2D In-Plane CuS/Bi2WO6 p-n Heterostructures with Promoted Visible-Light-Driven Photo-Fenton Degradation Performance

Photo-Fenton degradation of pollutants in wastewater is an ideal choice for large scale practical applications. Herein, two-dimensional (2D) in-plane CuS/Bi2WO6 p-n heterostructures have been successfully constructed by an in situ assembly strategy and characterized using XRD, XPS, SEM/TEM, EDX, UV-Vis-DRS, PL, TR-PL, ESR, and VB-XPS techniques. The XPS and the TEM results confirm the formation of CuS/Bi2WO6 heterostructures. The as-constructed CuS/Bi2WO6 showed excellent absorption in visible region and superior charge carrier separation efficiency due to the formation of a type-II heterojunctions. Under visible light irradiation, 0.1% CuS/Bi2WO6 heterostructure exhibited the best photo-Fenton-like catalytic performance. The degradation efficiency of Rhodamine B (RhB, 20 mg·L−1) can reach nearly 100% within 25 min, the apparent rate constant (kapp/min−1) is approximately 40.06 and 3.87 times higher than that of pure CuS and Bi2WO6, respectively. The degradation efficiency of tetracycline hydrochloride (TC-HCl, 40mg·L−1) can reach 73% in 50 min by employing 0.1% CuS/Bi2WO6 heterostructure as a photo-Fenton-like catalyst. The promoted photo-Fenton catalytic activity of CuS/Bi2WO6 p-n heterostructures is partly ascribed to its low carriers recombination rate. Importantly, CuS in CuS/Bi2WO6 heterostructures is conducive to the formation of heterogeneous photo-Fenton catalytic system, in which Bi2WO6 provides a strong reaction site for CuS to avoid the loss of Cu2+ in Fenton reaction, resulting in its excellent stability and reusability. The possible photo-Fenton-like catalytic degradation mechanism of RhB and TC-HCl was also elucidated on the basis of energy band structure analysis and radical scavenger experiments. The present study provides strong evidence for CuS/Bi2WO6 heterostructures to be used as promising candidates for photo-Fenton treatment of organic pollutants.


Introduction
In recent years, with the continuous development of modern economic society, the problem of water pollution is becoming more and more serious [1][2][3]. Especially organic dye and antibiotic wastewater discharged from factories have attracted people's attention [4][5][6][7]. The complex structure of the dye and antibiotic molecules containing amino groups, carboxyl groups, and benzene rings, lead to toxicity (teratogenic, carcinogenic, and mutagenic) and poor biodegradability [1][2][3][4][5][6][7]. If the dye and antibiotic-containing wastewaters are directly discharged into the freshwater without treatment, the human health and ecosystem will be threatened. Therefore, finding a simple, efficient, and low-cost water pollution remediation method has attracted the attention of researchers.

Preparation of Bi 2 WO 6 Nanosheets
The Bi 2 WO 6 nanosheets were synthesized by a modified hydrothermal method in accordance with previous report [33]. First, 1.94 g of Bi(NO 3 ) 3 ·5H 2 O was dissolved into 5 mL of 4 mol·L −1 HNO 3 aqueous solution. To this solution, 0.02 g of cetyltrimethylammonium bromide (CTAB) and 0.60 g of Na 2 WO 4 ·2H 2 O in 50 mL of deionized water was then slowly added dropwise to the above solution under magnetic stirring, and the solution of pH is adjusted to neutral with 1:1 concentrated ammonia water (NH 3 ·H 2 O), and finally stirred vigorously for 1 h to obtain a white precipitate. The whole mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and treated hydrothermally at 180 • C for 14 h. After cooling to room temperature, it was washed three times with deionized water and absolute ethanol, dried at 70 • C for 8 h, and calcined at 300 • C for 3 h in a muffle furnace to remove the surface residue.

Preparation of CuS/Bi 2 WO 6 Heterostructure
The CuS/Bi 2 WO 6 composites were synthesized by a hydrothermal method according to the previous literature with minor revision [31]. In brief, (0.0024 g, 0.0315 mmol) of thiourea was dissolved in 75 mL water, then 1 g of as-prepared Bi 2 WO 6 was added under ultrasonication for 10 min. Subsequently, (0.0025 g, 0.0103 mmol) of Cu(NO 3 ) 2 ·3H 2 O was added with another ultrasonication for 10 min. After stirring for 1 h, the resulting suspension were placed in a 100 mL stainless steel autoclave at 150 • C for 24 h. Finally, when the oven was naturally cooled to room temperature, the obtained products were separated by centrifugation and washed 3 times with deionized water and absolute ethanol, and dried at 70 • C in vacuum overnight to obtain 0.1% CuS/Bi 2 WO 6 composite (Herein, 0.1% represents the weight ratio of CuS to Bi 2 WO 6 ). Following the similar procedure, a series of X% CuS/Bi 2 WO 6 (X = 0.02%, 0.5%, 1% and 2%) heterostructures were prepared using the same procedure, only the amount of Cu(NO 3 ) 2 ·3H 2 O and thiourea was changed. For comparison, pure CuS was also prepared by the same procedure in the absence of Bi 2 WO 6 .

