Metal–organic framework-derived nano-CoS-enhanced photoelectrochemical water splitting performance of the BiVO₄ photoanode

Abstract

The utilization of metal–organic frameworks (MOFs) as oxygen evolution reaction (OER) co-catalysts for BiVO₄-based photoelectrochemical (PEC) water splitting is limited by their intrinsic low conductivity and low availability of accessible catalytic metal sites. In this work, we transformed the cobalt-based imidazole-based ZIF-67 catalyst into a MOF-derived CoS cocatalyst through a two-step vulcanization process to construct a novel M-CoS/BiVO₄ photoanode. The resulting M-CoS/BiVO₄ photoanode demonstrated a photocurrent density of 5.22 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode under AM 1.5G light irradiation. Both the experimental results and theoretical calculations indicated that the built-in electric fields of the CoS/BiVO₄ heterojunction effectively suppressed charge recombination in the bulk system. Furthermore, the MOF-derived CoS provided active sites with larger specific surface areas and increased the electronic conductivity, which effectively enhanced the charge separation and water oxidation kinetics, promoting the PEC water splitting performance. These findings indicate the potential of this new method in designing highly efficient photoanodes from MOFs.

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1. Introduction

Photoelectrochemical (PEC) water splitting is one of the promising strategies for producing clean and sustainable “green” hydrogen fuel, which can play an important role in solving the energy crisis.^1,2 The oxygen evolution reaction (OER) occurring on the photoanode for water oxidation, involving a four-electron process, is thermodynamically unfavorable, and it is the rate-determining step.^3–5 As a promising photoanode candidate, bismuth vanadate (BiVO₄) has received much attention because of its high theoretical photocurrent of 7.5 mA cm⁻² (under standard AM 1.5 light irradiation) and solar-to-hydrogen conversion efficiency of 9.2%.^6–12 Unfortunately, the low carrier mobility (0.044 cm² V⁻¹ s⁻¹) and short hole diffusion distance (70 nm) result in severe photogenerated charge recombination in BiVO₄.^13,14 In addition, the slow water oxidation kinetics (1.4 s⁻¹) limits the amount of photogenerated current and efficiency.¹⁵

In recent decades, metal–organic framework (MOF)-based OER co-catalysts have been used to increase the photoelectrochemical water oxidation properties of BiVO₄ photoanodes, passivating the surface states for suppressing charge recombination and exposing more active sites for decreasing the activation energy of the OER.^16–23 MOFs consist of transition metal ions/clusters and organic ligands, with the characteristics of an orderly structure, uniformly distributed active sites, ordered atom arrangement, and high stability.^24,25 For instance, a cobalt-based zeolitic imidazolate framework (Co-agZIF-62) was introduced on NiO/BiVO₄ photoanodes for promoting charge transfer and separation and accelerating the interfacial reaction kinetics, resulting in a photocurrent density of 5.34 mA cm⁻² at 1.23 V vs. RHE.²⁶ Furthermore, crystalline NiFc-MOFs were integrated into BiVO₄ photoanodes, resulting in a high photocurrent density of 4.34 mA cm⁻² at 1.23 V vs. RHE.²⁷ Although various MOFs have been applied to BiVO₄ photoanodes for PEC reactions with good results, the inherent low conductivity and instability of MOFs impede achieving a better photocurrent.²⁸ Therefore, researchers have modified MOFs as precursors to other substances via carbonization or oxidation, increasing their electrical conductivity.²⁹ Musa et al.³⁰ annealed MOFs at high temperatures to obtain MOF-derived RuO₂/N,S-TiO₂ heterojunctions, which demonstrated excellent photocatalytic activity for hydrogen generation. Moreover, MOF-derived Co₃O₄/BiVO₄ was obtained through calcination of Co-MOF/BiVO₄ in air, while the performance was still far from satisfactory (<2.5 mA cm⁻²).^31,32 High temperature calcination of MOFs is a common strategy to improve the electrical conductivity of MOFs, which destroys the overall framework of the MOFs and then reduces their surface area.^31,33 Therefore, a gentler modification method is needed to improve the electrical conductivity of MOFs and to provide a greater active area for reactions. Based on the above analysis, we propose obtaining small-structured CoS by the vulcanization of a MOF on BiVO₄ to enhance the PEC performance. The improvement in performance should arise as CoS has a low Gibbs free energy for hydrogen adsorption and the edge atoms are the catalytic active sites. Furthermore, CoS has high electrical conductivity and can effectively boost electron transfer.

