Fabrication of a novel 3D E-Fe2O3-Pi-MoS2 film with highly enhanced carrier mobility and photoelectrocatalytic activity

Electrochimica Acta 337 (2020) 135748

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Fabrication of a novel 3D E-Fe2O3-Pi-MoS2 film with highly enhanced carrier mobility and photoelectrocatalytic activity Yanqing Cong a, b, Wenchen Ding a, Wenhua Zhang a, Tongtong Zhang a, Qi Wang a, b, Yi Zhang a, b, * a b

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, 310018, China Institute of Urban Aquatic Environment, Zhejiang Gongshang University, Hangzhou, 310018, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 August 2019 Received in revised form 18 January 2020 Accepted 19 January 2020 Available online 23 January 2020

A novel 3D E-Fe2O3-Pi-MoS2 film was fabricated for water oxidation and environmental remediation. The nonmetal doping with phosphorus and vertically oriented MoS2 nanosheets were successfully constructed on Fe2O3 substrate, and the electrochemical modification further enhanced its photoelectrocatalytic (PEC) activity. The photocurrent density of E-Fe2O3-Pi-MoS2 for water oxidation was 30 times or 19.4 times higher than that of Fe2O3 under visible light or UVevis light illumination at 0.45 V vs. Ag/AgCl, respectively. E-Fe2O3-Pi-MoS2 film also showed excellent PEC activity for phenol degradation (93.73%) and good stability. Hydroxyl radicals and holes were main active species. The remarkable activity of E-Fe2O3-Pi-MoS2 film could be attributed to the synergistic effects of nonmetal doping with phosphorous, vertically oriented MoS2 nanosheets fabrication and the electrochemical modification treatment, which significantly improved the charge transfer and enhanced the separation efficiency of photogenerated carriers. This work provides an effective, environmentally friendly, and low-cost semiconductor materials for sustainable energy and environmental application. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Photoelectrocatalytic E-Fe2O3-Pi-MoS2 Visible light Water oxidation Organic pollutants

1. Introduction Solar energy is a clean and sustainable energy source which can alleviate our dependence on fossil fuels and resolve the problems of energy shortage and environmental pollution [1,2]. It is particularly vital but still challenging to establish effective conversion and storage of solar energy. Besides, the growing population and industrial production have led to a large amount of organic pollutants discharged into the water, causing serious water pollution [3,4]. As a representative toxic pollutant, phenol has been used extensively to produce pesticides, refine oil and pharmaceuticals [5]. It’s hazardous to humans which may lead to poisoning, carcinogenicity and even death [6], so it is necessary to develop an efficient and inexpensive method to deal with it. Electrochemical process and photocatalytic process show many innate advantages including ease of automation, simple operation, high energy efficiency and environmental safety in water oxidation and wastewater treatment

* Corresponding author. School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, 310018, China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.electacta.2020.135748 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

[7e9]. Photoelectrocatalysis has the combined advantages of electrocatalysis and photocatalysis and is one of the most important methods to convert solar energy into chemical energy through water splitting [10e12]. As the half reaction of water splitting, water oxidation has been considered to be the main obstacle for effective water splitting [13,14]. Photoanodes of metal oxides such as TiO2 [15e17], Bi2O3 [18e20] and a-Fe2O3 [21e23] were extensively investigated for water oxidation. a-Fe2O3 is a promising semiconductor material because of its high natural abundance, adequate visible light absorption and good stability [24,25]. However, its low electron mobility limits the photo-efficiency and the solar-to-hydrogen efficiency seems far below the theoretical limit [26e28]. Lots of efforts have been done to improve the photoelectrochemical (PEC) activity of a-Fe2O3 such as nanostructure engineering [29], doping [30,31] or surface modification [32,33]. Proton transfer was proved to play a pivotal role in water oxidation on a-Fe2O3 photoanodes [34], and the modification of the electronic structure via elemental doping seems to be an effective method to improve the photoactivity of a-Fe2O3 [35]. Previous researches focused on the doping of metal elements such as Ti [36,37], Co [38,39], Zn [40], Sn [41], etc. Some researches indicate

