Efficient charge separation and transfer of a TaON/BiVO₄ heterojunction for photoelectrochemical water splitting

Abstract

The separation and transfer of photogenerated electron–hole pairs in semiconductors is the key point for photoelectrochemical (PEC) water splitting. Here, an ideal TaON/BiVO₄ heterojunction electrode was fabricated via a simple hydrothermal method. As BiVO₄ and TaON were in well contact with each other, high performance TaON/BiVO₄ heterojunction photoanodes were constructed. The photocurrent of the 2-TaON/BiVO₄ electrode reached 2.6 mA cm⁻² at 1.23 V vs. RHE, which is 1.75 times as that of the bare BiVO₄. TaON improves the PEC performance by simultaneously promoting the photo-generated charge separation and surface reaction transfer. When a Co-Pi co-catalyst was integrated onto the surface of the 2-TaON/BiVO₄ electrode, the surface water oxidation kinetics further improved, and a highly efficient photocurrent density of 3.6 mA cm⁻² was achieved at 1.23 V vs. RHE. The largest half-cell solar energy conversion efficiency for Co-Pi/TaON/BiVO₄ was 1.19% at 0.69 V vs. RHE, corresponding to 6 times that of bare BiVO₄ (0.19% at 0.95 V vs. RHE). This study provides an available strategy to develop photoelectrochemical water splitting of BiVO₄-based photoanodes.

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Introduction

At present, the shortage of fossil fuels and increase of global warming have become very serious social and environmental problems. As a green energy source, solar energy is widely distributed. Hydrogen is also a type of clean energy with a high heating value. Therefore, the use of solar energy to produce hydrogen has attracted widespread attention.¹ Building a photoelectrochemical (PEC) cell to mimic the photosynthesis system is an effective method to realize the conversion of solar energy into hydrogen energy.^2–6 However, in a PEC system, the water oxidation reaction half reaction occurring in the photoanode involves light capture, charge separation, and complex surface reaction with multiple electron and proton transfer. Therefore, designing an efficient photoanode to accelerate the water oxidation process becomes the key to the development of PEC cells.

In recent years, semiconductors based on metal oxides, such as BiVO₄, TiO₂, and WO₃, have been widely used as photoanode materials.^7–11 BiVO₄, an n-type semiconductor with a band gap of 2.4 eV, is considered as one of the most promising alternative materials.¹² Under the irradiation of a sunlight AM 1.5 G, the solar-to-hydrogen efficiency (STH) can reach 9.2%,¹³ which is close to the solar-powered water decomposition technology target (STH efficiency 10%).¹⁴ However, the severe recombination of the electron–hole pair of BiVO₄ limits its development. Different strategies were adopted to enhance the PEC performance of BiVO₄,^15,16 such as elemental doping, nanostructure modification, loading cocatalysts and heterojunction construction. Among them, the construction of semiconductor heterojunctions with different materials is an effective approach to improve the PEC performance by enhancing the separation efficiency of photo-generated charges. Although there were numerous reports on heterojunction systems for BiVO₄-based semiconductors, such as CaFe₂O₄/BiVO₄,¹⁷ V₂O₅/BiVO₄,¹⁸ WO₃/BiVO₄ ^19–21 and TiO₂/BiVO₄,^22,23 the construction of a high-efficiency heterojunction photoanode still faces great challenges.

Here, TaON/BiVO₄ heterojunction photoanodes were constructed, which exhibited a greatly improved PEC water oxidation activity compared to the bare BiVO₄ photoanode. The photocurrent of the 2-TaON/BiVO₄ photoanode reached 2.6 mA cm⁻² at 1.23 V vs. RHE, which was 1.75-times that of BiVO₄. In order to enhance the surface water oxidation kinetics of the photoanode, the Co-Pi water oxidation catalyst was immobilized on the photoanode, and the PEC performance was further promoted. A high photocurrent density of 3.6 mA cm⁻² was obtained for Co-Pi/TaON/BiVO₄ at 1.23 V vs. RHE.

