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Article

Facile Synthesis of a Micro–Nano-Structured FeOOH/BiVO4/WO3 Photoanode with Enhanced Photoelectrochemical Performance

by
Ruixin Li
1,
Faqi Zhan
1,*,
Guochang Wen
1,
Bing Wang
1,
Jiahao Qi
1,
Yisi Liu
2,
Chenchen Feng
1,* and
Peiqing La
1
1
State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China
2
Institute of Advanced Materials, Hubei Normal University, Huangshi 415000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(11), 828; https://doi.org/10.3390/catal14110828
Submission received: 29 October 2024 / Revised: 13 November 2024 / Accepted: 14 November 2024 / Published: 17 November 2024
(This article belongs to the Special Issue Catalysts for Energy Storage)
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Versions Notes

Abstract

:
In the realm of photoelectrocatalytic (PEC) water splitting, the BiVO4/WO3 photoanode exhibits high electron–hole pair separation and transport capacity, rendering it a promising avenue for development. However, the charge transport and reaction kinetics at the heterojunction interface are suboptimal. This study uses the hydrothermal–electrodeposition–dip coating–calcination method to prepare a microcrystalline WO3 photoanode thin film as the substrate material and combines it with nanocrystalline BiVO4 to form a micro–nano-structured heterojunction photoanode to enhance the intrinsic and surface/interface charge transport properties of the photoanode. Under the condition of 1.23 V vs. RHE, the photoelectric current density reaches 1.09 mA cm−2, which is twice that of WO3. Furthermore, by using a simple impregnation–mineralization method to load the amorphous FeOOH catalyst, a noncrystalline–crystalline composite structure is formed to increase the number of active sites on the surface and reduce the overpotential of water oxidation, lowering the onset potential from 0.8 V to 0.6 V (vs. RHE). The photoelectric current density is further increased to 2.04 mA cm−2 (at 1.23 V vs. RHE). The micro–nano-structure and noncrystalline–crystalline composite structure proposed in this study will provide valuable insights for the design and synthesis of high-efficiency photoelectrocatalysts.

Graphical Abstract

1. Introduction

Due to its relatively narrow bandgap of ~2.7 eV, WO3 can absorb sunlight with wavelengths in the range of 410–500 nm, effectively covering the visible light spectrum and enhancing sunlight utilization efficiency. However, the rapid recombination of photogenerated carriers limits its energy conversion efficiency. The heterojunction formed between WO3 and BiVO4, which has a bandgap of ~2.4 eV, broadens the visible light absorption range and creates an internal electric field through their energy level alignment. This promotes the migration rate of photogenerated carriers, thereby enhancing photoelectric conversion efficiency [1,2,3]. Numerous studies have shown that the photogenerated charge separation mechanism of the BiVO4/WO3 heterojunction photoanode is type II, where photogenerated holes accumulate in BiVO4, while photogenerated electrons migrate to WO3 and further transfer to FTO conductive glass. BiVO4 primarily functions in the photoelectrocatalytic oxidation of water, while WO3 mainly handles the separation and transport of electrons [4,5,6,7,8]. Therefore, it is necessary to design high-specific-surface-area nanocrystalline BiVO4 and highly electron-conductive microcrystalline WO3 to construct a micro–nano-structural heterojunction. However, the current BiVO4/WO3 heterojunction photoanodes are all nanocrystalline mixtures [9,10,11,12], making it difficult to achieve this goal. Micro–nano-structured photoelectrodes can achieve a combination of high activity and high conductivity [13,14,15,16,17,18].
Additionally, the WO3/BiVO4 heterojunction photoanode exhibits poor surface reaction kinetics for water oxidation, significantly limiting its practical application. Many studies have demonstrated that iron (Fe), along with its oxides and hydroxides, is often used as a cocatalyst to reduce the overpotential of the water oxidation reaction, thereby enhancing the photoelectrochemical performance of photoanode materials. When Fe-based cocatalysts are loaded onto the surface of the photoanode, they can enrich photogenerated holes and effectively prevent their recombination. This facilitates the photoelectrochemical water splitting process at a lower potential, thereby reducing the onset potential of the reaction and increasing the efficiency of photoelectrochemical water splitting [19,20,21,22]. Currently, the main preparation methods for FeOOH cocatalysts include electrodeposition [23], hydrothermal methods [24], spin coating [25], and electrostatic adsorption [26]. These methods are relatively complex in terms of preparation processes, whereas the dipping method is relatively simple and can achieve surface loading of FeOOH.
This study prepared micro–nano-structured amorphous/crystalline FeOOH/BiVO4/WO3 composite photoanodes using hydrothermal, electrodeposition, and impregnation techniques and investigated their phase composition, micro-morphology, and structure. Meanwhile, the effects and mechanisms of composite amount and loading amount on the optical properties, band structure, and photoelectrochemical performance of the photoanodes were explored. The study found that, compared to single WO3 photoanodes and BiVO4/WO3 composite photoanodes, this ternary FeOOH/BiVO4/WO3 photoanode exhibited superior photoelectrochemical performance.

