Ni(II)-doped CuWO4 photoanodes with enhanced photoelectrocatalytic activity

CuWO4 has emerged in the last years as a ternary metal oxide material for photoanodes application in photoelectrochemical cells, thanks to its relatively narrow band gap, high stability and selectivity toward the oxygen evolution reaction, though largely limited by its poor charge separation efficiency. Aiming at overcoming this limitation, we investigate here the effects that Cu(II) ion substitution has on the photoelectrocatalytic (PEC) performance of copper tungstate. Optically transparent CuWO4 thin-film photoanodes, prepared via spin coating and containing different amounts of Ni(II) ions, were fully characterized via UV-Vis spectroscopy, XRD and SEM analyses, and their PEC performance was tested via linear sweep voltammetry, incident photon to current efficiency and internal quantum efficiency analyses. From tests performed in the presence of a hole scavenger-containing electrolyte, the charge injection and separation efficiencies of the electrodes were also calculated. Pure-phase crystalline and/or heterojunction materials were obtained with higher PEC performance compared to pure CuWO4, mainly due to a significantly enhanced charge separation efficiency in the bulk of the material.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

Oriented CuWO4 Films for Improved Photoelectrochemical Water Splitting

Hydrogen generation through photoelectrochemical (PEC) technology is one of the most appropriate ways for delivering sustainable fuel. Simultaneously, anisotropic properties will be exhibited by the materials with low crystal symmetry, allowing the tuning of the PEC properties by controlling the crystallographic orientation and exposed facets. Therefore, we synthesized copper tungstate films (CuWO4) with highly exposed (100) crystal facets by regulating anions in the precursor solution. According to experimental characterization and density functional theory calculations, the CuWO4 film with a high exposure ratio of the (100) crystal facet has promoted charge transport with trap-free mode and reduced recombination of electrons and holes. Meanwhile, the oxygen evolution reaction is promoted on the (100) facet because of the relatively low energy barrier. Compared to the CuWO4 with other mixed exposure facets, CuWO4 with a highly exposed (100) facet presents a twofold current density (0.38 mA/cm2) and one-fifteenth electron transit time (0.698 ms) and also has great stability (more than 6 h). These results provide an easy way to enhance the PEC performance by modulating the exposure facets of the film electrode.

Combinatorial Screening of Cu-W Oxide-Based Photoanodes for Photoelectrochemical Water Splitting

Metal oxide libraries for photoanodes for the oxygen evolution reaction (OER) were generated by printing a metal salt solution in an array layout, followed by calcination to yield 22 ternary metal oxide systems. The libraries included a ternary metal cation system based on CuWO₄ with one out of eight transition or posttransition metal ions Cr, Mn, Fe, Co, Ni, Zn, Bi, and Ga in different overall atomic ratios. The photocatalyst libraries were screened by scanning photoelectrochemical microscopy for the highest anodic photocurrents. Array elements that showed promising performance were printed in another set of eight libraries with smaller increments of overall composition. Improved performance with respect to CuWO₄ was found for Ga, Co, and Ni as the third element. A comparison of the most active composition of those arrays within one library showed the highest activity for Cu₄₈Ga₃W₄₉O_x. Printing spots of identical composition (Cu₄₈Ga₃W₄₉O_x, Cu₄₄Ni₉W₄₇O_x, and Cu₄₄Co₉W₄₇O_x) over a larger area facilitated further characterization by X-ray photoelectron spectroscopy ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction, scanning electron microscopy, chopped light voltammetry, and scanning electrochemical microscopy for the OER. High and stable steady-state photocurrents were generated in a photoelectrochemical cell for all three electrodes even at a low constant bias voltage. The best overall photoanode composition Cu₄₈Ga₃W₄₉O_x showed currents that were 36 times higher than the currents of the binary Cu₅₀W₅₀O_x system. Significant n-doping was found by UPS valence band spectra for Ga-containing materials.

Ultrafast Charge Carrier Dynamics in CuWO4 Photoanodes

CuWO₄ is a ternary metal oxide semiconductor with promising properties for photoelectrochemical (PEC) water splitting and solar light conversion, due to its quite low band gap (2.3 eV) and high stability in an alkaline environment. Aiming at understanding the origin of the relatively low PEC efficiency attained with CuWO₄ photoanodes, we here investigate transparent CuWO₄ electrodes prepared by a simple solution-based method through the combination of femtosecond transient absorption spectroscopy with electrochemical, PEC, and photochromic characterizations. The very fast recombination dynamics of the charge carriers photogenerated in CuWO₄, which is the reason for its low efficiency, is discussed in relation with its PEC performance and with the recently calculated band structure of this material, also in comparison with the behavior of other semiconductor oxides employed in PEC applications, in particular Fe₂O₃.