Herein, to prove our concept, a typical MOF (here, cobalt-imidazole-based ZIF-67) was simply integrated onto the BiVO₄ photoanode. Subsequently, the ZIF-67/BiVO₄ electrode was simply vulcanized to obtain the nanostructured CoS/BiVO₄ photoanode (M-CoS/BiVO₄). Such a structure could not only form a CoS/BiVO₄ heterojunction, but could also protects the stability of CoS through the framework of the MOF. In this design, the vulcanization process would not damage the structure of the MOF, effectively retaining a larger surface area and providing more active sites. In addition, the generated CoS have smaller particle sizes. The resulting M-CoS/BiVO₄ photoanode demonstrated a high photocurrent density of 5.22 mA cm⁻² at 1.23 V vs. RHE in potassium borate buffer. Experimental study and theoretical calculations revealed that the formation of the CoS/BiVO₄ heterojunction, the larger specific surface area and the more active sites could all effectively enhance the charge separation and water oxidation kinetics, thereby promoting the PEC water splitting performance. This study provides a new strategy for preparing MOF derivatives combined with semiconductors as photoanodes to promote PEC water oxidation.

2. Experimental section

Synthesis of M-CoS/BiVO₄

Bismuth vanadate was obtained according to our previous reports.^8,34 In brief, at room temperature, 30 mL aqueous solution containing 1 mmol Co(NO₃)₃·6H₂O was added to 30 mL aqueous solution containing 8 mmol 2-methylimidazole and the mixture was then stirred for 10 min. The prepared BiVO₄ was put into the above mixed solution for 3 min, and then the electrode was removed and washed with deionized water and dried in an oven at 60 °C for 60 min. Subsequently, the electrode was immersed in 0.03 M sodium sulfide solution for 10 min and then washed with deionized water and dried. Finally, the photoanode and 1 mg per mL thioacetamide ethanol solution were placed in a Teflon-lined stainless steel autoclave and heated at 120 °C for 5 h in an oven. Afterwards, after cooling to room temperature the final electrode was washed with ethanol and dried overnight in an oven at 60 °C and was denoted as M-CoS/BiVO₄. The CoS/BiVO₄ photoanode was prepared in the same way, except that 2-methylimidazole was not added.

Characterization

The phase of the samples were identified by X-ray diffraction (Supernova, Rigaku). The morphology was collected by SEM (Gemini SEM 500). High-resolution TEM images and EDS mapping were recorded on an FEI Tecnai G2F30 instrument. Raman spectroscopy analysis was performed with a 532 nm laser (inVia Qontor, RENISHAW). XPS (Thermo-VG Scientific, ESCALAB250) was carried out to analyze the valence states of the elements. UV-vis absorbance spectra and PL spectra were obtained using a Shimadzu UV-3600i Plus spectrometer and FLS1000 system from Edinburgh Instruments.

PEC performance measurements

All the PEC tests were carried out under simulated AM 1.5G irradiation with a 100 mW cm⁻² light intensity from a xenon lamp (PLS-FX300HU, Beijing Perfect light). A three-electrode cell was used, with the BiVO₄ electrode used as the working electrode, the Ag/AgCl electrode as the reference electrode, and Pt as the auxiliary electrode. The electrolyte was 0.5 M potassium borate solution (pH 9.5). The irradiation area of the working electrode is 1 cm². Additionally, the light passes through the insulating side of the FTO substrate.

3. Result and discussion

The nanostructures of the prepared BiVO₄ and M-CoS/BiVO₄ photoanodes were studied by scanning electron microscopy (SEM). As displayed in Fig. 1a, the pristine BiVO₄ sample displayed the accumulation of worm-like structures with a smooth surface. After introducing the MOF-derived CoS on the BiVO₄ surface, flocculation appeared on the surface (Fig. 1b). Furthermore, in-depth information on the morphology of M-CoS/BiVO₄ was obtained by transmission electron microscopy (TEM) analysis. As can be seen from Fig. 1c, there was a thin layer at the edge of BiVO₄. The high-resolution TEM (HRTEM) image shows a lattice fringe spacing of 0.308 nm, which corresponds to the (121) crystal plane of BiVO₄ (JCPDS No. 14-0688) (Fig. 1d).³⁵ The surface of BiVO₄ was covered with a layer of amorphous material, namely about 10 nm CoS particles enwrapping it. This shows that the vulcanization process converted the Co²⁺ in ZIF to CoS and preserved the MOF framework. Furthermore, the TEM-EDS (Fig. 1e) and SEM-EDS (Fig. S1†) element mappings of M-CoS/BiVO₄ both confirmed the uniform distribution of Co and S elements on the surface of BiVO₄, suggesting the MOF-derived had been CoS successfully grown on the surface of the BiVO₄ photoanodes.

