Eco-friendly synthesis of carbon dots derived from <i>Morus nigra</i> fruits with emphasis on photocatalytic degradation and relaxation time properties

This study reports the eco-friendly synthesis of carbon dots (C-dots) from Morus nigra fruits and evaluates their structural, optical, and photocatalytic properties. Transmission electron microscopy (TEM) analysis revealed predominantly spherical C-dots with an average diameter of 4.67 ± 1.6 nm and an interplanar spacing of 0.286 nm, confirming their crystalline nature. X-ray diffraction (XRD) revealed an amorphous structure with a broad peak at ~21.4°, characteristic of small sp² domains. X-ray photoelectron spectroscopy (XPS) confirmed the presence of carbon, oxygen, and nitrogen, with surface functional groups including C═O, C─N and O-C═O. The C-dots demonstrated effective photocatalytic activity, degrading methylene blue (MB) dye under solar irradiation with an efficiency of 95.8% at a 2.5 mg/L concentration, following first-order kinetics (rate constant of 0.0345 min−1). Photoluminescence (PL) studies showed excitation-dependent emission, with optimal performance at 350 nm, yielding the highest intensity and the longest average fluorescence lifetime of 21.88 ns. The CIE chromaticity coordinates further confirmed the excitation-dependent green luminescence. These results highlight the potential of Morus nigra-derived C-dots for environmental remediation, optical sensing and security-related applications.