A novel solar photo-Fenton system with self-synthesizing H2O2: Enhanced photo-induced catalytic performances and mechanism insights

Journal Pre-proofs Full Length Article A novel solar photo-Fenton system with self-synthesizing H2O2: Enhanced photo-induced catalytic performances and mechanism insights Yujing Wang, Hemei Song, Jian Chen, Shouning Chai, Limin Shi, Changwei Chen, Yanbin Wang, Chi He PII: DOI: Reference:

S0169-4332(20)30406-2 https://doi.org/10.1016/j.apsusc.2020.145650 APSUSC 145650

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 October 2019 12 January 2020 3 February 2020

Please cite this article as: Y. Wang, H. Song, J. Chen, S. Chai, L. Shi, C. Chen, Y. Wang, C. He, A novel solar photo-Fenton system with self-synthesizing H2O2: Enhanced photo-induced catalytic performances and mechanism insights, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145650

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A novel solar photo-Fenton system with self-synthesizing H2O2: Enhanced photo-induced catalytic performances and mechanism insights Yujing Wanga, Hemei Songa, Jian Chena, Shouning Chaib,*, Limin Shia, Changwei Chenb, Yanbin Wangc, Chi Heb,* a

Shaanxi Key Laboratory of Optoelectronic Functional Materials and Devices, School of

Materials Science and Chemical Engineering, Xi'an Technological University, Xi'an, Shaanxi, 710021, P.R. China b

Department of Environmental Science and Engineering, State Key Laboratory of Multiphase

Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, P.R. China c

School of Environment, Henan Key Laboratory for Environmental Pollution Control, Key

Laboratory for Yellow River and Huai River Water Environmental Pollution Control, Ministry of Education, Henan Normal University, Xinxiang, Henan, 453007, P. R. China

*Corresponding authors. E-mail addresses: [email protected] (S. Chai), [email protected] (C. He)

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Abstract: To solve problems of consecutive H2O2 external supplement and reutilization difficult of free Fe2+ ion in traditional homogeneous photo-Fenton, an efficient solar photo-Fenton system with self-supplying H2O2 was developed based on a heterogeneous catalyst of α-Fe2O3/g-C3N4. The microstructure, physicochemical and photoelectric properties of the optimal α-Fe2O3/g-C3N4 were characterized. It was found that the introduced α-Fe2O3 significantly enhanced the photoeletrochemical performances of g-C3N4. Rhodamine B and tetracycline hydrochloride were degraded in this solar photo-Fenton system with α-Fe2O3/g-C3N4, and the corresponding degradation rates reached to 96% (in 90 min) and 95% (in 150 min) at neutral pH circumstance respectively, which were much higher than that of single g-C3N4 and α-Fe2O3. The efficient degradation performance was attributed to the effective separation and rapid transfer of photo-generated charge carriers derived from classic Z-scheme type. Accordingly, it was deduced that H2O2 can produce on the g-C3N4 surface firstly under simulated solar irradiation, and the efficient removal of contaminant could be ascribed to oxidation by amount of ·OH predominantly derived from H2O2 decomposition on the α-Fe2O3, which was verified by trapping experiments and EPR analysis. In addition, ·O2− and h+ also played supporting roles. The main significance of this study gives a new insight into photo-Fenton.

Keywords: Photo-Fenton; Hydrogen peroxide; Carbon nitride; Iron oxide; Z-scheme.

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1. Introduction In recent years, Fenton reaction has been widely applied in the field of environmental remediation, especially for the decontamination of wastewaters containing chemically and biologically recalcitrant organic pollutants [1-3]. The conventional homogeneous Fenton process used dissolved Fe2+ and H2O2 to generate hydroxyl radicals (·OH), which is the second strongest oxidant only weaker than fluorine. Various organic pollutants could be attacked non-selectively by ·OH and decomposed to CO2, H2O and inorganic ions [4, 5]. However, the major disadvantages that hinder the practical application of the traditional Fenton process include: (i) optimum conditions at strong acidic pH (2.8-3.5); (ii) high iron sludge yield in the final effluent; (iii) high demand for H2O2 [6-8]. To overcome these drawbacks, heterogeneous photo-Fenton system has been developed as a promising alternative by virtue of widened application pH range, less iron leaching, and easy separation for cyclic utilization (Eq. 1~7) [9-12]. Photo-Fenton process involves sunlight or an artificial light irradiation, which increases the rate of contaminants degradation by increasing the production of ·OH from H2O2 decomposition and synchronously accelerating the reduction of FeIII to FeII (Eq. 2~5). Moreover, the photolysis of H2O2 also plays an important role in the photo-Fenton process to break the O-O bond for the production of ·OH (Eq. 6) [5, 13, 14]. FeII + H2O2 → FeIII + ·OH + OH−

(1)

FeIII + H2O2 → FeII + HO2· + H+

(2)

[FeIII(OH)]2+ + hν → FeII + ·OH

(3)

3

FeIII + H2O + hν → FeII +·OH + H+

(4)

FeIII(RCO2) + hν → FeII + CO2 + ·R

(5)

H2O2 + hν → ·OH

(6)

·OH + organic pollutants → CO2 + H2O

(7)

O2 + 2H++2e− → H2O2

(8)

As is well-known, it is generally considered that the continuous external supplement of H2O2 is indispensable to maintain the photo-Fenton reaction. In this regard, there still exist two problems hindering it further practical application of tradition photo-Fenton. On the one hand, high demand of commercial H2O2 will increase the operating cost. On the other hand, H2O2 is a powerful oxidizer, which is quite corrosive and unstable, inducing inconvenient and hazardous for transport and storage. So, it is urgent to develop some effective approaches to overcome these drawbacks. On the bright side, some interesting results were discovered that H2O2 may be generated on certain special semiconductor materials in aqueous solution under light irradiation [15, 16]. Compared with traditional methods, the photocatalytic synthesis of H2O2 has advantages of non-pollution and low energy consumption. Undoubtedly, the discovery is of significance for the development of photo-Fenton process. It is assumed that the cost of H2O2 consumption will be saved, and its transportation and storage risk also will be avoided if a proper photocatalyst is developed as an efficient H2O2 supplier for a heterogenous photo-Fenton system. g-C3N4 is a nonmetallic conjugated photocatalyst only consisting of carbon and nitrogen, which has a medium band gap, allowing it to utilize the visible light of the solar spectrum. It can be synthesized by one-step thermal polymerization method using melamine, dicyandiamide, urea, etc. as raw material [17-19]. In 2014, Shiraishi et al. [15] first reported that graphitic carbon nitride (g-C3N4) has a good ability to 4

produce H2O2 with very high selectivity (∼90%) due to the efficient formation of 1,4-endoperoxide species on the g-C3N4 surface. It revealed that the conduction band (CB) potential of g-C3N4 is more negative than the reduction potential of O2/H2O2, which thermodynamically guarantee the feasibility of O2 reduction reaction, and visible light irradiation on g-C3N4 in aqueous solution can successfully produces H2O2 via selective two-electron reduction of dissolved O2 by the conduction band electrons. Compared with conventional inorganic semiconductors, its merits of high hardness, low cost, reliable stability have endowed g-C3N4 as a reliable material for photocatalytic H2O2 generation [16]. However, the single g-C3N4 still confronts a couple of crucial challenges, including moderate visible-light absorption range and the fast recombination of photogenerated electrons (e−) and holes (h+) caused intrinsically by the π-π conjugated electronic system of g-C3N4 framework. Because the electrons transfer from the CB to O2 has been recognized as the rate determining step for the H2O2 production. If the recombination of photogenerated charge carriers is effectively suppressed, O2 can be reduced to H2O2 at a high rate [20]. A lot of studies have tried to broaden light absorption and improve charge carriers separation through constructing the C3N4 composites with other organic or inorganic substrates. Shiraishi et al. subsequently used graphene oxide, hexagonal boron nitride, and pyromellitic diimide, etc. combining with g-C3N4 to enhance the efficiency of selective two-electron reduction of O2 [21-23]. Peng and Zhang et al. [24] prepared a composite catalyst CoP/g-C3N4 with 1.76 wt% CoP loading amount and exhibited the best photocatalytic efficiency with a H2O2 production of 140 M in 2 h, which is about 4.6 and 23.3 times that of pure g-C3N4 and pure CoP, respectively. Zhao et al. [25] reported that the polyoxometalate cluster of [PW11O39]7− was successfully covalent combined with 3DOM g-C3N4 through the organic linker strategy, which 5

