Enhanced generation of reactive oxygen species under visible light

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Energy and the Environment

Enhanced generation of reactive oxygen species under visible light irradiation by adjusting the exposed facet of FeWO4 nanosheets to activate oxalic acid for organic pollutant removal and Cr(VI) reduction Jun Li, Chun Xiao, Kai Wang, Yuan Li, and Gaoke Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00641 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Environmental Science & Technology

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Enhanced generation of reactive oxygen species

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under visible light irradiation by adjusting the

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exposed facet of FeWO4 nanosheets to activate

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oxalic acid for organic pollutant removal and Cr(VI)

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reduction

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Jun Li‡, Chun Xiao‡, Kai Wang, Yuan Li and Gaoke Zhang*

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Hubei Key Laboratory of Mineral Resources Processing and Environment, State Key Laboratory

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of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road,

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Wuhan 430070, China

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*Corresponding author. E-mail: [email protected]; Phone/Fax: 86-27-87887445.

ACS Paragon Plus Environment

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ABSTRACT

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In this work, taking FeWO4 nanosheets as an example, the activation of oxalic acid (OA) based

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on facet engineering for the enhanced generation of active radical species is reported, revealing

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unprecedented surface Fenton activity for pollutant degradation. Density functional theory (DFT)

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calculations confirmed the more efficient generation of reactive oxygen species over FeWO4

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nanosheets with the {001} facet exposed (FWO-001) under visible light irradiation compared to

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the efficiency of FeWO4 nanosheets with the {010} facet exposed (FWO-010), which could be

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attributed to a higher density of iron and the efficient activation of OA on the {001} facet. The

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H2O2-derived •OH tended to diffuse away from the active sites of FWO-001 into solution to

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favor the continuous activation of OA into the active radicals for pollutant redox reactions, but

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preferred to remain on FWO-010 to hinder the further activation of OA on the {010} facet.

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Additionally, the generation of •CO2- endowed FeWO4 with a strong reduction ability. Compared

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with FWO-010, FWO-001 exhibited enhanced redox activity for the catalytic degradation of

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organic pollutants and Cr( Ⅳ ) in the optimized conditions. These findings can help in

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understanding the facet dependent surface Fenton chemistry of catalytic redox reactions and in

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designing efficient catalysts for environmental decontamination.


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INTRODUCTION

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The presence of persistent organic pollutants, such as industrial dyes, antibiotics, and

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hexavalent chromium Cr(VI), in the wastewater has caused great concern because of their

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incomplete elimination and persistent pollution of the environment.1-6 Due to the large volume of

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industrial and municipal wastewater, advanced water treatment techniques are essential for

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maintaining healthy water circulation. Advanced water treatment techniques for organic

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pollutant degradation rely on the Fenton reaction (Fe2+/H2O2) to generate •OH.7-10 Nevertheless,

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in a traditional Fenton reaction system, the free ferrous ions can be quickly expended, and this

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process requires the addition of Fe2+. The excess addition of Fe2+ can result in the accumulation

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of Fe2+. To avoid the disadvantages of traditional Fenton reactions, heterogeneous Fenton

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systems, such as Fe0, WS2, CuFeO2, α-FeOOH and maghemite/montmorillonite (MMT)

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composites, have been developed for environmental purification.11-15 Although numerous

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Fenton-like catalysts have been extensively investigated for contaminant removal, developing

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highly efficient, stable and low-cost catalysts is still challenging and a promising research field.

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Different from traditional Fenton systems, the Fe(III)-oxalate system, which can produce

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strong oxidized radicals to degrade organic pollutants under visible light irradiation without the

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addition of H2O2, has received considerable attention.16-18 The photochemical Fe(III)-oxalate

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system has been demonstrated to be more efficient for catalytic organic pollutant degradation

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than the Fenton reaction (Fe(II/III)/H2O2) due to the rapid cycling of iron and the generation of •

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OH.19-21 To date, numerous studies have been reported regarding the superior photo-Fenton

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catalytic activity and mechanism of the Fe(II/III)-oxalate system. Mazellier and Sulzberger22

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reported an α-FeOOH/oxalate system and systematically explored the mechanism of Fe(II)

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formation and the role of oxalate. Wei et al.23 studied a zero-valent iron (Fe0)/oxalate system for


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the simultaneous and rapid redox removal of chromium (Cr(VI)) and orange II and its relative

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mechanism. Recent studies of Fe/oxalate systems have focused on designing composite

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structures, different Fe-based compounds, etc. to improve the stability and activity of the

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system.24-27 Zhang et al.28 reported the hematite with exposed different facets for U(VI) removal.

