Metronidazole Degradation via Visible Light-Driven Z-Scheme BiTmDySbO₇/BiEuO₃ Heterojunction Photocatalyst

Abstract

This study presented the successful synthesis of a visible light responsive Z-scheme BiTmDySbO₇/BiEuO₃ heterojunction photocatalyst (BBHP) via the hydrothermal method, exhibiting outstanding removal efficiency for degrading the metronidazole (MNZ) in wastewater. The BBHP exhibited exceptional photocatalytic activity during the degradation process of the MNZ which was a widely detected pharmaceutical pollutant in aquatic environments. The key to the high photocatalytic activity of the BBHP was the formation of a Z-scheme photogenerated carrier transport channel which existed between BiTmDySbO₇ and BiEuO₃ within the heterojunction structure. This innovative structural design was experimentally confirmed for enhancing the separation efficiency of the photogenerated charge carriers significantly, thereby, the efficient photocatalytic activity of the BBHP was promoted. After visible light irradiation for 130 min, the BBHP achieved a removal efficiency of 99.56% for degrading MNZ and a mineralization rate of 98.11% for removing the total organic carbon (TOC) concentration. In contrast to a single photocatalyst, the removal rate of the MNZ by using the BBHP was 1.14 times that by using the BiEuO₃, 1.26 times that by using the BiTmDySbO₇, and 2.65 times that by using the nitrogen-doped TiO₂ (N-T) under visible light irradiation. The mineralization rate for removing the TOC concentration during the degradation process of the MNZ by using the BBHP was 1.17 times that by using the BiEuO₃, 1.29 times that by using the BiTmDySbO₇, and 2.86 times that by using the N-T under visible light irradiation. The photocatalytic degradation process of the MNZ by using the BBHP followed first-order kinetics model, concurrently, a dynamics rate constant of 0.0345 min⁻¹ was obtained. Furthermore, the BBHP demonstrated excellent stability and durability in accordance with multiple cyclic degradation experiments. According to the capturing radicals experiments and the electron paramagnetic resonance test experiments, it was determined that the hydroxyl radicals (•OH) and the superoxide anions (•O₂⁻) played key role during the photocatalytic degradation process of the MNZ by using the BBHP under visible light irradiation. Finally, the intermediate products that were produced during the degradation process of the MNZ were analyzed by using liquid chromatography-mass spectrometer, as a result, a potential degradation pathway for the MNZ was proposed. Overall, this study could provide valuable references for future research on composite photocatalysts and effectively maintain the safety and sustainable utilization of water resource.

1. Introduction

The rapid expansion of industries such as chemical engineering, animal husbandry, and medical care has unfortunately led to a significant increase in antibiotic pollution, which is derived from water systems [1,2,3,4]. These pollutants posed a serious threat to human health and the ecological systems, primarily because of their chemical stability, diverse origins, and collective tendency within the food chain [5,6]. Metronidazole (MNZ), a nitroimidazole antibiotic that was widely used in the treatment of infections caused by anaerobic bacteria and protozoa, exemplified this challenge due to its potent bactericidal properties [7,8]. Despite its therapeutic benefits, MNZ was structurally stable, highly water-soluble, and difficult to be degraded by using traditional methods, often leading to its persistence in wastewater which was discharged from pharmaceutical facilities [9]. Even at low concentrations, MNZ has been linked to potential human carcinogenicity and could contribute to the rise in the resistance genes that derive from antibiotics in organisms [10]. Therefore, effective methods for degrading MNZ in wastewater are urgently needed to protect water resources and ecological systems.

Traditional approaches for treating wastewater containing MNZ, such as the biological degradation approach, the chemical oxidation approach, and the physical adsorption approach, often cause some drawbacks [11]. The aerobic biological degradation approach or the anaerobic biological degradation approach often suffers from slow reaction rates and could be greatly affected by the environment [12,13]. While chemical oxidation approach could be effective and typically involved high costs for chemical reagents, thus chemical oxidation approach might cause secondary pollution [14,15]. The physical adsorption approach was mainly used in the pretreatment stage, and it also suffered from inconsistent treatment outcomes [16]. These limitations highlighted a pressing need for the development of more solid and valid degradation methods that could overcome the shortcomings of current technologies.

The emergence of the high-level semiconductor photocatalytic technology offered a compelling pathway for sustainable water purification. This photocatalytic technology utilized light energy for driving chemical reactions, and the principle was primarily based on solid band theory [17,18]. When a semiconductor photocatalyst (SP) such as titanium dioxide (TiO₂) was exposed to light irradiation with incident light energy higher than the band gap width of the SP, electrons would transit from the valence band (VB) of the SP to the conduction band (CB) of the SP. Accordingly, the photoinduced electrons stayed in the CB of the SP; concurrently, the holes would be left in the VB of the SP [19,20]. These photogenerated electrons and photoinduced holes then separated and migrated within or across the surface of the SP [21]. The electrons, acting as reducing agents, could convert oxygen into superoxide radicals [22,23]. Conversely, the holes that exhibited oxidizing properties could react with water or hydroxyl to produce highly reactive hydroxyl radicals. Ultimately, the generated radicals reacted with pollutants, breaking them down into innocuous chemical compounds such as carbon dioxide (CO₂) and water (H₂O) [24].

However, there was a fundamental challenge with single-component SP in balancing the utilization of light energy and the light stability. For instance, TiO₂, which obtained widespread use, possessed a wide indirect bandgap value of approximately 3.2 eV [25], which significantly limited its effective utilization of the solar energy, resulting in a photon quantum efficiency of less than 4% [26,27,28]. Consequently, the practical applications often necessitated supplementary strong ultraviolet light sources for boosting their photocatalytic performance. In order to enhance solar utilization and improve photostability, developing efficient visible light-responsive photocatalysts was imperative for real-world deployment.

Recent research has found that the A₂B₂O₇ compounds with perovskite structures exhibited excellent photocatalytic activity under visible light irradiation (VLI) [29,30,31]. For instance, Zhang et al. demonstrated that Sm₂Ti₂O₇, which owns the pyrochlore structure, displayed excellent photocatalytic activity [32]. Similarly, Naceur et al. achieved a removal rate of 90.16% for degrading rhodamine B within 60 min by using Bi_1.56Sb_1.48Co_0.96O₇, which was synthesized via the solid-state reaction method [33]. On the basis of previous findings, we aimed to select more advantageous elements for synthesizing the A₂B₂O₇ compounds. Murcia–López et al. found that the Bi-doped TiO₂ catalyst possessed a higher efficiency for degrading phenol than the pure phase TiO₂ catalyst under the condition of the same illumination time [34]. M. Mezyen et al. doped 5% Dy in SnO₂, which resulted in the removal rate of 90.99% for degrading methylene blue within three hours under sunlight irradiation [35], consequently, Dy-doped SnO₂ showed higher photocatalytic activity for degrading methylene blue compared with pure phase SnO₂. Rao et al. observed that Tm-doped TiO₂ exhibited improved degradation efficiency of acetaldehyde or o-xylene compared with the original pure phase TiO₂ [36]. Gandelman et al. observed that Sb-doped TiO₂ showed significant improvement in the photocatalytic degradation efficiency of acetaminophen compared to pure phase TiO₂ [37]. The above-mentioned evidence led us to propose the BiTmDySbO₇ photocatalyst as a novel candidate for a visible light-responsive photocatalyst. Ran et al. successfully prepared Eu doped TiO₂. Compared with the TiO₂, Eu doped TiO₂ exhibited higher separation efficiency of the photogenerated electrons and the photogenerated holes [38].

