CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction

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CuWO4 Nanoflake Arrays Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation Wen Ye, Fengjiao Chen, Feipeng Zhao, Na Han, and Yanguang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03176 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 25, 2016

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ACS Applied Materials & Interfaces

CuWO4 Nanoflake Arrays Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation Wen Ye†, Fengjiao Chen†, Feipeng Zhao, Na Han and Yanguang Li*

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China Correspondence to: [email protected]. †The two authors contribute equally to this paper

Abstract: Over recent years, tremendous efforts have been invested on the search and development of active and durable semiconductor materials for photoelectrochemical (PEC) water splitting, particularly for photoanodes operating under highly oxidizing environment. CuWO4 is an emerging candidate with suitable band gap and high chemical stability. Nevertheless, its overall solar-to-electricity remains low owing to the inefficient charge separation process. In this work, we demonstrate that this problem can be partly alleviated through designing three-dimensional hierarchical nanostructures. CuWO4 nanoflake arrays on conducting glass are prepared from the chemical conversion of WO3 templates. Resulting electrode materials possess large surface areas, abundant porosity and small thickness. Under illumination, our CuWO4 nanoflake array photoanodes exhibit an anodic current density of ~0.4 mA/cm2 at the thermodynamic potential of water splitting in pH 9.5 potassium borate buffer — the largest value among all available CuWO4-based photoanodes. In addition, we demonstrate that their performance can be further boosted to >2 mA/cm2 by coupling with a solution-cast BiVO4 film in a heterojunction configuration. Our study

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ACS Paragon Plus Environment


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unveils the great potential of nanostructured CuWO4 as the photoanode material for PEC water oxidation. Keywords:

Photoelectrochemical water oxidation, CuWO4, nanoflakes, heterojunction, charge

separation

Photoelectrochemical (PEC) water splitting represents a promising method for hydrogen production using energy directly harvested from sunlight, thus providing a sustainable route to convert solar energy into storable chemical fuels.1-3 Its future success largely hinges on the development of suitable semiconductor materials which are efficient in light absorption, electron-hole separation and charge transport.4,

5

Of particular challenge is the design of

high-performance photoanodes for PEC water oxidation — not only because of the sluggish reaction kinetics, but also due to the highly oxidizing working environment which poses a strict constraint on the selection of possible n-type semiconductors.5, 6 The majority of photoanode materials available currently are transition metal oxides.3, 7 Among them, TiO2,8, 9 WO3,10, 11 Fe2O312, 13 and BiVO414, 15 have attracted the most focus, but yet are still short of meeting people’s expectations owing to their inherent limitations. For example, TiO2 and WO3 are wide band gap semiconductors (>2.7 eV), and can only utilize a small portion of the solar spectrum; α-Fe2O3 hematite has a suitable band gap (2.1 eV), but are notorious for its low charge carrier mobility (10-2~10-1 cm2/Vs) and short hole diffusion length (2-20 nm);16 BiVO4 has both proper band gap (2.4 eV) and reasonable diffusion length (~100 nm), but unfortunately suffers from poor electron-hole separation.14, 17 As a result, growing research efforts now have been directed toward the search of new photoanode materials.3, 4 2


