Europium-Doped Y 2 O 3 -Coated Diatomite Nanomaterials: Hydrothermal Synthesis, Characterization, Optical Study with Enhanced Photocatalytic Performance

: Eu-doped Y 2 O 3 coated diatomite nanostructures with variable Eu 3+ contents were synthesized by a facile hydrothermal technique. The products were characterized by means of energy dispersive X-ray photoelectron spectroscopy (EDX), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET), UV-vis diffuse reﬂectance spectroscopy, and photoluminescence spectroscopy techniques. As claimed by PXRD, the particles were crystallized excellently and attributed to the cubic phase of Y 2 O 3 . The inﬂuence of substitution of Eu 3+ ions into Y 2 O 3 lattice caused a redshift in the absorbance and a decrease in the bandgap of as-prepared coated compounds. The pore volume and BET speciﬁc surface area of Eu-doped Y 2 O 3 -coated diatomite is greater than uncoated biosilica. The sonophotocata-lytic activities of as-synthesized specimens were evaluated for the degradation of Reactive Blue 19. The effect of various speciﬁcations such as ultrasonic power, catalyst amount, and primary dye concentration was explored.


Introduction
Advanced oxidation processes (AOPs) have achieved a substantial research interest for decolorization of toxic organic pollutants in different wastewater. The main mechanism of AOPs is the generation of •OH radicals which have greater oxidizing performance and led to attain faster and more effective decomposition of pollutants [1][2][3][4][5][6].
One kind of AOP operation is sonocatalysis that has recently been utilized for degradation of organic contaminants [7][8][9][10][11][12]. The chemical impact of ultrasonic irradiation is raised via the acoustic cavitation and results in collapse growth, and generation of bubbles inside a solution. Through collapsing the bubbles, localized hot spots are created with elevated temperature and pressure. Take into account such sever condition, the dissolved oxygen and water molecules can encounter straight thermal dissociation to produce particularly reactive radical species such as oxygen (•O), •OH, and hydrogen (•H) operating a substantial act to oxidize organic compounds in water [13][14][15][16].
The blend of photocatalysis and sono-catalysis seems to promote the decolorization share of organic contaminants due to synergistic results [17]. Besides the generation of reactive oxygen species (ROS) through cavitation, employing ultrasound in photocatalytic procedure has other satisfaction, such as boosting the functional surface area of the catalyst through blocking the aggregation of particles, incrementing the contaminants' mass transfer between the catalyst surfaces and solution, intercepting catalyst deactivation via constantly cleansing absorbed molecules from the catalyst surface through micro bubbling and micro streaming, and increasing the number of high temperature and pressure regions via breaking up micro-bubbles made via the ultrasound into smaller ones in the existence of catalyst nanoparticles [17][18][19]. Chemical Structure carrier material. Additionally, the porous structure of diatom made it a potential heterogeneous catalyst [20][21][22]. Yttrium oxide (Y2O3) is an essential engineering compound in various fields because of its physical and chemical properties, such as relatively large bandgap energy, high melting point, and high permittivity [23][24][25][26]. Previously, Eu-doped Y2O3 nanostructures were prepared through several processes such as the combustion method [27], hydrothermal method [28], laser vaporization [29], sol gel method [30] and co-precipitation method [31], Precipitation method [32], and evaporation method [33] for various applications of thermoluminescence, photoluminescence imaging and luminescence. Lanthanide-incorporated nano-structured semiconductors have freshly been employed as active sonocatalyst and photocatalysts. With empty 5d orbitals and partly occupied 4f orbitals, Ln 3+ cations could also remarkably boost the separation rate of photoinduced charge carriers within semiconductor catalysts and promote markedly the sonocatalytic performance [1,[34][35][36][37][38]. The photoluminescence properties of Eu 3+ -doped Y2O3 nanostructures have attracted much attention due to its red phosphors characteristics in the last decade [39][40][41].
This work deals with describing a simple hydrothermal route for the preparation of coated-diatom with europium-doped Y2O3 (EuxY2−xO3) nanomaterials. The nanoparticles' sonocatalytic activity was assessed for RB 19 (see Table 1). No report exists on the sonocatalytic degradation of Reactive Blue 19 (RB 19) in the presence of coated-diatom with europium-doped Y2O3. The sono-photocatalytic degradation of organic dye was examined and optimized and the influence of inorganic ions on the degradation effects of RB19 was investigated.

