Modeling and experimental analyses for Chitosan/Zinc oxide nanocomposite

The molecular structural and optical properties of chitosan (Cs) and its nanocomposites with zinc oxide (ZnO) are investigated using a combination of Density Functional Theory (DFT) calculations at the B3LYP/LANL2DZ level and experimental techniques (FTIR and UV-Vis diffuse reflectance spectroscopy). Different coordination modes (amine -NH₂, hydroxyl -OH, and oxygen-linkage) were modeled to describe Cs-ZnO interactions. The formation of Cs/ZnO and Cs/2ZnO complexes significantly increases the total dipole moment (TDM) from 5.884 Debye in pure Cs to 14.049 Debye in Cs/2ZnO via O-linkage and reduces the HOMO/LUMO energy gap (ΔE) from 6.908 eV in pure Cs to 2.239 ± 0.05 eV in Cs/2ZnO via OH indicating enhanced polarity, charge-transfer, and electronic reactivity. Global reactivity descriptors further confirm increased softness and electrophilicity upon ZnO incorporation. Electronic structure analyses (MESP, DOS/PDOS, and QTAIM) elucidate coordination and charge redistribution mechanisms. Experimentally, FTIR spectra reveal substantial interfacial interactions through shifts in N–H bending (from 1583 cm⁻¹ in pure Cs to lower wavenumbers with increasing ZnO content) and the appearance of Zn–O bands (424–600 cm⁻¹). UV-Vis diffuse reflectance spectroscopy and Tauc plot analysis show a redshift in optical bandgaps with rising ZnO content, attributed to defect states and band tailing; the direct bandgap decreases from 4.35 ± 0.05 eV (pure Cs) to 3.28 ± 0.04 eV (4 wt% ZnO), and the indirect bandgap from 3.19 ± 0.05 eV to 2.56 ± 0.04 eV. This study advances prior chitosan/ZnO research by providing comprehensive QTAIM-mapped analysis of specific Cs-ZnO binding modes (amine-H, O-linkage, OH) correlated with quantitative DFT reactivity descriptors, FTIR vibrational shifts, and defect-induced optical bandgap tuning in nanocomposites. These findings highlight the tunable electronic and optical properties of Cs/ZnO nanocomposites, positioning them as promising candidates for photocatalysis, optoelectronics, and sensors.