Influence of incorporation of gallium oxide nanoparticles on the structural and optical properties of polyvinyl alcohol polymer

In the present work, gallium oxide nanoparticles (nGa2O3) are synthesized via the thermal microwave combustion method, while nanocomposites of polyvinyl alcohol (PVA) polymer with various concentrations of nGa2O3 (0, 1, 2, 3, 4, and 5 wt%) are prepared by the casting technique. The structural characterization of nGa2O3, PVA, and films of PVA-Ga2O3 nanocomposites are studied using X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM), and Fourier-transform infrared spectroscopy. The HRTEM and XRD examinations showed that the prepared nGa2O3 has an average crystallite size of ~5.6 nm and particle size of ~0.9 μm. On another side, the optical transmission spectra were performed in the spectral range 250 to 2500 nm at room temperature. The refractive index, absorption coefficient, and optical bandgap (Eg) were determined using the WempleDiDomenico single oscillator model. It was shown that Eg slightly reduced from 3.61 to 3.55 eV with increasing the Ga2O3 content to 3 wt%, while raised again up to 3.58 eV for 5 wt% Ga2O3. Other optical characteristics such as the optical density, extinction coefficient, refractive index, optical susceptibility, thermal emissivity, optical sheet resistance for the PVAGa2O3 nanocomposites are investigated. The linear and nonlinear optical parameters together with their dependencies on the doping ratio reveals the qualification of PVA-Ga2O3 nanocomposites for nonlinear optical applications.


Introduction
One of the most exciting challenges in polymers science to the synthesis of polymer composites doped with nanomaterials of metals and semiconductors to improves their performance in industrial applications as sensors, light-emitting diodes, energy harvesting applications, etc. [1,2,3]. Polyvinyl alcohol (PVA) could be considered as a main applicable polymer in different filed of industry [4,5,6,7]. The incorporation of nanoparticles within polymers attracts many researchers due to the change in the physicochemical properties of the produced polymer composites [8] [9]. The PVA doped with metals and metal oxide nanoparticles, as examples, benefits biological and industrial applications [10,11]. For example, Ni and Co nanoparticles have been doped within PVA to be used for gas sensing applications [12]. Meanwhile, the composites of PVA that are doped with a carbon nanotube can be used in fuel cell applications amongst other applications [11]. Also, the doping of PVA by nano-titanium dioxide improves the mechanical and thermal properties of the PVA [13].
Similarly, the thermo-mechanical properties of the PVA are enhanced by blending with nanographene [14].
It generally observed the incorporation of the PVA with nanostructured materials, which is similar to other types of materials, affects the properties of the produced composites. To elaborate on that, the addition of nano-oxide materials to PVA caused degradation in PVA's main chain [15]. The degradation in the PVA was attributed to the strong interaction between the incorporated and the host material. As well, the degradation in the PVA's main chain produces a change in the intensity of IR absorption bands of the polymers which resulted in changes in its electrical and optical conductivities [13]. Meanwhile, adding nano-oxide elements as a dopant to the PVA films leads to a formation of a new carbonyl group C=O which plays an important role in the degradation of the PVA [16]. The optical conductivity of the PVA can be improved by blending the PVA with some metal oxides such as Fe2O3 and NiO to form the PVA composites [17].
Nanoparticles of Ga2O3 have significant structural and optical characterizations include long metastable state lifetime and high quantum efficiencies [18]. Accordingly, the Ga2O3 nanoparticles can be used as white-light emitters, for water purification, and emits photons in the visible region during down and up-conversion mechanisms [19,20,21]. Also, the presence of Ga2O3 nanoparticles within the PVA main chain matrix affects the optical quality and improves electronic properties for the treated polymer. The electronic transitions for Ga2O3 between 4f states are attributed to the absorption and emission of photons and characterized with sharp lines relative to host material [15]. Accordingly, the doping of Ga2O3 nanoparticles within PVA's main chains could affect the structural parameters and hence the optical characterization.
The present work is devoted to synthesizing Ga2O3, and various PVA-Ga2O3 nanocomposites. Besides investigating the influence of Ga2O3 content on the structural parameters and optical properties of the PVA-Ga2O3 nanocomposites compared to the pure PVA. The study is carried out using various techniques such as the XRD, FTIR, HRTEM, and double beam spectrophotometer.

Preparation of nGa2O3
The Ga2O3 nanoparticles (nGa2O3) are prepared by the thermal microwave combustion method, and more details available elsewhere [22]. Around 25 mL containing 0.2 M Ga(NO3)2⋅6H2O and 0.2 M Ga(NH2)2⋅6H2O is mixed in a round-bottom flask. The prepared product is put inside a microwave oven operating at 650 W (Nanjing Sanle General Electric Corporation, China) irradiated for 20 min. Finally, a fine powder with a bright green color of nGa2O3 is obtained.

