Interlayer bismuthene as a charge-transporter improves the photoelectrochemical water oxidation activity of a bismuth vanadate metal–organic framework

Abstract

The introduction of bismuthene as a charge transport layer between BiVO₄ and a cocatalyst (Ni-metal–organic framework, Ni-MOF) improves the photoelectrochemical water oxidation activity. The photoanode BiVO₄/Bi/Ni-MOF produces a photocurrent of 4.71 (±0.2) mA cm⁻² (at 1.23 V vs. RHE), which is 2.5 and 1.25 times higher than those of bare BiVO₄ and BiVO₄/Ni-MOF, respectively.

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Photoelectrochemical (PEC) water oxidation using semiconductors and sunlight offers a sustainable route for energy conversion.¹ The development of an efficient photoanode for the maximum utilization of visible light and to minimize the kinetic barrier of the sluggish oxygen evolution reaction (OER) is crucial for PEC water splitting.² BiVO₄ has garnered significant attention as a photoanode for the OER because of its suitable valence band maximum (VBM = 2.15 eV) and narrow band gap (2.42 eV).³ Additionally, the low cost and high chemical stability of BiVO₄ are suitable for practical applications.^4,5 However, the use of BiVO₄ for PEC-OER is limited by its sluggish water oxidation kinetics and poor charge carrier mobility (0.04 cm² V⁻¹ s⁻¹).⁶ Various approaches, including heteroatom doping, heterostructure formation, cocatalyst loading, etc., have been investigated to improve the water oxidation activity of BiVO₄.^5,7

Among these, cocatalyst loading has been extensively studied to modify the charge-transfer dynamics, to improve the water oxidation kinetics, transfer of holes, and access to active sites, and to increase the stability of the photoanode.⁸ The loading of cocatalysts such as metal oxides, hydroxides, metal–organic frameworks (MOFs), etc., on BiVO₄ has been found to improve the PEC activity of the photoanode.⁹ In this context, the use of MOFs as the cocatalyst offers extra advantages, such as providing abundant active sites, a high electrochemically active surface area, and improved charge carrier dynamics.¹⁰ For example, the fabrication of a CoNi-MOF on BiVO₄ was reported to achieve an increase in the photocurrent density compared to the bare BiVO₄ photoanode.¹¹ Similarly, the integration of Zn-MOF,⁷ Co-MOF,¹² CoFe-MOF,¹³etc., on BiVO₄ to improve its OER performance has been explored.

Additionally, heterostructure formation via layer-by-layer deposition of BiVO₄ and an electron/hole conductor has been found to improve the efficiency of the photoanode. In particular, the use of 2D materials such as graphene, phosphorene, bismuthene, etc., to attain high PEC activity has been explored.^6,14–16 Qiao et al. demonstrated the use of 2D phosphorene to enhance the charge separation efficiency of BiVO₄.¹⁴ The Park group reported that 2D phosphorene layers between BiVO₄ and NiOOH improved the PEC-OER kinetics.¹⁵ A functional interlayer of bismuthene nanosheets between BiVO₄ and NiFeOOH was also reported to enhance the PEC-OER activity.⁶ The construction of BiVO₄–reduced graphene oxide led to a 10-fold increase in PEC-OER efficiency compared to pure BiVO₄.¹⁶

The above studies inspired us to explore the combined effect of heterostructure formation and the use of Ni-MOF as the cocatalyst to improve the PEC water oxidation activity of the BiVO₄ photoanode. The interlayer deposition of bismuthene nanosheets as an electron-transporter between BiVO₄ and Ni-MOF was found to improve the PEC water oxidation activity.

Bismuthene with atomic-level thickness can improve charge separation and carrier mobility.⁶ The charge transfer between BiVO₄ and bismuthene results in upward band bending in the former and a shift of the CB to a more negative position. Consequently, the band gap of BiVO₄ expands, and the Fermi level relocates, leading to a reduction in the charge recombination. The partially oxidized bismuthene increases the density of oxygen vacancies on the photoanode surface, improves the n-type characteristics, and enhances charge separation and transport.⁶

Herein, physicochemical and (photo)electrochemical characterizations reveal the complementary roles of bismuthene and Ni-MOF to improve the PEC activity of BiVO₄. The deposition of Ni-MOF over the bismuthene layer is important for charge transfer and for increasing the number of surface active sites.

