Enhanced performance of NiF₂/BiVO₄ photoanode for photoelectrochemical water splitting

Abstract

The serious surface charge recombination and fatigued photogenerated carriers transfer of the BiVO₄ photoanode restrict its photoelectrochemical (PEC) water splitting performance. In this work, nickel fluoride (NiF₂) is applied to revamp pure BiVO₄ photoanode by using a facile electrodeposition method. As a result, the as-prepared NiF₂/BiVO₄ photoanode increases the dramatic photocurrent density by approximately 180% compared with the pristine BiVO₄ photoanode. Furthermore, the correlative photon-to-current conversion efficiency, the charge injection, and the separation efficiency, as well as the hydrogen generation of the composite photoanode have been memorably enhanced due to the synergy of NiF₂ and BiVO₄. This study may furnish a dependable guidance in fabricating the fluoride-based compound/semiconductor composite photoanode system.

Key words： BiVO₄; NiF₂; heterojunction; photoelectrochemical water splitting

1 Introduction

Conversion of solar power into chemical energy by photoelectrochemical (PEC) water splitting is a promising and efficient strategy for accommodating the growth of miscellaneous energy utilization and environment protection requirements [1–5]. Up to the present, the metal oxide semiconductor, including TiO₂ [6,7], WO₃ [8,9], ZnO [10], and α-Fe₂O₃ [11–13], with an impressive photoelectrochemical activity have been developed and investigated.

Bismuth vanadate (BiVO₄) is regarded as an expectant photoanode owing to its narrow energy gap (approximately 2.4 eV), suitable band edge position, low cost, good chemical stability, and relatively high theoretical solar-to-hydrogen conversion efficiency [14–16]. However, the PEC capacities and application of BiVO₄ is mainly restricted by its short carrier diffusion length, poor electron-hole separation efficiency, and slow water oxidation kinetics [17,18]. To overcome these suppression factors, all kinds of strategies, such as morphological control [19–21], element doping [22,23], oxygen evolution catalysts layer loading [24,25] and heterojunction engineering [26–28], have been committed to improvement of PEC performance in water splitting.

According to Refs. [29,30], the layer of formation metal fluoride-based materials on BiVO₄ photoanodes significantly improves the PEC performance, which principally benefits from the following preponderance. Fluoride-based materials can adsorb ample visible light to provide sufficient energy for the separation between photo-generated electron-hole pairs due to the distinct optical properties [31]. Owing to the fact that fluorine anion has the strongest electronegativity, the metal-fluorine bond possesses the characteristics of strong iconicity and high polarization, bringing about the raised electron transfer efficiency [32]. Meanwhile, the metal-fluorine bond easily dissociates to form metal oxides (or hydroxide) as a passivation layer, which can maintain the stable catalyst structure and facilitate the photoanode water oxidation reaction [33]. Additionally, the formation of heterojunctions between metal-fluoride and BiVO₄ can effectively increase the efficiency of the separation of electrons and holes [29]. Related research reports demonstrate that nickel fluoride (NiF₂) has relatively excellent performance in the field of electrocatalysis and batteries. Since there are related reports in the field of electrocatalysis and batteries that it has a relatively excellent performance, the research on nickel fluoride (NiF₂) has been continuously conducted [34,35]. Inspired by the results of electrochemical research, it may be a promising strategy to modify NiF₂ on the semiconductor photoanode to improve its PEC performance.

In this study, a novel hetero-structured photoanode was generated by electrodepositing NiF₂ onto the surface of BiVO₄ film. The as-prepared NiF₂/BiVO₄ photoanode demonstrates a dramatically improved PEC water oxidation property. Essentially, the outstanding PEC performance can be put down to the formed heterostructure between NiF₂ and BiVO₄, leading to the improved charge separation efficiency and the accelerative surface reaction kinetics. Furthermore, a feasible mechanism for explaining the increased phenomenon was reasonably speculated.