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The U(VI) adsorption site densities on the {012} and {110} facets of hematite were higher than

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that on the {001} facet, suggesting that the U(VI) adsorption activity by hematite was strongly

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dependent on the coordination type of U(VI) on the hematite facets. Like this, due to the

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difference of the surface structure of FeWO4 nanosheets with different exposed facets, the OA

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adsorption activity by FeWO4 nanosheets was determined by the coordination environment of

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OA on FWO-001 and FWO-010, which would affect the catalytic performance in

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FeWO4/OA/vis system. Typically, FeWO4 as a significant functional material, has been widely

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used in catalysis, dyes, pigments, sensors, preservatives and magnetic materials. However, a

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detailed study about the Fenton-like reaction by controlling exposed facets for pollutant removal

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has not been reported up to now.

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Herein, inspired by facet engineering, FeWO4 nanosheets with different exposed facets were

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successfully synthesized via a hydrothermal method, representing a promising approach for

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adding OA to a catalyst/aqueous organic pollutant system under visible light irradiation to

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produce active radicals and realizing the removal of pollutants. This system for the catalysts-

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assisted activation of oxalic acid Fenton catalysis was validated in FeWO4/acid red G (ARG),

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FeWO4/methyl orange (MO), FeWO4/4-nitrophenol (4-NP) and FeWO4/Cr( Ⅵ ) systems under

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visible light irradiation. Compared with FWO-010, FWO-001 exhibited improved H2O2 and •OH

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generation, resulting in a greatly enhanced redox activity for the degradation of ARG and Cr(VI)

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under optimal conditions. A possible mechanism for FeWO4 nanosheets with different exposed


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facets activating OA to generate active species was proposed. Our work provides atomic scale

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insights for understanding the influence of different facets on the adsorption and activation of

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oxalate, H2O2 dissociation and •OH generation behaviors.

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MATERIALS AND METHODS

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The Synthesis of FWO-001. Firstly, 0.014 mol of Na2WO4·2H2O was added to 100 mL of

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water with magnetic stirring until a homogeneous solution was formed. The pH of the Na2WO4

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solution was adjusted to 1.2 by using a 2 M HCl solution. Second, 0.035 mol C2H2O4·2H2O was

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added to the above solution, which was then diluted to 250 mL with continuous magnetic stirring

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for 30 min at room temperature. Finally, 30 ml of the above solution was measured and

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transferred into a 50 ml stainless steel polyphenylene (PPL)-lined autoclave, and simultaneously,

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0.012 mol of FeSO4·7H2O was added. The autoclave was sealed and heated to 220 °C for 24 h,

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and then naturally cooled to room temperature. The final products were collected by

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centrifugation, washed with distilled water, dried at 60 °C for 6 h in a vacuum drying chamber,

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and finally labelled as FWO-001.

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The Synthesis of FWO-010. First, 0.99 g of Na2WO4·2H2O and 1.176 g of

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Fe(NH4)2(SO4)2·6H2O were dissolved in 30 mL of deionized water with continuously magnetic

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stirring until a homogeneous solution was formed. Afterwards, 6 mL of a NaOH solution (1

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mol/L) was added to the above solution until a dark green solution formed. Finally, the

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suspension was transferred into a stainless steel PPL-lined autoclave, sealed and heated to 180

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°C for 12 h. The final products were collected by centrifugation, washed with distilled water,

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dried at 60 °C for 6 h in a vacuum drying chamber, and finally labelled as FWO-010.

