Facile construction of Z-scheme AgCl/Bi3TaO7 photocatalysts for effective removal of tetracycline under visible-light irradiation

A string of AgCl/Bi3TaO7 two-component composite was synthesized by hydrothermal and deposition-precipitation process initially. The photocatalytic activities of mixed-phase AgCl/Bi3TaO7 were evaluated toward the decomposition of tetracycline (TC). Among these as-prepared materials, AgCl/Bi3TaO7 nanocomposites when the molar ratio of baked materials between AgCl and Bi3TaO7 was 1:5 presented the optimal photocatalytic quantum efficiency for TC dissociation (86.82%) with visible-light exposure, which was 1.69 and 2.38 folders higher than that of single Bi3TaO7 and AgCl, respectively. What is more, it illustrated that the photo-generated carriers were markedly isolated on account of the formation of heterojunction confirmed by EIS analysis. Meanwhile, radical trapping experiments implied that the photo-induced holes (h+), hydroxyl radical (·OH), and superoxide radical (·O2−) were the major active species. The escalated photocatalytic activity could be ascribed to the unique construction of Z-scheme AgCl/Bi3TaO7 heterojunction, which could expedite charge separation and transmission, cement light absorption capability and retain the strong redox ability of photo-generated electrons and holes. Our finding suggests that AgCl/Bi3TaO7 nanocomposites possess great potential for photocatalytic oxidation of residual TC in the wastewater effluents and the reported strategy can contribute to the development of novel high-performance photocatalyst.


Introduction
Tetracycline (TC), the second most used antibiotic and the most adopted drug for livestock worldwide, has been frequently implemented in medicine, agriculture, and animal husbandry to boost the complete community's healthy level (Phoon et al 2022;Sharma et al. 2022). Whereas, the indiscriminate overuse and improper disposal of antibiotics would render more resistant mutations of bacteria and genes, further disturb the balance of ecosystem and endanger human health (Liu et al. 2022c;Xu et al. 2021b). Hence, it is essential and urgent to explore efficient methods to degrade sewage containing antibiotics. Currently, numerous methods for the antibiotics decomposition have been employed including adsorption (Xiang et al. 2020), chemical oxidation , biological degradation (Joo and Hussein 2022), and membrane treatment (Iborra-Torres et al. 2020). In comparison with these traditional treatments, the photocatalysis technique has stimulated growing interest in the dissociation of antibiotics in that low price, simple operation, maneuverability, and no secondary pollution (Hou et al. 2021). Notably, this strategy alleviates the conflict between energy depletion and sustainable development. Thus, photocatalysis has been recognized as an effective strategy to cope with residual pharmaceuticals.
To date, numerous significant effort has been worked to design and build efficient and speedy photocatalysts to dissociate antibiotics in the aqueous phase. The common materials mainly involved metal-based photocatalysts (such as Ag (Thao et al. 2022), non-noble metalbased photocatalysts (TiO 2 (Bai et al. 2022), ZnO (Faisal et al. 2021), MoS 2 (Liu et al. 2022a), CdS , Ta 3 N 5 (Xiao et al. 2021)), carbon-based photocatalysts Responsible Editor: Philippe Garrigues I have not submitted my manuscript to a preprint server before submitting it to Environmental Science and Pollution Research.
