Fluorescent onion-like carbons (OLCs) derived from diverse coal precursors

Abstract

Onion-like carbons (OLCs) or Carbon nano-onions (CNOs) have diverse applications across various fields of science and technology due to their remarkable properties, including active fluorescent centres, a high surface-to-volume ratio, and negligible toxicity. Coal, a carbon-rich and predominantly black solid material, was used as a precursor for the synthesis of fluorescent onion-like carbons (OLCs). This study highlights the formation mechanism of OLCs from four different grade/rank of coal precursors along with their fluorescent behaviours. Four distinct coal samples with varying petrographic and chemical compositions were used to synthesize OLCs by using an ultrasonic-assisted wet-chemical oxidation method. The resultant coal-derived OLCs exhibit a quasi-spherical shape with an onion-like morphology. The fluorescent behaviour of synthesized OLCs was systematically investigated with respect to the nature of the precursor materials and the surface functionalities present in the nanostructures. The results indicate that the fluorescence characteristics of OLCs are significantly influenced by the incorporation of heteroatoms in the carbon framework, as well as the overall elemental composition, particularly the relative content of carbon and nitrogen present in the precursor materials. The OLCs exhibit fluorescence quantum yields (FLQY) of 16.87, 2.53, 14.29, and 24.16 %, corresponding to average particle sizes of 36.49 ± 0.200, 39.14 ± 0.032, 50.24 ± 0.128, and 95.29 ± 0.246 nm, respectively, for the four distinct coal-derived OLCs. Furthermore, this study highlights the potential of employing coals of various rank as precursor materials for the synthesis of self-co-doped OLCs enriched with heteroatom functionalities. This approach offers a simple, facile, and cost-effective route for scalable production of high-value carbon nanomaterials derived from different grade/rank of coal precursors. © 2025 Elsevier Ltd