In order to further enhance the photocatalytic activity of the visible light-responsive catalysts, the strategy of constructing the heterojunction photocatalysts has become a widely adopted and effective approach [39,40,41]. The heterojunction photocatalysts were formed by combining two or more photocatalysts with different band structures to create a heterostructure photocatalyst. Upon light irradiation on the surface of the heterojunction photocatalyst, the differences in the CB positions of different photocatalysts and the VB positions of different photocatalysts led to the redistribution of electrons and holes at the interface, aiming to improve the photocatalytic efficiency of different photocatalysts [42,43,44]. For example, Wang et al. reported that the degradation rate of metronidazole by using the Zn₃In₂S₆/Bi₂MoO₆ heterojunction photocatalyst was 2.17 times that by using the single Zn₃In₂S₆ photocatalyst [45]. Wang et al. constructed a ZnSnO₃/CuBi₂O₄ heterojunction photocatalyst and found that the degradation rate of tetracycline by using ZnSnO₃/CuBi₂O₄ heterojunction photocatalyst was 3.1 times higher than that by using the single CuBi₂O₄ [46].

Therefore, based on the appropriate band structure, we successfully prepared a novel Z-Scheme BiTmDySbO₇/BiEuO₃ heterojunction photocatalyst (BBHP) via the solvothermal method. The BBHP was designed for the efficient degradation of MNZ in wastewater. The BBHP exhibited excellent stability and sustainability; concurrently, the BBHP reduced long-term operational costs. Although the initial preparation cost of the BBHP was high, the BBHP demonstrated strong stability and reusability, thereby, the BBHP lowered the overall cost in light of a long-term application perspective. Recycling experiments for degrading the MNZ indicated that after five consecutive degradation cycles, the BBHP maintained a removal efficiency of 94.71% for degrading the MNZ and a total organic carbon mineralization efficiency of 93.39%, simultaneously, the photocatalytic activity of the BBHP did not decay significantly. Compared with adsorption materials, which required frequent replacement or oxidation technologies, which consumed large amounts of chemical reagents, the BBHP operated without the need for additional reagents; meanwhile, the BBHP was driven solely under the condition of visible light irradiation, as a result, using the BBHP could save electrical energy and thermal energy. Furthermore, the BBHP was reusable and could offer better cost-effectiveness for long-term use. Additionally, the BBHP resolved the performance limitations of traditional treatment technologies; correspondingly, the BBHP also resolved the performance limitations of the single-component catalyst. Ultimately, the BBHP was beneficial for maintaining the sustainable development of the ecological environment. We confirmed the strong potential of BBHP as a visible light response catalyst by comprehensively analyzing the crystal structure, morphological characteristics, and optical properties of BBHP. Our investigation thoroughly explored BBHP’s photocatalytic activity, stability, and the underlying mechanism for degrading MNZ. This work highlighted a promising pathway toward developing highly effective and durable solutions for complex pollutant removal.

2. Materials and Methods

2.1. Materials and Reagents

The following chemicals were purchased from Aladdin Reagent Co., Ltd. in Shanghai, China: bismuth (III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O, purity 99.99%), europium (III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O, purity 99.99%), thulium (III) nitrate hexahydrate (Tm(NO₃)₃·6H₂O, purity 99.99%), dysprosium (III) nitrate hexahydrate (Dy(NO₃)₃·6H₂O, purity 99.99%), antimony (V) pentachloride (SbCl₅, purity 99.99%), ethanol (C₂H₅OH, purity ≥ 99.5%), ethylenediaminetetraacetic acid (EDTA, C₁₀H₁₆N₂O₈, purity ≥ 99.5%), isopropanol (IPA, C₃H₈O, purity ≥ 99.7%), p-benzoquinone (BQ, C₆H₄O₂, purity ≥ 98.0%), metronidazole (C₆H₉N₃O₃, purity ≥ 99.8%), ultrapure water (18.25 MU-cm), Polyvinylpyrrolidone (PVP, (C₆H₆NO)_n, purity ≥ 99.9%).

2.2. Preparation of BiEuO₃

BiEuO₃ photocatalyst was synthesized by the hydrothermal method. Bi(NO₃)₃·5H₂O and Eu(NO₃)₃·6H₂O were dissolved in deionized water with a stoichiometric ratio (Bi:Eu = 1:1). Next, the above mixed solution was stirred for 30 min to ensure complete dissolution. Subsequently, 2 mol/L sodium hydroxide solution was slowly added to adjust the pH value to 12. The resulting precursor solution was filled to 70% of the volume of the hydrothermal reactor, which was then well sealed. The sealed reactor was transferred to an oven and maintained at 180 °C for 1380 min. Following the reaction, the reactor was allowed to cool naturally up to room temperature. The precipitate was then collected and subjected to multiple washing cycles with deionized water and ethanol to remove any impurities. Finally, the thoroughly washed product was dried at 70 °C, followed by calcination in a muffle furnace at 600 °C for 4 h to enhance its crystallinity.

2.3. Preparation of BiTmDySbO₇

BiTmDySbO₇ photocatalyst was synthesized by the hydrothermal method. The precursor solution for the experiment was prepared using Bi(NO₃)₃·5H₂O, Tm(NO₃)₃·6H₂O, Dy(NO₃)₃·6H₂O, SbCl₅ and sodium hydroxide solution as raw materials. Subsequently, the reaction vessel was transferred to an oven and maintained at 200 °C for 1450 min. After the reaction, the product was collected, thoroughly washed, and dried at 70 °C. Ultimately, the dried product underwent calcination in a muffle furnace at 650 °C for 4 h to enhance its crystallinity.

2.4. Preparation of Nitrogen-Doped TiO₂

The complete production process of the nitrogen-doped TiO₂ was provided in the Supplementary Materials (Section S1).

2.5. Preparation of BiTmDySbO₇/BiEuO₃ Heterojunction Photocatalyst

The BiTmDySbO₇/BiEuO₃ heterojunction photocatalyst was prepared using a solvothermal method. Firstly, the prepared BiTmDySbO₇ powder and BiEuO₃ powder were mixed in a mass ratio of 1:1. Subsequently, ultrasonic dispersion was employed to treat the mixed powder and 0.5 g of polyvinylpyrrolidone in 30 mL of ethylene glycol for a duration of 30 min. The resulting well-dispersed mixed solution was shifted to a 50 mL high-pressure reaction vessel, which was sealed, and placed in an oven to react at 170 °C for 8 h. Following the reaction, the vessel was allowed to cool up to room temperature, and the product was centrifuged (8000 rpm, 10 min). Subsequently, the precipitate was alternately washed with deionized water and ethanol 3 times. Eventually, the precipitate was dried at 60 °C for 6 h to obtain the BiTmDySbO₇/BiEuO₃ heterojunction catalyst.

2.6. Feature Description

The specific feature description content was included in the Supplementary Materials (Section S2).

2.7. Photoelectrochemical Experiment

Electrochemical impedance spectroscopy (EIS) tests were conducted using a standard three-electrode system. It was composed of a platinum sheet counter electrode, an Ag/AgCl reference electrode, and a prepared glassy carbon working electrode.

