Peroxymonosulfate Activation by Bi2WO6/BiOCl Heterojunction Nanocomposites under Visible Light for Bisphenol A Degradation

The combination of peroxymonosulfate (PMS) activation and photocatalysis has proven to be effective for organic contaminants treatment. However, the construction of an efficient catalytic material is an important challenge. Herein, novel Bi2WO6/BiOCl heterojunction nanocomposites were successfully designed and fabricated using a facile and effective strategy for bisphenol A (BPA) photodegradation with PMS activation. The well-designed heterojunction with improvement of the contact area and interface microstructure was obtained through in situ growth of the Bi2WO6 on the surface of BiOCl. The Bi2WO6/BiOCl nanocomposites exhibit excellent catalytic performance in PMS activation for BPA degradation under visible light irradiation. A possible photocatalytic reaction mechanism was systematically revealed. The excellent catalytic performance is mainly attributed to the strong interaction between Bi2WO6 and BiOCl, resulting in an enhanced photoabsorption and a more efficient interfacial charge separation and transfer. This paper provides a novel strategy to design efficient catalytic materials for organic contaminants remediation with PMS activation.


Introduction
Peroxymonosulfate (PMS) activation processes have been recognized as promising methods for practical environmental remediation of emerging recalcitrant contaminants [1][2][3]. The SO 4 •− with superior oxidation ability and selectivity can efficiently degrade organic contaminants [1]. The metal ions, composites, and carbonaceous catalyst are frequently exploited to activate PMS [4,5]. However, concerns have been raised about the inevitable leaching of toxic metals and secondary contamination. The combination of PMS activation and photocatalyst has become a research focus because of their synergistic effect in the mineralization of organic contaminants [6,7]. Unfortunately, the main challenge faced by this technology is the lack of a highly efficient catalytic material [8,9]. Therefore, exploring efficient and environmentally benign materials for PMS activation is highly desirable.
In recent decades, Bi-based materials have attracted increasing interest in the areas of photocatalytic applications [10]. In particular, bismuth oxychloride (BiOCl) has attracted significant interest due to its unique crystallographic structure, which can establish an internal electric field for interlayer charge transfer [11,12]. However, the catalytic performance of BiOCl is restricted by the poor photogenerated charge separation ability and inactiveness of visible light utilization, which reduces the overall efficiency [13,14]. Therefore, various strategies, such as surface modification, defect engineering, morphology control, and heterojunction construction, have been widely adopted to tailor the photocatalytic activity of BiOCl [10,13]. The photocatalytic activity of BiOCl has been improved by these modification methods to some extent, but BiOCl-based catalysts are still unsatisfactory for environmental applications.
The light response and charge separation are two important factors to improve photocatalytic activity of semiconductors [15,16]. The construction of heterojunction composites has been studied in order to enhance the light absorption, the charge separation efficiency, and the lifetime of carriers [10,17]. Up to now, various BiOCl-based composites have been developed on the basis of this fundamental mechanism, such as BiOCl/g-C 3 N 4 [14], BiOCl/Bi 24 O 31 Cl 10 [18], and NiO/BiOCl [19]. It is well known that the heterojunction effect is established by an electronic coupling effect produced from the interfacial interactions of two components to enhance light-responsive capability and expedite interfacial charge carrier mobility [10,20]. However, the conventional large-sized materials could not construct perfect heterojunction interfaces. Recently, interface engineering has often been applied to explore the design of heterostructures [21,22]. The preparation process of a specific heterojunction is expensive and complicated in the context of mass production [23][24][25]. Therefore, exploring a facile strategy to create efficient heterojunction photocatalysts is highly challenging and appealing.
Herein, we have successfully designed a facile route to build Bi 2 WO 6 /BiOCl heterojunctions for organic pollutants degradation with PMS activation. The well-designed heterojunction with improvement of the contact area and interface microstructure was obtained through the growth of Bi 2 WO 6 on the surface of BiOCl nanoplates, which showed extremely high levels of photocatalytic activity and stability in bisphenol A (BPA) degradation with PMS activation. Furthermore, the possible reaction mechanism was revealed by radical trapping experiments, charge transportation, and band structures. These findings provide an efficient approach to design novel heterojunction materials for organic contaminants remediation.

