H2O2-free photo-Fenton degradation of organic pollutants on thermally exfoliated g-C3N4

Journal Pre-proof H2 O2 -free photo-Fenton degradation of organic pollutants on thermally exfoliated g-C3 N4 Yuangang Li, Ningdan Luo, Zimin Tian, Huajing Li, Mengru Yang, Weike Shang, Shen Yifeng, Mengnan Qu, Anning Zhou

PII:

S0927-7757(19)31183-5

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124190

Reference:

COLSUA 124190

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

15 September 2019

Revised Date:

1 November 2019

Accepted Date:

1 November 2019

Please cite this article as: Li Y, Luo N, Tian Z, Li H, Yang M, Shang W, Yifeng S, Qu M, Zhou A, H2 O2 -free photo-Fenton degradation of organic pollutants on thermally exfoliated g-C3 N4 , Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124190

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H2O2-free photo-Fenton degradation of organic pollutants on thermally exfoliated g-C3N4

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Yuangang Lia, b* [email protected], Ningdan Luo a, Zimin Tian a, Huajing Li a, Mengru Yang a, Weike Shang a, Shen Yifeng a, Mengnan Qu a, b and

a College

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Anning Zhou a, b

of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China.

Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, MLR., Xi’an, 710021, China

*

Corresponding author

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Graphical abstract

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A H2O2-free photo-Fenton reaction was proposed for degradation of organic pollutants on thermally exfoliated g-C3N4 using Rhodamine B (RhB) as a model organic compounds without external addition of H2O2. The photo-Fenton process conforms to pseudo-first-order kinetics while the photocatalytic reaction conforms to pseudo-zero-order kinetics according to RhB concentration. Detailed mechanism steps were established to give a satisfied explanation for the observed accelerated speed and different kinetics of photo-Fenton degradation of RhB compared with photocatalytic process.

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ABSTRACT

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A H2O2-free photo-Fenton reaction was proposed for degradation of organic pollutantswithout external addition of H2O2, for the first time. It was found that the photo-Fenton process conforms to pseudo-first-order kinetics while the photocatalytic reaction conforms to pseudo-zero-order kinetics. Detailed mechanism steps were established to give a satisfied explanation for the observed phenomena.

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Highlights

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A H2O2-free photo-Fenton reaction was proposed for degradation of organic pollutants on thermally exfoliated g-C3N4 using Rhodamine B (RhB) as a model organic compounds without external addition of H2O2. Compared with conventional photocatalytic degradation of RhB in absence of ferric ions, the photo-Fenton process exhibited accelerated speed and the remove efficiency reached ~100% within 2 h. More importantly, the kinetics of photo-Fenton reaction is different from that of photocatalytic process. The photo-Fenton degradation conforms to pseudo-first-order kinetics while the photocatalytic reaction conforms to pseudo-zero-order kinetics according to RhB concentration. Based on the results of

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photoluminescence (PL), electrochemical impedance spectra (EIS) and scavenger experiments, detailed mechanism steps were established to

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give a satisfied explanation for the observed accelerated speed and different kinetics of photo-Fenton degradation of RhB compared with

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photocatalytic process.

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Keywords: H2O2-free; Photo-Fenton; Graphitic carbon nitride (g-C3N4); Kinetics.

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1. INTRODUCTION

The treatment of toxic and harmful organic pollutants in water is a hotspot in the field of environmental protection [1, 2]. In recent years,

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different types of advanced oxidation processes such as Fenton, photocatalysis, and photo-Fenton reactions can partially meet the development requirements of industrial technology and low-carbon economy, so it has broad prospects in the degradation of refractory pollutants [3-5]. The conventional Fenton (Fe2+/H2O2) degradation can be achieved by producing one of the most effective strong oxidants hydroxyl radicals (•OH) [5, 6]. Currently, the photo-Fenton system that introduces visible light can increase the yield of •OH and the cycle of iron ion [7]. Both the Fenton and photo-Fenton systems require the addition and activation of H2O2 to produce oxygen-related free radicals (•O2-, •OH) for degradation of the

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contaminants [8, 9]. However, because H2O2 is need as a reagent in most of the photo-Fenton systems, its further application is limited by

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narrow pH range, high cost, low utilization of H2O2 and more complicated process [10, 11]. On the other hand, photocatalytic production of

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H2O2 as a self-starting button for photocatalytic pollutant degradation has been reported recently [12-16], but the photocatalytic process does not have a circulation pattern of iron species with H2O2 similar to that of photo-Fenton to activate hydrogen peroxide, so the less oxygen-related free

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radicals makes the degradation efficiency insufficient. With the knowledge of above mentioned, it is reasonable to develop a new type of H2O2-free photo-Fenton reaction in the help iron ion using H2O2 produced in situ by photocatalysis, which is beneficial to develop a simple and

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pH independent process with improved H2O2 utilization and reduced costs. Graphitic carbon nitride (g-C3N4) is considered to be a metal-free polycrystalline polymer with excellent intrinsic characteristics and

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adjustable band gap structure [17-21]. The pioneering work on visible-light photocatalytic water splitting by Wang and co-workers in 2009 has been a milestone in the use of g-C3N4 as photocatalysts [22]. Since g-C3N4 has attracted considerable attentions, the photocatalytic branches such as carbon dioxide reduction [23], nitrogen fixation [24, 25] and pollutant degradation [26-31] have been further rapidly expanded. However, the small specific surface area of pristine g-C3N4 and the weak van der Waals interaction between layers leads to high recombination rate of photogenerated electron-hole pairs, which limits its further practical application [32]. In order to overcome these drawbacks, different strategies

