Molybdenum doped CuWO4 nanoflake array films as an efficient photoanode for solar water splitting

Accepted Manuscript Molybdenum doped CuWO4 nanoflake array films as an efficient photoanode for solar water splitting Jingjing Yang, Chao Li, Peng Diao PII:

S0013-4686(19)30723-6

DOI:

https://doi.org/10.1016/j.electacta.2019.04.044

Reference:

EA 33992

To appear in:

Electrochimica Acta

Received Date: 17 January 2019 Revised Date:

6 April 2019

Accepted Date: 7 April 2019

Please cite this article as: J. Yang, C. Li, P. Diao, Molybdenum doped CuWO4 nanoflake array films as an efficient photoanode for solar water splitting, Electrochimica Acta (2019), doi: https://doi.org/10.1016/ j.electacta.2019.04.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Mo doping significantly improved the catalytic activity of CuWO4 nanoflake array film toward photoelectrochemical oxygen evolution reaction.

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Molybdenum doped CuWO4 nanoflake array films as an efficient photoanode for solar water splitting

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Jingjing Yang, Chao Li, and Peng Diao*

Key Laboratory of Aerospace Materials and Performance (Ministry of Education),

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School of Materials Science and Engineering, Beihang University, Beijing 100191,

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China. Corresponding author email: [email protected]

ABSTRACT

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We developed a two-step method to prepare Mo-doped CuWO4 NF array films, which involved a hydrothermal process to grow Mo-doped WO3 NF arrays and a follow-up thermal solid-state reaction to convert WO3 to CuWO4. The obtained Mo-doped

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CuWO4 NF array films retained the network structure of crosslinked CuWO4 NFs when the Mo-to-W atomic ratio in precursor solution is lower than 2 : 3. Mo doping not only

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narrowed the bandgap of CuWO4 but also significantly improved its electron density, both benefiting the photoelectrochemial (PEC) performance for oxygen evolution reaction (OER). The Mo-doped CuWO4 NF array films exhibited a high catalytic activity toward PEC OER, and the activity varied as a function of the Mo doping concentration. The highest activity was obtained on 32 at.% Mo-doped samples, with photocurrent densities of 0.62 mA·cm-2 and 1.23 mA·cm-2 at 1.23 V and 1.60 V vs RHE, respectively, which are the highest values among those reported in literature. Moreover, even at high OER overpotentials, the Mo-doped CuWO4 NF array films showed an excellent stability in 1

ACCEPTED MANUSCRIPT weak alkaline solution. This work indicates that cation doping is a effective strategy to prepare highly efficient CuWO4-based photoanodes for PEC OER.

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KEYWORDS copper tungstate, molybdenum doping, photoelectrochemistry, oxygen evolution reaction,

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solar water splitting.

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1. INTRODUCTION

Solar energy is a naturally available, inexhaustible, and environmentally benign energy source that may meet most of our future energy needs [1]. The photocatalytic splitting of water into hydrogen and oxygen by using sunlight is a promising strategy to

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collect and store solar energy in chemical bonds [2, 3]. Solar water splitting involves two half-cell reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Photoelectrochemical (PEC) cells have been widely used to

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investigate one half-cell reaction because they spatially separate OER and HER [2]. PEC OER have received great attention for years because it is more sluggish than HER due to

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its complexity and the involvement of four-electron charge transfer process [4], which makes OER the rate-determining half-cell reaction in solar water splitting. The photoanode material is the dominant factor determining the photo-to-chemical energy conversion efficiency of PEC OER. In principle, n-type semiconductors, whose valence band edge potential is more positive than the potential of O2/H2O redox couple (1.23 V vs standard hydrogen electrode (SHE)), can be used as photoanodes for OER. However, in practice only a limited number of semiconductors, such as TiO2 [5], ZnO [6-8], Fe2O3 [9, 10], WO3 [11, 12], BiVO4 [13], Ag3PO4 [14], and etc., have been demonstrated to be 2

ACCEPTED MANUSCRIPT effective in realizing PEC OER. Therefore, it is highly desirable to develop new photoanode materials that have high activity and stability for PEC OER. Among the above mentioned n-type semiconductors used for PEC OER, WO3 have been widely investigated due to its photoactivity [2, 11, 15-17], high chemical stability in acid solution [18],

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intrinsically good electron transport properties [19], ease preparation [20, 21], and most importantly the highly positive potential of its valence band edge (ca. 3.0 V vs NHE) [2]. However, the relatively large bandgap energy of WO3 (2.7 eV) makes it active only in the

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near UV-blue visible region (with wavelength shorter than 450 nm). Combining WO3 with another metal oxide to form binary oxides is an effective way to modulate the energy band

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structure while retaining the advantages of WO3. Therefore, on the basis of this idea, several WO3-based binary oxides such as CuWO4 [22], ZnWO4 [23], NiWO4 [24], and Bi2WO6 [25] have been successfully synthesized and all of them have been demonstrated to be active for PEC OER.

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Among these WO3-based binary oxides, CuWO4 has the smallest bandgap energy of ca. 2.25-2.45 eV depending on the preparation conditions.[22, 25-30] which ensures that CuWO4 absorbs more visible spectrum compared to WO3 (2.7 eV), ZnWO4 (3.3 eV),

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NiWO4 (2.6 eV), and Bi2WO6 (2.8 eV). Besides its relatively small bandgap, CuWO4 has

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several other merits as a catalysts for PEC OER: excellent chemical and photochemical stability in a wide pH range [25-27, 29, 30], highly positive potential of its valence band edge (ca. 2.8 V vs RHE) [26, 28] that provide a large driving force for photoinduced holes to oxidize water, high Faradic efficiency [27], and environmentally benign composition as well.

Accordingly, CuWO4 has been widely investigated as a promising photoanode

material for OER [22, 25-38] since the first report in 1982 [39]. Many approaches have been developed to prepare CuWO4 on fluorinated tin oxide (FTO) substrates, including electrochemical deposition [26], sol-gel synthesis followed by spin-coating and annealing

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ACCEPTED MANUSCRIPT [27, 28], atomic layer stack deposition [30, 37, 38], co-sputtering deposition [31], spray pyrolysis [32], drop-drying and annealing growth,[34] hydrothermal synthesis [33, 35], and sacrificial template method via solid phase reaction [25, 29].

