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[Hongjian Zhang](https://orcid.org/0009-0004-2835-596X), Qiansu Ma, [Guangqi An](https://orcid.org/0000-0003-2726-1369), [Yunxin Zhu](https://orcid.org/0000-0001-6070-7305), Xiang Sun, [Naoki Kawazoe](https://orcid.org/0000-0003-3916-0709), [Guoping Chen](https://orcid.org/0000-0001-6753-3678), [Yingnan Yang](https://orcid.org/0000-0001-8980-0634)

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[Development of ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 Z-scheme photocatalyst for high-efficiency tetracycline removal: Characterization, degradation pathway and toxicity assessments](https://mdr.nims.go.jp/datasets/4897583a-5bc4-4790-9d15-63d5918eb60f)

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Development of ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 Z-scheme photocatalyst for high-efficiency tetracycline removal: Characterization, degradation pathway and toxicity assessmentsEnvironmental Functional Materials 4 (2025) 21–32Contents lists available at ScienceDirectEnvironmental Functional Materialsjournal homepage: www.keaipublishing.com/en/journals/environmental-functional-materialsDevelopment of ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 Z-scheme photocatalystfor high-efficiency tetracycline removal: Characterization, degradationpathway and toxicity assessmentsHongjian Zhang a, Qiansu Ma a,b, Guangqi An a, Yunxin Zhu a,c, Xiang Sun a, Naoki Kawazoe d,Guoping Chen d, Yingnan Yang a,*a Graduate School of Life and Environmental Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8572, Japanb College of Chemistry and Biological Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Beijing, 100083, PR Chinac Faculty of Bioenvironmental Sciences, Kyoto University of Advanced Science, 1-1 Sogabecho Nanjo Otani, Kameoka, Kyoto, 621-8555, Japand Research Center of Macromolecules and Biomaterials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, JapanH I G H L I G H T S* Corresponding author.E-mail address: yo.innan.fu@u.tsukuba.ac.jp (Y.Peer review under the responsibility of editorial bhttps://doi.org/10.1016/j.efmat.2024.12.004Received 13 November 2024; Received in revised f2773-0581/© 2025 The Authors. Publishing servicelicense (http://creativecommons.org/licenses/by-ncG R A P H I C A L A B S T R A C T� A novel Ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 (ABC) photocatalyst was success-fully developed.� Optimized ABC ratio achieved highcrystallinity and enhanced photo-catalytic activity.� �O2�, hþ, �OH were detected to imply theformation of Z-scheme heterojunction inABC.� TC degradation pathway was proposedbased on intermediates and DFTcalculations.� ABC showed promise for antibiotictreatment under real environmentalconditions.A R T I C L E I N F OKeywords:Photocatalysisg-C3N4 dopingZ-scheme heterojunctionTetracycline degradationIntermediates identificationA B S T R A C TNowadays, tetracycline (TC) contamination of surface water has become a comprehensive environmental problemthat threatens the health and survival of animals and humans. It is essential to develop an efficient, and sus-tainable wastewater treatment strategy. This study introduces a novel Ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 (ABC) Z-scheme photocatalyst for high-efficiency TC removal under visible light. Optimized g-C3N4 doping facilitated theelectron transfer and provided more reaction sites for TC removal. It showed high efficiency, stable TC degra-dation activity and environmental adaptability. Moreover, the active reaction site on TC was predicted via DFTcalculation. In addition, through the analysis of the intermediates identified by LC-MS, the photo-decompositionpathways of TC were proposed. Based on the results of free radical trapping experiments and ESR spectroscopy,the mechanism of TC degradation was suggested. Finally, a toxicity assessment based on the molecular fractalsdata was carried out. The developed ABC showed higher visible light absorbance, better e�-hþ separation, andhigher phase purity and crystallinity. Overall, this study provides valuable insights into the development of Z-scheme heterojunctions composite for efficient organic pollution degradation.Yang).oard of Environmental Functional Materials.orm 20 December 2024; Accepted 23 December 2024s by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND-nd/4.0/).mailto:yo.innan.fu@u.tsukuba.ac.jphttp://crossmark.crossref.org/dialog/?doi=10.1016/j.efmat.2024.12.004&domain=pdfwww.sciencedirect.com/science/journal/27730581www.keaipublishing.com/en/journals/environmental-functional-materialshttps://doi.org/10.1016/j.efmat.2024.12.004http://creativecommons.org/licenses/by-nc-nd/4.0/https://doi.org/10.1016/j.efmat.2024.12.004https://doi.org/10.1016/j.efmat.2024.12.004H. Zhang et al. Environmental Functional Materials 4 (2025) 21–321. IntroductionSurface water contamination caused by antibiotics such as tetracy-cline (TC), ciprofloxacin, amoxicillin and norfloxacin, have posed seriousrisks to the environment and human health. Among these antibiotics, TC,widely used in agriculture and animal husbandry, is persistently releasedinto aquatic systems, leading to microbial resistance and ecological im-balances [1–3]. Given the challenges in treating antibiotic-laden waste-water, effective and sustainable treatment solutions are urgently needed.While methods such as adsorption, membrane separation, coagulationand biodegradation have been explored to remove TC from wastewater[4–6], problems such as high cost, low efficiency, and secondary pollu-tion limit their practical application. Moreover, the variability in envi-ronmental conditions such as pH, temperature, and light intensity furthercomplicates the efficiency of these methods [7,8].Photocatalysis has garnered attention as a promising and eco-friendlysolution for degrading organic pollutants under light irradiation [9].However, achieving efficient photocatalysis under realistic environ-mental conditions remains a challenge, necessitating innovations inmaterial design and functionality. Recently, researchers have found thatheterojunctions with appropriate semiconductor materials