Bi2WO6 composite for organic pollutant degradation

Journal Pre-proof Highly efficient visible-light-driven photo-Fenton catalytic performance over FeOOH/ Bi2WO6 composite for organic pollutant degradation Li Guo, Kailai Zhang, Xuanxuan Han, Qiang Zhao, Danjun Wang, Feng Fu, Yucang Liang PII:

S0925-8388(19)33806-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.152560

Reference:

JALCOM 152560

To appear in:

Journal of Alloys and Compounds

Received Date: 3 June 2019 Revised Date:

2 October 2019

Accepted Date: 4 October 2019

Please cite this article as: L. Guo, K. Zhang, X. Han, Q. Zhao, D. Wang, F. Fu, Y. Liang, Highly efficient visible-light-driven photo-Fenton catalytic performance over FeOOH/Bi2WO6 composite for organic pollutant degradation, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152560. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Entry for the Table of Contents

Bi2WO6 0.9% FeOOH/Bi2WO6

1.0 O2

e- e- e- eFeOOH QDs

0.8

C/C0

Amorphous FeOOH quantum dotsdecorated 2D Bi2WO6 nanosheet heterostructures were successfully fabricated and used as a highly efficient visible-light-driven photoFenton catalyst for the degradation of organic pollutants.

·O2- ·O2- H2O2

FeIII

0.6

solidFeII liquid FeIII 0.4 interface 0.2

FeOOH/Bi2WO6

0.0 H2O2

h+ h+ h+ h+

·OH H2O

Submitted to Journal of Alloys and Compounds

·OH

0 10 20 30 40 50 Irradiation time (min) Bi2WO6

Pollutants

FeOOH QDs CO2 + H2O +...

L. Guo, K. Zhang, X. Han, Q. Zhao, D. Wang,* F. Fu,* Y. Liang* …….. Page No. – Page No. Highly efficient visible-lightdriven photo-Fenton catalytic performance over FeOOH/Bi2WO6 composite for organic pollutant degradation Keywords: Quantum dots / photo-Fenton catalysis / ironbismuth photocatalyst / synergistic effect / pollutant degradation

1

Manuscript for Journal of Alloys and Compounds

Highly efficient visible-light-driven photo-Fenton catalytic performance over FeOOH/Bi2WO6 composite for organic pollutant degradation Li Guo,a Kailai Zhang,a Xuanxuan Han,a Qiang Zhao,a Danjun Wang,a* Feng Fu,a* Yucang Liangab* a

Shaanxi Key Laboratory of Chemical Reaction Engineering, School of Chemistry and Chemical Engineering, Yan’an University, Yan’an 716000, China b Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

* Corresponding authors: Dr. Yucang Liang Institut für Anorganische Chemie, Universität Tübingen Auf der Morgenstelle 18 72076 Tübingen, Germany Tel.: +49 07071 29 76216 Fax: +49 07071 29 2436 E-mail: [email protected] (Y. Liang) Prof. Dr. Danjun Wang, Prof. Dr. Feng Fu Shaanxi Key Laboratory of Chemical Reaction Engineering School of Chemistry and Chemical Engineering, Yan’an University Yan’an 716000, China Tel.: +86-911-2332037 E-mail: [email protected] (D. Wang), [email protected] (F. Fu)

Submitted to the Journal of Alloys and Compounds

1

FULL PAPER http://dx.doi.org/10.1016/((will be filled in by the editorial staff))

Highly efficient visible-light-driven photo-Fenton catalytic performance over FeOOH/Bi2WO6 composite for organic pollutant degradation Li Guo,a Kailai Zhang,a Xuanxuan Han,a Qiang Zhao,a Danjun Wang,*a Feng Fu,*a Yucang Liang*ab a

Shaanxi Key Laboratory of Chemical Reaction Engineering, School of Chemistry and Chemical Engineering, Yan’an University, Yan’an 716000, China b Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

Abstract Amorphous FeOOH quantum dots (QDs)-decorated Bi2WO6 nanosheet heterostructures (FeOOH/Bi2WO6) were successfully fabricated through a combined hydrothermal and impregnation approach, characterized by many modern techniques, and employed as an efficient visible-light-driven photo-Fenton catalyst for the degradation of organic pollutants including methylene blue, rhodamine B and tetracycline hydrochloride. The results showed that the photo-Fenton catalytic performance of FeOOH/Bi2WO6 composite is much higher than those of single FeOOH QDs, Bi2WO6 and their physical mixture. Especially, 0.9% FeOOH/Bi2WO6 composite indicated a highest photo-Fenton catalytic activity. This can be attributed to that the synergistic effects of FeOOH and Bi2WO6 can promote the rapid transfer of photo-generated electrons from Bi2WO6 to FeOOH QDs and accelerate the reduction of Fe (III) to Fe (II) by photo-generated electrons and the conversion of dissolved oxygen in water into the strong oxidant superoxide negative radicals (·O2-). The presence of hydrogen peroxide (H2O2) enables the oxidation of Fe (II) to Fe (III) and the formation of a great number of additional hydroxyl radicals (·OH). These active radicals ·OH and ·O2- confirmed by electron spinning resonance spectra greatly improved the degradation of organic pollutants. In addition, the dosage effect (catalyst, H2O2) and acidity-basicity effect was investigated and a reasonable mechanism of pollutant degradation was proposed. Finally, the superior stability and reusability of catalyst were confirmed by five-recycled use for a promising candidate for water remediation in industrial application. Keywords: Quantum dots, photo-Fenton catalysis, iron-bismuth photocatalyst, synergistic effect, pollutant degradation

In recent years, the rapid growth of population and the rapid development of modern industrial and agricultural production have led to a dramatic increase in wastewater discharge. Different kinds of organic pollutants including i) organic dyes (methylene blue (MB), congo red (CR), rhodamine B (RhB)), ii) antibiotics (sulfadiazine (SD), tetracycline hydrochloride (TC-HCl), ofloxacin (OFX), etc.), iii) pesticide (carbendazim (CD), chlorpyrifos (CP), fenarimol (FM), etc.), and iv) environmental hormone (bisphenol A (BPA), diphenyl ketone, polychlorinated biphenyl (PCB), and so on) are widely used and exceeded the minimum residue limit (MRL) in the environment. If the industrial and agricultural sewage is directly discharged into the rivers, lakes and seas without a proper treatment, the human health and the ecological environment will encounter a serious and unprecedentedly threat [1-3]. Therefore, the removal of the organic dyes and other pollutants from the wastewater has attracted widespread attention in academia and industry. Nowadays, the effective treatment of these pollutants has become a topic of common concern. Advanced oxidation processes (AOPs), especially Fenton oxidation technology as a kind of deep oxidation technology, was first reported by H. J. Fenton in 1894 [4,5], and after that the Fenton reaction is used to treat various refractory organic and biological pollutants in wastewater [6,7]. In the Fenton process, the degradation of pollutants is achieved by a chain reaction between Fe2+ ions and H2O2 to produce highly reactive ·OH for oxidizing and degrading various refractory

organic pollutants [8]. However, the Fenton oxidation performance is limited because of less exposed active sites of iron ions-based materials and low circulation of Fe (II) to Fe (III), thus hindering its large-scale promotion and use [9]. Therefore, the combination of various AOPs methods has become a research hot-spot to avoid these shortcomings. Photocatalytic technology has been extensively studied as a new advanced oxidation technology about nearly half a century [10-13]. Up to date, photocatalysis combined with Fenton catalysis as an environment-friendly technology to degrade organic pollutants has also been extensively studied [14-18]. Bismuth tungstate (Bi2WO6) with a layered structure consisting of alternating layers of (Bi2O2)2+ and (WO4)2-octahedral layers [19,20] is one of the simplest Aurivillius layered perovskites, which has been used as a potential photocatalyst to treat pollutants under visible light in recent decades [21-24]. Meanwhile, the coupling of Bi2WO6 and other compounds containing iron-based materials was also investigated as the photo- Fenton catalysts [25]. Among these materials, FeOOH as a widely-existing iron mineral in nature has attracted extensive attention because of its non-toxic, relatively low cost and corrosion-resistant properties. Moreover, a relatively narrow band gap of FeOOH with a strong visible light response is another cause to employ as a photo-Fenton catalyst for the degradation of organic pollutants [26]. Herein, as a novel and efficient photo-Fenton-like catalyst, 0D FeOOH QDs were in-situ prepared on the surface of 2D Bi2WO6 nanosheets to produce FeOOH/Bi2WO6 composite via a hydrothermal approach combined with an impregnation method. Such coupling composite can be used as a photo-Fenton catalyst to

Submitted to the Journal of Alloys and Compounds

2

1. Introduction

perform the degradation or decolorization of organic pollutants such as MB, RhB and TC-HCl. The results show a highly catalytic activity compared to that of pure Bi2WO6 and FeOOH QDs as well as those of previously reported catalysts. The enhanced photoFenton catalytic performance can be explained by the synergistic effect between FeOOH QDs and Bi2WO6. Moreover, catalyst dosage effect, acidity and basicity effect, H2O2 dosage effect on the photo-Fenton catalytic efficiency were investigated in detail. For a such excellent photo-Fenton catalyst, the stability, reusability and catalytic mechanism of FeOOH/Bi2WO6 composite were addressed during the photo-Fenton reaction process.

2. Experimental Section 2.1 Chemicals Ferric chloride hexahydrate (FeCl3·6H2O, 99.0%), sodium hydroxide (NaOH, 96.0%), RhB (C28H31ClN2O3, 96.0%), methylene blue trihydrate (C16H24ClN3O3S) and cetyltrimethylammonium bromide (C19H42NBr, 99.0%) were procured from Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China). Sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), and absolute alcohol (C2H5OH, 99.7%) were obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd (Tianjin, China). Ammonium bicarbonate (NH4HCO3, 98%) was purchased from Tianjin Tianli Chemical Reagent Co., Ltd (Tianjin, China). Hydrochloric acid (HCl, 36%), nitric acid (HNO3, 63%) and ammonia solution (NH3·H2O, 28%) were purchased form Sichuan Xilong chemical co., Ltd (Sichuan, China). TC-HCl (C22H25ClN2O8, Biotech Grade) were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). All the reagents were of AR grade except TC-HCl and used without any further purification. The deionized water was used as the reaction solvent for all the experiments. 2.2 Preparation of Bi2WO6 nanosheets The Bi2WO6 nanosheets were prepared via a modified hydrothermal method according to previous report [27]. Bi(NO3)3·5H2O (1.94 g) was dissolved into 5 mL of 4 M HNO3 to form solution A. Cetyltrimethylammonium bromide (CTAB, 0.02 g) and Na2WO4·2H2O (0.60 g) were dissolved in 50 mL of water under stirring to form solution B. The solution B was then slowly added into the solution A under stirring. The mixed solution was neutralized by ammonia aqueous solution (volume ratio of concentrated ammonia solution and water is 1 : 1) and stirred for 1 h. The resulting mixture was transferred into 100 mL Teflon-lined stainless-steel autoclave and kept at 180 °C for 14 h. After being naturally cooled down to room temperature, the precipitate was collected by filtration, washed with water for three times and with absolute ethanol for three times, and dried in a vacuum oven at 70 °C overnight and followed by calcination at 300 °C for 3 h to obtain pure-Bi2WO6 nanosheets. 2.3 Preparation of FeOOH/Bi2WO6 composite The deposition of amorphous FeOOH QDs onto the surface of the Bi2WO6 was performed using an impregnation procedure according to previous report [28]. Briefly, powdery Bi2WO6 (1.5 g) was dispersed in a 25 mL of absolute ethanol containing FeCl3·6H2O (0.0409 g, 0.1515 mmol) and sonicated for 10 min. NH4HCO3 (0.0359 g, 0.4545 mmol) was then added and magnetically stirred at room temperature for 8 h. The resulting solid was centrifuged and washed with deionized water, and then with absolute ethanol for several times, and dried at 40 °C for 24 h to obtain product denoted as 0.9% FeOOH/Bi2WO6. Similarly, 0.45% and 1.8% FeOOH/Bi2WO6 composites were also prepared by a same method by varying the quantity of the FeCl3·6H2O and

Submitted to the Journal of Alloys and Compounds

NH4HCO3. The FeOOH/Bi2WO6 composite is referred to x% FeOOH/Bi2WO6 composites in subsequent text where x represents the weight ratio of FeOOH to Bi2WO6. The preparation process of pure FeOOH QDs is the same as the above method except that Bi2WO6 is not added. For comparison, FeOOH QDs and Bi2WO6 was mixed to prepare a mixture with a weight ratio of 0.9% to 1, which is denoted as 0.9% FeOOH&Bi2WO6 mixture. 2.4 Characterization The powder X-ray diffractometer (PXRD) pattern of the assynthesized samples were performed on Shimadzu XRD-7000 by using CuKα radiation (λ = 0.15418 nm) and Ni filter on the reflected beam. The Fourier-transform infrared spectroscopy (FTIR) spectra were detected by an IRAffinity-1S FTIR spectrophotometer (Shimadzu, Japan). The X-ray photoelectron spectra (XPS) measurements were carried out on a PHI-5400 (America PE) 250 Xi system and the binding energies were calibrated with the C 1s (284.60 eV). The field emission scanning electron microscope (FESEM) images were taken on a JSM-6700F microscope (Japan electronics) to observe the surface appearance and size dimension of the sample. Energy disperse X-ray (EDX) analysis was performed on a field emission scanning electron microscope (JSM7610F). The transmission electron microscope (TEM) images and selected area electron diffraction (SAED) pattern were obtained with a JEM-2100 microscope (Japan electronics) at an accelerating voltage of 200 kV. The electron spin resonance (ESR) spectra were examined on a Bruker model ESR JES-FA200 spectrometer equipped with a quanta-ray Nd: YAG laser system as the irradiation source (λ ≥ 420 nm). The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the samples were collected on a UV2550 (Japan Shimadzu) from 200 nm to 800 nm with BaSO4 as a reference standard. Photoluminescence (PL) spectra were measured on a F-4600 fluorescence spectrophotometer (Hitachi, Japan). Time-resolved photoluminescence (TR-PL) spectra were conducted on a FLS920 fluorescence spectrometer (Edinburgh Analytical Instruments, UK). 2.5 Photocatalytic and photo-Fenton catalytic activity Photocatalytic and photo-Fenton catalytic activity of the FeOOH/Bi2WO6 composite was evaluated by oxidative degradation of typical organic pollutants MB (12 mg L-1), RhB (40 mg L-1) and TC-HCl (50 mg L-1) in the absence or presence of H2O2. A 300 W metal halide lamp equipped with wavelength cutoff filters (λ ≤ 420 nm) was chosen as the light source and the circulating cooling water was passed to maintain a constant temperature in order to ensure that the comparison is meaningful. In the typical photo-Fenton procedure, catalyst (0.02 g) was dispersed into 20 mL of simulated pollutant solution with a given pH (adjusted using 0.1 mM NaOH or 0.01 mM HCl aqueous solution). Before visible light irradiation, the mixture was stirred in the dark for 30 min to achieve adsorption-absorption equilibrium, and then 100 µL of H2O2 aqueous solution (30%) was added to the reaction mixture. At the predetermined intervals, 2 mL of supernatant was taken out and then centrifuged at a high rotation rate to remove catalyst particles. Then the concentration of supernatant would be obtained by monitoring the maximum characteristic absorption peak of MB, RhB and TC-HCl at 664 nm, 554 nm and 357 nm, respectively, over a UV-Vis spectroscopy (Shimadzu UV-2550). For RhB, 1 mL of the suspension was withdrawn and diluted in 1 mL of H2O prior to measurement.

3. Results and Discussion 3.1 Structure and physical properties

3

vibration of -OH groups of adsorbed water on Bi2WO6 [31]. For FeOOH QDs, the absorption peaks at 3390, 879, 790 and 611 cm-1 correspond to flexural vibrations of Fe-O-H [1,37-40], while the peaks at 1398, 1634 and 3420 cm-1 belong to the vibration modes of adsorbed carbonate and water on FeOOH surface [41,42]. For 0.9% FeOOH/Bi2WO6 composite, the typical characteristic vibrations of FeOOH QDs cannot be detected, probably due to the high dispersion and very low contents of the amorphous FeOOH QDs on Bi2WO6 [28].

Intensity (a.u.)

Bi2WO6

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40

50 2θ (°)

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70

40

Intensity (a.u.)

C 1s

Bi 4f W 4f W 4f

Bi 4f

C 1s

Bi2WO6

FeOOH

30

Bi 4d

(c) W 4f 0.9% FeOOH/Bi2WO6 W 4f7/2 W 4f5/2

0

Bi 4f5/2

Bi 4f Bi 4f7/2

Bi2WO6

166

164 162 160 158 Binding energy (eV)

(d) 0.9% FeOOH/Bi2WO6

Intensity (a.u.)

Intensity (a.u.)

0.45% FeOOH/Bi2WO6

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Bi 4p O 1s

1200 1200 800 800 400 400 Binding energy (eV)

0.9% FeOOH/Bi2WO6

(b) 0.9% FeOOH/Bi2WO6

Survey

O 1s Bi 4d

Bi2WO6

1.8% FeOOH/Bi2WO6

Bi 4p

Fe 2p

(a) 0.9% FeOOH/Bi2WO6

Intensity (a.u.)

The phase composition, crystallinity and purity of different samples were monitored by XRD and presented in Fig. 1. For the pure FeOOH QDs, weak and wide diffraction peaks were observed, confirming its amorphous structure.28 Moreover, the diffraction peaks of Bi2WO6 located at 2θ = 28.3, 32.7, 32.8, 32.9, 47.1, 55.7, 58.5, 76.1, 78.5° can be indexed as crystal planes (131), (060), (200), (002), (202), (191), (262), (333) and (204), respectively, showing an orthorhombic Bi2WO6 with a high purity and crystalline nature (JCPDS No. 39-0256) [29]. However, for a series of the FeOOH/Bi2WO6 composites, with increasing amount of FeOOH, no any influences on the phase composition and crystallization of Bi2WO6 are observed, but the intensity of the diffraction peaks gradually decreases due to influence of highly dispersive FeOOH QDs deposited on Bi2WO6 [30].

satellite

156

Fe 2p Fe 2p3/2

Fe 2p1/2

satellite

38 36 34 Binding energy (eV)

32 740

730 720 Binding energy (eV)

710

Fig. 3. (a) XPS survey scan spectra of as-prepared Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite, the high-resolution XPS spectra of Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite (b) Bi 4f, (c) W 4f, and the highresolution XPS spectrum of 0.9% FeOOH/Bi2WO6 composite (d) Fe 2p.

80

Fig. 1. Representative PXRD patterns of the FeOOH QDs, Bi2WO6 nanosheets and a series of FeOOH/Bi2WO6 composites with different FeOOH loadings.

(a)

(b)

Transmitance (a.u.)

Bi2WO6

1.0 µm (c)

0.9% FeOOH/Bi2WO6

(d)

FeOOH

1.0 µm 1.0 µm

4000

3500

400 nm

3000

2500 2000 1500 Wavenumber (cm-1)

1000

400 nm

500

Fig. 2. Representative FTIR spectra of the FeOOH QDs, Bi2WO6 nanosheets and 0.9% FeOOH/Bi2WO6 composite.

Fig. 4. The low-/high-magnification FE-SEM images of (a,b) Bi2WO6, and (c,d) 0.9% FeOOH/Bi2WO6 composite.

For the samples Bi2WO6, FeOOH QDs and 0.9% FeOOH/Bi2WO6, FTIR spectra were carried out to confirm the presence of FeOOH QDs. As shown in Fig. 2, the main characteristic peaks of pure Bi2WO6 sample appear at 422, 596 and 725 cm-1, which belongs to the stretching vibration of Bi-O, and W-O bonds, and the bridging stretching modes of W−O−W framework, respectively [31-33]. The peaks appear in the range of 1000 - 1410 cm-1, which correspond to the characteristic vibrations of nitrate ions (stemming from bismuth nitrate precursor) adsorbed on Bi2WO6 surface [34-36], while the peaks at 1634 and 3431 cm-1 are attributed to the bending vibration and symmetric stretching

The XPS spectra were used to detect the oxidation state of various elements presented in the samples. The survey scan XPS spectra of Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite shown in Fig. 3a indicate that the elements Bi, O, W and C exist in both samples and Fe element only can be observed in FeOOH/Bi2WO6. The corresponding high-resolution XPS spectra of Bi 4f and W 4f are shown in Figs. 3 (b and c), respectively. As can be seen in Fig. 3b, two characteristic binding energies of Bi 4f7/2 and Bi 4f5/2 of Bi3+ from sample Bi2WO6 respectively appeared at 159.16 and 164.47 eV with a spin orbit splitting of 5.31 eV [33,43]. After the deposition of FeOOH QDs, the binding energies of Bi 4f from sample FeOOH/Bi2WO6 did not change, implying that FeOOH QDs only located on the surface of Bi2WO6 and did not integrate into the interior of the lattice by the formation of chemical bonds.

Submitted to the Journal of Alloys and Compounds

4

As illustrated in Fig. 3c, the characteristic peaks observed at 35.41 and 37.58 eV were assigned to spin-orbit coupling effect of the W 4f7/2 and W 4f5/2 of W6+ from sample Bi2WO6, respectively [43]. For FeOOH/Bi2WO6 composite, the binding energies of W 4f7/2 and W 4f5/2 of W6+ remain identical as that of Bi2WO6. Due to deposition of FeOOH QDs, the characteristic peaks of binding energies of Fe 2p3/2 and Fe 2p1/2 of Fe3+ are observed at 710.8 and 724.8 eV, respectively (Fig. 3d) [44]. Moreover, due to charge transfer screening, the other two shake-up satellite peaks appear at 719.7 and 733.4 eV for Fe 2p3/2 and Fe 2p1/2 of Fe (III) species, respectively, showing the presence of Fe3+ in amorphous FeOOH QDs [44].

(a)

(b)

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2.5 µm

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Bi Mα1

(d)

W Lα1

O Kα1

2.5 µm

2.5 µm

(e)

Bi O W Fe

(f)

2.5 µm

Fe Kα1

2.5 µm

FeOOH and (f) 0.9% FeOOH/Bi2WO6 composite. The inset in (e) is SAED pattern of amorphous FeOOH. The FE-SEM and TEM techniques were used to observe and analyze the microscopic morphology, size and structure of the samples. The typical SEM images of the pure Bi2WO6 nanosheets shown in Fig. 4 (a and b) clearly reveal the formation of super thin 2D nanosheet structures with the thickness of about 20 nm. However, the deposition of amorphous FeOOH QDs did not substantially affect the morphology of the Bi2WO6 nanosheets due to its ultrafine size as shown in Fig. 4 (c and d). The spatial distribution of the elements Bi, O, W and Fe, the energy-dispersive X-ray (EDX) spectrum and the elemental mapping are shown in Fig. 5, which confirmed that the atoms Bi, O, W and Fe were uniformly distributed over the composite materials. As can be seen TEM images in Fig. 6, sample Bi2WO6 is a sheet-like structure (Fig. 6 (a and b)), which is consistent with that of SEM images observed, and 0.9% FeOOH/Bi2WO6 composite (Fig. 6 (c and d)) has a similar morphology as that of pure Bi2WO6. Moreover, owing to too low loading and highly and uniformly distributed small particle size of amorphous FeOOH QDs (Fig. 5), no any aggregations of FeOOH QDs are observed on the surface of the nanosheets. The high-resolution (HR) TEM image and selected area electron diffraction (SAED) of FeOOH shown in Fig. 6e clearly reveals an amorphous structure (No any lattice fringes were observed). For 0.9% FeOOH/Bi2WO6 composite, HRTEM image verified that amorphous FeOOH was almost uniformly distributed on the surface of crystalline Bi2WO6 and no aggregations were found. These phenomena observed are consistent with relevant literature about amorphous phase structure [1,26,28]. These unique surface and structure properties are propitious to the improvement of catalytic efficiency of FeOOH/Bi2WO6 composite [1, 21,33]. 3.2 Photo-absorption characteristics of samples

Fig. 5. (a) SEM image of 0.9% FeOOH/Bi2WO6 composite, (b) the spatial distribution of all elements and the elemental mapping of (c) Bi, (d) O, (e) W, and (f) Fe in 0.9% FeOOH/Bi2WO6 composite. (a)

(b)

400 nm

(c)

80 nm

(d)

400 nm

40 nm

The UV-Visible diffuse reflectance spectra (UV-Vis-DRS) were employed to investigate the optical absorption properties of the asprepared samples. Fig. 7a shows the UV-Vis-DRS of the pure Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite. Note that the absorption band edge of FeOOH/Bi2WO6 obviously shifted to low energy (redshift phenomenon) compared to pure Bi2WO6, indicating the introduction of amorphous FeOOH QDs broadened the visible light response range of Bi2WO6 and enhanced the utilization efficiency of visible light. The band gap of the photocatalyst is calculated according to the following formula: (αhν) = A(hν-Eg)n/2, in which α, n, A, h, ν and Eg are the absorption coefficient, the fate of transition, a proportionality constant, planck constant, photon frequency, and the band-gap energy, respectively [45,46]. Bi2WO6 is a direct transition semiconductor (n = 1), the band gap energies of Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite were calculated to be 2.66 and 2.36 eV, respectively, revealing that FeOOH QDs deposited onto the surface of Bi2WO6 is beneficial to broaden the visible light response range of catalyst and ultimately enhance the photo-Fenton catalytic activity. (a)

200

5 nm

5 nm

Fig. 6. TEM images of as-prepared samples (a,b) Bi2WO6, (c,d)

0.9% FeOOH/Bi2WO6 composite, and HRTEM images of (e)

Submitted to the Journal of Alloys and Compounds

(b)

Bi2WO6 0.9% FeOOH/Bi2WO6

Absorbance (a.u.)

(f)

Bi2WO6 0.9% FeOOH/Bi2WO6

(αhν)2

(e)

300

400 500 600 700 Wavelength (nm)

800 2.0

2.5

3.0

3.5

hν (eV)

Fig. 7. (a) UV-vis diffuses reflectance spectra of the Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite, (b) Tauc plots of (αhν)2 versus hν for the samples Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite.

5

3.3 Charge-carrier separation efficiency The photo-luminescent (PL) property can indirectly reflect the degree of recombination of photo-generated electrons and holes of a material [47]. In general, the lower PL intensity implies the lower photo-generated electron-hole recombination rate, and vice versa [48]. As shown in Fig. 8a, pure Bi2WO6 has a relatively strong emission peak at 400 nm compared to that of FeOOH/Bi2WO6, revealing that the deposition of FeOOH QDs onto Bi2WO6 surface can significantly weaken the recombination rate of photo-generated carrier and thereby greatly improve the separation ability of electron and hole on Bi2WO6. In order to further investigate the transfer process of photogenerated carriers in the photo-Fenton degradation process, the time-resolved fluorescence decay spectra were measured for the samples Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite (Fig. 8b). The fluorescence lifetimes were fitted using two-exponential decay model y = y0+A1·e-x/τ1 + e-x/τ2 and τav = (A1·τ12 + A2·τ22) / (A1·τ1 + A2·τ2). Decay times (τ1 and τ2) and PL aptitudes/intensities (A1 and A2) were presented in Table 1. The average photoelectron lifetimes of Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite are 0.718 ns and 1.129 ns, respectively. The increase of photoluminescence lifetime implied the decrease of the recombination rate of photogenerated electron-hole pairs. The photogenerated electrons excited by visible light transferred from the valence band of Bi2WO6 to the conduction band, then passed to FeOOH and reduced Fe3+ to Fe2+. The generated Fe2+ can activate H2O2 to generate ·OH, followed by oxidation of Fe2+ to form Fe3+ again, and thereby performed the effective cycle of conversion of Fe3+ to Fe2+ and Fe2+ to Fe3+. This process greatly improves the utilization efficiency of H2O2.

1.0

Bi2WO6

0.724

0.9% FeOOH/Bi2WO6

21.282

A1 7.75 ×1013 1.140 ×103

τ2/ns 0.712 1.129

A2 7.95 ×1013 5.71 ×1010

τ av/ns 0.718

0.6 0.4

3.4 Photocatalytic and photo-Fenton catalytic properties of samples The visible-light-driven photocatalytic and photo-Fenton activities of different samples were evaluated by the photodegradation of MB (12 mg·L-1). As shown in Fig. 9a, MB was degraded without catalyst or in the presence of only H2O2 under visible light irradiation, the decolorization efficiency could reach nearly 15% and 18% after visible light irradiation for 30 min. In the case of the presence of catalyst Bi2WO6 or 0.9% FeOOH/Bi2WO6, the photocatalytic efficiency of 0.9% FeOOH/Bi2WO6 composite was lower than that of pure Bi2WO6 for the decolorization of MB, probably due to that the deposition of amorphous FeOOH QDs on the surface of Bi2WO6 shielded the partially active sites of Bi2WO6, reduced the effective illumination area, and therefore declined photocatalytic activity. However, as

Submitted to the Journal of Alloys and Compounds

0.6 0.4 0.2

pure Bi2WO6 pure FeOOH 0.45% catalyst

0.0

0.0 0

6 12 18 24 Irradiation time (min)

30

0 5.0

2.0 (c) -30 min 0 min 6 min 12 min 18 min 24 min 30 min

1.5 1.0 0.5

6 12 18 24 Irradiation time (min)

30

pure Bi2WO6 pure FeOOH 0.45% catalyst 0.9% catalyst 1.8% catalyst 0.9% mixture

(d)

4.0 3.0 2.0 1.0 0.0

0.0 400 0.20

500 600 700 Wavelength (nm)

0.12 0.08

0.00

800

0 3.5

(e)

0.16

0.04

1.129

pure Bi2WO6 0.9% FeOOH/Bi2WO6 H2O2 only without catalyst

0.2

Kapp (min-1)

τ1/ns

0.9% catalyst 1.8% catalyst 0.9% mixture

(b)

0.8

0.8

Table 1. Lifetimes (τ) and PL intensities (A) of Bi2WO6 and 0.9% FeOOH/Bi2WO6.

Sample

1.0

(a)

C/C0

Fig. 8. (a) Photoluminescence spectra and (b) time-resolved fluorescence decay spectra of Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite.

In(C0/C)

50

Samples

Absorbance (a.u.)

40

0.9% FeOOH&Bi2WO6

30 Time (ns)

pure FeOOH

20

1.8% FeOOH/Bi2WO6

10

0.9% FeOOH/Bi2WO6

600

0.45% FeOOH/Bi2WO6

550

pure Bi2WO6

400 450 500 Wavelength (nm)

τ = 0.718 ns τ = 1.129 ns

can be seen in Fig. 9b, the photo-Fenton decolorization efficiency of MB over pure FeOOH and Bi2WO6 could reach 39% and 62% within 30 min, respectively. But the physically mixed 0.9% FeOOH&Bi2WO6 as the catalyst showed a slightly improved catalytic activity to compare to that of pure FeOOH, while higher than that of pure Bi2WO6 in initial 12 min then lower than that of pure Bi2WO6 after 13 min photocatalysis, and photocatalytic efficiency could reach 46% after 30 min, revealing that a simply physical mix of composite isn’t a good way to prepare a multicomposite photocatalyst with a highly catalytic performance in this case. For FeOOH/Bi2WO6 composite, with gradually increasing of FeOOH from 0.45 to 1.8%, note that that the photoFenton catalytic performance initially increased then decreased, in which 0.9% FeOOH/Bi2WO6 composite showed a highest catalytic performance (98%, 18 min) for the photodegradation of MB. These results imply that lower or higher loadings of in-situ formed FeOOH on Bi2WO6 can greatly influence photo-Fenton catalytic performance. This cause can be attributed to an optimal concentration of both Fe2+ ions and H2O2 during photo-Fenton catalysis. Based on above-mentioned results, a simply physical mixture of FeOOH and Bi2WO6 cannot effectively improve photoFenton catalytic performance of MB degradation, and in-situ prepared FeOOH QDs anchored on Bi2WO6 forms a nanostructured heterojunction FeOOH/Bi2WO6 composite, which can markedly enhance photo-Fenton catalytic performance.

C/C0

0.9% FeOOH/Bi2WO6

350

Bi2WO6 0.9% FeOOH/Bi2WO6

Absorbance (a.u.)

(b)

Bi2WO6 Counts (a.u.)

Intensity (a.u.)

(a)

3.0

6 12 18 24 Irradiation time (min)

30

(f)

2.5 2.0 1.5 1.0

-30 min 0 min 4 min 8 min 12 min 16 min 20 min

0.5 0.0 400

450 500 550 Wavelength (nm)

600

Fig. 9. (a) Photocatalytic MB degradation, (b) visible-light-driven photoFenton MB degradation over different catalysts, (c) UV-vis absorption spectra of MB with increasing visible light irradiation time using 0.9% FeOOH/Bi2WO6 as the catalyst during the photo-Fenton catalytic process, (d) the plots of ln(C0/C) versus visible light irradiation time, (e) the apparent rate constants (kapp/min-1) of the photo-Fenton decolorization of MB using different catalysts, (f) the absorption spectra of decolorization of RhB with increasing visible light irradiation time using 0.9% FeOOH/Bi2WO6 as the catalyst during the photo-Fenton catalytic process. For the decolorization of MB and RhB, reaction conditions are as follows: T = 25 °C, the dosage or concentration for H2O2 = 100 µL, MB = 12 mg·L-1, RhB = 40 mg·L-1, and catalyst = 1 g·L-1. Herein, 0.45% catalyst, 0.9% catalyst, 1.8% catalyst and 0.9% mixture respectively represent 0.45% FeOOH/Bi2WO6, 0.9% FeOOH/Bi2WO6, 1.8% FeOOH/Bi2WO6, and 0.9% FeOOH&Bi2WO6 in the figures.

As a representative, with increasing visible light irradiation time, the

6

1.0 (a)

1.0 (b)

0.8

0.8

0.6

C/C0

C/C0

0.9% FeOOH/Bi2WO6 Bi2WO6 H2O2 only Without catalyst

0.4 0.2

Bi2WO6 0.9% FeOOH/Bi2WO6

0.6 0.4 0.2 0.0

0.0 0

10 20 30 40 Irradiation time (min)

50

0

10 20 30 40 Irradiation time (min)

50

Fig. 10. (a) Photocatalytic oxidation of TC-HCl, (b) the visible-light-driven photo-Fenton oxidation of TC-HCl by using different catalysts. Reaction conditions are as follows: initial pH = 7, T = 25 ℃, the dosage of H2O2 = 100 µL, concentration of TC-HCl = 50 mg·L-1, and catalyst concentration = 1.0 g·L-1.

In order to confirm these advantages of the prepared photoFenton-like catalyst FeOOH/Bi2WO6 in various wastewater treatments, a refractory organic pollutant tetracycline hydrochloride (TC-HCl) was used as a simulated pollutant model to evaluate the visible-light-driven photocatalytic and photo-Fenton catalytic performance. As shown in Fig. 10a, under visible light irradiation, TC-HCl is hardly degraded without catalyst, indicating the stability of the antibiotic molecule, but the presence of only H2O2 made the degradation rate of TC-HCl to reach 16% in 50 min.

Submitted to the Journal of Alloys and Compounds

Moreover, the photocatalytic performance of catalyst 0.9% FeOOH/Bi2WO6 is obviously worse than that of pure Bi2WO6, which is consistent with the results of the decolonizing dye. However, as presented in Fig. 10b, the photo-Fenton catalytic performances of pure Bi2WO6 and 0.9% FeOOH/Bi2WO6 composite are quite different compared with their photocatalysis, indicating that 53% and 91% of TC-HCl under visible light irradiation were degraded after about 50 min, respectively. The results clearly verify the activation of Bi2WO6, the importance of FeOOH QDs and the synergistic effect between FeOOH QDs and Bi2WO6. Especially, synergistic effect acts as a critical role to perform the visible-light-driven Fenton oxidative degradation process for organic pollutants. 3.5 Various influence parameters on photo-Fenton degradation efficiency H2O2 dosage effect on the photo-Fenton catalytic activity over 0.9% FeOOH/Bi2WO6 composite was investigated. As can be seen in Fig. 11a, with increasing amount of H2O2 from 10 to 100 µL, the degradation efficiency of TC-HCl significantly improved. However, when further increased amount of H2O2 to 150 µL, the degradation efficiency decreased slightly. This result is probably caused by (i) an appropriate amount of H2O2 is beneficial for activation and decomposition into ·OH and ·O2-, and (ii) excess H2O2 can act as a capture agent of ·OH to form HO2· with a lower oxidation ability compared to ·OH. (a)

1.0 0.8

10 µL 50 µL 100 µL 150 µL

(b)

1.0 0.8

0.5g/L 1.0g/L 1.5g/L

0.6

0.6

C/C0

C/C0

UV-Vis absorption spectra of photo-Fenton degradation of MB using 0.9 % FeOOH/Bi2WO6 as photocatalyst is presented in Fig. 9c. As a result, the intensity of absorption peak at 664 nm for MB rapidly decreased and disappeared after 30 min in the photo-Fenton process under visible light irradiation, revealing that the presence of H2O2 greatly improved the reaction rate of photo-Fenton catalytic decolorization of MB. In this case, the variation of MB concentration [ln(C/C0)] versus irradiation time under visible light irradiation (λ > 420 nm) well conforms to a pseudo first-order kinetic model (Fig. 9d) [49]. If photocatalyst is replaced by pure FeOOH QDs, pure Bi2WO6 and a series of nanostructured heterojuction catalysts FeOOH/Bi2WO6, a similar trend is observed (Fig. 9d). According to the experimental data, the corresponding apparent rate constants kapp/min-1 are calculated to be 0.016, 0.034, 0.184, 0.195, 0.146, and 0.019 min-1 for FeOOH, Bi2WO6, 0.45% FeOOH/Bi2WO6, 0.9% FeOOH/Bi2WO6, 1.8% FeOOH/Bi2WO6 composite, and 0.9% FeOOH&Bi2WO6 mixture, respectively (Fig. 9e), indicating that the photo-Fenton catalytic activity of the 0.9% FeOOH/Bi2WO6 composite is the highest and nearly 5.7 times higher than that of pure Bi2WO6. On the basis of above-mentioned results, the highly photo-Fenton catalytic performance can be attributed to two aspects: (i) a certain amount of in-situ formed FeOOH QDs loading on Bi2WO6 can significantly improve the photo-Fenton catalytic activity due to the existence of the coupling interface between FeOOH QDs and Bi2WO6, (ii) the presence of H2O2 as a good electron acceptor is capable of trapping the catalyst surface e-, minimizing the recombination of holes and electrons and enabling more the photogenerated electrons to participate efficient the photo-Fenton process. These two aspects of factors ultimately drive and greatly enhance the photo-Fenton catalytic activity of FeOOH/Bi2WO6 for MB degradation. Furthermore, such optimal photocatalyst 0.9% FeOOH/Bi2WO6 can also be applied to the photo-Fenton decolorization of RhB. As shown in Fig. 9f, for the photo-Fenton decolorization of RhB, the temporal evolution of absorption peak clearly showed that the degradation of RhB was completed in 16 min, and photodegradation efficiency reached nearly 99%, fully demonstrating that the prepared photo-Fenton catalyst FeOOH/Bi2WO6 has a potential advantage to apply to the treatment of wastewater containing all kinds of dyes by heterogeneous Fenton reaction.

0.4

0.4

0.2

0.2

0.0

0.0 0

10 20 30 40 Irradiation time (min)

50

0

10 20 30 40 Irradiation time (min)

50

Fig. 11. The photo-Fenton catalytic activity for the removal of TC-HCl (50 mg·L-1) at T = 25 °C and initial pH = 7, (a) H2O2 dosage effect under a condition of the catalyst concentration of 1 g·L-1, (b) photocatalyst dosage effect under a condition of the dosage of H2O2 of 100 µL.

Moreover, photocatalyst dosage effect on the catalytic activity was also explored in the presence of H2O2. When amount of catalyst 0.9% FeOOH/Bi2WO6 increased from 0.5 to 1.0 g·L-1, the degradation of TC-HCl gradually accelerated (Fig. 11b) due to that the increase of the amount of catalyst provided more active sites while simultaneously generated more electrons which promote the conversion of Fe3+ to Fe2+, thereby further accelerating the decomposition of H2O2 and causing an increase on catalytic performance. However, when catalyst dosage reached to 1.5 g·L-1 the degradation efficiency of TC-HCl even decreased slightly. This reason may be attributed to an increasing in opacity and light scattering caused by excessive aggregation of the catalyst particles, as a result, the corresponding photo-Fenton catalytic activity slightly decreased. This investigation implies that use of appropriate amount of catalyst is beneficial to the improvement of catalytic performance, oppositely, excess catalysts correspondingly suppress exertion of activity of catalyst. Furthermore, the acidity and basicity effect of reaction system on the photo-Fenton catalytic performance of 0.9% FeOOH/Bi2WO6 composite for the degradation of TC-HCl was also investigated. The pH of reaction system changed from 5 to 9 under an identical condition. As can be seen in Fig. 12a, it is obvious to find that the reaction system changed from an acidic to a neutral, and finally to a basic condition, the catalytic activity of 0.9% FeOOH/Bi2WO6 composite slightly increased at the neutral condition and decreased at the basic condition for the removal rate of TC-HCl. This result

7

Fe2O3/rGO [17], β-FeOOH@GO [18], RGO/Fe3O4 [50] and so on (Table 2). This comparison clearly reveals that 0.9% FeOOH/Bi2WO6 composite is a promising candidate to apply to industry for removing organic pollutants under visible light irradiation. Degradation efficiency (%)

100 (a) 80 60 40 20 7 pH

60 40 20 1st

(c)

Bi 4f C 1s

Bi 4p

Intensity (a.u.)

(b)

80

0

9

Fresh After used

1200

100

900 600 300 Binding energy (eV)

0

2nd 3rd 4th Recycling time Fresh After used Fe 2p1/2

Intensity (a.u.)

5

W 4f

0

O 1s Bi 4d

Degradation efficiency (%)

reveals that the neutral condition is beneficial to maximally exert the highly catalytic activity of catalyst due to the formation of massive free radicals ·OH. Under an initial acidic condition, Clfrom HCl as solution acidification could react with HO· to generate the inorganic radical ion ClO-·H with a lower reactivity. Note that such species does not participate degradation process compared to radical HO· [50]. Hence, an initial acidic condition is not propitious for the formation of radical HO· [18], and thereby the degradation efficiency slightly decreased. Moreover, the rapid precipitation of iron hydroxide under a basic condition greatly weaken the formation of radicals ·OH as an oxidant, as a result, the photo-Fenton catalytic performance decreased. In addition, the stability of the catalyst 0.9% FeOOH/Bi2WO6 was evaluated by the photo-Fenton catalytic degradation of TCHCl after catalyst was run five times. The results shown in Fig. 12b reveal that the catalyst still maintained a high photo-Fenton catalytic activity, although activity of catalyst has a slight decrease, confirming higher stability and reusability of catalyst 0.9% FeOOH/Bi2WO6. In order to further verify unchanged composite and chemical state of catalyst after 5 cycling runs, XPS of catalyst before and after the photo-Fenton reaction was measured. According to the XPS survey scan spectra (Fig. 12c) and Fe 2p XPS spectra (Fig. 12d), the structure and composite of catalyst did not change significantly, clearly confirming a relatively high stability during the heterogeneous photo-Fenton reaction process. Based on above-mentioned investigations, no matter reaction time or catalytic efficiency, the 0.9% FeOOH/Bi2WO6 composite displayed a markedly enhanced photo-Fenton catalytic performance compared to previously reported catalysts for the decolorization and degradation of the dyes and antibiotics, such as Cu-FeOOH/g-C3N4 [1], GO-FePO4 [9], α-Fe2O3@SnO2 [14],

738

5th (d) Fe 2p3/2

732 726 720 714 Binding energy (eV)

708

Fig. 12. (a) Acidity and basicity effect on the photo-Fenton catalytic activity for TC-HCl removal, reaction conditions: T = 25 °C, the dosage of H2O2 = 100 µL, concentration of TC-HCl = 50 mg·L-1, and catalyst concentration = 1 g·L-1, (b) degradation efficiency of TC-HCl for every cycle, reaction conditions: initial pH = 7, T = 25 °C, the dosage of H2O2 = 100 µL, concentration of TC-HCl = 50 mg·L-1, and catalyst concentration = 1 g·L-1, and the XPS spectra of 0.9% FeOOH/Bi2WO6 composite before and after photo-Fenton reaction (c) survey scan and (d) Fe 2p.

Table 2. Comparison of photo-Fenton catalytic performance for removal of pollutants.

light source

photocatalyst

organic pollutant

sample a)

type Cu-FeOOH/g-C3N4 GO-FePO4 α-Fe2O3@SnO2 Fe2O3/rGO β-FeOOH@GO α-Fe2O3@TiO2 RGO/Fe3O4

Xe lamp Dy lamp Xe lamp Xe lamp mercury UV lamp Xe lamp Xe lamp

power (W) 500 300 300 300

MB RhB RhB RhB

10 10 25 15

125

MB

40

type

conc.

experimental conditions ηe (%) pH

H2O2

0.2 1.0 0.6 0.083

7.27 2.18 7.3 6.5

31.3 mM 1000 µL 0.8 mM 1000 µL

98.7 97 96.4 98

[1] [9] [14] [17]

0.25

60

2.67

1.10 mM

98

[18]

b)

conc.

c)

300 300

TC-HCl 50 0.5 90 5.45 120 µL 100 MB 20 0.25 120 6 10 mM 95 MB 12 1.0 18 98 metal 0.9% FeOOH/Bi2WO6 300 RhB 40 1.0 16 100 µL 99 halide lamp TC-HCl 50 1.0 50 7 91 a) Hg lamp were the UV-light sources, Xe lamp and metal halide lamp were the visible light source. b) The concentration of organic pollutant Photocatalyst concentration (g L-1). d) Reaction time (min). e) η is the total removal efficiency of organic pollutants.

Submitted to the Journal of Alloys and Compounds

DMPO-superoxide radical (a) 0.9% FeOOH/Bi2WO6 + H2O2 visible light

Bi2WO6 + H2O2

visible light Dark

318

320

322 324 B (mT)

326

Intensity (a.u.)

In order to explore the mainly active species during the photoFenton catalytic reaction process, the active free radicals were detected by electron spin resonance (ESR). As shown in Fig. 13 (a and b), the ESR signals of DMPO-·O2− and DMPO-·OH adducts were undetectable in the dark. However, under the photo-Fenton condition, the characteristic DMPO-·O2− adduct with the intensities of quartet signal of 1 : 1 : 1 : 1 was observed [49,51], while a fourline ESR signal of DMPO-·OH with intensities of 1 : 2 : 2 : 1 was also detected [52,53]. Note that it is obvious to find that the signal intensities of both ·OH and ·O2− generated by FeOOH/Bi2WO6 are much higher than those of pure Bi2WO6, clearly corroborating the

[46] [50] this work (mg L-1).

c)

highly efficient photo-Fenton catalytic performance of catalyst FeOOH QDs-loaded Bi2WO6.

Intensity (a.u.)

3.6 Photocatalytic and photo-Fenton catalytic mechanism

refs.

t d) (min) 40 120 60 120

DMPO-hydroxyl radical (b) 0.9% FeOOH/Bi2WO6 + H2O2 visible light

Bi2WO6 + H2O2

visible light Dark

328 318.1 318.2 318.3 318.4 318.5 318.6 318.7 B (mT)

Fig. 13. DMPO spin-trapping ESR spectra of catalysts for (a) DMPO- · O2-, and (b) DMPO-·OH.

8

Fig. 14. Schematic illustration for the proposed mechanism of photo-Fenton degradation of organic pollutant by 0.9% FeOOH/Bi2WO6 composite under visible light irradiation.

According to the aforementioned results, the proposed mechanism for the enhanced photo-Fenton activity of FeOOH/Bi2WO6 is schematically illustrated in Fig. 14 to elucidate the reaction mechanism of heterogeneous Fenton photocatalyst during photo-Fenton catalysis. First, the deposition of FeOOH QDs on the surface of Bi2WO6 nanosheets significantly broadens the visible light response range (Fig. 7). During the photocatalytic reaction, the electrons are excited from the valence band maximum (VBM) of Bi2WO6 to its minimum conduction band (CBM) (eq. 1). The photogenerated electrons are transferred from the CBM of Bi2WO6 to FeOOH QDs (eq. 1). This transfer greatly reduces the recombination rate of electrons and holes and prolongs the lifetime of photogenerated carriers (Fig. 8 and Table 1). On the one hand, the electrons migrated to the surface of the catalyst react with O2 to produce ·O2- with strong oxidizing ability (eq. 2), while Fe3+ is reduced to Fe2+ by the transferred electrons. On the other hand, the organic pollutants can also be directly oxidized by holes. If H2O2 attends to reaction system, the above-mentioned transferred electrons can also react with H2O2 to form radicals ·OH (eq. 3) which can oxidize organic pollutants. Simultaneously, FeOOH QDs on the surface of Bi2WO6 nanosheets can induce Fenton reaction in the presence of H2O2. It was known that Fe(II) plays a key role and radical ·OH is the main active species in such Fenton reaction [18,54]. Under acidic or neutral condition, H2O2 activated by photocatalyst FeOOH/Bi2WO6 may involve the formation of a complex between ≡FeIIIOOH/Bi2WO6 and H2O2 (eq. 4), where ≡FeIII stands for Fe species active sites [18]. Such complex species (H2O2≡FeIIIOOH/Bi2WO6) can convert to radical ·HO2, H+ and ≡FeIIOOH/Bi2WO6 species (eq. 5). The generated ·HO2 further react with ≡FeIII species to form ≡FeII species (eq. 6) that further react with H2O2 to generate active radical ·OH and FeIII species ≡FeIIIOOH/Bi2WO6 again (eq. 7) and thereby performing Fe(III)↔Fe(II) cycle. Finally, radicals ·OH and ·O2- act as main active species and react with organic pollutants molecules (RhB, MB and TC-HCl) to eventually decompose them into CO2 and H2O, etc. (eq. 8). Therefore, for FeOOH/Bi2WO6 composites, the synergistic effect of the Bi2WO6 and FeOOH QDs can effectively improve the photo-Fenton catalytic performance. All possible reactions are described as follows: FeIIIOOH/Bi2WO6→FeOOH/Bi2WO6 (h+ + e-) (1) O2 + e- → ·O2(2) H2O2 + e- → 2·OH (3) ≡FeIIIOOH/Bi2WO6 + H2O2 → H2O2≡FeIIIOOH/Bi2WO6 (4) H2O2≡FeIIIOOH/Bi2WO6 → ≡FeIIOOH/Bi2WO6 + ·HO2 + H+ (5) ≡FeIIIOOH/Bi2WO6 + ·HO2 → ≡FeIIOOH/Bi2WO6 + O2 + H+ (6) ≡FeIIOOH/Bi2WO6 + H2O2 → ≡FeIIIOOH/Bi2WO6 + ·OH + OH- (7) (8) Organic pollutants + ·O2-/ ·OH/h+ → ...→ CO2 + H2O + ...

4. Conclusions

Submitted to the Journal of Alloys and Compounds

In summary, a series of in-situ FeOOH QDs-modified Bi2WO6 nanosheets have been successfully constructed by a hydrothermal approach combined with an impregnation method. Such composite can be used as a visible-light-driven photo-Fenton photocatalyst for wastewater treatment and show highly photo-Fenton catalytic performance compared to 2D Bi2WO6 nanosheets and single FeOOH QDs as well as their physical mixture. The loadings of FeOOH QDs on Bi2WO6 have an influence on the photo-Fenton catalytic efficiency of FeOOH/Bi2WO6 composite, and 0.9% FeOOH/Bi2WO6 shows a highest catalytic activity for the degradation of MB. Such catalyst can be extensively applied to the photo-Fenton catalytic degradation of a series of color and colorless organic pollutants such as RhB and TC-HCl. The decolorization and degradation of the MB (12 mg·L-1), RhB (40 mg·L-1) and TC-HCl (50 mg·L-1) solution can be completed within 18, 16 and 50 min with photo-Fenton catalytic efficiency of 98%, 99% and 91%, respectively, showing the highly catalytic efficiency compared to those of previously reported catalysts, due to the synergistic effect of the FeOOH QDs and Bi2WO6 in the presence of H2O2 as a strong oxidant. In addition, catalyst dosage effect, acidity and basicity effect and H2O2 dosage effect are explored in detail for the photo-Fenton degradation of TC-HCl. For such photo-Fenton catalyst, its catalytic mechanism and reaction process were also proposed. Finally, as an important and key consideration for industrial application of catalyst, FeOOH/Bi2WO6 composite still keep a highly photo-Fenton catalytic efficiency after run 5 times, revealing a high stability and reusability. These results imply that 0.9% FeOOH/Bi2WO6 composite is a promising candidate for water remediation in industry such as the removal of color or colorless organic pollutants.

5. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21663030, 21666039) and the Project of Science & Technology Office of Shaanxi Province (No. 2018TSCXL-NY-02-01, 2015SF291) and the Project of Yan’an Science and Technology Bureau (No. 2018KG-04) and the Graduate Innovation Project of Yan'an University (YCX201988).

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Highlights • • • • •

Preparation of amorphous FeOOH quantum dots-decorated Bi2WO6-x composite. Highly photo-Fenton catalytic activity of FeOOH/Bi2WO6 composite for organic pollutant degradation, especially 0.9% FeOOH/Bi2WO6 photocatalyst. Highly efficient synergistic effects between FeOOH and Bi2WO6. A reasonable photo-Fenton catalytic mechanism under synergistic effect. Superior stability and reusability of FeOOH/Bi2WO6 photocatalyst.