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that the nonmetal element doping can enhance the electron mobility of the a-Fe2O3 due to the strong covalent interaction between Si and O or P and O, which can avoid the formation of deep electron trapping sites from the dopant [42,43]. This provides a new idea to the improvement of the PEC activity of a-Fe2O3, but the only doping of nonmetal elements is far from enough. Transitional chalcogenides have a special physical structure in which the interaction of d electrons can produce new physical phenomena [44,45]. Molybdenum disulfide (MoS2) is a typical transition metal chalcogenide composed of SeMoeS covalent bonds, which has a small band gap, good electrocatalytic activity and stability [45,46]. The different crystal orientation of MoS2 has a great influence on the light response [47e49]. A vertically oriented MoS2 can expose more functional sites and specific surface areas, which would significantly increase the visible light absorption and improve its PEC performance [50,51]. In this work, nonmetal doping with phosphorous and vertically oriented MoS2 were successfully fabricated on the Fe2O3 film by a simple impregnation and hydrothermal method. Nonmetal doping with phosphorous would improve the charge mobility of Fe2O3 film as electron carriers instead of electron trapping sites [43,52,53]. Vertically oriented MoS2 on Fe2O3 film was expected to further increase charge mobility since MoS2 is a two-dimensional layered material and vertical orientation is beneficial for the mobility of carriers [54,55]. In addition, a simple electrochemical treatment was carried out at room temperature and atmospheric pressure to achieve vertically oriented MoS2 which can provide more effective active sites without introducing foreign elements. The PEC activities for water oxidation and pollutants degradation under visible light were investigated. The photocurrent density of E-Fe2O3-PiMoS2 film for water oxidation was achieved 0.404 mA/cm2 under visible light at 0.45 V vs. Ag/AgCl, which is higher than some conventional anodes such as TiO2-NTs [56], Bi2O3 [57] or Fe2O3 [21]. The results indicated that the PEC performance of the composite EFe2O3-Pi-MoS2 film was significantly improved on account of the synergistic effects of nonmetal doping with phosphorous and vertically oriented MoS2.

2. Experimental 2.1. Chemicals The fluorine doped SnO2 (FTO) was purchased from Japan Plate Glass Co., Ltd. Ferrous chloride (FeCl2$4H2O) was purchased from Shanghai Macklin Biochemical Co., Ltd. Thiourea ((NH2)2CS) was purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium phosphate dibasic (Na2HPO4) and sodium phosphate monobasic (NaH2PO4) were provided by Shanghai Lingfeng Chemical Reagent Co., Ltd. Ethylene glycol ((CH2OH)2), boric acid (H3BO3) and acetone (CH3COCH3) were provided by Chengdu Kelon Chemical Reagent Factory. Anhydrous sodium sulfate (Na2SO4) was obtained from Xilong Scientific Co., Ltd. Ammonium molybdate ((NH4)6MoO24$4H2O), sodium hydroxide (NaOH), potassium hydroxide (KOH) and anhydrous ethanol (CH3CH2OH) were obtained from Yonghua Chemical Technology (Jiangsu) Co., Ltd. All chemicals were analytical purity and used without further purification.

2.2. Synthesis of E-Fe2O3-Pi-MoS2 FTO was used as substrate, and ultrasonically washed with acetone, ethanol and deionized water for 10 min. Fig. 1 is a schematic diagram of the preparation of E-Pi-Fe2O3eMoS2 thin film electrode.

(1) Phosphorus-doped Fe2O3 (Fe2O3-Pi) film: First, 0.3582 g of FeCl2$4H2O was dissolved in 10 mL of ethylene glycol and 80 mL of distilled water to obtain a Fe2þ electrodeposition solution with a concentration of 0.02 M. Electrodeposition was performed in a three-electrode system using the CHI660E electrochemical workstation. The pretreated FTO was used as the working electrode, graphite was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. Electrodeposition was carried out in Fe2þ solution at 70  C and 1.36 V for 5 min. The obtained films were soaked in 0.05 M phosphate buffer solution for 1 min and then dried in a vacuum oven at 80  C for 15 min. Finally, Fe2O3-Pi photocatalytic film was obtained by calcining at 630  C for 20 min with a heating rate of 3  C/min. (2) Fe2O3-Pi-MoS2 film: Fe2O3-Pi film was placed upward in 30 mL of a solution which contains 0.35 mmol H2NCSNH2 and 0.025 mmol (NH4)6Mo7O24$4H2O and maintained at 220  C for 2 h. When the reaction was ended and cooled to room temperature naturally, it was washed with distilled water and dried to obtain a Fe2O3-Pi-MoS2 photocatalytic film. (3) Electrochemical modification of Fe2O3-Pi-MoS2 (E-Fe2O3-PiMoS2) film: Electrochemical treatment is carried out in a three-electrode system. Fe2O3-Pi-MoS2 electrode was used as the working electrode, Pt was used as the counter electrode, and Ag/AgCl electrode was used as the reference electrode. A voltage from 0.9 V to 0.75 V was applied to the Fe2O3-Pi-MoS2 film within 300 s in a 1 M boric solution with a pH of about 9.5. The electrochemical treatment at 0.8 V for 150 s was found to be the optimized condition. Wash with distilled water and dry to obtain E-Fe2O3-Pi-MoS2 photocatalytic film.

