4.1. UV-vis. absorption results
Determination of the optical characteristics of polymers and metal oxides is of a great interest to investigate their different applications. Fig. 1 shows the absorbance spectra of pure Cs, Cs/CuO films and Cs/GO film versus wavelength collected in the optical range from 200 to 1000 nm. In certain cases, the absorption spectrum measurements of CuO nanoparticles produce an optimum absorption band at 242 nm and another band in the range of 400-600 nm (Rashad, M. et al. 2013) as illustrated in Fig. 1. There is another band at nearly 670 nm and its intensity increased with increasing CuO content. As presented in the figure, the characteristic absorption band of Cs appeared at nearly 214 nm which is attributed to the n-π* and π-π* transition of C=C and C=O, and refers to the presence of acetate group residuals. The absorption shoulder beyond 300 nm is attributed to the CuO nanoparticles.
The absorption spectra of CMC, CMC/TiO2 nanocomposite films that contains different amounts of TiO2 (2, 4, 6 and 8 wt%), and CMC/GO film, recorded in the range of 200-800 nm, are displayed in Fig. 2. As displayed in the figure, the absorbance of pristine CMC was strongly influenced by the addition of TiO2 and GO separately. As the CMC absorption edge undergoes a shift towards the higher wavelength region, the figure shows that the intensity of absorption increases with increasing TiO2 content in the samples which refers to the strong complexation between CMC, and TiO2 and GO separately. The characteristic absorption band of TiO2 nanoparticles undergoes a slight blue shift with decreasing the CMC content in the prepared nanocomposites. The characteristic absorption band observed at 207 nm was ascribed to n- π* transitions and that at 345 nm was attributed to π-π* which is characteristic for the TiO2 nanoparticles, and similarly for CMC/GO film (Ali H. E. et al. 2015; Alsulami Q. A. and Rajeh A. 2021). This means that the CMC/TiO2 nanocomposites have lower particle size.
Figure 3 shows the UV-Vis absorption spectra of pure Na-Alg, and Na-Alg/TiO2 and Na-Alg/GO nanocomposites. The characteristic absorption peak of TiO2 nanoparticles is clearly obvious in the Na-Alg/TiO2 nanocomposites absorption spectra. The redshift of the absorption peaks from 346 to 360 nm can be attributed to the particle size variation during the reaction. Size and morphology of the prepared nanoparticles control the electronic properties of the nanocomposites (Farea M. O. et al. 2020; Flores-Hernández C. G. et al. 2021). At low concentrations of TiO2, very broad absorption peak is observed with very low intensity which shows that the number of particles is small with larger particle size. As the concentration increases, the absorption peak becomes narrower with increment in the intensity together with a redshift in the wavelength, indicating that larger particles are formed.
4.2. Optical bandgap results
The optical absorption of polymer nanocomposites is the most important parameter through which the understanding of the band structure of all solids becomes possible. The Beer–Lambert law can be utilized to determine the absorption coefficient (α) of the prepared nanocomposites. The absorption coefficient can be determined directly using the following equation:
α = (2.303*A)/d (1)
Where, A is the absorbance and d is the sample thickness. It is known that the optical bandgap of the different materials can be determined from the electron transition between valance and conduction bands (the fundamental absorption). As reported by Tauc and Davis - Mott, the bandgap values, and the type of electron transition between the valence and conduction bands can be determined using the following equation:
(αhυ)r=B(hυ-Eg) (2)
Where hυ is the energy of the incident photons, B is a constant and r is a constant that depends on the type of transition. The constant r takes values of 2, 1/2, 2/3 and 1/3 for direct allowed, direct forbidden, indirect allowed or indirect forbidden transition respectively (Badry R. et al. 2021-a and b).
Based on previous work (Kocyigit A. et al. 2020) the type of electronic transition in Cs is direct allowed transition. The direct bandgap energy of pristine Cs, Cs/CuO nanocomposites and Cs/GO nanocomposites is plotted as a function of photon energy in Fig. 4- a, b, and c respectively. Based on the intercept of the linear part of the curves with the photon energy axis, the values of the direct bandgaps were calculated from the last three figures and recorded in Table 4.
From the figure, the direct bandgap energy value of pure Cs is determined and is equal to 2.66 eV, while those of Cs/CuO nanocomposites, as tabulated in Table 4, are 1.66, 1.73, 1.62 and 1.47 eV for 2, 4, 6 and 8 wt% respectively. On the other hand, for Cs/GO nanocomposites, two absorption shoulders were observed corresponding to two bandgaps. The direct allowed bandgaps of Cs/GO film were estimated as 1.22 and 5.68 eV. The reduction of the Cs bandgap can be attributed to the formation of chemical bonds between Cs and CuO nanoparticles that causes the localized states to generate energy between the bands of the highest occupied molecular orbital and the lowest unoccupied one, thus making the lower transitions feasible (Harun M.H. et al. 2009). Therefore, decreasing the direct allowed optical bandgap of pure Cs was achieved with the incorporation of CuO and GO. This result reflects that with increasing CuO content, the direct allowed energy bandgaps decrease in the Cs matrix by increasing the conjugation between Cs unsaturated bonds and CuO nanoparticles, aiming to reduce the energy bandgap. Previous work confirmed that organic polymers and their nanocomposites with reasonable optical bandgaps are imperative for photonics, and optoelectronic applications. It can then be concluded that Cs/CuO and Cs/GO nanocomposites can be used in optoelectronic devices.
