Cooperative Catalytic Behavior of SnO2 and NiWO4 over BiVO4 Photoanodes for Enhanced Photoelectrochemical Water Splitting Performance

n-BiVO4 is a favorable photoelectrode candidate for a photoelectrochemical (PEC) water splitting reaction owing to its suitable energy level edge locations for an oxygen evolution reaction. On the other hand, the sluggish water oxidation kinetics of BiVO4 photoanodes when used individually make it necessary to use a hole blocking layer as well as water oxidation catalysts to overcome the high kinetic barrier for the PEC water oxidation reaction. Here, we describe a very simple synthetic strategy to fabricate nanocomposite photoanodes that synergistically address both of these critical limitations. In particular, we examine the effect of a SnO2 buffer layer over BiVO4 films and further modify the photoanode surface with a crystalline nickel tungstate (NiWO4) nanoparticle film to boost PEC water oxidation. When NiWO4 is incorporated over BiVO4/SnO2 films, the PEC performance of the resultant triple-layer NiWO4/BiVO4/SnO2 films for the oxygen evolution reaction (OER) is further improved. The enhanced performance for the PEC OER is credited to the synergetic effect of the individual layers and the introduction of a SnO2 buffer layer over the BiVO4 film. The optimized NiWO4/BiVO4/SnO2 electrode demonstrated both enriched visible light absorption and achieves charge separation and transfer efficiencies of 23% and 30%, respectively. The photoanodic current density for the OER on optimized NiWO4/BiVO4/SnO2 photoanode shows a maximum photocurrent of 0.93 mA/cm2 at 1.23 V vs. RHE in a phosphate buffer solution (pH~7.5) under an AM1.5G solar simulator, which is an incredible five-fold and two-fold enhancement compared to its parent BiVO4 photoanode and BiVO4/SnO2 photoanodes, respectively. Further, the incorporation of the NiWO4 co-catalyst over the BiVO4/SnO2 film increases the interfacial electron transfer rate across the composite/solution interface.


Introduction
Solar-assisted electrolysis is recognized as a promising process for the commercial production of hydrogen from water. The US Department of Energy has estimated the hydrogen threshold cost at <4/Kg for future solar hydrogen production [1]. Numerous sustainable hydrogen production technologies are commercially available, and these can be categorized into three main types: thermal processes, electrolytic processes, and photolytic processes. Among the available photoelectrochemicals (PECs), water splitting is the most favorable technology for the sustainable manufacturing of clean and renewable fuels. However, the sluggish kinetics of the OER still presents a challenge [2,3]. Recently, Pinaud et al. assessed the feasibility of centralized facilities in a techno-economical way; the hydrogen production costs of a PEC system are in a range of $1.60-$10.40 per kg H 2 [4]. Moreover, it is evidenced that an industrial-scale PEC splitting system can be cost-efficient with fossil-based fuels. Besides, to compete commercially, highly effective and robust solar-assisted electrolysis reactors will need to be fabricated very cheaply [5]. Ideal photoelectrodes for PEC systems involve a small band gap to arrest visible-light photons, high conversion efficiency, and a good durability in aqueous environments [6].
During the past few decades, n-type semiconductors such as TiO 2 [7,8], Fe 2 O 3 [9,10], WO 3 [11,12], and BiVO 4 [13][14][15][16][17][18][19] have been surveyed as photoanode materials for the development of PEC water oxidation technology because their valence band positions are more positive than the potential for the O 2 /H 2 O couple. Among these n-type materials, BiVO 4 is recognized to be a favorable photoanode for the OER in PEC water splitting systems [20] owing to its suitable energy level positions, light absorption, cost-effectiveness, and easy accessibility in large quantities. The maximal theoretical photoanodic current for BiVO 4 films that is practicable with its narrow band gap of ∼2.4 eV, is ∼7.5 mA.cm −2 at 1.23 V RHE [21,22]. In particular, BiVO 4 films tends to possess relatively good stability, as they can reach a theoretical solar to hydrogen efficiency of 9.2% [23]. In this regard, the sluggish water oxidation kinetics, high exciton recombination rate, and also the lower conductivity of BiVO 4 are the major hurdles that must be overcome to realize this photocurrent value [24].
