Bi2WO6

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Energy, Environmental, and Catalysis Applications

Facile Hydrothermal Synthesis of Z-scheme Bi2Fe4O9/Bi2WO6 Heterojunction Photocatalyst with Enhanced Visible-Light Photocatalytic Activity Bisheng Li, Cui Lai, Guangming Zeng, Lei Qin, Huan Yi, Danlian Huang, Chengyun Zhou, Xigui Liu, Min Cheng, Piao Xu, Chen Zhang, Fanglong Huang, and Shiyu Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06128 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

Facile Hydrothermal Synthesis of Z-scheme Bi2Fe4O9/Bi2WO6 Heterojunction Photocatalyst with Enhanced Visible-Light Photocatalytic Activity Bisheng Li

a, b

, Cui Lai a, b, ∗, Guangming Zeng

a, b, ∗

, Lei Qin

a, b

, Huan Yi

a, b

, Danlian

Huang a, b, Chengyun Zhou a, b , Xigui Liu a, b, Min Cheng a, b, Piao Xu a, b, Chen Zhang a, b

a

, Fanglong Huang a, b, Shiyu Liu a, b

College of Environmental Science and Engineering, Hunan University, Changsha

410082, Hunan, PR China b

Key Laboratory of Environmental Biology and Pollution Control (Hunan University),

Ministry of Education, Changsha 410082, Hunan, PR China

∗ Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha, Hunan 410082, China. Tel.: +86–731– 88822754; fax: +86–731–88823701. E-mail address: [email protected] (C.Lai) and [email protected] (G.M. Zeng)

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ABSTRACT: An efficient binary Bi2Fe4O9/Bi2WO6 Z-scheme heterojunction was fabricated through facile hydrothermal route. The obtained Bi2Fe4O9/Bi2WO6 displays high catalytic activity for Rhodamine B (RhB) photodegradation, and 100% of RhB was photodegraded by Bi2Fe4O9 (7%)/Bi2WO6 within 90 min, which is much better than pure Bi2Fe4O9 and Bi2WO6. The effective photoinduced carriers separation, the broadened photoabsorption range, high oxidation capacity of hole and the high reduction power of electron are charge of the elevated catalytic activity because of the formed Z-scheme system. And the effects such as pollutant concentration, pH, inorganic anions and water sources exert on photocatalytic performance were also investigated, and the results suggest that Bi2Fe4O9/Bi2WO6 still possesses high photocatalytic performance. The free radical trapping experiments and electron spin resonance spin-trapping technology disclose that hole (h+), hydroxy radical (•OH) and superoxide radical (•O2-) are cardinal active radicals in catalytic system. In terms of above experimental analysis, a possible photodegradation mechanism of as-fabricated photocatalyst is thoroughly elucidated. And the possible RhB photodegradation pathway

is

also

raised

in

the

light

of

the

analysis

of

Liquid

Chromatography-Mass/Mass Spectrometry. In addition, Bi2Fe4O9/Bi2WO6 composite does not display dramatic reduction of catalytic performance after five recycles. Thus, this study reveals that as-obtained Bi2Fe4O9/Bi2WO6 catalyst has a great prospect for the environmental purification. KEYWORDS: Heterojunction, Bi2Fe4O9/Bi2WO6, Photocatalysis, Mechanism, Pathway, Environmental purification 2 ACS Paragon Plus Environment

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1. INTRODUCTION With the rapid economic growth, environmental contamination problems and energy shortages prove to be two major problems that contemporary society cannot help but to face, and these problems have seriously intimidated the survival of aquatic and terrestrial organisms.1-3 Environmentally benign fabrication of advanced materials and environment-friendly techniques to overcome these two problems have grabbed massive researchers’ attention.4-6 Among methods we know, semiconductor-based photocatalytic oxidation technique is regarded as a promising technique toward energy production and environment purification.7-8 Titanium dioxide (TiO2) is recognized as widely used catalyst in energy production and environment remediation owing to its great chemistry stability, excellent thermal stability, strong oxidation capacity, low cost and non-toxicity.9 But owing to its large energy gaps, TiO2 only responses to ultraviolet light (UV), that merely accounting for approximate 5% in sunlight, and thus its practical applications are limited in some extent.10-11 Therefore, exploring visible-light-driven photocatalysts is the top priority. Among diverse developed semiconductor photocatalysts, bismuth based photocatalyst have obtained gigantic focuses thanks to its high stability, nontoxicity, extraordinary

band

structures12

and

excellent

performance

for

pollutant

photodegradation.13 Moreover, they have wide solar energy response because their valence band was constituted by Bi (6s) and O (2p) orbitals.14 Among them, bismuth tungsten oxide (Bi2WO6) (BWO) is the typical and simplest Aurivillius oxides with extraordinary layered structure, which possess excellent physical and chemical 3 ACS Paragon Plus Environment


