A promising 3D flower-like CuWO4 photoanode modified with CdS

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Materials and Interfaces

A promising 3D flower-like CuWO4 photoanode modified with CdS and FeOOH for efficient photoelectrochemical water splitting Miao Zhou, Zhihua Liu, Xifei Li, and Zhifeng Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00358 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Industrial & Engineering Chemistry Research

A promising 3D flower-like CuWO4 photoanode modified with CdS and FeOOH for efficient photoelectrochemical water splitting Miao Zhou1, Zhihua Liu1#, Xifei Li2, Zhifeng Liu1∗ (1 School of Materials Science and Engineering, Tianjin Chengjian University, 300384, Tianjin, China. 2 Institute of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an, 710048, China) Abstract: This paper describes a novel promising film based on the flower-like CuWO4 structure, and applied on photoelectrochemical (PEC) water splitting as photoanode firstly. The growth mechanism and microstructure of CuWO4 are discussed in detail. The PEC measurements indicate that such flower-like CuWO4 exhibited a photocurrent density of 0.58 mA/cm2 at 0.8 V versus RHE. When coupled with CdS and FeOOH layers, the triple CuWO4/CdS/FeOOH photoanode exhibites further improved PEC performance with a higher photocurrent density about 2.05 mA/cm2 at 0.8 V versus RHE and excellent operation stability. The remarkable PEC performance stems from several crucial factors as following: (i) ideal band gap; (ii) improved light absorption; (iii) efficient charge-hole pairs separation and collection. Keywords: CuWO4; photoanode; photoelectrochemical; water splitting 1 Introduction Since Fujishima and Honda firstly report TiO2 photoanode in 1972, semiconductors have been found that they can directly harvest solar energy to produce a clean and storable hydrogen by photoelectrochemical (PEC) water splitting, which engrosses tremendous attention to solve both energy and environmental questions1-4. Numerous metal oxides such as TiO25,6, ZnO7, and α-Fe2O38,9 have been widely investigated due to their earth abundance, low cost, low toxicity, and high stability in aqueous environments3,10. Nevertheless, the overall efficiency is still limited by their inherent nature. For instance, TiO2 and ZnO own so wider band gaps (~3.2 eV) that only capture UV light what accounts for only a small proportion (4%) of the solar energy. α-Fe2O3 possesses a proper band gap (~2.2 eV) but is notorious for its low charge carrier mobility and short hole diffusion length11. Even though all kinds of

#

∗

corresponding author E-mail: [email protected] corresponding author Tel: +86 2223085236 Fax: +86 22 23085110 E-mail: [email protected] 1

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means have been attempted to improve their photocatalytic performances, the achievement of a high solar-to-hydrogen efficiency remains a challenge in PEC water splitting. Hence, it is still necessary to explore potential and high-performance photoanodes for PEC water splitting. Recently, ternary and quaternary oxides have attracted emerging interests, resulting from introducing some elements to binary oxides could create a narrower band gap with higher valence band position so as to absorb much visible light12-14. Benko firstly excavated some inherent properties of CuWO4 which benefit to PEC behavior in 1980s15. For example, CuWO4 is stable in neutral and slightly basic solutions under water oxidation conditions, which is necessary for PEC activities. What’s more, CuWO4 possesses a narrower band gap (2.2-2.4 eV) than that of WO3 (2.7 eV), due to the introducing of Cu element increases its valence band position through the interaction of Cu3d and O2p orbitals16-20. The narrow band gaps of photoelectrochemical water oxidation catalysts are crucial to obtain growing visible light and high photo-to-chemical conversion efficiency. On the basis of these characters of CuWO4, there have been many endeavors devoted to investigate the photocatalytic performance of it so far. For instance, Chang et al have produced CuWO4 by cosputtering with a photocurrent density of 400 µA/cm2 at 1.6 V vs SCE (saturated calomel electrode) in 0.33M H3PO4 (1.9 V vs RHE) under simulated solar irradiation21. Joseph et al have synthesized pure-phase CuWO4 photoanodes with 200 nm thickness by spin-casting, which are dramatically stable over a 12 h period of illumination in a 0.1 M KBi buffer at pH 722. Additionally, to optimize the photoelectrochemicial properties, much attention has been paid to the structure and morphology of semiconductors. Liu et al have demonstrated that one-dimensional (1D) WO3 nanorod displays nice PEC behaviours, due to 1D structure can provide a direct electrons transmission pathway23. Wen et al have prepared two-dimensional (2D) CuWO4 nanoflake arrays photoanodes from the chemical conversion of WO3 template with a photocurrent density of 0.4 mA/cm2 , owing to its large surface area and abundant porosity24. Note that photocatalysts that have the excellent photocatalystic activity also depend on their surface morphology characteristic. Compared with 1D and 2D structures, 3D flower-like structure displays unique properties including an enhanced incident light utilization efficiency by increasing the times of reflection and a shorten charge transport time as well as distance by special layered junctions25,26. However, to date, there have been few 2


