BiOBr@PZT Nanocomposite Membranes via Electrospinning-SILAR Technology: A Sustainable Green Material for Photocatalytic Degradation in Coloration-Related Wastewater Remediation

Abstract

The textile industry encounters serious environmental challenges from wastewater with persistent organic pollutants, demanding sustainable solutions for remediation. Herein, we report a novel green synthesis of flexible BiOBr@PZT nanocomposite membranes via electrospinning and successive ionic layer adsorption and reaction (SILAR) for visible-light-driven photocatalytic degradation. The hierarchical structure integrates leaf-like BiOBr nanosheets with PAN/ZnO/TiO₂ (PZT) nanofibers, forming a Z-scheme heterojunction. This enhances the separation of photogenerated carriers while preserving mechanical integrity. SILAR-enabled low temperature deposition ensures eco-friendly fabrication by avoiding toxic precursors and cutting energy use. Optimized BiOBr@PZT-5 shows exceptional photocatalytic performance, achieving 97.6% tetracycline hydrochloride (TCH) degradation under visible light in 120 min. It also has strong tensile strength (4.29 MPa) and cycling stability. Mechanistic studies show efficient generation of O₂⁻ and OH radicals through synergistic light absorption, charge transfer, and turbulence-enhanced mass diffusion. The material’s flexibility allows reusable turbulent flow applications, overcoming rigid catalyst limitations. Aligning with green chemistry and UN SDGs, this work advances multifunctional photocatalytic systems for scalable, energy-efficient wastewater treatment, offering a paradigm that integrates environmental remediation with industrial adaptability.

1. Introduction

The textile printing [1] and dyeing industry is a cornerstone of modern society, but it is under increasing environmental scrutiny. This industry uses massive amounts of water and generates toxic wastewater containing persistent organic pollutants (POPs) [2], such as synthetic dyes [3], heavy metals [4], and drug residues (such as tetracycline antibiotics [5]). If not effectively controlled, these pollutants can severely threaten aquatic ecosystems and human health through bioaccumulation and food-chain disruption. Conventional wastewater treatment methods (e.g., chemical coagulation [6] and activated carbon adsorption [7]) often have high operating costs, cause secondary pollution, or incompletely mineralize hard-to-degrade compounds. There is an urgent need for sustainable, low-energy alternative solutions [8].

In recent years, photocatalysis has emerged as a promising green solution in environmental remediation. It uses light-driven redox reactions to degrade pollutants into harmless CO₂ and H₂O [9,10]. However, traditional photocatalytic materials [11,12] have problems such as structural brittleness, low carrier separation efficiency [13], and energy-intensive synthesis processes, which contradict green chemistry and sustainable engineering principles. Semiconductor photocatalysis, as an economical and sustainable green technology, has shown great potential for degrading organic pollutants [14]. For example, TiO₂ is valued for its low cost and good chemical stability [15,16,17], and ZnO is noted for its high electron mobility [18,19,20]. However, their relatively wide band gaps limit visible light absorption and photocatalytic performance [21,22]. In addition, powder catalysts [23] are hard to separate and prone to suspension. Traditional immobilization methods (such as sintering [24]) can damage nanostructures, reducing activity and making recovery difficult. Among many photocatalysts, the layered semiconductor BiOBr [25,26], with a narrow band gap (2.7–2.8 eV), stands out for its excellent visible light response and stable photochemical properties, making it suitable for dye-wastewater treatment. However, its low conductivity [27] and fast photogenerated electron-hole recombination [28] have restricted practical applications. Constructing heterojunctions with metal oxides (such as CuO [29,30], ZnO [31,32], and TiO₂ [33,34]) has become an effective strategy to enhance carrier separation efficiency and broaden light absorption.

Meanwhile, the development of sustainable dyeing processes demands integrated materials that can reduce pollution and comply with circular economy principles. Electrospinning technology, with its simplicity and environmental friendliness, is seen as a versatile and efficient method for preparing organic flexible nanofiber felts with controllable diameters [14,35,36]. Among these, electrospun PAN nanofiber membranes offer advantages such as hydrophobicity, low density, and mechanical flexibility, making them ideal templates for catalyst immobilization [37,38]. Combining flexible substrates with heterojunction photocatalysts is a forward-looking strategy to achieve both mechanical resilience and enhanced catalytic activity [39].

Pioneering the first eco-friendly synthesis of its kind, this study introduces flexible BiOBr@PZT nanocomposite membranes fabricated through a synergistic combination of electrospinning and successive ionic layer adsorption and reaction (SILAR). The innovation lies in the novel hierarchical architecture: electrospun PAN/ZnO/TiO₂ (PZT) nanofibers, which uniquely integrate ZnO’s piezoelectric properties with TiO₂’s photocatalytic activity, are seamlessly coupled with vertically aligned leaf-like BiOBr nanosheets via SILAR. This design achieves a dual-functional Z-scheme heterojunction—enhancing visible-light-driven charge separation while preserving mechanical resilience—and enables turbulence-enhanced photocatalysis through the membrane’s inherent flexibility, a stark departure from rigid catalyst limitations. Crucially, the low-temperature SILAR protocol eliminates toxic precursors, minimizes solvent use, and reduces energy consumption, aligning with green chemistry principles. The optimized BiOBr@PZT-5 membrane exemplifies these breakthroughs, delivering unprecedented TCH degradation (97.6% under visible light), remarkable tensile strength (4.29 MPa), and robust cycling stability, outperforming conventional rigid photocatalysts in both efficiency and reusability.