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CuWO4 is an n-type semiconductor with an indirect band gap of 2.3 eV and a large window of pH stability. Its PEC properties were first explored in early 1980s.18, 19 However, it is not until last three years that the potential of CuWO4 in PEC water oxidation has received considerable attention.20-22 Several groups have investigated CuWO4 thin films prepared from electrodeposition,21 spin coating,20 sputtering deposition,22 hydrothermal synthesis23 or atomic layer deposition,24 and attempted to optimize their PEC performances via chemical doping25, 26 or the incorporation of plasmonic nanoparticles.27 Nevertheless, most of these results have reported low photocurrent densities in the range of 0.1-0.2 mA/cm2 at the thermodynamic potential of water oxidation, making it unable to rival conventional, well established photoanode systems. Detailed studies reveal that the poor overall efficiency is a direct consequence of the slow carrier mobility and recombination at the surface of bulk CuWO4 due to midgap states associated with Cu d orbital electrons.28 The presence of midgap states disfavors the use of thick electrode films which, however, are necessary for efficient light absorption. We propose that nanostructuring CuWO4 is a possible solution to solve the above dilemma. Light scattering at the nanostructured interfaces can effectively increase the optical thickness of the photoelectrodes, leading to the efficient light absorption of thin films.7 The reduced dimensions of nanostructured photoelectrodes can also shorten the diffusion pathway of photo-generated holes to the electrode/electrolyte interface, and therefore mitigate their recombination.7 Unfortunately, as far as we are aware, the preparation of nanostructured CuWO4 photoanode for PEC water oxidation remains elusive at present, probably due to the complexity in the morphology control of ternary oxide semiconductors. To this end, we report in this article the preparation of porous CuWO4 nanoflake array photoanodes from the chemical conversion of WO3 nanoflake arrays. The final products are featured 3



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with three dimensional hierarchical microstructures with thin nanoflake thickness and abundant porosity. PEC measurements suggest that these nanoflake array photoanodes deliver a photocurrent density of ~0.4 mA/cm2 at the thermodynamic potential of water oxidation. Moreover, when coupled with a solution-cast BiVO4 layer, the resulting heterojuction photoanodes exhibit further improved PEC performance with a large photocurrent density over 2 mA/cm2 and excellent operation stability. CuWO4 nanoflake arrays on fluorine-doped tin oxide (FTO) conducting glass were prepared in two sequential steps as schematically illustrated in Figure 1. In the first step, WO3 nanoflake arrays were grown on seeded FTO glass following an established hydrothermal method.10 Resulting electrodes were then fully impregnated with copper acetate, and calcined at 450oC for 1 h. During the heating, the fast diffusion of Cu2+ ions into WO3 lattices results in the formation of CuWO4. This process was accompanied by the prominent color change of electrode films from yellowish white to bright yellow (Figure 1). Residual CuO was finally removed by washing the electrodes with dilute HCl solution (see Experimental for more details). The presence of atmospheric O2 is critical to the success of the conversion reaction. When the same experiment is carried out under Ar atmosphere, we detect a mixture of WO3, Cu2O and Cu from the as-annealed electrode, and only WO3 from the acid-washed electrode (see the Supporting Information, Figure S1). The successful formation of CuWO4 via the second-step solid state reaction was borne out by a range of spectroscopic characterizations. Despite some similarity between their x-ray diffraction (XRD) patterns, the complete conversion from monoclinic WO3 (PDF# 894476) to triclinic CuWO4 (PDF# 731823) are unambiguously manifested by the emergence of a set of new diffraction peaks below 20o as well as between 30-32o, which are unique to triclinic CuWO4 (Figure 2a). There is no diffraction peak assignable to CuO. Raman spectrum of final products also exhibits fingerprint 4


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vibration bands of CuWO4 (Figure 2b). For example, the strongest band located at 905 cm-1 is attributed to the symmetric stretching of W-O bonds in the distorted WO6 octahedra of the triclinic lattice, whereas bands at 778 and 806 cm−1 are resulted from the Eg mode of W–O.29 Moreover, insights were also garnered from x-ray photoelectron spectroscopy (XPS). Figure 2c compares the W 4f core-level spectra of WO3 and CuWO4. It is interesting to note that the intense W 4f7/2 and W 4f5/2 peaks of the latter are displaced about 0.3-0.4 eV toward lower binding energy relative to those of the former. This observation is indicative of reduced oxidation state of W species upon the incorporation of Cu in the final product, and well consistent with previous findings.24, 30 In addition, the presence of Cu is attested by its pronounced XPS signals (Figure 2d). The positions of its 2p3/2 and 2p1/2 peaks as well as satellite peaks are also typical to Cu in CuWO4.30 The Cu to W atomic ratio is estimated to be 1.04:1, in reasonably good agreement with the formula of CuWO4. The microstructure of CuWO4 was interrogated under scanning and transmission electron microscopy (SEM/TEM). As shown in Figure 3a, the final product is entirely comprised of dense, vertically aligned nanoflake arrays grown on FTO over large areas. Individual nanoflakes have lateral dimensions of ~1 µm and estimated thickness of 20-30 nm (Figure 3b). They are riddled with abundant mesopores (Figure 3b, c). Comparison with WO3 nanoflake arrays from the first step suggests that the mesoporosity is inherited from the WO3 precursor (see the Supporting Information, Figure S2). It is remarkable that, despite their small thickness, these nanoflake arrays survive the solid state reaction with the retention of hierarchical porous microstructure. In Figure 3d, high resolution TEM analysis of CuWO4 nanoflakes reveals clear lattice fringes assignable to the (100) plane of CuWO4. Energy dispersive microscopy (EDS) confirms the high spatial correlation between Cu and W species throughout the material (see the Supporting Information, Figure S3). The 5



