Synergistic Effect of Charge Separation and Multiple Reactive Oxygen Species Generation on Boosting Photocatalytic Degradation of Fluvastatin by ZnIn2S4/Bi2WO6 Z-Scheme Heterostructured Photocatalytst

The application of semiconductor photocatalysts with narrow band gaps is hindered by the rapid recombination of electron–hole pairs and limitation of multiple reactive oxygen species (ROS) synchronous generation. A n–n-type direct Z-scheme heterostructured photocatalyst was constructed based on the staggered band alignment of bismuth tungstate (Bi2WO6) and indium zinc sulfide (ZnIn2S4) to reveal the synergistic effect of charge separation and multiple ROS synchronous generation on boosting photocatalytic performance. Under irradiation, electrons in the conduction band (CB) of Bi2WO6 and holes in the valence band (VB) of ZnIn2S4 recombined at interface to prolong the lifetime of electrons in the CB of ZnIn2S4 and holes in the VB of Bi2WO6. Meanwhile, the multiple ROS synchronously generated to oxidize pollutant due to the strong redox ability of electrons of ZnIn2S4 and holes of Bi2WO6, which was determined by the CB potential of ZnIn2S4 and VB potential of Bi2WO6. The results provided valuable insights for the application of photocatalysts with a narrow band gap in the field of water pollution control.


Introduction
Semiconductor photocatalytic technology converts light energy into chemical energy to oxidize and decompose organic pollutants, which is a sustainable technology to alleviate energy crises and environmental pollution [1,2]. When the photocatalyst is irradiated by incident light with energy equal to or larger than the band gap energy, the electrons in the valence band (VB) absorb photon energy and transit to the conduction band (CB) to generate electron (e − )-hole (h + ) pairs (namely charge carriers). On the one hand, organic pollutants are directly degraded by e − and h + [3]. On the other hand, the organic pollutants are oxidized by reactive oxygen species (ROS) generated through redox reaction by charge carriers, which is the main process for eliminating the harm of organic pollutants to the environment in the photocatalytic system [4,5]. Meanwhile, the recombination of e − and h + occurs through the internal and surface paths to inhibit the generation of ROS, which seriously reduces photocatalytic efficiency. Therefore, excitation of the photocatalyst to produce charge carriers is an important process of photocatalytic reaction, and the redox ability and separation capacity of charge carriers are key factors affecting the photocatalytic efficiency of the system.
Sunlight is the preferred light source for semiconductor photocatalytic technology, in which ultraviolet light accounts for 7% and visible light for 45.8%. Based on the basic principle of photocatalytic reaction, tuning the band gap to improve the response of visible light with low energy is a hotspot in the field of photocatalysis [6]. A bismuth (Bi)based photocatalyst has attracted attention due to the advantages of a narrow band gap, low cost, excellent stability and good photocorrosion resistance [7]. The electronic orbital arrangement of bismuth (Bi) is 6s 2 6p 3 , the orbitals of Bi 6s and Bi 6p constituting the VB and CB, respectively, which effectively reduce the band gap to realize the light harvesting of low-energy visible light [8]. The hybridization of the orbitals of Bi 6s and O 2p results in high dispersion of h + in the VB, which contributes to the transition of h + to the photocatalyst surface and inhibition of the charge carriers recombination [9]. In addition, the band gap structure of Bi-based photocatalysts can be tuned to initiate multiple redox reactions by appropriate preparation methods [10].
Although Bi-based photocatalysts show the excellent optical properties of visible light harvesting, the low photocatalytic efficiency still limits its application. Firstly, the VB of a Bi-based photocatalyst is more positive, which is conducive to the generation of hydroxyl radical (·OH) through oxidation of H2O and OH − by h + . However, the CB of a Bibased photocatalyst determines the low redox ability of e − to restrict the reduction of O2 and synchronous generation of multiple ROS. Secondly, the photocatalytic efficiency is negatively affected by the rapid recombination of charge carriers caused by the narrow band gap. Therefore, tuning the band gap structure of a Bi-based photocatalyst to enhance the redox ability and separation of charge carriers is the key problem to be solved in the field of engineering application.
The construction of a heterojunction is an effective way to tune the band gap structure of a Bi-based photocatalyst. The significant differences in transfer of charge carriers can be observed in the different types of heterojunction, which leads to different regulation mechanisms on charge carriers' separation and photocatalytic performance. Guan et al. synthesized a type Ι conventional heterojunction of BiOI/Bi5O7I to realize the e − and h + accumulation on the band gap of BiOI, which contributed to improve the photocatalytic performance [11]. The effective separation of charge carriers was achieved in the p-n heterojunction constructed by AgI decorated with Bi24O31Br10 [12]. Zhan et al. found that a Zscheme heterojunction improved photocatalytic performance through inhibition of charge carriers recombination [13]. Choosing a photocatalyst with suitable band gap structure to composite with a Bi-based photocatalyst is very important for the heterojunction construction.
Ternary metal sulfide photocatalysts can be synthesized by hydrothermal methods under mild conditions. The morphology, crystal structure and optical property can be controlled through changing the reaction conditions [14]. However, our previous research confirmed that the photocatalytic efficiency of the ternary metal sulfide photocatalyst was restricted by the low oxidation ability of h + because the VB potential was lower than the potential of ·OH generation [15]. Based on the characteristics of band gap structure, research on the construction heterojunction of a Bi-based photocatalyst and ternary metal sulfide photocatalyst was gradually carried out. The results showed that separation of charge carriers was facilitated to improve the photocatalytic performance for hydrogen production and pollutant degradation in the system with heterostructured Bi2WO6/ZnIn2S4 [16,17]. The degradation efficiency of dye by double Z-Scheme heterostructured BiVO4@ZnIn2S4/Bi2Sn2O7 photocatalyst with core-shell structure was 12-63 times higher than that of the single component photocatalyst [18]. However, photocatalytic performance of the heterostructure photocatalyst of a Bi-based photocatalyst and ternary metal sulfide photocatalyst has mainly been investigated in the published literature. Few pieces of research discussed the simultaneous regulation mechanism of heterostructures for the enhancement of redox ability and separation of charge carriers. Meanwhile, the synergistic effect of charge carriers separation and multiple ROS generation on boosting photocatalytic performance of the heterostructured photocatalyst had not been analyzed in depth. Consequently, it is necessary to construct a Bi-based/ternary metal sulfide heterojunction and carry out a comprehensive study to reveal the relationship between heterostructures with simultaneous enhancement of redox ability and charge carrier separation.
In the present work, bismuth tungstate (Bi2WO6) and indium zinc sulfide (ZnIn2S4) were selected as typical representatives of Bi-based photocatalysts and ternary metal sulfide photocatalysts to construct the heterojunction based on the staggered band alignment. The structural characteristics of the heterostructured photocatalyst, such as morphology, specific surface area, pore size distribution, and crystal phase were characterized by scanning electron microscopy (SEM), specific surface area analysis (BET) and X-ray diffraction (XRD). UV-Vis spectrophotometry (UV-Vis DRS) and electrochemical workstation were used to investigate the optical and photoelectrochemical properties of the heterostructured photocatalyst. The generation of ROS was analyzed by in-situ capture experiments, which was combined with pollutant removal to evaluate the photocatalytic performance of the heterostructured photocatalyst. The simultaneous regulation mechanism of the heterostructure for the enhancement of redox ability and separation of charge carriers was analyzed. The synergistic effect of charge carrier separation and multiple ROS generation on boosting photocatalytic performance of the heterostructured photocatalyst based on the staggered band alignment was revealed. The results will provide valuable insights for the performance improvement of photocatalysts with narrow band gaps and their application in the field of water pollution control.

