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Fabrication of Z-scheme Fe2O3-MoS2-Cu2O ternary nanofilm with significantly enhanced photoelectrocatalytic performance Yanqing Cong, Yaohua Ge, Tongtong Zhang, Qi Wang, Meiling Shao, and Yi Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04089 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Industrial & Engineering Chemistry Research

Fabrication of Z-scheme Fe2O3-MoS2-Cu2O ternary nanofilm with significantly enhanced photoelectrocatalytic performance Yanqing Cong†,‡, Yaohua Ge†, Tongtong Zhang†, Qi Wang†,‡, Meiling Shao†, Yi Zhang*,† †

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou

310018, China ‡

Institute of Urban Aquatic Environment, Zhejiang Gongshang University, Hangzhou 310018,

China

ABSTRACT: A ternary Fe2O3-MoS2-Cu2O nanocomposites were fabricated via electro-deposition and hydrothermal method. The as-prepared photocatalytic films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results indicated that MoS2 and Cu2O particles were successfully deposited onto the surface of Fe2O3 particles. MoS2 and Cu2O co-loading achieved a synergetic effect on the improvement of photoelectrochemical performance of Fe2O3 film. The highest photocurrent density was achieved on Fe2O3-MoS2-Cu2O film, which was 20, ~5.5, and 2 times that of Fe2O3,

Fe2O3-MoS2

and

Fe2O3-Cu2O

film,

respectively.

The

excellent

photoelectrocatalytic performance was attributed to the Z-scheme electron transfer mechanism, which result in the fast charge transfer and strong redox ability on the ternary composite. This work provides a promising Z-scheme ternary semiconductor for environmental purification and water oxidation.

To whom correspondence should be addressed. [email protected]. (Y. Zhang) Tel.: +86-571-28008211; Fax: +86-571-28008215 *

1

ACS Paragon Plus Environment

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

1. INTRODUCTION In recent decades, the development and application of environmentally friendly treatment technology and multicomponent composite photocatalysts have attracted much attentions.1-10 Photoelectrocatalysis (PEC) technique provides an effective method using abundant and clean solar light with a wide range of potential applications in elimination of pollution and production of energy.1-2 Up to now, various unitary materials,3 binary materials4-6 and ternary composites7-10 have been widely used in water splitting, pollutant degradation, carbon dioxide reduction and other applications, including TiO2,11-13 Bi2O3,14-15 graphene,16-17 CdS,18-20 Cu2O21-25 and Ta3N5.26-27 However, still more attempt is needed to develop highly efficient and stable photocatalytic material for energy and environmental application.28-29 α-Fe2O3 is an environmental friendly, abundant and cost-effective n-type semiconductor which can utilize the visible light since it has favorable band gap (2.0-2.2 eV). Nevertheless, its electron-hole recombination is severe and considerable efforts were carried out to enhance its photocatalytic activity including metal doping, fabrication of heterostructure, crystal engineering and so on.30-32 Fabrication of heterostructure can facilitate faster charge separation, but the redox ability always weaken after charge transfer because the photogenerated holes transfer to one semiconductor with a more negative valence band (VB) while electrons transfer to one with a more positive conduction band (CB).33-35 Weakened redox ability is unfavorable for the degradation of refractory pollutants and water splitting. Fabrication of Z-scheme composite is an effective method to simultaneously possess high charge separation efficiency and strong redox ability, in which the charge transfer directly quenches the weaker oxidative holes and reductive electrons. Obviously, Z-scheme transfer is more preferable to heterostructure transfer. To fabricate a highly efficient Z-scheme photocatalyst, it is important to chose a suitable connection mediators between two semiconductors such as reversible redox mediators or solid-state electron mediators. As a typical narrow-band gap layered transition-metal dichalcogenide, MoS2 has attracted tremendous attention because of its excellent chemical stability, electrocatalytic activity and accept/transport electrons 2


