Modeling and experimental analyses for Chitosan/Zinc oxide nanocomposite
Hanan Elhaes, Khaled S. Amin, Fawzy G. El Desouky, Medhat A. Ibrahim
TL;DR
This study explores how chitosan and zinc oxide form nanocomposites, revealing changes in their electronic and optical properties through both modeling and experiments.
Contribution
The paper introduces a detailed QTAIM-mapped analysis of specific chitosan-ZnO binding modes and correlates them with DFT reactivity and experimental data.
Findings
Cs/ZnO nanocomposites show increased dipole moments and reduced HOMO/LUMO gaps, indicating enhanced electronic reactivity.
FTIR and UV-Vis spectroscopy confirm interfacial interactions and optical bandgap tuning with ZnO content.
Defect states and band tailing cause redshifts in optical bandgaps, supporting potential applications in photocatalysis and sensors.
Abstract
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…
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Figure 9- —National Research Centre Egypt
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Taxonomy
TopicsNanocomposite Films for Food Packaging · Polymer Nanocomposites and Properties · Advanced Cellulose Research Studies
Introduction
Chitosan is a nontoxic, biocompatible, biodegradable polymer with low immunogenicity, making it suitable for various applications^1^^–^^3^. Chitosan has high molecular weight leading to low solubility in aqueous media, which limits its application in some fields, particularly medicine and food industry^4^. On the other hand, ZnO is a wide bandgap semiconductor (3.37 eV) with a large excitation binding energy (60 meV)^5^. It is used in electronic devices such as transistors, light-emitting diodes (LEDs), and solar cells^6,7^. ZnO is utilized in blue LEDs, lasers, UV detectors, and photovoltaic systems^8^. Both chitosan and ZnO exhibit strong antimicrobial properties. Chitosan acts as an excellent bacteriostatic agent, while ZnO is known for its antifungal and antibacterial activity^9,10^. Chitosan/ZnO nanocomposites have been used to preserve fruits like bananas and white grapes, maintaining their postharvest qualities and extending their shelf life^11^. The addition of ZnO nanoparticles to chitosan films enhances their mechanical properties, including increased tensile strength and elongation at break. This makes the films more durable and suitable for food packaging applications^12^. These nanocomposites are effective in dye degradation, microbial infection control, and environmental pollutant mineralization^13^. The interaction between chitosan and metal ions, including ZnO, occurs through the amine and hydroxyl groups present in chitosan, forming coordination bonds and ion-dipole interactions^14^. ZnO is classified as Generally Recognized as Safe (GRAS) by the FDA, indicating its safety for use in various applications, including food-contact materials^15^. Chitosan/ZnO nanocomposites are used in food packaging to coat polyethylene films, enhancing mechanical and barrier properties while maintaining food colors and extending shelf life^16^. Chitosan/Zn On anocomposites are eco-friendly and can be used to eliminate dyes from colored aqueous solutions, showcasing their versatility in environmental applications^17^. They offer improved mechanical strength, thermal stability, and resistance to discoloration, making them suitable for developing coated paper-based antimicrobial food packaging materials^18^. Overall, chitosan/ZnO nanocomposites provide a multifunctional solution for antimicrobial applications, food preservation, and environmental protection due to their enhanced properties and effectiveness^19^. One of the leading techniques for studying physical, chemical and vibrational properties of nanocomposite is molecular modeling^20^. This class of computational technique is important tool to study several systems in molecules covering several areas of applications^21^^–^^23^. Several parameters could be derived from molecular modeling to describe the interaction of molecules such as total dipole moment, TDM, HOMO/LUMO energy, molecular electrostatic potential MESP, reactivity descriptors, quantum theory for atoms in molecules QATIM beside vibrational frequencies^24^^–^^27^.
Recent studies on biopolymer–metal oxide interfaces, particularly Cs/ZnO systems, have demonstrated that motif-specific coordination between chitosan functional groups and metal oxide surfaces can be resolved using density functional theory and QTAIM analysis. These investigations revealed clear correlations between Cs–ZnO coordination motifs, FTIR band shifts associated with amino and hydroxyl groups, and changes in band edge positions derived from DRS Tauc analysis, highlighting the role of interfacial chemistry in tuning electronic and adsorption properties^27–29^.
Previous theoretical studies have successfully employed DFT to describe several systems and interfacial phenomena such as chitosan/graphene systems, doped graphene nanostructures, and polymer/metal oxide composites for sensing and adsorption applications^30–32^.
The objective of this study is to investigate the molecular, structural, and optical properties of chitosan (Cs) and its zinc oxide (ZnO) nanocomposites by integrating density functional theory (DFT) calculations at the B3LYP/LANL2DZ level with experimental FTIR and UV-Vis spectroscopy. The research systematically explores specific coordination modes including amine (-NH₂), hydroxyl (-OH), and oxygen-linkage to describe the interfacial interactions that lead to enhanced polarity, charge transfer, and electronic reactivity. The theoretical findings, were supported by QTAIM, MESP, DOS/PDOS and IR analyses.