Synthesis of ZnIn2S4
A total of 0.375 mmol Zn(NO3)2·6H2O, 0.75 mmol InN3O9·xH2O and excess C2H5NS (6 mmol) were mixed in 75 mL deionized water to completely dissolve under continuous stirring at room temperature. The mixture was transferred into a 100 mL stainless autoclave (Rui Quan Xing, Xi'an, China) with a Teflon liner. The autoclave was sealed and placed in a vacuum oven (Taiste Instrument Co.,Ltd, Tianjin, China) at 80 °C for 6 h of hydrothermal reaction, which was naturally cooled to room temperature. The yellow photocatalyst was obtained by vacuum filtration and washed with DI water for several times to be dried at 60 °C.

Synthesis of Composite Photocatalyst
A total of 0.02 mol Bi(NO3)3·5H2O, Na2WO4·2H2O and a certain amount of ZnIn2S4 were mixed in 150 mL deionized water and continuously stirred at room temperature until a uniform mixture was obtained. The mixture was transferred into a 200 mL stainless autoclave (Rui Quan Xing, Xi'an, China) with a Teflon liner to prepare composite photo-catalysts through hydrothermal reaction for 12 h at 160 °C in a vacuum oven (Taiste Instrument Co.,Ltd, Tianjin, China). The obtained sample was filtrated and washed several times to be dried at 60 °C. The samples fabricated with 0.6978 g, 1.3956 g and 2.0934 g of ZnIn2S4 were recorded as 0.1-Zn/Bi, 0.2-Zn/Bi and 0.3-Zn/Bi, respectively. The single component photocatalyst of ZnIn2S4 and Bi2WO6 were investigated as control.