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ability, which makes it potentially suitable as solid-state electron mediators for constructing Z-scheme composite photocatalysts.36,37 The binary composite of α-Fe2O3 and MoS2 had been studied for nitrite oxidation and energy application.38-40 However, to the best of our knowledge, Z-scheme ternary composites based on α-Fe2O3 and MoS2 have not been reported for refractory pollutant degradation or water splitting. In this work, we select MoS2 as solid-state electron mediator, n-type α-Fe2O3 and p-type Cu2O as components to fabricate an efficient Z-scheme ternary composites for refractory pollutants treatment. Cu2O is earth abundant, low cost and has good visible light absorption with the bandgap of 2.05 eV, which makes it suitable for solar driven environment protection and water splitting.41-43 MoS2 and Cu2O were deposited onto the surface of α-Fe2O3 by simple hydrothermal and electrophoretic deposition method. Phenol was used as a colorless model pollutant to confirm the removal performance of Fe2O3-MoS2-Cu2O for refractory pollutants. The generation, separation and transfer of photogenerated charges were systematically studied. The high charge separation efficiency and strong oxidation ability were simultaneously achieved via Z-scheme transfer. 2. EXPERIMENTAL METHODS 2.1. Chemicals. All chemicals were of analytical grades and used as purchased without further treatment. Iron dichloride (FeCl2·4H2O) was purchased from Mclean Biochemical Technology Co., Ltd (Sichuan, China). Ethylene glycol (CH2OH,  99.0%), ammonium molybdate ( (NH4)6Mo7O24·4H2O,  99.0%), copper acetate (Cu(CH3COO)2·H2O, 98.0%-102.0%), sodium hydroxide (NaOH,  96.0%), iodine (I2,  99.0%) and acetone (CH3COCH3,  99.0%) were obtained from Shanghai Lingfeng Chemical Co., Ltd. (Shanghai, China). Thiocarbamide (CH4N2S,  99.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Hangzhou, China). Ascorbic acid (C6H8O6,  99.5%) was purchased from Guangdong Guanghhua Chemical Co., Ltd. (Guangdong, China). 2.2. Synthesis of Fe2O3, Fe2O3-MoS2 and Fe2O3-MoS2-Cu2O photocatalysts. Fe2O3 films were prepared via a electrodeposition method as reported previously.44,45 3



Briefly, a 0.02 M precursor solution of FeCl2 in 10 mL ethylene glycol and 80 mL distilled water was prepared. The Fe2O3 films were prepared through a three-electrode system at a constant voltage of 1.36 V for 5 min onto F-doped tin oxide substrate (FTO, Nippon Sheet Glass, Japan), which was washed with acetone and distilled water repeatedly and dried in air at room temperature. Then, the films were heated to 500C for 2 h to produce Fe2O3 films in a muffle furnace. Fe2O3-MoS2 films were fabricated by a hydrothermal method (Scheme 1).46 0.124 g ammonium molybdate and 0.106 g thiocarbamide were dissolved in 60 mL distilled water. Then the Fe2O3 films were immersed in 15 mL precursor solution and were transferred into a Teflon-lined autoclave to be maintained at 220 C for several hours. After cooling to room temperature naturally, the films were thoroughly washed with distilled water and ethanol three times each. For the preparation of Fe2O3-MoS2-Cu2O photocatalysts, the precursor powder of Cu2O was first prepared. 0.01 M Cu(CH3COO)2, 0.1 M NaOH and 0.01 M ascorbic acid were dissolved in distilled water and keep stirring until the solution is mixed evenly. Subsequently, vacuum drying to obtain powder after centrifugation. Then, 0.02 g cuprous oxide powder and 0.005 g iodine were dissolved in 25 mL acetone. Cu2O layers were deposited by electrophoretic deposition method in the solution at a constant voltage of 5.0 V for several minutes. The Fe2O3-MoS2 films were used as cathode and clean FTO as anode, respectively.

Scheme 1. Schematic processes for the fabrication of Fe2O3-MoS2 and Fe2O3-MoS2-Cu2O films.