Calculation details
Gaussian 09 program^33^ was used for calculating the investigated structures at molecular modeling and Spectroscopy Laboratory, Centre of Excellence for Advanced Science, NRC., Egypt. The optimizations and frequency calculations for the structures were performed using the B3LYP functional, which combines Becke’s three-parameter exchange functional with the Lee-Yang-Parr correlation functional, and the Los Alamos National Laboratory 2 double ζ (LANL2DZ) basis set^34–36^, which combines an effective core potential for Zn with a double-ζ description for light atoms and is widely validated for ZnO clusters and organic–inorganic hybrids^37^. The total dipole moment (TDM), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), were calculated at the same level of theory. From the difference between HOMO and LUMO, the energy gap (∆E) was determined. Two valuable physical metrics that show a molecule’s capacity for molecular interaction are the TDM and the HOMO/LUMO energy gap (∆E). Convergence criteria were set to tight SCF (10⁻⁸ au) and maximum force/displacement thresholds of 0.00045 au. No empirical dispersion correction was applied, as the gas-phase cluster models do not require it for qualitative trends. The same theoretical method was used to map the molecular electrostatic potential (MESP) in order to identify the active sites. Global reactivity descriptors including Ionization Potential (I), Electronic Affinity (A), Electronic chemical potential (µ), Chemical hardness (η), Absolute softness (S) and Electrophilicity index (ω)^38^ where calculated using these formulas:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{I = -}}{{\mathrm{E}}_{{\mathrm{HOMO}}}}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{A} = \mathrm{E}_\mathrm{LUMO}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu ={\text{ }} - \left( {{\mathrm{I}}\,+\,{\mathrm{A}}} \right)/{2_{}}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upeta =\left( {{\mathrm{I}} - {\mathrm{A}}} \right)/2$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{S}}\,=\,1/\upeta$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upomega \,=\,{\upmu ^2}/2\upeta$$\end{document}To check the stability of the studied structures the quantum theory of atoms in molecules QTAIM calculations were conducted with both Multiwfn and VMD software^39,40^. Finally, the IR for the studied structures was computed at the same level of theory.
Materials and methods
Chemicals and reagents
Chitosan (medium molecular weight, ~ 100,000 Da, degree of deacetylation ≥ 75%) was obtained from Acros Organics (USA, Catalog No. ACROS-01). Zinc acetate was purchased from Sigma-Aldrich Company, Inc, USA. Sodium hydroxide and ethanol were purchased from El Nasr Pharmaceutical Chemicals Co., Cairo, Egypt. Sodium hydroxide (≥ 97%) was acquired from Fisher Chemical. All chemicals used were of analytical grade and used without further purification. Deionized water was used in all preparations.
Preparations of samples
Synthesis of zinc oxide nanoparticles (ZnO-NPs)
Zinc oxide nanoparticles were synthesized using the conventional precipitation method, as described in the literature^41^. Initially, a 1 M solution of zinc acetate dihydrate was prepared by dissolving the salt in 100 mL of deionized water and heating the mixture to 70 °C. Separately, a 2 M sodium hydroxide solution was also prepared in 100 mL of deionized water. The NaOH solution was then added dropwise to the zinc acetate solution under continuous stirring for 1 h. A white precipitate formed as a result of the reaction, which was collected by centrifugation at 1000 rpm. The precipitate was washed three times with deionized water to remove any residual ions, then dried overnight at 80 °C. Finally, the dried product was calcined at 500 °C for several hours in a muffle furnace to obtain pure ZnO nanoparticles.
Synthesis of Cs/ZnO composite
For Cs/ZnO membrane preparation using the casting method, 0.25 gm of Chitosan (Cs) are dissolved in 100 ml of distilled water containing 2% acetic acid. The solution was stirred using a magnetic stirrer under a fixed mechanical stirring for around 45 min at 70 °C until the Cs were dissolved completely. Then four samples were prepared in different Cs to ZnO weight ratios (0,2,4, and 6% wt.) then the mix were continued to stir at the same temperature for around 45 min, until the solution appeared uniform. The products were then drop-casted into plastic petri dishes. Finally, the blended solution left to dry at room temperature for 5 days. The final films had an average thickness of 65 ± 15 μm.
FTIR spectra analysis
The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra were obtained employing an FTIR spectrometer (Vertex 70, Bruker); the spectra are determined within a spectral range of 4000–400 cm^− 1^ with a spectral resolution of 4 cm^− 1^ collecting 64 scans per spectrum. Automatic baseline correction was applied using the OPUS software.
Optical diffuse reflection spectra investigations
The UV optical diffuse reflection spectra of the samples were recorded at ambient temperature using an optical spectrophotometer (Jasco model V-570) across a wavelength range of 190–2500 nm.
Results and discussion
Optimized modeled structures
Model structures of chitosan (Cs) and chitosan/zinc oxide (Cs/ZnO) complexes were constructed and fully optimized using Gaussian 09 software^33^. Chitosan was modeled using three repeating units, which is sufficient to capture the essential functional groups (amine and hydroxyl moieties)^42^ responsible for coordination and hydrogen-bonding interactions with ZnO, while maintaining computational feasibility. This truncated representation has been widely used to describe local polymer–inorganic interactions.
In the present models, chitosan amine groups were considered in their neutral (–NH₂) form. Although chitosan films were experimentally prepared under acidic conditions, where partial protonation of amine groups is expected, the neutral state was adopted to enable a direct comparison of intrinsic coordination tendencies between different functional groups. The influence of amine protonation on interaction strength and charge redistribution is therefore discussed as a limitation of the present model.