Therefore, this work highlights the crucial role of bismuthene as a charge-transporter between the semiconductor and cocatalyst to enhance the performance of the photoanode. The role of bismuthene as the charge transport layer is prominent, as the photoanode BiVO₄/Bi/Ni-MOF produces a photocurrent density of 4.71 (±0.2) mA cm⁻² (at 1.23 V vs. RHE), which is significantly higher than that of BiVO₄/Ni-MOF.

BiVO₄ was first deposited on a fluorine-doped tin oxide (FTO)-coated glass film, followed by the layer-by-layer deposition of bismuthene and Ni-MOF to form BiVO₄/Bi/Ni-MOF (Table S1, SI). The PXRD patterns revealed the monoclinic phase of BiVO₄ (JCPDS 014-0688) (Fig. 1a).¹³ With the loading of bismuthene and/or Ni-MOF on BiVO₄, the characteristic peaks of rhombohedral Bi with the R3̄m space group (JCPDS 05-0519) and Ni-MOF with the space group P31̄m were observed (Fig. 1a).^17,18 In the attenuated total reflection Fourier-transformed infrared (ATR-FTIR) spectrum of the nickel-chloranilate framework Ni-MOF, the C=O vibration of the chloranilate ligand was detected at 1652 cm⁻¹ (Fig. S1, SI).¹⁸

Fig. 1 (a) PXRD patterns of the photoanodes. (b) SEM image of BiVO₄. (c) SEM images of BiVO₄/Bi/Ni-MOF revealing the surface morphology and distribution of bismuthene and Ni-MOF in BiVO₄/Bi/Ni-MOF. (d) TEM image of the BiVO₄/Bi/Ni-MOF photoanode. (e) HR-TEM of BiVO₄/Bi/Ni-MOF, (e₁) Ni-MOF with d_(314̄) = 0.21 nm, (e₂) bismuthene with d_(s012) = 0.33 nm, and (e₃) BiVO₄ with d₍₁₁₀₎ = 0.47 nm.

The scanning electron microscopy (SEM) image of BiVO₄ showed a wormlike morphology with a porous structure (Fig. 1b). The SEM image of Ni-MOF revealed flower-like nanosheets (Fig. S2a, SI). In BiVO₄/Bi/Ni-MOF, the flower-like structure of Ni-MOF and the nanosheets of bismuthene were observed together (Fig. 1c and Fig. S2b, SI). The energy-dispersive X-ray (EDX) and elemental mapping of BiVO₄/Bi/Ni-MOF confirmed the elements present and their distribution (Fig. S3 and S4, SI).

The transmission electron microscopy (TEM) image of BiVO₄/Bi/Ni-MOF revealed transparent ultrathin nanosheets of bismuthene and nanosheets of Ni-MOF (Fig. 1d). High-resolution TEM (HR-TEM) detected the formation of the heterostructure (Fig. 1e). The HR-TEM revealed a lattice spacing of 0.21 nm,¹⁸ corresponding to the Ni-MOF (314̄) plane (Fig. 1e₁).¹⁸ A lattice spacing (0.33 nm) corresponding to the bismuthene (012) plane was also detected via HR-TEM (Fig. 1e₂).¹⁷ The (110) plane of BiVO₄ showed a lattice spacing of 0.47 nm (Fig. 1e₃).¹³ These results confirm the coexistence of the three different phases with intimate interfaces, which can enhance interfacial charge transfer and thus is beneficial for PEC performance.