Preparation of Materials
The BiOCl nanoplate was constructed via a hydrothermal method. In general, 2 mmol of cetyltrimethylammonium chloride (CTAC) was added to 20 mL of deionized water, and added into 2 mmol of Bi(NO 3 ) 3 •6H 2 O aqueous solution (40 mL) drop by drop. After 2 h of stirring, the resulting solution was poured into a 100 mL Teflon-lined autoclave and treated at 120 • C for 12 h. Then, the samples were washed several times with ultrapure water.
The Bi 2 WO 6 /BiOCl heterojunction was constructed by in situ growth strategy. Briefly, 0.26 g of BiOCl nanoplate dispersed ultrasonically in 60 mL of water for 1 h. A certain amount of Na 2 WO 4 ·2H 2 O was added into the suspension. After being stirred for another 2 h, the suspension were poured into a 100 mL Teflon-lined autoclave and treated at 120 • C for 12 h. The precipitation was obtained after being washed with ultrapure water.

Activity Evaluation
The activity evaluation of samples was conducted through the photodegradation of BPA with PMS activation under irradiation by 300 W Xe lamp with a 400 nm cut-off filter. Briefly, 50 mg of the sample was dispersed into a BPA solution (50 mL, 10 mg/L) under stirring for 30 min in the dark. After adding a specific amount of PMS, the lamp was triggered. The concentration of PMS was 1 mmol/L. The BPA residual concentration was analyzed using high-performance liquid chromatography (Agilent 1260, Santa Clara, CA, USA) with a C-18 column and UV wavelength at 276 nm. The mobile phase was 70% methanol and 30% water, the flow rate was 1.0 mL/min. Figure 1 presents the XRD patterns of the samples. The peaks of pure BiOCl are well assigned to the tetragonal phase (JCPDS no. 06-0249), implying the high purity of BiOCl nanoplates [13,14]. The peaks of Bi 2 WO 6 are assigned to the orthorhombic phase (JCPDS No. 39-0256) [26,27]. The main peaks of the nanocomposites could be well assigned to the tetragonal BiOCl. The diffraction peaks of Bi 2 WO 6 intensified with increasing mass content of Bi 2 WO 6 , evidencing the formation of the Bi 2 WO 6 in the nanocomposites. These results confirm that Bi 2 WO 6 /BiOCl nanocomposites were successfully prepared.

Activity Evaluation
The activity evaluation of samples was conducted through the photodegradation of BPA with PMS activation under irradiation by 300 W Xe lamp with a 400 nm cut-off filter. Briefly, 50 mg of the sample was dispersed into a BPA solution (50 mL, 10 mg/L) under stirring for 30 min in the dark. After adding a specific amount of PMS, the lamp was triggered. The concentration of PMS was 1 mmol/L. The BPA residual concentration was analyzed using high-performance liquid chromatography (Agilent 1260, Santa Clara, CA, USA) with a C-18 column and UV wavelength at 276 nm. The mobile phase was 70% methanol and 30% water, the flow rate was 1.0 mL/min. Figure 1 presents the XRD patterns of the samples. The peaks of pure BiOCl are well assigned to the tetragonal phase (JCPDS no. 06-0249), implying the high purity of BiOCl nanoplates [13,14]. The peaks of Bi2WO6 are assigned to the orthorhombic phase (JCPDS No. 39-0256) [26,27]. The main peaks of the nanocomposites could be well assigned to the tetragonal BiOCl. The diffraction peaks of Bi2WO6 intensified with increasing mass content of Bi2WO6, evidencing the formation of the Bi2WO6 in the nanocomposites. These results confirm that Bi2WO6/BiOCl nanocomposites were successfully prepared.   