2.3. Characterization. The morphology of Fe2O3-MoS2-Cu2O film was studied using a Hitachi Model S-4700 (II) field emission scanning electron microscope(SEM). The crystalline phases of the as-prepared samples were determined using an X-ray 4


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diffraction (XRD, with monochromatic Cu K radiation at 40 kV and 40 mA) with a Phillips PANalytical X’PERT diffractometer. In addition, Fe2O3-MoS2-Cu2O film was characterized by X-ray photoelectron spectroscopy (XPS, ESCA lab 220i-XL spectrometer with Al K at 1486.6 eV) to determine the valance states and atomic compositions, and the binding energies were calibrated to the C 1s peak (284.8 eV). 2.4. Photoelectrochemical (PEC) measurements. All the PEC properties were recorded in a three-electrode configuration including a working electrode, a platinum foil counter electrode and a reference electrode using a CHI660E instrument (Chenhua, Shanghai). The electrodes were irradiated with visible light from a xenon lamp with UV 420 nm cutoff filter. Photocurrents were measured in 0.1 M NaOH aqueous solution by linear sweep voltammetry (LSV) method. The incident light intensity (~100 mW/cm2) was detected by a power meter (model FZ-A). Electrochemical impedance spectroscopy (EIS) analyses were measured in 0.1 M NaOH aqueous solution under darkness and visible light irradiation with a scan rate of 5 mV/s. The incident monochromatic photon-to-current conversion efficiency (IPCE) was detected with the same xenon lamp mentioned above. 2.5. PEC degradation of organic pollutants. Phenol (10 mg/L, 50 mL) was used as a model of organic pollutants to evaluate the PEC degradation efficiency and stability of the as-prepared Fe2O3-MoS2-Cu2O photoelectrodes. The effective area of the working electrode was 2 cm × 2 cm, and the counter electrode (titanium) has the same active area. The PEC degradation experiments were carried out in a two-electrode glass cell under constant magnetic stirring using 0.2 M Na2SO4 solution containing phenol as the electrolyte. Before initiating the reactions, the electrode was immersed in the reaction solutions and placed in dark under constant magnetic stirring for 30 min to ensure the adsorption-desorption equilibrium. After optimization, the applied potential was 2.5 V and the visible light source used a xenon lamp with UV 420 nm cutoff filter. The concentration of phenol and its degradation intermediates was analyzed using high-performance liquid chromatography (HPLC, Agilent 1200) with a Diamonsil C18 column and wavelength detection at 254 nm. Deionized water/methanol (7/3, v/v) was used as mobile phase with a flow rate of 1 mL/min . 5



3. RESULTS AND DISCUSSION 3.1. Structure and morphology. The scanning electron microscopy (SEM) is utilized to observe the possible morphology of the samples, and the X-ray diffraction (XRD) patterns of samples were further carried out to investigate the crystallographic structures. As shown in Figure 1, comparing to pure Fe2O3 (Figure 1a) before the hydrothermal reaction, the Fe2O3-MoS2 (Figure 1b) nanofilms were arranged more neatly and denser, and these rice-shaped particles were  67 nm long and  40 nm wide. For Fe2O3-MoS2-Cu2O film (Figure 1c), small cubic particles ascribing to Cu2O were observed on Fe2O3-MoS2 film and the side length of Cu2O ranged from 100 to 200 nm. Besides, the intersectional SEM image for the thin film was shown in Figure S1. The thickness of Fe2O3-MoS2 was about 115 nm and the Cu2O was 178 nm. Figure 1d shows the XRD patterns of the as-prepared samples (Fe2O3, Fe2O3-MoS2 and Fe2O3-MoS2-Cu2O). Since FTO was used as the substrate, its XRD peaks were observed in all samples. As for Fe2O3, peaks at 33.3° and 35.6° were corresponded to (104) and (110) planes of -Fe2O3 (PDF 02-0915).47 When MoS2 film was loaded on the Fe2O3 film, peaks at 33.5° was observed, which belonged to the (104) crystal plane of MoS2.48 In addition, it can be easily observed the three peaks of Cu2O at 37.1°, 42.6° and 63.9° in the XRD pattern of Fe2O3-MoS2-Cu2O.49

