Study on the Construction and Performance Measurement of Tm₂FeSbO₇/BiYO₃ Heterojunction Photocatalyst and the Photocatalytic Degradation of Sulfamethoxazole in Pharmaceutical Wastewater Under Visible Light Irradiation

Abstract

A novel catalyst, Tm₂FeSbO₇, was synthesized by employing the solid-phase high-temperature sintering method, and, for the first time, it was utilized to create a Z-type heterojunction with BiYO₃. A direct Z-scheme Tm₂FeSbO₇/BiYO₃ heterojunction photocatalyst (TBHP) was successfully produced by employing the ball-milling technique. X-ray diffraction analysis results indicated that Tm₂FeSbO₇ crystallized in a cubic pyrochlorestructure which owned the Fd-3m space group, with a unit cell parameter of 10.1769 Å, whereas BiYO₃ displayed a fluorite structure in the Fm-3m space group, with a unit cell parameter of 5.4222 Å. The Mossbauer spectrum of Tm₂FeSbO₇ showed that Fe³⁺ ions might locate at octahedral sites. The measured bandgap widths for the TBHP, Tm₂FeSbO_7, and BiYO₃ were 2.14 eV, 2.21 eV, and 2.30 eV, respectively. Multiple experimental results demonstrated that the TBHP exhibited a higher valence band ionization potential, a narrower band gap width, and a higher removal efficiency of the sulfamethoxazole (SMX) compared with the Dy₂TmSbO₇/BiHoO₃ heterojunction photocatalyst. Under visible-light irradiation (VISLI) of 115 min, the TBHP showcased exceptional photocatalytic elimination performance; therefore, the elimination rate of the SMX and the total organic carbon (TOC) mineralization rate reached 99.51% and 98.10%, respectively. In contrast to single-component Tm₂FeSbO₇, BiYO_3, or conventional nitrogen-doped titanium dioxide (N-TiO₂) catalyst, the TBHP exhibited removal efficiency enhancement for degrading the SMX by 1.17 times, 1.31 times, or 4.06 times. Simultaneously, the matching mineralization rate for removing the TOC density by employing the TBHP was 1.20 times, 1.34 times, or 4.73 times higher than that by employing Tm₂FeSbO₇, BiYO₃, or conventional N-TiO₂. Above experimental results indicated that the mineralization efficiency for removing TOC density by employing the TBHP was higher than that by employing Tm₂FeSbO₇, BiYO₃, or N-TiO₂. Radicals trapping experiments and the electron paramagnetic resonance spectroscopy results revealed that hydroxyl radicals, superoxide anions, and photoinduced holes were the primary active species during the catalytic elimination course of the SMX by employing the TBHP under VISLI. The results demonstrated that the direct Z-scheme TBHP, which was developed in this study, exhibited the maximal removal efficiency for degrading the SMX in contrast to Tm₂FeSbO₇, BiYO₃, or N-TiO₂. Additionally, the possible elimination routes and elimination mechanisms of the SMX were proposed. Therefore, an important scientific foundation for developing high-performance heterojunction catalysts was established.

1. Introduction

Since the 21st century, the pharmaceutical industry has developed vigorously. The rapid accumulation of pharmaceutical wastewater has become a severe and unavoidable problem in environmental pollution control. Antibiotics have been detected in sewage and groundwater worldwide [1,2]. Nevertheless, antibiotics that enter the water environment not only disrupt ecological balance and reduce biodiversity, but also pose potential threats to human health [3]. Sulfamethoxazole (SMX) is a typical sulfonamide antibiotic. SMX is widely present in pharmaceutical wastewater and is difficult to degrade. The reason is that SMX contains sulfonamide bonds and isoxazole rings, which are stable chemical structures [4,5]. The key resolution procedure for decomposing the stable organic structures was the cleavage of the C-C bond and the C-H bond. Fukui index and liquid chromatograph mass spectrometer (LC-MS) analysis results revealed that the sulphonamide bond cleavage and isoxazole ring opening are the main detoxification pathways [6]. Therefore, these compounds require strong oxidative species such as •OH and h⁺ for degradation.

The emergence of high-level semiconductor photocatalytic technology provided a sustainable solution for water purification. This photocatalytic technology employed light energy for driving chemical reactions, with its principle, which was primarily based on solid-state band theory [7]. When semiconductor photocatalysts (SPC) such as titanium dioxide were exposed to incident light energy that exceeded the bandgap width of the SPC, electrons transferred from the valence band (VB) of the SPC to the conduction band (CB) of the SPC. Accordingly, the photo-induced electron remained in the CB of the SPC; meanwhile, the holes remained in the VB of the SPC [8]. These photo-induced electrons and holes were subsequently separated and migrated on or between the surfaces of the SPC [9]. The electrons, which were employed as a reducing agent, might reduce soluble oxygen into superoxide radicals [10]. On the contrary, the holes which possessed oxydic performance might respond with aquafer or hydroxyl groups for generating altus active hydroxyl radicals. Ultimately, the generated radicals reacted with the pollutants, decomposing the pollutants into harmless compounds such as carbon dioxide (CO₂) and water (H₂O) [11].

The advanced oxidation technology has been widely applied due to its more direct degradation mechanism. For example, the market-applied photocatalyst TiO₂ (e.g., Degussa P25) is currently used to treat aqueous contaminants under ultraviolet light irradiation [12]. In addition, Tatiana Makropoulou’s team had confirmed that ZnO could degrade SMX almost completely under ultraviolet light irradiation [13]. However, the conventional photocatalytic nanomaterials such as TiO₂ and ZnO have limitations such as weak response to visible light and low photogenerated electron mobility, which immensely restrict the enhancement of their catalytic property [14,15]. The semiconductor photocatalysis technologies, specifically those photocatalytic technologies that were represented by novel pyrochlore-type A₂BCO₇ (A³⁺, B³⁺, C⁵⁺, O^2-) metal oxide semiconductors, have provided new approaches for water pollution control. A loftier specific surficial area and stronger metal oxide interaction were achieved for promoting particle diffusion; accordingly, the photocatalytic activity and thermal stability were enhanced. These oxides belong to a class of semiconductor materials with layered structures and have gained significant attention owing to their unique crystal structures and photocatalytic activity [16]. Based on previous research outcomes, we aimed to further choose favorable elements for synthesizing the A₂BCO₇ compound.

