Photocatalytic Adsorption Synergistic Degradation of Tetracycline by Z-scheme Bi2WO6/ZIF-8

In this paper, in order to improved the photocatalytic activity of Bi 2 WO 6 , Bi 2 WO 6 and ZIF-8 were successfully combined by in-situ growth method for the rst time. The addition of ZIF-8 effectively inhibited the recombination of photogenerated electron hole pairs and further improved the electron utilization eciency, and superoxide anion was introduced to greatly improve the photocatalytic activity. The performance of Bi 2 WO 6 /ZIF-8 in the photodegradation of tetracycline (TC) was studied under different conditions of proportions of ZIF-8, dosage of catalyst and concentration of TC. The results indicated that B/Z/5/1 (10mg) had the best photocatalytic activity, and 97.8% of TC (20mg/L) could be degraded in 80 minutes under UV light, the rate constant (k) for TC degradation was almost 3 times that of Bi 2 WO 6 . The effects of pH, HA and inorganic anions on the degradation of TC were studied in simulated real water. Further, B/Z/5/1 could be reutilized up to ve cycles without reduction of eciency and catalysis performance. Therefore, Bi 2 WO 6 /ZIF-8 heterojunction composite material can be utilized as an ecient photocatalyst for remediation of environmental pollution. Due


Introduction
With the development of society, environmental pollutions, especially the spread of antibiotics in the environment, are becoming more and more serious. One of the most typical is tetracycline. Due to the limited absorption of tetracycline by human body, most of TC ow into the environment and return to human body through various channels, causing drug resistance and threatening human health [1]. The degradation rate of tetracycline in nature is very slow, and the intermediate products are more toxic. The existing process could not completely remove tetracycline [2], so it is urgent to establish the effective degradation methods in the eld of environmental protection. Semiconductor photocatalysis technology has unique advantages for pollutant degradation because of its green and high e ciency. Among many semiconductor materials, Bi 2 WO 6 is popular because of its high valence band potential energy and easyto-prepare. As mentioned in the literature review [3], there is a [WO] 2− octahedral structure between the (Bi 2 O 2 ) 2+ layers of Bi 2 WO 6 , and the electron transfer rate is fast, forming an electric eld that can inhibit the recombination of photogenerated electron hole pairs, so the photocatalytic activity is higher than that of general semiconductor materials. However, the speci c surface area of Bi 2 WO 6 is very small and the photogenerated carriers are unstable [4]. Therefore, how to improve the photocatalytic activity of Bi 2 WO 6 is worth studying.
Metal-organic frameworks (MOFs) are a new porous material composed of metal ions and organic ligands [5]. Because of the ordered structure [6], MOFs are good candidates for preparing composite semiconductor materials with large surface area [7]. In the previous report, Bi 2 WO 6 and MOFs (such as UiO-66-NH 2 ) were combined to construct heterojunction, which showed the enhanced dispersion and speci c surface area of Bi 2 WO 6 and effectively improved the separation and transfer of photogenerated carriers [8]. Compared with Bi 2 WO 6 , the degradation e ciency of Rhodamine B and Phenol was increased by 1.5 times. Among many MOFs, ZIF-8 has more outstanding performances, such as thermochemical stability, high porosity and large speci c surface area, core-shell g-C 3 N 4 @ZIF-8 was prepared by in-situ growth of ZIF-8 on g-C 3 N 4 [9], which has shown that the photocatalytic degradation rate of TC was 4.8 times higher than that of g-C 3 N 4 . In addition, compared with ZnInS nanosheets, the ZnInS@ZIF-8 composite exhibits higher photocatalytic e ciency, and the photocatalytic degradation e ciency of TC reach 73% in 5 minutes [10]. Hence, the heterojunction of semiconductor materials and MOFs is one of the effective methods [11] to improve the photocatalytic activity and stability of wastewater treatment. Among them, Z-scheme is the most typical, which can extend the range of material response to light, improve the charge transfer e ciency and enhance the photocatalytic activity [12]. However, up to date, there is no report on the construction of Z-scheme heterojunction through combination of Bi 2 WO 6 and ZIF-8 that shows the improved photocatalytic degradation e ciency of TC.
In this paper, we prepared a Bi 2 WO 6 /ZIF-8 Z-scheme with excellent photocatalytic activity by hydrothermal reaction of ZIF-8 with Bi 2 WO 6 precursor. It showed strong photocatalytic activity in the degradation of TC by UV light. Through a series of characterization techniques, the morphology and catalytic properties of the composites were studied, and the degradation of TC was further discussed.
Finally, the mechanism of photocatalysis was discussed. This research expands the practical application scenarios of MOF materials and may have important signi cance for the removal of tetracycline or other antibiotics in the future.  [14], and the given mass of ZIF-8 was added in the solution. The mixed reactants was sonicated for 1 h and then stirred for 1 h. After fully mixed, the mixed solution was poured into a 100 ml reactor and was heated at 140°C for 18 h. Then, the product was centrifuged with deionized water and ethanol twice respectively. After centrifugate, the product was dried at 60°C. The compound material with different molar mass ratios (Bi 2 WO 6 :ZIF-8 = 1:1, 2:1, 5:1 and 7:1) were abbreviated as B/Z/1/1, B/Z/3/1, B/Z/5/1 and B/Z/7/1. Under the above conditions, pure Bi 2 WO 6 was prepared as reference.

