New electrochemical synthesis methods were developed to produce copper hydroxy double salt(Cu-HDS) films with four different intercalated anions (NO3–, SO42–, Cl–, and dodecyl sulfate (DS)) as pure crystalline films as deposited (Cu2NO3(OH)3, Cu4SO4(OH)6, Cu2Cl(OH)3, and Cu2DS(OH)3). These methods are based on p-benzoquinone reduction, which increases the local pH at the working electrode and triggers the precipitation of Cu2+ and appropriate anions as Cu-HDS films on the working electrode. The resulting Cu-HDS films could be converted to crystalline Cu(OH)2 and CuO films by immersing them in basic solutions. Because Cu-HDS films were composed of 2D crystals as a result of the atomic-level layered structure of HDS, the CuO films prepared from Cu-HDS films have unique low-dimensional nanostructures, creating high surface areas that cannot be obtained by direct deposition of CuO, which has a 3D atomic-level crystal structure. The resulting nanostructures allowed the CuO films to facilitate electron–hole separation and demonstrate great promise for photocurrent generation when investigated as a photocathode for a water-splitting photoelectrochemical cell. Electrochemical synthesis of Cu-HDS films and their facile conversion to CuO films will provide new routes to tune the morphologies and properties of the CuO electrodes that may not be possible by other synthesis means.

LaFeO3 is a p-type oxide that has an ideal bandgap and band edge positions for overall solar water splitting. This study reports an electrochemical synthesis method to produce LaFeO3 as a high surface area, nanoporous photocathode. The resulting electrode generated a photocurrent density of −0.1 mA/cm2 at a potential as positive as 0.73 V vs RHE for photoelectrochemical oxygen reduction with a photocurrent onset potential very close to its flatband potential of 1.45 V vs RHE. Furthermore, without a protection layer, it showed stable photocurrent generation with no sign of photocorrosion. Due to the poor catalytic nature of the LaFeO3 surface for water reduction, the photocurrent obtained for water reduction was not substantial. However, its photostability and its ability to achieve a photovoltage for water reduction greater than 1.2 V encourage further studies on doping to enhance electron–hole separation as well as interfacing appropriate hydrogen evolution catalysts.

The development of new and inexpensive semiconductor electrodes that possess suitable band gap energies and band positions for solar water splitting is of great interest in the field of solar fuel production. In this study, n-type Cu11V6O26 that has a band gap energy of 1.9 eV was produced as a pure, high-quality photoanode, and its properties and stability for photoelectrochemical water splitting were systematically investigated in pH 9.2 and 13 solutions. As Cu11V6O26 photoanodes appeared to suffer from poor charge transport properties, Mo and W doping into the V site was also examined, which considerably improved the photocurrent generation of Cu11V6O26. The band gap energy, band edge positions, flatband potential, photocurrent generation, and photostability of pristine and doped Cu11V6O26 electrodes are discussed in comparison to elucidate the effect of Mo and W doping and to evaluate the promise and limitations of Cu11V6O26 as a photoanode for use in a water splitting photoelectrochemical cell.

Materials that can selectively store Na and Cl ions in the bulk of their structures and release these ions with good cycle stability can enable the construction of a high capacity, rechargeable desalination cell for use in seawater desalination. In this study, the ability of a nanocrystalline Bi foam electrode to serve as an efficient and high capacity Cl-storage electrode using its conversion to BiOCl was investigated. When Bi as a Cl-storage electrode was coupled with NaTi2(PO4)3 as a Na-storage electrode, a new type of rechargeable desalination cell, which is charged during desalination and discharged during salination, was constructed. The resulting Bi-NaTi2(PO4)3 cell was tested under various salination and desalination conditions to investigate advantages and potential limitations of using Bi as a Cl-storage electrode. Slow Cl– release kinetics of BiOCl in neutral conditions and an imbalance in Cl and Na storage (i.e., Cl storage requires three electrons/Cl, while Na storage requires one electron/Na) were identified as possible drawbacks, but strategies to address these issues were developed. On the basis of these investigations, optimum desalination and salination conditions were identified where the Bi/NaTi2(PO4)3 cell achieved a desalination/salination cycle at ±1 mA cm–2 with a net potential input of only 0.20 V. The kinetics of Cl– release from BiOCl was significantly improved by the use of an acidic solution, and therefore, a divided cell was used for the salination process. We believe that with further optimizations the Bi/BiOCl electrode will enable efficient and practical desalination applications.

