vital intermediate processes. Furthermore, we successfully demonstrated the application of multicolour-emitting GQDs derived from lignin in light-emitting diodes (LEDs). This synthetic strategy for producing multicolour-emitting GQDs from lignin opens up a new path for the conversion of sustainable biomass into high-value nanomaterials. "Lighting up" lignin: synthesis of multicolour-emitting GQDs with high crystallinity from lignin within 12 h and their successful application to multicolour LEDs.