The Auto-Combustion Method Synthesized Eu2O3- ZnO Nanostructured Composites for Electronic and Photocatalytic Applications

An efficient and environmentally friendly combustion technique was employed to produce ZnO nanopowders with different Eu concentrations (from 0.001 g to 5 g). The structural morphology of the Eu2O3-ZnO nanocomposites was examined using XRD, SEM, and infrared spectroscopy (FT-IR). In addition, UV-Vis diffuse reflectance spectroscopy was also used to investigate the effects of europium (Eu) dopant on the optical behaviors and energy bandgaps of nano-complex oxides. The photocatalytic degradation efficiency of phenol and methylene blue was investigated using all the prepared Eu2O3-ZnO nanostructured samples. Photocatalytic effectiveness increased when europium (Eu) doping ratios increased. After adding moderate Eu, more hydroxyl radicals were generated over ZnO. The best photocatalyst for phenol degradation was 1 percent Eu2O3-ZnO, while it was 0.5 percent Eu2O3-ZnO for methylene blue solutions. The obtained Eu2O3-doped ZnO nanostructured materials are considered innovative, promising candidates for a wide range of nano-applications, including biomedical and photocatalytic degradation of organic dyes and phenol.


Introduction
Recently, more attention has been paid to wastewater treatment for human society. Phenolics are particular contaminants frequently produced from a wide range of industrial products, including textiles, pesticides, gunpowder, dyes, and plastics [1,2]. Most of these

Material Growth
In this work, we synthesized ZnO nanoparticles (NPs) doped with europium rare earth metal (Eu 2 O 3 ) using a combustion process. Initially, 5 g of Zn(NO 3 ) 2 ·6H 2 O was mixed and well ground in ceramic crucibles with one gram of gum acacia. Next, eight concentrations of europium (III) nitrate elements ranging from 0.001 g to 5 g were added to the previous mixture, liquified in 5 mL of distilled water, and then kept at 100 • C for 48 h until a completely dried gel formed. After that, the dried gel's combustion process was applied without further purification at 600 • C for two hours before being cooled at ambient temperature. Gum acacia was used as a fuel in this preparation procedure to help convert the ZnO structure from crystallinity to nanoscale by expanding its elements within the matrix. Table 1 illustrates the abbreviations assigned to the prepared samples as S0 to S7, respectively.

Devices and Measurements
The crystal structure of the prepared samples was characterized using a Shimadzu LabX-XRD-6000 X-ray diffractometer, Kyoto, Japan, with CuK α = 1.54 Å radiation in the angle range from 5 • to 80 • . A JSM-6360 scanning electron microscopy (SEM) with an operating voltage of 20 kV (JEOL, Tokyo, Japan) was used to analyze the morphology of the prepared nanoparticles. Moreover, the ultraviolet-visible spectra of all prepared samples were measured using a UV-3600 UV-Vis spectrophotometer (Shimadzu, Kyoto, Japan), with a wavelength range from 2200 nm to 1600 nm and a step scan of 5 nm. A Thermo Scientific DXR FT-IR Spectrometer was used to study the Fourier transform infrared (FT-IR) spectra of Eu 2 O 3 -doped ZnO nanocomposites in the wavenumber range from 400 to 4000 cm −1 .

Photocatalytic Measurements
Different types of organic pollutants, such as methylene blue and phenol, were used in a thermostatic photoreactor fitted with a multi-position magnetic stirrer for photocatalytic measurements.

Design of the Ultraviolet-Visible Photoreactor
A wooden photoreactor was used to test the visible photocatalytic operation of all samples under investigation for industrial wastewater treatment under different experimental conditions [28]. The reactor is divided into two portions on the inside and outside. A hardwood frame of 100 cm in height, 95 cm in length, and 65 cm in width makes up the exterior component. In the inner part, seven white (400-700 nm) bulbs and seven blue (18 watts) lamps, each having a spectrum higher than 420 nm, are individually controlled. The wavelength of the visible lamp is larger than 420 nm and for the UVA lamp is 367 nm. The intensity of visible and UVA lamps is 18 watts each. The system is controlled primarily by ON/OFF buttons. The air fan is also attached to the photoreactor for an internal circulating air system to maintain the photoreactor temperature at room temperature. A magnetic stirrer with a multi-position was used to stir the prepared solution.

