Structural, morphological and optical bandgap analysis of multifunction applications of Y 2 O 3 -ZnO nanocomposites: Varistors and visible photocatalytic degradations of wastewater

In this study, a combustion method as an efficient, easy, low-cost, and eco-friendly technique was used to synthesize nano-ZnO as a matrix with different yttrium doping ratios with different doping concentrations. Not only X-ray diffraction (XRD), but also scanning electron microscopy (SEM), and Fourier transformation Infrared spectroscopy (FT-IR) technique employed to characterize the structural and surface morphology of the Y 2 O 3 -ZnO nanocomposites. The obtained results supported ZnO's growth from crystalline to satisfactory nanoparticle structure by changing the yttrium doping degradation of 100 % of phenol, Methylene Blue, and Rhodamine B solutions compared to 80% of photocatalysis for ZnO stand alone. The prepared Y 2 O 3 -ZnO nanostructured materials are considered novel potential candidates in broad nano-applications ranging from biomedical and photocatalytic degradation for organic dyes and phenol to environmental and varistor applications.

Importantly, the combination of ZnO with Y 3+ -ions as ZnO: Y nanostructures are deliberated to enhance photocatalytic activities, display transparent coatings, improve UVemission in photoluminescence (PL), etc. [24,25]. There are several research studies considering yttrium (Y 3+ -ions) as a rare earth element to be used in enhancing the optical properties of ZnO [26]; however, to the best of the current knowledge, there aren't many studies focus on Y2O3-ZnO being utilized as a photocatalyst, and varistors ceramic devices.
In this present study, Y 3+ -ions (at different concentrations) were doped with ZnO using the combustion method, which is a considerably low cost, simple, and effective technique. The prepared Y2O3-ZnO nanostructures are smart and multifunctional materials to be applied in various technological, environmental, and electronic applications, including varistors, and photocatalysis of waste materials. Here, X-ray diffraction (XRD) and scanning electron microscopy (SEM) was applied to characterize the structural and surface morphology of the Y2O3-ZnO nanocomposite materials. The enhancements in the optical properties of ZnO with yttrium (Y) with different concentrations were examined using UV-Vis optical diffuse reflectance spectroscopy (ODR) and Fourier transformation Infrared spectroscopy (FT-IR).
Besides, studying the electrical behaviors of Y2O3-ZnO nanocomposite materials as nanoscale ceramic varistors were investigated through current-voltage measurements and dielectric analysis. Visible photodegradation concerning illumination time of phenol, Rhodamine B, and Methylene Blue were used to evaluate ZnO and Y2O3-ZnO photocatalytic performance.

Synthesis of Y2O3 -ZnO nanocomposites.
The combustion method was used to prepare ZnO nanoparticles (NPs) doped with rareearth yttrium (Y 3+ -ions). First, 5 g of Zn(NO 3 ) 2 . 6H 2 O was mixed and grinding well in ceramic crucibles with one gram of gum acacia. Eight different yttrium (Y 3+ -ions) of different concentrations ranging from 0.001 g to 5 g (i.e., named as So, to S7 as illustrated in Table.1), respectively, were added separately to the previous mixture and dissolved in 5 mL of distilled water. The prepared mixtures of Y2O3-ZnO nanostructured were heated at 600 o C for 2 hours and then left to cold down to room temperature. In this preparation technique, gum acacia is considered a fuel to support the formation of ZnO nanostructure conversion from high crystallinity to nanoscale by enlarging their constituents inside the ZnO matrix. The final 5 obtained materials of ZnO-nanoparticles doped with yttrium-ions at various concentrations are shown in Fig. 1.

Table. 1.
Sample code of pure and doped ZnO nanostructures with different doping concentrations of yttrium Y 3+ -ions.

