Synthesis, Performance Measurement of Dy₂EuSbO₇/ZnBiDyO₄ Heterojunction Composite Catalyst and Photocatalytic Degradation of Chlorpyrifos within Pesticide Wastewater under Visible Light Irradiation

Abstract

For the first time, a novel catalyst named Dy₂EuSbO₇ was successfully synthesized via the high-temperature solid-state sintering method (HTSSM). Dy₂EuSbO₇/ZnBiDyO₄ heterojunction photocatalyst (DZHP) was fabricated through the HTSSM for degrading chlorpyrifos (CPS) in the pesticide wastewater under visible light irradiation (VSLID). Under VSLID, DZHP could effectively degrade CPS in pesticide wastewater. The experimental outcomes suggested that the kinetic curve with the Dy₂EuSbO₇/ZnBiDyO₄ heterojunction (DZH) as a photocatalyst for the reduction of CPS under VSLID conformed to the first-order kinetics (FOKT). After VSLID of 156 min, the photocatalytic degradation (PTD) removal rate of CPS using DZH as photocatalyst was 1.12 times, 1.21 times, or 2.96 times that using Dy₂EuSbO₇ as a photocatalyst, ZnBiDyO₄ as a photocatalyst, or nitrogen-doped titanium dioxide as a photocatalyst. After VSLID of 156 min for four cycle degradation tests (FCDTS) with DZH as a photocatalyst, the removal rate of CPS reached 98.78%, 97.66%, 96.59%, and 95.69%, respectively. Above results indicated that the DZHP possessed high stability. Experiments with the addition of trapping agents showed that hydroxyl radicals (•OH) owned the strongest oxidative removal ability for degrading CPS compared with superoxide anions (•O₂⁻) or holes (h⁺). The oxidation capacity of three oxidation radicals for eliminating CPS was ranked in the ascending order as follows: h⁺ < •OH < •O₂⁻. Lastly, the possible degradation pathway and degradation mechanism of CPS were discussed in detail. A visible light responsive heterojunction catalyst with high catalytic activity and a photocatalytic reaction system which were capable of efficiently removing toxic organic pollutants from pesticide wastewater were obtained.

1. Introduction

Pesticides perform a significant part in the procedure of grain cultivation, and they are mainly used to control pests and disease as well as to help increase food production. However, the overuse of pesticides poses a range of problems for the environment and human health [1,2]. The residual pesticides pollute the water source and the soil [3]. Long-term exposure or consumption of food containing pesticides increases the risk of cancer and neurological diseases [4,5,6]. Pesticides thereby endanger the biodiversity of the natural world.

Chlorpyrifos (CPS) is a widely used insecticide with a strong poisoning effect on insects and a long validity period; moreover, CPS is difficult to be degraded [7,8,9]. CPS affects the activity of soil microorganisms and changed the functions of the soil; thus, plant growth is affected. In addition, CPS could enter the human body through water sources or the food chain; thus, CPS could cause a potential threat to human health [10,11]. Therefore, the degradation of CPS is necessary.

Various methods are currently available to degrade CPS, which are derived from the pesticide wastewater, such as physical methods, chemical methods, and biological methods [12,13]. Physical methods include the adsorption method, ion exchange method, and membrane method. Chemical methods include the chemical precipitation method, electrochemical method, and chemical redox method. Biological methods are mainly divided into anaerobic methods and aerobic methods. Physical methods could achieve better results, but these technologies cannot completely eliminate CPS, and the material consumption of physical methods is large, which increases the treatment cost. The chemical redox method possesses a high cost and low efficiency; simultaneously, the chemical redox method also causes secondary pollution problems. Biological methods have long treatment cycles and are difficult to utilize for treating toxic and non-biodegradable organic contaminants. Obviously, the above-mentioned methods possess many disadvantages.

Some advanced oxidation techniques, such as the photocatalytic oxidation method, Feton, and supercritical water oxidation, possess incomparable advantages for the degradation of CPS due to their powerful oxidation capacity and extremely high reaction accuracy [14,15,16,17,18,19,20,21,22,23]. Photocatalytic technology utilizes sunlight as energy to activate photocatalysts; as a result, oxidized groups could be produced under the irradiation of light, and CPS could be effectively removed; meanwhile, the photocatalysts could be recycled. Therefore, the photocatalytic oxidation method is widely used in wastewater treatment [24]. The significant challenge for the industrialization application of photocatalytic technology is to develop optimal photocatalysts, which should possess four essential characteristics as follows: a high photocatalytic efficiency, a substantial specific surface area, comprehensive utilization of sunlight, and recyclability [25,26,27,28]. In addition, the photocatalytic activity is intricately associated with the dimensions, shape, bandgap, and crystalline configuration of the photocatalyst. The development of TiO₂, which belongs to the most commonly used metal oxide photocatalyst, is limited because TiO₂ is only able to absorb UV light, which makes up 5% of solar energy [29,30,31,32,33,34,35,36,37]; consequently, TiO₂ cannot effectively harness light energy. Therefore, developing visible light responsive photocatalysts and improving the photonic efficiency of photocatalysts are inevitable trends in the field of photocatalysis research.

As per earlier reports, it is generally believed that the metal oxide compounds ABO₃, ABO₄ and A₂B₂O₇ [38,39,40,41,42,43] have photocatalytic performance under visible light irradiation (VSLID). Tong et al. conducted photocatalytic experiments on BiFeO₃ wafers under VSLID and achieved good results [38]. Hu et al. investigated how the BiVO₄ catalyst affected the rate at which acetaminophen degraded under VSLID. [40]. Zhang et al. synthesized porous Sm₂Ti₂O₇ material using the sol-gel technique with PEG4000 as the template; consequently, the Sm₂Ti₂O₇ with a thermal feldspar structure promoted the crystallization of porous samples; thereby, the catalytic efficiency was improved [42]. In an effort to produce a photocatalyst with excellent performance and visible light response, Luan et al. prepared a Fe₂BiSbO₇ photocatalyst and explored its characteristics and photocatalytic activity; as a result, good results were achieved [43]. In our previous work [44], we found that Bi₂GaSbO₇ crystals possessed a thermal feldspar shape structure and could serve as efficient photocatalysts under VSLID. After changing the structure of Bi₂GaSbO₇, the novel photocatalyst had the capacity to increase the activity of the photocatalysts. Considering the analysis above, we can conclude that replacing Fe³⁺ with Bi³⁺ and replacing Bi³⁺ with Ga³⁺ in Fe₂BiSbO₇, as well as the synthetical Bi₂GaSbO₇, might enhance the carrier concentration. In a similar way, by replacing Bi³⁺ with Dy³⁺ and replacing Ga³⁺ with Eu³⁺ in Bi₂GaSbO₇, the synthetical Dy₂EuSbO₇ might also ultimately enhance the carrier concentration. Therefore, the novel Dy₂EuSbO₇ compound’s electrical transport and photophysical characteristics would undergo changes and improvements; as a result, advanced photocatalytic performance could be achieved.