Characterization
Using PXRD characterization at room temperature, the samples' crystal phase composition was determined via a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with an emission current of 30 mA, monochromatic high-intensity Cu Ka radiation (λ = 1.5406 Å), and accelerating voltage of 40 kV. Elemental analyses were obtained using a linked ISIS-300, (Oxford Instruments, Abingdon, UK) (energy dispersion spectroscopy) detector. Using an electron microscope (SEM, S-4200, Hitachi, Tokyo, Japan), the surface state and the morphology were observed. Moreover, the obtained SEM images were analyzed using manual microstructure distance measurement software (Nahamin Pardazan Asia Co., Iran) to determine the diameter size distribution of obtained samples. A diffuse reflectance UV-Vis spectrophotometer was used to evaluate the samples' optical absorption spectra (Varian Cary 3 Bio, Australia). Via a Spex FluoroMax-3 spectrometer (Longjumeau, France), photoluminescence spectra were acquired after dispersing a small amount of specimen in distilled water using ultrasound. The Specific surface areas were performed through a single point BET technique utilizing Micrometrics Gemini V and Gemini 2375. Cell para an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, an emission current of 30 mA, meters were calculated with Celref program from powder XRD patterns, and reflections have been determined and fitted using a profile fitting procedure with the Winxpow program. The reflections observed in 2θ = 10-70 • were used for the lattice parameter determination.

Preparation of Eu-Doped Y 2 O 3 Nanoparticles
Eu-doped yttrium oxide nanomaterials with various Eu contents (0-8 mol%) were acquired hydrothermally. In a typical synthesis, Eu (NO 3 ) 3 .5H 2 O and proper molar ratios of YCl 3 were dissolved first in 40 mL of deionized water. Therefore, EDTA (0.4 g) and 2 mmol NaOH were added drop-wise to the solution at moderate speed with stirring. The above solution was transferred into 100-mL autoclave with Teflon-lined stainless steel. Then it was settled in an oven at 250 • C for 72 h and finally allowed to cool to room temperature naturally. Gathering the as-prepared Eu x Y 2-x O 3 nanoparticles, they were rinsed several times with distilled water and absolute ethanol for eliminating remained impurities, and then dried for 2 h at 60 • C at vacuum. The result was a white powder.

Diatomite Coated with Eu-Doped Y 2 O 3 Nanoparticles
The diatom was weighed and added to 25 mL deionized water and was continuously stirred to spread evenly. Then, 1.5 g Eu-doped Y 2 O 3 compounds with various Eu content were added to the above solution to fully disperse. After 1 h of ultrasonic irradiation, the solution transferred into a 100-mL autoclave with Teflon-lined stainless steel, settled in an oven at 160 • C for 12 h, and then cools down naturally to room temperature. The final coated-diatom was obtained after washing and drying the precipitates at 60 • C.