The BiOBr@PZT nanocomposite film demonstrates theoretical potential and structural advantages when dealing with organic and inorganic pollutants in complex wastewater. Its Z-scheme heterojunction structure enables the generation of ·O₂⁻ and ·OH radicals under visible light, which can nonselectively attack the conjugated structures and functional groups of organic pollutants. This mechanism allows for the broad-spectrum degradation of dyes (e.g., methylene blue [40]) and drug residues (e.g., ibuprofen [41]). The hierarchical structure, composed of a flexible fibrous substrate and BiOBr nanosheets, enhances adsorption and mass transfer efficiency through a turbulence effect. Regarding inorganic pollutants, the ZnO/TiO₂ components in the material facilitate the photoreduction of heavy metal ions, such as Cr⁶⁺ to Cr³⁺ [42], and the BiOBr surface hydroxyl groups may adsorb Pb²⁺ cations [43]. However, the coexistence of competing pollutants in complex wastewater and the deactivation of catalysts during prolonged use may impact performance. These issues can be mitigated through surface charge regulation or process optimization. While this study focuses on tetracycline degradation, the radical-driven mechanism and structural characteristics offer a theoretical basis for treating diverse pollutants. Future research directions include expanding the range of pollutants for validation, synergizing composite processes, and conducting simulated real -wastewater studies. These steps are necessary to further clarify the industrial applicability and define the application boundaries of this approach.

By integrating hierarchical material design with eco-friendly synthesis, this study advances photocatalytic wastewater treatment and provides a paradigm for developing multifunctional green materials that balance environmental remediation and industrial scalability. Moreover, this visible light-driven photocatalytic system mimics natural photosynthesis. Using color tunable BiOBr nanosheets with a band gap of 2.70 eV, it enables efficient sunlight utilization and offers a “green-coloring” strategy for solar energy conversion. These findings not only propel the field of photocatalytic remediation but also provide a blueprint for designing multifunctional materials, in line with the United Nations’ Sustainable Development Goals (SDGs).

2. Materials and Methods

2.1. Materials

Polyacrylonitrile (PAN, MW = 150,000), N, N-Dimethylformamide (analysis pure, >99.5%), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), and potassium bromide (KBr) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China) Zinc oxide was purchased Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium oxide (anatase) was purchased from ShangHai D&B Biological Science and Technology Co., Ltd. (Shanghai, China). Tetracycline hydrochloride (TCH, 96%) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The TCH solution was prepared in pure water. All chemicals were used without further purification.

2.2. Preparation of the PAN/ZnO/TiO₂ Nanofiber Membrane

Firstly, dissolve 1 g of PAN powder in 10 mL of DMF at room temperature and stir for 2 h to obtain a 10 wt% PAN solution. Then weigh 0.8 g of ZnO and 0.8 g of TiO₂ powder and add them to the prepared PAN solution. After ultrasonication for 30 min, stir for 4 h to obtain the final spinning solution. Perform electrospinning (voltage: 19 V, feed rate: 40 × 10⁻⁴ mm s⁻¹) to obtain the PAN/ZnO/TiO₂ (PZT) nanofiber membrane.

2.3. Preparation of the BiOBr@PZT Nanofiber Membrane

Initially, aqueous solutions of Bi(NO₃)₃·5H₂O and KBr with fixed concentrations were separately prepared. A specific amount of Bi(NO₃)₃·5H₂O and KBr powder was weighed and separately added to containers holding deionized water, followed by uniform stirring for 30 min until the powder was completely dissolved and well mixed, thus obtaining Bi(NO₃)₃·5H₂O and KBr solutions both at a concentration of 5 mM. Then, a fiber membrane with uniform thickness and a smooth surface was soaked in the prepared Bi(NO₃)₃·5H₂O solution. During this process, Bi³⁺ ions were adsorbed onto the surface of the fiber membrane. Subsequently, the fiber membrane was rinsed with deionized water to remove unadsorbed Bi³⁺ ions and other potential impurities. After that, the Bi³⁺-adsorbed fiber membrane was immersed in the prepared KBr solution. Here, the Br⁻ ions in the solution reacted with the pre-adsorbed Bi³⁺ ions on the fiber membrane surface to form BiOBr nanosheets. Once again, the fiber membrane was washed with deionized water to eliminate unreacted ions and other possible impurities.

This procedure could be repeated multiple times to deposit varying amounts of BiOBr nanosheets on the fiber membrane. Fiber membranes could be fabricated with 1, 5, and 10 immersion cycles, respectively. After each soaking was performed, the fiber membrane was dried in an oven at 60 °C for 2 h to promote water evaporation and consolidate the attachment of the nanosheets. Finally, through the SILAR process with different numbers of cycles, BiOBr@TiO₂ nanofiber membranes with distinct BiOBr nanosheet contents were obtained. These membranes are denoted as BiOBr@PZT-1, BiOBr@PZT-5, and BiOBr@PZT-10, where the numbers indicate the number of SILAR cycles. The overall preparation process is illustrated in Figure 1.

2.4. Characterization

2.4.1. Morphology and Elements Test

The surface morphology of the samples was investigated using a scanning electron microscope (SEM, ZEISS Sigma 300, Carl Zeiss AG, Oberkochen, Germany), using energy dispersive X-ray spectrometer (EDS, Oxford Xplore, Oxford Instruments, Oxford, UK) to distinguish the chemical elemental composition of the BiOBr@PZT-5.

2.4.2. Fourier Transform Infrared Spectra and X-Ray Diffractometer

The FTIR characterization of the nanofiber membranes was performed using an infrared spectrometer (Spectrum Tow, PerkinElmer Instruments, Waltham, MA, USA). The spectral scan range was set from 4000 to 400 cm⁻¹, utilizing the X-ray diffractometer (XRD, Bruker D2 Phaser, Bruker Corporation, Billerica, MA, USA) with Cu Kα and a scanning speed of 2° min⁻¹ for the X-ray diffraction testing of the samples.

2.4.3. Mechanical Property Test

This study employs an electronic fabric strength tester (XS(08)F2-250, Shanghai Xusai Instrument Co., Ltd., Shanghai, China) to measure the tensile strength of four photocatalysts. Each type is tested five times.

2.4.4. Photocatalytic and Photoelectric Tests

To align with real-world environmental remediation needs, TCH was chosen as a model pollutant, and a 300 W xenon lamp (CEL-HXF300-T3, Beijing China Education Au-light Technology Co., Ltd., Beijing, China) provided visible light at 14 V and 20 A. Using a UV–vis spectrophotometer (UV2200) (Persee, Shanghai, China), the photocatalytic performance of four nanofiber membranes was measured. Each 2 × 2 cm² photocatalyst (10 mg) was immersed in 20 mL of 10 mg L⁻¹ TCH solution. Before reaction started, the solution was dark-adapted for 30 min for adsorption equilibrium. Then, the light source was turned on, and samples were taken every 30 min. Except for the blank, each sample underwent 2 min of centrifugation at 8000 rpm, followed by injection into a cuvette for absorbance measurement at 357 nm using the UV–vis spectrophotometer. Meanwhile, the absorption spectra of the tetracycline hydrochloride aqueous solution at different time points during the catalytic process were measured for the four photocatalysts using a UV–vis spectrophotometer (UV2200).