showed efficient catalytic performance (2.4 μmol·h−1) for light driven H2O2 production from H2O and O2 in the absence of organic electron donors. Therefore, introducing an appropriate guest catalyst to g-C3N4 host is an effective option to broaden the light adsorption range and promote the charge separation probably [26]. Among a variety of photocatalytic hybrids, direct all-solid-state Z-scheme systems only consisting of two components attracted more and more attention. Apart from achieve abovementioned two goals, maximum overpotentials can be obtained with this unique Z-scheme system, which benefits from the effective utilization of a high CB from one component and a low valence band (VB) from the other [27]. Hematite (α-Fe2O3), a natural abundant, inexpensive, and environment-friendly semiconductor material with narrow band gap (2.2 eV), has been extensively used in catalysis fields because of its peculiar physicochemical properties [28-30]. If α-Fe2O3 is selected to combine with the g-C3N4 to obtain α-Fe2O3/g-C3N4 hybrid, which not only play the key role as a co-catalyst constituents for enhancing visible light response and providing a VB with more positive potential, but also can act as a good heterogeneous iron-based photo-Fenton catalyst according to previous studies [6, 31, 32]. In this case, the photocatalytic produced H2O2 on g-C3N4 could be in situ decompose to generate ·OH in the condition of no external H2O2 addition. In addition, the holes on the VB of α-Fe2O3 can also oxidize OH− to ·OH. A two channel routes for ·OH production is very beneficial for efficient organic pollutants mineralization. At the same time, the reduction of FeIII to FeII maybe achieve via the routes of Eq. (2~4). Thus, α-Fe2O3 is an ideal candidate to combine g-C3N4 to construct a heterojunction structure, and charge carriers can be efficiently separated due to well-matched energy band, favoring the improvement of organic pollutants removal

6

efficiencies by means of both photo-Fenton with self-supplying H2O2 and direct photocatalysis routes. Herein, a facile strategy for the synthesis of direct all-solid-state Z-scheme composite α-Fe2O3/g-C3N4 was reported. The microstructure and physicochemical properties were characterized in detail. The photo-induced catalytic activity of the α-Fe2O3/g-C3N4 composite was evaluated by the degradation of Rhodamine B (RhB) and tetracycline hydrochloride (TC-H). Comparing with the pure g-C3N4, the introduction of α-Fe2O3 for g-C3N4 could increase specific surface area, broaden light absorption range, as well as accelerate the separation and transfer of photo-generated electrons and holes, which results in distinctly improving the photo-induced photo-Fenton and photocatalytic degradation performance. Finally, the degradation reaction mechanism of contaminants over the Z-scheme type α-Fe2O3/g-C3N4 heterojunction is also proposed. The obtained α-Fe2O3/g-C3N4 might be a promising material for application to wastewater treatment. 2. Experimental section 2.1. Reagents and materials All chemicals used in this study were analytical grade and purchased from Sinopharm (Shanghai, China). Double distilled Millipore water was used for all the experiments. 2.2. Preparation of catalysts The preparation of g-C3N4 is as follows: 3.0 g of melamine was added in a semi-closed ceramic crucible. The crucible with a lid was calcined in a muffle furnace at 520 °C for 2 h with heating rate at 5 °C·min-1 under air atmosphere. After the crucible was cooled to room temperature, the yellow product of g-C3N4 was collected [33]. 7

α-Fe2O3 was synthesized by a facile hydrothermal method. In a typical synthesis, 0.01 M fresh FeCl3·6H2O solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave after adjusting to pH=11 with 25 wt% aqueous ammonia, and maintained at 180 °C for 2 h. After that, the autoclave was cooled naturally and the obtained product was washed with deionized water and ethanol for three times, and then dried at 80 °C for 6 h in a vacuum oven [28]. α-Fe2O3/g-C3N4 composite was prepared by the impregnation–solvothermal method. The as-prepared g-C3N4 (0.4 g) was added into a certain volume 0.01 M fresh FeCl3·6H2O solution and ultrasonicated for 1 h. Then, the pH of above suspension was adjusted to pH=11 with 25 wt% aqueous ammonia, and subsequently transferred into a 100 mL Teflon-lined stainless autoclave and maintained at 160 °C for 12 h. The obtained products were washed and dried with the same procedure abovementioned. Four kinds of α-Fe2O3/g-C3N4 composites with different weight ratios of α-Fe2O3 and g-C3N4 (marked as Fe:C= 1:2, 1:1, 2:1, 3:1) can be obtained. 2.3. Characterizations The morphology of the samples was characterized by field-emission scanning electron microscope (SEM, JEOL JSM-7800F, Japan) with an accelerating voltage of 200 kV. The microstructure and elemental mapping were determined using a transmission electron microscope (TEM, JEOL JEM-2100Plus, Japan) equipped with an energy dispersive X-ray spectrometer (EDS) analysis system operated at 200kV. Fourier transform infrared (FTIR) spectra of the catalyst samples were recorded by a Tensor 37 spectrometer (Bruker Optics, Germany) with a potassium bromide flakelet method. Powder X-ray diffraction (XRD, Model D/max2550VB3+/PC, Rigaku) measurements were performed using a diffractometer with Cu Kα radiation, with the XRD working voltage at 40 kV, tube current of 30 mA, and scanning speed at 8

4 °·min-1. The chemical compositions were carried out on a multifunctional X-ray photoelectron spectrometer (XPS, Axis Ultra DLD, Kratos) with Mg-K radiation (hv = 1253.6 eV). Binding energies were calibrated according to the C 1s peak (284.8 eV) from adventitious carbon. UV–vis diffusive reflectance spectra (DRS) were determined using a JASCO V-550 UV–vis spectrometer. The specific surface area (SBET) of the samples was estimated by the BET adsorption-desorption isotherms of nitrogen at 77 K using a BET analyzer (TRISTAR 3000 Micromeritics, U.S.), and the samples were out gassed at 180 °C in vacuum for 6 h. Thermal stability of sample was studied by thermogravimetric analysis/differential scanning calorimetry (TGA/DSC, METILER TOLEDO) from ambient temperature at a rate of 10 °C·min-1 to 800 °C. The photoluminescence (PL) measurements were recorded on a fluorescence spectrometer (LS55, Perkin Elmer) at room temperature. Electrochemical tests were performed on CHI 660E (Chenhua Instruments Company, China) electrochemical workstation in a three-electrode cell system. A saturated calomel electrode served as the reference and Pt foil as the counter electrode. The electrolyte was 0.1 M Na2SO4 solution. The working electrodes were prepared as follows: 5.0 mg of the as-prepared catalysts were dispersed in a 1.0 mL mixed solvent containing isopropanol and water (1:3 v/v) by ultrasonic treatment, and 20 μL of Nafion solution (5 wt%) was added to form uniform suspension. Then, 200 μL of the suspension was spread on the pretreated FTO, and dried in air at room temperature to form catalyst modified electrodes. Electrochemical impedance spectroscopy (EIS) and photocurrent measurements were recorded by applying an AC voltage of 10 mV amplitude in the frequency range of 104 Hz –10−1 Hz with the initial potential (0 V). The working area of working electrode is 1 cm2. 2.4. Evaluation of catalytic activity 9

The catalytic activity of the as-prepared α-Fe2O3/g-C3N4 composite was evaluated by the degradation of RhB and TC-H. All the experiments were performed under irradiation of 300 W Xenon lamp. In all the degradation experiments, the catalyst (0.1 g) was suspended in 100 mL of 10 mg·L-1 RhB/TC-H aqueous solution. The stirring rate was about 500 rpm. At each sampling time point, 3 mL aqueous suspension was taken from the degraded solution and filtered through a PTEF filter (pore size of 0.22 μm) for analysis. The concentration of RhB and TC-H was determined via absorbance by UV-vis spectrophotometer analyzer (UV-2550, Shimadzu, Japan) at the maximum absorption wavelength 552 nm and 357 nm, respectively. The degradation efficiency of RhB/TC-H was calculated by the following equation: η=