2 Experimental

2.1 Chemicals

KI, C₄H₆O₂, VO(acac)₂, DMSO, Bi(NO₃)₂·5H₂O, Ni(NO₃)₂·6H₂O, NaF, NaCl, NaOH, and C₂H₅OH were purchased from Sinopharm Chemical Reagent Co., Ltd. All the above raw materials were analytical reagent (AR) grade and used without further purification. Fluorine-doped tin oxide (FTO) coated glasses substrates were obtained from Zhuhai Kaivo Electronic Components Co., Ltd.

2.2 Method

The synthesis pathway of the samples is schematically illustrated in Fig. 1. BiVO₄ photoanodes were fabricated by electrodeposition and the annealing method [36]. Primarily, 20 mmol of KI was dissolved in 50 mL of deionized water, whose pH value was then adjusted to 1.6 by appending 1 mol/L HNO₃. Afterwards, 5 mmol of Bi(NO₃)₃·5H₂O was added to the above mixed solution. Immediately, 20 mL of 0.23 mol/L p-benzoquinone ethanol solution was added to the above solution and stirred forcefully. A typical three-electrode system was used for electrodeposition which was conducted by cyclic voltammetry (CV) at a potential range of –0.13 to 0 V (versus Ag/AgCl), with a scan rate of 5 mV/s at room temperature. The obtained BiOI film was washed by deionized water and dried in an oven at 60°C. Subsequently, the 100 μL of 0.2 mol/L VO(acac)₂ DMSO solution was dropped onto the BiOI film and kept at 450°C for 2 h. To get rid of excess V₂O₅ on the surface of BiVO₄, the acquired film was immersed in 1 mol/L of NaOH solution for 30 min. Eventually, the as-prepared BiVO₄ electrode was adequately rinsed with deionized water and desiccated at air atmosphere.

The NiF₂ electrode and NiF₂/BiVO₄ electrode were electrodeposited by cyclic voltammetry in an electrochemical workstation with the three electrodes. The electrolyte was as follows: 5 mmol of Ni(NO₃)₂·6H₂O was dissolved into 1 mol/L hydrogen peroxide solution. Then 5 mmol of NaF and 5 mmol of NaCl were added to the above solution and stirred for 30 min to form a homogeneous solution. The as-prepared BiVO₄ electrode was placed in a precursor solution. Afterwards, potentials were swept from –0.52 to 0.41 V (versus Ag/AgCl) at a scan rate of 0.1 V/s for 5 cycles, denoted as NiF₂/BiVO₄. Meanwhile, pure NiF₂ electrode was synthesized by the similar approach but scanned for 20 cycles.

Fig.1 Schematic illustration of the synthesis approach of NiF₂/BiVO₄ composite electrodes.

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2.3 Characterization and PEC measurements

The X-ray diffraction (XRD) data patterns were measured on a Bragg-Brentano Rigaku D/MAX-2200/PCX-diffractometer with Cu Kα radiation (40 kV × 20 mA). The surface morphology of the electrodes was observed by using a JSM-6701F field emission scanning electron microscope (FE-SEM) and a JEOL JEM-2100 transmission electron microscopy (TEM). The X-ray photoelectron spectroscopy (XPS) was characterized by the PHI5702 photoelectron spectrometer. Using Ba₂SO₄ as the reflectance standard, the UV-vis diffuse reflectance spectra were observed by using a double-beam UV-vis spectrophotometer (UV-3100). The photoluminescence (PL) spectrum detection of samples was recorded on a PELS-55 luminescence/fluorescence spectrophotometer.

The PEC activity of photoanodes was evaluated on an electrochemical analyzer (CHI 760E) under a Xenon lamp illumination, which simulates sunlight resource AM 1.5 G (100 mW/cm²). The electrolyte was 0.5 mol/L of Na₂SO₄ aqueous solution (pH= 7.35). All measurements were conducted on the condition of light irradiates to the back side of the FTO glass. All the used voltage values were converted to a reversible hydrogen electrode (RHE) through the formula (E_RHE = E_Ag/AgCl + 0.197+ 0.059 × pH, of which 0.197 is the Ag/AgCl electrode with 3.5 mol/L KCl relative to the voltage value of the standard hydrogen electrode). The evolution of H₂ and O₂ gases were measured by using the GC-9560 gas chromatography.

