Ligand-assisted assembly of FeCo–phytate catalytic interfaces on WO₃ photoanodes for dual enhancement of charge separation and oxygen evolution kinetics

Zedong Bi† ^a, Baoxin Ge†^b, Chenming Yuan^a, Ziqian Chen^a, Xiaoyan Huang^a, Honglin Qian^a, Tianqi Sun^a, Yuxuan Hu^a, Jiantao Zhang^a, Haiyue Wang^a, Shangye Wen^a, Ye Liu^a, Teng Liu^a, Jie Zeng^a, Biyi Chen*^a, Changwei Shi*^a and Caijin Huang*^b
aState Key Laboratory of Green and Efficient Development of Phosphorus Resources, School of Chemical Engineering & Pharmacy, Wuhan Institute of Technology, Wuhan 430205, China. E-mail: cby@wit.edu.cn; ShiChangwei@wit.edu.cn
^bState Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, China. E-mail: cjhuang@fzu.edu.cn

First published on 25th July 2025

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1. Introduction

Photoelectrochemical (PEC) water splitting represents a highly potential approach toward a clean hydrogen economy, offering sustainability, zero carbon emissions, and modular design potential.¹ However, the slow kinetics of the oxygen evolution reaction (OER), driven by its complex four-electron transfer process, remains a major bottleneck limiting the PEC efficiency.² Consequently, the development of photoanode materials combining high light absorption, efficient charge separation, and stable interfacial reactions is of significant scientific and industrial importance.^3,4 Monoclinic tungsten trioxide (m-WO₃) has garnered considerable interest due to its unique physicochemical properties. Its narrow bandgap (2.6–2.8 eV) enables visible light absorption up to 476 nm. The valence band, which is primarily composed of O 2p orbitals, lies at approximately 3.1 V vs. NHE, imparting strong oxidation capability and facilitating the OER. The conduction band, dominated by W 5d orbitals, is situated around 0.5 V vs. NHE and is characterized by the enhanced mobility of charge carriers (∼12 cm² V⁻¹ s⁻¹).⁵ In particular, the distorted WO₆ octahedra induce anisotropic charge distribution, generating a built-in electric field that promotes charge separation.⁶ In contrast to conventional metal oxides such as TiO₂, m-WO₃ exhibits superior intrinsic stability in strongly acidic electrolytes, thus expanding its applicability in PEC systems.⁷ Despite these promising features, WO₃ still suffers from low charge separation efficiency, often due to surface recombination centers, leading to efficiencies of less than 40%.⁸ Additionally, its high OER overpotentials (>450 mV) hinder its PEC performance, maintaining it well below theoretical efficiency limits.

Various strategies, including element doping,⁹ oxygen vacancy engineering,¹⁰ and morphological and structural regulation,¹¹ have been proposed to address these challenges. Among these approaches, surface cocatalyst modification is particularly effective in directly enhancing interfacial reaction kinetics.^12,13 Conventional load-based nanoparticles (e.g., IrO₂ and Co–Pi) can lower OER overpotentials but often suffer from issues such as particle agglomeration, increased interfacial resistance, and poor long-term stability.^14,15 Recent studies have indicated that atomically uniform coatings can effectively mitigate these limitations.^16,17 First, continuous modification layers maximize the exposure of active sites and extend the heterojunction interfaces, thereby strengthening the built-in electric field.^18,19 Second, chemically bonded interfaces effectively prevent cocatalyst delamination, significantly improving the stability of the photoelectrode.^20,21 However, the design of these modification layers requires careful optimization to meet stringent criteria, including appropriate band alignment, chemical stability, and environmental compatibility.

Phytic acid, a naturally occurring and environmentally friendly organic phosphorus compound, offers a promising strategy for designing efficient and stable catalytic interfaces.²² Its fully phosphorylated myo-inositol ring strongly chelates metal ions (Fig. S1), forming stable coordination complexes with semiconductors, such as WO₃ and TiO₂. This interaction facilitates the formation of dense, uniform mono- or multilayer coatings on the photoelectrode surface, which enhances charge separation on the WO₃ surface and protects it from electrolyte-induced corrosion, thereby improving the stability of the photoelectrode. Moreover, the phosphate functionalities of phytic acid interact with electrocatalytically active metal ions, leading to the formation of stable octahedral complexes.²³ This coordination effectively tunes the electron density of the metal centers, while intermolecular hydrogen bonding networks further reinforce the structural stability of the modified layer.²⁴ When phytic acid binds to OER-active metal ions (e.g., Fe, Co, and Ni), synergistic effects facilitate electron transfer and lower the OER overpotential, significantly enhancing the kinetics of the OER.^25,26