Characterization
The XRD patterns of samples were recorded by a Shimadzu XRD-7000 using a Ni filtered Cu Kα (λ = 0.15418 nm) as X-ray source. The XRD data was obtained in 2θ range of 10-80 • at a scan rate of 8 • ·min −1 . The X-ray photoelectron spectra (XPS) patterns of samples were measured on a PHI-5400 (Physical Electronics PHI, MN, America ) 250 xi system with Al Kα X-rays as the excitation source. The morphology and microscopic size of the samples were carried out by a JSM-6700F (Japan electronics) and a (JEM-2100) (Japan electronics). Energy disperse X-ray (EDX) analysis was performed on a field emission scanning electron microscope (JSM-7610F). The optical properties of the samples were detected on a UV-2550 spectrophotometer (Japan Shimadzu, Kyoto, Japan) within 200 nm to 800 nm, using BaSO4 as a reflectance standard. The photoluminescence (PL) spectra of samples were conducted on a F-4600 spectrophotometer (Hitachi, Japan). Time-resolved photoluminescence (TR-PL) spectra were conducted on a FLS920 fluorescence spectrometer (Edinburgh Analytical Instruments, Edinburgh, UK). The electron spin resonance (ESR) spectra were obtained on a JES-FA300 model spectrometer (ESR, Japan JEOL, Tokyo, Japan) under visible light irradiation (λ ≥ 420 nm). The magnetic central field intensity, scanning width, scanning time, and microwave power are 324.006 mT, 10.00 mT, 60 s, and 0.99800 mW, respectively. The spectra is 4096 points. In a typical experiment for ·OH detection, 10 mg of photocatalyst was dispersed in the mixture solution of 20 mL of ultrapure water and 50 µL of H 2 O 2 under ultrasonic for 30 min. Then, 10 µL of mixture was sampled and 10 µL of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (10 ppm) was added with ultrasonic dispersion for 5 min. Thereafter, the obtained mixtures were sampled by a capillary tube and placed into a quartz ESR tube then subjected to test. The signals were collected after visible light irradiation for different time. Following a similar process-just replacing ultrapure water with methanol-·O 2 − was also detected.

Photocatalytic and Photo-Fenton Catalytic Activity Measurement
The prepared samples were used as photo and photo-Fenton like catalysts for the degradation of Rhodamine B (RhB, 20 mg·L −1 ) and tetracycline hydrochloride (TC-HCl, 40 mg·L −1 ). The photocatalytic degradation experiments were carried in photocatalytic reactor (Xujiang Mechanical and Electrical Factory, Nanjing, China, XPA-7). A 300 W metal halide lamp equipped with a 420 nm cut-off filter was used to simulate visible light source. The circulating cooling water was introduced to avoid liquid volatilization and keep the reaction temperature at 25 • C through the temperature control system. Moreover, 0.02 g of the catalyst was added to a 50 mL capacity reactor containing 20 mL of simulated contaminants aqueous solution adjusting with the determined pH (with NaOH or HCl aqueous solution), and then stirred under dark conditions for 2 h to ensure adsorption/desorption equilibrium, then a certain volume of H 2 O 2 (30%) was added when the light source was turned on. At a certain interval, 2 mL of the supernatant was removed for centrifugation, and then the maximum characteristic absorption peaks of RhB and TC-HCl at 554 nm and 357 nm were measured by UV-Vis spectroscopy (Shimadzu UV-2550), respectively.

Structure, Composition, and Morphology of Samples
The XRD patterns of as-synthesized Bi 2 WO 6 , CuS, and CuS/Bi 2 WO 6 samples are shown in Figure 1. For the Bi 2 WO 6 sample, all the diffraction peaks can be assigned to the orthorhombic phase Bi 2 WO 6 (JCPDS No. 39-0256) [34]. Besides this, the peaks observed for pure CuS are assigned to hexagonal covellite phase (JCPDS No.06-0464) [31]. For the 0.1% CuS/Bi 2 WO 6 heterostructure, the characteristic peaks of Bi 2 WO 6 are still retained, but there no characteristic peaks of CuS species and other phases appear in the XRD pattern, owing to the low loading amount of CuS. after visible light irradiation for different time. Following a similar process-just replacing ultrapure water with methanol-·O2 − was also detected.

Photocatalytic and Photo-Fenton Catalytic Activity Measurement
The prepared samples were used as photo and photo-Fenton like catalysts for the degradation of Rhodamine B (RhB, 20 mg·L −1 ) and tetracycline hydrochloride (TC-HCl, 40 mg·L −1 ). The photocatalytic degradation experiments were carried in photocatalytic reactor (Xujiang Mechanical and Electrical Factory, Nanjing, China, XPA-7). A 300 W metal halide lamp equipped with a 420 nm cut-off filter was used to simulate visible light source. The circulating cooling water was introduced to avoid liquid volatilization and keep the reaction temperature at 25 °C through the temperature control system. Moreover, 0.02 g of the catalyst was added to a 50 mL capacity reactor containing 20 mL of simulated contaminants aqueous solution adjusting with the determined pH (with NaOH or HCl aqueous solution), and then stirred under dark conditions for 2 h to ensure adsorption/desorption equilibrium, then a certain volume of H2O2 (30%) was added when the light source was turned on. At a certain interval, 2 mL of the supernatant was removed for centrifugation, and then the maximum characteristic absorption peaks of RhB and TC-HCl at 554 nm and 357 nm were measured by UV-Vis spectroscopy (Shimadzu UV-2550), respectively.