Characterization
The morphology was characterized by the Phenom LE field emission scanning electron microscope (FESEM, Phenom, Amsterdam, Holland). The as-prepared photocatalyst was uniformly dispersed on the sample stage to be sprayed gold and characterized at 15 kV. The crystal phase was analyzed by Ultiman IVX X-ray diffraction (XRD, Hitachi, Tokyo, Japan) with Cu Kα radiation in the scanning range of 5-70° at a speed of 10°/min. The voltage and current were 40 kV and 40 mA, respectively. The specific surface area and the pore diameter distribution were obtained from the V-Sorb 2800P BET analyzer (Gold App, Beijing, China). The as-prepared photocatalyst was measured at 77 K after degassing at 70 °C for 12 h under vacuum. The optical property was measured by U3310 UV-Vis spectrophotometry (UV-Vis DRS, Hitachi, Tokyo, Japan) at a wavelength range of 200-900 nm at speed of 600 nm/min.

Photoeletrochemical Measurement
The photoeletrochemical characterization was carried out on an electrochemical workstation (Chenhua Instrument Co.,Ltd, Shanghai, China) in a conventional three-electrode configuration, with Pt as the counter electrode and Ag/AgCl as the reference electrode. A 0.2 mol/L Na2SO4 aqueous solution was used as an electrolyte. A 500 W Xe lamp served as the light source. The working electrodes were prepared as follows: 10 mg of asprepared photocatalyst was dispersed in 1 mL ethanol solution, and 50 μL of Nafion ethanol solution was added to form a uniform suspension by ultrasound for 30 min. Next, 150 μL of suspension was coated on the surface of ITO glass and dried at room temperature for photoeletrochemical measurement. The reaction was conducted in a nitrogen stream. Transient photocurrent response was measured at 0.5 V with the irradiation/dark interval of 20 s. Electrochemical impedance spectroscopy (EIS) was measured at an AC voltage magnitude of 5 mV with a frequency range of 10 5 to 10 −2 Hz, with the initial potential of 0 V.

Photocatalytic Experiment
The photocatalytic experiment was conducted in a CEL-WLAX photochemical reactor (Education Au-light, Beijing, China) with a 500W Xe lamp emitting UV-Vis light (170-900 nm) with an average intensity of 230 W/m 2 . Fluvastatin at an initial concentration of 10 mg/L was used to evaluate the photocatalytic performance, and the dosage of photocatalyst was 0.2 g/L in all photocatalytic experiments. The adsorption reaction was carried out under dark conditions to achieve adsorption-desorption equilibrium between photocatalyst and pollutant, and then the photocatalytic reaction was carried out under irradiation. Samples were taken at the same time interval and filtered by 0.22 μm PES filter for concentration measurements. All experiments were repeated at least three times to evaluate reproducibility.

In-Situ Capture Experiment
An in-situ capture experiment was carried out to analyze the role of ROS generated during the photocatalytic reaction, in which different scavengers were introduced to capture corresponding ROS to evaluate the pollutant removal compared with condition of in the absence of a scavenger. In this study, isopropanol, TEMPOL and ammonium oxalate were used to capture hydroxyl radicals (·OH), superoxide radicals (·O2 − ) and holes (h + ), respectively. The concentration of isopropyl alcohol was 1 mL/L, and the concentration of ammonium oxalate and TEMPOL was 1 mmol/L. The process of in-situ capture experiment was consistent with photocatalytic experiments in addition to the introduction of a scavenger.

Probe Molecular Transformation
Nitroblue tetrazolium (NBT) was used as a probe molecule for quantitative analysis of ·O2 − . 0.025 mmol/L NBT was mixed with 0.2g/L 0.3-Zn/Bi composite photocatalyst in an aqueous solution to react under irradiation. The ·O2 − concentration was calculated by monitoring the change in the concentration of NBT on UV-2600 ultraviolet visible spectrophotometer (Unico, Shanghai, China) at a wavelength of 259 nm.
Terephthalic acid (TA) can be used as a probe molecule for quantitative analysis of ·OH. 0.5 mmol/L TA was prepared in 2 mmol/L NaOH solution, which was mixed with 0.2g/L 0.3-Zn/Bi composite photocatalyst to react under irradiation. The ·OH concentration was calculated by monitoring the concentration of 2-hydroxyterephthalic acid (2-HTA) on an F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at the excitation wavelength of 315 nm and emission wavelength of 425 nm, respectively.

Analysis Methods
The absorbance of the samples was measured at 234 nm by a UV-2600 UV-Visible spectrophotometer (Unico, Shanghai, China). The concentration of fluvastatin was calculated by the standard curve. The removal efficiency was calculated by Equation (1): where Ct is the concentration of fluvastatin at specific time intervals and C0 is the initial concentration of fluvastatin.

