Cobalt chloride-doped chitosan/sodium alginate biopolymer composite films with enhanced optical, dielectric, and electrical properties for dielectric applications

Abstract In this study, CoCl₂-doped chitosan (CS)/sodium alginate (SA) polymer composite films with composition x CoCl 2 -(100- x ) (0.6CS-0.40SA), where x = 0, 10, 20, 30, 40 wt%, were synthesized using a solution-casting method to investigate the role of Co ions modifying the structural, optical, dielectric, and electrical characteristics. X-ray diffraction patterns confirmed the amorphous nature of the undoped and CS/SA polymer films loaded with up to 30 wt% CoCl 2 . In contrast, the CS/SA polymer containing 40 wt% CoCl 2 exhibited partially crystalline peaks corresponding to the NaCl crystalline phase. FTIR analysis revealed systematic changes in the oxygen- and nitrogen-containing functional groups of CS and SA, consistent with polymer–ion interactions involving Co 2+ ions. UV–Vis absorption studies show a gradual red shift in the absorption edge with CoCl 2 addition, accompanied by reductions optical bandgap in both direct (4.76 to 3.93 eV) and indirect (3.29 to 2.76 eV) allowed transitions, and increases in refractive index (2.32 to 2.46), optical dielectric constant (5.39–6.07), and reflection loss (0.15–0.17), indicating enhanced electronic polarizability and stronger light–matter interaction. Scanning electron microscopy (SEM) was employed to examine the surfaces of the CS/SA-CoCl 2 composite films and confirm the XRD results. The electrical and dielectric properties were investigated using Broadband dielectric spectroscopy. The results revealed enhanced interfacial polarization and charge-relaxation behavior after CoCl 2 incorporation. The dc conductivity increased from 10⁻⁹ S cm⁻¹ for the CSCo-1 sample to 10⁻⁸ S cm −1 for the CSCo-5 sample. Dielectric constant increased from 10 3 to 10 4 . The relaxation time was $$\\:1.03\\:\\times\\:{10}^{-5}\\:$$ s for the CSCo-1 sample and $$\\:5.20\\:\\times\\:{10}^{-6}$$ s for the CSCo-5 sample. The overall findings elucidate the influence of CoCl 2 incorporation and provide insights into tailoring the optical, dielectric, and charge-relaxation properties of CS/SA biopolymer films. The developed films are promising dielectric materials for transient electroactive and polarization-based applications.