can enhancephotocatalytic performance by improving light utilization and electron(e�) and hole (hþ) transfer [10,11]. The Ag/Ag2O/BiPO4/Bi2WO6(Ag-BWO) heterojunction photocatalyst, in particular, offersvisible-light-driven photocatalytic capabilities [12]. In this photo-catalyst, the metallic Ag nanoparticles extend the visible light responserange of Bi-based photocatalysts through surface plasmon resonance(SPR) under visible light irradiation, significantly enhancing photo-catalytic efficiency [12]. Additionally, Ag2O was promisingvisible-light-driven photocatalysts due to their narrow band gaps,enabling efficient utilization of solar energy. Furthermore, Ag-basedsemiconductors can combine with Bi2WO6 and BiPO4 to form hetero-junction structures [13,14]. These heterojunctions improve the separa-tion of e�-hþ pairs, reducing recombination rates and enhancingquantum efficiency. Overall, these properties make Ag-BWO an attractivephotocatalyst for addressing organic pollutant degradation. However, toaddress the degradation of complex antibiotic like TC, further enhance-ment strategies are required, focusing on improving charge separationand catalytic redox capabilities.In recent years, the introduction of Z-scheme heterojunction haspresented a novel approach to boost photocatalytic redox capabilities[15–17]. In contrast to traditional heterojunctions, Z-scheme hetero-junction formation can remove more complex and resistant organicmatter by increasing photocatalysts’ redox ability and enhancingphotoinduced e�-hþ separation efficiency [18,19]. Graphitic carbonnitride (g-C3N4) has been increasingly utilized in Z-scheme photocatalystformation due to its optimal band gap and unique layered structure.Besides, g-C3N4 is characterized by its non-toxic nature,cost-effectiveness, ease of synthesis, commendable light absorption ca-pacity, advanced optical properties, and thermal stability. In addition, itssuitable valence potential (1.4 eV), promotes the generation of super-oxide radicals (⋅O2⁻) essential for effective oxidative degradation[20–22]. Exfoliation techniques allow the preparation of g-C3N4 nano-sheets, which can be easily immobilized with Bi and Ag nanoparticles toimprove the e� interaction and photocatalytic performance [23].Therefore, the advantages of introducing g-C3N4 well-suited forZ-scheme configurations that rely on the synergistic interaction of mul-tiple semiconductor materials [24].Despite recent progress, research on Z-scheme heterojunctionscombining Ag-BWO with g-C3N4 for TC degradation remains limited.Additionally, optimizing the g-C3N4 ratio is crucial for ensuring a highcrystallinity and mature structural growth, which are directly related tophotocatalytic performance. By achieving an ideal interaction between22Bi2WO6 and BiPO4 components in Ag-BWO, the enhanced Z-scheme Ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 (ABC) photocatalyst could facilitate effec-tive TC degradation. This study systematically investigated the impact ofg-C3N4 ratio on photocatalytic efficiency for TC degradation andexplored the structural and optical properties of the developed ABC.Characterizations such as UV–vis, photocurrent, and EIS assessmentsshed light on its photoelectron-chemical properties, while XRD, FTIR,XPS, and TEM were employed to analyze the structure and morphologyof ABC. Additionally, density functional theory (DFT) calculations iden-tified vulnerable sites on the TC molecule, aiding in the degradationpathway analysis and toxicity assessments of intermediates. Thisresearch provides insights into developing high-efficiency Z-schemephotocatalysts for antibiotic degradation under diverse environmentalconditions, suggesting a viable solution for sustainable wastewatertreatment.2. Materials and methods2.1. Preparation of the photocatalystsThe g-C3N4 was synthesized by calcining 10 g of urea at 550 �C for4.5 h with a heating rate of 5 �C/min. After cooling, the g-C3N4 powderwas ground uniformly and pre-treated with sonication in isopropanol(IPA) as the solvent.The Ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 was synthesized through atwo-step hydrothermal method with modifications. First, 45 mL ofNa2WO4 solution (10 μmol) was combined with 15 mL of Bi(NO3)3 so-lution (20 μmol in 1 mol/L HNO3) and stirred for 3 h at room tempera-ture. The pH was adjusted to 7 using NH3⋅H2O before transferring thesolution to a 100 mL Teflon-lined autoclave and heating at 160 �C for 18h. After several washes with ethanol and water, the sample was re-suspended in 45 mL ethanol. 2 μmol Agþ (Ag3PO4 and AgNO3) and0.14 μmol K2HPO4 were dissolved in 5 mL of 1 mol/L HNO3 solution andthen were added into the above mixture with 0.30 g-C3N4 (ABC(10)),0.12 g-C3N4 (ABC(25)), 0.06 g-C3N4 (ABC(50)) together, followed by 10-min sonification. This mixture was then reacted at 120 �C for 4 h, andthe resulting precipitates were collected, washed with ultrapure water,and dried at 85 �C for 24 h to obtain the ABC powders. A control Ag-BWOcomposite was prepared following the same two-step hydrothermalprocess, excluding the addition of g-C3N4.2.2. Evaluation of photocatalytic activityThe photocatalytic activity of the synthesized samples was assessedthrough TC degradation experiments under simulated solar light. Foreach test, 0.050 g of photocatalyst was dispersed in 50 mL of 10 mg/L TCaqueous solution, stirred in the dark for 1 h to achieve adsorption equi-librium, and then irradiated under simulated solar light (550 W/m2) at25 �C. Experiments were conducted in triplicate. Photocatalytic perfor-mance was evaluated comparatively, and degradation kinetics wereanalyzed using the Langmuir-Hinshelwood model:�ln�CtC0�¼ kt (1)where k is the degradation rate constant (min�1), t is the degradationtime. Total organic Carbon (TOC) was examined with a TOC analyzer(TOC-5000A, Shimadzu, Kyoto, Japan).2.3. Structural and photocatalytic characterizationThe morphology of as-prepared photocatalysts was characterized byScanning Electron Microscope (SEM) and energy dispersive X-RayFig. 