The photoluminescence (PL) spectra of the samples were measured using a steady-state/transient fluorescence spectrometer (Edinburgh FLS 1000, Edinburgh Instruments, Livingston, UK). The photovoltaic performance of BiOBr@PZT-5 was tested with an electrochemical workstation (CHI-660E, Chenhua Instrument Co., Ltd., Shanghai, China), with a platinum wire and Ag/AgCl electrode as the counter and reference electrodes, respectively. For the working electrode, 10 mg of powder was dispersed in 1 mL of ultrapure water/ethanol, mixed with 50 μL of Nafion solution, and sonicated for 30 min. Then, 100 μL of the suspension was dropped onto ITO and air-dried. During testing, 0.5 M Na₂SO₄ solution was used as the electrolyte.

2.4.5. Optical Test

The UV–vis diffuse reflectance spectra (DRS) were acquired by employing a spectrophotometer (Shimadzu UV-3600i Plus, Shimadzu Corporation, Kyoto, Japan) equipped with an integrating sphere.

2.4.6. Zeta Potential Test

Zeta potential was measured using a Zetasizer nano ZS (Malvern Instruments Ltd., Malvern, UK) via electrophoretic light scattering at pH 2, 4, 6, 8, 10, and 12, with pH adjustment using 0.1 M NaOH or 0.1 M HCl. The pH of each suspension was measured using pH paper.

2.4.7. Thermal Insulation Performance Test

Surface temperature distribution was measured using infrared thermography (FLIR T540, Teledyne FLIR LLC, Wilsonwille, OR, USA).

3. Results

3.1. Morphology and Elements

Figure 2 presents the SEM images of different samples. In Figure 2a, the fiber surface is smooth. In contrast, Figure 2b–d show that the fibers in these images have leaf-shaped nanosheets on their surfaces. As the number of SILAR cycles increases, the number of nanosheets on the fiber surface also increases. When the cycle number is only one, a few BiOBr nanosheets are visible on the fiber’s surface, with no obvious particle aggregation. Thus, the fiber may resemble PZT nanofibers and have little impact on the fiber diameter. When the SILAR cycle increases to five, more BiOBr nanosheets are present on the fiber surface, with a relatively uniform distribution, though local aggregation may still occur. When the cycle number reaches ten, BiOBr nanosheets almost entirely cover the original fiber surface, likely making the fiber surface very rough and causing the BiOBr nanosheets to form a continuous coating. The SEM images of BiOBr@PZT with three different cycle numbers all confirm the successful preparation of 2D BiOBr nanosheets, with the basic fiber structure remaining intact. Relevant experimental studies have demonstrated that the orientation and size of nanosheets significantly influence the construction of heterojunctions and photocatalytic performance [44]. When nanosheets can be arranged in an orderly manner and maintain close contact with the matrix fibers, they effectively shorten carrier transport pathways and reduce energy loss.

From Figure 3a, it can be seen that the BiOBr nanosheets are still evenly distributed in other regions of the BiOBr@PZT-5 nanofibers, further confirming the successful preparation of the samples. Figure 3b shows the elemental composition of BiOBr@PZT-5 and reveals the uniform distribution of C, O, Ti, Zn, Bi, and Br in the selected region. In the EDS elemental mapping of BiOBr@PZT-5, the distribution of Bi and Br on the fiber membrane surface was correlated with that of C, O, Ti, and Zn. Bi and Br were concentrated in the nanosheet regions, while C, O, Ti, and Zn were homogeneously distributed across the entire fiber membrane cross section. This elemental distribution pattern suggests that the BiOBr nanosheets primarily grew on the PZT fiber surface and exhibited good bonding with it. The homogeneous distribution of C and O may relate to the electrospinning process, and that of Ti and Zn reflects the uniform dispersion of ZnO and TiO₂ in PAN fibers, which helps form a uniform electric field distribution, promoting the separation and transfer of photogenerated carriers. There might be synergism between the Bi and Br elements in the BiOBr nanosheets and the Ti and Zn in PZT fibers, jointly constructing a Z-scheme heterojunction to enhance photocatalytic performance.

3.2. FTIR and XRD

To verify the successful preparation of the materials, FTIR spectroscopy was used to analyze four types of samples (Figure 4a). At 610 cm⁻¹, an increase in the intensity of the BiOBr characteristic peak with the number of SILAR cycles was observed [45]. This not only confirmed the successful loading of the BiOBr nanosheets but also indicated a positive correlation between the number of cycles and the loading amount of BiOBr, consistent with the results shown in Figure 2b–d. Additionally, compared with the PZT curve, the Ti-O/Zn-O peak in the BiOBr@PZT curve shifted to a higher wavenumber after BiOBr loading (e.g., from ~500 cm⁻¹ in PZT to ~520 cm⁻¹ in BiOBr@PZT-10), suggesting possible chemical bonding between Bi-O and Ti-O/Zn-O, forming heterojunctions such as Bi-O-Ti. This facilitates the separation of photogenerated carriers, thereby enhancing photocatalytic activity.

The crystalline structure was investigated using XRD patterns (Figure 4b). For the PZT nanofiber membranes, the diffraction peaks can be well indexed to anatase TiO₂ (JCPDS no. 99-0008) and zincite ZnO (JCPDS no. 99-0111). The other three fiber membranes loaded with BiOBr nanosheets exhibited strong diffraction peaks corresponding to BiOBr (JCPDS no. 09-0393) at 2θ angles of 10.9°, 25.2°, 31.7°, 32.2°, 40.4°, 46.2°, 57.1°, and 76.7°. The corresponding crystal planes were (001), (101), (102), (110), (103), (200), (212), and (310). Moreover, it can be observed that, as the number of cycles increases, the diffraction peak intensity of BiOBr gradually enhances, further confirming the successful loading of BiOBr nanosheets. Meanwhile, compared to other BiOBr crystal planes in the BiOBr@PZT nanofiber membrane, the (001) and (102) crystal planes exhibit higher intensities, indicating that BiOBr is well suited for growth and loading on the surface of PZT nanofibers.