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such as exfoliating [33], doping [12], constructing heterojunction [34-36] etc., have been employed. Now, photo-Fenton reaction on various

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iron-doped g-C3N4 catalysts such as FeOOH-QDs/g-C3N4, FeNx/g-C3N4, Fe2O3/g-C3N4 [37-39] and so on is well-acknowledged as an effective

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method for degrading refractory organic pollutants. In previous works, there appeared very few literatures about photo-Fenton reaction using in situ produced H2O2 under light irradiation for degradation of organic pollution. For example, Li et al. [40] reported a Fenton reaction at solid gas

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interface using alkalized g-C3N4 supported with iron species, in which •OH can be directly produced by hole oxidation of surface hydroxyl groups or by a multistep photoreduction of O2. Theoretically, the conduction band potential of g-C3N4 (-1.1 V) is more negative than the

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reduction potential of O2/H2O2 (0.69 V), so it is thermodynamically reasonable to reduce O2 into H2O2 under visible light irradiation of g-C3N4 [41]. Moreover, the valence band of g-C3N4 (1.57 V) is more positive than the redox potential of RhB (1.43 V), which indicates that it is

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energetically allowed to oxidize RhB by photo-generated holes at the same time [42, 43]. Clearly, g-C3N4 is one of the most suitable candidate catalysts for H2O2-free photo-Fenton pollutant degradation. In the context of the successful photo-Fenton degradation of organic pollutants using g-C3N4, nanosheets from g-C3N4 have once again attracted attention because of their high specific surface area, high aspect ratios and excellent optoelectronic properties, which consolidated the position of g-C3N4 in the field of photo-Fenton reaction [44]. In recent years, many research groups have invested a lot of energy to improve the

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physicochemical properties by optimizing the method for preparing nanosheets [45-47]. Compared with other methods, the most attractive one is

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thermal exfoliation and the successful thermal exfoliating of the bulk g-C3N4 material improves the charge transport channel or charge separation

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efficiency to achieve significant results [48].

Herein, we reported a H2O2-free photo-Fenton reaction to degrade organic pollutants using Rhodamine B (RhB) as a model organic

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pollutants and thermally exfoliated g-C3N4 as a photocatalyst in the presence of Fe3+. An efficient degradation of RhB by photo-Fenton reaction was completed on exfoliated g-C3N4 very quickly without the external addition of H2O2. It is the first time to directly propose the concept of

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H2O2-free photo-Fenton reaction, to the best of our knowledge. Based on the various benefits of H2O2-free photo-Fenton reaction beyond conventional Fenton reaction, we believe this work has unlimited potentials in the field of environmental remediation whether from the view of

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theory or application.

2. EXPERIMENTAL SECTION 2.1 Catalyst preparation

All chemicals and reagents used in this work were analytically pure without further purification. The procedure for preparing samples was as follows [49]: Melamine (20 g) was placed in a crucible and heated from 20 oC to 550 oC at a heating rate of 2.3 oC/min. It was kept at 550 oC

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for 4 h, and then the sample was naturally cooled to room temperature and the bulk g-C3N4 (B-g-C3N4) was obtained as a light yellow powder.

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The B-g-C3N4 powder (1.0 g) was suspended in 10 mL of concentrated H2SO4 solution and stirred at 25 oC for 8 h. The mixture was washed several times to remove unreacted sulfuric acid, and lyophilized under vacuum to obtain H2SO4-intercalated g-C3N4 (H-g-C3N4). Then, the

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H-g-C3N4 was further heat treated at 550 oC under N2 atmosphere for 2 h to obtain heat treated g-C3N4 (TE-g-C3N4) as shown in Fig. S1.

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2.2 Characterization

The crystallinity and crystal phase of as-prepared samples were characterized by X-ray diffraction (XRD) on a Bruker D8 Advance

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diffractometer with Cu Kα radiation (λ = 1.5418 Å) and the scanning angle ranged from 10° to 80° of 2θ. The morphology and structure of the material were observed by a Hitachi S-4800 scanning electron microscope (SEM) under an accelerating voltage of 15 kV. The surface chemical

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composition and chemical valence of the material were investigated by X-ray photoelectron spectroscopy (XPS) on Kratos Axis Ultra DLD PT002298 system (Himadzu, Japan) using Al Kα (hv =1486.69 eV). The structural information of the material was analyzed by a Bruker Tensor27 Fourier Infrared Spectroscopy (FTIR) using Attenuated total reflection (ATR) method. The light absorption properties of the material were analyzed using a Thermo Scientific Evolution 220 UV-Vis spectrophotometer with the UV-Vis diffuse reflectance spectra (UV-Vis DRS) in the wavelength range of 300-800 nm with BaSO4 as a reference. To characterize the transfer and recombination of photogenerated electrons in

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the catalytic material, fluorescence spectra (photoluminescence, PL) were analyzed using a Perkin Elmer LS55 spectrometer. The lamp source is

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a xenon lamp with an excitation wavelength of 325 nm. Samples were analyzed using a Micomeritics ASAP2020M automated gas adsorption

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specific surface area analyzer. The specific surface area and pore size distribution were calculated by the Brunauer-Emmett-Teller (BET) formula and the Barrett-Joyner-Halenda (BJH) formula, respectively.