Figure 2 shows the Raman spectra of samples. The pure BiOCl nanoplates exhibited the two peaks at 144 and 199.8 cm −1 due to the Bi-Cl stretching mode in BiOCl [12,28]. For pure Bi 2 WO 6 , the strong peak at 309 cm −1 is associated with the translational modes of Bi 3+ and WO 6 6− . A weak peak centered at 410 cm −1 is assigned to WO 6 bending (Eu) modes [29,30]. The broad band at 721 cm −1 is mainly assigned to the asymmetric stretching vibration of W plane and O [23,31]. The peaks at 798 and 824 cm −1 are associated with antisymmetric and symmetric Ag modes of O-W-O, respectively [23,29,30]. The characteristic peaks of Bi 2 WO 6 and BiOCl are both shown in the spectra of the nanocomposites. Furthermore, the peaks intensity of Bi 2 WO 6 gradually increased with increasing content of Bi 2 WO 6 , evidencing the presence of Bi 2 WO 6 in the nanocomposites. The results explicitly confirm that the coexistence of BiOCl nanoplates and Bi 2 WO 6 in the nanocomposites. Figure 2 shows the Raman spectra of samples. The pure BiOCl nanoplates exhibited the two peaks at 144 and 199.8 cm −1 due to the Bi-Cl stretching mode in BiOCl [12,28]. For pure Bi2WO6, the strong peak at 309 cm −1 is associated with the translational modes of Bi 3+ and WO6 6− . A weak peak centered at 410 cm −1 is assigned to WO6 bending (Eu) modes [29,30]. The broad band at 721 cm −1 is mainly assigned to the asymmetric stretching vibration of W plane and O [23,31]. The peaks at 798 and 824 cm −1 are associated with antisymmetric and symmetric Ag modes of O-W-O, respectively [23,29,30]. The characteristic peaks of Bi2WO6 and BiOCl are both shown in the spectra of the nanocomposites. Furthermore, the peaks intensity of Bi2WO6 gradually increased with increasing content of Bi2WO6, evidencing the presence of Bi2WO6 in the nanocomposites. The results explicitly confirm that the coexistence of BiOCl nanoplates and Bi2WO6 in the nanocomposites. The morphology of the samples was observed using FE-SEM. As shown in Figure 3a, pure BiOCl nanoplates displayed an ultrathin plate structure with a size ranging from 500 nm~2 um. The high-magnification SEM image (Figure 3b) reveals the plate had a smooth surface with thickness of about 20 nm. Furthermore, BC-3 nanocomposite preserved a wrinkled plate structure assembled from ultrathin nanosheets ( Figure 3c). Figure 3d clearly shows that the surfaces of the plates became rough, which can be ascribed to the growth of Bi2WO6 on the surface of BiOCl. This result reveals that Bi2WO6 has been successfully grown on the surface of BiOCl. The morphology of the samples was observed using FE-SEM. As shown in Figure 3a, pure BiOCl nanoplates displayed an ultrathin plate structure with a size ranging from 500 nm~2 µm. The high-magnification SEM image (Figure 3b) reveals the plate had a smooth surface with thickness of about 20 nm. Furthermore, BC-3 nanocomposite preserved a wrinkled plate structure assembled from ultrathin nanosheets ( Figure 3c). Figure 3d clearly shows that the surfaces of the plates became rough, which can be ascribed to the growth of Bi 2 WO 6 on the surface of BiOCl. This result reveals that Bi 2 WO 6 has been successfully grown on the surface of BiOCl.

Characterizations of Catalysts
The TEM and HRTEM images of the BC-3 nanocomposite are shown in Figure 4. An ultrathin plate morphology of the BC-3 nanocomposite can be clearly observed from Figure 4a,b. The HRTEM image ( Figure 4c) clearly reveals the clear lattice fringes. The lattice spacing was confirmed to be 0.272 nm, corresponding to (2 0 0) plane of orthorhombic Bi 2 WO 6 [32,33]. Moreover, the elemental mapping ( Figure 4d) and EDX spectrum ( Figure 4e) present the uniform distribution of Bi, O, Cl, and W elements in the nanocomposite. Those observations explicitly demonstrate that the in situ growth method resulted in a strong interfacial contact and a large contact area between the Bi 2 WO 6 and BiOCl.