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researches referring 3D

flower-like structure of CuWO4 to improve

its

photoelectrochemicial performances. Herein, 3D flower-like CuWO4 film was firstly prepared on FTO substrate via a simple hydrothermal method, and served as photoelectrode for PEC water splitting. The photocurrent density of the 3D flower-like CuWO4 was evaluated to be 0.58 mA/cm2 at 0.8 V versus RHE. In order to further enhance the PEC performances of CuWO4, CdS was chosen to design an ideal CuWO4/CdS heterojunction for their appropriate band gradient alignment, which can facilitate efficient directional separation of photogenerated charge carriers at their interface27-29. Furthermore, fabricating electrode/electrolyte interfaces to reduce the charge recombination rate is a one more step to increase the PEC efficiency. Hence, we modified CuWO4/CdS with FeOOH to achieve the above result. Because FeOOH is not only earth-abundant, chemical stable and environmental friendly, but also is an efficient oxygen evolution reaction (OER) catalyst30-34. When coupled with CdS and FeOOH layers, the resulting CuWO4/CdS/FeOOH photoanodes exhibit further improved PEC performance. In this paper, the detailed growth mechanisms of 3D flower-like CuWO4 were analyzed, the enhanced PEC properties of the CuWO4/CdS/FeOOH photoanode as well as CuWO4/CdS composite were discussed, and the possible photoelectrocatalytic mechanisms

were

also

explored

to

further

understand

the

specific

photoelectrocatalytic process. 2 Experimental section 2.1 Preparation of CuWO4 film The CuWO4 film was fabricated on fluorine-doped tin oxide (FTO) glass substrate through a hydrothermal process. All chemicals used in this study were of analytical grade. Fluorine-doped tin oxide (FTO) substrates were cleaned with distilled water, ethanol, and acetone. In a typical process, the precursor solution was prepared by mixing 0.1 mmol Ammonium metatungstate (NH4)6H2W12O40 and 0.1 mmol copper chloride dihydrate CuCl2·2H2O in deioned water and keeping stirring for 1h. Then, a transparent solution was obtained. Subsequently, The resultant solution was poured into a 25 mL Teflon-lined stainless steel autoclave, when it took up 80% of all capacity. After that the cleaned FTO glass substrate was immersed in the solution at a certain angle (45°) with the conductive surface down. Then the as-prepared autoclave was sealed and maintained at 180 °C for 4 h. After hydrothermal processing, the autoclave was cooled down to room temperature 3