The PEC OER

performance of the obtained CuWO4 photoanodes varied widely depending on the

energy band structure and charge carrier density of CuWO4.

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preparation methods and conditions, which determined the morphology, crystallinity,

The linear sweep voltammetry (LSV) is usually employed to investigate the activity

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of a photoelectrode under illumination. To rationally evaluate the activity of a photoelectrode, the photocurrent densities must be compared at the same potential (vs

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RHE), with the same potential sweep rate, and under the same incident light intensity, as increasing each of the three parameters will inevitably lead to an increase in photocurrent density and then may result in an overestimation of the activity. Therefore, people usually evaluated the activity of a photoanode material by comparing the photocurrent density at

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the following conditions: a fixed potential of 1.23 V vs RHE, a low potential sweep rate usually smaller than 25 mV⋅s-1 (especially 10 mV⋅s-1) and an incident light intensity of 100 mW⋅cm-2 [25-27, 29-32, 34, 35, 40, 41]. Among the reported CuWO4 photoanodes, the

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hydrogen-treated CuWO4 nanoflakes (NFs) array films, which possessed a network

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structure with single crystalline CuWO4 NFs intersected to each other, exhibited very high activity for PEC OER [29]. The photocurrent densities on CuWO4 NFs at potentials of 1.23 V and 1.57 V (vs RHE) in the LSV curve (obtained at potential sweep rate of 10 mV⋅s-1 under 100 mW⋅cm-2 illumination) were 0.45 mA⋅cm-2 and 0.82 mA⋅cm-2, respectively [29]. The high activity was attributed to the following two reasons: (1) the unique morphological and crystalline structure of CuWO4 NFs, which ensured a high specific area, a short hole transport distance, and a low grain boundary density, and (2) the hydrogen treatment, which induced oxygen vacancies in CuWO4 and then improved its

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ACCEPTED MANUSCRIPT conductivity. Although the CuWO4 NFs exhibited the largest photocurrent density for PEC OER among all CuWO4 photoanodes, its activity is still much lower compared to the theoretical prediction made on CuWO4 [22]. Reducing the bandgap and increasing the charge carrier density are two strategies that may further improve the PEC OER

latter enhances the charge transport properties.

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performance of CuWO4 because the former broadens the light absorption range and the

Doping a semiconductor with foreign atoms can not only alter the energy band

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structure of the semiconductor but also change its charge carrier density. Several metallic elements such as Mo [42, 43], Ti [44, 45], Ga [46], Mg [47], Nb [48], V [49] have been

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employed to dope WO3, and the resulting cation-doped WO3 exhibited an enhanced photocatalytic activity. It has been demonstrated that cation-doping led to the reduction of the bandgap energy of WO3 [42, 43, 46-48], and then significantly extended the light absorption band. Although there were much fewer work reported on the doping CuWO4

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for improving its photoactivity [22, 50, 51], it has been demonstrated that the formation of CuWxMo1-xO4 solid solution narrowed the bandgap of CuWO4 [50] and Fe-doping enhanced the charge carrier density [51], both benefiting the photoactivity of CuWO4.

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The purpose of this work is to further improve the PEC activity of the CuWO4 NF

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array photoanode toward OER by Mo-doping, even though the CuWO4 NF array photoanode already exhibited the highest activity reported in literature [29]. Herein, we report the preparation of Mo-doped CuWO4 NF array films that showed the same morphology and microstructure as the undoped films. We demonstrate that Mo-doping not only reduces the bandgap of CuWO4 NFs but also greatly improves its electron density. The obtained Mo-doped CuWO4 NFs exhibited significantly enhanced activity for PEC OER. The photocurrent density at 1.23 V and 1.58 V vs RHE were further improved 38% (to 0.62 mA⋅cm-2) and 50% (to 1.23 mA⋅cm-2), respectively. We believe this work may

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ACCEPTED MANUSCRIPT open a new avenue to develop highly efficient CuWO4-based photoanode materials for PEC OER.

2. EXPERIMENTAL SECTION

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2.1. Chemicals and substrates

Tungsten powder, tungstic acid (H2WO4), molybdenum trioxide (MoO3), hydrogen peroxide (H2O2), isopropanol (C3H8O), urea (H2NCONH2), acetonitrile (C2H3N), oxalic

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acid (H2C2O4), hydrochloric acid (HCl), and acetic acid (C2H4O2) were purchased from Beijing Chemical Works. Cupric nitrate trihydrate (Cu(NO)3·3H2O) was purchased from

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Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytically pure grade and were used without further purification. Fluorine-doped tin oxide substrates (FTO) (F:SnO2, 8 Ω⋅sq−1, transparency 80%) were purchased from Asahi Glass, Japan.

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2.2. Growth of undoped and Mo-doped WO3 nanoflakes array film on FTO substrate

The FTO substrates were ultrasonicated successively in 0.5M KOH, ethanol and

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deionized water each for fifteen minutes, and then dried in high purity N2 stream at room

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temperature. The WO3 NF array films were grown on FTO substrates by using a seed-assisted hydrothermal approach reported previously [29]. Briefly, the WO3 nanoseeds were prepared on FTO substrates via a double-potential deposition at -0.40 V (vs SCE) for 300 s and 0.20 V (vs SCE) for 15 s in a mixed electrolyte containing 2 g tungsten powder (dissolved in 10 mL of aqueous 30% H2O2), 30 mL of isopropanol and 100 mL of deionized water. Then the resulting FTO substrates were annealed in air for 30 min at 450 °C. The annealed

nanoseed-modified FTO substrates were employed to grow the

undoped WO3 NF array films by a hydrothermal method at 180 °C for 2 h in the mixed

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ACCEPTED MANUSCRIPT solution of 20 mg urea, 20 mg oxalic acid, 0.5 mL 6 M HCl, 12.5mL acetonitrile and 3.5 mL of 0.05 M H2WO4. While for the preparation of Mo-doped WO3 NF array films, the 0.05 M H2WO4 in the hydrothermal solution was replaced by a mixture of MoO3 and H2WO4 with different MoO3-to-H2WO4 molar ratio. The WO3 nanoseed-modified FTO

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substrates were placed at an angle against the inside wall of autoclave, with the conducting side facing down. After hydrothermal growth, the undoped and Mo-doped WO3 NF array films were rinsed with high purity water, and then dried at room temperature in a high

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purity N2 stream.