3 Results and discussion

The crystalline phases of all the as-prepared electrodes were investigated by XRD analysis. As shown in Fig. 2(a), the clear diffraction peaks stemmed from BiVO₄ can be well indexed to a monoclinic scheelite crystal system (JCPDS. No. 14-0688) [37]. Apart from the peaks of SnO₂ ingredient of FTO substrates (JCPDS No. 46-1088), no impurity diffraction peaks can be observed. However, in the pattern of NiF₂/BiVO₄, the characteristic diffraction peaks of NiF₂ nanoparticles cannot be distinctly observed (JCPDS No. 22-0749), which might be attributed to the diffraction peaks of NiF₂ overlap with that of BiVO₄ [38]. Yet, despite these, the TEM image of the NiF₂/BiVO₄ electrode indicates that a layer of fine NiF₂ nanoparticles was loaded on the BiVO₄ film, as presented in Figs. 2(b) and 2(c). More specifically, the high-crystalline structure with a lattice spacing of 0.16 nm and 0.18 nm corresponding to the (–121) plane of BiVO₄ and the (121) plane of NiF₂ respectively could be visibly observed in Fig. 2(d) [30]. In the meantime, it can be clearly seen that there is an obvious interface between BiVO₄ and NiF₂. These results attest that NiF₂/BiVO₄ has been successfully prepared. The surface morphology and microstructure of all the photoanodes are reflected by FE-SEM. From Fig. 2(e), it can be observed that a pure BiVO₄ with a wormlike structure and a smooth surface is coated on the surface of the FTO conductive glass. After electrodeposition, the NiF₂ nanoparticles are uniformly dispersed on the surface of BiVO₄, as demonstrated in Fig. 2(f). Additionally, it can be realized from the related element mappings of the NiF₂/BiVO₄ (Figs. 2(g)–2(k)) that the Bi, V, O, Ni, and F elements are scattered evenly in the composite electrode, which also indicates the successful preparation of the photoanode material.

Fig.2 Structure and morphology characterization.

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The chemical bonding state of the NiF₂/BiVO₄ photoanode was inquired by XPS, as exhibited in Fig. 3. The full spectrum of the composite photoanode is presented in Fig. 3(a). It can be precisely observed that the as-prepared sample is composed of Bi, V, O, Ni, and F elements, in accordance with the elemental mapping consequence. Figure 3(b) displays two different peaks located at 159.0 eV and 164.3 eV, which can be attributed to the Bi 4f_7/2 and Bi 4f_5/2, respectively [39]. Simultaneously, Fig. 3(c) demonstrates that the V 2p_3/2 and V 2p_1/2 orbital peaks are situated at 516.3 eV and 523.8 eV respectively. As depicted in Fig. 3(d), two peaks can be clearly identified in the O1s core level spectra. In detail, the peak at 531.0 eV is in accordance with the lattice oxygen while the peak at 532.7 eV can be ascribed to O 1s that is associated with hydroxy species of surface-adsorbed water molecules [40]. The characteristic peaks of Ni element were investigated, as manifested in Fig. 3(e). The two broad peaks at 861.3 and 878.9 eV are identified as shake-up satellites (marked as “Sat.”) of Ni 2p_3/2 and Ni 2p_1/2. Synchronously, the two peaks at 855.1 eV and 872.7 eV can be ascribed to Ni 2p_3/2 and Ni 2p_1/2 respectively, which is a convincible justification to the existence of the oxidation state of Ni²⁺ [41]. Figure 3(f) reveals that the peaks of F 1s core-levels are located at 684.2 eV, indicating a normal state of F⁻ in the electrode sample [30]. This further illustrates that the NiF₂ has been successfully loaded on the surface of the pristine BiVO₄.

Fig.3 XPS spectra.

