Visible light degradation of cationic dyes using carbon quantum dots/tin oxide composite

In this study, carbon quantum dots/tin oxide (CQDs/SnO₂) nanocomposite were used to degrade the cationic dyes such as Methylene Blue (MB) and Malachite Green (MG). Characterization studies reveals that the addition of CQDs does not alter the diffraction peak of SnO₂ and the crystallite size of the nanocomposite is 13.12 nm. The nanocomposite also has a carboxyl functional group along with a hydroxyl group, which proves that CQDs are decorated on the surface of SnO₂. In the study, for a 45 mg/100 mL of SnO₂, and for an initial dye concentration of 5 ppm, the degradation of dye was observed to be 48% for MB and 39% for MG, respectively, whereas with the CQDs decorated SnO₂ nanocomposite better efficiency was observed with respect to degradation of MB (87%) and MG (95%), respectively. Among the different catalyst dosage variations, 45 mg/100 mL exhibits higher degradation due to increased number of active sites on the photocatalysts surface. For different initial concentrations of pollutants (5–10 ppm) and for an optimum dosage of 45 mg/100 mL, it was observed that the maximum degradation of dyes was achieved only at 5 ppm and it was evident in both the dyes. At higher concentration of dyes, the solution filters light, allowing fewer photons to reach the surface and reduces the production of oxidizing species. With different pH studies conducted 89% degradation of MB was observed at pH 6.0 and for MG it was at pH 5.75 with 95% degradation.