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2 films by a polymer-assisted method
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Synthesis and photoelectrochemical study of BiWxV1LxO4Dx/2 films by a polymer-assisted method Xiaokang Wan, Mingtao Li, Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 West Xianning Street, Xi’an 710049, PR China
article info
abstract
Article history:
A series of BiWxV1xO4þx/2 films were coated on fluorine-doped tin oxide (FTO) glass by a
Received 4 May 2013
polymer-assisted method and examined as photoelectrodes for photoelectrochemical
Received in revised form
measurements under Xe lamp light irradiation in a 0.5 M Na2SO4 solution. The composi-
29 June 2013
tions, structural, optical and morphologic properties of the films were characterized by
Accepted 29 June 2013
XPS, XRD, UVevis and SEM. The results showed the successfully synthesized films and
Available online 29 July 2013
their photoelectrochemical activities, revealing that the amount of tungsten had an
Keywords:
highest incident photon to current conversion efficiency (IPCE) was obtained when
BiWxV1xO4þx/2
x equaled 0.1.
Film
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
important effect on the photoelectrochemical activities of BiWxV1xO4þx/2 films and the
Polymer-assisted method
reserved.
Photoelectrochemical water splitting
1.
Introduction
In the severe situation of growing energy crisis and environment pollution of the world, the development of clean, cheap and renewable energy is essential for human beings. Hydrogen production using solar energy via photoelectrochemical and photocatalytic water splitting has been studied for decades as a promising solution to the problems [1e5]. Thousands of semiconductor materials have been found and investigated for water splitting hydrogen evolution [6e9] since the pioneer work on TiO2 [10] by Fujishima and Honda in 1972. However, among these semiconductor materials, most of them harvest low activities under visible light while the relatively efficient materials usually cannot be stable because of photo-corrosion or too much cost [11,12].
Therefore, our goal is to develop clean, cost-effective and stable semiconductor materials with high activity for photoelectrochemical and photocatalytic water splitting hydrogen evolution. As is known to all, metal oxide is much more stable compared with metal sulfide. Bismuth based oxides have attracted much interest because of its high photoelectrochemical activity under visible light. It is reported that both Bi2WO6 [13e15] and the scheelite-monoclinic (s-m) structure BiVO4 [16e18] harvest high photoelectrochemical activities. Monoclinic BiVO4 is a promising material for visible light water splitting for oxygen evolution. The band gap of BiVO4 is w2.4 eV and the top of the conduction band (CB) is not negative enough for hydrogen evolution but available for oxygen evolution. Thus, more widely studies have focused on the applications of BiVO4 as promising
* Corresponding author. Tel.: þ86 29 82663895; fax: þ86 29 82669033. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.135
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 7 2 0 e1 1 7 2 6
photoanodes in photoelectrochemical (PEC) cells. The efficiency of pure BiVO4 is not very high so that much work has been focused on the modification of the photoanodes [19e23]. Ye et al. demonstrated that Bi/V/W oxide with a ratio of 4.5:5:0.5 by scanning electrochemical microscopy (SECM) showed w4.6 higher photocurrent than pure BiVO4 in 0.1 M Na2SO4 with 0.1 M Na2SO3 as a sacrificial reagent [24]. Bulk films by drop-casting were also studied to confirm the results. With the same SECM technique, Bard’s group also demonstrated that W/Mo-doped BiVO4 (Bi:V:W:Mo atomic ratio of 4.6:4.6:0.2:0.6) showed more than 10 times higher photocurrent density than pure BiVO4 in a solution of 0.1 M Na2SO4 and 0.1 M Na2SO3 [25]. By doping with 1% W and introducing a thin (w10 nm) interfacial layer of SnO2 in between the FTO and the BiVO4, highly improved quantum efficiencies were obtained by a factor of w7 [26]. In another study, Mo and W were incorporated into BiVO4 to show 7 and 8 times higher photocurrent densities than for pure BiVO4, respectively [27]. And 6% Mo, 2% W BiVO4 for co-incorporation showed 10 times higher photocurrent densities than for pure BiVO4. In this article, we synthesized a series of multi-composite bismuth based metal oxide BiWxV1xO4þx/2 films photoanodes by a polymer-assisted solegel method with a spincoating technique followed by annealing in air. The films prepared by this versatile method here possess porous nanostructure, which is beneficial to obtain better surface area for water splitting reaction. The structural, morphologic and optical properties and the photoelectrochemical activities of the films were characterized and investigated. The impact of the composition on the photoelectrochemical activities was discussed.
2.
Experimental section
2.1.
Synthesis
The BiWxV1xO4þx/2 films were synthesized by a polymerassisted solegel method with spin-coating technique [28,29]. Bi(NO3)3$5H2O and EDTA (Ethylene Diamine Tetraacetic Acid) were dissolved in nitric acid and aqueous ammonia respectively, and the resultant solutions were mixed and stirred in a mole ratio of Bi:EDTA ¼ 1:2. WO3 and NH4VO3 were dissolved in ammonia water respectively and the mixed with EDTA ammonia solution in a mole ratio of W:EDTA ¼ 1:2 and V:EDTA ¼ 1:2. Then the three solutions were mixed together with stirring and add appropriate amount of water to make the volume to be 100 mL in the mole ratio of Bi:W:V ¼ 1:x:1 x (x ¼ 0, 0.1,., 1). Meanwhile, 4 g of polyvinyl alcohol (PVA) was dissolved into 100 mL water. Add 5 mL of PVA solution into 5 mL of the mixed solution with stirring to obtain a precursor colloidal solution. PVA was used to obtain a colloidal solution during the preparation and the adhesion of the solution on substrates can be adjusted according to the concentration to optimize the experiments. Transparent conductive F-doped tin oxide (FTO) coated glasses were used as the substrates of the BiWxV1xO4þx/2 films. The FTO glass was cut into pieces ca. 2.5 2.0 cm2 in area and cleaned each for 30 min with detergent solution, acetone and ethanol successively under ultrasonic condition. The colloidal solution was applied on the
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FTO substrate after one small piece of transparent tape was pasted on the edge of the glass to leave a place for conducting. Then the glass was spin-coated at the speed of 700 rpm and 2000 rpm for 9 s and 30 s respectively and heated at 100 C for 10 min. Then the films were heated at 400 C for 2 h at the temperature raised rate of 1 C/min.