Structure, Composition, and Morphology of Samples
The XRD patterns of as-synthesized Bi2WO6, CuS, and CuS/Bi2WO6 samples are shown in Figure 1. For the Bi2WO6 sample, all the diffraction peaks can be assigned to the orthorhombic phase Bi2WO6 (JCPDS No. 39-0256) [34]. Besides this, the peaks observed for pure CuS are assigned to hexagonal covellite phase (JCPDS No.06-0464) [31]. For the 0.1% CuS/Bi2WO6 heterostructure, the characteristic peaks of Bi2WO6 are still retained, but there no characteristic peaks of CuS species and other phases appear in the XRD pattern, owing to the low loading amount of CuS. To ulteriorly investigate the surface composition and chemical state information of the CuS/Bi2WO6 heterostructure, XPS measurement was subsequently performed, and the binding energy of C 1s was calibrated at 284.6 eV. The surface survey spectrum (Figure 2a) showed the existence of Bi, W, and O elements in both the Bi2WO6 and 0.1% CuS/Bi2WO6 heterostructure, and exemplified only the existence of Cu, S, and C elements in the CuS, in which the binding energy of C 1s is ascribed to the adventitious carbon from the XPS instrument itself. The high-resolution spectra of Bi 4f, W 4f, Cu 2p, and S 2p are shown in Figure 2b-e, respectively. Figure 2b shows two peaks of 0.1% CuS/Bi2WO6 heterostructure are observed at around 164.57 and 159.25 eV, corresponding to the Bi 4f5/2 and Bi 4f7/2 energy states of Bi 3+ [35]. The strong peaks at 37.48 and 35.49 eV are assigned to the To ulteriorly investigate the surface composition and chemical state information of the CuS/Bi 2 WO 6 heterostructure, XPS measurement was subsequently performed, and the binding energy of C 1s was calibrated at 284.6 eV. The surface survey spectrum (Figure 2a) showed the existence of Bi, W, and O elements in both the Bi 2 WO 6 and 0.1% CuS/Bi 2 WO 6 heterostructure, and exemplified only the existence of Cu, S, and C elements in the CuS, in which the binding energy of C 1s is ascribed to the adventitious carbon from the XPS instrument itself. The high-resolution spectra of Bi 4f, W 4f, Cu 2p, and S 2p are shown in Figure 2b-e, respectively. Figure 2b shows two peaks of 0.1% CuS/Bi 2 WO 6 heterostructure are observed at around 164.57 and 159.25 eV, corresponding to the Bi 4f 5/2 and Bi 4f 7/2 energy states of Bi 3+ [35]. The strong peaks at 37.48 and 35.49 eV are assigned to the W 4f 5/2 and W 4f 7/2 for W 6+ (Figure 2c) [36]. The corresponding Cu 2p peaks are located at 952.42 and 932.02 eV, and are designated Cu 2+ 2p 1/2 and Cu 2+ 2p 3/2 ( Figure 2d) [30,31,37]. It is important to note that the binding energies of Bi 4f, W 4f, and Cu 2p in the 0.1% CuS/Bi 2 WO 6 heterostructure all exhibited slight shift to the binding energy compared with that of Bi 2 WO 6 or CuS (Figure 2b-d) due to the interaction between CuS and Bi 2 WO 6 nanosheets. As for S 2p, it can be seen that the standard peaks of CuS at 163.4 eV and 162.4 eV for S 2p 1/2 and S 2p 3/2 were observed, which are consistent with the reported binding energies in metal sulfide. (Figure 2e) [38]. On the contrary, two different peaks are unexpectedly similar to the binding energies of Bi 3+ at 159.10 and 164.30 eV in the 0.1% CuS/Bi 2 WO 6 heterostructure ( Figure 2e). It may be because the binding energy of S 2− is very similar to that of Bi 3+ [39]. The XRD and XPS results implied that 0.1% CuS/Bi 2 WO 6 heterostructure was authentically obtained.  (Figure 2c) [36]. The corresponding Cu 2p peaks are located at 952.42 and 932.02 eV, and are designated Cu 2+ 2p1/2 and Cu 2+ 2p3/2 ( Figure 2d) [30,31,37]. It is important to note that the binding energies of Bi 4f, W 4f, and Cu 2p in the 0.1% CuS/Bi2WO6 heterostructure all exhibited slight shift to the binding energy compared with that of Bi2WO6 or CuS (Figure 2b-d) due to the interaction between CuS and Bi2WO6 nanosheets. As for S 2p, it can be seen that the standard peaks of CuS at 163.4 eV and 162.4 eV for S 2p1/2 and S 2p3/2 were observed, which are consistent with the reported binding energies in metal sulfide. (Figure 2e) [38]. On the contrary, two different peaks are unexpectedly similar to the binding energies of Bi 3+ at 159.10 and 164.30 eV in the 0.1% CuS/Bi2WO6 heterostructure (Figure 2e). It may be because the binding energy of S 2− is very similar to that of Bi 3+ [39]. The XRD and XPS results implied that 0.1% CuS/Bi2WO6 heterostructure was authentically obtained.  The morphology and size of the as-synthesized Bi 2 WO 6 , CuS, and 0.1% CuS/Bi 2 WO 6 heterostructure were observed by SEM and TEM images. Figure 3a,b depict the nanosheet structures of Bi 2 WO 6 with the average thickness of about 20 nm. Figure 3c,d show the SEM image of CuS sample presenting spherical-shape morphology assembled by uniform nanosheets with thickness of approximately 50 nm. As can be seen from Figure 3e,f, the CuS nanosheets were well in-plate assembled on the Bi 2 WO 6 nanosheets, and the tight interfaces between Bi 2 WO 6 and CuS further confirmed the formation of CuS/Bi 2 WO 6 heterojunctions. Moreover, the energy-dispersive X-ray analysis of 0.1% CuS/Bi 2 WO 6 heterostructure was shown in Figure 3g-l, which confirms the uniform distribution of Bi, W, O, Cu, and S species over the 0.1% CuS/Bi 2 WO 6 heterostructure.