2.3. Characterization X-ray Diffraction (XRD) was used to characterize the phase structure and crystallinity of different catalytic films with a Cu-Ka radiation source, and a scanning range of 5 e70 . X-ray Photoelectron Spectroscopy (XPS) on a spectrometer (Thermo ESCALAB 250Xi) with Al-Ka radiation (1486.6 eV) was used to characterize the chemical composition and surface chemical states of catalytic films. The microstructure and surface morphologies of catalytic films were analyzed by field emission scanning electron microscope (FE-SEM, Gemini 500). UVevis spectra (UVevis) were carried out using TU-1901 UVeVis spectrophotometer. 2.4. PEC measurements for water oxidation PEC tests on the as-prepared photocatalytic films were carried out using CHI660E electrochemical workstation. The optimized photocatalytic film was used as working electrode. Pt foil and Ag/ AgCl electrode were acted as counter electrode and reference electrode, respectively. Incident visible light was obtained from a xenon lamp (CEL-300, Beijing Zhongjiao Jinyuan Co., Ltd.) with a 420 nm UV cutoff filter at an intensity of ~100 mW/cm2. Linear sweep voltammetry (LSVs) measurements were performed under alternating dark/light conditions in 0.1 M NaOH electrolyte solution with the scan rate of 10 mV/s. Electrochemical Impedance Spectroscopy (EIS) was measured in 0.1 M NaOH solution. The amplitude was 5 mV and the frequency range was 106 Hze0.01 Hz under dark and visible light illumination at 0 V vs. Ag/AgCl. Incident monochromatic photon-to-current conversion efficiency (IPCE) of thin films was measured on a Xe lamp with different monochromatic filters (400 nm, 430 nm, 450 nm, 475 nm, 500 nm,

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Fig. 1. Schematic diagram of E-Fe2O3-Pi-MoS2 film electrode preparation.

550 nm, 600 nm) in a mixed solution of 0.1 M Na2SO4 and 0.1 M Na2SO3 at 0.4 V vs. Ag/AgCl.

concentration of total Fe and Mo in solution after the reaction were detected by atomic absorption spectrometry (Thermo Scientific, ICE3300) to investigate the stability of electrodes.

2.5. PEC degradation of pollutants 3. Results and discussion Phenol was chosen as a model pollutant to evaluate the PEC degradation efficiency of as-prepared films. The PEC reaction system was constructed in the glass reactor in two electrode system with applied potential of 2.5 V (unless otherwise mentioned) and the intensity of ~100 mW/cm2 under the visible light irradiation (l > 420 nm). The as-prepared photocatalytic film was used as the anode with an effective area of 2 cm  2 cm and the titanium plate of the same effective area was used as the cathode. The reaction volume of the simulated phenol wastewater was set to 50 mL and the initial concentration was 10 mg/L 0.2 M Na2SO4 was the electrolyte solution. The pH of solution was adjusted by 0.1 M H2SO4 and 0.1 M NaOH. The dark reaction was carried out for 0.5 h before the PEC reaction in order to achieve the adsorption-desorption equilibrium between the phenol and the photocatalytic film. After the dark reaction, the appropriate voltage and light source intensity were set to continue the reaction. 2.6. Analytical methods High performance liquid chromatography (HPLC) was used to determine the concentrations of phenol. The column was a Diamonsil C18 reverse phase column (150 mm  4.6 mm, 5 mm) with a mobile phase of water/methanol (1/1, v/v) and the detection wavelength was 254 nm. The concentration of phenol and its degradation products were analyzed by external standard method. The retention time and peak area of each sample were determined and compared with the standard concentration peaks under the same conditions. Phenol removal rate was calculated according to formula (1):

hð%Þ ¼ ðC0 Ct Þ=C0  100

(1)

Where C0 and Ct represent the initial phenol concentration (mg/L) and the phenol concentration (mg/L) at reaction time t (min), respectively. A total organic carbon analyzer (Shimadzu Japan, TOC-L CPN) was used to evaluate the mineralization degree of phenol. The

