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• Photoelectrochemical (PEC) water splitting has the potential to convert solar energy into sustainable hydrogen energy using semiconductor photocatalysts, thus realizing an efficient, green and carbon-free pathway for green hydrogen production [1, 2]. Narrow-band-gap metal oxides such as WO₃ (~2.7 eV) [3-5], α-Fe₂O₃ (~2.2 eV) [5, 6] and BiVO₄ (2.4 eV) [7, 8] are frequently utilized as photoanodes due to their inexpensiveness, superior photoactivity and great photostability. In particular, monoclinic phase BiVO₄ is regarded as an excellent photoanode due to its non-toxicity, abundance, suitable band gap and valence band potential [9, 10]. Unfortunately, the low photogenerated electron-hole separation efficiency and slow surface water splitting rate on BiVO₄ photoanode surface constrain the further improvement of solar-to-hydrogen (STH) efficiency [1, 11]. Introducing oxygen evolution co-catalysts (OECs), especially binary or multi transition metal-based cocatalysts (such as NiFe₂O₄/BiVO₄, CoFe₂O₄/BiVO₄ [12] and ZnCo₂O₄/BiVO₄ [13]), helps to encourage the transfer and separation of charges, thereby enhancing charge injection efficiency and overall photocurrent density. However, the interface of OECs/BiVO₄ heterojunction usually acts as a charge recombination center, resulting in lower charge transfer efficiency and terrible long-term durability. Therefore, it is of particular interest to modify the interface between OECs and BiVO₄ photoanode that inhibits electron/hole pairs′ recombination and accelerating water oxidation.

  Introducing carrier shuttle bridge between OECs and BiVO₄ photoanode has shown to be a workable solution to eliminate the inherent charge transfer barriers [14-16]. For example, bridging the coordination of metal hydroxides and BiVO₄ photoanode with urea can achieve stable and efficient solar-driven PEC water splitting with a photocurrent density of 4.8 mA/cm² at 1.23 V vs. RHE [17]. This is due to the fact that metal ion and urea’s strong coordination connection may prevent electron-hole pairs from recombining at the OECs/BiVO₄ heterojunction, increasing the transmission of charge to the water oxidation reactive sites. However, the reported PEC water splitting performance of BiVO₄-based photoanodes with the built charge shuttle bridge is far from satisfactory when compared with the theoretical photocurrent density of BiVO₄ photoanode (7.5 mA/cm²) [18, 19]. Searching a more efficeint charge shuttle bridge is thus of significance and urgency. Aluminum oxide (Al₂O₃) is a polycrystalline structure which is composed by oxygen and aluminum atoms through ionic and covalent bonds [20-22]. Consequently, the NiFeO_x/BiVO₄ photoanode modified with Al-O bridge is expected to achiesubstantially improved PEC water splitting performance by virtue of the good light transmission and extremely high chemical stability [23, 24].

  Herein, we successfully bridged NiFeO_x and BiVO₄ with the Al-O unit through a facile impregnation method. Benefiting from the dual improved carrier charge transport by the co-loaded NiFeO_x and Al₂O₃, this novel Al-O bridged NiFeO_x/BiVO₄ photoanode achieved an impressive photocurrent density of 5.87 mA/cm² at 1.23 V vs. RHE. A stoichiometric hydrogen and oxygen evolution of 222.64 µmol/cm² and 110.31 µmol/cm² achieved over Al-O bridged NiFeO_x/BiVO₄ photoanode, which is 2.26 and 18.06 times more than NiFeO_x/BiVO₄ and pristine BiVO₄ photoanode, respectively. This effort delivers a novel perspective to manipulate the interface of OECs/BiVO₄ photoanode for highly efficient solar-driven PEC water splitting.

