Mangifera indica gum-mediated sol-gel synthesis of ZnO photocatalysts with tunable structure and controlled defect distribution for efficient metronidazole degradation

Zinc oxide photocatalysts were synthesized through a sol-gel route using Mangifera indica gum as a renewable macromolecular gelation and templating agent. Thermal analysis revealed three main degradation stages of the gum, including ~14% moisture loss up to ~150-170 °C, ~52.5% polysaccharide decomposition between ~170 and 400 °C, and an additional ~27% mass loss associated with carbonization between 400 and 600 °C, guiding the calcination of ZnO gels at 400 and 600 °C. Structural analysis confirmed the formation of pure hexagonal wurtzite ZnO, with lattice parameters a = 3.2491 (1) Å, c = 5.2044 (2) Å (400 °C) and a = 3.2484 (1) Å, c = 5.2029 (2) Å (600 °C). The average crystallite size increased from 86 (1) nm to 94(2) nm, while the dislocation density decreased from 1.35(3) × 10^-4 nm^-2 to 1.13 (7) × 10^-4 nm^-2, evidencing improved crystallinity. FTIR and Raman spectroscopy showed progressive disappearance of CO and CH vibrational modes and the sharpening of ZnO and E₂^(high) phonons, confirming enhanced lattice ordering and reduced structural disorder. Optical characterization indicated a slight bandgap narrowing from 3.22 (1) eV to 3.20(1) eV, associated with defect relaxation. Photoluminescence deconvolution revealed that oxygen vacancies-related emissions decreased from 66% to 53% of total intensity when the calcination temperature increased, indicating controlled defect distribution. Electron microscopy showed a morphological evolution from agglomerate clusters to spherical grains with uniform mapping. Photocatalytic assays conducted under UV irradiation showed that ZnO calcined at 400 °C achieved 75 (2) % degradation of metronidazole after 120 min, using a catalyst dosage of 0.5 g L^-1 and a pollutant concentration of 20 mg L^-1. This performance surpasses that of the 600 °C sample, which achieved only 41 (1) % degradation. Reactive-species trapping indicated that •OH, •O₂•^-, and photogenerated holes (h⁺) contribute significantly to MTZ degradation, as scavenging reduced the degradation from 75 (2) % to 52(1)-55(1) % (a 20-23 percentage-point decrease, corresponding to ~27-31% inhibition relative to the control). The catalyst maintained 53% efficiency after three reuse cycles without structural degradation. These results demonstrate that Mangifera indica gum enables the eco-friendly synthesis of structurally ordered ZnO with a tunable defect distribution, high photocatalytic efficiency, and long-term stability for the sustainable removal of pharmaceutical contaminants.