The Raman spectrum of pure bismuthene showed peaks at 119.0 cm⁻¹ and 87.8 cm⁻¹ corresponding to A_1g (out-of-plane mode) and E_g (in-plane mode) vibrations, respectively. The considerably higher wavenumber values for the E_g and A_1g vibrations compared to those of bulk Bi indicate the lowering of the dimensionality of the metal to form atomic-level thin nanosheets (Fig. 2a).^6,17 Further, the peak intensity ratio of A_1g to E_g was calculated to be 0.87, indicating the formation of few-layer bismuthene (Fig. 2a).^6,17

Fig. 2 (a) Raman spectra of BiVO₄, BiVO₄/Bi, and BiVO₄/Bi/Ni-MOF. (b) Bi 4f XPS of BiVO₄/Bi/Ni-MOF, showing the peaks of the Bi^III and Bi⁰ species. (c) V 2p XPS of BiVO₄ and BiVO₄/Bi/Ni-MOF.

The band at 306.0 cm⁻¹ corresponding to Bi–O indicates that the bismuthene is partially oxidized.⁶ The Raman spectra of Bi/BiVO₄ and BiVO₄/Bi/Ni-MOF showed a slight change in the Raman shifts of the bismuthene-related peaks compared to those of pure bismuthene due to charge transfer (Fig. 2a).

In the BiVO₄/Bi/Ni-MOF spectrum, the two peaks at 564 cm⁻¹ and 484 cm⁻¹ corresponded to the characteristic A_1g stretching and E_g bending modes of the Ni–O bond in Ni-MOF.¹⁹ Peaks corresponding to the C–C and C–O stretches of Ni-MOF were observed at 1351 and 1586 cm⁻¹, respectively.²⁰ Other peaks in the Raman spectra were assigned to the various asymmetric and symmetric stretching vibration modes of the [VO₄]³⁻ unit in BiVO₄ (Fig. 2a).

The Bi 4f X-ray photoelectron spectrum (XPS) of BiVO₄ was fitted into two peaks corresponding to Bi 4f_7/2 (158.9 eV) and Bi 4f_5/2 (164.3 eV), indicating the presence of only Bi(III) species. In BiVO₄/Bi/Ni-MOF, the Bi 4f_7/2 and Bi 4f_5/2 peaks were broadened due to the presence of both Bi(0) and Bi(III) species.¹⁷ Further, the Bi(III) peak was shifted to a higher binding energy (by 0.32 eV) than that of bare BiVO₄ due to electron transfer to Bi through the interface. The 4f_7/2 peaks at 158.8 eV and 157.6 eV were attributed to Bi–O and metallic Bi(0) species, respectively (Fig. 2b).^6,17

The V 2p XPS of BiVO₄ was fitted to V⁵⁺ and V⁴⁺ species (Fig. 2c). In BiVO₄/Bi/Ni-MOF, a significant decrease in the ratio of V⁵⁺/V⁴⁺ was observed because of the electron transfer from bismuthene to BiVO₄.^6,21

The O 1s XPS spectrum of BiVO₄ was fitted to three peaks (Fig. S5a, SI). The peaks at 531.2, 530.45, and 532.5 eV were attributed to oxygen vacancies (O_V), lattice oxygen (O_L), and chemisorbed oxygen species (O_C) from water molecules, respectively.⁶ The XPS spectrum of BiVO₄/Bi/Ni-MOF was fitted to three peaks with binding energies of 528.97 eV, 530.80 eV, 531.08 eV, and 532.11 eV corresponding to Ni–O, Bi–O, oxygen vacancies (O_v), and –CO groups,⁶ respectively (Fig. S5a, SI).

The Ni 2p XPS of BiVO₄/Bi/Ni-MOF showed two peaks at 854.93 eV and 872.34 eV, corresponding to Ni 2p_3/2 and Ni 2p_1/2, respectively. The peak at 854.93 eV confirmed the presence of Ni²⁺, whereas the peaks marked with an asterisk (*) were assigned as its satellites (Fig. S5b, SI).²²

UV-visible diffuse reflectance spectra (UV-vis DRS) were recorded to analyse the optical properties of the catalysts. The absorption band of pristine BiVO₄ was observed at 464 nm, and the loading of the Ni-MOF and bismuthene slightly changed the peak features (Fig. S6a, SI). A new peak (397 nm) corresponding to the metal-to-ligand charge transfer (MLCT) in Ni-MOF was observed in BiVO₄/Bi/Ni-MOF (Fig. S6a, SI), while bismuthene showed a broad absorption in the visible-light region (Fig. S6b, SI).