Zhang Bowen’s team employed rare earth element-doped TiO₂; as a result, in the activity test, the Tm-doped TiO₂ sample exhibited the highest photocatalytic activity for decomposing nicotinic insecticides, in contrast to pure TiO₂ [17]. Prof. Dr. Greta Patzke’s team pointed out that the Ni-based catalysts exhibited higher catalytic activity when they were doped with Fe³⁺ [18]. Moreover, Dr. Eleonora Aneggi et al. demonstrated that the Cu/Fe coupling effect not only exhibited extremely high elimination efficiency, but also maintained its reactivity over a wider pH range [19]. V. Ganesh et al. reported that Sb-doped ZnO achieved an 86% elimination efficiency for methylene blue dye [20]. Based on these findings, this study proposed to replace A³⁺ with Tm³⁺, replace B³⁺ with Fe³⁺, and replace C⁵⁺ with Sb⁵⁺ in the pyrochlore-type A₂BCO₇; as a result, novel Tm₂FeSbO₇ was designed with the expectation of increasing carrier concentration.

However, single-component Bi-based catalysts suffered from severe carrier recombination and limited active sites, prompting the development of heterojunction systems. Among these Bi-based catalysts, Z-scheme heterojunctions have emerged as a promising strategy for enhancing charge separation efficiency. In order to address the issue, binary heterojunction photocatalysts have gained popularity due to their interfacial electric fields that promote carrier separation. For instance, Yu et al. constructed an immediate Z-type ZnMoO₄/Cu₂O heterogeneous junction, which significantly improved the photodegradation efficiency of antibiotic pollutants under VISLI conditions [21]. Chen et al. developed an immediate Z-type BiVO₄/CdS heterogeneous junction, which demonstrated excellent photodegradation performance for quinolone antibiotics [22]. These results indicated that constructing Z-scheme heterojunctions was an effective strategy for enhancing the photocatalytic activity of the Z-type heterojunction catalysts.

Meanwhile, worldwide researchers have conducted extensive studies on Bi-based photocatalysts for the elimination of organic pollutants. Bi-based photocatalyzers typically exhibited a bandgap (Eg) which was lower than 3.0 eV; concurrently, due to the hybridization of Bi 6s orbit and O 2p orbit, these Bi-based photocatalyzers extended their light absorption waveband to the visible-light waveband region (400–700 nm) [23,24]. This hybridization effectively reduced the mass of photogenerated carriers and enhanced charge-separative efficiency and transport efficiency in Bi-based photocatalyzers. Consequently, Bi-based photocatalyzers were regarded as highly efficient catalysts. For example, BiYO₃, which was proposed by Zuzeng Qin, has also been confirmed to exhibit excellent photocatalytic activity [23].

Based on the above research, we creatively proposed to combine Tm₂FeSbO₇ with BiYO₃ for forming a heterostructure; accordingly, our objective was to increase carrier concentration and improve the degradation efficiency of the SMX for the sake of enhancing the separation efficiency of photo-induced electrons and photogenerated holes. The ball-milling technique was successfully employed for producing Tm₂FeSbO₇/BiYO₃ heterojunction photocatalyst (TBHP). In addition, we employed SMX as the degradation target for evaluating the photocatalytic activity of the Z-scheme TBHP. Meanwhile, we conducted photoelectrochemical experiments to analyze the degradation mechanism and potential degradation pathways of the SMX.

2. Results and Discussion

2.1. Structural and Photochemical Properties of Composition Photocatalysts

2.1.1. X-Ray Diffraction Patterns and Mössbauer Spectrum Analysis

Figure 1a–c displays the X-ray diffraction (XRD) patterns of the TBHP, Tm₂FeSbO_7, and BiYO₃. The prominent peaks that were perceived in the XRD pattern of TBHP (Figure 1a) matched those peaks that stemmed from Tm₂FeSbO₇ (Figure 1b) and BiYO₃ (Figure 1c), indicating that the successful synthesis of TBHP was realized. In order to further confirm the crystal structure, the XRD data of Tm₂FeSbO₇ and BiYO₃ were processed by employing the Pawley refinement method and Materials Studio software (version 2023) for improving data quality and obtaining more accurate crystal structure information. The refinement model included oxygen atoms, showing excellent agreement between the experimental diffraction peak intensities and calculated diffraction peak intensities (EDPI-CDPI) for Tm₂FeSbO₇ or BiYO₃. The refinement result for Tm₂FeSbO₇ is shown in Figure S1a. By comparing Figure S1 with the standard XRD card of γ-Fe₂O₃, it could be seen that Tm₂FeSbO₇ might contain a small amount of γ-Fe₂O₃ because the maximal diffraction peak of γ-Fe₂O₃ occurs at 2θ = 30.272 degrees [25]. However, the Pawley refinement results of Tm₂FeSbO₇ (R_P = 11.67%) confirmed a high consistency between EDPI-CDPI, demonstrating that Tm₂FeSbO₇ possessed a high precision in terms of the pyrochlore-type structure. Additionally, the results showed that Tm₂FeSbO₇ was a single-component cubic lattice with a space group of Fd-3m (No. 227). By analyzing the interplanar distance d, diffraction angle, and the wavelength (1.5406 Å) of the copper target in the X-ray diffractometer, the lattice parameter a of Tm₂FeSbO₇ was determined to be 10.1769 Å. Figure S1b shows the atomic structure of Tm₂FeSbO₇. The atomic positions and geometric factors of Tm₂FeSbO₇ are listed in Table S2. The refinement result for BiYO₃ compounds is shown in Figure S2a. The Pawley refinement results of BiYO₃ (R_P = 3.93%) confirmed a high consistency between EDPI-CDPI, indicating high precision in terms of the fluorite structure. The space group of BiYO₃ was Fm-3m (No. 225). Similarly, the lattice parameter a of BiYO₃ was determined to be 5.4222 Å. Figure S2b reveals the atomic structure of BiYO₃. The atomic positions and geometric factors of BiYO₃ are listed in Table S3. These observation results indicated that the synthesized Tm₂FeSbO₇ or BiYO₃ exhibited excellent structural stability and might serve as efficient photocatalysts in various scientific fields.