Many methods have been proven effective for improving photonic efficiency, such as ion doping, heterojunction construction, and photosensitization [45,46,47,48,49,50,51,52,53]. Among the various available methods, the construction of heterojunctions remains an actively promising research area within the domain of photocatalysis [54,55,56,57,58,59]. Heterojunction catalysts combine what comes out of one photocatalyst and results in higher light utilization efficiency, higher photocatalytic performance, and higher chemical stability of the composite system, as well as a longer service life. Xu et al. prepared n-n type heterojunction ZnFe₂O₄/BiVO₄ using chemical co-precipitation method; as a result, the experiments showed that the heterojunctions exhibited higher photocatalytic activity compared with pure BiVO₄ or ZnFe₂O₄ because heterojunctions could generate more photoinduced electrons (e⁻) [60]. Ma synthesized a p-n heterojunction NiO_x/Bi₂WO₆, and the enhanced photocatalytic performance of NiO_x/Bi₂WO₆ samples was attributed to the p-n junction between Bi₂WO₆ and NiO_x, which enhanced the photoinduced e⁻ and photoinduced hole (h⁺) separation efficiency [61]. By analyzing above results, we can conclude that constructing heterojunctions increased the photocatalysts’ redox performance [62,63,64,65,66]; ultimately, the reaction’s overall selectivity and activity were enhanced. Therefore, the Dy₂EuSbO₇/ZnBiDyO₄ heterojunction photocatalyst (DZHP) can also be synthesized, and the photocatalytic performance for CPS degradation is worth exploring.

This paper analyzed the structural characteristics of pure-phase ZnBiDyO₄ and single-phase Dy₂EuSbO₇ which were prepared by the high-temperature solid-state sintering method (HTSSM) through an X-ray diffractometer (XRD), UV-Vis spectrophotometer, Fourier transform infrared (FTIR) spectrometer, Raman spectrometer, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). In addition, the removal efficiency of CPS under VSLID was investigated using Dy₂EuSbO₇ or ZnBiDyO₄ or N-doped TiO₂ or Dy₂EuSbO₇/ZnBiDyO₄ heterojunction (DZH) as a photocatalyst. The purpose of this study was to prepare a novel heterojunction catalyst for the removal of CPS from pesticide wastewater under VSLID. The innovation idea of this study lies in the first synthesis of the new Dy₂EuSbO₇ nanocatalyst and DZHP using HTSSM. DZHP is a visible-light-responsive photocatalyst with high photocatalytic activity and can effectively remove CPS from pesticide wastewater. This study makes the removal of organic pollutants from pesticide wastewater more efficient and safer.

2. Results and Discussion

2.1. XRD Analysis

Figure 1 shows the XRD patterns of DZHP, Dy₂EuSbO₇, ZnBiDyO₄, and N-doped TiO₂. Data were analyzed using Materials Studio. Figure 2a shows the crystallization of Dy₂EuSbO₇, with an R_p factor of 5.20%. Figure 2b illustrates the atomic structure of Dy₂EuSbO₇. It could be found from Figure 2 that Dy₂EuSbO₇ possessed the perfect degree of crystallinity, meanwhile, Dy₂EuSbO₇ owned a pyrrhotite-type structure with a spatial group of Fd3m, belonged to a cubic crystal system, and possessed a lattice parameter of 10.4440423 Å. Figure 3a shows the crystallization of ZnBiDyO₄, with an R_p factor of 7.20%. Figure 3b illustrates the atomic structure of ZnBiDyO₄. It could be found from Figure 3 that ZnBiDyO₄ was a single-phase high-purity compound and owned perfect degree of crystallinity; simultaneously, ZnBiDyO₄ possessed a spinel structure with a spatial group of I41/Amd, belonged to a tetragonal system, and owned a lattice parameter of a = b = 10.541001 Å and c = 9.930823 Å. According to Figure 2 and Figure 3, the indices of crystallographic plane value for the principal diffraction peaks of Dy₂EuSbO₇ or ZnBiDyO₄ were calculated and labelled.

Table 1. Structural parameters of Dy₂EuSbO₇ prepared by the solid-phase sintering method.

Atom	x	y	z	Occupation Factor
Dy	0	0	0	1
Eu	0.5	0.5	0.5	0.5
Sb	0.5	0.5	0.5	0.5
O(1)	−0.185	0.125	0.125	1
O(2)	0.125	0.125	0.125	1

presents comprehensive details on the atomic coordinates and structural parameters of Dy₂EuSbO₇.

Table 2. Structural parameters of ZnBiDyO₄ prepared by the solid-phase sintering method.

Atom	x	y	z	Occupation Factor
Zn	0	0	0.5	1
Bi	0	0	0	1
Dy	0	0	0	1
O	0.76731	0.14013	0.08188	1

presents comprehensive details on the atomic coordinates and structural parameters of ZnBiDyO₄. Furthermore, as shown in Figure 1d, the peak of the nitrogen-doped TiO₂ sample prepared corresponds to the anatase TiO₂ phase. We have also successfully synthesized N-doped TiO₂.

Insights into the octahedral deformation of MO₆ (M = Eu³⁺ and Sb⁵⁺) could be derived from the x-coordinate value [58]. The x-value shifted from x = 0.375 [67], so the distortion of the MO₆ (M = Eu³⁺ and Sb⁵⁺) octahedral was clearly present in the crystal structure of Dy₂EuSbO₇. In order to prevent photogenerated h⁺ from recombining with photogenerated e⁻, achieving charge separation became imperative for the photocatalytic decomposition of CPS under VSLID. According to Inoue and Kudo, the key to reducing charge recombination and boosting photocatalytic effectiveness is the local distortion of MO₆ octahedra derived from photocatalysts such as BaTi₄O₉ and Sr₂M₂O₇ (M = Nb⁵⁺ and Ta⁵⁺). [68,69]. Therefore, the distortion of MO₆ (M = Eu³⁺ and Sb⁵⁺) octahedrals in the crystal structure of Dy₂EuSbO₇ could also be used to enhance photocatalytic activity. Dy₂EuSbO₇ consisted of a three-dimensional mesh structure of MO₆ (M = Eu³⁺ and Sb⁵⁺) octahedrals that shared angles. MO₆ (M = Eu³⁺ and Sb⁵⁺) octahedrals were linked into chains by Eu³⁺ ions. Two Dy-O bond lengths coexisted: six Dy-O(1) bond lengths (2.769 Å) were significantly longer than two Dy-O(2) bond lengths (2.472 Å). Six M-O(1) (M = Eu³⁺ and Sb⁵⁺) bond lengths were 2.569 Å long, and M-Dy (M = Eu³⁺ and Sb⁵⁺) bond lengths were 3.647 Å long. The M-O-M (M = Eu³⁺ and Sb⁵⁺) bond angle in the Dy₂EuSbO₇ crystal structure was 139.211°. The Dy-M-Dy (M = Eu³⁺ and Sb⁵⁺) bond angle was 135.425° in the Dy₂EuSbO₇ crystal structure. The Dy-M-O (M = Eu³⁺ and Sb⁵⁺) bond angle was 133.688° in the Dy₂EuSbO₇ crystal structure. Research on luminescence characteristics found that more excited states were delocalized the closer the M-O-M bond angle was to 180° [70]. The results showed that the angle between MO₆ (M = Eu³⁺ and Sb⁵⁺) octahedrals with shared angles, such as the M-O-M bond angle of Dy₂EuSbO₇, was of great significance for affecting the photocatalytic activity of Dy₂EuSbO₇.