  The surface morphology and microstructure of the prepared films were investigated by scanning electron microscopy (SEM). From the top view (Fig. 1a and Fig. S1 in Supporting information) and the corresponding side view (Fig. S2a in Supporting information), a uniform layer of BiVO₄ with irregular particles between 200 nm and 500 nm has been successfully grown uniformly on the FTO and in close contact with it. The surface and side SEM images of Al₂O₃/BiVO₄, NiFeO_x/BiVO₄, NiFeO_x/Al₂O₃/BiVO₄ photoanodes are shown in Figs. 1b-d and Figs. S2b-d (Supporting information), respectively, which show that they are highly similar to the morphology of BiVO₄ nanoparticles with or without the introduction of Al₂O₃ and NiFeO_x, indicating that the morphology of the original BiVO₄ nanoparticles is not changed by the simple preparation process. In order to further understand the microstructure of NiFeOx/Al₂O₃/BiVO₄ photoanodes, a detailed study was carried out using transmission electron microscopy (TEM) and high resolution transmission electron microscope (HRTEM). The BiVO₄ has a granular morphology in Fig. 1e and Fig. S3 (Supporting information) and an amorphous/disordered layer appears at the edge of BiVO₄, indicating that NiFeO_x and Al₂O₃ are coated on the surface of BiVO₄ particles. Further, a clearer HRTEM image of the NiFeOx/Al₂O₃/BiVO₄ photoanodes and the corresponding crystal plane spacing measurements were obtained in Fig. 1f by inverse Fourier transform, the spacing of the 10 ordered photoanodes was measured to be 3.1 nm, which represents the lattice stripe d = 0.31 nm matching the (121) crystal plane of BiVO₄. The component mapping and energy dispersive X-Ray spectroscopy (EDX) distribution of NiFeO_x/Al₂O₃/BiVO₄ shows that Ni, Fe, Al, Bi, V and O components are evenly scattered in the nanomorphology of the sample (Figs. 1g-l and Fig. S4 in Supporting information), which further confirms that we have successfully combined NiFeO_x/Al₂O₃/BiVO₄ photoanodes by a simple process.

  Figure 1

  

  Figure 1.  Top-view SEM images of (a) BiVO₄, (b) Al₂O₃/BiVO₄, (c) NiFeO_x/BiVO₄ and (d) NiFeO_x/Al₂O₃/BiVO₄ photoanodes. HRTEM images (e, f) and TEM-EDS mapping images of NiFeO_x/Al₂O₃/BiVO₄ film (g-l).

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  To further investigate the structures of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄, NiFeO_x/Al₂O₃/BiVO₄ samples, X-ray diffraction (XRD) analysis was performed in Fig. S5 (Supporting information). All samples give similar diffraction peaks as monoclinic phase scheelite BiVO₄ (JCPDS No. 14–0688), appearing at 2θ of 18.6°, 18.9°, 28.9°, 30.5°, 35.2°, 47.2° and 53.1°, corresponding to (110), (011), (121), (040), (002), (042), (222) crystallographic planes of BiVO₄, respectively [25]. No characteristic peaks of Al₂O₃ and NiFeO_x were observed, probably due to the thinness of the loaded Al₂O₃ and NiFeO_x layers or amorphous state, which is consistent with the TEM test results.

  To further reveal the surface chemistry and bonding state of the elements, X-ray photoelectron spectroscopy (XPS) analysis was performed on NiFeO_x/Al₂O₃/BiVO₄ and NiFeO_x/BiVO₄ photoanodes. According to Fig. S6 (Supporting information), the XPS spectrum reveals the following elements in the sample: Bi, V, O, Al, Ni and Fe. As far as XPS spectra are concerned, there are no noticeable change in position or intensity of Bi and V elements before and after the introduction of the Al₂O₃ interlayer in Figs. 2a and b. There are two peaks at 529.7 eV and 531.7 eV in the O 1s XPS spectra in Fig. 2c, corresponding to the binding energy of lattice oxygen and hydroxyl adsorbed oxygen [26, 27], respectively. The intensity of the lattice oxygen peak is significantly higher in NiFeO_x/Al₂O₃/BiVO₄ than in NiFeO_x/BiVO₄ sample, which is associated with Al₂O₃. Significantly, Al 2p signals were detected at 74.5 eV and 69.7 eV in Fig. 2d, which correspond to Al³⁺ in Al₂O₃ and Al³⁺ in tetrahedral positions [28, 29], respectively. The presence of NiFeO_x is further confirmed by the observation of faint Fe and Ni signals in Figs. 2e and f. Compared with the NiFeO_x/BiVO₄ sample, a shift of Fe and Ni binding energy to lower binding energy was observed in the NiFeO_x/Al₂O₃/BiVO₄ photoanode, revealing an off-domain effect due to the electron shuttle bridge Al₂O₃ to adjust the NiFeO_x/Al₂O₃/BiVO₄ to the lowest energy [24, 29, 30]. The XPS results suggest that Al₂O₃ was successfully integrated into the NiFeO_x/BiVO₄ anode as an efficient bridge for charge transfer and further validate the successful synthesis of NiFeO_x/Al₂O₃/BiVO₄ photoanode.