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Fig.4 Characterization of optical properties.

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Moreover, in order to assess the optical properties, the UV-vis diffuse reflectance spectra of the NiF₂, BiVO₄ and NiF₂/BiVO₄ electrodes were presented Fig. 4(a). It can be demonstrated that the absorption edge of NiF₂ and BiVO₄ are at the wavelength of 530 and 510 nm, respectively. Remarkably, the absorption intensity of BiVO₄ to light is enhanced after the deposition of NiF₂ nanoparticles. In the meantime, the absorption edge of NiF₂/BiVO₄ is located at approximately 525 nm, which indicates an obvious red-shift compared to the BiVO₄ photoanode. Meanwhile, the band gap of all the photoanodes could be calculated by the Tauc equation (Fig. 4(b)). According to Tauc relationship, the energies of the band gap are 2.46, 2.49, and 2.48 eV for the NiF₂, BiVO₄ and NiF₂/BiVO₄ electrodes, respectively. The slight decrease of enery gap (Eg) reflected the broadening of absorption spectrum range, which is beneficial to the enhancement of absorption capability [42]. The photoluminescence (PL) spectra technology is applied to effectively analyze the separation and recombination performance of photo-induced carriers [43]. It can be found in Fig. 4(c) that the peak intensity of the NiF₂/BiVO₄ electrode was weaker than that of the pristine BiVO₄ electrode, which indicates the tardy recombination rate of photogenerated electron-holes and the elevated charge separation efficiency.

The photoelectrochemical activities of as-prepared photoanodes for water splitting were appraised in 0.5 mol/L Na₂SO₄ under simulated solar light irradiation (AM 1.5 G). The linear sweep voltammetry (LSV) curves explicitly disclose that the photocurrent density of the pristine BiVO₄ is 1.0 mA/cm² at 1.23 V_RHE, as presented in Fig. 5(a). It is worth mentioning that the photocurrent density of the NiF₂/BiVO₄ composite photoanode is up to 2.8 mA/cm² at 1.23 V_RHE, which is 2.8 times that of BiVO₄ photoanode. Furthermore, the NiF₂/BiVO₄ electrode demonstrates a much lower onset potential and a steeper curve in comparison to the BiVO₄ electrode in Fig. 5(b). In detail, the onset potential of NiF₂/BiVO₄ electrode exhibits a negative shift of approximately 210 mV compared with the pure BiVO₄ electrode under dark condition. At the same time, the NiF₂ electrode possesses the highest current density and exhibits a favorable electrocatalytic performance, which improves the performance of the NiF₂/BiVO₄ electrode. These results reveal that the BiVO₄ electrode modified with NiF₂ demonstrates a better water oxidation ability.

Fig.5 LSV curves of NiF₂, BiVO₄, and NiF₂/BiVO₄ photoanodes.

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To further expound the separation course of photogenerated electrons and holes in detail, the electrochemical impedance spectroscopy (EIS) measurements of the bare BiVO₄ and NiF₂/BiVO₄ photoanodes with light illumination were conducted, as depicted in Fig. 5(c). Apparently, through the exhibition of the radius of the Nyquist diagram, a smaller arc radius appears for NiF₂/BiVO₄ in light irradiation condition compared with BiVO₄. Consequently, the relevant Randle equivalent circuit model can be obtained (the correlative Randle circuit values being presented in Table 1), where R_Ω represents the resistance corresponding to the charge transfer (including the resistance of the catalyst, the FTO substrate, the electrolyte, and the wire connection in the circuit), and the R_ct and C_ct respectively represent the resistance and capacitance related to charge transfer on the electrode/electrolyte interface [30,44,45]. This impedance decrement can be mainly put down to the swifter transference of photogenerated holes on the surface of NiF₂/BiVO₄ electrode.

