Enhanced optical and electrical properties of CeO2NPs/chitosan nanocomposites

Cerium oxide nanoparticles (CeO2NPs) of different ratios (x = 5, 10, 15, and 20 in wt%) are successfully incorporated into chitosan (CS) to synthesize CeO2NPs/CS nanocomposites by solution cast method. FTIR and XRD analysis confirmed the effective incorporation of CeO2NPs into chitosan nanocomposites. TGA and DTG showed that the thermal stability of the as-prepared nanocomposites is improved. The CeO2NPs/CS nanocomposites exhibited enhanced light absorption capacity in the UV-visible range as x increases, owing to the CeO2NPs’ large bandgap. The transmittance of UV decreased for x = 10 and 15 nanocomposites. Light scattering enhanced for x = 5 and 10 nanocomposites, increasing reflectance. Compared to CS (5.3 eV), the optical energy bandgap lowers to 4.94 eV and 5.1 eV, respectively. Impedance spectroscopy research validates the impedance spectroscopy parameters’ dependency on CeO2NPs concentrations. Because of the growth of multiple polarization types, generating interfaces of numerous defects, and space charge polarization, the dielectric constant increases with increasing x (up to x = 15). The dc conductivity (σ DC) and the frequency exponent (S) are estimated using the universal Josher’s power law and applied to the ac conductivity data (σ AC). Obviously, (S) decreases with increasing temperature, which refers to the electrical conductivity that follows the hopping mechanism. In addition, according to the CBH model, the Coulomb barrier of charge carriers (Um) is estimated, showing decreasing values as increasing x and recording the lowest value for x = 15 nanocomposites. Nyquist plots (Z″&Z′) indicate one semicircle arc behavior for all samples. As x rises, the radius of semicircular arcs reduces, suggesting that (σ DC) increases. The enhanced characteristics of CeO2NPs/CS nanocomposites make them suitable for future bio-applications.


Introduction
Nanotechnologies and nanomaterials are now topics of intense research activity due to the better specifications of nanoparticles in large-scale daily applications [1]. It strongly relates to the so-called quantum size effect, which improves the electrical characterizations of materials. The increase in the surface area of nanoparticles to volume ratio improves their transport and chemical interactions, resulting in distinct physical properties [2]. The incorporation of nanoparticles into biopolymers results in biopolymer nanocomposites. They are used in biosensors, solar cells, super-capacitors, drug delivery, and other disciplines [3]. Chitosan is a cationic polysaccharide obtained by alkaline N-acetylation of chitin, the second-most abundant natural polymer after cellulose [4]. Despite its specific structural qualities, CS lacks a majority of the physical properties such as optical, photovoltaic, electrical, and mechanical capabilities. The modulation of these specifications of CS could expand its employment in more applications. One of the most routes to modify the chemical structure of chitosan is mixing with inorganic fillers, such as clay [5], multi-walled carbon nanotubes [6], and TiO 2 [7], to produce new organic/inorganic composites. These emergent composites retain organic and inorganic components; therefore, they possess significant characteristics. The organic materials are characterized by flexibility, insulation, ductility, and curing capacity, while the inorganic fillers are characterized by hardness, thermal stability, and toughness. Metal-organic polymers are well-known composite materials that contain an inorganic metal ion and are based on organic materials. They have a wide variety of applications, including energy storage [8,9], sensor creation [10], catalysis [11], and gas separation. These biocompatible composite materials have been used in medicine delivery [12] and food packaging [13]. Evaluating the physical characteristics of such biopolymer nanocomposites is necessary for particular applications. CuS NPs reinforced chitosan show enhanced thermal stability, dielectric constant, ac conductivity, and mechanical characteristics [14]. Fe 2 O 3 NPs loaded to chitosan introduced improved thermal, optical, and dynamic mechanical properties, as reported in our previous work [15]. Similarly, CeO 4 ZrNPs incorporated into chitosan enhanced the thermal degradation parameters and the mechanical properties of the chitosan-based nanocomposites [16]. Depending on the doped amount of C 60 , the optical constants and optical energy bandgap of C 60 /chitosan composites have altered substantially [17]. CeO 2 NPs have been identified as a novel nano-filler that may be loaded into tissues to protect against a wide range of reactive oxygen species [18]. Furthermore, CeO 2 nanoparticles promote wound healing in vitro and in vivo [19][20][21][22][23]. CeO 2 NPs coupled with chitosan, on the other hand, boost antibacterial activity [24]. CeO 2 NPs exhibited no negative impacts on liver enzyme activity, liver tissue, kidneys, or blood parameters regarding health safety. CeO 2 nanoparticles, on the other hand, improved the blood redox state [25].
The simple casting approach is used in this work to efficiently incorporate CeO 2 NPs into chitosan at various concentrations and investigate their influence on the structural, thermal, optical properties, dielectric constant, and AC&DC conductivity of chitosan-based nanocomposites. Furthermore, the most significant regulated factors, such as the critical concentration of CeO 2 NPs for the best results, are analyzed.

