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Controllable one-pot synthesis of nest-like Bi2WO6/BiVO4 composite with enhanced photocatalytic antifouling performance under visible light irradiation† †

DOI: 10.1039/x0xx00000x

Peng Ju

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In this study, a novel visible-light-sensitive Bi2WO6/BiVO4 composite photocatalyst was controllably synthesized through a facile one-pot hydrothermal method. The Bi2WO6/BiVO4 composite exhibited a perfect nest-like hierarchical microsphere structure, which was constructed by the self-assembly of nanoplates with the assistance of polyvinylpyrrolidone (PVP). The growth mechanism of the Bi2WO6/BiVO4 composite and the effect of structure on photocatalytic performance was investigated and proposed. Experimental results showed that the Bi2WO6/BiVO4 composites displayed enhanced photocatalytic antifouling activities under visible light irradiation compared to pure Bi2WO6 and BiVO4. Bi2WO6/BiVO4-1 exhibited the best photocatalytic antifouling performance, and almost all (99.99%) of Pseudomonas Aeruginosa (P. aeruginosa), Escherichia Coli (E. coli) and Staphylococcus Aureus (S. aureus) could be killed within 30 min. Moreover, the Bi2WO6/BiVO4-1 composite exhibited an excellent stability and reusability in the cycled experiments. The photocatalytic antifouling mechanism was proposed based on the active species trapping experiments, revealing that the photo-induced holes (h+) and hydroxyl radicals (⋅OH) could attack the cell wall and cytoplasmic membrane directly and lead to the death of bacteria. The obviously enhanced photocatalytic activity of the Bi2WO6/BiVO4-1 composite could be mainly attributed to the formation of heterojunction, accelerating the separation of photo-induced electrons and holes. Furthermore, the large BET surface area combined with the wide photoabsorption region further improve the photocatalytic performance of the Bi2WO6/BiVO4-1 composite. This study provides a new strategy to develop novel composite photocatalysts with enhanced photocatalytic performances for marine antifouling and water purification.

a, b

a

a

a,

, Yi Wang , Yan Sun , Dun Zhang *

1. Introduction Natural and artificial substrata in the marine environment are quickly colonised by marine micro- and macroorganisms in a 1-3 process known as “biofouling’’. In the past few decades, marine biofouling has become one of the worldwide serious problems, resulting in significant economic losses, great energy waste, serious security risks to marine project facilities, 4, 5 and harmful ecological destruction around the world. So far, the most effective methods for biofouling control are using toxic antifouling coatings on the surface of marine installations, such as tributyltin (TBT), cuprous oxide and 4, 5 organic compounds. However, almost all of these conventional antifouling paints are contaminants to ecological environment and have a strong biotoxicity, leading to a serious 4-8 marine ecological crisis. Recently, TBT, tin-based and copperbased antifouling paints have been banned from using 4-8 worldwide. Therefore, there is an urgent and essential need

a.

Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, PR China b. University of Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, PR China † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

to develop novel, efficient, safe, nontoxic, and environmentally friendly marine antifouling materials and techniques to 1, 3, 5 prevent the harm of marine biofouling. Currently, a novel and green photocatalysis technology based on semiconductors, that facilitates the conversion of clean solar energy into chemical energy, has attracted a considerable concern for its potential applications in the degradation of organic pollutants, hydrogen production from 9-13 In addition, as a promising water and disinfection. alternative for solar energy utilization, photocatalysis has been regarded not only as a cost-effective technology, but also as a highly efficient method to decompose organisms completely 9-13 into harmless chemicals. Moreover, during the photocatalytic process, the excited electrons (e ) and photo+ induced holes (h ) on the surface of semiconductor will react + with water or air to generate active oxidative species (h , ·OH - 9-13 and ·O2 ). These oxidative radicals can attack the marine fouling microorganisms directly and then decompose them 14-17 completely, restricting the formation of biofilm on the surface of marine facilities and thus avoiding biofouling further. Therefore, there is a potential and bright prospect to apply photocatalysis technology in marine antifouling and water purification. To date, TiO2 has been the most popular catalyst in photocatalysis owing to its nice photocatalytic activity, good

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heterojunction photocatalyst based on a chemical etching method for photocatalytic antifouling, which exhibited an obviously enhanced photocatalytic antifouling performance under visible light irradiation compared to pure Bi2WO6 and pure BiOI. On the other hand, as a typical n-type semiconductor with a narrow band-gap (Eg = 2.3 eV) and excellent visible-light responsive ability, BiVO4 exhibits a fantastic photocatalytic 31, 45-47 activity and is widely used in photocatalysis. Especially, BiVO4 has been proved to be an effective visible-light-driven 46 photocatalyst in inactivation of Escherichia Coli. However, limited by the recombination of photoinduced charge carriers 47 in pure BiVO4, crystal facet engineering and coupled with 48 49 other semiconductors (BiVO4/TiO2, BiVO4/Bi2S3, 50 51 52 BiVO4/Bi2O3, BiVO4/BiOCl, and BiVO4/BiOI ) are usually used to further enhance its photocatalytic activity. Hence, given to the above background and the matching band edges, BiVO4 is appropriate to composite with Bi2WO6 to establish a composite with an enhanced photocatalytic antifouling 53 performances. Zhang et al. firstly reported the preparation of Bi2WO6/BiVO4 heterojunction with a higher photocatalytic activity than the individual components under visible light irradiation, and 81% of phenol could be degraded in 6 h, indicating that the combination of Bi2WO6 and BiVO4 could obviously enhance the photocatalytic activity of pure Bi2WO6. 54 55 56 Subsequently, Chaiwichian et al. , Xue et al. and Fan et al. prepared Bi2WO6/BiVO4 heterojunction photocatalysts via different preparation methods, respectively, displaying the enhanced visible light photocatalytic activity in the 57 degradation of dyes. Besides, in our previous reports, a calcined Bi2WO6/BiVO4 heterojunction photocatalyst with a multilayer plate-like structure was synthesized via a hydrothermal-calcination method, which exhibited an enhanced photocatalytic activity in the degradation of RhB, and the degradation efficiency could reach 100% within 30 min. However, though the study of BiVO4/Bi2WO6 heterojunction with an improved photocatalytic activity had attracted broad attentions of the researchers, considering the fact that the morphological diversity of inorganic materials has a significant impact on functional diversification and potential applications, there is still a huge potential for Bi2WO6/BiVO4 composite to improve its photocatalytic activity by optimizing the preparation process to control the structure, morphologies and surface properties. Besides, there is lack of complete and detailed explanation on the growth mechanism and the enhanced photocatalytic mechanism of the Bi2 WO6/BiVO4 in the previous works. And the application range of Bi2WO6/BiVO4 was limited to photocatalytic degradation of organic dyes and pollutants at present, it is necessary to extend its application areas to make Bi2WO6/BiVO4 a more bright prospect in photocatalysis. Herein, in this work a novel Bi2 WO6/BiOI composite photocatalyst with a perfect nest-like hierarchical microsphere structure was controllably synthesized through a facile one-pot hydrothermal method, which was further used in photocatalytic antifouling. The growth mechanism of the