In this study, an ∼2.5 nm phytic acid (PA)-coordinated Fe and Co (PA–FeCo) layer was deposited onto WO₃ photoanodes via interfacial coordination self-assembly to enhance surface charge separation and reduce the OER overpotential. PEC measurements demonstrated that the WO₃@PA–FeCo photoanode exhibited a photocurrent density of 1.93 mA cm⁻² and a surface charge separation efficiency of 77.7%, surpassing pristine WO₃ by a factor of 3.62 in photocurrent density (0.53 mA cm⁻²) and 1.42 times in charge separation efficiency (54.6%), accompanied by a negative shift of 150 mV on onset potential. Surface photovoltage (SPV) measurements revealed a significantly enhanced SPV signal in WO₃@PA–FeCo compared with pristine WO₃, indicating the presence of a stronger built-in electric field that effectively promotes surface charge separation. According to density functional theory (DFT) calculations, the Fe–Co bimetallic centers regulate the charge distribution at the phytate/WO₃ interface and optimize the potential-limiting step, significantly lowering the OER overpotential, as verified by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR). Moreover, owing to the intrinsic properties of phytate molecules, this interfacial self-assembly strategy exhibited great versatility, enhancing the PEC performance across a range of photoanodes, including BiVO₄, TiO₂, and Fe₂O₃. This study introduces natural phosphonic compounds as versatile tools for engineering atomically precise interfacial modification layers, offering a novel strategy for developing cost-effective, high-performance photoanodes.

2. Experimental

2.1 Reagents

Ferric sulfate (Fe₂(SO₄)₃), cobalt sulfate (CoSO₄), tungstic acid (H₂WO₄), sodium tungstate dihydrate (Na₂WO₄·2H₂O), hydrogen peroxide (30 wt% H₂O₂), phytic acid solution (C₆H₁₈O₂₄P₆ at 70% in H₂O) and concentrated hydrochloric acid (12 M HCl) were obtained from Sinopharm Chemical Reagent Co., Ltd. All reagents used in this study were of analytical grade. Deionized water was prepared using a UPT-ll-20T ultra-pure water system.

2.2 Synthesis of materials

The fabrication of the WO₃ photoanode comprises two primary steps: seed layer deposition and solvothermal growth.²⁷ To prepare the seed layer, 2.5 g of tungstic acid (H₂WO₄) was dissolved in 20 mL of 30 wt% hydrogen peroxide (H₂O₂) under magnetic stirring for 30 minutes until a clear solution was obtained. Subsequently, 0.5 mL of the precursor solution was uniformly coated onto a fluorine-doped tin oxide (FTO) glass substrate, followed by annealing in a muffle furnace at 500 °C for 2 hours. To enhance the robustness of the seed layer, this coating and annealing process was repeated three times. For the solvothermal growth step, the precursor solution was obtained by dissolving 8.25 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and 3.65 g of sodium chloride (NaCl) in 25 mL of deionized water. During stirring, 4.2 mL of 12 M hydrochloric acid was added dropwise, and the solution was subsequently diluted to a final volume of 250 mL. A volume of 20 mL of this solution was transferred to a 50 mL Teflon-lined autoclave containing the seeded FTO substrate, and the solvothermal reaction was conducted at 170 °C for 2 hours in a drying oven. The resulting electrode was then annealed at 500 °C for 2 hours in a muffle furnace to yield the final WO₃ photoanode.

The obtained WO₃ electrode was immersed in 150 mL of 10 mg mL⁻¹ phytic acid solution for 10 minutes. Subsequently, 25.2 mL of 10 mg mL⁻¹ Fe₂(SO₄)₃ solution was added to the same beaker, and the electrode was immersed for an additional 10 minutes. After immersion, a portion of the electrode was removed, rinsed with deionized water, and dried to yield the WO₃@PA–Fe photoanode. Next, 25.2 mL of 10 mg mL⁻¹ CoSO₄ solution was added to the beaker, and the remaining electrode was immersed for another 10 minutes. Finally, the electrode was removed, rinsed with deionized water, and dried to obtain the WO₃@PA–FeCo photoanode.