1. Photocatalytic degradation of tetracycline. (a) Degradation efficiency,(b) first-order kinetic plot, (c) total organic carbon (TOC) degradation rate and(d) cyclic degradation performance of ABC(25). (Irradiation intensity: 550 W/m2,tetracycline concentration: 10 mg/L, pH ¼ 7, Tetracycline absorption peak:357 nm).H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32spectroscopy (EDX) (FE-SEM (S-4800)þ EDX, Hitachi) and TransmissionElectron Microscopy (TEM) (JEOL JEM 2100F). The crystal phase wascharacterized by X-ray Diffraction (XRD) (RINT-Ultima III, Rigaku) withCu Kα radiation (k ¼ 1.54178 Å). The surface chemical environment wasanalyzed using the X-ray photoelectron spectroscopy (XPS) (JPS-9010TR,JEOL) and all the data were calibrated with C 1s binding energy of 284.8eV. Fourier transform infrared (FTIR) spectra were obtained using anFTIR infrared spectrometer (FTIR-6800, JASCO) with KBr as the refer-ence. UV–Vis diffuse reflectance spectra were obtained over a range of300–700 nm (V-750, JASCO). The Brunauer-Emmett-Teller (BET)method was used for the determination of surface area by BeckmanCoulter SA-3100. Photoelectrochemical performance, including electro-chemical impedance spectroscopy (EIS) and photocurrent response, wasmeasured with an Electrochemical Measurement System (HZ-7000,HOKUTO DENKO) using a platinum net as the counter electrode, astandard calomel reference electrode, ITO glass as the working electrode,and 1 mol/L Na2SO4 as the electrolyte. Photocurrent response wasmeasured at 0.6 V.2.4. Detection of reactive species and charge transfer mechanismsTo clarify the mechanism of TC degradation, radical trapping exper-iments were performed. For each experiment, 0.05 g of photocatalyst and1 mM of a specific scavenger were added to 50 mL of 10 mg/L TC so-lution. Ethylenediaminetetraacetic acid (EDTA), IPA, and 1,4-benzoqui-none were used as scavengers for h⁺, hydroxyl radicals (�OH), andsuperoxide radicals (�O2�), respectively. Electron spin resonance (ESR)spectroscopy was also conducted to detect the presence of h⁺, �O2�, and�OH under both light and dark conditions.2.5. DFT calculationStructural optimization and computational analyses of TC were con-ducted using Gaussian 16.0 under B3LYP/6-311G (d,2p) hybrid func-tional method, and analyzed by frequency recording to obtain the wavefunction of the stable structure. The frontier molecular orbitals (thehighest occupied molecular orbits (HOMO), the lowest unoccupied mo-lecular orbits (LUMO) and Fukui functions (the nucleophilic (fþ), elec-trophilic (f-) and radical attack (f0)) were then obtained and plotted byMultiwfn 3.8_dev and VMD (1.9.3 version) [25].2.6. Intermediates analysis and toxicity assessmentTC and its intermediates concentrations were quantified using LC-QTOF/MS (Bruker compact LC-MS System)with the Electrospray ioni-zation (ESI) as the ion resource. Chromatographic separation was per-formed on a C18 columnwith solution A: water phase (0.1 % formic acid)and solution B: organic phase (100%MeCN) with a gradient elution from5 % to 95 % MeCN for 10 min at a flow rate of 0.5 mL/min, by applyingthe following linear gradient: 0 min 1 % B, 4 min 10 % B, 11.50 min 20 %B, 13 min 30 % B, 15 min 45 % B, 18 min 50 % B, 23 min 1 % B, 28 min 1% B. The injection volume was 20 μL, and the oven temperature was setto 25 �C. Detection wavelength was set to 270 nm. With the results fromLC-QTOF/MS, the environmental toxicity of TC and its degradation in-termediates were simulated by quantitative structure-activity relation-ship (QSAR) method by applying the Toxicity Estimation Software Tool(T.E.S.T.) developed by EPA [26].3. Results and discussion3.1. Performance of optimized g-C3N4 mass ratio composite and itsenvironmental adaptabilityFig. 1a illustrated the photocatalytic performances of various g-C3N4mass ratio composite, Ag-BWO, and g-C₃N₄ for TC degradation. Amongthese photocatalysts, ABC(25) demonstrated the highest adsorption and23photocatalytic activity, achieving a degradation efficiency up to 96.8 %within 60 min. Additionally, as shown in Fig. 1b, ABC(25) exhibited thehighest k of 0.0525 min⁻1 compared to the other g-C3N4-based samples.In addition, TOC was further conducted to clarify the mineralization rateof TC (Fig. 1c). After 60 min of photocatalytic degradation process,H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32ABC(25) showed the best mineralization performance of 51.6 %, whichwas superior to other composites. Comparing to the other suboptimalcomposites (ABC(10) and ABC(50)), as well as the single loading of Ag-BWO and g-C3N4, the enhanced degradation efficiency of ABC(25) wasprobably attributed to the most advantageous g-C3N4 mass ratio, whichmay form an ideal interaction between Bi2WO6 and BiPO4. This config-uration promoted the generation of reactive species under solar light,thus facilitating TC removal. Table S1 showed the recent publications ondegradation of TC by Bi2WO6-based photocatalysts. Compared with mostreports, the developed ABC(25) sample exhibited the highest degradationefficiency to TC, which was about 1.75–3.88 times higher than otherreports [27–30]. Furthermore, recycled experiments were conducted toclarify the stability and reusability of synthesized ABC(25) photocatalyst.As shown in Fig. 1d, the ABC(25) maintained high photocatalytic activityabove 96.0 % after 5 cycles. Herein, the synthesized ABC(25) photo-catalyst showed significant promise for effective and sustainable TCpollution treatment.Fig. 2. Influence of environmental factors on TC degradation with ABC(25). (a)pH, (b) temperature, and (c) light irradiation density.24In light of its superior TC degradation ability, the potential of theABC(25) composite for wastewater treatment was further explored byevaluating its performance under various environmental conditions,including pH, temperature, and light intensity.The effects of initial pH on the photocatalytic treatment of TC in waterwas presented in Fig. 2a. ABC(25) exhibited the highest degradation ef-ficiency at a neutral pH of 7 under solar light. While it relatively declinedin both acidic and alkaline conditions. The observed degradation resultscould be ascribed to the different ionic states of TC at varying pH levels.The dissociation constants of TC are 3.32, 7.68, and 9.68 [31], indicatingthat TC