2.2. FTIR Analysis

In an effort to determine which chemical bonds and functional groups exist in DZHP, an FTIR spectrometer was used to produce the FTIR spectra of Dy₂EuSbO₇ and ZnBiDyO₄, as seen in Figure 4. DZHP, Dy₂EuSbO₇, and ZnBiDyO₄ all displayed many distinctive absorption peaks in their FTIR spectra. These included peaks related to Dy-O, Eu-O, Sb-O, Sb-O-Sb, Zn-O, and Bi-O bonds. The stretching vibration of Dy-O was observed at 613 cm⁻¹ [43,60], while the bending vibration of Eu-O occurred at 590 cm⁻¹ [71]. The stretching vibration of Zn-O occurred at 468 cm⁻¹ [72], while the bending vibration of Bi-O was associated with peaks at 428 cm⁻¹ [73,74], and the bending vibration of Sb-O was observed at 458 cm⁻¹ and 583 cm⁻¹ [75]. The bending vibration of Sb-O-Sb was represented by peaks at 658 cm⁻¹ [76]. Furthermore, the broad peak observed at 3431 to 3576 cm⁻¹ indicated the stretching vibrations of O-H groups from chemisorbed water molecules [77,78]. The peak at 1632 cm⁻¹ corresponded to the bending vibration mode of these O-H groups [79]. The multiple bands observed at 1379 to 1632 cm⁻¹ were attributed to the vibrations of C-H bonds from adsorbed water [80,81].

2.3. Raman Analysis

Raman spectra were obtained using a Raman spectrometer to analyze the interactions between various chemical bonds in DZHP, Dy₂EuSbO₇, and ZnBiDyO₄. The Raman spectra of ZnBiDyO₄, Dy₂EuSbO₇, and DZHP are shown in Figure 5. The Raman spectrum of Dy₂EuSbO₇ exhibited characteristic modes, including the E_1g main vibrational modes at 542 cm⁻¹, associated with the stretching vibration of Eu-O bonds, as well as the peaks at 199cm⁻¹ [82], associated with the stretching vibration of Dy-O bonds [83]. Additionally, peaks at 368 cm⁻¹, 654 cm⁻¹, and 719 cm⁻¹ could be assigned to the bend vibration of Sb-O and Sb-O-Sb [84,85]. The Raman spectra of ZnBiDyO₄ revealed a broad band at 128 cm⁻¹, 637 cm⁻¹, and 843 cm⁻¹, which could be attributed to the stretching vibrations of Dy-O, Bi-O, and Zn-O, respectively [83,84,85,86,87,88]. Notably, the Raman spectra of DZHP exhibited strong peaks encompassing the distinct absorption peaks of both Dy₂EuSbO₇ and ZnBiDyO₄, including peaks at 136 cm⁻¹, 192 cm⁻¹, 370 cm⁻¹, 533 cm⁻¹, 640 cm⁻¹, 713 cm⁻¹, and 839 cm⁻¹.

2.4. UV-Vis Diffuse Reflectance Spectra

The absorption spectra of DZHP, Dy₂EuSbO₇, and ZnBiDyO₄ sample are shown in Figure 6a. The absorption edges of DZHP, Dy₂EuSbO₇, and ZnBiDyO₄ were located at 595 nm, 430 nm, and 590 nm, which were all in the visible region of the spectrum. The band gap energies of the crystalline semiconductors could be determined by the intersection point between photon energy hν axis and the line extrapolated from the linear portion of the absorption edge of the so-called Kubelka–Munk function (1) (known as the re-emission function) [89]:

$${\displaystyle \frac{{\left[1-{\mathrm{R}}_{\mathrm{d}}\left(\mathrm{h}\mathsf{\nu }\right)\right]}^2}{2{\mathrm{R}}_{\mathrm{d}}\left(\mathrm{h}\mathsf{\nu }\right)}}={\displaystyle \frac{\mathsf{\alpha }\left(\mathrm{h}\mathsf{\nu }\right)}{\mathrm{S}}}$$

(1)

where S was the scattering factor, and R_d was the diffuse reflectance, and α represented absorption coefficient of radiation.

The optical absorption near the band edge of the crystalline semiconductors obeyed Equation (2) [90,91]:

αhν = A (hν − E_g)ⁿ

(2)

Here, A, α, E_g, and ν denote the proportional constant, absorption coefficient, band gap, and light frequency, respectively. Within this equation, n determined the character of the transition in a semiconductor. The E_g values of Dy₂EuSbO₇ and ZnBiDyO₄ were 2.68 eV and 1.78 eV, respectively. The E_g value of DZHP was 1.75 eV. As shown in Figure 6b, the estimated (n) value was roughly 0.5, which proves that the phototransition mode of the catalyst was a direct transition. Moreover, the absorption spectrum of N-doped TiO₂ is shown in Figure 6c. The absorption edge of N-doped TiO₂ was located at 421 nm, also in the visible region of the spectrum.

2.5. Property Characterization of Dy₂EuSbO₇/ZnBiDyO₄ Heterojunction Photocatalyst

To determine the surface chemical composition and valence states of DZHP, Dy₂EuSbO₇ and ZnBiDyO₄, X-ray photoelectron spectroscopy (XPS) was used. Figure 7 displays the XPS spectra of DZHP, Dy₂EuSbO₇, and ZnBiDyO₄. It was evident that Dy, Eu, Sb, Zn, Bi, and O elements were present in DZHP, indicating that heterojunction structures had been successfully constructed. As a reference for calibration, there was an observable carbon peak.

Figure 8a–f show the XPS spectra of Dy₂EuSbO₇ and ZnBiDyO₄ component elements. Based on the XPS analysis results, the peaks of various elements with specific binding energy were obtained, among which the Dy 4d_5/2 peak of Dy element was located at 159.09 eV, and the Eu 4d_5/2 peak of Eu element was located at 136.02 eV. The Sb 4d_3/2 peak of the Sb element was 34.96 eV. The Zn 2p_3/2 peak of Zn was 1022.13 eV. The Bi 4d_5/2 peaks of Bi elements were located at 442.21 eV. A slight shift of these peaks toward higher binding energy was observed when compared with the XPS spectrum of DZHP. These shifts indicate the presence of strong interfacial interactions between Dy₂EuSbO₇ and ZnBiDyO₄, which might be have been due to e⁻ transfer and delocalization between the two components in the heterojunction photocatalytic materials [92].

Figure 8f shows the O 1s spectra of DZHP and Dy₂EuSbO₇. They were located at 530.88 eV and 530.47 eV, respectively. The Sb 3d_5/2 peaks of Sb elements were located at 531.95 eV and 531.52 eV. The Sb 3d_3/2 peaks of Sb were located at 540.27 eV and 539.24 eV. Similarly, the spin-orbit separation value between Sb 3d_5/2 and Sb 3d_3/2 was observed to be 8.36 eV for Dy₂EuSbO₇ and DZHP, confirming the exclusive presence of Sb⁵⁺ [93,94].

From the XPS analysis findings, it is evident that the oxidation states of Dy, Eu, Zn, Bi, Sb, and O ions were +3, +3, +2, +3, +5, and −2, respectively. The surface elemental analysis results showed that the average atomic ratio of Dy:Eu:Sb:Zn:Bi:O was 1920:1010:890:650:390:5150. The atomic ratios of Dy:Eu:Sb and Zn:Bi:Dy in the samples of DZHP were 1.95:0.96:1.00 and 1.00:0.96:0.98, respectively. The high oxygen content was due to the large amount of oxygen adsorbed on the surface of DZHP. Apparently, no heteropeaks were observed in the XPS peak of the DZHP, suggesting that there were no other elements.