Characterization
The crystal phases was performed X-ray diffraction (XRD) on D/MAX2500 diffractometer (RIGAKU, Japan). Fourier transform infrared (FTIR) spectra were plotted by using Spectrum One FTIR spectrophotometer (PerkinElmer, USA). The ultraviolet-visible diffuse re ectance spectra (UV-vis DRS) were identi ed with a Shimadzu UV-2550. The Photoluminescence (PL) spectroscopy of samples were identi ed with an Edinburgh FL/FS900 spectrophotometer. The binding energy of each element (Bi, W, Zn, C, N and O) was measured by Thermo Scienti c K-Alpha. The images of crystal structure was obtained from FEI Talos 200S (TEM) and SUPRA55 (SEM) with energy-dispersive spectroscopy (EDS)..

Photocatalytic experiments
Before the photocatalytic experiments, 20 mg/L TC standard solution was prepared with deionized water.
10 mg catalyst was immersed in 50 mL (20 mg/L) TC solution, which was stirred in dark for 40 min until adsorption equilibrium. Then, the photocatalytic reaction was studied by a 300 W mercury lamp. During this reaction, samples were taken every 20 minutes for spectral analysis. The absorbance of TC at 357 nm was determined by UV-Vis spectrophotometer.

Characterizations
The XRD patterns of pure ZIF-8 in Figure. 1a was the same as that reported in literature [5], the diffraction peaks at 2θ = 7.  [15]. The crystal structure of Bi 2 WO 6 was not changed by adding ZIF-8, but the diffraction peak of ZIF-8 was weak compared to those of Bi 2 WO 6 and there were no characteristic peaks at low proportions (B/Z), due to uniform dispersion of ZIF-8 and the high crystallinity of Bi 2 WO 6 . The characteristic peaks of the two monomers could be clearly seen when the proportion was increased to 1/3. These results provided further support that Bi 2 WO 6 and ZIF-8 were combined successfully.
The infra-red (FT-IR) spectrum of the prepared sample was shown in Figure. 1b. The peaks at 3448 cm -1 , 3136 cm -1 and 2930 cm -1 were assigned to the stretching vibration of N-H bond, imidazole aromatics and C-H bond in the imidazole ring of ZIF-8. The in-plane bending of pyridine ring forms several peaks in the range of 902 ~ 1306 cm -1 . The peak near 421 cm -1 was assigned to the stretching vibration of Zn-N bond [16]. The absorption band at 1384 cm -1 , 729 cm -1 and 580 cm -1 were very strong, which may be caused by Bi-O, W-O and W-O-W stretching [15]. The peak at 1629 cm -1 were attributed to O-H bending of absorbed water moleculesmay. The corresponding peak could also be seen in the composite [17]. It is evident that the functional groups of the two monomers were retained in the composites.