of Bi2WO6 with the average thickness of about 20 nm. Figure 3c,d show the SEM image of CuS sample presenting spherical-shape morphology assembled by uniform nanosheets with thickness of approximately 50 nm. As can be seen from Figure 3e,f, the CuS nanosheets were well in-plate assembled on the Bi2WO6 nanosheets, and the tight interfaces between Bi2WO6 and CuS further confirmed the formation of CuS/Bi2WO6 heterojunctions. Moreover, the energy-dispersive X-ray analysis of 0.1% CuS/Bi2WO6 heterostructure was shown in Figure 3g-l, which confirms the uniform distribution of Bi, W, O, Cu, and S species over the 0.1% CuS/Bi2WO6 heterostructure. Figure 4a shows the in situ assembly strategy of 2D/2D CuS/Bi2WO6 heterostructures, which depicts the formation of CuS nanosheets onto Bi2WO6 nanosheets for assembling in-plane heterostructures. Figure 4b-d shows the TEM images of Bi2WO6, CuS and 0.1% CuS/Bi2WO6 composite. The TEM results further validated the nanosheet structures of Bi2WO6 and the microsphere structure of CuS assembled by nanosheets. The CuS and Bi2WO6 containing in 0.1% CuS/Bi2WO6 heterostructure were verified by HRTEM, as shown in Figure 4e. The lattice fringe spacing between adjacent crystal faces is 0.273 nm and 0.189 nm, which can be attributed to the (200) and (110) crystal plane of Bi2WO6 phase and CuS phase, respectively [40,41], leading to the formation of heterojunction.

Photocatalytic and Photo-Fenton Catalytic Performance
The photo-Fenton and photocatalytic degradation of organic pollutants are shown in Figure 5. As can be seen from Figure 5a, the concentration of initial RhB hardly changes without catalyst, which fully proves the stability of its own molecules. However, after the addition of H2O2 alone, the degradation rate of RhB can reach 14%. In addition, compared to pure Bi2WO6, the loading of CuS slightly decreased its photocatalytic activity, which may be due to CuS covering up the active sites, eventually resulting in a decrease in activity.
As can be seen from Figure 5b, RhB was only degraded by 10% when CuS is added as a catalyst alone, which may be due to its high electron-hole recombination rate as a photo-Fenton catalyst. It is important to note that the degradation performance of RhB by photo-Fenton process over CuS/Bi2WO6 heterostructures is obviously enhanced. The as-prepared 0.1% CuS/Bi2WO6 heterostructure represents the optimal amount relative to photo-Fenton activity. Figure 5c shows that the rate constant implemented by the equation ln (C0/C) = kapp·t, where kapp/min −1 means the apparent rate constant. The degradation efficiency of RhB (20 mg·L −1 ) can reach nearly 100% within 25 min, the apparent rate constant (kapp/min −1 ) is approximately 40.06 times and 3.87 times higher than that of pure CuS and Bi2WO6, respectively ( Figure 5d). Hence, it can be concluded that loading CuS nanosheets on Bi2WO6 nanosheets can remarkably promote photo-Fenton catalytic performance.
In order to show that the catalyst can handle a wide variety of wastewaters, the photocatalytic and photo-Fenton catalytic activity of the CuS/Bi2WO6 heterostructures in TC-HCl aqueous solution

Photocatalytic and Photo-Fenton Catalytic Performance
The photo-Fenton and photocatalytic degradation of organic pollutants are shown in Figure 5. As can be seen from Figure 5a, the concentration of initial RhB hardly changes without catalyst, which fully proves the stability of its own molecules. However, after the addition of H 2 O 2 alone, the degradation rate of RhB can reach 14%. In addition, compared to pure Bi 2 WO 6 , the loading of CuS slightly decreased its photocatalytic activity, which may be due to CuS covering up the active sites, eventually resulting in a decrease in activity.
As can be seen from Figure 5b, RhB was only degraded by 10% when CuS is added as a catalyst alone, which may be due to its high electron-hole recombination rate as a photo-Fenton catalyst. It is important to note that the degradation performance of RhB by photo-Fenton process over CuS/Bi 2 WO 6 heterostructures is obviously enhanced. The as-prepared 0.1% CuS/Bi 2 WO 6 heterostructure represents the optimal amount relative to photo-Fenton activity. Figure 5c shows that the rate constant implemented by the equation ln (C 0 /C) = k app ·t, where k app /min −1 means the apparent rate constant. The degradation efficiency of RhB (20 mg·L −1 ) can reach nearly 100% within 25 min, the apparent rate constant (k app /min −1 ) is approximately 40.06 times and 3.87 times higher than that of pure CuS and Bi 2 WO 6 , respectively ( Figure 5d). Hence, it can be concluded that loading CuS nanosheets on Bi 2 WO 6 nanosheets can remarkably promote photo-Fenton catalytic performance.
were also investigated. Figure 6a shows the variation of TC-HCl concentration of the degradation in different samples. It can be seen the photocatalytic effect of the 0.1% CuS/Bi2WO6 heterostructure is lower than that of pure Bi2WO6, which is the same as the activity of the comparison of the dye. At the same time, it is noted in Figure 6b that after the H2O2 is added, the photo-Fenton catalytic degradation efficiency was close to 73% in 50 min through the constructed 0.1% CuS/Bi2WO6 photo-Fenton system, which was obviously better than pure Bi2WO6.  In order to show that the catalyst can handle a wide variety of wastewaters, the photocatalytic and photo-Fenton catalytic activity of the CuS/Bi 2 WO 6 heterostructures in TC-HCl aqueous solution were also investigated. Figure 6a shows the variation of TC-HCl concentration of the degradation in different samples. It can be seen the photocatalytic effect of the 0.1% CuS/Bi 2 WO 6 heterostructure is lower than that of pure Bi 2 WO 6 , which is the same as the activity of the comparison of the dye. At the same time, it is noted in Figure 6b that after the H 2 O 2 is added, the photo-Fenton catalytic degradation efficiency was close to 73% in 50 min through the constructed 0.1% CuS/Bi 2 WO 6 photo-Fenton system, which was obviously better than pure Bi 2 WO 6 .
