Preparation of Nanoparticle Porous-Structured BiVO 4 Photoanodes by a New Two-Step Electrochemical Deposition Method for Water Splitting

: In the synthesis method of a BiVO 4 photoanode via BiOI ﬂakes, a BiOI ﬁlm is formed by electrochemical deposition in Step 1, and a vanadium (V) source solution is placed by drop-casting on the BiOI ﬁlm in Step 2. Following this, BiVO 4 particles are converted from the BiOI–(V species) precursors by annealing. However, it is challenging to evenly distribute vanadium species among the BiOI ﬂakes. As a result, the conversion reaction to form BiVO 4 does not proceed simultaneously and uniformly. To address this limitation, in Step 2, we developed a new electrochemical deposition method that allowed the even distribution of V 2 O 5 among Bi–O–I ﬂakes to enhance the conversion reaction uniformly. Furthermore, when lactic acid was added to the electrodeposition bath solution, BiVO 4 crystals with an increased (040) peak intensity of the X-ray diffractometer (XRD) pattern were obtained. The photocurrent of the BiVO 4 photoanode was 2.2 mA/cm 2 at 1.23 V vs. reversible hydrogen electrode (RHE) under solar simulated light of 100 mW/cm 2 illumination. The Faradaic efﬁciency of oxygen evolution was close to 100%. In addition, overall water splitting was performed using a Ru/SrTiO 3 :Rh–BiVO 4 photocatalyst sheet prepared by the BiVO 4 synthesis method. The corresponding hydrogen and oxygen were produced in a 2:1 stoichiometric ratio under visible light irradiation.


Introduction
Semiconductor photocatalysts and photoelectrodes can potentially be applied to a green technology that offers renewable energy alternatives to fossil fuels. One such technology is solar-driven hydrogen production by photoelectrochemical (PEC) water splitting. During PEC water splitting, hydrogen and oxygen are generated by p-and n-type semiconductor photoelectrodes, respectively. The advantage of the PEC system is that hydrogen production is separated from oxygen. Many n-type semiconductor photoelectrodes, especially the bismuth vanadate (BiVO 4 ) photoanode with favourable band edge positions, can also be utilised. The band gap of BiVO 4 is 2.4 eV and can absorb up to 11% of the solar spectrum assuming the theoretical photocurrent is 7.5 mA/cm 2 under AM1.5G one sun (100 mW/cm 2 ) illumination [1][2][3]. Its high flat band potential and being located close to the thermodynamic hydrogen evolution potential allow the water oxidation reaction to appear at a moderate external bias potential. Additionally, BiVO 4 can be easily synthesised and is relatively stable in aqueous solutions, non-toxic and inexpensive. The photocatalytic effect of BiVO 4 on water splitting under visible light was first reported by Kudo et al. [4]. Furthermore, Kudo et al. reported that scheelite monoclinic phase BiVO 4 showed higher visible light activity for water splitting than tetragonal phase BiVO 4 [5,6]. In 2003, Sayama et al. successfully demonstrated the PEC properties of a BiVO 4 thin film photoanode produced of charge carriers at the interface between a photoanode and an electrolyte. The mechanism of the effect of cobalt oxide on BiVO 4 is the same as that on hematite. Divalent cobalt can be oxidised into trivalent and tetravalent cobalt, which can be reduced back to divalent cobalt, and this catalytically assists water oxidation [22].
In this study, we synthesised a highly functional porous BiVO 4 photoelectrode by a new two-step electrochemical deposition method. Our strategy was to enhance the dispersibility of V species throughout the Bi species film and the balance of the Bi-V-O conversion reaction to form a uniform BiVO 4 film. To achieve this, we focused on the electrodeposition bath solution of the BiOI in Step 1. Moreover, in Step 2, amorphous V 2 O 5 was formed among Bi-O-I flakes by an electrochemical deposition method rather than a drop-casting method. Such electrochemical deposition method is attractive because electrochemical deposition is widely adopted in industries. The synthesis of BiVO 4 by electrochemical deposition offers easy scalability at a comparatively low cost. Here, we discuss the formation mechanism, morphology and characterisation of the BiVO 4 thin film photoelectrodes. Further, the benefit of film area up to ≥3 cm 2 was demonstrated to describe the shortcomings of large area photoelectrodes. The overall water splitting was also confirmed using a Ru/SrTiO 3 :Rh-BiVO 4 photocatalyst sheet prepared by the present BiVO 4 synthesis method as an application example. Choi et al. developed the two-step BiVO 4 synthesis method, which gave an excellent performance for PEC water oxidation [13,15]. In the Choi method, in Step 1, BiOI was formed by electrochemical deposition. In Step 2, the V source (e.g., V 2 O 5 , VO(acac) 2 , etc.) solution was placed by drop-casting on the BiOI film. Following this, the Bi-V-O reaction was processed by heating at high temperatures (350-650 • C) for 2-5 h in air to convert the BiOI-V precursor into BiVO 4 . In our tests, when using the method of placing a solution of vanadium species on the BiOI film by drop-casting, the BiVO 4 thin film obtained after the Bi-V-O heat treatment often broke. There is still room for improvement in the BiVO 4 crystal growth process for PEC properties. First, we focused on the components of the electrodeposition bath solution. It is well known that, in the field of electrochemical deposition research, lactic acid (C 3 H 6 O 3 ) in the deposition bath forms complexes with transition metals (e.g., Cu) and affects the crystal growth in a particular orientation [25]. Figure 1 shows the flowchart for preparing our BiVO 4 photoanode; the procedure in Step 1 is similar to the Choi method [13,15]. As a standard for this study, the aqueous deposition bath solution in Step 1 contained 60 mmol/L lactic acid. In Step 1, the complexes of Bi with iodide [BiI 4 ] − and lactic acid [Bi(C 3 H 4 O 3 ) 2 (C 3 H 5 O 3 )] 2− were first produced at pH 1.2 in the aqueous solution bath (Equations (1) and (2)). Then, the solution of the Bi 3+ species was mixed with an ethanol benzoquinone solution. The bath solution was stirred overnight for complex formation and pH stabilisation. Some of the benzoquinones were reduced to hydroquinone, thereby shifting the pH value towards alkalinity (pH 3.6-4.0). During Step 1, cathodic deposition, benzoquinone was actively reduced to hydroquinone (Equation (3)). At the same time, the concentration of the hydroxide ions (OH − ) increased on the working electrode and reacted with the bismuth iodide [BiI 4 ] − species to form BiOI (Equation (4)). Next, in our method, to reduce the hydrophobicity of the film surface, the BiOI film was heated to 450 • C for 1 h to generate penta-bismuth hepta-oxide iodide (Bi 5 O 7 I). In Step 2, an ethanol aqueous solution of vanadyl sulphate (VOSO 4 ) was utilised as the electrodeposition bath, and VO 2+ was oxidised to amorphous V 2 O 5 (Equation (5)) throughout the Bi 5 O 7 I film during the anodic deposition. This process allowed an aqueous solution of vanadium source to penetrate among the Bi 5 O 7 I flakes to produce V 2 O 5 . This process is different from previous reports. The as-deposited films were annealed at 475 • C for 1 h in air to convert the Bi 5 O 7 I-V 2 O 5 precursor to BiVO 4 . The pure BiVO 4 was obtained after washing with an aqueous KOH solution (0.5 mol/L) to remove residual V 2 O 5 . It

