Hemp-Derived Cellulose/NiFe₂O₄ Biocomposite Hydrogels for Flexible Optoelectronics

This study presents a systematic investigation into the optical and broadband dielectric behaviors of sustainable biocomposite hydrogels, synthesized from hemp biomass-derived cellulose and a poly-(AMPS/IA) network reinforced with varying concentrations of NiFe₂O₄ nanoparticles, hereafter denoted as NiFe₂O₄ biocomposite hydrogels (NiFH). The primary objective was to determine the suitability of these eco-friendly materials for flexible optoelectronic applications by elucidating the influence of filler loading on charge transport mechanisms. Structural and morphological characterizations via X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) confirmed the successful formation of a densely cross-inked amorphous matrix. The dispersion of the inorganic phase, without severe microscale agglomeration, was evidenced by the electron-dense domains resolved in the STEM images, while the characteristic spinel-type metal-oxygen vibrations identified by FTIR were consistent with the retention of the NiFe₂O₄ ferrite phase. Optical characterization revealed that the hydrogels exhibit tunable semiconductor properties, with direct band gap energies (E _g ^d ) modulated between 2.75 and 3.57 eV. While the NiFH2 formulation (0.5% NiFe₂O₄) showed the highest structural order, increasing the NiFe₂O₄ content to 1.0% (NiFH4) induced localized interfacial disorder, as evidenced by an elevated Urbach energy and a corresponding narrowing of the forbidden gap. Broadband dielectric spectroscopy demonstrated that the frequency-dependent response is governed by Maxwell-​Wagner-Sillars interfacial polarization and Koops' phenomenological models. Unlike conventional trends, the NiFH4 sample emerged as the optimal formulation, exhibiting peak dielectric permittivity and enhanced parallel capacitance due to the expanded electroactive surface area. This impedance analysis conclusively demonstrated that charge carriers encounter their primary impedance barriers at the microstructural interfaces and grain boundaries, which consistently exhibit significantly higher resistance than the bulk material. Moreover, increasing the NiFe₂O₄ concentration systematically diminished these internal resistances, thereby simultaneously enhancing both bulk and interfacial conductivity. The modeling further elucidated the capacitive dynamics of the system, revealing strong nonideal capacitive effects driven by interfacial polarization at low frequencies, which transitioned into highly stable dielectric behavior at higher frequencies. Ultimately, the synergistic combination of biomass-derived cellulose and magnetic nanoparticles positions these hydrogels as highly promising candidates for high-performance "green" energy storage systems and flexible bioelectronic sensing platforms.