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chemical stability, non-toxicity, and low cost. Since 21 Matsunaga and his coworkers firstly reported the high photocatalytic performance of TiO2 in disinfection under ultraviolet light (UV) light, estimating the photocatalytic antibacterial activity of TiO2 has aroused extensive concern. TiO2 is demonstrated to exhibit great antibacterial 22 23 performance for Escherichia Coli, Staphylococcus Aureus, 24 25 Sulfate Reducing Bacteria, Pseudomonas Lactobacilius, 26 27 Aeruginosa, and algae. However, the large band gap, UVresponse only and low quantum efficiency lead to a great inhibition of utilization of solar energy, which dramatically limit 18-20 the practical application of TiO2. Consequently, researchers are paying eager attentions to develop novel visible-light responsive photocatalysts and composites that can harness solar energy efficiently and promote the rapid separation of 28-34 charge carriers. Among the various of reported 29 photocatalysts, bismuth-based materials, such as Bi2O3, 30 31 32 33 34 Bi2S3, BiVO4, Bi2WO6, Bi2MoO6, and BiOX (X = Cl, Br, I), has brought about a widespread attention due to their particular layered structures, chemical and thermally stabilities, non-toxicity, environmental friendliness, and 28-34 excellent photocatalytic activities. In addition, in view of the interaction between Bi (6s) and O (2p) orbitals at the top of the valence band, most of bismuth-based materials reveal a 35-37 strong response to visible light, which give rise to the fantastic photocatalytic activities. As one of the typical Aurivillius oxides, Bi2WO6 is a typical n-type semiconductor with a direct band gap of about 2.8 eV, which has caused an extensive concern due to its excellent intrinsic physical and chemical property and photocatalytic 32, 35-39 40 What’s more, since Kudo and coauthors activity. reported its high photocatalytic performance for O2 evolution in AgNO3 solution under visible light irradiation, a variety of Bi2WO6 materials have been developed as visible-light-driven photocatalysts in the degradation of organic pollutants, water 32, 35-40 splitting and disinfection. However, the rapid recombination of photo-induced charge carriers and relative narrow absorption region of pure Bi2WO6 greatly restrict its 32, 35-40 photocatalytic activity. Therefore, in order to accelerate the separation of photoinduced charge carriers and further enhance the photocatalytic activity of Bi2WO6, Bi2WO6-based composite photocatalysts are constructed with a narrow band41-44 gap semiconductor with appropriate band position. Tian et 41 al. reported a Bi2WO6/TiO2 nanobelt heterostructure with an enhanced UV, visible and near-infrared photocatalytic activity, 42 realizing the effective utilization of solar energy. Wang et al. prepared a novel one-dimensional (1D) Bi2O3 nanorod-Bi2WO6 nanosheet p-n junction photocatalyst with a higher photocatalytic activity than single Bi2O3 nanorods or Bi2WO6 flowers, and the photocatalytic degradation efficiency of Rhodamine B (RhB) and phenol under solar/visible light could 43 reach 100% within 60 min. Li and his coworkers developed a robust single-step hydrothermal synthesis for the formation of hierarchically structured heterocatalysts of Bi2S3/Bi2 WO6 with a high yield (>95%), which exhibited excellent photocatalytic activity in remove toxic Cr(VI) ions under visible light irradiation. Recently, our group prepared a Bi2WO6/BiOI

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Bi2WO6/BiVO4-1 composite was presented according to the controllable preparation experiments. And the effect of BiVO4 contents on the structure and photocatalytic activity of the Bi2WO6/BiVO4 composite photocatalysts were investigated in detail. In addition, the photocatalytic antifouling activities, broad-spectrum antibacterial performance and reusability of the Bi2WO6/BiVO4 composites were evaluated and compared by photocatalytic disinfection of P. aeruginosa, E. coli and S. aureus under visible light irradiation. Besides, the destruction process of bacterial cells were studied and observed by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM). Furthermore, a possible enhanced photocatalytic antifouling mechanism was proposed based on trapping experiments and calculated energy bands.

2. Experimental 2.1. Materials Bi(NO3)3⋅5H2O, Na2WO4⋅2H2O, NH4VO3, polyvinylpyrrolidone (PVP), isopropanol (IPA), 4-hydroxy-2,2,6,6tetramethylpiperidinyloxy (TEMPOL), sodium oxalate, glutaraldehyde, nutrient agar, tryptone, acridine orange (AO), and other chemicals were all of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. Analytical reagent grade yeast extract was purchased from Oxoid Basingstoke Hampshire (England). All aqueous solutions were prepared with Milli-Q water (Millipore, USA). The strains employed in this work were the typical marine fouling microorganism Pseudomonas Aeruginosa (P. aeruginosa). Besides, the Gramnegative bacteria Escherichia Coli (E. coli) and the Grampositive bacteria Staphylococcus Aureus (S. aureus) were also selected as the model microorganism to evaluate the broadspectrum antibacterial performance. The bacteria were obtained from the Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences (Qingdao, China). Natural seawater collected from Huiquan Bay (Huanghai Sea, Qingdao, China) was used after sterilization by autoclaving for 20 min at 121 °C at 0.1 Mpa and filtration through a membrane filter (0.22 µm pore-size). 2.2. Preparation of the Bi2WO6/BiVO4 composite photocatalysts The Bi2WO6/BiVO4 composite photocatalyst was synthesized through a facile hydrothermal method based on our previous 57 paper. In a typical procedure, 3.0 mmol of Bi(NO3)3⋅5H2O was added into 30 mL of 2.0 M nitric acid under magnetic stirring to obtain a transparent solution A. Meanwhile, a proper amount of Na2WO4·2H2O and NH4VO3 (W/V molar ratios are 1:0.25, 1:0.5, 1:0.75, and 1:1 for the synthesis of Bi2WO6/BiVO4 with different molar ratios, respectively) were dissolved in 30 mL of 2.0 M NaOH solution under magnetic stirring, and then 0.5 g of PVP was added into the solution under continuously stirred to form a homogeneous mixture B. After that, the mixture B was subsequently added dropwise into the solution A slowly under vigorous stirring to form a final suspension, the pH value of which was then adjusted to 7.0 by 2.0 M NH3·H2O. The suspension was stirred for another 60 min and then transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 160 °C for 24 h, and

cooled to room temperature naturally. The yellow precipitates were collected and washed several times with Milli-Q water and absolute ethanol, respectively. Then the samples were dried at 60 °C in air for 6 h. The final composite products were denoted as Bi2WO6/BiVO4-0.25, Bi2WO6/BiVO4-0.5, Bi2WO6/BiVO4-0.75, and Bi2WO6/BiVO4 -1. Pure Bi2WO6 and pure BiVO4 were prepared under the same conditions mentioned above as controls. To investigate the growth mechanism of Bi2WO6/BiVO4-1, samples subjected to hydrothermal treatment for 0, 2, 6, 12, 24, and 36 h were also collected with other conditions fixed. 2.3. Characterization The crystal structure of the as-prepared samples was recorded by powder X-ray diffraction (XRD) measurements on a Rigaku Ultima IV powder X-ray diffractometer (Japan) under the following conditions: 40 kV, 30 mA, and graphite-filtered Cu Kα radiation (λ = 0.15406 nm). The structure and morphology of the as-prepared samples were observed by field-emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). Transmission electron microscopy (TEM) images, high resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns were carried out on a JEOL JEM-2100 field-emission transmission electron microscope (Japan) at an acceleration voltage of 200 kV, equipped with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., USA) equipped with an Al-anode at a total power dissipation of 150 W (15 kV, 10 mA), and the binding energies were referenced to the C 1s line at 284.60 eV from adventitious carbon. The specific surface areas of the samples were measured on the basis of the Brunauer-Emmett-Teller (BET) equation with a nitrogen adsorption instrument (ST-08A, China) at 77 K after a pretreatment at 473 K for 2 h. UV-visible diffuse reflectance spectra (UV-DRS) of the samples were obtained using a UV-visible spectrophotometer (Hitachi U4100, Japan) with BaSO4 as reference. 2.4. Culture of bacteria In this study, P. aeruginosa was chosen as a model marine fouling microorganism for antibacterial experiments. E. coli and S. aureus were used to assess the broad-spectrum antibacterial performances. All apparatus and materials were sterilized in an autoclave at 121 °C for 20 min before each microbiological experiment and all antibacterial tests were carried out in triplicate. Normally, P. aeruginosa was firstly incubated in Luria-Bertani (LB) nutrient solution at 37 °C overnight with shaking at150 rpm. Then the bacteria were separated by centrifuging at 4000 rpm for 10 min and sterilized seawater washing several times. The collected cells were diluted to the desired concentration or optical density in sterilized seawater. Bacterial cell numbers were estimated by the spread plate method, and the final suspension 8 concentration was around 8.5 × 10 colony forming units 8 (cfu/mL). The stock suspension of E. coli (3.0 × 10 cfu/mL) and 8 S. aureus (3.0 × 10 cfu/mL) were also prepared by the above