Results and Discussion
BiOI, Bi5O7I, and BiVO4 were synthesised according to the procedure shown in the flowchart in Figure 1. The absorbances of the BiOI, Bi5O7I, and BiVO4 films agreed with those in the previous literature [15,26] (Figure S1). BiOI and Bi5O7I produced by Step 1 were confirmed by X-ray diffraction (XRD) (Figure 2A). In the new synthesis Step 2 (Figure 2B), the as-deposited V2O5 was amorphous and could not be detected by XRD. How- BiOI, Bi 5 O 7 I, and BiVO 4 were synthesised according to the procedure shown in the flowchart in Figure 1. The absorbances of the BiOI, Bi 5 O 7 I, and BiVO 4 films agreed with those in the previous literature [15,26] (Figure S1). BiOI and Bi 5 O 7 I produced by Step 1 were confirmed by X-ray diffraction (XRD) (Figure 2A). In the new synthesis Step 2 ( Figure 2B), the as-deposited V 2 O 5 was amorphous and could not be detected by XRD. However, after mild heating for 1 h at 475 • C, the XRD pattern of a mixed pattern of BiVO 4 and V 2 O 5 was observed. Thereafter, the excess V 2 O 5 was removed with an aqueous KOH solution. In Figure 2 (Bv), the paired peaks, which are characteristic of the scheelite-structured monoclinic BiVO 4 , appeared around 19 • and 35 • of 2θ.
[BiI4] − + 2OH − → BiOI +H2O + 3I − (4) BiOI, Bi5O7I, and BiVO4 were synthesised according to the procedure shown in the flowchart in Figure 1. The absorbances of the BiOI, Bi5O7I, and BiVO4 films agreed with those in the previous literature [15,26] (Figure S1). BiOI and Bi5O7I produced by Step 1 were confirmed by X-ray diffraction (XRD) (Figure 2A). In the new synthesis Step 2 (Figure 2B), the as-deposited V2O5 was amorphous and could not be detected by XRD. However, after mild heating for 1 h at 475 °C, the XRD pattern of a mixed pattern of BiVO4 and V2O5 was observed. Thereafter, the excess V2O5 was removed with an aqueous KOH solution. In Figure 2(Bv), the paired peaks, which are characteristic of the scheelite-structured monoclinic BiVO4, appeared around 19° and 35° of 2θ. Regarding morphological observations, the BiOI film was relatively hydrophobic because of the physical structure of the 2D BiOI flake (Figures 3i and S2i). After the heat treatment at 450 °C for 1 h, the BiOI turned to Bi5I7O. The sharp BiOI flakes became the rounded tip of Bi5O7I, which caused a difference in surface height, and succeeded in reducing the hydrophobicity of the film surface (Figures 3ii and S2ii). The cross-sectional image of the Bi5O7I-V2O5 film (Figure 3iii) displays the Bi5O7I flakes surrounded by the as- Regarding morphological observations, the BiOI film was relatively hydrophobic because of the physical structure of the 2D BiOI flake (Figure 3i and Figure S2i). After the heat treatment at 450 • C for 1 h, the BiOI turned to Bi 5 I 7 O. The sharp BiOI flakes became the rounded tip of Bi 5 O 7 I, which caused a difference in surface height, and succeeded in reducing the hydrophobicity of the film surface (Figure 3ii and Figure S2ii The atomic distribution of Bi and V was performed by energy-dispersive X-ray spectroscopy (EDS) mapping to investigate whether the Bi-V-O conversion reaction could proceed throughout the thin film at the same time. In Figure 4, evidently, the EDS elemental mapping images of the Bi 5 O 7 I-V 2 O 5 film shows V atoms were densely and uniformly dispersed all over the Bi 5 O 7 I film. The atomic ratio of Bi to V was 1:2.17, which contained extra V 2 O 5 for BiVO 4 conversion.
During the annealing process at 475 • C for 1 h, BiVO 4 particles were formed by reacting the V 2 O 5 and the Bi 5 O 7 I flake on the FTO substrate. Since the Bi and V atoms were uniformly present, it is considered that the Bi-V-O conversion reaction occurred throughout the Bi species thin film and at the same time. Consequently, a porous structured thin film was created with nanosized BiVO 4 particles. In Figure 5, the shape of the BiVO 4 nanoparticles was not the same as the worm-like particles prepared by the Choi method [13,15,16]. These images indicate that the BiVO 4 nanoparticles were highly monodispersed. If extra agglomerates of BiVO 4 particles were formed on the surface by the excessive progress of Bi-V-O reaction during heating process, the agglomerates of BiVO 4 particles can act as carrier traps that might reduce the photocurrent [16]. However, Figure 5a shows that no unwanted BiVO 4 particle agglomerates on the surface of the BiVO 4 thin film. Furthermore, the EDS composition analysis of the pure BiVO 4 film resulted that the rational atomic ratio of Bi to V was 1:1.08 ( Figure S3). film. Furthermore, the EDS composition analysis of the pure BiVO4 film resulted that the rational atomic ratio of Bi to V was 1:1.08 ( Figure S3).