Electrodeposition is a widely used technique for electrode synthesis in various applications. Because of its low synthesis cost and easy scalability, electrodeposition is particularly attractive for the production of semiconductor and catalyst electrodes for use in solar fuel production. For researchers who are interested in learning about or utilizing electrodeposition, the current paper describes detailed methods for electrodeposition, which include procedures for preparing electrodes and plating solutions, determining deposition conditions, and performing electrodeposition. Postdeposition treatments that can be used to prepare electrodes of more diverse compositions and photodeposition procedures that can be used to place catalyst layers on semiconductor electrodes are also provided. The methods are described using the synthesis and modification of photoelectrodes as an example, but most principles and procedures explained in this paper are general and can be applied to electrodeposition of various electrodes. In addition to methods for electrochemically preparing photoelectrodes, methods for photoelectrochemical characterization, which include light setup and calibration, photoelectrochemical characterization, and efficiency calculations, are described along with rationale for each setup and procedure. This will improve understanding and performance of various experimental procedures used for photoelectrode evaluation.

Electrochemical synthesis methods were developed to produce CuBi2O4, a promising p-type oxide for use in solar water splitting, as high surface area electrodes with uniform coverage. These methods involved electrodepositing nanoporous Cu/Bi films with a Cu:Bi ratio of 1:2 from dimethyl sulfoxide or ethylene glycol solutions, and thermally oxidizing them to CuBi2O4 at 450 °C in air. Ag-doped CuBi2O4 electrodes were also prepared by adding a trace amount of Ag+ in the plating medium and codepositing Ag with the Cu/Bi films. In the Ag-doped CuBi2O4, Ag+ ions substitutionally replaced Bi3+ ions and increased the hole concentration in CuBi2O4. As a result, photocurrent enhancements for both O2 reduction and water reduction were achieved. Furthermore, while undoped CuBi2O4 electrodes suffered from anodic photocorrosion during O2 reduction due to poor hole transport, Ag-doped CuBiO4 effectively suppressed anodic photocorrosion. The flat-band potentials of CuBi2O4 and Ag-doped CuBi2O4 electrodes prepared in this study were found to be more positive than 1.3 V vs RHE in a 0.1 M NaOH solution (pH 12.8), which make these photocathodes highly attractive for use in solar hydrogen production. The optimized CuBi2O4/Ag-doped CuBi2O4 photocathode showed a photocurrent onset for water reduction at 1.1 V vs RHE, achieving a photovoltage higher than 1 V for water reduction. The thermodynamic feasibility of photoexcited electrons in the conduction band of CuBi2O4 to reduce water was also confirmed by detection of H2 during photocurrent generation. This study provides new understanding for constructing improved CuBi2O4 photocathodes by systematically investigating photocorrosion as well as photoelectrochemical properties of high-quality CuBi2O4 and Ag-doped CuBi2O4 photoelectrodes for photoreduction of both O2 and water.

2,5-Hexanedione (HD), which can be produced by reduction of 5-hydroxymethylfurfural (HMF), one of the most important biomass intermediates, can serve as a precursor to produce various biofuels and key building block chemicals. The conversion of HMF to HD requires reduction of both the alcohol and aldehyde groups to alkane groups as well as opening of the furan ring. In this study, a direct electrochemical conversion of HMF to HD at ambient pressure and temperature was demonstrated without using H2 or precious metal catalysts. Water was used as the hydrogen source and zinc was used as the catalytic electrode, which enabled hydrogenolysis and Clemmensen reduction coupled with furan ring opening. Optimum conditions to achieve high Faradaic efficiency (FE) and selectivity for HD production were investigated and plausible mechanisms were proposed. The environmentally benign one-step procedure to produce HD reported in this study will serve as a new route to valorize biomass intermediates.