𝐶0 ― 𝐶𝑡 𝐶0

× 100%

(9)

where C0 is the initial concentration of RhB/TC-H; Ct is the concentration of RhB/TC-H after light irradiation time t. Total organic carbon (TOC, Shimadzu TOC-VCPH) was also measured to evaluate the mineralization of RhB/TC-H during the photo-Fenton processes. H2O2 was determined by using titanium oxysulfate as the indicator. The absorbance of the complex compound solution was determined at the maximum absorption wavelength at 409 nm [34, 35]. For detecting the active species during photo induced degradation, some sacrificial agents, such as isopropanol (IPA), ethylenediamine tetraacetic acid disodium (EDTA-2Na), and benzoquinone (p-BQ) were used as the ·OH, h+, superoxide radicals (·O2−) scavengers, respectively [36, 37]. The experiment was similar to the former catalytic activity test with the addition of 0.1 mmol different radical scavengers in the presence of 10 mg·L-1 RhB. The electron paramagnetic resonance

10

(EPR) signals of radical species spin-trapped by 5, 5’-dimethyl-1-pirroline-N-oxide (DMPO) were recorded on Bruker E300 spectrometer. 3. Results and discussion 3.1. The physicochemical characteristics of α-Fe2O3/g-C3N4 According to the procedure in Fig. 1, α-Fe2O3/g-C3N4 composites with different Fe:C ratios were prepared. Powder XRD patterns of the as-prepared g-C3N4, α-Fe2O3 and four α-Fe2O3/g-C3N4 composites are presented in Fig. 2. It can be noticed that two distinct peaks at 12.8° and 27.4° of pure g-C3N4, which can be indexed as the (100) and (002) reflections, corresponding to the interlayer stacking peak of aromatic systems and the in-plane structural packing motif of tri-s-triazine units, respectively (JCPDS87-1526) [38]. For pure α-Fe2O3, the characteristic diffraction peaks at 24.1°, 33.3°, 35.6°, 40.9°, 49.6°, 54.2°, 57.5°, 62.5°, and 64.1° are obtained. These peaks are consistent with the diffraction of the (012), (104), (110), (113), (024), (116), (018), (214) and (300) planes of the hexagonal a-Fe2O3 (JCPDS89-8103), respectively [39]. For α-Fe2O3/g-C3N4 composites, the XRD pattern of α-Fe2O3/g-C3N4 (Fe:C=3:1) shows no obvious diffraction peak assigned to g-C3N4 because of the relatively low contents of g-C3N4. However, the composite with higher g-C3N4 loading clearly displays the characteristic diffraction peak of g-C3N4 at 27.4° when the Fe:C increase to 1:2. For the α-Fe2O3/g-C3N4 (Fe:C=1: 2), the dominant peaks belonging to both g-C3N4 and α-Fe2O3 phases are detectable, demonstrating they were combined together well. Furthermore, the lower XRD diffraction intensities of g-C3N4 in the composites also indicates that there are certain interactions between g-C3N4 and α-Fe2O3 in the α-Fe2O3/g-C3N4 composites, which maybe leads to destruction of long ranges ordered crystal structures of g-C3N4 partially [40].

11

The details of component optimization experiments are given in Fig. S1. A typical α-Fe2O3/g-C3N4 (Fe:C=1:2) shows the most outstanding photochemical degradation activity among four different mass ratio composites, so it is selected as the representative composite catalyst in the following study. The morphologies and microstructures of pure g-C3N4 and α-Fe2O3/g-C3N4 were characterized by SEM and TEM. The pure g-C3N4 sample displays an irregular particles characteristic of loose agglomeration in micron scale (Fig. 3a). But when it was treated by ultrasonic dispersion, it found that these agglomerates are composed of multilayer g-C3N4 nanosheets (Fig. 3c). From the SEM (Fig. 3b) and STEM images (Fig. 3d) of α-Fe2O3/g-C3N4, it can be seen that innumerable α-Fe2O3 spheres with about 1m diameter are embedded and dispersed in the g-C3N4 bulk uniformly. The structure is conducive to inhibiting the aggregation of α-Fe2O3 and forming much more the heterojunction interfaces in the catalyst, thus providing abundant active sites. Besides, the elemental mapping shown in Fig. 3 e-h also demonstrated the existence of elements C, N, Fe and O. To further investigate the groups and chemical bonding, the FTIR spectra was confirmed for α-Fe2O3, g-C3N4 and α-Fe2O3/g-C3N4. As shown in Fig. 4, the pristine α-Fe2O3 sample shows the two peaks in the fingerprint areas (546 cm-1 and 473 cm-1) are attributed to the stretching vibration of Fe-O in α-Fe2O3 [41, 42]. Regarding pure g-C3N4, the sharp band at 808 cm-1 was originated from the breathing vibration of tri-s-triazine units [43]. Several strong absorption bands in the range of 1200-1650 cm-1 was derived from the stretching vibration of aromatic C=N and the skeletal stretching of C-N heterocycles with peaks positioned at 1622, 1458, 1401, 1316 and 1234 cm-1. The broad peak ranging from 2500 to 3200 cm-1 is assigned to the N-H due to the free amino groups and adsorbed hydroxyl species [44, 45]. When 12

comparing the spectrum of α-Fe2O3/g-C3N4 with that of pure g-C3N4 and α-Fe2O3, it is obvious that the primary characteristic features present in the spectrum of α-Fe2O3/g-C3N4. It should be noted that the strong absorption bands of 1200-1650 cm-1, 546 cm-1 and 473 cm-1 shifted to higher wavenumbers slightly, which imply that the strength of C-N and Fe-O bonds became weaker and reveal the formation of an intimate interfacial contact between g-C3N4 and α-Fe2O3. The chemical states and the elemental composite of the constituent elements can be identified by XPS survey spectrum of the α-Fe2O3/g-C3N4 composite in Fig. S2, four strong peaks assigned to C, N, O and Fe can be clearly observed respectively, which suggests that those elements exist in this material, being coincident with the constituent of the composites. The C 1s spectrum (Fig. 5a) can be deconvoluted into three peaks at the binding energies of 288.1, 286.3 and 284.8.0 eV, which could be identified to the sp3 type C-N bonds, the sp2 type C=N bonds in the tri-s-triazine units, and C-C bonds in the turbostratic CN structure and adventitious carbon adsorbed in graphitic domains, respectively [40, 46]. In high-resolution N 1s spectra (Fig. 5b), four peaks could be observed. The dominated peak at 398.5 eV belongs to sp2-hybridized nitrogen atom (typical triazine rings C=N-C), while the peak at 399.5 eV is attributed to the tertiary nitrogen bonded to carbon atoms in the form of (N-(C)3) groups. The peak at 400.8 eV is ascribed to the terminal amino (C-N-H) groups originating from the imperfect polymerization. The weak peak at 404.2 eV may be attributed to π-excitations [47-49]. For the Fe 2p spectrum (Fig. 5c), the binding energy of 710.5 and 724.3 eV belonged to 2p3/2 and 2p1/2 of α-Fe2O3, respectively. The energy separation of 13.9 eV between the spin–orbit doublets further demonstrates the existence of the oxidation state of Fe3+ [50]. In Fig. 5d, the O 1s spectra exhibits two peaks at 533.1 and 531.8 eV, which corresponds to C=O and 13

C−O. Two extra peaks can be fitted at 530.6 and 529.0 eV, which originated from Fe−O−C and the lattice oxygen in α-Fe2O3. The existence of Fe−O−C indicates the strong interaction between g-C3N4 and α-Fe2O3, which maybe could serve as an electron channel to facilitate the electron transfer [50, 51]. Meanwhile, the binding energies of lattice oxygen and Fe core electrons displays a negative shift compared with pure α-Fe2O3 reported previous studies [49, 50], suggesting the electron transfer from g-C3N4 to α-Fe2O3 in the α-Fe2O3/g-C3N4 composite. These XPS results demonstrate that the synthesized α-Fe2O3/g-C3N4 composite is not a simple physical mixture but a heterostructure with strong interaction between the two components. The strong interaction is beneficial for the formation of heterojunction between α-Fe2O3 and g-C3N4, and consequently promotes the migration of photogenerated electron–hole pairs and the photo-Fenton catalytic performance. In order to study thermal stability of catalyst α-Fe2O3/g-C3N4, thermogravimetric analysis was performed from 35 °C to 800 °C in air atmosphere. As shown in Fig. S3, pure α-Fe2O3 shows no weight loss in the temperature range, which indicates that the structure of α-Fe2O3 is stable and its thermal stability is good. However, for α-Fe2O3/g-C3N4, the weight decreases imperceptibly at the temperature below 450 °C, because only the adsorbed water on the α-Fe2O3/g-C3N4 surface was removed. A significant weight loss is beginning at 530 °C due to the decomposition of g-C3N4. In addition, at the temperature 600 °C, total combustion of g-C3N4 was attained. In general, a larger surface area could offer more active adsorption and catalytic sites for surface redox reaction, thus favoring the enhancement in catalytic activity. The N2 adsorption-desorption isotherms and the pore size distributions of the pristine g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4 sample are shown in Fig.6 and the values of SBET, pore size and pore volume are summarized in Table S1. In according to 14