6


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(d)

FTO  Fe2O3  MoS2 Cu2O









Intensity

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Fe2O3-MoS2-Cu2O











Fe2O3-MoS2

Fe2O3

32

40 48 56 2 Theta/degree

64

Figure 1. SEM images of (a) Fe2O3 film, (b) Fe2O3-MoS2 film, and (c) Fe2O3-MoS2-Cu2O film; (d) XRD patterns of Fe2O3, Fe2O3-MoS2 and Fe2O3-MoS2-Cu2O film.

XPS spectra were used to investigate the chemical composition of Fe2O3-MoS2-Cu2O film. Survey spectra of the film was displayed in Figure 2a, which revealed obvious signals of Fe, Mo, Cu, S and O elements. In Figure 2b, five peaks of Fe 2p located at 709.80 eV (Fe 2p3/2), 711.82 eV (Fe 2p3/2), 717.02 eV (Fe 2p3/2,sat), 723.89 eV (Fe 2p1/2) and 732.39 eV (Fe 2p1/2,sat), respectively. The Fe 2p3/2 reveals that the samples have mixed phase of Fe2+ and Fe3+ because the peak of pure Fe2+ appears near to 709 eV, Fe3+ appears near to 711 eV.50,51 As for Mo 3d, Figure 2c illustrates that there are two peaks corresponding to MoS2 (232.19 eV for Mo 3d5/2 and 235.33 eV for Mo 3d3/2).52 XPS spectra of Cu 2p (Figure 2d) shows that the main peaks at 951.46 eV, 953.72 eV, 931.54 eV, and 933.80 eV corresponding to Cu 2p1/2 and Cu 2p3/2 of Cu2O,53 while existence of the two small peaks at 941.72 eV and 961.91 eV demonstrates the presence of Cu2+. In addition, peaks at 531.3 eV and 529.9 eV in O 1s (Figure 2a insert) indicates the presence of O2- in Cu-O and Fe-O.50 Therefore, the 7



(a)

Cu 2p

presence of Fe2O3, MoS2 and Cu2O can be further confirmed by XPS analysis. 529.86eV O 1s

(b) Intensity/a.u.

536

Fe 2p

528 530 532 534 Binding Energy/eV

O 1s

526

S 2p Mo 3d C 1s

Intensity/a.u.

531.26eV

Fe 2p3/2

711.82eV 709.80eV Fe 2p3/2 ,sat 717.02eV

Fe 2p1/2 723.89eV Fe 2p1/2 ,sat 732.39eV

Survey 0

200

(c)

400 600 800 Binding Energy/eV

Mo 3d5/2

Fe 2p

1000

710

720 730 Binding Energy/eV

740

(d)

232.19eV

Cu 2p1/2

931.54eV

235.33eV

Mo 3d3/2

Cu 2p3/2

Intensity/a.u.

Intensity/a.u.

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

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933.80eV

sat.

951.46eV

sat.

953.72eV

961.91eV

941.72eV

Mo 3d Cu 2p 225

230 235 Binding Energy/eV

930

240

940 950 960 Binding Energy/eV

970

Figure 2. XPS survey spectrum of (a) Fe2O3-MoS2-Cu2O, insert: XPS spectra of O 1s ; XPS spectra of (b) Fe 2p, (c) Mo 3d and (d) Cu 2p for Fe2O3-MoS2-Cu2O.