Devices & Measurements.
X-ray diffractometer (XRD) is a vital tool to investigate the prepared ZnO nanocomposites with different yttrium doping ratios. In this study, XRD analysis was achieved through Shimadzu LabX-XRD-6000, utilizing filtered radiation of CuKα (= 1.5406 Ǻ) at room

g Y-doped ZnO S4
1 g Y-doped ZnO S5 Moreover, The UV-visible diffused reflectance spectra of as-prepared Y2O3-ZnO nanostructured materials was verified through an integrated sphere device coupled to a UV-3600 UV/Vis spectrophotometer (Shimadzu, Japan). Here, the wavelength ranged from 200 nm to 1600 nm, the step scan was 5 nm, and the reference material was barium sulfate.  ) in the solution has been analyzed via the following absorbance at 269, 510, and 665 nm, respectively, using the JASCO V-550, Japan UV-Vis spectrophotometer in the wavelength range of (200-800 nm).

Results and discussion
3.1. Structural and morphological characterization of Y2O3-ZnO nanocomposites.

X-ray Diffraction patterns.
XRD patterns of Y2O3-ZnO nanocomposites with different doping ratios are illustrated in Fig. 1   Here, the crystallinity size (D) of the prepared Y2O3-ZnO nanocomposites were evaluated through the following Scherrer's equation, which was used to analyze the XRD data [29,30]: 8 Here, λ is the X-ray wavelength in the unit of nm, θ is the diffraction angle in the degree unit, and  is the full width at half maximum (FWHM) in the unit of a radian. Respectively, the values of dislocation density ( ) and lattice strain (  ) of Y2O3-ZnO nanocomposites were calculated from the following equations 2 and 3 [29,30]:

Scanning Electron Microscopy.
In this study, scanning electron micrographs were applied to investigate the as-prepared Y2O3-ZnO nanocomposites at different yttrium concentrations growth topographies and surface morphology. Fig. 3 showed the SEM images of the studied Y2O3-ZnO nanocomposites at different yttrium doing ratios (i.e., So to S7). The changes in the structural morphology of the pure ZnO and yttrium doped ZnO nanocomposites were directly noticeable. The SEM images of the studied Y2O3-ZnO nanocomposites showed mostly a uniform spherical nanoparticles distribution of Y2O3-ZnO nano-samples. It is clear that the yttrium ions create small-sized grains and encourage the crystal nucleation rate, while the trapping of yttrium grains prevents grain development. This issue could be due to the difference in ionic radius between zinc and yttrium [31].
As the yttrium doping concentrations increased, the shape and the size of the nanoparticles were varied. The obtained conclusion of SEM images agreed very well with the morphology characterizations of Y 3+ -doped TiO2 samples, a sol-gel prepared, by X. Niu et.al [21]. The grain sizes from SEM images were agreed well with the XRD results. The results of scanning electron microscopy (SEM) indicated that the crystalline size could be efficiently reduced with yttrium dopants [21].

Fourier transformation infrared (FT-IR) spectroscopy.
FT-IR spectroscopy was used to investigate the studied Y2O3-ZnO nanostructured samples' vibrational properties because the FT-IR is an extremely sensitive characterization technique and can contribute to XRD findings. The FT-IR spectra illustrate the relation between the light absorbance and the wavenumber within the wavenumber range from 400 cm -1 to 7000 cm -1 for the Y2O3-ZnO nanostructured at various doping concentrations, as shown in Fig. 4. It can be seen that the yttrium doping affects the shape and the intensity of the leading absorption bands, and the changes in the peak patterns were more noticeable in the Y2O3-ZnO curves than the undoped ZnO.
In the FT-IR spectral range, there is an overlapping of IR lines, which contributions to the phases of both ZnO and Y2O3. There were slight improvements in the optical transparency respecting the undoped ZnO sample in the samples of low yttrium concentrations. As the yttrium doping increased, the transmittance of ZnO-Y2O3 samples showed lower values, which might be due to the higher scattering. Therefore, the highest doping ratios of Y2O3 (i.e., S7) has the highest absorbance, which is in good agreement with the structure morphology results