2.2.
Characterization
X-ray diffraction patterns (XRD) of the resulting films were obtained with a PANalytical X’Pert PRO X diffractometer using a Ni-filtered Cu Ka (wavelength 1.5406 A) radiation. The morphology was characterized by a JSM6700F scanning microscope. X-ray photoelectron spectroscopic (XPS) analysis was made with an AXIS-ULTRA DLD XPS spectrometer. Optical transmission spectra of the films were measured on a HITACHI U4100 instrument.
2.3.
Photoelectrochemical experiments
Photoelectrochemical measurements were carried out in a three electrodes cell system, using a large area platinum plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The films prepared were employed as the work electrodes. An aqueous solution of 0.5 M Na2SO4 was used as electrolyte. A potentiostat 273A from Princeton Applied Research Company (PARC) and an analyzer Lock-in model 5210 from Signal Recovery were also used for the measurement. Photocurrent action spectra were recorded using lock-in technique. The absolute intensity of the incident light was recorded with an avaspec-2048 fiber optic spectrometer from AVNANTES. The area of the work electrodes was fixed at 0.785 cm2. The currentepotential properties and incident photon to current conversion efficiency (IPCE) were examined to evaluate the photoelectrochemical properties of the films.
3.
Results and discussion
In order to confirm the composition and oxidation states of the elements in the films, two samples were investigated by XPS. Fig. 1 shows the XPS peaks for BiWxV1xO4þx/2, x ¼ 0.1, 0.2. The elements ratios were shown in Table S1. We can find out that the mole ratio of BiW0.1V0.9O4.05 was Bi:W:V ¼ 1:0.118:0.443 and that of BiW0.2V0.8O4.1 was Bi:W:V ¼ 1:0.195:0.404, which revealed that the ratio of tungsten was almost the same as the feed ratio while the ratio of vanadium was apparently lower than the feed ratio. The possible explanation was that the element ratio examined by XPS was only precise on the surface of the films while information of the bulk could not be detected. The elements ratio on the surface might not be the same with the real ratio because of high temperature process during preparation. The C element showed at peak of 285.1 eV might come from the instrument or the organic precursors. The position of peaks also confirmed the element oxidation state. The Bi 4f5/2 and Bi 4f7/2 peaks located at 164.6 eV and 159.5 eV respectively confirmed the existence of Bismuth in the film and the oxidation state to be Bi3þ [22]. The V 2p1/2 and V 2p3/2 peaks
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Fig. 1 e XPS spectra of BiWxV1LxO4Dx/2 films, x [ 0.1: (a) wide; (b) Bi 4f; (c) V 2p; (d) W 4f.
located at 524.7 eV and 517.0 eV provided proof of the presence of V5þ. The peaks for tungsten located at 37.6 eV and 35.4 eV which were ascribed to be W 4f5/2 and W 4f7/2, revealing the presence of W6þ [24]. The XPS results showed expected composition and oxidation states of the films. Fig. 2 shows the X-ray diffraction patterns of the BiWxV1xO4þx/2 films. The films were very thin so that the diffraction peaks of the substrate were also shown. We can find out that BiVO4 had sharp diffraction peaks in the region of 2q from 10 to 80 . The peaks at 2q ¼ 18.7 , 28.9 , 30.5 , 39.7 , 42.4 and 46.8 , especially the splitting at 46.8 confirmed that it was the scheelite-monoclinic (s-m) BiVO4 (JCPDS 14e0688),
Fig. 2 e X-ray diffraction patterns of BiWxV1LxO4Dx/2 films. Asterisks (*): the peaks of FTO substrate.
which was reported to show high photocatalytic activity [30]. When x ¼ 0.1, the position of the peaks were almost the same, which indicated that W was successfully composited in the compound and the basic monoclinic structure was not changed. A new slight peak at about 34.8 emerged and no WO3 peaks were observed, which means tungsten was successfully incorporated. The peaks became weaker when x value increased, which was caused by the replacement of W to V. However, the peaks were too small to be investigated afterward when x became larger because the films were too thin. From the XRD patterns of the series of films, an obvious tendency can be noticed that with x increasing the diffraction peaks move to low angle direction. This can be explained by A) and V5þ (0.52 A). The the different ion radii of W6þ (0.65 6þ bigger ion radius of W enlarged the crystal lattice and led to the shift. The optical transmittance spectra of the BiWxV1xO4þx/2 films are shown in Fig. 3. The films prepared uniformly on FTO glass showed color from vivid yellow to light green with x value increasing. The change of color is also confirmed in the optical transmittance spectra and more detailed information can be observed. We can see an obvious monotonic blue-shift of the films transmittance spectra with x value increasing gradually from the figure, which influenced the band gap energy to increase in accordance gradually. BiVO4 possessed a highest visible light absorption while the visible light absorption of BiWO4.5 was the weakest. The band gap of BiVO4 was estimated to be 2.4 eV in literature. With the addition of tungsten, the band gap became larger. The change was caused by the addition of W that the absorption of visible light became weaker in order, which was a disadvantageous aspect for photoelectrochemical reaction. Thus the band gap of the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 7 2 0 e1 1 7 2 6
Fig. 3 e UVeVis transmittance spectra of BiWxV1LxO4Dx/2 films.