2.3 Characterization of material

X-ray diffraction (XRD, Rigaku) was employed to examine the crystallinity of the samples, with measurements conducted at a scan rate of 7° min⁻¹ in the angular range of 10°–80°. Raman measurements were performed using a Thermo Fisher DRX instrument equipped with a 532 nm semiconductor laser as the excitation source, and measurements were conducted on a temperature-controlled, anti-vibration optical platform at 25 °C. The spectra of UV-Vis absorption were recorded using a Shimadzu UV2450 spectrophotometer equipped with an integrating sphere, scanning across the 300–800 nm wavelength range at 1 nm intervals. The surface morphology was observed using a field emission scanning electron microscope (FESEM, JEOL, JSM-7001F) operated at an accelerating voltage of 5 kV. Microstructural analysis was performed using high-resolution transmission electron microscopy (HRTEM, FEI Talos) operated at 200 kV, coupled with an energy-dispersive spectrometer (EDS, Ultim Max) for elemental composition analysis. The surface electronic structure was investigated using X-ray photoelectron spectroscopy (XPS, Thermo Scientific 250Xi) with an Al Kα monochromatic light source. Charge correction was applied using the standard C 1s (284.8 eV) binding energy for all elements. SPV measurements were carried out in a sealed environment utilizing monochromatic illumination provided by a prism monochromator (SBP 300). Contact angle measurements were performed using a drop shape analyzer (Krüss DSA100), integrated with a high-precision injection system, a backlight LED cold light source (wavelength 520–550 nm) and a high-speed CMOS camera. In situ ATR-FTIR tests were performed using a Thermo Scientific Nicolet iS50 spectrometer equipped with a CaF₂ crystal and a liquid nitrogen-cooled MCT detector. The measurements were conducted in 1.0 M potassium borate electrolyte over an applied potential range of 0.9–1.6 V_RHE. All spectra were background-subtracted to enhance the signal resolution.

2.4 PEC measurement

The PEC performance was assessed using a CHI 760e electrochemical workstation. The photoelectrochemical cell was configured with three electrodes: a platinum sheet as the counter electrode, a Hg/HgO electrode as the reference and the prepared sample as the working electrode. A 1.0 M potassium borate solution served as the electrolyte (pH 9.5). A xenon arc lamp (PLS-SXE300+) provided simulated solar illumination (AM 1.5G, 100 mW cm⁻²) calibrated with a photometric sensor. Linear sweep voltammetry (LSV) was measured at a scan rate of 20 mV s⁻¹. Mott–Schottky plots were obtained in the dark using a 10 mV s⁻¹ scan rate at 1000 Hz. Electrochemical impedance spectroscopy (EIS) measurements were conducted ranging from 10⁻² to 10⁵ Hz under an open-circuit voltage. The reversible hydrogen electrode potential was calculated using the following equation:

ERHE (V) = 0.098 (V) + Eapplied (V) + 0.059 × pH,	(1)

where E_RHE represents the potential versus the reversible hydrogen electrode potential, E_applied is the applied bias during testing, pH denotes the pH value of the electrolyte, and 0.098 V stands for the Hg/HgO electrode's standard electrode potential. Mott–Schottky data processing follows eqn (2) and (3):

	(2)

	(3)

where C represents the capacitance per unit area (F cm⁻²); ε_r and ε₀ denote the dielectric constant of WO₃ (20)²⁸ and the vacuum permittivity (8.854 × 10⁻¹² F m⁻¹), respectively; V denotes the applied voltage; and q denotes the electronic charge (1.6 × 10⁻¹⁹ C). The equation for calculating the absorbed photon-to-current efficiency (ABPE) under an external bias is as follows:

	(4)

where j, E, and P represent the detected current density, externally applied bias voltage, and optical power intensity (100 mW cm⁻²), respectively. The charge separation efficiency (η_sur) is calculated using the following equation:

	(5)

where j(KBi) represents the current density when the electrolyte is 1 M Kbi and j(KBi + Na₂SO₃) represents the current density recorded with an electrolyte consisting of 1 M KBi and 0.5 M Na₂SO₃ used as a hole scavenger. The formula for the incident photon-to-current efficiency (IPCE) calculation is as follows:

	(6)

where λ represents the wavelength of monochromatic light (nm), i denotes the photoelectric current density under specific monochromatic illumination (mA cm⁻²), and P is the incident monochromatic light power density (mW cm⁻²). The transient decay time τ is related to the logarithm of parameter D, where D is calculated using the following equation:

	(7)

where I_t, I_st and I_in represent the photoelectric current, steady-state photoelectric current, and initial current at time t (s), respectively. τ is defined as the time when lnD = −1.

2.5 Computational details

First-principles calculations based on plane-wave pseudopotential density functional theory (DFT) were performed using the Vienna Ab initio Simulation Package (VASP). The exchange-correlation effects were described by the Perdew–Burke–Ernzerhof (PBE) functional within the framework of the generalized gradient approximation (GGA), complemented by projector-augmented wave (PAW) pseudopotentials. A plane-wave energy cutoff of 520 eV was implemented to ensure convergence accuracy. Structural optimization was carried out on a 1 × 1 × 1 gamma grid, while self-consistent calculations were performed using a 3 × 3 × 1 central grid. The convergence criteria included an energy threshold of 1 × 10⁻⁵ eV and an atomic force threshold of 0.02 eV Å⁻¹. van der Waals interactions were corrected using the DFT-D3 method with Becke–Johnson damping. A 25 Å vacuum layer was introduced to minimize periodic image interactions.