2. Results and Discussion

2.1. Phase and Chemical Composition of Composite Photoanodes

Detailed experimental procedures and characterization methods can be found in the Supplementary Materials [27,28]. The XRD of the FeOOH/BiVO4/WO3 film is shown in Figure 1a, where no characteristic peaks of FeOOH are observed because the deposited FeOOH layer is amorphous and noncrystalline with a low loading amount. Figure 1b shows the XRD patterns of the BiVO4/WO3 composite materials prepared with different electrochemical deposition times. The XRD patterns of the composite photoanode show the diffraction peaks of monoclinic WO3 and BiVO4 (PDF#14-0688), indicating the successful composite of WO3 and BiVO4 in the heterojunction. It can also be observed that the intensity of the characteristic peaks of BiVO4 increases with the increase in electrochemical deposition time, and the four crystal faces (110), (011), (121), and (040) are more obvious because with the increase in electrochemical deposition time, the number of BiVO4 particles deposited on WO3 also increases, and the (121) crystal face of BiVO4 shows a preferred orientation growth.
Further, the crystal structure characteristics of the photoanode thin film were studied through Raman and FTIR spectroscopy. As shown in Figure 2a, the characteristic peaks at 268 cm−1 and 320 cm−1 correspond to the O-W-O bending vibration mode in the WO3 crystal lattice, which weaken or even disappear after loading BiVO4 and FeOOH. The characteristic peaks at 709 cm−1 and 806 cm−1 reflect the stretching vibration mode of the O-W-O bond, consistent with WO3. After loading BiVO4, the peak at 709 cm−1 disappears, and the peak at 806 cm−1 weakens with increasing FeOOH loading. For the BiVO4/WO3 photoanode, the characteristic peaks at 328 cm−1 and 363 cm−1 are attributed to the bending vibration of the VO43− tetrahedron, and these peaks gradually weaken as FeOOH impregnation concentration increases. No additional characteristic peaks were observed in the Raman spectrum of FeOOH/BiVO4/WO3 impregnated with 15 mM, indicating that the FeOOH catalyst layer is amorphous. However, in the enlarged Raman image shown in Figure 2b, the characteristic peak at 806 cm−1 exhibits a shift and increases with more FeOOH, likely caused by the loading of FeOOH. Figure 2c shows the FTIR spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3. The peaks at 1400 cm−1 and 3128 cm−1 correspond to the stretching vibration of water molecules’ -OH groups [29]. From the figure, it can be seen that after loading the FeOOH catalyst, the intensity of the hydroxyl stretching vibration peak at 1400 cm−1 increases, which proves the successful loading of FeOOH.
To investigate the elemental composition and oxidation state of the obtained samples, XPS tests were conducted. The XPS total spectrum of the BiVO4/WO3 film is shown in Figure 3a, calibrated by the C 1s peak at 284.8 eV. From the spectrum, it can be seen that W, O, Bi, and V elements are present, but no other elements are detected. In Figure 3b, the W 4f spectrum detects four peaks of the W element. The peaks at 35.3 eV and 37.4 eV correspond to the binding energies of W6+ 4f7/2 and W6+ 4f5/2 orbitals, which correspond to the +6 oxidation state of W in the WO3 film. Minor W5+ 4f7/2 and 4f5/2 orbitals appear at 34.1 eV and 36.8 eV, due to some O being taken by BiVO4 in WO3, creating oxygen vacancies and forming WO3−x [30,31,32]. In Figure 3c, the O 1s spectrum shows a peak at 529.8 eV, corresponding to the O2− state in the WO3 crystal lattice [33], and a peak at 531.3 eV, corresponding to the characteristic peaks of water molecules and hydroxyl groups. In Figure 3d, the main peak of Bi is concentrated at 164.4 eV and 159.1 eV, corresponding to the binding energies of Bi 4f5/2 and Bi 4f7/2 orbitals, indicating the presence of Bi3+. In Figure 3e, the peaks at 524.2.1 eV and 516.7 eV correspond to the V 2p1/2 and V 2p3/2 orbitals [34], indicating a V5+ oxidation state. The XPS spectra prove the successful construction of the BiVO4/WO3 heterojunction.
The XRD patterns of the FeOOH/BiVO4/WO3 photoanode samples did not show any diffraction peaks of FeOOH, so XPS was used to analyze its elemental composition and oxidation state, as shown in Figure 4. Figure 4a shows the XPS total spectrum of the FeOOH/BiVO4/WO3 photoanode, where the characteristic peaks of W, O, Bi, V, and Fe elements can be observed. Figure 4b shows that the two characteristic peaks observed at binding energies of 37.5 eV and 35.4 eV correspond to the 4f5/2 and 4f7/2 orbitals of W6+, respectively. Smaller W5+ orbitals appear at 36.8 eV and 34.1 eV, attributed to minor O vacancies. Figure 4c shows that the two characteristic peaks observed at binding energies of 531.3 eV and 530.0 eV correspond to the lattice oxygen and adsorbed oxygen, respectively. Figure 4d shows that the two characteristic peaks observed at binding energies of 164.5 eV and 159.2 eV correspond to the 4f5/2 and 4f7/2 orbitals of Bi3+, respectively [35]. Figure 4e shows that the two characteristic peaks at binding energies of 524.3 eV and 516.8 eV correspond to the 2p1/2 and 2p3/2 orbitals of V5+, respectively, and a pair of minor peaks at 515.8 eV and 523.5 eV may be due to O vacancies in BiVO4. The Fe 2p characteristic peak can be observed in the total XPS spectrum of FeOOH/BiVO4/WO3, which includes the peaks of Fe2+ (715.3 eV, 728.6 eV) and Fe3+ (711.3 eV, 724.3 eV), as shown in Figure 4f, which is consistent with the literature report [36]. From Figure 4g,h, it can be seen that the W 4f and Bi 4f orbitals shifted to larger binding energies by 0.1 eV after loading FeOOH catalyst, indicating that there is electron transfer between FeOOH, BiVO4, and WO3, and a strong chemical bond has formed.