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2.3 Photocatalytic Activity Test

A 300W xenon lamp calibrated with the FZ400 optical power meter (Beijing Newbit Technology Co., Ltd.) was used as simulated solar

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light source, and the experimental light intensity was controlled at 100 mW/cm2 (AM 1.5 G). The photocatalytic activities of the samples were evaluated by the removal efficiency of RhB under light irradiation. The light source was

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placed on the quartz condensation cover, and the cooling water was introduced into the condensation cover to remove the heat released by the lamp. The distance between the light source and the surface of the reaction solution was 10 cm. 200 mg of the catalyst was dispersed in 200 mL of deionized water containing 25 mg of RhB and certain amount of Fe3+. Prior to irradiation, the mixed solution was magnetically stirred in the dark for 1 h to establish an adsorption-desorption balance. During the irradiation, an appropriate amount of the reaction solution was periodically taken out, and the absorbance of the supernatant after centrifugation at 554 nm was measured by a UV-Vis spectrophotometer. The removal

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efficiency of RhB was calculated by the concentration of dye (C/C0) according to the absorbance (A/A0), wherein C0 and A0 were the initial

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concentration and absorbance of dye, respectively.

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3. RESULTS AND DISCUSSION 3.1 Characterization

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The synthesis of TE-g-C3N4 was based on a previously reported literature and the experimental procedure was shown in Fig. S1. H2SO4 was firstly inserted into the B-g-C3N4 layer and further thermally decomposed at a high temperature, so that the generated sulfur oxides was released

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and caused the layered g-C3N4 exfoliated to form TE-g-C3N4. Fig. 1a is an XRD pattern of as-prepared samples. All of the g-C3N4 samples have two typical diffraction peaks at around 13.0° of (100) crystal plane and 27.5° of (002) crystal plane, indicating that the crystal structures of all

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samples were similar. Remarkably enough, after acid treatment and exfoliation, the diffraction peak of TE-g-C3N4 on (100) related to interplanar packing of the tri-s-triazine units almost disappeared. Nevertheless, the (002) peaks at around 27° ascribed to interlayer stacking of the conjugated aromatic structure broadened after exfoliated and the average grain size calculated according to the Scherrer formula is 26.95 nm for B-g-C3N4, 19.45 nm for H-g-C3N4 and 21.01 nm for TE-g-C3N4, respectively. Furthermore, these peaks shifted slightly from 27.5° for B-g-C3N4 to 27.0° for H-g-C3N4 and 27.1° for TE-g-C3N4, respectively, and the average interplanar space of the samples calculated by Bragg's equation is

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0.32, 0.33, 0.33 nm, respectively, indicating that the layer space increased as the crystal grains decreased. The above analysis shows that the layered g-C3N4 has been successfully thermally exfoliated as we expected [48]. (b)

(002)

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(a)

The bending vibration mode of the S=O bond

B-g-C3N4

H-g-C3N4

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B-g-C3N4

amine

TE-g-C3N4

10

20

40

CN cycle

50

2θ (degree)

200

(c)

100

0 0.0

3000

2500

0.2

0.4

0.6

0.8

Relative pressure (p/p0)

1.0

2000

1500

Wavenumber (cm-1)

1

tir-s-triazine

1000

500

B-g-C3N4 H-g-C3N4

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H-g-C N 3 4 TE-g-C N 3 4

50

3500

3 dV/dD (cm /g*nm)

150

4000

(d)

B-g-C N 3 4

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Volume absorbed (cm3/g)

30

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(100)

TE-g-C3N4

Intensity (a.u.)

Intensity (a.u.)

H-g-C3N4

TE-g-C3N4

The unit of Figure 1d has been modified to "cm3/g*nm"; 10

100

Pore diameter;

Pore diameter (nm)

Fig. 1. Wide-angle XRD patterns (a), FTIR spectra (b), Nitrogen adsorption-desorption isotherms (c) and Barret-Joyner-Halenda (BJH) pore size distribution plots (d) for B-g-C3N4, H-g-C3N4 and TE-g-C3N4 samples, respectively.

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Fig. 1b depicts that the FTIR spectra of the three samples are very similar, suggesting that all samples are similar in chemical structure.

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Obviously, the very sharp absorption peak at around 810 cm-1 is attributes to the breathing mode of the tri-s-triazine units [33]. Regarding the aromatic CN heterocycle, the weak absorption at 700-800 cm-1 region is designated as the bending vibration mode and the absorption peaks

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between 1200 and 1600 cm−1 should be ascribed to the stretching modes, including N-(C)3 and C-NH-C [28, 33, 37]. The broad peaks in the

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range of 3000 to 3600 cm-1 are indexed to N-H from the sample and O-H stretching absorption from adsorbed hydroxyl species [34]. We speculate that the peak of H-g-C3N4 at 1000 cm−1 can be classified as the stretching mode of S=O bond, which proves that the SO42- is

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successfully inserted between the layers [49], which will be proved by the XPS data further (Fig. S3). Correspondingly, the bending vibration mode of the S=O bond at 584 cm-1 also appears.