To improve the photon absorption and carrier transport of BiVO 4 photoanodes, numerous modifications have been reported. During the past few decades, significant research work has been carried out to incorporate oxygen evolution catalysts over BiVO 4 photoanodes. For instance, a more efficient approach includes the incorporation of oxygen evolution catalysts such as cobalt phosphate (CoPi), CoO x , and NiFeO x onto BiVO 4 photoanodes, thereby reducing surface recombination at the interface [25][26][27][28][29][30][31][32]. Very recently, we demonstrated the incorporation of silver phosphate (AgPi) [33] and nickel hydroxyl phosphate (Ni-OH-Pi) [34] over BiVO 4 photoanodes, and investigated their use as electrode materials for energy applications. Nano-structuring is an alternative process for performance enhancement of BiVO 4 photoanodes and other semiconductor materials with relatively poor carrier dynamics [35,36].
Another remarkable tactic was the incorporation of a new kind of metal oxide layer, namely WO 3 and SnO 2 , between BiVO 4 and a fluorine-doped tin oxide (FTO) conducting substrate, the widely used conducting parts for PEC cells [37][38][39]. In particular, the incorporation of a metal oxide at the interface between FTO and BiVO4 results in: (i) band bending of the BiVO 4 that assists the flow of suitable charge carriers and (ii) the passivation of trapping states, resulting in the suppression of charge recombination. For instance, BiVO 4 /SnO 2 electrodes displayed an improved PEC activity over a bare-BiVO 4 photoanode owing to the hole-mirroring nature of the SnO 2 layers [40]. Very recently, Byun et al. explored the influence of the thickness of the SnO 2 layer in BiVO 4 films on PEC water oxidation [41]. These reports demonstrate that the incorporation of a SnO 2 buffer layer is a crucial part of assembling photoanodes with improved performance based on BiVO 4 [38,39]. The incorporation of an added metal oxide (SnO 2 buffer layer) as well as an OER catalyst cooperatively boosts the catalytic efficiency of BiVO 4 photoanodes for PEC water oxidation. As stated earlier, coupling a semiconductor with an electrochemical OER co-catalyst is another way to promote the activity of photoanodes materials for water oxidation, because the co-catalyst can improve the charge transfer rate at the interface. In earlier reports, Sn-doping was found to be effective in promoting the electronic conductive nature of the photoanodes. In particular, the incorporation of Sn also dramatically reduces the resistivity of the electrodes [42][43][44][45][46]. In recent years, metal tungstates have been shown to be efficient co-catalysts that possess interesting properties [47][48][49] and have the potential to boost the performance of photoanode materials for PEC OER. In particular, NiWO 4 nanoparticles exhibit fascinating electrochemical behaviors, including electro-catalysis for OER [50]. Moreover, NiO x based materials have been widely considered as extraordinary hole-conducting protection layers for catalysts because of its hole-transfer and electron-blocking nature due to the relatively higher CB edge positions [51][52][53][54].
In this work, we demonstrate triple-layer NiWO 4 /BiVO 4 /SnO 2 photoelectrodes with superior PEC performance and light absorbance. The photoanodes were prepared through a simple electrodeposition process. Firstly, a SnO 2 nanoparticle film was loaded onto a FTO substrate. Afterwards, a BiVO 4 mid layer and a NiWO 4 top layer were electrodeposited. We found that the obtained NiWO 4 /BiVO 4 /SnO 2 nanocomposite demonstrated considerably enhanced performance for PEC OER. Further, the superior PEC performance of the triple-layer nanocomposite was ascribed to its high specific surface area and the enhanced electron-hole separation rate due to the NiWO 4 /BiVO 4 heterojunction. There have been a few reports concerning the incorporation of hole-blocking SnO 2 [41], but we believe that this is the first work to demonstrate the material properties of triple-layered NiWO 4 /BiVO 4 /SnO 2 films.