(such as g-C 3 N 4 (Hu et al. 2020), GO , rGO (Sun et al. 2022)), and MOFs (such as ZIFs (El Ouardi et al. 2022), UIOs (Li et al. 2021b)) based on the ingredients. Nevertheless, the poor quantum efficiency and rapid charge recombination are still the constraints derived from the photocatalysts in the photocatalytic practical reaction. Thus, new emerging photocatalytic materials have evoked growing awareness in recent years. Particularly, the oxidative potential of TC is 2.4 eV; therefore, the relative positive valence band and negative conduction band are required for photocatalysts (Zhang et al. 2019). Currently, tantalate photocatalysts have established substantial public attention in that the very positive VB involving tantalum (Ta) 5d orbital (Zhuk et al. 2022). Particularly, such as Bi 3 TaO 7 , the hybridized VB is priorly composed of O 2p and Bi 6 s orbitals, causing a shrank band gap energy and an ultrahigh oxidative activity (Bisht et al. 2021;Geng et al. 2022). Additionally, Bi 3 TaO 7 possesses a tuning band gap of 2.76-2.87 eV that makes it absorb visible light (Song et al. 2020). Furthermore, high-specific surface area and active sites could be generated due to the unique layered crystal structure of Bi 3 TaO 7 . Besides, it has been discovered that metal cations (the Bi ions) could generate a chelation effect with organic molecules like TC with strong coordination bonds (Salavati-Niasari et al. 2005). Nonetheless, Bi 3 TaO 7 subjects to the drawbacks of fast recombination and dissatisfied charge separation. To address the constraints, various strategies have been proposed and adopted. Among them, an eminent strategy is the construction of heterostructure by integration Bi 3 TaO 7 with other semiconductors.
AgX (X = Cl, Br, and I) have been adopted to couple with multiple semiconductors to construct heterojunctions that exhibit preferable quantum yields toward the elimination of organic pollutants. Particularly, AgCl has attracted the attention of scholars in the domain of photocatalysis . For instance, AgCl/WO 3 composite microrods was fabricated with p-n junction, which presented promising photocatalytic activity and stability (Yu et al. 2019). Direct Z-scheme Bi 3 O 4 Cl/AgCl heterojunction expressed 98.5% TOC removal rate of ceftriaxone (Du et al. 2020). S-scheme AgCl/MIL-100(Fe) heterojunction photocatalyst exhibited a complete removal efficiency (99.9%) of sulfamethazine (Ning et al. 2022). Based on the available literature, it is reasonable and feasible that the integration of AgCl and Bi 3 TaO 7 could accelerate the carriers separation effectively on account of the benign band potential matching between AgCl and Bi 3 TaO 7 for TC deterioration.
Herein, we made the first attempt to prepare a series of AgCl decorated AgCl/Bi 3 TaO 7 materials with varying proportions via facile hydrothermal method and deposition-precipitation method. The objective was to further promote the photocatalytic performance of single AgCl and Bi 3 TaO 7 . The results demonstrated that the Z-scheme heterojunction AgCl/Bi 3 TaO 7 photocatalyst had a markedly improved TC photodegradation efficiency exposed visible light, compared with baked AgCl and Bi 3 TaO 7 . The crystal phases, chemical compositions, morphologies, and optical characteristics of the as-obtained samples were investigated. Additionally, the photocatalytic performance of as-prepared binary metarials was accessed by regulating the molar ratio of AgCl and Bi 3 TaO 7 , solution pH, and initial TC concentration in detail. Subsequently, the possible mechanism for decent photocatalytic property was proposed in view of the corresponding testing data. This work may provide new possibilities for the development of novel photocatalysts and offer a strategy for tuning any catalysts from silent to highly reactive by carefully tailoring the chemical composition.

Preparation of Bi 3 TaO 7 composite
2.2548 g Bi(NO 3 ) 3 ⋅5H 2 O and 1.0000 g TaCl 5 were separately dispersed into 10 mL of ethanol with ultrasonic stirring for 30 min. Subsequently, TaCl 5 solution was continuously mixed in the Bi(NO 3 ) 3 solution, stirring for 30 min to gain a suspension. Afterward, KOH aqueous sample (7 M) was included into the mixed suspension until the value of pH was regulated to 10.0. Then, the resultant mixture was stirred for 30 min to obtain a yellow suspension. Thereafter, the yellow suspension was diverted into a 25 mL autoclave and reacted for 24 h at 200℃. Cooling down to ambient temperature, it was separated via centrifugation with distilled water and ethanol six times. Finally, pure Bi 3 TaO 7 nanoparticles were yielded by air oven drying at 60℃ for 12 h, denoted as BTO.