naturally. The resulting sample was obtained and washed with copious amount of alcohol and dried overnight at 60 °C in air. Afterward, the as-washed product was annealed at 500 °C for 2 h at a ramp rate of 2 °C/min in the air environment. Finally, the CuWO4 film was obtained. 2.2 Synthesis of CuWO4/CdS composite The CuWO4/CdS composite structure was synthesized through successive ionic layer adsorption and reaction (SILAR) method. Typically, 0.01 M Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O) and 0.01 M Sodium sulfide ninehydrate (Na2S·9H2O) were dissolved in soft water under stirring to form solution A and B, respectively. The FTO glass with CuWO4 was successively soaked in solution A and B for 20 s each. After each immersion process, the sample was rinsed several cycles with distilled water to remove impurity ions. The aforementioned process is one immersion cycle. After the immersion cycle was repeated by 15 times, the CuWO4/CdS composite structure was produced. 2.3 Fabrication of CuWO4/CdS/FeOOH photoanode The FeOOH was fabricated on the surface of CuWO4/CdS by conventional photoassisted electrodeposition method. A three electrode system was used, with the CuWO4/CdS samples serving as the working electrode, a platinum foil as the counter electrode and saturated Ag/AgCl as the reference electrode. A bias of 0.4 V vs Ag/AgCl was applied to the CuWO4/CdS working electrode in the electrolyte (pH=4) containing FeCl2 (0.1 M) while front illuminating with 100 mW/cm2 of AM1.5G light35.The PEC deposition was stopped after 0.03 C of charge had passed, which took approximately 60s. 2.4 Characterizations The crystalline structures of the samples are characterized by X-ray diffraction (XRD) (Rigaku-D/max-2500 with Cu Kα radiation) at 40 kV and 200 mA. The morphologies and microstructures of the samples are investigated by JEOL JSM-7800F scanning electron microscope (SEM) operated at the accelerating voltage of 10 kV. Meanwhile, the elemental mapping images of the samples are obtained using the energy dispersive spectrum (EDS) detector of the SEM. The Optical absorption capabilities of the photoelectrodes are studied by DU-8B UV-Vis double-beam spectrophotometer. The PEC performance of the samples is investigated in 0.1 M Na2SO4 solution (pH=6.8) using an electrochemical workstation (LK2005A, Tianjin, China), a 4


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standard three-electrode system, the samples employed as the working electrode, a platinum foil as the counter electrode and saturated Ag/AgCl as the reference electrode. The saturated Ag/AgCl electrode is converted to the reversible hydrogen electrode (RHE) in the recorded results according to the Nernst equation as follows: ERHE = EAg/AgCl + 0.0591× pH + 0.1976V

(1)

Additional, a solar simulator (CHF-XM500, 100 mW·cm-2) with an air mass 1.5 G (AM1.5G) filter is used as the illumination source. Electrochemical impedance spectroscopy (EIS) of the samples is carried out using the same electrochemical workstation. 3 Results and discussion The images of CuWO4 films are interrogated using SEM. The top view image of as-obtained sample is exhibited in Figure 1(a). It can be seen that large scale well-aligned CuWO4 was fabricated on FTO substrates vertically without any aggregation. Moreover, the enlarged view SEM image of Figure 1(a) is shown in Figure 1(b). It can be clearly seen that well-aligned CuWO4 displays approximately uniform flower-like structure. From the inserted SEM image in Figure 1(b), we can see clearly that the diameter of “flower” is about 5-12 µm. Additionally, a side view image of CuWO4 is revealed in Figure 1(c), it is indicated that the average thickness of CuWO4 film is about 13 µm, which also illustrates the successful preparation of film rather than powder. Subsequently, the formation mechanism of flower-like CuWO4 is analyzed from the micro level. First, from the reaction process, when (NH4)6H2W12O40 and CuCl2·2H2O are dissolved in an aqueous solution under high temperature and high pressure, the following reactions are produced: NH4+ + H2O ↔ NH3·H2O + H+ +

6H +

H2W12O406-

2-

+ 8H2O → 12WO4 + 24H

(2) +

(3)

Under acidic condition, the nucleation and growth of CuWO4 structures are produced by the following reaction: WO42- + Cu2+ → CuWO4

(4)