The Tauc plot revealed a slight change in the bandgap after heterostructure formation (2.53 eV for BiVO₄/Bi/Ni-MOF, 2.44 eV for BiVO₄/Bi, and 2.46 eV for BiVO₄) (Fig. S6c, SI). Bismuthene showed a narrow bandgap of 0.62 eV, supporting extended near-infrared absorption (Fig. S6d, SI).²³

The improvement in the charge transport in BiVO₄/Bi/Ni-MOF was confirmed by the photoluminescence (PL) study, as the PL-emission intensity of BiVO₄/Ni-MOF was found to be lower than that of BiVO₄/Bi and bare BiVO₄ (Fig. S7a, SI). The deposition of bismuthene on BiVO₄ improves the charge transport and hence minimizes the radiative charge carrier recombination, as reflected by the decrease in the PL intensity. Furthermore, Ni-MOF facilitates the interface electron transfer from bismuthene to the electrode surface for improved PEC activity (Fig. S7b, SI). A slight change in the emission peak λ_max was also observed with the loading of bismuthene and Ni-MOF on BiVO₄ because of the strong electronic interaction between the components.

A more comprehensive understanding of the band structure of the photoanodes was realized via Mott–Schottky (M–S) studies. BiVO₄ and bismuthene exhibited positive slopes in the M–S plots, indicating n-type semiconductor properties.²³ The flat band potentials of BiVO₄ and bismuthene were measured to be −0.26 V and −0.43 V vs. RHE, respectively. Hence, the CBM values of BiVO₄ and bismuthene were calculated to be −0.36 eV and −0.53 eV, respectively (Fig. 3a and b). The VBM of BiVO₄ was determined from the XPS valence band spectra to be 2.16 eV (Fig. 3c). Therefore, the bandgap calculated to be 2.52 eV closely matches the Tauc plot value (2.48 eV). Furthermore, the band alignment confirms the electron transfer from bismuthene to BiVO₄ (Fig. S6e).

Fig. 3 (a) and (b) Mott–Schottky plots of (a) bismuthene and (b) BiVO₄. (c) Valence band XPS of BiVO₄. (d) Depiction of the band in contact after reaching equilibrium in the presence of light. (e) Open-circuit potential (OCP) profiles of the photoanodes measured in the dark (gray background) and under illumination (yellow background).

The M–S plot confirmed the electron transfer from bismuthene to BiVO₄, which facilitates charge separation. This also results in an upward band bending and a shift of the CB of BiVO₄ to a more negative potential (Fig. 3d and e). As a result, the band gap of BiVO₄ widens, accompanied by repositioning of the Fermi level and suppression of charge recombination. The open-circuit potential (OCP) measurements under dark and light conditions revealed a pronounced shift in the OCP values of the bismuthene-modified photoanodes in the dark and similar values under light-saturated conditions (+0.36 ± 0.02 V vs. RHE), confirming that bismuthene modulates the interfacial charge transport properties (Fig. 3e).

The PEC water oxidation activity of the photoanodes was measured in phosphate buffer in the presence of visible light (Fig. 4a). Bare BiVO₄ showed the lowest photocurrent response (1.91 ± 0.2 mA cm⁻² at 1.23 V vs. RHE). After the deposition of bismuthene or Ni-MOF on BiVO₄, the photocurrent density increased. The best activity was recorded for BiVO₄/Bi/Ni-MOF, which achieved a photocurrent density of 4.71 ± 0.2 mA cm⁻² at 1.23 V vs. RHE. The linear sweep voltammetry (LSV) profiles detected poor current density in the dark, while under light irradiation, the photoanodes generated a significant photocurrent. Comparison of the PEC water oxidation activity of BiVO₄/Bi/Ni-MOF with that of the reported BiVO₄-based photoanodes revealed that it had higher activity than most of the reported catalysts (Table S2, SI). The loading of other Ni-based cocatalysts (NiO, Ni(OH)₂) on BiVO₄/Bi resulted in PEC activity superior to that of BiVO₄/Bi but inferior to that of BiVO₄/Bi/Ni-MOF, demonstrating the importance of the MOF as a cocatalyst (Fig. S8a and b, SI).