Photocatalytic Irradiation
The prepared samples of 0.01 g of each doping concentration were dissolved in 100 mL of organic solution, either MB or Ph, with the attention of 20 mg/L. The complete setup was kept in the dark for around 30 min to examine chemisorption until the equilibrium state was obtained. After a 10-min interval of irradiation, 1 mL of the solution sample was withdrawn and centrifuged at 3000 rpm to remove all powder suspension. The remainder of the mixture was exposed to visible light again. The sample activity was measured every 15 min during the irradiation. To investigate the main oxidant species of the photocatalytic reaction, photocatalytic experiments were also carried out on different scavengers. Moreover, following up on the dependency of the photocatalytic performance on light irradiation and its mechanism, trapping experiments with visible-light irradiation for the active species produced during the photodegradation reaction were notably studied. Different agents, including sodium chloride (Cl − ), sodium nitrate (NO 3 − ), isopropyl alcohol (IPA), and ascorbic acid (AA), were employed as the scavengers for h + , e − , • OH, and • O 2 − , respectively. Finally, the photodegradation process was detected using a UV-Vis spectrophotometer (200-800 nm).
The Scherrer equation was employed to investigate the XRD data and the crystallite size (D) of the prepared Eu 2 O 3 -doped ZnO nanocomposites [29,30].
Separately, the dislocation density (η), as well as lattice strain (ε) of the pure ZnO and Eu 2 O 3 -doped ZnO, were evaluated using the coming expressions [31,32]: where λ is the X-ray wavelength in the unit of nm, β is the full width at half maximum (FWHM) in terms of radians, and θ is the diffraction angle in the degree unit. Dislocations are imperfections in a crystal related to the misregister of the lattice in one part of the crystal concerning another part. Tauc's model was used to calculate the optical bandgap (E g ) of pure and Eu 2 O 3 -doped ZnO nanostructured samples using the following equations [29,33]: Here, F(R) represents the material reflectivity using the Kubelka-Munk model, R is reflectance, K is the molar absorption coefficient, and S is the scattering quantity. In Equations (5) and (6), α is the absorption index; t is the material thickness; ν is the photon frequency; h is the Planck constant; A is the band tailing factor, with values ranging from 1 × 10 5 cm −1 ·eV −1 to 1 × 106 cm −1 ·eV −1 ; and E g is a bandgap [34]. In addition, the values of n in Equation (6) are either n = 1 2 for direct optical bandgaps or n = 2 for indirect optical bandgaps of the studied samples determined by using the following relations: The catalytic efficiency of the prepared samples was estimated from the degradation (%) and the process rate [35]: where C 0 is the starting concentration of MB or Ph and C is noted as the concentration of the MB or Ph at various times. The photocatalytic degradation reaction is a virtual first-order reaction, as the pollutant's concentrations are within the millimolar range [35]: where k is the constant amount and t is the irradiation time in minutes. The reaction between electrons and the adsorbed H 2 O is illustrated in the following expressions [36][37][38][39][40]: Simultaneously, the VB photogenerated holes undergo trapping with the surface group of hydroxyls, reacting with ionized oxygen defects or reacting with adsorbed water (H 2 O 2 ) through transferring of interfacial charges, creating a highly radical reactive hydroxyl ( • OH) [41][42][43]: with the indices of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), and (2 0 1) respectively. The observed diffraction peaks for pure ZnO nanoparticles outstandingly matched the data reported by Aydn et al. [29] and were similar to the reported data by Wang, R. H., et al. [30]. In the case of doping samples, the Eu 2 O 3 -ZnO phase does not appear at low doping, as shown in Table 2 and Figure 1, but does occur as the dopant concentration increases in the pure ZnO matrix. The reason could be explained by dislocations which are crystal flaws in one section of the crystal that cause the lattice to misregister as compared with another part. Further, all the calculated structural parameters of pure and doped samples are depicted in Table 2.

Results and Discussions
Fascinating features can be seen in the XRD patterns of the Eu 2 O 3 -ZnO nanostructures with the phase (JSPDS 00-034-0392) with the indices of (2 2 2), (2 1 1), (4 0 0), and (6 2 2). The prominent peaks corresponded to (1 0 0) and (1 0 1) planes that have been detected in all prepared nanomaterials. A slight shift of the (0 0 2) plane towards a value to the left at a higher concentration of doping samples than undoped ZnO material reveals that the content of europium (Eu) dopant significantly affects the crystalline structure of pure material. It is also observed from the XRD graphs that there is a formation of the Eu 2 O 3 phase at higher concentrations, which suggests the effect of doping into the pure matrix. Furthermore, the mean crystallite size values of all prepared ZnO-doped Eu 2 O 3 nanocomposites are diverse with variations in doping, in the range of 13-42 nm. In this current study, the attained grain size was in agreement with the published results for silver-ZnO nanomaterial by Wang, R. H. et al. [30]. The Scherrer formula calculated the mean grain size from the XRD spectra, ranging between 20.9 nm and 22.1 nm [30]. The evaluated crystallinity size noticeably increased with an increase in the dopant ratios.
2 2). The prominent peaks corresponded to (1 0 0) and (1 0 1) planes that have been detected in all prepared nanomaterials. A slight shift of the (0 0 2) plane towards a value to the left at a higher concentration of doping samples than undoped ZnO material reveals that the content of europium (Eu) dopant significantly affects the crystalline structure of pure material. It is also observed from the XRD graphs that there is a formation of the Eu2O3 phase at higher concentrations, which suggests the effect of doping into the pure matrix. Furthermore, the mean crystallite size values of all prepared ZnO-doped Eu2O3 nanocomposites are diverse with variations in doping, in the range of 13-42 nm. In this current study, the attained grain size was in agreement with the published results for silver-ZnO nanomaterial by Wang, R. H. et al. [30]. The Scherrer formula calculated the mean grain size from the XRD spectra, ranging between 20.9 nm and 22.1 nm [30]. The evaluated crystallinity size noticeably increased with an increase in the dopant ratios.