Pyrochlore-type A₂BCO₇ compounds are well known for their outstanding structural stability. The x-coordinate of the O (1) atom played a crucial role in crystal structure analysis and was widely regarded as a fundamental parameter for characterizing structural properties of Tm₂FeSbO₇, highlighting its importance in compound characterization. An x-value of 0.375 indicated the equivalence of six A-O (1) connectivity dimensions and two A-O (2) connectivity dimensions [26]. Tm₂FeSbO₇ featured a three-dimensional interconnected framework which was composed of corner-sharing MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra, which were linked by Tm³⁺ ions to form a chain-like structure. When the x-value deviated from 0.375, it suggested distortion in the MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra, implying that a distortion in the crystal structure of Tm₂FeSbO₇ was realized. Carrier separation played a key role in the photocatalytic elimination of SMX under VISLI by effectively preventing the recombination of photo-induced electrons and photo-induced holes. Studies about certain photocatalytic compounds have found that local distortion of MO₆ octahedra could inhibit charge recombination; therefore, the photocatalytic performance of the catalysts could be enhanced [27,28]. Therefore, the distortion of MO₆ (M = Fe³⁺ and Sb⁵⁺) octahedra in Tm₂FeSbO₇ might contribute to the enhanced photocatalytic performance of Tm₂FeSbO₇.

In the crystal structure of Tm₂FeSbO₇, there were two Tm-O connectivity dimensions: six Tm-O (1) bond lengths (2.545 Å) were longer than two Tm-O (2) bond lengths (2.203 Å). The six M-O (1) (M = Fe³⁺ and Sb⁵⁺) bond lengths were 1.947 Å, and the M-Tm (M = Fe³⁺ and Sb⁵⁺) bond lengths were 3.598 Å. In the crystal structure of Tm₂FeSbO₇, the M-O-M (M = Fe³⁺ and Sb⁵⁺) interatomic angle was 135.080°, and the Tm-M-O (M = Fe³⁺ and Sb⁵⁺) interatomic angle was 137.072°. The angle of M-O-M caused the delocalization of the excited states. When the angle of M-O-M approached 180°, luminescence performance was enhanced [29]. The angle of M-O-M in Tm₂FeSbO₇ affected the transference of photoactivated electron-hole pairs; therefore, their moving ability for reaching the active centers on the photocatalyst surface was influenced; consequently, the photocatalytic performance of Tm₂FeSbO₇ was enhanced. Moreover, the larger interatomic angle of Tm-Fe-O or Tm-Sb-O, which originated from Tm₂FeSbO_7, also enhanced the photocatalytic activity of Tm₂FeSbO₇.

BiYO₃ exhibits the fluorite structure and the cubic crystal system with the space group Fm-3m. Doping with Bi₂O₃ for forming BiYO₃ could improve the photocatalytic performance of BiYO₃ by narrowing the bandgap width and introducing electronic crystal defects (e.g., oxygen vacancies). The electronic crystal defects could act as promoters for improving charge carriers’ locomotivity, extending the lifetime of charge carriers, and increasing the potential for reaching the crystal surface of BiYO₃. The above conclusion had been demonstrated by doping with metal elements such as Cu and other transition metal elements [29]. The structure of BiYO₃ consisted of a face-centered cubic close packing of A (Bi³⁺ and Y³⁺) ions, with B (O²⁻) ions occupying all tetrahedral voids. As to BiYO₃, this highly regular and symmetrical three-dimensional cubic lattice ensured the ordered arrangement of atoms and structural robustness; as a result, the fundamental geometric stability was provided. This structure of BiYO₃ made the crystal resistant to deformation or collapse under external forces (e.g., slight compression or vibration); accordingly, BiYO₃ was allowed to maintain its crystalline form stably under ambient foul environmental conditions.

In order to clarify the phase composition, iron ion valence, and coordination geometry of the prepared Tm₂FeSbO₇ samples, we used room-temperature ⁵⁷Fe Mössbauer spectroscopy for characterization. Figure 1d displays the Mössbauer spectrum of Tm₂FeSbO₇. The Mössbauer parameters of Tm₂FeSbO₇ are exhibited in Table S4. The spectral fitting results and the related parameters were obtained in accordance with Figure 1d and Table S4. In the Figure 1d the blue line (Sextet) represents the sextet from secondary hyperfine interaction, while the red line (Doublet) represents the doublet from quadrupole interaction. During the fitting process, the experimental data points were in high agreement with the calculated fitting curve, indicating that the fitting model was reliable.

In combination with the characteristics for Mössbauer parameters and the pyrochlore structure of Tm₂FeSbO₇, each fitting component was assigned. The hyperfine field (H) of 49.65 T and the isomer shift (IS) of 0.48 mm/s for the Sextet were consistent with the characteristics of high-spin Fe³⁺ in a magnetic order environment; therefore, it was speculated that the prepared Tm₂FeSbO₇ sample contained γ-Fe₂O_3, which occupied a relative content of 18.60%. The Doublet was the dominant component of Tm₂FeSbO₇ with a relative content of 81.40%. The IS of 0.46 mm/s and the quadrupole splitting (QS) of 0.86 mm/s were consistent with the characteristic parameters of Fe³⁺ in the Tm₂FeSbO₇ pyrochlore lattice; consequently, Fe³⁺ ions might be located at octahedral sites. Usually, the Mössbauer spectrum of pure single-phase Tm₂FeSbO₇ pyrochlore could only exhibit one set of doublets, which corresponded to lattice Fe³⁺. However, the sextet impurity phase represented the γ-Fe₂O₃ in this sample. This indicates that the prepared sample was not pure single-phase Tm₂FeSbO₇ but a multi-phase coexistence sample.

The iron ion valence, coordination geometry, and distribution of Fe³⁺ had a significant impact on the photocatalytic activity of Tm₂FeSbO₇. At the same time, the magnetic iron oxide impurity phase was endowed with certain magnetic separation properties, which facilitated the efficient recovery and reuse of the TBHP.