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Characterization. The phase and crystallinity of the catalysts were determined by powder X-

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ray diffraction (XRD, Japan) over the diffraction angle range from 5° to 70° at a step size of


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0.02°. The X-ray source was Cu Kα radiation. The morphologies and microstructures of the

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products were observed by scanning electron microscopy (SEM), transmission electron

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microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using a

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JEM 2100 F electron microscope at an accelerating voltage of 200 kV. X-ray photoelectron

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spectroscopy (XPS) analysis was performed using an ESCALAB 250Xi system (Thermo

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Scientific, USA), all binding energies were calibrated in reference to the binding energy of the C

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1s peak. Fourier transform infrared (FT-IR) spectroscopy (Thermo Nicolet, USA) was used to

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characterize the chemical bonds of the products.

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Pollutant Degradation. The degradation of acid red G (ARG) in water was performed to

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evaluate the photocatalytic activities of the samples. In a typical experiment, 35 mg of the

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catalyst was dispersed in 50 mL of an aqueous ARG solution (50 mg/L). To eliminate the effect

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of adsorption, the mixed suspensions were constantly stirred for 30 min in the dark to reach the

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adsorption equilibrium. Next, 6.3 mg of oxalic acid dihydrate was added to the above suspension

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and the suspension was irradiated with visible light. In this work, 100 W LED lamp with the

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wavelengths 420 nm was selected as visible light source and its light intensity of visible light

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sources was measured to be 55 mW•cm-2. Then, 7 mL of the suspension was collected after a

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certain time interval (15 min) and then centrifuged (5000 rpm, 5 min). Finally, the concentration

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of ARG was measured by using a UV-Vis spectrophotometer (Orion AquaMate 8000, China) at

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the maximum absorption wavelength of ARG (554 nm).

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Density Functional Theory Calculations. DFT calculations were performed using the Vienna

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Ab initio Simulation Package with projector-augmented wave (PAW) pseudopotentials.29 The

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generalized gradient approximation (GGA) in the Perdew, Burke and Ernzerhof (PBE)

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parametrization was selected as the exchange-correlation functional.30 A plane-wave basis set


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with a cutoff energy of 420 eV was set. The lattice parameters of FWO-001 were a = 4.75 Å, b =

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5.72 Å, c= 22.04 Å, and that of FWO-010 were a = 4.97 Å, b = 4.75 Å, and c= 22.18 Å. In

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addition, the k-point meshes of FWO-001 and FWO-010 were both selected to be 5 × 5 × 1. The

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residual force and iterative energy difference of all atoms were allowed to converge to 1×10-4 eV

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and 0.02 eV‧Å-1.

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RESULTS AND DISCUSSION

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To investigate the structures of the samples, a series of characterization methods, including

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XRD, FT-IR and XPS measurements, were performed. Figure 1a shows the XRD patterns of the

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samples. All the characteristic peaks agree well with those of the standard card for FeWO4

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(JCPDS: 46-1446), and no other peaks are found, indicating the high crystallinity and purity of

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the samples. Figure 1b shows the FTIR spectra of FWO-001 and FWO-010. The peak at 854 cm-

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band at 650 cm-1 can be assigned to the stretching vibration of W-O in the WO6 octahedral, and

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the asymmetric deformation vibration of Fe-O in FeO6 is observed at 508 cm-1.31-33 XPS

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measurements were then carried out to elucidate the chemical states of the samples. The two

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peaks at 709.7 and 723.9 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the

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presence of the Fe2+ species (Figure S1a).34 Figure S1b shows two obvious peaks located at 35.3

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and 37.4 eV, which can be attributed to W 4f7/2 and W 4f5/2, respectively, confirming the

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presence of W6+ in the FeWO4 structure.35 The O 1s region can be deconvoluted into two peaks

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corresponding to the lattice oxygen of the W-O bond (530.3 eV) and -OH (532.1 eV) (Figure

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S1c).36

corresponds to the symmetric vibration peak of the oxygen atom in Fe-W-O, the characteristic


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Figure 1. (a) XRD patterns, (b) FTIR spectra, (c, f) TEM images, (d, g) HRTEM images and (e,

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h) FFT images of FWO-001 and FWO-010.