The Evaluation of Catalytic Activity
The sonophotocatalytic activity of the diatom biosilica coated with Eu-doped yttria nanomaterial was examined to degrade RB19 as a model organic pollutant. In a general route, suspending the nanocatalyst (0.1 g) was carried out in an aqueous solution of the model dye (100 mL) of with a recognized initial concentration at pH = 6. So, the suspension was radiated into the ultrasonic bath via a 40 W compact fluorescent visible lamp armed with a cutoff filter to show visible light illumination (λ of higher than 420 nm). To determine the removal of dye, A UV-vis spectrophotometer was employed through the absorbance at λ max = 592 nm. The degradation effectiveness (DE) was calculated as follows: where C 0 and C are the dye's initial and ultimate concentrations within the solution (mg/L), respectively. To evaluate the nanocatalyst's reusability, the utilized catalyst was separated from the solution, rinsed with distilled water, and utilized in a new run after drying at 70 • C.  Figure 1 presents the PXRD pattern of uncoated diatomite, Y 2 O 3 , and diatom-coated with Eu-doped Y 2 O 3 . The diatomite PXRD pattern is indexed readily to SiO 2 (JCPDS card no. 00-001-0647) whilst the four broad and strong peaks with attributed reflections (222), (400), (440), (622) and 2θ = 29.1, 33.7, 48.6, and 58.2 • are related to the cubic structure of yttria, respectively (JCPDS card no. 41-1105) [42]. The indistinguishable forms of the PXRD patterns of the Y 2 O 3 and the coated with Eu x Y 2-x O 3 compounds propose that the surface of the diatom is equally covered. As illustrated in Figure 1, the substitution of yttria by Eu 3+ , only the peaks related to the Y 2 O 3 were still noticed and no other peaks corresponding to Y(OH) 3 , Eu 2 O 3 or other impurities were detected, which displays that the Eu 3+ ions substitute Y 3+ ions in yttria structure. A negligible shift was observed to lower diffraction for diatomite-coated with Eu-dopedY 2 O 3 samples. This could be attributed with the Y 2 O 3 lattice's contraction owing to the substitution of Eu 3+ ions, with a bigger radius (1.07 Å) in comparison to Y 3+ ions (1.02 Å) [7,35]. The XRD pattern of diatom-coated with 6%Eu-doped Y 2 O 3 is provided in Supplementary Materials as Figure S1.   The cell parameters of the synthesized materials were calculated from the PXRD patterns. With increasing dopant content (x), a parameter for Eu 3+ increases ( Figure 2). The trend for lattice constants can be correlated to the effective ionic radii of the Ln 3+ ions, which results in larger lattice parameters for Eu 3+ doped materials.