Optical band gap analysis using diffused reflectance spectroscopy.
The optical diffuse reflectance (ODR) measurement is considered a conventional 1.0x10   [34,35]. It was noted that the UV-vis diffuse reflectance measurements are in a great match with the J. Zhao et. al study of Y 3+ doped Bi5Nb3O15, which was synthesized via the sol-gel approach [23]. The obtained results of UV-vis characterization displayed that the absorbance edges of Y 3+ -Bi5Nb3O15 noteworthily move to the visible-light range [23].
The optical bandgaps Eg of the synthesized Y2O3-ZnO nanostructured were calculated using Tauc's model, as in the following equations [36,37]: 16 Here, F(R) represents the material reflectivity using the model of Kubelka-Munk, R is wellknown as the optical diffuse reflectance (ODR) that is immediately recorded using the UV-Vis spectrophotometer, K is noted as the absorption index, S is identified as the scattering quantity.
In Eq. (2),  is recognized as the absorption index, t is the thickness of the material, and as well as in equation (3), h is the photon energy of electromagnetic (EM) radiation,  is the frequency, h is Planck constant, A is the band tailing factor, and its values ranged from 1 × 10 5 to 1 × 10 6 cm −1 .eV −1 [38]. Therefore, the following formula is used to determine the material direct bandgap as illustrated below: While the following equation is expressed the material optical indirect bandgap as: where Eg is the optical bandgap energy of the studied material. Also, in the Eqs. (7&8), the values are either n = ½ for direct optical bandgaps or n = 2 for indirect bandgaps. Fig.7 and  Aydın et. al [37]. The calculated bandgap for pure ZnO semiconductor was around 3.19 eV, and as increasing Fe dopant concentration to 20% in the ZnO host matrix, the value of the energy bandgap decreases to reach 2.75 eV [37]. The optical bandgap is inversely proportional to the dopant concentration since new energy levels will be formed between the conduction and the valence bands as increasing the dopant ratios.

Dielectric properties and AC electrical conductivity of Y2O3 -ZnO nanocomposites.
The dielectric function clarifies the material's direct response to electromagnetic radiation (EM) and controls EM waves' propagation behavior in the studied medium. Thus, it is very critical to distinguish the nature and the origin of the dielectric function. For the 18 considered materials Y2O3-ZnO, the complex dielectric function could be expressed through the following equations [39]: and The part of the dielectric constant here is 1(), whereas 2() is the imaginary component.    could be understood through the interfacial polarization mechanism due to the charge carriers, which establish and limited by defects and grouped in the dielectric medium. At the higher frequency, the 1() begins to rise, taking the advantages of the dipole oscillation, which can quickly rotate [41].
It is essential to consider the frequency effects on AC electrical conductivity to explain the electronic conduction mechanism of the Y2O3-ZnO nanostructured materials at different doping ratios of yttrium-ions. To evaluate the AC conductivity of the studied materials, the set of the following equations were applied as [42][43][44][45]: Here, the whole AC electrical conductivity is , the impedance is Z, and A is a constant depends on temperature. Also, respectively, are the DC and AC electrical conductivities, where the angular frequency is  and the frequency exponent is donated as (s). Semiconductor materials represent semiconducting compounds that display values of AC electrical conductivity in the range between 10 -8 and 10 4  -1 .cm -1 [46]. The In this present study, s values were estimated to be almost around one (0.92) [42].
Commonly, the AC electric conductivity depends strongly on the photon frequency for either disarranged or structured, and for organic or inorganic materials [46]. Eq. 13 suggests that AC electrical conductivity plays a significant role in the interaction of many-body, which clarify the universal behavior of the power-law. The frequency exponent (s) is a vital parameter that illustrates the multi interaction between the material impurities and charge carriers, where the (s) value depends not only on the applied frequency but also on the temperature, where its value ranges from zero to one; it is equal to one for standard Debye type media. Furthermore, the (s) frequency parameter is correlated to either charge carriers or inessential electrical dipoles resulting from impurities. For conducting disordered mediums, the values of the frequency exponent range between 0.6 to 0.8, and it was around one for highly disordered dielectric mediums [47,48].  Fig. 11. AC electrical conductivity as a function of the applied frequency function of the studied Y2O3-ZnO nanostructured with different yttrium doping ratios.