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TEM images of pure FWO-001 and FWO-010 are shown in Figure 1c-h. It can be seen that

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FWO-001 and FWO-010 both exhibit nanosheet morphology. HRTEM images provide further

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insight into the microstructures of the as-prepared samples. Lattice fringe spacings of 0.37 nm

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for the FWO-001 nanosheet and 0.247 nm for the FWO-010 nanosheets correspond to the (011)

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and (021) lattice planes, respectively, and their corresponding fast Fourier transform (FFT)

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images reveal their single-crystalline nature. By analysis of the FFT images we can conclude that

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the FWO-001 nanosheets and the FWO-010 nanosheets were grown with the [001] and [010]


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orientations, respectively, as the main exposed facets, suggesting the main exposed facet are the

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{001} and {010} facets, respectively.

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The photo-Fenton activity of the FeWO4 nanosheets with different exposed facets in the

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presence of OA for visible-light-driven photo-Fenton catalysis was evaluated by the degradation

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of ARG and the results are shown in Figures 2a, b and S2. In the dark, with or without the

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addition of H2O2, FWO-001 showed weak adsorption performance and negligible activity toward

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ARG degradation. Under visible light irradiation, FWO-001/H2O2 system still exhibited

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ignorable performance toward ARG degradation. In the presence of both OA and visible light,

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the degradation ratio of ARG by FWO-001 is 98%, confirming that FWO-001 can significantly

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accelerate the degradation rate of ARG in the presence of OA under visible light irradiation. The

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reaction kinetic constant (k) for FWO-001 is more than 4 times greater than that of FWO-010 for

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the photo-Fenton catalytic oxidative degradation of ARG. Additionally, the photo-Fenton

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catalytic reductive degradation of Cr(VI) in an aqueous solution was also measured (Figure 2c

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and d). Only a 10% removal rate is observed for a Cr(VI) solution mixed with FWO-001 and

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C2H2O4 in the dark, showing the weak adsorption of Cr(VI) onto FWO-001. After adding oxalic

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acid or FWO-001, the removal rate of Cr(VI) is only 11% and 19%, respectively. A 100%

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removal ratio of Cr(VI) is achieved by FWO-001 in the presence of oxalic acid under visible

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light irradiation. However, a 38% removal rate is observed after adding FWO-010 to the aqueous

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solution under the same conditions. It can be seen that the reaction kinetic constant (k) of FWO-

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001 is approximately 10 times greater than that of FWO-010 for the photo-Fenton catalytic

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reductive degradation of Cr(VI). XPS measurement was further employed to analyze the surface

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species after Cr(VI) reduction (Figure S3). The peaks located at 579.3 and 588.4 eV could be

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assigned to the binding energies of Cr(VI) 2p3/2 and Cr(VI) 2p1/2, respectively. The peaks at


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577.8 and 586.9 eV could be ascribed to the binding energies of Cr(III) 2p3/2 and Cr(III) 2p1/2,

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respectively, suggesting the Cr(VI) was actually reduced into Cr(III) in the presence of the

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FWO-001 and OA under visible light irradiation. In comparison with previously reported

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catalysts/OA/vis systems (Table S1), the FWO-001/OA/vis system revealed superior or

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comparable catalytic performance for pollutants removal. The above results confirm that FWO-

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001 can more favorably activate OA for pollutant removal.