BiWxV1xO4þx/2 films could be modulated by changing the composition. The photoelectrochemical properties were also affected in consequence, which would be discussed afterward. The films obtained from the polymer-assisted solegel method possessed porous surface morphology nanostructure, which was advantageous for PEC water splitting reaction. Photo-generated charge carriers could be separated more easily so that the efficiency could be improved. Fig. 4 shows the field emission scanning electron microscopy images of some representative prepared BiWxV1xO4þx/2 films. It can be easily seen that the films were well crystallized and had the
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nanostructure of porous particles, which benefited the transportation of the electrons and holes to improve the photoelectrochemical activities for water splitting. Furthermore, the surface areas were improved with the porous structure, which promoted the contact with solution in the photocatalytic reaction. The advantageous porous structure benefited from the polymer-assisted method during synthesis. The series of films were very thin because of the preparation method and the sizes of all of the particles were all about 80e100 nm. The change in the composition did not affect the size of particles apparently. Maybe due to the poor electrical conductivity of the films, the slight difference of their sizes could hardly be identified. The thicknesses of the films were estimated to be about 100 nm from the cross section image (Fig. S1). It is worth mentioning that the thicknesses of the films can be adjusted by repeating the spincoating procedure or modifying the viscosity and concentration of the precursor. The thickness data of previous work on BiVO4 and modified BiVO4 were usually larger and ranged from 140 nm to 1 mm [20,23,27]. The thickness of the film had a fatal influence on the light absorption and the photoelectrochemical activity, which would be discussed in detail in the next part. To investigate the photoelectrochemical properties, photocurrent measurements in a conventional three-electrode cell were made. Fig. 5 shows the photocurrentepotential properties of representative coated films under visible light in 0.5 M Na2SO4 aqueous solution, in which the films were stable. From the figure we can find out that the films had obvious high photocurrent activities. The dark current densities were tiny and close to zero, which meant no photocurrent response detected. As to BiVO4, the photocurrent appeared at the
Fig. 4 e SEM images of BiWxV1LxO4Dx/2 films, (a) x [ 0, (b) x [ 0.1, (c) x [ 0.2, (d) x [ 0.4.
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incorporation of Mo and W. With a modified SECM technique, Ye et al. showed that Bi/V/W oxide with an optimal ratio of 4.5:5:0.5 had w4.6 times higher photocurrent density than pure BiVO4 in 0.1 M Na2SO4 with 0.1 M Na2SO3 as a sacrificial reagent. This resulted composition was relatively close to our results. In 0.1 M Na2SO4 without sacrificial reagent, the photocurrent density of Bi/V/W (4.5/5/0.5) oxide bulk film was shown to be w0.1 mA/cm2 at 0.5 V vs. Ag/AgCl. IPCE measurements were also made to investigate photoelectrochemical properties over the solar spectrum per incident photon flux [31]. The IPCE was calculated according to the formula as follows: IPCE ¼ Fig. 5 e Photocurrentepotential curves of BiWxV1LxO4Dx/2 films.
threshold potential of about 0.3 V vs. SCE under visible light and the photocurrent increased steadily with the applied potential rising. When x equaled 0.1, a remarkable increase of the photocurrent was observed, which proved that the addition of tungsten was quite beneficial for the photoelectrochemical activity. The photocurrent density was promoted by w6 times in comparison with that of BiVO4 and the value was w0.06 mA/cm2 at 0.5 V vs. SCE (w1.15 V vs. RHE). When x equaled 0.7 or larger, the photocurrent appeared to decrease and the photocurrent of BiWO4.5 was even smaller than that of BiVO4. The main structure of monoclinic BiVO4 did not exist so that the PEC performance fell down sharply. This could also be partly explained with the optical transmission spectra figure. The absorption of visible light became weak with x value increasing and the band gap was also larger so that the photoelectrochemical activities decreased with the x value increasing. Another important reason might be that too much tungsten led to the enhancement of recombination of charge carriers. As we know, the photocurrent activity of the photoelectrode could be greatly affected by the thickness of the material. For films with low thickness, the amount of the materials was not much and the absorption of light irradiation would be weak, which led to low efficiencies. For films with too high thickness, the long transport distance for charge carriers led to undesirable recombination. In our work, the films were synthesized by a facile polymer-assisted solegel method and the deposition process was not repeated so that the films were relatively thin (w100 nm). Although the photocurrent density was an important factor, more attention was paid on the influence by the composition. The results by Berglund and coworkers showed the optimal amount of W incorporation to be 2.5%, which raised the photocurrent density by 7 times [27]. The films were fabricated by simultaneous evaporation of Bi, V, Mo, and W in vacuum. Various ratios of W (2.5%, 5%, 7.5%, 10%, 15%, and 20%) were investigated and 2.5% W showed the highest photocurrent density of w0.1 mA/cm2 at 1.1 V vs. RHE in 0.1 M Na2SO4 and 0.1 M phosphate buffer solution (pH 6.8). PEC tests of the films under Na2SO3 and with electrocatalyst Pt were also investigated and more attention was paid on the co-
hc Iph e Pl
(1)
where Iph is the photocurrent density, P is the power intensity of the light, and h, c, e are Planck’s constant, speed of light in vacuum and elementary charge, respectively. Fig. S2 shows the photocurrent of BiWxV1xO4þx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution on the wavelength from 300 to 600 nm. All the films showed different levels of photoelectrochemical activities. The figure was in agreement with the optical transmittance spectra very well. The corresponding wavelength in the curve of BiVO4 to the photocurrent was about 470 nm and this wavelength shifted to shorter side with the x value increasing. However, when x equaled 0.8 or larger, the photoelectrochemical activities only appeared in the area of wavelength under 400 nm and the activities were thus very low. It could be obviously seen that the best photoelectrochemical activity was showed in the film of BiW0.1V0.9O4.05, of which the photocurrent was much higher than the others and covered a wide area. Fig. 6 shows the IPCE plots of BiWxV1xO4þx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution. As to BiVO4, it could be observed that the threshold wavelength of photocurrent was about 500 nm. As the x value increased, this threshold wavelength became smaller, which agreed well with the optical transmittance spectra. The curves were all in the tendency of descending, which was the result of optical absorption. The IPCE maintained at a relatively high
Fig. 6 e IPCE of BiWxV1LxO4Dx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution.
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and stable level in the range of 400e450 nm. Obviously, BiW0.1V0.9O4.05 showed the highest IPCE value. Compared with BiVO4, the addition of tungsten made a great enhancement on the photocurrent because W6þ promoted the density of charge carriers and the separation of the electrons and holes induced by light illumination so that the charge transfer was much easier. However, when x was larger, the charge recombination would be enhanced and the photo absorption of light irradiation became much weaker, which decreased the photoelectrochemical water splitting efficiency.
4.