Morphology
The morphology of as-prepared photocatalyst is shown in Figure 1. As shown in Figure 1a, ZnIn2S4 exhibited oblate morphology with a braided surface of 1-2 μm. In Figure  1b, Bi2WO6 is a petal-like microsphere assembled by a large number of nanosheets, with an average diameter of approximately 3 μm. It can be seen from Figure 1c-e that the morphology of the composite photocatalyst transited from a petal-like microsphere to a braided microsphere with the increase of the ZnIn2S4/Bi2WO6 ratio. The fashioned ZnIn2S4 was introduced into a hydrothermal reaction to act as the template to induce the assembly of Bi2WO6 nanosheets on the braided surface, which contributed to fabrication of the composite photocatalyst. The uniform morphology of 0.3-Zn/Bi was obviously observed in Figure 1e compared to the composite photocatalyst with a lower ratio of ZnIn2S4/ Bi2WO6 (Figure 1c,d), which was beneficial to obtain excellent adsorption and photocatalytic performance.

Crystal Phase
The XRD was used to characterize the crystal phase of the as-prepared photocatalysts and the results are shown in Figure 2. In the XRD pattern of ZnIn2S4, the diffraction peaks of 2θ at 21.5°, 27.6°, 32.4° and 47.2° were assigned to (006), (102), (105) and (110) crystallographic planes, which indicated the formation of pure hexagonal ZnIn2S4 [19]. For the XRD pattern of Bi2WO6, the obvious diffraction peaks at 28.3°, 32.9°, 47.1°, 56.0°, 58.5° and 68.8° were assigned to (131), (200), (202), (133), (262) and (400) crystallographic planes, which were consistent with the standard card JCPDS-01-033-1126 and declared the orthogonal phase [20]. As shown in Figure 2, the composite photocatalyst exhibited an orthogonal phase because the diffraction peaks of a composite photocatalyst with a different ZnIn2S4/Bi2WO6 ratio were same with that of Bi2aWO6, indicating that the crystal phase of Bi2WO6 was unchanged by the introduction of ZnIn2S4 into the hydrothermal reaction. Moreover, the characteristic diffraction peaks of ZnIn2S4 was absent in the XRD patterns of the composite photocatalyst, which may be caused by a low ratio of ZnIn2S4/Bi2WO6. Meanwhile, the diffraction peak intensity of the composite photocatalyst obviously decreased with the increase of ZnIn2S4/Bi2WO6 ratio. In the XRD pattern of 0.3-Zn/Bi, some new diffraction peaks with low intensities were observed. The results showed that the assembly of Bi2WO6 was affected by ZnIn2S4 during the hydrothermal reaction, which further confirmed that ZnIn2S4 and Bi2WO6 were successfully compounded to form the composite photocatalyst in a hydrothermal reaction. The narrow and sharp diffraction peaks represented high crystallinity of the composite photocatalyst compared to the single component photocatalyst.

Specific Surface Area and Pore Size Distribution
The specific surface area and pore size distribution of the as-prepared photocatalyst were analyzed through nitrogen adsorption-desorption isotherms ( Figure S1), and the related data is shown in Table 1. The nitrogen adsorption-desorption isotherms of ZnIn2S4 and Bi2WO6 were typical type IV isotherms with H3-type hysteresis loops under relatively high pressure, implying the presence of slit-like pores and capillary condensation in the mesoporous [21,22]. The hysteresis loop of ZnIn2S4 was significantly larger than that of Bi2WO6, indicating excellent adsorbability of ZnIn2S4 compared to Bi2WO6. As shown in Table 1, the BET surface area, pore volume and average pore size of ZnIn2S4 were 86.88 m 2 /g, 0.27 cm 3 /g and 9.99 nm, respectively. The corresponding data of Bi2WO6 were 19.64 m 2 /g, 0.51 cm 3 /g and 34.11 nm, respectively.
The type of nitrogen adsorption-desorption isotherms of the composite photocatalyst with different ZnIn2S4/Bi2WO6 ratio were consistent with that of the single component photocatalyst. However, the BET surface area of the composite photocatalyst dramatically increased with the increase of ZnIn2S4/Bi2WO6 ratio. The BET surface area of the 0.3-Zn/Bi composite photocatalyst reached 37.97 m 2 /g, which was twice that of Bi2WO6. Combined with the morphology analysis, the BET surface area of the composite photocatalyst was improved by the introduction of ZnIn2S4 with a porous braided surface. The larger specific surface area represented excellent photocatalytic performance because more active sites were provided for adsorption and photocatalytic reaction. Meanwhile, the porous structure provided pathways for separation and transfer of charge carriers to promote ROS generation, which played an important role in the enhancement of photocatalytic performance [23,24].