3.2. Photoelectrochemical characterizations. The deposition amount of MoS2 and Cu2O was optimized by controlling the hydrothermal time and electrophoretic deposition time. Figure 3 shows that the optimum deposition amount was obtained when MoS2 was prepared at 220 C for 2 h and Cu2O was deposited at 5.0 V for 5 min. The PEC performance of the as-prepared films was investigated by the linear sweep voltammetry (LSV). The photocurrent responses were compared in 0.1M NaOH solution at the potential range of -0.1 V to 0.4 V (vs. Ag/AgCl) and was shown in Figure 4. The Fe2O3 film shows quite low photocurrent under chopped visible light. When MoS2 or Cu2O was loaded on the surface of Fe2O3, the photocurrent density of Fe2O3-MoS2 or Fe2O3-Cu2O was improved 3 or 8 times relative to Fe2O3 at 0.38 V (vs. Ag/AgCl) under visible light irradiation. The highest photocurrent density was 8


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observed on Fe2O3-MoS2-Cu2O samples, which was 20 times to that of Fe2O3, more than 5.5 times to that of Fe2O3-MoS2 film, and 2 times to that of Fe2O3-Cu2O. Obviously, MoS2 and Cu2O co-loading achieved a synergetic effect on the improvement of PEC performance of Fe2O3 film. -0.3

(a)

-2

-0.10

-0.05

0.00 -0.1

0.0

(b) 1 min 3 min 5 min 8 min

-2

1h 2h 3h 4h

Current density/mA·cm

-0.15 Current density/mA·cm

-0.2

-0.1

0.0

0.1 0.2 0.3 Potential/V vs.Ag/AgCl

-0.1

0.0


0.4

Figure 3. Linear sweep voltammetric curves of (a) Fe2O3-MoS2 films (fabricated by a hydrothermal method); (b) Fe2O3-MoS2-Cu2O films (deposited by electrophoretic deposition

-2

method) under different time conditions in 0.1 M NaOH aqueous solution.

Current density/mA·cm

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


-0.3

(1) Fe2O3

light on

(2) Fe2O3-MoS2

(4)

light off

(3) Fe2O3-Cu2O

-0.2 (4) Fe2O3-MoS2-Cu2O

(1)

-0.1 0.0 0.1 -0.1

0.0


0.4

Figure 4. LSV of different composite film samples under chopped visible light irradiation in 0.1 M NaOH aqueous solution.

In order to study the charge separation properties of the prepared catalysts, the transient photocurrent measured at 0.4 V vs. Ag/AgCl (Figure 4) was calculated by the following formula:54 D = (I(t)- I(st)) / (I(in) - I(st))

(1)

where D is the normalized transient photocurrent, I(t) is the photocurrent at the 9



moment of t, I(in) represents the initial photocurrent, I(st) is the steady-state photocurrent, and t is the time. The normalized plots of photocurrent as a function of time for different as-prepared films were compared in Figure 5. When ln D = -1, the t value of Fe2O3-MoS2-Cu2O, Fe2O3-MoS2, Fe2O3-Cu2O and Fe2O3 film was 3.5 s, 1.6 s, 1.3 s, and 0.9 s, respectively. The higher t value indicated the slower recombination rate of photogenerated carriers. Obviously, the Fe2O3-MoS2-Cu2O ternary structure improved the charge separation properties. Fe2O3

Fe2O3-MoS2

Fe2O3-Cu2O

-0.5

Fe2O3-MoS2-Cu2O

-1.0

ln D

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

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-1.5 -2.0

0.1 M NaOH

-2.5 1

2

3

4

5

Time/s

Figure 5. Normalized plots of the photocurrent-time dependence for Fe2O3、Fe2O3-MoS2、 Fe2O3-Cu2O and Fe2O3-MoS2-Cu2O films at an applied potential of 0.4 V vs. Ag/AgCl.

EIS measurement was carried out under dark and visible light irradiation conditions to obtain the information about the charge transfer across the as-prepared films. As shown in Figure 6a and b, the arc radius of Fe2O3-MoS2-Cu2O composite electrode is always the smallest one under both dark and visible light conditions, indicating the smallest charge transfer resistance. Comparing to Fe2O3-MoS2 or Fe2O3-Cu2O, the fabrication of Fe2O3-MoS2-Cu2O ternary structure further enhanced the separation and transfer of photogenerated charges. Figure 6c is the equivalent circuit model. Rs and Rct represent the series resistance and interfacial charge transfer resistance, respectively. As shown in Figure 6d, the Rct had been significantly reduced with the addition of MoS2 and Cu2O, which indicated that the ternary material Fe2O3-MoS2-Cu2O greatly enhances charge transfer, resulting in enhanced photocurrent density.55