SEM Analysis
Scanning electron microscopy (SEM) was used to study the morphology of Eu 2 O 3 -ZnO nanostructures, as illustrated in Figure 2. The structural morphology changes in the pure-ZnO matrix and the europium (Eu)-doped ZnO nanocomposites were directly recognizable. The SEM pictures of Eu 2 O 3 -ZnO nanocomposite powder exhibited a general homogeneous dispersion of spherical nanoparticles. The europium ions strongly formed tiny grains and promoted crystal nucleation, whereas the trapping of Eu grains hindered grain development. The europium ions created grains of small size (ranging between 75 and 89 nm) and encouraged the crystal nucleation rate, while the trapping of europium grains prevented grain development. This trend could be due to the discrepancy in ionic radius between zinc and europium [44]. The form and size of the nanoparticles changed as the amount of Eu doped in them grew, as illustrated in Table 2. The obtained SEM results closely matched the results of Chao, L. C., et al. [45].  Figure 3 shows the FT-IR spectra of the prepared samples in 400-4000 cm −1 . The undoped ZnO sample's optical transparency improved marginally compared to Eu doped samples. As the doping ratio for europium (Eu) increased, the transmission of Eu2O3-ZnO nano samples decreased, possibly due to increased scattering. In the FT-IR spectra, the highly intense broad absorption peak at around 438 cm −1 of the wavenumber was credited to the stretching vibrations in the pure ZnO matrix [46]. In addition, a wide absorbance band at about 3450 cm −1 characterized the O-H group's stretching vibrations, while the small absorption peak at around 1635 cm −1 corresponded to the bending vibrations of the interlayered molecule. The surface caused the impurity bands of different higher wavenumber adsorbed organic matrices either in the synthesis procedure or the characterization. The impurity effects were extremely predominant for Eu nanoparticles because of the large ratio of surface to volume; however, those impurity bands gradually disappeared in the bulk materials [46]. As a result, the most excellent Eu2O3 doping ratios (i.e., 5 g) exhibit the highest absorbance, consistent with the structural morphology results from  Figure 3 shows the FT-IR spectra of the prepared samples in 400-4000 cm −1 . The undoped ZnO sample's optical transparency improved marginally compared to Eu doped samples. As the doping ratio for europium (Eu) increased, the transmission of Eu 2 O 3 -ZnO nano samples decreased, possibly due to increased scattering. In the FT-IR spectra, the highly intense broad absorption peak at around 438 cm −1 of the wavenumber was credited to the stretching vibrations in the pure ZnO matrix [46]. In addition, a wide absorbance band at about 3450 cm −1 characterized the O-H group's stretching vibrations, while the small absorption peak at around 1635 cm −1 corresponded to the bending vibrations of the interlayered molecule. The surface caused the impurity bands of different higher wavenumber adsorbed organic matrices either in the synthesis procedure or the characterization. The impurity effects were extremely predominant for Eu nanoparticles because of the large ratio of surface to volume; however, those impurity bands gradually disappeared in the bulk materials [46]. As a result, the most excellent Eu 2 O 3 doping ratios (i.e., 5 g) exhibit the highest absorbance, consistent with the structural morphology results from the XRD and SEM investigations. Furthermore, the observed effects were similar to those with the developments of Nd-doped ZnO nanocomposites reported by Chauhan et al. [46]. the XRD and SEM investigations. Furthermore, the observed effects were similar to those with the developments of Nd-doped ZnO nanocomposites reported by Chauhan et al. [46].