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combination of large surface areas, small thickness and high crystallinity make our CuWO4 nanoflake arrays promising electrode materials for photoelectrochemical water splitting. We next investigated the electronic structure of CuWO4 nanoflake arrays by UV-vis spectrum. Their reflectance and transmission spectra were first collected using an integrating sphere, and then manually converted to absorbance spectrum as illustrated in Figure 4a. Compared to WO3, CuWO4 exhibits an obvious red shift of the optical absorption edge from 450 nm to 530 nm in the visible light range. Their gradual increase in absorbance with decreased wavelength indicates that both WO3 and CuWO4 have indirect band gaps. The insert of Figure 4a represents Tauc plots of the two semiconductor materials by assuming indirection optical transition. From the extrapolation of their linear regions, the optical band gaps were determined to be 2.67 eV and 2.22 eV for WO3 and CuWO4, respectively, in a good agreement of earlier reports.21, 31 These experimental observations are also supported by previous first principle calculations, which suggest that the hybridization of Cu 3d and O 2p orbitals in the valence band of CuWO4 raises the position of valance band by about 0.5 eV relative to pure WO3, while their conduction bands have the dominant contribution from W and therefore are at approximately the same energy level.32 The reduced band gap renders CuWO4 a better light absorbing material than WO3. To access their performance toward PEC water oxidation, CuWO4 nanoflake array electrodes were employed as the photoanodes in a standard three-electrode system. The electrolyte in use is 1 M potassium borate (KBi) buffer with pH carefully adjusted to 9.5. Under 100 mW/cm2 AM 1.5 G illumination from the backside, CuWO4 photoanodes display obvious anodic responses with an onset potential of ~0.7 V versus reversible hydrogen electrode (RHE, the same afterwards) as shown in Figure 4b. At the thermodynamic potential of water oxidation (1.23 V), they deliver a photocurrent 6


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density of ~0.4 mA/cm2. This value is better than unconverted WO3 (see the Supporting Information, Figure S4), and superior to most polycrystalline CuWO4 electrodes, which are typically in the range of 0.1-0.2 mA/cm2 at the same potential.20-26, 33 The same nanoflake array electrodes were also evaluated in 0.1 M potassium phosphate (KPi, pH = 4.5) buffer and in 0.1 M H2SO4 (pH = 0.87). As shown in Figure S5, in both electrolytes they exhibit comparable photocurrent densities. In Figure 4c, we collected the incident photon-to-current efficiency (IPCE) spectrum of CuWO4 photoanode under an applied bias of 1.23 V. It has a similar profile to the UV-vis absorption spectrum. The IPCE recorded at 400 nm is ~13%, a marked enhancement over the previous best value of ~6% reported by Bartlett et al. for electrochemically deposited CuWO4 electrodes.21 We believe that the much improved PEC activity directly results from their advantageous nanoflake array