Prior to the test, a suspension of the photocatalyst was formed by dispersing 2 mg of the synthesized photocatalyst in a small sample tube, which contained 200 μL of ethanol and 5 μL of Nafion reagent. The mixture underwent ultrasonic dispersion for 90 min to ensure thorough and homogeneous mixing. In the meantime, a 500 mL mixed solution, which contained 0.05 mol of potassium ferricyanide, 0.1 mol of potassium ferrocyanide, and 0.1 mol of potassium chloride, was prepared as the electrolyte for the test. The glassy carbon working electrode was carefully polished with polishing powder and deionized water to ensure optimal conductivity. Subsequently, 25 μL of the prepared catalyst suspension was precisely pipetted onto the conductive surface of the cleaned glassy carbon electrode and allowed to air dry for 25 min. The EIS measurements were then performed to quantify the interfacial charge transfer resistance of the photocatalyst. This analysis provided further insights into the electrochemical performance of the BBHP.

2.8. Experimental Setup and Procedure

The photocatalyst solution of BiTmDySbO₇, BiEuO₃, or BBHP was prepared at a concentration of 0.4 g/L and added to 480 mL of metronidazole (MNZ) solution for testing the adsorption ability and photodegradation activities. All photodegradation experiments were conducted under VLI and provided by a 500-watt xenon lamp, which was equipped with a 420-nanometer cutoff filter (Changtuo Co., Ltd., Beijing, China).

During the entire experiment, 5 mL samples were collected from the suspension every 15 min. The samples were centrifuged at 7000 rpm for 15 min to separate the catalyst. The concentration of MNZ in the separated liquid was detected by using a liquid chromatography-mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA, Thermo Quest LCQ Duo), which was equipped with a Beta Basic-C18 liquid chromatography column. A 10 μL aliquot of the supernatant was analyzed chromatographically at a flow rate of 0.5 mL/min for MNZ quantification.

In order to assess the mineralization efficiency during the degradation process of MNZ, total organic carbon (TOC) analysis was performed by using a Shimadzu Corporation (Kyoto, Japan) TOC analyzer. Potassium phthalate (KHC₈H₄O₄) aqueous solution was used as the standard reference material for calibrating the TOC measurement. A calibration curve with a carbon concentration range of 0 mg/L to 100 mg/L was established by using potassium phthalate to ensure the accuracy and reliability of the TOC measurement.

For the purpose of characterizing the intermediate products of MNZ during the photocatalytic reaction process, a liquid chromatography mass spectrometry (LC-MS) was utilized. Following the photocatalytic reaction, 25 μL of the reaction solution was infused into the LC-MS system. A Beta Basic-C18 liquid chromatography column was used. Simultaneously, the mobile phase was methanol and water (60:40, v/v). The mass spectrometer scanning range was set from 50 m/z to 500 m/z. The wideband activation function was enabled to induce fragmentation of the intermediate products, thereby generating more structural information, which was crucial for the accurate identification of their chemical structures.

3. Results and Discussion

3.1. Characterization of Photocatalysts

3.1.1. X-Ray Diffraction (XRD) Pattern Analysis

Figure 1 shows the XRD patterns of BBHP, BiTmDySbO₇, and BiEuO₃. According to Figure 1, BiTmDySbO₇ exhibited distinct diffraction peak at 29.27°, 33.93°, 48.72°, 57.81°, 60.79°, 71.19°, 79.07°, 81.45° or 91.26°, which corresponded to the crystal plane of (222), (400), (440), (622), (444), (800), (662), (840) or (844), respectively; meanwhile, BiEuO₃ exhibited distinct diffraction peak at 28.18°, 32.16°, 46.71°, 55.53°, 58.41°, 68.44°, 75.38°, 77.88°, 86.95° or 93.76°, which corresponded to the crystal plane of (111), (200), (220), (311), (222), (400), (311), (420), (422) or (511). The diffraction peaks of BiTmDySbO₇ and the diffraction peaks of BiEuO₃ were all present in the diffraction peaks of BBHP, confirming that BiTmDySbO₇ and BiEuO₃ had successfully combined to form a heterojunction structure.

The weak impurity peak, which appeared at 30° in the XRD pattern of the BiTmDySbO₇ derived from the sample, corresponds to the carrier glass sheet in Figure 1 and Figure 2a. The main raw material for the glass sheet was SiO₂; thus, the weak impurity peak that appeared at 30° in the XRD pattern of the BiTmDySbO₇ was the SiO₂ peak. The Rietveld analysis of the XRD data for BiTmDySbO₇ or BiEuO₃ was conducted by using Materials Studio 2020 software. Figure 2a shows the Rietveld refinement results of BiTmDySbO₇ according to the XRD data of BiTmDySbO₇. Figure 2b displays the Rietveld refinement results of BiEuO₃ according to the XRD data of BiEuO₃. As shown in Figure 2a,b, the Rp value for BiTmDySbO₇ or BiEuO₃ was 7.91% or 4.87%, respectively, indicating that there was excellent agreement between the theoretical data and experimental data. The above results further confirmed that the BiTmDySbO₇ or the BiEuO₃ was a pure single-phase photocatalyst. The Rietveld refinement results of BiTmDySbO₇ and BiEuO₃ confirmed that BiTmDySbO₇ possessed a cubic perovskite structure and BiEuO₃ owned a face-centered cubic fluorite structure. Additionally, the space group of BiTmDySbO₃ was Fd3m, and BiTmDySbO₃ owned a lattice constant of 10.557 Å, concurrently, the lattice constant of BiEuO₃ was 5.489 Å, and the space group of BiEuO₃ was Fd3m. Atomic structure models, which are constructed based on these space groups, lattice constants, and refined atomic coordinates, are presented in Figure 2c,d for BiTmDySbO₇ and BiEuO₃. Table S1 provides the specific atomic coordinates and structural parameters of BiTmDySbO₇. Table S2 [47] provides the specific atomic coordinates and structural parameters of BiEuO₃.

Further examination of the BiTmDySbO₇ crystal structure revealed the presence of the distorted MO₆ octahedra (M = Dy³⁺ and Sb⁵⁺) [48,49]. This was due to the existence of two different A–O bond lengths (A = Bi³⁺ and Tm³⁺). One was the longer A–O (1) bond with a bond length of 2.286 Å, and the other was the shorter A–O (2) bond with a bond length of 1.988 Å. Additionally, the M–O–M (M = Dy³⁺ and Sb⁵⁺) bond angle within the BiTmDySbO₇ crystal structure was measured to be 139.624°. The unique structure of BiTmDySbO₇ resulted in a lower recombination rate of photogenerated charge carriers. Thus, the photocatalytic activity of BiTmDySbO₇ was enhanced [50,51].

3.1.2. Fourier Transform Infrared Spectroscopy Analysis

Fourier transform infrared (FTIR) spectra were trialed to define the functional groups and chemical bonds that were present in BiTmDySbO₇, BiEuO₃, and BBHP. The FTIR spectra of BBHP, BiTmDySbO₇, and BiEuO₃ are shown in Figure 3. The analysis result, as shown in Figure 3, revealed the presence of characteristic vibrational modes corresponding to Bi–O, Dy–O, Tm–O, Eu–O, Sb–O, or Sb–O–Sb bond.