Table 4
Direct allowed bandgap energy values for pure Cs and Cs substituted with 2, 4, 6 and 8 wt% CuO, and 2 wt% GO at room temperature.
Sample | Eg1 (eV) | Eg2 (eV) |
C1 | 2.66 | - |
C2 | 1.66 | - |
C3 | 1.73 | - |
C4 | 1.62 | - |
C5 | 1.47 | - |
C6 | 1.22 | 5.68 |
Regarding the CMC/TiO2 nanocomposite films, it was found that CMC has more than one direct bandgap due to its amorphousity, as presented in Figure 5, which are equal to 3.93 and 5.04 eV. Table 5 presents the different values of the optical bandgap for pristine CMC, CMC/TiO2 and CMC/GO. Figure 5 demonstrates that the addition of different weight percentages of TiO2 (2, 4, 6 and 8 wt%) to CMC reduces the optical bandgap of CMC. Based on the obtained results, it was found that all the prepared samples have two direct bandgaps and one indirect bandgap due to the formation of localized states within the valence band of CMC (Elkomy G. M. et al. 2016). For CMC/TiO2 samples, the first bandgap initially decreased until reaching 2.11 eV and then increased again. The reason for this decrease in the bandgap may be attributed to the decrease in the particle size due to the quantum effects. However, increasing the direct optical bandgap beyond 6 wt% of TiO2 refers to the formation of ion clusters which decreases the mobility, hence increasing the bandgap. These values are determined as presented in Figures 5 and 6. Therefore, it can be concluded that the sample containing 6 wt% TiO2 is the preferred one as it has the lowest bandgap energy. On the other hand, for CMC/GO nanocomposite film, the first bandgap of CMC decreased to 2.04 eV. This decrease in the CMC optical bandgap is attributed to the presence of a new energy levels within the optical bandgap, which facilitates the electrons movement from the valence band to these localized states within the conduction band, consequently increasing conductivity and decreasing the optical bandgap (Murri R., et al. 1992). However, the second bandgap corresponding to the second absorption edge in the low wavelength region changes irregularly due to the addition of TiO2 and GO to CMC.
Table 5
Direct (Edg1 and Edg2) and indirect (Eig) optical bandgap of pure CMC, CMC substituted with 2, 4, 6 and 8 wt% TiO2 and 2 wt% GO
Sample | Edg1 (eV) | Edg2 (eV) | Eig (eV) |
C1 | 3.93 | 5.04 | 4.66 |
C2 | 2.50 | 5.11 | 4.24 |
C3 | 2.30 | 5.34 | 4.33 |
C4 | 2.11 | 4.91 | 3.05 |
C5 | 2.55 | 4.76 | 1.86 |
C6 | 2.04 | 3.31 | - |
Figure 7 shows the plot of (αhυ)2 versus the photon energy (hυ) for pure Na-Alg, Na-Alg/TiO2 and Na-Alg/GO film. It is established that, for direct optical bandgap materials, the valence band top and the conduction band bottom have the same zero k-vector. All of the prepared samples possess two optical bandgaps; the onset bandgap equals 3.90 eV and the HOMO/LUMO gap equal 5.57 eV for pure Na-Alg. It is critical to show that the optical bandgap decreased from 3.90 eV for pure Na-Alg to 2.21 eV for Na-Alg doped with 6 wt% of TiO2 nanoparticles. Additionally, the HOMO/LUMO bandgap of pure Na-Alg (second bandgap) was decreased to 3.57 eV corresponding to 6 wt% of TiO2. However, the optical bandgap for all samples is still smaller than that for pure Na-Alg. Additionally, Na-Alg/GO nanocomposites have two optical bandgaps. However, the onset bandgap decreased strongly with the addition of GO when compared with that of both pure Na-Alg and Na-Alg/TiO2 samples. The small optical bandgap of the Na-Alg/TiO2 and Na-Alg/GO nanocomposites confirms its suitability for optoelectronics, photonics, and solar cell applications (Murri R., et al. 1992; Elkomy G. M. et al. 2016)
Table 6
Optical direct bandgap for pure Na-Alg and Na-Alg substituted with 2, 4, 6 and 8 wt% TiO2, and 2 wt% GO nanocomposite films
Sample | Eg1(eV) | Eg2(eV) |
N1 | 3.90 | 5.57 |
N2 | 2.31 | 5.42 |
N3 | 2.44 | 5.08 |
N4 | 2.21 | 3.57 |
N5 | 2.21 | 3.61 |
N6 | 2.10 | 5.52 |