Preparation of AgCl-Bi 3 TaO 7 composite
The deposition of AgCl on Bi 3 TaO 7 was obtained via a deposition-precipitation method. Typically, the resultant sample Bi 3 TaO 7 was dispersed into the distilled water (50 mL) 1 3 while being continuously agitated for uniform dispersion. A certain amount of NaCl and AgNO 3 were dissolved into Bi 3 TaO 7 solution and stirred vigorously for 120 min. Lastly, the obtained sediment was cleaned with ethanol and deionized water several times and dried at 60℃ for 12 h. AgCl/ Bi 3 TaO 7 nanopaticles were gathered that the molar ratios of AgCl and Bi 3 TaO 7 were 3:20, 4:20, 5:20, 6:20 and denoted as AB-1, AB-2, AB-3, AB-4, respectively. The preparation process of AgCl/Bi 3 TaO 7 samples was depicted using different amounts as illustrated in Fig S1.

Characterization of the photocatalysts
The phase composition and crystallinity of the as-synthesized samples were investigated based on powder X-ray diffraction (XRD, DX-27mini X-ray diffractometer) equipped with Cu-Kα beam source and the 2θ ranging from 5° to 90°, at a scanning speed of 2° per minute. The morphologies and structures of catalysts were detected using field emission scanning electron microscopy (SEM, Philips XL30) and transmission electron microscopy (TEM, JEM-ARM 200F). The surface elemental compositions were recorded by usinga monochrome Al X-ray resource (XPS, ESCALAB 250Xi). The photocatalytic experiments were examined on an XPA-7 photochemical reactor (Xujiang Electromechanical Plant). The specific surface areas were performed on the Brunauer-Emmett-Teller (BET, Micromeritics ASAP 2460). Photo-electrochemical measurements were implemented with electrochemical impendence spectroscopy (EIS, CHI660) in a sodium sulfate electrolyte solution. A standard platinum wire electrode, Ag/AgCl electrode and the as-obtained material acted as the auxiliary electrode, the reference electrode, and the working electrode, respectively. The working electrode was fabricated on the graphene with a thin film through a manual deposition process.

Assessment of photocatalytic performance
The photocatalytic performance of as-obtained catalysts was assessed for the TC decomposition by a 350 W Xe lamp with a 420 nm cutoff filter. During the experimental process, 15 mg of photocatalysts were put to 20 mg/L TC solution (50 mL). Before the irradiation, the mixture was magnetically stirred in dark for 90 min to assure the absorptiondesorption equilibrium between TC and as-obtained photocatalysts. Soon afterwards, at given intervals of illumination, 4 mL of mixture was sampled, coupled with centrifuging at 6000 rpm for 20 min and filtering to wipe off photocatalysts in the TC residual solution. The concentration of TC was defined on a UV-Vis spectrophotometer (UV8800s) at the absorption peak of 356 nm.

Radical-trapping experiment
To verify the pivotal active species during the photodegrading TC process, radical-trapping experiment was implemented by employing four quenchers, benzoquinone (BQ, a superoxide anion radical scavenger), ammonium oxalate (AO, a hole scavenger), tertiary butyl alcohol (TBA, a hydroxyl radical scavenger), and AgNO 3 (an electron scavenger). Different from the above photocatalytic process, 15 mg of AB-2 photocatalyst and 1 mmol of distinct radical scavengers were introduced to 20 mg/L of TC solution (50 mL).

Morphologies and structure of as-prepared photocatalysts
The crystalline structure of all as-obtained samples was performed by XRD analysis in the 2θ raging of 5-90°. As shown in Fig. 1, four characteristic peaks of pure Bi 3 TaO 7 at 2θ values of 28.32°, 32.27°, 46.93°, 55.67° were obviously observed, which matched with the (111) (Panchal et al. 2021). In the AgCl/Bi 3 TaO 7 composites, the characteristic peaks of AgCl and Bi 3 TaO 7 were found, demonstrating the formation of heterojunction. Meanwhile, the peaks of Bi 3 TaO 7 became Fig. 1 XRD patterns of bulk Bi 3 TaO 7 and AgCl-Bi 3 TaO 7 composites prepared at the various molar ratios more non-prominent and the diffraction peaks of AgCl grew stronger gradually as the proportion of AgCl increased. In addition, the peaks of AgCl/Bi 3 TaO 7 composites were explicit and without additional diffraction peaks, affirming the fine crystallinity, high purity, and interaction occurring at the interface in the binary components.