Initially, CuWO4 single crystalline crystals are formed by CuWO4 growth units during the primal homogeneous nucleation process, which has a relatively small number of defects. Subsequently, owing to a burst in homogeneous nucleation under hydrothermal condition, a large quantity of CuWO4 single crystalline crystals undergo a further growth process to form aggregated CuWO4 structures. It is noteworthy that 5



the CuWO4 crystals produced during initial growth stage have crystalline grains and boundaries, which are thermodynamically unstable and included more defects. Each grain of the initial CuWO4 crystals may have given priority to grow along one growth direction and a large number of growth units may result in secondary growth from the defects. Moreover, the crystal surfaces containing defects tend to further decrease their energy through surface reconstruction, providing active sites for secondary nucleation36. Thus, complex 3D flower-like CuWO4 structures are fabricated in the hydrothermal solution. The typical XRD patterns of as-prepared CuWO4 crystals are given in Figure 2. The pattern is recorded with 2θ from 10º to 60º. It can be seen from the curve that all diffraction peaks are exclusively corresponded to the triclinic phase structure of CuWO4 crystal with the lattice parameters of a = 4.703 Å, b = 5.839 Å, c = 4.878 Å，β = 92.47º(JCPDS Card No. 72-0616, excepting the peaks marked with “” which are from the FTO substrate. The labels above the peaks are the respective (hkl) indices, including (100), (110), (0-11), (011), (-1-11), (120), (-1-22),(202) which are indexed to the diffraction peaks in curve with 2θ value around 19.1°, 22.9°, 23.2°, 24.1°, 38.3°, 34.5°, 49.6°, 56.2°. As there are no characteristic peaks of other impurities detected in the pattern, a high purity of the samples are indicated. Furthermore, the sharp diffraction peaks manifest the good crystallinity of the prepared crystals. To further verify the component of CuWO4, the selected area elemental mapping spectra of CuWO4 film is conducted as shown in Figure 3. Obviously, the film is composed of three elements, Cu, W, and O without any impurities, which is consistent with XRD analysis above, suggesting the successfully formation of CuWO4 film based on hydrothermal process. The X-ray photoelectron spectroscopy (XPS) measurment is conducted to analyse the composition and chemical state of CuWO4 structure, as shown in Figure S1(a-c). From Figure S1(a), we can see clearly that the Cu 2p1/2 and Cu 2p3/2 lines are found at the binding energies of 954.5 and 934.03 eV with a peak separation of 20.47 eV, because of Cu2+ in the lattice sites of CuWO437. In Figure S1(b), The O 1s peak of the CuWO4 structure at binding energy of 530.33 eV, which agrees well with the value reported for CuWO438. Figure S1(c) indicates that there are one pair of peaks ( W 4f7/2 and W 4f5/2) for CuWO4 film at at binding energy of 35.27 and 37.6 eV. The binding energy of W 5p3/2 is 41.1 eV. These are in line with the data for the binding energies of W6+ oxidation states39. Ultimately, these results suggest that the 3D 6


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flower-like CuWO4 film were producted successfully via hydrothermal method. The UV-Vis optical absorption spectra of the CuWO4 films are presented in Figure 4(a). The date is recorded in the wavelength range of 300-900 nm and the absorption edge of the film is determined from the intersection of the tangent of sharply decreasing region and the baseline40. The absorption spectrum shows that the value of absorption edge of flower-like CuWO4 film approaches to 550 nm, which is wider than that of the previous report41. Such phenomenon may be due to the existence of more defects in the special flower-like structure of CuWO4[42]. Subsequently, as shown in the insert of Figure 4(a), the band gap (Eg) of flower-like CuWO4 film is calculated to be ∼2.24 eV according to the following equation (5)43: (αhν )n=A(hν-Eg)

(5)

where α represents the absorbance coefficient, h is the Planck constant, ν is the light frequency and A is a constant. n is 2 for the direct band gap and 1/2 for the indirect band gap. It is found that the band gap value of flower-like CuWO4 is fit for visible-light absorption. Furthermore, the inserted photograph of CuWO4 further demonstrates the product was coated on the FTO glass substrates successfully through hydrothermal chemical process. Subsequently, the PEC properties of the as-prepared samples are characterized by means of photocurrent measurements. As shown in Figure 4(b), the CuWO4 film shows a negligible background current in the dark condition. Specially, the photocurrent density of CuWO4 film is 0.58 mA/cm2 at 0.8 V vs. RHE under light-on condition. What is noteworthy is that this flower-like CuWO4 photoanode displays higher photocurrent density, compared to different CuWO4 morpholoys

