Preparation and Property Characterization of Eu 2 SmSbO 7 /ZnBiEuO 4 Heterojunction Photocatalysts and Photocatalytic Degradation of Chlorpyrifos under Visible Light Irradiation

: Eu 2 SmSbO 7 and ZnBiEuO


Introduction
In recent years, there has been an increasing demand for food supplies due to the growing global population [1][2][3].The use of pesticides is a valid method to increase crop yields [4][5][6][7].Therefore, pesticides are widely used in agricultural production.However, usually only a small amount of the pesticide works, and most of the pesticide remains on the crop or in the soil.Pesticide residues on crops can be harmful to creatures, and other residues can cause serious pollution of the soil and even the underground water [8][9][10][11].Chlorpyrifos, as one of the classical organophosphorus pesticides, has been widely used due to its low cost and effective ability to control pests, weeds, and diseases [12][13][14][15].However, the disadvantages of chlorpyrifos are also obvious; for example, chlorpyrifos is an organic pollutant that is difficult to degrade; at the same time, chlorpyrifos also affects the neurological system, leading to various diseases [16][17][18][19][20].Moreover, chlorpyrifos has adverse effects on the development of the brain and body, especially for children [21,22].Therefore, elimination of chlorpyrifos derived from wastewater is a meaningful goal, considering the healthy living environment for humans.
After preliminary physical filtration, some methods have been invented for the degradation of small particle organic pollutants that are easily dissolved in water, such as chlorination, electrocatalysis, and anaerobic-aerobic biological treatment [23][24][25][26][27][28][29][30][31].All the above methods have some drawbacks.For example, chlorination is hard to degrade completely.Moreover, electrocatalytic methods consume more energy, and, ultimately, the anaerobic-aerobic biological treatment requires complicated reaction conditions.Therefore, finding a method with higher degradation efficiency to solve the problem of organic pollutants is needed.
A promising method, photocatalytic technology, was reported to be an effective way of degrading organic pollutants [32][33][34][35][36][37][38][39].Photocatalytic technology absorbs solar energy to degrade water, generating free radicals with significant oxidizing and reducing properties.The free radicals can degrade organic pollutants without producing secondary pollution.Photocatalytic technology has become a favored choice due to its advantages of high efficiency, non-toxicity, low cost, and environmental friendliness [40][41][42].
At the beginning, TiO 2 became a popular research area in the field of photocatalysis due to its simple fabrication process, high photocatalytic activity, and excellent stability [43][44][45].With TiO 2 as a catalytic material, it was required that the incident photon energy must be higher than 3.2 eV, and only ultraviolet light would be able to satisfy this condition.At the same time, ZnO was also reported as the photocatalyst using ultraviolet light [46,47].The energy of ultraviolet light in sunlight was too small compared with visible light (which occupies 43% of the sunlight spectrum).Based on the reports of the researchers, the construction of the composite would have a wider light absorption range and higher photocatalytic efficiency than that of single metal oxides and composite catalysts [48][49][50][51].A 2 B 2 O 7 -type compounds and AB 2 O 4 -type compounds had been reported to have excellent catalytic performance under visible light irradiation (VLID).For example, Zou et al. found that Bi 2 InNbO 7 had a good effect in decomposing water to produce hydrogen [52].Based on our previous report, ZnBiYO 4 also exhibited excellent photocatalytic properties under VLID [53].It was speculated that Eu 2 SmSbO 7 and ZnBiEuO 4 could effectively degrade organic pollutants under VLID.
In the process of photochemical reactions, the photo-induced electrons and the photoinduced holes are generated after light irradiation on the surface of the photocatalytic material.The photo-induced electrons and the photo-induced holes participated in the subsequent reaction to generate superoxide anions and hydroxyl radicals.Superoxide radicals and hydroxyl radicals had strong redox capacity and therefore can degrade organic pollutants effectively.However, the photo-induced electrons and the photo-induced holes that do not participate in the reaction are recombined, affecting the generation of superoxide radicals and hydroxyl radicals, and thus affecting the photocatalytic efficiency.Therefore, the photocatalytic efficiency can be improved by inhibiting the recombination of the photo-induced electrons and the photo-induced holes [54,55].The catalytic efficiency can be improved by constructing heterojunction structures to inhibit the complexation of photo-induced electrons and photo-induced holes [56,57].For example, the heterojunction AgBr/BiPO 4 that was synthesized by Xu H. et [58,59].Thus, we synthesized the Eu 2 SmSbO 7 /ZnBiEuO 4 heterojunction photocatalyst (EZHP) and found that EZHP had excellent catalytic activity under VLID.The catalyst Ag/TiO 2 that was prepared by Fattah W. I. A. et al. achieved a conversion rate of chlorpyrifos of nearly 75% after light irradiation for 120 min [60].The conversion rate of chlorpyrifos reached 84.5% using EZHP under VLID of 120 min in our manuscript.In conclusion, the EZHP that was prepared by Jingfei Luan showed higher photocatalytic activity compared with the Ag/TiO 2 that was prepared by Fattah W. I. A. et al.The catalyst Cu/ZnO that was prepared by Pathania, D. et al. achieved a conversion rate of chlorpyrifos of 81% after solar light irradiation of 240 min [61].The conversion rate of chlorpyrifos reached 100% using EZHP under VLID of 160 min in our manuscript.In summary, the EZHP that was prepared by Jingfei Luan showed higher photocatalytic activity compared with the Cu/ZnO that was prepared by Pathania, D. et al.In this paper, the structural properties of pure-phase ZnBiEuO 4 and single-phase Eu 2 SmSbO 7 were analyzed using an X-ray diffractometer (XRD), a UV-Vis spectrophotometer, a Fourier transform infrared (FTIR) spectrometer, a Raman spectrometer, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and ultraviolet photoelectron spectroscopy (UPS).In addition, the conversion rates of chlorpyrifos under VLID were measured with pure-phase Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or Eu 2 SmSbO 7 /ZnBiEuO 4 as a photocatalyst.The aim of this study was to prepare a novel heterojunction photocatalyst for the removal of chlorpyrifos from pesticide wastewater under VLID.Our innovative research involved the first synthesis and characterization of a novel Eu 2 SmSbO 7 nanocatalyst, ZnBiEuO 4 , and EZHP.For the first time, EZHP was used to degrade chlorpyrifos and the conversion rates of chlorpyrifos under VLID were obtained.The results showed that EZHP had high photocatalytic activity and could effectively degrade chlorpyrifos.This paper will make a huge contribution to the structural construction of novel photocatalysts and to the study of pesticide degradation.

XRD Analysis
X-ray diffraction (XRD) uses the diffraction of X-rays in crystals to characterize the diffracted X-ray signals and obtain information about the structure of the crystalline material, the crystal size, and cell parameters.Figure 1 shows the XRD patterns of ZnBiEuO 4 , Eu 2 SmSbO 7 , and Eu 2 SmSbO 7 /ZnBiEuO 4 heterojunction photocatalyst (EZHP).As can be seen from Figure 1, the diffraction peaks of Eu 2 SmSbO 7 and ZnBiEuO 4 could be detected in the XRD spectra of EZHP without any other diffraction peaks that belonged to the impure phase, which indicated that the EZHP was successfully synthesized.
Catalysts 2024, 14, x FOR PEER REVIEW 3 of 35 EZHP under VLID of 120 min in our manuscript.In conclusion, the EZHP that was prepared by Jingfei Luan showed higher photocatalytic activity compared with the Ag/TiO2 that was prepared by Fattah W. I. A. et al.The catalyst Cu/ZnO that was prepared by Pathania, D. et al. achieved a conversion rate of chlorpyrifos of 81% after solar light irradiation of 240 min [61].The conversion rate of chlorpyrifos reached 100% using EZHP under VLID of 160 min in our manuscript.In summary, the EZHP that was prepared by Jingfei Luan showed higher photocatalytic activity compared with the Cu/ZnO that was prepared by Pathania, D. et al.
In this paper, the structural properties of pure-phase ZnBiEuO4 and single-phase Eu2SmSbO7 were analyzed using an X-ray diffractometer (XRD), a UV-Vis spectrophotometer, a Fourier transform infrared (FTIR) spectrometer, a Raman spectrometer, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and ultraviolet photoelectron spectroscopy (UPS).In addition, the conversion rates of chlorpyrifos under VLID were measured with purephase Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or Eu2SmSbO7/ZnBiEuO4 as a photocatalyst.The aim of this study was to prepare a novel heterojunction photocatalyst for the removal of chlorpyrifos from pesticide wastewater under VLID.Our innovative research involved the first synthesis and characterization of a novel Eu2SmSbO7 nanocatalyst, ZnBiEuO4, and EZHP.For the first time, EZHP was used to degrade chlorpyrifos and the conversion rates of chlorpyrifos under VLID were obtained.The results showed that EZHP had high photocatalytic activity and could effectively degrade chlorpyrifos.This paper will make a huge contribution to the structural construction of novel photocatalysts and to the study of pesticide degradation.

