Surface‐Dominated Bandgap and Structure–Property Correlation in Green‐Synthesized Fe₃O₄ Nanoparticles

This work examines how phytochemical-mediated reduction and capping during green synthesis govern the crystallite size, microstructural defects, and surface states of Fe₃O₄ nanoparticles, and how these structural features in turn determine the optical bandgap and antibacterial activity. The central hypothesis is that the optical bandgap of green-synthesized Fe₃O₄ is dominated by surface disorder and strain induced by the phytochemical capping layer rather than by quantum confinement. Fe₃O₄ nanoparticles were synthesized via co-precipitation using ferric chloride hexahydrate and Lagenaria siceraria leaf extract. Characterization employed UV-Visible spectroscopy, XRD, SEM, and EDX. Antimicrobial activity was tested against Serratia marcescens and Aspergillus flavus MTCC-3306 using agar well diffusion. XRD confirmed phase-pure inverse spinel Fe₃O₄ with an average crystallite size of 23.74 ± 1.52 nm, dislocation density of 3.98 × 10¹⁵ lines m⁻², and microstrain of 4.62 × 10⁻³. UV-Vis showed an absorption band at 280.83 nm, and Tauc plot analysis yielded an optical bandgap of 2.54 eV—substantially larger than bulk Fe₃O₄ (~0.1 eV). Since the crystallite size far exceeds the exciton Bohr radius (~2 nm), this bandgap widening is attributed to surface effects, oxygen vacancies, and lattice strain rather than quantum confinement. SEM revealed spherical nanoparticles (30–100 nm), and EDX gave a Fe:O atomic ratio of 0.38. Antibacterial activity against S. marcescens was weak and dose-dependent, with maximum inhibition zones of 0.5 ± 0.04 mm at 20 µg mL⁻¹ (vs. 13.0 ± 0.5 mm for streptomycin), and no antifungal activity against A. flavus . The weak activity is attributed to the phytochemical capping layer, nanoparticle aggregation, and the Gram-negative outer membrane. This work provides a quantitative structure–property–function correlation for a green-synthesized oxide ceramic, establishing a foundation for rational design of Fe₃O₄ nanoparticles, although magnetic, drug-delivery, and photocatalytic properties were not directly investigated.