Zinc oxide was modeled as a Zn–O unit representing a local coordination motif rather than an extended periodic surface. While this simplified cluster does not reproduce the full crystalline structure of ZnO, it allows a focused investigation of site-specific interactions between chitosan functional groups and Zn–O centers. The limitations associated with the non-periodic ZnO representation are addressed later in the manuscript.
Three possible coordination modes between chitosan and ZnO were considered. In the first configuration (Fig. 1a), isolated chitosan is shown for reference. In the second configuration (Fig. 1b), chitosan interacts with ZnO through hydrogen bonding between the amine hydrogen of chitosan and the oxygen atom of ZnO. In the third configuration (Fig. 1c), coordination occurs through the oxygen linkage of chitosan interacting with the ZnO unit. In the fourth configuration (Fig. 1d), interaction takes place between the hydroxyl group of chitosan and ZnO. All optimized structures were visualized using GaussView software^43^.
Fig. 1B3LYP/LANL2DZ optimized structures of: a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage of Cs, and d- Cs/ZnO through OH of Cs.
Physical parameters
The calculated total dipole moment (TDM) and HOMO/LUMO energy gap (ΔE) are listed in Table 1, chitosan (Cs) has a TDM of 5.884 Debye, as Cs forms Cs/ZnO complexes, TDM increased, ranging from 7.796 Debye (Cs/ZnO through H of amine) to 8.427 Debye (Cs/ZnO through OH). The interaction “through H of amine” (7.796 Debye) leads to a substantial increase in polarity compared to pure Cs. The “O linkage” (8.187 Debye) and “through OH” (8.427 Debye) interactions result in even higher TDM values. This suggests that the coordination of oxygen atoms (from the hydroxyl groups) with ZnO induces a greater charge separation and polarity in the complex compared to interactions involving the amine hydrogen.
The TDM increases further when two ZnO units interact with Chitosan. Thus, Cs/2ZnO through O linkage” has a TDM of 14.049 Debye. While, Cs/2ZnO through OH has a TDM of 10.498 Debye. This indicates that increasing the amount of ZnO interacting with Chitosan generally leads to a more polar composite material, likely due to enhanced charge redistribution and polarization at the interfaces.
Regarding the listed values for HOMO-LUMO Energy Gap. The most striking trend here is the significant decrease in the HOMO-LUMO energy gap when Chitosan interacts with ZnO. For Cs, it has a ΔE of 6.908 eV. This relatively large gap is typical for an insulating or semiconducting biopolymer. While, all Chitosan/ZnO complexes exhibit a much smaller ΔE. The Cs/ZnO through H of amine has a ΔE of 5.298 eV. The Cs/ZnO through O linkage” shows the smallest gap among the 1:1 complex at 5.247 eV. This suggests that oxygen-mediated interactions (coordination of hydroxyl or ether oxygen) are particularly effective at reducing the energy required for electron excitation. The Cs/ZnO through OH has a ΔE of 6.247 eV, which is higher than the “H of amine” and “O linkage” modes, but still lower than pure Chitosan. Finally, the ΔE values are dramatically reduced when two ZnO units interact with Chitosan. The Cs/2ZnO through O linkage” has a very small ΔE of 2.993 eV. Then, the Cs/2ZnO through OH has an even smaller ΔE of 2.239 eV. This is the lowest energy gap observed in the Table 1.
To ensure the reliability of the calculated electronic parameters, the HOMO-LUMO energy gap and Total Dipole Moment (TDM) of Cs-ZnO O-linkage of Cs were validated using the PBE0/LANL2DZ level of theory, as hybrid functionals like PBE0 often provide a more accurate description of the band gap in semiconductor-based nanostructures. The PBE0 calculation yielded an energy gap of 5.819 eV, which is slightly wider than the ∆E obtained via the B3LYP functional (5.247 eV). This shift is consistent with the known tendency of B3LYP to underestimate the energy gap in metal-oxide systems. Importantly, the calculated TDM remained nearly unchanged, with a value of 8.197 Debye compared to the B3LYP value of 8.187 Debye. This high level of consistency in the dipole moment confirms that the electronic structure and the charge distribution predicted by the B3LYP functional are robust. These findings are in good agreement with previous studies on ZnO nanostructures where hybrid functionals were employed to refine electronic property predictions^44^.
Table 1B3LYP/LANL2DZ calculated TDM in Debye and ΔE in eV for Cs, Cs/ZnO through H of amine, Cs/ZnO through O linkage of Cs, Cs/ZnO through OH of Cs and Cs/2ZnO.StructureTDM (Debye)∆E (eV) Cs 5.8846.908 Cs/ZnO through H of amine 7.7965.298 Cs/ZnO through O linkage of Cs 8.1875.247 Cs/ZnO through OH of Cs 8.4276.247 Cs/2ZnO through O linkage
Cs/2ZnO through OH 14.04910.4982.9932.239
HOMO/LUMO frontier orbitals
Figure 2a presented the DFT: B3LYB/LANL2DZ for HOMO/LUMO orbitals for the Cs. HOMO represents the region where electrons are most likely to be found and are most easily removed, which is often associated with a molecule’s nucleophilic character. For chitosan, the HOMO expected to have significant contributions from the lone pair on the nitrogen of the amine groups and/or oxygen atoms^27^, as these are electron-rich sites, especially in the left and middle chitons units. LUMO represents the region where a molecule is most likely to accept electrons. It’s associated with a molecule’s electrophilic character. For chitosan, the LUMO might be distributed over the carbon-oxygen or carbon-nitrogen bonds, or potentially involve antibonding orbitals across the glycosidic linkages, which is also noted in both left and middle units.