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Table 1. The SBET, pore volume, average pore size of the as-prepared materials.

Pore volume/cm g

Average pore size/nm

2 -1

BET area/m g

3 −1

B-g-C3N4

25.2

0.1

20.9

H-g-C3N4

47.1

0.2

17.0

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TE-g-C3N4

59.1

0.3

17.4

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Fig. 1c shows the N2 adsorption-desorption curve of B-g-C3N4, H-g-C3N4 and TE-g-C3N4. All samples have a type H3

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adsorption-desorption curve indicating weak adsorption interactions and porous structures of the sample [33]. Fig. 1d shows Barrett-Joyner-Halenda (BJH) pore-size distribution of as-prepared samples. These three samples have a pore distribution in the range of 5-100

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nm. There appears a new significant distribution at around 2-5 nm after acid and thermal exfoliating, which means that the average pore size is reduced. Table 1 is the specific surface area and pore volume of the three samples. The BET specific surface area (SBET) increases from 25.2

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m2g-1 of B-g-C3N4 to 59.1 m2g-1 of TE-g-C3N4, and the pore volume enlarges from 0.1 cm3g−1 of B-g-C3N4) to 0.3 cm3g−1 of TE-g-C3N4, respectively. The SBET and the pore volume of TE-g-C3N4 is nearly twice as high as that of the B-g-C3N4, suggesting that more abundant reaction

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sites and enhanced light-capturing ability is achieved by thermal exfoliating process [33].

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Fig. 2. SEM images of B-g-C3N4 (a), H-g-C3N4 (b) and TE-g-C3N4 (c, d).

The SEM images in Fig. 2 show the morphology change of as-prepared samples. As shown in Fig. 2a, the B-g-C3N4 sample prepared by high temperature polycondensation consisted of an amorphous sheet-like structure that is aggregated together to form huge particles with size of around several microns. The image of acid-embedded H-g-C3N4 sample shows many smooth and uniform small particles with size less than 1 micron on the surface (Fig. 2b). After thermal treatment, the generated sulfur oxides was released and caused the layered g-C3N4 exfoliated to

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form TE-g-C3N4. From Fig. 2c we can find that the size of the obtained TE-g-C3N4 particles is consistent with that of H-g-C3N4 but the surface

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obviously become rough compared with that of H-g-C3N4, which probably caused by the release of sulfur oxides. Through carefully observe the

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magnified image shown in Fig. 2d, it is found that the large TE-g-C3N4 sheets are consisted of many small particles with average size in the range of tens nanometers which is consistent with the results of XRD investigation.

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The elemental composition and surface electronic states of three g-C3N4 samples was tested by XPS experiment (Fig. 3, S2, S3). XPS survey spectra in Fig. 3a shows that C, N and O elements are present in B-g-C3N4, H-g-C3N4 and TE-g-C3N4, respectively. The small peak at

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169.5 eV from H-g-C3N4 is assigned to S 2p [50], which is derived from sulfuric acid inserted into B-g-C3N4 layer (Fig. S3). The high-resolution C 1s spectra of TE-g-C3N4 (Fig. 3b), B-g-C3N4 (Fig. S2a) and H-g-C3N4 (Fig. S2c) are deconvoluted into three peaks at binding energies of 288.3

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(288.5, 288.5), 285.8 (286.0, 286.6) and 284.6 eV, which are sequentially assigned as carbon-nitrogen sp2 hybridization (N-C=N), graphite carbon-carbon (C-C) and carbon-nitrogen N-(C)3, respectively [34]. Three deconvoluted peaks are presented in the N 1s spectra of TE-g-C3N4 (Fig. 3c), B-g-C3N4 (Fig. S2b) and H-g-C3N4 (Fig. S2d), respectively, and these peaks at binding energies of 398.6 (398.2, 398.5), 399.4 (398.7, 399.2), and 400.5 (400.2, 400.8) eV correspond to the sp2 hybrid aromatic N(C-N=C) bonded to the C atom in the tri-s-triazine (pyridinic N), sp3 N-(C)3 groups, and N-H groups, respectively [48]. It is noteworthy that the contents of pyridine N carefully calculated from the above data for

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the B-g-C3N4, H-g-C3N4 and TE-g-C3N4 catalysts are about 17.6%, 27.2% and 49.5%, respectively. The higher content of pyridine N, the more

(b)

C 1s

288.3

N 1S

H-g-C3N4

TE-g-C3N4

1200

1000

290

Binding Energy (eV)

403

402

401

400

399

398

Banding Energy (eV)

397

396

B-g-C3N4 H-g-C3N4 TE-g-C3N4 B-g-C3N4 H-g-C3N4 TE-g-C3N4

1/2

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400.5 C-N-H

y-axis.

1/2

49.5%

illustration is a K-M

282

284

(d)

N 1s

Absorbance (a.u.)

399.4

added

Banding Energy (eV)

398.6 C-N=C

N-(C)3

The

function diagram on the

286

288

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Intensity (a.u.)

(c)

3d:

C-C

C-(N)3

292

0

200

400

600

800

3a.3b.3c:Y-Intensity (a.u.); X-(eV);

284.6

285.8

Pr

B-g-C3N4

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N-C=N

Intensity (a.u.)

Intensity (a.u.)