The XPS spectra of the samples are illustrated in Figure 5. As exhibited in Figure 5a, the survey scan spectra indicate that BC-3 nanocomposites are mainly composed of Bi, O, Cl, and W elements. From the Bi 4f spectra of pure BiOCl (Figure 5b), the peaks of Bi 4f 5/2 and Bi 4f 7/2 at 164.9 and 159.6 eV indicate bismuth existed in the form of Bi 3+ , respectively [13,34]. Notably, the Bi 4f 7/2 and Bi 4f 5/2 in the BC-3 nanocomposites were located at about 164.6 and159.3 eV, respectively [35,36]. These shifts can be attributed to the chemical bonding actions among the components. The Cl 2p 3/2 and Cl 2p 1/2 in BiOCl ( Figure 5c) were located at 198.2 and 199.9 eV, which are characteristic positions of the Cl − [11,28]. These peaks changed to 198.0 and 199.7 eV after the construction of the Bi 2 WO 6 /BiOCl heterojunction, respectively. The XPS spectrum of O 1s in BiOCl (Figure 5d) displayed peaks at 531.0 and 530.1 eV, attributed to the lattice and surface oxygen [35,37,38]. Additionally, the O 1s spectrum of BC-3 nanocomposite was divided into three peaks at 531.0, 530.1, and 529.8 eV, corresponding to Bi-O, W-O, and hydroxyl groups, respectively [30,34]. From W 4f spectrum of BC-3 nanocomposite (Figure 5e), the peaks of W 4f at 37.5 and 35.4 eV are characteristic of the W 6+ cations in Bi 2 WO 6 [39,40]. The results indicate a strong interfacial contact between BiOCl and Bi 2 WO 6 in Bi 2 WO 6 /BiOCl nanocomposites.  The UV-vis DRS of the samples is displayed in Figure 6a. The pure BiOCl exhibited a strong absorption in UV light regions and an absorption edge of around 370 nm, consistent with the literature reports [28,41]. Obviously, pure Bi 2 WO 6 has an absorption edge of around 460 nm [42,43]. The Bi 2 WO 6 /BiOCl nanocomposites presented clear visible light absorption. Moreover, the absorption intensity gradually enhanced with increasing Bi 2 WO 6 content. Furthermore, the band gap energy of the samples was studied (Figure 6b) [18,30]. The band gap energy values of pure BiOCl, Bi 2 WO 6 , BC-1, BC-2, BC-3, and BC-4 were 3.31, 2.66, 2.74, 2.68, and 2.62 eV, respectively, which demonstrates that the band gap energy of Bi 2 WO 6 /BiOCl nanocomposite decreased with the increasing content of Bi 2 WO 6 . These results demonstrate that a strong interface interaction between BiOCl and Bi 2 WO 6 in the nanocomposites is effective to improve the visible light response.  The XPS spectra of the samples are illustrated in Figure 5. As exhibited in Figure 5a, the survey scan spectra indicate that BC-3 nanocomposites are mainly composed of Bi, O, Cl, and W elements. From the Bi 4f spectra of pure BiOCl (Figure 5b), the peaks of Bi 4f5/2 and Bi 4f7/2 at 164.9 and 159.6 eV indicate bismuth existed in the form of Bi 3+ , respectively [13,34]. Notably, the Bi 4f7/2 and Bi 4f5/2 in the BC-3 nanocomposites were located at about 164.6 and159.3 eV, respectively [35,36]. These shifts can be attributed to the chemical bonding actions among the components. The Cl 2p3/2 and Cl 2p1/2 in BiOCl ( Figure 5c  The UV-vis DRS of the samples is displayed in Figure 6a. The pure BiOCl exhibited a strong absorption in UV light regions and an absorption edge of around 370 nm, consistent with the literature reports [28,41]. Obviously, pure Bi2WO6 has an absorption edge of around 460 nm [42,43]. The Bi2WO6/BiOCl nanocomposites presented clear visible light absorption. Moreover, the absorption intensity gradually enhanced with increasing

Photocatalytic Activity Evaluation
The catalytic properties of the samples were examined by BPA degradation with PMS activation. Figure 7 shows the variations in BPA concentrations under various systems. The control experiment revealed that PMS could hardly degrade the BPA molecule. The removal rates of BPA were approximately 31.1% and 41.4% by the pure BiOCl and Bi2WO6 with PMS activation under visible light irradiation, respectively. As exhibited, the removal rate of BPA reached 97.4% by BC-3 under the same condition. Obviously, the removal rate of about 10.1% demonstrates that BC-3 cannot activate the PMS molecule without light irradiation. These results suggest that BC-3 exhibits an enhancement performance in BPA degradation with PMS activation under visible light irradiation. Furthermore, the effects of Bi2WO6 in the Bi2WO6/BiOCl nanocomposite on the BPA degradation with PMS activation were investigated, and the results are shown in Figure  8. As the content of Bi2WO6 increased from 0 to 75.0%, the removal efficiency firstly in-

Photocatalytic Activity Evaluation
The catalytic properties of the samples were examined by BPA degradation with PMS activation. Figure 7 shows the variations in BPA concentrations under various systems. The control experiment revealed that PMS could hardly degrade the BPA molecule. The removal rates of BPA were approximately 31.1% and 41.4% by the pure BiOCl and Bi 2 WO 6 with PMS activation under visible light irradiation, respectively. As exhibited, the removal rate of BPA reached 97.4% by BC-3 under the same condition. Obviously, the removal rate of about 10.1% demonstrates that BC-3 cannot activate the PMS molecule without light irradiation. These results suggest that BC-3 exhibits an enhancement performance in BPA degradation with PMS activation under visible light irradiation.