The interaction between Chitosan and ZnO can occur through various mechanisms, leading to changes in the electronic structure of Chitosan, which would be reflected in its HOMO and LUMO^45^. Figure 2b, indicated that, Cs/ZnO interacted through H of amine, the lone pair electrons on the nitrogen of chitosan’s amine groups can coordinate with the zinc atoms on the surface of ZnO. Similarly, the oxygen atoms of hydroxyl groups can also interact. This coordination can lead to a stabilization of the HOMO of chitosan (meaning its energy level might decrease) because the electrons are now shared or donated to ZnO. Another interaction through O-linkage, OH group as indicated in Fig. 2c and d both are almost similar to each other.
For interaction through O-linkage, it primarily refers to the involvement of the oxygen atoms from the hydroxyl (-OH) groups present in the glucosamine units of chitosan. These oxygen atoms possess lone pair electrons, which can act as Lewis bases and donate to electron-deficient sites on the ZnO surface. Other interaction through OH, the hydroxyl groups of chitosan can form hydrogen bonds with the oxygen atoms on the surface of ZnO. This type of interaction can also influence the electron density distribution and thus the orbital energies.
The data clearly demonstrates that the interaction of Chitosan with Zinc Oxide significantly alters its electronic properties, leading to more polar and electronically active hybrid materials. The specific mode of interaction and the number of ZnO units play a crucial role in determining the extent of these changes.
Fig. 2DFT calculated HOMO/LUMO orbitals for the structures using B3LYB/LANL2DZ and isovalue of 0.02 au where a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage, and d- Cs/ZnO through OH.
Table 2DFT calculated global reactivity descriptors using B3LYP/LANL2DZ in eV for Cs, Cs/ZnO through H of amine, Cs/ZnO through O linkage and Cs/ZnO through OH.StructureIAµΗSω Cs 5.931−0.977−2.4773.4540.2900.888 Cs/ZnO through H of amine 5.5250.227−2.8762.6490.3781.561 Cs/ZnO through O linkage 6.0820.835−3.4582.6230.3812.279 Cs/ZnO through OH 6.2830.035−3.1593.1230.3201.597
Table 2 presented the DFT calculated global reactivity descriptors for Cs, Cs/ZnO through H of amine, Cs/ZnO through O linkage and Cs/ZnO through OH. The ionization potential generally increases (becomes harder to remove electrons) for the oxygen-mediated interactions (O linkage, OH), meaning these complexes are less prone to oxidation compared to pure Chitosan or the H-amine interaction. The H-amine interaction actually makes it slightly easier to remove an electron.
All Cs/ZnO complexes show a positive electron affinity, meaning they can readily accept an electron, unlike pure Chitosan which requires energy for electron addition. This is a significant change, indicating a much stronger electron-accepting (electrophilic) character for the ZnO composites. The “O linkage” mode shows the highest electron affinity, suggesting it is the most capable of accepting electrons.
The chemical potential becomes more negative for all Cs/ZnO complexes compared to pure Chitosan. A more negative chemical potential indicates that the complexes are generally more stable and have a stronger tendency to attract electrons. The “O linkage” interaction leads to the most negative chemical potential, suggesting the most stable and electrophilic system.
The chemical hardness decreases for all Cs/ZnO complexes. This indicates that the complexes are “softer” than pure Chitosan, implying they are more reactive and less resistant to charge transfer. The “O linkage” complex is the softest, suggesting it would be the most reactive.
As expected, chemical softness increases for all Cs/ZnO complexes (being the inverse of hardness). This reinforces the idea that the composites are more reactive than pure Chitosan. The “O linkage” mode again stands out as the softest, and therefore potentially the most reactive.
The electrophilicity index significantly increases for all Cs/ZnO complexes. This is a crucial finding, as it indicates that the composites have a much stronger tendency to accept electrons. The “O linkage” interaction mode results in the highest electrophilicity, making this complex the strongest electron acceptor among those studied.
These findings are consistent with the observed reduction in the HOMO-LUMO gap from the previous table, which also points towards enhanced electronic activity. Such properties make Chitosan-ZnO composites highly promising for applications requiring electron transfer, such as in photocatalysis, sensors, and antimicrobial agents, where their ability to interact with and process electrons is key.
Charge distribution
To further quantify the charge redistribution at the chitosan–ZnO interface, Mulliken population analysis was performed on the optimized structures of the Cs/ZnO complexes. The results in Fig. 3 reveal a net electron transfer from the chitosan fragment to the ZnO unit, ranging from 0.12 to 0.28 e depending on the coordination mode (H-amine, O-linkage, or OH). This charge flow is most pronounced in the O-linkage configuration (~ 0.28 e), consistent with the highest observed increase in total dipole moment (TDM) and electrophilicity index. Such electron donation from the electron-rich amine/hydroxyl/oxygen sites of chitosan to the electron-deficient Zn centers enhances the polarity and charge-transfer capability of the nanocomposite, as evidenced by the increased TDM values and reduced HOMO–LUMO gaps. Although Mulliken charges are basis-set dependent, these findings qualitatively align with the trends in frontier orbital analysis and global reactivity descriptors, supporting the formation of interfacial interactions that improve the electronic properties of Cs/ZnO hybrids.