Characterizing the Synthesized Samples
. Figure 1. The PXRD pattern of diatomite (a), Y2O3 (b) and diatom-coated with 8%-Eu doped Y (c) nanoparticles.   Figure 3a. High porosity and the surface morphology of diatom can be employed as the template to enhance the potential of functional materials. Figure 3b represents the image of 6% Eu-doped Y 2 O 3 nanoparticles which diameter of these particles is around 20-60 nm. The size distribution of 6% Eu-doped Y 2 O 3 is provided in Supplementary Materials as Figure S2 indicating the particles size mainly around 20-60 nm. Following the immobilization of 6% Eu-doped yttrium oxide, the SEM image of diatomite is seen in Figure 3c,d. Changing the mass ratio of the substrate-diatomite and the coating material-doped-Y 2 O 3 controls the thickness of the coating layers. To achieve smooth, stable, and uniform coating, the optimum ratio is 1.5. Figure 4 displays the TEM and HRTEM images of 6% Eu-doped Y 2 O 3 -coated diatomite at different magnification. The SAED pattern of as-prepared 6% Eu doped samples shows good crystallinity of obtained compounds (Figure 4b). The HRTEM images also confirms the nanosized coated Eu-doped Y 2 O 3 on biosilica as shown in Figure 4c The elemental mapping of O, Y, Eu and Si in the diatom-coated with Eu-doped Y 2 O 3 sample was shown in Figure 5 confirming the XRD outcome and purity of sample. The atomic percent of as-prepared sample are provided in Table 2 displaying the composition, elements and there is no impurity in the prepared sample.     The elemental mapping of O, Y, Eu and Si in the diatom-coated with Eu-doped Y2O3 sample was shown in Figure 5 confirming the XRD outcome and purity of sample. The atomic percent of as-prepared sample are provided in Table 2 displaying the composition, elements and there is no impurity in the prepared sample.     Figure 6a shows the absorbance spectra of Eu-doped Y2O3 coated pounds. A redshift in absorbance spectra is seen with increasing dopant The as-synthesized compound's band-gap energy can be calculated ception of the obtained linear area with the energy axis according to (h (Tauc plot) (Figure 6b). Compared to the pure sample, the doped Y2O3 h  Figure 6a shows the absorbance spectra of Eu-doped Y 2 O 3 coated diatomite compounds. A redshift in absorbance spectra is seen with increasing dopant content.
The as-synthesized compound's band-gap energy can be calculated from the interception of the obtained linear area with the energy axis according to (hvα) 2 versus (hv) (Tauc plot) (Figure 6b). Compared to the pure sample, the doped Y 2 O 3 has lower Eg values, and it is deducted by incrementing the dopant. Table 3 lists the band-gap energy for Eu-doped Y 2 O 3 coated diatomite nanoparticles.     . Under UV irradiation, the samples present a strong emission centered at 614 nm, and other emission feature is an indication of europium cations in the cubic Y 2 O 3 host lattice. The emission spectra are consisting of Eu 3+ 5 D 0 -7 F J transitions (J = 0, 1, 2, 3 and 4) with the eminent peak 5 D 0 -7 F 2 (614 nm). Other peaks at 583 ( 5 D 0 -7 F 0) , 596 ( 5 D 0 -7 F 1 ), 633 (D 0 -7 F 3 ) and 711 nm ( 5 D 0 -7 F 4) were observed, respectively [43]. The schematic bandgap model for the luminescence of Eu 3+ doped in Y 2 O 3 lattice was provided in Supplementary Materials as Figure S8. Adsorption/desorption isotherm of nitrogen for uncoated diatomite and coated with 6% Eu 3+ -substituted Y2O3 are shown in Figure 8. In the case of both specimens, two isotherms with obvious hysteresis loop were noticed in the pressure range of 0.3-1.0 p/p0. The BET specific surface area of biosilica is 1.5 m 2 /g. Additionally, the BET specific surface area of diatomite coated with 6% Eu-substituted Y2O3 (8.7 m 2 /g) considerably exceeds that of uncoated diatom. A superior adsorption performance is expected for coated-diatom with europium-doped Y2O3 nanoparticles. Adsorption/desorption isotherm of nitrogen for uncoated diatomite and coated with 6% Eu 3+ -substituted Y 2 O 3 are shown in Figure 8. In the case of both specimens, two isotherms with obvious hysteresis loop were noticed in the pressure range of 0.3-1.0 p/p 0 . The BET specific surface area of biosilica is 1.5 m 2 /g. Additionally, the BET specific surface area of diatomite coated with 6% Eu-substituted Y 2 O 3 (8.7 m 2 /g) considerably exceeds that of uncoated diatom. A superior adsorption performance is expected for coated-diatom with europium-doped Y 2 O 3 nanoparticles.

Synergistic Effect of Photocatalysis and Sonocatalysis on the Degradation of RB19 Using Diatomite Coated with Eu-Doped Y 2 O 3 Nanoparticles
By comparative evaluations, the photo-, sono-, and sono-photocatalytic performance of the prepared diatomite coated with Eu-doped Y 2 O 3 were assessed, for which the results are displayed in Figure 9. The degradation percent was very low in the absence of catalyst nanoparticles under light irradiation, which demonstrates that photolysis doesn't contribute to the RB19 elimination from the solution.