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2.3. Preparation of undoped and Mo-doped CuWO4 NFs by a thermal solid-state reaction

The undoped and Mo-doped CuWO4 NF array films were prepared by a thermal solid-state reaction using the undoped and Mo-doped WO3 NF array films as the

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sacrificial templates, respectively. In detail, 0.725 g Cu(NO)3·3H2O was dissolved in 30 mL acetic acid, then the resulting Cu(NO)3 solution (100 µL) was drop-cast onto the undoped (or Mo-doped) WO3 NF array film (3 cm × 1 cm) and dried in air at room

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temperature. The drop-casting and drying procedure were repeated twice. Then, the

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Cu(NO)3 modified WO3 NF array films were annealed in air at 550 °C for 90 min to obtained a dark yellow film that was composed of CuWO4 (or Mo-doped CuWO4) and CuO. The CuWO4 (or Mo-doped CuWO4) NF array film was obtained by removing CuO in 0.5 M HCl for 30 min.

2.4. Characterization The crystal structure of the pure and Mo-doped CuWO4 NF array films were characterized by X-ray diffraction (XRD) (Rigaku, rint2000 advance theta-2theta powder

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ACCEPTED MANUSCRIPT diffractometer) with Cu Kα radiation. The morphological characterization and energy dispersive X-ray spectra (EDS) were performed on a Hitachi S-4800 field emission scanning electron microscope (FESEM) operating at an accelerating voltage of 10 kV. The Transmission electron microscope (TEM) and element mapping measurements were

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carried out on a field emission JEM-2010F microscope (JEOL Ltd., Japan) with an accelerating voltage of 200 kV. UV–vis diffuse reflection spectra were collected on a Shimadzu 3600 UV–vis–NIR spectrophotometer. X-ray photoelectron spectroscopic (XPS)

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data were recorded on a Kratos AXIS Ultra DLD (Kratos Analytical Ltd, Japan) at a power of 150 W using Mg-monochromatic X-ray. All the binding energies of XPS spectra

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were calibrated using the C 1s peak at 284.8 eV as the reference.

2.5. Electrochemical, photoelectrochemical, and O2 detection measurements All the electrochemical and PEC measurements were performed with a conventional

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three-electrode cell on a CHI 660D work station (CH Instruments Co.). The undoped (or Mo-doped) CuWO4 NF array films prepared on FTO substrates were employed as the working electrodes, and a coiled Pt wire and a saturated calomel electrode (SCE) were

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used as the counter and the reference electrodes, respectively. For PEC measurements, a 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) was used as the light source.

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The photoelectrodes were illuminated from the front side with an area of 0.21 cm2 exposed to illumination. The incident light intensity on electrode was calibrated to 100 mW⋅cm−2. All PEC measurements were performed either in 0.1 M sodium phosphate buffer solution (pH 7) or in 0.1 M sodium borate buffer solution (pH 9). For long-term stability measurements, the large undoped and Mo-doped CuWO4 NF array photoanodes (3.5 cm × 2.2 cm) were used with an area of 0.81 cm2 exposed to illumination. All potentials in this work are reported with reference to SCE and revisable hydrogen

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ACCEPTED MANUSCRIPT electrode (RHE). The potential vs SCE was converted into the potential vs RHE on the basis of the following equation: E(vs RHE) = E(vs SCE) + 0.244 + 0.059pH

(1)

The actually evolved amount of O2 was measured at 0.8 V vs SCE (1.46 V vs RHE) under

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illumination (100 mW⋅cm−2) in a gas-tight three-compartment PEC cell, from which the gas samples were taken at a time interval indicated below. The PEC cell was purged with high purity N2 for 40 min before illumination. The O2 detection measurements were first

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carried out in the dark to ensure the airtightness of the cell. Then after illumination for 20 min, the concentration of O2 gas samples was analyzed every 5 min by using a gas

3. RESULTS AND DISCUSSION

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chromatograph (Shimadzu GC-2014C, Japan).

CuWO4 NF array film

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3.1. Morphology, chemical composition and crystal structure of the Mo-doped

The strategy that we used to prepare Mo-doped CuWO4 NF array films is shown schematically in Fig. 1. Briefly, we first synthesis Mo-doped WO3 NF array films via a

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hydrothermal approach by using mixed solution of Mo(VI) and W(VI) precursors. Then

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the Mo-doped WO3 NFs were transformed to Mo-doped CuWO4 NFs by a thermal solid-state reaction between doped WO3 and CuO [29]. The Mo-doping amount in the obtained CuWO4 NF films could be tuned within a certain range by varying the molar ratio of Mo(VI) and W(VI) in the precursor solution. The actual Mo : W atomic ratio in doped CuWO4 was obtained by EDS and the results are summarized in Table 1. As shown in Table 1, the Mo : W atomic ratio first increased and then decreased with increasing the amount of Mo(VI) precursor in the hydrothermal solution. The maximum Mo : W atomic ratio in Mo-doped CuWO4 was

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ACCEPTED MANUSCRIPT 0.19 : 0.81, which was obtained at the Mo : W molar ratio of 2 : 3 in precursors solution. This is in good agreement with previous results that pure phase of CuMoxW1-xO4 solid solution with wolframite structure had a low Mo content (Mo : W atomic ratio < 0.30 : 0.7) [50, 52, 53]. It should be pointed out that, for ease expression of the doped CuWO4

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samples, we used the atomic percentage of Mo in hydrothermal solution to denote the Mo-doped CuWO4 samples, as listed in the rightest column in Table 1.