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properties.15 Besides, it possesses strong oxidation capacity due to the high valence band of BWO (the VB position at 3.26 eV in this manuscript). Because of these advantages, Bi2WO6 was chosen as the model photocatalyst and was applied to many fields such as pollutants degradation, O2 generation from water splitting.16 However, the visible photoabsorption spectrum range of bare BWO is shorter than 450 nm, it occupy a tiny fraction of sunlight.17 Moreover, the recombination of photoiexcited carriers in the reaction process will further limit its photocatalytic performance.18 Therefore, more work needs to be done to further promote separation of photogenerated carriers and broaden its photoabsorption range to make it satisfy the demand of environment application. Constructing heterojunction photocatalyst was regard as a more effective way to solve these shortcomings.18-24 For example, several heterojunction

like

BiOI@Bi12O17Cl2,25

BiOBr-BiOI,26

g-C3N4/AgBr,27

Ag2O/Ag2CO328 and Bi2O3/Bi2O2CO329 exhibited the high catalytic activity. Now, the question arises how to choose a matched semiconductor to construct heterojunction with BWO. In the past few years, Bi2Fe4O9 (BFO) has received extensive attention due to its dramatically magnetic,30 electronic, dielectric,31 non-toxicity and chemical stability.32 In addition, BFO has proved to be a promising photocatalyst for organic pollutants degradation. However, the low quantum yield and fast photoinduced carrier recombination also restrict its practical application.33 According to previous articles, BFO is reported as p-type photocatalyst,34 and BWO is known as n-type photocatalyst. Band structure of these two semiconductors is well matched, which indicates that they 4 ACS Paragon Plus Environment

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can be constructed into a heterojunction in theory. If heterojunction between BFO and BWO is constructed successfully, the photoexcited carriers are efficient split and absorption spectrum will further expand, the drawbacks of them are conquered perfectly, thereby the photocatalytic performance will enhance. And the novel Bi2Fe4O9/Bi2WO6 (BFWO) hybrid material will be taken as a promising photocatalyst to be applied to the environment purification. To our knowledge, there are no articles have reported about the fabrication of BFWO and investigating its catalytic properties under various conditions. In this manuscript, a novel BFWO heterogeneous photocatalyst was first fabricated via the facile and green hydrothermal route. Enhancing its photocatalytic activity was main target. The chemical constitution, structure, morphology and optical property were studied via different characterizations like X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectrometer (XPS), ultraviolet–visible diffuse reflection spectroscopy (UV-vis DRS). The catalytic performance was assessed by degrading RhB under illumination (>420 nm). Meanwhile, chemical stability was also assessed by cycle experiment. Furthermore, the possible mechanism of RhB photodegradation in the reaction process was proposed. This study provides a novel photocatalyst for environment remediation in practical application and a green technology to prepare different heterogeneous materials. 2. EXPERIMENTAL SECTION 5 ACS Paragon Plus Environment


2.1. Chemical reagents. Potassium hydroxide (KOH), Bismuth nitrate (Bi(NO3)3·5H2O),

Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na),

Nitric acid (HNO3), Benzoquinone (BQ) (C6H4O2) and tert-Butanol (TB) (C4H10O) bought from Sinopharm Chemical Reagent Co., Ltd. Sodium tungstate dehydrate (Na2WO4·2H2O) and Sodium hydroxide (NaOH) obtained from Jiangtian Chemical Technology Co., Ltd. Ferric nitrate (Fe(NO3)3·9H2O) acquired from Xilong Scientific Co., Ltd. The chemical reagents have not been further purified and ultrapure water is employed to prepare various solutions. 2.2 Fabrication of catalyst. 2.2.1. Fabrication of BFO. Typically, 4.8507 g of Bi(NO3)3 and 4.0400 g of Fe(NO3)3 were added into 0.015 L of solution comprising by 0.013 L of ultrapure water and 0.002 L of nitric acid. After being stirred for half an hour with high speed by the magnetic stirrer, 0.075 L of 8.0 M KOH solution was appended to forementioned solution. Immediately formed suspensions were poured into 100 mL autoclave and then heating at 200 °C for one day in an oven. When autoclave cooled to the room temperature, the solids were separated via air pump filtration, purging with ultrapure water and absolute alcohol for five times and drying at 60 °C for one day. 2.2.2. Fabrication of BWO and BFWO. BFWO photocatalyst was fabricated through the modified hydrothermal route. 0.9701 g of Bi(NO3)3 was appended to 0.02 L of 0.2 mol L-1 HNO3, a small quantity of BFO was appended to the above solution. 0.3297 g of Na2WO4 was dispersed in 0.02 L ultrapure water. Na2WO4 solution was added into forementioned solution and HNO3 and NaOH (0.1 mol L-1) was used to 6 ACS Paragon Plus Environment

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regulated pH of mixed solution to 4. After being stir for half an hour, the suspensions were transferred into 100 mL autoclave and reacted at 140 °C for one day. Production was obtained through filtration, washing five times with ultrapure water and absolute alcohol. The resulting production dried at 60 °C overnight. At last, the production with various mass ratio of BFO to BWO (2%, 5%, 7%, 10%, 20%) was obtained, which was recorded as BFWO-2, BFWO-5, BFWO-7, BFWO-10 and BFWO-20. For comparison, pure BWO was synthesized by the above method without adding BFO. 2.3. Characterization. Crystalline structures of various photocatalysts were assessed via XRD (Rigaku Rotaflex, Japan). Morphology and structure of the fabricated powders was exhibited through SEM (Quanta-200, Holland) and TEM (JEM-2010, JEOL, Japan) was employed to detect microstructures and crystal lattice parameters of various photocatalysts. The element constitution of photocatalysts was determined through an EDS technique. The surface elemental composition was acquired through XPS spectrometer (Thermo Fisher, USA). UV–vis DRS with region of 200-800 nm was recorded on UV–vis detector (Cary 300, USA) to explore the optical properties of various catalysts. 2.4. Photoelectrochemical test. Photoelectrochemical test was performed on electrochemical workstation, the working electrode was provided by photocatalyst, the platinum filament was used as counter electrode and the standard saturated calomel electrode was reference electrode. Working electrodes were manufactured as followed: 0.01 g of material was appended to 0.001 L of 0.5% Nafion liquid and 7 ACS Paragon Plus Environment