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The influences of the concentrations of the catalyst and oxalic acid and the solution pH on

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ARG degradation were examined. When the dosage of catalysts was increased from 0.3 to 1.5 g•

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L-1, more than 90 % degradation ratio of ARG was maintained within 60 min (Figure S4a, Table

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S2). The effect of OA concentration on the catalytic degradation of ARG was observed from 0.5

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to 2.0 mM. The results showed when the concentration of OA was 1.0 mM, FWO-001

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nanosheets exhibited the optimized catalytic activity (Figure S4b). The excess oxalate inhibited

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the degradation of ARG initially. The reason may be that oxalate can also act as a scavenger of

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·OH produced in the Fenton reaction. Figure S4c displayed the influence of initial solution pH on

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the degradation of ARG by FWO-001 nanosheets in a wide pH range of 2.2-9.6. As shown in

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Figure 3c, the catalytic activity of FWO-001 nanosheets increased with increasing pH between

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2.2 and 5.8. The pH-dependence of ARG degradation in the photo/ferrioxalate system can be

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explained by the dependence of Fe(II) speciation on pH value, which affects the rate of the

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Fenton reaction. At higher solution pH, the solubility of Fe(III) and Fe(II) strongly decreased and

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the main species were Fe(III)-OH and Fe(II)-OH which could form precipitations and lost

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photoactive. In order to observe the intermediate products of ARG degradation, the Liquid

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chromatography-mass spectrometer (LC-MS) was performed. According to the analysis of mass

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spectrometry, the main transformation products can be identified and products with m/z of 291,


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274, 231, 160 and 156 could be attributed to the degraded products of ARG in aqueous solution.

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The corresponding mass spectra and chemical structures of the possible intermediates are

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presented in Table S3. In addition, MO, RhB, 4-NP and tetracycline (TC) were also selected as

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target pollutants to generalize the effect of OA activation by the FeWO4 nanosheets (Figure S5),

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showing that the addition of OA is a promising approach that can significantly enhance visible-

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light-driven photo-Fenton catalysis for pollutant removal. The stability of FWO-001 was

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monitored over multiple runs (Figures 2e and S6). After six rums, the FWO-001 nanosheets

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retain their high photo-Fenton catalytic activity for ARG removal and Cr(VI) reduction. Figure

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2f shows the identical XRD spectra of the fresh and used FWO-001 nanosheets. It can be seen

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that their XRD spectra did not show significant changes, further indicating the structural stability

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of the FWO-001 nanosheets.

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Figure 2. Degradation activity curves for (a) ARG and (c) Cr( Ⅵ ) in aqueous solutions under

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different conditions. The corresponding kinetic curves for (b) ARG and (d) Cr(Ⅵ) degradation.

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(e) The cyclical performance of the FWO-001 nanosheet catalyst for the degradation of ARG in


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an aqueous solution. (f) XRD patterns of the catalyst before and after utilization. Initial

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conditions: CARG = 10.0 mg/L, Wcatalyst = 35 mg, COA = 1.0 mM.

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To identify the radical species involved in the pollutant degradation, trapping experiments of

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the active species were carried out.37-38 As shown in Figure 3a, the catalytic performance for the

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degradation of ARG can be inhibited in the presence of isopropanol and p-benzoquinone

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suggesting that •O2- play crucial roles in the degradation process. To identify the source of •O2-, a

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comparison experiment was employed by excluding dissolved O2 and conducting the experiment

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under a N2 atmosphere. The results reveal that the catalytic activity did not significantly

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decrease, confirming that ·O 2- was originated from the decomposition of ·C2O- 4 rather than a

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reaction between dissolved O2 and the electrons of the catalyst.

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Figure 3. (a) Photo-Fenton activities of FWO-001 for the degradation of ARG with different

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radical scavengers. Quantitative determination of the amounts of (b) H2O2 and (c) free •OH

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generated by the as-prepared samples in the presence of H2C2O4. ESR signals of (d) DMPO-•O2-

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and (e) DMPO-•OH for FWO-001 and FWO-010 after 2 min in the dark and under visible light


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irradiation, respectively. (f) Changes in the leached iron concentration at different irradiation

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times for FWO-001. Initial conditions: CARG = 10.0 mg/L, Wcatalyst = 35 mg, COA = 1.0 mM.