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Table 1 Summary of Mo and W concentration in precursor solution and the Mo : W atomic ratios in Mo-doped CuWO4 NF array films obtained by EDS.

Composition in film

Optical

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Composition in precursor solution Sample MoO3/mM

H2WO4/mM

Mo : W atomic ratio

bandgap/eV

undoped

0

5.0

0:1

2.37

8 at.% Mo-doped

0.4

4.6

16 at.% Mo-doped

0.8

4.2

32 at.% Mo-doped

1.6

3.4

40 at.% Mo-doped

2.0

66 at.% Mo-doped

3.3

100 at.% Mo-doped

5.0

0.04 : 0.96 2.34

0.15 : 0.85

2.27

3.0

0.19 : 0.81

2.22

1.7

0.14 : 0.86

-

0

0.05 : 0.95

-

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0.10 : 0.90

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Fig. 2 shows the typical SEM images of pure CuWO4 and 32 at.% Mo-doped CuWO4

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NF array films. As shown in Fig. 2a, the undoped CuWO4 NF array film had network structure composed of vertically-aligned NFs that were intersected and crosslinked with each other. The crosslinked NFs network structure ensured a large specific area, a well connection between NFs, and a high light absorption probability, all of which contributed to the high activity of CuWO4 NF array films toward PEC OER [29]. Fig. 2b clearly shows that, after Mo doping, the crosslinked NFs network structure kept almost unchanged, suggesting that the Mo-doped samples retained the structural advantages of CuWO4 NF array for PEC OER. The crystal structure of the undoped and 32 at.%

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ACCEPTED MANUSCRIPT Mo-doped CuWO4 NF array films was characterized by XRD, and the results are presented in Fig. 3. Only the diffraction patterns of triclinic wolframite CuWO4 are present in the XRD spectra, indicating that the samples had only a single phase regardless of the Mo-doping. Moreover, the dependence of XRD patterns on the Mo doping

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concentration indicates that there is no peak shift observed even if the Mo concentration was increased to 66 at.%. This observation suggests that doping of Mo into CuWO4 does not induce any detectable structural distortion, which is in agreement with previous

Å) is almost the same as that of W6+ (0.60 Å) [54].

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calculation [52]. This phenomenon is no surprise considering that the radius of Mo6+ (0.59

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Fig. 4a and b show the high resolution TEM (HRTEM) images of undoped and Mo-doped CuWO4 NFs, respectively. Both the pure and doped CuWO4 exhibit well resolved lattice fringes, indicating that both have a good crystallinity. The lattice spacings of undoped and Mo-doped CuWO4 are 0.368 nm and 0.367 nm, respectively, both

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corresponding to the (011) facet of triclinic CuWO4. The neglectable difference in lattice spacing before and after Mo-doping agrees well with the XRD results, confirming that Mo-doping does not bring about significant structural distortion in CuWO4 crystals. The

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EDS mapping was performed to explore the elemental composition and its distribution in

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the doped CuWO4 NFs, and the results are shown in Fig. 4c-f. Four elements, O, Cu, W, and Mo were present in EDS mapping with each element uniformly distributed in the sample. The mapping data provided solid evidence that Mo was homogeneously embedded into the crystal lattice of CuWO4, leading to the formation of uniformly Mo doped CuWO4.

3.2. Optical property and bandgap energy of the Mo-doped CuWO4 NF array film One purpose of Mo doping is to narrow the bandgap of CuWO4 so that the doped

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ACCEPTED MANUSCRIPT CuWO4 photoanode can absorb more light. As shown in the photographs of the Mo-doped CuWO4 (see the inset of Fig. 5a), the change of optical property of the samples could be observed even by naked eyes. When the Mo concentration in hydrothermal solution was increased from 0 to 40 at.%, the colour of the obtained CuWO4 NF array films turned

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from light yellow to yellow-orange, implying a Mo doping induced extension of light absorption range. However, further increasing the Mo concentration over 40 at.% would not lead to the deepening of film colour, instead, a very thin film with a colour of undoped

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CuWO4 was obtained. UV-vis diffuse reflectance spectroscopy was employed to confirm the extension of light absorption region, and the results are shown in Fig. 5a. The onset of

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light absorption was shifted to longer wavelength as the doping amount of Mo was increased.

Fig. 5b shows the Tauc plots of Mo-doped CuWO4 NF array films, from which the bandgap energy of 2.37 eV, 2.34 eV, 2.27 eV and 2.22 eV were obtained for pure, 16 at.%,

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32 at.%, and 40 at.% Mo-doped CuWO4 NF array films, respectively. These results confirm that Mo doping narrows the bandgap of CuWO4 and bandgap energy can be tuned

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to a certain extent by varying the Mo doping concentration.

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3.3. Electron density of the Mo-doped CuWO4 NF array film Another purpose of Mo doping is to increase the majority charge carrier density of

CuWO4 so that the doped photoanodes have a better conductivity. As is well known, the majority charge carrier density can be calculated on the basis of Mott-Schottky equation [55]: ଵ

మ ஼౩ౙ

=ఌ

ଶ

బ ఌே௘

ቀ‫ ܧ‬− ‫ܧ‬୊୆ −

௞் ௘

ቁ

(2)

where Csc is the capacitance of the space charge region of semiconductor electrode in µF⋅cm-2, E is the applied potential, EFB is the flat band potential, ε is the dielectric constant

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ACCEPTED MANUSCRIPT of semiconductor, ε0 is the permittivity of vacuum, N is the donor (or acceptor) density (for n-type semiconductors such as CuWO4, N equals to the electron density), e is the electron charge, k is the Boltzmann’s constant, and T is the absolute temperature. For an n-type semiconductor, equation (2) has a positive slope of 2/ε0εNe, while for a p-type equation

(2)

has

a

negative

slope.