The successful preparation of the samples was confirmed by FTIR and XRD tests. Anatase TiO₂, with its high specific surface area and numerous active sites, effectively adsorbs and degrades organic pollutants. Zincite ZnO, known for its high electron mobility, rapidly conducts photo-generated electrons, reducing electron–hole pair recombination and enhancing carrier separation efficiency. BiOBr, featuring a narrow band gap, efficiently harvests visible light, generating electron–hole pairs under irradiation and extending the photocatalyst’s spectral response range.

3.3. Flexibility and Mechanical Performance

Given the green and sustainable application environment of fiber membranes, flexibility and mechanical properties are crucial. As shown in Figure 5a, the fiber membranes possess excellent flexibility, maintaining their original state after folding and twisting, and can be folded at 180° without damage. Figure 5b presents the tensile strength of four samples (each set of data was obtained from five parallel experiments), clearly indicating that BiOBr@PZT-5 has the highest tensile strength of 4.29 MPa. This may be attributed to the uniform growth of BiOBr nanosheets on the fiber surface after five cycles (Figure 2c and Figure 3a), forming a leaf-shaped 2D structure that intertwines with PZT fibers to create a 3D reinforcing framework. In BiOBr@PZT-1, the single cycle results in insufficient BiOBr loading, with only a few nanosheets sparsely distributed on the surface, leading to limited enhancement of tensile strength. When the cycle number reaches ten, the slightly lower tensile strength of BiOBr@PZT-10 compared to BiOBr@PZT-5 may result from excessive coverage of BiOBr nanosheets on the fiber surface (Figure 2d), forming a thick and rigid outer layer that causes brittleness. The mechanical superiority of the composite system originates from distinct functional contributions of its components. The hierarchical blade-like structure formed by BiOBr nanosheets on PAN/ZnO/TiO₂ (PZT) nanofiber surfaces was demonstrated to enhance tensile strength through interfacial bonding with the PZT matrix. ZnO nanoparticles, which were embedded in PAN fibers via electrospinning, functioned as reinforcing fillers to improve structural rigidity and stability. In the PZT matrix, TiO₂ nanoparticles were identified to enhance thermal stability while effectively preventing PAN fiber deformation during successive ionic layer adsorption and reaction (SILAR) processing. The PAN polymer, serving as the electrospun fiber substrate, was observed to impart both flexibility and tensile strength to the composite material. Notably, the three-dimensional porous network structure enabled uniform stress distribution across the membrane, ensuring mechanical durability and fracture resistance during repeated utilization cycles.

The superior flexibility and mechanical properties of BiOBr@PZT-5 improve its process adaptability, photocatalytic efficiency, and recyclability, promoting the development of green and sustainable technology in the following ways: the elasticity of BiOBr@PZT facilitates the formation of turbulence and enhances the interaction between the catalyst and the pollutants. Specifically, when water flow impacts the flexible fibrous membrane, the swaying and bending of the fibers disrupt the initially ordered water flow, rendering it irregular and thereby promoting the formation of turbulence. This turbulence can break down the concentration boundary layer of pollutants on the catalyst surface, increase the collision frequency between pollutants and the active sites of the catalyst, and facilitate the adsorption and degradation of pollutants. Under turbulent conditions, traditional rigid catalysts, such as ceramic or metal-based materials, are prone to breakage, pulverization, or detachment of the active components due to hydraulic impact. In contrast, BiOBr@PZT-5 can withstand the dynamic shear force and periodic deformation in turbulence (as shown in Figure 5a), thus avoiding structural damage and maintaining the integrity of the active sites. Rigid materials usually have a relatively thick static boundary layer on their surface, through which pollutants have to diffuse slowly to reach the active sites, especially forming mass transfer bottlenecks in low-flow-rate regions. Unlike rigid catalysts, the flexible fibrous structure of BiOBr@PZT-5 can induce small-scale turbulence in flowing water, thereby enhancing the probability of contact with pollutants and boosting the efficiency of mass transfer. Moreover, the flexible structure allows for more uniform light absorption and distribution, thereby improving the utilization efficiency of photogenerated carriers.

The mechanical properties, especially the tensile strength, are of great significance for ensuring the stability and service life of the membrane under industrial conditions. The BiOBr@PZT membrane exhibits high tensile strength at 4.29 MPa, which enhances its advantages for industrial applications. In complex industrial settings, membrane materials are often subjected to various mechanical stresses, such as water flow impact, equipment vibration, and friction with other components. The high tensile strength enables the BiOBr@PZT membrane to maintain its structural integrity and resist rupture and deformation when exposed to these external forces. This ensures the stable performance of its photocatalytic activity. In contrast, membranes with lower tensile strength may be prone to damage even under relatively minor external forces, leading to a reduced exposure of active sites, decreased photocatalytic efficiency, and potentially frequent replacement requirements. This would increase maintenance costs and production downtime. Moreover, the excellent mechanical properties of the BiOBr@PZT membrane enhance its reliability and durability during installation, transportation, and long-term use. They allow it to adapt to different operating conditions and requirements in industrial production, thereby extending its service life. These advantages make BiOBr@PZT-5 a more economical and efficient solution for industrial wastewater treatment and other applications.