Fig. 1 SEM images of (a) BiVO₄ and (b) M-CoS/BiVO₄ samples. (c) TEM image, (d) HR-TEM image and (e) EDS elemental mapping images of the M-CoS/BiVO₄ sample.

To study the effect of M-CoS on the chemical structure of the electrode, BiVO₄ and M-CoS/BiVO₄ photoanodes were first analyzed by X-ray diffraction (XRD) measurements. As displayed in Fig. 2a, the XRD peaks of M-CoS/BiVO₄ photoanode were not different compared to pristine BiVO₄, agreeing well with the standard card of monoclinic BiVO₄ (JCPDS#14-0688) and the FTO substrate (SnO₂, JCPD#46-1088). No peaks belonging to CoS were observed, which suggests that the CoS was in the amorphous state, as also confirmed by the TEM results (Fig. 1d). Furthermore, the existence of CoS was also identified by the Raman spectra. As displayed in Fig. 2b, pristine BiVO₄ showed several peaks at 820, 707, 323, 210 and 120 cm⁻¹. The peaks at 120 and 210 cm⁻¹ were contributed by an external mode.³⁶ The peaks at 366 and 323 cm⁻¹ were assigned to the symmetric (δ_s) and antisymmetric (δ_as) bending modes of VO₄³⁻, respectively.³⁷ The peaks at 820 and 707 cm⁻¹ were attributed to the symmetric (υ_s) and antisymmetric (υ_as) V–O stretching modes, respectively.³⁸ Compared with pristine BiVO₄, a new peak at 473 cm⁻¹ appeared for M-CoS/BiVO₄, which could be assigned to the E_g mode of CoS,³⁹ indicating the successful construction of the CoS/BiVO₄ heterojunction. Subsequently, X-ray photoelectron spectroscopy (XPS) was performed to identify the chemical state and composition of M-CoS/BiVO₄ photoanode. As displayed in Fig. 2c, the Bi 4f XPS spectra of pristine BiVO₄ displayed two characteristic peaks at binding energies of 158.9 and 164.2 eV, corresponding to Bi 4f_7/2 and Bi 4f_5/2, respectively, implying the presence of Bi³⁺ in BiVO₄.^40,41 The Bi 4f peaks of M-CoS/BiVO₄ were slightly shifted to higher binding energies, indicating a strong interaction between BiVO₄ and CoS, which plays an important role in photogenerated carrier transfer. Interestingly, compared with BiVO₄, the V 2p and O 1s peaks of M-CoS/BiVO₄ also moved to higher binding energies. The peak at 532.5 eV for O 1s corresponding to dissociated oxygen species from water molecules (O_C) was increased,⁴² suggesting that M-CoS/BiVO₄ had a better hydrophilic ability and was conducive to the OER reaction. Furthermore, S 1s and Co 2p signals could be detected in M-CoS/BiVO₄ photoanode, further confirming the presence of CoS (Fig. 2e).⁴³ Moreover, the optical properties of the BiVO₄ and M-CoS/BiVO₄ photoanodes were recorded by UV-vis absorption spectroscopy (Fig. 2f). The pristine BiVO₄ and M-CoS/BiVO₄ photoanodes demonstrated a similar light absorption edge at 520 nm, while the light absorption intensity of M-CoS/BiVO₄ was much higher than that of BiVO₄, suggesting that the CoS could strengthen the light-absorption intensity.

Fig. 2 (a) XRD patterns, (b) Raman spectra obtained with a 532 nm laser, (c) Bi 4f, (d) V 2p and O 1s, and (e) S 1s and Co 2p XPS spectra, and (f) UV-vis diffuse reflectance spectra of BiVO₄ and M-CoS/BiVO₄ photoanodes.