Materials and methods
We purchase chitosan (CS) powder of average molecular weight (100,000-300,000) and molecular formula (C6H11NO4)n from Acros Organics, and Cerium oxide of molecular formula (CeO 2 ) and particle size of less than 50 nm from Sigma-Aldrich.

Preparation of CeO2 NPs/CS nanocomposites
We stirred a 50 ml distilled water mixture, 2% acetic acid, and 750 mg chitosan magnetically at 25°C for three hours, then x wt% of CeO 2 NPs (x=0, 5, 10, 15, 20), sonicated separately for two hours in 10 ml distilled water, was slowly added into the mixture separately and stirred for one hour. We distributed the solution into leveled hydrophobic polystyrene Petri dishes (10 cm diameter). The solution is left to dry for 24 h at 40°C. Nanocomposite films of 50 μm thickness on average were finally peeled off from the trays and placed in sealed containers to avoid moisture exchange.

Characterizations
X-ray diffraction (XRD) patterns were performed using a 1390 Philips diffractometer with filtered Cu K α radiation at 40 kV and 20 mA. XRD data were obtained for samples with dimensions of 2×2 cm 2 , 2θ-step of 0.02°, and measurement range of 5°-70°.
The prepared nanocomposite films' absorptance, transmittance, and reflectance were measured using a UV-visible-NIR spectrophotometer (JASCOV-670) in the range of 200-900 nm (for a 1×3 cm 2 sample area). FTIR spectra were obtained (JASCO, FTIR-300 E. Spectrophotometer) in the spectral range 400-4000 cm −1 , with a resolution of 4 cm −1 for a 1 cm 2 sample area. TGA analysis was carried out (Labsys Evo (France) thermal analyzer) with a heating rate of 10°C min −1 under a nitrogen atmosphere (for a 10 mg sample). To assess the dielectric constant and AC conductivity of the nanocomposites, a 3532-50 lCR HiTESTER (Hioki, Nagano, Japan) was functioned at 10 2 -106 Hz range under different temperatures (298K-333K) (for 1 cm 2 sample area).
The dielectric constant (ε′) and the dielectric loss (ε″) are obtained from the following relation [26]: where ε′ and ε″ are the real and imaginary parts of the complex permittivity, respectively. d is the thickness of the sample, A is the cross-sectional area, and the angular frequency is ω=2πf. The permittivity of vacuum is The real Z′(ω) and imaginary Z″(ω) parts of the complex impedance are calculated using the following mathematical expressions [26]: where G is the measured parallel conductance.

