Green synthesis of N-doped-carbon dots/ZnO for enhanced photocatalytic degradation of methylene blue dye: optimization of reaction parameters.

In this research work, nitrogen-doped carbon dots (N-CDs) adorned zinc oxide nanoparticles (N-CDs/ZnO) were successfully synthesized by a simple and cost-effective solution dispersion method and later on used as photocatalyst for decontamination of aqueous methylene blue (MB) dye on irradiation of UV light (300 W, 320-400 nm) at room temperature. Both the N-CDs and ZnO were prepared through green technique utilizing non-toxic, inexpensive and eco-friendly precursors, namely Foeniculum vulgare and Psidium guajava leaf extract, respectively. All the synthesized samples exhibited crystalline nature with average diameter of particle 4.42 nm, 12.38 nm and 14.11 nm corresponding to N-CDs, ZnO and N-CDs/ZnO, respectively. Further, band gap energy value (E_g) of 3.43, 2.76 and 2.49 eV for N-CDs, ZnO and N-CDs/ZnO, respectively, were obtained by using Tauc's plot. The photocatalytic capability of the sample N-CDs/ZnO was compared with bare ZnO nanoparticles, utilizing identical experimental conditions. The results demonstrated that the composite exhibited notably higher photocatalytic degradation efficiency than bare ZnO nanoparticles up to 15.54%. Lower band gap value of N-CDs/ZnO was the major factor for exhibiting this behaviour, decreasing the recombination rate and thus enhancing the efficiency. Furthermore, N-CDs/ZnO exhibited 98.17% MB degradation under optimized conditions (0.03 g, 5 ppm, pH 10). The resultant N-CDs/ZnO exhibited good stability and decontamination efficiency up to five cycles with efficiency loss of only 7.89%. Along with, trapping experiments was conducted to analyze the role of active species involved for deep understanding of mechanism. The order of efficiency of active constituents was observed to be: ^•O₂^-  > h⁺  > ^•OH. The study analyzed the non-toxic nature of treated water, revealing normal plant growth, suggesting its potential use in irrigation of parks and roadside areas. Overall, present research work obeys the green chemistry principles with the fabrication of highly efficient, eco-friendly, cost-effective photocatalyst N-CDs/ZnO by utilizing the green precursors for the whole research work.