2.2. Microscopic Morphology and Structure of Composite Photoanodes

The surface and cross-sectional microstructures of the photoanodes prepared by SEM were characterized. As shown in Figure 5a–c, the WO3 thin films all exhibit a plate-like structure perpendicular to the substrate with a width of about 200 nm and a thickness of about 1.3 μm. The SEM images of the optimal electrodeposited BiVO4/WO3 at 5 min, shown in Figure 5d–f, reveal a clear nanoparticle structure of BiVO4 and uniform attachment to WO3. The size of the BiVO4 nanoparticles is about 100 nm, and the film thickness increases to 1.5 μm compared to WO3. The EDS surface distribution of the BiVO4 nanoparticles deposited by the electrochemical method is shown in Figure 6, where the main elements of BiVO4/WO3 can be clearly observed. As shown in Figure 6f, the uniform coverage of BiVO4 on the WO3 photoanode can be seen, with a W content of 29 wt.% and a Bi content of 44 wt.%. The SEM images of the FeOOH/BiVO4/WO3 composite photoanode are shown in Figure 5g–i, where it can be observed that after depositing FeOOH, the surface of the BiVO4 particles becomes smooth and continuous, with a layer of amorphous material loaded on top. However, the overall morphology and thickness of the composite photoanode do not show obvious changes, indicating that the amount of deposited FeOOH is small and its thickness is thin. The EDS surface distribution map of the FeOOH-catalyst-loaded photoanode is shown in Figure 7, where it can be seen that Fe is evenly attached to the BiVO4 nanoparticles. Figure 7g determines the mass ratio of each element in the composite photoanode, with an Fe content of 4 wt.%. The optimal mass ratio of the FeOOH/BiVO4/WO3 photoanode obtained is 0.53:8.67:100. As seen in Figure 5j, the pore size distribution of BiVO4/WO3 is relatively uniform, with an average pore size of approximately 70 nm. In contrast, Figure 5k shows that after loading FeOOH, the pore size distribution becomes highly uneven, with the average pore size significantly increasing to 197 nm.
Further SEM tests were conducted on the five electrodeposited BiVO4 samples with different deposition times and the three FeOOH-impregnated samples with different concentrations. The results are shown in Figure 8, where it can be seen that as the electrodeposition time increases, the number of BiVO4 nanoparticles loaded onto the WO3 plate-like structure also increases. At 5 min, the BiVO4 nanoparticles are uniformly distributed, and as the deposition time increases, the deposited nanoparticles pile up and fill the hollow structure. Figure 8p–t show the pore size distribution of BiVO4/WO3 at different electrodeposition times, with no significant difference in average size. However, when deposited for 3 min and 7 min, there are more pores. During deposition for 4–5 min, fewer pores are present. Additionally, the maximum pore size of the sample deposited for 7 min is significantly higher than that of other samples. As the concentration of FeOOH impregnated on BiVO4/WO3 increases, the thickness of FeOOH loaded on it also increases (Figure 9). According to Figure 9j–l, it is found that under 10 mM FeOOH impregnation, the pore size distribution is relatively uniform, with an average size of 119 nm. Under 15 mM impregnation, the pore size distribution of the samples is uneven, and the average size is larger. Figure 10a shows the percentage of each atom based on EDS point scanning, which can be seen to show that as the electrodeposition time increases, the percentage of Bi atoms also increases. As shown in Figure 10b, the percentage of Fe atoms also increases with the increase in impregnated concentration.
To further investigate the microstructure and bonding of WO3 and BiVO4, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analyses on BiVO4/WO3 and FeOOH/BiVO4/WO3 were performed. The TEM and HRTEM images of BiVO4/WO3, along with the selected area electron diffraction (SAED) results, are shown in Figure 11. Consistent with the SEM results, the microstructure shows that BiVO4 nanoparticles are loaded on the sheet-like WO3 structure, with the WO3 plates measuring 7 μm in length and 5 μm in width, and the BiVO4 particles around 10 nm in size. Figure 11b shows clear lattice fringes, and measurements in Figure 11c,d indicate that the interplanar spacing of 0.338 nm corresponds to the (021) plane of WO3, and the spacing of 0.345 nm corresponds to the (−130) plane of BiVO4. The SAED pattern in Figure 11b shows diffraction rings corresponding to the respective planes of WO3 and BiVO4. Figure 12 shows the distribution of elements in BiVO4/WO3, with elements uniformly distributed on the photoanode, indicating that BiVO4 nanoparticles are successfully loaded on WO3.
The TEM image of the FeOOH/BiVO4/WO3 photoanode after loading the FeOOH cocatalyst is shown in Figure 13a. The overall morphology remains largely unchanged after loading the cocatalyst. In Figure 13b–d, the lattice spacing of 0.267 nm corresponds to the (–202) plane of WO3, while the spacings of 0.360 nm, 0.259 nm, and 0.274 nm correspond to the (–130), (200), and (040) planes of BiVO4, respectively. In the magnified image shown in Figure 13e, a 1 nm thick amorphous FeOOH layer can be observed attached to the surface of BiVO4/WO3. This amorphous layer structure is beneficial for the PEC water oxidation capability of the catalyst. The TEM-EDS surface distribution images of FeOOH/BiVO4/WO3 are shown in Figure 14a–g. In addition to W, O, Bi, and V, Fe is also detected to be uniformly distributed on the catalyst surface. The energy spectrum of FeOOH/BiVO4/WO3 is shown in Figure 14h, where the signal peak of Fe can be observed, with its atomic percentage being 0.45 wt.%.

2.3. The Optical Properties and Band Structure of the Photoanodes

The optical absorption properties of the photoanode were characterized using UV–Vis absorption spectroscopy. As shown in Figure 15a, the absorption range of WO3 is between 300 and 450 nm. After loading BiVO4 nanoparticles to form a heterostructure, the light absorption intensity of the BiVO4/WO3 photoanode increased significantly in the visible range, and the absorption edge of the BiVO4/WO3 photoanode exhibited a noticeable red shift, extending the maximum absorption wavelength to 500 nm. This demonstrates that forming a BiVO4/WO3 heterostructure broadens the visible light absorption range of the photoanode. However, after loading the FeOOH cocatalyst, compared to the BiVO4/WO3 photoanode, the overall light absorption curve did not show significant changes, except for a slightly higher light absorption intensity. Figure 15b shows the Tauc plot of the photoanode. It can be seen from the figure that the bandgap value of WO3 is 2.74 eV, which corresponds to the reported literature [37]. The formation of the heterostructure has a significant impact on the bandgap, with the bandgap of BiVO4/WO3 being 2.50 eV. When the FeOOH cocatalyst is loaded, the bandgap of FeOOH/BiVO4/WO3 is 2.48 eV, showing almost no change. The optical properties indicate that the heterostructure photoanode can effectively broaden the light absorption range and adjust the bandgap, while the surface modification with the FeOOH cocatalyst does not affect the optical properties.
Combining the XPS valence band spectra and optical absorption spectra, the band structure diagrams of the three catalysts were obtained, as shown in Figure 16. The valence bands (VBs) of WO3, BiVO4, and FeOOH/BiVO4/WO3 are 2.35 eV, 1.57 eV, and 1.60 eV, respectively, and the conduction bands (CB) are –0.39 eV, –0.93 eV, and –0.88 eV, respectively. Compared to the BiVO4/WO3 heterojunction, the valence band position of FeOOH/BiVO4/WO3 modified with the cocatalyst shifts positively, which is more favorable for the water oxidation reaction [38].