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sonicated for half an hour to generate uniform suspension, then coating onto 2.0 cm × 1.0 cm conductive glass, and then conductive glass was laid in an oven (110°C for 60 min). Sodium sulfate solution (0.2 M) was employed as electrolyte and light source was supplied by 300 W xenon lamp (>420 nm). The distance from light to reaction tubes adjusted approximately 15 cm and light intensity was determined as 155 mW cm-2. 2.5. Photocatalytic properties of photocatalysts. Catalytic properties of BWO, BFO and BFWO was assessed via photodegradation of RhB. Light source was supplied by a 300 W xenon lamp with 420 nm cutoff filter (155 mW cm-2). All experiments were performed in the same condition: catalyst (0.03 g) was added into 10 mg L-1 RhB (0.1 L) and magnetically stirring for halfhour to attain adsorption-desorption equilibrium, and then exposing to visible light illumination. After that, 0.004 L of solution was picked out from system and centrifuged to separate out catalyst. RhB concentrations were detected via employing Helious Betra UV–vis detector, the maximum absorbance wavelength locates at 554 nm.5 RhB photodegradation rate was acquired on the basis of following formula:

η =

RhB ini − RhB fin RhB ini

(1)

Where RhBini represents the initial RhB concentration, RhBfin represents RhB concentration after experiments finished. 3 RESULT AND DISCUSSION 3.1 Characterization. 3.1.1. XRD result. Crystallinity and composition of photocatalysts was detected via XRD. The distinct peaks of bare BWO at 2θ value of 8 ACS Paragon Plus Environment

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28.24°, 32.84°, 47.08° and 56.20° are well matched with (131), (002), (202) and (133) crystallographic planes of orthorhombic BWO (Figure 1a).35 There is no other peaks have been found, which indicates that the BWO was well crystallized and no impurity was formed in the preparation process. For original BFO, every peak is perfectly ascribed to orthorhombic phase of BFO, which retains consistency with previous article.33 But for BFWO composites, there are only BWO diffraction peaks are observed when content of BFO is lower than 10%, two main reasons can account for it :(1) the intensity of diffraction peaks is related with content of materials, when materials is too low, only weak characteristic diffraction peak can be formed, but it was hard to found;36 (2) the main characteristic peaks of BWO (28.24°, 32.84°, 47.10°, 58.50°) and BFO (27.96°, 32.84°, 46.80°, 56.48°) are relatively close and it cannot be accurately differentiated.37 However, when BFO content raises from 10 to 20%, the peaks at 2θ value of 28.78°, 33.54°, 37.38°, 49.30°, 51.70°, 51.98° and 56.48° attributed to BFO can be clearly observed in the BFWO composite (Figure 1b).From above information, it indicates that BFWO photocatalyst was prepared successfully.

Figure 1 XRD pattern of original BWO, BFO and BFWO with distrinct BFO content (2%, 5%, 7%, 10% and 20%) (a), and the amplified images of BFO and BFWO-20 9 ACS Paragon Plus Environment


(b).

3.1.2. SEM, EDS and TEM result. Particle sizes, morphologies, composition and micromorphology of bare BWO, BFO and BFWO were acquired via SEM, EDS and HRTEM. Figure 2a exhibits the original BFO image, which is seen that the geometric shape of pure BFO is similar to spherality. The surface of microsphere is a little rough and the microsphere is comprised by a substantial number of small particles. The pure BWO is exhibited as a rose-like microsphere formed by agglomeration of many 2D nanosheets (length: 0.5–3 µm: thickness 10-30 nm) (Figure 2b). SEM image of BFWO is showed in Figure 2c-d. When BFO content was 7%, there are a bit of big microsphere in the composite, which can be attributed to BFO, and some small microsphere (BWO) adhere to its surface. It suggests that the BFWO have been prepared successfully. However, when the content of BFO increases, the number of big microsphere also increase and some of them are barren, no small microsphere attaches on it (Figure 2d), it indicates that the content have exceed the optimum content. Furthermore, the HRTEM image of BFWO-7 is exhibited in the Figure S1. The crystalloid with interplanar distances of 0.193 and 0.319 nm attach to (202) and (121) planes of BWO and BFO, respectively. In addition, the EDS analysis indicates that element Bi, W, Fe and O coexist in the composite (Figures 2e), suggesting the as-prepared material is BFWO. The SEM, HRTEM and EDS results have a good consistency with the analysis of XRD.


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Figure 2 SEM of bare BFO (a), BWO (b), BFWO-7 (BFO/BWO mass ratio of 7%) (c), BFWO-20 (BFO/BWO mass ratio of 20%) (d) and the SEM EDS of BFWO-7(e).