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operations with sterilized phosphate buffer as the diluted solution. 2.5. Photocatalytic antifouling experiments The photocatalytic antifouling experiments were carried out using a 500 W Xe lamp as the light source with a 420 nm cutoff filter. A quartz glass container was used as the reaction vessel which was jacketed with water circulation in order to keep at room temperature, and the suspension was stirred with a magnetic stirrer throughout the reaction. Typically, 50 mg of photocatalysts were added into 49.5 mL of sterilized natural seawater or PBS, and then 500 µL of bacterial suspension were added subsequently. Prior to illumination, the suspension was magnetically stirred in the dark for 30 min to ensure an adsorption/desorption equilibrium between the photocatalysts and bacterial cells. Then the suspension was sequentially stirred and exposed to light irradiation. At given time intervals, 1 mL of the bacteria suspension was withdrawn and serially diluted with sterilized seawater or PBS. Then, 100 µL of the diluted sample suspension was immediately spread on LB agar plates and incubated at 37 °C for 24 h to determine the number of viable cells (in cfu). For comparison, control experiments were conducted along with the treatment experiments. The blank control was conducted without adding photocatalysts in bacterial suspension under visible light irradiation, and the reaction system without visible light irradiation was used as a dark control. The survival rate was determined as follows: Survival rate (%) = (Nsurvivor/Ncontrol) × 100, where Ncontrol and Nsurvivor are the numbers of viable cells in the blank control and after each photocatalytic reaction, respectively. Hence, the antibacterial rate could be defined as: Antibacterial rate (%) = 100 - Survival rate. The survival status and morphology change of P. aeruginosa were observed by fluorescence microscopy (Olympus BX-51 with image software of Cellsens, Japan), FESEM (Hitachi S-4800) and TEM (JEOL JEM-1200), respectively. The detailed operations were shown in the Supplementary Information. 2.6. Analysis of active species The active species generated in the photocatalytic reaction were detected by adding scavengers into the reaction solutions, in which the concentrations of IPA (a scavenger of hydroxyl radicals ⋅OH), of TEMPOL (a scavenger of superoxide + radical ⋅O2 ), and sodium oxalate (a scavenger of holes h ) were 58 10 mM, 2 mM and 10 mM, respectively. At given time intervals, 1 mL of the reaction solution was collected and then serially diluted to the required concentration. Then, 100 µL of the diluted solution was immediately spread on LB agar medium and incubated at 37 °C for 24 h. The number of viable cells was determined in terms of the formed colony and the diluted folds. For comparison, control experiments were carried out with scavenger alone in bacterial suspension under visible light irradiation to eliminate the toxic effect to the bacteria.

3. Results and discussion 3.1. Crystal structure and surface composition The crystal phase structure, crystallinity and purity of the asprepared samples were examined by XRD measurements. Fig.

1 shows the XRD patterns of Bi2WO6, BiVO4, and Bi2WO6/BiVO4 with different molar ratios. All of samples displayed narrow and sharp diffraction peaks, indicating the high crystallinity of the as-prepared samples. As seen in Fig. 1, all the diffraction peaks of Bi2WO6 can be indexed to the orthorhombic phased Bi2WO6 with lattice constants of a = 5.456 Å, b = 5.436 Å, and c = 16.426 Å, which are in good agreement with the reported data (JCPDS No. 73-1126). In addition, the diffraction peaks of BiVO4 matched very well with the monoclinic structure of BiVO4. The cell constants of BiVO4 were calculated to be a = 5.195 Å, b = 11.701 Å, and c = 5.092 Å, which are consistent with the literature value (JCPDS No. 14-0688). In addition, the addition of surface active agent PVP during the preparation process could suppress the formation of other crystalline phases and lead to the generation of a single monoclinic 59 crystal phase of BiVO4. As for the Bi2WO6/BiVO4 composites, all the characteristic diffraction peaks can be well indexed to monoclinic phase BiVO4 (JCPDS No. 14-0688) and orthorhombic phase Bi2WO6 (JCPDS No. 73-1126), indicating that Bi2 WO6 and BiVO4 composited together successfully. What’s more, with the increasing amount of BiVO4, the relative intensity of corresponding diffraction peaks of BiVO4 strengthened gradually. Besides, there were no significant shifts of the principal diffraction peaks occurring in the Bi2WO6/BiVO4 composites, implying that BiVO4 existed as a separate phase rather than being incorporated into the Bi2WO6 lattice. No peaks of other impurities were observed in the Bi2WO6/BiVO4 composites.

Fig. 1 XRD patterns of Bi2WO6, BiVO4, and Bi2WO6/BiVO4 with different molar ratios. To further investigate the surface compositions and the oxidation states of the Bi2WO6/BiVO4-1 composite, XPS analysis was performed typically, and the results are shown in Fig. 2(A)∼ ∼(E). The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing C 1s to 284.60 eV. As seen in the typical XPS survey spectrum (Fig. 2(A)), only Bi, W, V, O, and C elements were detected on the surface of samples, while the C 1s peak at around 284.6 eV could be attributed to the signal from carbon contained in the 50, 51, 60, 61 instrument and was used for calibration. Fig. 2(B) shows the high-resolution XPS spectrum of Bi 4f. The peaks

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with binding energy at about 159.0 eV and 164.0 eV were assigned to the Bi 4f7/2 and Bi 4f5/2, respectively, confirming that the bismuth species in the Bi2 WO6/BiVO4 composite were 3+ 50, 51, 60, 61 in the form of Bi . Moreover, it can be observed that the Bi 4f7/2 region could be deconvoluted into two peaks at around 158.3 eV and 159.3 eV, indicating that there were two 50, 51, 60, 61 types of Bi ions in the Bi2WO6/BiVO4 composite. Considering that the binding energy of Bi 4f7/2 in pure Bi2WO6 41, 60, 61 exists in the range of 158∼159 eV and that for BiVO4 in 50, 51 the range of 159∼160 eV, the peak at 158.3 eV could be 3+ ascribed to Bi in Bi2WO6, while the peak at 159.3 eV 3+ originated from Bi in BiVO4. Meanwhile, the Bi 4f5/2 region could be deconvoluted into two peaks at around 163.7 eV and 164.9 eV, which also demonstrated the existence of two types 50, 51, 60, 61 of Bi ions in the Bi2WO6/BiVO4 composite. According to the previous reports that the binding energy of Bi 4f7/2 in 41, 60, 61 pure Bi2WO6 exists in the range of 163∼164 eV and that 50, 51 for BiVO4 in the range of 164∼165 eV, the peak at 163.7 eV 3+ could be ascribed to Bi in Bi2WO6, while the peak at 164.9 eV 3+ originated from Bi in BiVO4. However, the binding energy values in the Bi2WO6/BiVO4 composite are different from that of pure Bi2WO6 or pure BiVO4 due to change of local environment and electron density of the elements in the 41, 50, 51, 60, 61 interfacial structure.