Effect of Adding Lactic Acid
To investigate the effect of the addition amount, BiVO4 films were prepared by changing the addition amount of lactic acid according to the procedure shown in Figure  1. The XRD patterns of the BiVO4 films all showed monoclinic scheelite-structure with a (040) peak of about 31° at 2θ ( Figure 6A). In particular, the XRD pattern of BiVO4 formed using the electrodeposition bath contained 60 mmol/L lactic acid exhibited increased (040) peak intensity. The (040) peak of the XRD pattern corresponds to the (010) facet. It is presumed that BiOI produced in the presence of an appropriate amount of lactic acid strongly

Effect of Adding Lactic Acid
To investigate the effect of the addition amount, BiVO 4 films were prepared by changing the addition amount of lactic acid according to the procedure shown in Figure 1. The XRD patterns of the BiVO 4 films all showed monoclinic scheelite-structure with a (040) peak of about 31 • at 2θ ( Figure 6A). In particular, the XRD pattern of BiVO 4 formed using the electrodeposition bath contained 60 mmol/L lactic acid exhibited increased (040) peak intensity. The (040) peak of the XRD pattern corresponds to the (010) facet. It is presumed that BiOI produced in the presence of an appropriate amount of lactic acid strongly adhered to the FTO in the specific orientation, hence the BiVO 4 (010) facets exposed favourably after Bi 5 O 7 I reacted with V 2 O 5 at 475 • C.
The BiVO 4 particles converted by the BiOI-(V species) precursors [15] on an indium tin oxide (ITO) substrate held a comparatively low light transmittance of ca. 30% at 600 nm [16]. This transmittance percentage was equivalent to the case without lactic acid in this study. As shown in Figure 6B, the BiVO 4 thin film prepared using the electrodeposition bath containing 60 mmol/L lactic acid shows the highest transmittance spectrum and has a transmittance of 52% at 600 nm. It is considered that the transmittance was improved because of atomic arrangement of the (010) oriented BiVO 4 . From the perspective of transmittance, our BiVO 4 photoanode will also be suitable for practical application of a PEC tandem cell system.

Effect of Adding Lactic Acid
To investigate the effect of the addition amount, BiVO4 films were prepared by changing the addition amount of lactic acid according to the procedure shown in Figure  1. The XRD patterns of the BiVO4 films all showed monoclinic scheelite-structure with a (040) peak of about 31° at 2θ ( Figure 6A). In particular, the XRD pattern of BiVO4 formed using the electrodeposition bath contained 60 mmol/L lactic acid exhibited increased (040) peak intensity. The (040) peak of the XRD pattern corresponds to the (010) facet. It is presumed that BiOI produced in the presence of an appropriate amount of lactic acid strongly adhered to the FTO in the specific orientation, hence the BiVO4 (010) facets exposed favourably after Bi5O7I reacted with V2O5 at 475 °C.
The BiVO4 particles converted by the BiOI-(V species) precursors [15] on an indium tin oxide (ITO) substrate held a comparatively low light transmittance of ca. 30% at 600 nm [16]. This transmittance percentage was equivalent to the case without lactic acid in this study. As shown in Figure 6B, the BiVO4 thin film prepared using the electrodeposition bath containing 60 mmol/L lactic acid shows the highest transmittance spectrum and has a transmittance of 52% at 600 nm. It is considered that the transmittance was improved because of atomic arrangement of the (010) oriented BiVO4. From the perspective of transmittance, our BiVO4 photoanode will also be suitable for practical application of a PEC tandem cell system. . Transmittance spectra were measured by a spectrophotometer with an integrating sphere that was operated in the transmission mode. Inset is a photo of these BiVO 4 films.

