Construction of Z-Scheme Heterojunction BiOCl/Bi₂WO₆ for Visible-Light Photocatalytic Degradation of Tetracycline Hydrochloride

Abstract

Tetracycline hydrochloride pollution poses a serious environmental threat; however, it is difficult to deal with by conventional methods. In this study, the Z-scheme BiOCl/Bi₂WO₆ composite was hydrothermally synthesized and evaluated for its ability to decompose tetracycline hydrochloride under visible light. The composite material was systematically characterized by XRD, SEM, TEM/HRTEM, XPS, FTIR, BET, PL, UV-Vis DRS, and EPR to analyze its structure, morphology, and optical/electrochemical properties. Characterization revealed that the composite featured a flower-ball structure with broader light absorption and higher solar energy efficiency. A narrow bandgap further facilitated charge separation, boosting photocatalytic performance. Among the synthesized materials, the 20% BiOCl/Bi₂WO₆ composite exhibited the best performance, removing 94% of tetracycline hydrochloride in 60 min, which was 5.2 times and 1.4 times higher than pure BiOCl and Bi₂WO₆, respectively. The rate constant was 10.8 times and 2.5 times higher than that of pure BiOCl and Bi₂WO₆. After five cycles, it maintained the 88.7% removal rate, with X-ray diffraction analysis confirming its structural stability and well mechanical properties. Electron paramagnetic resonance and radical scavenging experiments identified photogenerated holes (h⁺) and superoxide radicals (·O₂⁻) as the primary active species. This work highlights the fact that the prepared Z-scheme BiOCl/Bi₂WO₆ composite exhibited excellent photocatalytic performance in the degradation of tetracycline hydrochloride, demonstrating promising potential for practical applications.

1. Introduction

In recent years, due to the overuse of antibiotics in disease treatment and livestock, antibiotic residues are ubiquitous in the water body, leading to severe water pollution [1]. In particular, tetracycline hydrochloride (TCH) is persistent and accumulative, posing significant threats to human health and ecosystems [2]. To deal with TCH pollution, various strategies have been developed, including adsorption [3,4], biodegradation [5,6], membrane separation [7,8], Fenton processes [9,10], and photocatalysis. Among these methods, photocatalytic technology has gained extensive attention due to its properties of cost-effectiveness, high efficiency, and eco-friendliness [11,12]. For example, evidence shows that potassium peroxymonosulfate activation of WO₃/diatomite composites can effectively degrade TCH [13]; however, the great dependence on external oxidants significantly increases treatment costs. In addition, the rapid recombination of photogenerated carriers and the limited absorption of visible light significantly hinder the practical applications of photocatalysts [12]. Thus, the development of highly efficient photocatalysts is a matter of urgency.

Recently, bismuth-containing compounds have become a hotspot of research due to their unique properties, e.g., high photocatalytic performance, robust antimicrobial activity, non-toxicity, and strong absorption ability in the near-infrared region [11,14]. Bi₂WO₆ is an orthorhombic perovskite-structured material composed of (Bi₂O₂)²⁺ layers and perovskite-like octahedral (WO₄)_n²ⁿ⁻ layers [15], which is widely used in hydrogen production, CO₂ reduction, and the degradation of organic pollutants, due to its suitable bandgap, visible-light responsiveness, non-toxicity, and stability [16]. Wu et al. [17] enhanced hydrogen-production efficiency to 56.9 mol/g/h by adjusting the surface terminations to produce atomically thin two-dimensional layers of Bi₂WO₆. Jiang et al. [18] reported a fast photocatalytic conversion reaction of CO₂ using three-dimensional hollow-structured Bi₂WO₆ synthesized via a solvothermal method.

The halide perovskite BiOCl is composed of alternating Bi₂O₂ and double Cl⁻ layers, forming a layered network structure that facilitates the migration of photogenerated electron-hole pairs [19]; however, the large bandgap (approximately 3.2–3.5 eV) limits absorption to UV light, resulting in a suboptimal photocatalytic performance [20]. To effectively utilize the performance advantages of BiOCl, various modification strategies have been proposed [21,22,23,24]. Yang et al. [25] synthesized S-doped hydroxylated BiOCl catalysts via a one-pot high-pressure reaction with the kinetic constant being 15 times that of BiOCl. Hu et al. [26] developed a recrystallization method to fabricate three-dimensional multilevel defect-rich BiOCl, exhibiting hexagonal prismatic morphology and containing numerous oxygen vacancies. Moreover, constructing heterostructures is an effective method of improving charge separation. By combining various semiconductor materials through certain methods, each material can leverage its advantages, enabling the heterostructure to meet the practical application requirements [27]. For example, compared to pure components, the successful construction of the Cd_0.5Zn_0.5S/Bi₂WO₆ S-scheme heterojunction significantly improved the separation and stability of photogenerated electron-hole pairs in degrading tetracycline and reducing Cr(VI) [28].