10


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5

(a)

5x10

3

1.5x10

Fe2O3 Fe2O3-MoS2

5

4x10

(b)

Fe2O3 Fe2O3-MoS2

3

1.2x10

Fe2O3-Cu2O Fe2O3-MoS2-Cu2O

-Z''/ohm

5

6x10

-Z''/ohm

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


5

3x10

5

2x10

Fe2O3-Cu2O Fe2O3-MoS2-Cu2O

2

9.0x10

2

6.0x10

2

3.0x10

5

1x10

0 0.0

4

3.0x10

4

4

6.0x10 9.0x10 Z'/ohm

5

1.2x10

0.0

5

0

1.5x10

2

1x10

2

2

2x10 3x10 Z'/ohm

2

4x10

2

5x10

Figure 6. The EIS under (a) dark and (b) visible light conditions (0.1 M NaOH aqueous solution); (c) the equivalent circuit model and (d) Rct of different films.

To further confirm the high PEC activity of Fe2O3-MoS2-Cu2O ternary heterojunction, IPCE for the as-prepared films was measured in 0.1 M NaOH aqueous solution at 0.4 V vs. Ag/AgCl. The value of IPCE is defined by the following equation: IPCE (%)  1240  (

i ph

pin

)  100

(2)

where λ is the wavelength of incident monochromatic light (nm), iph is the photocurrent density (mA) under illumination at certain wavelength (λ, nm), and pin is the incident light intensity (mW) on the film at the selected wavelength. According to the IPCE spectra (Figure 7), it’s obvious that the photocurrent response of Fe2O3-MoS2-Cu2O film is much stronger than that of Fe2O3, Fe2O3-Cu2O and Fe2O3-MoS2 film in the range of 400-650 nm.

11



32

Fe2O3

28

Fe2O3-MoS2

24

IPCE/%

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

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Fe2O3-Cu2O

20

Fe2O3-MoS2-Cu2O

16 12 8 4 0 400

450

500

550

600

650

Wavelength/nm

Figure 7. The IPCE plots calculated in 0.1 M NaOH aqueous solution (pH 9.3)

3.3. PEC activity. PEC degradation experiments of phenol were carried out to study the performance of Fe2O3-MoS2-Cu2O for pollutants removal. As shown in Figure 8, Fe2O3-MoS2-Cu2O exhibited remarkable PEC activity for phenol degradation (Figure 8a). The highest phenol degradation rate was observed on PEC compared to electrochemical (EC) and photocatalytic (PC) processes. Besides, it can be observed that the remarkably difference among the tested samples (Figure 8b). The phenol degradation rate of Fe2O3-MoS2-Cu2O composite film was about 86%, which was ~2 times to that on Fe2O3 film. To exclude the possibility of phenol evaporation, the change of phenol concentration in the absence of any degradation activity was also investigated and the result was shown in Figure S2. Only 4% phenol was volatilized within five hours. The effect of phenol evaporation was almost negligible. The main intermediates of phenol degradation were analyzed by means of HPLC. Figure S3 shows that the main intermediates were benzoquinone and hydroquinone. The solutions pH before, during and after photoelectrocatalytic reaction was measured and the results were presented in Figure S4. There were no evident changes in the pH of the solutions.

12


Page 13 of 25

90

90

(a)

80

Phenol Degradation Rate/%


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


PC EC PEC

70 60 50 40 30 20 10 0

80

(b)

70

Fe2O3

60

Fe2O3-MoS2

50

Fe2O3-MoS2-Cu2O

40 30 20 10 0

0

50

100

150 200 Irradiation time /min

250

0

300

50

100

150

200

250

300

Irradiation time /min

Figure 8. (a) PEC degradation rate of phenol for different films under visible light irradiation; (b) Comparison of phenol degradation efficiency using Fe2O3-MoS2-Cu2O electrode under different processes.