C 1S

(αhv) ev cm

O 1S

-1/2

(a)

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favorable to binding with iron ions, probably provides more Fe-N species as possible active sites for photo-Fenton reaction [38, 51, 52].

1.6

2.0

2.4

2.8

3.2

3.6

4.0

hv (eV)

200

300

400

500

600

700

800

Wavelength (nm)

Fig. 3. XPS survey spectra of B-g-C3N4, H-g-C3N4 and TE-g-C3N4 (a), High-resolution XPS spectra of C 1s (b) and N 1s (c) of TE-g-C3N4, UV-Vis diffuse

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3.2 Optical properties and electrochemical analysis

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reflectance spectra (UV-Vis DRS) of g-C3N4 samples. The inset is a K-M function diagram on the y-axis (d).

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The optical properties of the samples are closely related to electronic structures, including band gap energy, charge separation and charge

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mobility [28]. Fig. 3d shows UV-Vis DRS of B-g-C3N4, H-g-C3N4 and TE-g-C3N4 samples, respectively, which are caused by the charge transfer response of g-C3N4 samples from the VB populated by N 2p orbitals to the CB formed by C 2p orbitals [22]. The absorption edges of the

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B-g-C3N4, H-g-C3N4 and TE-g-C3N4 are 470, 430, 450 nm respectively, and the inset is a plot of the corresponding band gaps energy values of (αhν)1/2 vs. hν (Kubelka-Munk function as a function of photon energy) for three samples obtained by extrapolation, consistent with previously

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reported literature [17, 22, 49, 55]. The detailed energy gap plots of these three samples as shown in Fig. S4, and band gaps energy values are 2.64, 2.88, 2.76 eV, respectively [53, 54]. The blue shifted absorption edges of H-g-C3N4 and TE-g-C3N4 makes the band gaps larger, which is consistent with the smaller grain size of the two samples proved by XRD and SEM observation. Fig. S5 depicts the PL spectra of B-g-C3N4, H-g-C3N4 and TE-g-C3N4 with an excitation wavelength of 325 nm. Due to the electronic transition of the π-π* orbital of the triazine ring, the photoluminescence characteristic peak is a photoluminescence band centered at 438 nm. Nevertheless, the photoluminescence characteristic

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peak positions of TE-g-C3N4 and H-g-C3N4 does not change significantly, but the PL intensity of the two samples is greatly reduced compared

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with that of B-g-C3N4, which indicates more obvious photochemical quenching of photogenerated carriers in the two samples. The average

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fluorescence lifetime of TE-g-C3N4 (Fig. S6) was prolonged to ~4.36 ns from ~4.2 ns of B-g-C3N4, suggesting that the recombination rate of photogenerated carriers in TE-g-C3N4 was reduced compared with pristine B-g-C3N4 (Table S1). The obvious PL quenching and the prolonged

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PL lifetime of TE-g-C3N4 demonstrated more efficient separation and migration of photogenerated carriers, which will be beneficial to better

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photocatalytic performance.

Fig. 4. Photo-responsive I-t (a) and electrochemical impedance spectra (EIS) Nyquist plots (b) of B-g-C3N4 and TE-g-C3N4, respectively.

In order to further confirm the efficient separation and migration of photogenerated carriers in TE-g-C3N4 compared to B-g-C3N4,

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photo-responsive I-t and EIS experiments were further analyzed [28]. The sample of B-g-C3N4 or TE-g-C3N4 is coated on the surface of the FTO

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to form a membrane catalyst electrode, and the photoelectric properties are measured in a mixture electrolyte solution of 0.1 M Na2SO4 and 10 mM triethanolamine. As shown in Fig. 4a, at the potential of 0 V vs. Ag/AgCl, the current densities of TE-g-C3N4 and B-g-C3N4 are 7 μA/cm2

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and 3 μA/cm2, respectively, suggesting that the TE-g-C3N4 catalyst electrode has more excellent optical response characteristics than the

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B-g-C3N4 without external voltage, which is very consistent with the results of PL quenching and PL lifetime. Notably, TE-g-C3N4 displays an obvious capacitive current when the lamp is switched on, and this phenomenon is also observed when the lamp is switched off. In sharp contrast,

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there did not appear this phenomenon in the photo-responsive I-t curve of B-g-C3N4. We speculate that this phenomenon may be closely related to the large number of defective sites present on the surface of TE-g-C3N4 [35]. Through the study of EIS, we can further understand the charge

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transport kinetics of the samples. As shown in Fig. 4b, compared with B-g-C3N4, H-g-C3N4 is somewhere in between (Fig. S14), the radius of semicircular Nyquist plots from TE-g-C3N4 is significantly decreased, which means the smaller electrochemical resistance. The inset is an equivalent circuit fitted according to experimental data, CPE is the constant phase component of the electrode/electrolyte interface, Rs is the resistance of the electrolyte, and Rct is the transfer resistance of the charge carriers at the electrode/electrolyte interface. More crucially, from the data in the inserted table, the Rs value is almost the same in accordance of experimental error which means the EIS results is very reliable

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because the electrolyte for the two measurements are the same, while the Rct of TE-g-C3N4 is about 27542 Ω much smaller than 35631Ω of

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B-g-C3N4. The reduced interfacial resistance on TE-g-C3N4 compared to that on B-g-C3N4 firmly proved more efficient separation of