The mechanism of the surface catalytic reactions, exemplified by the oxygen evolution reaction (OER), involves a four-step reaction on the WO₃ surface:

First:

OH− + * → e + *OH,	(8)

Second:

OH− + *OH → e + *O + H2O,	(9)

Third:

OH− + *O → e + *OOH,	(10)

Finally:

OH− + *OOH → e + O2 + H2O,	(11)

where * represents the adsorption site. The equations for calculating the Gibbs free energy change of the intermediate states related to the OER process are as follows:

	(12)

	(13)

	(14)

where E_*, E_*OH, E_*O and E_*OOH denote the total energies of pristine WO₃ and its hydroxyl adsorbed state, oxygen adsorbed state, and hydroperoxyl adsorbed state, respectively. E_H2O and E_H2 denote the energies of the gaseous free water molecules and hydrogen molecules, respectively. The Gibbs free energy calculation formulas are as follows:

ΔGi = ΔEi + Δ(ZPE)i − TΔSi, (i = 1, 2, 3)	(15)

ΔG4 = 4.92 eV − (ΔG1 + ΔG2 + ΔG3),	(16)

where ΔE_i (i = 1, 2, 3) denotes the energy derived from eqn (12)–(14), with the zero-point energy (ZPE) and entropy correction term TΔS obtained through vibrational frequency calculations. The overpotential calculation formula is as follows:

	(17)

where max(ΔG_i (i = 1, 2, 3, 4)) represents the maximum Gibbs free energy change among the four intermediate reactions. The work function (Φ) is calculated using eqn (18):

Φ = Evac − Ef,	(18)

where E_vac and E_f denote the energies of the stationary surface electrons near the vacuum level and the Fermi level, respectively.

3. Results and discussion

3.1 Structural characterization

The surface morphology of the photoanodes was analyzed via FESEM, as shown in Fig. 1(a)–(c). The pristine WO₃ photoanode displays a flake-like nanostructure, with individual nanoflakes approximately 500 nm in lateral dimension. This morphology provides a high specific surface area that increases the availability of surface-active sites. The surface morphology of the PA–metal-coated photoanodes remains unchanged, which is attributed to the ultrathin, conformal nature of the PA–metal coating. The HRTEM image in Fig. 1(d) reveals distinct lattice fringes with an interplanar spacing of 0.27 nm, matching the (022) plane of WO₃. An amorphous PA–FeCo layer (∼2.5 nm thick) encapsulates the WO₃ nanoflake, which effectively isolates the WO₃ photoanode from the electrolyte and mitigates electrolyte-induced corrosion, thereby improving stability. Additionally, the PA–FeCo layer, serving as a catalytically active component, directly interacts with the electrolyte, significantly enhancing OER efficiency. The polyphosphate groups in phytic acid impart strong hydrophilicity, facilitating the material exchange between the photoelectrode and electrolyte and further improving PEC performance. The inset of Fig. 1(d), showing the selected area electron diffraction (SAED) pattern, confirms the polycrystalline nature of WO₃, with two prominent diffraction rings corresponding to the (002) and (022) planes. As shown in Fig. 1(e), the EDS analysis verifies the incorporation of W, O, P, C, Fe and Co elements in the WO₃@PA–FeCo photoanode. Furthermore, the corresponding elemental mapping (Fig. 1(f)–(l)) reveals the uniform distribution of PA–FeCo constituents across the WO₃ surface. In summary, these findings confirm that a 2.5 nm amorphous PA–FeCo layer is uniformly coated on the WO₃ photoanode surface. It can be predicted that the hydrophilicity of the PA–FeCo layer and its abundant OER active sites significantly enhance the surface charge separation and OER kinetics for the WO₃ photoanode.