XRD Analysis
X-ray diffraction (XRD) uses the diffraction of X-rays in crystals to characterize the diffracted X-ray signals and obtain information about the structure of the crystalline material, the crystal size, and cell parameters.Figure 1 shows the XRD patterns of ZnBiEuO4, Eu2SmSbO7, and Eu2SmSbO7/ZnBiEuO4 heterojunction photocatalyst (EZHP).As can be seen from Figure 1, the diffraction peaks of Eu2SmSbO7 and ZnBiEuO4 could be detected in the XRD spectra of EZHP without any other diffraction peaks that belonged to the impure phase, which indicated that the EZHP was successfully synthesized.Figure 2a displays the structural properties of Eu 2 SmSbO 7 according to Rietveld analysis using the Materials Studio program.The results of Rietveld refinement for Eu 2 SmSbO 7 showed that the observed intensities and calculated intensities of the pyrochlore-type structure were in fine agreement.Eu 2 SmSbO 7 was a single-phase, cubic crystal system with a space group of Fd3m (modeled to include O atoms) [62].The atomic structure of Eu 2 SmSbO 7 is shown in Figure 2b, where the cell parameter a for Eu 2 SmSbO 7 is 10.5547 Å.Table 1 presents the atomic coordinates and the structural parameters of Eu 2 SmSbO 7 .The full-profile structural refinement of Eu 2 SmSbO 7 generated an unweighted R factor, R P = 2.14%.Figure 2a displays the structural properties of Eu2SmSbO7 according to Rietveld analysis using the Materials Studio program.The results of Rietveld refinement for Eu2SmSbO7 showed that the observed intensities and calculated intensities of the pyrochlore-type structure were in fine agreement.Eu2SmSbO7 was a single-phase, cubic crystal system with a space group of Fd3m (modeled to include O atoms) [62].The atomic structure of Eu2SmSbO7 is shown in Figure 2b, where the cell parameter a for Eu2SmSbO7 is 10.5547 Å.Table 1 presents the atomic coordinates and the structural parameters of Eu2SmSbO7.The full-profile structural refinement of Eu2SmSbO7 generated an unweighted R factor, RP = 2.14%.2) bonds (two), it can be assumed that x is equal to 0.375 [63].The value of x provides information about the octahedral distortion of MO6 (M = Sm 3+ and Sb 5+ ).Since the x value was shifted from x = 0.375, it was proved that there existed crystal structure distortions in Eu2SmSbO7 [63].The photocatalytic degradation of chlorpyrifos under VLID required charge separation to prevent photo-induced electron and photo-induced hole recombination.Deformation of MO6 octahedra has been reported to prevent charge recombination and enhance photocatalytic performance in some photocatalysts such as BaTi4O9 and Sr2M2O7 (M = Nb 5+ and Ta 5+ ) [64,65].Therefore, it was speculated that the deformation of MO6 (M = Sm 3+ and Sb 5+ ) octahedra in Eu2SmSbO7 could enhance the photocatalytic performance.Eu2SmSbO7 had a three-dimensional network structure which consisted of corner-sharing MO6 (M = Sm 3+ and Sb 5+ ) octahedra.Each Eu 3+ ion was  The x-coordinate of the O(1) atom reflects the change in crystal structure of A 2 B 2 O 7type compounds.If the lengths of the A-O(1) bonds (of which there are six) are identical to the lengths of the A-O(2) bonds (two), it can be assumed that x is equal to 0.375 [63].The value of x provides information about the octahedral distortion of MO 6 (M = Sm 3+ and Sb 5+ ).Since the x value was shifted from x = 0.375, it was proved that there existed crystal structure distortions in Eu 2 SmSbO 7 [63].The photocatalytic degradation of chlorpyrifos under VLID required charge separation to prevent photo-induced electron and photoinduced hole recombination.Deformation of MO 6 octahedra has been reported to prevent charge recombination and enhance photocatalytic performance in some photocatalysts such as BaTi 4 O 9 and Sr 2 M 2 O 7 (M = Nb 5+ and Ta 5+ ) [64,65].Therefore, it was speculated that the deformation of MO 6 (M = Sm 3+ and Sb 5+ ) octahedra in Eu 2 SmSbO 7 could enhance the photocatalytic performance.Eu 2 SmSbO 7 had a three-dimensional network structure which consisted of corner-sharing MO 6 (M = Sm 3+ and Sb 5+ ) octahedra.Each Eu 3+ ion was connected to two MO 6 octahedra to form a chain.Eu-O bonds were found in two lengths, with the six Eu-O (1) bond lengths (4.376 Å)  Many previous studies have indicated that the size of the bond angle affects the luminescent properties of the material [63,66].When the bond angles of M-O-M (M = Sm 3+ and Sb 5+ ) were close to 180 • , the produced photo-induced electrons and photo-induced holes were more easily accessible to the reaction sites on the catalyst surface.In addition, the larger the Eu-Sm-O bond angle or Eu-Sb-O bond angle of Eu 2 SmSbO 7 was, the higher the photocatalytic activity was.The degradation of chlorpyrifos under VLID with Eu 2 SmSbO 7 as a photocatalyst mainly depended on the crystal structure and electronic structure of Eu 2 SmSbO 7 .
Figure 3a displays the structural properties of ZnBiEuO 4 according to the Rietveld refinement analysis using the Materials Studio version 2.2 software.ZnBiEuO 4 had a tetragonal crystalline nature with the space group of I41/A [67].The atomic structure of ZnBiEuO 4 is shown in Figure 3b.It can be found from Figure 3b that the cell parameters for ZnBiEuO 4 were a = b = 10.5572Å and c = 10.0341Å.The atomic coordinates and structural parameters of ZnBiEuO 4 are listed in Table 2.The full-profile structural refinement of ZnBiEuO 4 generated an unweighted R factor, R P = 6.62%.
minescent properties of the material [63,66].When the bond angles of M-O-M (M = Sm 3+ and Sb 5+ ) were close to 180°, the produced photo-induced electrons and photo-induced holes were more easily accessible to the reaction sites on the catalyst surface.In addition, the larger the Eu-Sm-O bond angle or Eu-Sb-O bond angle of Eu2SmSbO7 was, the higher the photocatalytic activity was.The degradation of chlorpyrifos under VLID with Eu2SmSbO7 as a photocatalyst mainly depended on the crystal structure and electronic structure of Eu2SmSbO7.
Figure 3a displays the structural properties of ZnBiEuO4 according to the Rietveld refinement analysis using the Materials Studio version 2.2 software.ZnBiEuO4 had a tetragonal crystalline nature with the space group of I41/A [67].The atomic structure of ZnBiEuO4 is shown in Figure 3b.It can be found from Figure 3b that the cell parameters for ZnBiEuO4 were a = b = 10.5572Å and c = 10.0341Å.The atomic coordinates and structural parameters of ZnBiEuO4 are listed in Table 2.The full-profile structural refinement of ZnBiEuO4 generated an unweighted R factor, RP = 6.62%.
The crystallite size (L) could be calculated using the Scherrer Formula (1) [68][69][70]: is the shape constant, normally taken as 0.9.λ is the X-ray wavelength (in this study,  = 1.54056Å),  is the half-width of the diffraction peak at the maximum height, and  is the Bragg angle.Using the Scherrer formula, we could calculate the crystallite size of Eu2SmSbO7 (341 nm) and the crystallite size of ZnBiEuO4 (522 nm).The crystallite size of Eu2SmSbO7 or ZnBiEuO4 was 341 nm or 522 nm after the formation of the EZHP.The crystallite size of Eu2SmSbO7 and ZnBiEuO4 did not change after the formation of the EZHP.The crystallite size (L) could be calculated using the Scherrer Formula (1) [68][69][70]: K is the shape constant, normally taken as 0.9.λ is the X-ray wavelength (in this study, λ = 1.54056Å), β is the half-width of the diffraction peak at the maximum height, and θ is the Bragg angle.Using the Scherrer formula, we could calculate the crystallite size of Eu 2 SmSbO 7 (341 nm) and the crystallite size of ZnBiEuO 4 (522 nm).The crystallite size of Eu 2 SmSbO 7 or ZnBiEuO 4 was 341 nm or 522 nm after the formation of the EZHP.The crystallite size of Eu 2 SmSbO 7 and ZnBiEuO 4 did not change after the formation of the EZHP.

FTIR Analysis
The absorption peaks of functional groups and chemical bonds can be obtained from FTIR spectroscopy to determine the chemical composition of a substance.Figure 4 displays the FTIR spectra of Eu 2 SmSbO 7 , EZHP, and ZnBiEuO 4 , including the characteristic absorption peaks associated with the Eu-O, Sm-O, Sb-O-Sb, Zn-O, and Bi-O bonds.The stretching vibrations of Eu-O or Sm-O bonds appeared at 590 cm −1 or 508 cm −1 , respectively [71,72].The peaks at 728 cm −1 and 649 cm −1 were related to the bending vibrations of the Sb-O-Sb bond [73,74].The bending vibration of Zn-O bond appeared at 468 cm −1 and the bending vibration of Bi-O bond was associated with the characteristic peak at 429 cm −1 [75,76].The peak at 1631 cm −1 was related to the bending vibration of the O-H group [77].The multiple bands that were observed from 1361 cm −1 to 1631 cm −1 corresponded to the vibrations of the C-H bonds [78].

FTIR Analysis
The absorption peaks of functional groups and chemical bonds can be obtained from FTIR spectroscopy to determine the chemical composition of a substance.Figure 4 displays the FTIR spectra of Eu2SmSbO7, EZHP, and ZnBiEuO4, including the characteristic absorption peaks associated with the Eu-O, Sm-O, Sb-O-Sb, Zn-O, and Bi-O bonds.The stretching vibrations of Eu-O or Sm-O bonds appeared at 590 cm −1 or 508 cm −1 , respectively [71,72].The peaks at 728 cm −1 and 649 cm −1 were related to the bending vibrations of the Sb-O-Sb bond [73,74].The bending vibration of Zn-O bond appeared at 468 cm −1 and the bending vibration of Bi-O bond was associated with the characteristic peak at 429 cm −1 [75,76].The peak at 1631 cm −1 was related to the bending vibration of the O-H group [77].The multiple bands that were observed from 1361 cm −1 to 1631 cm −1 corresponded to the vibrations of the C-H bonds [78].

Raman Analysis
Raman spectroscopy is based on the interaction of chemical bonds within a substance and can reflect the chemical structure of the substance.The Raman spectra of ZnBiEuO4, Eu2SmSbO7, and EZHP are presented in Figure 5.The characteristic peaks of Zn-O, Eu-O, or Bi-O which were found in the Raman spectrum of ZnBiEuO4 appeared at 278 cm −1 , 438 cm −1 , or 631 cm −1 , respectively [79][80][81].In the Raman spectrum of Eu2SmSbO7, the characteristic peaks of Eu-O appeared at 366 cm −1 , 450 cm −1 , and 707 cm −1 ; at the same time, the characteristic peaks of Sm-O appeared at 168 cm −1 [80,82].The peaks at 224 cm −1 , 281 cm −1 , 472 cm −1 , and 527 cm −1 , which are displayed in Figure 5, belonged to Sb-O and Sb-

XPS Analysis
X-ray photoelectron spectroscopy can provide information on elemental composition, atomic valence, and elemental content.XPS analysis was carried out in order to study the chemical compositions and valence states of each element in Eu2SmSbO7, EZHP, and ZnBiEuO4.Figure 6 illustrates the XPS spectra of Eu2SmSbO7, EZHP, and ZnBiEuO4.From Figure 6 it can be seen that the prepared sample of Eu2SmSbO7, ZnBiEuO4, or EZHP was successfully synthesized.In addition, a carbon peak was observed and this carbon peak was attributed to adventitious hydrocarbon as a calibration reference.

XPS Analysis
X-ray photoelectron spectroscopy can provide information on elemental composition, atomic valence, and elemental content.XPS analysis was carried out in order to study the chemical compositions and valence states of each element in Eu 2 SmSbO 7 , EZHP, and ZnBiEuO 4 .Figure 6 illustrates the XPS spectra of Eu 2 SmSbO 7 , EZHP, and ZnBiEuO 4 .From Figure 6 it can be seen that the prepared sample of Eu 2 SmSbO 7 , ZnBiEuO 4 , or EZHP was successfully synthesized.In addition, a carbon peak was observed and this carbon peak was attributed to adventitious hydrocarbon as a calibration reference.O-Sb [83,84].Furthermore, the Raman spectrum of the EZHP included different absorption peaks which were derived from Eu2SmSbO7 and ZnBiEuO4, including peaks at 163 cm −1 , 225 cm −1 , 275 cm −1 , 288 cm −1 , 367 cm −1 , 440 cm −1 , 454 cm −1 , 471 cm −1 , 536 cm −1 , 625 cm −1 , and 706 cm −1 .The above results provide indirect evidence for our successful preparation of ZnBiEuO4, Eu2SmSbO7, and EZHP.