To investigate the effect of the MOF-derived CoS on the surface-active area, the electrochemical surface-active areas (ECSA) of the BiVO₄ and M-CoS/BiVO₄ photoanodes were calculated from the electrochemical double-layer capacitance (C_dl) obtained through cyclic voltammetry tests (Fig. S2†). As shown in Fig. S2, M-CoS†/BiVO₄ photoanode achieved a higher ECSA value (71.1 μF cm⁻²) than the BiVO₄ photoanode (58.4 μF cm⁻²), revealing that the active sites were significantly increased due to the exposure of the MOF-driven CoS on the surface of M-CoS/BiVO₄ photoanode, favoring better surface oxidation reactions.⁴⁴

Next, linear-sweep-voltammetry (LSV) was performed to evaluate the PEC water splitting performance of the pristine BiVO₄ and M-CoS/BiVO₄ photoanodes under AM 1.5G light irradiation and 0.5 M (pH 9.5) borate buffer solution electrolyte (0.05 V s⁻¹ scan rate). The photocurrent density of M-CoS/BiVO₄ photoanode at 1.23 V vs. RHE was 5.22 mA cm⁻², which was, as expected, much higher than that of BiVO₄ (1.95 mA cm⁻²) and ZIF/BiVO₄ (3.68 mA cm⁻²), confirming that M-CoS could effectively improve the PEC performance of BiVO₄ (Fig. 3a). Furthermore, compared with other reported BiVO₄-based photoanodes (Table S1†), the performance of M-CoS/BiVO₄ photoanode was also quite excellent, suggesting M-CoS/BiVO₄ is a promising PEC material. To verify the role of the MOF structure, we synthesized CoS/BiVO₄ electrodes without adding ligands for the MOF. However, the current density obtained was only 3.07 mA cm⁻² at 1.23 V vs. RHE (Fig. S3†), confirming that the MOF structure also plays an important role in the performance. Interestingly, we found that the results of the two-step vulcanization process were more significant than those of the separate vulcanization process (Fig. S4†). Meanwhile, the influence of the electrolyte pH value on the performance of M-CoS/BiVO₄ was also studied. M-CoS/BiVO₄ had the highest performance in the pH range at 9.5–11, so we controlled the electrolyte at pH 9.5 for PEC water splitting (Fig. S5†). The amounts of hydrogen and oxygen gases evolved from M-CoS/BiVO₄ photoanode were tested at 1.23 V vs. RHE and detected by gas chromatography. The total amounts of H₂ and O₂ reached 569.1 and 285.5 μmol cm⁻², respectively. The faradaic efficiency was about 91.5% (Fig. S6†).

Fig. 3 (a) LSV curves of BiVO₄, ZIF-BiVO₄ and M-CoS/BiVO₄ photoanodes at a 0.05 V s⁻¹ scan rate. (b) Charge-separation efficiency (η_sep), (c) charge transfer-efficiency (η_tran), (d) ABPE curves, (e) IPCE curves, and (f) i–t curves at 1.23 V vs. RHE of BiVO₄ and M-CoS/BiVO₄. Conditions: under AM 1.5G light irradiation and 0.5 M borate buffer solution (pH = 9.5).

To analyze the role of M-CoS on the charge separation and charge transfer of BiVO₄, LSV was performed with adding 0.5 M Na₂SO₃ as a hole-sacrificial agent in the electrolyte (Fig. S6†). Fig. 3b demonstrates the charge separation of the BiVO₄ and M-CoS/BiVO₄ photoanodes. The charge-separation efficiency of BiVO₄ was 54.0% at 1.23 V vs. RHE, while after loading M-CoS, the charge-separation efficiency reached 84.2%, confirming that the formation of the CoS/BiVO₄ heterojunction could effectively enhance the charge separation. Also, as expected, the charge-transfer efficiency of M-CoS/BiVO₄ increased, i.e. from 53.0% to 83.4%, revealing that M-CoS/BiVO₄ exhibited accelerated carrier-transfer dynamics. Moreover, the applied bias photon-to-current efficiencies (ABPEs) of BiVO₄ and M-CoS/BiVO₄ were calculated and are displayed in Fig. 3d. Clearly, M-CoS/BiVO₄ photoanode showed an excellent photoconversion efficiency and maximum ABPE value of 1.56% at 0.70 V. Additionally, the incident photon-to-current efficiency (IPCE) value of M-CoS/BiVO₄ photoanode was much higher than that of BiVO₄ over the entire window, suggesting that M-CoS/BiVO₄ photoanode had a better photons-to-electrons conversion efficiency (Fig. 3e).⁴⁵ Except for its performance, the stability of a photoanode under light irradiation is also an issue that needs to be considered in applications.⁴⁶ The stabilities of pristine BiVO₄ and M-CoS/BiVO₄ were not satisfactory (Fig. 3f). The current density of M-CoS/BiVO₄ decreased to only 3.87 mA cm⁻² after 7.6 h under light irradiation, which was mainly due to the instability of CoS under light irradiation. According to our previous research, loading a NiFeO_x cocatalyst can effectively enhance the stability of photoanodes.⁴⁷ Therefore, NiFeO_x was loaded on M-CoS/BiVO₄ electrode to improve its stability (labeled as NiFeO_x/M-CoS/BiVO₄). The photocurrent density of NiFeO_x/M-CoS/BiVO₄ was retained as 4.61 mA cm⁻² after 27 h light illumination, indicating that its stability had indeed been greatly improved (Fig. S7†).