Experimental section

Fabrication of the BiVO₄ photoanode

A BiVO₄ electrode was prepared according to the electrodeposition method reported in literature.²⁴ 150 mL of deionized water was taken in a small beaker, and nitric acid was added to adjust the pH to 1.7. Then, 2.91 g of bismuth nitrate and 9.96 g of potassium iodide were added to the above mixture, and sonicated for about 30 min to fully dissolve. The above solution was mixed with an ethanol solution containing 0.23 M of p-benzoquinone, and sufficiently stirred to prepare an electrodeposition solution. An appropriate amount of the above-mentioned deposition solution was placed in a small beaker, and a three-electrode cell was used for electrodeposition (FTO was used as a working electrode, Pt was used as a counter electrode, and Ag/AgCl was used a reference electrode). At room temperature, after depositing for about 300 s at an applied bias of −0.1 V vs. Ag/AgCl, we obtained a BiOI film, which was washed with deionized water and blown with nitrogen. Then, a DMSO solution-containing 0.2 M acetylacetonoxane was added dropwise to the deposited layer, and calcined at 450 °C for 2 h (2 °C min⁻¹) to convert BiOI into BiVO₄. Finally, it was immersed in a 1 M aqueous solution of sodium hydroxide for about 10 min in order to remove excess V₂O₅, rinsed with deionized water, and blown dry with nitrogen for use.

Fabrication of the TaON/BiVO₄ photoanode

TaON nanoparticles were prepared by heating Ta₂O₅ particles at 1123 K for 15 h under NH₃ flow (20 mL min⁻¹).²⁵ A certain amount of TaON (1, 0.02 g; 2, 0.04 g; 3, 0.06 g) was added to 100 mL of deionized water, sonicated to make a sol, and the BiVO₄ photoanodes were separately immersed in different concentrations TaON sol at 60 °C for 1 h, and calcined at 500 °C for 2 h under N₂ flow.

Fabrication of the Co-Pi/TaON/BiVO₄ photoanode

The Co-Pi cocatalyst was loaded by photoelectron deposition according to literature.²⁶ The 2-TaON/BiVO₄ photoanode was immersed in a phosphate buffer solution-containing a certain concentration of Co(NO₃)₂·6H₂O. Deposition was carried out at an applied bias of 0.1 V vs. Ag/AgCl and the sun illumination (100 mA cm⁻²) for 300 s.

Characterization of photoanodes

The XRD pattern was measured used a Bruker D8 QUEST. The SEM images were recorded on a SU8000 Schottky field emission scanning electron microscope (SFE-SEM) equipped with a Rontec EDX system. The UV-Vis absorbance spectra were recorded on a Lambda 35 UV-Vis spectrophotometer. X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific ESCALAB 250 instrument (150 W, spot size of 500 μm and Al Kα radiation at 1486.6 eV) to obtain the surface elements. Photoluminescence (PL) spectroscopy measurement was performed on a Cary Eclipse spectrophotometer.

Photoelectrochemical measurements

The PEC performance tests of the electrodes were carried out using a standard three-electrode cell using an Ag/AgCl (3.5 M KCl) as the reference electrode, a platinum wire as the counter electrode and samples as the working electrode. The electrolyte used was a 0.1 M potassium buffer solution (pH 7.0) that was a mixture of mono- and dibasic hydrophosphates KH₂PO₄ and K₂HPO₄. The light intensity of the solar simulator (AM 1.5 G) was calibrated to 100 mW cm⁻². The effective surface area of the electrodes was 1 cm². The electrochemical impedance spectroscopy (EIS) was measured with the range from 0.1 Hz to 100 000 Hz. The Mott–Schottky curves were obtained under dark conditions, and the potential range was from −0.6 V to +1.0 V vs. Ag/AgCl.

The incident photon-to-current efficiency (IPCE) at each wavelength was determined at 1.23 V vs. RHE using monochromatic light illumination from a 300 W Xe arc lamp and neutral density filters simulating sunlight. The IPCE values have been calculated as follows:

where J is the photocurrent density, λ is the incident light wavelength and P is the measured irradiance.