22,41,44

. The

better PEC performance may be due to the improved light absorption, which is discussed above. The special layered junctions of flower-like CuWO4 could shorten charge transport time and distance, which also contributes to the improvement of PEC performances26. To further improve the PEC properties of flower-like CuWO4 samples, on the one hand, CdS is loaded onto the CuWO4 to construct heterojunction via an appropriate band gradient alignment. On the other hand, FeOOH is deposited on the CuWO4/CdS compound to boost the separation of charge in electrode/electrolyte interfaces. The X-ray diffraction (XRD) patterns of the as-produced CuWO4 (a), CuWO4/CdS (b), CuWO4/CdS/FeOOH (c) are exhibited in Figure 5. Compared with the diffraction peaks of CuWO4, the diffraction peak in curve (b) with 2θ value 7



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around 27.3° could be visible, which can be indexed to the (101) plane of CdS (JCPDS Card No. 75-1545). However, there are not the diffraction peaks of FeOOH in curve (c), because FeOOH is amorphous phase30. The SEM images of the as-prepared CuWO4/CdS compound and CuWO4/CdS/FeOOH sample are deplayed in Figure 6(a) and Figure 6(b) respectively. It can be seen from the Figure 6(a) that abundant nanoparticals cover on the surface of CuWO4, which already become rough. The nanoparticals should be CdS according to the XRD patterns in Figure 5. From the Figure 6(b), we can see that there are numerous interconnected nanosheets growing on the surface of CuWO4/CdS composite, which are considered to be FeOOH. To further explore the elements of CuWO4/CdS/FeOOH compound, elemental mapping analysis under SEM observation is conducted and shown in Figure 7. It can be clearly seen that the as-prepared CuWO4/CdS/FeOOH compound is consisted of Cu, W, O, Cd, S and Fe. Obviously, the EDS results not only testify that the nanosheet shown in Figure 6(b) is the microstrcture of FeOOH, which is similar with the previous report45, but also demonstrate that CuWO4/CdS/FeOOH

compound was fabricated

successfully. To understand the optical properties of CuWO4-based photoanodes, the UV-Vis absorption capabilities of them are studied (Figure 8(a)). It can be seen that the absorption edge of CuWO4/CdS composite exhibits a tiny red shift, indicating the improved light absorption in visible light region. However, the light absorbance of CuWO4/CdS/FeOOH compound is slightly higher than CuWO4/CdS composite, suggesting that the modification of FeOOH awkwardly influences the light absorption of CuWO4/CdS composite. Next, the PEC properties of CuWO4-based photoanodes are investigated. As displayed in Figure 8(b), pure CuWO4, CuWO4/CdS and CuWO4/CdS/FeOOH photoanodes show negligible dark currents. Upon irradiation, it is worth noting that the photocurrent densities of the CuWO4/CdS composite and CuWO4/CdS/FeOOH photoanode are measured to be 1.02 mA/cm2 at 0.8 V versus RHE and 2.05 mA/cm2 at 0.8 V versus RHE, respectively. Apparently noticed that the photocurrent density of CuWO4/CdS composite is 1.76 times higher than that of pure CuWO4 (0.58 mA/cm2 at 0.8 V versus RHE), which may result from the fast separation and transportation of the photoexcited electron-hole pairs through the energy level gradient. What’s more, the little improved visible light harvesting and trapping after the modification of CdS, which is consistent with Figure 8(a), also has a certain

effect

on

the

enhanced

photocurrent

density.

8


In

contrast,

the

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CuWO4/CdS/FeOOH photoanode shows the highest photocurrent density of 2.05 mA/cm2 at 0.8 V versus RHE, which is nearly 2 and 3.53 times than CuWO4/CdS and CuWO4 respectively. This result demonstrates that CuWO4/CdS/FeOOH photoanode has high photoelectrocatalytic activity due to the modification of FeOOH has enhanced the photoelectrocatalytic efficiency successfully by reducing the combination

of

charge

in

electrode/electrolyte

interfaces.