Conclusion
A series of BiWxV1xO4þx/2 films have been successfully synthesized by a facile polymer-assisted solegel method and examined to show good photoelectrochemical activities. The films showed porous nanostructure property and the optical absorption was influenced by the ratio of W, which changed the band gap of the films. XPS results confirmed composition and oxidation states of the elements. XRD patterns showed the monoclinic structure of BiVO4 and the influence by W incorporation. The photoelectrochemical activities were investigated to be promoted by the addition of tungsten remarkably. The optimal composition BiW0.1V0.9O4.05 enhanced the photocurrent density by w6 times and the best IPCE at 450 nm reached 1.87%.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51121092) and the National Basic Research Program of China (Grant No. 2009CB220000).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.06.135.
references
[1] Weber MF, Dignam MJ. Splitting water with semiconducting photoelectrodes efficiency considerations. Int J Hydrogen Energy 1986;11:225e32. [2] Maeda K, Domen K. Photocatalytic water splitting: recent progress and future challenges. J Phys Chem Lett 2010;1:2655e61. [3] Licht S, Wang B, Mukerji S, Soga T, Umeno M, Tributsch H. Over 18% solar energy conversion to generation of hydrogen fuel; theory and experiment for efficient solar water splitting (reprinted from J Phys Chem B, vol 104, pg 8920e8924, 2000). Int J Hydrogen Energy 2001;26:653e9. [4] Delahoy AE, Gau SC, Murphy OJ, Kapur M, Bockris JOM. A one-unit photovoltaic electrolysis system based on a triple stack of amorphous-silicon (pin) cells. Int J Hydrogen Energy 1985;10:113e6.
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[5] Jing DW, Guo LJ, Zhao LA, Zhang XM, Liu HA, Li MT, et al. Efficient solar hydrogen production by photocatalytic water splitting: from fundamental study to pilot demonstration. Int J Hydrogen Energy 2010;35:7087e97. [6] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269e71. [7] Domen K, Kudo A, Onishi T. Mechanism of photocatalytic decomposition of water into H2 and O2 over NiOeSrTiO3. J Catal 1986;102:92e8. [8] Oregan B, Gratzel M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 1991;353:737e40. [9] Chen XB, Shen SH, Guo LJ, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503e70. [10] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [11] Liu MC, Jing DW, Zhao LA, Guo LJ. Preparation of novel CdSincorporated special glass composite as photocatalyst material used for catalyst-fixed system. Int J Hydrogen Energy 2010;35:7058e64. [12] Buhler N, Meier K, Reber JF. Photochemical hydrogenproduction with cadmium-sulfide suspensions. J Phys ChemUS 1984;88:3261e8. [13] Zhao X, Wu Y, Yao WQ, Zhu YF. Photoelectrochemical properties of thin Bi2WO6 films. Thin Solid Films 2007;515:4753e7. [14] Tang JW, Zou ZG, Ye JH. Photocatalytic decomposition of organic contaminants by Bi2WO6 under visible light irradiation. Catal Lett 2004;92:53e6. [15] Kudo A, Hijii S. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3þ with 6s2 configuration and d0 transition metal ions. Chem Lett 1999:1103e4. [16] Li MT, Zhao LA, Guo LJ. Preparation and photoelectrochemical study of BiVO4 thin films deposited by ultrasonic spray pyrolysis. Int J Hydrogen Energy 2010;35:7127e33. [17] Sayama K, Nomura A, Zou ZG, Abe R, Abe Y, Arakawa H. Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light. Chem Commun 2003:2908e9. [18] Yu JQ, Kudo A. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv Funct Mater 2006;16:2163e9. [19] Parmar KPS, Kang HJ, Bist A, Dua P, Jang JS, Lee JS. Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO4 photoanodes. Chemsuschem 2012;5:1926e34. [20] Zhong DK, Choi S, Gamelin DR. Near-complete suppression of surface recombination in solar photoelectrolysis by “CoPi” catalyst-modified W: BiVO4. J Am Chem Soc 2011;133:18370e7. [21] Seabold JA, Choi KS. Efficient and stable photo-oxidation of water by a bismuth vanadate photoanode coupled with an iron oxyhydroxide oxygen evolution catalyst. J Am Chem Soc 2012;134:2186e92. [22] Pilli SK, Furtak TE, Brown LD, Deutsch TG, Turner JA, Herring AM. Cobalt-phosphate (Co-Pi) catalyst modified Mo-doped BiVO4 photoelectrodes for solar water oxidation. Energy Environ Sci 2011;4:5028e34. [23] Luo WJ, Yang ZS, Li ZS, Zhang JY, Liu JG, Zhao ZY, et al. Solar hydrogen generation from seawater with a modified BiVO4 photoanode. Energy Environ Sci 2011;4:4046e51. [24] Ye H, Lee J, Jang JS, Bard AJ. Rapid screening of BiVO4-based photocatalysts by scanning electrochemical microscopy (SECM) and studies of their photoelectrochemical properties. J Phys Chem C 2010;114:13322e8.
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[25] Park HS, Kweon KE, Ye H, Paek E, Hwang GS, Bard AJ. Factors in the metal doping of BiVO4 for improved photoelectrocatalytic activity as studied by scanning electrochemical microscopy and first-principles densityfunctional calculation. J Phys Chem C 2011;115:17870e9. [26] Liang YQ, Tsubota T, Mooij LPA, van de Krol R. Highly improved quantum efficiencies for thin film BiVO4 photoanodes. J Phys Chem C 2011;115:17594e8. [27] Berglund SP, Rettie AJE, Hoang S, Mullins CB. Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation. Phys Chem Chem Phys 2012;14:7065e75. [28] Liu HM, Nakamura R, Nakato Y. Promoted photo-oxidation reactivity of particulate BiVO4 photocatalyst prepared by a
photoassisted sol-gel method. J Electrochem Soc 2005;152:G856e61. [29] Su JZ, Guo LJ, Yoriya S, Grimes CA. Aqueous growth of pyramidal-shaped BiVO4 nanowire arrays and structural characterization: application to photoelectrochemical water splitting. Cryst Growth Des 2010;10:856e61. [30] Tokunaga S, Kato H, Kudo A. Selective preparation of monoclinic and tetragonal BiVO4 with scheelite structure and their photocatalytic properties. Chem Mater 2001;13:4624e8. [31] Chen ZB, Jaramillo TF, Deutsch TG, Kleiman-Shwarsctein A, Forman AJ, Gaillard N, et al. Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols. J Mater Res 2010:3e16.