Fig. 4 (a) LSV profiles of the photoanodes in phosphate buffer in the presence of light (scan rate: 5 mV s⁻¹). (b) Photocurrent response at 1.23 V vs. RHE for all the photoanodes. (c) ABPE (%) and (d) IPCE (%) plots for the synthesized catalysts.

The transient photocurrent response provided insights into the charge separation behavior of photogenerated electron–hole pairs (Fig. 4b).⁷ The potential-dependent photocurrent also demonstrated the superior performance of the BiVO₄/Bi/Ni-MOF photoanode compared to the other synthesized catalysts (Fig. S9, SI). The applied bias photon-to-current efficiency (ABPE) of BiVO₄/Bi/Ni-MOF photoanode exhibited a peak of 2.01% at 0.59 V vs. RHE (Fig. 4c), which was significantly higher than that of pure BiVO₄ (0.73% at 0.582 V vs. RHE).

The BiVO₄/Bi/Ni-MOF photoanode showed remarkably improved incident photon-to-current conversion efficiency (IPCE) across the full 300–620 nm wavelength range (Fig. 4d), achieving a peak of 62.17% at 420 nm, which is 1.72 times greater than that of bare BiVO₄. As the light absorption capacity remained nearly constant after bismuthene and Ni-MOF loading, the substantial increase in the IPCE can be attributed to improved charge carrier transport and enhanced surface reaction kinetics (Fig. 4d).

The deposition of interlayer bismuthene enhances the electron transport properties, as revealed by electrochemical impedance spectroscopy (Fig. S10, SI). The loading of bismuthene on BiVO₄ reduced the charge-transfer resistance (R_ct) of BiVO₄, and BiVO₄/Bi/Ni-MOF showed the lowest R_ct value compared to other photoanodes.

Further, the electrical double-layer capacitance (C_dl) was determined to examine the electrochemically active surface area (which is proportional to the C_dl) (Fig. S11a–d, SI). The highest C_dl was observed for BiVO₄/Bi/Ni-MOF.

In addition, a 24 h photo-stability test at 1.23 V vs. RHE showed that the BiVO₄/Bi/Ni-MOF photoanode maintained a stable photocurrent, indicating that the introduction of bismuthene and the Ni-MOF layer significantly enhanced the stability of BiVO₄ (Fig. S12, SI).

In summary, we report the BiVO₄/Bi/Ni-MOF photoanode, in which bismuthene functions as an interlayer charge-transporter between BiVO₄ and the Ni-MOF cocatalyst, achieving a pronounced improvement in the PEC water oxidation activity. The effective electron transfer from bismuthene to BiVO₄ facilitates better charge carrier separation and suppresses recombination, while Ni-MOF offers abundant active sites for efficient hole extraction, leading to an enhancement in the charge separation and hence, the PEC-OER activity. The combined effect exerted by bismuthene and Ni-MOF results in an OER photocurrent of 4.76 mA cm⁻² at 1.23 V vs. RHE (ABPE of 2.01% at 0.59 V vs. RHE, and IPCE of 62.17% at 420 nm), which is far better than that of BiVO₄ and BiVO₄/Ni-MOF. The improved ABPE and IPCE establish a new interlayer-engineering paradigm for high-performance BiVO₄-based solar water-splitting systems.

This work is financially supported by ANRF, India (DST/C3E/MI2.0/CCUS/2K23/CALL/2023/71(G)/2). D. K. acknowledges UGC for providing the senior research fellowship.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: synthesis and characterization of the catalysts, details of the experiments. See DOI: https://doi.org/10.1039/d5cc04966k.