UV-Vis Diffuse Reflectance Spectra
UV-visible diffuse reflectance is a commonly used spectral analysis technique that can be used to determine the bandgap width of a material.Figure 8a illustrates the absorption spectrum of Eu 2 SmSbO 7. Figure 8b shows the plot of (αhν) 2 and hν for Eu 2 SmSbO 7 .Figure 9a illustrates the absorption spectrum of ZnBiEuO 4 .Figure 9b shows the plot of (αhν) 1/2 and hν for ZnBiEuO 4 .Figure 10a illustrates the absorption spectrum of EZHP. Figure 10b shows the plot of (αhν) 1/2 and hν for EZHP.The absorption edge of the novel photocatalyst Eu 2 SmSbO 7 was at 430 nm; at the same time, the absorption edge of EZHP was at 453 nm and the absorption edge of ZnBiEuO 4 was at 482 nm.The three above catalysts had absorption edges in the visible range.Through the point of intersection of the hν (photon energy)-axis with the prolonging of the linear part of the absorption edge of the Kubelka-Monk function (2) (re-emission function), the bandgap energy of the semiconductor could be determined [94,95].
Catalysts 2024, 14, x FOR PEER REVIEW 10 of 35       Photo-absorption near the band edge of the crystalloid semiconductor follows Equation (3) [96,97]: In this equation, A, , Eg, and ν represent the proportional constant, absorption coefficient, band gap, and light frequency, respectively.The value of n controls the transition property of the semiconductor.The values of Eg and n could be identified by following these three steps: (1) draw a graph of ln ℎ with ln ℎ  and estimate the In above equations, S, R d , and α represents the scattering quotiety, diffuse reflectance, and radiation absorption quotiety, respectively.
Photo-absorption near the band edge of the crystalloid semiconductor follows Equation (3) [96,97]: In this equation, A, α, E g , and ν represent the proportional constant, absorption coefficient, band gap, and light frequency, respectively.The value of n controls the transition property of the semiconductor.The values of E g and n could be identified by following these three steps: (1) draw a graph of ln(αhν) with ln hν − E g and estimate the approximate value of E g ; (2) based on the rate of slope of the graph, speculate about the value of n; (3) draw a graph of (αhν) 1/n with hν, refine the E g values, and prolong the region of the curve that approximates a straight line until it intersectsthebaseline.The energy bandwidths of Eu 2 SmSbO 7 , EZHP, and ZnBiEuO 4 could be estimated using the direct method (1240/transition wavelength λ).
The black dashed line in Figures 8b, 9b, and 10b is the extrapolation line of the linear part of the absorption edge; thus, the intersection of the black dashed line with the baseline is the bandgap width [98].According to the calculation that was conducted using above method, the E g value of Eu 2 SmSbO 7 was 2.881 eV and the E g value of ZnBiEuO 4 was 2.571 eV.Similarly, it could be concluded that the E g value of EZHP was 2.737 eV.All of the three above catalysts had a visible light response characteristic.The n value of Eu 2 SmSbO 7 was estimated to be 0.5, which indicated that the optical transition was directly allowed.The n value of ZnBiEuO 4 or EZHP was estimated to be 2, which indicated that the optical transition was indirectly allowed.

Property Characterization of Eu 2 SmSbO 7 /ZnBiEuO 4 Heterojunction Photocatalyst
Figure 11 shows the TEM image of Eu 2 SmSbO 7 .Figure 12 displays the TEM image of ZnBiEuO 4 .As can be seen from Figures 11 and 12, the particle size of Eu 2 SmSbO 7 was about 350 nm, while the particle size of ZnBiEuO 4 was about 520 nm, which was about the same as that of the XRD analysis.
Figure 13 shows the TEM image of EZHP. Figure 14 displays EDS elemental maps of EZHP (Eu, Sm, Sb, and O for Eu 2 SmSbO 7 , and Zn, Bi, Eu, and O for ZnBiEuO 4 ). Figure 15 illustrates the EDS spectrum of EZHP.From Figures 11-14, it can be noticed that the larger particles belonged to ZnBiEuO 4 while the smaller particles belonged to Eu 2 SmSbO 7 .ZnBiEuO 4 particles were surrounded by smaller Eu 2 SmSbO 7 particles, which indicated the successful synthesis of EZHP.
rectly allowed.The n value of ZnBiEuO4 or EZHP was estimated to be 2, which indicated that the optical transition was indirectly allowed.

Property Characterization of Eu2SmSbO7/ZnBiEuO4 Heterojunction Photocatalyst
Figure 11 shows the TEM image of Eu2SmSbO7.Figure 12 displays the TEM image of ZnBiEuO4.As can be seen from Figures 11 and 12, the particle size of Eu2SmSbO7 was about 350 nm, while the particle size of ZnBiEuO4 was about 520 nm, which was about the same as that of the XRD analysis.Figure 13 shows the TEM image of EZHP. Figure 14 displays EDS elemental maps of EZHP (Eu, Sm, Sb, and O for Eu2SmSbO7, and Zn, Bi, Eu, and O for ZnBiEuO4).Figure 15 illustrates the EDS spectrum of EZHP.From Figures 11-14, it can be noticed that the larger particles belonged to ZnBiEuO4 while the smaller particles belonged to Eu2SmSbO7.ZnBiEuO4 particles were surrounded by smaller Eu2SmSbO7 particles, which indicated the successful synthesis of EZHP.  Figure 13 shows the TEM image of EZHP. Figure 14 displays EDS elemental maps of EZHP (Eu, Sm, Sb, and O for Eu2SmSbO7, and Zn, Bi, Eu, and O for ZnBiEuO4).Figure 15 illustrates the EDS spectrum of EZHP.From Figures 11-14, it can be noticed that the larger particles belonged to ZnBiEuO4 while the smaller particles belonged to Eu2SmSbO7.ZnBiEuO4 particles were surrounded by smaller Eu2SmSbO7 particles, which indicated the successful synthesis of EZHP.Based on Figures 14 and 15, it could be proved that europium, samarium, antim zinc, bismuth, and oxygen elements were present in EZHP and there were no other i rities.It can be concluded that the EZHP that was synthesized in this study was o purity.According to the EDS spectrum of EZHP in Figure 15, the surface atomic co tration ratio of Eu:Sm:Sb:Zn:Bi:O was 9.36%:2.94%:2.87%:3.29%:3.31%:78.23%,whic approximately the same as the results of XPS analysis.Based on Figures 14 and 15, it could be proved that europium, samarium, antimony, zinc, bismuth, and oxygen elements were present in EZHP and there were no other impurities.It can be concluded that the EZHP that was synthesized in this study was of high purity.According to the EDS spectrum of EZHP in Figure 15, the surface atomic concentration ratio of Eu:Sm:Sb:Zn:Bi:O was 9.36%:2.94%:2.87%:3.29%:3.31%:78.23%,which was approximately the same as the results of XPS analysis.Based on Figures 14 and 15, it could be proved that europium, samarium, antimony, zinc, bismuth, and oxygen elements were present in EZHP and there were no other impurities.It can be concluded that the EZHP that was synthesized in this study was of high purity.According to the EDS spectrum of EZHP in Figure 15, the surface atomic concentration ratio of Eu:Sm:Sb:Zn:Bi:O was 9.36%:2.94%:2.87%:3.29%:3.31%:78.23%,which was approximately the same as the results of XPS analysis.