2.1.2. Fourier Transform Infrared Spectra and Raman Analysis

Through Fourier transform infrared (FTIR) spectra, we conducted an in-depth analysis of the chemical bond type and constitution characteristics of TBHP, Tm₂FeSbO_7, and BiYO₃. As shown in Figure 2a, in contrast to Tm₂FeSbO₇, BiYO₃ or TBHP exhibited a slight shift in the Fe-O vibration peak and Bi-O vibration peak, indicating that strong interfacial interactions between Tm₂FeSbO₇ and BiYO₃ were realized. According to the FTIR spectra analysis, the characteristic peak of BiYO₃ at 521 cm⁻¹ could match the stretching vibration about the Bi-O bond, while the characteristic peak at 895 cm⁻¹ was assigned to the in-plane flexural vibration about the Y-O bond, and the peak at 644 cm⁻¹ could match the out-of-plane flexural vibration [30]. The characteristic stretching vibration of Tm₂FeSbO₇ about the Fe-O bond was perceived at 556 cm⁻¹ [31]. The peak at 849 cm⁻¹ could be ascribed to the characteristic stretching vibration about the Tm-O bond [32,33]. The absorption peaks at 667 cm⁻¹ and 789 cm⁻¹ were thought to originate from the antisymmetric stretching vibration about the Sb-O-Sb bond in the SbO₆ octahedra, reflecting that the distortion of the pyrochlore structure was realized [20,34].

Notably, TBHP and Tm₂FeSbO₇ exhibited a common broad band in the range of 3232 cm⁻¹–3612 cm⁻¹, which arose from hydrogen-bonded O-H stretching vibration about adsorbed water molecules and surface hydroxyl groups, which were crucial for generating •OH radicals during the photocatalytic degradation course of the SMX. The characteristic absorption peaks at 1067 cm⁻¹, 1382 cm⁻¹ and 1665 cm⁻¹ could match the flexural vibration mode about H-O-H bond [35,36,37,38,39].

Figure 2b displays the Raman spectra of Tm₂FeSbO₇, BiYO₃, and TBHP. In the spectrum of Tm₂FeSbO₇, the strong peak which was perceived at 136 cm⁻¹ could be attributed to the translational vibrations about Tm and Fe, while the characteristic peak which was located at 301 cm⁻¹ was assigned to the flexural vibration mode about the Fe-O bond. The peak, which is located at 395 cm⁻¹, could match the stretching vibration mode of the Sb-O bond. Notably, the strongest peak, which was perceived at 519 cm⁻¹, was primarily ascribed to the stretching vibration mode about the Fe-O bond, reflecting the distortion phenomenon about the oxygen coordination structure around Fe³⁺ ions [40]. The peak, which is located at 655 cm⁻¹, originated from the antisymmetric stretching vibration about the Sb-O-Sb bond in the (SbO₆) octahedron, while the double peaks, which appeared at 741 cm⁻¹ and 782 cm⁻¹, were attributed to different vibrational modes about the Sb-O bond [41,42]. In the BiYO₃ spectrum, the peak located at 128 cm⁻¹ was assigned to the lattice vibration mode about the Bi-O bond, while the peak located at 636 cm⁻¹ could match the symmetric stretching vibration about the O-Y-O functional group [20].

2.1.3. Transmission Electron Microscopy and Energy Dispersive X-Ray Spectra Analysis

The microstructure and elemental composition of the TBHP were systematically characterized by employing transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectra. Figure 3 presents the TEM image and high-resolution TEM (HRTEM) image of the TBHP. The TEM image in Figure 3a clearly shows the coexistence of Tm₂FeSbO₇ nanoparticles and BiYO₃ nanoparticles in the TBHP. Figure 3b shows the HRTEM image of the marked region, which is shown in Figure 3a. As indicated in Figure 3a, the interfacial structure between Tm₂FeSbO₇ and BiYO₃, and the lattice fringes of Tm₂FeSbO₇ and BiYO₃ could be distinctly identified. The calculated interplanar distance of Tm₂FeSbO₇ was 0.293 nm, while the interplanar distance of BiYO₃ was 0.313 nm. Figure S3a reveals the EDX elemental scanning mapping of the TBHP. Figure S3a demonstrates the uniform distribution of Tm, Fe, Sb, Bi, Y, and O elements in the TBHP, directly confirming that the Tm₂FeSbO₇ phase and BiYO₃ phase coexisted. By comparing the intensity differences in the luminescent regions of Tm, Fe, Sb, Bi, and Y elements, it could be inferred that BiYO₃ primarily might match the larger-size particles, whereas Tm₂FeSbO₇ existed as smaller-size particles. Figure S3b displays the EDX composition content distribution image of the TBHP. As shown in Figure S3b, the EDX plot analysis revealed that the atomic ratio of the elements for TBHP was approximately Tm:Fe:Sb:Bi:Y:O = 175:53:61:54:48:609. Figure S4a exhibits the second EDX elemental scanning mapping of the TBHP. Figure S4b represents the second EDX composition content distribution image of the TBHP. Similarly, Figure S4a,b reveal that the atomic ratio of the elements for TBHP was approximately Tm:Fe:Sb:Bi:Y:O = 186:148:125:104:97:251.

Figure S4a confirmed that the particles that were rich in Tm, Fe, and Sb elements did not contain Bi and Y elements; correspondingly, the particles that were rich in Bi and Y elements did not include Tm, Fe, and Sb elements. Meanwhile, it could be found from Figure S4b that the particles that were rich in Tm, Fe, and Sb elements were observed to be in contact with the particles that were rich in Bi and Y elements, indicating that the TBHP was obtained successfully. Based on the above characterization results, it was confirmed that the TBHP could be prepared successfully by employing the ball milling method [43].