2.4. Photoelectrochemical Performance of Composite Photoanodes

To gain a deeper and more intuitive understanding of the role of BiVO4 nanoparticles and FeOOH cocatalysts loaded on WO3 films, PEC tests [39] were conducted using an electrochemical workstation with a three-electrode system. First, the electrodeposition time was investigated. As shown in Figure 17a, with the increase in electrodeposition time, the photocurrent density of BiVO4/WO3 initially increases and then decreases with the increasing amount of BiVO4 nanoparticles. The reason is that the moderate amount of BiVO4 nanoparticles on the photoanode surface formed a heterostructure with WO3 that can enhance the separation and transport efficiency of photogenerated carriers, thereby improving the photoelectrochemical performance of the photoanode. However, with the increase in electrodeposition time, the number of BiVO4 nanoparticles deposited on the WO3 surface also increases, reducing the specific surface area of BiVO4/WO3 and hindering the migration of photogenerated carriers, and excessive BiVO4 nanoparticles might act as charge recombination centers, thereby reducing the photoelectrochemical performance of the photoanode. It can be observed from Figure 17a that when the electrodeposition time is 5 min, the photocurrent density increases to 1.09 mA/cm2, which is higher than the photocurrent density of pure WO3 and BiVO4. Therefore, the electrodeposition time of 5 min is considered the optimal condition. Using the BiVO4/WO3 prepared with the optimal electrodeposition time as the basis for the next step of FeOOH cocatalyst loading, the effect of Fe ion concentration on the performance of the loaded FeOOH catalyst was investigated. The LSV performance is shown in Figure 17b. The results are similar to those of electrodeposition; as the impregnation solution concentration increases, the photocurrent density initially increases and then decreases. This phenomenon can be attributed to the thicker cocatalyst layer formed with higher impregnation solution concentrations, where excess FeOOH acts as recombination centers for photogenerated carriers, reducing the separation and migration efficiency of photogenerated carriers, thereby leading to a decline in photoelectrochemical performance. It can be observed that the maximum photocurrent density is exhibited when the impregnation solution concentration is 10 mM.
The photocurrent of the three samples gradually increased with the applied voltage, as shown in the LSV curve in Figure 17c. Compared with the standalone WO3 photoanode, the BiVO4/WO3 and FeOOH/BiVO4/WO3 heterojunction photoanodes exhibited a negative shift in onset potential and higher photocurrent density, with the FeOOH/BiVO4/WO3 photoanode showing the best performance. Under a bias of 1.23 V vs. RHE, the photocurrent density of the WO3 film was 0.69 mA/cm2, while the photocurrent densities of BiVO4/WO3 and FeOOH/BiVO4/WO3 reached 1.09 mA/cm2 and 2.04 mA/cm2, respectively. The performance improvement is due to the formation of a heterojunction between WO3 and BiVO4 at the interface, which, due to the difference in band structures, promotes the formation of an internal electric field. The presence of the internal electric field enables effective separation of photogenerated carriers at the heterojunction, thereby enhancing the photoelectrochemical performance of the BiVO4/WO3 heterojunction photoanode. After loading the cocatalyst, the FeOOH/BiVO4/WO3 electrode exhibited the highest photocurrent density, approximately three times that of WO3 and twice that of BiVO4/WO3. An appropriate amount of FeOOH cocatalyst increases the number of active sites on the photoanode surface, reduces overpotential, accelerates the reaction process, and adjusts the energy band structure of the photoanode, thereby promoting the separation and transfer of photogenerated charges and reducing their recombination rate, ultimately enhancing the photoanode’s photoelectric performance.
Through continuous light–dark switching of the samples over 3600 s, the I–t curves of the WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes were obtained, as shown in Figure 17d–f. The experimental results indicate that all three samples exhibit good photoelectric response characteristics. It can be observed from the figure that there is a significant peak in the transient photocurrent density curve at the moment of light exposure. This is due to the accumulation of a large number of carriers when the light source is switched on [40]. Furthermore, WO3 showed significant decay from the beginning, while the samples with heterojunction formation and cocatalyst loading did not exhibit significant decay, indicating that the BiVO4/WO3 and FeOOH/BiVO4/WO3 composite photoanodes have better photostability.
Table 1 presents a comparison of PEC performance of BiVO4/WO3 and FeOOH photoanodes in different studies. For BiVO4/WO3, the hydrothermal and electrodeposition methods have fewer adjustable parameters, which may limit control over structural growth. However, the BiVO4/WO3 prepared in this study still outperforms most other studies. Reports on FeOOH/BiVO4/WO3 materials are relatively scarce, but its PEC performance remains at a high level compared to other FeOOH photoanodes, and the synthesis method used in this experiment is relatively simple.
To further analyze the photoelectrochemical performance, applied bias photon-to-current efficiency (ABPE) of the photoanode materials was calculated at different biases based on the J–V curve data, as shown in Figure 18a. The FeOOH/BiVO4/WO3 photoanode exhibited the best photoelectric conversion efficiency, reaching 0.374% at 0.89 V vs. RHE. In contrast, WO3 showed a maximum value of 0.066% at 1.03 V vs. RHE, while BiVO4/WO3 had a maximum value of 0.083% at 1.06 V vs. RHE. This indicates that FeOOH as a cocatalyst can reduce the overpotential, requiring a smaller applied bias, which is consistent with the LSV results.
Figure 18b shows the electrochemical impedance spectroscopy (EIS) of the photoanode films under illumination, where all samples exhibit a semicircular arc in the high-frequency region. The composite samples that underwent electrodeposition of BiVO4 and loading of FeOOH cocatalyst showed a significant reduction in the radius of the semicircular arc. Using the Z-View software 2018 for circuit fitting, as shown in Table 2, CPE1 represents the constant-phase element at the interface between the electrode and the electrolyte, R1 is the series resistance, and R2 is the charge transfer resistance at the electrode–electrolyte interface. The charge transfer-resistances for WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 are 32.3 Ω and 486.0 Ω, 26.8 Ω and 306.5 Ω, and 22.9 Ω and 201.5 Ω, respectively. This indicates that the formation of the heterojunction facilitates charge transport within the bulk of the electrode, and the loading of the cocatalyst enhances charge transfer at the electrode–electrolyte interface. The FeOOH/BiVO4/WO3 photoanode has the lowest charge transfer resistance, with higher separation and mobility rates of photogenerated carriers.
Through Mott–Schottky analysis, the fundamental properties of semiconductor materials, such as their semiconductor type and carrier concentration, can be understood. As shown in Figure 18c, the slopes of the Mott–Schottky plots for WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes are all positive, indicating that these three are n-type semiconductors with electrons as the main charge carriers. It is well known that the slope is inversely proportional to the carrier concentration; a smaller slope indicates a higher carrier concentration. The carrier concentrations for WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 are 2.3 × 1023 cm−3, 3.3 × 1023 cm−3, and 6.3 × 1023 cm−3, respectively. The FeOOH/BiVO4/WO3 photoanode has the smallest slope, indicating that loading FeOOH as a cocatalyst improves the separation efficiency of photogenerated charges within the photoanode, thereby enhancing the photoelectrochemical performance of the FeOOH/BiVO4/WO3 photoanode.
By calculating the charge separation and injection efficiency, the separation and transport of charge carriers in the WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes were further analyzed. Figure 19a shows the photocurrent density of the three photoanodes in sodium sulfite electrolyte, with the FeOOH/BiVO4/WO3 photoanode exhibiting the highest photocurrent density. Figure 19b,c, respectively, display the bulk charge separation efficiency (ηbulk) and surface charge injection efficiency (ηsurface) of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes. At 1.23 V vs. RHE, the surface charge injection efficiency and bulk charge separation efficiency of the FeOOH/BiVO4/WO3 photoanode are higher than those of the BiVO4/WO3 and WO3 photoanodes. This indicates that FeOOH as a cocatalyst can enhance the surface charge transfer capability, thereby improving the surface charge injection efficiency of the photoanode and enhancing the photoelectrochemical performance. From the dark current curves of the three photoanodes (Figure 19d), it can be seen that after loading the FeOOH cocatalyst, the dark current of FeOOH/BiVO4/WO3 significantly increases, and the onset potential shifts negatively, indicating that the built-in electric field of the heterojunction photoanode effectively reduces the overpotential and that the conductivity of the photoanode is enhanced after loading the cocatalyst.
During PEC water splitting, the electrochemical active surface area (ECSA) of the photoanode is an important reference for the number of active reaction sites. Figure 20a–c, respectively, show the cyclic voltammetry (CV) curves of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes at different scan rates between 0.47 and 0.57 V vs. RHE. Through CV testing, the double-layer capacitance of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 samples can be fitted, and the relative sizes of the ECSA of different photoanodes can be calculated, as shown in Figure 20d: WO3 is 0.17 mF/cm2, BiVO4/WO3 is 0.26 mF/cm2, and FeOOH/BiVO4/WO3 reaches 0.54 mF/cm2. The results indicate that the ECSA of the FeOOH/BiVO4/WO3 photoanode is significantly enhanced compared to the other two photoanodes. This is attributed to the loading of FeOOH as a cocatalyst, which broadens the double-layer capacitance of the photoanode and increases the electrochemical active surface area and the number of reaction sites, thereby improving the photoelectrochemical performance.