Crystal Growth and Design of Disk/Filament ZnO-Decorated 1D TiO2 Composite Ceramics for Photoexcited Device Applications

Disk- and filament-like ZnO crystals were decorated on one-dimensional TiO2 nanostructures (TiO2-ZnO) through various integrated physical and chemical synthesis methods. The morphology of the ZnO crystals on TiO2 varied with the chemical synthesis method used. ZnO nanodisks decorated with TiO2 nanorods (TiO2-ZnO-C) were synthesized using the chemical bath deposition method, and ZnO filament-like crystals decorated with TiO2 nanorods (TiO2-ZnO-H) were synthesized through the hydrothermal method. Compared with the pristine TiO2 nanorods, the as-synthesized TiO2-ZnO composites exhibited enhanced photophysiochemical performance. Furthermore, because of their fast electron transportation and abundant surface active sites, the ZnO nanodisks in the TiO2-ZnO-C composite exhibited a higher photoactivity than those in the TiO2-ZnO-H composite. The morphology and crystal quality of the ZnO decoration layer were manipulated using different synthesis methods to realize disk- or filament-like ZnO-decorated TiO2 composites with various photoactive performance levels.

Enhanced Photocatalytic Activity of CuWO4 Doped TiO2 Photocatalyst Towards Carbamazepine Removal under UV Irradiation

Abatement of contaminants of emerging concerns (CECs) in water sources has been widely studied employing TiO2 based heterogeneous photocatalysis. However, low quantum energy yield among other limitations of titania has led to its modification with other semiconductor materials for improved photocatalytic activity. In this work, a 0.05 wt.% CuWO4 over TiO2 was prepared as a powder composite. Each component part synthesized via the sol-gel method for TiO2, and CuWO4 by co-precipitation assisted hydrothermal method from precursor salts, underwent gentle mechanical agitation. Homogenization of the nanopowder precursors was performed by zirconia ball milling for 2 h. The final material was obtained after annealing at 500 °C for 3.5 h. Structural and morphological characterization of the synthesized material has been achieved employing X-ray diffraction (XRD), Fourier transform infra-red (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET) N2 adsorption–desorption analysis, Scanning electron microscopy-coupled Energy dispersive X-ray spectroscopy (SEM-EDS), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-Vis diffuse reflectance spectroscopy (UV-vis DRS) for optical characterization. The 0.05 wt.% CuWO4-TiO2 catalyst was investigated for its photocatalytic activity over carbamazepine (CBZ), achieving a degradation of almost 100% after 2 h irradiation. A comparison with pure TiO2 prepared under those same conditions was made. The effect of pH, chemical scavengers, H2O2 as well as contaminant ion effects (anions, cations), and humic acid (HA) was investigated, and their related influences on the photocatalyst efficiency towards CBZ degradation highlighted accordingly.

Charge-Carrier Dynamics at the CuWO4/Electrocatalyst Interface for Photoelectrochemical Water Oxidation

Unraveling the charge-carrier dynamics at electrocatalyst/electrode interfaces is critical for the development of efficient photoelectrochemical (PEC) water oxidation. Unlike the majority of photoanodes investigated for PEC water oxidation, the integration of electrocatalysts with CuWO₄ electrodes generally results in comparable or worse performance compared to the bare electrode. This is despite the fact that the surface state recombination limits the water oxidation efficiency with CuWO₄ electrodes, and an electrocatalyst ought to bypass this reaction and improve performance. Here, we present results that deepen the understanding of the energetics and electron-transfer processes at the CuWO₄/electrocatalyst interface, which controls the performance of such systems. Ni_0.75Fe_0.25O_y (denoted as Ni75) was chosen as a model electrocatalyst, and through dual-working electrode experiments, we have been able to provide significant insight into the role of the electrocatalyst on the charge-transfer process at the CuWO₄/Ni75 interface. We have shown a lack of performance improvement for CuWO₄/Ni75 relative to the bare electrode to water oxidation. We attribute this surprising result to water oxidation on the CuWO₄ surface kinetically outcompeting hole transfer to the Ni75 electrocatalyst interface.