2.1.4. X-Ray Photoelectron Spectra Analysis

This study systematically characterized the surface chemical composition and valence states of Tm₂FeSbO₇, BiYO₃, and TBHP by employing X-ray photoelectron spectra (XPS), with specific analytical results shown in Figure 4. The full-spectrum scan revealed the presence of Tm, Fe, Sb, Bi, Y, and O elements in TBHP; concurrently, the carbon peak might serve as a calibration reference. Comparative analysis results stemming from BiYO₃ demonstrated significant enhancement of the characteristic peaks of Bi³⁺ and Y³⁺ in TBHP, confirming that the Bi element and Y element were present. Comparative analysis results, which originated from Tm₂FeSbO₇, demonstrated significant enhancement of the characteristic peaks of Tm³⁺, Fe³⁺, and Sb⁵⁺ in TBHP, confirming that the Tm element, Fe element, and Sb element were present. Figure 5a–f displays the XPS of Tm 4d, Fe 2p, Sb 4d, Bi 4f, Y 3p, Y 3d, and O 1s, which stem from Tm₂FeSbO₇, BiYO₃, and TBHP.

As shown in Figure 5a, the binding energy of the Tm 4d_5/2 peak in TBHP was 177.25 eV, which exhibited a distinct high binding energy shift compared with Tm₂FeSbO₇. In Figure 5b, the Fe 2p spectrum of TBHP showed peaks at 726 eV and 711 eV, which could match the 2p_1/2 orbit and 2p_3/2 orbit of Fe³⁺, respectively. Figure 5c revealed the Sb 4d spectrum of TBHP at 35.7 eV. In Figure 5d, the characteristic peaks of Bi 4f in TBHP appeared at 159.80 eV and 164.90 eV, which could match the 4f_7/2 and 4f_5/2 orbits of Bi³⁺, respectively, with a spin–orbit splitting energy of 5.1 eV, which was consistent with the characteristics of Bi³⁺. Notably, the matching peaks in BiYO₃ shifted toward higher binding energies at 164.55 eV and 159.55 eV.

For the Y 3d_5/2 spectrum shown in Figure 5d and the Y 3p spectrum shown in Figure 5e, peaks located at 158.75 eV, 312.60 eV, and 300.75 eV indicate the presence of Y in the form of Y³⁺ within BiYO₃. After the combination of Tm₂FeSbO₇ with BiYO₃, the binding energy of Y 3d_5/2 turned into 159.10 eV, and the binding energies of Y 3p_3/2 turned into 312.95 eV and 300.35 eV compared with standalone BiYO₃. These binding energy shifts suggested possible electron transfer and delocalization phenomena between Tm₂FeSbO₇ and BiYO₃. Such interfacial interactions enhanced the overall efficiency and performance of the photocatalyzers [44].

Figure 5f presents the peak fitting results of the O 1s spectrum for TBHP, Tm₂FeSbO_7, and BiYO₃. Peaks located at 529.8 eV, 530 eV, and 530.7 eV could match lattice oxygen, while peaks located at 531.15 eV and 531.7 eV were attributed to surface adsorbed oxygen. The characteristic peak, located at 532.05 eV, indicated the presence of surface hydroxyl groups. Notably, in Figure 5f, the binding energy of Sb 3d_3/2 in TBHP (540.61 eV) was significantly higher than that in Tm₂FeSbO₇. The result confirmed the presence of Sb⁵⁺.

2.1.5. Ultraviolet-Visible Diffuse Reflectance Spectra and Analysis

In order to thoroughly investigate the energy band structure characteristics of the samples, this study thoughtfully analyzes the ultraviolet visible diffuse reflectance (UV-Vis DR) spectra of Tm₂FeSbO₇, BiYO₃, and TBHP; accordingly, the above results are revealed in Figure S5a. As illustrated in Figure S5a, the absorption edges of Tm₂FeSbO₇ and BiYO₃ appeared at 560 nm and 565 nm, respectively, while the absorption edge of TBHP was perceived at 590 nm. TBHP exhibited a significant red shift in contrast to Tm₂FeSbO₇ or BiYO₃, indicating that TBHP possessed broader spectral response characteristics. This suggested that TBHP exhibits superior visible-light absorption capability in contrast to Tm₂FeSbO₇ and BiYO₃.

The Kubelka–Munk function is exhibited in Formula (1). Simultaneously, the E_g value of semiconductor catalysts can be accurately determined by counting the tangential line within the rectilineal range of the diffused reflective spectra absorption edge and extending it to the crosspoint with the horizontal axis [45,46].

$${\displaystyle \frac{{[1-R_d\left(hv\right)]}^2}{{2R}_d\left(hv\right)}}={\displaystyle \frac{\alpha \left(hv\right)}{S}}$$

(1)

In this function, S, R_d, and α represent the scattering coefficient, diffuse reflectance, and radiation absorption coefficient, respectively. Moreover, the photoabsorption character near the band brink of the compound conforms to the arithmetical relation of Formula (2) [47,48]:

$${\left(\alpha hv\right)}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}}=A(hv-E_g)$$

(2)

In the equation, A is the proportionality constant, α is the absorption coefficient, E_g is the band gap, and ν is the photon frequency. The parameter n characterizes the type of electronic transition after photoexcitation, where the n value of 1/2 can match a direct transition, and the n value of two indicates an indirect transition. By analyzing the data shown in Figure S5b, the bandgap widths of Tm₂FeSbO₇ and BiYO₃ were measured to be 2.21 eV and 2.30 eV, respectively, with both n values pressing towards two, indicating that their interband transitions were mediated. The calculated band gap width for TBHP was 2.14 eV, exhibiting characteristics of an indirect transition. The significant reduction in the bandgap width of TBHP was consistent with the enhanced light absorption performance, which certified the research trend of heterojunctions for improving photoelectric conversion efficiency.