We further explored the effect of H 2 O 2 concentration, catalyst dosage, and initial pH value on the degradation performance of TC-HCl by 0.1% CuS/Bi 2 WO 6 heterostructure under visible light irradiation. Firstly, the effect of different H 2 O 2 concentrations on the degradation of TC-HCl is shown in Figure 7a. As the initial H 2 O 2 concentration increased from 50 µL to 100 µL, the degradation efficiency increased gradually from 68.1% to 72.8%, while the initial H 2 O 2 concentration further increased from 100 µL to 200 µL, the degradation efficiency increased slightly. From an economic point of view, the optimal concentration of H 2 O 2 was chosen as 100 µL. This indicates that adding the appropriate amount of H 2 O 2 is advantageous for increasing the photo-Fenton catalytic activity. Secondly, the effect of the catalyst dosage on the degradation of TC-HCl is shown in Figure 7b. It can be seen that there is an optimum amount of catalyst and the photo-Fenton activity is highest at this time. When the optimum amount is exceeded, the activity is gradually decreased, which may be because too much catalyst blocks the light, and it eventually leads to a decrease in photo-Fenton catalytic activity. Finally, the effect of pH on the degradation of TC-HCl is shown in Figure 7c. As the pH increased from 5 to 7, the degradation efficiency of TC-HCl increased, and then continued to increase pH to 9, and the degradation activity declined instead, which indicated that it has better photo-Fenton activity under natural pH conditions. This is because under natural conditions, the photo-Fenton reaction is more likely to generate enough active free radicals to degrade pollutants.  We further explored the effect of H2O2 concentration, catalyst dosage, and initial pH value on the degradation performance of TC-HCl by 0.1% CuS/Bi2WO6 heterostructure under visible light irradiation. Firstly, the effect of different H2O2 concentrations on the degradation of TC-HCl is shown in Figure 7a. As the initial H2O2 concentration increased from 50 μL to 100 μL, the degradation efficiency increased gradually from 68.1% to 72.8%, while the initial H2O2 concentration further increased from 100 μL to 200 μL, the degradation efficiency increased slightly. From an economic point of view, the optimal concentration of H2O2 was chosen as 100 μL. This indicates that adding the appropriate amount of H2O2 is advantageous for increasing the photo-Fenton catalytic activity. Secondly, the effect of the catalyst dosage on the degradation of TC-HCl is shown in Figure  7b. It can be seen that there is an optimum amount of catalyst and the photo-Fenton activity is highest at this time. When the optimum amount is exceeded, the activity is gradually decreased, which may be because too much catalyst blocks the light, and it eventually leads to a decrease in photo-Fenton catalytic activity. Finally, the effect of pH on the degradation of TC-HCl is shown in Figure 7c. As the pH increased from 5 to 7, the degradation efficiency of TC-HCl increased, and then continued to increase pH to 9, and the degradation activity declined instead, which indicated that it has better photo-Fenton activity under natural pH conditions. This is because under natural conditions, the photo-Fenton reaction is more likely to generate enough active free radicals to degrade pollutants. The cyclic experiments of TC-HCl were also carried out four times over 0.1% CuS/Bi2WO6 heterostructure (Figure 8a). It can be seen that the catalyst still maintains a high degradation efficiency after each cycle. Figure 8b presents the XRD patterns of fresh and reused 0.1% CuS/Bi2WO6 The cyclic experiments of TC-HCl were also carried out four times over 0.1% CuS/Bi 2 WO 6 heterostructure (Figure 8a). It can be seen that the catalyst still maintains a high degradation efficiency after each cycle. Figure 8b presents the XRD patterns of fresh and reused 0.1% CuS/Bi 2 WO 6 heterostructure in the photo-Fenton reaction. No impurity peaks were presented and only the peak intensity slightly decreased, which suggested that the catalyst has relatively stability. This showed that Cu 2+ leaching problem in the traditional homogeneous Fenton reaction can be significantly restrained by employing CuS/Bi 2 WO 6 heterostructures. According to the above results, the 0.1% CuS/Bi 2 WO 6 heterostructure exhibited significantly enhanced catalytic performance compared with the previously reported catalysts for the degradation of dyes and antibiotics (Table 1), for instance, Bi 2 WO 6 /Fe 3 O 4 [27], g-C 3 N 4 /CuS [29], TiO 2 /Fe 3 O 4 [42], etc. This comparison clearly reveals that the 0.1% CuS/Bi 2 WO 6 heterostructure is a potential candidate for treating dye and antibiotic wastewater under visible light irradiation. heterostructure in the photo-Fenton reaction. No impurity peaks were presented and only the peak intensity slightly decreased, which suggested that the catalyst has relatively stability. This showed that Cu 2+ leaching problem in the traditional homogeneous Fenton reaction can be significantly restrained by employing CuS/Bi2WO6 heterostructures. According to the above results, the 0.1% CuS/Bi2WO6 heterostructure exhibited significantly enhanced catalytic performance compared with the previously reported catalysts for the degradation of dyes and antibiotics (Table 1), for instance, Bi2WO6/Fe3O4 [27], g-C3N4/CuS [29], TiO2/Fe3O4 [42], etc. This comparison clearly reveals that the 0.1% CuS/Bi2WO6 heterostructure is a potential candidate for treating dye and antibiotic wastewater under visible light irradiation.  