Optical Properties
The optical properties of a semiconductor photocatalyst is the key factor affecting photocatalytic efficiency. The light harvest of the composite photocatalyst was measured by UV-vis DRS and the spectras of as-prepared photocatlyst are exhibited in Figure 3a. The strong response to UV-Vis and steep absorption edge at 600 nm of ZnIn2S4 were confirmed due to inherent narrow band gap. In the spectra of Bi2WO6, the absorption edge around 450 nm and poor absorptivity of visible light were observed, because only light at short-wavelength with relatively high energy could be absorbed determined by the high band gap energy of Bi2WO6. The absorption edge of the composite photocatalyst with a different ZnIn2S4/Bi2WO6 ratio exhibited slight red shift, demonstrating that enhancement of visible light harvest can be realized by the construction of the composite photocatalyst [25]. The band gap energy of as-prepared photocatalyst can be calculated by Equation (2): where α is absorption coefficient in UV-Vis diffuse reflection, hν is photon energy, Eg is band gap energy, and A is constant. The value of n was 1/2, 2, 3/2 and 3, for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively [26].
The band gap energy of as-prepared photocatalyst was determined by the plots of (αhν) 1/n versus hν, in which n was equal to 2 because ZnIn2S4 and Bi2WO6 were indirect transition semiconductors [26,27]. From Figure 3b, the band gap energy of ZnIn2S4 and Bi2WO6 were 1.88 eV and 2.72 eV, respectively. The band gap energy of the composite photocatalyst slightly decreased compared with Bi2WO6, but the difference of band gap energy between the composite photocatalyst with different ZnIn2S4/Bi2WO6 ratio was negligible. The band gap energy of 0.1-Zn/Bi, 0.2-Zn/Bi and 0.3-Zn/Bi composite photocatalysts were determined to be 2.56 eV, 2.65 eV and 2.62 eV, respectively. The results showed that the introduction of ZnIn2S4 to composite with Bi2WO6 could reduce the band gap energy to a certain extent and contribute to improve the utilization rate of light with low energy. The elemental composition of ZnIn2S4 and Bi2WO6 was analyzed by XPS and EDS and the results were displayed in Figure S2. The molar ratio of Zn, ln and S for ZnIn2S4 was 1:2:3.9, and the molar ratio of Bi, W and O for Bi2WO6 was 2.3:1:5.8. The CB potential and VB potential of single component photocatalysts were calculated based on Equations (3) and (4) [28]: where χ is the absolute electronegativity of the photocatalyst recorded as the geometric mean value of the absolute electronegativity for constituent atoms; Ee is free electron energy (4.5 eV vs. NHE); and Eg is band gap energy of the photocatalyst. The calculation results are demonstrated in Table 2.

Photoelectrochemical Property
The photocatalyst was excited by light with sufficient energy to generate ROS through the redox reaction of charge carriers. Therefore, the separation and transfer of charge carriers were a crucial process to determine photocatalytic efficiency. The transient photocurrent response and EIS were performed to investigate the separation efficiency of charge carriers. The photocurrent-time (I-T) curves of as-prepared photocatalyst under irradiation for several on-off cycles are displayed in Figure 4a. ZnIn2S4 and Bi2WO6 exhibited a very low photocurrent density, which was 1.32 μA and 1.84 μA, respectively. The results demonstrated the fast recombination of charge carriers in a single component photocatalyst, which can be attributed to e − in an unstable state easily returning to the VB to recombine with h + because of narrow band gap structure. Based on the band gap analysis, the band gap of ZnIn2S4 was obviously more narrow than that of Bi2WO6, resulting in faster recombination of charge carriers in ZnIn2S4. Therefore, the photocurrent density of ZnIn2S4 was lower than that of Bi2WO6.
The composite photocatalyst showed higher photocurrent response than that of the single component photocatalyst and the photocurrent density increased with the increase of ZnIn2S4/Bi2WO6 ratio. The photocurrent density of 0.3-Zn/Bi composite photocatalyst reached 4.42 μA, representing the maximum separation and transfer efficiency of charge carriers. The results revealed more efficient separation and transfer of charge carriers for the composite photocatalyst. The longer lifetime of charge carriers is conducive to ROS generation to improve the photocatalytic efficiency of the composite photocatalyst [30]. According to the results of optical property analysis, the difference of band gap energy between Bi2WO6 and the composite photocatalyst could be negligible. Therefore, the efficient separation and transfer of charge carriers were attributed to the formation of heterostructure by staggered band alignment of Bi2WO6 and ZnIn2S4. In the heterostructure, the transfer pathway of charge carriers was changed to inhibit the recombination of e − and h + , which was conducive to improving the photocatalytic efficiency [31].  EIS is a useful index to characterize transfer of charge carriers through strong correlations between the radius of EIS and transfer performance of charge carriers. The smaller radius of EIS represented smaller resistance, which demonstrated efficient transfer of charge carriers. It could be observed from the EIS of the as-prepared photocatalyst in Figure 4b that the radius of ZnIn2S4 was larger than that of Bi2WO6, while the radius of the composite photocatalyst was smaller than that of the single component photocatalyst and the radius decreased with the increase of ZnIn2S4/Bi2WO6 ratio. The results further confirmed that the heterostructure was successfully constructed by ZnIn2S4 and Bi2WO6, which could promote the separation and transfer of charge carriers to enhance the generation of ROS. The 0.3-Zn/Bi composite photocatalyst showed the highest transfer efficiency of charge carriers, which was in agreement with the results of transient photocurrent response.