Fig. 3DFT calculated Mulliken Population analysis for the structures using B3LYP/LANL2DZ where a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage, and d- Cs/ZnO through OH.
Density of States DOS/PDOS
The total density of states (DOS) and atom-projected density of states (PDOS) were calculated at the B3LYP/LANL2DZ level to elucidate the electronic structure and orbital contributions. All plots are aligned with the Fermi level set to 0 eV for consistent energy referencing.
For pure chitosan (Cs) (Fig. 4a), the total DOS shows a clear separation between occupied and unoccupied states, with the valence band edge (HOMO) at approximately − 5.7 eV. The PDOS reveals dominant contributions from C 2p, N 2p, O 2p, and H 1 s orbitals to the occupied states, particularly near the HOMO. The absence of states in the gap region confirms the insulating character of Cs, with an approximate DOS-derived gap of ~ 5.7–6.9 eV, in good agreement with the frontier orbital HOMO–LUMO gaps reported in Table 1.
In the Cs/ZnO complexes (Figs. 4b–d), Zn incorporation introduces new states primarily from Zn 3 d and O 2p orbitals, which are strongly localized in the lower energy region (below − 5 eV). The occupied valence states remain dominated by C, N, O, and H contributions from chitosan, with only minor mixing from Zn near the band edges. The DOS-derived gaps remain approximately 5.7 eV in the 1:1 complexes (H-amine and OH modes), consistent with the frontier orbital values in Table 1. Minor numerical differences arise from plotting resolution and orbital overlap effects. The O-linkage mode exhibits a slightly narrower effective gap due to enhanced hybridization between chitosan and ZnO states.
These DOS/PDOS results as indicated in Fig. 4 quantitatively support the observed reduction in HOMO–LUMO gaps upon ZnO coordination (Table 1) by demonstrating the introduction of Zn-derived states that modify the electronic structure and enhance reactivity. The consistent energy alignment (Fermi level at 0 eV) across all figures and the close match between DOS edges and frontier orbital energies resolve any apparent discrepancies. The systematic underestimation of gaps by the B3LYP functional (due to self-interaction error) is expected for ZnO-based systems and does not affect the relative trends.
Fig. 4DFT calculated DOS and PDOS for the structures using B3LYB/LANL2DZ where a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage, and d- Cs/ZnO through OH.
Molecular electrostatic potential MESP
The molecular electrostatic potential (MESP) maps were calculated at the B3LYP/LANL2DZ level to visualize the charge distribution and potential interaction sites.
In pure chitosan (Fig. 5a), negative potential regions (red/orange) are localized around oxygen atoms of the –OH groups and glycosidic linkages, as well as nitrogen atoms of the –NH₂ groups, indicating electron-rich sites. Positive potential regions (blue) appear around the hydrogen atoms of the hydroxyl and amine groups, marking electron-poor sites.
For the Cs/ZnO complex through amine H (Fig. 5b), the MESP shows negative potential (red) concentrated around oxygen atoms of ZnO and chitosan functional groups, while positive potential (blue) is present around the hydrogen atoms of the amine group. This pattern is consistent with the modeled hydrogen bonding, where the positive region on the amine hydrogen aligns toward the negative oxygen of ZnO.
In the Cs/ZnO complex through O-linkage (Fig. 5c), negative potential (red) is observed around oxygen atoms of chitosan (including the glycosidic linkage) and ZnO, with positive potential (blue) around hydrogen atoms of nearby groups.
For the Cs/ZnO complex through OH (Fig. 5d), the hydroxyl oxygen exhibits strong negative potential (red), while positive potential (blue) appears around the hydrogen of the OH group.
Overall, the MESP maps illustrate the electron density distribution in pure Cs and the Cs/ZnO complexes, with negative potential localized on oxygen atoms (including ZnO oxygen) and positive potential on hydrogens of NH and OH groups, consistent with the modeled coordination and hydrogen-bonding modes.
Fig. 5DFT calculated MESP for the structures using B3LYB/LANL2DZ where a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage, and d- Cs/ZnO through OH.
Quantum theory of atoms in molecules QTAIM
The topological properties of the electron density were analyzed using the Quantum Theory of Atoms in Molecules (QTAIM) to characterize the nature of bonding at the chitosan (Cs)–ZnO interface. Bond paths, bond critical points (BCPs), and key topological descriptors — electron density ρ(r), Laplacian ∇²ρ(r), and total energy density H(r) — provide a rigorous classification of interactions in accordance with established criteria.
Figure 6 illustrates the molecular graphs for pure chitosan and the three Cs/ZnO coordination modes. In pure chitosan (Fig. 6a), the QTAIM analysis reveals a network of strong covalent bonds (solid lines connecting C, O, N, and H atoms) forming the molecular skeleton, along with numerous intramolecular hydrogen bonds (dashed lines with BCPs) between hydroxyl hydrogens and adjacent oxygen atoms, as well as amine hydrogens and oxygen atoms. These hydrogen bonds, together with the cyclic sugar units (indicated by yellow ring critical points), contribute significantly to the conformational rigidity, crystallinity, and film-forming ability of chitosan.