Increased photocurrent of CuWO4 photoanodes by modification with the oxide carbodiimide Sn2O(NCN)

Tin(ii) oxide carbodiimide is a novel prospective semiconductor material with a band gap of 2.1 eV and lies chemically between metal oxides and metal carbodiimides. We report on the photochemical properties of this oxide carbodiimide and apply the material to form a heterojunction with CuWO₄ thin films for photoelectrochemical (PEC) water oxidation. Mott-Schottky experiments reveal that the title compound is an n-type semiconductor with a flat-band potential of -0.03 V and, as such, the position of the valence band edge would be suitable for photochemical water oxidation. Sn₂O(NCN) increases the photocurrent of CuWO₄ thin films from 32 μA cm^-2 to 59 μA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE) in 0.1 M phosphate buffer (pH 7.0) under backlight AM 1.5G illumination. This upsurge in photocurrent originates in a synergistic effect between the oxide and oxide carbodiimide, because the heterojunction photoanode displays a higher current density than the sum of its individual components. Structural analysis by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveals that Sn₂O(NCN) forms a core-shell structure Sn₂O(NCN)@SnPO_x during the PEC water oxidation in phosphate buffer. The electrochemical activation is similar to the behavior of Mn(NCN) but different from Co(NCN).

Insights into the multiple effects of oxygen vacancies on CuWO4 for photoelectrochemical water oxidation

For CuWO₄, oxygen vacancies can shorten the electron transfer time and boost the water oxidation kinetics, but they aggravate the charge recombination on the surface.

Comparative Study of Photocarrier Dynamics in CVD-deposited CuWO4, CuO, and WO3 Thin Films for Photoelectrocatalysis

The temporal evolution of photogenerated carriers in CuWO4, CuO and WO3 thin films deposited via a direct chemical vapor deposition approach was studied using time-resolved microwave conductivity and terahertz spectroscopy to obtain the photocarrier lifetime, mobility and diffusion length. The carrier transport properties of the films prepared by varying the copper-to-tungsten stoichiometry were compared and the results related to the performance of the compositions built into respective photoelectrochemical cells. Superior carrier mobility was observed for CuWO4 under frontside illumination.

Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting

Solar water splitting using photoelectrochemical cells (PECs) has emerged as one of the most promising routes to produce hydrogen as a clean and renewable fuel source. Among various semiconductors that have been considered as photoelectrodes for use in PECs, oxide-based photoanodes are particularly attractive because of their stability in aqueous media in addition to inexpensive and facile processing compared to other types of semiconductors. However, they typically suffer from poor charge carrier separation and transport. In the past few years, there has been tremendous progress in developing ternary oxide-based photoelectrodes, specifically, photoanodes. The use of ternary oxides provides more opportunities to tune the composition and electronic structure of the photoelectrode compared to binary oxides, thus providing more freedom to tune the photoelectrochemical properties. In this article, we outline the important characteristics to analyze when evaluating photoanodes and review the major recent progress made on the development of ternary oxide-based photoanodes. For each system, we highlight the favorable and unfavorable features and summarize the strategies utilized to address the challenges associated with each material. Finally, by combining our analyses of all the photoanodes surveyed in this review, we provide possible future research directions for each compound and an outlook for constructing more efficient oxide-based PECs. Overall, this review will provide a critical overview of current ternary oxide-based photoanodes and will serve as a platform for the design of future oxide-based PECs.

Ag-Functionalized CuWO4/WO3 nanocomposites for solar water splitting

A new plasmonic Ag-functionalized CuWO₄/WO₃ hetero-structured photoanode was successfully prepared via a PVP-assisted sol–gel (PSG) route and electrophoretic deposition which reveals 4 times enhanced photocurrent density compared with pristine WO₃.

Light induced formation of a surface heterojunction in photocharged CuWO4 photoanodes

Photocharging of CuWO₄ photoanodes enhances its water oxidation kinetics as a result of improved charge separation near the electrode/electrolyte interface post photocharging.

Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets

The crystal facet of the BiVO₄ photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO₄ polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi₂O₃ template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO₄ film with a {121} monohigh-index facet, the BiVO₄ film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm² was achieved on the multihigh-index facet BiVO₄ photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na₂SO₄, which is about 200% improved over the normal BiVO₄ photoanode (0.61 mA/cm² at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO₄ photoanode (0.22 V vs RHE) relative to the normal BiVO₄ photoanode (0.52 V vs RHE) in 0.1 M Na₂SO₄. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO₄ film were boosted obviously. Theory calculations reveal that the adsorption of H₂O molecules on BiVO₄{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO₄ is changed to the step of two protons reacting with •O to form •OOH species (•O + H₂O_(l) + 2H⁺ + 2e^- → •OOH + 3H⁺ + 3e^-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H₂O to •OH (2H₂O_(l) + • → •OH + H₂O_(l) + H⁺ + e^-). In addition, the overpotential of water oxidation limiting step on BiVO₄{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.

An MnNCN-Derived Electrocatalyst for CuWO4 Photoanodes

CuWO₄ is a photoanode candidate in neutral pH, and manganese-based oxygen evolution reaction electrocatalysts are of high interest due to their low price and low toxicity. Considering the unexplored chemistry of transition-metal carbodiimides/cyanamides for the PEC water oxidation, we investigated MnNCN as an electrocatalyst for CuWO₄ under AM 1.5G illumination in potassium phosphate electrolyte (pH 7). Surface functionalization of CuWO₄ photoanodes with MnNCN increased the photocurrent from 22 to 30 μA cm^-2 at 1.23 V vs RHE. Complementary structural analysis by means of XRD and XPS revealed that MnNCN forms a core-shell structure MnNCN@MnPO _x in phosphate electrolyte and mimics a manganese phosphate electrocatalyst. As such, the surface chemistry of MnNCN significantly differs from previous studies on the cobalt analogue (CoNCN). A separately prepared MnNCN electrode developed a small but detectable photocurrent due to photogenerated holes inside the semiconducting carbodiimide core of the MnNCN@MnPO _x structure.

Quantitative hole collection for photoelectrochemical water oxidation with CuWO4

The hole collection efficiency of water oxidation was evaluated for CuWO₄ electrodes from comparisons of the photocurrent of H₂O₂ and Na₂SO₃ oxidation as well as intensity modulated photocurrent spectroscopy (IMPS) measurements. We found current multiplication using H₂O₂, however use of Na₂SO₃ and IMPS revealed quantitative water oxidation at 1.23 V vs. RHE.

Visible-enhanced photocatalytic performance of CuWO4/WO3 hetero-structures: incorporation of plasmonic Ag nanostructures

A new plasmonic Ag hybridized CuWO₄/WO₃ heterostructured nanocomposite was successfully synthesized via a ligand-assisted sol gel method and the photocatalytic activity was evaluated by photo-degradation of methylene blue (MB) under visible light irradiation.

Structural evolution, growth mechanism and photoluminescence properties of CuWO4 nanocrystals

Copper tungstate (CuWO₄) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100°C and 200°C have water molecules in their lattice (copper tungstate dihydrate (CuWO₄·2H₂O) with monoclinic structure), when the crystals are calcinated at 300°C have the presence of two phase (CuWO₄·2H₂O and CuWO₄), while the others heat treated at 400°C and 500°C have a single CuWO₄ triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet-Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300°C for 1h, which have a mixture of CuWO₄·2H₂O and CuWO₄ phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions.

Elucidation of CuWO4 Surface States During Photoelectrochemical Water Oxidation

Electrochemical, photoelectrochemical, and impedance spectroscopy measurements were performed to investigate the role of CuWO₄ surface states during water oxidation. We found that a capacitive feature related to a surface state is clearly observable under water oxidation conditions. The magnitude of the surface state capacitance is light intensity-dependent, with a peak potential that coincides with the water oxidation onset potential region. The surface state is not observed in the dark nor in contact with nonaqueous solvents. These results strongly support our assignment of this surface state as the buildup of water oxidation intermediate species at the surface of CuWO₄ photoanodes, not a permanent or intrinsic state as previously reported. We suggest this is a general feature that controls the behavior and efficiency of solar water splitting reactions.