Photocatalytic Activity
Figure 16a shows the concentration variation curves of chlorpyrifos during the photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , nitrogen-doped titanium dioxide (N-doped TiO 2 ), or without a catalyst.N-doped TiO 2 is a widely recognized visible light responsive photocatalyst; thus, we chose to use N-doped TiO 2 for comparing the photocatalytic activity with that of other photocatalysts [99,100].It can be obviously seen that the concentration of chlorpyrifos in pesticide wastewater gradually decreased with the extension in VLID time.
Catalysts 2024, 14, x FOR PEER REVIEW 14 of 35 Figure 16a shows the concentration variation curves of chlorpyrifos during the photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu2SmSbO7, ZnBiEuO4, nitrogen-doped titanium dioxide (N-doped TiO2), or without a catalyst.N-doped TiO2 is a widely recognized visible light responsive photocatalyst; thus, we chose to use N-doped TiO2 for comparing the photocatalytic activity with that of other photocatalysts [99,100].It can be obviously seen that the concentration of chlorpyrifos in pesticide wastewater gradually decreased with the extension in VLID time.The conversion rates of chlorpyrifos could be calculated by ( 1) × 100%, where C represents the instantaneous concentration of chlorpyrifos and C0 represents the initial concentration of chlorpyrifos.For a better comparison in all experiments, the VLID time was set to 160 min.By analyzing the data in Figure 16a, it can be seen that the conversion rate of chlorpyrifos during degradation of chlorpyrifos using EZHP was 100% after VLID of 160 min, with the reaction rate of 3.33 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0700%.When Eu2SmSbO7 was used as the photocatalyst, the conversion rate of chlorpyrifos was 88.16%, with a reaction rate of 2.94 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0617%.The conversion rate of chlorpyrifos that was derived from pesticide wastewater with ZnBiEuO4 as the photocatalyst was 84.03%; at the same time, the reaction rate of 2.80 × 10 −9 mol•L −1 •s −1 and the photonic efficiency of 0.0588% were achieved.When N-doped TiO2 was used as the photocatalyst, the conversion rate of chlorpyrifos was 35.25%, with a reaction rate of 1.18 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0247%.Finally, when chlorpyrifos was photodegraded directly under VLID without the addition of any catalysts, the conversion rate of chlorpyrifos was 4.72%, with a reaction rate of 0.16 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0033%.According to the above findings, it could be concluded that the descending order of photodegradation efficiency was as following: EZHP > Eu2SmSbO7 > ZnBiEuO4 > N-doped TiO2 under the same conditions.Comparing the conversion rates of chlorpyrifos after VLID of 160 min, it could be found that the conversion rate of chlorpyrifos using EZHP was 1.13 times higher than that with Eu2SmSbO7 as the photocatalyst, 1.19 times higher than that with ZnBiEuO4 as the photocatalyst, and 2.84 times higher than that with Ndoped TiO2 as the photocatalyst.
Figure 16b presents the concentration variation curves of total organic carbon (TOC) during the photocatalytic degradation of chlorpyrifos within pesticide wastewater under VLID using EZHP, Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or without a catalyst.As can be seen from Figure 16b, the concentration of TOC decreased gradually with increasing VLID time.16a, it can be seen that the conversion rate of chlorpyrifos during degradation of chlorpyrifos using EZHP was 100% after VLID of 160 min, with the reaction rate of 3.33 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0700%.When Eu 2 SmSbO 7 was used as the photocatalyst, the conversion rate of chlorpyrifos was 88.16%, with a reaction rate of 2.94 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0617%.The conversion rate of chlorpyrifos that was derived from pesticide wastewater with ZnBiEuO 4 as the photocatalyst was 84.03%; at the same time, the reaction rate of 2.80 × 10 −9 mol•L −1 •s −1 and the photonic efficiency of 0.0588% were achieved.When N-doped TiO 2 was used as the photocatalyst, the conversion rate of chlorpyrifos was 35.25%, with a reaction rate of 1.18 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0247%.Finally, when chlorpyrifos was photodegraded directly under VLID without the addition of any catalysts, the conversion rate of chlorpyrifos was 4.72%, with a reaction rate of 0.16 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0033%.
According to the above findings, it could be concluded that the descending order of photodegradation efficiency was as following: EZHP > Eu 2 SmSbO 7 > ZnBiEuO 4 > Ndoped TiO 2 under the same conditions.Comparing the conversion rates of chlorpyrifos after VLID of 160 min, it could be found that the conversion rate of chlorpyrifos using EZHP was 1.13 times higher than that with Eu 2 SmSbO 7 as the photocatalyst, 1.19 times higher than that with ZnBiEuO 4 as the photocatalyst, and 2.84 times higher than that with N-doped TiO 2 as the photocatalyst.
Figure 16b presents the concentration variation curves of total organic carbon (TOC) during the photocatalytic degradation of chlorpyrifos within pesticide wastewater under VLID using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst.As can be seen from Figure 16b, the concentration of TOC decreased gradually with increasing VLID time.
By analyzing the data that are shown in Figure 16b, it was demonstrated that after VLID of 160 min, the conversion rates of TOC that was derived from pesticide wastewater were 98.02%, 84.04%, 80.12%, 30.23%, and 4.14% when chlorpyrifos was degraded using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without catalyst, respectively.In other words, the descending order of the conversion rates of TOC during the degradation of chlorpyrifos was as follows: EZHP > Eu 2 SmSbO 7 > ZnBiEuO 4 > N-doped TiO 2 .The above results indicated that EZHP had a higher mineralization rate compared with Eu 2 SmSbO 7 , ZnBiEuO 4 , or N-doped TiO 2 during the degradation of chlorpyrifos.
Figure 17 demonstrates the effect of different photocatalyst doses on the conversion rates when using EZHP to degrade chlorpyrifos under VLID.Beginning with the increase in the initial concentration of EZHP, the conversion rates of chlorpyrifos increased after VLID of 160 min.The conversion rate of chlorpyrifos reached 100% at an initial concentration of EZHP of 0.75 g/L.After that, the conversion rates of chlorpyrifos decreased with the increase in the initial concentration of EZHP.The reason for the decrease in the conversion rates of chlorpyrifos could be the aggregation of a high concentration of EZHP, which resulted in the decrease in active sites on the surface of the catalyst [101].The above results indicated that the optimum conversion rate was achieved at an initial EZHP concentration of 0.75 g/L.Table 3 demonstrates the effect of different ratios of different catalysts to chlorpyrifos on the conversion rates of chlorpyrifos.From the data in Table 3, it can be seen that the optimum conversion rate of chlorpyrifos was achieved at a ratio of catalyst to chlorpyrifos of 66.96.
Catalysts 2024, 14, x FOR PEER REVIEW 15 of 35 By analyzing the data that are shown in Figure 16b, it was demonstrated that after VLID of 160 min, the conversion rates of TOC that was derived from pesticide wastewater were 98.02%, 84.04%, 80.12%, 30.23%, and 4.14% when chlorpyrifos was degraded using EZHP, Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or without catalyst, respectively.In other words, the descending order of the conversion rates of TOC during the degradation of chlorpyrifos was as follows: EZHP > Eu2SmSbO7 > ZnBiEuO4 > N-doped TiO2.The above results indicated that EZHP had a higher mineralization rate compared with Eu2SmSbO7, ZnBiEuO4, or N-doped TiO2 during the degradation of chlorpyrifos.
Figure 17 demonstrates the effect of different photocatalyst doses on the conversion rates when using EZHP to degrade chlorpyrifos under VLID.Beginning with the increase in the initial concentration of EZHP, the conversion rates of chlorpyrifos increased after VLID of 160 min.The conversion rate of chlorpyrifos reached 100% at an initial concentration of EZHP of 0.75 g/L.After that, the conversion rates of chlorpyrifos decreased with the increase in the initial concentration of EZHP.The reason for the decrease in the conversion rates of chlorpyrifos could be the aggregation of a high concentration of EZHP, which resulted in the decrease in active sites on the surface of the catalyst [101].The above results indicated that the optimum conversion rate was achieved at an initial EZHP concentration of 0.75 g/L.Table 3 demonstrates the effect of different ratios of different catalysts to chlorpyrifos on the conversion rates of chlorpyrifos.From the data in Table 3, it can be seen that the optimum conversion rate of chlorpyrifos was achieved at a ratio of catalyst to chlorpyrifos of 66.96.The concentration variation curves of chlorpyrifos during four cyclic degradation experiments using EZHP under VLID are shown in Figure 18a.It can be deduced from Figure 18a that after VLID of 160 min using EZHP, the conversion rates of chlorpyrifos reached 98.16%, 97.03%, 96.03%, or 95.06%, respectively.From the four cyclic degradation experiments, the corresponding photon efficiencies were 0.0687%, 0.0679%, 0.0672%, or 0.0666%, respectively.Figure 18b illustrates the concentration variation curves of TOC during the photocatalytic degradation of chlorpyrifos using EZHP under VLID.It can be concluded from Figure 18b that the conversion rates of TOC were 96.94%, 95.72%, 94.68%, or 93.66% using EZHP after VLID of 160 min.According to the four consecutive cyclic degradation experiments using EZHP, the conversion rates of chlorpyrifos decreased by 1.84%, 1.13%, 1%, or 0.97% after each cyclic degradation experiment, respectively.It can be concluded from Figure 18b that the conversion rates of TOC decreased by 1.08%, 1.22%, 1.04%, or 1.02% after each cyclic degradation experiment using EZHP under VLID.The above results demonstrated that EZHP had excellent structural stability and reusability.
The concentration variation curves of chlorpyrifos during four cyclic degradation experiments using EZHP under VLID are shown in Figure 18a.It can be deduced from Figure 18a that after VLID of 160 min using EZHP, the conversion rates of chlorpyrifos reached 98.16%, 97.03%, 96.03%, or 95.06%, respectively.From the four cyclic degradation experiments, the corresponding photon efficiencies were 0.0687%, 0.0679%, 0.0672%, or 0.0666%, respectively.Figure 18b illustrates the concentration variation curves of TOC during the photocatalytic degradation of chlorpyrifos using EZHP under VLID.It can be concluded from Figure 18b that the conversion rates of TOC were 96.94%, 95.72%, 94.68%, or 93.66% using EZHP after VLID of 160 min.According to the four consecutive cyclic degradation experiments using EZHP, the conversion rates of chlorpyrifos decreased by 1.84%, 1.13%, 1%, or 0.97% after each cyclic degradation experiment, respectively.It can be concluded from Figure 18b that the conversion rates of TOC decreased by 1.08%, 1.22%, 1.04%, or 1.02% after each cyclic degradation experiment using EZHP under VLID.The above results demonstrated that EZHP had excellent structural stability and reusability.
To demonstrate the significance of our study in the field of photocatalytic degradation of chlorpyrifos, a comparative overview of relevant studies in this field is presented in Table 4.The data in Table 4 show that the photocatalytic activity of EZHP was better than that of other catalysts.These results indicate that EZHP can increase the conversion rates of chlorpyrifos and make a significant contribution to the field of photocatalysis.In conclusion, XRD and XPS analyses demonstrated the excellent structural stability of EZHP during the photocatalytic degradation of chlorpyrifos under VLID.To demonstrate the significance of our study in the field of photocatalytic degradation of chlorpyrifos, a comparative overview of relevant studies in this field is presented in Table 4.The data in Table 4 show that the photocatalytic activity of EZHP was better than that of other catalysts.These results indicate that EZHP can increase the conversion rates of chlorpyrifos and make a significant contribution to the field of photocatalysis.In conclusion, XRD and XPS analyses demonstrated the excellent structural stability of EZHP during the photocatalytic degradation of chlorpyrifos under VLID.The first-order kinetic plots that correspond to chlorpyrifos concentration and VLID time during photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst, are shown in Figure 19a.Figure 19b presents the first-order kinetic curves that correspond to TOC concentration and VLID time during photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst.The kinetic constants were calculated from the equation ( ln C 0 C = k C t and ( ln TOC 0 TOC = k TOC t .In the equation, C 0 is the initial saturation of chlorpyrifos, C is the reactive saturation of chlorpyrifos, TOC 0 represents the initial saturation of total organic carbon, and TOC represents the reactive saturation of total organic carbon.It can be concluded from Figure 19a that the kinetic constant value K C that was derived from the dynamic curves for EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst, was 0.0202 min −1 , 0.0090 min −1 , 0.0075 min −1 , 0.0020 min −1 , and 0.0003 min −1 under VLID, and its corresponding linear correlation coefficient (R 2 ) was 0.8511, 0.8741, 0.8524, 0.9919, and 0.9773, respectively.Moreover, it can be deduced from Figure 19b that the kinetic constant value K TOC that originated from the dynamic curves for EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst, was 0.0182 min −1 , 0.0078 min −1 , 0.0067 min −1 , 0.0017 min −1 , and 0.0002 min −1 under VLID, and its corresponding R 2 was 0.8512, 0.8775, 0.8562, 0.9962, and 0.9745, respectively.According to the above studies, it was found that the mineralization efficiency during photocatalytic degradation of chlorpyrifos using EZHP was higher than that using Eu 2 SmSbO 7 or ZnBiEuO 4 under VLID.Furthermore, the K TOC value of the above four photocatalysts was lower than the K C value of the above four photocatalysts, which indicates that the intermediate products might be formed during the photocatalytic degradation of chlorpyrifos.