Fig. 2 XPS spectra of Bi2WO6/BiVO4-1: survey spectrum (A), high resolution spectra of Bi 4f (B), W 4f (C), V 2p (D), and O1s (E). As shown in Fig. 2(C), the two peaks located at 34.1 eV and 36.2 eV could be assigned to W 4f7/2 and W 4f5/2, 6+ respectively, suggesting that W existed in W oxidation 61 state. V 2p orbital showed the splitting peaks at binding energies of approximately 523.5 and 515.9 eV (Fig. 2(D)), which were assigned to V 2p1/2 orbital and V 2p3/2 orbital, 5+ in the respectively, corresponding to the surface V 50, 51 composite. It is well known that the O1s peak is broad and complicated due to the existence of several types of nonequivalent lattice O atoms. In Fig. 2(E), the O 1s region could be divided into two peaks at about 529.3 and 531.2 eV, which could be attributed to the oxygen species of lattice

2-

2+

oxygen (O ) of Bi2O2 and that of hydroxyl oxygen and adsorbed oxygen on the surface of the composite, 41, 50, 51, 60, 61 respectively. Therefore, the XPS results verified the existence of BiVO4 along with Bi2WO6 in the Bi2WO6/BiVO4 composite, which is in good accordance with the XRD analysis. 3.2. Morphology and microstructure

Fig. 3 FESEM images of the as-prepared photocatalysts: (A) Bi2WO6, (B) BiVO4, (C) Bi2WO6/BiVO4-0.25, (D) Bi2WO6/BiVO40.5, (E) Bi2WO6/BiVO4-0.75, and (F) Bi2WO6/BiVO4-1. The morphology and microstructure of the as-prepared samples were investigated by FESEM. As shown in Fig. 3(A), the pure Bi2WO6 samples were flower-like microspheres with diameters about 1 µm. These flower-like microspheres possessed superstructures assembled hierarchically by many two-dimensional (2D) nanoplates with a thickness of about 20 nm and an average side length of about 100 nm. Fig. 3(B) shows that the shape of pure BiVO4 samples had irregular rodlike structures with the average sizes about 1 µm, and some leaf-like shaped products were also seen. As for the Bi2WO6/BiVO4 composites, a number of uniform and hierarchical three-dimensional (3D) microspheres were observed in Fig. 3(C)∼ ∼(F) ranging from 1 to 4 µm in diameter, which were constructed by various thin 2D nanoplates. However, the morphologies of these composites were different from each other due to the different contents of BiVO4 in the composites. Compared to the loose flower-like structure of pure Bi2WO6, the Bi2WO6/BiVO4-0.25 composites exhibited uniform underdeveloped flower-like structures (looks like pancakes) with the average size of about 2 µm (Fig. 3(C)). It can be seen clearly that each ‘‘pancake’’ was assembled by many nanoplates with a thickness of about 20 nm and an average side length of about 100 nm, and it had a concave with several perpendicular nanoplates at its center (inset in Fig. 3(C)). The imperfect pancake-like structures were commonly observed during the formation of flower-like 59, 62 structures, which could further evolve to the perfect flower-like structures. Thus, with the increase of BiVO4 content in the composite, more nanoplates were accumulated on the pancake-like structure and bigger flower-like structures were constructed tightly by these thin nanoplates, as shown in Fig. 3(D) and Fig. 3(E). Besides, with further increasing the content

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of BiVO4, a more perfect and tighter nest-like structure of Bi2WO6/BiVO4-1 was observed in Fig. 3(F), which was derived from the sequential assembling of the flower-like microsphere by more and more nanoplates. These tiny nanoplates were attached side by side into integrated sheets and aligned to the spherical surface with clearly oriented layers, pointing toward a common center, as seen in the inset of Fig. 3(F). In addition, it can be seen that the size of the composites increased along with the increase of BiVO4 content in the composite, and the average size of the Bi2WO6/BiVO4-1 nest-like structure was about 4 µm. Therefore, based on the FESEM results, it can be deduced that the flower-like microspheres were composed of Bi2WO6 and BiVO4 nanoplates, and Bi2WO6 and BiVO4 grew together with assistance of PVP to form nanoplates and further leaded to the construction of hierarchical nest-like structures accumulated by the well-ordered and oriented nanoplates. These results indicated that the morphology and microstructure of the Bi2 WO6/BiVO4 composites could be facilely adjusted by controlling the molar ratio of the two constituents.

diffraction ring, which confirmed that the Bi2WO6/BiVO4-1 composite was a polycrystalline structure in nature and the nest-like microspheres were consisted of single-crystal 59, 63-66 subunits. Fig. 4(C) shows the HRTEM image of the Bi2WO6/BiVO4-1 composite corresponding to the orange square region displayed in Fig. 4(B), in which the interface of orthorhombic Bi2WO6 and that of monoclinic BiVO4 could be clearly observed. Moreover, the HRTEM image reveals the high crystalline nature of the composite. By measuring the lattice fringes, the interplanar distances at 0.272 nm and 0.292 nm agreed well with the fringe spacing of the (200) lattice plane of 41, 50, 51, 59 Bi2WO6 and the (040) lattice plane of BiVO4, which are in good accordance with the XRD results. Therefore, it is apparent that a well-defined heterojunction structure was formed between Bi2 WO6 and BiVO4. To further investigate the chemical composition and element distribution of the Bi2WO6/BiVO4-1 composite, TEMEDX elemental mapping was examined when operating the TEM in scanning mode (STEM). Fig. 4(D) shows TEM-EDX image of the Bi2WO6/BiVO4-1 composite, from which strong signals of Bi, W, V, and O elements could be clearly observed without exhibiting other elements, confirming the presence of Bi, W, V, and O elements in the composite. In addition, quantitative results gave the atomic ratio of Bi:W:V:O in Bi2WO6/BiVO4-1 was 20:7:6:67, which coincided well with the theoretical molar ratio of 1:1 for Bi2WO6 and BiVO4 in Bi2WO6/BiVO4-1 considering the instrumental error. Hence, the EDX results are in well agreement with the XPS analysis, indicating that Bi2WO6 and BiVO4 composited with the molar ratio 1:1 in the Bi2WO6/BiVO4-1 composite. The STEM mapping images shown in the inset of Fig. 4(D) clearly reveal that the Bi, O, W, and V elements were distributed uniformly and had the same shape and locations in the composite, confirming the certain existence of Bi2 WO6 and BiVO4 in the Bi2WO6/BiVO4 -1 composite. Therefore, these results gave a solid evidence that Bi2WO6 and BiVO4 were successfully composted together and assembled to form a hierarchical nest-like structure. 3.3. Growth mechanism

Fig. 4 TEM images in low magnification (A) and high magnification with the SAED image (inset) (B), the corresponding HRTEM image (C), and the EDX spectrum (D) (inset: EDX elemental mapping images) of Bi2WO6/BiVO4-1. To get more detailed information about the crystalline structure of the Bi2WO6/BiVO4-1 composite, TEM, HRTEM and SAED characterizations were conducted. The lowmagnification TEM image (Fig. 4(A)) presents the Bi2WO6/BiVO4-1 composite a nest-like hierarchical morphology, assembling by thin nanoplates, which is consistent with the FESEM results. Fig. 4(B) is the enlarged TEM image of marked area by an orange rectangle in Fig. 4(A), showing the thin nanoplates around the sides of the microsphere. Moreover, the SAED pattern of this part (inset of Fig. 4(B)) shows the arc-like symmetry spots rather than a

Fig. 5 FESEM images of Bi2WO6/BiVO4-1 obtained at different reaction times: (A) 0 h, (B) 2 h, (C) 6 h, (D) 12 h, (E) 24 h, and (F) 36 h.