Tab.1 Fitted results of EIS data using Randle equivalent circuit

Sample	RΩ/Ω (Error/%)	Rct/Ω (Error/%)	Cct/F (Error/%)
BiVO4	27.8 (0.37)	622.6 (0.43)	5.236e–5 (1.50)
NiF2/BiVO4	28.5 (0.38)	298.2 (0.45)	8.082e–5 (2.01)

Simultaneously, to explore the possible cause for the result in the different PEC performance of the bare BiVO₄ and NiF₂/BiVO₄ photoanodes, the transfer kinetics of these electrodes were illustrated by the variation of open-circuit voltage (V_oc) [46]. The open-circuit voltage decay rate is a significant criterion of applying to understand the situation of the electron recombination [47]. As shown in Fig. 5(d), the V_oc curve of the NiF₂/BiVO₄ photoanode demonstrates a slower decay rate than that of the BiVO₄ photoanode under the identical condition, which reveals that the recombination of the photogenerated carriers are efficiently suppressed.

Moreover, the applied bias photo-to-current efficiency (ABPE) value of the NiF₂/BiVO₄ composite photoanode reaches up to 0.56% at 0.88 V_RHE, while the pristine BiVO₄ photoanode only achieves an efficiency of 0.1% at 1.0 V_RHE, which is consistent with the value of their photocurrent density (Fig. 6(a)). Based on the performances above, it can be concluded that the surface modification of NiF₂ could efficaciously improve the PEC water oxidation activity and inhibit the recombination of the photogenerated electron-hole pairs of the pristine BiVO₄ photoanode. The photoanode competence can be evaluated by the incident photo-to-current conversion efficiency (IPCE) [48]. When the light wavelength ranges from 360 to 520 nm, the IPCE magnitudes of NiF₂/BiVO₄ photoanode is higher than that of the pure BiVO₄ photoanode (Fig. 6(b)). To be specific, the NiF₂/BiVO₄ photoanode achieves a maximum IPCE value of 30% at 380 nm, which is 3 times higher than that of the pure BiVO₄ photoanode at the alike wavelength. Meanwhile, the promotion of the light harvesting efficiency (LHE) of the composite photoanode will give rise to the increase of the photo-to-electron efficiency (Fig. 6(c)). Consequently, the absorbed photo-to-current conversion efficiency (APCE) of the NiF₂/BiVO₄ photoanode is computed as 34% at 380 nm, while the pristine BiVO₄ photoanode can only reach 13% at the identical wavelength, as shown in Fig. 6(d). Sequentially, it can be concluded that the NiF₂/BiVO₄ photoanode may make use of the absorbed light more effectively.

As displayed in Fig. 7(a) and 7(b), by integrating IPCE on the AM 1.5 G solar spectrum, the photocurrent densities of BiVO₄ and NiF₂/BiVO₄ are calculated as 0.69 mA/cm² and 2.35 mA/cm², respectively. These estimated values are very close to the actual measured values in the LSV curve, indicating that all measurement procedures are trustworthy [49].

Fig.6 Photoelectric conversion efficiency.

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Fig.7 Solar currents.

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Whether the measured photoanode has a significant PEC performance can be reflected by the proportion of holes participating in the entire water splitting reaction. At the same time, the proportion of holes which participates in the PEC reaction are estimated by the charge injection efficiency, which is obtained by dividing the photocurrent density for the water oxidation by the photocurrent density for the Na₂SO₃ oxidation under identical circumstances [50,51]. As presented in Fig. 8(a), the charge injection efficiency of the pure BiVO₄ photoanode is maintained 19% at 1.23 V_RHE, implying that a large proportion of the generated holes are unutilized because of the bulk recombination. In contrast, after modifying with NiF₂ on the surface of BiVO₄, the charge injection efficiency of the composite photoanode is attained to 48% at 1.23 V_RHE, which is 2.5 times higher than that of the pristine BiVO₄ photoanode. In the meantime, the charge separation efficiency also appears to be an analogous situation as the charge injection efficiency, as demonstrated in Fig. 8(b). It further confirms the promoted separation of photogenerated electrons and holes after bringing in NiF₂.

Fig.8 Charge injection and separation efficiency.