Optical Diffused Reflectance (ODR) and Absorption Index
Diffuse reflectance (DR) measurement is a well-known technique for obtaining information on the absorption properties of nanostructured materials. Figures 4 and 5 show the optical diffused reflectance (ODR) and absorption index (k) of the Eu2O3-doped ZnO nanocomposites from 200 nm to 700 nm. From Figure 4, it can be observed that, in the range from 200 nm to 370 nm, no change in ODR data was monitored. However, a substantial increase in the ODR was seen from 370 nm to 410 nm, producing the optical bandgap. Within the wavelength range from 410 to 700 nm, the diffuse optical reflectance (ODR) spectra showed virtually straight curves with few variations.  The calculated ODR agreed with the reported optical results reported by Chaudhary et al. [23]. The optical properties showed that the absorption edges shifted toward the visible region [23]. The optical investigation of sol-gel prepared Nd-doped ZnO nanoparticles reported by Chao   The calculated ODR agreed with the reported optical results reported by Chaudhary et al. [23]. The optical properties showed that the absorption edges shifted toward the visible region [23]. The optical investigation of sol-gel prepared Nd-doped ZnO nanoparticles reported by Chao, L. C., et al. also demonstrated durable emission of band edge placed at the wavelength of 380 nm, where the defect emission was related to the deep The calculated ODR agreed with the reported optical results reported by Chaudhary et al. [23]. The optical properties showed that the absorption edges shifted toward the visible region [23]. The optical investigation of sol-gel prepared Nd-doped ZnO nanoparticles reported by Chao, L. C., et al. also demonstrated durable emission of band edge placed at the wavelength of 380 nm, where the defect emission was related to the deep level at about 620 nm wavelength [47]. In conclusion, the absorbed light generates a bandgap of optical absorption via the deliberate samples, which can be explained due to non-absorbent surface area and the predisposition of Eu 2 O 3 -ZnO nanocomposite materials to incident light [48,49]. Figure 5 shows a slightly small absorption index (k) of the examined Eu 2 O 3 -doped ZnO nano-samples, ranging between 1 × 10 −4 and 8 × 10 −4 . The most outstanding absorption values are at a light wavelength of roughly 380 nm. The light will cause electrons to move from their ground state orbitals to higher-energy, excited state or antibonding orbitals. If the molecule has transitions in the ultraviolet (UV) or visible ranges of the electromagnetic spectrum, ultraviolet-visible spectroscopy can be used to determine the electronic transitions. Many molecular electronic transitions exist, i.e., σ → σ * , π → π*, n → σ * , n → π * . Especially in the region of 200-800 nm, the three responsible transitions, i.e., π → π*, n → σ * , n → π * , [50,51].  Table 3. The present samples' direct and indirect bandgap values agree with the recently reported Fe-ZnO nanostructures by Aydın et al. [29] and Chauhan et al. [45]. The decreasing bandgap in the samples could be explained due to the new generation of energy state between the conduction and the valence levels.

Photodegradation of Organic Compounds under Investigation Using Prepared Samples
The photocatalytic activities of pure and Eu 2 O 3 -doped ZnO were examined under visible irradiation using methylene blue as a colored dye and phenol as a colorless organic compound. The absorption spectra of the aqueous MB and Ph solution at various time intervals at irradiation were recorded. The photodegradation was estimated from the reduction in the maximum intensity of the absorption peak. Photocatalytic degradation of MB and Ph is distinguished from the concentration plot of MB or Ph versus irradiation time (min). Figure 7a

Photodegradation of Organic Compounds under Investigation Using Prepared Samples
The photocatalytic activities of pure and Eu2O3-doped ZnO were examined under visible irradiation using methylene blue as a colored dye and phenol as a colorless organic compound. The absorption spectra of the aqueous MB and Ph solution at various time intervals at irradiation were recorded. The photodegradation was estimated from the reduction in the maximum intensity of the absorption peak. Photocatalytic degradation of MB and Ph is distinguished from the concentration plot of MB or Ph versus irradiation time (min). Figure 7a,b shows the decrease in the MB solution and Ph solution concentrations at various illumination times for pure and Eu2O3-doped ZnO samples.

Kinetic Studies of the Photocatalytic Degradation Process
The weight concentrations of Eu increased, and the % of degradation of both MB and Ph increased, as shown in Figure 8a,b. The relationship between ln (C/C o ) and the reaction time is illustrated in Figure 9a,b. The spectrum fitting of photocatalytic degradation of MB or Ph was accomplished to obtain the best straight line. The obtained coefficients of linear regression (R2) were estimated at around one for all the measured samples, indicating that the photodegradation obeyed the pseudo-first-order kinetics. The rate constants for the degradation of MB ranged from 0.017 to 0.07 min −1 . At the same time, it varied from 0.03 to 0.09 min −1 for the phenol degradation for different doping concentrations.
The degradation factors for MB with 0.5% Eu-doped ZnO nanocomposites increase the reaction rate by around four times compared with pure ZnO samples. However, the reaction rate for Ph degradation constants with 1% Eu 2 O 3 -doped ZnO is three times more than that for pure ZnO. Generally, the performance of photocatalyst activities was well-defined through the recombination rate delay of photo-created electron-hole pairs. Consequently, the correct mole fraction of Eu 3+ ions-doped ZnO nanostructures would enhance the photocatalytic activities due to increased electron-hole lifetime.
The degradation factors for MB with 0.5% Eu-doped ZnO nanocomposites increase the reaction rate by around four times compared with pure ZnO samples. However, the reaction rate for Ph degradation constants with 1% Eu2O3-doped ZnO is three times more than that for pure ZnO. Generally, the performance of photocatalyst activities was welldefined through the recombination rate delay of photo-created electron-hole pairs. Consequently, the correct mole fraction of Eu 3+ ions-doped ZnO nanostructures would enhance the photocatalytic activities due to increased electron-hole lifetime.   The degradation factors for MB with 0.5% Eu-doped ZnO nanocomposites increase the reaction rate by around four times compared with pure ZnO samples. However, the reaction rate for Ph degradation constants with 1% Eu2O3-doped ZnO is three times more than that for pure ZnO. Generally, the performance of photocatalyst activities was welldefined through the recombination rate delay of photo-created electron-hole pairs. Consequently, the correct mole fraction of Eu 3+ ions-doped ZnO nanostructures would enhance the photocatalytic activities due to increased electron-hole lifetime.    Table 4 summarizes the first-order rate coefficient (k) values extracted from the linear graphs. The defined k values for pure ZnO and Eu 2 O 3 -doped nanostructures indicated increased rate parameters with increasing Eu dopants. Therefore, the improvement in the Eu-ZnO photocatalytic activities could increase the concentration of Eu nanoparticles, as confirmed through absorption and XRD spectra. Additionally, doping Eu NPs could present in the ZnO lattice both point defects and trapping sites, which results in increased separation among electron-hole pairs, thus, increasing the ZnO photocatalytic efficiency [17]. Moreover, a photo-created electron in the conduction band could be recognized via Eu +3 ions. Then, the generated Eu +2 could be related to the dissolved O 2 and produce the radical anion of superoxide O 2 − [22].