Former studies investigated Bi-based heterostructures for photocatalytic applications that rely on conventional type-I or type-II configurations [29], therefore limiting charge-separation efficiency. To improve charge separation and preserve redox potential, a Z-scheme BiOCl/Bi₂WO₆ heterostructure was synthesized in this study utilizing hydrothermal and deposition methods, supplemented by in situ growth techniques. This synthetic strategy offers an efficient and sustainable route to develop advanced photocatalysts due to its simplicity, cost-effectiveness, and eco-friendly nature. Comprehensive characterizations were conducted to measure the structure, morphology, optical, and electrochemical properties of materials using XRD, SEM, TEM/HRTEM, XPS, FTIR, BET, PL, UV-Vis DRS, and EPR techniques. The photocatalytic degradation of TCH under visible-light irradiation was systematically investigated to assess the influence of various parameters on the photocatalytic performance. The main active species involved in the photocatalytic processes were identified through radical scavenging experiments. This study also delves into the underlying mechanisms responsible for the enhancement of photocatalytic activity, providing insights into the development of more effective photocatalytic systems for environmental remediation.

2. Materials and Methods

2.1. Synthesis of Bi₂WO₆, BiOCl, and BiOCl/Bi₂WO₆ Composites

Firstly, 2 mmol of Bi(NO₃)₃·5H₂O and 1 mmol of Na₂WO₄·2H₂O were dissolved into deionized water and transferred to a 100 mL Teflon-lined stainless-steel autoclave. The mixture underwent a hydrothermal reaction at 180 °C for 12 h to obtain Bi₂WO₆.

Next, 2 mmol of KCl and 1 mmol of Bi(NO₃)₃·5H₂O were dissolved in 40 mL of deionized water and stirred for 1 h. Different amounts of Bi₂WO₆ were added to the above-mentioned solution and stirred for another 1 h. Then, the solution was transferred into a 100 mL Teflon-lined autoclave to react at 180 °C for 18 h to obtain BiOCl/Bi₂WO₆. The mass proportions of BiOCl in the BiOCl/Bi₂WO₆ composites were 10%, 20%, 30%, and 40%, denoted as 10-BOC/BWO, 20-BOC/BWO, 30-BOC/BWO, and 40-BOC/BWO, respectively. Pure BiOCl was synthesized by omitting Bi₂WO₆ in these steps. Additionally, to confirm the improved performance results from true heterojunction formation, a physical mixture of BiOCl and Bi₂WO₆ with 20% BiOCl by mass (20-BOC+BWO) was synthesized under the same conditions.

2.2. Characterization of the Synthesized Samples

The absorbance of the samples was analyzed using an ultraviolet-visible spectrophotometer (UV-5100, Yuanxi Instruments Co., Ltd., Shanghai, China). The crystal phase structure was characterized by powder X-ray diffraction (XRD, TD-3500, Dandong Tongda Technology Co., Ltd., Dandong, China) with a scan range from 10° to 80°. Morphological features were examined by a field-emission scanning electron microscope (SEM, Gemini 300, Carl Zeiss AG, Oberkochen, Germany). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed to assess microcrystalline structures (JEM-2100F, JEOL Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo ESCALAB 250Xi (Thermo Fisher Scientific Inc., Waltham, MA, USA). Surface functional groups of BiOCl, Bi₂WO₆, and BiOCl/Bi₂WO₆ were analyzed with Fourier transform infrared spectroscopy (FT-IR, Nicolet Nexus 670, Thermo Nicolet Corporation, Waltham, MA, USA). Brunauer–Emmett–Teller (BET) tests for nitrogen adsorption–desorption isotherms were conducted by a Quantachrome Autosorb-iQ. Photoluminescence (PL) spectra were recorded to measure the recombination of photo-generated electron-hole pairs using a fluorescence spectrophotometer (PL, FLS980, Edinburgh Instruments Ltd., Livingston, UK). The optical-absorption properties were measured using a UV-vis diffuse reflectance spectrometer (UV-vis DRS, UV3600, Shimadzu Corporation, Kyoto, Japan).