Photocatalytic Activity Evaluation
The catalytic properties of the samples were examined by BPA degradation with PM activation. Figure 7 shows the variations in BPA concentrations under various systems The control experiment revealed that PMS could hardly degrade the BPA molecule. Th removal rates of BPA were approximately 31.1% and 41.4% by the pure BiOCl and Bi2WO with PMS activation under visible light irradiation, respectively. As exhibited, the re moval rate of BPA reached 97.4% by BC-3 under the same condition. Obviously, the re moval rate of about 10.1% demonstrates that BC-3 cannot activate the PMS molecule with out light irradiation. These results suggest that BC-3 exhibits an enhancement perfor mance in BPA degradation with PMS activation under visible light irradiation.  Furthermore, the effects of Bi 2 WO 6 in the Bi 2 WO 6 /BiOCl nanocomposite on the BPA degradation with PMS activation were investigated, and the results are shown in Figure 8.
As the content of Bi 2 WO 6 increased from 0 to 75.0%, the removal efficiency firstly increased and then decreased. Notably, the BC-3 nanocomposite showed the highest photocatalytic activity for PMS activation. This can be attributed to the successful construction heterojunctions between the Bi 2 WO 6 and BiOCl, accelerating the charge separation. However, the excessive Bi 2 WO 6 might decrease the interface effect. The kinetic behavior of the samples for BPA degradation was investigated. The reaction kinetics could be fitted well by the principle of kinetics pseudo first-order reaction [44,45]. The rate constants for BPA degradation by BiOCl, Bi 2 WO 6 , BC-1, BC-2, BC-3, and BC-4 with PMS activation were 0.010, 0.017, 0.024, 0.040, 0.102, and 0.044 min −1 , respectively. So, the BC-3 displayed better photocatalytic activity than pure BiOCl and Bi 2 WO 6 , evidencing the synergistic effect between BiOCl and Bi 2 WO 6 . These results suggest that the content of Bi 2 WO 6 is important to optimize the catalytic activities of the nanocomposites. creased and then decreased. Notably, the BC-3 nanocomposite showed the highest photocatalytic activity for PMS activation. This can be attributed to the successful construction heterojunctions between the Bi2WO6 and BiOCl, accelerating the charge separation. However, the excessive Bi2WO6 might decrease the interface effect. The kinetic behavior of the samples for BPA degradation was investigated. The reaction kinetics could be fitted well by the principle of kinetics pseudo first-order reaction [44,45]. The rate constants for BPA degradation by BiOCl, Bi2WO6, BC-1, BC-2, BC-3, and BC-4 with PMS activation were 0.010, 0.017, 0.024, 0.040, 0.102, and 0.044 min −1 , respectively. So, the BC-3 displayed better photocatalytic activity than pure BiOCl and Bi2WO6, evidencing the synergistic effect between BiOCl and Bi2WO6. These results suggest that the content of Bi2WO6 is important to optimize the catalytic activities of the nanocomposites. The stability and reusability of the catalysts are crucial for their practical applications [10,31]. Thus, the catalytic stability of the Bi2WO6/BiOCl nanocomposites was explored through the recycling reaction of BPA degradation. From Figure 9, a slight decrease in the catalytic activity of the BC-3 nanocomposite was observed after four cycles, illustrating that the nanocomposites have robust stability to activate PMS for BPA degradation and great potential in their practical application. The stability and reusability of the catalysts are crucial for their practical applications [10,31]. Thus, the catalytic stability of the Bi 2 WO 6 /BiOCl nanocomposites was explored through the recycling reaction of BPA degradation. From Figure 9, a slight decrease in the catalytic activity of the BC-3 nanocomposite was observed after four cycles, illustrating that the nanocomposites have robust stability to activate PMS for BPA degradation and great potential in their practical application.