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Synthesis and photoelectrochemical study of BiWxV1LxO4Dx/2 films by a polymer-assisted method Xiaokang Wan, Mingtao Li, Liejin Guo* International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 West Xianning Street, Xi’an 710049, PR China
article info
abstract
Article history:
A series of BiWxV1xO4þx/2 films were coated on fluorine-doped tin oxide (FTO) glass by a
Received 4 May 2013
polymer-assisted method and examined as photoelectrodes for photoelectrochemical
Received in revised form
measurements under Xe lamp light irradiation in a 0.5 M Na2SO4 solution. The composi-
29 June 2013
tions, structural, optical and morphologic properties of the films were characterized by
Accepted 29 June 2013
XPS, XRD, UVevis and SEM. The results showed the successfully synthesized films and
Available online 29 July 2013
their photoelectrochemical activities, revealing that the amount of tungsten had an
Keywords:
highest incident photon to current conversion efficiency (IPCE) was obtained when
BiWxV1xO4þx/2
x equaled 0.1.
Film
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
important effect on the photoelectrochemical activities of BiWxV1xO4þx/2 films and the
Polymer-assisted method
reserved.
Photoelectrochemical water splitting
1.
Introduction
In the severe situation of growing energy crisis and environment pollution of the world, the development of clean, cheap and renewable energy is essential for human beings. Hydrogen production using solar energy via photoelectrochemical and photocatalytic water splitting has been studied for decades as a promising solution to the problems [1e5]. Thousands of semiconductor materials have been found and investigated for water splitting hydrogen evolution [6e9] since the pioneer work on TiO2 [10] by Fujishima and Honda in 1972. However, among these semiconductor materials, most of them harvest low activities under visible light while the relatively efficient materials usually cannot be stable because of photo-corrosion or too much cost [11,12].
Therefore, our goal is to develop clean, cost-effective and stable semiconductor materials with high activity for photoelectrochemical and photocatalytic water splitting hydrogen evolution. As is known to all, metal oxide is much more stable compared with metal sulfide. Bismuth based oxides have attracted much interest because of its high photoelectrochemical activity under visible light. It is reported that both Bi2WO6 [13e15] and the scheelite-monoclinic (s-m) structure BiVO4 [16e18] harvest high photoelectrochemical activities. Monoclinic BiVO4 is a promising material for visible light water splitting for oxygen evolution. The band gap of BiVO4 is w2.4 eV and the top of the conduction band (CB) is not negative enough for hydrogen evolution but available for oxygen evolution. Thus, more widely studies have focused on the applications of BiVO4 as promising
* Corresponding author. Tel.: þ86 29 82663895; fax: þ86 29 82669033. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.135
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photoanodes in photoelectrochemical (PEC) cells. The efficiency of pure BiVO4 is not very high so that much work has been focused on the modification of the photoanodes [19e23]. Ye et al. demonstrated that Bi/V/W oxide with a ratio of 4.5:5:0.5 by scanning electrochemical microscopy (SECM) showed w4.6 higher photocurrent than pure BiVO4 in 0.1 M Na2SO4 with 0.1 M Na2SO3 as a sacrificial reagent [24]. Bulk films by drop-casting were also studied to confirm the results. With the same SECM technique, Bard’s group also demonstrated that W/Mo-doped BiVO4 (Bi:V:W:Mo atomic ratio of 4.6:4.6:0.2:0.6) showed more than 10 times higher photocurrent density than pure BiVO4 in a solution of 0.1 M Na2SO4 and 0.1 M Na2SO3 [25]. By doping with 1% W and introducing a thin (w10 nm) interfacial layer of SnO2 in between the FTO and the BiVO4, highly improved quantum efficiencies were obtained by a factor of w7 [26]. In another study, Mo and W were incorporated into BiVO4 to show 7 and 8 times higher photocurrent densities than for pure BiVO4, respectively [27]. And 6% Mo, 2% W BiVO4 for co-incorporation showed 10 times higher photocurrent densities than for pure BiVO4. In this article, we synthesized a series of multi-composite bismuth based metal oxide BiWxV1xO4þx/2 films photoanodes by a polymer-assisted solegel method with a spincoating technique followed by annealing in air. The films prepared by this versatile method here possess porous nanostructure, which is beneficial to obtain better surface area for water splitting reaction. The structural, morphologic and optical properties and the photoelectrochemical activities of the films were characterized and investigated. The impact of the composition on the photoelectrochemical activities was discussed.
2.
Experimental section
2.1.
Synthesis
The BiWxV1xO4þx/2 films were synthesized by a polymerassisted solegel method with spin-coating technique [28,29]. Bi(NO3)3$5H2O and EDTA (Ethylene Diamine Tetraacetic Acid) were dissolved in nitric acid and aqueous ammonia respectively, and the resultant solutions were mixed and stirred in a mole ratio of Bi:EDTA ¼ 1:2. WO3 and NH4VO3 were dissolved in ammonia water respectively and the mixed with EDTA ammonia solution in a mole ratio of W:EDTA ¼ 1:2 and V:EDTA ¼ 1:2. Then the three solutions were mixed together with stirring and add appropriate amount of water to make the volume to be 100 mL in the mole ratio of Bi:W:V ¼ 1:x:1 x (x ¼ 0, 0.1,., 1). Meanwhile, 4 g of polyvinyl alcohol (PVA) was dissolved into 100 mL water. Add 5 mL of PVA solution into 5 mL of the mixed solution with stirring to obtain a precursor colloidal solution. PVA was used to obtain a colloidal solution during the preparation and the adhesion of the solution on substrates can be adjusted according to the concentration to optimize the experiments. Transparent conductive F-doped tin oxide (FTO) coated glasses were used as the substrates of the BiWxV1xO4þx/2 films. The FTO glass was cut into pieces ca. 2.5 2.0 cm2 in area and cleaned each for 30 min with detergent solution, acetone and ethanol successively under ultrasonic condition. The colloidal solution was applied on the
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FTO substrate after one small piece of transparent tape was pasted on the edge of the glass to leave a place for conducting. Then the glass was spin-coated at the speed of 700 rpm and 2000 rpm for 9 s and 30 s respectively and heated at 100 C for 10 min. Then the films were heated at 400 C for 2 h at the temperature raised rate of 1 C/min.
2.2.