2.3. Photocatalytic Degradation Experiment

The degradation experiment was carried out using a xenon light source (PLS-SXE300+, Beijing Perfectlight Technology Co., Ltd., Beijing, China) to simulate sunlight. The experiments were conducted in a photoreactor with a circulating water-cooling system to maintain a constant temperature of 25 °C. First, 100 mg of the photocatalyst was ultrasonically dispersed in 100 mL of TCH solution (20 mg/L). The solution was stirred in the dark for 30 min to achieve adsorption-desorption equilibrium before light exposure. During the photocatalytic degradation under simulated sunlight (350 W xenon lamp, λ > 420 nm), samples were collected every 10 min and filtered through a 0.22 μm membrane to measure TCH concentration using a UV-visible spectrophotometer (357 nm). The remaining TCH solution containing photocatalyst was centrifuged to collect powders for 4 repetitions of the photocatalytic experiment in order to facilitate an assessment of the stability of the photocatalyst.

Electron paramagnetic resonance spectroscopy (EPR, Bruker A300, Bruker Corporation, Karlsruhe, Germany) was utilized to measure the oxidative active species mainly involved in the reaction. The active species superoxide radicals (·O₂⁻), photogenerated holes (h⁺), and hydroxyl radicals (·OH) were quenched by adding p-benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), and isopropanol (IPA), respectively, to investigate the photocatalytic mechanism. Moreover, Cl⁻ and HCO₃⁻ were added to examine the influence of co-ions on TCH degradation.

3. Results and Discussion

3.1. Characterization of Catalysts

XRD analysis (Figure S1) confirmed the successful synthesis of Bi₂WO₆ and BiOCl; the characteristic diffraction peaks at 2θ values corresponded to orthorhombic Bi₂WO₆ (JPCDS card No.79-2381) and tetragonal BiOCl (JPCDS card No.85-0861). No significant shifts in the diffraction peaks of the BiOCl/Bi₂WO₆ composites were observed, indicating that the presence of BiOCl did not cause lattice distortion in Bi₂WO₆ [30]. Peaks became more prominent as the BiOCl content increased, confirming the successful integration. However, the intensity of the (002) and (003) planes of BiOCl was reduced in the composites, suggesting that Bi₂WO₆ suppressed BiOCl crystal growth [30].

SEM images (Figure 1) show that BiOCl consists of irregular, dispersed nanosheets, while Bi₂WO₆ forms flower-like structures with voids [31]. The composite shows BiOCl nanosheets covering the Bi₂WO₆ surface and maintaining a similar flower-like morphology, confirming successful formation of composite. TEM and HRTEM images (Figure 2a–e) show irregular nanosized particles; these images aligned with the SEM results. As magnification increased (Figure 2b,c), particle details and crystal boundaries became clearer. The fast Fourier transform analysis (Figure 2d,e) confirmed the presence of clear lattice fringes in the BiOCl/Bi₂WO₆, reflecting the high crystallinity and ordered crystal structure. The crystal planes in regions I–IV indicated the presence of both BiOCl and Bi₂WO₆ and further confirmed the successful synthesis of the BiOCl/Bi₂WO₆ composite. Energy-dispersive X-ray spectroscopy (EDX) (Figure 2f) revealed the presence of Bi, W, Cl, and O elements in the composites.

XPS spectra (Figure S2a) further confirmed the presence of Bi, W, Cl, and O elements in the BiOCl/Bi₂WO₆ composite. The spectra were calibrated using the C1s peak at 284.8 eV (Figure S2b) [32]. High-resolution Bi 4f spectra (Figure S2c) indicated chemical-state changes in Bi during composite formation, while O 1s spectra (Figure S2d) showed peaks for W-O, Bi-O, and adsorbed oxygen [33,34]. The Cl 2p spectra (Figure S2e) revealed peaks of Cl 2p_1/2 and Cl 2p_3/2, with a shift in the Cl 2p peak of the BiOCl/Bi₂WO₆ heterojunction. This shift indicated a strong interaction between Cl⁻ and WO₄²⁻ interactions and an increase in binding energy due to electron loss in BiOCl [35]. W 4f spectra (Figure S2f) confirmed the presence of W⁶⁺; moreover, the shifts of binding energy indicated electron accumulation in Bi₂WO₆ and an increase in surface-charge density. These changes confirmed electron transfer from BiOCl to Bi₂WO₆, establishing an internal electric field and successful heterojunction formation [36].