Conversion Temperature from Bi 5 O 7 I-V 2 O 5 Precursor to BiVO 4
The effect of temperature on the conversion of Bi 5 O 7 I-V 2 O 5 precursors of Sample (iii) in Figure 1 to BiVO 4 crystals is especially important. The annealing process of the Bi-V-O conversion reaction was investigated from 350 to 550 • C. In the XRD patterns ( Figure 7 and Figure S4), all the thin films exhibited the (040) peak after heat process at 350-550 • C. For the paired peak, a feature of the scheelite-structured monoclinic BiVO 4 , the required temperature for the Bi-V-O annealing process was 450 • C or higher. A tetragonal BiVO 4 was established by heating at 400 • C or lower. The BiVO 4 thin film prepared at 350 • C exhibited relatively low crystallinity.
In the SEM images ( Figure S5), it was observed that the particle size of BiVO 4 increased as the temperature was increased. These particles exhibited high uniformity in the size and shape of each film. Furthermore, there was no extra agglutination of BiVO 4 particles on the surface of these films. At the process temperature from 350 to 475 • C, the nanoscale particles created porous structured layer on the FTO substrate. Noticeably, these thin films possessed a relatively large surface area. Contrarily, the aggregation of the BiVO 4 particles was initiated at 500 • C, and the porous structure became non-porous. At 550 • C, completely large particles were remodelled into a non-porous-layered structure. Therefore, when the conversion temperature was extremely high, the surface area where the water oxidation Catalysts 2021, 11, 136 9 of 21 reaction occurs decreased. To prevent the agglutination of ultrafine particles, it is vital to complete the conversion reaction in a short time such as 1 h. Additionally, the light scattering between the large particles occurred in the 500 and 550 • C treated thin films, and the absorbance increased in the long-wavelength region ( Figure S6). The samples treated at 450 and 475 • C possessed the appropriate band gaps for BiVO 4 and enabled the absorption of visible light of up to 520 nm. creased as the temperature was increased. These particles exhibited high uniformity in the size and shape of each film. Furthermore, there was no extra agglutination of BiVO4 particles on the surface of these films. At the process temperature from 350 to 475 °C, the nanoscale particles created porous structured layer on the FTO substrate. Noticeably, these thin films possessed a relatively large surface area. Contrarily, the aggregation of the BiVO4 particles was initiated at 500 °C, and the porous structure became non-porous. At 550 °C, completely large particles were remodelled into a non-porous-layered structure. Therefore, when the conversion temperature was extremely high, the surface area where the water oxidation reaction occurs decreased. To prevent the agglutination of ultrafine particles, it is vital to complete the conversion reaction in a short time such as 1 h. Additionally, the light scattering between the large particles occurred in the 500 and 550 °C treated thin films, and the absorbance increased in the long-wavelength region ( Figure  S6). The samples treated at 450 and 475 °C possessed the appropriate band gaps for BiVO4 and enabled the absorption of visible light of up to 520 nm.

Effect of Adding Lactic Acid
The photocurrent density-potential (J-V) curves of BiVO 4 films grown in the deposition bath with varying lactic acid concentrations were measured (Figure 8). From the photocurrent density, it was observed that the optimum concentration of lactic acid added to the deposition bath was 60 mmol/L. This result may be related to the XRD findings in Figure 6A. BiVO 4 crystals with exposed (010) facets corresponding to the (040) peak of the XRD pattern were reported to exhibit higher photocatalytic activity for water oxidation than without (040) peak [27]. However, even with high peak intensities, the improvement in water oxidation reaction did not increase proportionally [27].
On the other hand, the (010) and (110) crystal facets of monoclinic BiVO 4 have been reported to be active reducing and oxidising sites, respectively [28]. The large (010) facet provided rapid charge separation and suppressed both bulk and surface recombination [28]. In this study, the (010) oriented BiVO 4 crystals enhanced the photocurrent density by improving electron transport. than without (040) peak [27]. However, even with high peak intensities, the improvement in water oxidation reaction did not increase proportionally [27].
On the other hand, the (010) and (110) crystal facets of monoclinic BiVO4 have been reported to be active reducing and oxidising sites, respectively [28]. The large (010) facet provided rapid charge separation and suppressed both bulk and surface recombination [28]. In this study, the (010) oriented BiVO4 crystals enhanced the photocurrent density by improving electron transport.

Bi-V-O to BiVO 4 Conversion Temperature
The temperature-dependent PEC properties of the Bi-V-O annealing process were investigated from 350 to 550 • C ( Figure 9A). The BiVO 4 photoanodes processed by the Bi-V-O annealing at 450 and 475 • C showed excellent photocurrent densities. Figure 9B shows the applied bias photo-to-current efficiency (ABPE) calculated from the corresponding J-V curves. Generally, the average maximum ABPE of bare BiVO 4 photoanode is 0.3% or less in the range of about 0.9-1.0 V vs. RHE [29,30]. In the present study, for the Bi-V-O annealing process at 475 • C, the ABPE of the bare BiVO 4 photoanode reached 0.37% at 0.9 V vs. RHE, which was the greatest of our tests. This is probably because the porous-structured BiVO 4 photoanode was a highly crystalline scheelite-structured monoclinic system.
Furthermore, the open circuit (OC) potential of each photoanode was measured in the dark and under illumination ( Figure S7A) because the difference in the OC potentials between dark and light conditions is related to the photovoltage. Further, the onset potential is considered to be related to the light OC potential. The BiVO 4 photoanode prepared by Bi-V-O annealing process at 475 • C recorded the most negative light OC potential and the biggest difference ( Figure S7B). It can be inferred that the photoanode produced at 475 • C possessed a higher photovoltage than the other temperatures. These characteristics discussed above are consistent with those of the PEC photoresponse. the dark and under illumination ( Figure S7A) because the difference in the OC potentials between dark and light conditions is related to the photovoltage. Further, the onset potential is considered to be related to the light OC potential. The BiVO4 photoanode prepared by Bi-V-O annealing process at 475 °C recorded the most negative light OC potential and the biggest difference ( Figure S7B). It can be inferred that the photoanode produced at 475 °C possessed a higher photovoltage than the other temperatures. These characteristics discussed above are consistent with those of the PEC photoresponse.