2.2. Optical Characterization of TBHP, Tm₂FeSbO₇, and BiYO₃

2.2.1. Photoelectrochemical Analysis

Using short wavelength excitation for TBHP, Tm₂FeSbO₇, and BiYO₃ were detailed in the supporting information (Section S9 and Figure S6). Figure 6a presents the time-resolved photoluminescence (TRPL) spectra of Tm₂FeSbO₇, BiYO₃, and TBHP. Further confirmation was achieved by verifying that TBHP exhibited stronger photocatalytic efficacy compared with Tm₂FeSbO₇ and BiYO₃. A double exponential decay Formula (3) was employed for obtaining the fitting results of the TRPL spectra, as shown in Figure 6a [49]:

$$\left(t\right)=I_0+A_1\exp \left(-{\displaystyle \frac{t}{{\tau }_1}}\right)+A_2\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{t}{{\tau }_2}})$$

(3)

According to the given equation, $A_1$ and $A_2$ represent the matching weighting coefficients of the first-order and second-order decay times for each decay channel, respectively. The average photogenerated carrier lifetime (${\tau }_{ave}$) was calculated by employing Equation (4) [50]:

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

(4)

The construction of heterojunctions facilitated the separation velocity of photogenerated charge carriers; as a result, TBHP attained a significantly prolonged lifetime of 14.9 ns for ${\tau }_{ave}$ compared with the lifetime ${\tau }_{ave}$ of 11.6 ns for Tm₂FeSbO₇ or the lifetime ${\tau }_{ave}$ of 8.7 ns for BiYO₃. Relevant studies demonstrated that forming heterojunction structures, such as combining Bi₂O₃ with other semiconductor catalysts, could effectively enhance the separative efficiency of photo-induced electrons and photo-induced holes; therefore, the photocatalytic performance of the heterostructure catalysts might be enhanced [46]. For instance, the research about the preparation of WS₂/ZnIn₂S₄ heterojunction and its photocatalytic properties revealed that heterostructure catalysts could promptly and efficiently promote the separative efficiency of photogenerated electron-hole pairs; accordingly, the catalytic activity of the heterostructure catalysts was enhanced [51].

Figure 6b shows the transient photocurrent intensity variation spectra of TBHP, Tm₂FeSbO_7, and BiYO₃. In the light of Figure 6b, the photocurrent density of TBHP was significantly higher than that of Tm₂FeSbO₇ or BiYO₃; therefore, the reason that TBHP had a higher photocurrent density could be attributed to the efficient separation and rapid transport velocity of photogenerated electron-hole pairs, which stemmed from the heterojunction structure of TBHP. This phenomenon indicated that the composite structure of Tm₂FeSbO₇ and BiYO₃ effectively promoted the separative efficiency of photogenerated carriers [52,53,54]. To further validate this conclusion, Figure 6c presents the electrochemical impedance spectra (EIS) of the TBHP, Tm₂FeSbO₇, and BiYO₃. It can be found from Figure 6c that a smaller arc semidiameter could typically match the reduced charge transfer resistance [55,56]. The arc semidiameter about TBHP was significantly smaller than that of single-component compounds such as Tm₂FeSbO₇ or BiYO₃, confirming that TBHP possessed more efficient charge disassociation competency. This result was highly consistent with the conclusions drawn from the PL spectra, the transient photocurrent response test spectra, and the TRPL spectra. The observation results suggested that TBHP obtained enhanced photocatalytic activity by more efficiently separating photogenerated carriers and improving interfacial charge mobility.

2.2.2. Photocatalytic Elimination Analysis of the SMX

This study systematically evaluated the degradation elimination efficiency of SMX by employing TBHP, Tm₂FeSbO₇, BiYO₃, and N-TiO₂ in SMX solution under VISLI. As shown in Figure 7a, the experimental groups that contained catalysts exhibited prominent decreases in SMX concentration, whereas the blank control group, which was utilized for the photolysis experiment without a catalyst, did not display a notable change in SMX concentration, confirming that the photocatalysts play a dominant role in the photocatalytic elimination of SMX. The elimination efficiency of SMX was calculated by employing the formula of ($1-C_t/C_0$) × 100%, where $C_t$ and $C_0$ represented the instantaneous concentration of SMX and the initial concentration of SMX, respectively. The experimental result demonstrated that after VISLI of 115 min, the removal rate of SMX reached as high as 99.51% by employing TBHP, significantly surpassing the removal rates achieved by employing Tm₂FeSbO₇ (85.41%) and BiYO₃ (75.63%) alone. Comparative analysis results revealed that the removal rate of SMX by employing TBHP was 1.17 times, 1.31 times, and 4.06 times higher than that by employing Tm₂FeSbO₇, BiYO₃, and N-TiO₂, respectively, underscoring that TBHP possesses superior catalytic performance compared with Tm₂FeSbO₇, BiYO₃, or N-TiO₂. Additionally, by comparison, the removal rate for degrading the SMX reached 89.86% or 94.30% [57] by employing Dy₂TmSbO₇/BiHoO₃ or molecular imprinted (MI) TiO₂@Fe₂O₃@g-C₃N₄ [57] after VISLI of 115 min; obviously, TBHP exhibited a superior photocatalytic activity for degrading the SMX compared with Dy₂TmSbO₇/BiHoO₃ or MI TiO₂@Fe₂O₃@g-C₃N₄. Similarly, by comparison, the removal rate for degrading the SMX reached 63.1% [57] or 48.9% [57] by employing MI-polymers TiO₂@g-C₃N₄ [57] or MI-polymers TiO₂@Fe₂O₃ [57] after 120 min of VISLI. Obviously, TBHP exhibited a superior photocatalytic activity for degrading the SMX compared with MI-polymers TiO₂@g-C₃N₄ or MI-polymers TiO₂@Fe₂O₃ [57].

Figure 7b further demonstrates the mineralization efficiency for removing the total organic carbon (TOC) concentration by employing TBHP, Tm₂FeSbO₇, BiYO_3, and N-TiO₂ in SMX solution under VISLI. The mineralization rate for removing TOC concentration (MRRTC) was calculated by employing the formula ($1-{TOC}_t/{TOC}_0$) × 100%, where ${TOC}_t$ and ${TOC}_0$ represented the instantaneous concentration of TOC and the initial concentration of TOC, respectively. The results revealed that after VISLI of 115 min, the MRRTC was 98.10%, achieved by employing TBHP. The MRRTC by employing TBHP was significantly higher than that by employing Tm₂FeSbO₇ (81.97%), BiYO₃ (73.03%), or N-TiO₂ (20.75%). Comparative analysis results revealed that the MRRTC by employing TBHP was 1.20 times, 1.34 times, or 4.73 times higher than that by employing Tm₂FeSbO₇, BiYO₃, or N-TiO₂, respectively, further confirming that TBHP possesses superior capability for the deep mineralization of organic pollutant SMX.