As shown in Figure S3, the FTIR spectra of BiOCl, Bi₂WO₆, and BiOCl/Bi₂WO₆ reveal functional group changes before and after composite formation. The peaks at 520 cm⁻¹ and 1382 cm⁻¹ in BiOCl represent the Bi-O vibrations and Bi-Cl stretching, respectively [37]. In Bi₂WO₆ and BiOCl/Bi₂WO₆, the peaks at 573 cm⁻¹ and 731 cm⁻¹, as well as 818 cm⁻¹ and 1383 cm⁻¹, represent Bi-O, W-O, and W-O-W bonds, respectively [38,39].

Figure S4a displays nitrogen adsorption–desorption isotherms, revealing type-IV isotherms for Bi₂WO₆ and BiOCl/Bi₂WO₆, characterized by H3 hysteresis loops [40,41]. In contrast, BiOCl exhibits a type-III isotherm, typical of non-porous or microporous materials with minimal surface area. Pore-size distribution (Figure S4b–d), determined by the Barrett–Joyner–Halenda (BJH) and density functional theory (DFT) methods, shows predominant pore sizes of 9–15 nm for Bi₂WO₆ and BiOCl/Bi₂WO₆. As summarized in Table S1, BiOCl/Bi₂WO₆ exhibits the highest specific surface area, pore volume, and pore size, indicating that it is favorable for pollutant adsorption [42]. The optical-absorption characteristics of the prepared samples were analyzed using UV-Vis diffuse reflectance spectroscopy. Figure S5a shows that BiOCl has an optical-absorption edge at 370 nm, whereas Bi₂WO₆ and BiOCl/Bi₂WO₆ display light-absorption edges between 440 and 460 nm.

The bandgap (E_g) of photocatalysts is determined by the Kubelka–Munk equation:

$${\mathrm{Ah}\mathsf{\upsilon }}^{1/\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)$$

(1)

where α is the absorption coefficient, h is the Planck constant, $\upsilon$ is the photon frequency, E_g is the bandgap, A is a constant, and n is 1/2 and 2 for direct and indirect bandgap semiconductors, respectively. Both pure BiOCl and Bi₂WO₆ are classified as indirect bandgap semiconductors, meaning n is 2 [30]. As illustrated in Figure S5b, the E_g of BiOCl, Bi₂WO₆, and BiOCl/Bi₂WO₆ are 3.29 eV, 2.56 eV, and 2.51 eV, respectively. The reduced E_g of the composite material facilitates carrier migration within the BiOCl/Bi₂WO₆ heterojunction, enhancing light-absorption efficiency and photocatalytic activity [43].

PL spectroscopy (Figure 3) was employed to evaluate the separation efficiency of photo-generated electron-hole pairs. The BiOCl/Bi₂WO₆ composite shows lower fluorescence intensity compared to BiOCl, suggesting that the heterojunction structure effectively suppresses the recombination of electron holes. The enhanced carrier separation prolongs their lifetime and improves charge-transfer efficiency, ultimately enhancing photocatalytic performance. However, the observed reduction in PL intensity for the BiOCl/Bi₂WO₆ composite is relatively modest, implying that single PL may not be sufficient to fully validate the improvement in carrier-separation efficiency.

3.2. Photocatalytic Activity

In Figure 4a, the TCH solution containing photocatalyst was equilibrated in the dark for 30 min to achieve adsorption–desorption equilibrium. The dark treatment led to variations in the initial (t = 0) concentrations across samples, which can be attributed to the difference in the ability of the composites to absorb the TCH pollution during the treatment. As shown in Figure S4 and Table S1, BiOCl/Bi₂WO₆ composites possessed a higher surface area and pore volume compared to the pure BiOCl and Bi₂WO₆, leading to greater TCH adsorption at equilibrium. After irradiation for 60 min, the concentration of TCH decreased with time. The BiOCl/Bi₂WO₆ composite, particularly 20-BOC/BWO, exhibited superior degradation efficiency compared to pure BiOCl and Bi₂WO₆. The removal efficiencies of BiOCl and Bi₂WO₆ were 18.2% and 66.8%, respectively, while the removal efficiencies of BiOCl/Bi₂WO₆ composites ranged from 83.1% (10-BOC/BWO) to 94.0% (20-BOC/BWO). Notably, the photocatalytic TCH degradation rate of 20-BOC/BWO showed significant improvement. Additionally, the degradation efficiency of 20-BOC+BWO was only 73.4%, which was significantly lower than that of 20-BOC/BWO (94.0%), demonstrating that the performance enhancement stems from genuine heterojunction formation. The apparent quantum yield (AQY) serves as a critical tool for evaluating the efficiency of photon utilization in photocatalytic processes [44,45], defined as the ratio of the number of degraded molecules to the number of incident photons. All experiments were performed under the same visible-light irradiation source and intensity; the number of incident photons remained constant. Therefore, the differences in TCH degradation performance among the photocatalysts directly reflect their relative AQY values. The 20% BiOCl/Bi₂WO₆ composite exhibited the highest degradation efficiency, indicating its superior photocatalytic capability compared to the other synthesized materials, particularly pure BiOCl and Bi₂WO₆.