Removal of Fluvastatin
Fluvastatin was selected as a pollutant to investigate the photocatalytic performance of the as-prepared photocatalyst, and the results are demonstrated in Figure 5a. The removal of fluvastatin by ZnIn2S4 under dark conditions was negligible, indicating poor adsorption performance of ZnIn2S4 for fluvastatin because of electrostatic repulsion between negatively charged fluvastatin and ZnIn2S4 [15]. A total of 58.36% of fluvastatin was removed by ZnIn2S4 under irradiation, indicating that ZnIn2S4 possessed photocatalytic performance. Fluvastatin could be removed by Bi2WO6 through adsorption and photocatalytic reaction, with the removal efficiency being 57.89% in an 120 min photocatalytic reaction. In the system with the composite photocatalyst, the removal of fluvastatin by adsorption and photocatalysis was significantly improved compared to the system with the single component photocatalyst. The removal efficiency of fluvastatin increased from 60.84% to 75.47% when the ratio of ZnIn2S4/Bi2WO6 increased from 0.1 to 0.3. For the composite photocatalyst, the improvement of adsorption performance was mainly due to the change of morphology and specific surface area by the introduction of ZnIn2S4, which provided abundant active sites for fluvastatin adsorption. The high adsorption of fluvastatin on 0.2-Zn/Bi was observed in Figure 5a, because of the largest pore volume and average pore size. The photocatalytic performance was enhanced through the construction of the heterostructure to inhibit the recombination of charge carriers and generate multiple ROS.
In conclusion, the photocatalytic performance of the composite photocatalyst was significantly improved and 0.3-Zn/Bi composite photocatalyst exhibited excellent photocatalytic performance. The stability and lifetime were key factors to evaluate the performance of photocatalysts in engineering applications. The removal of fluvastatin by 0.3-Zn/Bi composite photocatalyst in four cycle experiments was conducted to evaluate the stability of the composite photocatalyst. As shown in Figure 5b, the high removal efficiency of fluvastatin could be observed in the cycle experiments, which remained at 65.61% after four cycles, only decreasing by 7.6% compared to that of the first cycle. The results confirmed that the com-posite photocatalyst exhibited high stability of photocatalytic performance. The main reasons for the decrease of removal efficiency were the deactivation of photocatalysts caused by active site occupation and leakage of powder photocatalysts in the recovery process. In general, the composite photocatalyst showed high stability of photocatalytic performance, suggesting a possibility for the engineering application.