In the Cs/ZnO complexes (Figs. 6b–d), the QTAIM molecular graphs clearly show direct coordination between Zn atoms and heteroatoms (O or N) of chitosan, evidenced by bond paths and BCPs connecting Zn to O/N. The quantitative topological parameters at selected BCPs are summarized in Table 3.
Across all configurations, the Zn–O and Zn–N interactions exhibit positive ∇²ρ(r) values combined with negative H(r) values — a hallmark of partially covalent (dative) bonding with significant closed-shell character typical of metal–oxide/metal–organic coordination^28,29^. The highest electron density (ρ(r) = 0.1085 a.u.) and most negative H(r) (–0.0218 a.u.) occur at the Zn(70)–O(71) BCP in the O-linkage mode (Cs/ZnO through O linkage), indicating that glycosidic oxygen serves as the strongest and most stable anchoring site for ZnO.
The OH-coordination mode (Fig. 6d) shows a more diverse coordination profile, involving both O and N atoms, with the N(20)–Zn(71) interaction also displaying partial covalent character. In the H-amine mode (Fig. 6b), Zn primarily coordinates to oxygen atoms, while the amine group contributes via hydrogen bonding. Additionally, a weak non-covalent interaction (Zn(70)–H(47)) is detected in the O-linkage configuration (positive H(r)), suggesting auxiliary electrostatic or van der Waals contributions to overall stability.
These QTAIM results confirm that the Cs/ZnO interface is stabilized by a combination of partially covalent coordination bonds and intramolecular hydrogen bonding within the chitosan chain.
Table 3. Topological parameters (a.u.) at the BCPs of Cs/ZnO complexes.StructureBCP (A–B)ρ(r)∇2ρ(r)H(r)Interaction Nature Cs/ZnO through H of amine O(23)–Zn(70)0.04440.2091−0.0079Partially CovalentO(71)–Zn(70)0.09610.6009−0.0183Partially CovalentZn(70)–O(32)0.09430.5907−0.0174Partially Covalent Cs/ZnO through O linkage of Cs Zn(70)–O(71)0.10850.6996−0.0218Partially CovalentO(12)–Zn(70)0.09090.5150−0.0183Partially CovalentZn(70)–H(47)0.01240.0481+ 0.0009Non-covalent Cs/ZnO through OH of Cs N(20)–Zn(71)0.05430.2077−0.0139Partially CovalentZn(71)–O(70)0.10420.6707−0.0205Partially CovalentO(12)–Zn(71)0.02840.1201−0.0039Partially Covalent
Fig. 6DFT calculated QTAIM for the structures using B3LYB/LANL2DZ where a- Cs, b- Cs/ZnO through H of amine, c- Cs/ZnO through O linkage, and.
FTIR spectra analysis
Figure 7 is the FTIR absorbance spectra for the Cs and Cs/ZnO composites, for Cs (pure), the band at 3311 cm^− 1^ corresponds to O–H stretching vibration and the N–H extension vibration of the polysaccharide moieties of chitosan^46,47^. FTIR bands assignment for Cs and Cs/ZnO with different concentration of ZnO are recorded in Table 4. The band at 2926 ~ 2870 cm-1 is associated with symmetric or asymmetric CH_2_ stretching vibration^46^. The band at 1650 cm^− 1^ is attributed to C = O in amide group (amide I band) and at 1561 cm^− 1^ is NH-bending vibration in amide group^47^. The band at 1583 cm^− 1^ is attributed to N–H amide II bending and C-N stretching vibrations^48^. The band at 1375 cm^− 1^ is likely stretching vibrations of carbodiimides and CH_3_ in amide group of chitosan^49^. The band at 1024 cm^− 1^ corresponds to C–O stretching^47,48^. For Cs/ZnO, the band at 424 ~ 600 cm^− 1^ is the finger print for ZnO which correspond to Zn–O is in the range of 400 to 700 cm^− 1^ as reported by^31,50,51^. The band at 1583 cm^− 1^ in pure Cs exhibited a gradual shift to lower wavenumbers upon increasing ZnO percentage. In the Cs/ZnO (98:2) %, the band shifted to 1579 cm^− 1^ with an increase in intensity. This shift continued to 1559 cm^− 1^ in the Cs/ZnO (96:4) % composite, accompanied by a further increase in intensity. Notably the band remained at 1559 cm^− 1^ in the Cs/ZnO (94:6) % composite, while the intensity increased. Meanwhile, the O–H band at 3311 cm^− 1^, the C–O stretching band at 1024 cm^− 1^, and the Zn–O band in the range of 424 ~ 600 cm^− 1^ persisted in the Cs/ZnO composites, exhibiting an increase in intensity with increasing Cs/ZnO ratio. The systematic redshift of the amide II band (1583 cm^− 1^ to 1559 cm^− 1^) and the intensification of the Zn–O region (424–600 cm^− 1^) suggest strong interfacial interactions. These changes correlate with the coordination-induced electronic redistribution predicted by DFT, suggesting that the interface is stabilized by a synergy of hydrogen bonding and partial covalent Zn–O/Zn–N coordination, as further confirmed by QTAIM analysis.