  Figure 2

  

  Figure 2.  (a) Bi 4f, (b) V 2p, (c) O 1s, (d) Al 2p, (e) Fe 2p and (f) Ni 2p XPS spectra of NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄.

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  By using 1 mol/L potassium borate buffer (KPi, pH 9.5) and AM 1.5G (100 mW/cm²), the PEC performance of the as-prepared BiVO₄ photoanodes was tested by linear scanning voltammetry (LSV). The photocurrent density of unmodified BiVO₄ was 1.65 mA/cm² at 1.23 V vs. RHE in Fig. 3a. Such low current density was ascribed by the severe charge recombination and low surface charge transport [25, 31]. The photocurrent density rose to 3.26 mA/cm² when BiVO₄ was loaded by Al₂O₃ (Fig. S7 in Supporting information). The photocurrent densities of the prepared NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes were significantly increased after the NiFeO_x co-catalyst was loaded, which is backed up by the recent literature claim that NiFeO_x co-catalyst can improve BiVO₄ photoanode PEC water oxidation activity [32, 33]. Differently, the prepared NiFeO_x/Al₂O₃/BiVO₄ film exhibit a significant photocurrent density of 5.87 mA/cm² at 1.23 V vs. RHE, which is nearly 1.4 times greater than that of NiFeO_x/BiVO₄ (4.21 mA/cm²) and 3.5 times higher than that of pure BiVO₄. The presence of the Al₂O₃ intermediate layer and the synergistic effect of the NiFeO_x co-catalyst might be the cause of the higher photocurrent density in the NiFeO_x/Al₂O₃/BiVO₄ system. To further reveal the response of the photoanode to light under illumination with the transfer and separation of carriers, we performed transient photocurrent tests. The rapid change of photocurrent density implies a very fast charge transfer in the photoanode. It is clearly observed that the current density of NiFeO_x/Al₂O₃/BiVO₄ photoanode is the strongest and more stable after compounding NiFeO_x and Al₂O₃ in Fig. 3b, which is the same as the previous experimental results of LSV. Assumedly, photogenerated holes on the electrode surface can be 100% injected into the electrolyte because Na₂SO₃ is oxidized by holes so quickly [34]. The LSV curves of the prepared BiVO₄ photoanodes were tested under the conditions of 1 mol/L phosphate buffer (KPi, pH 9.5) electrolyte with the addition of Na₂SO₃ in Fig. 3c. The trends of the photocurrent densities were the same and increased for all photoanodes, indicating that the photoanode suffered from a severe recombination of electrons and holes for PEC water splitting performance.

  Figure 3

  

  Figure 3.  PEC performance of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes: (a) Linear-sweep voltammogram curves at scan rate of 50 mV/s. (b) Transient photocurrent (i-t) spectra at 1.23 V vs. RHE, (c) LSV curves in the presence of 1 mol/L Na₂SO₃ with a scan rate of 50 mV/s. H₂ (d) and O₂ (e) gas evolution from PEC water splitting in a 1.0 mol/L potassium borate buffer solution electrolyte. (f) Long-time i-t spectra of H₂ and O₂ production of NiFeO_x/Al₂O₃/BiVO₄ photoanode.

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  To further demonstrate that the measured photogenerated currents were used for the PEC water splitting of hydrogen and oxygen production, the hydrogen and oxygen produced by BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes were quantified using gas chromatography. The Figs. 3d and e demonstrate that the gas precipitation from all photoanodes showed an increasing trend with time. After 120 min of continuous light exposure to the PEC decomposition water reaction, the yields of H₂ and O₂ from BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes were 12.33 and 5.66 µmol/cm², 72.14 and 35.06 µmol/cm², 98.42 and 49.07 µmol/cm² and 222.64 and 110.31 µmol/cm², respectively (Fig. S9 in Supporting information). The NiFeO_x/Al₂O₃/BiVO₄ photoanode for PEC decomposition of water performance was nearly 18 times that of the blank BiVO₄, which verified the feasibility of our design using Al₂O₃ as an electron shuttle bridge and the introduction of NiFeO_x co-catalyst for improving BiVO₄′s PEC performance. In the meanwhile, the photoelectrodes’ stability test revealed that after 120 min of exposure to light at 1.23 V vs. RHE, the photocurrent density of the NiFeO_x/Al₂O₃/BiVO₄ electrode was greater than that of BiVO₄, indicating a significant enhancement of the photostability of NiFeO_x/Al₂O₃/BiVO₄ (Fig. 3f). Meanwhile, the XRD pattern and SEM in Fig. S10 (Supporting information) showed little change in the NiFeO_x/Al₂O₃/BiVO₄ photoanode morphology before and after the test, further confirming the enhanced stability of the BiVO₄ photoanode after deposition of NiFeO_x and Al₂O₃. Therefore, the NiFeO_x/Al₂O₃/BiVO₄ thin film not only can avoid the oxidation of BiVO₄, but also can promote the transfer of carriers to the electrolyte solution, thus promoting the release of hydrogen and oxygen.