3.1. Characterization of photocatalytic films The surface morphology of several composite films was identified by SEM. As shown in Fig. 2a and b, the prepared Fe2O3 has a small and dense structure with the width of ~25 nm and the length of ~40 nm. After doping with phosphorus, Fe2O3-Pi becomes more compact. More gaps disappear, and some phosphoric acid crystals exist in the upper layer of Fe2O3 particle (Fig. 2c and d). Fig. 2e and f shows the morphology of Fe2O3-Pi-MoS2. Some vertically oriented two-dimensional MoS2 was fabricated on Fe2O3-Pi film, but the amount was relatively small. After simple electrochemical treatment, dense vertically oriented MoS2 nanosheets were produced and constructed an open three-dimensional (3D) framework (Fig. 2g and h), which facilitated mass transport through the vertical high-porosity network structure. The thickness of MoS2 nanosheets is ~20 nm and their surfaces are quite smooth, which could supply more specific surface area and was expected to bring about high PEC performance. Besides, the cross sectional SEM images for the thin films were shown in Fig. S1. The thickness of Fe2O3-Pi was 104.4 nm and the E-Fe2O3-Pi-MoS2 was 82.8 nm. The reduced thickness with doping was probably because hydrothermal and electrochemical treatment removed some materials from the previous films. X-ray diffraction technique (XRD) was used to analyze the phase structure and crystallinity of the obtained photocatalytic film electrode. As shown in Fig. 3, five distinct characteristic peaks (26.6 , 37.9 , 51.8 , 61.8 and 65.6 ) in the XRD patterns of several catalytic materials are characteristic diffraction peaks of FTO (PDF 99e0024, indicated by the "*" symbol). The diffraction peaks at 2q ¼ 33.3 and 35.7 (indicated by the “F" symbol) correspond to the (104) and (110) crystal planes of Fe2O3 (PDF 02e0915), which indicated that Fe2O3 exists in all four catalytic materials. The three characteristic diffraction peaks (34.13 , 39.34 and 58.25 ) in Fe2O3Pi-MoS2 and E-Fe2O3-Pi-MoS2 films correspond to (012), (103) and (104) crystal planes of standard MoS2 (PDF 17e0744), respectively [58,59]. The enlarged patterns of MoS2 were shown in Fig. S2.

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Fig. 2. SEM images of (a, b)Fe2O3, (c, d)Fe2O3-Pi, (e, f)Fe2O3-Pi-MoS2 and (g, h)E-Fe2O3-Pi-MoS2 film.

Therefore, it indicated that the MoS2 phase was loaded in the Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 composite film. XPS spectra were performed to further investigate the chemical composition and surface chemical state of the E-Fe2O3-Pi-MoS2 film. The XPS survey spectrum in Fig. 4a confirms that the asprepared E-Fe2O3-Pi-MoS2 composite film consists of Fe, O, P, Mo and S elements. In Fig. 4b, Fe 2p3/2 and Fe 2p1/2 with binding energies of 710.9 and 724.5 eV corresponding to a-Fe2O3 can be found [60]. The Fe 2p2/3 peak was found to be well fitted by several different peaks, which is related to the multiplet splitting of Fe3þ in a-Fe2O3 [61]. There are also two satellite peaks of Fe 2p with binding energies of 718.7 and 732.7 eV. The O 1s spectrum shown in

Fig. 4c includes four distinct diffraction peaks with binding energies of 529.8, 530.8, 531.8 and 532.6 eV which can be ascribed to FeeO [62], P]O [63], eOH [64] and PeO [63]. Fig. 4d shows the diffraction peak of phosphorus. The binding energy of phosphorus is 133.1 eV corresponding to P 2p which can be ascribed to P5þ atom in PO34 anions [65]. Fig. 4e shows that the spectrum of Mo 3d had the main peaks of Mo 3d3/2 and Mo 3d5/2 located at 235.3 eV and 232.2 eV, which was ascribed to Mo4þ in MoS2 [66]. There are also two small shoulder of Mo 3d3/2 and Mo 3d5/2 with binding energies of 234.2 eV and 231.0 eV corresponding to Modþ (0
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Fig. 3. XRD patterns of (a)FTO, Fe2O3, Fe2O3-Pi films; (b)Fe2O3-Pi, Fe2O3-Pi-MoS2, E-Fe2O3-Pi-MoS2 films.