Structural studies
XRD was implemented to examine the crystal structure of the formed phase(s) in Ga2O3, PVA, and PVA-Ga2O3 composites since it is an essential method for the study of the order of crystallization and orientation.  The FTIR spectroscopy is used to determine the various kinds of interaction between PVA and Ga2O3 nanoparticles based on investigating their vibration modes [26]. The FTIR spectrum of PVA and PVA-Ga2O3 nanocomposites films are shown in Fig. 3. In the case of pristine PVA film, the spectrum exhibits a broad peak between 3000 and 3600 cm -1 and attributed to the OH stretching vibration mode. The observation of the broad peak indicates the presence of many free OH groups. The position of the bottom of the OH transmission band for all PVA-Ga2O3 samples shifts to a higher frequency with the increase in Ga2O3 concentration.
The bottom position of the OH band was found at 3291.5, 3292.16, 3293, 3293.8, and 3297.8 cm -1 for pure PVA, and PVA-Ga2O3 contains 1, 2, 3, and 4 wt% of nGa2O3, respectively. This result demonstrates the reaction of Ga2O3 molecules with the OH groups in the PVA matrix.
The complex formation between Ga2O3 molecules and PVA structure is expected. Therefore, hydrogen bonding formations play an important factor in that process. The band located at

Optical studies
The optical properties of PVA and PVA-Ga2O3 nanocomposites films are evaluated by applying the Wemple-DiDomenico (WDD) model [29]. The optical transmittance (T) for the PVA and PVA-Ga2O3 samples are shown in Fig. 5. From the figure, the pure PVA sample is highly transparent while the increase of Ga2O3 content reduces the transmission of the fabricated PVA-Ga2O3 films. This observation may be related to the change of the local configurations of Ga 3+ ions, as reported by Zhao et al. [30]. Besides, for λ<320 nm, the transmittance is closed to zero for all the studied films. This phenomenon reveals the high absorption of incident photons at a shorter wavelength region. For a wavelength equals to 520, 710, and 980 nm, there are 3 peaks related to the scattering that may happen within the prepared composites films.
The important features for understanding the optical absorption and the refractive index of materials to design various optoelectronic devices. The absorption coefficient (α) of pure PVA and PVA-Ga2O3 nanocomposites are calculated using the following equation [31]: where x is the sample thickness which is about 0.08 mm for all the investigated samples. The optical energy for the bandgap is calculated using the following expression [32]: where b is a constant called the band tailing parameter, Eg is the energy bandgap, and A is a constant that characterizes the transition process. For an allowed direct transition, A equals 2, while for indirect allowed transition equals ½. Figure 7a shows plots of (αhν) 1/2 versus the photon energy (hυ) for pure PVA and PVA-Ga2O3 nanocomposite films. According to Eq. 2, the numerical value Eg can be estimated from the slope and the intercepts with the x-axis in   [37,38]. It is notified that Eg decreases from 3.61 eV to 3.55 eV as the Ga2O3 contents increased up to 3 wt%, while increased again to 3.58 for PVA-Ga2O3 composite contain 5 wt% Ga2O3. The observation deceased in Eg could be attributed to a high electronegativity of Ga +3 which creates additional lone-pair electron energy, and the additional lone pair energies will increase the valance band and decrease the optical band gap within studied composites.
In the lower absorption region, the values of α(λ) are obeyed to the Urbach relation and the Urbach energy or the Urbach tail width can be determined from [39]: where 0 is a constant. The plots of ln(α) against hυ for the PVA-Ga2O3 composite are presented in Fig. 7b. The evaluated Ee for various samples is calculated from the inverse of the slope in The estimated values of the refractive index can be used to estimate other optical parameters including dielectric constant ( 0 ), the linear susceptibility (χ (1) ), third-order susceptibility (χ (3) ), and nonlinear refractive index (n2) from the next equations, respectively [43] [44] [45]: where B=1.7×10 -10 for χ (3) in the electrostatic system of units (esu). The determined values of all these parameters for the PVA-Ga2O3 nanocomposites are listed in Table 2. The numerical value of 0 for PVA-Ga2O3 nanocomposites between 5.47 and 5.52 and their change behavior is similar to 0 .
In this section, we have discussed two important parameters that influence by λ or hν which can be deduced from the obtained optical parameters. The first one is the optical surface resistance (Rs) as a function of λ and given by [45]: where c is the speed of light in a vacuum, and n is the refractive index of the investigated material. Fig. 9 shows the plots of Rs versus λ for PVA-Ga2O3 nanocomposites irradiated for various doses. It is observed the trend of with λ is quite similar to the trend of T(λ). For example, Rs is almost constant for λ˂ 340 nm, then rapidly increased (more than 10 times) to remain constant again for the rest of λ. Besides, the change in reveals a reduction as the ratio of Ga2O3 increased up to 3 wt%, while increased again for a further increase in Ga2O3 content.

Conclusion
The   Figure 1: The XRD charts for pure Ga2O3 nanoparticles, pure PVA, and PVA-Ga2O3 nanocomposites.