# Efficient Photoelectrochemical Water Oxidation by Metal-Doped Bismuth Vanadate Photoanode with Iron Oxyhydroxide Electrocatalyst

Authors: Eun Jin Joo; Gisang Park; Ji Seon Gwak; Jong Hyeok Seo; Kyu Yeon Jang; Kyung Hee Oh; Ki Min Nam
Journal: Journal of Nanomaterials
 (2016)

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## Abstract

Intensive attention has been currently focused on the discovery of semiconductor and proficient cocatalysts for eventual applications to the photoelectrochemical water splitting system. A W-Mo-doped BiVO4 semiconductor was prepared by the surfactant-assisted thermal decomposition method on a fluorine-doped tin oxide conductive film. The W-Mo-doped BiVO4 films showed a porous morphology with the grain sizes of about 270 nm. Because the hole diffusion length of BiVO4 is about 100 nm, the W-Mo-doped BiVO4 film in this study is an ideal candidate for the photoelectrochemical water oxidation. Iron oxyhydroxide (FeOOH) electrocatalyst was chemically deposited on the W-Mo-doped BiVO4 to investigate the effect of the electrocatalyst on the semiconductor. The W-Mo-doped BiVO4/FeOOH composite electrode showed enhanced activity compared to the pristine W-Mo-doped BiVO4 electrode for water oxidation reaction. The chemical deposition is a promising method for the deposition of FeOOH on semiconductor.


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## Body

## 1. Introduction

Photoelectrochemical (PEC) water splitting using semiconductor electrode is a promising method of converting solar energy to chemical fuel [1, 2]. Among the various semiconductor materials, metal oxides such as TiO2, WO3, Fe2O3, and BiVO4 have gained significant interest owing to their photochemical stability and low cost. However, they have a low PEC efficiency compared to the theoretical values, because of significant electron-hole recombination and slow surface kinetics [3, 4]. While a multitude of methods such as doping, morphology control, making composite structure, and adding electrocatalysts have been investigated for the improvement in PEC water splitting [5–8], achievement of theoretical conversion efficiency is still far from being reached.BiVO4 has been intensively studied as a photoanode (n-type semiconductor) for PEC water oxidation, because it absorbs a large portion of the visible light and has a favorable valence band edge [9–11]. However, the slow carrier mobility in the bulk as well as fast recombination at the surface contributes to the poor water oxidation efficiency of BiVO4. The introduction of dopant such as W and Mo into BiVO4 has been found to enhance the PEC performance [12, 13]. The dopant in BiVO4 can increase n-type conductivity and could significantly enhance the PEC activity. Furthermore, W and Mo codoping (W-Mo-doped BiVO4) has shown better performance than W or Mo alone for the BiVO4 [14]. Nanostructure can also enhance the kinetic parameters of the water oxidation reactions through the discrimination of bulk recombinations. For efficient PEC water oxidation, BiVO4 requires both particles smaller than its hole diffusion length (~100 nm) [11] and the introduction of proper dopants. However, those are still not sufficient to overcome the low surface kinetic of BiVO4.Recently, proficient electrocatalysts for eventual PEC applications have been intensively studied, but there is no guarantee that the best electrocatalysts will perform equally when integrated into a PEC water splitting system [15, 16]. The source of catalytic improvement of electrocatalyst on semiconductor is not yet fully understood [17, 18]. The nature of the loaded catalysts and their interaction with the semiconductor are important to further study the PEC water splitting. Recently, a number of studies have focused on the potential applications of iron oxyhydroxide (FeOOH) as the cocatalysts [15]. Unfortunately, most of the researches considered to date have only focused on the photodeposition or electrodeposition method [7, 15].In this study, we report a facile formation of W-Mo-doped BiVO4 films on fluorine-doped tin oxide (FTO) for the PEC water oxidation. The W-Mo-doped BiVO4 films showed a porous morphology with the grain sizes of about 270 nm. Because the hole diffusion length of BiVO4 is about 100 nm, the W-Mo-doped BiVO4 film in this study is an ideal candidate for effective charge separation. Furthermore, FeOOH cocatalyst was chemically deposited on the W-Mo-doped BiVO4 films by the oxidation of FeSO4 to investigate the effect of electrocatalysts on the semiconductor surface. The W-Mo-doped BiVO4/FeOOH composites showed enhanced PEC water oxidation performance.