The photoelectron-hole (E-H) recombination rate of the composite was analyzed by PL spectroscopy. The higher the PL peak strength, the lower the separation e ciency of E-H [13]. It could be seen from Figure.1c that the peak intensity of pure ZIF-8 was the strongest, and it showed that the recombination rate of E-H was very fast. The peak intensity of the composites was lower than that of Bi 2 WO 6 , which indicates that the separation rate of photoexcited interface charge of the composite was improved [18], and it also con rmed that the recombination of Bi 2 WO 6 and ZIF-8 inhibits the recombination e ciency of photogenerated electron hole pairs, thus enhancing the photocatalytic activity. And the E-H recombination rate of B/Z/5/1 was the lowest.
The structure of the composite was studied by scanning electron microscope and transmission electron microscope. In Figure. 3a, ZIF-8 presented regular dodecahedron structure, which was consistent with that reported in the literature [19]. The Bi 2 WO 6 in Figure 3b showed nano akes with the size of 50-200 nm [20]. Figure. 3C was the SEM image of B/Z/5/1. It could be seen that Bi 2 WO 6 was tightly loaded on the surface of ZIF-8, which was further con rmed by high power transmission electron microscopy ( Figure. 3d and 3e). The successful combination of the two materials could also be con rmed by EDS. From Figure. g, the chemical composition of both ZIF-8 and Bi 2 WO 6 could be clearly seen, including C, O, W, Bi and Zn.
High power TEM photographs of B/Z/5/1 showed that the structure and lattice fringes of the sample were very clear, and the measured lattice spacing was 0.315 nm [15], which matched with the crystal plane (131) of Bi 2 WO 6 [21]. B/Z/5/1 is expected to effectively inhibit the rapid recombination of carriers and improve the photocatalytic activity due to the close contact between Bi 2 WO 6 and ZIF-8. Hence, the above analysis could prove that the preparation of Bi 2 WO 6 and ZIF-8 composite were successfully.
The chemical composition and bonding con guration of B/Z/5/1 were determined by X-ray photoelectron spectroscopy (XPS). Figure. 4a was full spectrum XPS scan of B/Z/5/1 and Bi 2 WO 6 , the elements, including Bi, W, Zn, C and O, could be clearly seen, and due to the recombination of ZIF-8, the main peaks of Bi and W elements were weakened. Figure. 4b presented the high-resolution XPS spectrum of Bi 4f. The two peaks at 158.48 eV and 163.68 eV correspond to the photoelectron responses of Bi 4f 7/2 and Bi 4f 5/2 in Bi 2 WO 6 [22], and it proved that there was a combination of Bi of trivalence state in Bi 2 WO 6 [23]. Figure. 4c presented the high-resolution XPS spectrum of W. The two peaks at 35.05 eV and 37.17 eV correspond to the photoelectron responses of W 4f 7/2 and W fp 5/2 in Bi 2 WO 6 , which is indicative of the existence of W 6+ in Bi 2 WO 6 [24]. In Figure. 4d, the peaks of 1021.6 eV (Zn 2p 3/2 ) and 1044.8 eV (Zn 2p 1/2 ) were caused by ZIF-8 [6], the peak of B/Z/5/1 near the binding energy of 1027.6 eV was very weak. In Figure. 4f, two dominant peaks at 284.3 eV and 285.2 eV correspond to C=C and N=C-N bonds, respectively [25]. The results of XPS indicated that Bi 2 WO 6 and ZIF-8 coexist in the composites.