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To reveal the formation process of the nest-like Bi2WO6/BiVO41 hierarchical microspheres, products formed at different growth stages were collected and determined by XRD and FESEM. At the initial stage of the reaction, numerous small nanoparticles with irregular sizes and morphologies were formed when the solutions were mixed together, as presented in Fig. 5(A). However, these products exhibited weak peaks according to the XRD patterns (Fig. 6), indicating their poor crystalline and amorphous characteristics. After hydrothermal treatment for 2 h (Fig. 5(B)), these primary nanocrystals tended to aggregate together to form imperfect microspheres with a diameter of about 2 µm, along with many aggregated nanoparticles. Moreover, it can be seen that some nanoparticles gradually transformed to 2D nanoplates. Moreover, the peaks shown in Fig. 6 can be ambiguously designated as the orthorhombic phased Bi2WO6, indicating that Bi2 WO6 would crystallize and precipitate firstly at the initial reaction stage. On the other hand, there was a competition in nucleation and crystallization between Bi2WO6 and BiVO4 in the reaction, slowing down the formation of flower-like Bi2WO6 microspheres compared to the previous 56 reports. Then BiVO4 nanoparticles gradually crystallized and precipitated during the reaction process, and formed 2D nanoplates in a further crystallization process. Subsequently, Bi2WO6 and BiVO4 would grow together. When the reaction time increased to 6 h, some hierarchical pretty flower-like microspheres with a diameter of about 3 µm gradually appeared with the disappearance of the small nanoparticles under the Ostwald ripening process (Fig. 5(C)). It is apparently that these hierarchical flower-like microspheres were assembled by many thin nanoplates, which interconnected with each other and aligned with clearly oriented layers, pointing toward a common center. Thus, concaves were formed on the top of these microspheres. Moreover, some weak peaks exist in Fig. 6, which can be indexed to the monoclinic phase BiVO4, confirming the existence of BiVO4 during the reaction process. Meanwhile, the characteristic peaks of Bi2WO6 became sharp and strong, indicating that the crystal structure of the composite gradually tended to be complete. When further elongate the reaction time to 12 h, the number of layers on the flower-like microspheres gradually increased while the size of concaves at the top of the microspheres decreased, and the nest-like microspheres constructed by lots of more compactly packed nanoplates can be observed in Fig. 5(D). What’s more, the XRD patterns in Fig. 6 show that all the characteristic diffraction peaks can be well indexed to monoclinic phase BiVO4 (JCPDS No. 14-0688) and orthorhombic phase Bi2WO6 (JCPDS No. 73-1126), and the diffraction peaks of BiVO4 became stronger as prolonging the reaction time, confirming the coexistence of BiVO4 and Bi2WO6 at this stage. Finally, the perfect and tight nest-like microspheres with a diameter of about 4 µm were obtained when the reaction is further prolonged to 24 h (Fig. 5(E)), which were also assembled by many thin and tiny nanoplates. In addition, the morphology and XRD pattern of the products obtained at 36 h reveal almost the same to that of 24 h, as

shown in Fig. 5(F) and Fig. 6, proving that further prolonging the reaction time could not affluence the crystallization process of the Bi2WO6/BiVO4-1 composite, leading to no significant change in the morphology and crystal structure.

Fig. 6 XRD patterns of Bi2WO6/BiVO4-1 obtained at different reaction times. Therefore, based on the above analysis, combined with the crystal structure and chemical composition results, the growth mechanism of Bi2 WO6/BiVO4-1 nest-like hierarchical microspheres was proposed, as is presented in Scheme 1. It was a typical Ostwald ripening process, and the crystal anisotropy and the face-inhibitor function of PVP played key roles in the formation of nest-like hierarchical microspheres via selective adsorption on various crystallographic planes to 59, 66 partially passivate the surfaces of crystal. Initially, when the solution containing Bi(NO3)3⋅5H2O and HNO3 was mixed with the solution containing Na2WO4⋅2H2O, NH4VO3 and PVP, many irregular nanoparticles appeared, which would further serve as seeds for anisotropic growth in the subsequent 59, 62-66 process via Ostwald ripening mechanism. Then, the polymer molecule PVP would be absorbed on these nanoparticles. Driven by the minimization of the total energy of the system and van der Waals interactions between the 59, 64 polymer molecules, these small primary nanocrystals would aggregate into spherical structure with many PVP molecules absorbing on its surface after hydrothermal treatment for 2 h, which would provide many high-energy sites for nanocrystalline growth. However, there was a competition in nucleation and crystallization between Bi2WO6 and BiVO4, which resulted in the different formation order. According to the XRD analysis shown in Fig. 6, Bi2WO6 crystallized and precipitated firstly at the initial reaction stage. Then, the crystallization of Bi2WO6 began from these nucleation sites on the surface anchoring with PVP spontaneously. And the microspheres gradually grew inside the unstructured polymer aggregates and subsequently dissolved the nanocrystal from the inner side of the aggregates toward the outside. Meanwhile, these nanoparticles tended to grow into 2D nanoplates due to the Ostwald ripening process and the high 59, 62-66 intrinsic anisotropic properties of Bi2WO6. The further

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crystal growth of Bi2WO6 was strongly related to the intrinsic crystal structure of Bi2WO6. Orthorhombic Bi2WO6 is constructed by a corner-shared WO6 octahedral layer and 2+ [Bi2O2] atom layers sandwiched between WO6 octahedral 66 layers, which is parallel to the (001) facets. Thus the (200) and (020) faces have a much higher chemical potential than other facets, leading to a faster growth of the (200) and (020) faces along the layer. Then, the crystallization of 2D Bi2WO6 nanoplates began from these nucleation sites on the surface anchoring with PVP spontaneously. The free polymer molecules PVP would preferentially be absorbed on the primary nanoplates and functioned as potential crystal face inhibitors, which formed polymer interlayers on the nanoplates and benefited the formation of oriented 59, 66 nucleation. On the other hand, BiVO4 gradually crystallized and precipitated during the reaction process, and then also formed 2D nanoplates in a further crystallization process due to its high anisotropy characteristics. As a result, Bi2WO6 and BiVO4 would then grow together with the assistance of Ostwald ripening process and PVP.

Then, the number of layers on the flower-like microspheres gradually increased, further leading to the thickening of the microspheres. Finally, the flower-like microspheres further evolved to the perfect and tight nest-like microspheres with a smaller concave, which were constructed by many wellordered and oriented nanoplates. Therefore, the Bi2WO6/BiVO4-1 nest-like hierarchical microspheres were obtained by self-assembly of the in situ formed nanoplates with the assistance of PVP. 3.4. BET analysis The BET specific surface area is considered as an important influencing factor of the photocatalytic performance of photocatalyst, which can be measured by nitrogen adsorption method. Hence, the BET specific surface areas were 2 2 2 2 determined as 12.83 m /g, 7.51 m /g, 14.94 m /g, 17.98 m /g, 2 2 22.13 m /g, and 26.53 m /g for Bi2WO6, BiVO4, Bi2WO6/BiVO40.25, Bi2 WO6/BiVO4-0.5, Bi2WO6/BiVO4-0.75, and Bi2WO6/BiVO4-1, respectively. Clearly, the Bi2WO6/BiVO4 composites exhibited much higher BET specific surface areas than pure Bi2WO6 and BiVO4. In addition, for the Bi2WO6/BiVO4 composites, the BET specific surface areas increase as the contents of BiVO4 increase, and the Bi2WO6/BiVO4-1 composite with the nest-like hierarchical structure displayed the highest the BET specific surface area among these photocatalysts. These results coincide well with the morphologies observed by the FESEM images. As is known, a larger BET specific surface area will provide more active sites and better absorption property for the photocatalyst, promoting the electron-hole separation efficiency and further enhancing the photocatalytic 31, 32, 67 activity. Therefore, it is reasonable to expect that the asprepared the Bi2WO6/BiVO4-1 nest-like hierarchical microspheres will show excellent photocatalytic activities in photocatalysis. 3.5. Optical absorption property analysis

Scheme 1 Growth mechanism of Bi2WO6/BiVO4-1 nest-like hierarchical microspheres. With further extending the reaction time, the spherical aggregates further dissolved and recrystallized, and these tiny nanoplates were gradually self-assembled in an edge-to-edge way by an oriented attachment process, during which the asformed nanoplates gradually bended outward due to the 59, 62-66 Thus, the shape growth of new nanoplates in the spaces. of the composite gradually evolved to 3D hierarchical flowerlike microspheres with a concave on the top. With elongating the reaction time sequentially, more new tilted nanoplates would generate on the notched flower-like microspheres.