3.4. The Photocatalytic and Photoelectric Properties

In this study, to investigate the photocatalytic performance, TCH solutions at concentrations of 5, 10, 15, 20, and 25 mg L⁻¹ were prepared, and their absorbance was measured to obtain the fitted curve, as shown in Figure 6a. The photocatalytic degradation efficiency was characterized by the C/C₀ ratio, which is the ratio of the TCH concentration at different times to the initial concentration. The equation for C is as follows:

C = (A + 0.003)/0.0342

(1)

where A is the absorbance of the TCH solution at 357 nm at different times. As indicated in Figure 6b, without a catalyst, the TCH concentration remained nearly constant. However, upon addition of a catalyst, a significant enhancement in photocatalytic activity was observed. After 120 min of visible light irradiation, the degradation rates of TCH by PZT, BiOBr@PZT-1, BiOBr@PZT-5, and BiOBr@PZT-10 nanofiber membranes were 60.8%, 67.8%, 97.6%, and 76.8%, respectively. These results demonstrate that the loading of BiOBr nanosheets effectively improved the photocatalytic performance of PZT nanofiber membranes. As can be seen from Figure 6b, there is minimal change in the PZT curve between 90 and 120 min. This may be because, during the prolonged photocatalytic reaction, despite the stability of the PZT nanofibrous membrane, some physical or chemical changes, such as catalyst aggregation, may still occur. Additionally, the intermediate products generated during the degradation process may have adsorptivity that occupies the active sites of PZT, thereby inhibiting further reactions. However, these factors do not affect the overall stability of the catalyst.

To further confirm the optimal photocatalytic performance of BiOBr@PZT-5 nanofiber membranes, kinetic analysis of TCH degradation was conducted using the Langmuir–Hinshelwood pseudo-first-order kinetic equation:

−ln(C₀/C_t) = kt

(2)

where k (min⁻¹) is the degradation rate constant. As shown in Figure 6c, the k values for PZT, BiOBr@PZT-1, BiOBr@PZT-5, and BiOBr@PZT-10 nanofiber membranes were 0.0077, 0.0079, 0.02789, and 0.01057, respectively. It was found that the k value of BiOBr@PZT-5 was 3.53 times that of BiOBr@PZT-1 and 2.64 times that of BiOBr@PZT-10, indicating that BiOBr@PZT-5 nanofiber membranes exhibited the best degradation performance for TCH under visible light.

Although the BiOBr@PZT-5 membrane demonstrates remarkable TCH degradation efficiency (97.6%) under visible light, further TOC/COD analysis and toxicity assessment are crucial for validating complete mineralization and ecological safety. These studies will be prioritized in subsequent large-scale trials to bridge the gap between laboratory innovation and industrial application.

The measurement wavelength range for the absorption spectrum of tetracycline hydrochloride photocatalytic degradation was 250–400 nm, covering the region of the tetracycline dual-absorption band. As the photocatalytic degradation time was prolonged from 30 to 120 min, a general downward trend was observed in the absorbance of the absorption spectra in each figure. This indicates that the concentration of tetracycline decreased during the photocatalytic degradation process. By monitoring the changes in absorbance at the characteristic main absorption peak of tetracycline (357 nm) (Figure 7), a TCH degradation efficiency of 97.6% was achieved by BiOBr@PZT-5 within 120 min. Compared to the main peak at 357 nm, the rate of absorbance decrease at the subsidiary peak (270 nm) slowed down (e.g., PZT and BiOBr@PZT-1), possibly due to competitive adsorption of intermediate products (e.g., benzoic-acid-like substances). However, these substances were ultimately completely mineralized, as evidenced by the absence of new absorption peaks. The synchronous attenuation of the dual-absorption band confirmed that the Z-scheme heterojunction effectively disrupted the conjugated structure and benzene ring groups of tetracycline through the synergistic action of ·OH and ·O₂⁻.

The excellent photocatalytic performance of BiOBr@PZT-5 is attributed to the synergistic effects of its components. BiOBr, as the main visible-light absorber, generates electron–hole pairs. In a Z-scheme, electrons transfer from the conduction band of BiOBr to ZnO/TiO₂, while holes remain in the valence band of BiOBr. This maintains the redox capability for O₂ reduction (generating ·O₂⁻) and H₂O oxidation (generating ·OH), driving efficient tetracycline degradation. ZnO enhances photocatalytic activity by promoting charge separation and extending light absorption. The piezoelectric property of PZT may induce an additional electric field under mechanical stress (e.g., turbulence), further suppressing charge recombination. Although TiO₂ has lower visible-light activity, it stabilizes the heterojunction and prolongs carrier lifetimes. PAN, while non-photocatalytic, provides a fibrous structure to immobilize BiOBr/ZnO/TiO₂, preventing nanoparticle loss.

In this study, PL spectra were used to characterize the recombination rate of photogenerated electron–hole pairs in the four samples. As shown in Figure 8a, the PZT nanofiber membrane exhibited a strong fluorescent peak in the 450–550 nm range. The BiOBr@PZT-1 nanofiber membrane showed a slightly lower fluorescent intensity compared to PZT, indicating that the introduction of BiOBr reduced carrier recombination through a heterojunction. The BiOBr@PZT-5 sample had the lowest fluorescent intensity, suggesting the highest carrier separation efficiency and optimal photocatalytic performance. In contrast, BiOBr@PZT-10 showed a slight increase in fluorescent intensity, possibly due to the excessive stacking of BiOBr nanosheets, which increased interfacial defects and provided recombination sites for carriers. Based on these results, BiOBr@PZT-5, with the best photocatalytic performance, was selected for transient photocurrent response testing. As shown in Figure 8b, under visible light irradiation, the photocurrent of BiOBr@PZT-5 rapidly increased and stabilized after the xenon lamp was turned on, and it continued to grow with time. The photoresponse was cycled three times to confirm its stability.

3.5. Optical Properties

The UV–visible absorption spectra of three BiOBr@PZT nanofiber membranes with different cycle numbers were measured to investigate their light absorption properties. As shown in Figure 9a, the absorbance in the visible light region increased with the number of cycles, but the absorbance of BiOBr@PZT-10 was lower than that of BiOBr@PZT-5, which is consistent with the results in Figure 6b. The band gaps of the prepared BiOBr@PZT were evaluated using the following equation:

αhν = A(hν − Eg)^n/2

(3)

where α is the absorption coefficient, hν is the discrete photon energy, Eg is the band gap, A is a constant, and n is 4 for indirect transitions. As shown in Figure 9b–d, the band gaps of BiOBr@PZT-1, BiOBr@PZT-5, and BiOBr@PZT-10 nanofibers were evaluated as 2.75 eV, 2.70 eV, and 2.67 eV, respectively. This indicates that as the loading of BiOBr nanosheets increased, the band edge of the nanofiber membranes slightly red-shifted, which is beneficial for enhancing their photocatalytic performance.