International Union of Pure and Applied Chemistry (IUPAC) classification, three curves belonged to type IV with an H3 hysteresis loop at P/P0=0.50-0.99, revealing the existence of mesopores structure [52]. It can be found that the specific surface area of α-Fe2O3/g-C3N4 (13.65 m2/g) is larger than that of pure g-C3N4 (9.04 m2/g) because the α-Fe2O3 nanoparticles dispersed on the g-C3N4 nanosheets and decreased the interlayer stacking of g-C3N4. From the inset of Fig.6, it can be seen that the pore size distribution of α-Fe2O3/g-C3N4 are very broad, confirming the presence of mesopores and macropores. Compared to pristine g-C3N4 (67.2 nm) and α-Fe2O3 (92.2 nm) pore diameter, the α-Fe2O3/g-C3N4 pore diameter (36.0 nm) was smaller, which can supply more active sites and make charge carriers transport easier, leading to an improvement of the catalytic performance [53]. 3.2. Catalytic performances RhB and TC-H were selected as probe organic pollutant to evaluate the catalytic activity of α-Fe2O3/g-C3N4, which could be served as the representative contaminant for dyes and antibiotics in the wastewater respectively. Fig. 7a, c show the photo-induced catalytic performance of the pure g-C3N4, α-Fe2O3 and α-Fe2O3/g-C3N4 under simulated solar irradiation. Two blank tests without using any photocatalyst demonstrates

that

the

self-photolysis

of

RhB

are

negligible,

while

the

photodecomposition is obvious for the TC-H, approximately 20% after 150 min. In addition, 10% of RhB and 15% of TC-H could be removed when achieving adsorption equilibrium after 60 min. Obviously, the α-Fe2O3/g-C3N4 displays distinctly higher catalytic activity than the single components, α-Fe2O3 and g-C3N4. After 90 min irradiation, 96% of RhB decomposed in the presence of α-Fe2O3/g-C3N4 hybrid catalyst, while only 59% and 84% are decomposed at α-Fe2O3 and g-C3N4 within the same time, respectively. Although a longer period of time is needed to 15

remove the TC-H completely comparing to RhB, the similar tendency could be observed. After 150 min irradiation, 95% of TC-H decomposed in the presence of α-Fe2O3/g-C3N4, while only 85% and 70% are removed within the same time using pure g-C3N4 and α-Fe2O3, respectively. Despite of the narrow band gap and broad spectral absorption range, the α-Fe2O3 exhibits the lowest degradation rate, which is mainly attributed to its more positive conduction band potential, which is disadvantage of production of oxidizing species derived from electrons. It is different from the g-C3N4, the photo excited electrons generated from α-Fe2O3 is could not reduce O2 to •O2− or •HO2, because the standard redox potentials of E(O2/•O2−) (−0.33 eV vs. NHE) and E (O2/•HO2) (−0.05 eV vs. NHE) are more negative than their conduction band edge potential ECB (α-Fe2O3) (0.39 eV vs. NHE) [52]. Moreover, the production of H2O2 is also less. For α-Fe2O3/g-C3N4 catalyst, it is speculated that the excellent catalytic activity is ascribed to synergic effect of photo-Fenton with self-supplying H2O2 and photocatalysis on the catalyst interface. The kinetics of RhB and TC-H decay is also analyzed. The catalytic degradation of RhB is fitted by the pseudo-first-order kinetic model ln (Ct/C0) = −kt. The linear relationship is shown in Fig. 7b, d. The apparent rate constant of α-Fe2O3/g-C3N4 is 0.036 min−1 for RhB degradation, which is 1.8 and 3.6 times that of single g-C3N4 and α-Fe2O3, respectively. Meanwhile, the corresponding kinetic constant of α-Fe2O3/g-C3N4 (0.020 min−1) for TC-H is also much higher than that of pure g-C3N4 and α-Fe2O3. To further evaluate the photo-Fenton process, TOC analysis was carried out to understand the organic carbon removal efficiency of the catalyst. Fig. S4 shows the time-dependent TOC amount for RhB and TC-H during solar photo-Fenton processes. From the figure, it is observed that a mineralization of 92% and 86% is achieved at 180 min by α-Fe2O3/g-C3N4, respectively. Obviously, the complete mineralization for 16

dyes and antibiotics would be need much longer time and consume more energy comparing to the primary organic pollutants decomposing. The TOC results clearly indicate that the catalyst could mineralize the organics in this photo-Fenton process, demonstrating that the combination of g-C3N4 and α-Fe2O3 and the heterojunction formation between of them play crucial roles for enhancing the catalytic activity under simulated solar irradiation, which is beneficial to having some outstanding photoelectric properties. Besides the catalytic activity, the stability is another important factor from the view of practical application. The stability of the α-Fe2O3/g-C3N4 is evaluated by degrading RhB for five cycles. The results are shown in Fig. S5, it is found that the removal ratio of RhB in 90 min doesn't show obvious decrease after five successive experimental runs, and the values are all no less than 90%. The used catalyst was analyzed by XRD (in Fig. S6). The pattern demonstrates that the position of characteristic diffraction peaks doesn't change. The XRD intensities of α-Fe2O3 decay in a certain degree while that of g-C3N4 remains well. It may be ascribed to the hydroxylation and crystallinity decrease of α-Fe2O3 component in α-Fe2O3/g-C3N4 composite due to participating in photo-Fenton reaction. So, the α-Fe2O3/g-C3N4 has a good recycle performance and could be considered as a stable catalyst. In order to demonstrate the occurrence of photo-Fenton, the photocatalytic production of H2O2 over three catalysts has been determined in a saturated O2 solution at room temperature. As shown in Fig. 8, the H2O2 can be rapidly produced at α-Fe2O3/g-C3N4 and the amount of H2O2 reaches 12 mol at 60 min. The yield of H2O2 at single g-C3N4 (10 mol) or α-Fe2O3 (4 mol) is lower than that at α-Fe2O3/g-C3N4 within the same period. However, the concentration of light-driven H2O2 over g-C3N4 is higher than that over -Fe2O3/g-C3N4 when extending the 17

reaction time, which probably due to the heterogeneous Fenton reaction rate gradually accelerate and H2O2 decompose to ·OH with the -Fe2O3 component of -Fe2O3/g-C3N4 with the H2O2 concentration increasing. Moreover, the product ·OH might be also further react with a portion of H2O2 as Eq. 1 and slow down the H2O2 production consequently. For the pure α-Fe2O3, the low formation rate of H2O2 maybe due to its positive conduction band potential comparing to the reduction potential E(O2/H2O2 = 0.68 eV, vs. NHE), which result in the weak reduction ability of photo-generated electrons to molecular O2 [20]. In addition, the self-catalytic Fenton reaction maybe also exist on the single α-Fe2O3 interface. 3.3. Plausible mechanism of enhanced photo-induced catalytic activity A series of optical and photoelectric characterizations were performed to study the optical absorption property and charge transfer behaviors. UV-vis DRS were employed to determine the optical absorption of three catalysts (Fig. 9 (a)). The bare -Fe2O3 shows a weak absorption peak at ~350 nm and a strong absorption peak at ~540 nm. -Fe2O3 has an absorption edge at 610 nm, whereas pure g-C3N4 shows absorption edge only at 440 nm. After the introduction of α-Fe2O3 onto g-C3N4, the visible light absorption of the composite is expanded to ac. 600 nm, and the absorption ability also increases in the whole range of 200-800 nm compare to single g-C3N4. The higher absorption ability of the hybrids may lead to utilize more solar energy, which is beneficial to their photocatalytic activity. Besides, based on the UV–vis DRS data, the band gap energy (Eg) of the photocatalysts could be calculated using the following Tauc equation:

hv=A(hv-Eg)n

(10)