Photocatalytic Reaction Mechanisms
To explore the underlying mechanism, the active radical species generated in the photocatalytic process were tested by radical trapping experiments with benzoquinone (BQ), methanol (MeOH), isopropanol (IPA), and ethylenediamine tetraacetic acid disodium salt (EDTA) as radical scavengers, respectively [2,6,26]. Figure 10 shows that the degradation efficiency of BPA was slightly reduced with the addition of IPA, indicating that •OH has a negligible effect on BPA degradation. Furthermore, the degradation of BPA could be significantly limited by EDTA and BQ. Therefore, the holes (h + ) and superoxide radical (•O 2 − ) play pivotal roles in BPA degradation. Furthermore, the degradation of BPA was partially limited by methanol, which could be attributed to the rapid conversion of sulfate radicals. Therefore, the h + , SO 4 •− , and •O 2 − are important active radicals in the degradation process.

Photocatalytic Reaction Mechanisms
To explore the underlying mechanism, the active radical species generated in the photocatalytic process were tested by radical trapping experiments with benzoquinone (BQ), methanol (MeOH), isopropanol (IPA), and ethylenediamine tetraacetic acid disodium salt (EDTA) as radical scavengers, respectively [2,6,26]. Figure 10 shows that the degradation efficiency of BPA was slightly reduced with the addition of IPA, indicating that •OH has a negligible effect on BPA degradation. Furthermore, the degradation of BPA could be significantly limited by EDTA and BQ. Therefore, the holes (h + ) and superoxide radical (•O2 − ) play pivotal roles in BPA degradation. Furthermore, the degradation of BPA was partially limited by methanol, which could be attributed to the rapid conversion of sulfate radicals. Therefore, the h + , SO4 •− , and •O2 − are important active radicals in the degradation process.

Photocatalytic Reaction Mechanisms
To explore the underlying mechanism, the active radical species generated in th photocatalytic process were tested by radical trapping experiments with benzoquinon (BQ), methanol (MeOH), isopropanol (IPA), and ethylenediamine tetraacetic acid diso dium salt (EDTA) as radical scavengers, respectively [2,6,26]. Figure 10 shows that th degradation efficiency of BPA was slightly reduced with the addition of IPA, indicatin that •OH has a negligible effect on BPA degradation. Furthermore, the degradation o BPA could be significantly limited by EDTA and BQ. Therefore, the holes (h + ) and super oxide radical (•O2 − ) play pivotal roles in BPA degradation. Furthermore, the degradatio of BPA was partially limited by methanol, which could be attributed to the rapid conver sion of sulfate radicals. Therefore, the h + , SO4 •− , and •O2 − are important active radicals i the degradation process.  Generally, the specific surface area is a key factor influencing the catalytic ability of the catalyst [10,19]. Hence, N 2 adsorption-desorption measurements were taken to reveal BET surface area and pore structure. The nitrogen sorption isotherms (Figure 11a) revealed that all the samples belonged to type IV isotherms with H3 loops, illustrating mesoporous structures [31,40]. The pore size distribution curves illustrate the coexistence of abundant mesopores and macropores (Figure 11b). The BC-3 nanocomposites showed larger specific surface area than pure BiOCl (5.394 m 2 /g), but lower than pure Bi 2 WO 6 ( Table 1). Therefore, the specific surface areas and pore size distributions are not the main reason for the enhanced photocatalytic performance of the samples.
BET surface area and pore structure. The nitrogen sorption isotherms (Figure 11a) revealed that all the samples belonged to type IV isotherms with H3 loops, illustrating mesoporous structures [31,40]. The pore size distribution curves illustrate the coexistence of abundant mesopores and macropores (Figure 11b). The BC-3 nanocomposites showed larger specific surface area than pure BiOCl (5.394 m 2 /g), but lower than pure Bi2WO6 (Table 1). Therefore, the specific surface areas and pore size distributions are not the main reason for the enhanced photocatalytic performance of the samples. Figure 11. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of pure BiOCl, Bi2WO6, and BC-3 nanocomposite. The separation efficiency and kinetic barrier of Bi2WO6/BiOCl were revealed by EIS and photocurrent response. Sensitive and reproducible photocurrent responses of the sample under on/off visible light irradiation can be observed from Figure 12a. The BC-3 nanocomposite had higher photocurrent responses than pure BiOCl and Bi2WO6, illustrating an efficient charge transmission [30,42]. Furthermore, Figure 12b shows that the arc radius of the BC-3 nanocomposite was smaller than pure BiOCl and Bi2WO6, which suggests a lower charge transfer resistance [16,31]. Thus, the heterojunction interface between BiOCl and Bi2WO6 could provide a driving force for charge separation and transfer.  The separation efficiency and kinetic barrier of Bi 2 WO 6 /BiOCl were revealed by EIS and photocurrent response. Sensitive and reproducible photocurrent responses of the sample under on/off visible light irradiation can be observed from Figure 12a. The BC-3 nanocomposite had higher photocurrent responses than pure BiOCl and Bi 2 WO 6 , illustrating an efficient charge transmission [30,42]. Furthermore, Figure 12b shows that the arc radius of the BC-3 nanocomposite was smaller than pure BiOCl and Bi 2 WO 6 , which suggests a lower charge transfer resistance [16,31]. Thus, the heterojunction interface between BiOCl and Bi 2 WO 6 could provide a driving force for charge separation and transfer. The TRPL spectra was used to further identify the charge transfer dynamics. As depicted in Figure 13, the curves are fitted well based on a two exponential decay function [25,31,46]. The average PL lifetimes of pure BiOCl, Bi2WO6, and BC-3 nanocomposite were calculated to be 1.44, 1.18, and 1.62 ns, respectively. Obviously, the average lifetimes of BC-3 nanocomposites are longer than those of pure BiOCl and Bi2WO6, which further indicates the most efficient migration of photoexcited charges [25,31]. As a result, a notable synergetic effect between BiOCl and Bi2WO6 can optimize the electronic structure and suppress the recombination of charge carriers, thus promoting the catalytic performance. The TRPL spectra was used to further identify the charge transfer dynamics. As depicted in Figure 13, the curves are fitted well based on a two exponential decay function [25,31,46]. The average PL lifetimes of pure BiOCl, Bi 2 WO 6 , and BC-3 nanocomposite were calculated to be 1.44, 1.18, and 1.62 ns, respectively. Obviously, the average lifetimes of BC-3 nanocomposites are longer than those of pure BiOCl and Bi 2 WO 6 , which further indicates the most efficient migration of photoexcited charges [25,31]. As a result, a notable synergetic effect between BiOCl and Bi 2 WO 6 can optimize the electronic structure and suppress the recombination of charge carriers, thus promoting the catalytic performance. Figure 12. The EIS (a) and photocurrent response (b) of pure BiOCl, Bi2WO6, and Bi2WO6/BiOCl nanocomposites.
The TRPL spectra was used to further identify the charge transfer dynamics. As depicted in Figure 13, the curves are fitted well based on a two exponential decay function [25,31,46]. The average PL lifetimes of pure BiOCl, Bi2WO6, and BC-3 nanocomposite were calculated to be 1.44, 1.18, and 1.62 ns, respectively. Obviously, the average lifetimes of BC-3 nanocomposites are longer than those of pure BiOCl and Bi2WO6, which further indicates the most efficient migration of photoexcited charges [25,31]. As a result, a notable synergetic effect between BiOCl and Bi2WO6 can optimize the electronic structure and suppress the recombination of charge carriers, thus promoting the catalytic performance. Figure 13. The TRPL spectra of pure BiOCl, Bi2WO6, and Bi2WO6/BiOCl nanocomposites.
The Mott-Schottky plots of the samples are shown in Figure 14. The flat-band potential (Efb) could be determined based on the Mott-Schottky curves [16,47]. The positive slope of the curves suggests that the BiOCl and Bi2WO6 are n-type semiconductor. Notably, the Efb values of BiOCl and Bi2WO6 were determined to be −0.47 and −0.98 V (vs. Ag/AgCl), respectively. Therefore, the CB potential values of the BiOCl and Bi2WO6 were −0.25 and −0.76 V (vs. NHE, pH = 7), respectively [23,31]. Based on these results, the valence band (VB) positions were determined to be 3.06 and 1.90 V. These results indicate Figure 13. The TRPL spectra of pure BiOCl, Bi 2 WO 6 , and Bi 2 WO 6 /BiOCl nanocomposites.
