Optical band-edge engineering and dielectric enhancement in chitosan films through green-synthesized ZnO nanocrystals

Sustainable routes to engineer optical constants in recyclable films are central to low-impact photonics and sensing. Here we develop fully aqueous, solution-cast chitosan (CS) films loaded with green-synthesized zinc oxide (ZnO) nanoparticles at 4, 8, and 12 wt% (CSZ4–CSZ12), and ZnO was prepared by a plant-mediated aqueous route and calcination to phase-pure wurtzite, then embedded in CS to produce self-supported films. Structure–property relationships were established by FTIR, X-ray diffraction (XRD), FESEM/EDS, and UV–Vis spectroscopy with extraction of the extinction and refractive indices k ( λ ) and n ( λ ), band-gap estimates (Tauc/ASF), Urbach energy E U , and Wemple–DiDomenico (W–D) dispersion parameters, also FTIR revealed redistribution/red-shifts within the O–H/N–H and amide I/II envelopes, consistent with hydrogen bonding/coordination between CS functional groups and surface Zn–O sites. XRD showed the semicrystalline CS halo near 2θ ≈ 20° with composition-dependent broadening, while no resolvable ZnO reflections were observed at the investigated loadings. Together with FESEM observations of well-distributed bright ZnO-rich domains within the polymer matrix and EDS confirmation of zinc incorporation, these results suggest good ZnO dispersion within the CS matrix. Optically, increasing ZnO content enhanced k(λ) across the near-UV/visible, strengthened refractive-index dispersion, and produced a systematic bathochromic edge shift, Tauc/ASF analyses indicated band-gap reduction, while E U ​ rose, signaling increased static/dynamic disorder from interfacial coupling. W–D fitting yielded higher dispersion energy E d with ZnO loading, linking microstructural softening to elevated polarizability and a more responsive dielectric function ( ε 1 ,ε 2 ), so these results demonstrate that green ZnO nanocrystals provide a precise, water-processed lever to tune the dielectric response of CS films, recommending CS/ZnO nanocomposites as eco-friendly candidates for film photonic elements, and optical sensing layers.