3.6. Zeta Potential

Figure 10 presents the relationship between the zeta potential of BiOBr@PZT-5 nanofibrous membranes and pH. When the pH is 4.95, the zeta potential is 0, indicating that the sample’s pH of zero point charge (pHpzpc) is 4.95. Therefore, the sample surface is negatively charged when the solution pH exceeds 4.95. Tetracycline carries a negative charge under specific pH conditions, particularly within the experimental pH range for tetracycline adsorption studies. Since the sample surface and tetracycline are both negatively charged (at pH > 4.95), the adsorption of tetracycline onto the sample is reduced due to electrostatic repulsion. This pHpzc value offers a theoretical basis for understanding the adsorption behavior of tetracycline on the sample surface, aligning with the experimentally observed low adsorption capacity.

3.7. Photocatalytic Mechanism

As shown in Figure 11, the process of visible-light photocatalysis using BiOBr@PZT-5 involves four key steps: “light absorption, carrier separation, radical generation, and pollutant mineralization”. Initially, the photocatalyst absorbs light: the 2.70 eV bandgap of BiOBr@PZT-5 (Figure 9c) allows visible light photons (energy > 2.70 eV) to excite electrons from the valence band (VB) to the conduction band (CB), creating holes in the VB. Next, the separation of electron–hole pairs is crucial. BiOBr’s layered structure spatially separates the electrons (migrating to the CB) and holes (remaining in the VB), reducing recombination. Simultaneously, ZnO and TiO₂ in PZT act as electron acceptors, forming heterojunctions that further facilitate carrier separation. Subsequently, the separated electrons and holes initiate redox reactions. Conduction-band electrons react with O₂ to produce superoxide radicals (·O₂⁻), and valence-band holes react with H₂O or OH⁻ to generate hydroxyl radicals (·OH). These highly oxidative radicals are pivotal for pollutant degradation: they oxidize organic compounds such as TCH, mineralizing them into CO₂, H₂O, and other harmless substances, thereby achieving environmental remediation.

Figure 12 details the mechanism of the Z-scheme heterojunction formed by BiOBr and ZnO/TiO₂ for the visible-light-driven degradation of tetracycline hydrochloride. Firstly, the narrow bandgap of BiOBr (2.70 eV) endows it with the ability to absorb visible light (λ > 460 nm). Its layered structure, equipped with an intrinsic electric field, facilitates the separation of charges. Moreover, the exposed (001)/(102) crystal planes enhance the surface reactivity. TiO₂, with its wide bandgap (3.2 eV), predominantly absorbs ultraviolet light and its conduction band aligns with the energy levels of ZnO, which promotes the transfer of electrons to O₂, resulting in the generation of superoxide radicals (·O₂⁻). The hydrophobicity of PAN intensifies the adsorption of pollutants on the fiber surface, thereby concentrating tetracycline in the vicinity of the catalytic active sites. Moreover, its chemical inertness obviates the occurrence of side reactions.

The formation mechanism of the Z-scheme heterojunction is based on the band alignment and optimized charge transfer pathways between BiOBr and PZT (PAN/ZnO/TiO₂) components. The Z-scheme heterojunction enhances the separation efficiency of photoinduced charges and photocatalytic performance under visible light through its band alignment, interface chemical bonding, and enhanced visible light absorption and redox capabilities. The band alignment is characterized by the CB of BiOBr being more positive than the VB of ZnO/TiO₂, and the VB of BiOBr being more negative than the CB of ZnO/TiO₂. Upon visible light excitation, the electrons in BiOBr are promoted to its CB, while the holes remain in the VB of ZnO/TiO₂. In the Z-scheme mechanism, the high-energy electrons in the CB of BiOBr transfer to the VB of ZnO/TiO₂ and recombine with the holes there. Meanwhile, the holes in the VB of BiOBr and the electrons in the CB of ZnO/TiO₂ are retained, effectively suppressing the recombination of electron–hole pairs. The interface chemical bonding is achieved through Bi–O–Ti/Zn bonds formed between BiOBr nanosheets deposited via the SILAR technique and Ti/Zn–O on the surface of PZT fibers. This bonding promotes the directional transfer of photoinduced charge carriers from BiOBr to ZnO/TiO₂. The enhanced visible light absorption and redox capabilities are due to the narrow bandgap of BiOBr. The VB holes of BiOBr can directly oxidize H₂O/OH⁻ to generate hydroxyl radicals (·OH), and the CB electrons of ZnO/TiO₂ can reduce O₂ to produce superoxide radicals (·O₂⁻). The Z-scheme heterojunction promotes directional charge transfer via its built-in electric field. When two semiconductors come into contact, a built-in electric field forms due to their different Fermi levels. In the BiOBr@PZT system, upon contact between BiOBr and PZT (ZnO/TiO₂), electrons spontaneously transfer from the CB of BiOBr to the VB of PZT, with holes migrating in the opposite direction. This forms a Z-scheme charge transfer path, effectively separating electrons and holes and reducing their recombination probability. The Z-scheme heterojunction’s band structure is stepwise. This design retains high-energy electrons (in BiOBr’s CB) and strong oxidizing holes (in PZT’s VB) on opposite sides of the heterojunction. It avoids the redox capability loss of traditional type-II heterojunctions and prolongs carrier lifetime through spatial separation. The sustained response of BiOBr@PZT-5 in transient photocurrent tests (Figure 7b) confirms this mechanism. The TCH under visible light relies on the Z-scheme heterojunction photocatalysis of the BiOBr@PZT nanocomposite membrane. With a band gap of 2.70 eV, this material absorbs visible light, exciting electrons from the valence band of BiOBr to its conduction band, creating electron–hole pairs. The Z-scheme heterojunction facilitates carrier separation, enabling electron transfer to the conduction band of PZT and hole retention in the valence band of BiOBr. The separated electrons react with oxygen to form ·O₂⁻, while the holes generate ·OH by reacting with H₂O/OH⁻. These radicals synergistically attack the conjugated structure and functional groups of TCH, fragmenting and mineralizing it into CO₂ and H₂O. UV–vis spectroscopy shows the synchronous decay of dual-absorption bands, confirming this process. In the experiments, photo current and PL analysis also verify the efficient separation of carriers and the dominant role of radicals, highlighting the Z-scheme heterojunction-driven efficient photocatalytic degradation mechanism.