Figure 9a,b show the TEM images of DZHP, while Figure 9c,d show the TEM images of Dy₂EuSbO₇ and the electron diffraction patterns of the selected regions. Figure 9e,f show the TEM image of ZnBiDyO₄ and the electron diffraction pattern of the selected region. Figure 10 shows the EDS elemental mapping of DZHP (Dy, Eu, Sb, O from Dy₂EuSbO₇ and Zn, Bi, Dy, O from ZnBiDyO₄). Figure 11 presents the EDS spectra of DZHP. Based on Figure 9 and Figure 10, the larger tetragonal particles were identified as Dy₂EuSbO₇, and the smaller circular-shaped particles were identified as ZnBiDyO₄. Figure 9 and Figure 10 show Dy₂EuSbO₇ particles with a rhombic dodecahedral shape surrounded by smaller ZnBiDyO₄ particles. This close encapsulation indicates the successful synthesis of DZHP. It is widely recognized that the structural growth of photocatalysts is regulated by the different surface energies of microcrystalline faces [87,88], resulting in the rhombic dodecahedral morphology of Dy₂EuSbO₇. The experimental results shown in Figure 9 indicate that ZnBiDyO₄ exhibited a regular spherical morphology and uniform particle distribution. The particle size of ZnBiDyO₄ was approximately 420 nm, whereas the particle size of Dy₂EuSbO₇ was about 798 nm. The lattice parameters of Dy₂EuSbO₇ have been proven to be a = b = c = 1.0444 nm. The lattice parameters of ZnBiDyO₄ have been proven to be a = b = 1.0541 nm, c = 0.99308 nm. According to the calculation results in Figure 9, the same diffraction peaks as the main diffraction peaks of Dy₂EuSbO₇ in Figure 2 and ZnBiDyO₄ in Figure 3 could be found. Additionally, high-resolution TEM (HRTEM) images in Figure 9b reveal that the interplanar spacings (d-values) for Dy₂EuSbO₇ and ZnBiDyO₄ were 0.303 nm and 0.196 nm, respectively, and that the two interfaces were in close contact. This further confirms the successful synthesis of DZHP.

The TEM-EDS analysis showed that no other impurity elements were detected in DZHP compound. In addition, the measured pure phase of Dy₂EuSbO₇ aligned with the X-ray diffraction analysis results presented in Figure 1. By observing Figure 10 and Figure 11, it could be clearly identified that the DZHP contained dysprosium, europium, antimony, zinc, bismuth, and oxygen. These findings were consistent with the XPS results of DZHP, as shown in Figure 7 and Figure 8. Further analysis of the EDS spectra of DZHP showed that the atomic ratio of Dy:Eu:Sb:Zn:Bi:O was 1920:1010:890:650:390:5150, which agreed with the DZHP XPS findings. In addition, we also found that the atomic ratio of Dy₂EuSbO₇:ZnBiDyO₄ was close to 1.9:1. The findings of the study above allow us to infer that under the current preparation conditions, DZHP had high purity.

2.6. Photocatalytic Activity

Figure 12a shows the concentration change curves of CPS after photocatalytic degradation (PTD) of CPS under VSLID conditions using DZH, Dy₂EuSbO₇, ZnBiDyO₄, or N-doped TiO₂ as the photocatalyst. From Figure 12a, it is evident that the concentration of CPS within pesticide wastewater gradually decreased with increasing VSLID time. The findings derived from Figure 12a demonstrate that 99.81% of the CPS in pesticide wastewater was removed, the rate of reaction was 3.41 × 10⁻⁹ mol·L⁻¹·s⁻¹, and the photonic efficiency was 0.0716% with DZHP as the catalyst after VSLID of 156 min. And all the other experiments followed the same VSLID time of 156 min. When Dy₂EuSbO₇ was used as photocatalyst, the removal rate of CPS reached 89.09%, the rate of reaction was 3.05 × 10⁻⁹ mol·L⁻¹·s⁻¹, and the photonic efficiency was 0.0641%. The removal rate of CPS within pesticide wastewater reached 82.16%, the rate of reaction was 2.81 × 10⁻⁹ mol·L⁻¹·s⁻¹, and the photonic efficiency was 0.0590% with ZnBiDyO₄ as the photocatalyst. The removal rate of CPS reached 33.69%, the rate of reaction was 1.15 × 10⁻⁹ mol·L⁻¹·s⁻¹, and the photonic efficiency was 0.0242% with N-doped TiO₂ as the photocatalyst. The aforementioned findings demonstrate that, after a 156 min VSLID, the removal rate of CPS using DZHP was 1.12 times, 1.21 times, or 2.96 times greater than that using Dy₂EuSbO₇, ZnBiDyO₄, or N-doped TiO₂ as photocatalysts. Figure 12b shows the concentration change curve of CPS after nonilluminated adsorption by DZH, Dy₂EuSbO₇, ZnBiDyO₄, or N-doped TiO₂ under dark conditions.

Table 3. Specific surface area (SSA), total pore volume (V), and average pore size (D) of the samples, determined by nitrogen adsorption.

Samples	SSA (m2/g)	V (mL/g)	D (nm)
DZHP	1.7311	0.0124	28.6523
Dy2EuSbO7	1.9762	0.0123	24.8963
ZnBiDyO4	2.6984	0.0288	42.6920
N-doped TiO2	45.1082	0.0212	6.9609

shows the specific surface area (SSA), total pore volume (V), and average pore size (D) of the samples determined by nitrogen adsorption. After 45 min of no light adsorption, the adsorption rates of DZH, Dy₂EuSbO₇, ZnBiDyO₄, and N-doped TiO₂ for CPS were 4.81%, 5.28%, 5.47%, and 8.28%, respectively. It could be seen that N-doped TiO₂ had the highest dark adsorption rate, which could be attributed to its higher specific surface area.

Figure 13 shows the change of total organic carbon (TOC) concentration when CPS in pesticide wastewater was degraded by different photocatalysts under VSLID conditions. As VSLID time increased, the concentration of CPS decreased gradually. After 156 min of VSLID, for DZHP, the TOC removal rates of Dy₂EuSbO₇, ZnBiDyO₄ and N-doped TiO₂ were 97.32%, 84.56%, 77.63% and 28.57%, respectively. DZHP had the highest mineralization rate during CPS degradation.

Figure 14 shows the concentration curve of CPS under VSLID during PTD of DZHP, and four-cycle degradation tests (FCDTS) were performed. The removal rate of CPS reached 98.78% or 97.66% or 96.59% or 95.69% after irradiation with DZHP for 156 min after irradiation with DZHP after completing FCDTS of CPS. Figure 15 shows the concentration curve of TOC during PTD of CPS and DZHP under VSLID, and FCDTS were performed. As illustrated in Figure 15, the removal rates of TOC were 96.25%, 95.18%, 94.16%, and 93.25%, respectively. The experimental results from Figure 15 and Figure 16 indicate that DZHP exhibited high stability.