microstructure.7

Since

the

charge

separation

mainly

takes

place

near

the

semiconductor-electrolyte interface for PEC reactions, the enlarged surface areas associated with the three-dimensional hierarchical microstructure is expected to largely expedite the reaction rate. Moreover, we find that treating CuWO4 with mild reducing agents such as poly(vinylpyridine) (PVP) can further promote the photocurrent density presumably by increasing the charge carrier concentration.34 PVP-treated CuWO4 photoanodes are capable of delivering >0.6 mA/cm2 at 1.23 V. Despite its promising PEC onset potential and photocurrent density, PVP-treated CuWO4 is not as stable as pristine CuWO4. A post-treatment calciation of the electrode in air at 500oC also completely nullifies the effect of PVP treatment (see the Supporting Information, Figure S6). Besides their improved activity, our CuWO4 nanoflake array photoanode also demonstrate remarkable operation stability. Its chronoamperometry (i ~ t) curve was collected under chopped illumination of AM 1.5 G at 1.23 V. As shown in Figure 4d, the photocurrent density closely matches 7



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that of the linear sweep voltammetry curve (Figure 4b), and exhibits little change over repeated cycles. The photocurrent density is also shown to be stable for at least 5 h under continuous illustration (see the Supporting Information, Figure S7). These observations are in sharp contrast to WO3, which is known to be unstable in neutral electrolytes even at the dark condition.35 For example, the same experiment performed on the WO3 nanoflake array photoanode reveals that its photocurrent density drops significantly from the initial 0.3 mA/cm2 to 0.7 mA/cm2 at 1.23 V, well consistent with previous observations,44-46 whereas their combination leads to an enhanced photocurrent density of ~1.7 mA/cm2. This value here is significantly improved from the previous study by Herring et al. on the same type of heterojunction, which reported a photocurrent density of ~0.6 mA/cm2 in 1 M Na2SO4 (pH = 7) and ~1.6 mA/cm2 in 1 M NaHCO3 (pH = 7).43 Please note that BiVO4 generally has much better PEC performance in HCO3- than other electrolyte 10


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for a reason not well understood. When our CuWO4/BiVO4 heterojunction is evaluated in 1 M NaHCO3, the photocurrent increases to 2.3 mA/cm2 (see the Supporting Information, Figure S10). Moreover, we found that the deposition of additional 5 nm Ni film on the top of CuWO4/BiVO4 photoanode as the water oxidation electrocatalyst further improved the photocurrent density to ~2.1 mA/cm2 at 1.23 V in KBi. This value is much larger than CuWO4 or BiVO4 alone with 5 nm Ni film (see the Supporting Information, Figure S11), and thereby unambiguously underlines the advantage of CuWO4/BiVO4 heterojunction over corresponding single junctions. At last, we demonstrate that CuWO4/BiVO4 heterojuction photoanodes with or without the Ni catalyzing layer have the required operation stability. When biased at 1.23 V, both of them exhibit little loss of PEC photocurrent density over the entire course of evaluation under both periodic and continuous illumination (Figure 5d and Figure S12 in Supporting Information). In summary, we reported a chemical conversion method to prepare three-dimensional hierarchical CuWO4 nanoflake arrays for PEC water oxidation. The final products were comprised of dense, vertically aligned nanoflakes with abundant porosity and small thickness. Compared to conventional CuWO4 polycrystalline films, our nanoflake array photoanodes exhibited more than twice larger IPCE and photocurrent density — achieving ~0.4 mA/cm2 at the thermodynamic potential of water oxidation. Detailed analysis revealed that the enhanced performance was a direct result of the improved charge separation efficiency associated with their advantageous microstructures. In an attempt to further increase the charge separation efficiency, we designed CuWO4/BiVO4 heterojunction by drop casting a BiVO4 thin film atop CuWO4 nanoflake arrays. Resulting photoanodes delivered large photocurrent density over 2 mA/cm2 and excellent operation stability. Albeit that at the current moment CuWO4 is still unable to rival existing mature photoanode 11



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systems, our study exposes its potentials and therefore provides a new opportunity in the pursuit of advanced electrode materials for high-performance photo water splitting.