Fig. 7 UV-DRS of the as-prepared photocatalysts (A) and Plots 2 of (αhν) versus hν for the Eg of the as-prepared photocatalysts (B). The photocatalytic activity of a semiconductor is mainly determined by the photoabsorption property and the diffusion of the photo-induced electrons and holes, which are relevant to the energy band structure of the semiconductor. Thus, UVDRS were used to illustrate the photoabsorption property of the samples, as can be seen in Fig. 7(A). According to the spectra, both Bi2WO6 and BiVO4 had a good optical absorption

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in visible light, and all of the Bi2WO6/BiVO4 composites showed a strong absorption from the UV light to visible light region, implying the possibility of photocatalytic activity over these materials under visible light irradiation. In addition, the absorption edges of the different Bi2WO6/BiVO4 samples were gradually red-shifted with the increase of BiVO4 content due to the strong visible light response of BiVO4, and Bi2WO6/BiVO4-1 presented the best visible-light absorption property. These results further confirmed the existence of BiVO4 in the Bi2WO6/BiVO4 composite, and the introduction of BiVO4 in Bi2WO6 greatly improved the visible-light absorption property of the Bi2 WO6/BiVO4 composite. Besides, for a crystalline semiconductor, the band gap energy can be calculated based n/2 31, 32 on the spectra by the equation αhν = A(hν − Eg) , where α, h, ν, Eg, and A are the absorption coefficient, the Planck constant, the light frequency, the band gap, and a constant, respectively. Among them, the value of n depends on the type of optical transition of a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). As a direct transition semiconductor, the n value of both Bi2WO6 and 31, 32 2 From the plot of (αhν) versus hν in Fig. 7(B), BiVO4 is 1. the Eg of Bi2WO6, BiVO4, Bi2WO6/BiVO4-0.25, Bi2WO6/BiVO40.5, Bi2WO6/BiVO4-0.75, and Bi2WO6/BiVO4-1 were calculated to be 2.70 eV, 2.30 eV, 2.18 eV, 2.14 eV, 2.12 eV, and 2.07 eV, respectively. Thus the results indicated that the Bi2WO6/BiVO41 composite had a wider photoabsorption region and more suitable band gap, making it a potential candidate as visiblelight-driven photocatalyst. 3.6. Photocatalytic antifouling performance

6

Fig. 8 (A) Temporal course of P. aeruginosa (8.5 × 10 cfu/mL) survival curve in aqueous dispersions containing 1 mg/mL photocatalysts under visible light irradiation and (B) Photocatalytic antibacterial rate of different photocatalysts in dark and irradiated by visible light for 30 min. The photocatalytic antifouling performances of the asprepared samples were evaluated by the antibacterial activities for marine fouling microorganism P. aeruginosa (8.5 6 × 10 cfu/mL) under visible light irradiation. As seen in Fig. 8(A), the blank control experiment (without photocatalysts under visible light irradiation) shows little loss of P. aeruginosa, which could be almost ignored. And the dark control test (with photocatalyst in dark) also displayed a neglectable effect on P. aeruginosa, indicating the non-toxicity of the photocatalysts to P. aeruginosa. However, when

irradiated by visible light, the as-prepared samples exhibited high photocatalytic antifouling activities. It can be seen in Fig. 8(A) that the Bi2WO6/BiVO4 composites revealed obviously enhanced photocatalytic antifouling activities in comparison with pure Bi2WO6 and pure BiVO4 in the temporal course of P. aeruginosa survival curve. In addition, with the increase of the BiVO4 molar ratio in the composite, the photocatalytic antifouling properties of the Bi2WO6/BiVO4 composites improved apparently, indicating that the introduction of BiVO4 into Bi2WO6 significantly enhanced the photocatalytic activities of the composite. Among the as-prepared composites, Bi2WO6/BiVO4-1 exhibited the best photocatalytic antifouling activity and only about 2.1 log of P. aeruginosa survived after 30 min reaction compared with about 6.9 log of P. aeruginosa survived before the reaction. What’s more, almost all of the bacterial cells were killed as the reaction time was extended to 60 min. In addition, Fig. 8(B) displays that the antibacterial rate of Bi2WO6/BiVO4-1 could achieve 99.99% after 30 min reaction, far exceeding those of pure Bi2WO6, pure BiVO4 and other Bi2WO6/BiVO4 composites. Furthermore, compared to other reported antifouling photocatalysts (the detail antibacterial activities are shown in Table S1 in the Supplementary 23 25 46 68 Information), such as TiO2, Ag@TiO2, BiVO4, Bi2WO6, 69 70 71 Ag2S/Bi2S3, Bi2O2CO3/Bi3NbO7, and AgBr-Ag-Bi2WO6, the Bi2WO6/BiVO4-1 composite photocatalyst in this study also showed a superior photocatalytic antifouling activity. Furthermore, the fluorescence microscopy images of P. aeruginosa during the photocatalytic process for the Bi2WO6/BiVO4-1 composite further demonstrated the excellent photocatalytic antifouling performance of the Bi2WO6/BiVO4-1 composite under visible light irradiation (Fig. S1). Therefore, the above results revealed the superior and enhanced photocatalytic antifouling performance of the Bi2WO6/BiVO4-1 composite, which could be mainly attributed to the formation of heterostructure between Bi2WO6 and BiVO4, accelerating the separation of photoinduced electrons 2 and holes. Furthermore, a large BET surface area (26.53 m /g) due to the more perfect structure that brought about more active sites, combined with the wide photoabsorption region, further gave rise to the excellent photocatalytic performance of Bi2WO6/BiVO4-1, making it a potential marine antifouling material in practical application. 3.7. Destruction process of bacterial cell To investigate the destruction progress of bacteria cells visually in the Bi2WO6/BiVO4-1 photocatalytic system under visible light irradiation, the morphology and structure changes of P. aeruginosa at different stages during the photocatalytic reaction were observed by FESEM and TEM. Fig. 9(A) shows a representative FESEM image of untreated P. aeruginosa cells, which had a regular and intact cellular structure with a welldefined cell wall, indicating that the cells were living without any environmental disturbance. After P. aeruginosa cells were mixed with the photocatalyst in dark for 30 min, it can be seen in Fig. 9(B) that the bacterial cells without obvious damage were adhered to the surface of the photocatalysts owing to their large sizes and specific surface areas. The close contact between bacterial cells and the interface of Bi2WO6/BiVO4-1

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3.8. Stability and reusability

Fig. 10 Recycling photocatalytic antibacterial experiments for 6 P. aeruginosa (8.5 × 10 cfu/mL) (A); XRD pattern (B), EDX spectrum (C), and FESEM image (D) of Bi2WO6/BiVO4-1 after six recycling experiments under visible light irradiation.