164.1 eV, which corresponds to S 2p of MoS2 [68]. Based on the above analysis, the presence of Pi and MoS2 in E-Fe2O3-Pi-MoS2 film can be further determined. 3.2. PEC performance on water oxidation The PEC activities of the as-synthesized photoanodes are investigated in 0.1 M NaOH solution by the linear sweep voltammetry (LSV) technique under visible light and UVevis light irradiation. As shown in Fig. 5, the photocurrent response of Fe2O3-Pi film and Fe2O3-Pi-MoS2 film were 8 times and 17.5 times higher than that of pure Fe2O3 at 0.45 V (vs. Ag/AgCl) under visible light irradiation, respectively. The photocurrent of E-Fe2O3-Pi-MoS2 film under visible light and UVevis light was 30 times and 19.4 times higher than that of Fe2O3 film at 0.45 V vs. Ag/AgCl in 0.1 M NaOH aqueous solution, respectively. Obviously, the facile electrochemical treatment dramatically increased the PEC activities of the as-synthesized photoanodes. XPS analysis on Fe2O3-Pi-MoS2 samples before and after electrochemical treatment was performed. As shown in Fig. S3, the binding energy of Fe, Mo, S and O atoms did not change significantly after electrochemical treatment, indicating that the electrochemical treatment did not change the valence state of the material. In Fig. S3d, the peak at 169.1 eV before electrochemical treatment can be ascribed to SO24 , which may come from the oxidation of part of the S element in thiourea during the hydrothermal preparation of MoS2. It disappears after electrochemical treatment, probably due to the repulsion of the charges which made the SO24 discharged from the material. The SEM image in Fig. 2g and h shows that electrochemical treatment can transform the morphology of MoS2. Liu et al. induced the transformation of hematite nanorods into ultrathin hematite nanosheets by electrochemical treatment at a negative potential (1.2 V vs. Ag/AgCl) [69]. It is possible that appropriate electrochemical treatment can act as a driving force for morphology transformation that leading the vertical growth of MoS2 nanosheets and expose more surfaces, thereby improving the PEC performance of Fe2O3-Pi-MoS2 film. In order to further optimize the performance of composite catalysts, several factors affecting the preparation and modification of thin film electrode were studied and shown in Fig. 6. The photocurrent of Pi-Fe2O3 film drastically increased with the calcination temperature (Fig. 6a), which may be related to the diffusion of Sn with the increasing temperature [70]. The XPS depth profile of the Fe2O3-Pi film was tested and the Sn 3d spectrum shown in Fig. S4 confirms that the Sn diffused into the material. While the conductive surface of FTO substrate may be destroyed at 650  C due

to the melting point limitation of the conductive glass. Therefore, 630  C is selected as the optimum calcination temperature. Fig. 6b shows that the electrochemical treatment has a significant improvement in the PEC activity of Fe2O3-Pi-MoS2 film, which may be attributed to the changes of the crystal orientation and nanomorphology of MoS2 which provide more active sites for water splitting. The voltage and treatment time are important parameters in electrochemical treatment and could largely affect the performance of treated Fe2O3-Pi-MoS2 film. A voltage from 0.9 V to 0.75 V was applied to treat the Fe2O3-Pi-MoS2 film for 5 s in a 1 M boric solution with a pH of 9.5. LSV test was performed under visible light irradiation of 100 mW/cm2 to investigate the photocurrent density of as-prepared films. The photocurrent density at 0.3 V vs. Ag/AgCl was selected for comparison to determine the optimum voltage. As shown in Fig. 6c, the optimized voltage for electrochemical treatment is 0.8 V vs. Ag/AgCl. Thus the voltage of 0.8 V was used to further investigate the effect of electrochemical treatment time. As shown in Fig. 6d, the optimized treatment time is 150 s. Therefore, the optimized condition for electrochemical treatment is at 0.8 V for 150s. Electrochemical impedance spectroscopy (EIS) was performed to further study the charge transfer process and photocatalytic activity of E-Fe2O3-Pi-MoS2. It can be seen from Fig. 7 that the impedance ring radius of several electrodes exhibits a change trend of E-Fe2O3-Pi-MoS2 < Fe2O3-Pi-MoS2 < Fe2O3-Pi < Fe2O3 under dark and visible light irradiation. Smaller radius of the impedance loop indicated the smaller charge transfer resistance of electrode material. Obviously, E-Fe2O3-Pi-MoS2 has the smallest charge transfer resistance among the as-prepared materials, indicating that its carrier mobility could be improved. In addition, all Nyquist plots were modeled using the equivalent circuit diagram (inset in Fig. 7), where Rs and Rct (Table S1 and Table S2) represent the series resistance and interface charge transfer resistance in the PEC system, respectively. As listed in Table S2, the Rct value of Fe2O3-Pi, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 anodes are reduced by 2.9, 8.8 and 25 times relative to Fe2O3 under visible light illumination, respectively. This indicates that the MoS2 composite and electrochemical modification greatly enhance the interface charge transfer of E-Fe2O3-Pi-MoS2 anode, thereby increasing the photocurrent density. IPCE is used to characterize the photoelectric conversion efficiency of semiconductor photoelectrodes under different monochromatic light illumination. The calculation formula is shown in Eq. (2):

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Fig. 4. XPS spectra of E-Fe2O3-Pi-MoS2 film electrode (a) survey spectrum and high-resolution core spectrum for (b)Fe 2p, (c)O 1s, (d)P 2p, (e)Mo 3d, (f)S 2p.