The applied bias photon-to-current efficiency (ABPE) was calculated from a J–V curve using the following equation:

where J is the photocurrent density, V_bias is the applied bias and P_in is the incident illumination power density (AM 1.5 G, 100 mW cm⁻²).

The total water oxidation photocurrent J_H2O was determined by the following expression:

J_H2O = J_abs × η_sep × η_trans

where J_abs is the photocurrent density when the absorbed photons completely converted into current, η_sep is the charge separation efficiency, and η_trans is the surface charge transfer efficiency.

To obtain η_trans, Na₂SO₃ was added into the 0.1 M PBS electrolyte, which was an efficient hole scavenger. Therefore, the η_trans efficiency equal to 1, and the photocurrent could be described as J_Na2SO3 = J_abs × η_sep. So, the η_sep could be described as:

η_sep = J_Na2SO3/J_abs.

The η_trans was calculated using the equation:

η_trans = J_H2O/J_Na2SO3

where J_H2O is the photocurrent density of the TaON/BiVO₄ electrode in 0.1 M PBS, and J_Na2SO3 is the photocurrent density of the TaON/BiVO₄ electrode in 0.1 M PBS containing 0.1 M Na₂SO₃.

Results and discussion

BiVO₄ films were fabricated via the electrochemical-deposition method. TaON nanoparticles were loaded onto the BiVO₄ surface, as shown in Scheme 1. By controlling the amount of TaON nanoparticles, different TaON/BiVO₄ electrodes were fabricated (denoted as 1-TaON/BiVO₄, 2-TaON/BiVO₄ and 3-TaON/BiVO₄, respectively). The microstructure of the pristine BiVO₄ is shown in Fig. 1a. A porous structure could be seen clearly in the whole area, which guaranteed the combination with TaON nanoparticles. The SEM images of TaON/BiVO₄ electrodes containing different amounts of TaON are shown in Fig. 1. The distribution density of TaON nanoparticles increased with the amount of TaON (Fig. 1b–d). The crystal structures of the TaON, BiVO₄ and TaON/BiVO₄ samples were tested by X-ray diffraction (XRD) patterns, as shown in Fig. S1,† confirming that TaON has been successfully modified on the BiVO₄ electrode.

Scheme 1 An illustration of the preparation of TaON/BiVO₄ and Co-Pi/2-TaON/BiVO₄ electrodes.

Fig. 1 SEM images of different electrodes. (a) BiVO₄, (b) 1-TaON/BiVO₄, (c) 2-TaON/BiVO₄, (d) 3-TaON/BiVO₄.

X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical states of the TaON/BiVO₄ electrodes. Ta, O, N, Bi and V elements were detected and shown in Fig. 2. Compared to the pristine BiVO₄, the characteristic signals of TaON can be observed in the spectrum of the TaON/BiVO₄ electrode. The characteristic peaks of N 1s and Ta 4p are shown in Fig. 2a. The binding energies of 529.7 eV and 532.0 eV are assigned to the O 1s of BiVO₄, and the O 1s peak of TaON appears at 530.6 eV (Fig. 2b). The binding energies of 159.1 eV and 164.4 eV were assigned to the 4f_7/2 and 4f_5/2 of the Bi element, confirming that the Bi element was present as Bi³⁺ (Fig. 2c).²⁷ The binding energies for the V 2p_3/2 and V 2p_1/2 located at 516.6 and 524.2 eV were typical values for V⁵⁺ (Fig. 2d).²⁸ The Bi 4f and V 2p peaks of the TaON/BiVO₄ electrode shifted positively compared to that of the BiVO₄ electrode, indicating an interaction between TaON and BiVO₄.

Fig. 2 XPS spectra of BiVO₄ and TaON/BiVO₄ electrodes: (a) N 1s, Ta 4p, (b) O 1s, (c) Bi 4f, (d) V 2p.