## 2. Experimental Procedures

### 2.1. Materials

Fluorine-doped tin oxide (FTO, TEC 15, WY-GMS) coated glass was used as the substrate for the thin film electrodes. (NH4)6H2W12O40·xH2O (≥99.0%, Sigma-Aldrich), Bi(NO3)3·5H2O (99.999%, Sigma-Aldrich), (NH4)6Mo7O24·4H2O (99.98%, Sigma-Aldrich), and VCl3 (99%, Alfa-Aesar) were used as the metal precursor salts and used as received. In addition, Nafion (5%, Sigma-Aldrich) and NaOCl (10%), Na2SO4, Na2SO3, Na2HPO4, NaH2PO4, ethylene glycol (99.0%), acetone (99.0%), and ethanol (99.5%) were purchased from Daejung Chemicals (Korea). Deionized (DI) water was used as the solvent in electrochemical experiments.


### 2.2. Preparation of W-Mo-Doped BiVO4 and Undoped BiVO4 Electrodes

FTO substrates were first cleaned in deionized water and ethanol and then sonicated in ethanol for at least 1 h. A drop-casting technique was used to create the thin film electrodes. Here, 10 mM W-Mo-doped BiVO4 precursor (the atomic ratio in between Bi, V, W, and Mo was 4.6 : 4.6 : 0.2 : 0.6) in ethylene glycol solution was prepared. Nafion solution was added to the precursor solution (volume ratio between precursor and Nafion solution was 1 : 5) and then applied onto an FTO substrate. The prepared films were annealed at 500°C for 3 h (with a 3 h ramp time) in air to form the W-Mo-doped BiVO4 thin film. The existence of Nafion in precursor solution tends to give reproducible growth on FTO substrate. For undoped BiVO4 precursor, the atomic ratio in between Bi and V was 1 : 1 in ethylene glycol.


### 2.3. Chemical Deposition of FeOOH on W-Mo-Doped BiVO4 Film

Chemical deposition of FeOOH was carried out by adding 30 mL of 1.5 M NaOCl to 15 mL of 1.0 M FeSO4 solution. The solution was kept at 30°C for 3 h in air in the presence of W-Mo-doped BiVO4 film, and the resulting W-Mo-doped BiVO4/FeOOH electrode was washed with ethanol and DI water. FeOOH was also deposited on undoped BiVO4 with the same method.


### 2.4. Photodeposition of FeOOH on W-Mo-Doped BiVO4 Film

Photodeposition of FeOOH on the W-Mo-doped BiVO4 was carried out in a 0.1 M FeSO4 solution using a three-electrode cell setup. For the photodeposition, an external bias of 0.3 V versus Ag/AgCl was applied. The light was illuminated through the FTO side (backside) with the light intensity of 100 mW/cm2. Photodeposition was performed for 30 min, and the electrode was washed with ethanol and DI water.


### 2.5. Electrochemical Characterization of Electrodes

Electrochemical characterization was performed in a specially designed cell in a three-electrode configuration with the thin film as the working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The working electrode with the actual geometric area of 0.28 cm2 was exposed to electrolyte solution. A 150 W xenon lamp (ABET Technologies) was used as the light source in the PEC characterization step, and light illumination area was 0.28 cm2. Chopped light linear sweep voltammetry (LSV) was utilized to obtain the photocurrent responses using a DY2321 potentiostat (Digi-Ivy). The light chopping frequency was set at 2 Hz and the PEC measurements were performed by backside illumination in aqueous solutions of 0.1 M Na2SO4 with a phosphate buffer (pH 7) for water oxidation. In all tests, the intensity of the lamp on the sample was measured to be 100 mW/cm2 using a Si solar cell (AIST). A 425 nm long-pass filter was used to cut the UV portion of the spectrum and to provide only visible light illumination. A monochromator (ORIEL) was used to obtain the action spectra of photoresponse as a function of wavelength. Because the preparation of W-Mo-doped BiVO4 electrode is reproducible, it always shows the same photocurrents of each sample.


### 2.6. Materials Characterization of Electrodes

UV-Vis absorption spectra were acquired with a Lambda 3B Spectrophotometer (Perkin-Elmer) for wavelengths from 300 to 900 nm. The thin film electrodes were characterized by scanning electron microscopy (SEM, Philips XL30SFEG operated at 10 and 30 kV). The X-ray diffraction data was measured using CuK
α radiations at 40 kV and 100 mA (Rigaku, Dmax-RB diffractometer). X-ray photoelectron spectroscopy (XPS) measurements were taken using a K
α spectrometer with an X-ray source of Al K
α and at a pass energy level of 40 eV.