Specifically, bending vibrations which were attributed to the Bi–O bond were observed at 439 cm⁻¹ and 485 cm⁻¹ [52,53]. Stretching vibration of Dy–O, Eu–O, or Tm–O occurred at 516 cm⁻¹, 595 cm⁻¹, or 600 cm⁻¹ [54,55,56]. In addition, the bending vibration of Sb–O or Sb–O–Sb occurred at 661 cm⁻¹ or 745 cm⁻¹ [57,58]. The broad peak at 3463 cm⁻¹ corresponded to the stretching vibration of the O–H group, which derived from chemically adsorbed water molecules [59]. In the FTIR spectrum of the BiTmDySbO₇, BiEuO₃, or BBHP, differences were observed in OH- groups. The difference was primarily attributed to the varying hydrophilicity of the BBHP, BiTmDySbO₇, or BiEuO₃ and the distinct water adsorption capacities of the BBHP, BiTmDySbO₇, or BiEuO₃. The bending oscillation of the surface H–O–H group was related to the absorption peak at 1606 cm⁻¹ [60]. The spectral band around 1361 cm⁻¹ was due to the vibration of the C–H bond [61]. Additionally, variations which appeared in the C–H bonds were also noted in the FTIR spectrum of the BiTmDySbO₇, BiEuO₃, or BBHP. However, the absence of the C–H bonds was not a key characteristic for distinguishing the BiTmDySbO₇, BiEuO₃, and BBHP. The slight difference that derived from the C–H bond was related to the organic adsorbate, which appeared on the sample surface of the BiTmDySbO₇, BiEuO₃, or BBHP.

3.1.3. Raman Spectroscopy Analysis

The Raman spectra of BBHP, BiTmDySbO₇, and BiEuO₃ are presented in Figure 4. For BiTmDySbO₇, seven distinct characteristic peaks were observed according to Figure 4. The vibrational modes of the Bi–O, Tm–O, and Dy–O bonds were identified from the characteristic peaks at 176 cm⁻¹, 390 cm⁻¹, and 727 cm⁻¹ [62,63,64,65]. The stretching vibration of the Sb–O bond gave rise to the peak at 455 cm⁻¹ or 501 cm⁻¹ [66]. Furthermore, the Raman bands at 238 cm⁻¹ and 295 cm⁻¹ were characteristic of the Sb–O–Sb stretching vibration [66,67].

The Raman spectrum of BiEuO₃ exhibited two major characteristic peaks. The Bi–O bond stretching vibration manifested as a peak at 135 cm⁻¹, whereas the Eu–O bond stretching vibration gave rise to the peak at 627 cm⁻¹ [68,69]. Moreover, the Raman spectrum of BBHP showed characteristic peaks at 135 cm⁻¹, 176 cm⁻¹, 238 cm⁻¹, 298 cm⁻¹, 390 cm⁻¹, 455 cm⁻¹, 501 cm⁻¹, 627 cm⁻¹, and 727 cm⁻¹. These peaks were in perfect agreement with those peaks that existed in the Raman spectra of BiTmDySbO₇ and BiEuO₃. The above analysis result once again confirmed the successful preparation of BBHP.

3.1.4. Morphology and Elemental Analysis

Figure 5a shows the morphology image of the BBHP using transmission electron microscopy (TEM). The TEM morphology analysis results, which were obtained according to Figure 5a, indicated a close contact between the BiEuO₃ particle and the BiTmDySbO₇ particle. The TEM morphology image of the BBHP indicated the presence of a small number of the BiEuO₃ particles, which were not attached to the BiTmDySbO₇ particles or were free-standing. However, the TEM morphology analysis image of the BBHP was derived only from a specific localized area of the BBHP. The distribution of the BiEuO₃ particles, which appeared in Figure 5a, was somewhat stochastic and did not fully represent the overall distribution of the BiEuO₃ particles that existed in the BBHP. Based on the comprehensive characterization results, the BiEuO₃ particles are generally uniformly distributed on the surface of the BiTmDySbO₇. Figure 5b displays high-resolution TEM (HRTEM) images of BiTmDySbO₇ and BiEuO₃. The high-resolution TEM image of BiTmDySbO₇ or BiEuO₃ provided detailed insights into the crystal lattice. It revealed a lattice fringe spacing of 0.305 nm for BiTmDySbO₇, which was consistent with the (222) crystal plane index of BiTmDySbO₇. The crystal lattice fringe of BiEuO₃ with a value of 0.316 nm was consistent with the (111) crystal plane index. Figure 5c manifests the TEM morphology image of the energy dispersive spectroscopy (EDS) measurement region, which belonged to BBHP. Figure 5d displays the EDS composition content distribution image of BBHP. Figure 5e is the EDS facial scanning element distribution image of BBHP. Figure 5c–e proved that the Bi element, Tm element, Dy element, Sb element, Eu element, and O element were uniformly distributed within BBHP. As shown in Figure 5d, the system peak, which was found from 8.5 KeV to 9.0 KeV, came from the copper mesh of the specimen stage, which was used for providing electroconductivity of the photocatalyst sample. The peak energy of the copper element was approximately 8.90 keV. Elemental surface analysis result indicated that the average atomic ratio of Bi, Tm, Dy, Sb, Eu, and O was approximately 823:411:412:411:413:4521, which was consistent with the result determined by spot scan of EDS. The atomic ratio of Bi, Tm, Dy, and Sb was calculated to be 0.98:0.99:1.01:1.00 in the BBHP sample. This precisely aligned with the proposed chemical formula and provided strong evidence for the successful and stoichiometric preparation of the BBHP.

3.1.5. X-Ray Photoelectron Spectroscopy Analysis

Figure 6a shows the full scan X-ray photoelectron spectroscopy (XPS) spectrum of BBHP. The Bi, Tm, Dy, Sb, Eu, or O were all identified in the full-scan XPS spectrum of BBHP, revealing the successful incorporation of BiEuO₃ and BiTmDySbO₇. The high-resolution XPS spectra of Bi 4f, Tm 4d, Dy 4d, Eu 3d, Sb 3d, and O 1s were presented in Figure 6b–f. The Bi 4f spectrum of BiTmDySbO₇ exhibited two peaks at 169.58 and 164.96 eV in Figure 6b, corresponding to the Bi 4f_7/2 and Bi 4f_5/2 states. This was consistent with the +3 oxidation state of Bi [70,71]. A characteristic Bi 4f doublet was identified in BiEuO₃, with the 4f_7/2 and 4f_5/2 located at 158.81 eV and 164.20 eV. In BBHP, the Bi 4f_7/2 peak and Bi 4f_5/2 peak shifted to 159.16 eV and 164.48 eV. The Tm 4d_5/2 peak in BiTmDySbO₇ was located at 177.46 eV according to Figure 6c. However, in BBHP, the Tm 4d_5/2 peak shifted to 176.56 eV. Similarly, the Sb 3d_5/2 peak and Sb 3d_3/2 peak in BiTmDySbO₇ were at 532.62 eV and 540.14 eV, respectively, while in BBHP, the Sb 3d_5/2 peak and Sb 3d_3/2 peak shifted to 531.89 eV and 539.49 eV according to Figure 6d. The Eu 3d_3/2 peak of BiEuO₃ was at 1165.23 eV, and the Eu 3d_5/2 peak was at 1135.81 eV in accordance with Figure 6e. In BBHP, the Eu 3d_3/2 peak and Eu 3d_5/2 peak shifted to 1165.05 eV and 1135.47 eV. Table S3 shows the XPS peak position values of various elements that derive from the BiTmDySbO₇ or the BBHP. Table S4 displays the XPS peak position values of various elements that originate from the BiEuO₃ or the BBHP. From the analysis of the peak positions for different elements in the BiTmDySbO₇, BiEuO₃, and BBHP, it could be seen that the Bi 4f peak, Tm 4d peak, Dy 4d peak, and Sb 3d peak in BBHP all shifted to the right direction compared to those in BiTmDySbO₇. The Bi 4f peak and Eu 3d peak in BBHP shifted to the left direction compared with those in BiEuO₃. These systematic shifts within peak positions were confirmatory for the formation of complex electronic interactions within the BBHP, which were crucial for effectively enhancing the catalytic performance of BBHP [72].