PEC Properties of the Optimised BiVO4 Photoanode
The optimised bare BiVO4 photoanode was obtained by using the electrodeposition bath containing 60 mmol/L lactic acid and treating the Bi-V-O conversion process at 475 °C. The photocurrent density of water oxidation attained 2.2 mA/cm 2 at 1.23 V vs. RHE ( Figure 10A). In addition, using an electrolyte with hole scavenger K2SO3, the J-V curve shifted to the negative potential region and the photocurrent density increased significantly. It is considered that the oxidation of K2SO3 reduced the charge recombination on the surface of BiVO4.
To quantify the contributions of the suppressing bulk or surface recombination, charge separation efficiency (ŋsep) and surface transfer efficiency (ŋtrans) of the BiVO4 photoanode were calculated by the following equation: [22,31]

PEC Properties of the Optimised BiVO 4 Photoanode
The optimised bare BiVO 4 photoanode was obtained by using the electrodeposition bath containing 60 mmol/L lactic acid and treating the Bi-V-O conversion process at 475 • C. The photocurrent density of water oxidation attained 2.2 mA/cm 2 at 1.23 V vs. RHE ( Figure 10A). In addition, using an electrolyte with hole scavenger K 2 SO 3 , the J-V curve shifted to the negative potential region and the photocurrent density increased significantly. It is considered that the oxidation of K 2 SO 3 reduced the charge recombination on the surface of BiVO 4 .
To quantify the contributions of the suppressing bulk or surface recombination, charge separation efficiency (η sep ) and surface transfer efficiency (η trans ) of the BiVO 4 photoanode were calculated by the following equation: [22,31] η sep = J K 2 SO 3 /J absorbed (8) where J H 2 O is the measured photocurrent density and J absorbed is the photon absorption rate (expressed as a photocurrent density). The calculated J absorbed of the BiVO 4 photoanode was specified as 4.45 mA/cm 2 [15]. J H2O and J K 2 SO 3 are the photocurrent densities collected in the 0.5 mol/L potassium borate electrolyte without and with 0.2 mol/L K 2 SO 3 , respectively. When K 2 SO 3 is added into an electrolyte, it acts as a hole scavenger. In the presence of K 2 SO 3 in electrolyte, the surface transfer efficiency is assumed to be 100%. For comparison, in general, the average charge separation efficiency (η sep ) of bare BiVO 4 photoanode was lower than 45% at 1.23 V vs. RHE [30,31], and the average sur-face transfer efficiency (η trans ) was ca. 35% at 1.23 V vs. RHE [30,32,33]. As shown in Figure 10B, the η sep and η trans of the bare BiVO 4 increased to 56% and 86% at 1.23 V vs. RHE, respectively. This is likely due to the uniform particle size and composition of the morphologically controlled BiVO 4 crystals, which increased the efficiency of the water photooxidation reaction.
The exposed (010) facets are able to provide rapid electron transfer, thus improving the PEC water oxidation on BiVO 4 by smooth charge separation [27,28]. Our results indicate that lactic acid contributed to the growth of (010) oriented crystals. In addition, by controlling the Bi-V-O reaction temperature, particles of appropriate size were supplied. The hole diffusion length of BiVO 4 is estimated as 70 nm [8]; nanosized particles are more advantageous for the movement of holes to the surface. Consequently, the charge recombination was reduced, thereby resulting in an increased photocurrent density even with frontal illumination.
The photocurrent from the back illumination was slightly lower than that from the front illumination ( Figure 10C). In many reports, large BiVO 4 crystals size and film thickness resulted in higher photocurrents at the back exposure than at the front [34,35]. In contrast, our BiVO 4 film was composed of nanosized crystals, was thin, and showed high transmittance. In the case of the back illumination, there was probably a loss of light by the reflection on the glass surface of the FTO substrate.

Effect of Cocatalyst
It is known that surface modification of BiVO4 with a cocatalyst can reduce the recombination of the surface charges and accelerate the rate of oxygen generation [21,24]. It was confirmed that BiVO4 utilised in this study was also influenced by the loading of cobalt (II, III) oxide (Co3O4) cocatalyst ( Figure 11A). The BiVO4/Co3O4 photoelectrode was obtained by drop-casting an ethanol cobalt nitrate solution to the BiVO4 photoanode surface and heating it to 350 °C for 1 h in air. The cobalt nitrate decomposed into Co3O4 during heating process at 350 °C [23].

Effect of Cocatalyst
It is known that surface modification of BiVO 4 with a cocatalyst can reduce the recombination of the surface charges and accelerate the rate of oxygen generation [21,24]. It was confirmed that BiVO 4 utilised in this study was also influenced by the loading of cobalt (II, III) oxide (Co 3 O 4 ) cocatalyst ( Figure 11A). The BiVO 4 /Co 3 O 4 photoelectrode was obtained by drop-casting an ethanol cobalt nitrate solution to the BiVO 4 photoanode surface and heating it to 350 • C for 1 h in air. The cobalt nitrate decomposed into Co 3 O 4 during heating process at 350 • C [23].
The photocurrent of the bare BiVO 4 and BiVO 4 /Co 3 O 4 photoelectrodes were 2.2 and 2.9 mA/cm 2 at 1.23 V vs. RHE, respectively ( Figure 11A). The Co 3 O 4 cocatalyst exerted a great influence on the negative potentials shift of the J-V curve, which was a characteristic of the effect of cocatalysts. Moreover, when using the electrolyte with K 2 SO 3 of a hole scavenger, a higher photocurrent of 3.8 mA/cm 2 at 1.23 V vs. RHE was achieved.
Owing to the effect of the Co 3 O 4 cocatalyst, the maximum ABPE of water oxidation increased from 0.37% at 0.9 V to 1.03% at 0.6 V vs. RHE ( Figure 11B). The Co 3 O 4 cocatalyst effectively enhanced the kinetic water oxidation on the BiVO 4 photoanode, and a higher efficiency was obtained with moderate bias.
The incident photon-to-current conversion efficiency (IPCE) measurements were performed by monochromatic lights between 370 and 560 nm to ascertain the light conversion efficiency ( Figure 12). Typically, the average IPCE value at 420 nm performed at 1.23 V vs. RHE is lower than 20% for bare BiVO 4 [11,35,36]. In the present study, the IPCEs of the bare BiVO 4 photoanode were 7.5% and 23.2% at 0. 6  photoanodes with and without Co3O4 cocatalyst presented photoresponse in the visible light region, which was consistent with the absorption spectrum of BiVO4.