From Figure 14 and Figure 15, it could be concluded that after FCDTS, under VSLID, the removal rate of CPS by DZHP was reduced by 4.12%, and the removal rate of TOC was reduced by 4.07%. The degradation efficiencies in the aforementioned four cycles did not show significant differences, indicating that DZHP was highly stable.

Figure 16 illustrates the first-order kinetic (FOKT) plots for CPS PTD using various photocatalysts (DZHP, Dy₂EuSbO₇, ZnBiDyO₄, and N-doped TiO₂) under VSLID conditions. The kinetic constants k for DZHP, Dy₂EuSbO₇, ZnBiDyO₄, and N-doped TiO₂ were 0.02437 min⁻¹, 0.00879 min⁻¹, 0.00707 min⁻¹, and 0.00195 min⁻¹, respectively. The kinetic constants K for TOC concentration were 0.01489 min⁻¹, 0.00719 min⁻¹, 0.00595 min⁻¹, and 0.00163 min⁻¹, respectively. The K_TOC values for CPS degradation by the same catalyst were lower than the K_C values, indicating that there were intermediate products in the process of photodegradation. DZHP exhibited higher mineralization efficiency.

Figure 17 illustrates the FOKT curve for the four-cycle degradation of CPS under VSLID using DZH as the photocatalyst. The results indicate that the kinetic constants k were 0.01788 min⁻¹, 0.01536 min⁻¹, 0.01384 min⁻¹, and 0.01270 min⁻¹, respectively. In the four-cycle degradation experiments, the kinetic constants k for DZHP were 0.01331 min⁻¹, 0.01221 min⁻¹, 0.01124 min⁻¹, and 0.01047 min⁻¹ for TOC. It was consistent with the FOKT.

Figure 18 illustrates the impact of various free radical scavengers, such as benzoquinone (BQ) isopropanol (IPA) or ethylenediamine tetraacetic acid (EDTA) on the removal of CPS by DZHP under VSLID conditions. Initially, at the onset of the photocatalytic experiment, different free radical scavengers were introduced into the CPS solution to identify the active species involved in its degradation. IPA was used to trap hydroxyl radicals (•OH), BQ to capture superoxide anions (•O₂⁻), and EDTA to bind to h⁺. The concentration of IPA, BQ, or EDTA was set at 0.15 mmol L⁻¹, and the dosage was 1 mL. As depicted in Figure 19, the addition of BQ, IPA, or EDTA to the CPS solution reduced the removal rate by 44.22%, 35.68%, and 25.90%, respectively, compared to the control group. Hence, •OH, h⁺ and •O₂⁻ were all active free radicals during CPS degradation. Figure 19 indicates that under VSLID conditions, •OH dominated in the CPS solution when DZHP was used as a photocatalyst for CPS degradation. Experiments with added scavengers revealed that •OH were more effective at oxidatively removing CPS from pesticide wastewater than •O₂⁻ or h⁺. The order of scavengers’ effectiveness in oxidatively removing CPS was •O₂⁻ > •OH > h⁺.

We used electron paramagnetic resonance (EPR) studies to look into the production of •O₂⁻ and •OH free radicals during the PDP process [95]. Figure 19 shows the EPR spectra of DMPO•O₂⁻ and DMPO•OH with DZHP as the photocatalyst. After 10 min of visible light exposure, a significant DMPO•O₂⁻ signal appeared, with four peaks in a 1:1:1:1 ratio, indicating •O₂⁻ radicals. The spectra likewise showed a four-line DMPO•OH signal in a 1:2:2:1 ratio after light exposure, suggesting the generation of •OH radicals. A higher radical production was indicated by these signals’ higher relative intensities, with •O₂⁻ yield being higher than •OH. These results align with previous free radical scavenger experiments.

The Photoluminescence (PL) spectrum and Time-Resolved Photoluminescence (TRPL) spectra of Dy₂EuSbO₇, ZnBiDyO₄, and DZHP are depicted in Figure 20 and Figure 21a–c. Figure 20 shows the PL spectra of prepared Dy₂EuSbO₇ catalyst, the ZnBiDyO₄ catalyst, and the constructed DZHP. All samples showed an emission peak at 470 nm. With the lowest emission peak intensity and hence the lowest e⁻ and h⁺ recombination rate, the DZHP had the maximum catalytic activity. The TRPL spectra of Dy₂EuSbO₇, ZnBiDyO₄, and DZHP in Figure 21a–c were fitted by the following double-exponential decay Equation (3) [95]:

$$I\left(t\right){=I}_0+A_{1}exp\left(-{\displaystyle \frac{t}{{\mathsf{\tau }}_1}}\right)+{\mathrm{A}}_{2}exp\left(-{\displaystyle \frac{t}{{\mathsf{\tau }}_2}}\right)$$

(3)

In the provided equation, τ₁ and τ₂ represent the first and second-order decay times, and A₁ and A₂, were weighting coefficients of each decay channel [96]. DZHP (τ₁ = 1.37 ns, τ₂ = 1.26 ns, τ_ave = 1.25 ns) presented a much higher photogenerated charge carrier lifetime (PCC) than Dy₂EuSbO₇ (τ₁ = 1.61 ns, τ₂ = 1.21 ns, τ_ave = 1.21 ns) and ZnBiDyO₄ (τ₁ = 1.46 ns, τ₂ = 0.978 ns, τ_ave = 0.97 ns), which indicates that DZHP has the most excellent photocatalytic effect. To determine the average PCC lifetime (τ_ave), the following Equation (4) was utilized [97]:

$${\mathsf{\tau }}_{\mathrm{a}\mathrm{v}\mathrm{e}}=\left({\mathrm{A}}_1{\mathsf{\tau }}_1^2+A_2{\mathsf{\tau }}_2^2\right)/\left({\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2\right)$$

(4)

This study investigated the impact of prepared samples on the separation efficiency of PCC using photocurrent and electrochemical impedance spectroscopy (EIS). Figure 22a illustrates the transient photocurrent responses of DZHP, as well as individual samples of Dy₂EuSbO₇ and ZnBiDyO₄. The findings reveal that DZHP exhibited the highest photocurrent response intensity compared to Dy₂EuSbO₇ and ZnBiDyO₄. The effective diffusion of photoexcited e⁻ and the quick transfer of photoexcited h⁺ across ZnBiDyO_4’s surface may be the causes of this increased photocurrent responsiveness. This phenomenon was facilitated by the potential difference existing between the valence bands of Dy₂EuSbO₇ and ZnBiDyO₄ within the DZHP composites. Consequently, the heightened photocurrent response in DZHP indicates that PCC was effectively separated during photodegradation, thereby extending its lifespan. These observations further elucidate the superior photocatalytic efficiency achieved by DZHP [98,99].

EIS serves as another critical characterization of the migration process of photogenerated e⁻ and h⁺ at the solid/electrolyte interface. The smaller the arc radius is, the higher the transmission efficiency of photogenerated e⁻ and h⁺ is. Figure 22b illustrates the corresponding EIS diagrams for the prepared DZHP, Dy₂EuSbO₇, and ZnBiDyO₄ photocatalysts. The figure reveals that the arc radius sequence was ZnBiDyO₄ > Dy₂EuSbO₇ > DZHP. These findings suggest that the prepared DZHP exhibited a more efficient separation of photogenerated e⁻ and h⁺, along with a faster interfacial charge transfer.