As

the

capacitance

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semiconductor,

of

semiconductor/solution interface (C) is composed of Csc and the double layer capacitance

ଵ

஼

ଵ

ଵ

ೞ೎

೏೗

=஼ +஼

(3)

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(Cdl) in series, the value of C can be expressed according to the following equations:

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For a semiconductor electrode, Csc is much smaller than Cdl, and so Csc ≈ C. Therefore, the variation of Csc as a function of applied potential could be obtained by measuring C at different potentials via electrochemical impedance spectroscopy, and the obtained Mott-Schottky plots of undoped and Mo-doped CuWO4 NF array films are shown in Fig. 6. Both the pure and Mo-doped samples have a positive slope, indicating that both are

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n-type semiconductors. On the basis of equation (2), the electron density of the undoped and doped CuWO4 could be calculated out from the slope of the corresponding linear part

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of Mott-Schottky plot. Taking the ε value of CuWO4 as 83 [56], the calculated electron densities of the undoped and 32 at.% Mo-doped CuWO4 are 4.27×1019 cm-3 and 3.04×1020

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cm-3, respectively, with the latter 7 times higher than the former. To elucidate why Mo doping increases the majority charge carrier density of CuWO4,

XPS measurements was carried out because XPS spectra provide not only the elemental composition of the film but also the chemical state of elements, which were closely related with the formation of oxygen vacancies [29, 57]. Fig. 7a shows the high resolution W4f XPS spectrum of 32 at.% Mo-doped CuWO4 NF array film. Deconvolution of the spectra demonstrates that the W4f XPS spectrum (Fig. 7a) is dominated by the spin-orbit doublet peaks of the W4f7/2 and W4f5/2 states of W6+. The appearance of doublet peaks of W5+ in

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ACCEPTED MANUSCRIPT XPS spectra indicates the presence of W5+ in the film. As the W5+ (or Mo5+) in WO3 and CuWO4 can induce oxygen vacancies that are regarded as shallow donors [29, 57-59], a higher concentration of W5+ will result in a larger electron density in WO3 [57] and CuWO4 [29, 35]. On the basis of the XPS peak areas of W6+ and W5+ in Fig. 7a, the

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atomic percentage of W5+ in total W is calculated out to be 2.5%. Compared to the pure CuWO4, Mo doping has little influence on the concentration of W5+ in the film. Fig. 7b shows core-level deconvoluted Mo3d XPS spectra of the doped CuWO4 film, which also

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have two sets of doublet peaks. The doublet peaks at 232.4 eV and 235.6 eV correspond to 3d5/2 and 3d3/2 of Mo6+, while the doublet peaks at 231.6 eV and 234.8 eV arise from 3d5/2

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and 3d3/2 of Mo5+ [43, 60]. The atomic percentage of Mo5+ in total Mo is calculated to be 5.3%, which is over 2 times higher than that of W6+ (2.5%). This result indicates that Mo doping can induce more oxygen vacancies and then improve the electron density in CuWO4. The UV-vis spectra, the Mott-Schottky plots, and the XPS spectra provided solid

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evidence that the Mo doping not only narrowed the bandgap of CuWO4 but also doubled its electron density, both benefiting the activity of CuWO4 toward PEC OER.

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3.4. Activity of the Mo-doped CuWO4 NF array film toward PEC OER

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Fig. 8 shows the linear potential sweep response of the undoped and 8 at.% Mo-doped CuWO4 NF array photoanodes in 0.1 M sodium phosphate buffer solution (pH 7) under chopped illumination (100 mW cm-2). When the light was cut off by the chopper, no oxidation current could be observed on either the undoped or the Mo-doped samples within the potential sweep range from 0.66 V to 1.86 V vs RHE (0 V to 1.20 V vs SCE), indicating that both the pure and doped CuWO4 exhibited no electrochemical activity toward OER. However, when the light was projected on the photoanodes, an oxidation photocurrent was immediately observed on both the undoped and doped CuWO4

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ACCEPTED MANUSCRIPT photoanodes at potentials higher than 0.80 V vs RHE (0.14 V vs SCE). Moreover, the photocurrent density increased significantly with increasing the applied potentials. This photocurrent comes from the oxidation of water under illumination [22, 25-38, 50, 51] and can be used to evaluate the activity of the photoanode toward OER. Fig. 8 clearly shows

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that, compared to the undoped CuWO4 photoanode, the 8 at.% Mo-doped photoanode exhibited a significantly improved OER photocurrent density especially at high potential region. This result strongly supports the idea that Mo-doping can enhance the catalytic

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activity of CuWO4 for PEC OER.

To investigate the effect of Mo-doping concentration on the activity of the doped

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CuWO4, we compared the linear sweep voltammetric (LSV) curves recorded under continuous illumination on CuWO4 samples with different Mo doping concentration, and the results are shown in Fig. 9a. The corresponding LSV curves under chopped illumination were also obtained and presented in Fig. S-1 in the Electronic Supplementary

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Material.

Fig. 9a and Fig. S-1 (in the Electronic Supplementary Material) clearly demonstrate that the activity for PEC OER depended greatly on the Mo doping concentration. The

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photocurrent density does not increase monotonically with increasing the Mo

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concentration in CuWO4. To clearly show the influence of the Mo doping amount on the PEC activity, the photocurrent density of each CuWO4 sample at 1.23 V and 1.60 V vs RHE are plotted against the Mo doping concentration and the results are shown in Fig. 9b. It is clearly seen from Fig. 9b that the photocurrent density first increased and then decreased as the concentration of Mo was increased from 0 at.% to 100 at.%. The highest activity for PEC OER was obtained on the 32 at.% Mo-doped sample. With a slow potential sweep rate of 10 mV⋅s-1, the maximum photocurrent densities were 0.62 mA·cm-2 and 1.23 mA·cm-2 at 1.23 V and 1.60 V vs RHE, respectively. These values are

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ACCEPTED MANUSCRIPT the highest among those reported in literature on CuWO4 [22, 25-39, 50, 51]. The incident photon-to-current efficiency (IPCE) measurements were also performed was also performed on pure and 32 at.% Mo-doped CuWO4 at a potential bias of 1.23 V vs RHE, and the results are presented in Fig. 9c. The IPCE of the pure CuWO4 NFs is ca.