  UV–vis absorption spectra were used to check the synthetic photoanodes’ capacity to absorb light. In accordance with other results [18, 31, Fig. 4a demonstrates that BiVO₄ exhibits a considerable light absorption at a wavelength of around 500 nm. The light absorption of BiVO₄ photoanode has been slightly broadened after the introduction of Al₂O₃ and NiFeO_x. Correspondingly, the optical bandgaps of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ calculated by Tauc method are 2.47 eV, 2.46 eV, 2.46 eV and 2.43 eV, respectively (Fig. S11 in Supporting information). As a result, the NiFeO_x/Al₂O₃/BiVO₄ photoanode’s improvements in PEC water splitting activity are not primarily influenced by the energy band structure of the photoanodes. PEC activity and incident light wavelength can be related to one other using observations of incident photon to current conversion efficiency (IPCE) in Fig. 4b. Pure BiVO₄ shows the lowest IPCE value in the 300–550 nm. However, the IPCE value increased and reached 45% at 430 nm for Al₂O₃/BiVO₄. Similarly, the IPCE values of NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ show a further increase. In particular, the NiFeO_x/Al₂O₃/BiVO₄ shows the largest IPCE value of 58% at 430 nm, demonstrating that NiFeO_x and Al₂O₃ do not expand the light absorption, but can significantly increase the efficiency of solar energy conversion and the interfacial separation of photogenerated carriers. The applied bias photon-to-current efficiency (ABPE) was computed from the LSV curve to describe the solar water splitting half-reaction hydrogen precipitation efficiency. Accordingly, the NiFeO_x/Al₂O₃/BiVO₄ photoanode has a higher photoelectric conversion efficiency in the whole applied bias range and attains an ABPE of 2.19% at 0.67 V vs. RHE in Fig. 4c, further supporting the claim that combining NiFeO_x and Al₂O₃ is a successful method for enhancing BiVO₄’s PEC performance with higher photoelectric conversion efficiency at lower potentials. In comparison, this was found to be an extremely high level among the many photoanodes reported so far (Fig. 4d) [35-38].

  Figure 4

  

  Figure 4.  The Optical properties and PEC properties of BiVO₄, Al₂O₃/BiVO₄, NiFeOx/BiVO₄ and NiFeOx/Al₂O₃/BiVO₄ photoanodes: (a) the UV–vis absorption spectra; (b) IPCE curves at 1.23 V vs. RHE; (c) ABPE curves and (d) the ABPE of different photoelectrodes under AM1.5G illumination.

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  Based on the aforementioned experimental results, the values of surface charge injection efficiency (η_inj) and bulk phase charge separation efficiency (η_sep) of the samples were determined. Compared with the pure BiVO₄ photoanode, the η_inj of Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ have significantly improved in the overall bias voltage range in Fig. S12a (Supporting information), especially for the NiFeO_x/BiVO₄ electrode. The η_inj of NiFeO_x/BiVO₄ shows a decreasing trend when the bias voltage is > 1.3 V vs. RHE, indicating that the water oxidation reaction at the interface has almost reached saturation at this time [39], further demonstrating the effectiveness of NiFeO_x in accelerating the water oxidation reaction kinetics. The η_sep follows a similar pattern with η_sep reaching 74.4% for NiFeO_x/BiVO₄ and 62.4% for Al₂O₃/BiVO₄ at 1.23 V vs. RHE in Fig. S12b (Supporting information), both of which are much greater than BiVO₄ (59.3%). After introducing Al₂O₃ between NiFeO_x/BiVO₄, the η_sep of NiFeO_x/Al₂O₃/BiVO₄ is increased to 85.2% at 1.23 V vs. RHE, indicating that Al₂O₃ forms a fast transport channel for photogenerated electrons, reducing their chance of recombination in the bulk phase. Electrochemical impedance spectroscopy (EIS) measurements were carried out in Fig. S12c (Supporting information) under visible light irradiation to acquire a better understanding of the electron transfer kinetics. From the impedance diagrams of Fig. S12c and simulated impedance values of Table S1 (Supporting information), the impedances of Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes are all reduced compared to the original BiVO₄ photoanode, suggesting that the addition of both Al₂O₃ and NiFeO_x helps to improve the electrical conductivity of the BiVO₄ photoanode while the addition of Al₂O₃ optimizes the contact between NiFeOx and BiVO₄ substrate, which may be one of the reasons for its high photocurrent density. To demonstrate the charge transport mechanism of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes, the room temperature steady-state PL were characterized in Fig. S12d (Supporting information). As a consequence, BiVO₄ showed a signal peak in the wavelength range around 500 nm, which may be due to the emission of the BiVO₄ band gap jump. Likewise, the signal peaks of Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ showed no change in position but the intensity of the emission peak was significantly reduced, indicating that the NiFeO_x/Al₂O₃/BiVO₄ photoanode could prevent the photogenerated electron-hole pair recombination.