Further insight into the chemical environment of oxygen was gained from the deconvoluted O 1s XPS spectra of BBHP, BiTmDySbO₇, and BiEuO₃ in accordance with Figure 6f. The BBHP, BiTmDySbO₇, and BiEuO₃ exhibited three distinct oxygen species. For BBHP, the peak, which is located at 529.32 eV, corresponds to the lattice oxygen. The peak, which is located at 530.06 eV, corresponds to the adsorbed oxygen. The peak, which is located at 530.91 eV, was consistent with the surface hydroxyl group [73,74]. Similarly, the peak located at 530.35 eV, 531.13 eV, or 531.77 eV in BiTmDySbO₇ and the peak located at 529.93 eV, 531.05 eV, or 531.74 eV in BiEuO₃ also corresponded to the above three oxygen species. The secondary phases were not detected within the XPS peaks of BBHP, BiTmDySbO₇, and BiEuO₃. These XPS results indirectly confirmed the existence of a heterostructure in the BBHP and further verified the chemical interaction between BiTmDySbO₇ and BiEuO₃.

3.1.6. Optical Properties Analysis

Figure 7a shows the ultraviolet and visible diffuse reflection spectra of BBHP, BiTmDySbO₇, and BiEuO₃. The optical absorption characteristic of BBHP, BiTmDySbO₇, or BiEuO₃ was investigated by using the ultraviolet and visible spectrophotometer. The analysis result of the diffuse reflection spectra, which were derived from Figure 7a, revealed that the absorption onset point of BBHP, BiTmDySbO₇, or BiEuO₃ was approximately at 570 nm, 445 nm, or 640 nm. The transition ranges of BBHP, BiTmDySbO₇, and BiEuO₃ were located within the visible light spectrum. Figure S1 shows the ultraviolet and visible diffuse reflection spectrum of the N-T. In contrast, the absorption onset of the N-T was observed at approximately 450 nm. These results indicated that the N-T exhibited a very limited capacity for visible light absorption.

Figure 7b displays the corresponding graphs of (αhν)^1/2 and hν for BBHP, BiTmDySbO₇, and BiEuO₃. Combined with Equation (1), the band gap energy of BBHP, BiTmDySbO₇, or BiEuO₃ was calculated to preliminarily determine the visible light absorption capability of BBHP, BiTmDySbO₇, or BiEuO₃ [75,76,77]. The calculation results are shown in Figure 7b. The band gap energy of BBHP was 2.170 eV. Meanwhile, the band gap energy of BiTmDySbO₇ or BiEuO₃ was 2.784 eV or 1.933 eV. This indicated that the BBHP possessed a strong absorption capacity in the visible light region.

$${\left(\alpha h\nu \right)}^{{\displaystyle \frac{1}{2}}}=A\left(h\nu -Eg\right),$$

(1)

In Equation (1), A was the absorbance factor, α was the absorption coefficient, Eg was the band gap energy, and ν was the photon energy.

Figure 7c shows the photoluminescence (PL) spectra of BBHP, BiTmDySbO₇, and BiEuO₃. Figure 7c revealed the recombination rate of the photoinduced electrons and the photoinduced holes (PEPH), and clarified the Difference in Photocatalytic activity for BBHP, BiTmDySbO₇, and BiEuO₃ [78,79]. A higher PL intensity corresponded to a higher recombination rate of PEPH, while a lower PL intensity indicated a lower recombination rate of PEPH; thus, a more efficient separation efficiency of PEPH was obtained when the PL intensity was low [80,81,82]. However, solely relying on the PL spectrum of the BBHP, BiTmDySbO₇, or BiEuO₃ for inferring that the BBHP possessed the highest separation efficiency of the PEPH during the photocatalytic degradation process of the MNZ was somewhat lacking in evidence. According to the results which were obtained by using multiple experimental methods, it was demonstrated that the BBHP possessed the highest separation efficiency of the PEPH. The PL experiment, TRPL experiment, photocurrent intensity variation experiment, and electrochemical impedance experiment collectively demonstrated that the separation efficiency of the PEPH by using the BBHP was superior to that by using BiTmDySbO₇ or BiEuO₃. As observed in Figure 7c, BBHP, BiTmDySbO₇, and BiEuO₃ exhibited the highest PL intensity in the wavelength range, which was from 460 nm to 480 nm. However, a significant difference in PL intensity of BBHP, BiTmDySbO₇, and BiEuO₃ was observed; therefore, the PL intensity of BBHP was lower than that of BiTmDySbO₇ or BiEuO₃. The above observation led to the conclusion that the BBHP owned a significantly accelerated separation rate of PEPH, which directly contributed to the strong photocatalytic performance of PEPH.

The time-resolved photoluminescence (TRPL) spectra of BBHP, BiEuO₃, and BiTmDySbO₇ are presented in Figure 7d–f. The average lifetime of the photoinduced electrons derived from BBHP, BiEuO₃, or BiTmDySbO₇ was calculated based on the data gained from Equation (2) [83,84]. Table S5 exhibits the time constant components and average fluorescence lifetime of BiEuO₃, BiTmDySbO₇, and BBHP.

The average lifetime, derived from BiTmDySbO₇ or BiEuO₃, was 2.201 ns or 4.771 ns. The average lifetime, derived from BBHP, was 9.485 ns. The lifetime of the photoinduced electrons derived from BBHP was extended compared with that derived from BiTmDySbO₇ or BiEuO₃. The above phenomenon provided strong evidence that PEPH, which originated from the BBHP, was effectively separated. The high separation efficiency of PEPH was a key factor for enhancing the photocatalytic efficiency of BBHP.

$${\tau }_{ave}=\left(A_1{\tau }_1^2+A_2{\tau }_2^2\right)/\left(A_1{\tau }_1+A_2{\tau }_2\right),$$

(2)

Figure 8a shows the effect of the photocurrent density on VLI time with BBHP, BiEuO₃, or BiTmDySbO₇ as the photoelectrode. The photocurrent response was a direct indicator of the density of PEPH [85,86]. The results achieved from Figure 8a revealed that the photocurrent density with BBHP as the photoelectrode was significantly higher than that with BiTmDySbO₇ or BiEuO₃ as the photoelectrode. The above observation was consistent with the enhanced separation efficiency of PEPH, which originated from the PL spectra results and TRPL spectra results.

Figure 8b displays the EIS of BiTmDySbO₇, BiEuO₃, and BBHP. In accordance with Figure 8b, the Nyquist plot analysis result of the EIS for BiTmDySbO₇, BiEuO₃, or BBHP was further consistent with the conclusion derived from the above PL spectra results, the above TRPL spectra results, or the above photocurrent density variation results. The smaller the radius which was observed from the Nyquist plot was, the smaller the resistance of charge transfer was [83]. The radius of the EIS for BBHP was smaller than that for BiTmDySbO₇ or BiEuO₃, indicating that the charge transfer resistance was smaller and the separation efficiency of PEPH is higher when using BBHP.