In order to evaluate the actual durability and actual reusability of TBHP, this study conducted five cyclic experiments for degrading SMX by employing TBHP under VISLI, with the results shown in Figure 7c,d. The experimental data indicated that after five consecutive photocatalytic reactions for degrading SMX, the removal rates of SMX by employing TBHP in the five cyclic experiments were 99.51%, 97.99%, 96.68%, 95.32%, and 93.92%, which could match the MRRTC of 98.1%, 96.56%, 95.28%, 93.77%, and 92.48%. After five cyclic experiments, the TBHP demonstrated outstanding stability, with only a slight decrease of 5.59% in the removal rate of SMX and a 5.62% reduction in MRRTC under VISLI. The results fully confirmed the application potential of TBHP as a photocatalyst in wastewater remediation.

The first-order kinetic curve for degrading the SMX (Figure S7a), removing the TOC concentration (Figure S7b), degrading the SMX concentration by employing TBHP over five cyclic experiments (Figure S8a), and removing the TOC concentration by employing TBHP over five cyclic experiments (Figure S8b) are detailed in the Supplementary Materials (Section S10).

2.2.3. Radicals Assay Experiment

Figure 7e illustrates the influence of radical inhibitors benzoquinone (BQ), isopropanol (IPA), and ethylenediaminetetraacetic acid (EDTA) on the catalytic degradation efficiency of SMX in SMX solution by employing TBHP under VISLI. The experimental results determined the directed scavenging effectiveness of •OH, •O₂⁻, and h⁺ active radicals by separately adding IPA, BQ, and EDTA. In Figure 7f, comparative analysis results revealed that the introduction of IPA reduced the degradation efficiency of the SMX by an average of 56.95%; accordingly, the single effect of •OH could cause a removal rate of 56.95% for degrading SMX by employing TBHP after VISLI of 115 min. Correspondingly, the introduction of BQ reduced the degradation efficiency of the SMX by an average of 33.70%; resultingly, the single effect of •O₂⁻ could cause a removal rate of 33.70% for degrading SMX by employing TBHP after VISLI of 115 min. Analogically, the introduction of EDTA diminished the degradation efficiency of the SMX by an average of 13.02%; consequently, the single effect of h⁺ could cause a removal rate of 13.02% for degrading SMX by employing TBHP after VISLI of 115 min. It was thus inferred that •OH played a dominant role for the photocatalytic degradation of SMX by employing TBHP; as a result, the descending order of contribution importance might be •OH > •O₂⁻ > h⁺.

In order to further verify the generation characteristics of active radicals during the photocatalytic degradation of SMX by employing TBHP, electron paramagnetic resonance (EPR) technology was employed to detect the •O₂⁻ and •OH, which were produced during the photocatalytic elimination process. Figure 8a displays the EPR signal characteristics of DMPO•O₂⁻ and DMPO•OH adducts by employing TBHP. Figure 8a was related to the EPR spectral analysis of the spin adducts of DMPO with •O₂⁻ and •OH during EPR detection. After VISLI of 10 min, a quartet of equal intensity peaks (intensity ratio 1:1:1:1), which were consistent with the characteristic signal of DMPO•O₂⁻ [46] appeared in the EPR spectrum, confirming the existence of •O₂⁻ radicals. Simultaneously, a quartet of peaks with an intensity ratio of 1:2:2:1 was detected, matching the characteristic signal of DMPO•OH adducts [58], indicating that the generation of •OH radicals was realized under VISLI conditions. Notably, the relative intensity analysis of the EPR signals revealed that the production amount of •OH was significantly higher than that of •O₂⁻. This result was highly consistent with the conclusions drawn from the radicals trapping experiments, providing direct evidence for the synergistic participation of •O₂⁻ and •OH in the degradation reaction.

2.2.4. Ultraviolet Photoelectron Spectrogram and Degradation Mechanism Analysis

Figure 8b,c show the ultraviolet photoelectron spectrogram test results of Tm₂FeSbO₇ and BiYO₃, providing crucial data support for determining the ionization potentials of the compounds. The initial binding energy (E_i) and cutoff binding energy (E_cutoff) of Tm₂FeSbO₇ were 0.91 eV and 19.01 eV, and the matching values for BiYO₃ were 1.56 eV and 21.41 eV [59]. By combining the excitation light source energy 21.20 eV, the ionization potentials of Tm₂FeSbO₇ and BiYO₃ could be calculated as 3.10 eV and 1.35 eV, respectively [60]. Both Tm₂FeSbO₇ and BiYO₃ exhibited light absorption characteristics in the visible-light waveband region, thereby generating the photoexcited charge carriers.

Based on the energy band structure characteristics of Tm₂FeSbO₇ and BiYO₃, this study thoroughly investigated the photocatalytic reaction mechanism of the TBHP. As illustrated in Figure 9a,b, two possible charge transfer modes are proposed: the conventional Type-II heterojunction and the direct Z-scheme heterojunction. The conventional Type-II structure heterojunction mechanism is depicted in Figure 9a. The conventional Type-II structure is detailed in the supporting information (Section S11). In this paper, the heterojunction is synthesized as a Z-scheme in Figure 9b. In the Z-scheme TBHP, the photoinduced electrons migrated from the CB of Tm₂FeSbO₇ (E_CB = 0.87 eV) to the VB of BiYO₃ (E_VB = 1.35 eV); as a result, the separation efficiency of the photogenerated charge carriers was significantly improved. Due to the structural nature of the Z-scheme heterojunction, the recombination of charge carriers with lower redox potentials is facilitated. Consequently, highly reactive charge carriers are preserved, which is crucial for enhancing the photocatalytic performance of the catalysts. During the photocatalytic reaction of the SMX, the photo-induced electrons which existed in the CB of BiYO₃ (E_CB = −0.95 eV) could react with O₂ for generating •O₂⁻ because the electrochemic potential of the CB for BiYO₃ (E_CB = −0.95 eV) was more negative than the reductive potential of O₂/•O₂⁻ (−0.33 eV vs. NHE); meanwhile, the photogenerated holes which existed in the VB of Tm₂FeSbO₇ (E_VB = 3.10 eV) could oxidize OH⁻ for producing •OH because the ionizing potential of the VB for Tm₂FeSbO₇ (E_VB = 3.10 eV) was more positive than the oxidative potential of OH⁻/•OH (2.38 eV vs. NHE). TBHP possessed a higher valence band ionization potential compared with the Dy₂TmSbO₇/BiHoO₃ heterojunction photocatalyst. TBHP was endowed with a stronger capability for efficiently generating •OH, which was presumably the primary reason that TBHP had higher photocatalytic activity during the efficient degradation process of the SMX compared with the Dy₂TmSbO₇/BiHoO₃ heterojunction photocatalyst. A portion of photogenerated holes that existed in the VB of Tm₂FeSbO₇ could serve as excellent oxidants for degrading the SMX, indicating that Tm₂FeSbO₇ might exhibit significant photocatalytic reactivity. Therefore, the photogenerated holes derived from the VB of Tm₂FeSbO₇ had strong oxidative capacity and could directly participate in the oxidative degradation of the SMX. The synergistic effects of •OH⁻, •O₂, and h⁺ active radicals contributed to pollutant degradation, indicating that the above results were consistent with the results derived from the radicals trapping experiment and the EPR detection. The aforementioned mechanistic analysis results not only elucidated the photocatalytic degradation pathway of the SMX but also highlighted the notable advantages of the TBHP for environmental remediation. As a result, the theoretical innovation for the efficient photocatalytic degradation of SMX was provided.