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The generation and decomposition of H2O2 in photo-Fenton systems determine the reaction

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rate. The generation of H2O2 over FWO-010/OA and FWO-001/OA systems was evaluated as

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shown in Figure 3b. As the irradiation time increases, the concentration of H2O2 gradually

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increases in the presence of FeWO4 and OA. Compared with that of FWO-010, FWO-001

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exhibits a relative higher H2O2 generation efficiency of 160 μmol/L. Similar phenomena are also

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observed for the generation of •OH (Figure 3c). The high rate of H2O2 production is positively

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correlated to the surface iron ion concentration, which is in agreement with the following DFT

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study. The generation of H2O2 was further demonstrated by the color variation of KI paper, as

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shown in Figure S7. DMPO-ESR spectra were further used to detect the active species. The

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characteristic signals of DMPO-·O 2- and DMPO-·OH can be obviously detected for the FWO-

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001 and FWO-010 samples under visible light irradiation (Figure 3d-e). The ·O 2- and ·OH

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signals of FWO-001 are stronger than those of FWO-010, indicating the amount of ·O2- and ·OH

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generated by the FWO-001 activation of OA significantly exceeds that produced by the FWO-

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010 activation of OA. Figure 3f shows the changes in the Fe2+ and Fe3+ concentrations during the

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catalytic process. Fe2+ and Fe3+ have the same variation tendencies and their maximum

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concentrations are less than 2.5 mg/L, suggesting low Fe leaching rates and high stabilities for

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the samples. A comparison of the homogeneous and heterogeneous Fenton catalytic performance

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for the same Fe content was also performed, indicating that Fe leaching has little influence on the

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activity of the heterogeneous Fenton catalytic degradation of pollutants in this work (Figure

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S4d).


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In-situ IR experiments were performed to observe the relative information on both adsorption

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modes and the relative affinities of OA/H2O2 to the two facets. It can be seen in Figure S8 that as

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the oxalic acid concentration increased from 0 to 2.0 mM, the wavenumbers at 677 and 1948 cm-

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1

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FWO-010 surface. The possible coordination types of Fe-OA on FWO-001 surface were given in

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Figure S9. In this work, FWO-001 revealed better catalytic activity for removal of Cr(Ⅵ) and

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pollutants in the presence of OA under visible light irradiation. According to previous reports,39-

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40

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removal of Cr(Ⅵ) and pollutants. Therefore, the second coordination type in Figure S9b was

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regarded as the optimized Fe-OA coordination type.

appeared, suggesting the different Fe-OA formed different coordination type on FWO-001 and

the second coordination type (Figure S9b) of Fe-OA on FWO-001 surface was helpful to the

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To gain further insight into the effect of the exposed facet and iron concentration on the

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catalytic activity of the FeWO4 nanosheets, density functional theory (DFT) calculations were

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performed to compare the {001} and {010} facets. The representative relaxed atomic geometries

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of each slab are shown in Figure 4a and c. The calculated surface energies of the {001} and

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{010} facets are summarized in Table S4. The surface energies are 1.86 and 2.13 J/m2 for the

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{001} and {010} facets, respectively, indicating that the {010} facet of FeWO4 nanosheets is the

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more active facet. In general, crystals prefer to expose thermodynamically stable facets during

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the crystal growth process, resulting in minimization of the surface energy. Therefore, the

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exposed percentage of the {001} facet is 97%, while the exposed percentage of the {010} facets

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is only 52%. Considering the effect of specific surface area of the samples on their catalytic

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activity, the BET measurements were carried out (Figure S10). Baaed on the IUPAC

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classification, the N2 adsorption-desorption isotherm of FWO-001 and FWO-010 are both of

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type IV with obvious hysteresis hoops of type H3. The specific surface areas of FWO-001 and


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FWO-010 are 10.879 and 17.641 m2/g, respectively (shown in Table S5). The results showed

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that FWO-001 with smaller specific surface area, but had the better catalytic activity as

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compared to FWO-010. Therefore, it can be concluded that the Fe density on different facets of

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FeWO4 may dominate the catalytic activity. Therefore, it is necessary to investigate the Fe

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charge density in the FeWO4 nanosheets with different exposed facets. Figure 4b and d reveal the

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calculated charge densities of the Fe atoms in the {001} and {010} facets, respectively. As such,

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our DFT results confirm that an overwhelming majority of the Fe is distributed in the {001}

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facet, which allows for a higher carrier density as well as more efficient carrier transport. The

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higher concentration of Fe in the {001} facet provides a greater number of active sites in the

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photo-Fenton reaction system, favoring a highly efficient catalytic reaction.