exists in distinct ionic forms across different pH level range.Below pH 3.32, TC is predominantly in its cationic form (TCH3þ),reducing its ability to react with radicals due to H⁺ consumption. AbovepH 9.68, TC becomes an anion (TCH� or TC2�), resulting in electrostaticrepulsion and poor adsorption on the negatively charged photocatalystsurface. Between pH 3.32 and 7.68, TC exists as a zwitterion (TCH2�),allowing optimal degradation without competitive reactions or electro-static issues, consistent with theoretical analysis [32]. The results indi-cated that ABC(25) was effective for TC degradation across a wide pHrange from 3 to 10, and suitable under the neutral pH condition. Thisaligns well with the pH of most real-world pollutant environments [33,34], demonstrating its strong environmental adaptability.The influence of temperature on the degradation of TC under solarlight is presented in Fig. 2b. The results showed that the photocatalyticremoval efficiency of TC increases as the temperature raised from 20 �Cto 30 �C. At higher temperatures (30 �C), e�-hþ recombination isreduced, leading to a greater generation of radical species with higheroxidative potential, promoting the TC decomposition. Even at the lowertemperature of 20 �C, the degradation efficiency remained significantunder solar light. In natural environments, the actual water temperaturetypically ranges from 20 �C to 30 �C, depending on geographical location,season, and weather conditions [35]. This temperature range aligns withthe experimental conditions in this study, further supporting the appli-cability of ABC(25) for real-world water treatment scenarios.The effect of light intensity on the photocatalytic efficiency of ABC(25)is shown in Fig. 2c. The results clearly indicated that TC degradationefficiency increased proportionally with light intensity, ranging from 400to 700 W/m2. Specifically, the TC degradation rates at 400, 550, and 700W/m2 were 94.9 %, 96.7 %, and 99.7 %, respectively. The enhancedphotocatalytic performance at higher light intensity (700 W/m2) can beattributed to the increased generation of excited e� and hþ on the pho-tocatalyst surface. Even at lower light intensity (400 W/m2), the TCdegradation efficiency remained high, demonstrating the strong redoxcapability of ABC under low-light conditions. Typically, natural lightintensity is around 550 W/m2, further supporting the applicability ofthese findings in real water conditions [36]. Therefore, the photo-catalytic degradation ratio was positively correlated with the incidentlight irradiation intensity, which is fit for the degradation results. Allthese results concerning about environmental factors highlighted therobust degradation performance of ABC(25) consistently exhibited acrossa variety of typical environmental conditions, making it applicable forreal-world utilization.3.2. Optical and photoelectron-chemical performanceIn order to understand the reasons behind the superior photocatalyticperformance of the optimized ABC(25), systematic characterizationmethods including light absorption, charge recombination, e� transfer,and photocurrent analysis were employed. As illustrated in Fig. 3a, theUV–Vis spectrum of ABC(25) showed the highest absorption intensity.Notably, ABC(25) exhibited an increasing absorption peak from 450 nm to600 nm, with a marginal redshift towards longer wavelengths. On theother hand, ABC(10) showed reduced absorption due to excessive g-C3N4loading, which hindered the growth and development of the Bi2WO6base material. Similarly, ABC(50) showed lower visible light absorption,indicating that lower amounts of g-C3N4 showed no significant improvedFig. 3. (a) UV–Vis absorption spectra, (b) photocurrent density, and (c) EISspectra of different mass ratios of ABC, Ag-BWO and g-C3N4.H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32effect compared with the optimized ABC(25) These pronounced redshiftsarose from the charge-transfer transition between g-C3N4 and Ag-BWO,facilitating the efficient transferring ability of solar energy. Further-more, the band gap energy (Eg) of the composites was calculated usingthe equation Eg ¼ 1240/λg (eV) [37], where λg represents the absorptionedge derived from the intersection between the tangent of the absorptioncurve and the abscissa. As shown in Table S2, the Eg of g-C3N4, Ag-BWO,ABC(10), ABC(25), and ABC(50) are 2.62, 2.70, 2.61, 2.30, 2.48eV,25respectively. The lowest band gap of ABC(25) signified the enhanced theutilization of visible light due to the incorporation of g-C3N4, therebyleading to the improved the efficacy of photocatalytic reactions.Additionally, the photocatalytic properties of ABC composites weredetected through the conduction of transient photocurrent responses andEIS. As shown in Fig. 3b, the photocurrent densities of the as-synthesizedcatalysts exhibited rapid increases and sharp decreases upon switchingthe light on and off, respectively, indicating their outstanding photo-responsive abilities [38–40]. Typically, ABC(25) achieved the highesttransient photocurrent density, which was associated with the longestcharge pair lifetime, indicating excellent photocatalytic activity. ABC(10)exhibited a similar photocurrent density to g-C3N4, suggesting that alower ratio of g-C3N4 negligibly affects the charge transfer. On the otherhand, ABC(50) showed a lower photocurrent density compared to ABC(25),which may be attributed to the excessive modification of g-C3N4. Thepure Ag-BWO composite exhibited a weaker photocurrent responsivity tovisible light and faster charge recombination. In comparison, all the ABCcomposites demonstrated enhanced photocurrent intensities, evidencingthe improvement of e� chemical properties by introducing g-C3N4.Notably, the average photocurrent intensity of ABC(25) was about twiceas that of Ag-BWO. These findings suggested that the optimized ratio ofg-C3N4 in ABC(25) may promote the effective growth of photocatalystwith higher crystallinity. This enhancement resulted in more effectivelyseparated photo-generated e�-hþ pairs, thereby leading to the improvedphotocatalytic performance [41]. In Fig. 3c, the semicircle radius ofABC(25) was smaller than the