To gain a deeper understanding of the reasons for the enhanced PEC performance of M-CoS/BiVO₄ photoanode, the electron-transport mechanism was studied through measuring the band positions of CoS and BiVO₄. The band positions of CoS and BiVO₄ are shown in Fig. S8,† revealing that a type II heterojunction could be formed between CoS and BiVO₄.⁴⁸ This structure can effectively separate photogenerated electrons and holes, thereby improving the charge-transfer efficiency (Fig. 4a).⁴⁹ The charge mobility process was studied by chopped chronoamperometry measurements at 1.23 V vs. RHE (Fig. S9†) and the transient decay time (τ) could be obtained at the point where ln D = −1, which represents the charge lifetime of the bulk semiconductor.⁵⁰

where I_t, I_in and I_st are the photocurrent at time t and the initial state and steady state currents, respectively. Impressively, M-CoS/BiVO₄ demonstrated a large decay time τ value of 2.3 s, which was higher than that of BiVO₄ (1.2 s), which could be ascribed to the holes on the catalyst surface easily transferring to the electrolyte to prolong the carrier lifetime (Fig. 4b). Furthermore, the charge transfer and separation characteristics of photoanodes can be also observed by electrochemical impedance spectroscopy (EIS) (Fig. 4c).^51,52 Here, under AM 1.5G light illumination, M-CoS/BiVO₄ exhibited a smaller charge-transfer resistance, suggesting that the CoS/BiVO₄ heterojunction could boost the charge-transfer ability at the photoanode/electrolyte interface for water oxidation.⁵³

Fig. 4 (a) Schematic of the charge transfer at CoS/BiVO₄ heterojunction. (b) Plots of ln D versus time, (c) EIS curves under 1.23 V vs. RHE within the 0.1–10⁵ Hz frequency range in 0.5 M KBi buffer solution, (d) time-resolved photoluminescence spectra, (e) normalized decay profiles, (f) OCP-derived carrier lifetimes, (g) M-S plots without light irradiation with 1 kHz frequency in 0.5 M KBi buffer solution, (h) delay of the cathodic photocurrent curves, (i) plot of the charge-storage capability against the applied bias of BiVO₄ and M-CoS/BiVO₄ photoanodes.

The charge behaviors of the BiVO₄ and M-CoS/BiVO₄ photoanodes were further examined by steady-state PL spectra.⁵⁴ As displayed in Fig. S10,† pristine BiVO₄ displayed a very strong PL peak, which demonstrated the severe electron–hole recombination (whereby holes in the O 2p band recombine with electrons in the V 3d band).^55,56 After loading CoS, the intensity of the PL peak was significantly reduced, suggesting that the CoS/BiVO₄ heterojunction could reduce the recombination of charge carriers.⁵⁷ Furthermore, time-resolved photoluminescence spectra were used to analyze the charge-carrier lifetimes of the BiVO₄ and M-CoS/BiVO₄ photoanodes with a wavelength of 540 nm (Fig. 4d). The average charge lifetimes (τ_avg) of M-CoS/BiVO₄ were calculated to be 3.2 ns, which were higher than that of BiVO₄ (1.9 ns), indicating that charge-carrier recombination was effectively suppressed in M-CoS/BiVO₄ photoanode.