Figs. 12(a-h) displayed the voltage-current (V-I) curves of the ZnO varistor ceramics
with different yttrium contents. The curves show that the conduction characteristics are divided into two zones: a high impedance linear region below the knee-point voltage and a low 22 impedance nonlinear region above the knee point voltage. Nahm C. W. [49] studied the effect of sintering temperature on nonlinear electrical properties and stability against the DC accelerated aging stress of (CoO, Cr2O3, La2O3)-doped ZnO-Pr6O11-based varistors. It is known that the sharper the knee-point of the curves between the linear region and the breakdown field is, the better the nonlinear characteristic is, in other words, the threshold voltage Vss, the nonlinear coefficient α, the grain boundary resistance Rgb from the Cole-Cole impedance measurements and the leakage current IR is determined by the V-I curves. Current   0.0 3.0x10 -6 6.0x10 -6 9.0x10 -6 (h) Fig. 12(a-h). V-I characteristics of the studied Y2O3-ZnO nanostructured with different yttrium doping ratios.
The AC impedance parameters have been analyzed the complex impedance (Z*=Z'+iZ'') by the following functions [50]: where is the chosen angular frequency, ( ', '' = ' tanδ) are the real and imaginary portions of the dielectric permittivity, and co is the vacuum capacitance. The real and the imaginary components of this impedance were calculated as follows [50]: The average square-method used for simulating the resistance Rb, the CPE value indicated by Q, and the parameter c through the difference between experimental and theoretical results.
The impedance of CPE is ZCPE =1/A0(iω) c .  It is noticed that the tested-ceramic varistor systems have a quasi-semicircle, which is related to the grain boundary contribution. Also, with increasing the yttrium doping, it is noticeable that it produces a decrease in the high-frequency quasi-semicircle and becoming almost imperceptible for the studied ceramic varistor systems. This change in the high-frequency impedance data results from the increase of the ZnO matrix's charge carriers due to incorporating yttrium contents. In this way, the equivalent circuit model using the Eqs. (15&16) were used to determine the grain boundary resistance (Rgb) and capacitance (Q) in parallel.  Figs. 14 (a-c). Influence of yttrium contents on V-I fitted parameters of the studied Y2O3-ZnO nanostructured with different yttrium doping ratios.
The varistor action takes place in the breakdown region. In this region, the applied voltage is a highly nonlinear function of the current and can be described by the empirical law [52]: where V is the applied voltage, I is current, K is a constant depending on the geometry and manufacturing process, and α is the nonlinear coefficient. The value of nonlinear coefficient α is obtained from the inverse of the ln V-ln I curves' slope at any voltage value, as shown in boundaries in the specimen (see Fig. 14(b)). A graphical description of the nonlinear coefficient α and the threshold voltage Vss according to ZnO varistors' yttrium contents is illustrated in Fig. 14 (c).  Rgb obtained from the relevant impedance data, and the leakage current for the studied ceramic 28 varistor systems as represented in Fig. 14(b). The leakage current results from most of the electrons passing over the Schottky barrier at the grain boundaries. With increasing the yttrium contents from (S3) to (S7) with increasing yttrium contents, the leakage current is very low and only shows little change. It is believed that the decrease in the leakage current can be attributed to an increase in the average energy needed for electrons to overcome the Schottky barrier and the homogeneous distribution in the ceramic varistor samples. As a result, it is necessary to ensure a high level of practical barriers to the grain boundaries, increase its resistance, and avoid leakage currents to achieve higher values with a nonlinear coefficient. Reducing the leakage current of Y2O3-ZnO ceramic-based varistors' can make these materials an excellent candidate in power systems.

Photocatalytic degradation of color and colorless using Y2O3-doped ZnO
Nanocomposites.

