Green synthesis of ZnO nanoparticles using enzymatically hydrolyzed Sargassum fusiforme extracts: Characterization and enhanced photocatalytic performance under UV and solar irradiation

This study presents an eco-friendly approach for the green synthesis of ZnO nanoparticles (ZnO NPs) using enzymatically hydrolyzed Sargassum fusiforme extracts (SFEE). A comprehensive suite of analytical techniques, including UV-visible absorption spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS), were employed to systematically characterize the structural, morphological, surface chemical, and optical properties of the synthesized ZnO NPs. UV-visible absorption spectroscopy confirmed the formation of nanoparticles through a distinct absorption peak at 375 nm. FTIR and XPS analyses revealed that functional groups derived from SFEE participate in nanoparticle formation and surface stabilization. XRD, FESEM, and HRTEM results demonstrated that SFEE-ZnO nanoparticles possess a well-defined hexagonal wurtzite structure with uniform dispersion, a reduced average particle size of 75.23 nm, and a smaller crystallite size (21.22 nm) compared with chemically synthesized ZnO (C-ZnO) and ZnO prepared using hot aqueous extracts (SFAE-ZnO). UV-Vis DRS analysis revealed a red shift in the absorption edge of SFEE-ZnO, corresponding to a narrowed bandgap of 3.20 eV, while PL analysis indicated suppressed electron-hole recombination. These synergistic structural and electronic features contribute to enhanced photocatalytic activity. The photocatalytic performance of SFEE-ZnO was evaluated by degrading methylene blue (MB, 15 mg L -1 ) using 1 mg mL -1 of catalyst under both UV and solar irradiation. SFEE-ZnO exhibited superior degradation efficiencies of 97.65% under UV light and 99.48% under solar light within 30 min, significantly outperforming C-ZnO and SFAE-ZnO. These results highlight the potential of SFEE-ZnO as an efficient and environmentally sustainable photocatalyst for solar-driven wastewater remediation.