Impact of the Photoelectrode Area
Scale-up of photoelectrodes for the practical application of hydrogen generation from PEC water splitting is a key issue. In many reports, the photoelectrodes used were often smaller than 1 cm 2 . These could not satisfy the actual requirements. According to previous reports [18,19], the decrease in photocurrent density by electrode scale-up is mainly due to the resistance of the FTO substrate. Here, to find the size that causes the resistance loss

Impact of the Photoelectrode Area
Scale-up of photoelectrodes for the practical application of hydrogen generation from PEC water splitting is a key issue. In many reports, the photoelectrodes used were often smaller than 1 cm 2 . These could not satisfy the actual requirements. According to previous reports [18,19], the decrease in photocurrent density by electrode scale-up is mainly due to the resistance of the FTO substrate. Here, to find the size that causes the resistance loss of the FTO substrate in this study, we probed the PEC water oxidation on the BiVO 4 electrodes with larger than 1 cm 2 from 1.3 to 3.9 cm 2 . The current-potential (I-V) curves of the bare BiVO 4 photoanodes are shown in Figure 13A. Photocurrents increased with the area of the photoelectrodes. The maximum photocurrent reached 9 mA at 1.5 V vs. RHE with the 3 × 1.3 cm 2 photoanode. Nevertheless, the 3 × 1.3 cm 2 photoanode showed a little decrease in its photocurrent density ( Figure 13B). The 1 × 1.3 and 1.8 × 1.3 cm 2 photoelectrodes were both retained at high photocurrent densities of 2.2 mA/cm 2 at 1.23 V vs. RHE. No photocurrent loss was observed in the FTO glass substrate with an area up to 2 cm 2 when using our BiVO 4 synthesis method.

Solar-Driven Water Oxidation on the Bare BiVO4 Photoanode
To confirm that the photocurrent actually causes water oxidation without any other side reaction on the bare BiVO4 photoanode, PEC oxygen generation was measured by an online gas chromatography system when applying a constant potential of 1.23 V vs. RHE under solar simulated light of 100 mW/cm 2 irradiation. Six hours after the reaction, the

Solar-Driven Water Oxidation on the Bare BiVO 4 Photoanode
To confirm that the photocurrent actually causes water oxidation without any other side reaction on the bare BiVO 4 photoanode, PEC oxygen generation was measured by an online gas chromatography system when applying a constant potential of 1.23 V vs. RHE under solar simulated light of 100 mW/cm 2 irradiation. Six hours after the reaction, the stable photocurrent density dropped slightly from about 2 to 1.7 mA/cm 2 ( Figure 14A). The corresponding oxygen generated was more than 100 µmol/cm 2 in 6 h, agreeing with the calculated value of the photocurrent density ( Figure 14B), and its Faradaic efficiency was almost 100%. Thus, it was confirmed that the photocurrent was consistent with the solar-driven water oxidation on the bare BiVO 4 photoanode.

Overall Water Splitting on the Ru/SrTiO3:Rh-BiVO4 Sheet
Our BiVO4 is a promising candidate for unassisted water splitting on p-n semiconductor photocatalyst sheet because of its high PEC photocurrent and transmittance. Here, as an example of the application of the present BiVO4 synthesis method, visible-lightdriven water splitting was performed with a Ru/SrTiO3:Rh-BiVO4 sheet. The details of the preparation of the photocatalyst sheet are presented in the experimental Section 3. Materials and Methods.
The XRD patterns of the prepared Ru (0.7 wt%)/SrTiO3:Rh (1%)-BiVO4 sheet was measured and matched with the reference data [37] ( Figure S8). The morphological observations indicated that the electrochemically deposited BiVO4 particles adhered well on the surface of the Ru/SrTiO3:Rh particles to form the Ru/SrTiO3:Rh-BiVO4 sheet ( Figure 15). The weight of Ru/SrTiO3:Rh plus BiVO4 was an average of 3 mg/cm 2 and the sheet area was 12 cm 2 . Figure 16 shows a time course of water splitting using the Ru/SrTiO3:Rh-BiVO4 sheet. Evidently, the visible-light-driven water splitting occurred on the Ru/SrTiO3:Rh-BiVO4 sheet in deionised water without a redox mediator. It was confirmed that the stoichiometric ratio of hydrogen and oxygen was 2:1. After 22 h of water splitting, there was no change in the appearance before and after the reaction.