Fig. 9 FESEM images of untreated P. aeruginosa cells (A) and untreated P. aeruginosa cells in dark with Bi2 WO6/BiVO4-1 for 30 min (B), treated P. aeruginosa cells under visible light irradiation with Bi2 WO6/BiVO4-1 for 15 min (C) and 30 min (D). Therefore, these results indicated that the destruction process of bacterial cell was beginning from the cell wall to cellular components, which could be mainly attributed to the + attack of the oxidative active species (⋅OH, ⋅O2 , and h ) 69-75 generated during the photocatalytic process. During the photocatalytic reaction, the oxidative active species would firstly attack the cell wall and cytoplasmic membrane of bacterial cell due to the close absorption of bacterial cell on the surface of Bi2WO6/BiVO4-1. After the cell wall was ruptured, a large amount of diffusing oxidative active species would enter the cell to further degrade the left intracellular 69-75 components of the cell, and decompose the cell completely. Therefore, these results proved the advantages including efficient, environmentally friendly, and nonsecondary pollution of photocatalysis technique.

To evaluate the stability and reusability of the photocatalyst, the repeating photocatalytic antifouling experiments for P. 6 aeruginosa (8.5 × 10 cfu/mL) were conducted with recycled use of Bi2WO6/BiVO4 -1 under visible light irradiation. After each cycle, the photocatalysts were collected by centrifuging, washed with Milli-Q water several times, dried, and reused in the next run. It can be seen in Fig. 10(A) that the antibacterial rate did not exhibit significant loss and could reach 99.21% after six successive cycles with each reaction lasting for 30 min, indicating that the Bi2WO6/BiVO4-1 composite was relatively effective and stable. Though the antibacterial rate reduces from 99.99% to 99.21% after six successive reactions, the antibacterial rate is still higher than 99%, exhibiting a good stability and reusability considering the loss of photocatalysts during the recycle process and impurities absorption on the photocatalysts. The crystal structure of Bi2WO6/BiVO4-1 after six circulating runs was analyzed by XRD, as shown in Fig. 10(B). The XRD pattern of Bi2WO6/BiVO4-1 after six successive photocatalytic reactions revealed that the crystal structure and peaks’ intensity showed no obvious change after cycled use. In addition, the chemical composition of Bi2WO6/BiVO4-1 after six cycles was also characterized by EDX. It can be seen in Fig. 10(C) that there was no significant change in the EDX spectra of Bi2 WO6/BiVO4-1 after used six times. The composite showed strong signals of Bi, W, V, and O elements with the atomic ratio of 17:6:5:69 for Bi:W:V:O, which was close to the ideal value of 1:1 for Bi2WO6/BiVO4 considering instrumental error, indicating the good stability of Bi2WO6/BiVO4-1. As seen Fig. 10(D), there was no apparent change in the morphology of Bi2WO6/BiVO4-1 after six circulating photocatalytic reactions, indicating that the hierarchical nest-like microspheres were

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would be beneficial to the antibacterial process. When irradiated under visible light for 15 min, great changes in morphology and structure of the P. aeruginosa cell are observed in Fig. 9(C). Flagella existed in some cells compared to the untreated cells due to the stress response, and some cells were subject to mass-missing on the cell wall and the cell membrane, which would further develop as deep ‘holes’. These results verified that P. aeruginosa cells were gradually destroyed during the photocatalytic reaction. The formation of flagella and holes in bacteria is in good agreement with some 72-75 previous reports. With further extension of the reaction time up to 30 min, the cell structure shows much severer damage in Fig. 9(D). The cell wall and the cell membrane were greatly ruptured with an even more severe leakage of the + intracellular components such as K , indicating that the bacterial cells were completely killed by the Bi2WO6/BiVO4-1 composite. The destruction progress of P. aeruginosa cells was also observed TEM, which are consistent with the FESEM results, as shown in Fig. S2.

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stable enough that they could be hardly decomposed to disperse nanoplates after long time’s use. Therefore, these results indicated that the Bi2WO6/BiVO4 -1 composite exhibited an excellent stability, reusability, and less photocorrosion, which favored a long-term use in photocatalytic antifouling in marine environment. 3.9. Broad-spectrum antibacterial performance for E. coli and S. aureus

Fig. 11 Photocatalytic antibacterial rate of different 6 photocatalysts for E. coli (3.0 × 10 cfu/mL) and S. aureus (3.0 × 6 10 cfu/mL) in dark and irradiated by visible light for 30 min (A), TEM images of E. coli untreated (B) and treated for 30 min (C), and TEM images of S. aureus untreated (D) and treated for 30 min (E) with Bi2WO6/BiVO4-1 under visible light irradiation. In order to assess the broad-spectrum antibacterial performance of Bi2WO6/BiVO4-1, the photocatalytic antibacterial experiments were carried out against the Gram6 negative bacteria E. coli (3.0 × 10 cfu/mL) and the Gram6 positive bacteria S. aureus (3.0 × 10 cfu/mL) using 1 mg/mL Bi2WO6/BiVO4-1. As observed in Fig. 11(A), Bi2WO6/BiVO4-1 exhibited a superior antibacterial activity compared to pure Bi2WO6 and pure BiVO4. The antibacterial rate for Bi2WO6/BiVO4-1 could achieve 99.99% after 30 min reaction in both E. coli and S. aureus reaction system. What’s more, the blank control experiment (without photocatalysts under visible light irradiation) and the dark control test (with photocatalyst in dark) showed little effect on E. coli and S. aureus, which could be almost ignored. Furthermore, the morphology of E. coli and S. aureus before and after the photocatalytic reaction was investigated, as presented in Fig. 11(B)∼ ∼Fig. 11(E). It can be seen that both of the E. coli and S. aureus cells displayed intact cellular structures with well-defined cell walls and good living conditions before the photocatalytic reaction. When irradiated under visible light in the presence of Bi2WO6/BiVO41, the cell walls of both E. coli and S. aureus were destructed after 30 min reaction, leading to the leakage of the intracellular components and further death of the cells. All the observations indicated that the rupture of cell walls could be ascribed to the damage of photo-induced oxidative active species in the photocatalytic process, followed by the further degradation of intracellular constituents and complete decomposition of the bacterial cells finally. Therefore, the results confirmed the highly efficient and broad-spectrum antibacterial performance of the Bi2WO6/BiVO4-1 composite

under visible light irradiation, making it a potential application in not only marine antifouling but also other water purification areas. 3.10. Photocatalytic antifouling mechanism + As is known, various active species (⋅OH, ⋅O2 , and h ) generated during the photocatalytic process played key roles in killing the bacteria. Therefore, to ascertain the dominant active species in the photocatalytic antifouling process, different scavengers were added into the reaction solutions in the presence of Bi2WO6/BiVO4-1 under visible light irradiation by using IPA as ⋅OH scavengers, TEMPOL as ⋅O2 scavengers, + 58 and sodium oxalate as h scavengers, respectively. It can be observed in Fig. 12 that the antibacterial rate for P. aeruginosa could achieve 99.99% after 30 min reaction by Bi2WO6/BiVO4-1 without the addition of a scavenger. In addition, there was no obvious effect on the antibacterial rate with the addition of TEMPOL (a scavenger of ⋅O2 ), indicating that ⋅O2 was not the major active species involved in the photocatalytic antifouling 57 process. On the other hand, when IPA (a scavenger of ⋅OH) + and sodium oxalate (a scavenger of h ) were added, the antibacterial rate for P. aeruginosa decreased apparently both in the two systems, and especially in the solution with sodium + oxalate. Thus, ⋅OH radicals and h were proved to be the dominant active species for Bi2WO6/BiVO4-1, playing key roles in the photocatalytic antifouling process. And holes are the major contributing for generation of active radicals. What’s more, the control experiments showed that the addition of the scavengers did not result in any toxic effect to P. aeruginosa within 30 min, which could be almost ignored.