Furthermore,

CuWO4/CdS/FeOOH exhibits a remarkable cathodic shift of nearly 100 mV in the onset potential for water oxidation, which is significantly requisite for reducing the external bias needed for water oxidation reaction46. Whereafter, the incident photon-to-current efficiency (IPCE) are conducted under an applied bias of 0.8 V versus RHE (Figure 8(c)) to explore the relationship between incident light absorption and photocurrent. The values are calculated with the following equation (6): IPCE = [1240×J /(λ× Ilight)]×100%

(6)

Where J is the photocurrent density, Ilight is the incident light irradiance, and λ is the incident light wavelength47. From Figure 8(c), we can see that the photocurrent response of CuWO4/CdS/FeOOH photoelectrode is higher than those of other photoanodes, which is in line with Figure 8(b). In order to further elucidate the reasons for the photoelectrochemical behavior above, the electrochemical impedance spectroscopy (EIS) measurements were implemented with an electrochemical workstation. And recorded at frequency range of 10-100 kHz. The EIS Nyquist plots of the CuWO4, CuWO4/CdS composite, and CuWO4/CdS/FeOOH photoelectrode are shown in Figure 8(d). The EIS data are fitted into dotted line using the simplified equivalent circuit model (inset of Figure 8(d)). Herein, Rss is the series resistance of the external circuit, Rint represents the charge transfer

resistance

values

reflecting

to

the

charge

recombination

at

photoanode/electrolyte interface, which consistents with the larger semicircle in the low frequency region, Rpt expresses the charge transfer resistance of the counter electrode/electrolyte interface, corresponding to the smaller semicircle in the high frequency region48-50. the fitted values of Rint for CuWO4, CuWO4/CdS and CuWO4/CdS/FeOOH photoelectrode are 1135.06, 731.74, and 547.80 Ω, respectively. Obviously, the CuWO4/CdS composite electrode presents the smaller impedance than that of pristine CuWO4, indicating that it has faster charge transport rate and the longer service life of charge carriers, which can be attributed to the formed heterojunction interfaces. The further decreased impedance after the CuWO4/CdS was 9



decorated by FeOOH, manifesting that FeOOH can accelerate the kinetic of surface water oxidation by increasing the holes transfer from the electrode surface to electrolyte. These results agree well with the above explanation of the raised photocurrent and in Figure 8(b). Besides, since the external circuit and the counter electrode/electrolyte interface are the same in the entire system, the values of Rss and Rpt have little difference as listed in Table S1. The photostability of photoanodes are also very important for practical applications of the PEC cell. Thus, the stability of the synthesized photoanodes are studied through 1h water-splitting in 0.1 M sodium sulfate solution with pH = 6.8 under simulated sunlight illumination. As suggested by Figure 9, the extremely high stability of 3D CuWO4/CdS/FeOOH photoelectrode is shown over the entire duration with perfect photocurrent retention after 1 h light illumination. By contrast, the pristine CuWO4 film retains a steadystate current density of 0.35 mA/cm2 at 0.8 V versus RHE after 3600 s, which is about 39% loss. For CuWO4/CdS composite, the photocurrent density decreases gradually with time, and is, after 1 h light illumination, 58% of its maximum value. This photocurrent decay is due to the poor antioxidation ability of the CdS in the photohole surroundings which could induce the self-oxidative decomposition of CdS46. To further explore the degree of decomposition of CdS after stability test, the EDS are performed on CuWO4/CdS and CuWO4/CdS/FeOOH photoelectrodes before and after the light stability test, as shown in Figure S2. Distinctly, for CuWO4/CdS, the atomic percentage of CdS reduces from 11.39% to 7.77% after 1h light stability test, which agrees well with the self-oxidative decomposition of CdS in the photohole environment. On the contrast, for CuWO4/CdS/FeOOH the atomic percentage of CdS displays insignificant variation, after 1h light stability test, conresponding to the stable photocurrent in Figure 9. These results indicate that the introduction of FeOOH cocatalyst on CuWO4/CdS composite for driving water oxidation is also efficient for resisting the photocorrosion due to the largely improvement of the kinetic transport of photogenerated holes and further protection of FeOOH nanosheets51. Hence, FeOOH nanosheets not only act as a cocatalyst, but also a protective layer against photocorrosion. Based on the aforementioned discussions, a schematic diagram of 3D CuWO4/CdS/FeOOH composite applied in overall water splitting is proposed in Figure 10. Both of the CuWO4 and CdS absorb the photons and generate electron-hole pairs under light illumination. The photogenerated electrons can easily move from the 10