Fabrication of Triple-Layer Photoanodes
As schematically shown in Figure 1, compact but porous SnO 2 buffer layers comprising of SnO 2 nanoparticles were loaded onto FTO substrates by means of an electrodeposition process. Subsequently, BiVO 4 and NiWO 4 photoanodes were loaded over these SnO 2 /FTO layers through an electrodeposition process and the fabricated photoanodes were annealed to obtain NiWO 4 /BiVO 4 /SnO 2 photoanodes. Figure 1 illustrates the various phases of the synthesis process used to obtain the triple-layer NiWO 4 /BiVO 4 /SnO 2 photoanodes. For comparison, pure BiVO 4 photoanodes were also fabricated by means of an electrodeposition process on bare FTO substrates.

Material Characterization of NiWO 4 /BiVO 4 /SnO 2 Photoanodes
The phase compositions of the fabricated photoanodes were explored by X-ray diffraction (XRD). Figure 2 shows the XRD patterns for the bare FTO substrates (curve (i)) and after the incorporation of SnO 2 onto FTO substrates (SnO 2 /FTO) by electrodeposition (curve (ii)). There were no considerable variations in the diffraction patterns of SnO 2 over FTO, demonstrating that there was no creation of new kinds of phases in the as-synthesized photoanodes. Moreover, BiVO 4 was also deposited over FTO substrates (curve (iii)) and SnO 2 /FTO (curve (iv)) using the same technique, which was subsequently subjected to an annealing process at 300 • C for 1 h. Further, a NiWO 4 layer was electrodeposited over the BiVO 4 /SnO 2 photoanodes using the same electrodeposition technique. For the as-synthesized BiVO4 (curves (iii) and (iv)), obtained sharp peaks credited to the BiVO4 film, well matched with the JCPDS # 00-014-0688, were evidenced. After the incorporation of NiWO 4 , Figure 2 curve (iv) displayed peaks corresponding to the NiWO 4 layer, matching the standard JCPDS pattern (00-015-0755) of pure NiWO 4 . The XRD patterns displayed in Figure 2 provide evidence of the generation of pure phases of SnO 2 , BiVO 4 " and NiWO 4 . Hence, we can conclude that the NiWO 4 /BiVO 4 composite was successfully prepared by means of electrodeposition.  Figure 3a,b, FTO substrates were obviously covered by a dense, pin-hole free buffer layer of SnO 2 crystal grains, in which the massive but irregular lumps of ∼150 nm are comprised of small nanoparticles. Planview FE-SEM images of bare-BiVO 4 as well as BiVO 4 layers deposited on SnO 2 are shown in Figure 3c,d, and Figure S1. In BiVO 4 films with a buffer SnO 2 layer, a BiVO 4 particle size of 200 ± 20 nm was observed (Figure 3c,d) from the FESEM images and these particles were much smaller in comparison than those observed in the bare-BiVO 4 films (500 ± 20 nm), where the morphological nature of BiVO 4 was not considerably influenced by a SnO 2 layer underneath, and was similar to that of other samples. Figure 3e,f show microscope images of the deposited NiWO 4 layer (charge: 30 mC cm −2 ) over the BiVO 4 /SnO 2 films. In particular, the FE-SEM of NiWO 4 /BiVO 4 /SnO 2 evidences that the NiWO 4 layer incorporation was mostly uniform as well as thin layer to see the substantial changes in the morphological nature of the bare-BiVO 4 films. The cross sectional FESEM shows a well-connected SnO 2 layer of approximately 310 nm thick with an increase in film thickness to 520 nm after deposition of the BiVO4 layer and the results are shown in Figure S2. Changes in the elemental distributions of the fabricated photoanodes were studied by energy dispersive X-ray analysis (EDAX). These investigations evidenced the existence of Sn, Bi, V, Ni, W, and O in the fabricated photoanodes, as shown in the Figure S3. Diffuse reflectance ultraviolet-visible (DRS UV-Vis) was employed to determine the optical bandgap and absorption properties of SnO 2 , BiVO 4 , BiVO 4 /SnO 2 , and NiWO 4 /BiVO 4 /SnO 2 layers deposited over FTO substrates; the obtained spectra are shown in Figure 4a. The spectrum of the BiVO 4 /SnO 2 film deposited on a FTO substrate exhibits a different absorption profile than that of the FTO/BiVO 4 film, thus resulted