Synthesis of carbon quantum dots using unutilized low-grade lignite coal

The large deposits of unutilized coal in Nigeria present both environmental and economic challenges, including disposal issues and underutilization of a potentially valuable resource. A resource recovery approach, such as converting low-grade coal into high-value products, can offer a sustainable solution with industrial scalability and innovation in nanomaterial synthesis. In this study, carbon quantum dots (CQDs) were synthesized via a one-step oxidation process using lignite coal from Enugu, Nigeria. Elemental analyses (EDX and XPS) confirmed that the CQDs consist of over 90% carbon, containing dopants of sulfur, silicon, nitrogen, and oxygen. These CQDs exhibited direct and indirect allowed band gaps of 2.61 and 2.68 eV, respectively. The produced CQDs exhibited redshifted fluorescence emission spectra at various excitation wavelengths, and an ultraviolet-visible (UV-Vis) absorption maximum around 400 nm. XRD analysis revealed a predominantly amorphous structure (68%) and a crystallite size of 0.650 nm. Structural analyses highlighted quantum confinement effects and significant lattice imperfections, including average microstrain ≈ 0.2. Nitrogen adsorption studies indicated microporous and mesoporous structures with DFT surface area of 52 m²/g and average pore diameter of 2.297 nm. With moderate surface area, multi-elemental doping, and tunable optical properties, the coal-derived CQDs demonstrated strong potential for industrial applications in environmental remediation, catalysis, optoelectronics, and biomedical imaging. This approach not only supports sustainable nanomaterial production but also promotes economic revitalization of the coal sector and environmental management through innovative reuse of local resources.