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conduction band of CdS to the conduction band of CuWO4, as the conduction band of CdS is more negative than the corresponding band of CuWO4. And then the excited electrons in CuWO4 transform onto the conductive FTO substrate and flow to the Pt electrode via external circuit to generate hydrogen. Concomitant with this, the photogenerated holes from the valence band of CuWO4 migrate to that of CdS. In addition, after combined the CuWO4/CdS electrode with FeOOH cocatalyst, the driving force of water oxidation reaction of CuWO4/CdS/FeOOH composite is accelerated due to the holes from CdS can transform to FeOOH leading to the separation of carriers. And accumulated holes in FeOOH catalyze water oxidation to form oxygen thanks to its valence band potential is more positive than the water oxidation potential. In this way, the accumulated holes could shift to the electrode/electrolyte interface quickly to reduce the electron-hole recombination rate, finally, the photoelectrochemistry performance is increased. 4 Conclusion In summary, the special 3D flower-like CuWO4 architecture was fabricated on FTO substrate through a simple hydrothermal chemical method. It was founded that the flower-like CuWO4 exhibits higher photocurrent density of 0.58 mA/cm2 at 0.8 V versus RHE in contrast to some reported CuWO4 photoanodes for PEC water splitting. When coupled with CdS and FeOOH layers, the resulting CuWO4/CdS/FeOOH photoanodes exhibit further improved PEC performance with a large photocurrent density about 2.05 mA/cm2 at 0.8 V versus RHE and excellent operation stability. The extraordinary PEC performance may be arising from multiple factors as follows: (i) ideal band gap, (ii) improved light absorption, (iii) efficient charge-hole pairs separation and collection, and (iv) the unique features of the flower-like CuWO4 contribute to the excellent PEC activity for water splitting. In addition, the composite CuWO4/CdS/FeOOH as a photoanode will offer new insights into PEC water splitting. Acknowledgements The authors gratefully acknowledge financial support from Science Funds of Tianjin for Distinguished Young Scholar (No. 17JCJQJC44800) and Natural Science Foundation of Tianjin (16JCYBJC17900).

11



Supplemental Information Figure S1 The XPS spectra of pure CuWO4 structure: (a) Cu(2p), (b) O(1s), (c) W(4f), respectively Figure S2 EDS analysis spectrums of the (a) CuWO4/CdS, (c) CuWO4/CdS/FeOOH composites before 1 h stability test and (b) CuWO4/CdS, (d) CuWO4/CdS/FeOOH photoanodes after 1 h stability test Table S1 EIS fitting results for CuWO4, CuWO4/CdS and CuWO4/CdS/FeOOH photoanodes as shown in Figure 8(d)

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Photoelectrochemical Properties of Reactively Cosputtered Copper Tungstate for 13



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Lists of the Figures: Figure 1 Typical top view SEM image (a), Large-view SEM image and (inset) enlarged-view image (b), side view SEM image (c) of as-obtained CuWO4 film with hydrothermal time for 4 h at 180 °C Figure 2 XRD of flower-like CuWO4 structure with hydrothermal time for 4 h at 180 °C Figure 3 SEM image and element distribution mapping of flower-like CuWO4 structure with hydrothermal time for 4 h at 180 °C Figure 4 UV-Vis absorbance spectra (a), the derived Tauc plot corresponding to allowed indirect (inset) and Photocurrent density-voltage curve (b) of flower-like CuWO4 measured at 0.8 V vs. RHE in 0.1 M Na2SO4 Figure 5 XRD patterns of flower-like CuWO4 (a), CuWO4/CdS (b) and CuWO4/CdS/FeOOH (c) Figure