Table 5 presented the DFT calculated IR spectra for Cs, Cs/ZnO scaled appropriately for B3LYB/LANLD2Z method and compared with FTIR spectra. The vibrational frequencies calculated at the B3LYP/LANL2DZ level were scaled by a uniform scaling factor of 0.9612. This factor is widely accepted and frequently applied for B3LYP/LANL2DZ calculations to correct for systematic overestimation of vibrational frequencies^52^. The scaled frequencies are presented in Table 5 and show significantly improved agreement with the experimental FTIR bands.
Fig. 7ATR**-** FTIR absorbance spectra for a- Cs (100) %, b- Cs/ZnO (98:2) %, c- Cs/ZnO (96:4) % and d- Cs/ZnO (94:6) %.
Table 4FTIR bands assignment for Cs and Cs/ZnO samples.StructureFIIRAssignmentCs (pure)3311O–H stretching vibrationN–H extension vibration2926 ~ 2870symmetric or asymmetric CH_2_ stretching vibration1650C = O in amide I1583N–H amide II1561NH-bending vibration1375stretching vibrations of carbodiimides and CH_3_1024C–O stretching.Cs/ZnO (98:2)1579N-H amide II424 ~ 600Zn–OCs/ZnO (96:4)1559N-H amide II424 ~ 600Zn–OCs/ZnO (94:6)1559N-H amide II424 ~ 600Zn–O
Table 5FTIR bands compared with DFT-calculated vibrational frequencies (unscaled and scaled) for Chitosan (Cs) and Cs/ZnO composites at the B3LYP/LANL2DZ level.Unscaled LANL2DZ: IRScaled LANL2DZ: IRFTIRAssignment350533693311O–H stretching365235103311N–H extension3046 ~ 31262928 ~ 30052926 ~ 2870CH2 stretching167016051583N–H amide II106710261024C–O stretching475 ~ 669457 ~ 643424 ~ 600Zn–O stretching
Optical properties
Figure 8-a illustrates the spectra recorded from diffuse reflectance measurements conducted using UV-Vis spectroscopy on Cs and Cs/ZnO nanocomposite films across a wavelength range of approximately 200–1000 nm. Figure 8-b demonstrates the Kubelka-Munk function F(R) vs. wavelength that displays several absorption peaks, especially inside the UV spectrum^53–55^.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:F\left(R\right)=\frac{{(1-R)}^{2}}{2R}\:.\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$\end{document}In this principle, R stands for the diffuse reflectance, and F(R) signifies the absorption coefficients.
These peaks are highly informative and may be physically associated with both the Cs and ZnO constituents of the composite system. The maximum at around 250–280 nm exhibits significant absorption. Blank Cs shows a slight absorption between 250 and 270 nm, mainly due to the movement of electrons in the carbonyl or amino groups found in glucosamine units which are attributed to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:n-\pi\:*$$\end{document} inter-band transitions^55,56^. ZnO possesses a substantial straight band gap of approximately 3.3 eV, equating to around 375 nm. The exciton absorption peak, attributed to bound electron-hole pairs, often appears at 250–280 nm in nanoscale ZnO, shifted from the bulk edge due to quantum confinement in diminutive nanoparticles^57^. Peaks between 300 and 330 nm signify charge transfer and defect states. This absorption may be associated with flaws or oxygen vacancies in ZnO nanoparticles. ZnO made in different ways often shows absorption below the bandgap due to surface defects, extra zinc atoms, and oxygen vacancies^58,59^. These defect states can absorb photons with energies below the bandgap; hence, they manifest in this region^60^.
The interaction between Cs and ZnO matrices, possibly via hydrogen bonding or electrostatic interactions involving -NH₂ groups in Cs and Zn²⁺, modifies the electronic environment of ZnO as confirming by ATR-FTIR data, and DFT calculations^61^.
Fig. 8a-The diffuse reflectance spectroscopy (DRS) and b-Kubelka-Munk function F(R) as a function of wavelength for Cs and Cs with different concentration of ZnO.
The amount of disorder content throughout the energy gaps in polymers can be realized through the application of Urbach’s energy. The absorption coefficient, establishes an exponential relationship to photon energy in the vicinity of the optical band edge in both amorphous besides crystalline substances. The formulation proposed by Urbach^58,59^ pronounces this relationship.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:F\left(R\right)={F\left(R\right)}_{O\:}\mathrm{exp}\frac{h\vartheta\:}{{E}_{u}}$$\end{document}The constant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{F\left(R\right)}_{O\:}$$\end{document} is the thickness of the band tails of localized states within the characteristically prohibited band gap in addition Eu is the Urbach energy. This width is a product of the amorphous features of the solids. Upon analyzing equation above, it develops seeming that the plot of ln \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:F\left(R\right)$$\end{document} against hυ should presentation a linear trend as detected in Fig. 9. The Eu values for the band tails are attained by investigating the inclines of the lines shown in Fig. 9 and are showed in Table 6. The values of Eu show a clear and dependable growth, starting at 0.344 eV for Cs(100%) and accomplishment 0.485 eV for Cs/ZnO (96:4%). This growth signifies a supplemented level of disorder with the rise of ZnO content.
Fig. 9. Ln F(R) versus photon energy for Cs and Cs with different concentration of ZnO.