## 2.1. Materials

Fluorine-doped tin oxide (FTO, TEC 15, WY-GMS) coated glass was used as the substrate for the thin film electrodes. (NH4)6H2W12O40·xH2O (≥99.0%, Sigma-Aldrich), Bi(NO3)3·5H2O (99.999%, Sigma-Aldrich), (NH4)6Mo7O24·4H2O (99.98%, Sigma-Aldrich), and VCl3 (99%, Alfa-Aesar) were used as the metal precursor salts and used as received. In addition, Nafion (5%, Sigma-Aldrich) and NaOCl (10%), Na2SO4, Na2SO3, Na2HPO4, NaH2PO4, ethylene glycol (99.0%), acetone (99.0%), and ethanol (99.5%) were purchased from Daejung Chemicals (Korea). Deionized (DI) water was used as the solvent in electrochemical experiments.


## 2.2. Preparation of W-Mo-Doped BiVO4 and Undoped BiVO4 Electrodes

FTO substrates were first cleaned in deionized water and ethanol and then sonicated in ethanol for at least 1 h. A drop-casting technique was used to create the thin film electrodes. Here, 10 mM W-Mo-doped BiVO4 precursor (the atomic ratio in between Bi, V, W, and Mo was 4.6 : 4.6 : 0.2 : 0.6) in ethylene glycol solution was prepared. Nafion solution was added to the precursor solution (volume ratio between precursor and Nafion solution was 1 : 5) and then applied onto an FTO substrate. The prepared films were annealed at 500°C for 3 h (with a 3 h ramp time) in air to form the W-Mo-doped BiVO4 thin film. The existence of Nafion in precursor solution tends to give reproducible growth on FTO substrate. For undoped BiVO4 precursor, the atomic ratio in between Bi and V was 1 : 1 in ethylene glycol.


## 2.3. Chemical Deposition of FeOOH on W-Mo-Doped BiVO4 Film

Chemical deposition of FeOOH was carried out by adding 30 mL of 1.5 M NaOCl to 15 mL of 1.0 M FeSO4 solution. The solution was kept at 30°C for 3 h in air in the presence of W-Mo-doped BiVO4 film, and the resulting W-Mo-doped BiVO4/FeOOH electrode was washed with ethanol and DI water. FeOOH was also deposited on undoped BiVO4 with the same method.


## 2.4. Photodeposition of FeOOH on W-Mo-Doped BiVO4 Film

Photodeposition of FeOOH on the W-Mo-doped BiVO4 was carried out in a 0.1 M FeSO4 solution using a three-electrode cell setup. For the photodeposition, an external bias of 0.3 V versus Ag/AgCl was applied. The light was illuminated through the FTO side (backside) with the light intensity of 100 mW/cm2. Photodeposition was performed for 30 min, and the electrode was washed with ethanol and DI water.


## 2.5. Electrochemical Characterization of Electrodes

Electrochemical characterization was performed in a specially designed cell in a three-electrode configuration with the thin film as the working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The working electrode with the actual geometric area of 0.28 cm2 was exposed to electrolyte solution. A 150 W xenon lamp (ABET Technologies) was used as the light source in the PEC characterization step, and light illumination area was 0.28 cm2. Chopped light linear sweep voltammetry (LSV) was utilized to obtain the photocurrent responses using a DY2321 potentiostat (Digi-Ivy). The light chopping frequency was set at 2 Hz and the PEC measurements were performed by backside illumination in aqueous solutions of 0.1 M Na2SO4 with a phosphate buffer (pH 7) for water oxidation. In all tests, the intensity of the lamp on the sample was measured to be 100 mW/cm2 using a Si solar cell (AIST). A 425 nm long-pass filter was used to cut the UV portion of the spectrum and to provide only visible light illumination. A monochromator (ORIEL) was used to obtain the action spectra of photoresponse as a function of wavelength. Because the preparation of W-Mo-doped BiVO4 electrode is reproducible, it always shows the same photocurrents of each sample.