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12%, which is in good agreement with previous report [41]. After Mo doping, the IPCE is increased to ca. 19%, a clear improvement as compared to previously values [26, 41].

It should be pointed out herein that, for samples with doping amount larger than 8

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at.%, the photocurrents at low overpotential region were smaller than the undoped CuWO4 (Fig.9a) This observation was usually observed on semiconductor photoelectrodes after

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doping or other treatments [29, 50, 57]. Although doping (or other treatment) may narrow the bandgap and then results in the generation of more electron-hole pairs under illumination, it can also change the surface states, and then lead to a slow electron-hole separation rate. As is well known that, at very low overpotentials, the driving force to

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separate photoinduced electron-hole pairs is very small, and therefore the photocurrent is mainly dominated by surface condition of the semiconductor. This is the reason why, at low overpotentials, smaller photocurrents were usually obtained on doped samples than on

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undoped ones. While at high overpotentials, doping induced extension of light absorption

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region and increase of high absorption dominate the increase of photocurrent.

3.5. Mechanism of the enhancement of the activity of CuWO4 by Mo-doping All PEC measurements clearly show that the doping of CuWO4 with Mo can

significantly improve its activity for OER. To understand the Mo-doping induced enhancement of activity, there are two important questions needed to be addressed: (1) What is the mechanism for the enhancement of activity by Mo-doping? (2) Why did the 32 at.% Mo-doped CuWO4 sample exhibit the highest activity for PEC OER?

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ACCEPTED MANUSCRIPT As for the first question, we believe that the narrowing of bandgap and the increasing of charge carrier density (electron density) due to Mo-doping are the two main reasons for the enhancement of the activity for OER. As shown in Fig. 5, the bandgap energy of CuWO4 was reduced from 2.37 eV to 2.22 eV as the Mo-doping concentration was

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increased. As a result, the onset wavelength of absorption was red-shifted, which means that Mo-doped CuWO4 can absorb more light as compared with undoped CuWO4. In addition, the light absorption intensity of CuWO4 was also improved after Mo-doping (see

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Fig. 5a), indicating that Mo-doping not only extended the light absorption region but also increased the light absorption efficiency. Both the extension of light absorption region and

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the increase of light absorption efficiency facilitated the generation of electron-hole pairs and then benefited the improvement of the PEC activity for OER. As discussed in section 3.3, Mo-doping can generate large amounts of oxygen vacancies (see Fig. 7 and discussion in section 3.3), which will result in a larger electron density in CuWO4.

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Mott-Schottky measurement (Fig. 6) clearly demonstrates that the electron density of the 32 at.% Mo-doped CuWO4 (3.04×1020 cm-3) is over 7 times higher than that of the undoped CuWO4. High electron density ensured the Mo-doped CuWO4 a better

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conductivity and a higher transportation rate of photoinduced holes, which also favor the

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improvement of the PEC activity of CuWO4 toward OER. As for the second question, the effect of Mo doping concentration on the

microstructure of the CuWO4 films was characterized to provide clues why the 32 at.% Mo-doped CuWO4 sample exhibit the highest activity for OER. Fig. 10 shows the evolution of the morphology of Mo-doped CuWO4 with the increase of the concentration of Mo precursors in hydrothermal solution. It is seen clearly that the network structure of crosslinked NFs deteriorated when the Mo concentration was higher than 32 at.%. Moreover, further increasing the Mo concentration over 40 at.% completely destroyed the

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ACCEPTED MANUSCRIPT NF network structure, leading to the formation of a very thin CuWO4 film composed of irregularly shaped nano- and micro-particles (see Fig. 10d and e). The thickness of these films was so small that FTO substrate can be clearly identified from SEM images in Fig. 10d and e. EDS analysis was conducted to provide information of the actual Mo : W

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atomic ratio in the Mo-doped CuWO4. Fig. 10f shows the variation of the actual Mo : W atomic ratio in the doped film as a function of Mo concentration in hydrothermal solution. It is surprising to see that the actual Mo : W ratio reaches a maximum (0.19 : 0.81) when

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the Mo concentration in hydrothermal solution was 40 at.%. Further increasing the Mo concentration over 40 at.% in precursor solution would significantly decrease the actual

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Mo : W ratio in the doped CuWO4 films. The EDS results together with SEM images suggest that, during the hydrothermal reaction, a very high Mo concentration (over 40 at.%) in the precursor solution did not lead to the growth of highly Mo-doped WO3 films but resulted in the cease of WO3 growth on FTO substrate. The thin films observed in high

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Mo concentration (Fig. 10d and e) were CuWO4 films, which were converted from WO3 nanoparticles pre-seeded on FTO substrate before hydrothermal reaction. According to the above discussion, we can safely conclude that increasing the Mo concentration in

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precursor solution resulted in the following two effects: (1) the increase of Mo : W atomic

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ratio (within 0.19 : 0.81) in CuWO4 films and (2) the deterioration of the NF array structure. The former favors whereas the latter blocks the enhancement of the photoactivity of the CuWO4 films. Therefore, the highest activity of the 32 at.% Mo-doped CuWO4 NF array is the result of the balance between these two opposite effects.

3.6. Stability and O2 evolving amount of the Mo-doped CuWO4 NF array film In addition to activity, the stability is another key factor influencing the performance of photoanode material for PEC OER. Accordingly, we explored the stability of both the

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ACCEPTED MANUSCRIPT undoped and doped CuWO4 photoanodes via chronoamperometric measurements at 0.80 V vs SCE in two solutions with pH values of 7.0 and 9.0, and the results are shown in Fig. 11. It should be pointed out herein that, to ensure a meaningful OER rate, a relatively large

9.0 solutions, respectively) was applied to the photoanodes.