XPS was conducted to investigate the chemical bonding types and variations in the chemical states of the elements on the photoanode surface. As shown in Fig. 2(a), the W 4f_7/2 and 4f_5/2 peaks of pristine WO₃ appear at 36.06 eV and 38.2 eV, respectively.²⁹ In contrast, for the PA–metal-modified WO₃ photoanodes, these peaks shift negatively to 35.82 eV and 35.54 eV for W 4f_7/2, and to 37.97 eV and 37.72 eV for W 4f_5/2. This suggests that electrons transfer from the PA–Fe and PA–FeCo layers to the WO₃, leading to a partial reduction of the W. This electron migration increases the surface dipole moment and induces an interface electric field between WO₃ and the PA–metal complex, thus contributing to surface charge separation.³⁰ Fig. 2(b) shows that the O 1s spectrum of pristine WO₃ presents two distinct peaks located at 529.8 eV and 531.98 eV, which can be attributed to lattice oxygen and adsorbed oxygen species, respectively.³¹ Upon PA–metal modification, the O 1s spectra of WO₃@PA–Fe and WO₃@PA–FeCo display distinct characteristic peaks at 530.79 eV and 531.08 eV, respectively. These peaks are attributable to the characteristic oxygen species in the H₂PO-4 groups, which confirms the successful anchoring of PA onto photoanode surfaces.³² Furthermore, the PO binding energy in the WO₃@PA–FeCo shifts to higher values compared with the WO₃@PA–Fe. The observed binding energy shift originates from Co incorporation, where Co–O coordination bonding elevates the metal oxidation state. This electronic perturbation enhances polarization of the oxygen electron cloud, ultimately inducing the PO peak's positive shift. Fig. 2(c) presents the P 2p peaks at 133.8 eV for the WO₃@PA–Fe and 134.61 eV for the WO₃@PA–FeCo. The higher phosphorus binding energy in the WO₃@PA–FeCo suggests that the incorporation of Co facilitates enhanced electron withdrawal from P. Fig. 2(d) further reveals a positive shift in the Fe 2p peaks for the WO₃@PA–FeCo relative to WO₃@PA–Fe, consistent with the enhanced electron delocalization induced by Co. Fig. 2(e) displays distinct characteristic peaks of Co 2p, confirming the successful coordination of Co within the PA–FeCo bimetallic complex.³³ Fig. 2(f) presents the Raman spectra of the three photoanodes. The peaks observed at 272 cm⁻¹ and 327 cm⁻¹ are attributed to the bending vibrations of bridging O–W–O bonds, while those at 716 cm⁻¹ and 806 cm⁻¹ arise from the stretching vibrations of the bonds, confirming the monoclinic phase structure of the sample.³⁴ Additionally, a phosphate stretching vibration peak at 947 cm⁻¹ further confirms the successful incorporation of phytate.³⁵ Fig. 2(g) presents the XRD patterns of the three photoanodes. For the pristine WO₃ photoanode, diffraction peaks at 23.5°, 23.9°, 24.7°, 33.6°, and 34.5° correspond to the (002), (020), (200), (022), and (202) crystal planes, respectively (JCPDS 43-1035). The PA–metal-coated photoanodes exhibit an unchanged diffraction pattern compared with the pristine WO₃ owing to the thin and amorphous structure of the PA–metal layer. Fig. 2(h) and (i) show the water contact angle measurements for pristine WO₃ and WO₃@PA–FeCo. The WO₃@PA–FeCo exhibits a significant enhancement in hydrophilicity (20.0°) compared with pristine WO₃ (48.7°). This improvement in hydrophilicity is primarily due to the incorporation of numerous hydrophilic phosphate oxy and hydroxyl groups via PA modification. These groups facilitate material exchange between the photoanode and electrolyte, which aligns with the HRTEM image analysis. Electrochemical active surface area measurements further confirm that the introduction of the PA–FeCo layer substantially increases the number of OER active centers, enhances the electrical conductivity of the PA coating, and improves electrolyte access (Fig. S2).

