Bio-derived carbon dots from black mulberry fruits: experimental synthesis and computational insights with ethylenediamine for photocatalytic degradation performance

This study integrates experimental synthesis (Hydrothermal) and computational modelling to elucidate the structure-property relationships of bio-derived carbon dots (C-dots) from black mulberry fruits passivated with ethylenediamine. Excellent agreement is observed between experimental data and time-dependent density functional theory (TD-DFT) simulations. The close agreement between the experimental and DFT-simulated FT–IR vibrational spectra confirm the reliability of the structural models, with matching bands corresponding to O–H, C–H, C=O, N–H/C=C, and C=C functional groups. The strong agreement between experimental Photoluminescence (PL) spectra and DFT-simulated Frontier molecular orbital (FMO) analysis confirms excitation-dependent emission and charge-transfer (CT) transitions influenced by surface C=O and COOH groups and FMO results showing HOMOs localized on edge-bound CO/NH₂ and LUMOs on the conjugated core. TD‑DFT calculations revealed high oscillator strengths and enhanced CT states upon surface passivation, aligning with the experimentally observed high quantum yield (~74.9%). Photocatalytically, the C-dots exhibited optimal methylene blue degradation at 0.2 mol/m³ under solar light, driven by effective light absorption and radical (•OH) generation. Computational predictions of CT excited-state behavior further confirmed efficient charge separation and reduced recombination. Overall, both experimental and theoretical results underscore the central role of surface functional groups and π-conjugated cores in governing the optical, photophysical, and photocatalytic properties of C-dots.