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potential of 0.80 V vs SCE (corresponding to 1.46 V and 1.58 V vs RHE in pH 7.0 and pH

As shown in Fig. 11a, a stable photocurrent of ca. 0.50 mA·cm-2 and 0.80 mA·cm-2 was obtained on the undoped and doped CuWO4 photoanodes, respectively, with little

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decay after 1 hour’s reaction in 0.1 M sodium phosphate buffer solution (pH 7.0). These results indicate that both the undoped and the Mo-doped CuWO4 photoanodes exhibited

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good stability in neutral solution [26, 27]. Moreover, as shown in Fig. 11b, in a weak alkaline solution (0.1 M sodium borate buffer solution with pH 9.0), the photocurrent density of the undoped and the doped CuWO4 stabilized at 0.65 mA·cm-2 and 0.96 mA·cm-2, respectively, with almost no decay being observed during one hour’s OER.

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These results clearly demonstrate that the stability of the pure and Mo-doped CuWO4 is even better in weak alkaline solution than in neutral solution. This result is in good agreement with the report that CuWO4 exhibited an excellent stability in alkaline solution

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[25, 29, 30]. Besides, regardless of solution pH, the stable photocurrent density on the

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Mo-doped samples is much higher than that on the undoped samples, further confirming that Mo doping is an efficient approach to improving the OER performance of CuWO4 photoanodes.

The actually evolved O2 amount during PER OER could be measured by gas

chromatography [61] or by O2 fluorescence detector [14, 17, 25, 27]. The former usually needs long reaction time and thereby requires a good stability of the photoanode, whereas the latter was employed when the concentration of O2 is low duo to short reaction time, low activity or unstability of the photoanode. The high stability of CuWO4 for OER

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ACCEPTED MANUSCRIPT provided a solid basis for the detection of O2 by gas chromatography [61] We measured the amount of evolved O2 in a gas-tight PEC cell, from which the gas samples were taken at a time interval of 5 min. The PEC cell was purged with high purity N2 for 40 min before OER and the O2 amount was also measured to ensure the airtightness. The first gas sample

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was analyzed after reaction for 20 min. Fig. 12 compares the theoretical amount of O2 with the actual amount of O2 produced on both undoped and Mo-doped CuWO4 NFs with a light projected area of 1.33 cm2. The theoretical amount of O2 was calculated from

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photocurrent by assuming that all photocurrent was originated from OER. Only a very small deviation of the actual O2 amount from the theoretical values is observed in Fig. 12,

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suggesting that nearly all photocurrents came from water oxidation. Faradaic photocurrent-to-O2 conversion efficiency can be obtained from the actual-to-theoretical ratio of the slopes in Fig. 12 [17], and the values for both pure and 32 at.% Mo-doped CuWO4 are ca. 96%. In addition, the slope of the line reflects the OER rate. The larger

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slope of Mo-doped sample, as compared to the pure CuWO4, also provided a strong evidence that Mo doping significantly improved the activity of CuWO4.

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4. CONCLUSIONS

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The Mo-doped CuWO4 nanoflake (NF) array films were prepared via a two-step approach that involved a hydrothermal growth of the Mo-doped WO3 NF array followed by a thermal solid-state reaction to convert WO3 to CuWO4. The Mo concentration in the precursor solution had a great effect on the formation and morphology of the Mo-doped CuWO4 films. When the Mo concentration was lower than 40 at.%, the obtained Mo-doped CuWO4 film resembled the microstructure of the undoped CuWO4 NF array film, which was composed of crosslinked single crystalline NFs. By increasing the Mo doping concentration, the bandgap energy of the doped CuWO4 could be reduced from

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ACCEPTED MANUSCRIPT 2.37 eV to 2.22 eV and the electron density could be increased over 7 times. As compared to undoped samples, the Mo-doped CuWO4 NF array films exhibited a significantly improved activity for PEC OER. The photocurrent densities obtained on 32 at.% Mo-doped samples were 0.62 mA·cm-2 and 1.23 mA·cm-2 at applied potentials of 1.23 V

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and 1.60 V vs RHE, respectively. These values are the highest among those reported in literature for CuWO4 photoanodes. We attributed the high activity of the Mo-doped samples to the following three factors: (1) the crosslinked NF network structure, which

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ensures a high specific area, a good electrical connection between NFs, and a high light absorption probability as well, (2) the narrowing of energy bandgap via Mo doping, which

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extends the light absorption range, and (3) the Mo doping induced high electron density, which favors the improvement of the conductivity.

The Mo-doped CuWO4 NF array

films also exhibited a good stability in neutral solution and an excellent stability in weak alkaline solution. Our work demonstrates that cation-doping is an efficient way to enhance

Acknowledgements

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the performance of CuWO4 as a promising photoanode material for solar water splitting.

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and 51872015).

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This work was supported by the National Natural Science Foundation of China (51672017,

Supplementary Material: Supplementary material (linear potential sweep curves obtained under chopped illumination on the Mo-doped CuWO4 films with different Mo doping concentration) is available in the online version of this article at …(automatically inserted by the publisher).

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(2011) 2512-2515.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Schematic representation of the preparation process of the Mo-doped CuWO4 NF arrays on the FTO substrate.

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Fig. 2. Typical SEM images of (a) CuWO4 NFs array and (b) Mo-doped CuWO4 NFs array.

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Fig. 3. XRD patterns of the FTO supported CuWO4 and 32 at.% Mo-doped CuWO4 NFs

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array films.

Fig. 4. Typical HRTEM images of pure (a) and Mo-doped CuWO4 NF (b). Elemental mapping images of a Mo-doped CuWO4 NF (c)-(f). The inset in (c) is the TEM image of

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the doped NF for mapping.

Fig. 5. (a) UV-vis absorption spectra of the pure and Mo-doped CuWO4 NF array films. The inset shows the optical images of (from left to right) the undoped, 16 at.%, 32 at.%,

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and 40 at.% Mo-doped CuWO4 NF array films. (b) Tauc plots of the pure and Mo-doped

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CuWO4 NF array films.

Fig. 6. Mott-Schottky plots of the undoped and 32 at.% Mo-doped CuWO4 NF array films. The variation of Csc as a function of applied potential was obtained on 0.1 M sodium phosphate buffer solution (pH 7) by electrochemical impedance spectroscopy at a frequency of 1000 Hz.