Proposed Mechanism of Photodegradation of MB and Ph
In the literature, investigations of the photocatalytic mechanism of semiconductor materials and the effects of rare earth ions have been extremely recognized [61][62][63][64]. In this study, the VB electrons absorb energy from visible light to irradiate ZnO. Then, those electrons are excited to the conduction band (CB), creating electron-hole pairs (e − /h + ). The reactions of oxidation are initiated from those (e − /h + ) pairs as follows: The dissolved oxygen is reduced to an anion of superoxide radical (O 2 − ), and the H 2 O molecule is oxidized to (OH) radicals. Here, h + , O 2 − , and . OH, radicals are very significant in contributing to the organic dye's decomposition to CO 2 and H 2 O. Even though one challenging factor in an investigation of photocatalytic efficiency is the recombination of the created (e − /h + ) pairs [64]. The degradation mechanism of Eu 2 O 3 -ZnO photocatalytic is illustrated in Scheme 1.
The 0.5 g of Eu 2 O 3 -doped ZnO (S4) displayed 100% MB removal in 80 min. Furthermore, 1 g Eu-doped ZnO degraded 100% phenol in 60 min. The more considerable photocatalytic activity of Eu 2 O 3 -doped ZnO nanocomposites is primarily due to electron movement among the valence bands of Eu, Eu 2+ , and Eu 3+ [65]. Since europium (Eu) can have two and three valence electrons, Eu 3+ and Eu 2+ could trap those electrons and give them oxygen to form radical ions of superoxide (O 2 − ). It was conveyed that the potential energy of Eu (Eu 3+ /Eu 2+ ) reduction is negative about -0.35 eV further than E (O 2 − ) with +0.12 eV [66]. Therefore, the oxygen absorption on the Eu 2 O 3 -doped ZnO surface could be decreased to reach O 2 − . As seen in Scheme 1, the capturing and transferring of electrons increases the Eu 2 O 3 -doped ZnO photocatalytic activities, which results in the effective separation of electron-hole pairs. them oxygen to form radical ions of superoxide (O2 − ). It was conveyed that the potential energy of Eu (Eu 3+ /Eu 2+ ) reduction is negative about -0.35 eV further than E (O2 − ) with +0.12 eV [66]. Therefore, the oxygen absorption on the Eu2O3-doped ZnO surface could be decreased to reach O2 − . As seen in Scheme 1, the capturing and transferring of electrons increases the Eu2O3-doped ZnO photocatalytic activities, which results in the effective separation of electron-hole pairs. Scheme 1. Degradation mechanism for MB and phenol using Eu2O3-ZnO nanocomposites.
Thus, rapid exciton recombination could enormously reduce the efficiency of the Eu2O3-ZnO photocatalytic activity. In the case of Eu 3+ dopants, the electrons are trapped in the energetically favorable Eu 3+ ions, as in Equation (13). As a result, electrons are inhibited from recombination with holes to generate superoxide radicals (O2 •− ), as seen in Equation (14) [67]. Equation (15) illustrates that the transferring of electrons to ionize oxygen vacancies (V0 ++ ) leads to the reaction by adsorbate oxygen (O2) to produce radicals of superoxide (O2 •− ) [33]. As ZnO nanostructures are doped with Eu 3+ ions, the larger quantity of the created radicals (O2 •− ) increases the photocatalytic activity. In the degradation process, organic compounds are initially oxidized via the sequential hole transfer and/or the radical attack of hydroxyl • OH [68].
Furthermore, the Eu 3+ dopants could agree to take the CB electrons of ZnO nanostructures to generate Eu 2+ , which could move one electron for dissolving O2 to create radical anions of superoxide for preventing the electron-hole recombination, as demonstrated in Scheme 1 [69,70]. As a result, Eu 3+ on the ZnO surface could be considered a scavenger of electrons. Additionally, Eu 3+ -ZnO nanostructures have a narrow energy bandgap as compared with undoped ZnO semiconductors, which could cause an increase in the stability of electron-hole pairs and enhance the efficiency of photocatalytic degradation. Nevertheless, Eu 3+ ions charging above their ideal level could perform as a recombination focus on photogenerated electron-hole pairs due to their interaction, which reduces the efficiency of photocatalytic [67,71]. Jinrui Li et al. reported that Eu doping on ZnO exhibited the highest photocurrent density, which indicated that the electron-hole lifetime at the interface of the catalyst was extended [72].
Similarly, different researchers have reported the electron transfer and involvement of superoxide radicals in photodegradation by Eu ions [73]. Paola Franco et al. showed the photocatalytic activities of manufactured materials investigated in EBT dye photodegradation using UV and visible irradiation [57]. The experimentally obtained results illustrated that the Eu-ZnO structures synthesized using the supercritical antisolvent SAS (Eu-ZnO-SAS) technique could assure the highest photocatalytic efficiency. Finally, Phuruangrat et al. investigated the photocatalytic activity of manufactured ZnO and Eu-ZnO nanostructures using MB degradation in aqueous solutions within UV irradiation [58]. Thus, rapid exciton recombination could enormously reduce the efficiency of the Eu 2 O 3 -ZnO photocatalytic activity. In the case of Eu 3+ dopants, the electrons are trapped in the energetically favorable Eu 3+ ions, as in Equation (13). As a result, electrons are inhibited from recombination with holes to generate superoxide radicals (O 2 •− ), as seen in Equation (14) [67]. Equation (15) illustrates that the transferring of electrons to ionize oxygen vacancies (V 0 ++ ) leads to the reaction by adsorbate oxygen (O 2 ) to produce radicals of superoxide (O 2 •− ) [33]. As ZnO nanostructures are doped with Eu 3+ ions, the larger quantity of the created radicals (O 2 •− ) increases the photocatalytic activity. In the degradation process, organic compounds are initially oxidized via the sequential hole transfer and/or the radical attack of hydroxyl • OH [68].
Furthermore, the Eu 3+ dopants could agree to take the CB electrons of ZnO nanostructures to generate Eu 2+ , which could move one electron for dissolving O 2 to create radical anions of superoxide for preventing the electron-hole recombination, as demonstrated in Scheme 1 [69,70]. As a result, Eu 3+ on the ZnO surface could be considered a scavenger of electrons. Additionally, Eu 3+ -ZnO nanostructures have a narrow energy bandgap as compared with undoped ZnO semiconductors, which could cause an increase in the stability of electron-hole pairs and enhance the efficiency of photocatalytic degradation. Nevertheless, Eu 3+ ions charging above their ideal level could perform as a recombination focus on photogenerated electron-hole pairs due to their interaction, which reduces the efficiency of photocatalytic [67,71]. Jinrui Li et al. reported that Eu doping on ZnO exhibited the highest photocurrent density, which indicated that the electron-hole lifetime at the interface of the catalyst was extended [72].
Similarly, different researchers have reported the electron transfer and involvement of superoxide radicals in photodegradation by Eu ions [73]. Paola Franco et al. showed the photocatalytic activities of manufactured materials investigated in EBT dye photodegradation using UV and visible irradiation [57]. The experimentally obtained results illustrated that the Eu-ZnO structures synthesized using the supercritical antisolvent SAS (Eu-ZnO-SAS) technique could assure the highest photocatalytic efficiency. Finally, Phuruangrat et al. investigated the photocatalytic activity of manufactured ZnO and Eu-ZnO nanostructures using MB degradation in aqueous solutions within UV irradiation [58]. The MB photocatalytic efficiency of 3% Eu-ZnO reached 90.51% in 300 min, more extensive than undoped ZnO semiconductor. Yanqing Zong et al. found that the efficiency of MO photodegradation for undoped ZnO was just 75.7% in 180 min, while the efficiency reached 95.3% for 1.0 mol% dopants Eu-ZnO [60]. Conspicuously, as the Eu dopant concentrations increases, the efficiency of photodegradation initially upsurges and, after that, declines. Balachandran et al. used the hydrothermal process to prepare Eu 2 O 3 -ZnO nanoclusters, where the composite was comprised of 91% ZnO and 9% Eu 2 O 3 [60]. The synthesized Eu 2 O 3 -ZnO nanostructures displayed the more considerable photocatalytic activity of 99.2% in the Rhodamine B (RhB) photodegradation under sunlight in 75 min under neutral pH. Numerous researchers have considered hydroxyl radicals the most vital active materials for impurity degradation. The previous comparison proved the influences of the preparation method, weight concentrations of Eu, and conditions of photocatalytic procedure on the photocatalytic activities of prepared Eu 2 O 3 -doped ZnO materials.
In this present work, 0.5 g Eu-ZnO (0.05 Eu/Zn molar%) nanocomposites displayed the highest MB photocatalytic degradation in 80 min of visible irradiation to reach 100% degradation. However, 1 g Eu-doped ZnO indicated 100% phenol degradation after 60 min. This behavior may be attributed to the oxidation procedure resulting from the electron injection into ZnO NPs, with just one exciting organic compound level.