Optical properties
UV-vis absorption spectra of CS and CeO 2 NPs/CS nanocomposites are represented in figure 4(a). CS films have modest absorption across the 200-800 nm wavelength range, but CeO 2 NPs/CS films demonstrate improved absorption as CeO 2 NPs concentration increases. It might be due to the CeO 2 NPs' broad energy bandgap up to 500 nm, resulting in significant UV absorption for the CeO 2 NPs/CS nanocomposites. This data shows that the CeO 2 NPs/CS nanocomposites preserved the CeO 2 NPs' intrinsic optical characteristics. Furthermore, as seen in the transmittance spectra in figure 4, CeO 2 NPs/CS nanocomposites virtually completely block UV light below 300 nm for x=5 and extensively block UV light for x=10 and 15 nanocomposites. Furthermore, the reflectance of the prepared CeO 2 NPs/CS nanocomposites films shows a substantial influence on CeO 2 NPs concentrations, as shown in figure 4(c). The nanoparticles will unavoidably  scatter light and increase reflectance, as seen for x=5 and 10, before declining again as CeO 2 NPs rise to 15% [35]. The UV blocking activity and low transparency of CeO 2 NPs/CS nanocomposites are attractive features in various applications [36]. The energy bandgap (E g ) of CS and CeO 2 NPs/CS nanocomposites is estimated by extending the straight-line in the plot of (αhν) 1/2 against (hν) shown in figure 5. (E g ) of CS is in the range of  Figure 6 depicts the variation of ε′ with f (10 2 -10 5 Hz) for CeO 2 NPs/CS nanocomposites films (x=0, 5, 15, and 20) at various temperatures (297 K-333 K). It is obvious that ε′ can be modified at different temperatures and frequencies by adding varying amounts of CeO 2 NPs into the CS film. For x=15 samples, the greatest value of ε′ (270) is obtained at 100 Hz and 333 K. Because of the accumulating effect of CeO 2 NPs for x>15 nanocomposites, it seems that the ε′ range grows for x=5 and 15 nanocomposites, then decline for x=20 nanocomposites. This improvement in ε′ can be attributed to the creation of various forms of polarization due to the integration of CeO 2 NPs, which produce interfaces [37,38]. Under an external electric field, the interfaces mentioned above have a significant number of defects with uneven charge distribution and, as a result, space charge polarization [39]. ε′ is substantially enhanced at lower frequencies due to dipoles' inclination to align with the electric field. However, when f grows, the space charge polarization response cannot follow the change in the electric field, resulting in a decrease in the polarization contribution to ε′. The free carriers and ionic conductivity increase as the temperature rises [40,41]. AC spectra at various temperatures are taken to explore the dependence of the ionic conductivity of the produced materials on x. In this context, electrical conduction may be defined by observing the behavior of the dispersion zones and using the well-known Josher's universal power low (equation (3)), which can be used to determine the nature of the ionic dynamics by computing the frequency exponent (S) [42].

Dielectric constant and AC&DC conductivities
is the angular frequency, and (S) is the frequency exponent. Figure 7 depicts the dependency of log (σ AC ) on log (ω) at various temperatures, along with the fitting curves for x=0, 5, 15, and 20 nanocomposites. The estimated values of (S) presented in table 2 drop noticeably as temperature rises, suggesting that the electrical conductivity of these materials may follow the hopping process. For x = 15 and 20, the electrical conductivity rises. This increase in electrical conductivity by increasing CeO 2 NPs content in CeO 2 NPs/CS nanocomposites attributes to the function of metal oxide nanoparticles in bridging the gap between two localized states, lowering the potential barrier and allowing for a more straightforward charge carrier transfer [43]. The Coulomb barrier of charge carriers (Um) is calculated using the Correlated Barrier Hopping (CBH) model (equation (4)), as shown in table 2 [42].  These results demonstrated the role of CeO 2 NPs in enhancing the dc electrical conductivity by reducing the Coulomb barrier at high temperatures, which suggests that thermal induction could increase the degree of overlap of potential Colombian barriers at local locations [44].
The Nyquist plots reveal one semicircle arc behavior for all samples, notably at high temperatures. A lowfrequency tiny spike was detected for x=15 and 20 nanocomposites at T = 333 K due to the greatest resistivity resulting in the lowest ionic concentration and mobility (figure 9). The x=20 nanocomposites have a longer spike length than the x=15 nanocomposites. The electrical bulk resistance is calculated by intersecting the imperfect semicircular arc with the Z′ axis. Semicircular arcs with decreasing diameters confirmed the increase in σ DC for x=15 and 20 nanocomposites.

Conclusions
CeO 2 NPs/CS nanocomposites were successfully prepared and analyzed by XRD, FTIR, TGA, and DTG. The optical properties and ac & dc conductivities of CeO 2 NPs/CS were investigated in order to assess the impact of included CeO 2 NPs on the characterizations of the chitosan nanocomposites. FTIR and XRD results confirmed the efficient incorporation of CeO 2 NPs into chitosan nanocomposites. TGA and DTG results show that thermal stability has improved. According to the transmittance spectra, the UV radiation shielding performance of CeO 2 NPs/CS nanocomposites strengthened. The optical energy bandgap (Eg) decreases, particularly for x=5 and 10 nanocomposites. The enhancement of σ AC and σ DC for CeO 2 NPs/CS nanocomposites relates to the ability of the CeO 2 NPs to lower the Coulomb barrier. The lowering of (S) with temperature indicates that ac conductivity follows the hopping process. The semicircular arcs observed in the Nyquist plots show decreasing diameters as the CeO 2 NPs concentration increases. These results demonstrate the rise in σ DC , particularly for x = 15 nanocomposites. CeO 2 NPs/CS nanocomposites are promising for bio-applications due to their superior structural, optical, and electrical characteristics.