  IPCEð%Þ ¼ 1240  JPEC =l  Plight  100

(2)

where l is the wavelength (nm) of monochromatic light, JPEC is the photocurrent (mA) of the catalytic film at a specific wavelength, and Plight is the incident light intensity (mW) on the surface of the catalytic film at the selected wavelength. The IPCE of Fe2O3, Fe2O3Pi, Fe2O3eMoS2, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 films is presented in Fig. 8. The calculated IPCE value for E-Fe2O3-Pi-MoS2 electrode is up to 43% at 400 nm, which is higher than that of Fe2O3

(7%), Fe2O3-Pi (13%), Fe2O3eMoS2 (17%), and Fe2O3-Pi-MoS2 (19%) films. It further indicates that the loading of vertically oriented MoS2, nonmetal doping with phosphorous and electrochemical modification enhance the photoelectric conversion performance. Photoelectric transient response test of the as-prepared film electrodes was carried out to investigate the carrier recombination of the film material. The conversion calculation is obtained by the following formula:

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Fig. 5. Linear sweep voltammetric curves of Fe2O3, Fe2O3-Pi, Fe2O3eMoS2, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 films under (a)chopped visible, (b)UVevisible, light irradiation in 0.1 M NaOH aqueous solution.

Fig. 6. (a)Linear sweep voltammetric curves of Fe2O3-Pi film at different calcination temperatures, (b)Fe2O3-Pi-MoS2 film electrochemical modification under chopped visible light irradiation in 0.1 M NaOH solution; (c) Effect of voltage variation on Fe2O3-Pi-MoS2 film electrochemical process and (d) Effect of electrochemical processing time variable on current density in 0.1 M NaOH solution.

D ¼ ðIðtÞ IðstÞ Þ=ðIðinÞ IðstÞ Þ

(3)

where D is the normalized instantaneous photocurrent, I(t) is the photocurrent at time t, I(in) is the initial photocurrent, and I(st) is the steady state photocurrent. Fig. 9 shows the normalized instantaneous photocurrent of Fe2O3, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2

electrodes at 0.4 V vs. Ag/AgCl. When the ln D ¼ 1, the corresponding t value of E-Fe2O3-Pi-MoS2, Fe2O3-Pi-MoS2 and Fe2O3 film is 2.5 s, 1.7 s and 0.65 s, respectively. The larger t value indicates the slower photogenerated carrier recombination rate in the electrode [58,71]. Therefore, it can be concluded that the nonmetal doping with phosphorous, vertically oriented MoS2 fabrication and the

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Fig. 7. Nyquist plots of Fe2O3, Fe2O3-Pi, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 films under (a)dark condition and (b)visible light irradiation in 0.1 M NaOH aqueous solution. The inset shows the equivalent circuit using to fit the Nyquist plots.

Fig. 8. IPCE plots of Fe2O3, Fe2O3-Pi, Fe2O3eMoS2, Fe2O3-Pi-MoS2 and E-Fe2O3-Pi-MoS2 films calculated from the photocurrent at applied potential of 0.4 V vs. Ag/AgCl.

electrochemical modification treatment highly reduces the photogenerated carrier recombination rate, and enhanced carrier mobility.

3.3. PEC activity for organic pollutants removal Phenol was chosen as a colorless model pollutant to investigate the PEC activity of the as-prepared films for organic pollutants removal. As shown in Fig. 10a, the removal rate of phenol in EFe2O3-Pi-MoS2 film is 93.73% after 300 min PEC degradation, while the degradation rates of Fe2O3, Fe2O3-Pi and Fe2O3eMoS2 film are 48.94%, 51.18% and 68.58%, respectively. The kinetic data of phenol removal was fitted using a pseudo-first-order kinetic reaction model. As listed in Table S3, E-Fe2O3-Pi-MoS2 electrode has the highest reaction rate constant, which is 4.24 and 2.22 times higher than Fe2O3 and Fe2O3eMoS2, respectively. A total organic carbon analyzer (TOC) was used to evaluate the mineralization degree. The TOC removal rate of phenol wastewater by E-Fe2O3-Pi-MoS2 film is 22.9%, which indicates the low mineralization of phenol. The reaction products were analyzed by high performance liquid chromatography. As shown in Fig. S5, the main degradation products of

Fig. 9. Normalized plots of the photocurrent-time dependence for Fe2O3, Fe2O3-PiMoS2 and E-Fe2O3-Pi-MoS2 composite films at an applied potential of 0.4 V vs. Ag/AgCl in 0.1 M NaOH aqueous solution.