Detection of Active Species Responsible for the Photocatalytic Degradation Process
The rare-earth dopants can enhance photocatalytic activities to a substantial degree via the mechanism of charge separation [74]. This significant recombination can be decreased via metal dopant, which professionally traps the charge carriers and increases their lifetime. In this case, Eu 3+ behaves as traps for electrons, Eu 3+ ions in the material take the CB electrons, condensed to get Eu 3+ . These produced Eu 3+ can move those electrons toward adsorbed O 2 − molecules, thus, creating O 2 − radicals. Throughout this procedure, Eu 3+ would be oxidized as Eu 2+ . Both trapping and the releasing process of electrons by Eu 3+ significantly decrease the recombination percentage of e − -h + pairs [59]. The effects of such scavenging agents have been studied to clarify the main reactive types (photogenerated electrons and holes, superoxide/OH radicals) in the photodegradation of MB and phenol molecules shown in Figure 10a,b. As a hole scavenger, the degradation of 200 mM sodium chloride substantially reduces the catalyst activity against photodegradation of MB and Ph.
MO photodegradation for undoped ZnO was just 75.7% in 180 min, while the efficiency reached 95.3% for 1.0 mol% dopants Eu-ZnO [60]. Conspicuously, as the Eu dopant concentrations increases, the efficiency of photodegradation initially upsurges and, after that, declines. Balachandran et al. used the hydrothermal process to prepare Eu2O3-ZnO nanoclusters, where the composite was comprised of 91% ZnO and 9% Eu2O3 [60]. The synthesized Eu2O3-ZnO nanostructures displayed the more considerable photocatalytic activity of 99.2% in the Rhodamine B (RhB) photodegradation under sunlight in 75 min under neutral pH. Numerous researchers have considered hydroxyl radicals the most vital active materials for impurity degradation. The previous comparison proved the influences of the preparation method, weight concentrations of Eu, and conditions of photocatalytic procedure on the photocatalytic activities of prepared Eu2O3-doped ZnO materials.
In this present work, 0.5 g Eu-ZnO (0.05 Eu/Zn molar%) nanocomposites displayed the highest MB photocatalytic degradation in 80 min of visible irradiation to reach 100% degradation. However, 1 g Eu-doped ZnO indicated 100% phenol degradation after 60 min. This behavior may be attributed to the oxidation procedure resulting from the electron injection into ZnO NPs, with just one exciting organic compound level.