The surface composition and chemical states of AgCl, BTO, and AB-2 nanocomposite were conducted by XPS for the sake of more structural information. The XPS survey spectrum from Fig. 2 identified that AgCl and BTO were compounded from corresponding elements of Ag, Cl, and elements of Bi, Ta, and O, respectively, withal AB-2 composite was consistent with five elements including Bi, Ta O, Ag, and Cl, demonstrating the coexistence in AB-2 powder. Figure 3(a-f) displayed the high-resolution XPS spectra of AgCl, BTO and AB-2 composite. Thereinto, the C 1 s peak of 284.8 eV was regarded as the calibration value since the XPS instrument itself received the adventitious hydrocarbon (Al-Kandari et al. 2021). As displayed in Fig. 3(a), the binding energies at 159.07 and 164.31 eV were classified as Bi 4f 7/2 and Bi 4f 5/2 for the bulk BTO, suggesting Bi presented as trivalent oxidation state, while it was located at 158.79 eV and 164.09 eV for the AB-2, respectively (Bisht et al. 2021). The individual peaks at 26.06, 27.76, and 29.23 eV were elucidated as Ta 4f 7/2 , Ta 4f 5/2 of Ta 5+ , and Ta 4f 5/2 of Ta 4+ oxidation state ( Fig. 3(b)), respectively (Xu et al. 2021a). The primary peak of 530.16 eV belonged to the lattice oxygen and another peak of 531.7 eV was correlated with the hydroxy oxygen of the material (Fig. 3(c)) (He et al. 2020). The peak of C 1 s was observed at 284.9 eV (Fig. 3(d)) which stemmed from accidental carbon-containing compounds and was introduced as the calibrating binding energy of other elements. From the XPS spectra of Ag (Fig. 3(e)), the peaks located at 368.1 eV and 374.01 eV were in line with Ag 3d 5/2 and Ag 3d 3/2 , which originated from Ag + of AgCl and AB-2 (Aletayeb et al. 2020). Likewise, Ag presented in the silver ions form with no metallic silver in the AgCl and AB-2 composite. From the Fig. 3(f), the characteristic peaks around 198.32 and 199.63 eV were associated with Cl 2p 3/2 and Cl 2p 1/2 orbits, respectively (Fan et al. 2021). Compared with pristine AgCl and BTO, the peaks of AB-2 shifted slightly with a certain degree to higher binding energy, implying the strong interactions and the possible electron transfer between AgCl and BTO. In light of the above evidence, the electron transfer in AB-2 contributed to the heterojunction formation with built-in electric field direction between two components.
To observe clearly the morphology and structure of pure BTO, AgCl, and AB-2 nanocomposite, SEM, SEM-EDS, TEM, and HRTEM were investigated. In Fig. 4(a), the pure phase AgCl showed the sphere-like morphology inclined to agglomerate with diameter of about 100 nm and relatively smooth surface. The SEM observation displayed that Bi 3 TaO 7 consisted of staggered stacks with rough surface (Fig. 4(b)). Figure 4(c) proofed that the AgCl particles deposited on the surface of BTO nanosheets to form a substantial interaction. In addition, the energy dispersive X-ray spectroscopy (EDS) was also utilized to determine the element distributions of AB-2 ( Fig. 4(d-e)). It was evident that each element of BC-2 material was uniformly distributed over the whole surface, identifying the successful synthesis of AgCl/Bi 3 TaO 7 heterojunction. The above results were in great agreement with the result of XRD and XPS analysis, which further illustrated that the AB-2 composite was composed of AgCl and BTO.