in the blue-shifted absorption edge, i.e., on the way to a larger band gap, as the SnO 2 film was loaded on the FTO substrate. The spectrum of the bare-BiVO 4 film shows absorption at wavelengths up to~520 nm, corresponding to a band-gap of 2.4 eV. Because the bandgap of SnO 2 is 3.6 eV, the absorption of SnO 2 was not observed in this spectrum. Additionally, the incorporation of NiWO 4 over BiVO 4 /SnO 2 films resulted in considerably greater light absorption compared to other electrodes in the wavelength ranging from 300-500 nm (Figure 4a). The triple layer NiWO 4 /BiVO 4 /SnO 2 films revealed optimal light absorption, demonstrating that the Figure 4b presents the plotting of (αhν) 1/2 versus the photon energy of BiVO 4 , BiVO 4 /SnO 2 , and NiWO 4 /BiVO 4 /SnO 2 , with band-gaps of 2.35, 2.40, and 2.25 eV, correspondingly. Lastly, the observed changes in the band gap of BiVO 4 /SnO 2 are in agreement with the XRD measurements, since the tetragonal phase is recognized to have a larger band-gap [55].

Photo-Electrochemical Behavior of Photoanodes
Three-electrode J-V analysis were performed for PEC measurements under stimulated conditions in the presence of 0.1 M PBS. The electrochemical conditions during the deposition of NiWO 4 were adjusted by varying the total charge engaged for Ni electrodeposition (30 mC·cm −2 ) as well as the immersion time in the tungstate solution to obtain triple-layer NiWO 4 /BiVO 4 /SnO 2 photoanodes over FTO substrates. The distinctive J-V measurements under illumination condition presented in Figure 5a confirm that the optimized triple-layered NiWO 4 /BiVO 4 /SnO 2 photoanodes exhibited superior photocurrents compared to BiVO 4 /SnO 2 , bare-BiVO 4 , and bare-SnO 2 . At a potential of 1.23 V RHE , the photoanodic current densities of the bare-BiVO 4 , BiVO 4 /SnO 2 , and NiWO 4 /BiVO 4 /SnO 2 photoanode films were 0.18, 0.56, and 0.93 mA·cm −2 , correspondingly. Further, the photocurrent density of the triple-layered NiWO 4 /BiVO 4 /SnO 2 films increased considerably with applied potential, reaching nearly 0.93 mA·cm −2 at 1.23 V RHE , which is a five-fold enhancement of the photocurrent density of the bare-BiVO 4 photoanode (0.18 mA·cm −2 ). This significant photocurrent improvement is complemented by a substantial cathodic shift in the photocurrent onset potential of~200 mV. Further, the J-V curve of the BiVO 4 /SnO 2 photoanode shown in Figure 5a exhibits a three-fold enhancement in photocurrent density in comparison with bare-BiVO 4 . An appreciable photocurrent measurements was acquired in the lower potential area (0.6 V RHE ) with all samples, as displayed in Figure 5b. Further, Figure 5c displays the J-V plots of the NiWO 4 /BiVO 4 /SnO 2 , BiVO 4 /SnO 2 , and bare-BiVO 4 electrodes under chopped illumination. This investigation has elucidated that the construction of the BiVO 4 /SnO 2 heterojunction is a major factor causing the enhancement of the activity toward PEC water oxidation. As predicted by earlier reports in the literature [39], the incorporation of a SnO 2 buffer layer between FTO and BiVO 4 resulted in enhanced PEC behaviors. This is credited to the downward band bending within the BiVO 4 and hole-mirroring effect of SnO 2 . The plots of calculated applied bias photon to current efficiency (ABPE) with respect to applied bias are displayed in Figure 5d. While the optimum photoconversion efficiencies were only 0.01% at 0.8 V RHE and 0.03% at 0.8 V RHE for bare-SnO 2 and bare-BiVO 4 electrodes, correspondingly, a much higher optimal conversion efficiency of 0.08% at 0.8 V RHE was attained for the BiVO 4 /SnO 2 photoanode. In addition, the optimum conversion efficiency of BiVO 4 /SnO 2 photoanode was further enhanced to 0.21% at 0.8 V RHE by electrodeposition of NiWO 4 on the BiVO 4 surface. From these PEC investigations, it is obvious that the combination of a SnO 2 buffer layer and NiWO 4 nanoparticles synergistically improved the PEC performance enough to meet the requirements for superior efficiency as well as electrochemical stability.