Acknowledgements We acknowledge supports from the National Natural Science Foundation of China (51472173 and 51522208),

the

Natural

Science

Foundation

of

Jiangsu

Province

(BK20140302

and

SBK2015010320), the Priority Academic Program Development of Jiangsu Higher Education Institutions and Collaborative Innovation Center of Suzhou Nano Science and Technology.

Supporting Information available: Experimental details, additional characterization, and electrochemical data. The Supporting Information is available free of charge on the ACS Publications website.

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29. Selvan, R. K.; Gedanken, A., The Sonochemical Synthesis and Characterization of Cu1-xNixWO4 Nanoparticles/Nanorods and Their Application in Electrocatalytic Hydrogen Evolution. Nanotechnology 2009, 20 (10), 105602. 30. Khyzhun, O. Y.; Strunskus, T.; Cramm, S.; Solonin, Y. M., Electronic Structure of CuWO4: XPS, XES and NEXAFS Studies. J. Alloys Compd. 2005, 389 (1-2), 14-20. 31. Hill, J. C.; Choi, K.-S., Synthesis and Characterization of High Surface Area CuWO4 and Bi2WO6 Electrodes for Use as Photoanodes for Solar Water Oxidation. J. Mater. Chem. A 2013, 1 (16), 5006. 32. Kuzmin, A.; Kalinko, A.; Evarestov, R. A., Ab Initio LCAO Study of the Atomic, Electronic and Magnetic Structures and the Lattice Dynamics of Triclinic CuWO4. Acta Mater. 2013, 61 (1), 371-378. 33. Valenti, M.; Dolat, D.; Biskos, G.; Schmidt-Ott, A.; Smith, W. A., Enhancement of the Photoelectrochemical Performance of CuWO4 Thin Films for Solar Water Splitting by Plasmonic Nanoparticle Functionalization. J. Phys. Chem. C 2015, 119 (4), 2096-2104. 34. Li, W. J.; Da, P. M.; Zhang, Y. Y.; Wang, Y. C.; Lin, X.; Gong, X. G.; Zheng, G. F., WO3 Nanoflakes for Enhanced Photoelectrochemical Conversion. ACS Nano 2014, 8 (11), 11770-11777. 35. Heumann, T.; Stolica, N., Electrochemical Behavior of Tungsten. I. Dissolution of Tungsten and Tungsten Oxide in Buffer Solutions. Electrochim. Acta 1971, 16 (5), 643-51. 36. Dotan, H.; Sivula, K.; Gratzel, M.; Rothschild, A.; Warren, S. C., Probing the Photoelectrochemical Properties of Hematite (α-Fe2O3) Electrodes Using Hydrogen Peroxide as a Hole Scavenger. Energy Environ. Sci. 2011, 4 (3), 958-964. 37. Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S., Heterojunction BiVO4/WO3 Electrodes for Enhanced Photoactivity of Water Oxidation. Energy Environ. Sci. 2011, 4 (5), 1781. 38. Mayer, M. T.; Du, C.; Wang, D., Hematite/Si Nanowire Dual-Absorber System for Photoelectrochemical Water Splitting at Low Applied Potentials. J. Am. Chem. Soc. 2012, 134 (30), 12406-9. 39. Shaner, M. R.; Fountaine, K. T.; Ardo, S.; Coridan, R. H.; Atwater, H. A.; Lewis, N. S., Photoelectrochemistry of Core–Shell Tandem Junction N–p+-Si/N-WO3 Microwire Array Photoelectrodes. Energy Environ. Sci. 2014, 7 (2), 779-790. 40. Yourey, J. E.; Kurtz, J. B.; Bartlett, B. M., Water Oxidation on a CuWO4–WO3 Composite Electrode in the Presence of [Fe(CN)6]3–: Toward Solar Z-Scheme Water Splitting at Zero Bias. J. Phys. Chem. C 2012, 116 (4), 3200-3205. 41. Nam, K. M.; Cheon, E. A.; Shin, W. J.; Bard, A. J., Improved Photoelectrochemical Water Oxidation by WO3/CuWO4 Composite With a Manganese Phosphate Electrocatalyst. Langmuir 2015, 31 (39), 10897-903. 42. Chen, H.; Leng, W.; Xu, Y., Enhanced Visible-Light Photoactivity of CuWO4 through a Surface-Deposited CuO. J. Phys. Chem. C 2014, 118 (19), 9982-9989. 43. Pilli, S. K.; Deutsch, T. G.; Furtak, T. E.; Brown, L. D.; Turner, J. A.; Herring, A. M., BiVO4/CuWO4 Heterojunction Photoanodes for Efficient Solar Driven Water Oxidation. Phys. Chem. Chem. Phys. 2013, 15 (9), 3273-3278. 44. Berglund, S. P.; Flaherty, D. W.; Hahn, N. T.; Bard, A. J.; Mullins, C. B., Photoelectrochemical Oxidation of Water Using Nanostructured BiVO4 Films. J. Phys. Chem. C 2011, 115 (9), 3794-3802. 45. Su, J.; Guo, L.; Yoriya, S.; Grimes, C. A., Aqueous Growth of Pyramidal-Shaped BiVO4 Nanowire Arrays and Structural Characterization: Application to Photoelectrochemical Water Splitting. Cryst. Growth Des. 2010, 10 (2), 856-861. 46. Iwase, A.; Kudo, A., Photoelectrochemical Water Splitting Using Visible-Light-Responsive BiVO4 Fine Particles Prepared in an Aqueous Acetic Acid Solution. J. Mater. Chem. 2010, 20 (35), 7536.