Detection of Active Species Responsible for the Photocatalytic Degradation Process
The rare-earth dopants can enhance photocatalytic activities to a substantial degree via the mechanism of charge separation [74]. This significant recombination can be decreased via metal dopant, which professionally traps the charge carriers and increases their lifetime. In this case, Eu 3+ behaves as traps for electrons, Eu 3+ ions in the material take the CB electrons, condensed to get Eu 3+ . These produced Eu 3+ can move those electrons toward adsorbed O2 − molecules, thus, creating O2 − radicals. Throughout this procedure, Eu 3+ would be oxidized as Eu 2+ . Both trapping and the releasing process of electrons by Eu 3+ significantly decrease the recombination percentage of e − -h + pairs [59]. The effects of such scavenging agents have been studied to clarify the main reactive types (photogenerated electrons and holes, superoxide/OH radicals) in the photodegradation of MB and phenol molecules shown in Figure 10a,b. As a hole scavenger, the degradation of 200 mM sodium chloride substantially reduces the catalyst activity against photodegradation of MB and Ph. The photogenerated holes can react and emit chlorine radicals with the chloride anion, which can respond and form radicals of dichloride anions (Cl2) [61]. To produce chlorine and chloride-free anions, two radicals of dichloride anion can efficiently react with each other. To generate radicals of chloro and hydroxyl, the reaction between radicals of dichloride anion and water molecules is also accomplished to get ClOH •− . Chloride has been referred to as a scavenger of OH radicals. IPA can also scavenge a moderately slower rate of . H radicals than OH radicals. OH can be effectively captured by nitrate ions, while • H and photogenerated electrons can be scavenged to a lesser degree [59]. Ascorbic acid (H2A) showed the most extreme decline in MB and Ph degradation as an O2 − scavenger. The reaction between H2A and superoxide radicals could be completed under acidic environments to create ascorbate radicals and H2O2 with a second-order response. The resulting ascorbate was a relatively non-reactive reaction that decomposed into H2A and dehydroascorbic acid through a disproportionate reaction [75].
As confirmed before, the following pattern was found to reduce the photocatalytic behavior of the studied composites in the existence of trapping agents. This pattern, AA > IPA > chloride > nitrate, suggests that the highly significant positions were in MB and phenol photodegradation using the prepared materials, superoxide radicals, and then OH, h + and e − . The arrived photons will excite both semiconductors based on these findings, resulting in e − /h + pair output in both semiconductors. Degradation in the presence of scavengers reveals the prominent role of superoxide radicals generated by the longer lifetime of electrons caused by the presence of Eu ions. As seen in the scavenger test results, superoxide and holes were the main reactive species that caused the degradation of MB and Ph. In addition, the bandgap energy of this coupled oxide is more minor than ZnO, which makes the catalyst widen its spectral absorption range, extending to the visible region. Eu2O3-ZnO may have intermittent bands, which causes a reduction in bandgap energy, and this improves its visible light absorption. The photogenerated electrons-hole separation over ZnO/Eu2O3 heterojunctions enhances the Eu2O3-doped ZnO performance to be larger than either Eu2O3 or ZnO nanoparticles. The photogenerated holes can react and emit chlorine radicals with the chloride anion, which can respond and form radicals of dichloride anions (Cl 2 ) [61]. To produce chlorine and chloride-free anions, two radicals of dichloride anion can efficiently react with each other. To generate radicals of chloro and hydroxyl, the reaction between radicals of dichloride anion and water molecules is also accomplished to get ClOH •− . Chloride has been referred to as a scavenger of OH radicals. IPA can also scavenge a moderately slower rate of . H radicals than OH radicals. OH can be effectively captured by nitrate ions, while • H and photogenerated electrons can be scavenged to a lesser degree [59]. Ascorbic acid (H 2 A) showed the most extreme decline in MB and Ph degradation as an O 2 − scavenger. The reaction between H 2 A and superoxide radicals could be completed under acidic environments to create ascorbate radicals and H 2 O 2 with a second-order response. The resulting ascorbate was a relatively non-reactive reaction that decomposed into H 2 A and dehydroascorbic acid through a disproportionate reaction [75].