Kinetic Studies.
The The initial concentration Co, concentration is Ct at time t with constant rate k.
The kinetic rate is attained through a plot of Ph, MB, and RhB concentration versus time of exposure, as shown in Figs. 18 (a-d). Table. 3 also showed that the rate constant was affected by yttrium dopant concentration. The kinetic rate using pure ZnO is 0.00262, 0.00112, and 0.00178 min -1 for phenol, MB, and RhB solutions. It was 0.01079, 0.00887, and 0.01061 min -1 of phenol, MB, and RhB solution, respectively, using 1% Y2O3 -doped ZnO. These findings reveal that Y2O3 -doped ZnO has greater photocatalytic activity than pure ZnO. As the number of oxygen vacancies and surface defects increases, electron-hole regeneration, and charge trapping efficiency will be high. As the photocatalytic operation of ZnO-ion doping is boosted, there is no distinct phase at the lower concentration of Y 3+ -ions as the Y-dissolution 29 will occur in the ZnO lattice [54]. On the other side, Y 3+ -distorts the crystallinity of ZnO at high concentrations of more than 1% [55]. Photocatalytic activity of the tested samples is increasing by doping as shown in Fig. 17. Hemalatha et al. revealed the increasing photocatalytic efficiency of Y-doped ZnO because the electron-holes produced are weakly recombined, leading to subsequent growth in activity canters [56]. Such factors help to improve the operation of photocatalysis.

Comparative Study Between Rare Earth Doped ZnO Photocatalysts
For enhancing the properties of ZnO nanostructures, we have to add impurity or defect at the time of synthesis of nanoparticles. The addition of foreign atoms or impurities (doping) to a compound by creating a defect that can enhance its physical properties. Here, doping is required to modify the physical properties of nanostructures [57,28] [72]. Moreover, Gd-doping significantly impacts the particle size, and Gd-doping ZnO has a much smaller particle size than ZnO. In the photocatalytic process, the aqueous solutions of methyl blue (MB) under the sunlight are applied for degradation by ZnO and Gd-doped ZnO nanocrystalline powders. These results showed that an increase in photocatalytic activity by increasing Gd-loading [73].  [59].
Additionally, the photodegradation of organic compounds by modified doped-ZnO with materials that contain higher oxygen defects is outstanding. The ferromagnetic characteristics of rare-earth-doped ZnO are determined by the energy level or charge transfer between states [67].

Photodegradation Mechanism
The proposed Y2O3-doped ZnO photocatalytic mechanism is visible irradiation is presented via scheme 1. As light is emitted, holes and electrons are formed on the Y2O3doped ZnO surface. The electrons excited from the valence to the conducting band contribute to hole creation. The electron-hole recombination occurred, causing the heat release. Afterward, the excited electrons react with the adsorbed oxygen, leading to O2 . , while the valence band hole with OHor H2O molecules leads to the hydroxyl radical's formation [83,84]. The HO2 . is then generated due to the conjunction between H + and O2 . .

Reusability Studies
The recovered catalyst 1g of Y (S5) doped ZnO was reused for photodegradation of phenol, MB, and RhB. This process has been repeated for 4 cycles. The findings are represented in Figs. 19 (a-d). The reusability of the catalysts occurs as the photocatalytic efficiency of the catalysts, especially ZnO, decreases during re-use due to photo-corrosion [85,86]. Since 1 g Y doped ZnO ~ 98% activity (S5) didn't decrease significantly after four cycles, it was confirmed that Y 3+ -doping improved the stability and anti-photo-corrosive nature of ZnO. Particularly ZnO, photocatalytic efficiency decreases because of photo-corrosion throughout reusing [85,86]. On the contrary, 1g Y2O3 doped ZnO -98% (S5) was no longer significantly reduced after 4 cycles by the action of 1 g of Y 3+ (S5). These data revealed the stability and anti-photocorrosive quality of ZnO by Y2O3 doping.

Conclusion
It is found that using the combustion method is a very reasonable, low cost, effective, and eco-friendly approach for successful preparing nano-ZnO doped with yttrium element of different concentrations. Both X-ray diffraction (XRD) and scanning electron microscopy