Photocurrent-potential analysis was also carried out with the H 2 O 2 , where the charge collection efficiency can be assumed to be at its maximum. In particular, this investigation will assist us in estimating the efficiency of the NiWO 4 incorporation and subsequently assessing the charge separation (η CS ) as well as charge-transfer efficiencies (η CT ). Hence, photocurrent-potential data were acquired with and without H 2 O 2 to examine the limitations of the PEC behavior of the bare-BiVO 4 and NiWO 4 /BiVO 4 /SnO 2 photoanodes (Figure 6a). Photocurrent-potential curves of NiWO 4 /BiVO 4 /SnO 2 photoanodes in the presence of H 2 O 2 showed a substantial, but predicted, increase in the onset potential (~0.16 V vs. RHE) as well as the photocurrent density (3.1 mA·cm −2 ). Remarkably, these enhancements match well with the PEC performance of the triple-layered NiWO 4 /BiVO 4 /SnO 2 photoanodes (Figure 5a). These PEC comparative studies evidenced that the introduction of nanosized NiWO 4 particles is necessary to meet the requirements for superior PEC performances as well as stability. The triple-layered NiWO 4 /BiVO 4 /SnO 2 photoanodes were further analyzed by examining their η CT and η CS efficiencies at different applied biases, and the results are displayed in Figure 6b,c. The bare-BiVO 4 films yielded η CT values <10% (Figure 6b), even at an applied bias as high as 1.23 V RHE , where the strong electric field hampers surface recombination. After incorporation of NiWO 4 over BiVO 4 /SnO 2 , the η CT of the NiWO 4 /BiVO 4 /SnO 2 triple-layered photoanode was increased to ∼30% at 1.23 V RHE , signifying improved charge-transfer kinetics and representing a nearly three-fold enrichment with respect to the bare-BiVO 4 . Besides, the SnO 2 buffer layer also improves the charge-transfer behavior of BiVO 4 by restricting the probable recombination that can happen at the interface between the BiVO 4 /SnO 2 films. This behavior is consistent with the literature [41,56,57]. When the NiWO 4 layer was added between BiVO 4 /SnO 2 , the charge separation can be further enriched, as seen from Figure 5a, where the photocurrent increased after the insertion of a NiWO 4 layer. As further evidence of this fact, the η CS of the triple-layered NiWO 4 /BiVO 4 /SnO 2 samples was assessed to be 23% at 1.23 V RHE , which indicates a considerable enrichment relative to the bare-BiVO 4 photoanodes (18.2% at 1.23 V RHE ). The incorporation of NiWO 4 produces an energetically promising interface with BiVO 4 as well as water, as evidenced by the enriched separation efficiency. In any case, additional investigation is required to better understand this issue.