Next, the normalized open-circuit potential (OCP) decay profiles of the BiVO₄ and M-CoS/BiVO₄ photoanodes were calculated and are displayed in Fig. 4e. As expected, M-CoS/BiVO₄ photoanode demonstrated faster OCP decay compared with BiVO₄, confirming that charge was rapidly transferred to the surface and injected into the electrolyte for water oxidation reaction.⁵⁸ The charge lifetime was calculated based on the following equation:⁵⁹

where τ is the charge lifetime, e is 1.602 × 10⁻¹⁹ C. k is 1.38 × 10⁻²³ J K⁻¹, T is the temperature (K), and dOCP/dt is the derivative of the OCP transient decay. As displayed in Fig. 4f, M-CoS/BiVO₄ photoanode (0.052 s) demonstrated smaller carrier lifetime values than BiVO₄ (0.13 s), revealing that M-CoS/BiVO₄ offered advanced charge separation via the internal electric field. To quantify the charge-carrier concentrations in the BiVO₄ and M-CoS/BiVO₄ photoanodes, Mott–Schottky (M-S) curves were recorded.^60,61 As shown in Fig. 4g, the positive slopes revealed that the BiVO₄ and M-CoS/BiVO₄ photoanodes were n-type semiconductors. Moreover, the charge-carrier density (N_d) was calculated and the values are displayed in Table S1.† The N_d of M-CoS/BiVO₄ photoanode reached 1.55 × 10¹⁸ cm⁻³, which was much higher than that of BiVO₄ (1.08 × 10¹⁸ cm⁻³), suggesting that the CoS/BiVO₄ heterojunction effectively increases the charge-carrier concentration and charge-transfer efficiency. Simultaneously, the flat band potential of M-CoS/BiVO₄ was shifts negatively compared with pristine BiVO₄, revealing that the band bending at the interface between the photoanode and the electrolyte was sharper and more conducive to water oxidation.

The charge-storage capability of M-CoS/BiVO₄ was studied through the transient-state photocurrent method, in which the cathodic photocurrent peaks were measured under chopped illumination. The trapped holes on the surface were reduced when the light was cut off, representing the cathodic current formation. Fig. 4h exhibits the delay of the cathodic photocurrent curves, which is analyzed from the transient cathodic current in Fig. S11.† The delay in the steady-state cathode current indicated that the holes were stored on the electrode surface. Therefore, M-CoS/BiVO₄ photoanode had more holes in the surface, suggesting its excellent hole-transfer ability. The number of storage holes can be represented by the integration of the transient photocurrent and the steady-state photocurrent.⁶² The number of storage holes of M-CoS/BiVO₄ photoanode was obviously higher than that of BiVO₄, especially at low bias (Fig. 4i). The stored holes can eliminate the injection barrier to allow entry of the electrolyte and its participation in water oxidation at high potential, resulting in the reduction of the stored holes.

Next, Kelvin probe force microscopy (KPFM) was performed to understand the mechanism of the enhanced PEC water splitting activity of M-CoS/BiVO₄ photoanode through the internal electrical field.⁶³ As shown in Fig. 5a, the surface potential of pristine BiVO₄ was measured under dark and light irradiation. It was found that the surface potential of BiVO₄ did not significantly change under light irradiation, which indicated severe photogenerated electron–hole pair recombination. As expected, the surface potential of M-CoS/BiVO₄ photoanode significantly increased under light irradiation, definitely confirming that the internal electrical field was beneficial for charge separation. Furthermore, to disclose the charge behavior on the surface of the different components (such as BiVO₄ and CoS) in M-CoS/BiVO₄ photoanode, in situ XPS was carried out with/without light illumination (500 mW, 405 nm, 5 min). The high-resolution XPS spectra of Bi 4f, V 2p and O 1s for pristine BiVO₄ under light and without light illumination are displayed in Fig. S12.† The peak spectra were not changed, suggesting that severe photogenerated electron–hole pair recombination occurred for pristine BiVO₄ after light illumination. Interestingly, both peaks of V 2p and O 1s for M-CoS/BiVO₄ photoanode were also unchanged, while the Bi 4f and Co 2p peaks moved to a higher binding energy direction under light irradiation. These experimental results confirm that more photogenerated holes were transferred to the Co sites through the Bi sites under light irradiation.⁶⁴

Fig. 5 Surface potential images of (a) BiVO₄ and (b) M-CoS/BiVO₄ photoanodes from KPFM measurements in the dark and under light irradiation. XPS spectra of (c) Bi 4f, (d) V 2p and O 1s, (e) Co 2p of M-CoS/BiVO₄ photoanode under light irradiation.