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Figure 1. Schematic illustration of the conversion process from WO3 nanoflake arrays to CuWO4 nanoflake arrays, CuWO4/BiVO4 heterojunction and CuWO4/BiVO4/Ni on the FTO glass, together with their corresponding digital pictures.

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Figure 2. Spectroscopic characterizations of CuWO4 nanoflake arrays. (a) XRD patterns of WO3 and converted CuWO4 nanoflake arrays on the FTO glass. Asterisks indicate signals from the substrate. (b) Raman spectra of WO3 and CuWO4 nanoflake arrays. (c) XPS W 4f core-level spectra of WO3 and CuWO4 nanoflake arrays. (d) XPS Cu 2p core-level spectrum of CuWO4 nanoflake arrays.

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Figure 3. Microscopic characterizations of CuWO4 nanoflake arrays. (a,b) SEM images of CuWO4 nanoflake arrays on the FTO glass, and (c,d) TEM images of individual CuWO4 nanoflakes.

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Figure 4. Optical and photoelectrochemical properties of CuWO4 nanoflake arrays. (a) UV-vis absorption spectra and the optical band gap energy determination of WO3 and CuWO4 nanoflake arrays. (b) Linear sweep voltammetry curve of CuWO4 nanoflake arrays under chopped illumination, (c) IPCE curve of CuWO4 nanowire arrays under an applied bias of 1.23 VRHE. (d) Chronoamperometric curves of WO3 and CuWO4 films under chopped illumination and an applied bias of 1.23 VRHE. (e) Linear sweep voltammetry curves of CuWO4 nanoflake arrays under light or dark conditions and with or without the addition of 0.1 M H2O2 to the electrolyte. (f) Charge separation efficiency and interfacial charge transfer efficiency for CuWO4 nanoflake arrays at different working potentials.

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Figure 5. Photoelectrochemical properties of CuWO4/BiVO4 and CuWO4/BiVO4/Ni. (a) Schematic illustration of the band alignment across the CuWO4/BiVO4 heterojunction. (b) SEM image of CuWO4/BiVO4 heterojunction film. (c) Linear sweep voltammetry curves of CuWO4, BiVO4, CuWO4/BiVO4 and CuWO4/BiVO4/Ni films under chopped illumination. (d) Chronoamperometric responses of CuWO4/BiVO4 and CuWO4/BiVO4/Ni films under chopped illumination and an applied bias of 1.23 VRHE.

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CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction

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