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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 2012 Jun 13;109(29):11564-11569. doi: 10.1073/pnas.1204623109

Facile fabrication of an efficient BiVO4 thin film electrode for water splitting under visible light irradiation

Qingxin Jia a, Katsuya Iwashina a, Akihiko Kudo a,b,1
PMCID: PMC3406872  PMID: 22699499

Abstract

An efficient BiVO4 thin film electrode for overall water splitting was prepared by dipping an F-doped SnO2 (FTO) substrate electrode in an aqueous nitric acid solution of Bi(NO3)3 and NH4VO3, and subsequently calcining it. X-ray diffraction of the BiVO4 thin film revealed that a photocatalytically active phase of scheelite-monoclinic BiVO4 was obtained. Scanning electron microscopy images showed that the surface of an FTO substrate was uniformly coated with the BiVO4 film with 300–400 nm of the thickness. The BiVO4 thin film electrode gave an excellent anodic photocurrent with 73% of an IPCE at 420 nm at 1.0 V vs. Ag/AgCl. Modification with CoO on the BiVO4 electrode improved the photoelectrochemical property. A photoelectrochemical cell consisting of the BiVO4 thin film electrode with and without CoO, and a Pt counter electrode was constructed for water splitting under visible light irradiation and simulated sunlight irradiation. Photocurrent due to water splitting to form H2 and O2 was confirmed with applying an external bias smaller than 1.23 V that is a theoretical voltage for electrolysis of water. Water splitting without applying external bias under visible light irradiation was demonstrated using a SrTiO3∶Rh photocathode and the BiVO4 photoanode.

Keywords: hydrogen production, solar energy conversion, visible light response

The development of powdered photocatalysts and semiconductor photoelectrodes for water splitting have been studied extensively in view of utilization of solar energy, since the report of the Honda-Fujishima effect (1). There are advantageous and disadvantageous points for water splitting using the powdered photocatalysts and photoelectrochemical cells. Although a powdered photocatalyst system is simple, H2 and O2 are produced as a mixture. In contrast to it, photoelectrochemical cells give H2 separately from O2 gas. Moreover, even if powdered photocatalysts do not possess band potentials suitable for water splitting (the bottom of the conduction band < 0 V and the top of the valence band > 1.23 V vs. NHE at pH 0), water splitting may be achieved by application of these powdered photocatalysts to photoelectrodes with applying some external bias. However, the electrical conductivity of semiconductor electrode is indispensable, resulting in that the number of the photoelectrode materials is limited.

It has been reported that many metal oxides are active photocatalysts for water splitting into H2 and O2 stoichiometrically under UV light irradiation (2). Photoelectrodes, such as TiO2 (1, 3, 4), SrTiO3 (5, 6), BaTiO3 (7), and KTaO3 (8) have been reported for photoelectrochemical water splitting under UV light irradiation. Development of a photoelectrode material with visible light response has been sought for efficient utilization of solar energy. It has been reported that Fe2O3 (9–11), WO3 (12–14), BiVO4 (15–24), and SrTiO3∶Rh (25) of metal oxide electrodes respond to visible light. Recently, some (oxy) nitride materials such as TaON (26, 27), Ta3N5 (27, 28), SrNbO2N (29), and Ta0.9Co0.1Nx (30) have also been found to be visible light responsive photoelectrodes for water splitting.

BiVO4 has three crystal systems: Scheelite structure with monoclinic (s-m) and tetragonal (s-t) phases, and zircon structure with tetragonal (z-t) phase (31–34). Among them, the scheelite monoclinic BiVO4 shows the highest photocatalytic activity for O2 evolution from an aqueous AgNO3 solution under visible light irradiation (19, 34–36). However, BiVO4-powdered photocatalyst is inactive for overall water splitting into H2 and O2, because the bottom of conduction band is more positive than the reduction potential of H2O to form H2. BiVO4, with an n-type semiconductor character, shows an excellent photoelectrochemical property (15). The photoelectrochemical cell consisting of BiVO4 can split water under visible light irradiation (16, 19). There are mainly two methods for the preparation of BiVO4 electrodes: BiVO4 powder-loaded electrodes and BiVO4-coated electrodes using metal organic precursors. The BiVO4 photoelectrode prepared by the metal-organic decomposition method gives 20% of IPCE at 420 nm with applying 0.6 V vs. Ag/AgCl for water splitting (15). It also works with 1.6 V of an externally applied bias vs. a counter electrode (C.E.) under visible light irradiation (16). A BiVO4 photocatalyst electrode that is readily prepared by pasting BiVO4 nanoparticle prepared in an acetic acid onto an FTO substrate can split water with an externally applied bias smaller than 1.23 V vs. a Pt counter electrode under simulated sunlight irradiation (19). It is important to develop a simple and large-scale producible preparation method for the photoelectrode.

Surface modification is a useful method to improve photoelectrochemical properties. Modification with Co3O4 by precipitation method increases the photocurrent of BiVO4 electrode (18). Recently, cobalt phosphate (CoPi) prepared by an electro-deposition has been reported as an oxygen evolving electrocatalyst (37). Photoelectrochemical properties of Si (37, 38), Fe2O3 (39–41), WO3 (42), BiVO4 (43, 44), and BiVO4 doped with Mo and W (45, 46) electrodes are improved with the surface modification with the CoPi cocatalyst. Modification with FeOOH is also effective (47).