Advancing the Chemistry of CuWO4 for Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) cells are an ongoing area of exploration that provide a means of converting solar energy into a storable chemical form (molecular bonds). In particular, using PEC cells to drive the water splitting reaction to obtain H2 could provide a clean and sustainable route to convert solar energy into chemical fuels. Since the discovery of catalytic water splitting on TiO2 photoelectrodes by Fujishima and Honda, significant efforts have been directed toward developing high efficiency metal oxides to use as photocatalysts for this reaction. Improving the efficiency of PEC cells requires developing chemically stable, and highly catalytic anodes for the oxygen-evolution reaction (OER). This water oxidation half reaction requires four protons and four electrons coupling in two bond making steps to form O2, which limits the rate. Our group has accelerated efforts in CuWO4 as a candidate for PEC OER chemistry. Its small band gap of 2.3 eV allows for using visible light to drive OER, and the reaction proceeds with a high degree of chemoselectivity, even in the presence of more kinetically accessible anions such as chloride, which is common to seawater. Furthermore, CuWO4 is a chemically robust material when subjected to the highly oxidizing conditions of PEC OER. The next steps for accelerating research using this (and other), ternary phase oxides, is to move beyond reporting the basic PEC measurements to understanding fundamental chemical reaction mechanisms operative during OER on semiconductor surfaces. In this Account, we outline the process for PEC OER on CuWO4 thin films with emphasis on the chemistry of this reaction, the reaction rate and selectivity (determined by controlled-potential coulometry and oxygen-detection experiments). We discuss key challenges with CuWO4 such as slow kinetics and the presence of an OER-mediating mid-gap state, probed by electrochemical impedance spectroscopy. We propose that this mid-gap state imparts the observed chemoselectivity of OER on CuWO4. We introduce insights into the chemical mechanism of PEC OER on CuWO4 using Tafel analysis of electrochemical polarization. We measure Tafel slopes of ∼161 mV/dec, showing that PEC OER proceeds at a slower rate on CuWO4 than on common electrocatalysts for this reaction. Moreover, the observed photocurrent is independent of the borate buffer concentration, signaling that the buffer plays no role in the rate-determining elementary step of the reaction. Finally, we explore some recent developments in doping this material with Co (a known electrocatalytically active metal) and in coupling it with a transparent manganese phosphate (MnPO) electrocatalyst. We find that introducing Co into the wolframite structure leads to detrimental recombination of photogenerated charge carriers. However, coupling CuWO4 with MnPO increases the photocurrent density. Despite some of these challenges, CuWO4 proves to be a robust, visible light absorbing photoanode that can oxidize water with a high degree of selectivity and is therefore worthy of further exploration. Even if new compositions emerge that show better reactivity, this material serves as an excellent proving ground for the common challenges in developing ternary-phase oxides and other compositionally complex materials.

CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation

Over recent years, tremendous efforts have been invested in the search and development of active and durable semiconductor materials for photoelectrochemical (PEC) water splitting, particularly for photoanodes operating under a highly oxidizing environment. CuWO4 is an emerging candidate with suitable band gap and high chemical stability. Nevertheless, its overall solar-to-electricity remains low because of 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/cm(2) 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/cm(2) by coupling with a solution-cast BiVO4 film in a heterojunction configuration. Our study unveils the great potential of nanostructured CuWO4 as the photoanode material for PEC water oxidation.

Copper(II) tungstate nanoflake array films: sacrificial template synthesis, hydrogen treatment, and their application as photoanodes in solar water splitting

We report the preparation of CuWO4 nanoflake (NF) array films by using a solid phase reaction method in which WO3 NFs were employed as sacrificial templates. The SEM, TEM and XRD results demonstrated that the obtained CuWO4 films possessed a network structure that was composed of single crystalline NFs intersected with each other. The CuWO4 NF films showed superior photoelectrochemical (PEC) activity to other CuWO4 photoanodes reported recently for the oxygen evolution reaction (OER). We attributed the high activity to the unique morphological and crystalline structure of the CuWO4 film, which enhanced the photoactivity by providing a large specific area, a short hole transport distance from the inside of CuWO4 to the CuWO4/solution interface, and a low grain boundary density. Hydrogen treatment by annealing the CuWO4 NF film in mixed gases of H2 and Ar could further enhance the photoactivity, as hydrogen treatment significantly increased the electron density of CuWO4 by generating oxygen vacancy in the lattice. The photocurrent density for OER obtained on the hydrogen-treated (H-treated) CuWO4 NF film is the largest ever reported on CuWO4 photoanodes in the literature. Moreover, the CuWO4 photoanodes exhibit good stability in weak alkaline solution, while the H-treated CuWO4 photoanodes exhibit acceptable stability. This work not only reveals the potential of CuWO4 as a photoanode material for solar water splitting but also shows that the construction of nanostructured CuWO4 photoanodes is a promising method to achieve high PEC activity toward OER.