3.8. Thermal Insulation Performance

Figure 13a,b present infrared camera images of four samples (each with a thickness of 2 mm) placed on ice for three minutes. It can be seen that the greater the number of cycles, the greater the change in the surface temperature of the fibrous membrane. The surface temperature of BiOBr@PZT-10 is around 13 °C, that of BiOBr@PZT-5 is around 10 °C, that of BiOBr@PZT-1 is around 6 °C, and that of PZT is around 4 °C. Figure 13c,d show infrared camera images of the four samples (each with a thickness of 2 mm) placed on a heating stage and heated to 55 °C for three minutes. It can be seen that the greater the number of cycles, the lower the surface temperature of the sample. The surface temperature of BiOBr@PZT-10 is around 40 °C, that of BiOBr@PZT-5 is around 43 °C, that of BiOBr@PZT-1 is around 48 °C, and that of PZT is around 50 °C.

It has been demonstrated that the increased amount of BiOBr nanosheets attached reduces the thermal conductivity of the fibrous membrane. An appropriate proportion of BiOBr and suitable doping elements can work together to optimize the material’s performance, enhancing its stability and photocatalytic efficiency in complex environments. Taking all the properties into account, the BiOBr@PZT-5 material can achieve efficient and stable operation under extreme conditions, providing a scientific basis for its practical application in environmental remediation and industrial wastewater treatment, among other fields.

4. Discussion

This study demonstrates the successful preparation of a flexible BiOBr@PZT nanocomposite membrane through environmentally benign electrospinning and SILAR techniques, thereby presenting a sustainable resolution to the pivotal obstacles encountered in photocatalytic wastewater treatment. The Z-scheme heterojunction of BiOBr nanosheets and PZT nanofibers has been shown to provide a comprehensive enhancement of the photocatalytic process efficiency. This configuration not only augments visible light absorption but also substantially diminishes electron–hole recombination, a persistent issue that has hampered the performance of traditional photocatalytic materials. Furthermore, the mechanical robustness of the membrane has been significantly improved, achieving a tensile strength of 4.29 MPa, a critical attribute for practical implementation. The optimized BiOBr@PZT-5 configuration has achieved a notable 97.6% degradation of TCH under visible light within a 120 min timeframe, a performance that clearly surpasses that of conventional rigid catalysts. This exceptional efficacy is attributed to the unique material combination and the innovative fabrication methodology employed in this study. The sustainable synthesis approach has minimized energy consumption and eliminated toxic precursors, thereby aligning with the principles of green chemistry and sustainable engineering.

The hierarchical structure of the BiOBr@PZT nanocomposite membrane has played a pivotal role in its photocatalytic performance. The hierarchical structure of BiOBr@PZT-5 consists of a flexible PAN/ZnO/TiO₂ (PZT) nanofibrous substrate and vertically grown BiOBr nanosheets (Figure 2c and Figure 3a). The synergistic effect of these components increases the active surface area for pollutant adsorption and photocatalytic degradation. The primary structure, the PZT fiber substrate, forms a 3D porous network between fibers, providing a high specific surface area that facilitates initial pollutant adsorption and mass transfer. The secondary structure, BiOBr nanosheets grown via the SILAR technique, vertically coat the fiber surface in a “leaflet-like” manner (Figure 2c). Their open structure offers abundant adsorption sites. As 2D nanosheet structures, the uniform distribution of BiOBr nanosheets on the PZT nanofibers significantly increases the contact area between the catalyst and pollutants compared to bare PZT nanofibers, providing more active sites for pollutant adsorption. In the experiments, BiOBr@PZT-5 shows much better photocatalytic degradation of tetracycline hydrochloride (TCH) under visible light compared to PZT. This is largely due to its larger specific surface area, which offers more sites for TCH adsorption and thus improves degradation efficiency. The integration of leaf-like BiOBr nanosheets with PAN/ZnO/TiO₂ (PZT) nanofibers to form a Z-scheme heterojunction has effectively enhanced the separation of photogenerated carriers. This structural design has provided an extensive surface area for pollutant adsorption and has facilitated the transfer of photogenerated electrons and holes, leading to an elevated degradation efficiency. When compared to previous studies, in which the construction of heterojunctions was constrained by preparation methods and material compatibility, the approach detailed in this study offers a more versatile and efficient strategy to enhance photocatalytic performance.

The low-temperature deposition facilitated by SILAR technology represents another significant aspect of this study. In addition to ensuring an eco-friendly fabrication process, this method has preserved the structural integrity of the nanocomposite membrane. The low-temperature SILAR process optimizes the crystallinity, morphology, and adhesion of BiOBr layers. XRD analysis (Figure 4b) reveals sharp characteristic peaks of BiOBr layers prepared via low-temperature SILAR, matching the standard BiOBr crystal structure (JCPDS no. 09-0393). As SILAR cycles increase, the peak intensity grows, indicating enhanced crystallinity. This is because the low-temperature SILAR process involves layer-by-layer adsorption and reaction, avoiding the structural defects caused by high temperatures. Under low-temperature conditions, BiOBr precursor ions can uniformly adsorb and orderly arrange on the PZT nanofiber surface, forming well-crystallized BiOBr nanosheets. This process also allows precise control over the thickness and deposition rate of BiOBr layers, further refining their crystalline state. In terms of morphology, BiOBr exhibits a typical layered structure on the PZT nanofiber surface, as shown in Figure 2. With increasing SILAR cycles, the size and number of BiOBr nanosheets increase, and they become more uniformly distributed. This is attributed to the controlled concentration of precursor solutions, immersion time, and reaction temperature during the SILAR process. The low temperature facilitates slow diffusion and uniform adsorption of BiOBr precursor ions on the fiber surface, promoting the formation of regular nanosheet structures. This uniform morphology increases the active surface area and enhances the separation and transfer efficiency of photogenerated charge carriers, thereby improving photocatalytic performance. Regarding adhesion, FTIR analysis (Figure 4a) shows a shift in the Ti-O/Zn-O peak position from ~500 to ~520 cm⁻¹ in BiOBr@PZT, indicating the formation of Bi-O-Ti/Zn bonds. These bonds are formed through the coordination reaction of Bi³⁺ with hydroxyl groups on the PZT surface, strengthening the adhesion between BiOBr layers and the substrate.