Figure 9 shows the UV−Vis DRS spectra of Bi2WO6, CuS and 0.1% CuS/Bi2WO6 heterostructure. Obviously, CuS exhibits a stronger absorption band in visible region, while Bi2WO6 exhibits weak absorption in visible region. After assembling CuS nanosheets on the surface of Bi2WO6 nanosheets, the absorption of visible region by the composite catalyst was slightly enhanced. Nevertheless, when light and H2O2 were simultaneously introduced, the heterogeneous photo-Fenton system formed by CuS and Bi2WO6 exhibited higher catalytic performance in the case of extremely low Cu 2+ loading.   Figure 9 shows the UV−Vis DRS spectra of Bi 2 WO 6 , CuS and 0.1% CuS/Bi 2 WO 6 heterostructure. Obviously, CuS exhibits a stronger absorption band in visible region, while Bi 2 WO 6 exhibits weak absorption in visible region. After assembling CuS nanosheets on the surface of Bi 2 WO 6 nanosheets, the absorption of visible region by the composite catalyst was slightly enhanced. Nevertheless, when light and H 2 O 2 were simultaneously introduced, the heterogeneous photo-Fenton system formed by CuS and Bi 2 WO 6 exhibited higher catalytic performance in the case of extremely low Cu 2+ loading. heterostructure in the photo-Fenton reaction. No impurity peaks were presented and only the peak intensity slightly decreased, which suggested that the catalyst has relatively stability. This showed that Cu 2+ leaching problem in the traditional homogeneous Fenton reaction can be significantly restrained by employing CuS/Bi2WO6 heterostructures. According to the above results, the 0.1% CuS/Bi2WO6 heterostructure exhibited significantly enhanced catalytic performance compared with the previously reported catalysts for the degradation of dyes and antibiotics (Table 1), for instance, Bi2WO6/Fe3O4 [27], g-C3N4/CuS [29], TiO2/Fe3O4 [42], etc. This comparison clearly reveals that the 0.1% CuS/Bi2WO6 heterostructure is a potential candidate for treating dye and antibiotic wastewater under visible light irradiation.  Figure 9 shows the UV−Vis DRS spectra of Bi2WO6, CuS and 0.1% CuS/Bi2WO6 heterostructure. Obviously, CuS exhibits a stronger absorption band in visible region, while Bi2WO6 exhibits weak absorption in visible region. After assembling CuS nanosheets on the surface of Bi2WO6 nanosheets, the absorption of visible region by the composite catalyst was slightly enhanced. Nevertheless, when light and H2O2 were simultaneously introduced, the heterogeneous photo-Fenton system formed by CuS and Bi2WO6 exhibited higher catalytic performance in the case of extremely low Cu 2+ loading.    The recombination rate of photogenerated electrons and holes of the materials were analyzed by photoluminescence (PL) spectroscopy. In general, the weak fluorescence intensity of the samples indicates that the recombination rate of photogenerated electrons and holes in the material is relatively low. It can be seen from Figure 10a that the Bi 2 WO 6 has a higher recombination rate of photogenerated electrons and holes than other samples. The intensity of the diffraction peak of the 0.1% CuS/Bi 2 WO 6 heterostructure is much lower than that of the Bi 2 WO 6 alone, suggesting that the combination of Bi 2 WO 6 and CuS can effectively inhibit the recombination of electrons and holes. This is beneficial for the photo-Fenton catalytic reaction involving H 2 O 2 . The time-resolved photoluminescence (TR-PL) spectroscopy of Bi 2 WO 6 and 0.1% CuS/Bi 2 WO 6 heterostructure are also used to detect the carrier lifetime, and the results were shown in Figure 10b. The photoelectron lifetimes conform to the two-exponential decay model. Decay times (τ 1 and τ 2 ) and PL aptitudes (A 1 and A 2 ) are given in Table 2. Compared to the Bi 2 WO 6 (0.903 ns), the photogenerated charge carriers lifetime of 0.1% CuS/Bi 2 WO 6 heterostructure is significantly prolonged (1.360 ns). The increased carriers lifetime can effectively promote the cycle of Cu 2+ to Cu + , and then Cu + can react with H 2 O 2 to generate ·OH with strong oxidation performance, thus improving the photo-Fenton catalytic performance. The recombination rate of photogenerated electrons and holes of the materials were analyzed by photoluminescence (PL) spectroscopy. In general, the weak fluorescence intensity of the samples indicates that the recombination rate of photogenerated electrons and holes in the material is relatively low. It can be seen from Figure 10a that the Bi2WO6 has a higher recombination rate of photogenerated electrons and holes than other samples. The intensity of the diffraction peak of the 0.1% CuS/Bi2WO6 heterostructure is much lower than that of the Bi2WO6 alone, suggesting that the combination of Bi2WO6 and CuS can effectively inhibit the recombination of electrons and holes. This is beneficial for the photo-Fenton catalytic reaction involving H2O2. The time-resolved photoluminescence (TR-PL) spectroscopy of Bi2WO6 and 0.1% CuS/Bi2WO6 heterostructure are also used to detect the carrier lifetime, and the results were shown in Figure 10b. The photoelectron lifetimes conform to the two-exponential decay model. Decay times (τ1 and τ2) and PL aptitudes (A1 and A2) are given in Table 2. Compared to the Bi2WO6 (0.903 ns), the photogenerated charge carriers lifetime of 0.1% CuS/Bi2WO6 heterostructure is significantly prolonged (1.360 ns). The increased carriers lifetime can effectively promote the cycle of Cu 2+ to Cu + , and then Cu + can react with H2O2 to generate ·OH with strong oxidation performance, thus improving the photo-Fenton catalytic performance.  