Synergistic Effect of Photocatalysis and Sonocatalysis on the Degradation of R Diatomite Coated with Eu-Doped Y2O3 Nanoparticles
By comparative evaluations, the photo-, sono-, and sono-photocatalytic of the prepared diatomite coated with Eu-doped Y2O3 were assessed, for wh are displayed in Figure 9. The degradation percent was very low in the absen nanoparticles under light irradiation, which demonstrates that photolysis do ute to the RB19 elimination from the solution.
The findings of the assay conducted in dark condition present that surfa has no remarkable effect on degrading the dye solution. The degradation eff photocatalytic route was less than 64%. The degradation efficiency through degradation (52%) was higher than sonolysis (7%). The degradation percen standingly boosted in the sonophotocatalytic process (96.45%) owing to sy pacts between photo-and sono-catalysis. This can be simplified as: firstly, th The findings of the assay conducted in dark condition present that surface adsorption has no remarkable effect on degrading the dye solution. The degradation efficiency by the photocatalytic route was less than 64%. The degradation efficiency through sonocatalytic degradation (52%) was higher than sonolysis (7%). The degradation percentage was outstandingly boosted in the sonophotocatalytic process (96.45%) owing to synergistic impacts between photo-and sono-catalysis. This can be simplified as: firstly, the production of several ROSs through the integrated cavitation effects and photocatalytic procedure, secondly, the elevated mass transfer rates, thirdly, the catalyst nanoparticles disaggregation by ultrasound and the larger active surface areas, and fourthly, the creation of more hot spots through catalyst nanoparticles [17][18][19]. Figure 10 shows the spectra of the lamp used for catalytic process. . Figure 10. The spectrum of the lamp used for catalytic process.
As displayed in Figure 11, the graph of (−ln (C/C0) vs. time indicates linear trend for all three sono-photocatalytic, photocatalytic and sonocatalytic processes, displaying pseudo-first-order kinetics [1,44,45]. It is apparent that the pseudo-first-order kinetic constant and degradation ability in sono-photocatalysis (kobs,sono-photo) are bigger than those for sonocatalysis (kobs,sono) and photocatalysis (kobs,photo). These outcomes totally introduce the concept of a synergistic impact.  . Figure 10. The spectrum of the lamp used for catalytic process.
As displayed in Figure 11, the graph of (−ln (C/C0) vs. time indicates linear trend for all three sono-photocatalytic, photocatalytic and sonocatalytic processes, displaying pseudo-first-order kinetics [1,44,45]. It is apparent that the pseudo-first-order kinetic constant and degradation ability in sono-photocatalysis (kobs,sono-photo) are bigger than those for sonocatalysis (kobs,sono) and photocatalysis (kobs,photo). These outcomes totally introduce the concept of a synergistic impact. As displayed in Figure 11, the graph of (−ln (C/C 0 ) vs. time indicates linear trend for all three sono-photocatalytic, photocatalytic and sonocatalytic processes, displaying pseudo-first-order kinetics [1,44,45]. It is apparent that the pseudo-first-order kinetic constant and degradation ability in sono-photocatalysis (k obs,sono-photo ) are bigger than those for sonocatalysis (k obs,sono ) and photocatalysis (k obs,photo ). These outcomes totally introduce the concept of a synergistic impact. Inorganics 2021, 9, x FOR PEER REVIEW 13 of 20 To examine the decolorization process's mechanism and to reveal the chief oxidative species, assays were conducted in the presence of proper scavengers of active species. As stated by Figure 12, the addition of t-BuOH (a scavenger of hydroxyl radicals) results in decreasing of 14% in the decomposition percent. By addition of oxalate (a scavenger of h + VB), the decolorization efficiency reduced to 54%. In the case of Cl − and I − (scavenger of hole) it reaches to 27 and 33% respectively. The dye degradation was inhibited remarkably as benzoquinone (BQ) was added (a scavenger of superoxide radicals). These results display that the superoxide radicals and h + VB were the substantial oxidative species in degradation of organic dye. In addition, the hydroxyl radicals also affect the degradation. Considering the synergistic impacts of photo-and sono-catalysis, a possible mechanism for the decolorization procedure can be proposed [17,19]: .  To examine the decolorization process's mechanism and to reveal the chief oxidative species, assays were conducted in the presence of proper scavengers of active species. As stated by Figure 12, the addition of t-BuOH (a scavenger of hydroxyl radicals) results in decreasing of 14% in the decomposition percent. By addition of oxalate (a scavenger of h + VB ), the decolorization efficiency reduced to 54%. In the case of Cl − and I − (scavenger of hole) it reaches to 27 and 33% respectively. The dye degradation was inhibited remarkably as benzoquinone (BQ) was added (a scavenger of superoxide radicals). These results display that the superoxide radicals and h + VB were the substantial oxidative species in degradation of organic dye. In addition, the hydroxyl radicals also affect the degradation. Considering the synergistic impacts of photo-and sono-catalysis, a possible mechanism for the decolorization procedure can be proposed [17,19]: To examine the decolorization process's mechanism and to reveal the chief oxida species, assays were conducted in the presence of proper scavengers of active species stated by Figure 12, the addition of t-BuOH (a scavenger of hydroxyl radicals) resul decreasing of 14% in the decomposition percent. By addition of oxalate (a scavenge h + VB), the decolorization efficiency reduced to 54%. In the case of Cl − and I − (scavenge hole) it reaches to 27 and 33% respectively. The dye degradation was inhibited remark as benzoquinone (BQ) was added (a scavenger of superoxide radicals). These results play that the superoxide radicals and h + VB were the substantial oxidative species in de dation of organic dye. In addition, the hydroxyl radicals also affect the degradation. C sidering the synergistic impacts of photo-and sono-catalysis, a possible mechanism the decolorization procedure can be proposed [17,19]: .
(3) Water molecules or hydroxyl anions can be oxidized by the photogenerated holes to produce hydroxyl radicals: (4) The pyrolysis of water molecule can be boosted by the US to generate hydrogen and hydroxyl radicals: (5) Eventually, the molecules of dye can be decomposed by the produced active spices: Reactive blue 19 + reactive oxygen species (ROSs) → degrading dye molecules