Where  h, and ν are absorption coefficient, Planck's constant, and the light frequency, respectively. A represents a constant number, and n equals 1/2 for -Fe2O3 18

and g-C3N4 (direct band gap material) [54]. Fig. 9 (b) shows the plots of (αhν)2 vs. hν. The Eg could be approximately estimated by measuring the x-axis value of inflection point near the absorption band edge. Thus, the values of Eg for g-C3N4, -Fe2O3, -Fe2O3/ g-C3N4 are 2.95 eV, 2.10 eV, 2.06 eV, respectively, which consistent well with the previously studies. The result indicates that the combination of -Fe2O3 reduce the Eg for g-C3N4. To determine the band structure of the -Fe2O3/g-C3N4, the band structure of g-C3N4 and α-Fe2O3 was investigated based on the Mott-Schottky measurements, as depicted in Fig. 10. The positive slopes indicate that g-C3N4 and α-Fe2O3 are both n-type semiconductors. The flatband potential (Vfb) of g-C3N4 and α-Fe2O3 can be deduced to be −0.75 and 0.31 V (vs. NHE) on the basis of the extrapolation of the x-intercept of the linear region in the Mott–Schotty curves, respectively. According to the previous study, the valance band maximum of the g-C3N4 and α-Fe2O3 locate at ac. 2.30 and 2.18 eV below the Fermi level [55]. Additionally, the potential of Fermi level is close to Vfb for n-type semiconductor. Thus, the valence band positions of g-C3N4 and α-Fe2O3 are estimated at 1.55 V and 2.49, respectively. By virtue of the value of Eg, the conduction band potential could be calculated to be −1.40 and 0.39 V for g-C3N4 and α-Fe2O3, respectively. According to these results, the band structure of α-Fe2O3/g-C3N4 is presented in Fig. 14. In order to investigate migration, transfer and recombination rate of photogenerated electron-hole pairs in the composite material, the PL emission spectra of α-Fe2O3, g-C3N4 and α-Fe2O3/g-C3N4 composites were determined. As shown in Fig. 11, the emission peak of the bulk g-C3N4 appears at 440 nm, attributed to the band-band recombination of the charge carriers with emission photon energy equal to its band gap energy [56]. The emission peak of α-Fe2O3/g-C3N4 hybrid is much weaker than that of pure g-C3N4, suggesting that the recombination of electron-hole pairs might be 19

effectively inhibited. Besides, no obvious emission peak of pure α-Fe2O3 is observed as the intensity is much weaker than pure g-C3N4 and α-Fe2O3/g-C3N4 hybrid [52]. It is clearly seen that PL emission intensity decreases when the α-Fe2O3 combination loading on the g-C3N4, which could provide the strong evidence for significantly inhibiting recombination of photogenerated charge carriers and improving photo-induced degradation activity of RhB. The electron–hole separation ability of direct Z-scheme α-Fe2O3/g-C3N4 catalyst was investigated by electrochemical impedance spectroscopy (EIS) and photocurrent (i-t) measurements. Figure 12 (a) shows the EIS response of g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4 in dark and under solar irradiation. In general, the size of arc radius on the Nyquist plot is relevant to the charge transfer process at the corresponding electrode/electrolyte interface with a smaller radius corresponding to the lower charge transfer resistance. The Nyquist plot of g-C3N4 displays the smallest arc radius in comparison to α-Fe2O3 and α-Fe2O3/g-C3N4 in dark, confirming the lowest charge transfer resistance. The low charge transfer is in favor of achieving a more efficient photo-generated electron–hole separation. It could be also seen that the arc radius of three catalysts all decrease and the α-Fe2O3/g-C3N4 becomes the minimum when they are irradiated, indicating that their resistances decrease and the α-Fe2O3/g-C3N4 has the best electronic conductivity under solar light irradiation. Furthermore, the photo-generated charge carriers are investigated by transient photocurrent responses under intermittent light illumination for several periodic on-and-off cycle mode in neutral Na2SO4 solution. The larger photocurrent often means the stronger light absorption (in Fig. 12 (b)) and higher separation efficiency of holes and electrons. Once the light is turned on, the transient photocurrent increases instantaneously, and the signals sharply decline on three catalysts when the light is turned off. It is found 20

that α-Fe2O3/g-C3N4 shows much higher photocurrent density than that of single g-C3N4 and α-Fe2O3, and the increment reaches 6.4 μA·cm2, which is ac. 1.6 and 23 times that of g-C3N4 and α-Fe2O3, respectively. This result confirms that the loading of α-Fe2O3 on the g-C3N4 facilitate to enhance the light harvesting efficiency, suppress charge recombination, and improve the stability of photo-responses. It indicates that Z-scheme type construction is preferable for charge separation and transfer, which makes more photo-generated electrons participate in the reduction of O2 to produce H2O2 and more holes proceed the direct oxidation, thereby enhances the photo-Fenton and photocatalytic activities towards organic dyes. Based on the above observation, it is reasonable to conclude that the efficient RhB removal and mineralization with α-Fe2O3/g-C3N4 composite is mostly due to the enhanced photo-Fenton and photocatalytic oxidation ability. According to previous researches, several main active species including ·OH, h+, and superoxide radicals (·O2−) may be produced and oxidize the organic pollutants in aqueous solution when the semiconductors is irradiated [36, 37]. To further elucidate the degradation mechanism, three reactive species were examined by the radical scavenger experiments, as shown in Fig.13. IPA, EDTA-2Na, and p-BQ were selected as the ·OH, h+, and ·O2− scavengers, respectively. It is well known that the redox ability of photo-induced active species is directly related to its redox potential. The CB potential of g-C3N4 (−1.40 eV, vs. NHE) is more negative than E(O2/·O2−), and that of α-Fe2O3 (0.39 eV, vs. NHE) is more positive, so the photo-generated electrons of g-C3N4 can reduce O2 to form ·O2− easily while α-Fe2O3 is difficult. Therefore, the addition of p-BQ notably suppresses RhB degradation, which indicates that ·O2− is predominant oxidative species for the g-C3N4. For the α-Fe2O3, the degradation efficiency decreases maximally when EDTA-2Na is 21

added, indicating the h+ is the main active species and can oxidize RhB directly owing to its high VB potentials (2.49 eV, vs. NHE). Additionally, it can be seen that the degradation of RhB is inhibited significantly with the existence of IPA for the α-Fe2O3/g-C3N4 and the decrement of 47% is much higher than those of single g-C3N4 (12%) and α-Fe2O3 (5%), suggesting the ·OH plays the most important role. Hereby, there are two principle channels for the production of ·OH. On the one hand, ·OH is derived from a photo-Fenton process without external H2O2 addition, namely, when the α-Fe2O3/g-C3N4 is irradiated, abundant of H2O2 can be produced on the interface of g-C3N4 component firstly via a two-electron reduction reaction of O2 from the photo-generated electrons on the CB of g-C3N4 as depicted in Eq. (8), because the CB potential is more negative than E(O2/H2O2) and have a high potential difference. Consequently, ·OH is produced form H2O2 decomposition through a heterogeneous Fenton in situ on the other component of α-Fe2O3 (Eq. 1), which is consistent with the result of relatively low H2O2 production on the α-Fe2O3/g-C3N4 abovementioned. On the other hand, the photo-generated h+ from VB of α-Fe2O3 can oxidize OH− to ·OH due to its VB potential is more positive than E(·OH/OH− = 2.38 eV, vs. NHE) [57]. However, the pathway is blocked for g-C3N4 because of its more negative VB potential. Undoubtedly, the UV portion in solar light also accelerates the conversion between FeIII and FeII and activates the H2O2 and iron complexes (Eq. 3~6). To further verify the generation of •OH, DMPO was used as the capture agent to detect the EPR signal of the samples in aqueous dispersion. The test results (Fig. 14a) displays that the signals of DMPO-•OH with a peak-intensity ratio of 1:2:2:1 appear and increase with the increase of time from 0 to 15 min only over α-Fe2O3/g-C3N4 [40], proving that the generation of •OH in the process of photo-Fenton. However, there is no DMPO-•OH signal over pure α-Fe2O3 and g-C3N4 (Fig. 14b). In addition, 22