2.7. Analysis of Possible Photocatalytic Degradation Mechanisms

Figure 23 displays the ultraviolet photoelectron spectroscopy (UPS) spectra of Dy₂EuSbO₇ and ZnBiDyO₄. The ionization potentials of Dy₂EuSbO₇ and ZnBiDyO₄ were determined using UPS. The onset (Ei) and cutoff (Ecutoff) binding energies for the two semiconductor samples are presented in Figure 23. The measured values for Dy₂EuSbO₇ and ZnBiDyO₄ were 1.523 eV and 17.175 eV, respectively, while the values for ZnBiDyO₄ were 0.124 eV and 17.625 eV, respectively. By calculating the excitation energy (approximately 21.2 eV), the ionization potentials of Dy₂EuSbO₇ and ZnBiDyO₄ were determined to be 2.791 eV and 1.217 eV, respectively [100]. The results indicate that the CB potentials of Dy₂EuSbO₇ and ZnBiDyO₄ were 0.111 eV and −0.563 eV, respectively. These results further validate the mechanism model proposed in this study.

Figure 24 shows the possible PTD mechanism of CPS and DZHP under VSLID. It could be found that when the DZHP was irradiated with visible light, both Dy₂EuSbO₇ and ZnBiDyO₄ could absorb visible light and generated e⁻-h⁺ pairs internally. Due to the CB of the ZnBiDyO₄ redox potential position (−0.563 eV) than Dy₂EuSbO₇ (0.111 eV) being more negative, light-induced e⁻ on the CB of ZnBiDyO₄ could be transferred to the CB of Dy₂EuSbO₇. The redox potential position of VB of Dy₂EuSbO₇ (2.791 eV) was more correct than that of VB of ZnBiDyO₄ (1.217 eV), and light-induced h⁺ on Dy₂EuSbO₇ VB could be transferred to VB of ZnBiDyO₄. Therefore, the coupling of Dy₂EuSbO₇ and ZnBiDyO₄ to produce DZHP could effectively reduce the recombination rate of photogenerated e⁻ and photogenerated h⁺, thereby reducing the internal resistance, prolonging the lifetime of photogenerated e⁻ and photogenerated h⁺, and improving the interfacial charge transfer efficiency [101]. Therefore, more oxidized radicals, such as •OH or •O₂⁻, could be produced to improve the degradation efficiency of CPS. In addition, the CB potential of ZnBiDyO₄ was −0.563 eV, which was greater than the negative value of O₂/•O₂⁻ (−0.33 eV vs. NHE), indicated that e⁻ within the CB of ZnBiDyO₄ could absorb oxygen to produce O, thereby degrading CPS (as shown in path ① in Figure 24). At the same time, the VB potential of Dy₂EuSbO₇ was 2.791 eV, which was corrected for OH⁻/•OH (2.38 eV vs. NHE), indicated that the VB h⁺ of Dy₂EuSbO₇ could oxidize H₂O or OH⁻ to •OH to degrade CPS, as shown in path ②. Finally, the light-induced h⁺ in VB of ZnBiDyO₄ or Dy₂EuSbO₇ could directly oxidize CPS and degrade CPS due to their strong oxidizing capacity, which is shown as path ③. In summary, the excellent photocatalytic activity of DZHP for CPS degradation was mainly attributed to the efficient e⁻-h⁺ separation induced by DZHP.

LC-MS was used to identify the intermediate products produced during the CPS degradation process in order to investigate the degradation mechanism of CPS. The intermediate products which were obtained during the PTD of CPS were identified as C₇H₇Cl₃NO₃PS (m/z = 322), C₅H₃Cl₃NO₃PS (m/z = 294), C₅H₂Cl₃NO (m/z = 198), C₉H₁₁Cl₃NO₄P (m/z = 334), and C₇H₇Cl₃NO₄P (m/z = 306). Based on the detected intermediate products, the PTD pathway of CPS was proposed. Figure 25 shows the suggested PTD pathway scheme of CPS under VSLID with DZH as the photocatalyst. Figure 25 shows that oxidation and hydroxylation reactions occurred during the PTD of CPS. Ultimately, CPS was converted into CO₂ and water.

3. Experimental Section

3.1. Materials and Reagents

In this study, Dy₂O₃ (purity 99.99%), Eu₂O₃ (purity 99.99%), Sb₂O₅ (purity 99.99%), Bi₂O₃ (purity 99.99%), ZnO (purity 99.99%), Ethylenediaminetetraacetic acid (purity of 99.5%), Isopropyl alcohol (purity greater than 99.7%), P-benzoquinone (purity greater than 98.0%), anhydrous ethanol (purity greater than 99.5%), chlorpyrifos (purity greater than 99.0%), and Ultra-pure water (18.25 MU cm) were used. All chemicals were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China) or Aladdin Group Chemical Reagent Co., Ltd. (Shanghai, China) or Tianjin Bodi Chemical Co., Ltd. (Tianjin, China) and were of analytical grade or gas chromatography grade.

3.2. Preparation Method of Dy₂EuSbO₇

Dy₂EuSbO₇ was synthesized via the HTSSM.

(1) We mixed the Dy₂O₃ powder, Eu₂O₃ powder and Sb₂O₅ powder evenly at a molar ratio of Dy₂O₃:Eu₂O₃:Sb₂O₅ = 2:1:1, then added them into a ball mill and grind for 120 min to obtain the powder.

(2) We put the above powder into an oven and heat it at 180 °C for 180 min. After drying, we pressed the powder into a sheet and placed it in an alumina crucible. Then, the alumina crucible containing the powder was placed in a high-temperature sintering furnace (HTSF) and sintered at room temperature (RT) to 450 °C in the range of 9.5 °C/min and then held at 450 °C for 5 h. Then, the temperature was raised to 700 °C in the range of 8.5 °C/min and kept at 700 °C for 14 h. Then, the temperature was raised to 1050 °C in the range of 7.5 °C/min and kept at 1050 °C for 32 h. Finally, the temperature was lowered from 1050 °C to RT at a cooling rate of 6.3 °C/min, resulting in the first sintered tablet.

(3) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample-pressed sheet from the alumina crucible after the first sintering, and we put it into an agate mortar for crushing and grinding for 60 min. We put the ground powder into an alumina crucible, pressing it into a sheet, and then placed the alumina crucible containing the sample-pressed sheet into a HTSF for a second sintering. The temperature was raised from RT to 550 °C in the range of 9.6 °C/min and then held at 550 °C for 5 h. Then, the temperature was raised to 1000 °C in the range of 8.2 °C/min and kept at 1000 °C for 16 h. Then, the temperature was raised to 1150 °C in the range of 7.2 °C/min and kept at 1150 °C for 26 h. Finally, the temperature was lowered from 1150 °C to RT at a cooling rate of 6.1 °C/min to obtain the second sintered tablet.

(4) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed from the second sintering in the alumina crucible, and we put it into an agate mortar for crushing and grinding for 80 min. We put the ground powder into an alumina crucible, and then we pressed it into a sheet to obtain a sample-pressed sheet. Then, the alumina crucible containing the sample-pressed sheet was placed in a HTSF for a third sintering. The temperature was raised to 700 °C at RT in the range of 8.8 °C/min and held at 700 °C for 7 h. Then, the temperature was raised to 1100 °C in the range of 8.1 °C/min and kept at 1100 °C for 24 h. Then, the temperature was raised to 1300 °C in the range of 6.8 °C/min and held at 1300 °C for 42 h. Finally, the temperature was lowered from 1300 °C to RT at a cooling rate of 6.3 °C/min to obtain the third sintered tablet.