Effect of Eu 3+ Content of Diatom Coated Eu-Doped Y 2 O 3 Nanoparticles
The removal effectiveness of RB 19 over diatomite coated Eu-doped Y 2 O 3 sonocatalysts during 70 min of sonophotolysis is shown in Figure 13. The doped samples with proper content of Eu 3+ ion had much better catalytic performance compared with uncoated biosilica. For diatom coated 6% Eu doped Y 2 O 3 nanoparticles, the highest decolorization efficiency was achieved. Two mechanisms can explain these findings. First, rare-earth cations such as Eu 3+ restrain the electron-hole recombination and act as an electron scavenger. Second, the new energy level under the conduction band edge of Y 2 O 3 is created by substituting of Eu 3+ into yttria lattice [46,47]. Considering the standard redox potentials of E 0 (Eu 3+ /Eu 2+ ) = −0.35 V and E 0 (O 2 /O 2 − ) = 0.338 V, the existence of Eu 3+ in yttria structure can improve the following reactions [48]: • OH, H 2 O 2 and O 2 • radicals are well-known powerful oxidants for the degrading of organic dye [49].
The UV-vis absorption spectra of RB 19 at various irradiation periods for the sonophotocatalytic process are shown in Figure 14. The reducing concentration of RB 19 during the catalytic process is used to examine the performance of the catalyst. The UV-vis absorption spectra of RB 19 at various irradiation periods for the sonophotocatalytic process are shown in Figure 14. The reducing concentration of RB 19 during the catalytic process is used to examine the performance of the catalyst.
. Figure 14. Degradation of Reactive blue 19 under ultrasonic irradiation using diatomite coated with 6% Eu-doped yttrium oxide. The UV-vis absorption spectra of RB 19 at various irradiation periods for the sonophotocatalytic process are shown in Figure 14. The reducing concentration of RB 19 during the catalytic process is used to examine the performance of the catalyst.