## 2.6. Materials Characterization of Electrodes

UV-Vis absorption spectra were acquired with a Lambda 3B Spectrophotometer (Perkin-Elmer) for wavelengths from 300 to 900 nm. The thin film electrodes were characterized by scanning electron microscopy (SEM, Philips XL30SFEG operated at 10 and 30 kV). The X-ray diffraction data was measured using CuK
α radiations at 40 kV and 100 mA (Rigaku, Dmax-RB diffractometer). X-ray photoelectron spectroscopy (XPS) measurements were taken using a K
α spectrometer with an X-ray source of Al K
α and at a pass energy level of 40 eV.


## 3. Results and Discussion

### 3.1. Preparation of W-Mo-Doped BiVO4 Electrode

For the facile preparation of W-Mo-doped BiVO4 structure, thin film electrodes were prepared by surfactant-assisted thermal decomposition method on an FTO substrate. Figure 1(a) shows the scanning electron microscopy (SEM) of the W-Mo-doped BiVO4 thin film electrode, indicating a porous network with the grain sizes of 274.8 ± 63.7 nm. The cross section SEM image of the W-Mo-doped BiVO4 shows the film with a thickness of about 1.1 μm (inset). The porous structures can allow the electrolyte to easily diffuse within the BiVO4, increasing the contact area and shortening the hole diffusion distance [13]. Because the hole diffusion length of BiVO4 is about 100 nm [11], the W-Mo-doped BiVO4 thin film in this study is ideal for effective charge separation. Notably, the precursor solution without Nafion increased the grain sizes of the W-Mo-doped BiVO4 and irregularly formed on the FTO substrate (351.5 ± 82.8 nm, see Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2016/1827151). The existence of Nafion in the precursor solution tends to provide small grain sizes as well as uniform growth on the FTO substrate (Figure 1(b) inset). The X-ray diffraction (XRD) peaks corresponded to the monoclinic structure of BiVO4 (Figure 1(b)). Any secondary phase in the XRD patterns was not observed. However, a shift and merging of the XRD peaks at 34, 47, and 59° were observed, indicating that W and Mo were well dissolved in the BiVO4 solid solution [14].Figure 1
(a) SEM top-view and cross section image (inset) of the W-Mo-doped BiVO4, (b) XRD patterns of the W-Mo-doped BiVO4 (red line) and FTO substrate (blue line), and photographic image (inset) of the W-Mo-doped BiVO4.
(a)
(b)


### 3.2. PEC at the W-Mo-Doped BiVO4 Electrode

The PEC performance of the W-Mo-doped BiVO4 thin film electrode was studied by linear sweep voltammetry (LSV) for both sulfite oxidation (0.1 M Na2SO3 + 0.1 M Na2SO4) and water oxidation (0.1 M Na2SO4 + 0.1 M phosphate buffered, pH 7). The LSV was conducted from −0.6 to +0.8 V versus Ag/AgCl at a scan rate of 20 mV/s with chopped light under UV-visible and visible (>425 nm) irradiations (Figure 2). The W-Mo-doped BiVO4 electrode successfully generated anodic photocurrents (n-type character). Because the sulfite oxidation has extremely fast oxidation kinetics, the surface recombination is negligible [14, 15]. The sulfite oxidation is thermodynamically and kinetically favorable, and thus it has a more negative onset potential compared to that of water oxidation (Figure 2). An early onset potential (−0.5 V versus Ag/AgCl) and a rapid increase in photocurrent of 1.6 mA/cm2 (0.6 V versus Ag/AgCl ) for sulfite oxidation on the W-Mo-doped BiVO4 electrode indicated an excellent fill factor. However, nanosized structures are also associated with significant disadvantages, such as an increased number of grain boundaries and a reduced space-charge region [19], resulting in much lower efficiency of the W-Mo-doped BiVO4 electrode than the theoretical value (7.5 mA/cm2) [20]. Furthermore, the photocurrent from the W-Mo-doped BiVO4 electrode for water oxidation is far lower than that of sulfite oxidation. The significant reduction in photocurrent demonstrates that the water oxidation on the W-Mo-doped BiVO4 electrode is mainly limited by poor water oxidation kinetics on the electrode surface. This result indicates that a considerably improved photocurrent can be possible when the W-Mo-doped BiVO4 electrode is coupled with a proper water oxidation cocatalyst.Figure 2
LSVs for the sulfite oxidation (0.1 M Na2SO3 + 0.1 M Na2SO4) and water oxidation (0.1 M Na2SO4 + 0.1 M phosphate buffered, pH 7) using the W-Mo-doped BiVO4 under (a) UV-visible and (b) visible light irradiation. Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.
(a)
(b)