3.1.3. XPS result. Elemental state of pure BFO, BWO and BFWO-7 composite were provided by XPS technology. The full survey spectrum (0-1300 eV) is displayed in Figure 3a, it is not hard to find that elements carbon, bismuth, tungsten, ferrum and oxygen coexisted in the BFWO-7 composite. The appearance of carbon element belongs to XPS instrument itself (Figure S2). In order to accurately analyze, the amplified spectrum of the Bi 4f, W 4f, Fe 2p and O 1s are employed. Two main peaks of Bi 4f at 164.55 eV and 159.30 eV belong to Bi 4f5/2 and Bi 4f7/2, corresponding to Bi3+ in BFO and BWO crystalline structure (Figure 3b). In the high-resolution W 4f 11 ACS Paragon Plus Environment


spectrum, the binging energy at 37.7 eV and 35.6 eV belong to W 4f5/2 and W 4f7/2, indicating that element W exhibits a hexavalent (W6+) chemical state in the composite (Figure 3c). The binding energy situates at 724.50 eV and 710.25 eV are Fe 2p1/2 and Fe 2p3/2, it can be in line with Fe3+ species and no marked reduced state (Fe2+ and Fe0) in the composite (Figure 3d-e).38 In high-resolution O 1s spectrum, the peaks are split into two bands at 530.20 and 531.55 eV, corresponding to lattice oxygen (Bi-O and Fe-O) and O-H bonds (Figure 3f).39-40 Comparing with pure BWO, the main XPS peaks of the Bi 4f, W 4f and Fe 2p have minor change but not very obvious, indicating chemical circumstance of BFWO-7 was changed due to interaction between BFO and BWO (Figure 3b-f). The XPS result has a great consistency with XRD result, which further verifies the BFWO-7 composite have been successfully fabricated.


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Figure 3 XPS result of pure BWO, BFO and BFWO-7: Survey scan (a), Bi 4f (b), W 4f (c), Fe 2p (d-e) and O 1s (f).

3.1.4. Optical property and energy gap analysis. Optical properties of as-prepared bare BWO, BFO and BFWO hybrids materials with different content of BFO were characterized through DRS (Figure 4a). The optical absorption spectrum of pure BWO approximately ranges from 200 to 450 nm and BFO ranges from 200 to 600 nm. Comparing with pure BWO, the light absorption spectrum has a distinct red 13 ACS Paragon Plus Environment


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shift when BFO content in hybrids materials increases, the increased visible light absorption can be originated from the interaction between BWO and BFO, which further enhances the photoadsorption of BFWO. The abrupt shape of the spectrum reveals that photoabsorption is not because of impurity level transition but owing to energy gap transition. The energy gap of various photocatalysts can be acquired according to Kubelka-Munk equation:41 αhv = 𝐴𝐴(ℎ𝑣𝑣 − 𝐸𝐸𝑔𝑔 )𝑛𝑛⁄2

(2)

Where α denotes absorption coefficient, 𝑣𝑣 denotes light frequency, Eg

represents energy gap, A denotes proportionality constant, h denotes Planck constant

and n relies on the optical transition type of photocatalysts (for direct transitions n=1 and for indirect transitions n=4). The n values for BFO and BWO are 4, indicating these two photocatalysts are indirect transitions. Energy gap of BWO, BFO and BFWO-7 are computed as 2.73, 2.09 and 2.54 eV (Figure 4b). It is palpable that BFWO-7 has a smaller energy gap than BWO. Furthermore, the valence band (VB) and conduction band (CB) of photocatalysts also obtaine through following equation:42 1

𝐸𝐸𝐶𝐶𝐶𝐶 = X − 𝐸𝐸 𝑒𝑒 − 2 𝐸𝐸𝑔𝑔

𝐸𝐸𝑉𝑉𝑉𝑉 = 𝐸𝐸𝐶𝐶𝐶𝐶 + 𝐸𝐸𝑔𝑔

(3) (4)

Where X denotes the electronegativity of crystalline semiconductors, which is geometric average value of the absolute electronegativity of all atoms in the materials. Ee is the energy of free electrons on the hydrogen scale (~4.50 eV vs NHE). Eg represents energy gap of the photocatalyst. Herein, on the basis of the above equation, 14 ACS Paragon Plus Environment

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CB and VB of BWO (BFO) are determined to be +0.53 (-0.90) and +3.26 eV (+1.19 eV).33

Figure 4 UV–vis DRS of bare BWO, BFO and BFWO with various BFO content (a), energy gaps of the pure BWO, BFO and BFWO-7 determined by Tauc plot (b).

3.1.5. Electrochemistry properties. Separation efficiency of photoinduced carriers exerts a vital part in catalytic process, which can determine photocatalytic activity of catalyst. Photogenerated electron migrates from VB to CB with remaining photogenerated hole in the VB alone, and then the transient photocurrent formed. Therefore, the transient photocurrent is believed as useful tool to assess separation efficiency of photoinduced carriers. Photocurrent responses of bare BWO, BFO and BFWO-7 hybrids composite is exhibited in Figure 5a, the current density of bare BWO, BFO are 0.015 and 0.05 µA cm-2 with light on, while the current density of BFWO-7 has a great enhancement, it can reach to 0.1725 µA cm-2, which is 12 times and 4 times of bare BWO and BFO. It indicates that inhibited efficiency of photogenerated carrier recombination of BFWO-7 is much higher than bare BWO and BFO. The faster separation efficiency of photogenerated carriers achieves, the better 15 ACS Paragon Plus Environment


catalytic property obtains. EIS is another tool to assess the electrochemical properties of photocatalyst. EIS was used to evaluate electron transfer efficiency and the result is showed in Figure 5b. The arc radius represents the electron transfer efficiency, the smaller the radius is, the better electron-transfer rate get, and vice versa. The arc radius of BFWO-7 is smaller than BWO, indicating electron-transfer rate of BFWO-7 is faster than BWO. But the interesting is that the arc radius of BFWO-7 is larger than BFO, this is because that BFO is a multi-ferrous material with ferroelectricity, its conductivity is strong and the transmission resistance of electrons is smaller.38 In order to further confirm the BFWO is favor for photoinduced carrier separation, photoluminescence spectra (PL) of various catalysts was determined. The higher PL intensity suggests the less photoexcited carriers join in catalytic reaction. It can be found that BFWO-7 possesses the lowest photoluminescence intensity comparing with BWO and BFO, which indicates that BFWO-7 heterojunction is better for charge separation (Figure 5c).