Primary Dye Concentration Effect
Various initial dye concentrations in the range of 20 to 40 mg/L were used in this study. As seen in Figure 15, DE % reduced from 96.54 to 49.21% in the case of initial concentration 20 to 40 mg/L. Filling the energetic sites on the catalyst's surface by pollutants molecules at high concentration results in an astounding decrease in the degradation effectiveness.
The prevention of diffusion of light to the catalyst's surface can led to unsatisfactory performance at dense concentration of dye solution [50].

Primary Dye Concentration Effect
Various initial dye concentrations in the range of 20 to 40 mg/L were used in this study. As seen in Figure 15, DE % reduced from 96.54 to 49.21% in the case of initial concentration 20 to 40 mg/L. Filling the energetic sites on the catalyst's surface by pollutants molecules at high concentration results in an astounding decrease in the degradation effectiveness. The prevention of diffusion of light to the catalyst's surface can led to unsatisfactory performance at dense concentration of dye solution [50].

The Amount of Catalyst Effect
The effect of catalyst value on the removal percentage of RB 19 is seen in Figure 16. The sonication period and primary dye concentration were 70 min and 20 mg/L, respectively. At concentration of 0.5, 0.75, 1.0, 1.25 and 1.5 g/L, the color removal ratio was 78.43, 85.92, 90.76 and 96.45%, sequentially. The DE% increased from 0.5 to 1.25 g/L, and then reduced. This could be explained by the aggregation of catalyst beyond 1.25 g/L which led to reducing ultrasound scattering and decreasing the number of active sites [7,17].

The Amount of Catalyst Effect
The effect of catalyst value on the removal percentage of RB 19 is seen in Figure 16. The sonication period and primary dye concentration were 70 min and 20 mg/L, respectively. At concentration of 0.5, 0.75, 1.0, 1.25 and 1.5 g/L, the color removal ratio was 78.43, 85.92, 90.76 and 96.45%, sequentially. The DE% increased from 0.5 to 1.25 g/L, and then reduced. This could be explained by the aggregation of catalyst beyond 1.25 g/L which led to reducing ultrasound scattering and decreasing the number of active sites [7,17].
Various initial dye concentrations in the range of 20 to 40 mg/L were used in this study. As seen in Figure 15, DE % reduced from 96.54 to 49.21% in the case of initial concentration 20 to 40 mg/L. Filling the energetic sites on the catalyst's surface by pollutants molecules at high concentration results in an astounding decrease in the degradation effectiveness. The prevention of diffusion of light to the catalyst's surface can led to unsatisfactory performance at dense concentration of dye solution [50].

The Amount of Catalyst Effect
The effect of catalyst value on the removal percentage of RB 19 is seen in Figure 16 The sonication period and primary dye concentration were 70 min and 20 mg/L, respectively. At concentration of 0.5, 0.75, 1.0, 1.25 and 1.5 g/L, the color removal ratio was 78.43 85.92, 90.76 and 96.45%, sequentially. The DE% increased from 0.5 to 1.25 g/L, and then reduced. This could be explained by the aggregation of catalyst beyond 1.25 g/L which led to reducing ultrasound scattering and decreasing the number of active sites [7,17].

Effect of Sonication Energy
The decolorization percent of RB19 under different ultrasonic power in the range 60 to 150 W was also investigated. Employing the 50 to 150 W/L sonication power, the degradation percentage is increased from 84.17% to 96.45% (Figure 17). Enhanced degra-dation ratio was achieved through increasing the ultrasonic power which the production of •OH radicals increases. Moreover, the disturbance of the solution was elevated with significant power and accordingly promotes the mass transfer rate of dye, reactive species and intermediates between catalyst's surface and the bulk solution [51]. In order to save energy, the experiments set on 120 W due to slight contrast of degradation percentage between 120 and 150 W.