### 3.3. Chemical Deposition of FeOOH on the W-Mo-Doped BiVO4 Electrode

Efficient PEC water splitting requires both highly active semiconductor photoelectrode and proficient electrocatalyst, that is, cocatalyst. Catalyst-modified BiVO4 enhanced PEC efficiency and also noticeably improved the stability [7, 21]. Recently, a number of studies have focused on the potential applications of FeOOH as the cocatalyst [15]. Unfortunately, most of the researches considered to date have only used the photodeposition or electrodeposition method on the semiconductor [7, 15].To improve water oxidation kinetics, a thin layer of FeOOH catalyst was chemically deposited. The chemical deposition of FeOOH on the W-Mo-doped BiVO4 electrode was carried out in a 1.0 M FeSO4 with 1.5 M NaOCl solution. FeSO4 was oxidized to FeOOH by the NaOCl reduction reaction [22] and then deposited on the W-Mo-doped BiVO4 electrode. Fe3+ ions are insoluble in an aqueous medium [23] and thus precipitated as FeOOH on the W-Mo-doped BiVO4 electrode. As-deposited FeOOH film was amorphous. To determine the chemical state of the film, X-ray photoelectron spectroscopy (XPS) was performed (Figure 3). In the Fe 2p1/2 and Fe 2p3/2 region, the spectra have three major peaks assigned at 724, 718, and 712.5 eV for Fe3+ [24, 25]. In the O 1s region, the lowest binding energy peak at 529.7 eV can be assigned to oxygen atoms in the iron oxide lattice, O 1s (Fe-O), and the peak at 532.1 eV is assigned to lattice hydroxyl group, O 1s (Fe-OH), that matched well with FeOOH spectra [24]. Figure 4 shows the SEM image of FeOOH on the W-Mo-doped BiVO4 (W-Mo-doped BiVO4/FeOOH), indicating that FeOOH was uniformly covered on the electrode surface, while maintaining the shape of the W-Mo-doped BiVO4. This method is simple and cost effective compared to electrodeposition or photodeposition.Figure 3
XPS spectra of the FeOOH on the W-Mo-doped BiVO4 in the (a) Fe 2p and (b) O 1s regions.
(a)
(b)Figure 4
SEM image of FeOOH on the W-Mo-doped BiVO4.