ABC(10) and ABC(50) composites. Normally,EIS arc possessing a significantly smaller radius indicates a lowercharge-transfer resistance during the photocatalytic process [42].ABC(25) with a smaller radius indicated a reduced interface resistance forcarrier transfer, which resulted from the enhanced electron mobility,While ABC(10) and ABC(50) exhibited higher interface resistance andlower e� mobility compared to ABC(25). These results consistent with thephotocurrent results highlighted that the optimized ABC(25) exhibited theproper optical and photoelectron-chemical performance.3.3. Structure and morphology analysisTo further investigate the performance disparity of the different g-C3N4 ratios, characterizations of their crystal structures and compositioncomponents were enforced. The crystal phases and functional groups ofthe photocatalysts were analyzed by XRD and FTIR (Fig. 4). For pure g-C3N4, a distinct higher diffraction peak was detected around 27.4�(Fig. 4a), which was indexed to (0 0 2) plane (JCPDS No. 87–1526) [43].The diffraction peaks at 28.3�, 32.8�, 47.0�, 55.8� and 58.5� in Ag-BWOand all the ABC samples belonged to (1 3 1), (2 0 0), (2 0 2), (3 3 1) and (26 2) planes of Bi2WO6, respectively (JCPDS No.39-0256) [44]. However,no characteristic peaks of g-C3N4 were observed in all the ABC samples,likely due to the overlap between peaks of Bi2WO6 and g-C3N4. Inaddition, no other diffraction peaks were found, indicating the highpurity of the prepared photocatalysts [22]. Furthermore, the XRD pat-terns were used to calculate the crystallite size of the as-prepared sam-ples. Among the samples, ABC(25) exhibited the largest crystallite size of26.3 nm, while ABC(10) (17.4 nm), ABC(50) (12.0 nm), Ag-BWO (17.6nm), and g-C3N4 (3.7 nm) showed smaller sizes (Table S3). This resultsuggested that the ABC(25) exhibited the highest phase purity and themost mature crystal structure.Further characterizations were conducted through FTIR analysis(Fig. 4b). The peak observed around 810 cm⁻1 in the ABC samples wasattributed to the vibration of s-triazine units, a signature of g-C3N4, whilepeaks between 1200 and 1800 cm⁻1 corresponded to the stretching of C-N or C¼N bonds [45]. The broad peak between 3000 and 3500 cm�1 wasascribed to the stretching vibration of N-H or O-H. In the spectra of ABC,the characteristic peaks at 590 cm�1 and 730 cm�1 were attributed tostretching vibrations of Bi-O and W-O bands, indicating the presence ofBi2WO6 in ABC. The peak around 1380 cm�1 could be explained by thecharacteristic vibration of P¼O band in BiPO4. Notably, eachFig. 4. (a) XRD patterns, and (b) FTIR spectra of different mass ratio of ABC, Ag-BWO and g-C3N4.H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32characteristic peak observed in ABC samples originated from Ag-BWOand g-C3N4, providing partial evidence for the successful modificationof g-C3N4 into Ag-BWO. Additionally, shifts existed in the C-N and C¼Npeaks (1200~1800 cm�1) in ABC, compared to pure g-C3N4(1000~1800 cm�1) [46], suggested the interaction between the g-C3N4and other components in the ABC composite. Consequently, the XRD andFTIR results indicated the successful synthetization of ABC.Furthermore, the morphology of the synthesized photocatalysts wasvisualized by SEM and TEM. As shown in the SEM image (Fig. 5a), the g-C3N4 displayed an irregular flake structure, while Ag-BWO exhibited amulti-layered sheet-like structure, which was consistent with previousstudies [47]. Upon modifying g-C3N4 into Ag-BWO, the Ag-BWO wererandomly distributed over the g-C3N4 nanosheets, resulting in the for-mation of irregularly stacked sheets. Additionally, TEM image (Fig. 5b)provided a clear visualization of the ABC morphology, revealing a tightcontact between g-C3N4 and Ag-BWO. Furthermore, the TEM analysisdeclared the observation of crystal lattice parameters of approximately0.334 nm, 0.347 nm, 0.235 nm, 0.236 nm, corresponding to (1 3 1) planeof Bi2WO6, (0 2 0) plane of BiPO4, (1 1 1) plane of Ag and (2 0 0) plane ofAg2O, respectively [48,49]. These findings further confirmed the suc-cessful construction of the ABC composite, as corroborated by the FTIRdata. Moreover, the EDX mapping in Fig. 5c indicated a homogeneousdistribution of elements including C, N, Ag, O, Bi, P and W in the ABCcomposite, further supporting the successful synthesis of the g-C3N426modified Ag-BWO composite.To gain further insight into the surface elemental composition andchemical states of the photocatalysts, XPS analysis was carried out. Asshown in Fig. 6, the XPS spectra of ABC proved the existence of C, N, O,Ag, Bi, and W elements, which are consistent with the EDX results. Thehigh resolution of C 1s spectra (Fig. 6a) exhibited two peaks at approx-imately 284.5 and 288.2 eV, corresponding to ubiquitous carboncontamination and sp2 hybridized carbon in aromatic rings [50]. InFig. 6b, two distinct peaks at 398.4 and 399.7 eV were observed, whichcan be attributed to sp2-bonded N atoms in C-N-C and tertiary N atoms inN-(C)3, respectively. In addition, a weak peak at 404.8 eV, assigned toπ-excitations, was detected in all ABC samples, indicating the interactionbetween Ag-BWO and g-C3N4 through the π-electrons of g-C3N4 hetero-cycles [51,52]. The O 1s spectra (Fig. 6c) showed peaks at 529.5, 530.8,531.1, and 533.3 eV, representing W-O or Ag-O, Bi-O, P-O in the com-posite, respectively [53]. Furthermore, peaks at 374.1 and 367.9 eV(Fig. 6d) belonging to Ag 3d were attributed to the binding energies of Ag3d3/2 and Ag 3d5/2, demonstrating the Ag0 and Ag þ state in ABC [54].Moreover, two peaks at 164.9 and 159.8 eV (Fig. 6e), corresponded to Bi4f5/2 and Bi 4f7/2, respectively, indicating a state of Bi3þ [55]. Moreover,the patterns of W 4f showed peaks at 37.7 and 35.9 eV (Fig. 6f), whichwere ascribed to W 4f5/2 and W 4f7/2, respectively [56]. In conclusion,based on the characterization of XRD, FTIR, SEM and TEM images, EDXmapping and XPS spectra, it could be confirmed that with g-C3N4 effec-tively incorporated into the Ag-BWO structure, the ABC composite wassuccessfully constructed.Moreover, a comprehensive analysis of BET surface area and pore sizedistribution for g-C3N4, Ag-BWO, and