6

SEM

images

of

CuWO4/CdS

composite

structure

(a)

and

CuWO4/CdS/FeOOH photoanode (b) Figure 7 SEM image and element distribution mapping of CuWO4/CdS/FeOOH composite structure Figure 8 Photoelectrochemical performance of flower-like CuWO4, CuWO4/CdS composite and 3D CuWO4/CdS/FeOOH. (a) UV-vis absorption spectrum (b) photocurrent density versus voltage curves measured at 0.8 V vs. RHE, (c) incident photon-to-current efficiency plots and (d) electrochemical impedance spectroscopy Figure

9

Steady-state

photocurrent curves

of

CuWO4,

CuWO4/CdS and

CuWO4/CdS/FeOOH photoelectrodes measured in 0.1 M sodium phosphate electrolyte (pH = 6.8) at 0.8 V versus RHE under consistent one sun illumination for 3600 s Figure 10 Schematic diagram of 3D CuWO4/CdS/FeOOH photoanode for PEC water splitting

17



Figure 1 Typical top view SEM image (a), Large-view SEM image and (inset) enlarged-view image (b), side view SEM image (c) of as-obtained CuWO4 film with

(110) (0-11) (011)

hydrothermal time for 4 h at 180 °C

♦ ♦

(202)

(120)

(-1-22)

♦

♦ FTO

(-1-11)

(100)

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

JCPDS NO.72-0616

10

20

30 40 2θ(degree)

50

60

Figure 2 XRD of flower-like CuWO4 structure with hydrothermal time for 4 h at 180 °C

Figure 3 SEM image and element distribution mapping of flower-like CuWO4 structure with hydrothermal time for 4 h at 180 °C

18


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Figure 4 UV-Vis absorbance spectra (a)，the derived Tauc plot corresponding to allowed indirect (inset) and Photocurrent density-voltage curve (b) of flower-like

(c)

♥ CdS

♥

(110) (b)

♦

♦

(-1-22)

♦

(120)

♥

(202)

(a)

♦ FTO

(101) (101)

(110)

CuWO4 measured at 0.8 V vs. RHE in 0.1 M Na2SO4

(100) (110) (0-11) (011) (-1-11)

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


Intensity(a.u.)

Page 19 of 22

JCPDS NO.72-0616

10

20

30 40 2θ(degree)

50

60

Figure 5 XRD patterns of flower-like CuWO4 (a), CuWO4/CdS (b) and CuWO4/CdS/FeOOH (c)

Figure 6 SEM images of CuWO4/CdS composite structure (a) and CuWO4/CdS/FeOOH photoanode (b)

19



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w

o

Cd

Cu

S

Fe

Figure 7 SEM image and element distribution mapping of CuWO4/CdS/FeOOH composite structure

Figure 8 Photoelectrochemical performance of flower-like CuWO4, CuWO4/CdS composite and 3D CuWO4/CdS/FeOOH. (a) UV-vis absorption spectrum (b) photocurrent density versus voltage curves measured at 0.8 V vs. RHE, (c) incident photon-to-current efficiency plots and (d) electrochemical impedance spectroscopy

20


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


Photocurrent density(mA/cm2)

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2.4 2.0 CuWO4/CdS/FeOOH

1.6 1.2 0.8

CuWO4/CdS

0.4 CuWO4

0.0 0

600

1200 1800 2400 3000 3600 Time (s)

Figure 9 Steady-state photocurrent curves of CuWO4, CuWO4/CdS and CuWO4/CdS/FeOOH photoelectrodes measured in 0.1 M sodium phosphate electrolyte (pH = 6.8) at 0.8 V versus RHE under consistent one sun illumination for 3600 s

Figure 10 Schematic diagram of 3D CuWO4/CdS/FeOOH photoanode for PEC water splitting

21



110x62mm (72 x 72 DPI)


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A promising 3D flower-like CuWO4 photoanode modified with CdS

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