Biosynthesis of ZnO and Ag-doped ZnO nanoparticles using Citrus sinensis (orange) peel extract: A comprehensive investigation of the effect of silver (Ag) on the structural, photocatalytic, and antibacterial properties

Pure ZnO and Ag-doped ZnO (0.5%, 3%, and 8.5%) nanoparticles (NPs) were synthesized using a facile and eco-friendly route using Citrus sinensis (orange) peel extract as a reducing and stabilizing agent. Comprehensive structural, morphological, optical, thermal, antibacterial, and photocatalytic analyses were performed. The XRD analysis confirmed a hexagonal wurtzite phase, while the size–strain plot (SSP) model provided the most accurate crystallite size estimates (R² = 0.80–0.99): 37.41, 41.49, 28.67, and 24.24 nm for pure and Ag-doped ZnO (0.5%, 3%, 8.5%), respectively. FESEM images revealed elongated ZnO NPs, while Ag doping resulted in distorted spheres with sizes varying around 40-70 nm. The EDX analysis confirms the presence of the desired elements in the synthesized NPs. Ag incorporation induced a redshift in absorption and narrowed the bandgap from 3.19 to 3.09 eV. TG/DT analysis indicated enhanced thermal stability of ZnO with increasing Ag content. The presence of different functional groups was determined from the FT-IR spectra. Antibacterial tests showed that 3% Ag–ZnO exhibited the strongest inhibition zones (12 mm for Escherichia coli , 16 mm for Staphylococcus aureus ). Moreover, 3% Ag–ZnO achieved the highest photocatalytic degradation efficiency-36.37% for methylene blue (MB) and 88.48% for methyl orange (MO) within 90 min-following pseudo-first-order kinetics with rate constants of 0.00471 and 0.02513 min⁻¹, respectively. Excessive Ag loading (8.5%) diminished photocatalytic activity compared to 3% Ag–ZnO but still outperformed pristine ZnO. • ZnO and Ag-doped ZnO (0.5%, 3%, 8.5%) was biosynthesized using waste Citrus sinensis peel extract • ZnO and Ag-doped ZnO nanoparticles possess hexagonal wurtzite structure. • Nanoparticles have elongated and distorted spherical shaped morphology • FTIR confirms the presence of different functional groups • Ag:ZnO possesses enhanced thermal stability behavior • Ag doping generates the redshift peak at the absorption spectra of the ZnO NPs • 3% Ag-doped ZnO nanoparticles exhibited superior inhibition zone of 12 mm and 16 mm against E. coli and S. aureus respectively. • ZnO and Ag-doped ZnO NPs demonstrated time-dependent methylene blue dye degradation