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photo-induced charge carriers on the TE-g-C3N4 sample, which have been proved by PL experiments already. 3.3 H2O2-free Photo-Fenton reaction on various g-C3N4 samples

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Using RhB as a model pollutant, the photo-Fenton or photocatalytic activity of the samples was evaluated by the removal efficiency of RhB with or without addition of Fe3+ ions under light irradiation. The removal efficiency of RhB was calculated by the relative concentration of RhB

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solution (C/C0) according to the relative absorbance (A/A0). Fig. 5a shows the absorption spectra of an RhB solution at different illumination time using TE-g-C3N4 and Fe3+ (supplied by FeCl3) as catalysts. After 1 h of stirring in the dark, the absorbance of the RhB solution slightly

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decreased, presumably due to the absorption of a small amount of RhB by catalyst. As the illumination time increased, the absorbance of the RhB solution gradually decreased, demonstrating that RhB was continuously degraded. After 2 h of illumination, the solution becomes colorless contrast compared to the initial strong fresh pink colored RhB solution as shown in the inserted image in Fig. 5a, which imply the completely degradation of RhB by photo-Fenton reaction. For clear comparison, the relative concentration (C/C0) was plotted versus illumination time in Fig. 5b. The removal efficiency reached 90% within 1 h and ~100% within 2 h of light illumination using TE-g-C3N4 with Fe3+ as catalyst,

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respectively. With strong contrast, the removal efficiency of RhB using TE-g-C3N4 without Fe3+ is just about 55% after 1 h and 95% after 2 h of

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illumination, respectively. From the curves in Fig. 5b, it is clearly found that the photo degradation of RhB was sharply accelerated by addition of Fe3+ without any change of other conditions. We attribute this effect to the well-known Fenton reaction through which strong oxidant •OH can

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be produced by Fe3+/Fe2+ redox cycling with H2O2 (will be further proved in the following section). It should be noted that the removal

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efficiency of RhB without Fe3+ is linearly related to time, while that is non-linear with addition of Fe3+, which means the kinetic mechanisms are very different. To get more information about the kinetics behind, the two curves were fitted with zero-order kinetic model [35] or first-order

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kinetic model [36, 56, 57] and the results were shown in Fig. 5d, respectively. The linear correlation coefficients (R2) reach 0.987 and 0.991 for zero-order kinetic without Fe3+ and first-order kinetic with Fe3+ [58], respectively, which means the RhB degradation kinetics are very different

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in absence or in presence of Fe3+. The RhB degradation reaction conforms to pseudo-zero-order kinetics in absence of Fe3+ or pseudo-first-order kinetics or in presence of Fe3+, respectively. From the kinetic fitting, the rate constant could be obtained and the results were shown in Table S2. Very similar phenomena were observed using untreated B-g-C3N4 as catalyst (Fig. S7). The reaction rate of photo degradation of RhB was sharply accelerated by addition of Fe3+ and the RhB degradation reaction conforms to pseudo-zero-order kinetics in absence of Fe3+ or pseudo-first-order kinetics in presence of Fe3+, respectively (Fig. S8, S9). It is noteworthy to mention that although the regularity is very similar,

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the absolute degradation speed is lower on B-g-C3N4 than that on TE-g-C3N4 either in absence or in presence of Fe3+. As shown in Table S2, the -1

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rate constants on B-g-C3N4 and TE-g-C3N4 are 0.145 mol/L and 0.271 mol/L for pseudo-zero-order kinetics without Fe3+, 0.015 min and 0.037 -1

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min for pseudo-first-order kinetics with Fe3+, respectively. In both cases, the rate constants are only about one a half using B-g-C3N4 compared with that on TE-g-C3N4, which can be easily explained by more difficult separation of charge carriers on B-g-C3N4 already proved by PL and Finally the control experiment in absence of catalysts proved RhB itself is very stable under these conditions

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electrochemical experiments.

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because the degradation of RhB after 4 h illumination was negligible (Fig. 5b, S7).

before after

0.6

0.2

550

600

650

TE-g-C3N4 TE-g-C3N4 Fe

0.6

0.2

500

f RhB only

0.4

450

700

0.0 -60

-30

0.0

(c)

1.0

(d)

1st

2nd

3rd

4th

0

30

60

C/C0

120

5

TE-g-C3N4

-C/C0

0.8

4

3

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-1.0

0.4

3+ TE-g-C3N4 Fe

2

4

6

8

10

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0

Time (h)

2

Linear fit

-1.5

0.2 0.0

90

Time (min)

Linear fit y=0.008 * x - 0.926 2 R = 0.987

5th

-0.5

0.6

3+

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Wavelength (nm) 1.2

light irradiation

0.8

0.4

0.0 400

dark

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0.8

(b)

1.0

e-

1.0

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-60 min 0 min 20 min 40 min 60 min 80 min 100 min 120 min

C/C0

Absorbance (a.u.)

(a)

Ln (C0/C)

1.2

1.2

y=0.037 * x + 0.083 2 R = 0.992

-2.0

0

60

1

0 120

Time (min)

Fig. 5. UV-Vis absorbance spectral changes of RhB as function of time over TE-g-C3N4, the inset is the color contrast before and after degradation (a); Removal efficiency of RhB using TE-g-C3N4 (b) and model kinetic models (d); Cycling runs of H2O2-free photo-Fenton degradation of RhB on TE-g-C3N4 (c). Reaction conditions: 125 mg/L RhB, 1 g/L catalysts, 250 mg/L Fe3+, 25 oC.

