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

Intensive attention has been currently focused on the discovery of semiconductor andproficient cocatalysts for eventual applications to the photoelectrochemical water splitting system. A W-Mo-doped BiVO 4 semiconductor was prepared by the surfactantassisted thermal decomposition method on a fluorine-doped tin oxide conductive film. The W-Mo-doped BiVO 4 films showed a porous morphology with the grain sizes of about 270 nm. Because the hole diffusion length of BiVO 4 is about 100 nm, the W-Mo-doped BiVO 4 film in this study is an ideal candidate for the photoelectrochemical water oxidation. Iron oxyhydroxide (FeOOH) electrocatalyst was chemically deposited on theW-Mo-doped BiVO 4 to investigate the effect of the electrocatalyst on the semiconductor.TheW-Mo-doped BiVO 4 /FeOOH composite electrode showed enhanced activity compared to the pristineW-Modoped BiVO 4 electrode for water oxidation reaction.The chemical deposition is a promising method for the deposition of FeOOH on semiconductor.


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 TiO 2 , WO 3 , Fe 2 O 3 , and BiVO 4 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][6][7][8], achievement of theoretical conversion efficiency is still far from being reached.
BiVO 4 has been intensively studied as a photoanode (ntype semiconductor) for PEC water oxidation, because it absorbs a large portion of the visible light and has a favorable valence band edge [9][10][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 BiVO 4 .The introduction of dopant such as W and Mo into BiVO 4 has been found to enhance the PEC performance [12,13].The dopant in BiVO 4 can increase n-type conductivity and could significantly enhance the PEC activity.Furthermore, W and Mo codoping (W-Mo-doped BiVO 4 ) has shown better performance than W or Mo alone for the BiVO 4 [14].Nanostructure can also enhance the kinetic parameters of the water oxidation reactions through the discrimination of bulk recombinations.For efficient PEC water oxidation, BiVO 4 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 BiVO 4 .
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 BiVO 4 films on fluorine-doped tin oxide (FTO) for the PEC water oxidation.The W-Mo-doped BiVO 4 films showed a porous morphology with the grain sizes of about 270 nm.Because the hole diffusion length of BiVO 4 is about 100 nm, the W-Mo-doped BiVO 4 film in this study is an ideal candidate for effective charge separation.Furthermore, FeOOH cocatalyst was chemically deposited on the W-Mo-doped BiVO 4 films by the oxidation of FeSO 4 to investigate the effect of electrocatalysts on the semiconductor surface.The W-Mo-doped BiVO 4 /FeOOH composites showed enhanced PEC water oxidation performance.O (99.98%, Sigma-Aldrich), and VCl 3 (99%, Alfa-Aesar) were used as the metal precursor salts and used as received.In addition, Nafion (5%, Sigma-Aldrich) and NaOCl (10%), Na 2 SO 4 , Na 2 SO 3 , Na 2 HPO 4 , NaH 2 PO 4 , 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.

Preparation of W-Mo-Doped BiVO 4 and Undoped BiVO 4
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 BiVO 4 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 BiVO 4 thin film.The existence of Nafion in precursor solution tends to give reproducible growth on FTO substrate.For undoped BiVO 4 precursor, the atomic ratio in between Bi and V was 1 : 1 in ethylene glycol.

Chemical Deposition of FeOOH on W-Mo-Doped BiVO 4
Film.Chemical deposition of FeOOH was carried out by adding 30 mL of 1.5 M NaOCl to 15 mL of 1.0 M FeSO 4 solution.The solution was kept at 30 ∘ C for 3 h in air in the presence of W-Mo-doped BiVO 4 film, and the resulting W-Mo-doped BiVO 4 /FeOOH electrode was washed with ethanol and DI water.FeOOH was also deposited on undoped BiVO 4 with the same method.

Photodeposition of FeOOH on W-Mo-Doped BiVO 4 Film.
Photodeposition of FeOOH on the W-Mo-doped BiVO 4 was carried out in a 0.1 M FeSO 4 solution using a threeelectrode 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/cm 2 .Photodeposition was performed for 30 min, and the electrode was washed with ethanol and DI water.

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 cm 2 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 cm 2 .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 Na 2 SO 4 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/cm 2 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-Modoped BiVO 4 electrode is reproducible, it always shows the same photocurrents of each sample.

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.BiVO 4 Electrode.For the facile preparation of W-Mo-doped BiVO 4 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 BiVO 4 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 BiVO 4 shows the film with a thickness of about 1.1 m (inset).The porous structures can allow the electrolyte to easily diffuse within the BiVO 4 , increasing the contact area and shortening the hole diffusion distance [13].Because the hole diffusion length of BiVO 4 is about 100 nm [11], the W-Mo-doped BiVO 4 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 BiVO 4 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 BiVO 4 (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 BiVO 4 solid solution [14].).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 BiVO 4 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/cm 2 (0.6 V versus Ag/AgCl ) for sulfite oxidation on the W-Mo-doped BiVO 4 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 spacecharge region [19], resulting in much lower efficiency of the W-Mo-doped BiVO 4 electrode than the theoretical value (7.5 mA/cm 2 ) [20].Furthermore, the photocurrent from the W-Mo-doped BiVO 4 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 BiVO 4 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 BiVO 4 electrode is coupled with a proper water oxidation cocatalyst.

Chemical Deposition of FeOOH on the W-Mo-Doped
BiVO 4 Electrode.Efficient PEC water splitting requires both highly active semiconductor photoelectrode and proficient electrocatalyst, that is, cocatalyst.Catalyst-modified BiVO 4 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 BiVO 4 electrode was carried out in a 1.0 M FeSO 4 with 1.5 M NaOCl solution.FeSO 4 was oxidized to FeOOH by the NaOCl reduction reaction [22] and then deposited on the W-Mo-doped BiVO 4 electrode.Fe 3+ ions are insoluble in an aqueous medium [23] and thus precipitated as FeOOH on the W-Mo-doped BiVO 4 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 Fe 3+ [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 BiVO 4 (W-Mo-doped BiVO 4 /FeOOH), indicating that FeOOH was uniformly covered on the electrode surface, while maintaining the shape of the W-Mo-doped BiVO 4 .This method is simple and cost effective compared to electrodeposition or photodeposition.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 BiVO 4 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 BiVO 4 is essentially the same.
To assess the stability of both the W-Mo-doped BiVO 4 and W-Mo-doped BiVO 4 /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 BiVO 4 /FeOOH was stabilized at a steady-state value of 0.3 mA/cm 2 at 0.3 V versus Ag/AgCl.The presence of FeOOH electrocatalyst effectively suppresses the photochemical deactivation of the W-Mo-doped BiVO 4 .This result demonstrates the promise of chemically deposited FeOOH electrocatalyst for improving the photocurrent as well as the stability of the W-Mo-doped BiVO 4 .Furthermore, when FeOOH catalyst  was deposited on undoped BiVO 4 , the photocurrent also showed significantly enhanced PEC efficiency (Figure 8).
For comparison, a FeOOH layer was photodeposited on the W-Mo-doped BiVO 4 electrode.The photocurrents for water oxidation from the resulting W-Mo-doped BiVO 4 /FeOOH electrode also showed enhanced activity compared to that of the W-Mo-doped BiVO 4 electrode (Figure S3).The photocurrent of chemically deposited FeOOH on the W-Mo-doped BiVO 4 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.