Traditional synthesis methods often involve high temperatures and toxic precursors, which can result in structural damage and increased energy consumption. By circumventing these issues, the method employed in this study provides a more sustainable pathway for the synthesis of photocatalytic materials, thereby meeting the growing demand for environmentally benign technologies within the field of photocatalysis. The flexibility and mechanical properties of the BiOBr@PZT-5 membrane are also of significant importance. The membrane’s capacity to maintain structural integrity after folding and twisting, along with its high tensile strength, renders it highly suitable for practical wastewater treatment applications. This flexibility permits the membrane to accommodate the deformation of wastewater treatment equipment, thereby preventing the loss of active sites commonly associated with rigid catalysts. Additionally, the flexible fiber membrane can generate a turbulence effect within water, increasing the contact area between the wastewater and the photocatalyst, and consequently enhancing photocatalytic efficiency. This feature extends the replacement cycle of the membrane, reduces costs, and improves the overall sustainability of the wastewater treatment process.

Within the broader context of environmental remediation, this study provides a scalable and flexible platform for industrial wastewater treatment. The integration of environmental sustainability with practical engineering represents a significant advancement in the field of photocatalytic materials. Compared to conventional synthesis methods such as high-temperature sintering and toxic solvent-based approaches, the electrospinning/SILAR technique offers significant advantages for environmental and industrial applications. This method uses low-temperature solution deposition (drying at 60 °C) and room-temperature electrospinning, eliminating the need for high temperatures and toxic precursors. It achieves green synthesis with non-toxic reagents and produces flexible membrane materials that are recyclable, reducing secondary pollution. In industrial applications, the flexible PAN/ZnO/TiO₂ substrate fabricated by this technique provides a tensile strength of 4.29 MPa, enabling the material to accommodate equipment deformation and enhance mass transfer through turbulence effects. The number of SILAR cycles can be controlled to adjust the loading amount, precisely optimizing catalytic performance (e.g., BiOBr@PZT-5 achieves a degradation rate of 97.6%), while the Z-scheme heterojunction improves carrier separation efficiency. Unlike traditional methods that suffer from high energy consumption, brittleness, and significant batch-to-batch variations, this combined technique is both environmentally sustainable and industrially adaptable, offering an efficient solution for wastewater treatment. This research not only advances the development of efficient photocatalytic materials for pollutant degradation but also provides promising research directions for large-scale industrial wastewater treatment. For instance, in pharmaceutical wastewater treatment, the high levels of organic drug residues and antibiotics are difficult to degrade using conventional methods. However, the excellent photocatalytic performance of BiOBr@PZT makes it an ideal material for this application. In textile wastewater treatment, which involves large amounts of synthetic dyes and organic additives, BiOBr@PZT can effectively degrade these pollutants, ensuring that the treated water meets discharge standards. This is in line with the green development trend of the textile industry. As for petrochemical wastewater treatment, the complex organic compounds and heavy metal ions present in the wastewater can be simultaneously treated by the multifunctional BiOBr@PZT, improving treatment efficiency. When combined with other treatment technologies, such as chemical precipitation and adsorption, the treatment effectiveness can be further enhanced.

The BiOBr@PZT nanocomposite membrane can be integrated with solar energy-harvesting devices or smart sensors to create hybrid systems that offer innovative solutions for wastewater treatment. In a solar-driven setup, the visible-light-responsive BiOBr@PZT can be paired with photovoltaic panels to form a self-sustained photocatalytic reactor. The photovoltaic panels supply power to auxiliary components, such as water pumps or UV LEDs for nighttime operation, while the BiOBr@PZT membrane directly uses sunlight to degrade pollutants, minimizing external energy input. The flexibility of BiOBr@PZT also allows it to be deployed in floating photocatalytic platforms. These platforms can utilize natural water-body illumination and water flow for passive restoration. In smart systems, BiOBr@PZT can be integrated with real-time sensors (e.g., pH, turbidity, or optical sensors) to dynamically monitor pollutant concentrations and adjust treatment parameters automatically. For example, smart sensors can trigger the photocatalytic activity of BiOBr@PZT based on detected pollutant levels. They can also use adaptive LED arrays to adjust light intensity for optimal energy efficiency. Machine learning algorithms further enhance process control by predicting degradation kinetics and optimizing reactor operation. These hybrid systems align with the Internet of Things paradigm and support remote monitoring and scalable applications in industrial or decentralized settings. Although further research is needed to address system-engineering design and long-term stability issues, the inherent flexibility, durability, and high photocatalytic efficiency of BiOBr@PZT make it an ideal candidate for next-generation smart and solar-driven wastewater treatment technologies.

Future research can focus on optimizing the composition and structure of BiOBr@PZT nanocomposite membranes to enhance long-term stability and recyclability. One approach is interfacial bond strengthening through the introduction of a functional intermediate layer (e.g., polydopamine [46]) between BiOBr nanosheets and PZT nanofibers to enhance interfacial bonding. This can anchor BiOBr nanosheets via chemical bonding or physical adsorption, reducing detachment risks during extended use. Alternatively, incorporate interfacial crosslinking agents (e.g., polyethylenimine [47]) during SILAR deposition to minimize BiOBr nanosheet loss during recycling. Another approach is improved recyclability through magnetic composites by incorporating magnetic nanoparticles (e.g., Fe₃O₄ [48]) into the BiOBr@PZT membrane. This allows for rapid separation and recycling using an external magnetic field without significantly affecting photocatalytic performance, thereby improving operational efficiency during recycling.