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Figure 5 Transient photocurrent responses of BWO, BFO and BFWO-7 (a), EIS spectra of BWO, BFO and BFWO-7 (b) and PL spectra of BWO, BFO and BFWO-7 (c).

3.2 Photocatalytic performance of photocatalyst. 3.2.1. Photocatalytic behavior of different materials. For the purpose of evaluating the photocatalytic property of all as-obtained catalysts, RhB was selected as a representative contaminant with the maximum absorption wavelength at 554 nm. Figure 6a illustrates the photocatalytic activity of sole BWO, BFO and BFWO-7 hybrids materials with different content of BFO under visible light illumination. The blank experiment shows that RhB is hardly removed without photocatalyst, which indicates that photolysis of RhB has negligible influence on catalytic degradation. And catalytic performance of hybrid materials enhance when BFO content increase. The BFWO-7 17 ACS Paragon Plus Environment


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hybrid materials possesses the best performance and almost 100% of RhB is degraded within 90 min, which is about 1.6 times of bare BWO. The excellent catalytic performance is assigned to rapid carriers separation rate. However, when content of BFO exceeds 7%, the photocatalytic activity will descend. This is because that the redundant BFO cannot form heterojunction with BWO (Figure 2d) and catalytic performance and adsorption of bare BFO is poor, so the performance of hybrid materials decreases. And BFWO-7 shows outstanding catalytic performance comparing with some conventional photocatalyst (TiO2, CdS, NiO and ZnO) (Figure S3). In addition, another refractory organic pollutants tetracycline was chosen to evaluate the catalytic activity performance of bare BFO, BWO and BFWO with different content of BFO, and the result suggests that BFWO-7 also has an excellent photocatalytic for tetracycline degradation (Figure S4). All results indicate that this Z-scheme photocatalyst not only has a good degradation performance on RhB, but also has high photocatalytic activity on other refractory organic pollutants. The pseudo-first-order equation is selected to quantitative study the photocatalytic degradation kinetics of RhB. 𝐶𝐶

ln � 𝐶𝐶0 � = kt 𝑡𝑡

(5)

Where C0 and Ct denote RhB concentration at time 0 and t. The reaction rate constant k of pure BWO, BFO and hybrid materials with BFO content at 2%, 5%, 7%, 10% and 20% are 0.0122, 0.0015, 0.0133, 0.0258, 0.0380, 0.0242 and 0.0065 min-1, respectively (Figure 6b). The BFWO-7 hybrids material possesses the high degradation efficiency (0.0380 min-1). 18 ACS Paragon Plus Environment

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Figure 6 Photodegradation of RhB by (a) bare BWO, BFO and BFWO with different BFO content under visible light illumination and the corresponding rate constants (k) (b).

3.2.2. Effect of RhB concentrations on photocatalytic process. In the practical wastewaters, the contaminants concentration may vary enormously and pose a threat to photocatalytic activity. As depicted in Figure 7a, a set of initial RhB concentrations (from 10 to 60 mg L-1) were selected to explore influence of pollutant concentrations on photocatalytic activity. Results show that the photodegradation efficiency decreases (from 100% to 75%) accordingly when RhB concentrations increase (from 10 to 60 mg L-1). Two reasons are responsible for this phenomenon: for the first one, the higher RhB concentrations prevent photon from entering RhB solution owing to the increase of the pathway length and reducing the number of photons on the catalyst surface.42 For another, there are some intermediates producing in the photocatalytic process and these intermediates competed with RhB for limited reactive sites 43. The competition between intermediates and RhB will soar with the increase of RhB concentrations because more and more intermediates produce in high RhB 19 ACS Paragon Plus Environment


concentration. Therefore, in order to attain better photodegradation efficiency, a dilution process should be needed to conduct to cut down the high contaminant concentrations in the practical application. And 10 mg L-1 of RhB is chosen as the optimal initial concentration in whole experiments. 3.2.3. Effect of pH on photocatalytic process. Degradation efficiency of RhB for BFWO-7 with different initial pH value of RhB solution is exhibited in Figure 7b. It is not hard to found that degradation efficiency decreases when initial pH increases and the efficiency has a dramatic reduction when initial pH increases from 5 to 7, which indicates that neutral or alkaline environment exhibits negative influence on photocatalytic process. The phenomenon is attributed to that the influence of different RhB pH on photocatalytic process of BFWO-7 should be related to the isoelectric point of BFWO-7.44 Therefore, the Zeta potential of BFWO-7 composite was tested under different pH value of solution and the result is displayed in Table 1. The result shows that BFWO-7 composite has negative surface charge at pH 3-7 and the negative surface charge becomes weaker and weaker, and when pH of RhB adjust to 9-11, the BFWO-7 composite has positive charge. While RhB is positively charged, so the adsorption ability of BFWO-7 for RhB is worse and worse when increase the pH value of RhB solution. And the result shows that 31.24%, 12.97%, 5.80% and 4.17% of RhB are adsorbed at the initial pH equal to 3, 5, 7 and 9 and only 2.61% of RhB is absorbed on the surface of catalyst when pH equal to 11 (Figure 7b), which is in line with the result of Zeta potential. The absorption ability of photocatalyst has an important role in photocatalytic reaction, the lower absorption will seriously hinder 20 ACS Paragon Plus Environment