References

1. S. Zhao, B. Liu, K. Li, S. Wang, G. Zhang, Z. J. Zhao, T. Wang and J. Gong, Nat. Commun., 2024, 15, 2970.
2. I. J. Teh, L. K. Putri, C. S. Yaw, W. C. Ng, M. N. Chong and S. P. Chai, Coord. Chem. Rev., 2025, 541, 216773.
3. X. Li, J. Wu, C. Dong, Y. Kou, C. Hu, J. Zang, J. Zhu, B. Ma, Y. Li and Y. Ding, Appl. Catal., B, 2024, 353, 124096.
4. J. Chi, Z. Jiang, J. Yan, A. Larimi, Z. Wang, L. Wang and W. Shangguan, Mater. Today Chem., 2022, 26, 101060.
5. Y. An, C. Lin, C. Dong, R. Wang, J. Hao, J. Miao, X. Fan, Y. Min and K. Zhang, ACS Energy Lett., 2024, 9, 1415–1422.
6. J. Cui, M. Daboczi, M. Regue, Y. C. Chin, K. Pagano, J. Zhang, M. A. Isaacs, G. Kerherve, A. Mornto, J. West, S. Gimenez, J. S. Kim and S. Eslava, Adv. Funct. Mater., 2022, 32, 2207136.
7. H. Bai, F. Wang, Z. You, D. Sun, J. Cui and W. Fan, Colloids Surf., A, 2022, 640, 128412.
8. S. Wang, D. Cui, W. Hao and Y. Du, Energy Fuels, 2022, 36, 11394–11403.
9. Y. Zhang, B. Liu, L. Xu, Z. Ding, R. Yan and S. Wanag, ChemSusChem, 2025, 18, e202401420.
10. M. Jia, W. Xiong, Z. Yang, J. Cao, Y. Zhang, Y. Xiang, H. Xu, P. Song and Z. Xu, Coord. Chem. Rev., 2021, 434, 213780.
11. S. Zhou, K. Chen, J. Huang, L. Wang, M. Zhang, B. Bai, H. Liu and Q. Wang, Appl. Catal., B, 2020, 266, 118513.
12. S. Zhong, B. Kang, X. Cheng, P. Chen and B. Fang, ACS Sustainable Chem. Eng., 2024, 12, 1233–1246.
13. X. Y. Yang, Z. W. Chen, X. Z. Yue, X. Du, X. H. Hou, L. Y. Zhang, D. L. Chen and S. S. Yi, Small, 2023, 19, 2205246.
14. J. Qiao, X. Kong, Z. X. Hu, F. Yang and W. Ji, Nat. Commun., 2014, 5, 4475.
15. K. Zhang, B. Jin, C. Park, Y. Cho, X. Song, X. Shi, S. Zhang, W. Kim, H. Zeng and J. H. Park, Nat. Commun., 2019, 10, 2001.
16. Y. H. Ng, A. Iwase, A. Kudo and R. Amal, J. Phys. Chem. Lett., 2010, 1, 2607–2612.
17. D. Kumar, L. Gu, A. D. Chowdhury and A. Indra, Inorg. Chem., 2025, 64, 11610–11617.
18. A. Yadav, T. Ansari, P. Mannu, B. Singh, A. K. Singh, Y. C. Huang, V. Kumar, S. Singh, C. L. Dong and A. Indra, J. Mater. Chem. A, 2024, 12, 29072–29080.
19. S. Ghosh, D. Bagchi, I. Mondal, T. Sontheimer, R. V. Jagadeesh and P. W. Menezes, Adv. Energy Mater., 2024, 14, 2400696.
20. J. A. DeGayner, I. R. Jeon, L. Sun, M. Dincă and T. D. Harris, J. Am. Chem. Soc., 2017, 139, 4175–4184.
21. H. T. T. Nguyen, D. Jung, C. Y. Park and D. J. Kang, Mater. Chem. Phys., 2015, 165, 19–24.
22. A. Indra, A. Acharjya, P. W. Menezes, C. Merschjann, D. Hollmann, M. Schwarze, M. Aktas, A. Friedrich, S. Lochbrunner, A. Thomas and M. Driess, Angew. Chem., Int. Ed., 2017, 56, 1653–1657.
23. Z. Niu, W. Shan, X. Wang, X. Zhang, A. Shi, Y. Zhang and X. Niu, J. Phys. Chem. Lett., 2025, 16, 4057–4065.

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