Characterization
X-ray diffraction patterns (XRD) of the resulting films were obtained with a PANalytical X’Pert PRO X diffractometer using a Ni-filtered Cu Ka (wavelength 1.5406 A) radiation. The morphology was characterized by a JSM6700F scanning microscope. X-ray photoelectron spectroscopic (XPS) analysis was made with an AXIS-ULTRA DLD XPS spectrometer. Optical transmission spectra of the films were measured on a HITACHI U4100 instrument.
2.3.
Photoelectrochemical experiments
Photoelectrochemical measurements were carried out in a three electrodes cell system, using a large area platinum plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The films prepared were employed as the work electrodes. An aqueous solution of 0.5 M Na2SO4 was used as electrolyte. A potentiostat 273A from Princeton Applied Research Company (PARC) and an analyzer Lock-in model 5210 from Signal Recovery were also used for the measurement. Photocurrent action spectra were recorded using lock-in technique. The absolute intensity of the incident light was recorded with an avaspec-2048 fiber optic spectrometer from AVNANTES. The area of the work electrodes was fixed at 0.785 cm2. The currentepotential properties and incident photon to current conversion efficiency (IPCE) were examined to evaluate the photoelectrochemical properties of the films.
3.
Results and discussion
In order to confirm the composition and oxidation states of the elements in the films, two samples were investigated by XPS. Fig. 1 shows the XPS peaks for BiWxV1xO4þx/2, x ¼ 0.1, 0.2. The elements ratios were shown in Table S1. We can find out that the mole ratio of BiW0.1V0.9O4.05 was Bi:W:V ¼ 1:0.118:0.443 and that of BiW0.2V0.8O4.1 was Bi:W:V ¼ 1:0.195:0.404, which revealed that the ratio of tungsten was almost the same as the feed ratio while the ratio of vanadium was apparently lower than the feed ratio. The possible explanation was that the element ratio examined by XPS was only precise on the surface of the films while information of the bulk could not be detected. The elements ratio on the surface might not be the same with the real ratio because of high temperature process during preparation. The C element showed at peak of 285.1 eV might come from the instrument or the organic precursors. The position of peaks also confirmed the element oxidation state. The Bi 4f5/2 and Bi 4f7/2 peaks located at 164.6 eV and 159.5 eV respectively confirmed the existence of Bismuth in the film and the oxidation state to be Bi3þ [22]. The V 2p1/2 and V 2p3/2 peaks
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Fig. 1 e XPS spectra of BiWxV1LxO4Dx/2 films, x [ 0.1: (a) wide; (b) Bi 4f; (c) V 2p; (d) W 4f.
located at 524.7 eV and 517.0 eV provided proof of the presence of V5þ. The peaks for tungsten located at 37.6 eV and 35.4 eV which were ascribed to be W 4f5/2 and W 4f7/2, revealing the presence of W6þ [24]. The XPS results showed expected composition and oxidation states of the films. Fig. 2 shows the X-ray diffraction patterns of the BiWxV1xO4þx/2 films. The films were very thin so that the diffraction peaks of the substrate were also shown. We can find out that BiVO4 had sharp diffraction peaks in the region of 2q from 10 to 80 . The peaks at 2q ¼ 18.7 , 28.9 , 30.5 , 39.7 , 42.4 and 46.8 , especially the splitting at 46.8 confirmed that it was the scheelite-monoclinic (s-m) BiVO4 (JCPDS 14e0688),
Fig. 2 e X-ray diffraction patterns of BiWxV1LxO4Dx/2 films. Asterisks (*): the peaks of FTO substrate.
which was reported to show high photocatalytic activity [30]. When x ¼ 0.1, the position of the peaks were almost the same, which indicated that W was successfully composited in the compound and the basic monoclinic structure was not changed. A new slight peak at about 34.8 emerged and no WO3 peaks were observed, which means tungsten was successfully incorporated. The peaks became weaker when x value increased, which was caused by the replacement of W to V. However, the peaks were too small to be investigated afterward when x became larger because the films were too thin. From the XRD patterns of the series of films, an obvious tendency can be noticed that with x increasing the diffraction peaks move to low angle direction. This can be explained by A) and V5þ (0.52 A). The the different ion radii of W6þ (0.65 6þ bigger ion radius of W enlarged the crystal lattice and led to the shift. The optical transmittance spectra of the BiWxV1xO4þx/2 films are shown in Fig. 3. The films prepared uniformly on FTO glass showed color from vivid yellow to light green with x value increasing. The change of color is also confirmed in the optical transmittance spectra and more detailed information can be observed. We can see an obvious monotonic blue-shift of the films transmittance spectra with x value increasing gradually from the figure, which influenced the band gap energy to increase in accordance gradually. BiVO4 possessed a highest visible light absorption while the visible light absorption of BiWO4.5 was the weakest. The band gap of BiVO4 was estimated to be 2.4 eV in literature. With the addition of tungsten, the band gap became larger. The change was caused by the addition of W that the absorption of visible light became weaker in order, which was a disadvantageous aspect for photoelectrochemical reaction. Thus the band gap of the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 1 7 2 0 e1 1 7 2 6
Fig. 3 e UVeVis transmittance spectra of BiWxV1LxO4Dx/2 films.
BiWxV1xO4þx/2 films could be modulated by changing the composition. The photoelectrochemical properties were also affected in consequence, which would be discussed afterward. The films obtained from the polymer-assisted solegel method possessed porous surface morphology nanostructure, which was advantageous for PEC water splitting reaction. Photo-generated charge carriers could be separated more easily so that the efficiency could be improved. Fig. 4 shows the field emission scanning electron microscopy images of some representative prepared BiWxV1xO4þx/2 films. It can be easily seen that the films were well crystallized and had the
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nanostructure of porous particles, which benefited the transportation of the electrons and holes to improve the photoelectrochemical activities for water splitting. Furthermore, the surface areas were improved with the porous structure, which promoted the contact with solution in the photocatalytic reaction. The advantageous porous structure benefited from the polymer-assisted method during synthesis. The series of films were very thin because of the preparation method and the sizes of all of the particles were all about 80e100 nm. The change in the composition did not affect the size of particles apparently. Maybe due to the poor electrical conductivity of the films, the slight difference of their sizes could hardly be identified. The thicknesses of the films were estimated to be about 100 nm from the cross section image (Fig. S1). It is worth mentioning that the thicknesses of the films can be adjusted by repeating the spincoating procedure or modifying the viscosity and concentration of the precursor. The thickness data of previous work on BiVO4 and modified BiVO4 were usually larger and ranged from 140 nm to 1 mm [20,23,27]. The thickness of the film had a fatal influence on the light absorption and the photoelectrochemical activity, which would be discussed in detail in the next part. To investigate the photoelectrochemical properties, photocurrent measurements in a conventional three-electrode cell were made. Fig. 5 shows the photocurrentepotential properties of representative coated films under visible light in 0.5 M Na2SO4 aqueous solution, in which the films were stable. From the figure we can find out that the films had obvious high photocurrent activities. The dark current densities were tiny and close to zero, which meant no photocurrent response detected. As to BiVO4, the photocurrent appeared at the
Fig. 4 e SEM images of BiWxV1LxO4Dx/2 films, (a) x [ 0, (b) x [ 0.1, (c) x [ 0.2, (d) x [ 0.4.