compared with DMPO-•OH, no obvious peak of DMPO-•O2− could be observed over all catalyst samples in methanol dispersion, maybe attributing to relative lower concentration and rapid quenching of •O2− (Fig. S7). Therefore, EPR results are also proved that •OH is the dominate reactive oxidation species and the combination of α-Fe2O3 and g-C3N4 is favorable to the generation of •OH, which is agreement with reactive oxidation species trapping experiments. In the light of the band structures of g-C3N4 and α-Fe2O3, there are two possible pathways for charge carrier transfer in the composite catalyst, including the traditional type-II and direct Z-scheme, as shown in Fig. 15. Provided that the photo-generated charge carriers transfer follow traditional type-II, the electrons in CB of g-C3N4 would transfer into the CB of α-Fe2O3, while the holes would transfer from the VB of α-Fe2O3 to that of g-C3N4 under solar light illumination. If so, the charge carries are spatially separated but their redox abilities are weakened, the catalytic activity for producing ·OH of α-Fe2O3/g-C3N4 would inevitably decrease compared with pure g-C3N4, which is opposite with above results [58]. At the same time, the band bending and built-in electronic field in a type-II heterojunction would be hinder the continual charge carries transfer by reason of needing more energy [59]. Thus, on basis of the actual experimental results and analysis, it is more reasonable if α-Fe2O3/g-C3N4 conforms to direct Z-scheme pattern for enhancing catalytic performance in RhB degradation. When both of components in the α-Fe2O3/g-C3N4 absorb amounts of photons with enough energy, electrons would be excited from their respective VB to CB (Eq. 11). The photo-generated electrons in the CB of α-Fe2O3 transfer into the VB of

g-C3N4

and

recombine

with

residual

holes,

consequently

inhibiting

self-recombination of photo-generated charge carries in one component. In this case, electrons can accumulate in the CB of g-C3N4 while holes are restrained in the VB of 23

α-Fe2O3. Accordingly, the α-Fe2O3/g-C3N4 could maintain both strong reduction capability of g-C3N4 and strong oxidation capability of α-Fe2O3, supplying a great driving force for RhB degradation reaction. The photo-generated electrons accumulated in the CB of g-C3N4 would trigger O2 reduction reaction and participate in the iron species transformation, generating a series of active species such as ·O2−, ·HO2, H2O2, and ·OH etc. (Eq. 12~16). Besides that, the photo-generated holes retained in the VB of α-Fe2O3 would participate in water and RhB oxidation. Owing to large amount of various oxidative species, especially ·OH exhibited strong and non-selective oxidation capability, they could degrade various organic contaminants. Ultimately, the RhB and intermediates were gradually decomposed into carbon dioxide and water, as in Eq. 18. Obviously, the direct Z-scheme type α-Fe2O3/g-C3N4 significantly improves charge separation efficiency and enhancing the photo-induced catalytic degradation performance [60]. α-Fe2O3/g-C3N4 + hν → g-C3N4(h+) + α-Fe2O3(e−)

(11)

e− + O2 →·O2−

(12)

·O2− + H+ → ·HO2

(13)

e− + ·HO2 + H+ → H2O2

(14)

e− + H2O2 →·OH + OH−

(15)

h+ + OH− → ·OH

(16)

FeIII + e− → FeII

(17)

·OH/·O2−/h+ + RhB → CO2 + H2O

(18)

4. Conclusions In conclusion, as a heterogeneous Fenton catalyst, the narrow band gap semiconductor of α-Fe2O3 was introduced for g-C3N4 to fabricate an efficient direct 24

solid-state

Z-scheme

type

α-Fe2O3/g-C3N4

catalyst.

Through

the

specific

characterizations of SEM, FTIR, XRD, XPS, and DSR, the morphology and physicochemical properties of α-Fe2O3/g-C3N4 were investigated. The composite catalyst exhibited an enhanced photo-induced catalytic activity for RhB and TC-H degradation under simulated solar light irradiation and had a stable recycle performance. The experimental results demonstrated that the efficient removal of contaminant is triggered by the predominant photo-Fenton process and the subordinate photocatlysis. The enhancement was ascribed to the formation of direct Z-scheme structure, which not only facilitates the separation of photo-generated electron-hole pairs, but also maintains the strong redox ability derived from the electrons in CB of g-C3N4 and the holes in VB of α-Fe2O3. The α-Fe2O3/g-C3N4 could be used as a promising bi-functional catalyst for organic wastewater degradation by synergetic processes of photo-Fenton with self-supplying H2O2 and traditional photocatalysis. Acknowledgments This work was supported jointly by the National Natural Science Foundation of China (Grant Nos. 51508435, 51671150, and 21507103), the Natural Science Basic Research Plan in Shaanxi Province of China (2017JM5057), the Shaanxi Key Science and Technology Innovation Team Project (No. 2017KCT-19-02). References [1] Y. Zhang, M.H. Zhou, A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values, J. Hazard. Mater. 362 (2019) 436-450. [2] H. Li, J. Shang, Z.P. Yang, W.J. Shen, Z.H. Ai, L.Z. Zhang, Oxygen vacancy associated surface Fenton chemistry: Surface structure dependent hydroxyl radicals 25

generation and substrate dependent reactivity, Environ. Sci. Technol. 51 (2017) 5685-5694. [3] J.R. Steter, E. Brillas, I. Sires, Solar photoelectro-Fenton treatment of a mixture of parabens spiked into secondary treated wastewater effluent at low input current, Appl. Catal. B: Environ. 224 (2018) 410-418. [4] X.J. Yang, X.M. Xu, J. Xu, Y.F. Han, Iron oxychloride (FeOCl): An efficient Fenton-like catalyst for producing hydroxyl radicals in degradation of organic contaminants, J. Am. Chem. Soc. 135 (2013) 16058-16061. [5] X.C. Liu, Y.Y. Zhou, J.C. Zhang, L. Luo, Y. Yang, H.L. Huang, H. Peng, L. Tang, Y. Mu, Insight into electro-Fenton and photo-Fenton for the degradation of antibiotics: Mechanism study and research gaps, Chem. Eng. J. 347 (2018) 379-397. [6] H. Lan, A. Wang, R. Liu, H. Liu, J. Qu, Heterogeneous photo-Fenton degradation of acid red B over Fe2O3 supported on activated carbon fiber, J. Hazard. Mater. 285 (2015) 167-172. [7] Y. He, Y. Ma, J. Meng, X. Zhang, Y. Xia, Dual electrochemical catalysis of Bi2Mo3O12/Ti cathode for hydrogen peroxide production in electro-Fenton system, J. Catal. 373 (2019) 297-305. [8] E. Brillas, I. Sires, M.A. Oturan, Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry, Chem. Rev. 109 (2009) 6570-6631. [9] Y. Wang, G. Zhao, S. Chai, H. Zhao, Y. Wang, Three-Dimensional homogeneous ferrite-carbon aerogel: One-pot fabrication and enhanced electro-Fenton reactivity, ACS Appl. Mat. Interfaces 5 (2013) 842-852. [10] W. Song, M. Cheng, J. Ma, W. Ma, C. Chen, J. Zhao, Decomposition of hydrogen peroxide driven by photochemical cycling of iron species in clay, Environ. 26