Overall Water Splitting on the Ru/SrTiO 3 :Rh-BiVO 4 Sheet
Our BiVO 4 is a promising candidate for unassisted water splitting on p-n semiconductor photocatalyst sheet because of its high PEC photocurrent and transmittance. Here, as an example of the application of the present BiVO 4 synthesis method, visible-light-driven water splitting was performed with a Ru/SrTiO 3 :Rh-BiVO 4 sheet. The details of the preparation of the photocatalyst sheet are presented in the experimental Section 3. Materials and Methods.
The XRD patterns of the prepared Ru (0.7 wt%)/SrTiO 3 :Rh (1%)-BiVO 4 sheet was measured and matched with the reference data [37] ( Figure S8). The morphological observations indicated that the electrochemically deposited BiVO 4 particles adhered well on the surface of the Ru/SrTiO 3 :Rh particles to form the Ru/SrTiO 3 :Rh-BiVO 4 sheet ( Figure 15). The weight of Ru/SrTiO 3 :Rh plus BiVO 4 was an average of 3 mg/cm 2 and the sheet area was 12 cm 2 . Figure 16 shows a time course of water splitting using the Ru/SrTiO 3 :Rh-BiVO 4 sheet. Evidently, the visible-light-driven water splitting occurred on the Ru/SrTiO 3 :Rh-BiVO 4 sheet in deionised water without a redox mediator. It was confirmed that the stoichiometric ratio of hydrogen and oxygen was 2:1. After 22 h of water splitting, there was no change in the appearance before and after the reaction.  Visible-light-driven overall water splitting on the Ru/SrTiO3:Rh-BiVO4 sheet (area = 12 cm 2 ) in deionised water (120 mL). The substrate was an FTO glass. Light source: 300 W Xe lamp with a long-pass filter (λ > 420 nm).
The same setup (as in Step 1) was employed for Step 2 deposition. To deposit the amorphous V 2 O 5 on the Bi 5 O 7 I film, an ethanol/water solution (ratio 1:1) of 0.22 mol/L vanadyl sulphate hydrate (VOSO 4 ·nH 2 O; >55%; Kanto Chemical Co., INC., Tokyo, Japan) was utilised as the electrodeposition bath. The formation of amorphous V 2 O 5 can be achieved on the Bi 5 O 7 I film or an FTO glass by anodic deposition, e.g., at a current of 0.5-2 mA/cm 2 and a certain time. In this study, V 2 O 5 was deposited on the Bi 5 O 7 I film at a constant current of 1 mA/cm 2 for 15 min. The as-deposited films were annealed in air at varying temperatures at 350, 400, 450, 475, 500, and 550 • C (ramping rate = 5 • C/min) for 1 h. In the annealing process, Bi 5 O 7 I and the amorphous V 2 O 5 reacted to form crystallised BiVO 4 and V 2 O 5 . To remove the residual V 2 O 5 on BiVO 4 , the BiVO 4 photoelectrodes were immersed in a 0.5 mol/L KOH aqueous solution for 30 min with gentle stirring. Following this, these films were washed with deionised water and wiped with tissue paper (KimWipes s-200, Kimberly Clark Corp., Irving, Texas, USA) and then air dried.
PEC measurements. The PEC properties of the BiVO 4 films were measured under AM1.5G type solar simulated light (Asahi spectra, HAL320, Tokyo, Japan) with a spotlight area of 3 × 3 cm 2 . The light output was calibrated to 100 mW/cm 2 by a power meter (Asahi spectra, CS-20, Tokyo, Japan) The PEC performance of the photoanodes was evaluated in a three-electrode configuration utilising a potentiostat (Hokuto Denko, HSV110, Tokyo, Japan). The reference electrode was Ag/AgCl in saturated KCl, and a 2 cm 2 Pt gauze was utilised as the counter electrode. As a standard, the films were illuminated from the BiVO 4 side (front). The scan rate for linear sweep voltammograms was set to 20 mV/s. The electrolyte comprised an aqueous solution of 0.5 mol/L boric acid that was pH-adjusted to 9.5 with potassium hydroxide. All the potentials in this work were reported against the RHE calculated from those relative to the Ag/AgCl reference electrode by Nernst equation, E RHE = E Ag/AgCl vs. NHE + E Ag/AgCl + 0.059 pH. Here, E RHE is the converted potential vs. RHE. E Ag/AgCl vs. NHE is the correction coefficient for Ag/AgCl reference electrode regarding normal hydrogen electrode (NHE) and was taken as 0.197 at 25 • C. E Ag/AgCl is the experimentally measured potential against the Ag/AgCl reference.
Further, ABPE (%) was calculated using Equation (10): where ∆J (J light − J dark (mA/cm 2 )) is the measured photocurrent density, V (vs. RHE) is the applied bias, and P is the incident light intensity (100 mW/cm 2 ). The incident photon-to-current conversion efficiency (IPCE) measurements were carried out with coloured filters from 370 to 560 nm by measuring the photocurrent produced under chopped monochromatic light irradiation. The monochromatic light intensities were measured by a photodiode power sensor (OPHIR Japan Ltd., Tokyo, Japan). The IPCE (%) was then calculated using Equation (11): where λ is the wavelength of incident monochromatic light (nm), j(λ) is the difference between dark current density and photocurrent density under illumination at wavelength λ (mA/cm 2 ), and I 0 (λ) is the incident light intensity at wavelength λ (mW/cm 2 ). Measurement of the oxygen produced by PEC water splitting. PEC water splitting on the BiVO 4 film was performed at an applied potential of 1.23 V vs. RHE under solar simulated light (Asahi spectra, HAL320, Tokyo, Japan) at 100 mW/cm 2 irradiation (Asahi spectra, CS-20). The PEC properties were evaluated using a potentiostat (Hokuto Denko, HZ-7000, Tokyo, Japan). An airtight three-electrode configuration with a compartmental H-type cell that was separated by a membrane (DuPont, Nafion 117, Wilmington, Delaware, USA) was employed. Incidentally, using a membrane separation cell could prevent the occurrences of reverse reactions. The reference electrode was Ag/AgCl in saturated KCl, and a Pt wire (diameter = 0.5 cm, length = 6 cm) was utilised as the counter electrode. Furthermore, 0.5 mol/L potassium borate (buffered at pH 7.5) was utilised as an electrolyte. The amount of evolved oxygen on the BiVO 4 photoanode was recorded by an online gas chromatograph (GC) (Shimadzu GC-8A, Kyoto, Japan), TCD detector, MS-5 Å column, oven at 70 • C, argon carrier). Argon was the carrier gas from the reactor to the GC instrument at a flow rate of 15 mL/min.
Preparation and water splitting of the photocatalyst sheet. Rh-doped SrTiO 3 powder was prepared by a previously reported solid-state method [37]. In a typical procedure, SrCO 3 (Kanto Chemical, 99.9%, Tokyo, Japan), TiO 2 (High Purity Chemicals, 99.9%, Tokyo, Japan), and Rh 2 O 3 (Wako Pure Chemical, Tokyo, Japan) were mixed in the Sr/Ti/Rh ratio of 1.07:0.93:0.01. The mixture was calcined in air for 1 h at 900 • C, after which the powder was mingled well and calcined again for 10 h at 1100 • C. Ru was utilised as a cocatalyst to SrTiO 3 for hydrogen evolution. Ru (0.7 wt%) was loaded on SrTiO 3 :Rh by photo-assisted reduction in an aqueous 10 vol% methanol solution containing RuCl 3 (Wako Pure Chemical, 85.0%). The Ru/SrTiO 3 :Rh photocatalyst was collected by filtration and washed with distilled water. The photocatalyst powder (100 mg) was added to 0.3 mL of a mixture of 1:2 acetylacetone (Kanto Chemical, 99.5%) and deionised water, and then sonicated for 5 min. The resulting paste was placed on an FTO glass substrate. Next, to remove the organic solvent, the photocatalyst sheet was heated in air for 2 h at 200 • C. By the method described above (Figure 1), BiVO 4 was deposited on the Ru/SrTiO 3 :Rh sheet. The overall water-splitting experiment on the Ru/SrTiO 3 :Rh-BiVO 4 sheet was performed in 120 mL of deionised water without a redox mediator. A top-irradiation cell with a Pyrex window was utilised after degassing by applying a vacuum and purging with argon. The setup was returned to reduced pressure (70 Torr) and irradiated with a 300 W Xe arc-light source (PerkinElmer; CERMAX PE300BF, Waltham, Massachusetts, USA) with a long-pass filter (λ > 420 nm). The amounts of evolved hydrogen and oxygen were determined by an online GC (Shimadzu; GC-8A, TCD, MS-5Å column, oven at 50 • C, argon carrier).