3.2. Examination of Photocatalytic Efficiency

Figure 9a shows the curves of C_t/C₀ versus VLI time for the photocatalytic degradation of the MNZ by using BBHP, BiTmDySbO₇, BiEuO₃, or nitrogen-doped TiO₂ (N-T). C_t meant the concentration of the MNZ at a specific VLI time; correspondingly, C₀ referred to the initial concentration of the MNZ. Prior to the photodegradation of the MNZ, the MNZ solution was stirred in the dark for 45 min to establish the adsorption–desorption equilibrium. Figure S4 displays the relationship curve between the irradiation light wavelength and absorbance for the photocatalytic degradation of the MNZ by using the BBHP under different conditions of the visible light irradiation time. As shown in Figure S4, the maximal absorption wavelength of the MNZ was found to be 320 nm. Simultaneously, it could be found from Figure S4 that the absorbance intensity that corresponded to the MNZ concentration decreased gradually with the extension of the VLI time. The experimental results, which were exhibited in Figure S4, were consistent with the experimental results which is shown in Figure 9a. As shown in Figure 9a, the concentration of the MNZ remained essentially unchanged without the presence of a photocatalyst, indicating the resistance to natural photodegradation of the MNZ. In contrast, the newly synthesized BBHP demonstrated the most significant degradation efficiency of the MNZ compared with BiTmDySbO₇, BiEuO₃, or N-T. Figure 9b presents the fitting results of ln(C_t/C₀) versus VLI time by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. As a result, an excellent correlation coefficient R² was calculated to be 0.9966, 0.9981, 0.9947, or 0.9954 by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T.

In order to assess the complete mineralization efficiency of the MNZ, the Reduction in TOC concentration was monitored. Figure 9c presents the curves of TOC_t/TOC₀ versus VLI time for the photocatalytic degradation of the MNZ by using BBHP, BiTmDySbO₇, BiEuO₃, or N-T. TOC_t referred to the TOC concentration in the system at a specific VLI time; correspondingly, TOC₀ meant the initial TOC concentration. BBHP exhibited a superior mineralization efficiency of the TOC concentration during the degradation process of the MNZ, and possessed the strongest ability for obtaining the highest photocatalytic degradation rate of the MNZ compared with BiTmDySbO₇, BiEuO₃, or N-T. Figure 9d shows the fitting results of ln(TOC₀/TOC_t) versus VLI time during the degradation process of MNZ by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. Above TOC data, which were gained during the degradation process of the MNZ by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T, also fitted a first-order kinetic model, which was evidenced by the linear plots of ln(TOC₀/TOC_t) versus VLI time and the high correlation coefficient value of R², which was achieved according to Figure 9d.

Figure 9e evaluates the photodegradation rate of the MNZ and the kinetic constants after VLI of 130 min by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. It could be found from Figure 9e that the degradation rate of the MNZ by using BBHP was significantly higher compared with that by using BiEuO₃, BiTmDySbO₇, or N-T. The removal rate of the MNZ was 99.56%, 87.22%, 79.22%, or 37.59% by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. The removal rate of the MNZ by using BBHP was 1.14 times that by using BiEuO₃, 1.26 times that by using BiTmDySbO₇, and 2.65 times that by using N-T. Four photocatalysts exhibit different removal rates for degrading the MNZ, with the descending order as follows: BBHP > BiEuO₃ > BiTmDySbO₇ > N-T. In addition, the reaction rate of degrading the MNZ was 3.61 × 10⁻⁹ mol·L⁻¹·s⁻¹, 3.12 × 10⁻⁹ mol·L⁻¹·s⁻¹, 2.77 × 10⁻⁹ mol·L⁻¹·s⁻¹, or 1.54 × 10⁻⁹ mol·L⁻¹·s⁻¹ by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. The irradiation photon flux was 4.76 × 10⁻⁶ Einstein L⁻¹·s⁻¹ after VLI was measured by a radiometer. In accordance with Equation (3), the calculated photonic efficiency (PHEY) value for degrading the MNZ was 0.0758%, 0.0655%, 0.0581% or 0.0323% by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. The first-order kinetic constant k_C was calculated to be 0.0345 min⁻¹, 0.0131 min⁻¹, 0.0098 min⁻¹, or 0.0031 min⁻¹ during the degradation process of the MNZ by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. Obviously, the value of the kinetic constant k_C, which was acquired during the degradation process of the MNZ by using BBHP, was higher compared with that by using BiEuO₃, BiTmDySbO₇, or N-T.

The PHEY was calculated by using Equation (3):

$$\mathit{\varphi }=\mathit{R}/{\mathit{I}}_0$$

(3)

The Φ was the PHEY (%), simultaneously, the R was the degradation velocity of the MNZ (mol·L⁻¹·s⁻¹), and the I₀ was the irradiation photon flux (Einstein L⁻¹ s ⁻¹).

Similarly, Figure 9f presents the mineralization efficiency of the TOC concentration and the kinetic constants after VLI of 130 min by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. The removal rate of the TOC concentration was 98.11%, 84.02%, 75.93% or 34.27% during the degradation process of the MNZ by using BBHP, BiEuO₃, BiTmDySbO₇, or N-T. The mineralization rate of the TOC concentration during the degradation process of MNZ by using BBHP was 1.17 times that by using BiEuO₃, 1.29 times that by using BiTmDySbO₇, and 2.86 times that by using N-T; thereby, after VLI of 130 min, BBHP demonstrated superior mineralization efficiency of the TOC concentration compared with BiEuO₃, BiTmDySbO₇, or N-T. These results unequivocally demonstrated that BBHP played a crucial role in degrading the MNZ and complete mineralization of the TOC concentration. When the BiTmDySbO₇/BiEuO₃ heterojunction, BiEuO₃, BiTmDySbO₇, or N-T was utilized as the catalyst for degrading the MNZ, the kinetic constant k_TOC was 0.0257 min⁻¹, 0.0118 min⁻¹, 0.0093 min⁻¹, or 0.0027 min⁻¹. Therefore, the value of the kinetic constant k_TOC during the degradation process of the MNZ by using the BBHP was the highest compared with that by using BiEuO₃, BiTmDySbO₇, or N-T.