Reductive biomass conversion has been conventionally conducted using H2 gas under high-temperature and -pressure conditions. In this study, efficient electrochemical reduction of 5-hydroxymethylfurfural (HMF), a key intermediate for biomass conversion, to 2,5-bis(hydroxymethyl)furan (BHMF), an important monomer for industrial processes, was demonstrated using Ag catalytic electrodes. This process uses water as the hydrogen source under ambient conditions and eliminates the need to generate and consume H2 for hydrogenation, providing a practical and efficient route for BHMF production. By systematic investigation of HMF reduction on the Ag electrode surface, BHMF production was achieved with the Faradaic efficiency and selectivity nearing 100%, and plausible reduction mechanisms were also elucidated. Furthermore, construction of a photoelectrochemical cell (PEC) composed of an n-type BiVO4 semiconductor anode, which uses photogenerated holes for water oxidation, and a catalytic Ag cathode, which uses photoexcited electrons from BiVO4 for the reduction of HMF to BHMF, was demonstrated to utilize solar energy to significantly decrease the external voltage necessary for HMF reduction. This shows the possibility of coupling electrochemical HMF reduction and solar energy conversion, which can provide more efficient and environmentally benign routes for reductive biomass conversion.Keywords: 2,5-bis(hydroxymethyl)furan; 5-hydroxymethylfurfural; aldehyde reduction; biomass conversion; electrochemical hydrogenation; photoelectrochemical cell; solar biomass conversion

Electrochemical synthesis conditions using nonaqueous solutions were developed to prepare highly transparent (T > 90%) and crystalline ZnO and Al-doped ZnO (AZO) films for use in solar energy conversion devices. A focused effort was made to produce pinhole-free films in a reproducible manner by identifying a key condition to prevent the formation of cracks during deposition. The polycrystalline domains in the resulting films had a uniform orientation (i.e., the c-axis perpendicular to the substrate), which enhanced the electron transport properties of the films. Furthermore, electrochemical Al doping of ZnO using nonaqueous media, which was demonstrated for the first time in this study, effectively increased the carrier density and raised the Fermi level of ZnO. These films were coupled with an electrodeposited p-type Cu2O to construct p-n heterojunction solar cells to demonstrate the utilization of these films for solar energy conversion. The resulting n-ZnO/p-Cu2O and n-AZO/p-Cu2O cells showed excellent performance compared with previously reported n-ZnO/p-Cu2O cells prepared by electrodeposition. In particular, replacing ZnO with AZO resulted in simultaneous enhancements in short circuit current and open circuit potential, and the n-AZO/p-Cu2O cell achieved an average power conversion efficiency (η) of 0.92 ± 0.09%. The electrodeposition condition reported here will offer a practical and versatile way to produce ZnO or AZO films, which play key roles in various solar energy conversion devices, with qualities comparable to those prepared by vacuum-based techniques.

BiVO4 photoanodes have mainly been investigated under neutral conditions because BiVO4 gradually dissolves under extreme pH conditions. In this study, the possibility of utilizing ZnFe2O4 as a protection layer to stabilize BiVO4 in a 0.1 M KOH solution was investigated. A 10–15 nm thick ZnFe2O4 layer was conformally placed on a nanoporous BiVO4 electrode by photodepositing a FeOOH layer, followed by drop casting a zinc nitrate solution and annealing. The resulting BiVO4/ZnFe2O4 electrode generated a photocurrent density of >2 mA/cm2 at 1.23 V versus RHE with a significantly improved stability compared with the pristine BiVO4 electrode. The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron–hole separation. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO4-based photoanodes in basic media.

This review focuses on introducing and explaining electrodepostion mechanisms and electrodeposition-based synthesis strategies used for the production of catalysts and semiconductor electrodes for use in water-splitting photoelectrochemical cells (PECs). It is composed of three main sections: electrochemical synthesis of hydrogen evolution catalysts, oxygen evolution catalysts, and semiconductor electrodes. The semiconductor section is divided into two parts: photoanodes and photocathodes. Photoanodes include n-type semiconductor electrodes that can perform water oxidation to O2 using photogenerated holes, while photocathodes include p-type semiconductor electrodes that can reduce water to H2 using photoexcited electrons. For each material type, deposition mechanisms were reviewed first followed by a brief discussion on its properties relevant to electrochemical and photoelectrochemical water splitting. Electrodeposition or electrochemical synthesis is an ideal method to produce individual components and integrated systems for PECs due to its various intrinsic advantages. This review will serve as a good resource or guideline for researchers who are currently utilizing electrochemical synthesis as well as for those who are interested in beginning to employ electrochemical synthesis for the construction of more efficient PECs.