The efficient charge transfer in the fabricated samples was examined further by photoelectrochemical impedance spectroscopic (PEIS) assessment, as presented in Figure 7. The PEIS Nyquist measurements of the fabricated electrodes investigated under the irradiation conditions at 1 V RHE and their resultant equivalent circuit are presented in Figure 7. Obviously, the radius of the arc of the Nyquist plots of the NiWO 4 /BiVO 4 /SnO 2 triple-layered films is comparatively lesser with respect to those of the bare BiVO 4 and BiVO 4 /SnO 2 films, indicating quick interfacial charge-transfer and also the effective separation of induced charge carriers. In particular, the observed constant capacitance and reduction in RCT strongly suggest that NiWO 4 assists as an energetic electrocatalytic material and thereby improves the charge-transfer kinetics and it effectively decreases surface recombination. The long-term stability of bare-BiVO 4 , BiVO 4 /SnO 2 , and NiWO 4 /BiVO 4 /SnO 2 photoanodes were relatively assessed in phosphate buffer solution over 3 h at 1.23 V vs RHE under constant illumination conditions and the results was shown in Figure 8. The photocurrent density of bare-BiVO 4 declined from 0.21 mA/cm 2 to 0.14 mA/cm 2 after constant illumination for 2.5 h, because the bare-BiVO 4 hurt from not only a continuous photocorrosion by means of illumination but also the chemical corrosion from H 2 O 2 affected by oxygen reduction on BiVO 4 surface [34]. A similar behavior was ever observed in earlier reported studies [30,33,34]. The triple-layer films showed a considerable photocurrent density of~0.93 mA·cm −2 , superior to other photoanodes, and there was certainly no apparent decay in the photocurrent, demonstrating their considerable long-term stability. After an early transient decrease of the photocurrent of triple-layered photoanodes over the course of the reaction, the photocurrent steadily improved, reaching ∼78% of its original value in 2 h. When coating a NiWO 4 layer on BiVO 4 /SnO 2 photoanodes, the photostability was greatly enhanced. This may have demonstrated that the photocurrent of triple layer was prone to be stable after a sharp decline in the initial 60 s. On the other hand, considerable photocurrent deterioration was noticed for the BiVO 4 /SnO 2 film within 15 min, partially credited to the chemical nature and PEC uncertainty. The observed photocurrent decay is credited to the photoanodic corrosion behavior of BiVO 4 , which in turn is attributed to the interfacial hole accumulation as a consequence of the sluggish transfer kinetics of interfacial holes in BiVO 4 during water oxidation. The XRD patterns of SnO 2 /BiVO 4 /NiWO 4 were acquired at 3 h to investigate the mass loss of fabricated photoanodes during the J-t measurement. The XRD pattern of SnO 2 /BiVO 4 /NiWO 4 obtained at 3 h exhibited no obvious changes in comparison with that of SnO 2 /BiVO 4 /NiWO 4 acquired before 3 h ( Figure S4). Further, XPS was used to examine the surface composition of SnO 2 /BiVO 4 /NiWO 4 photoelectrodes before and after the 3 h of irradiation ( Figure S5). Supplementary Figure S5 displays two main peaks with binding energies at 861.8 eV and 856.1 eV, matching well with the Ni 2p 1/2 and Ni 2p 3/2 spin-orbit peaks of the NiWO 4 phase, correspondingly [58]. Further, the W 4d peaks were considered instead of the high intensity W 4f peaks. This is because the binding energy of W4f lies close to V 3p, and is also not very far from the Sn 4d and Bi 5d peaks. Therefore, W 4d were chosen in order to unambiguously confirm the NiWO 4 phase [59,60]. Moreover, for SnO 2 /BiVO 4 /NiWO 4 photoelectrodes, no significant change can be found from XPS spectra of the Ni, and W elements of the NiWO 4 photocatalyst before and after the irradiation. Thus the XPS investigations provide crucial information for the compositions and oxidation states of the NiWO 4 in fabricated SnO 2 /BiVO 4 /NiWO 4 photoelectrodes. The above investigation evidenced that conformal deposition of SnO 2 and incorporation of NiWO 4 cocatalyst could efficiently enhance the PEC performances and the stability of NiWO 4 /BiVO 4 /SnO 2 triple layer photoanode.