### 3.4. PEC at the W-Mo-Doped BiVO4/FeOOH Electrode

The photocurrents for water oxidation from the resulting W-Mo-doped BiVO4/FeOOH electrode were significantly higher than that of the W-Mo-doped BiVO4 electrode (Figure 5). The W-Mo-doped BiVO4/FeOOH composite electrode attained almost 2-fold higher photocurrent than the W-Mo-doped BiVO4 for water oxidation reaction at +0.6 V versus Ag/AgCl. The onset potential of W-Mo-doped BiVO4/FeOOH electrode is slightly shifted to the negative direction indicating reduced surface recombination processes at the small overpotential value. The action spectra of the W-Mo-doped BiVO4/FeOOH electrode show typical photocurrents at +0.3 V versus Ag/AgCl depending on the wavelength with a 10 nm interval (Figure 6). The bandgaps were determined from the wavelengths for the onset of photocurrent. The W-Mo-doped BiVO4/FeOOH showed the same onset wavelength as that of the W-Mo-doped BiVO4 (540 nm), indicating that the bandgap of W-Mo-doped BiVO4/FeOOH did not change (Figure S2(a)). The bandgap can also be estimated from the onset of the UV-visible absorbance spectrum (Figure S2(b)). From the absorbance data, the W-Mo-doped BiVO4 sample showed direct transitions with the bandgaps of ~2.4 eV. The bandgap obtained from the absorbance agrees well with the action spectrum data, and the onset wavelength of the W-Mo-doped BiVO4 is essentially the same.Figure 5
LSVs of the W-Mo-doped BiVO4 and W-Mo-doped BiVO4/FeOOH electrodes under UV-visible illumination in phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.Figure 6
Action spectrum of the W-Mo-doped BiVO4/FeOOH electrode at an applied potential of 0.3 V versus Ag/AgCl under UV-visible illumination in phosphate buffer (pH 7).To assess the stability of both the W-Mo-doped BiVO4 and W-Mo-doped BiVO4/FeOOH electrodes over time, chronoamperometry was carried out at +0.3 V versus Ag/AgCl under UV-visible irradiation (Figure 7). After an initial drop, the photocurrent of the W-Mo-doped BiVO4/FeOOH was stabilized at a steady-state value of 0.3 mA/cm2 at 0.3 V versus Ag/AgCl. The presence of FeOOH electrocatalyst effectively suppresses the photochemical deactivation of the W-Mo-doped BiVO4. This result demonstrates the promise of chemically deposited FeOOH electrocatalyst for improving the photocurrent as well as the stability of the W-Mo-doped BiVO4. Furthermore, when FeOOH catalyst was deposited on undoped BiVO4, the photocurrent also showed significantly enhanced PEC efficiency (Figure 8).Figure 7
Current-time response curves of the W-Mo-doped BiVO4, and W-Mo-doped BiVO4/FeOOH electrodes at an applied potential of 0.3 V versus Ag/AgCl in phosphate buffer (pH 7).Figure 8
LSVs of BiVO4 and BiVO4/FeOOH electrodes under UV-visible illumination in phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.For comparison, a FeOOH layer was photodeposited on the W-Mo-doped BiVO4 electrode. The photocurrents for water oxidation from the resulting W-Mo-doped BiVO4/FeOOH electrode also showed enhanced activity compared to that of the W-Mo-doped BiVO4 electrode (Figure S3). The photocurrent of chemically deposited FeOOH on the W-Mo-doped BiVO4 showed a slightly higher value than that of photodeposited FeOOH sample, indicating that the chemical deposition can be an alternative method for the preparation of semiconductor-FeOOH composite for PEC water oxidation. This result indicates that the chemically deposited FeOOH is promising for improving the PEC activity for water oxidation.


## 3.1. Preparation of W-Mo-Doped BiVO4 Electrode

For the facile preparation of W-Mo-doped BiVO4 structure, thin film electrodes were prepared by surfactant-assisted thermal decomposition method on an FTO substrate. Figure 1(a) shows the scanning electron microscopy (SEM) of the W-Mo-doped BiVO4 thin film electrode, indicating a porous network with the grain sizes of 274.8 ± 63.7 nm. The cross section SEM image of the W-Mo-doped BiVO4 shows the film with a thickness of about 1.1 μm (inset). The porous structures can allow the electrolyte to easily diffuse within the BiVO4, increasing the contact area and shortening the hole diffusion distance [13]. Because the hole diffusion length of BiVO4 is about 100 nm [11], the W-Mo-doped BiVO4 thin film in this study is ideal for effective charge separation. Notably, the precursor solution without Nafion increased the grain sizes of the W-Mo-doped BiVO4 and irregularly formed on the FTO substrate (351.5 ± 82.8 nm, see Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2016/1827151). The existence of Nafion in the precursor solution tends to provide small grain sizes as well as uniform growth on the FTO substrate (Figure 1(b) inset). The X-ray diffraction (XRD) peaks corresponded to the monoclinic structure of BiVO4 (Figure 1(b)). Any secondary phase in the XRD patterns was not observed. However, a shift and merging of the XRD peaks at 34, 47, and 59° were observed, indicating that W and Mo were well dissolved in the BiVO4 solid solution [14].Figure 1
(a) SEM top-view and cross section image (inset) of the W-Mo-doped BiVO4, (b) XRD patterns of the W-Mo-doped BiVO4 (red line) and FTO substrate (blue line), and photographic image (inset) of the W-Mo-doped BiVO4.
(a)
(b)