2.5. Phase Changes of the FeOOH/BiVO4/WO3 Photoanode After PEC Application

Figure 21a shows the XRD comparison of the FeOOH/BiVO4/WO3 composite photoanode before and after PEC application. After application, a slight decrease in the intensity of the (002), (020), and (200) crystal plane peaks of WO3 can be observed, as well as a slight reduction in the intensity of the (121) crystal plane peak of BiVO4. Since FeOOH is amorphous, its diffraction peaks cannot be observed. Additionally, the catalyst did not undergo any significant phase change. Figure 21b shows the Raman spectra comparison of FeOOH/BiVO4/WO3 before and after application. After application, the stretching vibration peak intensity of O-W-O slightly decreases, with no other changes. These results indicate that the BiVO4 and WO3 in the photoanode are relatively stable.
Figure 22a shows the TEM morphology image of the FeOOH/BiVO4/WO3 photoanode after PEC application, with BiVO4 still loaded on the surface of the flaky WO3. Figure 22b shows the HRTEM image of the sample, with measurements of lattice spacings in three different regions shown in Figure 22c–e. In Figure 22c, the lattice spacing is 0.345 nm, identified as the (011) plane of WO3. In Figure 22d, the lattice spacing is 0.259 nm, corresponding to the (200) plane of BiVO4. In Figure 22e, there is a thin amorphous FeOOH layer on the outermost surface, indicating that the FeOOH layer remains after PEC application. Figure 22f shows the SAED pattern of the sample, corresponding to the (200), (021), and (221) planes of WO3 and the (200) and (211) planes of BiVO4.