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ACCEPTED MANUSCRIPT Fig. 7. High resolution XPS spectra of (a) W 4f and (b) Mo 3d obtained with 32.at% Mo-doped CuWO4 NF array films.

Fig. 8. Current density-potential responses of the undoped and 8 at.% Mo-doped CuWO4

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NF array photoanodes in 0.1 M sodium phosphate buffer solution (pH 7) under chopped illumination (100 mW cm-2). The potential sweep rate is 10 mV⋅s-1.

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Fig. 9. The effect of Mo doping concentration on the activity of the doped CuWO4 NF photoanodes for PEC OER. (a) Linear potential sweep curves of the doped CuWO4 NF

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photoanodes with different Mo doping concentration. (b) The variation of the current density at potentials of 1.23 V and 1.60 V vs RHE (labeled by the dash-dot line in (a)) as a function of Mo doping concentration. (c) The IPCE of undoped and 32 at.% Mo-doped CuWO4 NFs. All PEC measurements were carried out in 0.1 M sodium phosphate buffer

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solution (pH 7) under continuous illumination (100 mW cm-2).

Fig. 10. Typical SEM images of doped CuWO4 films with different Mo-doping

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concentration. (a) 8 at. %, (b) 16 at.%, (c) 40 at.%, (d) 66 at.%, (e) 100 at.%. (f) The

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variation of atomic fraction of Mo in doped CuWO4 films as a function of the atomic fraction of Mo in precursor solution.

Fig. 11. Current density-time responses of the undoped and 32 at.% Mo-doped CuWO4 photoanodes at a constant potential of 0.8 V vs SCE under continuous illumination (100 mW cm-2) in (a) 0.1 M sodium phosphate buffer solution with pH 7.0 and (b) 0.1 M sodium borate buffer solution with pH 9.0.

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ACCEPTED MANUSCRIPT Fig. 12. Comparison of the theoretical amount of O2 with the actual amount of O2 produced on the undoped and 32 at.% Mo-doped CuWO4 NFs. The theoretical amount of O2 was calculated from photocurrents that were obtained in 0.1 M sodium phosphate

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buffer solution pH 7 at 1.46 V vs RHE under continuous illumination (100 mWcm-2).

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Fig. 1. Schematic representation of the preparation process of the Mo-doped CuWO4 NF

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arrays on the FTO substrate.

Fig. 2. Typical SEM images of (a) CuWO4 NFs array and (b) Mo-doped CuWO4 NFs

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array.

Fig. 3. XRD patterns of the FTO supported CuWO4 and 32 at.% Mo-doped CuWO4 NFs array films.

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Fig. 4. Typical HRTEM images of pure (a) and Mo-doped CuWO4 NF (b). Elemental

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mapping images of a Mo-doped CuWO4 NF (c)-(f). The inset in (c) is the TEM image of

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the doped NF for mapping.

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Fig. 5. (a) UV-vis absorption spectra of the pure and Mo-doped CuWO4 NF array films.

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The inset shows the optical images of (from left to right) the undoped, 16 at.%, 32 at.%, and 40 at.% Mo-doped CuWO4 NF array films. (b) Tauc plots of the pure and Mo-doped

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CuWO4 NF array films.

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Fig. 6. Mott-Schottky plots of the undoped and 32 at.% Mo-doped CuWO4 NF array films. The variation of Csc as a function of applied potential was obtained on 0.1 M sodium

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phosphate buffer solution (pH 7) by electrochemical impedance spectroscopy at a

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frequency of 1000 Hz.

Fig. 7. High resolution XPS spectra of (a) W 4f and (b) Mo 3d obtained with 32.at% Mo-doped CuWO4 NF array films.

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Fig. 8. Current density-potential responses of the undoped and 8 at.% Mo-doped CuWO4 NF array photoanodes in 0.1 M sodium phosphate buffer solution (pH 7) under chopped

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illumination (100 mW cm-2). The potential sweep rate is 10 mV⋅s-1.

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Fig. 9. The effect of Mo doping concentration on the activity of the doped CuWO4 NF photoanodes for PEC OER. (a) Linear potential sweep curves of the doped CuWO4 NF

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photoanodes with different Mo doping concentration. (b) The variation of the current density at potentials of 1.23 V and 1.60 V vs RHE (labeled by the dash-dot line in (a)) as a

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function of Mo doping concentration. (c) The IPCE of undoped and 32 at.% Mo-doped CuWO4 NFs. All PEC measurements were carried out in 0.1 M sodium phosphate buffer solution (pH 7) under continuous illumination (100 mW cm-2).

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Fig. 10. Typical SEM images of doped CuWO4 films with different Mo-doping concentration. (a) 8 at. %, (b) 16 at.%, (c) 40 at.%, (d) 66 at.%, (e) 100 at.%. (f) The

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variation of atomic fraction of Mo in doped CuWO4 films as a function of the atomic

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fraction of Mo in precursor solution.

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Fig. 11. Current density-time responses of the undoped and 32 at.% Mo-doped CuWO4

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photoanodes at a constant potential of 0.8 V vs SCE under continuous illumination (100 mWcm-2) in (a) 0.1 M sodium phosphate buffer solution with pH 7.0 and (b) 0.1 M

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sodium borate buffer solution with pH 9.0.

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Fig. 12. Comparison of the theoretical amount of O2 with the actual amount of O2

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produced on the undoped and 32 at.% Mo-doped CuWO4 NFs. The theoretical amount of O2 was calculated from photocurrents that were obtained in 0.1 M sodium phosphate

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buffer solution pH 7 at 1.46 V vs RHE under continuous illumination (100 mWcm-2).

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Highlights： ：
Mo-doped CuWO4 nanoflake networks were prepared on FTO substrate.

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Mo doping narrows the bandgap of CuWO4 and improves its electron density.
Mo-doped CuWO4 exhibits a greatly enhanced activity for solar water oxidation.


Doping amount significantly influences photoelectrochemical activity of CuWO4.

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Doped CuWO4 shows an excellent stability in neutral and weak alkaline solution.