3.2 PEC performance measurement

As shown in Fig. 3(a), the LSV curves demonstrate a marked enhancement in the photocurrent density of WO₃ photoanodes modified with PA–metal cocatalysts relative to pristine WO₃, confirming their effectiveness in boosting PEC water-splitting performance. Among the modified photoanodes, the WO₃@PA–FeCo photoanode demonstrates the most significant improvement, reaching a photocurrent density of 1.93 mA cm⁻² at 1.23 V_RHE, which represents an enhancement of approximately 3.62-fold compared with the pristine WO₃. This enhancement is likely attributed to the formation of bimetallic active sites within the PA–FeCo cocatalyst, which facilitates improved surface charge separation and transfer. Additionally, the onset potential of the WO₃@PA–FeCo shifts negatively by 150 mV relative to the pristine WO₃, suggesting a substantial enhancement in OER activity and PEC efficiency facilitated by the PA–FeCo cocatalyst. An immersion time of 10 minutes in the PA–metal solution was found to achieve optimum performance, as confirmed by additional tests (Fig. S3). Fig. 3(b) presents the ABPE measurements for the three photoanodes. The WO₃@PA–FeCo photoanode reaches the highest ABPE value of 1.27% at 0.54 V_RHE, indicating that the PA–FeCo composite layer enhances hole migration, thereby promoting efficient surface charge separation and OER kinetics. The Tafel slopes of the photoanodes are shown in Fig. 3(c). Compared with pristine WO₃, the PA–metal-coated WO₃ photoanodes exhibit significantly lower Tafel slopes. A lower Tafel slope signifies that current density increases more rapidly at lower overpotentials, providing further evidence that the cocatalyst accelerates OER kinetics. Fig. 3(d) compares the transient photocurrent responses of the samples. The WO₃@PA–FeCo photoanode exhibits the highest photocurrent density, consistent with its LSV performance. This enhancement is likely attributed to its superior surface charge separation efficiency, which mitigates recombination losses during carrier transport. Transient decay kinetics analysis (Fig. 3(e)) reveals that WO₃@PA–FeCo achieves a carrier lifetime (τ) of 2.47 s, exceeding the pristine WO₃ (0.36 s) by nearly an order of magnitude. The extended τ value correlates with the PA–FeCo cocatalyst's ability to establish efficient charge extraction pathways, suggesting that the WO₃@PA–FeCo possesses superior charge separation capabilities, consistent with previous findings. Fig. 3(f) shows that the surface charge separation efficiency of the WO₃@PA–FeCo reaches 77.7%, which is significantly higher than that of the pristine WO₃ (54.6%). The charge injection efficiency profiles show similar behavior (Fig. S4). Fig. 3(g) shows the IPCE curves of the three samples. The PA–metal-coated photoanodes exhibit significantly higher IPCE values compared with the pristine WO₃, indicating improved light energy utilization. This enhancement is likely due to enhanced surface charge separation or accelerated OER kinetics in the modified photoanodes. This further confirms that the PA–FeCo cocatalyst substantially enhances charge separation efficiency. Fig. 3(h) presents the Nyquist plots of the samples along with their corresponding equivalent circuits. Upon introducing the PA–metal cocatalyst, the arc radius of the WO₃ photoanodes decreases, indicating a reduction in charge transfer resistance. This suggests that the cocatalyst enhances the transport and separation of photogenerated electron–hole pairs in the WO₃ photoanodes, improving charge transfer efficiency. The equivalent circuit diagram consists of R_s, R_ct and CPE, which correspond to the resistance between the FTO substrate and catalyst, charge transfer resistance and constant phase angle element, respectively. The fitted values are summarized in Table S1. Specifically, the R_ct values for the pristine WO₃ and WO₃@PA–FeCo are 3437 Ω and 1273 Ω, respectively, demonstrating that the PA–FeCo composite layer significantly reduces the interfacial resistance encountered during charge transfer between the electrolyte and electrode surface, which in turn accelerates OER kinetics. Based on the Bode phase diagram results, the electronic lifetime of WO₃@PA–FeCo is approximately 2.64 times longer than that of pristine WO₃, as shown in Fig. S5. Fig. 3(i) displays the Mott–Schottky characteristics of the three photoanodes. The flat-band potentials (V_fb) of the pristine WO₃, WO₃@PA–Fe, and WO₃@PA–FeCo are 0.74 V, 0.55 V, and 0.32 V, respectively. The lowest V_fb of the WO₃@PA–FeCo suggests that the PA–FeCo cocatalyst shifts the Fermi level closer to the conduction band bottom, enhancing band bending and increasing the driving force for charge transport. Moreover, the N_d values of the three photoanodes are 4.34 × 10²⁰ cm⁻³, 5.31 × 10²⁰ cm⁻³ and 10.7 × 10²⁰ cm⁻³, respectively, further confirming that the PA–metal layer enhances photogenerated charge transfer. Open-circuit potential (OCP) measurements (Fig. S6) reveal that the WO₃@PA–FeCo photoanode shows a larger negative OCP shift, indicating stronger optical response characteristics. This shift corresponds to a 0.15 V higher photovoltage than the pristine WO₃, which enhances the driving force for the OER reaction and promotes efficient photogenerated charge migration. The stability test (Fig. S7) indicates that the WO₃@PA–FeCo photoanode retains 75% of its initial photocurrent density after 4.5 hours of continuous illumination, while the pristine WO₃ exhibits nearly undetectable photocurrent. This indicates that the PA–FeCo layer acts as a protective coating, preventing direct contact between the WO₃ photoanode and the electrolyte. This isolation reduces WO₃ corrosion and dissolution, significantly enhancing the stability of the photoanode. Furthermore, Fig. S8 compares the reported photocurrent densities of WO₃ photoanodes at 1.23 V vs. RHE in recent years, highlighting the improvements achieved with the PA–metal modification.