2.6. Mechanism of PEC Water Splitting Using FeOOH/BiVO4/WO3 Photoanode

Based on the experimental results mentioned above, we propose the mechanism of heterojunction- and cocatalyst-enhanced FeOOH/BiVO4/WO3 photoanodes (see Figure 23). The significant enhancement of PEC performance can be attributed to the following synergistic effects: (1) From a microstructural perspective, WO3 microcrystals facilitate charge transport due to fewer defects such as grain boundaries; BiVO4 nanocrystals increase the specific surface area; amorphous FeOOH is conducive to the adsorption and activation of water molecules. (2) In terms of optical properties, the porous and interconnected framework of BiVO4/WO3 enhances the light absorption range, improving the separation and migration efficiency of photogenerated carriers. (3) From the energy level structure perspective, the type-II heterojunction constructed between WO3 and BiVO4 significantly promotes charge separation of photogenerated carriers. Because the conduction band (CB) and valence band (VB) of BiVO4 are more negative than those of WO3, photogenerated electrons transfer from the CB of BiVO4 to the CB of WO3, and holes transfer from the VB of WO3 to the VB of BiVO4, thereby reducing the recombination of photoinduced carriers and enhancing charge separation efficiency. (4) From a reaction kinetics perspective, the holes enriched in FeOOH rapidly oxidize water to form oxygen through the Fe2+/Fe3+ valence cycle, while the accumulated electrons in WO3 transfer via FTO to the counter electrode for the hydrogen evolution reaction, thereby achieving photoelectrochemical water splitting.

3. Conclusions

A micro–nano- and amorphous–crystalline-structured FeOOH/BiVO4/WO3 photoanode was constructed using hydrothermal–electrodeposition–impregnation methods. The effects of electrodeposition time and impregnation solution concentration on the microstructure and photoelectrochemical performance of the prepared composite photoanodes were studied. The results showed that when the Bi electrodeposition time was 5 min and the Fe impregnation solution concentration was 10 mM, the FeOOH/BiVO4/WO3 photoanode possessed an optimal mass ratio of 0.53:8.67:100 and exhibited the best photoelectrochemical performance, with a photocurrent density of 2.04 mA/cm2. The main reasons for the performance improvement were attributed to the high conductivity of microcrystalline WO3, the high active area of nanocrystalline BiVO4, and the high active sites of amorphous FeOOH. Meanwhile, the construction of the heterojunction enhanced the built-in electric field, promoting charge separation; the cocatalyst reduced surface polarization, lowered the overpotential of the water oxidation reaction, and improved reaction kinetics. This study provides new ideas and references for the design and preparation of PEC water splitting photoanode materials.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14110828/s1, Figure S1. Experimental roadmap.

Author Contributions

Conceptualization, F.Z. and C.F.; data curation, R.L., G.W. and B.W.; formal analysis, R.L., Y.L. and C.F.; funding acquisition, F.Z.; methodology, R.L. and G.W.; project administration, F.Z. and P.L.; resources, R.L., G.W., B.W. and J.Q.; software, G.W.; supervision, P.L.; validation, J.Q.; writing—original draft, F.Z.; writing—review and editing, Y.L. and C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Gansu Province Science and Technology Major Project (22ZD6GA008), the Gansu Provincial Department of Education Young Doctor Support Project (2023QB-036), the China Postdoctoral Science Foundation (2022MD723787), and the Tamarisk Outstanding Young Talents Program of Lanzhou University of Technology (062202).

Data Availability Statement

The data supporting the findings of this study are available upon request from the corresponding author.

Conflicts of Interest

There are no conflicts to declare.