ABC(25) was conducted to clarifytheir structural characteristics and their relationship with photocatalyticproperties. The N2 adsorption-desorption isotherms for all samples arecharacterized as type IV with H3 hysteresis loops, indicating the presenceof mesoporous structures. The results of the BET surface area analysisrevealed that ABC(25) exhibits the highest surface area of 22.524 m2/g,followed by Ag-BWO with 12.041 m2/g, and g-C3N4 with the lowestsurface area of 6.331 m2/g (Fig. S1a). A higher surface area facilitatesgreater adsorption of reactant molecules, which is beneficial forenhancing photocatalytic efficiency [12]. Additionally, the pore sizedistribution (Fig. S1b) demonstrated that ABC(25) has a broader pore sizerange and a higher total pore volume compared to Ag-BWO and g-C3N4.This feature further supported the photocatalytic performance of ABC(25)due to improved adsorption for reactants and products. In summary, thehigher specific surface area and well-developed mesoporous structure ofABC(25) contribute to its enhanced photocatalytic performance in moreefficient light absorption, greater surface reactivity, and improved chargecarrier dynamics.3.4. Z-scheme mechanisms of ABC photocatalystAccording to the above results, inducing an optimized mass ratio of g-C3N4 into Ag-BWO leads to excellent photocatalytic activity for thedegradation of TC, as well as improved crystallinity and acceleratedcharge carrier transfer. To further elucidate the mechanism of TC pho-todegradation by ABC(25), radical trapping experiments and ESR spectrawere conducted to identify the active species involved in the photo-catalytic process. EDTA, IPA and 1–4 benzoquinone were used as thescavengers of hþ, �OH and �O2�, respectively [57]. As shown in Fig. 7a,compared to TC degradation by ABC(25) without scavengers, the photo-catalytic activity decreased significantly in the presence of all threescavengers. Specifically (Fig. 7b), after 60 min of irradiation, thedegradation rates were 70 %, 85 %, and 62 % with EDTA, IPA, and 1,4-benzoquinone, respectively. The lowest degradation rate with 1,4-ben-zoquinone indicated that �O2� was the primary active species in thephotocatalytic process of ABC(25). Additionally, h⁺ and �OH contributedto the reaction, with the order of active species effectiveness following�O2� > h⁺ > �OH. To further confirm the generation of active species inABC(25), DMPO was employed in ESR analysis. Notably, the DMPO-�O2⁻Fig. 5. (a) SEM image, (b) HRTEM image, and (c) EDX elemental mapping of ABC(25) composite.Fig. 6. XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s, (d) Ag 3d, (e) Bi 4f and (f)W 4f of ABC(25) composite.H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32and DMPO-�OH signals were quantified under light irradiation in thephotocatalyst composites, as shown in Fig. 7c and d. The DMPO-�O₂⁻signal increased significantly after 15 and 30 min of light exposure,confirming the generation of superoxide radicals during the photo-catalytic process. Similarly, the DMPO-�OH signal showed a clear riseunder light conditions, reflecting the active participation of hydroxylradicals. No detectable signals were observed in the dark, indicating thelight-driven property of the radical generation. These results highlightthat �O2⁻, h⁺, and �OH are the primary active species driving the pho-tocatalytic degradation process of ABC(25), thereby contributing to itsenhanced redox species inactivation performance.The Mott-Schottky analysis was conducted to explore the bandstructure of ABC(25). As illustrated in Fig. 7e, the positive slopes of theMott-Schottky plots confirm that ABC(25) exhibits n-type semiconductor27characteristics [58,59]. Generally, the CB level of an n-type semi-conductor is more about 0.1 eV negative than that of flat band potential.Thus, the CB level of ABC(25) was estimated to be �0.60 V vs. NHE(Fig. 7e). Regarding the calculation ECB ¼ EVB-Eg and Eg value accordingto UV–VIS-DRS, VB level of ABC(25) was calculated to be þ1.70 V vs.NHE. The oxidative ability of the photogenerated holes is closely relatedto their valence band position, while positive VB position is beneficial tooxidation reaction. VB potential of ABC(25) (þ1.7 V vs. NHE) shows morepositive than the water oxidation level (þ1.23 V vs. NHE), indicating ahighly capable of oxidizing H2O to O2. CB position of �0.60 V vs. NHEsuggests the producing of O2/�O2- (�0.33 V vs. NHE) by ABC(25).Based on the band structure analysis and scavenger results, a possiblephotocatalytic mechanism for the charge transfer of ABC under simulatedsolar light was proposed in Fig. 8. When exposed to solar light, theFig. 7. (a, b) Radical trapping experiment for tetracycline degradation on ABC(25) with different scavengers, (c) ESR spectra of radical adducts trapped by DMPO-�O2�and (d) ESR spectra of radical adducts trapped by DMPO-�OH. (e) Mott�Schottky plots on ABC(25) with different frequency.Fig. 8. The proposed schematic for the photocatalytic mechanism of ABC(25) composite.H. Zhang et al. Environmental Functional Materials 4 (2025) 21–3228H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32photocatalysts absorb sufficient energy, causing e� in the valence bands(VB) of Bi2WO6, Ag-Ag2O, g-C3N4 and BiPO4 to become excited andmoveto their respective conduction bands (CB), leaving hþ in the VB. Since theCB potentials of g-C3N4 (�1.3 eV vs. NHE), Bi2WO6 (0.48 eV vs. NHE),BiPO4 (�0.64 eV vs. NHE) and Ag2O (0.2 eV vs. NHE) are more negativethan the Fermi level of Ag (0.99 eV vs. NHE), the photogenerated e� intheir CBs can readily transfer to the Ag and Ag2O nanoparticles, facili-tating effective charge carrier separation [60]. Furthermore, due to themore negative CB potentials of BiPO4 (�0.64 eV vs. NHE) and g-C3N4(�1.3 eV vs. NHE) relative to the reduction potential of O2/�O2� (�0.33eV vs. NHE), e� in the CB of BiPO4 and g-C3N4 can react with O2 togenerate �O2�, as confirmed by radical trapping experiments and ESRspectroscopy. Meanwhile, hþ from the VB of Bi2WO6 and BiPO4 cantransfer to the VB of g-C3N4 and Ag2O, owing to the more positive VBpotential of Bi2WO6 (3.26 eV vs. NHE) and BiPO4 (3.24 eV vs. NHE)compared to