In order to assess the long-term viability of the synthesized BBHP, a cyclic performance test was conducted. Figure 10a displays the effect of the MNZ concentration on VLI time during five cyclic experiments by using the BBHP. As shown in Figure 10a, the ratio variation of C_t/C₀, which was derived from the contribution of the remaining MNZ concentration at facultative illumination time and the initial concentration, was monitored over multiple cyclic experiments. Figure 10b presents the fitting results of ln(C₀/C_t) versus VLI time during the cyclic degradation process of the MNZ by using BBHP. The outcomes indicated that the above variation curves of ln(C₀/C_t) versus VLI time conformed to the first-order kinetic model during the cyclic degradation process of the MNZ by using BBHP. Figure 10c displays the change curves of the TOC concentration during five cyclic experiments for degrading MNZ by using the BBHP. Figure 10d shows the fitting results of ln(TOC₀/TOC_t) versus VLI time during the cyclic degradation process of the MNZ by using BBHP. The kinetic analysis results of the cyclic degradation process of the MNZ by using BBHP are shown in Figure 10d. The kinetic analysis results confirmed that the changes in the kinetic constants, which were derived from the contribution of ln(TOC0/TOCt) and VLI time, continued to obey the first-order kinetic model across all cyclic experiments. Figure 10e displays the removal efficiencies of MNZ and the kinetic constants for degrading MNZ during five cyclic experiments by using the BBHP. After five cycles of photocatalytic degradation of the MNZ by using BBHP, the removal rate of the MNZ decreased by only 4.85%. Figure 10f presents the mineralization efficiency of the TOC concentration and the kinetic constants derived from the contribution of ln(TOC₀/TOC_t) and VLI time during five cyclic experiments by using the BBHP. After five cycles of photocatalytic degradation of the MNZ by using BBHP, the removal rate of the TOC concentration decreased by merely 4.72%. Figure 10e,f provided a clear summary of the performance for BBHP over five consecutive cyclic experiments. After five repeated photodegradation processes of MNZ, BBHP was able to maintain a degradation efficiency of 94.71% and a mineralization efficiency of 93.39%. Figure S2 shows the XRD spectrum of the BBHP after the quintic cyclic degradation process of the MNZ by using the BBHP under the condition of VLI. Therefore, the XRD pattern, which was shown in Figure S2, confirmed that the crystal structure of the BBHP remained unchanged after the quintic cyclic degradation process of the MNZ. Figure S3 presents the ultraviolet and visible diffuse reflection spectrum of the BBHP after the quintic cyclic degradation process of the MNZ by using the BBHP under the condition of VLI. The intrinsic transition edge of the absorption spectrum, which belonged to the BBHP, almost did not exhibit a change and remained within the visible light region. The above result provided strong evidence for the high durability and robust reusability of BBHP, confirming that BBHP possessed excellent stability for the sustainable removal of the MNZ from wastewater. Meanwhile, after VLI of 130 min, the removal rate of the MNZ by using BBHP was 1.14 times that by using the BiEuO₃, 1.26 times that by using the BiTmDySbO₇, and 2.65 times that by using the N-T. The above experimental results demonstrated that the BBHP possessed outstanding Z-scheme performance and photocatalytic activity.

For the purpose of identifying the key reactive species involved in the photocatalytic degradation of the MNZ by using BBHP, a series of radical scavenging experiments was conducted. BQ, IPA, or EDTA was utilized as a scavenger for capturing superoxide anions (•O₂⁻), hydroxyl radicals(•OH), or holes [87]. Figure 11a shows the change curves that derive from the effect of the MNZ concentration on the VLI time with the addition of the radical scavenger, such as IPA, BQ, or EDTA. Figure 11b presents the removal rate of the MNZ during the degradation process of the MNZ by using BBHP with IPA, BQ, or EDTA as scavengers under the condition of VLI. Due to the removal of the •OH by using the IPA, the removal rate for degrading the MNZ by using the BBHP had decreased by 47.90% under the condition of VLI in the free radical scavenging experiment. Additionally, the removal rate for degrading the MNZ by using the BBHP had decreased by 32.78% under the condition of VLI because the BQ captured the •O₂⁻ in the pharmacy wastewater reaction solution system. Eventually, the removal efficiency for degrading the MNZ by using the BBHP was reduced by 11.32% under the condition of VLI due to the scavenging of h⁺ by using the EDTA. The above results, which were derived from the free radical scavenging experiment, indicated that •OH, •O₂⁻, and h+ are involved in the degradation process of the MNZ by using the BBHP under the condition of VLI. The contribution descending order of the active species was as follows: •OH > •O₂⁻ > h⁺. The •OH played a major role compared with •O₂⁻ or h⁺ in the pharmacy wastewater reaction solution system. The OH⁻ underwent oxidation reaction with h⁺ for generating the •OH. However, the capture of the h⁺ by using the EDTA did not significantly reduce the removal rate of the MNZ. In the free radical scavenging experiment, most of the h⁺ was consumed for the oxidation of the OH⁻ to generate •OH, thereby, the consumed h⁺ which was utilized for generating the •OH was not captured by EDTA, as a result, only a small amount of the free h⁺ which was not used to oxidize OH⁻ for generating the •OH was captured by EDTA. Therefore, the removal rate for degrading the MNZ by using the BBHP decreased by only 11.31% after capturing the h⁺ with EDTA under the condition of VLI.

Figure 12 shows the electron paramagnetic resonance (EPR) spectrum of DMPO·O₂⁻ or DMPO·OH by using BBHP during the degradation process of the MNZ by using BBHP under the condition of VLI. It could be found from Figure 12 that the unique signal of •OH was detected in the EPR spectrum, which possessed a four-peak intensity ratio of 1:2:2:1, which indicated the generation of •OH during the degradation process of the MNZ by using BBHP under the condition of VLI [87,88]. Moreover, the signal peak of •O₂ was also detected from the EPR spectrum, which had four peak intensity ratios of 1:1:1:1, which implied the formation of •O₂⁻ during the degradation process of the MNZ. Figure S5 shows the EPR spectrum of DMPO•O₂⁻ or DMPO•OH during the photocatalytic degradation process of the MNZ by using the BiEuO₃ photocatalyst under the condition of VLI. According to the experimental results, which were gained from Figure S5, the detected •OH was observed and generated during the photocatalytic degradation process of the MNZ by using the BiEuO₃ photocatalyst under the condition of VLI, in contrast, the detected •O₂⁻ was not generated during the photocatalytic degradation process of the MNZ by using the BiEuO₃ photocatalyst under the condition of VLI. Additionally, Figure S6 displays the EPR spectrum of DMPO•O₂⁻ or DMPO•OH during the photocatalytic degradation process of the MNZ by using the BiTmDySbO₇ photocatalyst under the condition of VLI. According to the experimental results, which were obtained from Figure S6, the detected •O₂⁻ was observed and generated during the photocatalytic degradation process of the MNZ by using the BiTmDySbO₇ photocatalyst under the condition of VLI, by contrast, the detected •OH was not generated during the photocatalytic degradation process of the MNZ by using the BiTmDySbO₇ photocatalyst under the condition of VLI. The EPR experimental results, which were obtained according to Figure 12, confirmed that •OH and •O₂⁻ played major roles during the photocatalytic degradation process of the MNZ by using BBHP, and provided crucial insight into the photocatalytic degradation mechanism.

3.3. Research on the Photocatalytic Mechanism of BBHP

In order to gain a deeper understanding of the degradation mechanism of the BBHP, the VB edge potential of BiEuO₃ or BiTmDySbO₇ was determined by using ultraviolet photoelectron spectroscopy (UPS). Figure 13a shows the UPS spectrum of BiTmDySbO₇. Figure 13b displays the UPS spectrum of BiEuO₃. As shown in Figure 13a, the starting value of the binding energy, which originated from BiTmDySbO₇, was 1.048 eV; concurrently, the ending value of the binding energy, which derived from BiTmDySbO₇, was 19.955 eV. At the same time, as shown in Figure 13b, the starting value of the binding energy, which originated from BiEuO₃, was 0.721 eV; Meanwhile, the ending value of the binding energy, which derived from BiEuO₃, was 18.711 eV. Given the excitation energy of 21.2 eV [89,90,91], the ionization potential of VB for BiTmDySbO₇ was 2.293 eV; simultaneously, the ionization potential of VB for BiEuO₃ was 3.215 eV. Combined with the band gap energy values, which were calculated according to the previous ultraviolet and visible diffuse reflection spectra that belonged to BiTmDySbO₇ or BiEuO₃, we could determine that the electrochemical potential of the CB for the BiTmDySbO₇ photocatalyst or the BiEuO₃ photocatalyst was −0.491 eV or 1.282 eV [91].