CuO/TiO2
Visible light 90 Chlorpyrifos 60 [105] Ni The first-order kinetic plots that correspond to chlorpyrifos concentration and VLID time during photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or without a catalyst, are shown in Figure 19a.Figure 19b presents the first-order kinetic curves that correspond to TOC concentration and VLID time during photocatalytic degradation of chlorpyrifos under VLID using EZHP, Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or without a catalyst.The kinetic constants were calculated from the equation (   and (   .In the equation, C0 is the initial saturation of chlorpyrifos, C is the reactive saturation of chlorpyrifos, TOC0 represents the initial saturation of total organic carbon, and TOC represents the reactive saturation of total organic carbon.It can be concluded from Figure 19a that the kinetic constant value KC that was derived from the dynamic curves for EZHP, Eu2SmSbO7, ZnBiEuO4, Ndoped TiO2, or without a catalyst, was 0.0202 min −1 , 0.0090 min −1 , 0.0075 min −1 , 0.0020 min −1 , and 0.0003 min −1 under VLID, and its corresponding linear correlation coefficient (R 2 ) was 0.8511, 0.8741, 0.8524, 0.9919, and 0.9773, respectively.Moreover, it can be deduced from Figure 19b that the kinetic constant value KTOC that originated from the dynamic curves for EZHP, Eu2SmSbO7, ZnBiEuO4, N-doped TiO2, or without a catalyst, was 0.0182 min −1 , 0.0078 min −1 , 0.0067 min −1 , 0.0017 min −1 , and 0.0002 min −1 under VLID, and its corresponding R 2 was 0.8512, 0.8775, 0.8562, 0.9962, and 0.9745, respectively.According to the above studies, it was found that the mineralization efficiency during photocatalytic degradation of chlorpyrifos using EZHP was higher than that using Eu2SmSbO7 or ZnBiEuO4 under VLID.Furthermore, the KTOC value of the above four photocatalysts was lower than the KC value of the above four photocatalysts, which indicates that the intermediate products might be formed during the photocatalytic degradation of chlorpyrifos.Figure 20a displays the first-order kinetic plots that correspond to chlorpyrifos concentration and VLID time during four cyclic degradation experiments for photocatalytic degradation of chlorpyrifos using EZHP.Figure 20b shows the first-order kinetic plots that correspond to TOC concentration and VLID time during four cyclic degradation experiments for photocatalytic degradation of chlorpyrifos using EZHP.Based on the analysis of the data in Figure 20a, the kinetic constant value KC that was derived from the Figure 20a displays the first-order kinetic plots that correspond to chlorpyrifos concentration and VLID time during four cyclic degradation experiments for photocatalytic degradation of chlorpyrifos using EZHP.Figure 20b shows the first-order kinetic plots that correspond to TOC concentration and VLID time during four cyclic degradation experiments for photocatalytic degradation of chlorpyrifos using EZHP.Based on the analysis of the data in Figure 20a, the kinetic constant value K C that was derived from the dynamic curves using EZHP for the four cyclic degradation experiments was 0.0186 min −1 , 0.0161 min −1 , 0.0144 min −1 , and 0.0131 min −1 under VLID, and its corresponding R 2 was 0.8527, 0.8605, 0.8604, and 0.8562, respectively.It can be deduced from Figure 20b that the kinetic constant value K TOC that originated from the dynamic curves using EZHP for the four cyclic degradation experiments was 0.0158 min −1 , 0.0137 min −1 , 0.0125 min −1 , and 0.0110 min −1 under VLID, and its corresponding R 2 was 0.8608, 0.8517, 0.8509, and 0.8503, respectively.During the cycling experiments for degrading chlorpyrifos, we used the catalyst that was extracted from the previous degradation experiment of chlorpyrifos for the next degradation experiment of chlorpyrifos; as a result, K C and K TOC decreased with the increasing number of cycling experiments.According to the above results, it could be concluded that the photocatalytic degradation of chlorpyrifos that was derived from pesticide wastewater using EZHP followed the first-order reaction kinetics under VLID.dynamic curves using EZHP for the four cyclic degradation experiments was 0.0186 min −1 , 0.0161 min −1 , 0.0144 min −1 , and 0.0131 min −1 under VLID, and its corresponding R 2 was 0.8527, 0.8605, 0.8604, and 0.8562, respectively.It can be deduced from Figure 20b that the kinetic constant value KTOC that originated from the dynamic curves using EZHP for the four cyclic degradation experiments was 0.0158 min −1 , 0.0137 min −1 , 0.0125 min −1 , and 0.0110 min −1 under VLID, and its corresponding R 2 was 0.8608, 0.8517, 0.8509, and 0.8503, respectively.During the cycling experiments for degrading chlorpyrifos, we used the catalyst that was extracted from the previous degradation experiment of chlorpyrifos for the next degradation experiment of chlorpyrifos; as a result, KC and KTOC decreased with the increasing number of cycling experiments.According to the above results, it could be concluded that the photocatalytic degradation of chlorpyrifos that was derived from pesticide wastewater using EZHP followed the first-order reaction kinetics under VLID.Figure 21 shows the XRD pattern of EZHP before and after the photocatalytic degradation reaction of chlorpyrifos.The crystal structure of EZHP before and after the photocatalytic degradation of chlorpyrifos under VLID did not change significantly, as can be seen in Figure 21. Figure 22 displays the XPS spectra of EZHP before and after the photocatalytic degradation reaction of chlorpyrifos.From Figure 22, it can be seen that the elemental composition and atomic valence of EZHP did not change remarkably before and after the photocatalytic degradation of chlorpyrifos under VLID.Figure 21 shows the XRD pattern of EZHP before and after the photocatalytic degradation reaction of chlorpyrifos.The crystal structure of EZHP before and after the photocatalytic degradation of chlorpyrifos under VLID did not change significantly, as can be seen in Figure 21. Figure 22 displays the XPS spectra of EZHP before and after the photocatalytic degradation reaction of chlorpyrifos.From Figure 22, it can be seen that the elemental composition and atomic valence of EZHP did not change remarkably before and after the photocatalytic degradation of chlorpyrifos under VLID.
Figure 23a shows the effect of different pH values on the conversion rates of chlorpyrifos during photocatalytic degradation of chlorpyrifos using EZHP under VLID.Analysis of the data in Figure 23a showed that the conversion rates of chlorpyrifos reached 99.2%, 100%, and 98.8% after VLID of 160 min at pH 3, 7, and 11, respectively.Therefore, it could be concluded that different pH values have little effect on the conversion rates of chlorpyrifos using EZHP under VLID.Figure 23a shows the effect of different pH values on the conversion rates of chlorpyrifos during photocatalytic degradation of chlorpyrifos using EZHP under VLID.Analysis of the data in Figure 23a showed that the conversion rates of chlorpyrifos reached 99.2%, 100%, and 98.8% after VLID of 160 min at pH 3, 7, and 11, respectively.Therefore, it could be concluded that different pH values have little effect on the conversion rates of chlorpyrifos using EZHP under VLID.Figure 23b displays the effect of different metal ions species on the conversion rates of chlorpyrifos during photocatalytic degradation of chlorpyrifos using EZHP under VLID.Ultrapure water containing Mg 2+ , Zn 2+ , or Ba 2+ was introduced into the photocatalytic reaction system.It was found that the concentration of chlorpyrifos in pesticide wastewater decreased gradually with the increase in VLID time.In addition, by analyzing the data in Figure 23b, it could be seen that the conversion rate of chlorpyrifos using EZHP without the addition of metal ions after VLID of 160 min was 100%.Under the same conditions, the introduction of 1 mmol/L Mg 2+ , 1 mmol/L Zn 2+ , or 1 mmol/L Ba 2+ metal ions resulted in the conversion rates of chlorpyrifos to 99.75%, 98.94%, and 99.37%, respectively.Overall, the addition of Mg 2+ , Zn 2+ , or Ba 2+ metal ions did not significantly inhibit the photocatalytic degradation of chlorpyrifos using EZHP under VLID.
Figure 23c presents the effect of different anion species on the conversion rates of Figure 23b displays the effect of different metal ions species on the conversion rates of chlorpyrifos during photocatalytic degradation of chlorpyrifos using EZHP under VLID.Ultrapure water containing Mg 2+ , Zn 2+ , or Ba 2+ was introduced into the photocatalytic reaction system.It was found that the concentration of chlorpyrifos in pesticide wastewater decreased gradually with the increase in VLID time.In addition, by analyzing the data in Figure 23b, it could be seen that the conversion rate of chlorpyrifos using EZHP without the addition of metal ions after VLID of 160 min was 100%.Under the same conditions, the introduction of 1 mmol/L Mg 2+ , 1 mmol/L Zn 2+ , or 1 mmol/L Ba 2+ metal ions resulted in the conversion rates of chlorpyrifos to 99.75%, 98.94%, and 99.37%, respectively.Overall, the addition of Mg 2+ , Zn 2+ , or Ba 2+ metal ions did not significantly inhibit the photocatalytic degradation of chlorpyrifos using EZHP under VLID.
Figure 23c presents the effect of different anion species on the conversion rates of chlorpyrifos during photocatalytic degradation of chlorpyrifos using EZHP under VLID.Ultrapure water containing NO 3 − , SO 4 2− , CO 3 2− , or Cl − was introduced into the photocatalytic reaction system.It was found that the concentration of chlorpyrifos in pesticide wastewater decreased gradually with the increase of VLID time.Furthermore, by analyzing the data in Figure 23c, it could be seen that the conversion rate of chlorpyrifos using EZHP without the addition of anions after VLID of 160 min was 100%.Under the same conditions, the introduction of 1 mmol/L NO 3 − , 1 mmol/L SO 4 2− , 1 mmol/L CO 3 2− , or 1 mmol/L Cl − anions resulted in the conversion rates of chlorpyrifos of 96.56%, 98.13%, 73.44%, and 62.50%, respectively.The Cl − ion has a stronger inhibitory effect on the degradation of chlorpyrifos.The Cl − ion consumes •OH and photo-induced holes, leading to the reduction of free radicals and thus hindering the photocatalytic reaction.The inhibition of chlorpyrifos degradation by NO 3 − or SO 4 2− ions was small.This is due to the fact that NO 3 − or SO 4 2− only consume the photo-induced holes in the photocatalytic reaction system, and thus the inhibition is weaker.
Figure 24 shows the effect of the addition of isopropanol (IPA), benzoquinone (BQ), or ethylenediaminetetraacetic acid (EDTA) on the conversion rates of chlorpyrifos using EZHP under VLID.In order to identify the active species involved in the photocatalytic reaction during chlorpyrifos photocatalytic degradation, different radical scavengers, as described above, were introduced at the beginning of the photocatalytic experiments.IPA captured hydroxyl radicals (•OH), while BQ captured superoxide anions (•O 2 − ) and EDTA captured holes (h + ).The volume of IPA, BQ, or EDTA added was 1 mL at a concentration of 0.15 mmol L −1 .Compared with the control group, the conversion rates of chlorpyrifos in the presence of IPA, BQ, or EDTA decreased by 47.62%, 35.66%, and 22.97% after VLID of 160 min.Therefore, it could be concluded that •OH, h + , and •O 2 − acted as reactive radicals during the photocatalytic degradation of chlorpyrifos.The above experiments with the addition of scavengers showed that the oxidative removal capacities that were from the largest to the smallest for the three above oxidizing radicals during photocatalytic degrading of chlorpyrifos obeyed the following order: •OH > •O 2 − > h + .Thus, we could conclude that •OH possessed the strongest oxidative removal ability for photocatalytic degradation of chlorpyrifos within pesticide wastewater.
Catalysts 2024, 14, x FOR PEER REVIEW 21 of 35 inhibition of chlorpyrifos degradation by NO3 − or SO4 2− ions was small.This is due to the fact that NO3 − or SO4 2− only consume the photo-induced holes in the photocatalytic reaction system, and thus the inhibition is weaker.
Figure 24 shows the effect of the addition of isopropanol (IPA), benzoquinone (BQ), or ethylenediaminetetraacetic acid (EDTA) on the conversion rates of chlorpyrifos using EZHP under VLID.In order to identify the active species involved in the photocatalytic reaction during chlorpyrifos photocatalytic degradation, different radical scavengers, as described above, were introduced at the beginning of the photocatalytic experiments.IPA captured hydroxyl radicals (•OH), while BQ captured superoxide anions (•O2 − ) and EDTA captured holes (h + ).The volume of IPA, BQ, or EDTA added was 1 mL at a concentration of 0.15 mmol L −1 .Compared with the control group, the conversion rates of chlorpyrifos in the presence of IPA, BQ, or EDTA decreased by 47.62%, 35.66%, and 22.97% after VLID of 160 min.Therefore, it could be concluded that •OH, h + , and •O2 − acted as reactive radicals during the photocatalytic degradation of chlorpyrifos.The above experiments with the addition of scavengers showed that the oxidative removal capacities that were from the largest to the smallest for the three above oxidizing radicals during photocatalytic degrading of chlorpyrifos obeyed the following order: •OH > •O2 − > h + .Thus, we could conclude that •OH possessed the strongest oxidative removal ability for photocatalytic degradation of chlorpyrifos within pesticide wastewater.Photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra can reflect the complexation rate of photo-induced electrons and photo-induced holes, and provide the electronic lifetime of the photocatalyst.Figure 25a displays the PL spectra of EZHP, Eu2SmSbO7, and ZnBiEuO4.Figure 25b-d show the TRPL spectrum of Eu2SmSbO7, ZnBiEuO4, and EZHP, respectively.The lower the relative intensities of the vertical coordinates corresponding to the PL spectra, the more difficult it was to combine Photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra can reflect the complexation rate of photo-induced electrons and photo-induced holes, and provide the electronic lifetime of the photocatalyst.Figure 25a displays the PL spectra of EZHP, Eu 2 SmSbO 7 , and ZnBiEuO 4 .Figure 25b-d show the TRPL spectrum of Eu 2 SmSbO 7 , ZnBiEuO 4 , and EZHP, respectively.The lower the relative intensities of the vertical coordinates corresponding to the PL spectra, the more difficult it was to combine the photo-induced electrons and photo-induced holes.The above results would lead to the prolongation of the survival life of photo-induced electrons and photo-induced holes; as a result, the numbers of •O 2 − and •OH would be increased.Ultimately, the photocatalytic activity was improved [107,108].According to Figure 25a, it can be seen that the value of the relative intensity of the longitudinal coordinate of the PL spectrum corresponding to EZHP was lower than the value of the relative intensity of the longitudinal coordinate of the PL spectrogram corresponding to Eu 2 SmSbO 7 .At the same time, the value of the relative intensity of the longitudinal coordinate of the PL spectrum corresponding to Eu 2 SmSbO 7 was lower than the value of the relative intensity of the longitudinal coordinate of the PL spectrum corresponding to ZnBiEuO 4 .Therefore, based on above results, it could be concluded that the photocatalytic activity was ranked from high to low as follows: EZHP > Eu 2 SmSbO 7 > ZnBiEuO 4 .The above results suggest that the compounding rate of the photo-induced electrons and the photo-induced holes could be reduced by constructing a heterojunction structure.The TRPL spectrum of Eu 2 SmSbO 7 , ZnBiEuO 4 , and EZHP in Figure 25b-d was fitted by the double-exponential decay shown in Equation ( 4) [109]: Catalysts 2024, 14, x FOR PEER REVIEW 22 of 35 EZHP > Eu2SmSbO7 > ZnBiEuO4.The above results suggest that the compounding rate of the photo-induced electrons and the photo-induced holes could be reduced by constructing a heterojunction structure.The TRPL spectrum of Eu2SmSbO7, ZnBiEuO4, and EZHP in Figure 25b-d was fitted by the double-exponential decay shown in Equation ( 4) [109]: In above equations,  or  denote the fast and slow attenuation components, respectively.It is usually assumed that the  component is attributed to non-radiative recombination involving defects or traps, while the  component corresponds to radiative recombination within the photocatalyst.In order to obtain the average electron lifetime ( ), Equation ( 5) could be used as follows [110]: Table 5 lists the calculated lifetimes and the corresponding parameters.By comparing the data in Table 5, it was observed that the average electron lifetime of EZHP ( = 3.0910 ns) was higher than the average electron lifetime of Eu2SmSbO7 ( = 1.5094 ns) and the average electron lifetime of ZnBiEuO4 ( = 1.1692 ns).The above results again indicate that EZHP had a better photocatalytic activity than Eu2SmSbO7 or ZnBiEuO4.In above equations, τ 1 or τ 2 denote the fast and slow attenuation components, respectively.It is usually assumed that the τ 1 component is attributed to non-radiative recombination involving defects or traps, while the τ 2 component corresponds to radiative recombination within the photocatalyst.In order to obtain the average electron lifetime (τ ave ), Equation ( 5) could be used as follows [110]: Table 5 lists the calculated lifetimes and the corresponding parameters.By comparing the data in Table 5, it was observed that the average electron lifetime of EZHP (τ ave = 3.0910 ns) was higher than the average electron lifetime of Eu 2 SmSbO 7 (τ ave = 1.5094 ns) and the average electron lifetime of ZnBiEuO 4 (τ ave = 1.1692 ns).The above results again indicate that EZHP had a better photocatalytic activity than Eu 2 SmSbO 7 or ZnBiEuO 4 .Electrochemical impedance spectroscopy (EIS) reflects the relationship between the impedance of the electrode interface and the frequency of the change in the applied voltage or current under light conditions.At the same time, the migration process of photo-induced electrons and photo-induced holes at the interfaces of Eu 2 SmSbO 7 and ZnBiEuO 4 , which constituted the EZHP, could be understood based on the EIS.The radius of the arc in the Nyquist impedance diagram can be used to compare the compounding rates of photoinduced electrons and photo-induced holes in the catalysts; thus, the length of the survival lifetimes of the photo-induced electrons and photo-induced holes can be determined.Figure 26 illustrates the Nyquist impedance plot of EZHP, Eu 2 SmSbO 7 , and ZnBiEuO 4 .It should be known that the smaller the arc radius of the Nyquist impedance plots profile of the catalyst, the more difficult it was to combine the photo-induced electrons and photoinduced holes, which led to the longer survival life of the photo-induced electrons and photo-induced holes [67].As can be seen from Figure 26, the descending order of the arc radius that was derived from the Nyquist impedance plot was as follows: ZnBiEuO 4 > Eu 2 SmSbO 7 > EZHP.The above results show that the prepared EZHP had a longer survival lifetime of photo-induced electrons and photo-induced holes and higher interfacial charge mobility; thus, EZHP had better photocatalytic activity.Electrochemical impedance spectroscopy (EIS) reflects the relationship between the impedance of the electrode interface and the frequency of the change in the applied voltage or current under light conditions.At the same time, the migration process of photoinduced electrons and photo-induced holes at the interfaces of Eu2SmSbO7 and ZnBiEuO4, which constituted the EZHP, could be understood based on the EIS.The radius of the arc in the Nyquist impedance diagram can be used to compare the compounding rates of photo-induced electrons and photo-induced holes in the catalysts; thus, the length of the survival lifetimes of the photo-induced electrons and photo-induced holes can be determined.Figure 26 illustrates the Nyquist impedance plot of EZHP, Eu2SmSbO7, and ZnBiEuO4.It should be known that the smaller the arc radius of the Nyquist impedance plots profile of the catalyst, the more difficult it was to combine the photo-induced electrons and photo-induced holes, which led to the longer survival life of the photo-induced electrons and photo-induced holes [67].As can be seen from Figure 26, the descending order of the arc radius that was derived from the Nyquist impedance plot was as follows: ZnBiEuO4 > Eu2SmSbO7 > EZHP.The above results show that the prepared EZHP had a longer survival lifetime of photo-induced electrons and photo-induced holes and higher interfacial charge mobility; thus, EZHP had better photocatalytic activity.