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Figure 4. (a, c) Surface structures of the relaxed stoichiometric {001} and {010} facets and (b,

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d) their corresponding charge density distributions. (e) Calculated adsorption energy profiles for


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H2O2 dissociation over FWO-001 and FWO-010. (f) Schematic illustration of the •OH

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generation process over FWO-001.

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The above theoretical calculation results indicate that the higher concentration of Fe in FWO-

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001 provides a greater number of active sites in the photo-Fenton reaction system, which

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contributes to its highly efficient catalytic reactivity. In a catalyst/OA/visible light system, the

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catalytic performance is closely related to the adsorption and activation of OA, the adsorption

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and dissociation of H2O2, and the diffusion of •OH. To further understand these processes on the

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different facets of the FeWO4 nanosheets, the adsorption energies of OA and H2O2 on the

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different facets of the FeWO4 nanosheets were calculated. Tables S6 and S7 show the calculated

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adsorption energies for the different facets with OA (or H2O2) molecules. In a

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catalyst/OA/visible light system, the OA molecules are first adsorbed onto the FeWO4

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nanosheets, forming an FeII-OA complex and generating H2O2. Once the OA has been activated,

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OA and H2O2 competitively adsorb onto the exposed iron atoms. Due to the higher adsorption

298

energy of H2O2 on the {010} facet of the FeWO4 nanosheets than that of OA on the {010} facet

299

of the FeWO4 nanosheets, the generated H2O2 molecules will preferentially adsorb onto the

300

{010} facet of the FeWO4 nanosheets. The preferential adsorption of H2O2 molecules onto the

301

{010} facet of the FeWO4 nanosheets will occupy the abundant active sites, further hindering the

302

adsorption and activation of OA on the {010} facet of the FeWO4 nanosheets and limiting the

303

catalytic reaction. Furthermore, the slow diffusion of •OH from the {010} facet of the FeWO4

304

nanosheets affects its reactivity with pollutants (Figure 4e). The process for OA activation by

305

FWO-001 is presented in Figure 4f. For FWO-001, the adsorption energy of OA on the {001}

306

facet of the FeWO4 nanosheets is more negative than that of the adsorption energy of H2O2 on

307

the {001} facet of the FeWO4 nanosheets, indicating that OA molecules are preferentially


16

Page 17 of 28


308

adsorbed onto the {001} facet of the FeWO4 nanosheets compared to H2O2, which favors the

309

activation of OA and the diffusion of H2O2. In addition, the decomposition products of H2O2 can

310

easily diffuse into solution to oxidize the pollutants. Subsequently, the freed active sites can

311

participate in the next reaction step. Therefore, the FWO-001 nanosheets can exhibit a better

312

catalytic performance for pollutant degradation in the activated OA system than can the exposed

313

{010} facet of the FeWO4 nanosheets. Obviously, controlling the activation of OA, adsorption

314

and dissociation of H2O2, and diffusion of •OH by facet engineering is a promising strategy to

315

control their catalytic reactivity for pollutant removal in catalyst/OA systems.

316

According to the reported work and the experimental results, a possible mechanism for the

317

catalytic degradation of organic pollutants and Cr(VI) in the FeWO4/OA system under visible

318

light irradiation is proposed (Figure 5). In the reaction system, FeII and OA can form an FeII-OA

319

complex and then transform into an [FeIII(C2O4)3]3- complex, which is a reciprocal reaction

320

(reaction 1) that has a high photochemical reactivity. The excitation of [FeIII(C2O4)3]3- under

321

visible light irradiation involves an intramolecular electron transfer from oxalate to Fe(III),

322

forming ·C2O4- (reaction 2), and then ·C2O4- can be decomposed into ·CO2- and ·O2- radicals

323

(reaction 3-4), which can react with Fe(II) to generate H2O2 (reactions 5-6). In this process,

324

visible light irradiation promotes the generation of the intermediate free radicals, contributing to

325

the catalytic performance, and then, [FeII(C2O4)2]2- reacts with H2O2 to produce ·OH (reaction 7).