Next, density functional theory (DFT) calculations were performed to better understand the role of nano CoS in electron transfer and in Gibbs free energy H₂O oxidation.⁶⁵ First, theoretical structural models of CoS_(nano)/BiVO₄ and CoS_(solid)/BiVO₄ were constructed (Fig. S13†). The electron density difference (EDD) models of CoS_(nano)/BiVO₄ and CoS_(solid)/BiVO₄ are displayed in Fig. 6a and b. The more closely stacked electrons around BiVO₄ in CoS_(nano)/BiVO₄ confirmed that nano CoS could facilitate charge transfer between CoS and BiVO₄. Furthermore, the density of states (DOS) of the d-band centers of CoS_(nano)/BiVO₄ and CoS_(solid)/BiVO₄ were determined and are displayed in Fig. 6c and d. CoS_(solid)/BiVO₄ mainly involved the d-band of Co interacting with H₂O, while CoS_(nano)/BiVO₄ involved the d-band of V and Co interacting with H₂O. In addition, the d-bands of V and Co were closer in CoS_(nano)/BiVO₄, which would facilitate electron transport. The standard Gibbs free energy diagrams for the OER were calculated and are displayed in Fig. S14,† where the formation of *OOH from *O was the potential rate-determining step with a low overpotential (η) of 180 mV. The introduction of nano CoS boosted the generation of *OOH to enhance the OER kinetics. In addition, the transient electric field distributions of BiVO₄ and CoS_(nano)/BiVO₄ are shown in Fig. 6e. CoS_(nano)/BiVO₄ displayed the stronger transient electric field, revealing that the nano CoS boosted the charge transfer and water oxidation kinetics, thereby increasing the photocurrent density.

Fig. 6 Electron density differences for (a) CoS_(nano)/BiVO₄ and (b) CoS_(solid)/BiVO₄. Calculated DOS of the d-band centers for (c) CoS_(nano)/BiVO₄ and (d) CoS_(solid)/BiVO₄. (e) Transient electric field distribution simulated using MATLAB simulations of BiVO₄ and CoS_(nano)/BiVO₄ (−0.1 V vs. RHE).

4. Conclusions

This study demonstrated a novel metal–organic framework (MOF)-derived strategy for fabricating CoS/BiVO₄ heterostructured photoanodes with exceptional photoelectrochemical performance. Through controlled ZIF-67 loading and subsequent vulcanization treatment, we successfully constructed a hierarchical M-CoS/BiVO₄ architecture. The optimized photoanode achieved a record photocurrent density of 5.22 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode (RHE) in borate buffer (pH 9.5) under AM 1.5 G illumination. Systematic investigations revealed three synergistic enhancement mechanisms: (1) the strong built-in electric field at the heterointerface promoted the charge-carrier separation efficiency (η_sep = 84.2%), (2) the MOF structure offered a larger specific surface area and more active sites for OER reaction, and (3) CoS enhanced the electrical conductivity and accelerated the surface reaction kinetics for water oxidation. This MOF-mediated synthesis approach opens new avenues for designing efficient photoelectrodes for solar fuel generation.

Data availability

Data supporting the conclusions of this study are included in the manuscript and in the ESI.† These data are also available from the corresponding authors upon reasonable request.

Author contributions

Dongqin Li: investigation, writing – original draft, funding acquisition. Xinchao Chen: investigation, methodology. Mingshi Shao: investigation, data curation. Shushi Hou: investigation, data curation. Jingyi Guo: data curation. Haoyu Jin: data curation. Hao Yang: software. Guikui Chen: formal analysis, writing – review & editing. Yongchao Huang: formal analysis, writing – review & editing, resources, supervision, project administration.

Conflicts of interest

There are no conflicts to declare

Acknowledgements

This work was supported by the Guangzhou basic research planning for basic and applied basic research project (2024A04J3256); Special fund for scientific innovation strategy construction of high level Academy of Agriculture Science (R2023YJ-YB3005), Guangdong Provincial Modern Agricultural Industry Common Key Technologies Research and Development Innovation Team Construction Project (Common Key Technologies for Agricultural Product Quality and Safety) under the Agricultural Sector Framework (2024CXTD18).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02162f

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