Sci. Technol. 40 (2006) 4782-4787. [11] S.S. Lin, M.D. Gurol, Catalytic decomposition of hydrogen peroxide on iron oxide: Kinetics, mechanism, and implications, Environ. Sci. Technol. 32 (1998) 1417-1423. [12] X. Huang, X. Hou, J. Zhao, L. Zhang, Hematite facet confined ferrous ions as high efficient Fenton catalysts to degrade organic contaminants by lowering H2O2 decomposition energetic span, Appl. Catal. B: Environ. 181 (2016) 127-137. [13] L. Clarizia, D. Russo, I. Di Somma, R. Marotta, R. Andreozzi, Homogeneous photo-Fenton processes at near neutral pH: A review, Appl. Catal. B: Environ. 209 (2017) 358-371. [14] S. Giannakis, A review of the concepts, recent advances and niche applications of the (photo) Fenton process, beyond water/wastewater treatment: Surface functionalization, biomass treatment, combatting cancer and other medical uses, Appl. Catal. B: Environ. 248 (2019) 309-319. [15] Y. Shiraishi, S. Kanazawa, Y. Sugano, D. Tsukamoto, H. Sakamoto, S. Ichikawa, T. Hirai, Highly selective production of hydrogen peroxide on graphitic carbon nitride (g-C3N4) photocatalyst activated by visible light, ACS. Catal. 4 (2014) 774-780. [16] Y.X. Li, S.X. Ouyang, H. Xu, X. Wang, Y.P. Bi, Y.F. Zhang, J.H. Ye, Constructing

solid-gas-interfacial

Fenton

reaction

over

alkalinized-C3N4

photocatalyst to achieve apparent quantum yield of 49% at 420 nm, J. Am. Chem. Soc. 138 (2016) 13289-13297. [17] F. K. Kessler, Y. Zheng, D. Schwarz, C. Merschjann, W. Schnick, X.C. Wang, M.J. Bojdys, Functional carbon nitride materials design strategies for electrochemical devices, Nat. Rev. Mater. 2 (2017). [18] Y.X. Fang, X.C. Wang, Metal-free boron-containing heterogeneous catalysts, 27

Angew. Chem. Int. Ed. 56 (2017) 15506-15518. [19] Y. Pi, H. Gao, Y. Cao, R. Cao, Y. Wang, J. Sun, Cobalt ferrite supported on carbon nitride matrix prepared using waste battery materials as a peroxymonosulfate activator for the degradation of levofloxacin hydrochloride, Chem. Eng. J. 379 (2020). [20] Z. Wei, M.L. Liu, Z.J. Zhang, W.Q. Yao, H.W. Tan, Y.F. Zhu, Efficient visible-light-driven

selective

oxygen

reduction

to

hydrogen

peroxide

by

oxygen-enriched graphitic carbon nitride polymers, Energy Environ. Sci. 11 (2018) 2581-2589. [21] Y. Shiraishi, S. Kanazawa, Y. Kofuji, H. Sakamoto, S. Ichikawa, S. Tanaka, T. Hirai, Sunlight-driven hydrogen peroxide production from water and molecular oxygen by metal-free photocatalysts, Angew. Chem. Int. Ed. 53 (2014) 13454-13459. [22] Y. Kofuji, Y. Isobe, Y. Shiraishi, H. Sakamoto, S. Tanaka, S. Ichikawa, T. Hirai, Carbon nitride-aromatic diimide-graphene nanohybrids: Metal-free photocatalysts for solar-to-hydrogen peroxide energy conversion with 0.2% efficiency, J. Am. Chem. Soc. 138 (2016) 10019-10025. [23] Y. Kofuji, Y. Isobe, Y. Shiraishi, H. Sakamoto, S. Ichikawa, S. Tanaka, T. Hirai, Hydrogen peroxide production on a carbon nitride-boron nitride-reduced graphene oxide hybrid photocatalyst under visible light, Chemcatchem 10 (2018) 2070-2077. [24] Y.L. Peng, L.Z. Wang, Y.D. Liu, H.J. Chen, J.Y. Lei, J.L. Zhang, Visible-light-driven photocatalytic H2O2 production on g-C3N4 loaded with CoP as a noble metal free cocatalyst, Eur. J. Inorg. Chem. (2017) 4797-4802. [25] S. Zhao, X. Zhao, H. Zhang, J. Li, Y.F. Zhu, Covalent combination of polyoxometalate and graphitic carbon nitride for light-driven hydrogen peroxide production, Nano Energy 35 (2017) 405-414. [26] J.W. Fu, J.G. Yu, C.J. Jiang, B. Cheng, g-C3N4-based heterostructured 28

photocatalysts, Adv. Energy. Mater. 8 (2018). [27] Q.L. Xu, L.Y. Zhang, J.G. Yu, S. Wageh, A.A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: Principles, synthesis, and applications, Mater. Today 21 (2018) 1042-1063. [28] Y.F. Jian, T.T. Yu, Z.Y. Jiang, Y.K. Yu, M. Douthwaite, J.Y. Liu, R. Albilali, C. He, In-depth understanding of the morphology effect of alpha-Fe2O3 on catalytic ethane destruction, ACS Appl. Mat. Interfaces 11 (2019) 11369-11383. [29] C. He, J. Cheng, X. Zhang, M. Douthwaite, S. Pattisson, Z.P. Hao, Recent Advances in the catalytic oxidation of volatile organic compounds: A review based on pollutant sorts and sources, Chem. Rev. 119 (2019) 4471-4568. [30] Y. Zhao, F. Pan, H. Li, T. Niu, G. Xu, W. Chen, Facile synthesis of uniform alpha-Fe2O3 crystals and their facet-dependent catalytic performance in the photo-Fenton reaction, J. Mater. Chem. A 1 (2013) 7242-7246. [31] L. He, L. Jing, Y. Luan, L. Wang, H. Fu, Enhanced visible activities of -Fe2O3 by coupling N-doped graphene and mechanism insight, ACS. Catal. 4 (2014) 990-998. [32] L. Wang, M. Cao, Z. Ai, L. Zhang, Dramatically enhanced aerobic atrazine degradation with Fe@Fe2O3 core-shell nanowires by tetrapolyphosphate, Environ. Sci. Technol. 48 (2014) 3354-3362. [33] G. Zhang, Z.-A. Lan, L. Lin, S. Lin, X. Wang, Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents, Chem. Sci. 7 (2016) 3062-3066. [34] H.Y. Zhao, X.Q. Shen, Y. Chen, S.N. Zhang, P. Gao, X.J. Zhen, X.H. Li, G.H. Zhao, A COOH-terminated nitrogen-doped carbon aerogel as a bulk electrode for completely selective two-electron oxygen reduction to H2O2, Chem. Commun. 55 29

(2019) 6173-6176. [35] Z.H. Zhang, H.S. Meng, Y.J. Wang, L.M. Shi, X. Wang, S.N. Chai, Fabrication of graphene@graphite-based gas diffusion electrode for improving H2O2 generation in Electro-Fenton process, Electrochim. Acta 260 (2018) 112-120. [36] J.J. Zhang, X. Zhao, Y.B. Wang, Y. Gong, D. Cao, M. Qiao, Peroxymonosulfate-enhanced visible light photocatalytic degradation of bisphenol A by perylene imide-modified g-C3N4, Appl. Catal. B: Environ. 237 (2018) 976-985. [37] Y.B. Wang, M. Liu, X. Zhao, D. Cao, T. Guo, B. Yang, Insights into heterogeneous catalysis of peroxymonosulfate activation by boron-doped ordered mesoporous carbon, Carbon 135 (2018) 238-247. [38] Y.Z. Wu, J. Ward-Bond, D.L. Li, S.H. Zhang, J.F. Shi, Z.Y. Jiang, g-C3N4@alpha-Fe2O3/C photocatalysts: Synergistically intensified charge generation and charge transfer for NADH regeneration, ACS. Catal. 8 (2018) 5664-5674. [39] Q.S. Peng, H.Y. Zhao, L. Qian, Y.B. Wang, G.H. Zhao, Design of a neutral photo-electro-Fenton system with 3D-ordered macroporous Fe2O3/carbon aerogel cathode: High activity and low energy consumption, Appl. Catal. B: Environ. 174 (2015) 157-166. [40] Z. Jin, R. Hu, H. Wang, J. Hu, T. Ren, One-step impregnation method to prepare direct Z-scheme LaCoO3/g-C3N4 heterojunction photocatalysts for phenol degradation under visible light, Appl. Surf. Sci. 491 (2019) 432-442. [41] G.K. Zhang, Y.Y. Gao, Y.L. Zhang, Y.D. Guo, Fe2O3-pillared rectorite as an efficient and stable Fenton-like heterogeneous catalyst for photodegradation of organic contaminants, Environ. Sci. Technol. 44 (2010) 6384-6389. [42] M. Ba-Abbad, M.S. Takriff, A. Benamor, A.W. Mohammad, Size and shapecontrolled of alpha-Fe2O3 nanoparticles prepared via sol-gel technique and their 30

