Fish scale-based carbon dots with g-C3N4 as a photo-catalyst for oxidation of herbicide 2,4-dichloro phenoxy acetic acid under visible light: Experimental and theoretical studies

2,4-dichloro phenoxy acetic acid (2,4-D) has been employed as an herbicide to increase crop yields and ensure large-scale food production; however, it is highly toxic and persistent in the environment. This work deals with the green chemistry preparation of carbon quantum dots (NCDs) from fish scales and then combines them with graphitic carbon nitride to oxidize 2,4-D under visible light. The samples, C-NCDs, S-NCD, g-C 3 N 4 , C-NCD/g-C 3 N 4, and S-NCD/g-C 3 N 4 were characterized by different analytical methods (XRD, SEM, TEM, HRTEM, XPS, and fluorescence/UV–visible spectra). C-NCD/g-C 3 N 4 and S-NCD/g-C 3 N 4 are thermally stable, and the average size of the particles was ∼5.0–15 nm. SEM/TEM show that quasi-spherical particles (7–20 nm) of C-CQD and S-NCD are homogeneously dispersed, and the fluorescence emission spectra show distinct profiles as C-NCDs (429 nm) and S-NCDs (318 nm and 404 nm), caused by hetero atoms from the fish scales that are less pronounced for C-NCDs. This is consistent with the photoluminescence spectra that C-NCDs and S-NCDs exhibit a green visible emission at 575 and 526 nm, respectively, as a typical behavior of carbon quantum dots. The bandgap energy of the samples (2.77–2.91 eV) is consistent with those from the Density Functional Theory (DFT). The adsorption of 2,4-D by S-NCDs/g-C 3 N 4 is highly efficient; for example, q m = 2.85 × 10 −5 mol/g for C-NCD/g-C 3 N 4 , 2.02 × 10 −5 mol/g for S-NCD/g-C 3 N 4 or 2.43 × 10 −5 mol/g for g-C 3 N 4. The photocatalytic oxidation of 2,4-D by the samples reveals that there is considerable photocatalytic oxidation and follows first-order kinetics in the substrate as follows: k visible = 4.3 × 10 −3 mM s −1 and k solar = 7.5 × 10 −3 mM s −1 for C-NCDs/g-C 3 N 4 ; k visible = 5.4 × 10 −3 mM s −1 and k solar = 10.4 × 10 −3 mM s −1 for S-NCDs/g-C 3 N 4 . In the cell images, although C-NCD and S-NCD exhibit dispersed fluorescence, adding C-NCD or S-NCD results in a significant fluorescence increase. • Fish scale-based carbon dots with g-C 3 N 4 perform as a photo-catalyst for degradation herbicide. • The semiconducting nature of C-NCDs/g-C 3 N 4 bandgap energy determination by DFT. • Enhancement of photoluminescent properties of C-NCDs/g-C 3 N 4 by Fish scale-based carbon dots.