Optoelectronic modulation and dielectric enhancement in copper oxide–chitosan nanocomposites fabricated via green route

Developing environmentally friendly materials with tunable optical characteristics is essential for next-generation photonic technologies. In this study, copper oxide–chitosan nanocomposite films were fabricated using a completely green and water-based process. Copper oxide nanoparticles were synthesized via a polyphenol-assisted route using green-tea extract and subsequently calcined at 500 °C to obtain pure monoclinic tenorite. The nanoparticles were then incorporated into chitosan matrices through solution casting to produce flexible nanocomposite films containing 0, 4, 8, and 12 wt% copper oxide. Structural, morphological, and optical characterizations were conducted using X-ray diffraction, Fourier-transform infrared spectroscopy, field-emission scanning electron microscopy with energy-dispersive X-ray analysis, and UV–visible spectroscopy. The results revealed strong interfacial interactions between chitosan functional groups (–OH and –NH₂) and copper-oxygen sites, leading to reduced crystallinity and increased microstrain within the polymer network. Optical studies demonstrated a red shift in absorption, a progressive decrease in band-gap energy from 4.98 eV to 3.80 eV, and an increase in Urbach energy, indicating enhanced structural disorder and polarizability. The refractive index, dielectric constant, and dispersion energy all increased with copper oxide content, confirming improved light–matter interaction and optical density. Overall, this work presents a sustainable route to engineer biodegradable chitosan-based nanocomposite films with controllable optical and dielectric properties, making them promising candidates for ultraviolet-shielding coatings, photonic sensors, and optoelectronic devices.