Hydrothermal Synthesis of ZnO@MnO₂-Montmorillonite Nanocomposites: Influence of Molarity on Structural, Optical, and Photocatalytic Performance toward Ciprofloxacin Degradation under Variable Conditions

In this work, a series of ZnO@MnO₂-montmorillonite nanocomposites was successfully synthesized via a hydrothermal route using NaOH solutions at varying molarities (3-9 M) and evaluated for their structural, optical, morphological, and photocatalytic properties, with specific application in ciprofloxacin degradation. Structural investigation confirmed the coexistence of pure ZnO (hexagonal), MnO₂ (tetragonal), and montmorillonite phases and showed molarity-dependent variations in crystallite size, from 34 nm (ZMM5) to 7 nm (ZMM9), and dislocation density, which ranged from 0.0009 to 0.0180 nm^-2. Fourier-transform infrared spectroscopy (FTIR) spectra evidenced the progressive occupation of montmorillonite OH sites by metal oxides. At the same time, photoluminescence deconvolution revealed a shift in dominant intrinsic defects: ZMM3 exhibited 49% Zn vacancies (<i>V</i> _Zn), whereas ZMM7 and ZMM9 showed higher concentrations of oxygen vacancies (<i>V</i> _O ⁺ up to 40.7%). The band gap narrowed from 3.294 eV (ZMM3) to 2.945 eV (ZMM9), indicating an increase in defect states and enhanced light absorption. Textural analysis revealed that ZMM3 had the highest surface area (35.1 m² g^-1) and microporosity, whereas samples synthesized at higher molarities exhibited mesoporous structures with reduced surface areas (∼18-22 m² g^-1). Photocatalytic tests under ultraviolet (UV) irradiation for 120 min showed excellent degradation performance for all nanocomposites: ZMM3 (60.1%), ZMM5 (61.0%), ZMM7 (59.7%), and ZMM9 (59.5%), indicating robust activity independent of structural variations. Operational parameter studies confirmed that ZMM5 maintained high activity across different catalyst dosages (25-100 mg (47-61%)), pollutant concentrations (10-30 mg L^-1 (45-61%)), and pH levels (3-9 (48-61%)), achieving degradation rates between 45 and 61%. Scavenger tests confirmed hydroxyl radicals (•OH) and holes (h⁺) as the main active species. Reusability tests revealed a 28% drop after three cycles (from 59 to 31%), while X-ray diffraction (XRD) confirmed structural stability with no formation of secondary phases. These results demonstrate the robustness, tunability, and applicability of the ZnO@MnO₂-montmorillonite system for real-world water treatment under variable conditions.