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In order to assess the stability and reusability of the catalyst in the H2O2-free photo-Fenton reaction, recycling experiment was operated for

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five cycles. After each cycle, the catalyst was centrifuged and then dried overnight at 80 oC. After five consecutive cycles, the removal efficiency

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of RhB is still more than 90% within 2 h, indicating that the catalyst is very stable (Fig. 5c) and can be reused after simple treatment. Moreover, we performed characterization and performance testing of the recycled catalyst, similar to the results before the cycle, Fig. S15 shows that the

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TE-g-C3N4 sample has two typical diffraction peaks at 13.0° on the (100) plane and 27.5° on the (002) plane, and the corresponding FTIR spectra (Fig. S16) also maintains a stable chemical structure, and the shape is basically unchanged (Fig. S17). As shown in Fig. S18, the

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absorption spectrum of the RhB solution at different irradiation times of TE-g-C3N4 and Fe3+ as catalysts after the cycle. This indicates that the cyclic process did not destroy its structure and its effect on performance was negligible. In addition, the optimized Fe3+ concentration was

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achieved by investigating the RhB remove efficiency using various Fe3+ concentrations (Fig. S10). However, as the concentration of Fe3+ increases, the removal efficiency of RhB using the TE-g-C3N4 catalyst first increases and then decreases. One of the reasons is that the excess Fe3+ may increase the light opacity, thereby preventing TE-g-C3N4 from the effective light absorption. When the concentration of Fe3+ reach 250 mg/L, optimized RhB remove efficiency was obtained.

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3.4 Mechanisms of H2O2-free photo-Fenton degradation of RhB

It is well-known that Fenton reaction is based on the production of strong oxidant •OH by Fe3+/Fe2+ redox cycling with H2O2. To prove the

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photodegradation of RhB in presence of Fe3+ is a photo-Fenton reaction, it is prior to measure the production of H2O2 by in situ photocatalytic

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process. Fig. S11 shows the results of KMnO4 fading reaction of the photo-illuminated aqueous solution in absence and in presence of TE-g-C3N4, respectively. The right sample shows that the KMnO4 solution fade completely when mixed with the photo-illuminated aqueous

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solution with TE-g-C3N4 even the catalyst was removed completely by centrifugation after illumination, while the left one shows an obvious purple color of the pristine KMnO4 solution in sharp contrast when mixed with the photo-illuminated aqueous solution without TE-g-C3N4 [12,

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40]. This is a strong evidence to prove the in situ formation of H2O2 through photo-illumination on TE-g-C3N4 photocatalyst. In order to further understand the mechanism of photo-Fenton reaction on TE-g-C3N4 catalysts, we carried out several scavenger experiments and electron spin-resonance spectroscopy (ESR) captured by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (Fig. S12). In general, •OH, h+ and •O2− are the main active species contributing to the degradation of organic compounds in the photocatalytic oxidation process, which can be determined according to the changes in retention efficiency of RhB by addition of corresponding quenching reagents [35, 36, 40]. The corresponding quenching

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scavengers for •OH, h+ and •O2− are isopropanol (IPA), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and p-benzoquinone (p-BQ),

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respectively [28, 59, 60]. From the results of scavenger experiments shown in Fig. 6a, it is obvious that the retention efficiencies of RhB reached

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52%, 32%, 10% after 1 hour of illumination in presence of IPA, EDTA-2Na or p-BQ, respectively, while the retention efficiency in the control experiment in absence of any scavenger is 8%. It is easily observed that IPA significantly inhibits the photodegradation of RhB, which clearly

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reveals the dominant position of •OH in the photocatalytic process. The addition of EDTA-2Na also reduced the photodegradation to an obvious extent, suggesting that h+ also functions. In contrast, the photoderadation activity is only slightly reduced by the addition of p-BQ, indicating the

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least importance of •O2− species. The retention efficiency of RhB using the scavenger is consistent with the removal efficiency when no scavenger is added, indicating that the degradation process is affected by three factors and the critical role in the degradation of RhB by active

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species is •OH> h+>> •O2−. The obvious signals appeared in the ESR spectra (Fig. S12) further provide solid evidence for the formation of both •OH and •O2− radicles under photo illumination. On the other hand, the scavenger experiments based on the system in absence of Fe3+ (Fig. S13a) proved that photogenerated holes play the most important role in the process of photoctatlytic RhB degradation and the sequence of the critical role in the RhB degradation by active species is h+ >> •OH > •O2−, which imply very different mechanism of RhB degradation using TE-g-C3N4 in absence of Fe3+ compared with that in presence of Fe3+ discussed above.