## 3.2. PEC at the W-Mo-Doped BiVO4 Electrode

The PEC performance of the W-Mo-doped BiVO4 thin film electrode was studied by linear sweep voltammetry (LSV) for both sulfite oxidation (0.1 M Na2SO3 + 0.1 M Na2SO4) and water oxidation (0.1 M Na2SO4 + 0.1 M phosphate buffered, pH 7). The LSV was conducted from −0.6 to +0.8 V versus Ag/AgCl at a scan rate of 20 mV/s with chopped light under UV-visible and visible (>425 nm) irradiations (Figure 2). The W-Mo-doped BiVO4 electrode successfully generated anodic photocurrents (n-type character). Because the sulfite oxidation has extremely fast oxidation kinetics, the surface recombination is negligible [14, 15]. The sulfite oxidation is thermodynamically and kinetically favorable, and thus it has a more negative onset potential compared to that of water oxidation (Figure 2). An early onset potential (−0.5 V versus Ag/AgCl) and a rapid increase in photocurrent of 1.6 mA/cm2 (0.6 V versus Ag/AgCl ) for sulfite oxidation on the W-Mo-doped BiVO4 electrode indicated an excellent fill factor. However, nanosized structures are also associated with significant disadvantages, such as an increased number of grain boundaries and a reduced space-charge region [19], resulting in much lower efficiency of the W-Mo-doped BiVO4 electrode than the theoretical value (7.5 mA/cm2) [20]. Furthermore, the photocurrent from the W-Mo-doped BiVO4 electrode for water oxidation is far lower than that of sulfite oxidation. The significant reduction in photocurrent demonstrates that the water oxidation on the W-Mo-doped BiVO4 electrode is mainly limited by poor water oxidation kinetics on the electrode surface. This result indicates that a considerably improved photocurrent can be possible when the W-Mo-doped BiVO4 electrode is coupled with a proper water oxidation cocatalyst.Figure 2
LSVs for the sulfite oxidation (0.1 M Na2SO3 + 0.1 M Na2SO4) and water oxidation (0.1 M Na2SO4 + 0.1 M phosphate buffered, pH 7) using the W-Mo-doped BiVO4 under (a) UV-visible and (b) visible light irradiation. Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.
(a)
(b)


## 3.3. Chemical Deposition of FeOOH on the W-Mo-Doped BiVO4 Electrode

Efficient PEC water splitting requires both highly active semiconductor photoelectrode and proficient electrocatalyst, that is, cocatalyst. Catalyst-modified BiVO4 enhanced PEC efficiency and also noticeably improved the stability [7, 21]. Recently, a number of studies have focused on the potential applications of FeOOH as the cocatalyst [15]. Unfortunately, most of the researches considered to date have only used the photodeposition or electrodeposition method on the semiconductor [7, 15].To improve water oxidation kinetics, a thin layer of FeOOH catalyst was chemically deposited. The chemical deposition of FeOOH on the W-Mo-doped BiVO4 electrode was carried out in a 1.0 M FeSO4 with 1.5 M NaOCl solution. FeSO4 was oxidized to FeOOH by the NaOCl reduction reaction [22] and then deposited on the W-Mo-doped BiVO4 electrode. Fe3+ ions are insoluble in an aqueous medium [23] and thus precipitated as FeOOH on the W-Mo-doped BiVO4 electrode. As-deposited FeOOH film was amorphous. To determine the chemical state of the film, X-ray photoelectron spectroscopy (XPS) was performed (Figure 3). In the Fe 2p1/2 and Fe 2p3/2 region, the spectra have three major peaks assigned at 724, 718, and 712.5 eV for Fe3+ [24, 25]. In the O 1s region, the lowest binding energy peak at 529.7 eV can be assigned to oxygen atoms in the iron oxide lattice, O 1s (Fe-O), and the peak at 532.1 eV is assigned to lattice hydroxyl group, O 1s (Fe-OH), that matched well with FeOOH spectra [24]. Figure 4 shows the SEM image of FeOOH on the W-Mo-doped BiVO4 (W-Mo-doped BiVO4/FeOOH), indicating that FeOOH was uniformly covered on the electrode surface, while maintaining the shape of the W-Mo-doped BiVO4. This method is simple and cost effective compared to electrodeposition or photodeposition.Figure 3
XPS spectra of the FeOOH on the W-Mo-doped BiVO4 in the (a) Fe 2p and (b) O 1s regions.
(a)
(b)Figure 4
SEM image of FeOOH on the W-Mo-doped BiVO4.