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Figure 1. XRD patterns of (a) WO3, BiVO4, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes; (b) XRD patterns of BiVO4/WO3 photoanodes with electrodeposition times of 3, 4, 5, 6, and 7 min.
Figure 1. XRD patterns of (a) WO3, BiVO4, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes; (b) XRD patterns of BiVO4/WO3 photoanodes with electrodeposition times of 3, 4, 5, 6, and 7 min.
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Figure 2. (a) Raman spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 samples immersed in different concentrations of FeOOH. (b) Local amplification of the Raman spectra. (c) Infrared spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 (10 mM) photoanodes.
Figure 2. (a) Raman spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 samples immersed in different concentrations of FeOOH. (b) Local amplification of the Raman spectra. (c) Infrared spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 (10 mM) photoanodes.
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Figure 3. (a) XPS survey spectrum of BiVO4/WO3 photoanode, (b) W 4f, (c) O 1s, (d) Bi 4f, and (e) V 2p.
Figure 3. (a) XPS survey spectrum of BiVO4/WO3 photoanode, (b) W 4f, (c) O 1s, (d) Bi 4f, and (e) V 2p.
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Figure 4. XPS spectra of FeOOH/BiVO4/WO3 photoanode: (a) survey spectrum of FeOOH/BiVO4/WO3, (b) W 4f, (c) O 1s, (d) Bi 4f, (e) V 2p, (f) Fe 2p, (g) W 4f shift, and (h) Bi 4f shift.
Figure 4. XPS spectra of FeOOH/BiVO4/WO3 photoanode: (a) survey spectrum of FeOOH/BiVO4/WO3, (b) W 4f, (c) O 1s, (d) Bi 4f, (e) V 2p, (f) Fe 2p, (g) W 4f shift, and (h) Bi 4f shift.
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Figure 5. (a,b) SEM morphology of WO3 at different magnifications, (c) cross-sectional morphology of WO3 flake thickness, (d,e) SEM morphology of BiVO4/WO3 at different magnifications, (f) cross-sectional morphology of the film after BiVO4 deposition, (g,h) SEM images of FeOOH/BiVO4/WO3 at different magnifications, (i) cross-sectional morphology after FeOOH deposition, and pore size distribution statistics of (j) BiVO4/WO3 and (k) FeOOH/BiVO4/WO3.
Figure 5. (a,b) SEM morphology of WO3 at different magnifications, (c) cross-sectional morphology of WO3 flake thickness, (d,e) SEM morphology of BiVO4/WO3 at different magnifications, (f) cross-sectional morphology of the film after BiVO4 deposition, (g,h) SEM images of FeOOH/BiVO4/WO3 at different magnifications, (i) cross-sectional morphology after FeOOH deposition, and pore size distribution statistics of (j) BiVO4/WO3 and (k) FeOOH/BiVO4/WO3.
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Figure 6. Element distribution of the BiVO4/WO3 photoanode: (a) morphology image, (b) W, (c) O, (d) Bi, (e) V, and (f) EDS result.
Figure 6. Element distribution of the BiVO4/WO3 photoanode: (a) morphology image, (b) W, (c) O, (d) Bi, (e) V, and (f) EDS result.
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Figure 7. Element distribution of the FeOOH/BiVO4/WO3 photoanode: (a) morphology image, (b) Fe, (c) W, (d) O, (e) Bi, (f) V, and (g) EDS result.
Figure 7. Element distribution of the FeOOH/BiVO4/WO3 photoanode: (a) morphology image, (b) Fe, (c) W, (d) O, (e) Bi, (f) V, and (g) EDS result.
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Figure 8. SEM images of BiVO4/WO3 at different electrodeposition times: electrodeposition for 3 min (a,b) surface morphology, (c) cross-sectional morphology; electrodeposition for 4 min (d,e) surface morphology, (f) cross-sectional morphology; electrodeposition for 5 min (g,h) surface morphology, (i) cross-sectional morphology; electrodeposition for 6 min (j,k) surface morphology, (l) cross-sectional morphology; electrodeposition for 7 min (m,n) surface morphology, (o) cross-sectional morphology; and pore size distribution of samples obtained at different electrodeposition times: (p) 3 min, (q) 4 min, (r) 5 min, (s) 6 min, and (t) 7 min.
Figure 8. SEM images of BiVO4/WO3 at different electrodeposition times: electrodeposition for 3 min (a,b) surface morphology, (c) cross-sectional morphology; electrodeposition for 4 min (d,e) surface morphology, (f) cross-sectional morphology; electrodeposition for 5 min (g,h) surface morphology, (i) cross-sectional morphology; electrodeposition for 6 min (j,k) surface morphology, (l) cross-sectional morphology; electrodeposition for 7 min (m,n) surface morphology, (o) cross-sectional morphology; and pore size distribution of samples obtained at different electrodeposition times: (p) 3 min, (q) 4 min, (r) 5 min, (s) 6 min, and (t) 7 min.
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Figure 9. SEM images of FeOOH/BiVO4/WO3 at different impregnation concentrations: (a,b) surface morphology at 5 mM impregnation concentration, (c) cross-sectional morphology; (d,e) surface morphology at 10 mM impregnation, (f) cross-sectional morphology; (g,h) surface morphology at 15 mM impregnation, (i) cross-sectional morphology; and pore size distribution of samples at different impregnation concentrations: (j) 5 mM, (k) 10 mM, and (l) 15 mM.
Figure 9. SEM images of FeOOH/BiVO4/WO3 at different impregnation concentrations: (a,b) surface morphology at 5 mM impregnation concentration, (c) cross-sectional morphology; (d,e) surface morphology at 10 mM impregnation, (f) cross-sectional morphology; (g,h) surface morphology at 15 mM impregnation, (i) cross-sectional morphology; and pore size distribution of samples at different impregnation concentrations: (j) 5 mM, (k) 10 mM, and (l) 15 mM.
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Figure 10. (a) EDS atomic percentage of Bi in samples with different electrodeposition times and (b) EDS atomic percentage of Fe in samples with different Fe ion dipping concentrations.
Figure 10. (a) EDS atomic percentage of Bi in samples with different electrodeposition times and (b) EDS atomic percentage of Fe in samples with different Fe ion dipping concentrations.
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Figure 11. (a) TEM of BiVO4/WO3, (b) HRTEM image, (c) WO3 lattice spacing, and (d) BiVO4 lattice spacing. The inset of (b) is SAED.
Figure 11. (a) TEM of BiVO4/WO3, (b) HRTEM image, (c) WO3 lattice spacing, and (d) BiVO4 lattice spacing. The inset of (b) is SAED.
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Figure 12. (a) HAADF, elemental distribution of (b) W, (c) O, (d) Bi, and (e) V of BiVO4/WO3.
Figure 12. (a) HAADF, elemental distribution of (b) W, (c) O, (d) Bi, and (e) V of BiVO4/WO3.
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Figure 13. (a) TEM image of FeOOH/BiVO4/WO3, (b) HRTEM image, (c) WO3 lattice spacing, (d) BiVO4 lattice spacing, (e) FeOOH amorphous layer (dashed line), and (f) SAED image.
Figure 13. (a) TEM image of FeOOH/BiVO4/WO3, (b) HRTEM image, (c) WO3 lattice spacing, (d) BiVO4 lattice spacing, (e) FeOOH amorphous layer (dashed line), and (f) SAED image.
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Figure 14. (a) HAADF image, and element distribution of (b) all elements, (c) W, (d) O, (e) Bi, (f) V, (g) Fe, and (h) elemental EDS of FeOOH/BiVO4/WO3.
Figure 14. (a) HAADF image, and element distribution of (b) all elements, (c) W, (d) O, (e) Bi, (f) V, (g) Fe, and (h) elemental EDS of FeOOH/BiVO4/WO3.
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Figure 15. (a) UV–Vis diffuse reflectance spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3; (b) Tauc plots.
Figure 15. (a) UV–Vis diffuse reflectance spectra of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3; (b) Tauc plots.
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Figure 16. Valence band spectra of (a) WO3, (b) BiVO4/WO3, (c) FeOOH/BiVO4/WO3, and (d) band structure.
Figure 16. Valence band spectra of (a) WO3, (b) BiVO4/WO3, (c) FeOOH/BiVO4/WO3, and (d) band structure.
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Catalysts 14 00828 g017
Figure 17. LSV curves of (a) BiVO4/WO3 with different electrodeposition times, (b) FeOOH/BiVO4/WO3 with different Fe ion concentrations, (c) WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes under a 100 mW/cm2 light source. I–t curves for (d) WO3, (e) BiVO4/WO3, and (f) FeOOH/BiVO4/WO3 at 1.23 V vs. RHE.
Figure 17. LSV curves of (a) BiVO4/WO3 with different electrodeposition times, (b) FeOOH/BiVO4/WO3 with different Fe ion concentrations, (c) WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes under a 100 mW/cm2 light source. I–t curves for (d) WO3, (e) BiVO4/WO3, and (f) FeOOH/BiVO4/WO3 at 1.23 V vs. RHE.
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Catalysts 14 00828 g018
Figure 18. (a) ABPE curves, (b) EIS curves, and (c) M–S curves of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3.
Figure 18. (a) ABPE curves, (b) EIS curves, and (c) M–S curves of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3.
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Catalysts 14 00828 g019
Figure 19. (a) LSV curves in sacrificial reagent electrolyte, (b) ηbulk curves, (c) ηsurface curves, and (d) dark current curves of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes.
Figure 19. (a) LSV curves in sacrificial reagent electrolyte, (b) ηbulk curves, (c) ηsurface curves, and (d) dark current curves of WO3, BiVO4/WO3, and FeOOH/BiVO4/WO3 photoanodes.
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Catalysts 14 00828 g020
Figure 20. CV curves of (a) WO3, (b) BiVO4/WO3, and (c) FeOOH/BiVO4/WO3 photoanodes and (d) plots of current density difference versus scan rate.
Figure 20. CV curves of (a) WO3, (b) BiVO4/WO3, and (c) FeOOH/BiVO4/WO3 photoanodes and (d) plots of current density difference versus scan rate.
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Catalysts 14 00828 g021
Figure 21. (a) XRD tests and (b) Raman tests of composite photoanodes before and after PEC application.
Figure 21. (a) XRD tests and (b) Raman tests of composite photoanodes before and after PEC application.
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Catalysts 14 00828 g022
Figure 22. (a) TEM morphology images, (b) HRTEM images, (c,d) lattice spacing images of WO3 and BiVO4, (e) HRTEM of amorphous FeOOH, and (f) SAED images of the FeOOH/BiVO4/WO3 photoanode after PEC application.
Figure 22. (a) TEM morphology images, (b) HRTEM images, (c,d) lattice spacing images of WO3 and BiVO4, (e) HRTEM of amorphous FeOOH, and (f) SAED images of the FeOOH/BiVO4/WO3 photoanode after PEC application.
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Catalysts 14 00828 g023
Figure 23. Schematic mechanism diagram of FeOOH/BiVO4/WO3 for PEC water splitting.
Figure 23. Schematic mechanism diagram of FeOOH/BiVO4/WO3 for PEC water splitting.
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Table 1. Comparison of PEC performance of BiVO₄/WO₃ and FeOOH cocatalyst photoanodes.
Table 1. Comparison of PEC performance of BiVO₄/WO₃ and FeOOH cocatalyst photoanodes.
Serial
Number		SamplesPreparation MethodElectrolyteJ (mA/cm2)Ref.
1BiVO4/WO3Spin coating0.1 M phosphate buffer0.8 at 1.2 V vs. RHE[41]
2BiVO4/WO3Spin coating0.5 M Na2SO41.0 at 1.23 V vs. RHE[42]
3BiVO4/WO3WO3: hydrothermal
BiVO4: spin coating		0.1 M phosphate buffer0.80 at 1 V vs. Pt[43]
4BiVO4/WO3Dip coating0.1 M Na2SO40.25 at 1.0 V vs. Ag/AgCl[44]
5BiVO4/WO3Electrodeposition0.1 M Na2SO40.70 at 0.8 V vs. Ag/AgCl[45]
6BiVO4/WO3WO3: hydrothermal
BiVO4: electrodeposition		0.5 M Na2SO41.09 at 1.23 V vs. RHEThis work
7FeOOH/BiVO4-OvHydrothermal0.2 M Na2SO41.18 at 1.23 V vs. RHE[46]
8NiOOH/FeOOH/BiVO4/WO3 Microwave annealing0.5 M Na2SO41.50 at 1.2 V vs. RHE[47]
9FeOOH/WO3Electrodeposition0.5 M K2SO41.90 at 1.8 V vs. RHE[48]
10FeOOH/BiVO4/WO3Impregnation0.5 M Na2SO42.04 at 1.23 V vs. RHEThis work
Table 2. Parameter values of each component in the equivalent circuit diagram.
Table 2. Parameter values of each component in the equivalent circuit diagram.
PhotoanodesR1 (Ω)R2 (Ω)
WO332.33486.0
BiVO4/WO326.85306.5
FeOOH/BiVO4/WO322.99201.5
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MDPI and ACS Style

