Green Synthesis of ZnO@SiO₂, CuO@SiO₂, and Al₂O₃@SiO₂ Nanocomposites Using <i>Calligonum comosum</i> L. Extract: Synthesis, Characterization, Photocatalysis, and Antioxidant Applications

ABSTRACT Addressing the challenge of removing toxic pollutants and pathogens from water, this study explores the synthesis and performance of shell–core bimetallic oxide nanocomposites (ZnO@SiO 2 , CuO@SiO 2 , Al 2 O 3 @SiO 2 ) alongside individual nanoparticles (ZnO, CuO, Al 2 O 3 , SiO 2 ). A green synthesis approach utilizing Calligonum comosum L. extract was employed, followed by characterization using UV–Visible spectroscopy, FTIR, XRD, and SEM. The band gap energies of the shell–core nanocomposites were significantly reduced compared to their individual counterparts (ZnO@SiO 2 : 2.15 eV, CuO@SiO 2 : 1.06 eV, Al 2 O 3 @SiO 2 : 1.80 eV) versus (ZnO: 2.30 eV, CuO: 1.42 eV, Al 2 O 3 : 2.50 eV). These reductions, combined with smaller crystallite sizes, contributed to improved optical and structural properties. Photocatalytic efficiency was tested with Methylene Blue (MB) and Rose Bengal (RB) dyes. Nanocomposites outperformed pure oxides, achieving MB degradation of 99.3% (ZnO@SiO 2 ), 97.57% (CuO@SiO 2 ), and 99.2% (Al 2 O 3 @SiO 2 ), and RB degradation of 98.0%, 84.8%, and 92.4%, respectively. SiO 2 enhanced surface area, charge separation, and reduced electron–hole recombination and agglomeration, improving stability. Antioxidant activity, via DPPH assay, peaked in CuO nanoparticles (IC 50 = 0.1313 mg/mL), with further gains from SiO 2 . This shows the nanocomposites' superior photocatalytic and antioxidant performance, highlighting SiO 2 's synergistic role and potential for environmental and biomedical uses.