Analysis of Possible Degradation Mechanisms
The conceivable photocatalytic degradation mechanism of chlorpyrifos using EZHP

Analysis of Possible Degradation Mechanisms
The conceivable photocatalytic degradation mechanism of chlorpyrifos using EZHP under VLID is illustrated in Figure 27.The electrochemical potential of the conduction band (CB) in semiconductor catalysts could be calculated according to Equation ( 6) [111].According to Equation (7), the electrochemical potential of the valence band (VB) in the semiconductor catalyst could be calculated [111].Equations ( 6) and ( 7) are as follows: where E g is the band gap of the semiconductor and E e is the energy of the free electrons on the hydrogen scale (about 4.5 eV).X is the electronegativity of the semiconductor.X is the average of the ionization energy (I) and the electron affinity energy (A), expressed through Equation ( 8) [112].The X of the photocatalyst could be calculated using Equation ( 9) [113].
Catalysts 2024, 14, x FOR PEER REVIEW 25 of 35 The basic principle of the ultraviolet photoelectron spectroscopy (UPS) is the photoelectric effect, with ultraviolet light as the excitation light source.The ionization potential of the photocatalyst and the electrochemical potential of the VB could be obtained by UPS.The UPS spectrum of Eu2SmSbO7 is illustrated in Figure 28a.It can be deduced from Figure 28a that the measured onset (Ei) energy and cut-off (Ecutoff) energy of Eu2SmSbO7 were 0.496 eV and 19.011 eV, respectively.Figure 28b presents the UPS spectrum of ZnBiEuO4.It can be deduced from Figure 28b that the onset (Ei) energy and cut-off (Ecutoff) energy of ZnBiEuO4 were measured to be 0.159 eV and 19.907 eV, respectively.The ionization potentials of Eu2SmSbO7 and ZnBiEuO4 could be calculated by decreasing the width of the UPS spectrum by the excitation energy, which was about 21.2 eV [115].According to the above approach, the VB ionization potential of Eu2SmSbO7 and ZnBiEuO4 was calculated to be 2.685 eV and 1.452 eV, respectively, which was consistent with the VB potential that was derived from the above simulation calculation results.
The electrochemical potential of O2/•O 2− was -0.33 eV and the absolute electrochemical potential value of OH − /•OH was 2.38 eV.The ionization potentials of the VB for A, B, and C and a, b, and c are the corresponding element and atomic numbers in the semiconductor photocatalysts.The electronegativity of Eu 2 SmSbO 7 and ZnBiEuO 4 was calculated to be 5.745 and 4.667, respectively.Based on above equations, the VB potential and CB potential of Eu 2 SmSbO 7 were estimated to be 2.685 eV and −0.196 eV, respectively; contemporaneously, the VB potential and CB potential of ZnBiEuO 4 were estimated to be 1.452 eV and −1.119 eV, respectively.Eu 2 SmSbO 7 and ZnBiEuO 4 were both able to absorb visible light energy and produced electron-hole pairs when Eu 2 SmSbO 7 or ZnBiEuO 4 was exposed to VLID.The redox potential of the CB for ZnBiEuO 4 (−1.119eV) was more negative than the redox potential of the CB for Eu 2 SmSbO 7 (−0.196eV).At the same time, the redox potential position of the VB of Eu 2 SmSbO 7 (2.685 eV) was more positive than the redox potential position of the VB for ZnBiEuO 4 (1.452 eV).Therefore, the photoinduced electrons on the CB of ZnBiEuO 4 could be transferred to the CB of Eu 2 SmSbO 7 , while the photo-induced holes on the VB of Eu 2 SmSbO 7 could be transferred to the VB of ZnBiEuO 4 .Therefore, the composition of EZHP effectively increased the separation rate of photo-induced electrons and photo-induced holes; as a result, the above results improved the interfacial charge transfer efficiency [114].More oxidizing radicals (•OH and •O 2 − ) participated during the photocatalytic reaction process of degrading chlorpyrifos, thereby improving the degradation efficiency.
In addition, as shown in pathway 1 of Figure  27.In conclusion, the construction of EZHP could enhance the separation rate of the photo-induced electrons and the photo-induced holes; therefore, EZHP exhibited excellent photocatalytic performance during the process of degrading chlorpyrifos.The basic principle of the ultraviolet photoelectron spectroscopy (UPS) is the photoelectric effect, with ultraviolet light as the excitation light source.The ionization potential of the photocatalyst and the electrochemical potential of the VB could be obtained by UPS.The UPS spectrum of Eu 2 SmSbO 7 is illustrated in Figure 28a.It can be deduced from Figure 28a that the measured onset (Ei) energy and cut-off (Ecutoff) energy of Eu 2 SmSbO 7 were 0.496 eV and 19.011 eV, respectively.Figure 28b presents the UPS spectrum of ZnBiEuO 4 .It can be deduced from Figure 28b that the onset (Ei) energy and cut-off (Ecutoff) energy of ZnBiEuO 4 were measured to be 0.159 eV and 19.907 eV, respectively.The ionization potentials of Eu 2 SmSbO 7 and ZnBiEuO 4 could be calculated by decreasing the width of the UPS spectrum by the excitation energy, which was about 21.2 eV [115].According to the above approach, the VB ionization potential of Eu 2 SmSbO 7 and ZnBiEuO 4 was calculated to be 2.685 eV and 1.452 eV, respectively, which was consistent with the VB potential that was derived from the above simulation calculation results.The intermediates produced during chlorpyrifos degradation by LC-MS were identified to investigate the mechanism of chlorpyrifos degradation.The intermediate products were identified as C5H2Cl3NO (m/z = 196), C5H2Cl3N (m/z = 180), C5H3Cl2NO (m/z = 162), C5H3Cl2N (m/z = 146), C5H5N (m/z = 79), C4H11O3PS (m/z = 170), C4H11O4P (m/z = 154), C2H7O4P (m/z = 126), and H3O4P (m/z = 97).Based on above-mentioned detected intermediate products, we could deduce the degradation pathway of chlorpyrifos.Figure 29 shows the degradation pathway of chlorpyrifos.From Figure 29, it can be seen that chlorpyrifos underwent oxidation and hydroxylation reactions during photocatalytic degradation.Eventually, the above results revealed that chlorpyrifos was converted to CO2, H2O, SO4 2− , NO 3− , and PO4 3− .The electrochemical potential of O 2 /•O 2− was -0.33 eV and the absolute electrochemical potential value of OH − /•OH was 2.38 eV.The ionization potentials of the VB for Eu 2 SmSbO 7 and ZnBiEuO 4 were calculated to be 2.685 eV and 1.452 eV, respectively.Thus, it can be deduced from Figure 28  Figure 29 shows the degradation pathway of chlorpyrifos.From Figure 29, it can be seen that chlorpyrifos underwent oxidation and hydroxylation reactions during photocatalytic degradation.Eventually, the above results revealed that chlorpyrifos was converted to CO