3.3 Mechanism for PEC performance enhancement

The UV-Vis spectra of the three photoanodes (Fig. S9) show nearly identical curves, indicating that the PA–metal cocatalyst slightly affects the light absorption characteristics of the WO₃ photoanode. This suggests that the observed enhancement in PEC performance is primarily due to improved charge separation efficiency or increased OER activity. Work function (Φ) calculations (Fig. 4(a) and (b)) reveal that the PA–FeCo cocatalyst has a work function of 2.08 eV, which is significantly lower than that of the pristine WO₃ (7.06 eV), suggesting that electrons preferentially move from the PA–FeCo coating to the WO₃ surface. Upon reaching equilibrium, an electric potential difference forms between WO₃ and PA–FeCo layers, establishing an interfacial electric field. Given the encapsulated structure of PA–FeCo and WO₃, this electric field extends over a larger area, facilitating charge separation at the photoanode surface. Fig. 4(c) and (d) illustrates charge density difference analysis, which shows that the PA–FeCo-coated WO₃ exhibits significantly enhanced charge polarization compared with the PA–Fe-coated WO₃. This effect of charge delocalization and confinement significantly strengthens the internal electric field at the interface between WO₃ and PA–FeCo composites, thereby improving charge separation efficiency at the WO₃ surface. Fig. 4(e) presents the SPV test curves for the three photoanodes. In the 300–355 nm range, the WO₃@PA–FeCo photoanode displays the strongest SPV intensity, followed by the WO₃@PA–Fe, while the pristine WO₃ shows the lowest intensity. This trend indicates that PA–FeCo-coated WO₃ generates the highest number of photogenerated carriers, demonstrating the most efficient surface charge separation. This finding provides strong evidence that the synergistic effect of the bimetallic components enhances the surface charge separation efficiency, which agrees with the work function calculations. Fig. 4(f) shows the planar-average charge density calculations for WO₃@PA–Fe and WO₃@PA–FeCo. It is evident that the electrostatic potential difference in the z direction for WO₃@PA–FeCo is significantly higher than that for WO₃@PA–Fe, which thoroughly demonstrates the establishment of a stronger internal electric field between WO₃ and the PA–FeCo layer. The calculated adsorption energies reveal that the hydroxyl (*OH) intermediate preferentially binds to the Co site, indicating that Co serves as the primary active center for this step (Fig. S10). Gibbs free energy calculations (Fig. 4(g)) reveal that the rate-determining step (RDS) in OER for pristine WO₃ primarily involves the formation of the *OOH intermediate. After modification with PA–Fe and PA–FeCo layers, the theoretical overpotential for this step decreases significantly from 0.79 V to 0.66 V and 0.40 V. This suggests that the synergistic effect of the bimetallic system plays a crucial role in reducing OER overpotentials and enhancing OER kinetics, which aligns with the observed negative shift in onset potential in the LSV tests. Furthermore, in situ ATR-FTIR measurements were conducted to elucidate the evolution mechanism of OER intermediates on the WO₃@PA–FeCo photoanode (Fig. 4(h)). The absorption band near 1267 cm⁻¹ corresponds to the O–O stretching vibration of *OOH species,³⁶ while the broad band in the range of 3320–3560 cm⁻¹ is assigned to the O–H stretching vibration of *OH intermediates,³⁷ thereby confirming the adsorbate evolution mechanism of the OER process³⁸ (Fig. 4(i)). Notably, as the applied potential increases, the intensity of the *OOH characteristic band is markedly enhanced, indicating an accelerated formation rate of the *OOH intermediate. This observation suggests the facilitation of the RDS, which agrees with the DFT calculation results. Overall, the PA–FeCo cocatalyst improves interfacial charge separation efficiency by generating a strong interfacial electric field at the WO₃ photoanode surface. It also reduces the OER reaction energy barrier, accelerates the RDS in the OER, and significantly enhances the PEC water splitting efficiency.

3.4 Charge transfer mechanism

Under illumination, electrons in the valence band of pristine WO₃ gain sufficient energy to be excited into the conduction band. These electrons subsequently traverse the WO₃ thin film, enter the external circuit through the FTO substrate, and eventually reach the platinum film electrode functioning as the photocathode. Therefore, they participate in the reduction reaction to convert protons into hydrogen gas. Nevertheless, the extensive bulk and surface recombination of photogenerated electron–hole pairs severely restrict the PEC performance of the WO₃ photoanode. As shown in Fig. 5, the introduction of the PA–FeCo layer generates a strong internal electric field arising from the work function disparity between PA–FeCo and WO₃. This internal electric field effectively promotes charge separation at the surface, driving photogenerated holes toward the metal active sites and substantially improving the utilization efficiency of these holes. The accumulated holes then participate in the OER, leading to the evolution of oxygen gas. Additionally, the PA–FeCo cocatalyst enhances the utilization efficiency of photogenerated holes by significantly lowering the overpotential of the O → OOH rate-determining step during OER, thereby achieving improved reaction kinetics.