Figure 14 displays the schematic diagram of the photodegradation mechanism of the MNZ under the condition of VLI by using BBHP. The possible charge transfer pathways within the BBHP could be found according to Figure 14. Based on the determination value of the UPS spectrum, which was derived from BiTmDySbO₇ or BiEuO₃, the ionization potential of VB and the electrochemical potential of CB for BiTmDySbO₇ or BiEuO₃ were achieved; thus, it could be inferred from the above results that the heterojunction structure of BBHP might conform to either a Z-scheme or a Type II mechanism [92,93,94,95,96]. The Z-scheme heterojunction structure for BBHP was identified as the most plausible pathway. Based on the EPR experiments and the free radical capture experiments, it was determined that the primary active species which were responsible for degrading the MNZ by using the BBHP were •OH and •O₂⁻. In order to utilize the photoinduced electrons for reducing O₂ into •O₂⁻, the CB potential of the photocatalyst had to be more negative than the standard reduction potential of −0.33 eV (vs. NHE). Similarly, for the sake of utilizing the photoinduced holes for oxidizing OH^— into •OH, the VB potential of the photocatalyst should be more positive than the standard oxidizing potential of 2.38 eV (vs. NHE). When BBHP absorbed visible light energy, the electrons were excited from the VB of BiEuO₃ or BiTmDySbO₇ to the CB of BiEuO₃ or BiTmDySbO₇. In the Z-scheme heterojunction structure for BBHP, the photogenerated electrons, which are located in the CB of BiEuO₃, combine with holes, which are located in the VB of BiTmDySbO₇. This specific recombination pathway effectively preserved the photogenerated electrons, which possessed the strongest reducing power, and the holes, which possessed the strongest oxidizing power, which played a crucial role in improving the photocatalytic activity of BBHP. This left the electrons in the CB of BiTmDySbO₇, and left the holes in the VB of BiEuO₃ for driving the redox reactions. The CB potential of BiTmDySbO₇ was −0.491 eV, which was more negative than the standard potential of O₂/•O₂⁻ (−0.33 eV vs. NHE) [97]. The above potential was sufficient to reduce oxygen for forming •O₂⁻. Concurrently, the VB potential of BiEuO₃ was 3.215 eV, which was more positive than the standard potential of OH⁻/•OH (2.38 eV vs. NHE), allowing the holes that existed in the VB of BiEuO₃ to oxidize hydroxide ions for generating highly reactive •OH [98,99]. The VB potential of the BiTmDySbO₇ was 2.293 eV, which was insufficient for generating the •OH through the photocatalytic oxidative reaction with the photoinduced holes. The CB potential of the BiEuO₃ was 1.282 eV, which could not reduce O₂ for forming •O₂⁻ by using the photoinduced electrons. Consequently, when the BBHP absorbed visible light, the photoinduced electrons migrated from the CB position of the BiEuO₃ to the VB position of the BiTmDySbO₇ and recombined with the photoinduced holes, which existed in the VB position of the BiTmDySbO₇. The generation of these key active species, such as •O₂⁻ and •OH, was consistent with our experimental findings, which were derived from the free radical scavenging experimental results and the EPR experimental results. Alternatively, a Type II mechanism for BBHP would suggest that the photogenerated electrons are transferred from the higher-energy CB of BiTmDySbO₇ to the lower-energy CB of BiEuO₃. However, this pathway would not produce the highly reactive radical species, which were observed in our experiments; as a result, the Z-scheme for BBHP became the more probable mechanism.

3.4. Degradation Pathway of the MNZ

In the light of the LC-MS results, three possible degradation pathways for the MNZ (m/z = 171) were proposed. The detailed path diagram is shown in Figure 15 under the condition of VLI by using BBHP. Figure S7 indicates the mass spectra and the retention time (rt) of the possible intermediate products for the metronidazole during the photocatalytic degradation process of the metronidazole by using the BBHP under the condition of VLI. The degradation of the MNZ was initiated by the attack of the reactive species, which was produced by using BBHP; thereby, the attack of the reactive species could lead to transformations such as hydroxyethyl cleavage, denitration, and oxidation of the MNZ molecule [100,101,102,103,104]. The first degradation pathway of the MNZ was initiated by hydroxyethyl cleavage, which formed an intermediate product with an m/z = 127. This intermediate product then underwent ring opening to generate oxalic acid (m/z = 91). The second degradation pathway of the MNZ involved the denitration of MNZ, resulting in the formation of 1H-imidazole-2,5-diol (m/z = 101). This intermediate product subsequently underwent hydroxyethyl cleavage; as a result, the oxalic acid was directly formed. The third degradation pathway of the MNZ began with the oxidation of MNZ to form (2-methyl-5-nitroimidazol-1-yl) acetic acid (m/z = 185). This product then followed one of two routes: either denitration played a crucial role in producing 2-(5-hydroxy-2-methyl-1H-imidazol-1-yl) acetic acid (m/z = 157), or ring opening played an important role in producing acetylglycine (m/z = 118). Ultimately, all intermediate products, including oxalic acid, 2-(5-hydroxy-2-methyl-1H-imidazol-1-yl) acetic acid, and acetylglycine, were further degraded into the final mineralized products: carbon dioxide (CO₂), water (H₂O), and nitrate (NO₃⁻) [105,106,107,108,109].

4. Conclusions

In this study, a novel Z-scheme BiTmDySbO₇/BiEuO₃ heterojunction photocatalyst was successfully synthesized via a hydrothermal method. Comprehensive characterization by using XRD, FTIR, Raman spectroscopy, XPS, and various microscopy techniques such as TEM, HRTEM, and EDS confirmed the successful synthesis of the BiTmDySbO₇/BiEuO₃ heterojunction and the existence of a Z-scheme structure, indicating that the BBHP possessed high photocatalytic activity potential. The superior charge carrier dynamics of the BBHP were verified by the photoelectrochemical analysis results and the optical analysis results, including the PL spectra results, the TRPL spectra results, the ultraviolet and visible diffuse reflection spectra results, the photocurrent intensity variation results, and the EIS results, obviously, above experimental results provided strong evidence for the high photocatalytic activity of the BBHP. Under VLI, the BBHP demonstrated exceptional photocatalytic activity for the degradation of the metronidazole compared with the BiTmDySbO₇ or the BiEuO₃. After VLI of 130 min, the degradation rate of the MNZ by using the BBHP was found to be 1.14 times that by using the BiEuO₃, 1.26 times that by using the BiTmDySbO₇, and 2.65 times that by using the N-T. Similarly, the mineralization rate for removing the TOC concentration during the photocatalytic degradation process of the MNZ by using the BBHP was 1.17 times that by using the BiEuO₃, 1.29 times that by using the BiTmDySbO₇, and 2.86 times that by using the N-T. Both the degradation process of the MNZ and the mineralization process for removing the TOC concentration by using the BBHP, the BiTmDySbO₇, or the BiEuO₃ conformed to a first-order kinetic model. Furthermore, the cyclic experiments by using the BBHP confirmed that the BBHP possessed strong stability and reusability. Studies that included the radical scavenging experiments and the EPR test experiments clarified that the •OH and the •O₂⁻ were the main active species that provided a dedication for the photocatalytic degradation of the MNZ. Eventually, in accordance with the LC-MS results and the existing research results, three potential degradation pathways for the MNZ were proposed. In conclusion, the BBHP not only offered a highly effective solution for purifying the wastewater which was contaminated by MNZ, but also represented a major devotion that was related to the progress of the Z-scheme photocatalyst for environmental applications.