phenol were benzoquinone and a small amount of hydroquinone. The concentration of benzoquinone increase gradually in 3 h reaction and then decrease with time. Because it consumes more energy and time to mineralize pollutants into CO2, the E-Fe2O3-PiMoS2 electrode may be more suitable for selective oxidation relative to the mineralization of contaminants. The phenol degradation performance of E-Fe2O3-Pi-MoS2 electrode was further studied under photocatalytic (PC), electrocatalytic (EC) and photoelectric synergistic catalysis (PEC) conditions. As shown in Fig. 10b, phenol degradation efficiency of EFe2O3-Pi-MoS2 electrode in PEC process is significantly higher compared to PC or EC process. The PEC reaction rate is 30 and 9 times of PC and EC (Table S4), respectively. It is obvious that there is synergistic effect between PC and EC. Considering water oxidation may occur during the phenol degradation, we performed LSV measurement until 2.5 V in 0.1 M Na2SO4 aqueous solution with or without phenol in a two-electrode system. As shown in Fig. S6, the current density of E-Fe2O3-Pi-MoS2 film at 2.5 V is 0.27 mA/cm2 in 0.1 M Na2SO4 aqueous solution under visible light irradiation. When phenol was added to the 0.1 M Na2SO4 solution, the current density of E-Fe2O3-Pi-MoS2 film

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Fig. 10. (a) PEC degradation of phenol for Fe2O3, Fe2O3-Pi, Fe2O3eMoS2 and E-Fe2O3-Pi-MoS2 films under visible light irradiation; (b) Comparison of different catalytic processes on phenol degradation over E-Fe2O3-Pi-MoS2 film.

increased to 0.40 mA/cm2. The onset potential of phenol oxidation was 0.27 V, while that of water oxidation was 1.50 V, which indicated that phenol oxidation is easier than water oxidation. Therefore, phenol oxidation takes precedence over water oxidation during the entire reaction process. Water oxidation may exist in phenol degradation, but it is not the main reaction. Effect of applied potential on phenol removal rate was studied in the PEC degradation using E-Fe2O3-Pi-MoS2 as photoanode. According to Fig. 11a and Table S5, the applied potential has a significant effect on the PEC degradation when the potential increased to 2.0 V. Fig. 11b shows the changes of the electrochemical enhancement factor (E) and the reaction rate constant k with the applied bias voltage. The electrochemical enhancement factor (E) is calculated by Eq. (4):

Eð%Þ ¼ ðkPEC kPC Þ=kPEC  100

(4)

where kPEC and kPC represent the reaction rate constant in PEC and PC, respectively. As shown in Fig. 11b, when the applied potential reaches to 2.5 V, the increasing trend of the reaction rate constant k and electrochemical enhancement factor E becomes slower, and the E value reaches 95%. Considering the cost and the pollutant removal effect, the applied potential in the PEC degradation of phenol is selected as 2.5 V.

Effect of pH value on the removal efficiency of phenol was presented in Fig. S7 and Table S6. When the initial pH value is 4.0 and 6.4, the phenol removal rate and the reaction rate constant of EFe2O3-Pi-MoS2 photoanode were significantly higher than those at pH ¼ 10. Fig. S8 and Table S7 show the effect of initial phenol concentration on the degradation of phenol by E-Fe2O3-Pi-MoS2 electrode. Higher phenol removal rate and the reaction rate constant were achieved at lower initial phenol concentration. Therefore, the E-Fe2O3-Pi-MoS2 electrode is suitable for the degradation of low concentration pollutants in neutral or subacid medium. The stability of E-Fe2O3-Pi-MoS2 film was studied by successive cyclic PEC degradation experiments. It can be seen from Fig. 12 that the degradation of phenol was maintained at a high efficiency and observed negligible loss after 4 cycles, indicating that E-Fe2O3-PiMoS2 film has good PEC stability. Leaching experiments were carried out to test the concentration of metal dissolved in the solution after the cyclic reaction by atomic absorption spectrometry. Neither Fe nor Mo was detected. 3.4. Possible mechanism Several kinds of trapping agents (tert-butanol, oxalic acid, benzoquinone and L-histidine) were selected to investigate the main active species generated in the PEC degradation of phenol by

Fig. 11. (a) Effects of different applied potential and (b) reaction rate constants and electrochemical enhancement factor (E%) on phenol removal using E-Fe2O3-Pi-MoS2 as photoanode under visible light irradiation.