(5) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed from the alumina crucible after the third sintering, and we put it into an agate mortar for crushing and grinding for 80 min. Finally, the Dy₂EuSbO₇ powder catalyst was obtained.

3.3. Preparation Method of ZnBiDyO₄

ZnBiDyO₄ was synthesized via the HTSSM.

(1) We mixed the Bi₂O₃ powder, Dy₂O₃ powder and ZnO powder evenly at a molar ratio of Bi₂O₃:Dy₂O₃:ZnO = 1.15:1:2, then added them into a ball mill and grind for 120 min to obtain the powder.

(2) We put the above powder into an oven and heated it at 180 °C for 180 min. After drying, we pressed the powder into a sheet and placed it in an alumina crucible. Then, the alumina crucible containing the powder was placed in a HTSF and sintered at RT to 450 °C in the range of 9.5 °C/min and then held at 450 °C for 5 h. Then, the temperature was raised to 700 °C in the range of 8.5 °C/min and kept at 700 °C for 14 h. Then, the temperature was raised to 1000 °C in the range of 7.5 °C/min and kept at 1000 °C for 32 h. Finally, the temperature was lowered from 1000 °C to RT at a cooling rate of 6.3 °C/min, resulting in the first sintered tablet.

(3) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample-pressed sheet from the alumina crucible after the first sintering, and we put it into an agate mortar for crushing and grinding for 60 min. We put the ground powder into an alumina crucible, pressing it into a sheet, and then place the alumina crucible containing the sample-pressed sheet into a HTSF for a second sintering. The temperature was raised from RT to 550 °C in the range of 9.6 °C/min and then held at 550 °C for 5 h. Then, the temperature was raised to 800 °C in the range of 8.2 °C/min and kept at 800 °C for 16 h. Then, the temperature was raised to 1100 °C in the range of 7.2 °C/min and kept at 1100 °C for 26 h. Finally, the temperature was lowered from 1100 °C to RT at a cooling rate of 6.1 °C/min to obtain the second sintered tablet.

(4) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed from the second sintering in the alumina crucible, and we put it into an agate mortar for crushing and grinding for 80 min. We put the ground powder into an alumina crucible and then pressed it into a sheet to obtain a sample-pressed sheet. Then, the alumina crucible containing the sample-pressed sheet was placed in a HTSF for a third sintering. The temperature was raised to 600 °C at RT in the range of 8.8 °C/min and held at 600 °C for 7 h. Then, the temperature was raised to 1000 °C in the range of 8.1 °C/min and kept at 1000 °C for 24 h. Then, the temperature was raised to 1250 °C in the range of 6.8 °C/min and held at 1250 °C for 42 h. Finally, the temperature was lowered from 1250 °C to RT at a cooling rate of 6.3 °C/min to obtain the third sintered tablet.

(5) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed from the alumina crucible after the third sintering, and we put it into an agate mortar for crushing and grinding for 80 min. Finally, the Zn₂BiDyO₄ powder catalyst was obtained.

3.4. Synthesis of N-Doped TiO₂

Using tetrabutyl titanate as a precursor and ethanol as a solvent, nitrogen-doped titanium dioxide (N-doped TiO₂) catalysts were successfully prepared by the sol-gel method. The specific preparation process is as follows:

Firstly, we thoroughly mixed 17 mL of tetrabutyl titanate with 40 mL of anhydrous ethanol to form solution A. We mixed 40 mL anhydrous ethanol, 10 mL glacial acetic acid, and 5 mL double-distilled water evenly to obtain solution B. Subsequently, under strong magnetic stirring, solution A was added dropwise to solution B to form a transparent and uniform colloidal suspension.

Subsequently, under the condition of magnetic stirring, ammonia water with a N/Ti ratio precisely controlled at 8 mol% was added to the transparent colloid suspension and stirred continuously for 1 h. After this step, the colloidal suspension was aged for 2 days to form a stable dry gel.

After that, the obtained dry gel was ground into powder and calcined at 500 °C for 2 h. After the calcination was completed, the calcined products were further ground in an agate mortar and finely screened through a vibrating screen to obtain N-doped TiO₂ (NT) powder with uniform particle size and high purity.

3.5. Synthesis of Dy₂EuSbO₇/ZnBiDyO₄ Heterojunction Photocatalysts

DZHP was synthesized via the HTSSM.

(1) We mixed the Dy₂EuSbO₇ powder and the ZnBiDyO₄ powder evenly at a molar ratio of 1:1, then added them into a ball mill and grind for 120 min to obtain the powder.

(2) We put the above powder into an oven and heat it at 180 °C for 180 min. After drying, we pressed the powder into a sheet and placed it in an alumina crucible. Then, the alumina crucible containing the powder was placed in a HTSF and sintered at RT to 350 °C in the range of 9.5 °C/min and then held at 350 °C for 5 h. Then, the temperature was raised to 700 °C in the range of 8.5 °C/min and kept at 700 °C for 14 h. Then, the temperature was raised to 900 °C in the range of 7.5 °C/min and kept at 900 °C for 32 h. Finally, the temperature was lowered from 900 °C to RT at a cooling rate of 6.3 °C/min, resulting in the first sintered tablet.

(3) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed sheet from the alumina crucible after the first sintering and put it into an agate mortar for crushing and grinding for 60 min. We put the ground powder into an alumina crucible, pressing it into a sheet, and then placed the alumina crucible containing the sample pressed sheet into a HTSF for a second sintering. The temperature was raised from RT to 350 °C in the range of 9.6 °C/min and then held at 350 °C for 5 h. Then, the temperature was raised to 800 °C in the range of 8.2 °C/min and kept at 800 °C for 16 h. Then, the temperature was raised to 1000 °C in the range of 7.2 °C/min and kept at 1000 °C for 26 h. Finally, the temperature was lowered from 1000 °C to RT at a cooling rate of 6.1 °C/min to obtain the second sintered tablet.

(4) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample-pressed from the second sintering in the alumina crucible and put it into an agate mortar for crushing and grinding for 80 min. We put the ground powder into an alumina crucible, and then we pressed it into a sheet to obtain a sample-pressed sheet. Then, the alumina crucible containing the sample-pressed sheet was placed in a HTSF for a third sintering. The temperature was raised to 500 °C at RT in the range of 8.8 °C/min and held at 500 °C for 7 h. Then, the temperature was raised to 900 °C in the range of 8.1 °C/min and kept at 900 °C for 24 h. Then, the temperature was raised to 1100 °C in the range of 6.8 °C/min and held at 1100 °C for 42 h. Finally, the temperature was lowered from 1100 °C to RT at a cooling rate of 6.3 °C/min to obtain the third sintered tablet.

(5) After the chamber of the HTSF was cooled to RT, the alumina crucible was removed from the chamber of the HTSF. Then, we removed the sample pressed from the alumina crucible after the third sintering, and we put it into an agate mortar for crushing and grinding for 80 min. Finally, the DZHP material was obtained.