The morphology of AB-2 was performed by TEM tests to further explore morphological features. As depicted in Fig,  5(a-c), it was distinct that AgCl particles were anchored on the surface of BTO in the AB-2 composite through deposition-precipitation route. This implied that they are in intimate contact with each other, which was unanimous in SEM results. The HRTEM image of AB-2 ( Fig. 5(d)) demonstrated the interplanar distance of 0.273 nm and 0.277 nm were gathered for BTO and AgCl, respectively, matching well with (200) plane of Bi 3 TaO 7 ) and (200) plane of AgCl, separately (Liu et al. 2022b). As a result, it elucidated that the co-existed binary phases of BTO and AgCl formed an intimate interface, indicating a good interaction.
The light absorption properties of AgCl, BTO, and AB composites were performed with UV-Vis DRS. The band gap energy (E g ) was measured using the following equation ): (1) αhv = A(hv − Eg) n∕2 Fig. 2 The survey spectrum of AgCl, BTO, and AB-2 where α, hv, A, and E g referred to the optical absorption, the photonic energy, the proportionality coefficient constant and the band gap energy of various catalysts, respectively. Thereinto, n/2 or 1/2 for a direct or indirect band gap material (Panchal et al. 2021;Zhu et al. 2019). From Fig. 6(a), BTO distinctly absorbed with wavelengths less than 400 nm in UV region, conversely, the opposite was the case with AgCl. In comparison with their composites, a stronger absorption capacity of visible light could be observed. Figure 6(b) depicted that the absorption edge of BTO, AgCl, and AB-2 were around 2.8 eV, 3.02 eV, and 2.83 eV, respectively. With the decoration of AgCl, the band gap energy of AgCl/Bi 3 TaO 7 composites became gradually shrunken by compositing with pristine AgCl and BTO. Summarily, the phenomenon ravealed that the modification of AgCl extended the light absorption region by forming a binary phase heterojunction and further developed the visible-light photocatalytic capacity.
It was well known that specific surface area was a momentous factor to finetune the physicochemical properties and escalate the photocatalytic performance with the modification of single material. N 2 adsorption-desorption isotherm and the pore distribution of AgCl, BTO, and AB-2 composite were carried out. As illustrated in Fig S2, BTO and AB-2 corresponded to a type IV isotherm and a type H 3 hysteresis loop, while the bulk AgCl belonged to a type of V isotherm and a type H 1 hysteresis loop, coupling with the mesoporous structure (Zhao et al. 2022). The specific surface area of AB-2 was 43.3249 m 2 ·g −1 , which sharply augmented by comparison with baked AgCl (1.3993 m 2 ·g −1 ) and raw BTO (28.0337 m 2 ·g −1 ), respectively. In addition, the pore size of AB-2 was bigger than the sole component, manifesting the superior active sites. Decent surface area and pore structure were capable to photo-degrade efficiently which donated more active sites to the target pollutant.

Photocatalytic activity for TC removal
The photocatalytic activity of resultant samples was elucidated toward TC degradation in aqueous medium under visible-light illumination with 15 mg of AgCl, BTO, and AB composites, severally. On the other hand, several vital affecting factors in the photocatalytic degradation including pH and initial concentration were also investigated. Prior to the photoreaction, the adsorption-desorption equilibrium was achieved with magnetic stirring in darkness. The controlling trial was allotted to rule out the weak impact of selfdegradation in TC solution simultaneously. From Fig. 7(a), the TC decomposition ratio was only 36.5% and 51.4% for baked AgCl and pristine Bi 3 TaO 7 under 3 h of visible-light exposure, respectively. Above all, the TC degradation rate of AgCl/Bi 3 TaO 7 composites dramatically improved relative to that of raw AgCl and BTO. Among them, the optimized AgCl-Bi 3 TaO 7 sample with a molar ratio of 1:5 exhibited superior degradation of TC, coming up to 86.8%, which was 1.67 and 2.38 times higher than the level of pure Bi 3 TaO 7 and original AgCl. Noticeably, the photocatalytic efficiency was increased by rising of the content of AgCl. The eminent photocatalytic ability of AB-2 composite could be credited with the construction of heterojunction, which sped up charge separation and transportation. While the content of AgCl was loaded excessively, the TC removal efficiency decreased marginally due to the shading effect and the inhibited visible-light absorption.