ROS Analysis
The removal efficiency of fluvastatin by the as-prepared photocatalyst in the presence of various scavengers is expressed in Figure 6.
In the photocatalytic system with ZnIn2S4, the removal efficiency of fluvastatin decreased to 34.81% after the addition of TEMPOL to capture ·O2 − , demonstrating significant inhibition of photocatalytic degradation of fluvastatin compared to the system without a scavenger. When ·OH was captured by isopropyl alcohol, the removal efficiency of fluvastatin also decreased, but was higher than that in the presence of TEMPOL. The effect of the addition of ammonium oxalate to capture h + on photocatalytic degradation of fluvastatin was negligible. The results suggested that fluvastatin was removed by the synergistic oxidation of ·O2 − and ·OH, in which ·O2 − was the crucial ROS and ·OH played an auxiliary role. The ROS generation in the photocatalytic system was determined by the potential of CB and VB. The CB potential and VB potential of ZnIn2S4 were −0.60 eV vs. NHE and +1.28 eV vs. NHE, respectively. O2 could be reduced by e − in the CB to generate ·O2 − , but h + in VB could not oxidize OH − and H2O to generate ·OH. The production of ·OH relied on a series of reactions associated with ·O2 − , such as when ·O2 − reacted with e − or H2O to produce H2O2, which further decomposed into ·OH. Therefore, ·O2 − and ·OH played vital and auxiliary roles in the system with ZnIn2S4. The ESR analysis in the published literature demonstrated that ·O2 − and ·OH were generated in the photocatalytic reaction of ZnIn2S4, but generation of ·OH was inhibited after addition of superoxide dismutase to react with ·O2 − , suggesting that ·OH was generated based on ·O2 − . The results were consistent with the conclusions obtained in the present work [32]. For the system in the presence of Bi2WO6, the removal efficiency of fluvastatin dramatically decreased to 28.52% and 40.30% when h + and ·OH were captured by ammonium oxalate and isopropyl alcohol, respectively, while the fluvastatin removal did not change after the addition of TEMPOL compared with that of without a scavenger. Fluvastatin could be directly oxidized by h + . Meanwhile, OH − and H2O were oxidized by h + to generate ·OH, because the VB potential of Bi2WO6 (+3.16 eV vs. NHE) was more positive than the potential of OH − /·OH (+1.99 eV vs. NHE) and H2O/·OH (2.27 eV vs. NHE). Fluvastatin was also oxidized by ·OH with strong oxidative ability. However, the reduction of O2 by e − to generate ·O2 − was prevented due to the CB potential of Bi2WO6 (+0.44 eV vs. NHE) being more positive than the potential of O2/·O2 − (−0.28 eV vs. NHE) [33]. Therefore, h + and ·OH were important ROS for the removal of fluvastatin in the photocatalytic system with Bi2WO6. According to the results of ROS analysis, the mechanisms of ROS generation in the system with ZnIn2S4 and Bi2WO6 were completely opposite because of different band gap structure. However, the staggered band alignment provided an opportunity for the construction of heterostructured photocatalysts to generate multiple ROS. The ROS generation of the composite photocatalyst was analyzed by 0.3-Zn/Bi composite photocatalyst. The obvious difference of ROS generation between the composite photocatalyst and single component photocatalyst can be observed in Figure 6. The removal efficiency of fluvastatin by 0.3-Zn/Bi composite photocatalyst was 75.47% absence of scavenger, which decreased to 32.39%, 58.22% and 62.73% when h + , ·OH and ·O2 − were captured, respectively. The results demonstrated that the joint oxidation of fluvastatin was achieved by h + , ·OH and ·O2 − in the order of h + > ·OH >·O2 − . The concentration of ROS in the system with the composite photocatalyst was quantitatively analyzed by the probe molecular transformation. NBT can react with ·O2 − to generate insoluble purple product as shown in Equation (5), and the concentration of ·O2 − in the system can be calculated by the change of the NBT concentration. TA can be used as a probe molecule for quantitative analysis of ·OH because the fluorescent product 2-HTA is generated during the reaction, as shown in Equation (6). The concentration of ·OH in the system can be calculated by the concentration of 2-HTA.
NBT + 4·O2 − +2H + → formazan + O2 (5) TA + ·OH → 2-HTA The concentration of ·OH and ·O2 − in the system with 0.3-Zn/Bi composite photocatalyst is expressed in Figure 7. The results showed that ROS continuously generated during the reaction because the concentration of ·OH and ·O2 − increased with the increase of reaction time. After 120min of photocatalytic reaction, the concentration of ·OH and ·O2 − were 1.26 mmol/L and 2.26 × 10 −4 mmol/L, respectively. Obviously, ·OH played a more important role than ·O2 − in the system presence of the composite photocatalyst because the concentration of ·OH was higher than that of ·O2 − . The results were consistent with the conclusions obtained in the in-situ capture experiments. The goal of the present study was the construction of heterostructured photocatalyst based on staggered band alignment to initiate multiple ROS generation. The results of ROS analysis confirmed that the chain reaction for the generation of ·OH and ·O2 − could be synchronously initiated due to the CB potential of ZnIn2S4 and VB potential of Bi2WO6, which demonstrated that the idea about heterostructure construction was feasible.