Fig. 12 Effect of scavengers on photocatalytic antibacterial rate of Bi2WO6/BiVO4-1 for P. aeruginosa under visible light irradiation. In order to further investigate the enhanced photocatalytic antifouling activity of Bi2WO6/BiVO4 heterojunction, the relative band positions of Bi2WO6 and BiVO4 should be confirmed, since the band potential positions played an important part in determining the migration direction of the photogenerated charge carriers in the 41-44, 48-57 heterojunction structure. The conduction band (CB) edge potential of semiconductor at the point of zero charge

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31, 32

C

can be calculated by the empirical equation: ECB = χ − E − 0.5Eg, where χ is the absolute electronegativity of the semiconductor, which is the geometric mean of the absolute electronegativity of the constituent atoms (6.360 eV for 32 31 C Bi2WO6 and 6.035 eV for BiVO4, respectively), E is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap energy of the semiconductor (Eg is 2.70 and 2.30 eV/NHE for Bi2WO6 and BiVO4, respectively). Besides, the valance band (VB) edge potential can be determined by the equation: EVB = Eg + ECB. Hence, according to the equations above, the calculated ECB and EVB of Bi2WO6 are 0.51 eV and 3.21 eV/NHE, respectively. And the calculated ECB and EVB are 0.385 eV and 2.685 eV/NHE for BiVO4, respectively. Thus a difference of band potentials existed between the two semiconductors. When Bi2WO6 and BiVO4 were in contact, the heterojunction structure was established, and then an inner 41-44, 48-57 electric field was built in the interface leading to the efficient separation of photoinduced electrons and holes between Bi2WO6 and BiVO4.

Scheme 2 Photocatalytic antifouling mechanism of the Bi2WO6/BiVO4-1 composite. Hence, on the basis of the calculated energy bands and above results, a possible photocatalytic antifouling mechanism of the Bi2WO6/BiVO4-1 composite was proposed, as shown in Scheme 2. When the composite was illuminated with visible light, Bi2 WO6 and BiVO4 were excited simultaneously and the electrons (e ) in the VB were excited to the CB, leaving the + same amount of holes (h ) on VB in both Bi2WO6 and BiVO4. Meanwhile, the photoinduced electrons in BiVO4 would be easily injected from the CB of BiVO4 to the CB of Bi2WO6 due to the more negative CB of BiVO4 than that of Bi2 WO6. On the

other hand, because of the lower VB of Bi2WO6 compared to that of BiVO4, the holes on the VB of Bi2WO6 will transfer to that of BiVO4. Therefore, an efficient separation of photogenerated electrons and holes accrued on the n-n heterojunction, which could reduce the recombination of the 41-44, 48-57 photogenerated charge carriers greatly. Consequently, more separated photogenerated electrons and holes were then free to initiate reactions with the reactants adsorbed on the surface of the photocatalyst, leading to an enhanced photocatalytic performance compared to pure Bi2WO6 and pure BiVO4. These results are consistent with some previous studies on the transition of photo-induced charge carriers 41-44, 48-57 between semiconductors. In addition, because the ECB value of Bi2WO6 (0.51 eV/NHE) was more positive than the redox potential of O2/⋅O2 44, 50, 58 (-0.046 eV/NHE), the electrons located on the CB of Bi2WO6 could not reduce O2 to yield ⋅O2 . However, the twoelectron reduction of O2 to form H2O2 could occur on Bi2WO6 owing to that ECB value of Bi2WO6 (0.51 eV/NHE) was more negative compared to the redox potential of O2/H2O2 (0.70 76 eV/NHE) and H2O2/⋅OH (0.71 eV/NHE). Hence, the electrons located on the CB of Bi2WO6 could react with O2 to form H2O2 and form ⋅OH at last. These ⋅OH radicals would further attack the bacterial cells and lead to the destruction of the cells. At the same time, the EVB value of BiVO4 (2.685 eV/NHE) was much more positive than the standard redox potentials of 44, 50, 58 ⋅OH/OH (1.99 eV/NHE) and OH /⋅OH (2.38 eV/NHE), exhibiting a strong oxidative ability. Thus, the photo-induced holes could oxidize H2O or OH to form ⋅OH radicals, which would further attack the bacterial cells and then lead to the destruction of the cells. Moreover, the photo-induced holes could oxidize the bacterial cells directly. Hence, based on the active species trapping experiments and the analysis of redox potentials, it can be deduced that holes are the major contributing for generation of active radicals. As shown in Scheme 2, when the bacterial cells were in contact with Bi2WO6/BiVO4-1, lots of cells were absorbed on the surface of the composite due to its large BET specific surface area. Then the photoinduced holes combined with ⋅OH radicals would attack the cell wall and cytoplasmic membrane directly, leading to the severe rupture of cell wall and release of the interior component, which would also facilitate the entry of ⋅OH radicals to further damage the interior DNA molecules, 70-75 degrade the left intracellular components, and then decompose the cell completely. Therefore, the photoinduced holes and ⋅OH radicals were demonstrated to play major roles in the photocatalytic antibacterial process for the Bi2WO6/BiVO4-1 composite. The efficient separation of photoinduced electrons and holes due to the formation of heterojunction, combined with the large BET specific surface area and wide photoabsorption region, leaded to the excellent and enhanced photocatalytic antifouling performance of the Bi2WO6/BiVO4-1 composite.

Conclusions

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In summary, a novel Bi2WO6/BiVO4 composite photocatalyst was successfully synthesized by a facile hydrothermal method. The growth mechanism of the Bi2 WO6/BiVO4 composite was studied by XRD and FESEM, revealing that the nest-like hierarchical microspheres were constructed by the selfassembly of nanoplates with the assistance of PVP under the anisotropic growth and Ostwald ripening process. The different contents of BiVO4 in Bi2WO6/BiVO4 composite would result in different morphologies, surface properties and even the photocatalytic activities. Photocatalytic antifouling experiments showed that the Bi2WO6/BiVO4 composites exhibited enhanced photocatalytic activities compared to pure Bi2WO6 and BiVO4. Especially, the Bi2WO6/BiVO4-1 composite showed the best photocatalytic antifouling performance, and almost all (99.99%) of P. aeruginosa, E. coli and S. aureus could be killed within 30 min. Moreover, the Bi2WO6/BiVO4-1 composite exhibited excellent stability and long-time reusability. The antibacterial process were observed by FESEM and TEM, indicating that the death of bacteria were mainly ascribed to the attack of photoinduced radicals to the cell wall and cytoplasmic membrane, leading to the rupture of cell wall. Based on the active species trapping experiments, a possible photocatalytic antifouling mechanism was proposed, and ⋅OH + radicals and h were proved to play key roles in the photocatalytic reaction. The obviously enhanced photocatalytic activity of the Bi2WO6/BiVO4-1 composite could be mainly attributed to the effective separation of electronhole pairs at the interface of the heterojunction, leading to the low recombination rate of photo-induced charge carriers. Moreover, a large BET specific surface area and wide photoabsorption property also favored the excellent photocatalytic antifouling performance. More importantly, this study not only provides a new insight into developing novel and broad-spectrum antibacterial materials, but also introduces a highly efficient visible-light-driven photocatalyst for marine antifouling and water purification.

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Acknowledgements

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This work was supported by National Natural Science Foundation of China (Grant No. 41476068 and 51131008) and National Key Basic Research Program of China (Grant No. 2014CB643304).

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Notes and references

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Novel visible-light-sensitive Bi2WO6/BiVO4 nest-like hierarchical microsphere was

Dalton Transactions Accepted Manuscript

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controllably synthesized for photocatalytic antifouling

BiVO4 composite with enhanced photocatalytic antifouling performance under visible light irradiation.

In this study, a novel visible-light-sensitive Bi2WO6/BiVO4 composite photocatalyst was controllably synthesized through a facile one-pot hydrothermal...
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