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strong covalent interaction between P and O, which provide more electron carriers and avoid to form deep electron trapping sites [28,42]. The electrochemical treatment could generate the dense vertically oriented MoS2 nanosheets (Fig. 2g and h). Vertically oriented MoS2 is favorable for increasing the visible light absorption and providing more active sites since MoS2 is a two-dimensional layered material. In addition, vertically oriented MoS2 could reduce the adhesion of as-formed gas bubbles (such as oxygen bubbles in water oxidation) or intermediates in pollutants removal on the surface of MoS2, which is beneficial to provide a constant active surface. Therefore, the composite E-Fe2O3-Pi-MoS2 has excellent carrier mobility and PEC activity. The holes accumulated on the valence band directly oxidize the phenol or generate HO radicals to indirectly oxidize the phenol. The main reactions of phenol degradation are as follows:

E  Fe2 O3 Pi  MoS2 þ hv/E  Fe2 O3 Pi  MoS2 ðe þhþ Þ (5) Fig. 12. Cyclic degradation of phenol using E-Fe2O3-Pi-MoS2 film.

E-Fe2O3-Pi-MoS2 film. As shown in Fig. 13a and Table S8, when 1 mM tert-butanol (HO trapping agent) was added, the degradation efficiency of phenol was greatly decreased, and the reaction rate constant of phenol decreased from 0.0089 min1 to 0.0041 min1, indicating that HO was the major active species. Oxalic acid (hþ trapping agent) had a moderate inhibitory effect to the removal of phenol, and the reaction rate constant of phenol dropped from 0.0089 min1 to 0.0065 min1, indicating that hole also participated in the degradation of phenol. The influence of benzoquinone 1 (O$2 trapping agent) and L-histidine ( O2 trapping agent) is almost neglectable [72,73]. Therefore, the main active species of E-Fe2O3Pi-MoS2 electrode are HO and hþ in the PEC degradation of phenol. The charge transfer process of E-Fe2O3-Pi-MoS2 composite film is shown in Fig. 13b. The conduction bands of Fe2O3 and MoS2 are 0.44 V and 0.13 V, and the band gaps are 2.15 eV and 2.0 eV, respectively [13,74]. Under visible light illumination (l > 420 nm), electrons are excited from the valence band of MoS2 to its conduction band, forming photogenerated electron-hole pairs. Then, the electrons can be transferred to the conduction band of a-Fe2O3, and the holes can migrate to the valence band of MoS2, which achieves the effective separation of the photo-generated carrier. The P-doping also could improve the carrier mobility because of the

2e þO2 þ2Hþ /H2 O2

(6) (7) (8)

hþ þ phenol/degredation production

(9) (10)

4. Conclusions A novel E-Fe2O3-Pi-MoS2 electrode was synthesized by a simple impregnation and hydrothermal method. The nonmetal doping with phosphorous and vertically oriented MoS2 nanosheets were successfully constructed on Fe2O3 film substrate, which greatly improved the PEC performance of Fe2O3. The E-Fe2O3-Pi-MoS2 electrode exhibited remarkably high PEC activity for water oxidation and organic pollutants removal. The photocurrent of E-Fe2O3Pi-MoS2 film increased by 30 times compared with pure Fe2O3 film at 0.45 V (vs. Ag/AgCl) under visible light irradiation. The degradation efficiency on simulated phenol wastewater using E-Fe2O3Pi-MoS2 electrode can reach to 93.73% in PEC process under visible

Fig. 13. (a) Effect of different trapping agents on degradation efficiencies of phenol using E-Fe2O3-Pi-MoS2 film under PEC; (b) Schematic diagram of the charge transfer process for E-Fe2O3-Pi-MoS2 under visible light irradiation.

Y. Cong et al. / Electrochimica Acta 337 (2020) 135748

light irradiation, while it is only 48.94% for Fe2O3 electrode under the same conditions. In addition, good stability of E-Fe2O3-Pi-MoS2 electrode was also observed in a 20-h long-term degradation experiment. The excellent PEC performance of E-Fe2O3-Pi-MoS2 was attributed to the synergistic effects of nonmetal doping with phosphorous, vertically oriented MoS2 nanosheets fabrication and the electrochemical modification treatment, which highly reduced the photo-generated carrier recombination rate and enhanced carrier mobility. The E-Fe2O3-Pi-MoS2 film synthesized in this work is a promising material for water oxidation and pollutants removal. CRediT authorship contribution statement

[13] [14]

[15]

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Yanqing Cong: Conceptualization, Data curation, Writing - review & editing, Funding acquisition. Wenchen Ding: Writing original draft, Investigation, Data curation, Formal analysis. Wenhua Zhang: Methodology, Validation. Tongtong Zhang: Investigation, Software, Methodology, Data curation. Qi Wang: Writing review & editing, Resources. Yi Zhang: Writing - review & editing, Project administration.

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Acknowledgement

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This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (LY18B060003, LR18B070001, LY16B060001), the National Natural Science Foundation of China (21876154, 21576237).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135748.

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