The Mott-Schottky plots of the samples are shown in Figure 14. The flat-band potential (E fb ) could be determined based on the Mott-Schottky curves [16,47]. The positive slope of the curves suggests that the BiOCl and Bi 2 WO 6 are n-type semiconductor. Notably, the E fb values of BiOCl and Bi 2 WO 6 were determined to be −0.47 and −0.98 V (vs. Ag/AgCl), respectively. Therefore, the CB potential values of the BiOCl and Bi 2 WO 6 were −0.25 and −0.76 V (vs. NHE, pH = 7), respectively [23,31]. Based on these results, the valence band (VB) positions were determined to be 3.06 and 1.90 V. These results indicate that the construction of a heterostructure interface between the BiOCl and Bi 2 WO 6 can significantly improve photoinduced carrier separation efficiency and photocatalytic efficiency due to the matching energy levels. that the construction of a heterostructure interface between the BiOCl and Bi2WO6 can significantly improve photoinduced carrier separation efficiency and photocatalytic efficiency due to the matching energy levels. Based on the above analysis, the possible mechanism for BPA degradation is portrayed in Figure 15. The heterojunction structure is obtained at the interface between Bi-OCl and Bi2WO6. Under light irradiation, Bi2WO6 is excited to form electrons and h + [38,43]. The photogenerated electrons flow to the heterojunction interface, then to the CB of the BiOCl. The accumulation of electrons at the CB of the BiOCl reduce O2 and PMS in the solution to generate •O2 − and SO4 •− radicals [5,48]. Meanwhile, the generated h + in the VB of BiOCl can easily accumulate at the surface of Bi2WO6 to directly degrade organic pollutants [40]. The •OH cannot be produced by h + in the VB of Bi2WO6 due to its low oxidiz-− Figure 14. Mott-Schottky curves of the pure BiOCl and Bi 2 WO 6 .
Based on the above analysis, the possible mechanism for BPA degradation is portrayed in Figure 15. The heterojunction structure is obtained at the interface between BiOCl and Bi 2 WO 6 . Under light irradiation, Bi 2 WO 6 is excited to form electrons and h + [38,43]. The photogenerated electrons flow to the heterojunction interface, then to the CB of the BiOCl.
The accumulation of electrons at the CB of the BiOCl reduce O 2 and PMS in the solution to generate •O 2 − and SO 4 •− radicals [5,48]. Meanwhile, the generated h + in the VB of BiOCl can easily accumulate at the surface of Bi 2 WO 6 to directly degrade organic pollutants [40]. The •OH cannot be produced by h + in the VB of Bi 2 WO 6 due to its low oxidizing capacity, whereas a small amount of •OH is formed by the contribution of •O 2 − or PMS [26,42]. This result agrees well with the reactive species trapping experiments. Based on the above analysis, the possible mechanism for BPA degradation is portrayed in Figure 15. The heterojunction structure is obtained at the interface between Bi-OCl and Bi2WO6. Under light irradiation, Bi2WO6 is excited to form electrons and h + [38,43]. The photogenerated electrons flow to the heterojunction interface, then to the CB of the BiOCl. The accumulation of electrons at the CB of the BiOCl reduce O2 and PMS in the solution to generate •O2 − and SO4 •− radicals [5,48]. Meanwhile, the generated h + in the VB of BiOCl can easily accumulate at the surface of Bi2WO6 to directly degrade organic pollutants [40]. The •OH cannot be produced by h + in the VB of Bi2WO6 due to its low oxidizing capacity, whereas a small amount of •OH is formed by the contribution of •O2 − or PMS [26,42]. This result agrees well with the reactive species trapping experiments.

Conclusions
In summary, novel Bi2WO6/BiOCl heterojunction nanocomposites were successfully designed and fabricated by a facile and effective strategy. The heterojunction was fabricated by in situ growth of Bi2WO6 on the surface of BiOCl. The Bi2WO6/BiOCl nanocomposites possess excellent performance in PMS activation for BPA degradation under visi- Figure 15. A proposed mechanism for BPA degradation via Bi 2 WO 6 /BiOCl nanocomposites with PMS activation.

Conclusions
In summary, novel Bi 2 WO 6 /BiOCl heterojunction nanocomposites were successfully designed and fabricated by a facile and effective strategy. The heterojunction was fabricated by in situ growth of Bi 2 WO 6 on the surface of BiOCl. The Bi 2 WO 6 /BiOCl nanocomposites possess excellent performance in PMS activation for BPA degradation under visible light irradiation. The Bi 2 WO 6 /BiOCl composite with 75.0% of Bi 2 WO 6 exhibited the optimal catalytic performance. A possible photocatalytic mechanism for BPA degradation with PMS activation over Bi 2 WO 6 /BiOCl nanocomposites was systematically investigated. The improved catalytic performance of the Bi 2 WO 6 /BiOCl is attributed to the strong interaction between the Bi 2 WO 6 and BiOCl, resulting in an enhanced photoabsorption and a more efficient charge interfacial separation and transfer. This work might provide new inspiration for designing efficient composites for the environmental remediation of organic contaminants.