Analysis of the photocatalytic reactions Adsorption experiments
Due to the strong adsorption properties of the two monomers, a separate study on the adsorption properties of the materials was undertaken. Before studying the photocatalytic performance of the material, the adsorption studies of samples were performed in the dark to determine the reaction time when the material reached the adsorption equilibrium.10 mg of catalyst was added into 25 mg/L TC solution and the dark reaction time was prolonged to 1 hour. As shown in Figure. 5a, the adsorption e ciency of Bi 2 WO 6 for TC was only 6%, and that of ZIF-8 for TC was as high as 50%. The adsorption properties of the composites were improved, and increased with the increasing proportion of ZIF-8. It could be seen from Figure. 5a that the adsorption of TC on all materials tends to be stable in about 40 minutes, and it could be determined that the adsorption equilibrium was reached.
Photocatalytic degradation of TC 20 mg/L TC solution was used as the target, and 10 mg of catalytic material was added respectively. First, 40 minute dark reaction was carried out to complete the adsorption process of the material. As shown in Figure. 5b, in the absence of catalyst, TC hardly degraded after 80 minutes of UV irradiation.
Compared with the two monomers, the photocatalytic performance of the composites was improved, especially for B/Z/5/1. After adsorption for 40 min and UV irradiation for 80 min, the degradation e ciency of TC reached 97.8%. One of the reasons should be ascribed to the improved speci c surface area and surface active sites [26]. Another reason was that the heterojunction formed in the composite could change electron transfer routes and improve electron utilization e ciency, which thus improves the photocatalytic activity [12,27,28].
As shown in Figure. 5c and 6d, the kinetics of TC degradation and the calculated rate constant were match with the experimental degradation results. It was con rmed that the degradation e ciency of B/Z/5/1 was the best. The effect of different catalytic dosage on the degradation of TC was within the scope of assumption. The excessive catalyst dosage did not help the degradation of TC too much. The dosage of 10 mg was the most suitable. The degradation e ciency of different TC concentration was different. When TC concentration was 10 mg/L, the degradation rate was the most rapid. When TC concentration was 100 mg/L, the degradation rate was the lowest, which indicates the absorption of UV light by TC would affect the absorption of the material.

Cyclic experiments
Cyclic experiment was the main way to evaluate the stability of materials. After each photocatalytic experiment, the material was centrifuged, dried and recycled. This process was repeated ve times and the photocatalytic e ciency of each experiment was recorded. As shown in Figure 6, after ve cycles of experiments, the photocatalytic activity of B/Z/5/1 was still very high (88.7%). The XRD patterns of the fth experiment and the rst experiment have no obvious change. The results suggested that the crystal structure of the composite is highly stable and shows no obvious change during the photocatalytic process [29].

Analysis of photocatalytic degradation of TC
The optical absorption characteristics of the three materials was analyzed by UV-Vis absorption spectra. In Figure. 7a, the absorption edge of Bi 2 WO 6 was 468 nm, and the result was consistent with the literature [30,31]. The absorption edge of ZIF-8 was about 250 nm and it had no response to visible light [32]. The VB and CB could be calculated by equation (1) and (1) In formula, α, h, A and v refer to absorption coe cient, Planck constant, proportion constant and optical frequency, respectively. If the material has direct band gap, then n was 2. Conversely, if the material has indirect band gap, then n was 1/2 [35].

Photoelectrochemical analysis
The separation and transfer of charge carriers was studied by electrochemical characterization of the materials. As shown in Figure. 8a, the transient photocurrent density of B/Z/5/1 was much higher than that of ZIF-8 and Bi 2 WO 6 , which showed that the combination of ZIF-8 and Bi 2 WO 6 improved the separation of photo-excited carriers and photocatalytic performance [36]. It was consistent with the results of PL.
The charge separation effect of the material was studied by electrochemical impedance spectroscopy (EIS). The Nyquist curve in Figure. 8b showed that the arc radius of the composite was signi cantly smaller than that of ZIF-8 and Bi 2 WO 6 . The radius on the Nyquist curve representd the separation and transfer velocity of optically excited electron hole pairs. The smaller the radius, the faster the separation and transfer velocity [37]. Therefore, the addition of ZIF-8 was bene cial to inhibit charge recombination.
Simulating real water quality Water quality in nature is very complex, which will affect the degradation of TC. By adjusting the pH value, added HA and anions, the change of degradation e ciency of B/Z/5/1 in simulated water quality was explored. The pH value had great in uence on the photocatalytic degradation, therefore, the pH value of photocatalytic water matrix was adjusted, and the degradation e ciency of TC by B/Z/5/1 at different pH values was studied. As shown in Figure. 9, the degradation of TC by B/Z/5/1 was highly pH dependent. Under acidic conditions, the degradation e ciency of TC by B/Z/5/1 was inhibited with the decreasing pH. The absorbance intensity of TC was affected by pH value, the lower the pH value, the weak the absorbance. Therefore, the extinction e ciency decreased with the decreasing pH [38]. In addition, the dissociation morphology of TC changed at different pH values. With the increasing pH, TC gradually transformed into ions that were easy to be degraded due to deprotonation reaction [39]. Therefore, under alkaline conditions, the adsorption performance and photocatalytic e ciency of the material for TC were greatly improved. HA could reduce the degradation e ciency of TC, which may be induced by the reaction of HA with ·OH in aqueous solution. The capture experiment demonstrated that the key factor of TC degradation was ·OH [40]. Secondly, HA could absorb ultraviolet light in aqueous solution, which was fatal in UV photocatalytic degradation system [41]. HCO 3 was helpful to the degradation of TC. Because under neutral conditions, HCO 3 would generate CO 3 2-· [42]. Clalso reduced photocatalytic e ciency because Clwould react with ·OH during photocatalysis [43].