Effect of Sonication Energy
The decolorization percent of RB19 under different ultrasonic power in the range 60 to 150 W was also investigated. Employing the 50 to 150 W/L sonication power, the degradation percentage is increased from 84.17% to 96.45% (Figure 17). Enhanced degradation ratio was achieved through increasing the ultrasonic power which the production of •OH radicals increases. Moreover, the disturbance of the solution was elevated with significant power and accordingly promotes the mass transfer rate of dye, reactive species and intermediates between catalyst's surface and the bulk solution [51]. In order to save energy, the experiments set on 120 W due to slight contrast of degradation percentage between 120 and 150 W.
.  Figure 18 displays the reusability of diatomite coated 6% Eu-doped yttrium oxide, which was tested at a reaction period of 70 min, a dye concentration of 20 mg/L and 1.25 g/L of catalyst powder. These settings are the optimum operational parameters values verified through experiments. After five continuous tests, the reduction in the sono-photocatalytic procedure was slight, which validates the prepared particles' high potential stability and reusability.  Figure 18 displays the reusability of diatomite coated 6% Eu-doped yttrium oxide, which was tested at a reaction period of 70 min, a dye concentration of 20 mg/L and 1.25 g/L of catalyst powder. These settings are the optimum operational parameters values verified through experiments. After five continuous tests, the reduction in the sono-photocatalytic procedure was slight, which validates the prepared particles' high potential stability and reusability.

Effect of Sonication Energy
The decolorization percent of RB19 under different ultrasonic power in the range 60 to 150 W was also investigated. Employing the 50 to 150 W/L sonication power, the degradation percentage is increased from 84.17% to 96.45% (Figure 17). Enhanced degradation ratio was achieved through increasing the ultrasonic power which the production of •OH radicals increases. Moreover, the disturbance of the solution was elevated with significant power and accordingly promotes the mass transfer rate of dye, reactive species and intermediates between catalyst's surface and the bulk solution [51]. In order to save energy, the experiments set on 120 W due to slight contrast of degradation percentage between 120 and 150 W.
[Catalyst] = 1.25 g/L, pH = 6 and [dye] = 20 mg/L. Figure 18 displays the reusability of diatomite coated 6% Eu-doped yttrium oxide, which was tested at a reaction period of 70 min, a dye concentration of 20 mg/L and 1.25 g/L of catalyst powder. These settings are the optimum operational parameters values verified through experiments. After five continuous tests, the reduction in the sono-photocatalytic procedure was slight, which validates the prepared particles' high potential stability and reusability. In comparison to other routine semiconductors such as TiO 2 , ZnO, CdS, etc., Eudoped Y 2 O 3 coated-diatom displays astonishing photocatalytic activity toward degradation of organic dye. It is notable that the time of degradation process is 70 min and the decolorization efficiency is over 90 percent competing with previous ordinary catalysts.

Conclusions
Europium-doped Y 2 O 3 -coated diatomite nanomaterials were prepared via facile hydrothermal route and employed for degradation of reactive blue 19 in an aqueous solution. Substitution of Eu 3+ into yttrium oxide lattice led to a redshift in absorbance spectra and reduction of bandgap respectively. The PL spectra of as-prepared Eu-doped Y 2 O 3 coated diatomite are composed of 5 D 0 -7 F J transitions. The decolorization effectiveness of the as-synthesized compounds was much bigger in sono-photocatalytic procedure than that of other routes. 6% Eu 3+ -incorporated Y 2 O 3 coated diatom exhibits the best degradation ratio. Radical scavengers reduce the sonophotocatalytic degradation percentage of R B 19 particularly subject to 1,4 benzoquinone. The results revealed that diatomite coated with Eu 3+ -incorporated yttrium oxide nanoparticle can be employed in numerous experimental runs with no notable drop in photocatalytic performance.