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Tab.2 PEC measurement results of BiVO₄ and NiF₂/BiVO₄ photoanodes

Sample	BiVO4	NiF2/BiVO4
Current density(mA/cm2 at 1.23 VRHE)	1.0	2.8
Onset potential (VRHE)	2.14	1.93
ABPE/%	0.1	0.56
IPCE/%	11	30
APCE/%	13	34
Charge injection efficiency/%	19	48
Charge separation efficiency/%	27	36

Hydrogen and oxygen can be acquired through the participation of NiF₂/BiVO₄ electrode in the photoelectrochemical water splitting process. As shown in Fig. 9(a), after three hours of photoelectric reaction, the generated magnitudes of H₂ to O₂ conform to the molar ratio of approximately 2:1. Synchronously, the calculated average Faradaic efficiencies of H₂ and O₂ reactions are both approximately 90%. As a result, it sufficiently elucidates that the photogenerated charges are almost completely used for decomposing water to obtain hydrogen and oxygen. Stability is an important indicator to evaluate the catalytic performance and the application value of the photoanode. As presented in Fig. 9(b), the NiF₂/BiVO₄ photoanode can maintain 83% of the initial photocurrent density under 3 h of constant light illumination, indicating that it has a relatively favorable stability.

Fig.9 Effect of H₂ and O₂ production, stability of the photoanode, and Mott-Schottky curves.

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As a consequence, based on the above characterization and properties, a possible mechanism of the NiF₂ effectively which improved the PEC water splitting performance of the BiVO₄ photoanode was proposed. Initially, based on the correlative energy gap and Mott-Schottky curves of the NiF₂ and BiVO₄ (Figs. 9(c) and 9(d)), it can be concluded that both of the semiconductors exhibit the n-type behavior. Based on Fig. 10, it can be comprehended that the positions of the conduction band (CB) and the valence band (VB) of BiVO₄ and NiF₂ are (0.21, 2.70 eV) and (–0.12, 2.34 eV), respectively [52]. These results demonstrate that they possess more appropriate band matching, which promotes a more efficient separation of the photogenerated electrons and holes [53,54]. Specifically speaking, benefited from the n-n heterostructure formed between the NiF₂ and BiVO₄, the photogenerated electrons in the CB of NiF₂ migrate to the CB of BiVO₄. These electrons will eventually be transferred to the Pt counter electrode for the reduction of H⁺ into H₂. Meanwhile, the photogenerated holes at the VB of BiVO₄ migrate to the CB of NiF₂ and further oxidize water molecules to acquire O₂ [30,55–57]. Consequently, the heterostructure NiF₂/BiVO₄ composite photoanode can efficiently inhibit the charge recombination and enhance the PEC performance. Furthermore, NiF₂ as an electrocatalyst, not only can efficaciously decrease the overpotential, but also can suitably accelerate the surface reaction kinetics and facilitate the electrons and holes separation [40,58].

Fig.10 Possible mechanism schematic depiction of NiF₂/BiVO₄ photoelectrode for PEC water splitting application.

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4 Conclusions

This paper demonstrated a facile electrodeposition approach to fabricate the NiF₂/BiVO₄ heterojunction photoanode for photoelectrochemical water splitting. The NiF₂/BiVO₄ composite photoanode was found to display a higher photocurrent density of 2.8 mA/cm² at 1.23 V_RHE under AM 1.5 G illumination, which was 2.8 times than that of the pristine BiVO₄ photoanode. In addition, further systematized exploration revealed that the photogenerated electrons-holes separation and surface reaction kinetics were effectually improved when NiF₂ was assembled onto the surface of the BiVO₄ photoanode. Consequently, this research thoroughly explored the impact of metal fluoride on the PEC ability of BiVO₄ photoanode, providing a profound insight into the construction of a high-performance heterojunction photoanode system.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2017YFC0602306), the National Natural Science Foundation of China (Grant No. 21808189), and National Natural Science Foundation of Gansu Province (Grant No. 20JR5RA523).

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