  Mott-Schottky (M-S) experiments were performed in Fig. 5a to further study the intrinsic electronic structure of the photoanodes. The positive slopes of the M-S curves show that these photoanodes are n-type semiconductors with a high proportion of migrating electrons. The carrier concentrations of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes were calculated to be 2.91 × 10²¹, 3.12 × 10²¹, 3.21 × 10²¹ and 3.97 × 10²¹ cm⁻³ in Table S2 (Supporting information), respectively. The carrier concentration did not change significantly and also verified that the Al, Ni and Fe ions were not doped into the BiVO₄ photoanode during the loading of Al₂O₃ and NiFeO_x. The OCP curves were obtained by rapidly switching off the light source after reaching steady state at the open circuit voltage to analyze the mechanism of charge separation. In Fig. 5b, two decay process are observed after switching off the light. The instantaneous rise in voltage indicates that part of the electrons has returned to the semiconductor’s valence band (VB), while the other part is captured and recombinated with holes. The later, slowly decaying process represents the injection of charge into the electrolyte. Compared to the other samples, the NiFeO_x/Al₂O₃/BiVO₄ photoanode has a more positive potential value of 0.21 V vs. RHE, while the BiVO₄ film has a potential value of 0.38 V vs. RHE, further demonstrating that the co-addition of NiFeO_x and Al₂O₃ can accelerate the aggregation of photogenerated electrons at the conduction band (CB) of BiVO₄, which in turn induces the NiFeO_x/Al₂O₃/BiVO₄ surface electron-hole pair separation.

  Figure 5

  

  Figure 5.  Carrier transport performance of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes: (a) Mott-Schottky curves under dark; (b) Open circuit potential measurements.

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  The energy band structure of the BiVO₄ and NiFeO_x/BiVO₄ samples was investigated to study and clarify the reaction mechanism for PEC water splitting. In the M-S curve, the intersection of the tangent line with the x-axis (y = 0) is the flat-band potential. The flat-band potential is 0.2 V higher than the CB potential for n-type semiconductors, so the CBs of BiVO₄ and NiFeO_x/BiVO₄ photoanodes are −0.06 V vs. NHE and −0.17 V vs. NHE (E_RHE = E_NHE + pH × 0.059). After combining with the previously obtained data on the forbidden band width of the films, a schematic energy band structure of the as-prepared films is obtained. From the Fig. 6, it can be found that the CB and VB of BiVO₄ films are gradually shifted to lower energy levels with the addition of NiFeO_x. Such a structure not only allows the excited holes of VB of BiVO₄ to be transferred to the aqueous solution, but also keeps the photogenerated electrons in BiVO₄’s CB.

  Figure 6

  

  Figure 6.  The mechanism of NiFeO_x/Al₂O₃/BiVO₄ photoanode for PEC water splitting.