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incorporation of Mo and W. With a modified SECM technique, Ye et al. showed that Bi/V/W oxide with an optimal ratio of 4.5:5:0.5 had w4.6 times higher photocurrent density than pure BiVO4 in 0.1 M Na2SO4 with 0.1 M Na2SO3 as a sacrificial reagent. This resulted composition was relatively close to our results. In 0.1 M Na2SO4 without sacrificial reagent, the photocurrent density of Bi/V/W (4.5/5/0.5) oxide bulk film was shown to be w0.1 mA/cm2 at 0.5 V vs. Ag/AgCl. IPCE measurements were also made to investigate photoelectrochemical properties over the solar spectrum per incident photon flux [31]. The IPCE was calculated according to the formula as follows: IPCE ¼ Fig. 5 e Photocurrentepotential curves of BiWxV1LxO4Dx/2 films.
threshold potential of about 0.3 V vs. SCE under visible light and the photocurrent increased steadily with the applied potential rising. When x equaled 0.1, a remarkable increase of the photocurrent was observed, which proved that the addition of tungsten was quite beneficial for the photoelectrochemical activity. The photocurrent density was promoted by w6 times in comparison with that of BiVO4 and the value was w0.06 mA/cm2 at 0.5 V vs. SCE (w1.15 V vs. RHE). When x equaled 0.7 or larger, the photocurrent appeared to decrease and the photocurrent of BiWO4.5 was even smaller than that of BiVO4. The main structure of monoclinic BiVO4 did not exist so that the PEC performance fell down sharply. This could also be partly explained with the optical transmission spectra figure. The absorption of visible light became weak with x value increasing and the band gap was also larger so that the photoelectrochemical activities decreased with the x value increasing. Another important reason might be that too much tungsten led to the enhancement of recombination of charge carriers. As we know, the photocurrent activity of the photoelectrode could be greatly affected by the thickness of the material. For films with low thickness, the amount of the materials was not much and the absorption of light irradiation would be weak, which led to low efficiencies. For films with too high thickness, the long transport distance for charge carriers led to undesirable recombination. In our work, the films were synthesized by a facile polymer-assisted solegel method and the deposition process was not repeated so that the films were relatively thin (w100 nm). Although the photocurrent density was an important factor, more attention was paid on the influence by the composition. The results by Berglund and coworkers showed the optimal amount of W incorporation to be 2.5%, which raised the photocurrent density by 7 times [27]. The films were fabricated by simultaneous evaporation of Bi, V, Mo, and W in vacuum. Various ratios of W (2.5%, 5%, 7.5%, 10%, 15%, and 20%) were investigated and 2.5% W showed the highest photocurrent density of w0.1 mA/cm2 at 1.1 V vs. RHE in 0.1 M Na2SO4 and 0.1 M phosphate buffer solution (pH 6.8). PEC tests of the films under Na2SO3 and with electrocatalyst Pt were also investigated and more attention was paid on the co-
hc Iph e Pl
(1)
where Iph is the photocurrent density, P is the power intensity of the light, and h, c, e are Planck’s constant, speed of light in vacuum and elementary charge, respectively. Fig. S2 shows the photocurrent of BiWxV1xO4þx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution on the wavelength from 300 to 600 nm. All the films showed different levels of photoelectrochemical activities. The figure was in agreement with the optical transmittance spectra very well. The corresponding wavelength in the curve of BiVO4 to the photocurrent was about 470 nm and this wavelength shifted to shorter side with the x value increasing. However, when x equaled 0.8 or larger, the photoelectrochemical activities only appeared in the area of wavelength under 400 nm and the activities were thus very low. It could be obviously seen that the best photoelectrochemical activity was showed in the film of BiW0.1V0.9O4.05, of which the photocurrent was much higher than the others and covered a wide area. Fig. 6 shows the IPCE plots of BiWxV1xO4þx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution. As to BiVO4, it could be observed that the threshold wavelength of photocurrent was about 500 nm. As the x value increased, this threshold wavelength became smaller, which agreed well with the optical transmittance spectra. The curves were all in the tendency of descending, which was the result of optical absorption. The IPCE maintained at a relatively high
Fig. 6 e IPCE of BiWxV1LxO4Dx/2 films electrodes at applied potential of 0.5 V vs. SCE in the aqueous 0.5 M Na2SO4 solution.
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and stable level in the range of 400e450 nm. Obviously, BiW0.1V0.9O4.05 showed the highest IPCE value. Compared with BiVO4, the addition of tungsten made a great enhancement on the photocurrent because W6þ promoted the density of charge carriers and the separation of the electrons and holes induced by light illumination so that the charge transfer was much easier. However, when x was larger, the charge recombination would be enhanced and the photo absorption of light irradiation became much weaker, which decreased the photoelectrochemical water splitting efficiency.
4.