Conclusions
We developed a highly efficient BiVO 4 photoanode synthesis method for PEC water oxidation by a simple electrochemical procedure. According to our method, the electrochemical deposited amorphous V 2 O 5 was able to spread densely among the precursor Bi 5 O 7 I flakes. Therefore, during the appropriate annealing process of Bi-V-O conversion at 475 • C for 1 h, the BiVO 4 was formed along the shape of the Bi 5 O 7 I flakes by converting with the amorphous V 2 O 5 , thereby creating a porous structured film. Since the Bi-V-O reaction occurred throughout the thin film as the same time, the BiVO 4 particles were highly monodispersed nanoscales.
It is considered that the BiVO 4 formation control by the addition of lactic acid strengthened the atomic arrangement of the (010) orientation and improved the transmittance of the BiVO 4 photoanode thin film. The (010) facets were the active sites for photoelectron migration to the FTO substrate, which enhanced charge separation and reduced the electron−hole pair recombination. Thus, high PEC properties were accomplished even when the light irradiation was from the front of the BiVO 4 photoanode.
The optimised BiVO 4 photoanode in this study showed a photocurrent density of 2.2 mA/cm 2 at 1.23 V vs. RHE under one sun light illumination. The maximum ABPE was 0.37% at 0.9 V vs. RHE. The IPCE at 420 nm achieved 23.2% at 1.23 V vs. RHE. Both the enhancement in ABPE and IPCE were higher than the values reported recently. After 6 h of reaction the PEC water oxidation appeared to have excellent stability. The Faradaic efficiency of ca. 100% and high oxygen evolution of over 100 µmol/cm 2 were obtained. Furthermore, increasing the BiVO 4 photoanode area to 2 cm 2 did not reduce the photocurrent density. When the area reached 3 cm 2 , the photocurrent densities decreased a little. The BiVO 4 photoanode has high light transmittance and will be suitable for practical use of PEC tandem cell system. It was also found that even higher efficiency can be obtained from the effect of surface modification of the Co 3 O 4 cocatalyst. The photocurrent density of the BiVO 4 /Co 3 O 4 photoelectrode was 2.9 mA/cm 2 at 1.23 V vs. RHE under one sun light illumination. The maximum APBE was 1.03% at 0.6 V vs. RHE, and IPCE at 420 nm carried out at 1.23 V vs. RHE was 37.5%.
In addition, it was confirmed that the BiVO 4 formed by our new two-step method could attach firmly to a Ru/SrTiO 3 :Rh photocatalyst sheet. The Ru/SrTiO 3 :Rh-BiVO 4 photocatalyst sheet successfully performed visible-light-driven overall water splitting in deionised water for 22 h.