326

H2O2 can also be trapped by Fe3+ on the surface of the catalyst, forming Fe2+ and completing the

327

cycle of Fe2+/Fe3+. Finally, the produced active species can attack the pollutants causing the

328

degradation of the organic pollutants (reactions 8) and the reduction of Cr(VI) into Cr(III)

329

(reactions 9-10). The possible reaction pathway is shown in reactions 1-1017, 19, 41-42:


17


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330 331

Figure 5. Schematic illustration for the activation of OA in the FeWO4/OA system under visible

332

light irradiation for the degradation of organic pollutants and Cr(VI).

333

[FeII(C2O4)2]2-↔ [FeIII(C2O4)3]3-

(1)

334

[FeIII(C2O4)3]3- + hν → ·C2O- 4 + [FeII(C2O4)2]2-

(2)

335

C2O- 4·→ ·O2- + CO2

(3)

336 337

C2O- 4·→ ·CO2- + CO2 (4)

338

H+ + ·O2-→ ·HO2

339 340

FeII + ·O- 2/HO2· + H+→ FeIII + H2O2 (6)

341

[FeII(C2O4)2]2- + H2O2 →[FeIII(C2O4)2]+ + ·OH + OH-

(7)

342

OH/·O2- + organic pollutants → CO2 + H2O

(8)

343

[FeII(C2O4)2]2- + Cr(VI)→ [FeIII(C2O4)2]+ + Cr(III)

(9)

344

·CO- 2 + Cr(VI)→ CO2 + Cr(III)

(10)

(5)

345

Environmental Implications. In this work, the effect of the exposed facet of FeWO4

346

nanosheets on oxalic acid activation and the catalytic performance for pollutant degradation was

347

investigated for the first time. In FeWO4/OA/vis system, once the OA has been activated, OA

348

and H2O2 competitively adsorb on the surface of catalysts. Compared to FWO-001, the

349

preferential adsorption of H2O2 molecules on FWO-010 will occupy the abundant active sites,

350

further hindering the continuous adsorption and activation of OA to generation H2O2 and


18

Page 19 of 28


351

limiting the generation of •OH and other active radicals. The easier generation of active radicals

352

on FWO-001 resulted in the efficiently catalytic performance toward the degradation of organic

353

pollutants, especially for Cr(VI) reduction. Therefore, regulating the exposed facet of a catalyst

354

allows for controlling the adsorption and activation of oxalic acid and the generation and

355

decomposition of H2O2. Our findings provide comprehensive insights into facet-dependent

356

surface Fenton chemistry for environmental decontamination.

357

ASSOCIATED CONTENT

358

Supporting Information: Details for the time dependent UV-visible absorption spectra of ARG

359

in an aqueous solution; COD removal; effect of catalyst concentration, C2H2O4 concentration

360

and initial pH on the catalytic performance of FWO-001; the catalytic degradation of MO, RhB,

361

4-NP and TC in the FeWO4/OA system under visible light irradiation; and the adsorption

362

energies of the OA and H2O2 molecules on the FeWO4 nanosheets with different exposed facets.

363

AUTHOR INFORMATION

364

*Corresponding

365

Phone/fax: +86-27-87887445. E-mail: [email protected]

366

Author Contributions

367

‡These authors contributed equally.

368

Notes

369

The authors declare no competing financial interest.

370

ACKNOWLEDGMENTS

Author


19


Page 20 of 28

371

This work was supported by NSFC (No. 51472194), National Program on Key Basic Research

372

Project of China (973 Program) 2013CB632402 and the NSF of Hubei Province (2016CFA078).


20

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374 375


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Enhanced generation of reactive oxygen species under visible light

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