3.6. Characterizations

We conducted an in-depth analysis of the crystal structures of Dy₂EuSbO₇ and ZnBiDyO₄ using X-ray diffractometer (XRD, Shimadzu, XRD-6000, Cu Kα radiation, λ = 1.54184 Å, sampling pitch of 0.02°, preset time of 0.3 s step−1, Kyoto, Japan). In order to further reveal the conformational and microstructural characteristics of DZHP, we characterized it using a transmission electron microscope (TEM, Talos F200X G2, Thermo Fisher Scientific, Waltham, MA, USA) and determined its constituent elements using energy dispersive spectroscopy (EDS). In order to accurately obtain Dy₂EuSbO₇, we tested the band width information of ZnBiDyO₄ and DZHP using an UV-Vis spectrophotometer (UV-Vis DRS, Shimadzu, UV-3600, Kyoto, Japan). To clarify the functional groups and chemical bond types present in DZHP, we utilized a Fourier infrared spectrometer (FTIR, WQF-530A, Beifen-Ruili Analytical Instrument (Group) Co., Ltd., Beijing, China) to obtain the FTIR spectral data of Dy₂EuSbO₇ and ZnBiDyO₄. In addition, to reveal DZHP, we used a Raman spectrometer (INVIA0919-06, RENISHAW plx, Wotton-under-Edge, Gloucestershire GL 12 8JR, UK) to test the interaction relationships between various chemical bonds in Dy₂EuSbO₇ and ZnBiDyO₄. In order to have a comprehensive understanding of DZHP, we conducted a detailed analysis of the surface chemical composition and valence state information of Dy₂EuSbO₇ and ZnBiDyO₄ using an X-ray photoelectron spectrograph (XPS, PHI 5000 VersaProbe, UlVAC-PHI, Maoqi city, Japan) with an Al-kα X-ray source. To determine the ionization potentials of Dy₂EuSbO₇ and ZnBiDyO₄, we conducted relevant measurements using ultraviolet photo-electron spectroscopy (UPS, Escalab 250 xi, Thermo Fisher Scientific, Waltham, MA, USA). The related information about the free radicals in the samples was detected using an electron paramagnetic resonance spectrometer (EPR, A300, Bruker Corporation, Karlsruhe, Germany). The characteristics of PL and TRPL were analyzed using a fluorescence spectrophotometer (FLS980, Edinburgh Instruments Ltd., Edinburgh, UK).

3.7. Photoelectrochemical Experiments

The electrochemical impedance spectroscopy (EIS) experiment was conducted at the CHI660D electrochemical station (Chenhua Instruments Co., Shanghai, China). The experiment used a standard three-electrode system, with a prepared catalyst for the working electrode, a platinum plate for the counter electrode, and an Ag/AgCl electrode for the reference electrode. A 0.5 mol/L Na₂SO₄ aqueous solution was selected as the electrolyte for the experiment, and a 500 W Xe lamp equipped with a UV cut-off filter was used as the visible light source. When preparing the working electrode, the sample was mixed with chitosan and dissolved in dimethylformamide. After ultrasonic treatment, a uniform suspension was obtained, which was dropped onto ITO conductive glass and dried at a constant temperature of 80 °C for 10 min.

3.8. Experimental Setup and Procedure

This experiment was conducted in the photocatalytic reactor (XPA-7, Xujiang Electromechanical Plant, Nanjing, China). The experiment was conducted at a constant temperature of 20 °C, and the temperature was controlled through a circulating cooling water system to avoid temperature changes interfering with the results. We used a 500 W xenon lamp equipped with a 420 nm cut-off filter to simulate sunlight, ensuring the stability and accuracy of the light source. We used 12 quartz tubes of the same specification as reaction vessels, and each tube contained 40 mL of reaction solution, totaling 480 mL. Dy₂EuSbO₇ ZnBiDyO₄, and DZHP were used as photocatalysts with a unified dosage of 0.75 g/L. We introduced CPS with a concentration of 0.025 mmol/L to simulate actual pesticide wastewater treatment conditions. We regularly extracted suspension samples and filtered them with a 0.22 μm PES membrane to remove catalyst particles. We analyzed organic pollutants in the filtered solution using Agilent 200 high-performance liquid chromatography (Santa Clara, CA, USA). The mobile phase was a mixed solution of 50% CH3CN and 50% distilled deionized water. Before the photodegradation reaction, magnetic stirring was performed in the dark for 45 min to allow the photocatalyst to CPS and atmospheric oxygen reach adsorption/desorption equilibrium. Then, under visible light irradiation, we stirred the suspension at a speed of 500 rpm.

This article used a TOC analyzer (TOC-5000 A, Shimadzu Corporation, Kyoto, Japan) to measure CPS mineralization experimental data. We used potassium phthalate or anhydrous sodium carbonate as standard reagents to detect the concentration of TOC during PTD. We prepared standard solutions with multiple concentration gradients (0–100 mg/L) for calibrating the TOC analyzer. Six samples of 45 mL reaction solution were used to measure the TOC concentrations in each experiment to understand the changes in TOC concentration during PTD.

This study used advanced liquid chromatography–mass spectrometry (LC-MS, Thermo Quest LCQ Duo, Thermo Fisher Scientific Corporation, Waltham, MA, USA. Beta Basic-C18 HPLC column: 150 × 2.1 mm, ID of 5 μm, Thermo Fisher Scientific Corporation, Waltham, MA, USA) to analyze CPS and its degradation products. Here, the 20 μL solution obtained from the photocatalytic reaction was automatically injected into the system, with a mobile phase of 60% methanol and 40% ultrapure water, and a flow rate of 0.2 mL/min. The mass spectrometry analysis used the electric spray ionization interface. The capillary temperature, voltage, and spray voltage were set at 27 °C, 19.00 V, and 5000 V, respectively, and the sheath gas flow rate remained constant. Obtaining spectra in negative ion scanning mode, the m/z range was from 50 to 600.

The incident photon flux Io measured by a radiometer (Model FZ-A, Photoelectric Instrument Factory Beijing Normal University, Beijing, China) was determined to be 4.76 × 10⁻⁶ Einstein L⁻¹ s⁻¹ under VSLID.

4. Conclusions

For the first time, Dy₂EuSbO₇ and DZHP with high photocatalytic activity were synthesized using a HTSSM. The photophysical properties and photocatalytic performance were characterized using SEM-EDS, XRD, UV-vis, FTIR, Raman, and XPS techniques. The results revealed that Dy₂EuSbO₇ was a single phase with a pyrochlore structure and a cubic crystal system, categorized under the space group Fd3m. The lattice parameter and the band gap of Dy₂EuSbO₇ were a = 10.440423 Å and 2.680 eV, respectively. DZHP demonstrated high efficacy as a photocatalyst for the removal of CPS from wastewater, achieving CPS and TOC removal rates of 99.81% and 97.32%, respectively, after 156 min of visible light irradiation (VSLID). The CPS removal efficiency with DZHP as the photocatalyst was 1.12 times, 1.21 times, and 2.96 times greater than that with Dy₂EuSbO₇, ZnBiDyO₄, and N-doped TiO₂ catalysts, respectively, after 156 min of VSLID. Consequently, it could be inferred that DZHP is a highly effective photocatalyst for treating CPS-contaminated wastewater. Finally, potential photodegradation pathways for CPS were proposed.