As shown in Fig. 7(b), the kinetic curves of TC removal over AgCl-Bi 3 TaO 7 samples were simulated for further insight into the photocatalytic oxidation process. The second-order model was employed to analyze the kinetics of TC decomposition. The apparent rate constants for TC degradation with AgCl, BTO, AB-1, AB-2, AB-3, and AB-4 were 0.0234, 0.0313, 0.1043, 0.1168, 0.1142, and 0.1096 min −1 , respectively. The optimal rate constant over AB-2 was 3.73 and 4.99 times as high as relative to sole BTO and AgCl.
The value of pH would affect the TC molecules on account of the protonation-deprotonation change. Thus, the pH of TC solution made a momentous difference on the adsorption and photocatalytic capability. Typically, HNO 3 or KOH (1 M) were added to monitor the initial pH in the TC solution. As depicted in Fig. 7(c), it should be noted that there was an eminent photocatalytic performance at pH = 7, 9, and 11 as the value of pH accelerated TC molecules to adhere on the surface of AB-2. Nevertheless, when the initial pH was at 3 and 5 in acidic media, the TC adsorption was restricted remarkably owing to the decreased active species since·O 2 − reacted with h + to form H 2 O 2 under acidic conditions (Dong et al. 2018). Furthermore, the TC removal efficiency was the highest with visible light when the value of pH was 9, which was assigned to the synergistic effect of ·OH, ·O 2 − , and h + (Wang et al. 2018a). TC initial concentration was a crucial factor to research deeply the photocatalytic performance of AB-2 as well. The  Fig. 7d, the degradation rate tended to descende gradually from 95.3 to 44.12% during the TC degradation process with the concentration of TC monitored in the ranging of 10 to 50 mg L −1 . There were the major factors for the decreased photocatalytic property by the raising of TC concentration. On the one hand, it was difficult to degrade with higher initial concentration since the high chromaticity could decrease the transmission light path length (Wang et al. 2018b). Furthermore, it possessed less reactive species of TC when the catalyst/organic pollutant ratio decreased in higher TC concentration. Beside that, the increase of TC concentration could inescapably cause more intermediate products, which enhanced adsorption competition between TC molecules and the intermediates . As a consequence, lower TC concentration was more favorable to reaching a higher TC removal rate.

Possible photocatalytic mechanism of TC degradation
Electrochemical impedance (EIS) was performed to validate the charge separation and migration rate of BTO, AgCl, and AgCl-Bi 3 TaO 7 nanoparticles. The arc radius was in accordance with the charge transfer resistance and the smaller arc radius could own higher charge transfer resistance, resulting in better separation of carriers. As illustrated in Fig. 8, AgCl/ Bi 3 TaO 7 composites presented much smaller radiuses of semicircle arc relating to single AgCl and BTO and the molar ratio of 1:5 exhibited the minimum radius of semicircle arc among them, confirming the highest charge separation capacity. The phenomenon was owing to the heterojunction between BTO and AgCl, which impelled the charge carriers to migrate in opposite directions and further dedicate to the photocatalytic process.
The dominant active species were ascertained through radical capture experiments for the photocatalytic reaction process that various scavengers added into the TC solution with AB-2 sample. In detail, 1 mM isopropyl alcohol (TBA) was employed for quenching ·OH, p-benzoquinone (BQ) for ·O 2 − , AgNO 3 for e − , and Na 2 C 2 O 4 (AO) for h + . Fig S5 explained the inhibitory effect of distinct scavengers for TC degradation over AB-2. In the existence of AgCl, TC removal was slightly declined from 86.8 to 81.4%, indicating that e − produced a weak effect negligibly. Comparatively, the photo-degrading efficiency of TC prominently decreased by introducing TBA into the solution, demonstrating that ·OH impelled the action of TC degradation. Moreover, h + caused the prior inhibition of TC degradation with efficiency from 86.8 to 44.4%, when AO was added. Furthermore, the photocatalytic performance of TC was severely decreased while BQ was integrated into AB-2, indicating the remarkedly negative effect. Consequently, ·OH, ·O 2 − , and h + had a major effect on TC decomposition. Additionally, the VB-XPS spectra were detected to explore the interaction between multivariant semiconductors. As displayed in Fig. 9, the VB potentials of AgCl, BTO, and AB-2 were calculated as 2.94, 2.05, and 1.83 eV, separately.