Kinetic analysis reveals that TCH degradation follows a pseudo-first-order model:

ln(C₀/C_t) = kt

(2)

where C₀, C_t, k, and t represent the concentration of TCH (mg/L) at initial and t time, the apparent reaction-rate constant, and time (min), respectively.

Figure 4b shows the pseudo-first-order kinetic fitting curves and apparent rate constants for each sample under visible light. In particular, the reaction-rate constant of the BiOCl/Bi₂WO₆ composite was significantly higher than that of pure BiOCl and Bi₂WO₆. Specifically, the degradation rate reached 0.0312 min⁻¹ when the proportion of BiOCl in the composite was 20%, which was 2.5 times and 10.8 times higher than that of pure Bi₂WO₆ and BiOCl, respectively. These results confirmed that the introduction of BiOCl enhanced the photocatalytic performance of the BiOCl/Bi₂WO₆ composite.

Figure S6a,b illustrates the influence of initial TCH concentrations on the photocatalytic degradation performance of the 20-BOC/BWO composite material. As shown in Figure S6a, the pollutant-removal rate decreased as initial TCH concentration increased. Figure S6b demonstrates that the apparent rate constants declined as the pollutant concentration increased. Figure S6c demonstrates that photocatalytic efficiency increased with catalyst mass; when the dosage of catalyst was 0.1 g, maximum degradation efficiencies achieved 93%; however, the efficiency declined when the dosage of catalyst exceeded 0.1 g. Excessive catalyst dosage can cause turbidity, blocking light paths and reducing light absorption [46]. Additionally, increased catalyst content may lead to particle aggregation, decreasing the effective reaction surface area. The pseudo-first-order kinetic curves and apparent rate constants in Figure S6d show that the rate constants initially increased and then decreased with catalyst dosage. According to the previous studies, this behavior may be attributed to factors such as light scattering and reduced light penetration, which inhibited the effective utilization of the photocatalyst surface [47,48].

To further assess photocatalytic performance, 100 mg of 20-BOC/BWO was applied to degrade 100 mL of congo red (CR, 100 mg/L), rhodamine B (RhB, 50 mg/L), and TCH (20 mg/L) under simulated visible light. As shown in Figure S6e, 99.7% CR was degraded within 20 min, highlighting its high efficiency. After 40 min, the degradation rate of RhB also reached 99.7%, while the degradation rate of TCH increased to 93.9% over 60 min. Overall, 20-BOC/BWO composite demonstrated excellent versatility to degrade CR, RhB, and TCH. Figure S6f illustrates the influence of anions in the solution on the photocatalytic degradation of TCH. The results indicate that the addition of Cl⁻, SO₄²⁻, and NO₃⁻ reduced the degradation rate; moreover, the inhibition strengthened as the concentration of these anions increased.

To evaluate the stability of the 20-BOC/BWO composite photocatalyst, TCH degradation experiments were conducted over five consecutive cycles under identical conditions. As shown in Figure S7a, despite a slight decrease in efficiency after the five cycles, the degradation rate remained 88%, showing strong stability and mechanical robustness. Figure S7b presents the XRD results of the 20-BOC/BWO composite catalyst before and after the cycling experiments. The positions and intensities of the diffraction peaks were unchanged, confirming that the crystal structure remained stable after five cycles. These results further demonstrate that the 20-BOC/BWO composite catalyst is suitable for repeated use in photocatalytic applications.