New electrochemical synthesis methods have been developed to obtain layered potassium niobates, KNb3O8 and K4Nb6O17, and perovskite-type KNbO3 as film-type electrodes. The electrodes were synthesized from aqueous solutions using the redox chemistry of p-benzoquinone and hydroquinone to change the local pH at the working electrode to trigger deposition of desired phases. In particular, the utilization of electrochemically generated acid via the oxidation of hydroquinone for inorganic film deposition was first demonstrated in this study. The layered potassium niobates could be converted to (H3O)Nb3O8 and (H3O)4Nb6O17 by cationic exchange, which, in turn, could be converted to Nb2O5 by heat treatment. The versatility of the new deposition method was further demonstrated for the formation of CuNb2O6 and AgNbO3, which were prepared by the deposition of KNb3O8 and transition metal oxides, followed by thermal and chemical treatments. Considering the lack of solution-based synthesis methods for Nb-based oxide films, the methods reported in this study will contribute greatly to studies involving the synthesis and applications of Nb-based oxide electrodes.

Mo-doped BiVO4 electrodes were prepared by an electrochemical route for use as photoanodes in a photoelectrochemical cell. The purpose of Mo-doping was to improve the electron transport properties, which in turn can increase the electron–hole separation yield. The poor electron–hole separation yield was known to be one of the main limiting factors for BiVO4-based photoanodes. The electrochemical route provided an effective way of doping BiVO4, and the optimally doped sample, BiV0.97Mo0.03O4, increased the electron–hole separation yield from 0.23 to 0.57 at 0.6 V vs. RHE, which is a record high separation yield achieved for BiVO4-based photoanodes. As a result, BiV0.97Mo0.03O4 generated impressive photocurrents, for example, 2 mA cm−2 at a potential as low as 0.4 V vs. RHE for sulfite oxidation, which has fast oxidation kinetics and, therefore, the loss of holes by surface recombination is negligible. For photooxidation of water, BiV0.97Mo0.03O4 was paired with FeOOH as an oxygen evolution catalyst (OEC) to improve the poor catalytic ability of BiV0.97Mo0.03O4 for water oxidation. The resulting BiV0.97Mo0.03O4/FeOOH photoanodes generated a significantly improved photocurrent for water oxidation compared to previous reported results, but the photocurrent of BiV0.97Mo0.03O4/FeOOH for water oxidation could not reach the photocurrent of BiV0.97Mo0.03O4 for sulfite oxidation. In order to examine the cause, the effects of Mo-doping on the interaction between BiVO4 and FeOOH and the effects of FeOOH on the electron–hole separation yield of BiV0.97Mo0.03O4 were investigated in detail, which provided new insights into semiconductor–OEC interactions.

A new electrochemical synthesis route was developed to prepare spinel-type ZnCo2O4 and Co3O4 as high quality thin film-type electrodes for use as electrocatalysts for oxygen evolution reaction (OER). Whereas Co3O4 contains Co2+ in the tetrahedral sites and Co3+ in the octahedral sites in the spinel structure, ZnCo2O4 contains only Co3+ in the octahedral sites; Co2+ in the tetrahedral sites is replaced by Zn2+. Therefore, by comparing the catalytic properties of ZnCo2O4 and Co3O4 electrodes prepared with comparable surface morphologies and thicknesses, it was possible to examine whether Co2+ in Co3O4 is catalytically active for OER. The electrocatalytic properties of ZnCo2O4 and Co3O4 for OER in both 1 M KOH (pH 13.8) and 0.1 M phosphate buffer (pH 7) solutions were investigated and compared. The results suggest that the Co2+ in Co3O4 is not catalytically critical for OER and ZnCo2O4 can be a more economical and environmentally benign replacement for Co3O4 as an OER catalyst.Keywords: Co3O4; electrocatalyst; oxygen evolution reaction; spinel; water oxidation; ZnCo2O4;