Recycling of the Prepared Samples
As confirmed before, the following pattern was found to reduce the photocatalytic behavior of the studied composites in the existence of trapping agents. This pattern, AA > IPA > chloride > nitrate, suggests that the highly significant positions were in MB and phenol photodegradation using the prepared materials, superoxide radicals, and then OH, h + and e − . The arrived photons will excite both semiconductors based on these findings, resulting in e − /h + pair output in both semiconductors. Degradation in the presence of scavengers reveals the prominent role of superoxide radicals generated by the longer lifetime of electrons caused by the presence of Eu ions. As seen in the scavenger test results, superoxide and holes were the main reactive species that caused the degradation of MB and Ph. In addition, the bandgap energy of this coupled oxide is more minor than ZnO, which makes the catalyst widen its spectral absorption range, extending to the visible region. Eu 2 O 3 -ZnO may have intermittent bands, which causes a reduction in bandgap energy, and this improves its visible light absorption. The photogenerated electrons-hole separation over ZnO/Eu 2 O 3 heterojunctions enhances the Eu 2 O 3 -doped ZnO performance to be larger than either Eu 2 O 3 or ZnO nanoparticles.

Recycling of the Prepared Samples
The reusability and recyclability of the 0.5 g and 1 g E-ZnO (S4, S5) photocatalysts are correspondingly estimated for MB and phenol photodegradation, respectively. As illustrated in Figure 11a,b, no loss was significantly presented in the efficiency of MB and Ph photodegradation, even after five succeeding series, which confirms the excellent reusability of the photocatalyst. Additionally, recovering the photocatalyst could be efficiently completed using the photoreaction mixtures with just the centrifugal separation approach. The effectively obtained reusability and recyclability of Eu 2 O 3 -doped ZnO photocatalysts could potentially be advantageous in wastewater treatment technological applications.
The reusability and recyclability of the 0.5 g and 1 g E-ZnO (S4, S5) photocatalysts are correspondingly estimated for MB and phenol photodegradation, respectively. As illustrated in Figure 11a,b, no loss was significantly presented in the efficiency of MB and Ph photodegradation, even after five succeeding series, which confirms the excellent reusability of the photocatalyst. Additionally, recovering the photocatalyst could be efficiently completed using the photoreaction mixtures with just the centrifugal separation approach. The effectively obtained reusability and recyclability of Eu2O3-doped ZnO photocatalysts could potentially be advantageous in wastewater treatment technological applications.

Conclusions
In the present work, the combustion technique is a suitable, inexpensive, efficient, and environmentally friendly approach to preparing Eu2O3-doped ZnO (from 0.001 g to 5 g). The SEM studies confirm an increase in grain size of doped samples compared with

Conclusions
In the present work, the combustion technique is a suitable, inexpensive, efficient, and environmentally friendly approach to preparing Eu 2 O 3 -doped ZnO (from 0.001 g to 5 g). The SEM studies confirm an increase in grain size of doped samples compared with pure ZnO nanostructures. The Eu 2 O 3 -ZnO nanocomposites' optical energy bandgap was between 3.31 eV and 3.24 eV, representing the catalyst absorption edge subject to the ZnO structure. This proposed research concludes that the performance of photocatalytic degradation increases with doping concentration and reaches 100% degradation of MB after 80 min for 0.5 g. Similarly, for 1 g Eu, 100% phenol degradation is reached within 60 min under visible irradiation, indicating the excellent and selective photocatalytic behavior of the produced Eu 2 O 3 -doped ZnO nanocomposites towards the degradation of phenols and methylene blue dyes. The primary role of . O 2 − made from the longer lifetime of electrons due to Eu ions was confirmed to have the highest responsibility for degradation via various trapping agents. Furthermore, this catalyst was recyclable for several runs. Therefore, the nanostructures, innovative and multifunctional materials of Eu 2 O 3 -doped ZnO production, are promising candidates for wide-scale technological, environmental, and biomedical applications, such as optoelectronics and photocatalysis.

Conflicts of Interest:
The authors declare no conflict of interest.