Electrodeposition of SnO 2 onto FTO Substrates
In this work, electrodeposited SnO 2 nanoparticles were loaded over FTO substrates (Hartford glass 15 Ω·cm −2 ) by using a solution of 0.02 M SnCl 4 .2H 2 O in EG subjected to purging under an argon for 15 min. Afterwards, the electrochemical deposition process was performed in an undivided electrochemical cell via an Autolab PGSTAT302 potentiostat. In particular, a classical 3-electrode setup was employed, which comprised of an FTO working electrode, a Ag/AgCl (3 M KCl) reference electrode, and a Pt counter electrode. Sn was electrodeposited from the 0.02 SnCl 4 .2H 2 O solution by continuously applying a bias at −2.0 V Ag/AgCl , the solution was stirred, and the electrodeposition process was iterated. This cycle was repeated for a different number of times (2 to 8 times) to pass a total charge between 200 and 800 mC·cm −2 and control the film thickness. The resulting film was annealed in an oven at 450 • C for 1 h in air atmosphere (rate = 2 • C/min) to convert Sn into SnO 2 . The optimized charge density was found to be 500 mC·cm −2 in order to obtain a SnO 2 /FTO substrate.

Preparation of NiWO 4 on BiVO 4 /SnO 2 Photoanodes
After electrodeposition of the SnO 2 and BiVO 4 films over the BiVO 4 photoanodes, NiWO 4 films were incorporated using an electrodeposition process. Initially, metallic Ni was deposited from a 20 mM NiCl 2 solution dispersed in DMSO. In particular, cathodic deposition was performed potentiostatically at −2.0 V Ag/AgCl , and the most optimized procedure was achieved by changing the electrodeposition charge from 10 to 50 mC·cm −2 . In this regard, the best optimal charge density was assessed to be 30 mC·cm −2 . Subsequently, for the electrochemical deposition of the greatly dispersed metallic Ni particles over the BiVO 4 film, the fabricated electrode was dipped in a 20 mM Na 2 WO 4 .H 2 O solution at 25 • C without stirring. The optimized immersion time was assessed to be 20 min to obtain triple-layered NiWO 4 /BiVO 4 /SnO 2 photoanodes over FTO substrates.

Materials Characterization
The morphological nature and chemical composition of the fabricated electrodes were investigated via field effect SEM (FE-SEM; JEOL JSM-7000F). DR UV-Vis spectra were measured in a Shimadzu UV-3600 spectrophotometer. XRD analysis was carried out in a Rigaku XtaLAB Mini II benchtop system. XPS measurements were carried out using an X-ray Photoelectron Spectrometer (JEOL JPS-9030, Japan) with Alkα radiation source (1486.7 eV). PEC analysis of the fabricated electrodes were performed through an electrochemical workstation (PGSTAT30) in the dark as well as under AM 1.5G simulated solar illumination. Moreover, 0.5 M H 2 O 2 (30%) was used to estimate the charge collection properties of the fabricated photoanodes.
The ABPE is given by: where J PEC is the photocurrent density, Vbias is the applied bias, and Pin is the incident illumination power density (AM 1.5G).

Conclusions
In summary, we demonstrated a simple electrochemical deposition approach for developing a triple-layered NiWO 4 /BiVO 4 /SnO 2 electrode for PEC water oxidation reaction. The electrodeposition approach produced a highly uniform NiWO 4 layer that exhibited considerable resistance to chemical dissolution in a phosphate buffer solution. The optimized NiWO 4 /BiVO 4 /SnO 2 photoanodes exhibited a photocurrent density of~0.93 mA·cm −2 at 1.23 V RHE and a nearly five-fold improvement in comparison with bare-BiVO 4 . The charge-transfer and charge separation data, together with the PEC measurements, demonstrated that the NiWO 4 layer boosts the photoanodic current by increasing the photon absorption nature and efficient charge separation of the BiVO 4 electrodes.