The PEC measurements of BiVO₄ and TaON/BiVO₄ were carried out using a three-electrode system under simulated solar light 100 mW cm⁻². The linear sweep voltammetry (LSV) curves of the sample electrodes are shown in Fig. 3a. The photocurrent density of TaON/BiVO₄ electrodes was significantly higher than that of the bare BiVO₄ electrode. The 2-TaON/BiVO₄ photoanode showed the highest photocurrent density, reaching 2.6 mA cm⁻² at 1.23 V vs. RHE, which was 1.75-times that of bare BiVO₄. However, the photocurrent density of 3-TaON/BiVO₄ was lower than that of 2-TaON/BiVO₄, suggesting that excessive TaON nanospheres hindered the charge transfer. The applied bias photon-to-current efficiencies (ABPEs) of BiVO₄ and TaON/BiVO₄ electrodes were calculated by the LSV curves (Fig. 3b). The maximum value of the 2-TaON/BiVO₄ electrode reached 0.61% at 0.74 V, about 3-times of the BiVO₄ electrode (0.19% at 0.95 V).

Fig. 3 (a) LSV curves in the PBS solution, (b) ABPE values, (c) LSV curves in the PBS solution containing 0.1 M Na₂SO₃ and (d) the charge transfer efficiencies of different electrodes.

In order to measure the charge recombination of the TaON/BiVO₄ electrodes, 0.1 M Na₂SO₃ was added into the electrolyte as a hole scavenger for the PEC measurement.²⁹ The introduction of Na₂SO₃ can eliminate the surface charge recombination. The PEC measurement results are shown in Fig. 3c. Compared to the bare BiVO₄ electrode, all the TaON/BiVO₄ electrodes showed higher photocurrent, which indicated that the combination of the two semiconductors produced a better charge separation. The 2-TaON/BiVO₄ electrode showed the highest photocurrent density of 5.6 mA cm⁻² at 1.23 V vs. RHE. Based on the LSV curves, the separation and surface charge transfer efficiencies of the sample electrodes are calculated and shown in Fig. S4† and 3d, respectively. All the TaON/BiVO₄ electrodes exhibited higher η_sep and η_trans than the bare BiVO₄. 2-TaON/BiVO₄ showed the highest values. These results indicated that the construction of TaON/BiVO₄ could promote both charge separation and surface charge transfer.

The incident photon-to-current conversion efficiency (IPCE) values are shown in Fig. S2.† In the visible spectrum, TaON/BiVO₄ showed a higher IPCE value than BiVO₄. At 520 nm, the IPCE value dropped to 0, which was consistent with the absorption spectra (Fig. S3†).

In order to understand the charge transfer characteristics of TaON/BiVO₄ and BiVO₄ electrodes, electrochemical impedance spectroscopy (EIS) tests were performed in 0.1 M PBS at 1.23 V. Smaller semicircles represent better charge transfer capabilities and faster surface reaction kinetics. As shown in Fig. 4a, the BiVO₄ electrode exhibited the largest semicircle among all electrodes, indicating the highest interface charge transfer barrier. Three sample TaON/BiVO₄ electrodes showed smaller charge transfer resistance than the BiVO₄ electrode, further confirming that TaON was beneficial to improve the charge transfer. The interface charge transfer resistance (R_ct) of the sample electrodes is shown in Table S1.† 2-TaON/BiVO₄ showed the smallest charge transfer resistance, which was in accordance with the photocurrents.

Fig. 4 (a) Electrochemical impedance spectroscopy (EIS) of different electrodes. (b) PL measurements of the BiVO₄ and 2-TaON/BiVO₄ electrode (exciting source: 360 nm). (c) The Mott–Schottky plots of BiVO₄ and TaON. (d) The charge transfer mechanism of the TaON/BiVO₄ electrode.

To investigate the charge transfer behavior between BiVO₄ and TaON, photoluminescence (PL) measurements were performed. As shown in Fig. 4b. BiVO₄ electrode exhibited a high-intensity characteristic emission, suggesting an obvious radiative charge recombination. However, the TaON/BiVO₄ electrode exhibited much lower emission intensity. The obvious photoluminescence quenching for TaON/BiVO₄ electrode fully proved the formation of TaON/BiVO₄ heterojunctions.