Mechanism for the Enhancement of Photocatalytic Performance
In general, photocatalytic performance is determined by the redox ability and lifetime of charge carriers. The key process for the enhancement of photocatalytic performance includes initiating charge carriers, promoting charge carrier separation to prolong its lifetime and inducing the redox reactions of charge carriers. The construction of heterostructured photocatalysts is an effective way to improve the mentioned process. According to the results obtained in the present work, the mechanism for the photocatalytic performance enhancement of ZnIn2S4/Bi2WO6 heterostructured photocatalyst was proposed as shown in Figure 8.
ZnIn2S4 and Bi2WO6 were intimately contacted during the hydrothermal reaction to construct an n-n-type heterojunction due to staggered band alignment. The free e − of ZnIn2S4 with a higher Fermi level (Ef1) transferred to Bi2WO6 with a lower Fermi level (Ef2) through interface to balance their Fermi level. The directional movement of free e − led to positive charge of the ZnIn2S4 side and negative charge of the Bi2WO6 side at the interface to establish an internal electric field (IEF). Meanwhile, the band edge of Bi2WO6 at the interface bent downwards due to accumulation of free e − , while the band edge of ZnIn2S4 at the interface bent upwards due to a decrease of e − density (Figure 8b).
Under light irradiation, e − in the VB of ZnIn2S4 and Bi2WO6 absorbed photon energy and transited to the CB to form e − and h + pairs (namely charge carriers). e − in the CB of Bi2WO6 and h + in the VB of ZnIn2S4 transferred to interface under IEF and recombined rapidly with each other to form n-n-type direct Z-scheme heterojunctions. The transfer pathway of charge carriers was changed in the heterostructure to prolong the lifetime of e − in the CB of ZnIn2S4 and h + in the VB of Bi2WO6, which provided more opportunities for generation of ROS [36]. Meanwhile, the potential barrier caused by band bending prevented e − and h + transferring to opposite directions, ensuring the formation of a direct Zscheme heterojunction rather than a Type II heterojunction. As the CB potential of ZnIn2S4 was more negative than the potential of O2/·O2 − , O2 was reduced by e − to generate ·O2 − . In a similar way, generation of ·OH was achieved through oxidation of OH − and H2O by h + in the VB of Bi2WO6, which potential was more positive than that of OH − /·OH and H2O/·OH. Differing from the single component photocatalyst, h + , ·O2 − and ·OH were synchronously generated in the system with the heterostructured photocatalyst to synergistically oxidize pollutant, effectively enhancing the photocatalytic performance of the system (Figure 8c). In summary, the mechanism for the photocatalytic performance enhancement of ZnIn2S4/Bi2WO6 heterostructured photocatalyst included two points as follows. Firstly, the well-matched staggered band alignment of ZnIn2S4 and Bi2WO6 was utilized to form an n-n direct Z-scheme heterojunction to promote the separation and utilization of charge carriers through changing the transfer pathway of charge carriers. Secondly, the synchronous generation of multiple ROS was achieved to effectively improve the redox ability relying on the advantage of the CB potential of ZnIn2S4 and VB potential of Bi2WO6. Therefore, the proposed strategy for the construction of a heterostructured photocatalyst based on the staggered band alignment of a Bi-based photocatalyst and a ternary metal sulfide photocatalyst was a feasible way to obtain an efficient and stable photocatalyst with visible light response.

Conclusions
In this study, an n-n direct Z-scheme heterostructured photocatalyst was constructed by hydrothermal method based on staggered band alignment of a Bi-based photocatalyst and ternary metal sulfide photocatalyst, whose morphology, crystal phase, specific surface area, pore size distribution, optical property, photoelectrochemical property, photocatalytic performance and ROS generation mechanism were systematically investigated. Moreover, the mechanism for the enhancement of photocatalytic performance was discussed. The main conclusions could be summarized as follows.
The morphology of the heterostructured photocatalyst transited from a petal-like microsphere to a braided microsphere with the increase of the ZnIn2S4/Bi2WO6 ratio. The uniform morphology of the heterostructured photocatalyst was conducive to obtain efficient and stable photocatalytic performance. The crystal phase of Bi2WO6 did not change with the introduction of ZnIn2S4 into the hydrothermal reaction, and the heterostructured photocatalyst exhibited high crystallinity of the orthogonal phase, which was the same as that of Bi2WO6. As the ratio of ZnIn2S4/Bi2WO6 increased, the BET surface area of the heterostructured photocatalyst increased dramatically. The BET surface area of 0.3-Zn/Bi composite photocatalyst was twice than that of Bi2WO6, which provided more active sites for adsorption and photocatalytic reaction to improve the photocatalytic performance. The difference of visible light response between the heterostructured photocatalyst and single component photocatalyst was negligible. However, the transient photocurrent response of the heterostructured photocatalyst was significantly improved and the EIS of the heterostructured photocatalyst was obviously decreased, confirming that charge carriers were effectively separated and transferred to prolong its lifetime under irradiation.
The photocatalytic performance of heterostructured photocatalysts was significantly improved, and 75.47% of fluvastatin was removed, by a 0.3-Zn/Bi composite photocatalyst with high stability. In the system with a heterostructured photocatalyst, the joint oxidation of fluvastatin was achieved by multiple ROS, in the order of h + > ·OH >·O2 − . The mechanism for the photocatalytic performance enhancement of ZnIn2S4/Bi2WO6 heterostructured photocatalyst included two points as follows. Firstly, the well-matched staggeredband alignment of ZnIn2S4 and Bi2WO6 was utilized to form an n-n direct Z-scheme heterojunction to inhibit recombination of charge carriers and improve the utilization of e − and h + through changing the transfer pathway of charge carriers. Secondly, the synchronous generation of multiple ROS was achieved to effectively improve the redox ability, relying on the advantage of the CB potential of ZnIn2S4 and VB potential of Bi2WO6.
In conclusion, the proposed strategy for the construction of a heterostructured photocatalyst based on the staggered band alignment of a Bi-based photocatalyst and ternary metal sulfide photocatalyst was a feasible way to obtain efficient and stable photocatalysts with visible light response. The results provided valuable insights for the performance improvement of photocatalysts with narrow band gaps and their application in the field of water pollution control.