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Based on the above experiments and discussions, a mechanism speculation is conducted as shown in the Fig. 6b. As a typical photochemical

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primary process in common photocatalytic reactions, photogenerated charge carriers were produced by light excitation of electrons from the VB

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to the CB on the photocatalyst (Eq. (1)). Because the potential of electrons located above the g-C3N4 CB (-1.1 V) is more negative than the reduction potential of O2/H2O2 (0.69 V) [41] , so there is a big probability in thermodynamics for reduction of O2 to H2O2 by photogenerated

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electrons on g-C3N4 (Eqs. (2-3)). The produced H2O2 can be possibly further reduced by an electron to produce very oxidative •OH species (Eq. (4)) with very high reactive activity to degrade RhB into colorless and harmless products (Eq. (9)). Meanwhile as the oxidation potential of holes

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located below g-C3N4 VB (1.57 V) is more positive than the redox potential of RhB (1.43 V) [42, 43], the photogenerated holes can be easily consumed by the oxidation process of RhB at the same time (also Eq. (9)). In the presence of Fe3+, the reduction of Fe3+ into Fe2+ (Eq. (5)) can

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be achieved easily for the reductive potential of the CB bottom (-1.1 V) is also negative than the reduction potential of Fe3+/Fe2+ (0.77 V) [37]. Followed by the production of Fe2+, the Fenton process of Fe3+/Fe2+ redox cycling with H2O2 (Eqs. (6-8)) was booted up automatically, which accelerate the production of •OH dramatically. From the proposed mechanism of the RhB degradation on TE-g-C3N4 with Fe3+, it was found that the process started with the light excitation of g-C3N4, followed up with the in situ production of H2O2 by photogenerated electrons and accelerated by the Fenton process of Fe3+/Fe2+ redox cycling with H2O2. However, there existed obvious difference for the mechanism (Fig.

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S13b) of RhB degradation on TE-g-C3N4 without Fe3+ (Eqs. (S1-S5)). Although the processes of light excitation and the in situ H2O2 production

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remained the same, there was lack of the accelerating process from Fe3+/Fe2+ redox cycling with H2O2 for the absence of iron species, which can be used to well explaining the slower degradation speed and the less critical role of •OH in the absence of Fe3+.

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The difference of the detailed mechanism between the degradation reactions with Fe3+ and that without is consistent very much with the fact

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that the photo-Fenton degradation of RhB with Fe3+ is a pseudo-first-order reaction, while the photocatalytic degradation of RhB without Fe3+ is a pseudo-first-order reaction. Because the reaction of active species (•OH and h+) with RhB (Eq. (S5)) is relative faster than the processes of

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•OH and h+ production and the rate-limiting step should be the slower process for •OH and h+ production, which only determined by the intensity of illumination, so the overall reaction conforms to pseudo-zero-order kinetics according to RhB concentration. However, the mechanism was

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changed a lot with the addition of Fe3+. The main difference comes from the fact that there formed a redox cycling of Fe3+/Fe2+ with H2O2 (Eqs. (5-8)), in which the production of •OH was sharply accelerated. With the production of •OH being accelerated by Fe3+/Fe2+ redox cycling with H2O2, the reaction of RhB with active species (•OH and h+) become relatively slower than the process of active species formation and the rate-determing step switch to the RhB degradation step (Eq. (9)), which can be well explained that the overall reaction changed from pseudo-zero-order reaction into pseudo-first-order reaction according to RhB concentration.

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g-C3N4 + hv → e- + h+ (1);

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O2 + e- → •O2- (2); •O2- + 2H+ + e- → H2O2 (3)

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H2O2 + e- → •OH + OH- (4);

H2O2 + Fe2+ → Fe3+ + •OH + OH- (6);

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Fe3+ + e- → Fe2+ (5);

HO2• → H+ + •O2- (8);

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H2O2 + Fe3+ → Fe2++ HO2• + H+ (7);

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•O2-/ h+ /•OH + RhB → degradation products (9);

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Fig. 6. Effects of a series of scavengers on the degradation efficiency of TE-g-C3N4 with Fe3+ (the dosage of EDTA-2Na and p-BQ = 13 mmol/L, the dosage of IPA =

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1 mol/L, illumination time t = 60 min) (a); Possible degradation mechanism (b).

4. Conclusion

In conclusion, a H2O2-free photo-Fenton reaction was proposed for the first time using thermally exfoliated g-C3N4 as photocatalyst and RhB as model organic pollutants, in the presence of iron species in the solution. The reaction rate of photo-Fenton degradation in the presence of

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ferric ions is sharply accelerated compared with that of photocatalytic degradation in the absence of ferric ions. The remove efficiency of RhB

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was achieved ~100% within 2 h of treatment. And the behind kinetics of photo-Fenton reaction is different from that of photocatalytic reaction.

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The photo-Fenton reaction conforms to pseudo-first-order kinetics and photocatalytic reaction conforms to pseudo-zero-order kinetics, respectively. Mechanism investigation proposed possible detailed steps for the photo-Fenton reaction and photocatalytic reaction, respectively.

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The accelerated speed and different kinetics of the photo-Fenton reaction compared to the photocatalytic reaction can be well explained by the proposed mechanism. The expansion of this concept with other catalyst is undergoing in our lab and it is believed that the results of this work

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will inspire strong interests in the field of photocatalysis and environment remediation.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Acknowledgements

This research is supported by the National Natural Science Foundation of China (No. 51674194, 51074122), the financial support from

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Beijing National Laboratory for Molecular Sciences (BNLMS201825) and the CAS Key Lab of Colloids, Interfaces and Thermal Dynamics. The authors appreciate the program of the Youth Innovation Team of Shaanxi Universities. We also thank Mr. Xiangyang Yan (School of Chemistry

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and Chemical Engineering, Shaanxi Normal University) for his technical supports in XPS measurements.

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