The influence of different potentials applied in the PEC degradation of phenol was studied. As shown in Figure 9a, with the increase of applied bias voltage, phenol degradation rate was correspondingly increased. The removal rate of phenol at 1.5 V increased  3 times than that at 1.0 V. When the applied bias reached 2.5 V, the increase trend slowed down. In addition, the electrochemical enhancement value (E, in %) were calculated according to Equations (3) to further verify the effect of applied potential.54 As shown in Figure 9b, the k and E value also increased with the increase of voltage. When the voltage increased to 2.5 V, the enhancement trend slowed down. Thus, 2.5 V was selected for the purpose of energy conservation.

E

k PEC  k PC  100% k PEC

(3)

13



(a)

0.006

1V 1.5V 2V 2.5V 3V

50 40

80

30

0.004

60

0.003

40

E/%

60

(b)

0.005 -1

70

k/min


80

100

0.007

90

0.002

20

20

0.001

10 0

0.000 0

50

100

150

200 250 Irradiation time /min

300

0 1.0

1.5

2.0

2.5

3.0

Applied potential /V

Figure 9. (a) Phenol degradation rate and (b) the rate constants k and E value for the PEC degradation of phenol under visible light irradiation.

3.4. Stability of Fe2O3-MoS2-Cu2O film electrode. The PEC activity is a vital

factor for the optimal film. In addition, the stability in cyclic PEC degradation of pollutants is another important factor. Five cyclic runs were conducted to identify the stability of Fe2O3-MoS2-Cu2O composite on phenol degradation, and the film was taken out and rinsed with distilled water for the next cycle after each cycle. As shown in Figure 10a, the degradation rate of phenol was observed negligible loss after five cycles, and the LSV of them (Figure 10b) almost remained unchanged. -0.30

(a) Run 2

Run 3

Run 4

Run 5

Current density/mA•cm

Run1

-2

100

Degradation efficiency /%

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

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80 60 40 20

-0.25 -0.20 -0.15

5

10

15 20 Irradiation time /h

R0 R1 R2 R3 R4 R5

-0.10 -0.05 0.00 -0.1

0 0

(b)

25

0.0


0.4

Figure 10. (a) Cyclic degradation of phenol and (b) their LSV using Fe2O3-MoS2-Cu2O film under visible light irradiation at 2.5 V applied potential. Reaction condition: 10 mg/L phenol, 0.2 M Na2SO4.

3.5. Proposed mechanism. The trapping experiments using different scavengers

were investigated to seek the formation of possible active species (O2-, HO and h+) 14


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in the degradation process. As shown in Figure 11a, the degradation of phenol was greatly inhibited when 1 mM tertiary butanol (HO scavengers) was added, which suggested that HO was the major reactive species in the PEC reaction. The scavenger of h+ (1 mM oxalic acid) had the medium influence, indicating photogenerated hole was also involved in the degradation reaction. Phenol degradation rate only slightly decreased with the addition of benzoquinone (BQ), indicating that O2- was minimally involved. Therefore, the main active species generated on Fe2O3-MoS2-Cu2O ternary composite were HO and h+ in PEC degradation of phenol. The band gap of Fe2O3, MoS2 and Cu2O was determined to be 2.2, 1.9 and 2.05 eV, respectively.46,56,57 Under visible light irradiation, the valence band (VB) electrons were excited up to the conduction band (CB) and photogenerated electron-hole pairs could transfer according to heterojunction mechanism or Z-scheme mechanism (Figure 12). If electron-hole pairs transfer followed the heterojunction mechanism (Figure 12a), the accumulated holes in the VB of Cu2O could not oxidize H2O to HO because the VB potential of Cu2O (0.36 V vs. NHE) is more negative than the standard redox potential E0(H2O/HO) (2.4 V vs. NHE).57,58 This could not explain the experiment results that HO was the major reactive species. If MoS2 serve as a charge transport center to form a Z-scheme Fe2O3-MoS2-Cu2O composite (Figure 12b), the remained holes in the VB of Fe2O3 (2.48 V vs. NHE) could oxidize H2O to generate HO, resulting in the promoted redox ability which agreed well with the experiment results. On the other hand, for the Z-scheme mechanism, the remained electrons in the CB of Cu2O (-1.69 V vs. NHE) could reduce O2 to O2- because it’s potential is more negative than the standard redox potential E0(O2/O2-) (0.33 V vs NHE).59,60 However, Figure 11a shows that O2- was minimally involved in the PEC degradation of phenol. This is because the remained electrons in the CB of Cu2O in Fe2O3-MoS2-Cu2O anode would be driven to the cathode under the applied bias potential in PEC process. To further investigate whether O2 participate in the PEC degradation of phenol, the comparative experiments with or without O2 were carried out. As shown in Figure 11b, when pure O2 was passed into the reaction solution, phenol removal rate was 90.47% after 5h. When pure N2 was passed into the solution, phenol removal rate was 15