The standard hydrogen electrode potential (E VB-NHE ) could be counted up by underneath formula: where ψ represented the electron work function of the XPS analyzer, and E VB-XPS was VB value, which was determined by VB-XPS plots Li et al. 2020;Zhong et al. 2022). According to Eq. (2), the E VB-NHE potentials of AgCl and BTO were 3.47 and 2.58 eV. Conmbining with UV-Vis DRS analysis, the band gap energy (E g ) of pure AgCl and BTO were 3.02 and 2.80 eV, respectively. According to Eq. (3), the E CB positions of baked AgCl and BTO were 0.45 and − 0.22 eV, respectively.
In light of the above experimental and theoretical research, the possible mechanism for TC dissociation upon visible-light exposure was reasonably constructed and proposed in Fig. 10. Furthermore, the degrading process of TC over AgCl/Bi 3 TaO 7 composite was explained in detail as below: (2) E VB−NHE = ψ + E VB−XPS − 4.44 (3) E VB = E CB + Eg AgCl∕Bi 3 TaO 7 + hv → AgCl∕Bi 3 TaO 7 (h + ) + AgCl∕Bi 3 TaO 7 (e − ) e − + AgCl → AgCl(e − ) AgCl(e − ) + O 2 → ⋅O − 2 h + + Bi 3 TaO 7 → Bi 3 TaO 7 (h + ) Bi 3 TaO 7 (h + ) + H 2 O∕OH − → ⋅OH Bi 3 TaO 7 (h + ) + ⋅O − 2 + ⋅OH → Intermediate products + H 2 O + CO 2 Fig. 8 Electrochemical impedance spectrum (EIS) of AgCl, BTO, and AB composites 1 3 The binary photocatalysts were excited to produce photo-generated electrons (e − ) and holes (h + ). After that, due to the presence of built-in electric field force, the electrons in the CB of AgCl recombined with the holes in the VB of BTO. Holes in the VB of AgCl could oxidize H 2 O or OH − into ·OH owing to its promising oxidative ability. Apart from that, electrons in the CB of BTO could reduce O 2 into ·O 2 − owing to the more positive CB potential of BTO (−0.22 eV vs. NHE) than O 2 /O 2 − potential (− 0.046 eV vs. NHE) (Guo et al. 2022).Furthermore, holes could participate directly in the degradation process due to superior oxidative ability. In this case, ·O 2 − could be produced during the TC degradation process and along with h + and ·OH react with TC molecules. There was no difference in the analysis of the radical-trapping experiments. In Table S1, it was found that the photocatalytic activity of AB-2 was still higher than that of the reported photocatalysts.Therefore, the novel Z-scheme heterojunction photocatalysts possessed an impressive photocatalytic property.

Conclusions
A novel Z-scheme AgCl/Bi 3 TaO 7 photocatalyst was initially fabricated via hydrothermal and deposition-precipitation method. By modulating the molar ratio of AgCl and Bi 3 TaO 7 , the optimal degradation rate reached up to 86.8% under visible light exposure, when the molar ratio of AgCl to Bi 3 TaO 7 composite was 1:5. Moreover, the radical trapping measurements suggested that ·OH, ·O 2 − , and h + took the positive effects on the photocatalytic process for TC elimination. The advanced property was mainly determined by the formation of heterojunction, which could maintain the high redox potential and impede sharply the charge recombination. This work opened up whole new vistas in