3.3. Mechanism of Enhanced Photoactivity

EPR experiments further identified h⁺ and ·O₂⁻ as the dominant active species in the BOC/BWO photocatalytic system. Figure 5a shows that the TEMPO-h⁺ signal peak intensity decreased under illumination, indicating increased h⁺ generation due to effective electron-hole pair separation [43]. In Figure 5b, characteristic quartet signals for DMPO-·O₂⁻ adducts appeared only under illumination, reinforcing the fact that h⁺ and ·O₂⁻ were key active species in the degradation process.

Radical scavenging experiments were conducted to elucidate the photocatalytic degradation mechanism of TCH and identify the primary active species involved. IPA, BQ, and EDTA-2Na were utilized as scavengers for ·OH, ·O₂⁻, and h⁺, respectively. As shown in Figure 6a, IPA, BQ, and EDTA-2Na decreased the photocatalytic degradation efficiencies by 7.1%, 57.2%, and 88.4%, respectively, indicating that ·OH played a minor role. Conversely, the removal of h⁺ and ·O₂⁻ significantly hindered photocatalytic degradation, showing that these active species were vital and played a key role in the degradation of TCH.

The valence band (VB) and conduction band (CB) potentials of the photocatalysts are calculated as Equation (3) [49]:

E_g = E_VB − E_CB

(3)

where E_VB and E_CB are the potentials of VB and CB, respectively.

According to the XPS valence band spectra of the BiOCl and Bi₂WO₆ photocatalysts in Figure 6b, the E_VB values of p-type BiOCl and n-type Bi₂WO₆ are 2.12 eV and 2.28 eV, respectively. The Eg of BiOCl and Bi₂WO₆ are 3.29 eV and 2.56 eV, resulting in E_CB values for BiOCl and Bi₂WO₆ of −1.17 eV and −0.28 eV, respectively.

To explain the enhanced photocatalytic capabilities of the binary system, two mechanisms are proposed in Figure 7a,b: a traditional type-II mechanism and a direct Z-scheme mechanism. As shown in Figure 7a, the E_CB of BiOCl is more negative than that of Bi₂WO₆, while the E_VB of Bi₂WO₆ is more positive than that of BiOCl. In the traditional mechanism, photo-induced electrons from the CB of BiOCl transfer to Bi₂WO₆. However, the reductive capacity of electrons in the CB of Bi₂WO₆ is weaker than the O₂/·O₂⁻ potential (−0.33 eV), thus preventing ·O₂⁻ radical generation [50]. This contradicts the trapping experiments and EPR results that clearly indicate the formation of ·O₂⁻ radicals. Therefore, the traditional type-II mechanism cannot accurately explain the observed photocatalytic activity, demonstrating that it is inappropriate to adopt this mechanism.

In contrast, the direct Z-scheme model illustrated in Figure 7b provides a clearer explanation for the enhanced photocatalytic activity in decomposing TCH [51]. In this mechanism, photo-generated electrons transfer from the CB of BiOCl to the VB of Bi₂WO₆, effectively separating electrons and holes, generating the active radicals necessary for TCH degradation. These radicals, characterized by their high reactivity, drive a series of photoredox reactions. The detailed reactions are outlined in Equations (4)–(8).

BiOCl/Bi₂WO₆ + hν → BiOCl (h⁺ + e⁻)/Bi₂WO₆ (h⁺ + e⁻)

(4)

BiOCl (h⁺)/Bi₂WO₆ (e⁻) → BiOCl/Bi₂WO₆ (recombination)

(5)

Bi₂WO₆ (h⁺) + TCH → degradation products

(6)

BiOCl (e⁻) + O₂ → ·O₂⁻

(7)

·O₂⁻ + TCH → degradation products

(8)

4. Conclusions

In this study, Z-scheme heterojunction BiOCl/Bi₂WO₆ was successfully synthesized via a hydrothermal method and utilized to decompose TCH. The results indicated that the BiOCl/Bi₂WO₆ composite material exhibited a significantly higher carrier-migration rate and visible-light-absorption ability, greatly outperforming the pure Bi₂WO₆ and BiOCl in the degradation of TCH. EPR and radical scavenging tests showed that h⁺ and·O₂⁻ are the key active species in the photocatalytic degradation of TCH. This study presents the Z-scheme BiOCl/Bi₂WO₆ composite as a promising and efficient photocatalyst for TCH degradation, offering great potential for practical applications. Future research can explore its performance in complex water bodies and investigate large-scale production for environmental cleanliness.