Synthesis of N-Doped TiO 2
Nitrogen-doped titanium dioxide (N-doped TiO 2 ) used in this research was synthesized by the sol-gel method using tetrabutyl titanate as the precursor and ethanol as the dissolvent.N-doped TiO 2 was synthesized as follows: In the first step, mixed Solution 1 (17 mL tetrabutyl titanate and 40 mL absolute ethanol) and mixed Solution 2 (5 mL double-distilled water, 10 mL glacial acetic acid, and 40 mL absolute ethanol) were prepared, and mixed Solution 1 was added to mixed Solution 2 by dropwise addition under continuous stirring.At this time, a transparent colloidal suspension was obtained.In the second step, ammonia (N/Ti ratio of 8 mol%) was added to the above solution to form a dry gel after 48 h of aging.Finally, the dry gel was ground into powder and calcined at 500 • C for 2 h and then ground again and passed through a vibrating sieve to obtain N-TiO 2 powder.

Synthesis of Eu 2 SmSbO 7 /ZnBiEuO 4 Heterojunction Photocatalyst
Eu 2 SmSbO 7 and ZnBiEuO 4 were prepared by the hydrothermal method.Quantities of 0.30 mol/L Eu(NO 3 ) 3 •6H 2 O, 0.15 mol/L Sm(NO 3 ) 3 •6H 2 O, and 0.15 mol/L SbCl 5 were mixed and stirred for 20 h and then transferred to a Teflon-lined autoclave and heated at 200 • C for 15 h.Subsequently, the resulting powder was calcined in a tube furnace at 790 • C for 10 h under nitrogen to obtain Eu 2 SmSbO 7 powder.
To prepare ZnBiEuO 4 , 0.15 mol/L Zn(NO 3 ) 2 •6H 2 O, 0.15 mol/L Bi(NO 3 ) 3 •5H 2 O, and 0.15 mol/L Eu(NO 3 ) 3 •6H 2 O were mixed and stirred for 20 h.The solution was transferred to a Teflon-lined autoclave and heated at 200 • C for 15 h and then calcined in a tube furnace under nitrogen at 770 • C for 10 h.
In this study, the solvothermal method was used to synthesize EZHP.The synthesized Eu 2 SmSbO 7 (1 mol) and ZnBiEuO 4 (1 mol) were added to 300 mL of octanol (C 8 H 18 O) and dispersed for 1 h using an ultrasonic wave.Then, in order to make it easier for ZnBiEuO 4 to adhere to the surface of Eu 2 SmSbO 7 , it was heated and refluxed at 150 v for 2 h under vigorous stirring.Finally, the product was collected by centrifugation after cooling to room temperature.The collected EZHP was washed several times with a hexane/ethanol mixture to purify the EZHP.The pure EZHP was dried at 60 • C for 6 h and stored.

Photoelectrochemical Experiments
The electrochemical impedance spectroscopy (EIS) of Eu 2 SmSbO 7 , ZnBiEuO 4 , and EZHP was experimentally derived from a CHI660D electrochemical bench (Shanghai Zhenhua Instrument Co., Ltd., Shanghai, China) equipped with standard three electrodes.The working electrodes were fabricated by mixing 0.03 g of photocatalyst (Eu 2 SmSbO 7 , ZnBiEuO 4 , or EZHP) and 0.01 g chitosan with 0.45 mL of dimethylformamide, and then ultrasonicated for 1 h.The processed solution was added dropwise to a conductive glass (10 mm × 20 mm) made with indium tin oxide (ITO) and then dried at 80 • C for 10 min.A platinum plate was used as the counter electrode and an Ag/AgCl electrode was used as the reference electrode.Na 2 SO 4 aqueous solution (0.5 mol/L) was used as the electrolyte.A 500 W Xe lamp with a 420 nm cut-off filter was used as the experimental light source.

Experimental Setup and Procedure
The photocatalytic experiments were carried out at a constant temperature of 20 • C, and the visible light in the experiments was simulated using a xenon lamp (500 W) with a cut-off filter (420 nm) in a photochemistry chamber (CEL-LB70, China Education Au-Light Technology Co., Ltd., Beijing, China).
Twelve identical quartz tubes with a volume of 40 mL were used for the individual reaction solutions.The total reaction volume of the pesticide effluent was 480 mL.The amount of catalyst (Eu 2 SmSbO 7 or ZnBiEuO 4 ) was 0.75 g/L.The amount of EZHP was 0.35 g/L, 0.45 g/L 0.55 g/L, 0.65 g/L 0.75 g/L, 0.85 g/L, or 0.95 g/L.The initial concentration of chlorpyrifos was 0.032 mmol/L.
The concentration of chlorpyrifos in the solution was determined by liquid chromatography (Agilent Technologies, Palo Alto, CA, USA) after filtering out the catalyst from 3 mL of the suspension at the corresponding time according to the experimental requirements.The mobile phase consisted of one-half CH 3 CN and one-half distilled deionized water.The injection volume of the test solution was 10 µL, and the flow rate was 1 mL•min −1 .
In order to establish a reaction system with an equilibrium of adsorption/desorption of the photocatalyst, chlorpyrifos, and atmospheric oxygen, the solution containing the photocatalyst and chlorpyrifos was magnetically stirred for 45 min under dark conditions before VLID.Stirring was conducted at 500 rpm/min during the desorption of chlorpyrifos under VLID.
In order to determine the concentration of TOC during the photocatalytic degradation of chlorpyrifos, the solution was analyzed using a TOC analyzer (TOC-5000 A, Shimadzu Corporation, Kyoto, Japan).Potassium acid phthalate (KHC 8 H 4 O 4 ) solutions of known carbon concentration (0 to 100 mg/L) were prepared as standard reagents for comparison.The TOC concentration of six reaction solutions of 45 mL was measured.
A liquid chromatograph mass spectrometer (LC-MS, Thermo Quest LCQ Duo, Thermo Fisher Scientific Corporation, Waltham, MA, USA) was used to identify the intermediates produced during the photocatalytic degradation of chlorpyrifos.After the photocatalytic degradation of chlorpyrifos, 20 µL of the solution was automatically injected into the instrument for testing.The mobile phase consisted of 60% methanol and 40% ultrapure water at a flow rate of 0.2 mL/min.The relevant parameter conditions for the experiment were set to a capillary temperature of 27 • C, a voltage of 19.00 V, a spray voltage of 5000 V, and a constant sheath gas flow rate.The m/z range of the test spectrum was 50 to 600.
The incident photon flux I o was 4.76 × 10 −6 Einstein L −1 s −1 under VLID measured with a radiometer (Model FZ-A, Photoelectric Instrument Factory Beijing Normal University, Beijing, China).The incident photon flux could be adjusted by varying the distance between the photoreactor and the xenon arc lamp.
The photonic efficiency could be calculated by Equation (10): In the above equations, φ, R, and I o represent the photonic efficiency (%), conversion rate of chlorpyrifos (mol L −1 s −1 ), and incident photon flux (Einstein L −1 s −1 ), respectively.