The major limitation to investigating a variety of ternary oxides for use in solar energy conversion is the lack of synthesis methods to prepare them as high-quality electrodes. In this study, we demonstrate that Bi-based n-type ternary oxides, BiVO4, Bi2WO6, and Bi2Mo3O12, can be prepared as high-quality polycrystalline electrodes by mild chemical and thermal treatments of electrodeposited dendritic Bi films. The resulting oxide films have good coverage, adhesion, and electrical continuity, allowing for facile and accurate evaluation of these compounds for use in solar water oxidation. In particular, the BiVO4 electrode retained the porosity and nanocrystallinity of the original dendritic Bi film. This feature increased the electron–hole separation yield, making this compound more favorable for use as a photoanode in a photoelectrochemical cell.Keywords: Bi2Mo3O12; Bi2WO6; BiVO4; electrodeposition; photoanode; solar water splitting;

Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very attractive and desirable way to solve the energy challenge. Many efforts have been made to find appropriate materials and systems that can utilize solar energy to produce chemical fuels. One of the most viable options is the construction of a photoelectrochemical cell that can reduce water to H2 or CO2 to carbon-based molecules. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode that oxidizes water to O2 in these cells. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this review, the crystal and electronic structures that are closely related to the photoelectrochemical properties of BiVO4 are described first, and the photoelectrochemical properties and limitations of BiVO4 are examined. Subsequently, the latest efforts toward addressing these limitations in order to improve the performances of BiVO4-based photoanodes are discussed. These efforts include morphology control, formation of composite structures, composition tuning, and coupling oxygen evolution catalysts. The discussions and insights provided in this review reflect the most recent approaches and directions for general photoelectrode developments and they will be directly applicable for the understanding and improvement of other photoelectrode systems.

Molybdenum-rich solid solutions of CuWO4 and CuMoO4 (i.e., CuW1−xMoxO4, x > 0.4) having a wolframite structure were prepared as thin-film type electrodes using a new electrochemical route. The synthesis of Mo-rich CuW1−xMoxO4 was not previously achieved because the CuMoO4 phase that is isostructural to CuWO4 is not thermodynamically stable. The resulting solid solution, CuW0.35Mo0.65O4, exhibited a significantly reduced optical bandgap (Eg = 2.0 eV), compared to CuWO4 (Eg = 2.3 eV). Since both CuW0.35Mo0.65O4 and CuWO4 are n-type semiconductors, their photoelectrochemical properties were compared for possible use as photoanodes in water splitting photoelectrochemical cells. CuW1−xMoxO4 showed enhanced photon absorption not only in the 2.0 eV ≤ E ≤ 2.3 eV region but also above 2.3 eV, compared to CuWO4, which directly resulted in enhanced photocurrent generation. Ab initio calculations were performed to understand the origin of the bandgap reduction by Mo incorporation. The calculations showed that the conduction band edge of CuWO4 is mainly composed of W 5d and O 2p hybrid orbitals. Therefore, when Mo atoms with 4d orbitals, which are lower in energy than W 5d orbitals, occupy W sites, the conduction band edge is shifted to lower energy. These results suggest that there may be many unexplored new compositions in ternary oxide systems that possess ideal bandgap energies for solar energy conversion.

Two n-type W-containing ternary oxides, CuWO4 and Bi2WO6, were prepared as high surface area electrodes and characterized for use as photoanodes in a water-splitting photoelectrochemical cell. The synthesis involved electrochemical preparation of porous WO3 electrodes and annealing them with Cu2+- or Bi3+-containing solutions on their surfaces to form the respective electrodes. The resulting CuWO4 electrode had a bandgap of 2.3 eV, and showed excellent photostability and photocurrent-to-O2 conversion efficiency (ca. 100%) in 0.1 M borate buffer solution (pH 9). Bi2WO6 had a bandgap of 2.8 eV but, regardless of its higher bandgap energy, Bi2WO6 showed an earlier photocurrent onset and much higher photocurrent than CuWO4 due to its more favorable CB edge and flatband potential position for water splitting. Bi2WO6 also showed chemical stability over a wide pH range (−0.26 ≤ pH ≤ 9.0). The photocurrent-to-O2 conversion efficiency of Bi2WO6 was in the range of 50–75% and its photocurrent decayed over time, indicating photocorrosion. However, stable photocurrent was obtained when H2O2, which has faster oxidation kinetics than water, was introduced into the electrolyte as a hole scavenger. This suggests that the photocorrosion of Bi2WO6 can be suppressed when an oxygen evolution catalyst is placed on its surface to improve interfacial hole transfer kinetics. With proper oxygen evolution catalysts and improved charge transport properties, both CuWO4 and Bi2WO6 have the possibility of achieving better photoelectrochemical performances than WO3.