To investigate the catalytic mechanism, an in situ ESR study was employed to monitor the formation of active radicals. When DMPO (a spin-trapping reagent for·OH and O2 − ) was added to the reaction systems, no signal of DMPO-·O 2 − and DMPO-·OH adducts can be detected in the dark (Figure 11), suggesting that negligible OH and·O 2 − radical concentrations were present in the solution. However, under the visible light irradiation, both pristine Bi2WO6 and CuS/Bi2WO6 heterostructure can produce ESR signals ( Figure 11). The characteristic signals of DMPO-·O H with a four-line ESR signal, relative intensities of 1:2:2:1, and αH = αN = 1.5 mT (Figure 11a) were observed [43,44]. Similarly, a doublet of triplets ESR signal with relative intensities of 1:1:1:1 characteristic signals of DMPO-·O 2 − (Figure 11b) was also observed [45,46]. However, it is worth noting that the coupling constants of DMPO-·O 2 -adduct are aH = 0.92 mT, aN =1.4 mT, respectively. The value of aH does not match with the normal value, which may be ascribed to the effects of the solution environment such as pH and solvent. According to the literature reported and our experimental result, the doublet of triplets ESR signal was assigned to DMPO-·O 2 − adduct rather than those centerd on hetero atoms such as O or S [47,48]. Furthermore, compared to pristine Bi2WO6, the highly intensive signals were observed for·OH and·O2 − adducts in the 0.1% CuS/Bi2WO6  To investigate the catalytic mechanism, an in situ ESR study was employed to monitor the formation of active radicals. When DMPO (a spin-trapping reagent for·OH and O 2 − ) was added to the reaction systems, no signal of DMPO-·O 2 − and DMPO-·OH adducts can be detected in the dark ( Figure 11), suggesting that negligible OH and·O 2 − radical concentrations were present in the solution.

Photocatalytic and Photo-Fenton Catalytic Mechanism
However, under the visible light irradiation, both pristine Bi 2 WO 6 and CuS/Bi 2 WO 6 heterostructure can produce ESR signals ( Figure 11). The characteristic signals of DMPO-·OH with a four-line ESR signal, relative intensities of 1:2:2:1, and α H = α N = 1.5 mT (Figure 11a) were observed [43,44]. Similarly, a doublet of triplets ESR signal with relative intensities of 1:1:1:1 characteristic signals of DMPO-·O 2 − (Figure 11b) was also observed [45,46]  The band structure of CuS/Bi2WO6 heterostructure was studied by Tauc plots and XPS valence band spectrum analysis. The measured band gap diagram shows that Bi2WO6 and CuS, correspond to band gap energies of 2.65 eV and 1.54 eV according to the equation of Kubelka-Munk function, respectively [49], as shown in Figure 12a,b. In addition, according to the intercept with X axis obtained from XPS results, the position of valence band of the material is further determined, indicating that valence band (VB) of Bi2WO6 and CuS are 2.24 and 1.46 V, respectively (Figure 12c,d). Therefore, following the empirical equation: ECB = EVB − Eg, the conduction band minimum (CBM) of Bi2WO6 and CuS are confirmed to be −0.41 and −0.08 V, respectively.
In order to further investigate the role of CuS/Bi2WO6 heterostructures in photo-Fenton catalysis system, the enhancement mechanism of photo-Fenton activity is discussed, and the schematic diagram is shown in Figure 13a,b. Due to the n-type nature of Bi2WO6 and p-type nature of CuS, the Fermi levels of Bi2WO6 and CuS are located nearer to the CBM and VBM, respectively. Upon contact between Bi2WO6 and CuS, due to Fermi level equilibrium the energy level of CuS is upshifted and Bi2WO6 is downshifted leading to the formation of p-n heterojunction [41]. The p-n junction also led to the generation of an internal electric field where the CuS region is negatively charged and the Bi2WO6 region is positively charged. This internal field is responsible for efficient separation of the charge carriers. When irradiated with light, both Bi2WO6 and CuS are capable of absorbing visible light photons to produce e − /h + pairs. Due to favorable band alignment, the photogenerated electrons can migrate from the CB of CuS to Bi2WO6, whereas the migration of holes can take place in reverse direction in the VB of both semiconductor components. This cyclic movement of photogenerated carriers is characteristic of a type-II p-n heterojunction, which leads to their efficient space separation. In the heterogeneous visible light photo-Fenton system constructed in this experiment, the active species involved in the oxidative degradation of organic pollutants mainly include photogenerated holes, O2 − and ·OH radical. The band structure of CuS/Bi 2 WO 6 heterostructure was studied by Tauc plots and XPS valence band spectrum analysis. The measured band gap diagram shows that Bi 2 WO 6 and CuS, correspond to band gap energies of 2.65 eV and 1.54 eV according to the equation of Kubelka-Munk function, respectively [49], as shown in Figure 12a,b. In addition, according to the intercept with X axis obtained from XPS results, the position of valence band of the material is further determined, indicating that valence band (VB) of Bi 2 WO 6 and CuS are 2.24 and 1.46 V, respectively (Figure 12c,d). Therefore, following the empirical equation: E CB = E VB − E g , the conduction band minimum (CBM) of Bi 2 WO 6 and CuS are confirmed to be −0.41 and −0.08 V, respectively.   Figure 12. Tauc plots of (αhν) 1/2 vs. hν for the samples Bi 2 WO 6 (a) and CuS (b) and XPS valence band spectra of Bi 2 WO 6 (c) and CuS (d).