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  Based on the above experimental results, we propose the mechanism of charge separation and transfer at NiFeO_x/Al₂O₃/BiVO₄ electrodes in the PEC water splitting process (Fig. 6). The energy band structure of NiFeO_x was adopted from previous reports [40, 41]. The electrons stored in the BiVO₄ photoanode are also rapidly released and transmitted to the Pt photocathode, where they reduce water to produce H₂. The holes remaining in the VB of the BiVO₄ photoanode will be rapidly transferred to the NiFeO_x surface via the Al-O bridge, then transferred to the electrolyte solution for the oxidation reaction. At the same time, the Al-O bridge acts as a channel to speed up the transmission of holes from the BiVO₄ photoanode to the NiFeO_x/Al₂O₃/BiVO₄ photoanode surface. Therefore, the excellent PEC water splitting performance of NiFeO_x/Al₂O₃/BiVO₄ photoanode depends on the dual improvement of carrier transfer both within the photoanode and between the photoanode and electrolyte when NiFeO_x and Al₂O₃ are co-loaded.

  To address the shortcomings of BiVO₄ photoanode that electrons and holes are easily compounded, NiFeO_x/Al₂O₃/BiVO₄ photoanode was successfully designed and prepared by compounding the interlayer Al₂O₃, which significantly improved the PEC performance of BiVO₄. The NiFeO_x/Al₂O₃/BiVO₄ photoanode not only maintains high chemical stability, but also achieves a water oxidation photocurrent density of 5.87 mA/cm² (1.23 V vs. RHE) and an ABPE value of 2.19% (0.67 V vs. RHE), respectively, a 3.5-fold improvement over BiVO₄ (1.65 mA/cm²) and 8.1 times better than BiVO₄ (0.27%). By modifying the Al₂O₃ layer with a NiFeO_x co-catalyst, the holes are rapidly transferred to the NiFeO_x surface for the water oxidation reaction, which reduces the interfacial charge recombination, regulates carrier concentration, effectively promotes charge separation and improves the stability of PEC water splitting. This research offers fresh perspectives and takeaways for the practical use of solar-powered PEC water splitting systems.

  Declaration of competing interest

  All authors declared that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

  CRediT authorship contribution statement

  Lina Wang: Writing – original draft, Writing – review & editing. Hairu Wang: Data curation, Writing – review & editing. Qian Bu: Writing – review & editing. Qiong Mei: Data curation, Methodology, Writing – review & editing. Junbo Zhong: Supervision, Writing – review & editing. Bo Bai: Resources, Supervision. Qizhao Wang: Funding acquisition, Resources, Supervision, Writing – review & editing.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. 52173277), the Fundamental Research Funds for the Central Universities of Chang’an University (No. 300102299304), the Innovative Research Team for Science and Technology of Shaanxi Province (No. 2022TD-04) and the open program of Key Laboratories of Fine Chemicals and Surfactants in Sichuan Provincial Universities (No. 2023JXZ03).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110139.

  
  
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  Figure 1  Top-view SEM images of (a) BiVO₄, (b) Al₂O₃/BiVO₄, (c) NiFeO_x/BiVO₄ and (d) NiFeO_x/Al₂O₃/BiVO₄ photoanodes. HRTEM images (e, f) and TEM-EDS mapping images of NiFeO_x/Al₂O₃/BiVO₄ film (g-l).

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  Figure 2  (a) Bi 4f, (b) V 2p, (c) O 1s, (d) Al 2p, (e) Fe 2p and (f) Ni 2p XPS spectra of NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄.

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  Figure 3  PEC performance of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes: (a) Linear-sweep voltammogram curves at scan rate of 50 mV/s. (b) Transient photocurrent (i-t) spectra at 1.23 V vs. RHE, (c) LSV curves in the presence of 1 mol/L Na₂SO₃ with a scan rate of 50 mV/s. H₂ (d) and O₂ (e) gas evolution from PEC water splitting in a 1.0 mol/L potassium borate buffer solution electrolyte. (f) Long-time i-t spectra of H₂ and O₂ production of NiFeO_x/Al₂O₃/BiVO₄ photoanode.

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  Figure 4  The Optical properties and PEC properties of BiVO₄, Al₂O₃/BiVO₄, NiFeOx/BiVO₄ and NiFeOx/Al₂O₃/BiVO₄ photoanodes: (a) the UV–vis absorption spectra; (b) IPCE curves at 1.23 V vs. RHE; (c) ABPE curves and (d) the ABPE of different photoelectrodes under AM1.5G illumination.

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  Figure 5  Carrier transport performance of BiVO₄, Al₂O₃/BiVO₄, NiFeO_x/BiVO₄ and NiFeO_x/Al₂O₃/BiVO₄ photoanodes: (a) Mott-Schottky curves under dark; (b) Open circuit potential measurements.

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  Figure 6  The mechanism of NiFeO_x/Al₂O₃/BiVO₄ photoanode for PEC water splitting.

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