## 3.4. PEC at the W-Mo-Doped BiVO4/FeOOH Electrode

The photocurrents for water oxidation from the resulting W-Mo-doped BiVO4/FeOOH electrode were significantly higher than that of the W-Mo-doped BiVO4 electrode (Figure 5). The W-Mo-doped BiVO4/FeOOH composite electrode attained almost 2-fold higher photocurrent than the W-Mo-doped BiVO4 for water oxidation reaction at +0.6 V versus Ag/AgCl. The onset potential of W-Mo-doped BiVO4/FeOOH electrode is slightly shifted to the negative direction indicating reduced surface recombination processes at the small overpotential value. The action spectra of the W-Mo-doped BiVO4/FeOOH electrode show typical photocurrents at +0.3 V versus Ag/AgCl depending on the wavelength with a 10 nm interval (Figure 6). The bandgaps were determined from the wavelengths for the onset of photocurrent. The W-Mo-doped BiVO4/FeOOH showed the same onset wavelength as that of the W-Mo-doped BiVO4 (540 nm), indicating that the bandgap of W-Mo-doped BiVO4/FeOOH did not change (Figure S2(a)). The bandgap can also be estimated from the onset of the UV-visible absorbance spectrum (Figure S2(b)). From the absorbance data, the W-Mo-doped BiVO4 sample showed direct transitions with the bandgaps of ~2.4 eV. The bandgap obtained from the absorbance agrees well with the action spectrum data, and the onset wavelength of the W-Mo-doped BiVO4 is essentially the same.Figure 5
LSVs of the W-Mo-doped BiVO4 and W-Mo-doped BiVO4/FeOOH electrodes under UV-visible illumination in phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.Figure 6
Action spectrum of the W-Mo-doped BiVO4/FeOOH electrode at an applied potential of 0.3 V versus Ag/AgCl under UV-visible illumination in phosphate buffer (pH 7).To assess the stability of both the W-Mo-doped BiVO4 and W-Mo-doped BiVO4/FeOOH electrodes over time, chronoamperometry was carried out at +0.3 V versus Ag/AgCl under UV-visible irradiation (Figure 7). After an initial drop, the photocurrent of the W-Mo-doped BiVO4/FeOOH was stabilized at a steady-state value of 0.3 mA/cm2 at 0.3 V versus Ag/AgCl. The presence of FeOOH electrocatalyst effectively suppresses the photochemical deactivation of the W-Mo-doped BiVO4. This result demonstrates the promise of chemically deposited FeOOH electrocatalyst for improving the photocurrent as well as the stability of the W-Mo-doped BiVO4. Furthermore, when FeOOH catalyst was deposited on undoped BiVO4, the photocurrent also showed significantly enhanced PEC efficiency (Figure 8).Figure 7
Current-time response curves of the W-Mo-doped BiVO4, and W-Mo-doped BiVO4/FeOOH electrodes at an applied potential of 0.3 V versus Ag/AgCl in phosphate buffer (pH 7).Figure 8
LSVs of BiVO4 and BiVO4/FeOOH electrodes under UV-visible illumination in phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.For comparison, a FeOOH layer was photodeposited on the W-Mo-doped BiVO4 electrode. The photocurrents for water oxidation from the resulting W-Mo-doped BiVO4/FeOOH electrode also showed enhanced activity compared to that of the W-Mo-doped BiVO4 electrode (Figure S3). The photocurrent of chemically deposited FeOOH on the W-Mo-doped BiVO4 showed a slightly higher value than that of photodeposited FeOOH sample, indicating that the chemical deposition can be an alternative method for the preparation of semiconductor-FeOOH composite for PEC water oxidation. This result indicates that the chemically deposited FeOOH is promising for improving the PEC activity for water oxidation.


## 4. Conclusions

A W-Mo-doped BiVO4 semiconductor was prepared by Nafion-assisted thermal decomposition method on an FTO substrate. The W-Mo-doped BiVO4 electrodes showed a porous network with the grain sizes of ~270 nm. Because the hole diffusion length of BiVO4 is about 100 nm, the BiVO4 film in this study was found to be ideal for effective charge separation. FeOOH electrocatalyst was chemically deposited by the oxidation of FeSO4 on the W-Mo-doped BiVO4. The W-Mo-doped BiVO4/FeOOH composite electrode attained at least 2-fold higher photocurrent at 0.3 V (versus Ag/AgCl) than that of the W-Mo-doped BiVO4 for water oxidation reaction. Furthermore, the W-Mo-doped BiVO4/FeOOH composite showed high photochemical stability. This result demonstrates that the chemically deposited FeOOH is promising for improving the activity as well as the stability of the water oxidation reaction.



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*Source: 1827151-2016-05-18.xml*