Harvesting energy directly from sunlight as nature accomplishes through photosynthesis is a very attractive and desirable way to solve the energy challenge. Many efforts have been made to find appropriate materials and systems that can utilize solar energy to produce chemical fuels. One of the most viable options is the construction of a photoelectrochemical cell that can reduce water to H2 or CO2 to carbon-based molecules. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode that oxidizes water to O2 in these cells. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this review, the crystal and electronic structures that are closely related to the photoelectrochemical properties of BiVO4 are described first, and the photoelectrochemical properties and limitations of BiVO4 are examined. Subsequently, the latest efforts toward addressing these limitations in order to improve the performances of BiVO4-based photoanodes are discussed. These efforts include morphology control, formation of composite structures, composition tuning, and coupling oxygen evolution catalysts. The discussions and insights provided in this review reflect the most recent approaches and directions for general photoelectrode developments and they will be directly applicable for the understanding and improvement of other photoelectrode systems.

Two n-type W-containing ternary oxides, CuWO4 and Bi2WO6, were prepared as high surface area electrodes and characterized for use as photoanodes in a water-splitting photoelectrochemical cell. The synthesis involved electrochemical preparation of porous WO3 electrodes and annealing them with Cu2+- or Bi3+-containing solutions on their surfaces to form the respective electrodes. The resulting CuWO4 electrode had a bandgap of 2.3 eV, and showed excellent photostability and photocurrent-to-O2 conversion efficiency (ca. 100%) in 0.1 M borate buffer solution (pH 9). Bi2WO6 had a bandgap of 2.8 eV but, regardless of its higher bandgap energy, Bi2WO6 showed an earlier photocurrent onset and much higher photocurrent than CuWO4 due to its more favorable CB edge and flatband potential position for water splitting. Bi2WO6 also showed chemical stability over a wide pH range (−0.26 ≤ pH ≤ 9.0). The photocurrent-to-O2 conversion efficiency of Bi2WO6 was in the range of 50–75% and its photocurrent decayed over time, indicating photocorrosion. However, stable photocurrent was obtained when H2O2, which has faster oxidation kinetics than water, was introduced into the electrolyte as a hole scavenger. This suggests that the photocorrosion of Bi2WO6 can be suppressed when an oxygen evolution catalyst is placed on its surface to improve interfacial hole transfer kinetics. With proper oxygen evolution catalysts and improved charge transport properties, both CuWO4 and Bi2WO6 have the possibility of achieving better photoelectrochemical performances than WO3.

Mo-doped BiVO4 electrodes were prepared by an electrochemical route for use as photoanodes in a photoelectrochemical cell. The purpose of Mo-doping was to improve the electron transport properties, which in turn can increase the electron–hole separation yield. The poor electron–hole separation yield was known to be one of the main limiting factors for BiVO4-based photoanodes. The electrochemical route provided an effective way of doping BiVO4, and the optimally doped sample, BiV0.97Mo0.03O4, increased the electron–hole separation yield from 0.23 to 0.57 at 0.6 V vs. RHE, which is a record high separation yield achieved for BiVO4-based photoanodes. As a result, BiV0.97Mo0.03O4 generated impressive photocurrents, for example, 2 mA cm−2 at a potential as low as 0.4 V vs. RHE for sulfite oxidation, which has fast oxidation kinetics and, therefore, the loss of holes by surface recombination is negligible. For photooxidation of water, BiV0.97Mo0.03O4 was paired with FeOOH as an oxygen evolution catalyst (OEC) to improve the poor catalytic ability of BiV0.97Mo0.03O4 for water oxidation. The resulting BiV0.97Mo0.03O4/FeOOH photoanodes generated a significantly improved photocurrent for water oxidation compared to previous reported results, but the photocurrent of BiV0.97Mo0.03O4/FeOOH for water oxidation could not reach the photocurrent of BiV0.97Mo0.03O4 for sulfite oxidation. In order to examine the cause, the effects of Mo-doping on the interaction between BiVO4 and FeOOH and the effects of FeOOH on the electron–hole separation yield of BiV0.97Mo0.03O4 were investigated in detail, which provided new insights into semiconductor–OEC interactions.