Conclusion
A series of BiWxV1xO4þx/2 films have been successfully synthesized by a facile polymer-assisted solegel method and examined to show good photoelectrochemical activities. The films showed porous nanostructure property and the optical absorption was influenced by the ratio of W, which changed the band gap of the films. XPS results confirmed composition and oxidation states of the elements. XRD patterns showed the monoclinic structure of BiVO4 and the influence by W incorporation. The photoelectrochemical activities were investigated to be promoted by the addition of tungsten remarkably. The optimal composition BiW0.1V0.9O4.05 enhanced the photocurrent density by w6 times and the best IPCE at 450 nm reached 1.87%.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51121092) and the National Basic Research Program of China (Grant No. 2009CB220000).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.06.135.
references
[1] Weber MF, Dignam MJ. Splitting water with semiconducting photoelectrodes efficiency considerations. Int J Hydrogen Energy 1986;11:225e32. [2] Maeda K, Domen K. Photocatalytic water splitting: recent progress and future challenges. J Phys Chem Lett 2010;1:2655e61. [3] Licht S, Wang B, Mukerji S, Soga T, Umeno M, Tributsch H. Over 18% solar energy conversion to generation of hydrogen fuel; theory and experiment for efficient solar water splitting (reprinted from J Phys Chem B, vol 104, pg 8920e8924, 2000). Int J Hydrogen Energy 2001;26:653e9. [4] Delahoy AE, Gau SC, Murphy OJ, Kapur M, Bockris JOM. A one-unit photovoltaic electrolysis system based on a triple stack of amorphous-silicon (pin) cells. Int J Hydrogen Energy 1985;10:113e6.
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[5] Jing DW, Guo LJ, Zhao LA, Zhang XM, Liu HA, Li MT, et al. Efficient solar hydrogen production by photocatalytic water splitting: from fundamental study to pilot demonstration. Int J Hydrogen Energy 2010;35:7087e97. [6] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269e71. [7] Domen K, Kudo A, Onishi T. Mechanism of photocatalytic decomposition of water into H2 and O2 over NiOeSrTiO3. J Catal 1986;102:92e8. [8] Oregan B, Gratzel M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 1991;353:737e40. [9] Chen XB, Shen SH, Guo LJ, Mao SS. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503e70. [10] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [11] Liu MC, Jing DW, Zhao LA, Guo LJ. Preparation of novel CdSincorporated special glass composite as photocatalyst material used for catalyst-fixed system. Int J Hydrogen Energy 2010;35:7058e64. [12] Buhler N, Meier K, Reber JF. Photochemical hydrogenproduction with cadmium-sulfide suspensions. J Phys ChemUS 1984;88:3261e8. [13] Zhao X, Wu Y, Yao WQ, Zhu YF. Photoelectrochemical properties of thin Bi2WO6 films. Thin Solid Films 2007;515:4753e7. [14] Tang JW, Zou ZG, Ye JH. Photocatalytic decomposition of organic contaminants by Bi2WO6 under visible light irradiation. Catal Lett 2004;92:53e6. [15] Kudo A, Hijii S. H2 or O2 evolution from aqueous solutions on layered oxide photocatalysts consisting of Bi3þ with 6s2 configuration and d0 transition metal ions. Chem Lett 1999:1103e4. [16] Li MT, Zhao LA, Guo LJ. Preparation and photoelectrochemical study of BiVO4 thin films deposited by ultrasonic spray pyrolysis. Int J Hydrogen Energy 2010;35:7127e33. [17] Sayama K, Nomura A, Zou ZG, Abe R, Abe Y, Arakawa H. Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light. Chem Commun 2003:2908e9. [18] Yu JQ, Kudo A. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv Funct Mater 2006;16:2163e9. [19] Parmar KPS, Kang HJ, Bist A, Dua P, Jang JS, Lee JS. Photocatalytic and photoelectrochemical water oxidation over metal-doped monoclinic BiVO4 photoanodes. Chemsuschem 2012;5:1926e34. [20] Zhong DK, Choi S, Gamelin DR. Near-complete suppression of surface recombination in solar photoelectrolysis by “CoPi” catalyst-modified W: BiVO4. J Am Chem Soc 2011;133:18370e7. [21] Seabold JA, Choi KS. Efficient and stable photo-oxidation of water by a bismuth vanadate photoanode coupled with an iron oxyhydroxide oxygen evolution catalyst. J Am Chem Soc 2012;134:2186e92. [22] Pilli SK, Furtak TE, Brown LD, Deutsch TG, Turner JA, Herring AM. Cobalt-phosphate (Co-Pi) catalyst modified Mo-doped BiVO4 photoelectrodes for solar water oxidation. Energy Environ Sci 2011;4:5028e34. [23] Luo WJ, Yang ZS, Li ZS, Zhang JY, Liu JG, Zhao ZY, et al. Solar hydrogen generation from seawater with a modified BiVO4 photoanode. Energy Environ Sci 2011;4:4046e51. [24] Ye H, Lee J, Jang JS, Bard AJ. Rapid screening of BiVO4-based photocatalysts by scanning electrochemical microscopy (SECM) and studies of their photoelectrochemical properties. J Phys Chem C 2010;114:13322e8.
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[25] Park HS, Kweon KE, Ye H, Paek E, Hwang GS, Bard AJ. Factors in the metal doping of BiVO4 for improved photoelectrocatalytic activity as studied by scanning electrochemical microscopy and first-principles densityfunctional calculation. J Phys Chem C 2011;115:17870e9. [26] Liang YQ, Tsubota T, Mooij LPA, van de Krol R. Highly improved quantum efficiencies for thin film BiVO4 photoanodes. J Phys Chem C 2011;115:17594e8. [27] Berglund SP, Rettie AJE, Hoang S, Mullins CB. Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation. Phys Chem Chem Phys 2012;14:7065e75. [28] Liu HM, Nakamura R, Nakato Y. Promoted photo-oxidation reactivity of particulate BiVO4 photocatalyst prepared by a
photoassisted sol-gel method. J Electrochem Soc 2005;152:G856e61. [29] Su JZ, Guo LJ, Yoriya S, Grimes CA. Aqueous growth of pyramidal-shaped BiVO4 nanowire arrays and structural characterization: application to photoelectrochemical water splitting. Cryst Growth Des 2010;10:856e61. [30] Tokunaga S, Kato H, Kudo A. Selective preparation of monoclinic and tetragonal BiVO4 with scheelite structure and their photocatalytic properties. Chem Mater 2001;13:4624e8. [31] Chen ZB, Jaramillo TF, Deutsch TG, Kleiman-Shwarsctein A, Forman AJ, Gaillard N, et al. Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols. J Mater Res 2010:3e16.