Conclusions
In this study, Eu 2 SmSbO 7 with high photocatalytic activity was prepared via the hydrothermal method for the first time.The new photocatalyst, Eu 2 SmSbO 7 , was proven to be a pure phase with a pyrochlore structure, cubic crystal system, and Fd3m space group.Various characterization techniques, such as XRD analysis, FT-IR, Raman spectrometry, UV-Vis, XPS, and TEM-EDS, were utilized to detect the properties of the newly prepared photocatalysts.The lattice parameter and band gap of Eu 2 SmSbO 7 were a = 10.5547Å and 2.881 eV, respectively.EZHP was successfully prepared using the solvothermal method for the first time.The results demonstrated that EZHP was effective for removing chlorpyrifos from pesticide wastewater.After VLID of 160 min to degrade chlorpyrifos using EZHP, the conversion rate of chlorpyrifos reached 100% and the conversion rate of TOC was 98.02%.The kinetic constant value K C that was derived from the dynamic curves for EZHP was 0.0202 min −1 .After VLID of 160 min to degrade chlorpyrifos using Eu 2 SmSbO 7 , the conversion rate of chlorpyrifos reached 88.16% and the conversion rate of TOC was 84.04%.The kinetic constant value K C that was derived from the dynamic curves for Eu 2 SmSbO 7 was 0.0090 min −1 .After VLID of 160 min, the conversion rates of chlorpyrifos using EZHP were 1.13 times, 1.19 times, and 2.84 times higher than the conversion rates of chlorpyrifos with Eu 2 SmSbO 7 , ZnBiEuO 4 , or N-doped TiO 2 as the photocatalyst.Therefore, it can be concluded that the use of EZHP was an effective method for treating chlorpyrifos that was derived from pesticide wastewater.Finally, the possible photodegradation pathways of chlorpyrifos was hypothesized.Chlorpyrifos was degraded into inorganic compounds such as CO 2 , H 2 O, SO 4 2− , NO

Figure 2 .
Figure 2. (a) XRD and Rietveld refinement of Eu2SmSbO7: Simulated XRD data (blue solid line), experimental XRD data (red dotted line), difference between experimental XRD data and simulated XRD data (black solid line), and observed diffraction peaks (green perpendicular lines).(b) Atom construction of Eu2SmSbO7.(Red atom: O, green atom: Eu, purple atom: Sm or Sb).
coordinate of the O(1) atom reflects the change in crystal structure of A2B2O7type compounds.If the lengths of the A-O(1) bonds (of which there are six) are identical to the lengths of the A-O(

Figure 2 .
Figure 2. (a) XRD and Rietveld refinement of Eu 2 SmSbO 7 : Simulated XRD data (blue solid line), experimental XRD data (red dotted line), difference between experimental XRD data and simulated XRD data (black solid line), and observed diffraction peaks (green perpendicular lines).(b) Atom construction of Eu 2 SmSbO 7 .(Red atom: O, green atom: Eu, purple atom: Sm or Sb).

Figure 3 .Figure 3 .
Figure 3. (a) XRD and Rietveld refinement of ZnBiEuO4: simulated XRD data (blue solid line), experimental XRD data (red dotted line), difference between experimental and simulated XRD data

Figure 8 .
Figure 8.(a) The UV-Vis diffuse reflectance spectrum of Eu2SmSbO7; (b) plots of (αhν) 2 and hν for Eu2SmSbO7 (black dashed line: the linear fit of the correlative diagram; red dashed line: base line).

Figure 16 .
Figure16.Concentration variation curves of (a) chlorpyrifos and (b) TOC during the photodegradation of chlorpyrifos using EZHP, Eu 2 SmSbO 7 , ZnBiEuO 4 , N-doped TiO 2 , or without a catalyst under VLID.The conversion rates of chlorpyrifos could be calculated by (1 − C C 0 ) × 100%, where C represents the instantaneous concentration of chlorpyrifos and C 0 represents the initial concentration of chlorpyrifos.For a better comparison in all experiments, the VLID time was set to 160 min.By analyzing the data in Figure16a, it can be seen that the conversion rate of chlorpyrifos during degradation of chlorpyrifos using EZHP was 100% after VLID of 160 min, with the reaction rate of 3.33 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0700%.When Eu 2 SmSbO 7 was used as the photocatalyst, the conversion rate of chlorpyrifos was 88.16%, with a reaction rate of 2.94 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0617%.The conversion rate of chlorpyrifos that was derived from pesticide wastewater with ZnBiEuO 4 as the photocatalyst was 84.03%; at the same time, the reaction rate of 2.80 × 10 −9 mol•L −1 •s −1 and the photonic efficiency of 0.0588% were achieved.When N-doped TiO 2 was used as the photocatalyst, the conversion rate of chlorpyrifos was 35.25%, with a reaction rate of 1.18 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0247%.Finally, when chlorpyrifos was photodegraded directly under VLID without the addition of any catalysts, the conversion rate of chlorpyrifos was 4.72%, with a reaction rate of 0.16 × 10 −9 mol•L −1 •s −1 and a photonic efficiency of 0.0033%.

Figure 17 .
Figure 17.Effect of different photocatalyst doses on degradation of chlorpyrifos using EZHP under VLID.

Figure 18 .
Figure 18.Concentration variation curves of (a) chlorpyrifos and (b) TOC during four cyclic degradation tests of chlorpyrifos that was derived from pesticide wastewater under VLID using EZHP.

Figure 18 .
Figure 18.Concentration variation curves of (a) chlorpyrifos and (b) TOC during four cyclic degradation tests of chlorpyrifos that was derived from pesticide wastewater under VLID using EZHP.

Figure 20 .
Figure 20.First-order kinetic plots of (a) chlorpyrifos and (b) TOC that were observed during four cyclic photodegradation experiments for chlorpyrifos under VLID using EZHP.

Figure 20 .
Figure 20.First-order kinetic plots of (a) chlorpyrifos and (b) TOC that were observed during four cyclic photodegradation experiments for chlorpyrifos under VLID using EZHP.

Catalysts 2024 , 35 Figure 21 .Figure 21 .
Figure 21.The XRD pattern of EZHP before photocatalytic degradation reaction of chlorpyrifos (red line), and the XRD pattern of EZHP after photocatalytic degradation reaction of chlorpyrifos (blue line).

Figure 21 .
Figure 21.The XRD pattern of EZHP before photocatalytic degradation reaction of chlorpyrifos (red line), and the XRD pattern of EZHP after photocatalytic degradation reaction of chlorpyrifos (blue line).

Figure 22 .
Figure 22.The XPS spectrum of EZHP before photocatalytic degradation reaction of chlorpyrifos (red line), and the XPS spectrum of EZHP after photocatalytic degradation reaction of chlorpyrifos (blue line).

Figure 22 . 35 Figure 23 .
Figure 22.The XPS spectrum of EZHP before photocatalytic degradation reaction of chlorpyrifos (red line), and the XPS spectrum of EZHP after photocatalytic degradation reaction of chlorpyrifos (blue line).Catalysts 2024, 14, x FOR PEER REVIEW 20 of 35

Figure 23 .
Figure 23.The effect of different (a) pH values, (b) metal ion species, and (c) anion species on the degradation of chlorpyrifos using EZHP under VLID.

Figure 24 .
Figure 24.(a) Effect of different scavengers such as BQ, IPA, or EDTA on removal efficiency of chlorpyrifos using EZHP under VLID; (b) effect of different scavengers on chlorpyrifos conversion rates during photocatalytic degradation of chlorpyrifos using EZHP under VLID.

Figure 24 .
Figure 24.(a) Effect of different scavengers such as BQ, IPA, or EDTA on removal efficiency of chlorpyrifos using EZHP under VLID; (b) effect of different scavengers on chlorpyrifos conversion rates during photocatalytic degradation of chlorpyrifos using EZHP under VLID.

Catalysts 14 ,
x FOR PEER REVIEW 23 of 35
that the photo-induced holes in the VB of Eu 2 SmSbO 7 possessed the ability to oxidize H 2 O or OH − to form •OH. In addition, the CB potential of Eu 2 SmSbO 7 and ZnBiEuO 4 was −0.196 eV and −1.119 eV, respectively, which indicated that the photo-induced electrons in the conduction band of ZnBiEuO 4 could absorb O 2 to form •O 2 − .Based on above studies, EZHP possessed the ability to produce •OH and •O 2 − under VLID, which could degrade chlorpyrifos efficiently within pesticide wastewater.The intermediates produced during chlorpyrifos degradation by LC-MS were identified to investigate the mechanism of chlorpyrifos degradation.The intermediate products were identified as C 5 H 2 Cl 3 NO (m/z = 196), C 5 H 2 Cl 3 N (m/z = 180), C 5 H 3 Cl 2 NO (m/z = 162), C 5 H 3 Cl 2 N (m/z = 146), C 5 H 5 N (m/z = 79), C 4 H 11 O 3 PS (m/z = 170), C 4 H 11 O 4 P (m/z = 154), C 2 H 7 O 4 P (m/z = 126), and H 3 O 4 P (m/z = 97).Based on above-mentioned detected intermediate products, we could deduce the degradation pathway of chlorpyrifos.

Figure 29 .
Figure 29.Suggested photocatalytic pathway scheme for the degradation of chlorpyrifos under VLID with EZ heterojunction as photocatalyst.

Figure 29 .
Figure 29.Suggested photocatalytic pathway scheme for the degradation of chlorpyrifos under VLID with EZ heterojunction as photocatalyst.
al. possessed higher catalytic activity than pure BiPO 4 , and the heterojunction Ag 2 MoO 4 /Bi 4 Ti 3 O 12 by Cheng T. T. et al. had higher catalytic activity than pure Ag 2 MoO 4 or pure Bi 4 Ti 3 O 12

Table 3 .
The effect of different ratios of catalyst to chlorpyrifos on conversion rates of chlorpyrifos.

Table 3 .
The effect of different ratios of catalyst to chlorpyrifos on conversion rates of chlorpyrifos.

Table 4 .
Comparison of the photocatalytic activity of EZHP with that of other reported photocatalysts during photocatalytic degradation of chlorpyrifos.

Table 4 .
Comparison of the photocatalytic activity of EZHP with that of other reported photocatalysts during photocatalytic degradation of chlorpyrifos.
27, because the CB potential of ZnBiEuO 4 (−1.119eV) was more negative than the potential of O 2 /•O 2 − (−0.33 eV), the photoinduced electrons within the CB of ZnBiEuO 4 could absorb oxygen to produce •O 2 − .•O 2 − was used to degrade chlorpyrifos.Similarly, as illustrated in pathway 2 of Figure 27, because the VB potential of Eu 2 SmSbO 7 (2.685 eV) was more positive than the potential of OH − /•OH (2.38 eV), the photo-induced holes in the VB of Eu 2 SmSbO 7 could oxidize H 2 O or OH − into •OH, which could degrade chlorpyrifos.Finally, the photo-induced holes within the VB of ZnBiEuO 4 or Eu 2 SmSbO 7 could directly oxidize chlorpyrifos, as illustrated in Path 3 of Figure
3− , and PO 4 3− .J.L. (Jingfei Luan): conceptualization, data curation, formal analysis, investigation, methodology, software, visualization, writing-original draft preparation, writing-review and editing, validation.Y.W.: software, data curation, methodology, writing-original draft preparation, validation.Y.Y.: formal analysis, investigation, writing-original draft preparation, validation, investigation.L.H.: software, visualization, validation.J.L. (Jun Li): formal analysis, methodology, writing-original draft preparation.Y.C.: software, validation, data curation.All authors have read and agreed to the published version of the manuscript.This study was supported by the Free Exploring Key Item of the Natural Science Foundation of the Science and Technology Development Plan Project of Jilin Province, China (Grant No. YDZJ202101ZYTS161).
Author Contributions:Funding: