Mechanisms for highly-efficient mineralization of bisphenol A by

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Mechanisms for highly-efficient mineralization of bisphenol A by heterostructured Ag2WO4/Ag3PO4 under simulated solar-light Tengfei Li, Haoran Wei, Hanzhong Jia, Tianjiao Xia, Xuetao Guo, Tiecheng Wang, and Lingyan Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05794 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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ACS Sustainable Chemistry & Engineering

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Mechanisms for highly-efficient mineralization of bisphenol A by

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heterostructured Ag2WO4/Ag3PO4 under simulated solar-light

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Tengfei Li†, Haoran Wei‡, Hanzhong Jia†,§, Tianjiao Xia†,§, Xuetao Guo†,§, Tiecheng

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Wang†,§*, Lingyan Zhu†, ‡, §*

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†College

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Taicheng Road, Yangling, Shaanxi Province 712100, PR China

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‡Key

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Education, Tianjin Key Laboratory of Environmental Remediation and Pollution

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Control, College of Environmental Science and Engineering, Nankai University,

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No.38 Tongyan Road, Jinnan District, Tianjin 300071, PR China

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§Key

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Ministry of Agriculture, No.3 Taicheng Road, Yangling, Shaanxi 712100, PR China

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*Corresponding author: Tiecheng Wang, Lingyan Zhu

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E-mail: [email protected] (Tengfei Li)

of Natural Resources and Environment, Northwest A&F University, No.3

Laboratory of Pollution Processes and Environmental Criteria, Ministry of

Laboratory of Plant Nutrition and the Agri-environment in Northwest China,

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[email protected] (Haoran Wei)

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[email protected] (Hanzhong Jia)

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[email protected] (Tianjiao Xia)

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[email protected] (Xuetao Guo)

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[email protected] (Tiecheng Wang)

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[email protected] (Lingyan Zhu)

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT:

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The application of Ag3PO4 as a commonly used photocatalyst is limited by its high

3

recombination potency and electron-hole pairs with weak redox potential. It is

4

significant to enhance the photocatalytic activity of Ag3PO4 by coupling with a wide

5

band-gap semiconductor. In this study, Ag2WO4 was selected to promote the

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photocatalysis of Ag3PO4 giving that it has wide band-gap energy and

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strong-redox-potential. A facile chemical precipitation method was applied to

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assemble Ag2WO4 nanoparticles on the surface of Ag3PO4 to form Ag2WO4/Ag3PO4

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heterojunction. The Ag2WO4/Ag3PO4 heterojunction containing 7.5% molar mass of

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WO42- (7.5W) displayed the most superior photocatalytic efficiency under simulated

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solar-light irradiation: 93% of bisphenol A was degraded just within 10 min and

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above 75% was mineralized within 30 min. The degradation reaction rate constant

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was three times higher than the pure Ag3PO4. The excited high-level-energy electrons

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on the conduction band of Ag3PO4 would transfer thermodynamically to the

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conduction band of Ag2WO4 and the generated valance band holes on Ag2WO4 easily

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shifted to the low-energy valence band of Ag3PO4, resulting in high separation of

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electron-hole pairs. The photogenerated holes and superoxide radical species played

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predominant roles in the reaction system.

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KEYWORDS: Ag2WO4/Ag3PO4; Photocatalysis; Highly-efficient mineralization;

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Solar-light; Bisphenol A.

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INTRODUCTION

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Bisphenol A (BPA) has been extensively used as an additive in household and

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commercial plastic products to improve their hardness.1 The annual production of

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BPA is more than 2 billion pounds worldwide and large amounts of them are finally

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released into aquatic environment.2 Great concerns have been paid to the potential

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risks of BPA to aquatic organisms and human health, because of its endocrine

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disruption effect,3 immunotoxicity,4 and embryo toxicity.5 Recently, many

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remediation methods have been attempted to eliminate BPA from aquatic

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environment, including physical adsorption,6 chemical oxidation7 and biological

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degradation.8 Among these methods, photocatalytic degradation is considered as the

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most promising option due to its environment-friendliness, high-efficiency, relatively

12

low cost, and facile operation conditions.9 Nano-TiO2, as a most widely used

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photocatalyst, has been successfully employed to eliminate BPA from wastewater

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under ultraviolet irradiation.10 However, the wide band gap energy of nano-TiO2

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restrains its photocatalytic activity under visible light irradiation, limiting its practical

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applications.11 Therefore, it is of great significance to develop high-efficient and

17

visible-light driven photocatalysts.

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Silver orthophosphate (Ag3PO4) has received lots of concerns due to its visible

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light absorption and high photocatalytic activity.12-13 However, the high

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recombination potency severely depresses the catalytic activity of pure Ag3PO4.14 In

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addition, the narrow energy gap and low band position of Ag3PO4 would result in its

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poor redox ability. Modification of Ag3PO4 by forming heterojunction is proved to be 3



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1

an efficient strategy to improve its photocatalytic activity. Wang et al.15 prepared a

2

heterostructured Ag3PO4/AgBr/Ag plasmonic photocatalyst with strong photocatalytic

3

activity for degradation of dyes in wastewater. Yang et al.16 reported that the

4

Ag3PO4-graphene

5

performance under visible-light irradiation than pure Ag3PO4.

composite

photocatalyst

exhibited

higher

photocatalytic

6

Silver tungstate (Ag2WO4) is a semiconductor photocatalyst with a wide band

7

gap of 3.1 eV, and often used to decompose organic pollutants under ultraviolet

8

light.17 The more negative/positive conduction/valence band of Ag2WO4 would

9

generate electrons/holes with stronger redox potentials. Thus, it was usually coupled

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with other photocatalysts with narrow band gaps to improve their photocatalytic

11

activities. Zhu et al.18 and Rajamohan et al.19 prepared g-C3N4/Ag2WO4 and

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Fe3O4/Ag2WO4 heterojunction hybrids, respectively, which displayed higher

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photocatalytic activities than the individual photocatalysts. It was also reported that

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double Ag-based binary complexes could significantly improve the photocatalytic

15

properties and stabilities.20

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Inspired by the above analysis, it is hypothesized that modifying Ag3PO4 with

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Ag2WO4 to form heterojunction composite may be a novel strategy to improve the

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photocatalytic activity of Ag3PO4, and thus degrade BPA in water efficiently. To our

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knowledge, there is no any published work investigating the Ag2WO4/Ag3PO4 hybrid

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and its structure-activity relationship.

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Therefore, in this study a series of Ag2WO4/Ag3PO4 heterojunction hybrids were

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prepared via a facile chemical precipitation. Their morphologies, crystal structures, 4


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compositions and optical properties were systematically investigated using SEM,

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TEM, XRD, XPS, UV-vis DRS and so on. The photocatalytic activities of the hybrid

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catalysts for BPA degradation were evaluated under simulated sunlight irradiation.

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Furthermore, the possible mechanism for the improved activity of Ag2WO4/Ag3PO4

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heterojunction was explored via radical trapping experiments, photoluminescence

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measurement and transient photocurrent responses.

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EXPERIMENTAL

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Materials and Reagents. Analytical grade silver nitrate (AgNO3) and sodium

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tungstate dihydrate (Na2WO4·2H2O) were purchased from Kermel Chemical Reagent

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Co. Ltd. (Tianjin, China). BPA (purity>99%) was supplied by Shanghai Aladdin

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Biochemical Technology Co. Ltd. All other reagents were of analytical grade and

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used without further purification. All the solutions were prepared with ultrapure

13

water.

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Preparation of the Catalysts. The Ag2WO4/Ag3PO4 heterojunctions were

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prepared as follows. 0.4 g of AgNO3 was dissolved in 80 mL of pure water, and a

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certain amount of Na2WO4 was added followed by stirring in dark for 15 min.

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Subsequently, 20 mL of 0.04 M Na2HPO4 solution was added into the above mixed

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solution and stirred for 30 min. The precipitates were collected by centrifugation,

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washed several times with pure water and dried overnight, and finally an

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Ag2WO4/Ag3PO4 heterojunction was obtained. To investigate the effects of molar

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ratio of Ag2WO4 and Ag3PO4 on the photocatalytic performance, five hybrid catalysts

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were prepared by adding different amounts of Na2WO4 in the solution of AgNO3. The 5



1

as-prepared photocatalysts were denoted as pure Ag3PO4, 5W, 7.5W, 10W, 12.5W,

2

where W refers to the molar mass percentage of WO42- in the hybrid photocatalysts.

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For example, 5W was the hybrid photocatalyst containing 5% molar mass of WO42-.

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Pure Ag2WO4 was prepared following the same procedure but skipping the step of

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Na2HPO4 addition.

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Photocatalytic Experiments. The photocatalytic experiments were conducted in

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a XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China).

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In a typical experiment, a certain amount of catalysts were added in 40 mL of 20 mg

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L-1 BPA solution. Prior to irradiation, the mixture was magnetically stirred for 60 min

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in dark to achieve adsorption-desorption equilibrium. Subsequently, a 350 W Xe lamp,

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simulating sunlight source, was applied to irradiate the reaction solution. At given

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intervals, 500 μL of target liquid was sampled for residual BPA concentration

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measurement.

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Catalyst characterization and BPA analysis. The crystalline information of the

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catalysts was obtained by X-ray diffractometer (XRD, Ulitma IV, Japan) with Cu-Kα

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radiation under 40 kV and 150 mA. The scanning range was from 20 ° to 80 ° and the

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scanning step was 0.02 °. The morphology and elemental compositions of the catalyst

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were analyzed on a field emission scanning electron microscopy (FESEM, 1530vp,

19

Germany) and a high-resolution transmission electron microscopy (HRTEM,

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JEM-2010FEF, Japan). X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI,

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USA) was applied to analyze the chemical compositions of the catalysts. UV-vis

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diffuse reflectance spectrometer (UV-DRS, Hitachi U-3010) was used to measure the 6


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optical absorption property of the catalyst, with BaSO4 as reflectance standard.

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Fourier transform infrared spectroscopy (FTIR, TENSOR37, Bruker) was employed

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to determine the binding states of the catalysts. Recombination potency of the

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photogenerated

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spectrophotometer (PL, Hitachi F-4500).

electron-hole

pairs

was

measured

by

a

Fluorescence

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The transient photocurrent responses of the catalyst were obtained on a

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CHI660D Electrochemical Workstation (Shanghai Chenhua Instrument Co. Ltd.), in

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which ITO/photocatalyst electrode, platinum wire, and saturated calomel electrode

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were employed as the working electrode, counter electrode, and reference electrode,

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respectively. Na2SO4 solution (0.1 mol L-1) was used as the electrolyte and a 500 W

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Xe lamp was applied as the incident light source. The photocurrent responses were

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recorded along with light switching on and off at certain intervals.

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BPA concentration was measured on a high performance liquid chromatograph

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(HPLC, Agilent 1260) equipped with a Fluorescence Detector (with excitation and

15

emission wavelengths at 230 and 315 nm, respectively), and the column was an

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Agilent XDB_C18. The mobile phase consisted of 65% methanol and 35% water at a

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flow rate of 0.15 mL/min. Total organic carbon analyzer (TOC MultiN/CUV,

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Analytic Jena, Germany) was used to measure the content of total organic carbon in

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the reaction solution.

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RESULTS AND DISCUSSION

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Characterization of As-Prepared Photocatalysts. Fig. 1 displayed the XRD

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patterns of Ag2WO4, Ag3PO4 and Ag2WO4/Ag3PO4 composites. All the identified 7



1

peaks of prepared Ag2WO4 and Ag3PO4 could be well indexed to the crystal planes of

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Ag2WO4 (JCPDS No. 70-1719)21 and Ag3PO4 (JCPDS No. 06-0505)22, respectively,

3

indicating that pure Ag2WO4 and Ag3PO4 crystals were successfully synthesized. For

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the 12.5W hybrid photocatalyst, the characteristic peaks of Ag2WO4 were observed

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although with very low intensities, suggesting that Ag3PO4 was successfully

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impregnated with Ag2WO4. The diffraction peaks of Ag2WO4 were rarely detected in

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other hybrids, which might be attributed to the high homogeneity and low content of

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Ag2WO4.23 In all the as-prepared composite photocatalysts, the diffraction peaks of

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Ag3PO4 were in agreement with the standard, implying impregnation of Ag2WO4 did

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not affect the crystal structure of Ag3PO4.

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Fig. 2 illustrated the morphologies of the catalysts. The FESEM images showed

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that Ag3PO4 (Fig. 2a) exhibited uniform spherical-like particles with smooth surface.

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Four elements (C, O, P, and Ag) appeared in the Ag3PO4 catalyst as shown in Fig. S1a,

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supporting that pure Ag3PO4 was obtained. Noticeably, the C element was mainly

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derived from the FESEM grid.24 Compared with pure Ag3PO4, the 7.5W product still

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maintained spherical-like structure but with rough surface (Fig. 2b), which could be

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attributed to anchoring of Ag2WO4 on the surface of Ag3PO4. The appearance of W

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element in EDS (Fig. S1b) further demonstrated successful impregnation of Ag2WO4.

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Some small Ag2WO4 particles were observed to adhere to the surface of Ag3PO4, as

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shown in Fig. 2c. The crystal structure of the interface between Ag2WO4 and Ag3PO4

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was characterized by HRTEM and the result is shown in Fig. 2d. The two lattice

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fringe spacings at 0.246 and 0.230 nm could be well indexed to the (211) plane of 8


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Ag3PO425 and (421) plane of Ag2WO4,21 respectively. The EDS elemental mapping in

2

Fig. 2e illustrated that Ag, O, W and P uniformly distributed in 7.5W photocatalyst

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within the select area. All the above results demonstrated that Ag2WO4 was evenly

4

impregnated on Ag3PO4 to form heterojunction structure, which would be conducive

5

for electron transfer and charge separation.26

6

The FTIR spectra of Ag2WO4, Ag3PO4 and 7.5W composite are displayed in Fig.

7

3. A broad and strong absorption band at around 3100 ~ 3500 cm−1 and a weak peak at

8

1663 cm−1 were observed in the FTIR spectra, which could be assigned to the

9

vibrations of O-H of water molecules adsorbed on the surface.27 It was reported that

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the adsorbed water molecules could be transformed into strong oxidizing hydroxyl

11

radicals for organic pollutant degradation.28 Besides, two sharp absorption peaks at

12

about 1076 and 544 cm−1 appeared, which could be attributed to vibrations of PO43-.29

13

The characteristic vibration peaks of Ag2WO4 at about 449, 561, 685, 783 and 827

14

cm-1 corresponded to W-O-W and O-W-O asymmetric stretching vibration modes.30-32

15

Meanwhile, a peak at around 829 cm-1 emerged in the spectrum of 7.5W, suggesting

16

the existence of WO42-.27 These results confirmed that Ag3PO4 and Ag2WO4 were

17

successfully coupled to form the 7.5W heterojunction.

18

XPS was applied to analyze the surface chemical compositions and chemical

19

status of the as-prepared composites, as shown in Fig. 4. All the peak positions were

20

corrected using C1s peak of aliphatic carbon at 284.8 eV. The full XPS spectrum of

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7.5W (Fig. S2) illustrated that Ag, O, W, P and C elements were present, where the

22

C1s was from the XPS instrument itself.24 This result was in accordance with that of 9



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EDS. The high-resolution XPS spectra of different elements are presented in Fig. 4a-d.

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The Ag 3d spectrum of pure Ag3PO4 (Fig. 4a) was composed of two individual peaks

3

at 368.0 and 374.0 eV, which corresponded to the Ag 3d5/2 and Ag 3d3/2 binding

4

energies of Ag (Ⅰ) species,26 respectively. Compared to the pure Ag3PO4, the binding

5

energies of Ag 3d in 7.5W heterojunction decreased slightly, which might be due to

6

the electron injection induced by the strong interfacial interactions between Ag3PO4

7

and Ag2WO4. For the P 2p spectrum (Fig. 4b), the two deconvoluted peaks at 132.34

8

and 133.09 eV in pure Ag3PO4, respectively corresponding to the electron orbitals of

9

P 2p3/2 and P 2p1/2 of P (Ⅴ),33-34 also shifted to lower binding energies at 132.18 and

10

132.95 eV in the 7.5W heterojunction. Similar phenomenon was reported by Ruan et

11

al.35 The high-resolution XPS spectrum of O 1s in Fig. 4c could be further

12

deconvoluted into two individual peaks at 530.64 and 531.99 eV for pure Ag3PO4, and

13

530.48 and 531.64 eV for 7.5W heterojunction, which could be originated from the

14

crystal oxygen and oxygen in –OH group of the absorbed water molecules.36 The new

15

emerged W 4f spectrum in 7.5W heterojunction (Fig. 4d) could be split into two

16

spin-orbit components, W 4f7/2 and W 4f5/2 at 35.20 and 37.30 eV, respectively; which

17

was attributed to the binding energies of W (VI) in WO42-.18,

18

further elucidated that Ag2WO4 and Ag3PO4 were successfully assembled with

19

chemical rather than physical interactions, which would be advantageous for electron

20

transfer between Ag2WO4 and Ag3PO4 and photocatalytic activity enhancement.24, 37

23

These XPS results

21

The UV-vis DRS was applied to examine the optical properties of pure Ag2WO4,

22

Ag3PO4 and Ag2WO4/Ag3PO4 composites. As shown in Fig. 5a, bare Ag2WO4 10


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1

presented a quite weak visible light response with an absorption edge at around 410

2

nm, implying that Ag2WO4 scarcely utilized visible light to generate electrons and

3

holes. Whereas Ag3PO4 displayed a remarkable absorption in visible light region and

4

its absorption edge was around 535 nm.15, 36 The composite photocatalysts exhibited

5

very similar absorption feature to those of Ag3PO4, implying that impregnation of

6

Ag2WO4 did not change the internal structure of Ag3PO4.38 The light absorption

7

capacity of the composite was only slightly weaker than that of Ag3PO4. For a

8

crystalline semiconductor, the optical band gap energy was estimated based on the

9

classic Tauc approach by the following equation: αhν=A(hν–Eg)n/2, in which α, h, ν, A

10

and Eg were the absorption coefficient, Planck constant, light frequency, absorption

11

constant, and band gap energy, respectively. It was worth noting that Ag3PO4 was a

12

semiconductor with indirect band gap (n=4)39 while Ag2WO4 was a direct band gap

13

semiconductor (n=1).40 The Eg values of Ag3PO4 and Ag2WO4 were individually

14

determined to be 2.16 and 3.05 eV from the intercepts of tangent to plot of (αhν)2/n

15

versus photon energy (hν) in Fig. 5b, which were consistent with those reported by

16

Zhao et al.41 and Li et al.42 As for the heterojunction composites, because of their

17

uncertain optical transition types, the band gap energies could be determined using the

18

equation Eg=1240/λg, where λg corresponds to the wavelength of absorption edge.38

19

The calculated Eg values of heterojunction composites with different Ag2WO4

20

contents were in the range of 2.34 ~ 2.35 eV.

21 22

Meanwhile, the specific band structures of Ag2WO4 and Ag3PO4 could be determined by the following equations:43 11



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1

EVB = X – Ee + 0.5Eg

(1)

2

ECB = EVB – Eg

(2)

3

Where EVB and ECB are the valence band and conduction band edge potentials,

4

respectively; X is the geometric mean of the absolute electronegativity of the

5

constituent atoms;44 Ee is the energy of free electrons on the hydrogen scale (about

6

4.5 eV vs NHE); Eg is the band gap energy. The calculated energy band parameters of

7

Ag2WO4 and Ag3PO4 are listed in Table 1.

8

Photocatalytic Activities of the Catalysts. The photocatalytic activities of the

9

as-prepared catalysts were evaluated via BPA degradation under simulated sunlight

10

irradiation. As shown in Fig. 6a, BPA was very stable under sunlight irradiation, and

11

only a small percentage of BPA was decomposed when Ag2WO4 was applied as the

12

catalyst. Ag3PO4 exhibited a good photocatalytic activity and about 92% of BPA was

13

decomposed within 30 min irradiation. As the molar mass percentage of Ag2WO4

14

increased from 0 to 7.5%, the degradation efficiency increased with the Ag2WO4

15

content, and 7.5W presented the best performance with nearly 93% of BPA removal

16

within 10 min irradiation. The 7.5W heterojunction displayed higher removal rate

17

than the physical mixture with the same molar ratio of Ag3PO4 and Ag2WO4. This

18

enhancement could be due to the formation of heterojunction, which improved the

19

separation of photoinduced electron-hole pairs and then generated more amounts of

20

reactive radicals to decompose the organic pollutants.

21

However, BPA removal efficiency decreased gradually as the Ag2WO4 content

22

further increased. This could be attributed to the decrease of reactive sites of Ag3PO4, 12


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1

which might be occupied by additional Ag2WO4. The pseudo-first-order reaction

2

kinetic model was selected to describe the BPA decomposition process45 and the

3

fitting results are shown in Fig. 6b. Specially, 7.5W heterojunction presented the

4

maximum reaction rate constant, which was three times of that of pure Ag3PO4.

5

Efficient mineralization of organic compounds was of significance to avoid

6

secondary pollution in wastewater treatment. TOC removal rate was analyzed and the

7

results are shown in Fig. 6c. Approximately 68% and 2% of TOC was eliminated in

8

the Ag3PO4 and Ag2WO4 photocatalytic system, respectively, after 30 min irradiation,

9

whereas it was enhanced to 75 ~ 77% for the Ag2WO4/Ag3PO4 heterojunctions. Only

10

38% of TOC was removed during BPA degradation by traditional TiO2 photocatalyst

11

under visible light irradiation for 12 h.46 Therefore, the Ag2WO4/Ag3PO4

12

photocatalyst displayed an outstanding performance for BPA mineralization under

13

visible light irradiation.

14

Possible Mechanisms. PL spectrum is usually used to investigate the ability of

15

charge separation. The typical PL spectra of pure Ag3PO4 and 7.5W heterojunction

16

are illustrated in Fig. 7a. Obviously, Ag3PO4 exhibited a broad and strong PL signal at

17

around 510 nm, which could be attributed to the rapid recombination of electron-hole

18

pairs; whereas the PL intensity of 7.5W heterojunction significantly decreased. These

19

indicated that introduction of Ag2WO4 into Ag3PO4 efficiently facilitated the transfer

20

of photogenerated charges and inhibited their rapid recombination.47 Furthermore, the

21

transient photocurrent responses of pure Ag3PO4 and 7.5W heterojunction are

22

recorded in Fig. 7b. The photocurrent response of 7.5W heterojunction was higher 13



1

than that of pure Ag3PO4, which further indicated a lower recombination probability

2

of electron-hole pairs in the 7.5W heterojunction.48

3

Strong oxidizing radicals, such as ·OH, h+ and ·O2-, are the major species for

4

degradation of organic pollutants in aquatic phase.49 In order to evaluate the roles of

5

different radicals in the photodegradation process, radical trapping experiments were

6

conducted and the results are presented in Fig. 8. Ammonium oxalate (AO),

7

p-benzoquinone (p-BQ) and methanol (MT) were employed as the scavengers of

8

h+, ·O2- and ·OH, respectively.14, 50 Obviously, the photocatalytic activity was greatly

9

inhibited in the presence of AO and p-BQ but not MT. The effects of various

10

quenchers on the removal efficiency are presented in Fig. S3a-c. The results suggested

11

that h+ and ·O2- played vital roles in the photocatalytic process with 7.5W under

12

simulated sunlight irradiation.

13

Fig. 9 manifested the possible photocatalytic mechanism of the Ag2WO4/Ag3PO4

14

heterojunction composite under simulated solar light (λ > 290 nm). As depicted in Fig.

15

9, the charge transfer between Ag2WO4 and Ag3PO4 significantly facilitated

16

separation of the generated electron-hole pairs, which was different from previous

17

studies.51-52 It is well known that the energy potential difference between valence and

18

conduction band determines the potency of photocatalytic reactions under a certain

19

wavelength. Since the Eg of Ag3PO4 was calculated to be 2.16 eV, according to the

20

equation: λ=1240/Eg, it was reasonable that the light with wavelength λ < 574 nm

21

could be absorbed by Ag3PO4 and the electrons in its VB were excited to different

22

energy-levels of CB, including high-energy region and low-energy one.53 Similarly, 14


Page 14 of 37

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1

Ag2WO4 could absorb the irradiation light < 406 nm to generate electron-hole pairs.

2

When the incident light λ was in the range of 406 ~ 574 nm (a), Ag3PO4 was able to

3

be activated and the produced electrons on Ag3PO4 corresponded to the redox

4

potential of 0.38 ~ -0.51 eV, in which the low-level-energy electrons would relax

5

quickly to the CB bottom and recombine with its holes; while the excited

6

high-level-energy electrons with < -0.02 eV would transfer thermodynamically to the

7

CB of Ag2WO4. A similar phenomenon was also reported in previous studies.54-56 For

8

the irradiation light λ within the range of 290 ~ 406 nm (b), both Ag2WO4 and

9

Ag3PO4 could be excited to generate electron-hole pairs. In this case, the excited

10

electrons in Ag3PO4 with high-level-energy (-0.51 ~ -1.73 eV) inclined to shift

11

energetically to the CB of Ag2WO4, which was equipped with excited electrons at

12

lower potential of -0.02 ~ -1.24 eV. Meanwhile, the generated valance band holes on

13

Ag2WO4 with energy of 3.03 eV easily shifted to the low energy valence band of

14

Ag3PO4. Thus, the separation of electron-hole pairs was greatly improved. Similar

15

thermodynamic electronic transfer mechanisms in other hybrid photocatalysts were

16

reported by Chang et al.37 and Xie et al.57 The reformed high-level-energy electrons in

17

CB of Ag2WO4 had more negative potential than the standard redox potential of

18

O2/•O2− (-0.33 eV),58 could reacted with O2 to form •O2−, which was in agreement

19

with those reported by Chen and Vignesh et al.59-60 Additionally, the generated holes

20

stored in the valence band of Ag3PO4 oxidized OH− into •OH due to its more positive

21

potential (2.54 eV) than the standard redox potential of OH−/•OH (1.99 eV).61 The

22

generated •O2−, •OH and holes with strong oxidizabilities, could degrade BPA into 15



1

CO2 and H2O. As a result, the Ag2WO4/Ag3PO4 heterojunction displayed strong

2

photocatalytic activity and mineralization capacity to BPA.

3

CONCLUSIONS

4

A novel heterojuncted Ag2WO4/Ag3PO4 composite was firstly synthesized by

5

facile chemical precipitation method. Various catalyst characterization of the prepared

6

Ag2WO4/Ag3PO4 heterojunctions confirmed that Ag2WO4 was successfully anchored

7

on the surface of Ag3PO4 with chemical interactions. The photocatalytic activity of

8

Ag3PO4 was greatly enhanced for BPA degradation as well as its mineralization under

9

simulated sunlight irradiation. The 7.5W heterojunction displayed the highest

10

photocatalytic activity among the composites with percentage of Ag2WO4 in the range

11

of 0 ~ 12.5%. Due to the successful anchoring of Ag2WO4 on Ag3PO4, the electrons in

12

Ag3PO4 generated by simulated solar light irradiation migrated efficiently to the CB

13

of Ag2WO4, while the produced holes on Ag2WO4 preferred to shift to the valence

14

band of Ag3PO4, resulting accelerated separation of electron-hole pairs. As a

15

consequence, large amounts of •O2−, •OH and hole were generated and efficiently

16

degraded and mineralized BPA.

17 18

ASSOCIATED CONTENT

19

Supporting Information

20

The Supporting Information is available free of charge on the ACS Publications

21

website. EDS results of pure Ag3PO4 and 7.5W heterojunction, full XPS spectrum of

22

7.5W heterojunction, effects of different amount of AO, p-BQ, MT on BPA 16


Page 16 of 37

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1

degradation by 7.5W heterojunction.

2 3

AUTHOR INFORMATION

4

Corresponding Authors:

5

* E-mail: [email protected]

6

* E-mail: [email protected]

7

Notes

8

The authors declare no competing financial interest.

9 10

ACKNOWLEDGEMENTS

11

This work was supported by the National Natural Science Foundation of China

12

(21737003,

13

(2014CB932001), and Yangtze River scholar program, and 111 program, Ministry of

14

Education, China (T2017002).

21577067),

Ministry

of

Science

and

Technology

of

China

15 16

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induce

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1 2

Figures

3

4 5 6

Figure 1 XRD patterns of the as-prepared Ag2WO4, Ag3PO4, and Ag2WO4/Ag3PO4

7

composites with different Ag2WO4 contents.

27



Page 28 of 37

(a)

(b)

(c)

(d)

1

(421) Ag2WO4 (211) Ag3PO4

2

(e)

3 4 5

Figure 2 FESEM and TEM images of the as-prepared catalysts (a. FESEM of pure

6

Ag3PO4; b. FESEM of 7.5W heterojunction; c. TEM of 7.5W heterojunction; d.

7

HRTEM of 7.5W heterojunction; e. Mapping image of 7.5W heterojunction.

28


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1 2 3

Figure 3 FTIR spectra of the as-prepared pure Ag3PO4, Ag2WO4 and 7.5W

4

heterojunction.

5 6 7 8 9

29



Page 30 of 37

(a)

(b)

(c)

(d)

1

2 3 4

Figure 4 High-resolution XPS spectra of the as-prepared photocatalysts. (a-c. Ag 3d,

5

P 2p, and O 1s of pure Ag3PO4 and 7.5W heterojunction, respectively; d. W 4f

6

spectrum of 7.5W heterojunction).

7

30


Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

1

(b)

2 3 4

Figure 5 UV-vis DRS of as-prepared Ag2WO4, Ag3PO4, and Ag2WO4/Ag3PO4

5

composite with different Ag2WO4 contents (a), the band gaps of Ag2WO4 and Ag3PO4

6

(b).

7

31



1

(a) 2

(b)

3

(c)

4 5 6

Figure 6 Photocatalytic degradation of BPA (a), pseudo-first-order reaction kinetic (b)

7

and TOC removal efficiency (c) by the different catalysts under simulated sunlight

8

irradiation. 32


Page 32 of 37

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(a)

1

(b)

2 3 4

Figure 7 PL spectra (a) and photocurrent curves (b) of the as-prepared Ag3PO4 and

5

7.5W heterojunction.

6

33



1

2 3 4

Figure 8 Effects of different radicals scavengers on the photodegradation of BPA by

5

7.5W heterojunction under simulated solar light irradiation.

34


Page 34 of 37

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1

(a)

2

(b)

3 4 5

Figure

9

Schematic

diagram

of

possible

photocatalytic

6

Ag2WO4/Ag3PO4 heterojunction for BPA degradation under simulated light

7

irradiation with 406 nm < λ < 574 nm (a) and 290 nm < λ < 406 nm (b). 35


mechanism

of


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Table

1 2

Semiconductor

Eg (eV)

X (eV)

EVB (eV)

ECB (eV)

Ag2WO4

3.05

6.00

3.03

-0.02

Ag3PO4

2.16

5.96

2.54

0.38

3 4

Table 1 The calculated energy band parameters of Ag2WO4 and Ag3PO4.

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 36


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1

Abstract Graphic

2 3 4

Synopsis: Novel Ag2WO4/Ag3PO4 heterojunction was synthesized successfully and

5

displayed super photocatalytic activity to BPA as well as the excellent mineralization.

37


Mechanisms for highly-efficient mineralization of bisphenol A by

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