only 58.12%. This indicated that O2 indeed participated in the PEC degradation of phenol. When BQ was added into solution bubbled with pure N2, phenol removal rate was further enhanced, which indicated that BQ has a positive effect on the reaction process. It is well known that BQ and hydroquinone are a redox couple. As they could promote the HO  production and pollutants removal via quinone redox cycling in environmental microorganism,61,62 BQ and hydroquinone might serves as a shuttle to enhance the PEC performance of Fe2O3-MoS2-Cu2O Z-scheme composite after the added BQ were transformed to hydroquinone. This also explained why the CB of Cu2O (-1.69 V vs. NHE) is more negative than E0(O2/O2  -) in Z-scheme mechanism mode but the addition of O2- scavenger (BQ) hadn’t decreased the phenol degradation rate. Therefore, the Z-scheme mechanism in Figure 12b is consistent with the results and Fe2O3-MoS2-Cu2O composite is a Z-scheme photocatalyst under our experimental conditions.

(a) None

100 Benzoquinone

80 Oxalic acid

60

Tert-butyl alcohol

40 20 0

Blank

O2•HO• Trapping agent type

+

h


100 Phenol Degradation Rate/%

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

Page 16 of 25

(b) 80 60 40

Pure N2 Pure O2

20

Pure N2+BQ Pure O2+BQ

0 0

50

100

150

200

250

300

Irradiation time /min

Figure 11. (a) PEC degradation rate of phenol for different scavengers and (b) different gases in electrolyte solution.

16


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Figure 12. Proposed mechanism of charge transfer in Fe2O3-MoS2-Cu2O under visible light. (a) Heterojunction mechanism; (b) Z-scheme mechanism.

4. CONCLUSIONS

The highly efficient Fe2O3-MoS2-Cu2O ternary composite was obtained via simple electro-deposition and hydrothermal method. The photocurrent response of Fe2O3-MoS2-Cu2O was 20 times that of Fe2O3 at 0.38 V (vs. Ag/AgCl) in 0.1 M NaOH aqueous solution under visible light irradiation. The phenol degradation rate of Fe2O3-MoS2-Cu2O composite film was ~2 times to that on Fe2O3 film. More efficient charge separation and transfer, and the reduced charge transfer resistance were achieved on Fe2O3-MoS2-Cu2O ternary structure due to the synergistic effect of the visible-light-active components (Fe2O3 and Cu2O) and the excellent solid-state electron mediator (MoS2). HO  and h+ were the main active species generated on Fe2O3-MoS2-Cu2O ternary composite. The high charge separation efficiency and strong oxidation ability were ascribed to the Z-scheme transfer mechanism. This work provides a simple, efficient and promising technique for constructing Z-scheme visible-light photocatalysts with simultaneously prompted charge transfer and redox ability. SUPPORTING INFORMATION

Figures S1-S4. ACKNOWLEDGEMENT

This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (LY18B060003, LY16B060001, LR18B070001), the National 17



Science Foundation of China (21576237, 21477114), and Graduate Innovation Foundation of Zhejiang Gongshang University (15020000334). REFERENCES

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