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the photocatalytic efficiency. Therefore, in the practical wastewater treatment, the initial pH of solution should be adjusted to lower than 5 to promote adsorption capacity of BFWO-7 and further enhance catalytic performance property. Table 1 The corresponding Zeta potentials of BFWO-7 composite under different solution pH. pH Potential (mV)

3

5

7

-7.94±0.57 -5.23±0.43 -1.76±0.22

9 0.75±0.17

11 3.85±0.45

3.2.4. Effects of coexisting ions on photocatalytic process. As we know, many coexisting ions are existed in the real wastewater such as Na+, Cl- and SO42-, and they may pose a threat to attain the satisfying catalytic property. Herein, it is urgent to explore the effect of various ions on RhB photodegradation. Figure 7c displays result of photocatalytic performance under four model inorganic salts (NaCl, NaNO3, Na2SO4 and Na2CO3) with the concentration of 0.1 M and corresponding photodegradation efficiency sequence is: NaNO3> NaCl> Na2SO4> Na2CO3. It observes that NaNO3 and NaCl have negligible effect on RhB removal during the photocatalytic process, which might be ascribed to that NaNO3 and NaCl aqueous solution was neutral, an appropriate amount of them adding into system did not change the reaction environment. A slight reduction of removal efficiency originates from competition between Na+ and RhB molecule for limited catalytic sites. Besides, it is obvious that NaCl displays the much stronger inhibition effect than NaNO3, this is because that a small amount of •OH can be produced by photolysis of low concentration NO3-, and they can oxidize some pollutants non-selectively.45 21 ACS Paragon Plus Environment


Because Na2SO4 aqueous solution is weakly alkaline, the pH will increase when Na2SO4 adding into the system, and thus have a negative effect on photocatalytic process. However, Na2CO3 shows the striking inhibition to the photocatalytic degradation of RhB, two aspects can apply to explicate the phenomenon: for one, a certain amount of OH- and HCO3- are generated by hydrolysis of Na2CO3, the pH of the aqueous solution will enhance and then photocatalytic efficiency decrease, for another, it is known that CO32- and HCO3- are an effective h+ scavengers, the presence of CO32- and HCO3- will consume h+, resulting in poor photocatalytic activity. 3.2.5. Effects of water sources on photocatalytic process. The influence of various water sources on the removal of RhB is displayed in Figure 7d. After a whole photocatalytic degradation process, a sharply removal efficiency reduction can be found in the RhB-contained river wastewater and tap water and only 37.89% and 41.34% of RhB is degraded, respectively. This is because that RhB photodegradation by BFWO-7 strongly depends on pH of solution and neutral or alkali conditions have negative effect on RhB removal. However, the initial pH value of river wastewater and tap water are 7.5 and 7.27, so it contributes to poor photocatalytic performance. Moreover, a small amount of coexisting ions are common in the river wastewater and tap water, they also pay partly responsible for the poor photocatalytic performance.


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Figure 7 Effects of initial RhB concentration (a), initial solution pH (b), coexisting ions (c) and water sources (d) on RhB photodegradation performance by BFWO-7 under visible light irradiation

3.2.6. The stability of catalyst. Photostability of as-prepared catalyst is a crucial parameter to assess its practical application. The cycling experiment for RhB degradation by BFWO-7 was preformed to evaluate its stability. The result reveals after five recycling runs for RhB removal, the photocatalytic performance of BFWO-7 does not display any palpable reduction for RhB removal (Figure 8). It confirms that the BFWO-7 heterojunction is not easily photocorroded during the reaction process, which further suggests that the BFWO-7 is considerable stable. Based on above results, BFWO-7 will be an effective photocatalyst in environment remediation.



Figure 8 Cycling experiments for RhB degradation by BFWO-7 (conducting in the same experiments condition).

3.2.7. The mineralization capacity of photocatalyst. The mineralization capacity is a vital parameter to evaluate the catalytic property of various photocatalyst. The mineralization capacity of BFO, BWO and BFWO-7 was investigated by TOC analysis. Figure 9 shows that mineralization efficiency achieves about 8.64%, 36.98% and 75.91% for pure BFO, BWO and BFWO-7 composite by degradation of RhB within 90 min, respectively. It is easy to find that photodegradation efficiency is higher than mineralization efficiency, which indicates that there were intermediates generated in the photocatalytic reaction. The acquired result indicates that BFWO-7 composite possesses high mineralization ability comparing with pure BFO and BWO, it can directly mineralize RhB into CO2 and H2O or mineralize RhB into some small intermediates and further decompose into CO2 and H2O with extension of reaction time.


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Figure 9 TOC removal ratio by pure BFO, BWO and BFWO-7 composite under 90 min visible light irradiation.

3.3 Possible photodegradation mechanism. Reactive oxygen species possess high catalytic activity, which take crucial role in the photocatalytic reaction.46 The free radical trapping experiment was conducted to study active radical produced in photocatalytic reaction. In this work, TB, BQ and EDTA-2Na are applied as quenchers of •OH, •O2- and h+.47 When 1 mmol of TB put into photocatalytic process, RhB removal rate decreases from 100% to 82.74 %, indicating a certain amount of •OH take part in RhB photodegradation. Similarly, a more loss of removal efficiency (from 100% to 69.44%) can be observed when adding 1 mmol of TB into the system, indicating that •O2- playes a relatively important part in the RhB photo-oxidation (Figure 10a). However, the removal efficiency of RhB evidently declines from 100% to 26.45% with addition of 1 mmol of EDTA-2Na, confirming that h+ is a cardinal factor in RhB removal. Namely, photocatalytic reaction system is impacted by •OH, •O2- and h+ in different extent. Furthermore, the free radical 25 ACS Paragon Plus Environment


trapping experiments of pure BWO and BFO reveal that only h+ and •OH participated in the reaction process in BWO system, and only h+ and •O2- took part in the photodegradation process in BFO system (Figure 11a-b).

Figure 10 The free radical trapping experiments of BFWO-7 (a) and the corresponding DMPO spin-trapping ESR measurement of BFWO-7: DMPO-•O2- (b) and DMPO-•OH (c).

ESR technology was used to further verify the above-mentioned active radical produced in catalytic reaction.48 Experiment was first conducted under dark and then irradiated under visible light. The results display in Figure 10b, four distinctly signal peaks over BFWO-7 are found under irradiation and signal peaks intensity became higher with extension of time in methanol solution, which could be attributed to 26 ACS Paragon Plus Environment

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DMPO-•O2-. However, there are no signals can be observed in the same experiment conditions under dark. Meanwhile, the same phenomenon is appeared in Figure 10c. These results confirm that •OH and •O2- are formed in catalytic system. In addition, the ESR measurement of BWO and BFO confirm that only •OH generated in BWO system and only •O2- produced in BFO system. This result is in accordance with trapping experiment and DRS analysis (Figure 11c-f).

Figure 11 The free radical trapping experiments of pure BFO (a) and BWO (b) and the corresponding DMPO spin-trapping ESR measurement of BFO (c and e) and 27 ACS Paragon Plus Environment


BWO (d and f)

Based on above results, two possible mechanisms for photodegrading RhB by BFWO-7 can be raised and presented in Scheme 1. In view of the calculation, the CB of BWO and BFO are +0.53 and -0.90 eV, and the corresponding VB are +3.26 and +1.19 eV, respectively. Therefore, one possible mechanism displays in Scheme 1A. Under illumination, the photoexcited carriers are formed in BWO and BFO due to their narrow bang gap. The photoexcited hole from VB of BWO migrate to VB of BFO, and photoinduced electron from CB of BFO migrated to CB of BWO, then carriers can be effectively separated. However, the VB (+1.19 eV) of BFO is more negative than oxidation potential of OH-/•OH (+2.40 eV), promulgating that photoexcited hole cannot oxidize H2O to •OH in theory. Meanwhile, •O2- is also not be generated in catalytic reaction because reduction potential of O2/•O2- (-0.33 eV) is more negative than CB potential (+0.53 eV) of BWO.42 But in view of free radical trapping experiments and ESR spin-trap technique, •OH and •O2- were tested. Therefore, the traditional photoinduced carriers transferring and separation mode does not fit this possible mechanism. Another possible mechanism is displayed in Scheme 1B. On the basis of calculation, energy gap (0.67 eV) of photogenerated electron migrates from CB of BWO to VB of BFO is much smaller than that energy gap (1.43 eV) for photoinduced electron from CB of BFO to CB of BWO. Thus, it is easier for photogenereted electron in CB of BWO to migrate to VB of BFO, then combining with photoexcited hole originated from VB of BFO. Meanwhile, the electron 28 ACS Paragon Plus Environment

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generated in CB of BFO can easily reduce O2 to •O2- and hole generated in VB of BWO oxidize H2O to •OH because CB of BFO is more negative than reduction potential of O2/•O2- and VB potential of BWO is more positive than oxidation potential of OH-/•OH.49 Subsequently, the formed h+, •OH and •O2- take part in RhB photodegradation system (eqs 6-12). This photoinduced carrier transferring process exactly corresponds with result of free radical trapping experiments and ESR spin-trap technique. Thus, photoexcited carriers transfer process fit the Z-scheme charge transfer system. Because of transfer process, the photoinduced carriers is effectively separated and then significantly enhance photodegradation efficiency. BWO + hv → eCB–(BWO) + hVB+(BWO)

(6)

BFO + hv → eCB–(BFO) + hVB+(BFO)

(7)

eCB–(BWO) + hVB+(BFO) → recombination

(8)

eCB–(BFO) + O2 → •O2–(BFO)

(9)

hVB+(BWO) + H2O → •OH(BWO) + H+

(10)

hVB+(BWO) + OH– → •OH(BWO)

(11)

(h+, •O2-(BFO) and •OH(BWO)) + RhB → photodegradation production


(12)


Scheme 1 Schematic diagram of possible mechanism toward carrier migration and photocatalytic reaction of BFWO heterojunction photocatalyst: conventional mode (A) and Z-Scheme heterojunction system (B).

3.5 RhB photodegradation pathway. Figure 12a displays emporal UV-vis spectrum of RhB under illumination. Intensity of maximum peak (554 nm) RhB decreases with extension of reaction time, and maximum peak has a slightly blue-shifts. The hypsochromic shifts is attributed to the intermediates generated by the stepwise deethylation of RhB.50 In order to obtain more detailed information about intermediates, LC-MS/MS was employed (Figure S6). The HPLC spectra profiles of RhB solution with different reaction time (0, 30, 60, 90 and 120 min) is displayed in Figure 12b. In the original solution, only RhB peak can be found in the spectrum. However, other four peaks also appeare with the extension of reaction time, which are attributed to N, N-diethyl-N-ethylrhodamine (DER), N-ethyl-N-ethylrhodamine 30 ACS Paragon Plus Environment

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(EER), N, N-diethylrhodamine (DR), Nethyl rhodamine (ER) and rhodamine (R), respectively (Table 2). Among them, EER and DR has identical molecular weight because they are isomers. The intensity of these four peaks increase firstly and the decrease with the decrease of RhB peak. It indicates that the photodegradation of RhB is ascribed to the abruption of chromophore structure as a result of the deethylation.

Figure 12 Temporal UV-vis adsorption spectrum changes of RhB under irradiation (a) and the LC-MS/MS spectrum changes of RhB and its intermediates photodegraded by BFWO-7 under different reaction time (b).

Table 2 The detailed information of RhB and its intermediates photodegraded by BFWO-7. Molecular formula C28H31N2O3

Retention time (min) 3.45

m/z 443.3000

Proposed structure CH3

H 3C N

O

N+

H 3C

CH3 COOH



C26H27N2O2

2.36

415.3000

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H 3C

CH3 N

NH

O

+

H 3C COOH

C24H23N2O2

1.50

387.2000

CH3

H 3C HN

NH+

O

COOH

C24H23N2O2

1.38

387.2000

H 3C N

O

H 3C

NH2+

COOH

C22H18N2O2

0.98

359.2000

H 3C HN

O

NH2+

COOH

C20H13N2O2

0.85

331.2000

H 2N

O

NH2+

COOH

Based on the photodegradation mechanism and the results of LC-MS/MS, a possible RhB photodegradation pathway is proposed and showed in Scheme 2. The N-ethyl group on the original RhB molecule is continuous attacked by the active radicals (•OH, h+ and •O2-), the electronic delocalization of N-ethyl became weaker and weaker and an ethyl group detached from the molecule, then DER is formed. However, because of the instability of DER, another ethyl group will also detaches 32 ACS Paragon Plus Environment

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from different part of DER, and then two isomers (EER and DR) are formed. After further N-de-ethylation process, R and ER are generated due to the third and the fourth ethyl group fall off. With the continuous attack by active radicals, the opening-ring process will conduct subsequently, some products produce and degrade into some smaller substances with the extension of time. And the generated substances will be completely mineralized to CO2, NO3-, NH4+ and H2O.

Scheme 2 Schematic diagram of proposed RhB photodegradation pathway by BFWO-7.

4. CONCLUSION In conclusion, an efficient binary BFWO Z-scheme hybrid material was prepared through a facile hydrothermal route. When BFO content reaches 7%, the BFWO-7 has the highest photocatalytic activity comparing with pure BWO, BFO and other BFWO contained different BFO content. The highest photocatalytic performance of BFWO-7 33 ACS Paragon Plus Environment


is attributed to the effective photoinduced carriers separation, the broadened photoabsorption range, high oxidation capacity of hole and the high reduction ability of electron. Furthermore, some effects like RhB concentration, coexisting ions and water sources on photocatalytic activity of BFWO-7 were investigated. And in view of trapping experiments and ESR testing, the possible photodegradation mechanism is proposed: the primary radicals taking part in the RhB degradation were h+, •O2- and •OH. In addition, the stability experiment shows that the BFWO-7 photocatalyst is quite stable under irradiation. Thus, this work proposes a facile route for fabricating highly active and stable binary heterojunction photocatalyst and applying it into practical environment remediation. ASSOCIATED CONTENT Supporting Information Available: HRTEM image of as-prepared BFWO-7, high resolution XPS spectrum of BFWO-7 of C 1s, the photocatalytic performance of some traditional photocatalysts (TiO2, ZnO, CdS and NiO), the detailed preparation method of some traditional photocatalysts (TiO2, ZnO, CdS and NiO), photocatalytic degradation of TC and -ln(C/C0) versus time curves of TC, VSM patterns of as-obtained BFO and BFWO-7, and the detailed LC-MS/MS spectrums of RhB and its intermediates photodegraded by BFWO-7. AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected]. Tel: +86-731-88822754 *E-mail address: [email protected]. 34 ACS Paragon Plus Environment

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ORCID Cui Lai : 0000-0002-1585-1911 Guangming Zeng: 0000-0002-4230-7647 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This study was financially supported by the Program for the National Natural Science Foundation of China (51779090, 51709101, 51408206, 51579098, 51521006, 51278176, 51378190), Science and Technology Plan Project of Hunan Province (2017SK2243), the National Program for Support of Top–Notch Young Professionals of China (2014), the Program for New Century Excellent Talents in University (NCET-13-0186), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), and Hunan Provincial Science and Technology Plan Project (No.2016RS3026), the Fundamental Research Funds for the Central Universities (531107050978, 531107051080).

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