In order to further investigate the role of CuS/Bi 2 WO 6 heterostructures in photo-Fenton catalysis system, the enhancement mechanism of photo-Fenton activity is discussed, and the schematic diagram is shown in Figure 13a,b. Due to the n-type nature of Bi 2 WO 6 and p-type nature of CuS, the Fermi levels of Bi 2 WO 6 and CuS are located nearer to the CBM and VBM, respectively. Upon contact between Bi 2 WO 6 and CuS, due to Fermi level equilibrium the energy level of CuS is upshifted and Bi 2 WO 6 is downshifted leading to the formation of p-n heterojunction [41]. The p-n junction also led to the generation of an internal electric field where the CuS region is negatively charged and the Bi 2 WO 6 region is positively charged. This internal field is responsible for efficient separation of the charge carriers. When irradiated with light, both Bi 2 WO 6 and CuS are capable of absorbing visible light photons to produce e − /h + pairs. Due to favorable band alignment, the photogenerated electrons can migrate from the CB of CuS to Bi 2 WO 6 , whereas the migration of holes can take place in reverse direction in the VB of both semiconductor components. This cyclic movement of photogenerated carriers is characteristic of a type-II p-n heterojunction, which leads to their efficient space separation. In the heterogeneous visible light photo-Fenton system constructed in this experiment, the active species involved in the oxidative degradation of organic pollutants mainly include photogenerated holes, O 2 − and ·OH radical.   When visible light irradiated on the surface of the CuS/Bi2WO6 heterostructure, electrons on the Bi2WO6 valence band were excited to the conduction band (Equation (1)). Both in the process of degrading RhB and TC-HCl, some electrons in the CB of Bi2WO6 could reduce O2 into ·O 2 − because When visible light irradiated on the surface of the CuS/Bi 2 WO 6 heterostructure, electrons on the Bi 2 WO 6 valence band were excited to the conduction band (Equation (1)). Both in the process of degrading RhB and TC-HCl, some electrons in the CB of Bi 2 WO 6 could reduce O 2 into ·O 2 − because the CB potential of Bi 2 WO 6 (−0.41 eV vs. NHE) was more negative than O 2 /·O 2 − potential (−0.33 or −0.046 eV vs. NHE) (Figure 13a,b; Equation (2)). The other electrons could transfer to the surface of CuS to react with H 2 O 2 to generate ·OH via the photo-Fenton reaction (Equations (4)- (7)), while the photogenerated holes left on the valence band of Bi 2 WO 6 could directly degrade RhB and TC-HCl or react with OH − to generate ·OH (Equation (3)). During the degrading of RhB, the VB holes of Bi 2 WO 6 could oxidize OH − into OH because the VB potential of Bi 2 WO 6 (2.24 eV vs. NHE) was more positive than that of ·OH/OH − potential (1.99 eV vs. NHE) (Figure 13a), but could not oxidize OHinto ·OH during the degradation of TC-HCl because the VB potential of Bi 2 WO 6 (2.24 eV vs. NHE) was more negative than ·OH/OHpotential (2.38 eV vs. NHE) (Figure 13b). This resulted in a heterogeneous photo-Fenton process to degrade the organic pollutants via the generated active species (Equation (8)).

Conclusions
In summary, we have developed a simple in situ assembly strategy to construct the two-dimensional (2D) in-plane CuS/Bi 2 WO 6 sheet-on-sheet p-n heterostructures and used as photo-Fenton catalysts. A series of techniques were employed to characterize the physical and chemical properties of as-synthesized CuS/Bi 2 WO 6 heterostructures. Photo-Fenton catalytic treatment of organic pollutants was successfully achieved by employing CuS/Bi 2 WO 6 heterostructures as photocatalysts in the presence of H 2 O 2 . Photo-Fenton catalytic activity and stability can be significantly promoted by assembling CuS nanosheets on Bi 2 WO 6 nanosheets. The excellent photo-Fenton catalytic performance can be attributed to the high-efficiency charge separation and migration of CuS/Bi 2 WO 6 p-n heterostructures. The photo-Fenton reaction of CuS on the surface of the heterogeneous catalyst with H 2 O 2 produces a large number of·O 2 − and ·OH, which are responsible for the degradation of the organic pollutants. The effects of different factors such as H 2 O 2 concentration, catalyst dosage, and initial pH value on TC-HCl degradation were systematically investigated, and the enhanced catalytic oxidation mechanism of 2D/2D CuS/Bi 2 WO 6 p-n heterogeneous photo-Fenton system was also elucidated. The present work offers a simple strategy to construct 2D in-plane heterostructures with efficient photo-generated electron-hole separation and high stability, and can be used for wastewater purification via photo-Fenton reaction.
Author Contributions: L.G. designed the experiments and wrote the paper; K.Z. wrote the paper; X.H. and Q.Z. performed the experiments and analyzed the data; D.W. and F.F. designed the experiments. All authors commented and approved the final manuscript.