The Mott–Schottky plots of BiVO₄ and TaON were also measured. As shown in Fig. 4c, it can be seen that the CB positions of BiVO₄ and TaON are 0.15 eV and 0.05 eV. The band gap width of BiVO₄ and TaON obtained from the absorption spectrum were 2.41 eV and 2.16 eV, respectively (Fig. S5†). The valence band positions of BiVO₄ and TaON can be calculated by the formula using E_CB = E_VB − E_g, which were 2.56 eV and 2.21 eV. The CB edge potential of TaON was more negative than that of BiVO₄. Thus, a difference of band potentials existed between the two materials, and a contact electric field was built at the interface of the TaON and BiVO₄. When electrons and holes were photogenerated, as shown in Fig. 4d, driven by the contact electric field, electrons transferred from TaON to BiVO₄ and holes transferred from BiVO₄ to TaON, thereby leading to an enhancement both in photogenerated charge separation and transfer.

In order to further enhance the PEC performance, Co-Pi, which is well-used as highly efficient catalyst in PEC water oxidation, was equipped onto the TaON/BiVO₄ electrode. The SEM image of the Co-Pi/TaON/BiVO₄ electrode showed that Co-Pi appeared to be discontinuous particles (Fig. 5a). The EDS-mapping (Fig. 5b–h) suggested the existence of Bi, V, O, Ta, N, Co, and P elements, indicating the successful fabrication of the Co-Pi/TaON/BiVO₄ electrode. According to the LSV curves shown in Fig. 6a, an obviously increased photocurrent density was obtained for the Co-Pi/TaON/BiVO₄ photoanode, achieving 3.6 mA cm⁻² at 1.23 V vs. RHE, which was about 3-times higher than that of the BiVO₄ electrode. During a potentiostatic electrolysis at 0.8 V vs. RHE, the bare BiVO₄ electrode always exhibited very low photocurrent density. For Co-Pi/TaON/BiVO₄ high activity and stability was observed for more than 6000 s (Fig. S6†). The half-cell photoconversion efficiency of the Co-Pi/TaON/BiVO₄ electrode achieved 1.19% at 0.69 V (Fig. 6a), approximately 6-times compared to that of BiVO₄ electrode.

Fig. 5 SEM image of the Co-Pi/TaON/BiVO₄ electrode (a) and the EDS-mapping element of the Co-Pi/TaON/BiVO₄ electrode. (b) Bi, (c) V, (d) O, (e) Ta, (f) N, (g) Co, (h) P.

Fig. 6 LSV curves (a) and ABPE values (b) of BiVO₄ and Co Pi/TaON/BiVO₄ electrodes.

Conclusions

In summary, we have successfully fabricated TaON/BiVO₄ heterojunction electrodes by integrating TaON nanoparticles onto BiVO₄ electrodes. The as-prepared TaON/BiVO₄ electrodes exhibited significantly higher performances for PEC water oxidation. The 2-TaON/BiVO₄ electrode showed the highest photocurrent, reaching 2.6 mA cm⁻² at 1.23 V vs. RHE, 1.75 times that of the bare BiVO₄ electrode. In order to further increase the photocurrent, a Co-Pi water oxidation catalyst was attached onto the TaON/BiVO₄ electrode. A high photocurrent density as high as 3.6 mA cm⁻² (1.23 V vs. RHE) was obtained, and the half-cell photoconversion efficiency achieved 1.19% at 0.69 V, approximately 6-times that of the BiVO₄ photoanode. This study provides a promising route to achieve highly efficient PEC water splitting in designing solar-to-fuel conversion devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21671089), the Liaoning Revitalization Talents Program (XLYC1807210), the Natural Science Foundation of Liaoning Province of China (2020-YKLH-22) and the Scientific Research Fund of Liaoning Provincial Education Department (L2020002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra00974e

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