Utilized Tauc plots to analyze UV-Vis diffuse reflectance spectroscopy (DRS) data, establishing the optical bandgap energies of pure Cs and Cs/ZnO nanocomposite films.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{\left[F\right(R)\cdot\:h\nu\:]}^{n}=A\left(h\nu\:-Eg\right)$$\end{document}where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:F\left(R\right)$$\end{document} Kubelka-Munk function (derived from reflectance data), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\:h\nu\:$$\end{document} Photon energy, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:Eg$$\end{document} band gap energy, A constant.
We employed the Kubelka-Munk function \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:F\left(R\right)$$\end{document} to determine the absorption coefficient. The presented it as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\left[F\right(R\left)hv\right]^2$$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\left[F\right(R\left)h\nu\:\right]^1/^2$$\end{document} vs. photon energy ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:h\nu\:$$\end{document} ) as direct and indirect band gap transition^63,64^ as shown in Fig. 10. The accompanying figures demonstrate that extending the linear segment of each curve to the energy axis revealed a consistent redshift in bandgap values with increasing ZnO content. The direct band gap decreased from 4.35 to 3.4 eV for pure Cs to 4.1–3.28 eV with 4 wt% ZnO incorporation. The indirect band gap decreased from 3.19 eV to 2.56 eV throughout the same interval as recorded in Table 6. The interaction between ZnO nanoparticles and the Cs matrix generates defect levels and localized states within the band structure. This procedure facilitates the occurrence of lower energy transitions, resulting in a gradual reduction of the bandgap. As mentioned before^62–64^, the band tailing we see might be caused by the creation of states below the bandgap and some effects of partial quantum confinement.
Regarding the discrepancies between DFT results and UV-Vis spectroscopy, the Kohn–Sham gaps ∆E represent single-particle electronic transition energies within the DFT framework. These values are typically underestimated by hybrid functionals such as B3LYP due to residual self-interaction errors and the inherent neglect of excitonic effects (electron-hole interactions). In contrast, the experimental Tauc-derived optical bandgaps incorporate many-body interactions, excitonic binding energies, defect states, Urbach band tailing, and potential quantum confinement effects within the nanocomposite. These combined physical factors explain why the calculated ∆E values (ranging from 2.2 to 6.9 eV) deviate systematically from the measured optical gaps, which fall within the 2.5–4.3 eV range^65–67^.
Table 6. The estimated direct indirect optical band gaps and Urbach energy of the samples.SampleDirect Band Gap (eV)Indirect Band Gap (eV)Urbach energy (eV)R squaredCs (100%)4.35–3.43.190.3442 ± 0.0990.9862Cs/ZnO (98:2%)4.31–3.53.5–2.910.3637 ± 0990.9916Cs/ZnO (96:4%)4.1–3.282.910.4852 ± 0.12980.9445Cs/ZnO (94:6%)4.35–4.383.3–2.560.3423 ± 0.16850.9615
Fig. 10 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\left[F\right(R\left)hv\right]^2$$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\left[F\right(R\left)h\nu\:\right]^1/^2$$\end{document} vs. photon energy ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:h\nu\:$$\end{document} ) as direct and indirect band gap transition of the samples.
Conclusion
This study demonstrates that the interaction of chitosan (Cs) with zinc oxide (ZnO) nanoparticles induces significant structural, electronic, and optical modifications, as revealed by DFT calculations (B3LYP/LANL2DZ) and experimental FTIR and UV-Vis diffuse reflectance spectroscopy. The formation of Cs/ZnO complexes markedly increases the total dipole moment (from 5.884 Debye in pure Cs to 14.049 Debye in Cs/2ZnO via O-linkage) and reduces the HOMO–LUMO energy gap (from 6.908 eV to as low as 2.239 eV in Cs/2ZnO via OH), indicating enhanced polarity, charge transfer, and electronic reactivity. Global reactivity descriptors further confirm increased softness, reduced hardness, and higher electrophilicity, with the O-linkage mode exhibiting the strongest electron-accepting character. DOS/PDOS, MESP, and QTAIM analyses provide a detailed mechanistic picture: Zn states localize in lower energy regions, while interfacial binding involves partially covalent Zn–O/N coordination and hydrogen bonding, stabilizing the hybrid structure. Experimentally, FTIR confirms interfacial interactions through systematic N–H bending shifts and the emergence of Zn–O bands, while UV-Vis DRS and Tauc analysis show redshifted optical bandgaps (direct: 4.35 eV to 3.28 eV; indirect: 3.19 eV to 2.56 eV at 4 wt% ZnO), attributed to ZnO-induced defect states and band tailing.
Limitations
The computational model employs small ZnO clusters (monomer/dimer) rather than periodic surfaces, limiting representation of extended crystalline effects. Chitosan is truncated to a 3-unit oligomer, which may overlook long-chain conformational influences. Additionally, the Kohn–Sham HOMO–LUMO gaps are systematically underestimated by B3LYP due to self-interaction error, leading to substantial differences from experimental optical bandgaps. These approximations, while sufficient for relative trends and local coordination mechanisms, should be considered when extrapolating to bulk properties.
Outlook
Future work should explore periodic DFT models for more realistic surface interactions, incorporate protonated chitosan (NH₃⁺) to better simulate acidic casting conditions, and include advanced characterization (XRD, SEM/TEM) alongside photocatalytic or antimicrobial performance testing. Such refinements will further elucidate the potential of Cs/ZnO nanocomposites as multifunctional materials for photocatalysis, sensing, and biocompatible optoelectronics.
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