3.5 Versatility of PA-coordinated metal complexes in boosting PEC performance

This study demonstrates that the strong chelating interaction with OER-active metals of phytic acid and its inherent electron donor characteristics and hydrophilicity can significantly enhance the OER performance of photoanodes. Moreover, the strong built-in electric field formed at the interface between the PA–FeCo layer and the photoanode substrate promotes efficient surface charge separation. These advantages are significantly independent of the specific photoanode material, suggesting that the PA-coordinated metal complexes reported here could be extended to a broad range of photoanode systems to improve their PEC performance. To assess the versatility of the method, the performances of the prevalent photoanodes without and with PA–metal modification were evaluated. As illustrated in Fig. 6, the photocurrent density and charge transfer resistance of TiO₂, BiVO₄, and Fe₂O₃ photoanodes were optimized to varying extents under both PA–single metal and PA–bimetal modifications. Furthermore, Fig. S11 shows a comparison of the photocurrent densities for TiO₂, BiVO₄, and Fe₂O₃ photoanodes prepared using the strategy proposed in this study at 1.23 V vs. RHE, alongside literature data from recent years. These findings validate the broad versatility of the PA–metal system in photoanode modification and underscore its substantial potential to improve PEC performance across a range of materials.

4. Conclusion

In summary, we demonstrate a coordination-driven self-assembly strategy to construct ultrathin PA–FeCo cocatalysts on WO₃ photoanodes, effectively addressing inefficient surface charge separation and sluggish OER kinetics in PEC water splitting. The incorporation of the PA–FeCo layer enhanced the photocurrent density of WO₃ to 1.93 mA cm⁻², accompanied by a negative shift of 150 mV in the onset potential, indicating significantly improved interfacial reaction kinetics. Additionally, the efficiency of the surface charge separation in WO₃@FeCo increased to 77.7%. Owing to the intrinsic hydrophilicity of phytic acid, the water contact angle of WO₃@PA–FeCo decreased to 20.0°, compared with 48.7° for the pristine WO₃, which substantially promoted mass transfer between the photoanode surface and the electrolyte. DFT calculations and in situ ATR-FTIR further indicated that the dual-metal centers of Fe and Co accelerate the kinetics of the OER via lowered energy barriers for the O → OOH rate-determining step. Moreover, PA-coordinated metal complexes exhibit great versatility in promoting surface charge separation and OER kinetics, as confirmed in prevailing photoanodes, such as TiO₂, BiVO₄ and Fe₂O₃. Our findings establish PA-coordinated metal complexes as a green and versatile cocatalyst layer, presenting a new strategy for designing multifunctional cocatalysts that simultaneously address charge transport and catalytic bottlenecks in solar fuel systems.

Author contributions

Zedong Bi: methodology, formal analysis, writing-original draft, review and editing. Baoxin Ge: DFT calculations. Chenming Yuan: methodology and formal analysis. Ziqian Chen: methodology. Xiaoyan Huang: methodology. Honglin Qian: methodology. Tianqi Sun: methodology. Yuxuan Hu: methodology. Jiantao Zhang: methodology. Haiyue Wang: methodology, Shangye Wen: methodology, Ye Liu: methodology. Teng Liu: methodology. Jie Zeng: methodology. Biyi Chen: investigation, formal analysis, writing-original draft, review and editing, and supervision. Changwei Shi: supervision. Caijin Huang: supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data in this study are available from the corresponding author upon reasonable request.

Phytic acid molecular structure, CV curves and C_dl results, LSV curves, charge injection efficiency profiles, fitted data values of the equivalent circuit, bode phase plots, OCP measurements, stability tests, UV-Vis spectra, calculation of adsorption energies, and comparison with previous works. See DOI: https://doi.org/10.1039/d5ta04107d.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22205051, 22301228), the Natural Science Foundation of Hubei Province (2024AFB267), the Hubei Provincial Department of Education Scientific Research Plan Key Project (F2023007) and the Foundation of China Innovation and Entrepreneurship Training Program funded by the Wuhan Institute of Technology (202410490003).

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Footnote

† These authors contributed equally to this work.

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