2.2.5. Degradation Pathway Analysis

In order to investigate the degradation pathways of the SMX, this study systematically analyzed the intermediate products during the photocatalytic elimination process of the SMX by employing a literature review and LC-MS detection. Figure 10 represents the possible elimination pathways of the SMX by employing TBHP. Based on above findings, we proposed the possible photocatalytic elimination mechanisms of the SMX: (1) the sulfonamide group -SO₂-NH- which was contained in the SMX molecules underwent the S-N bond cleavage under the synergistic attack of the •OH and the •O₂⁻, yielding the intermediate products SMX1 (m/z = 173) and SMX2 (m/z = 99); (2) the double bond contained in the SMX isoxazole ring was oxidized and underwent the substitution by hydroxyl radicals, forming intermediate products such as SMX3 (m/z = 268), SMX4 (m/z = 288) and SMX7 (m/z = 190); (3) the •OH abstracted the hydrogen atoms from the amino group -NH₂, which was contained on the aromatic ring, followed by hydroxylation for generating the intermediate products such as SMX5 (m/z = 270), SMX6 (m/z = 280) and SMX8 (m/z = 133). During the photocatalytic elimination course of the SMX, the •O₂⁻ and the •OH worked cooperatively for driving the ring-opening reactions; meanwhile, the •O₂⁻ continuously generated H₂O₂ through disproportionation reactions; therefore, the radical chain reaction was sustained for ensuring a continuous •OH supply. Ultimately, pollutants were mineralized into water and carbon dioxide [61,62,63,64,65,66].

3. Materials and Methods

3.1. Preparation of Photocatalysts

The experimental instruments were detailed in the supporting information (Table S1). The complete raw chemical reagents were detailed in the supporting information (Section S1); the complete preparation course of Tm₂FeSbO₇ was detailed in the supporting information (Section S2); the complete preparation course of BiYO₃ was detailed in the supporting information (Section S3); the complete preparation course of TBHP was detailed in the supporting information (Section S4); the complete preparation course of nitrogen-doped titanium dioxide (N-TiO₂) was detailed in the supporting information (Section S5).

3.2. Degradation Elimination Experiment of SMX

The complete preparation course of the SMX solution was detailed in the supporting information (Section S6); the complete course of the photoelectrochemical experiment was detailed in the supporting information (Section S7); the complete course for the degradation elimination experiment of SMX was detailed in the supporting information (Section S8).

4. Conclusions

This study pioneered the successful preparation of a direct Z-type TBHP by employing the ball milling method, and single-component Tm₂FeSbO₇ and BiYO₃ were prepared by employing a solid-state synthesis and high-temperature sintering method. The first comprehensive property characterization spectrograms for TBHP, Tm₂FeSbO_7, and BiYO₃ encompassed the XRD patterns, the Mössbauer spectrum, the FTIR spectra, the Raman spectra, the UV-Vis DR spectra, the XPS, the TEM images, the EDX spectra, the photoluminescence spectra, the transient photocurrent intensity variation spectra, the EIS, the ultraviolet photoelectron spectrograms, and the EPR testing spectra. As a result, the precise crystal structures, microstructures, chemical compositions, and electrochemical properties for TBHP, Tm₂FeSbO_7, and BiYO₃ were revealed. The experimental results indicated that Tm₂FeSbO₇ crystallized in a cubic pyrochlore-type structure; BiYO₃ displayed a fluorite structure. TBHP exhibited remarkable separative efficiency of photo-induced electrons and photogenerated holes, absorption performance, and higher degradation efficiency of the SMX with a narrower bandgap. After VISLI of 115 min, TBHP showcased exceptional photocatalytic degradation performance; therefore, the elimination rate of the SMX with the TBHP was 99.51%. Simultaneously, the mineralization rate for eliminating the TOC density during the photocatalytic elimination course of the SMX with the TBHP reached 98.10%. In contrast to Tm₂FeSbO₇, BiYO₃, or N-TiO₂, TBHP exhibited enhanced degradation efficiency for SMX and a higher mineralization rate for TOC removal. Cyclic stability tests confirmed the promising practical applications of the TBHP. These enhancements were attributed to the Z-scheme electron transport mechanism, which effectively preserved the redox capacities of both Tm₂FeSbO₇ and BiYO₃. The trapping radicals experiments and the EPR testing results revealed that hydroxyl radicals, superoxide radicals, and photoinduced holes were the primary active species (•OH > •O₂⁻ > h⁺) during the catalytic elimination course of the SMX by employing the TBHP under VISLI. Moreover, the possible elimination pathways and the elimination mechanism of SMX were proposed. Notably, this research not only advanced the development of high-efficiency Z-type heterostructure photocatalysts but also demonstrated the practical applicability of the prepared heterojunction photocatalyst for effectively decomposing SMX. The results held significant theoretical value and practical value for environmental rehabilitation applications.