Li, R.; Zhan, F.; Wen, G.; Wang, B.; Qi, J.; Liu, Y.; Feng, C.; La, P. Facile Synthesis of a Micro–Nano-Structured FeOOH/BiVO4/WO3 Photoanode with Enhanced Photoelectrochemical Performance. Catalysts 2024, 14, 828. https://doi.org/10.3390/catal14110828

AMA Style

Li R, Zhan F, Wen G, Wang B, Qi J, Liu Y, Feng C, La P. Facile Synthesis of a Micro–Nano-Structured FeOOH/BiVO4/WO3 Photoanode with Enhanced Photoelectrochemical Performance. Catalysts. 2024; 14(11):828. https://doi.org/10.3390/catal14110828

Chicago/Turabian Style

Li, Ruixin, Faqi Zhan, Guochang Wen, Bing Wang, Jiahao Qi, Yisi Liu, Chenchen Feng, and Peiqing La. 2024. "Facile Synthesis of a Micro–Nano-Structured FeOOH/BiVO4/WO3 Photoanode with Enhanced Photoelectrochemical Performance" Catalysts 14, no. 11: 828. https://doi.org/10.3390/catal14110828

APA Style

Li, R., Zhan, F., Wen, G., Wang, B., Qi, J., Liu, Y., Feng, C., & La, P. (2024). Facile Synthesis of a Micro–Nano-Structured FeOOH/BiVO4/WO3 Photoanode with Enhanced Photoelectrochemical Performance. Catalysts, 14(11), 828. https://doi.org/10.3390/catal14110828

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