those of g-C3N4 (1.4 eV vs. NHE) and Ag2O (1.4 eV vs. NHE).Recent studies indicate that a Z-scheme charge transfer pathway isfeasible for g-C3N4-modified materials [61]. In this Z-scheme system,Ag-Ag2O served as an e� mediator, facilitating e� transfer and recom-bination between photogenerated e� from the CB of Ag2O and hþ fromthe VB of g-C3N4. Additionally, due to the potential difference betweenthe CB of Ag2O and the Fermi level of Ag, e� accumulating in the CB ofAg2O can easily transfer to Ag and subsequently recombine with h⁺ fromthe VB of Ag2O. Due to the effect of e� mediator of Ag-Ag2O, the e� onthe CB of BiPO4 and Bi2WO6 could transfer to Ag-Ag2O and recombinewith hþ from the VB of g-C3N4, accelerating efficient charge separation ofdifferent components. The formation of �OH radicals occurred due to thesuitable redox potential of �OH/H2O (2.68 eV vs. NHE), as verified byscavenger experiments and ESR spectroscopy.In summary, e� generated in the CB of BiPO4 and Bi2WO6 transfer toAg-Ag2O, where they recombined with hþ from the VB of g-C3N4.Remaining e� in the CB of g-C3N4 react with O2 to form �O2�, while re-sidual h⁺ in the VB of BiPO4 and Bi2WO6 oxidize H2O or OH� to produce�OH. Additionally, hþ remaining in the VB of BiPO4 and Bi2WO6contribute to reductive reactions. These reactive species collectivelydrove the degradation process. Therefore, introducing g-C3N4 to Ag-BWOto form a Z-scheme system can significantly enhance light utilization,promote charge separation and transfer, and improve redox capacity.Based on the results above, the proposed reaction mechanism of ABCdegradation on TC could be summarized as reactions (2)–(6), as follows:ABC(25) þ visible light → e�þ hþ (2)(e�) þ Ag-Ag2O → Ag-Ag2O (e�) (3)g-C3N4 (hþ) þ Ag-Ag2O (e�) → Ag-Ag2O þ Energy (4)(e�) þ O2 → �O2� (5)Fig. 9. The frontier molecular orbitals (a) HOMO, (b) LUMO, (c) nucleophilic attackoptimized structure of tetracycline under B3LYP/6-311G (d,2p) methods.29(hþ) þ H2O/OH� → H2Oþ�OH (6)3.5. DFT calculations and reaction site prediction intermediatesdegradation pathwayIn addition to the photocatalytic mechanism, understanding thedegradation pathway is crucial for the photocatalytic removal of TC.Here, the frontier orbitals and Fukui Index of TC were computed usingDFT calculations to elucidate the possible degradation pathway. Struc-turally, TC comprises a quadra-ring framework, which includes a ben-zene ring, a ketone group, and an enol group. The optimized HOMO andLUMO orbitals of TC are shown in Fig. 9a and b, highlighting its pho-tostability and resistance to self-degradation. The HOMO is primarilylocalized on the benzene ring, while the LUMO is concentrated on aketone group and an enol group. According to frontier orbital theory,atoms with higher LUMO components are more susceptible to nucleo-philic reactions, while those with higher HOMO components tend toundergo electrophilic reactions [62]. Based on the LUMO and HOMOdistributions, the Fukui function was applied to predict the nucleophilicand electrophilic sites (Fig. 9c–e). Atoms with higher fþ values are moreprone to nucleophilic attack by �O2�. While atoms with elevated f- valuesare more susceptible to electrophilic attack by hþ. Similarly, atoms withhigher f0 values are more vulnerable to free radical attack by �OH. InTable S4 and Fig. 9c, atoms 22 (O), 17 (C) and 15 (C) show the highest fþvalues (0.1075 0.0937 and 0.0893, respectively), indicating that the[-N(CH3)2] group is particularly venerable to nucleophilic attack byactive species in the dismember process. The highest f0 and f� value arefound at 20 (O) position with an iso-surface intensity of 0.108 (Table S4),indicating that the C¼O bond is particularly prone to electrophilic attackby hþ (Fig. 9d and e). The combined results of radical trapping experi-ments and Fukui function analysis suggested that nucleophilic attack ismore likely to occur than electrophilic or free radical attack, supporting astepwise degradation pathway for TC.3.6. Intermediates degradation pathway and toxicity assessmentLC-MS analysis was performed to elucidate the step-by-step pathwaysfor the photocatalytic degradation of TC, revealing a diverse array ofintermediate products. Based on the identification of these intermediatesby LC-MS, the proposed structures of the TC degradation products arepresented in Table S5, and the degradation pathways are delineated inFig. 10. Initially, nucleophilic sites in the TC molecule were attacked,leading to deamination and subsequent nucleophilic reactions, formingintermediates P1 (m/z ¼ 428) and P2 (m/z ¼ 396). Following this,sites (f⁺), (d) electrophilic attack sites (f⁻), and (e) radical attack sites (f⁰) withFig. 10. Tetracycline degradation pathway on ABC(25) exposure (a: nucleophilicattack; b: free radical and electrophilic attack).H. Zhang et al. Environmental Functional Materials 4 (2025) 21–3230intermediates from P1 (m/z¼ 428) to P8 (m/z¼ 194) underwent a seriesof reactions, including ring-opening, which resulted in the degradation ofthe amino group. P2 (m/z¼ 396) transformed into P4 (m/z¼ 346) as theintermediate's ring structure broke, and the amino group was releasedfrom TC. In the progression from P4 (m/z ¼ 346) to P8 (m/z ¼ 194), thedegradation primarily involved electrophilic reactions and free radicalattacks. Some of P4 (m/z ¼ 346) further degraded to P10 (m/z ¼ 160),releasing one O atom and breaking the carbon-oxygen double bond, ul-timately forming P11 and P12 (m/z¼ 147). Subsequently, a dehydration-condensation reaction occurred, producing P15 (m/z ¼ 85) and, even-tually, P16 (m/z ¼ 60). Concurrently, an alternative pathway from P8(m/z ¼ 194) included a series of ring-opening reactions, resulting in in-termediates P8 (m/z ¼ 194), P9 (m/z ¼ 167), P13 (m/z ¼ 154), and P14(m/z¼ 118). This pathway caused a reduction in the carbon chain length,ultimately generating smaller organic molecules, CO2, and H2O. Tovalidate the degradation pathway, TOC analysis was conducted (Fig. 1c).Notably, the TOC degradation rate of TC by ABC(25) was high after 60min of irradiation, indicating that most of the TC was effectively brokendown into smaller organic molecules, CO2, and H2O [63]. This obser-vation indicated the effectiveness of the degradation process in breakingdown complex contaminants into simpler, less harmful substances.The TC and its intermediates may present potential ecological risks,therefore, it is essential to predict their ecotoxicity. In this study, fourrepresentative toxicity factors were investigated, including Daphniamagna LD50 and Fathead minnow LD50, which indicate acute toxicity,along with mutagenicity and developmental toxicity as chronic toxicity.As shown in Fig. 11a, the initial acute toxicity of TC to Fathead minnowwas categorized as toxic, with an LD50 of 0.90 mg/L. After the initialstages of degradation, while the intermediate product P2 still exhibitedsome toxicity (LD50 of 0.47 mg/L), most subsequent intermediatesshowed a significant reduction in toxicity. For instance, the LD50 of P14increased to 2014.77 mg/L, categorizing it was not harmful. Similarly,the acute toxicity for Daphnia magna showed a significant decrease aftertreatment with the optimized ABC(25) photocatalyst (Fig. 11b). Most in-termediates exhibited progressively higher LD50 values, indicating aconsistent reduction in toxicity. Notably, P9 exhibited an LD50 of 684.35mg/L, indicating its negligible toxicity. This overall trend suggested thatmost TC degradation intermediates became less harmful with the pho-tocatalytic degradation process.In terms of chronic toxicity, mutagenicity and developmental toxicitywere also analyzed (Fig. 11c and d). Except for slightly elevated toxicityfor P2 in the early degradation stages, all the other intermediatesexhibited mutagenicity/developmental toxicity negative results, indi-cating there was no significant mutagenic or developmental toxicity.Additionally, the TOC results (Fig. 1c) further confirmed the minerali-zation of TC into less harmful products by ABC(25), with a portion of thedegradation byproducts being completely converted to CO2 and water.This substantial reduction in both intermediate toxicity and TOC levelshighlights the overall detoxification was achieved through the ABC(25)photocatalytic process. These findings demonstrated that the majority ofintermediates formed during the degradation of TC were either non-toxicor exhibited reduced toxicity. This supported the conclusion that theABC(25) photocatalyst efficiently degrades TC while ensuring a high levelof ecological safety. Overall, the optimized ABC(25) photocatalyst pro-vides an environmentally friendly and sustainable method for treatingTC-containing wastewater, offering great potential for practicalapplications.4. ConclusionThis study presents the successful synthesis of a novel ABC photo-catalyst with remarkable TC degradation efficiency under solar irradia-tion. The optimized ABC(25) composite, harnessing an idea Z-schemeheterojunction, exhibited superior crystallinity, favorable structure andphotocatalytic properties. These advantageous characteristics contributeto a 96.8 % TC degradation efficiency and high mineralization rate acrossFig. 11. Acute toxicity of (a) Daphnia magna LC50, and (b) Fathead minnowLC50, chronic toxicity of (c) mutagenicity and (d) developmental toxicity oftetracycline and degradation intermediates.H. Zhang et al. Environmental Functional Materials 4 (2025) 21–32various environmental conditions. DFT calculations and toxicity assess-ments outlined the TC degradation pathway with reduced toxicity of TCintermediates, underscoring an environmentally safer process for anti-biotic treatment. Therefore, the merits of high efficiency, robust envi-ronmental adaptability and stability make ABC(25) composite as an eco-friendly material for sustainable water treatment, addressing antibioticcontamination in real-world scenarios.31CRediT authorship contribution statementHongjian Zhang: Writing – original draft, Methodology, Investiga-tion, Formal analysis, Data curation. Qiansu Ma: Writing – review &editing, Validation, Investigation, Formal analysis. Guangqi An:Writing– review & editing, Investigation, Formal analysis. Yunxin Zhu: Writing– review & editing, Validation. Xiang Sun: Formal analysis. NaokiKawazoe: Writing – review & editing, Resources. Guoping Chen:Writing – review & editing, Supervision, Resources. Yingnan Yang:Writing – review & editing, Supervision, Resources, Project administra-tion, Methodology, Funding acquisition, Data curation,Conceptualization.Declaration of competing interestThe authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influencethe work reported in this paper.AcknowledgementsThis research was supported by Scientific Research (B) 22H03778and Grant-in-Aid for Exploratory Research 21k19628 from Japan Societyfor the Promotion of Science. The authors would like to thank NationalInstitute of Materials Science (NIMS) for their technical support withcharacterization of photocatalyst materials.Appendix A. Supplementary dataSupplementary data to this article can be found online at https://doi.org/10.1016/j.efmat.2024.12.004.References[1] A. Mann, K. Nehra, J.S. Rana, T. Dahiya, Antibiotic resistance in agriculture:perspectives on upcoming strategies to overcome upsurge in resistance, Curr. 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Development of ag/Ag2O/BiPO4/Bi2WO6/g-C3N4 Z-scheme photocatalyst for high-efficiency tetracycline removal: Characterizatio ... 1. Introduction 2. Materials and methods 2.1. Preparation of the photocatalysts 2.2. Evaluation of photocatalytic activity 2.3. Structural and photocatalytic characterization 2.4. Detection of reactive species and charge transfer mechanisms 2.5. DFT calculation 2.6. Intermediates analysis and toxicity assessment 3. Results and discussion 3.1. Performance of optimized g-C3N4 mass ratio composite and its environmental adaptability 3.2. Optical and photoelectron-chemical performance 3.3. Structure and morphology analysis 3.4. Z-scheme mechanisms of ABC photocatalyst 3.5. DFT calculations and reaction site prediction intermediates degradation pathway 3.6. Intermediates degradation pathway and toxicity assessment 4. Conclusion CRediT authorship contribution statement Declaration of competing interest Acknowledgements Appendix A. Supplementary data References