Free radical capture experiment
In exploring the mechanism of photocatalysis, different corresponding active substances were added to capture hydroxyl radicals, holes and superoxide anions to study the photocatalytic reaction process [39][40][41]. As shown in Figure. 10, in presence of BQ and TBA, the adsorption performance and photocatalytic e ciency of the material for TC decreased signi cantly. The presence of EDTA-2Na had little effect on the reaction. The results of capture experiments con rmed that superoxide anion and hydroxyl radical were crucial in the degradation of TC.

Photocatalytic mechanism
The mechanism of photocatalytic reaction was analyzed by characterization and experimental results. anion. Secondly, due to the VB potential of Bi 2 WO 6 (3.38 eV) was higher than the oxidation potential of water (2.4 eV) [44]. Therefore, some of the holes degraded TC by oxidizing water to hydroxyl groups. The capture experiments indicated that hydroxyl radicals played a more important role than holes in the whole photocatalytic process, so it could be inferred that most of the holes generated on the VB of Bi 2 WO 6 reacted with water to form hydroxyl radicals. Thus, TC was effectively degraded. According to these data, it could infer that superoxide anion and hydroxyl radical were crucial in the whole photocatalytic process.
This was consistent with the experimental results. The conclusion obtained by analyzing the photocatalytic process matched with the photocatalytic mechanism of Z-scheme [12]. The composite of Bi 2 WO 6 and ZIF-8 could effectively inhibit the recombination of photogenerated electron-hole pairs, thus enhanced the photocatalytic activity. The possible process was displayed in formulas (4)

Conclusion
In summary, the composite of Bi 2 WO 6 and ZIF-8 was successfully prepared, which promoted the degradation of TC. Transient photocurrent response and EIS revealed the photoelectrochemical properties of B/Z/5/1, the addition of ZIF-8 could effectively inhibit the recombination of electron and hole, increase the utilization e ciency of light, and greatly improve the photocatalytic activity. The conduction band potential of ZIF-8 was lower than the redox potential of oxygen. The ·O 2 − produced by ZIF-8 was the main factor to improved the photocatalytic activity. At the same time, the strong adsorption performance of ZIF-8 attracted TC molecules to contacted with active substances. Among composites of different proportions, B/Z/5/1 had the optimal catalytic activity and high cycle stability, which was consistent with the Z-type photocatalytic mechanism. In addition, the photocatalytic degradation for the composite with B/Z/5/1 was highly dependent on pH value, and the degradation e ciency was the maximum under Neutral pH conditions, and B/Z/5/1 still had a good degradation e ciency of TC in simulated real water. Finally, this work showed that ZIF-8 can enhance the e ciency of Bi 2 WO 6 photocatalytic degradation of TC, and as an excellent photocatalytic composite, it is expected to provide an important reference for future research.

Declarations
CRediT authorship contribution statement     The degradation e ciency of photocatalyst B/Z/5/1 on TC under ultraviolet radiation, different pH (a), different HA concentration (b), different HCO3-concentration (c) and different Cl-concentration (d).