photocatalytic activity, J. Sol-Gel Sci. Technol. 81 (2017) 880-893. [43] Y.J. Zhang, T. Mori, L. Niu, J.H. Ye, Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion, Energy Environ. Sci. 4 (2011) 4517-4521. [44] J.G. Yu, S.H. Wang, J.X. Low, W. Xiao, Enhanced photocatalytic performance of direct Z-scheme g-C3N4-TiO2 photocatalysts for the decomposition of formaldehyde in air, PCCP 15 (2013) 16883-16890. [45] L.M. Sun, Y. Qi, C.J. Jia, Z. Jin, W.L. Fan, Enhanced visible-light photocatalytic activity of g-C3N4/Zn2GeO4 heterojunctions with effective interfaces based on band match, Nanoscale 6 (2014) 2649-2659. [46] Y. Li, X.Y. Wu, J. Li, K. Wang, G.K. Zhang, Z-scheme g-C3N4@CsxWO3 heterostructure as smart window coating for UV isolating, Vis penetrating, NIR shielding and full spectrum photocatalytic decomposing VOCs, Appl. Catal. B: Environ. 229 (2018) 218-226. [47] J.H. Liu, T.K. Zhang, Z.C. Wang, G. Dawson, W. Chen, Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity, J. Mater. Chem. 21 (2011) 14398-14401. [48] K. Li, Z.Y. Huang, X.Q. Zeng, B.B. Huang, S.M. Gao, J. Lu, Synergetic effect of Ti3+ and oxygen doping on enhancing photoelectrochemical and photocatalytic properties of TiO2/g-C3N4 heterojunctions, ACS Appl. Mat. Interfaces 9 (2017) 11577-11586. [49] R. Ma, S. Zhang, L. Li, P. Gu, T. Wen, A. Khan, S. Li, B. Li, S. Wang, X. Wang, Enhanced visible light induced photoactivity of Type-II CeO2/g-C3N4 nanosheet toward organic pollutants degradation, ACS Sustain. Chem. Eng. 7 (2019) 9699-9708. [50] Y. Wu, J. Ward-Bond, D. Li, S. Zhang, J. Shi, Z. Jiang, g-C3N4@alpha-Fe2O3/C 31

Photocatalysts: Synergistically intensified charge generation and charge transfer for NADH regeneration, ACS Catal. 8 (2018) 5664-5674. [51] Y. Zheng, X. Zhang, J. Zhao, P. Yang, Assembled fabrication of alpha-Fe2O3/BiOCl heterojunctions with enhanced photocatalytic performance, Appl. Surf. Sci. 430 (2018) 585-594. [52] J.S. Lu, Y.J. Wang, F. Liu, L. Zhang, S.N. Chai, Fabrication of a direct Z-scheme type WO3/Ag3PO4 composite photocatalyst with enhanced visible-light photocatalytic performances, Appl. Surf. Sci. 393 (2017) 180-190. [53] M.J. Tian, C. He, Y.K. Yu, H. Pan, L. Smith, Z.Y. Jiang, N.B. Gao, Y.F. Jian, Z.P. Hao, Q. Zhu, Catalytic oxidation of 1,2-dichloroethane over three-dimensional ordered meso-macroporous Co3O4/La0.7Sr0.3Fe0.5Co0.5O3: Destruction route and mechanism, Appl. Catal. A: Gen. 553 (2018) 1-14. [54] S.D. Yan, Y. Shi, Y.F. Tao, H. Zhang, Enhanced persulfate-mediated photocatalytic oxidation of bisphenol A using bioelectricity and a g-C3N4/Fe2O3 heterojunction, Chem. Eng. J. 359 (2019) 933-943. [55] Z.F. Jiang, W.M. Wan, H.M. Li, S.Q. Yuan, H.J. Zhao, P.K. Wong, A hierarchical Z-scheme alpha-Fe2O3/g-C3N4 hybrid for enhanced photocatalytic CO2 reduction, Adv. Mater. 30 (2018). [56] J. Theerthagiri, R.A. Senthil, A. Priya, J. Madhavan, R.J.V. Michael, M. Ashokkumar, Photocatalytic and photoelectrochemical studies of visible-light active alpha-Fe2O3-g-C3N4 nanocomposites, RSC. Adv. 4 (2014) 38222-38229. [57] L.H. Yu, X.Y. Zhang, G.W. Li, Y.T. Cao, Y. Shao, D.Z. Li, Highly efficient Bi2O2CO3/BiOCl photocatalyst based on heterojunction with enhanced dye sensitization under visible light, Appl. Catal. B: Environ. 187 (2016) 301-309. [58] K. Geng, Y. Wu, G.D. Jiang, K.L. Liu, L.L. Jiang, RuC@g-C3N4(H+)/TiO2 32

visible active photocatalyst: Facile fabrication and Z-scheme carrier transfer mechanism, Mol. Catal. 458 (2018) 33-42. [59] Q.L. Xu, B.C. Zhu, C.J. Jiang, B. Cheng, J.G. Yu, Constructing 2D/2D Fe2O3/g-C3N4 direct Z-scheme photocatalysts with enhanced H2 generation performance, Sol. RRL 2 (2018). [60] J.W. Wang, X.J. Zuo, W. Cai, J.W. Sun, X.L. Ge, H. Zhao, Facile fabrication of direct

solid-state

Z-scheme

g-C3N4/Fe2O3

heterojunction:

a

cost-effective

photocatalyst with high efficiency for the degradation of aqueous organic pollutants, Dalton Trans. 47 (2018) 15382-15390.

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Figure captions: Fig.1. Schematic illustration of the α-Fe2O3/g-C3N4 preparation. Fig.2. XRD patterns of pristine g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4 composites with different Fe:C ratios. Fig.3. SEM images of (a) g-C3N4 and (b) α-Fe2O3/g-C3N4, (c) TEM image of g-C3N4, (d) STEM images of α-Fe2O3/g-C3N4 and corresponding elemental mapping of (e) C, (f) N, (g) Fe and (h) O, insert: corresponding SEM image. Fig.4. FTIR spectra of pristine g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4. Fig.5. XPS high-resolution spectra for the α-Fe2O3/g-C3N4, (a) C 1s, (b) N 1s, (e) Fe 2p, and (d) O 1s. Fig. 6. N2 adsorption–desorption isotherms and pore size distribution (inset) of g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4. Fig. 7. Concentration change of contaminants as the function of the processing time and the corresponding decay kinetics for RhB (a, b), and TC-H (c, d). Fig. 8. The yields of H2O2 at different catalysts under simulated solar light illumination. Fig. 9. (a) UV−vis diffuse reflectance spectra. (b) Calculated band gap patterns based on UV−vis diffuse reflectance spectra. Fig. 10. Mott-Schottky plots of (a) g-C3N4 and (b) α-Fe2O3 in 0.1 M Na2SO4 solution. Fig. 11. PL spectra of pristine g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4. Fig.12. (a) Transient photocurrent responses and (b) electrochemical impedance spectroscopy of g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4. Fig. 13. Effects of different scavengers on the RhB degradation efficiencies over g-C3N4, α-Fe2O3 and α-Fe2O3/g-C3N4 in 60 min. Fig. 14. EPR spectra of different photo-Fenton systems with (a) α-Fe2O3/g-C3N4, (b) 34

α-Fe2O3 and g-C3N4 using DMPO as spin trapping agent in aqueous dispersion. Fig. 15. Schematic illustration of photo-generated charges transfer and degradation mechanism of RhB over α-Fe2O3/g-C3N4.

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520 ℃ 2h

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Graphical Abstract:

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Highlights:

 A heterogeneous photo-Fenton system without external H2O2 supplement was developed.  An efficient all-solid-state Z-scheme -Fe2O3/g-C3N4 was fabricated.  The dominant oxidative species of ·OH was generated from self-produced H2O2 decomposition.  96% of organic pollutant removal was achieved after 90 min.  A new reasonable photo-Fenton mechanism was proposed.

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Declarations of Interest：None

Corresponding author: Dr. Shouning Chai E-mail addresses: [email protected] Oct 21, 2019

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Credit Author Statement Yujing Wang: Investigation, Writing - Review & Editing, and Supervision. Hemei Song: Investigation, Writing - Original Draft, and Validation. Jian Chen: Resources and Data Curation. Shouning Chai: Formal analysis, Writing - Review & Editing, Resources, and Supervision. Limin Shi: Formal analysis. Changwei Chen: Investigation and Data Curation. Yanbin Wang: Formal analysis. Chi He: Writing- Reviewing and Editing, Resources, and Supervision.

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