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

Joo, Eun Jin; Park, Gisang; Gwak, Ji Seon; Seo, Jong Hyeok; Jang, Kyu Yeon; Oh, Kyung Hee; Nam, Ki Min

doi:https://doi.org/10.1155/2016/1827151

Journal of Nanomaterials

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Research Article | Open Access

Volume 2016 | Article ID 1827151 | https://doi.org/10.1155/2016/1827151

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

Eun Jin Joo,¹Gisang Park,¹Ji Seon Gwak,¹Jong Hyeok Seo,¹Kyu Yeon Jang,¹Kyung Hee Oh,¹and Ki Min Nam¹

Academic Editor: Vincenzo Baglio

Received18 Feb 2016

Accepted27 Apr 2016

Published18 May 2016

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.

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 Cu radiations at 40 kV and 100 mA (Rigaku, Dmax-RB diffractometer). X-ray photoelectron spectroscopy (XPS) measurements were taken using a spectrometer with an X-ray source of Al 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].

(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.

(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 2p_1/2 and Fe 2p_3/2 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.

(a)

(b)

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.

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).

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.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02037373).

Supplementary Materials

Additional materials, including SEM, LSV, UV-vis, and action spectrum results of composite electrodes can be found in the supporting information.

Supplementary Material

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Copyright © 2016 Eun Jin Joo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

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

Abstract

1. Introduction

2. Experimental Procedures

2.1. Materials

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

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

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

2.5. Electrochemical Characterization of Electrodes

2.6. Materials Characterization of Electrodes

3. Results and Discussion

3.1. Preparation of W-Mo-Doped BiVO4 Electrode

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

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

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

4. Conclusions

Competing Interests

Acknowledgments

Supplementary Materials

References

Copyright

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

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

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

3.1. Preparation of W-Mo-Doped BiVO₄ Electrode

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

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

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