In the present study, we developed a facile fabrication of an efficient BiVO4 thin film coated on FTO electrode using inorganic salts of bismuth and vanadium. The formation mechanism of the present BiVO4 thin film electrode was discussed. Photoelectrochemical property of the obtained BiVO4 thin film electrode with and without surface modification with cobalt oxide was examined. Water splitting was also demonstrated using photoelectrochemical cells consisting of a Pt counter electrode or a SrTiO3∶Rh photocathode under visible light and simulated sunlight irradiation.

Results and Discussion

Preparation of BiVO4 Thin Film Electrode.

Fig. 1 shows XRD patterns of the obtained BiVO4 thin film electrodes prepared under different condition. The diffraction pattern of BiVO4 (s-m) was observed for the sample calcined at 673 K for 2 h after drying an FTO substrate treated with an aqueous nitric acid precursor solution dissolving Bi(NO3)3 and NH4VO3, in addition to the XRD pattern of SnO2 derived from the FTO substrate (Fig. 1A). Thus, BiVO4 thin film with scheelite structure with monoclinic phase (s-m) that showed high photocatalytic activity was obtained by this simple preparation method. The crystal structure of BiVO4 thin films prepared at various calcination temperatures was examined (Fig. 1 B–E). A soda-lime glass was used as a substrate for these XRD measurements, because the (002) diffraction peak of BiVO4 was overlapped with the (101) diffraction peak of FTO. BiVO4 (s-m) was obtained by calcination at 573 and 623 K as a major component, in addition to a minor product of zircon structure tetragonal phase BiVO4 (z-t). A single phase of BiVO4 (s-m) was obtained by calcination at 673 and 723 K. Fig. 2 shows SEM images of surface and cross-section of BiVO4 thin films prepared at various calcination temperatures. An FTO substrate without the BiVO4 thin film was treated with 2 mmol L-1 of an aqueous nitric acid solution and subsequently calcined at 673 K for 2 h as a reference sample (Fig. 2A). It was observed that an FTO substrate was dense aggregates of crystal with bumpy surface. The morphologies of the BiVO4 thin films changed with calcination temperature. The thin film without calcination was sponge-like and orange. Calcination at 573 K gave network structure for the thin film. The thin film began to sinter by calcination at 623 K and became dense and porous by calcination at 673 K. The cross-sectional image revealed that the thickness of the BiVO4 porous film was several hundred nanometers. This morphology was quite different from that of powder-loaded electrodes (19). These network and porous structures were considered to be formed by volatilization of water and HNO3, and decomposition of some by-products, during the calcination as mentioned later. The thin film considerably sintered to form necked nano-particles by calcination at 723 K. Thus, this simple preparation method gave a porous BiVO4 (s-m) thin film well attached on the FTO substrate. UV-vis. transmission spectra of BiVO4 thin films showed that the thickness was controllable by changing the concentration (20–300 mmol L-1) of Bi(NO3)3 and NH4VO3 dissolving in an aqueous nitric acid precursor solution as shown in Fig. S1. The wavy spectra observed in the wavelength range of 500–800 nm was due to light interference.

Fig. 1.

Fig. 1.

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X-ray diffraction patterns of BiVO4 thin films formed on (A) FTO and (B–E) soda-lime glass substrates. Preparation conditions, calcined at (B) 573 K, (C) 623 K, (A, D) 673 K, and (E) 723 K for 2 h.

Fig. 2.

Fig. 2.

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Scanning electron microscope images of (A) FTO substrate, (B–F) surface and (G) cross-section of BiVO4 thin film electrodes. Preparation conditions, (B) dried and calcined at (C) 573 K, (D) 623 K, (E, G) 673 K, and (F) 723 K for 2 h.

Mechanism of Formation of BiVO4 Thin Film.

The formation process of BiVO4 thin films from an aqueous nitric acid precursor solution containing Bi(NO3)3 and NH4VO3 was examined with XPS and TG. The precursor solution covered on an FTO substrate became an orange film after drying, and a bright yellow film after calcination. The chemical state of the orange film was examined by XPS and TG-DTA. XPS spectra of each element of the orange film were different from those of the BiVO4 thin film as shown in Fig. 3. The binding energies of Bi4f7/2 (159.0 eV), V2p3/2 (516.7 eV), and O1s (529.7 eV) of the BiVO4 thin film calcined at 673 K were coincident with literature’s data of BiVO4 (s-m) (48, 49). The deconvoluted O1s peak at 531.6 eV was assigned to surface hydroxyl groups (50, 51). Inline graphic was almost decomposed by the calcination. Binding energies of Bi4f (159.5 eV) and V2p (517.5 eV) of the orange film without calcination were larger than those of the calcined BiVO4 thin film, whereas they were close to those of (BiO)2Cr2O7 at 159.8 eV (52) and NaVO3 at 517.0 eV (53), respectively. It suggested that Bi and V in the orange film took the chemical states similar to [BiO]+ and [VO3]- species, respectively. The XPS spectra of N1s at 401.8 eV and 406.7 eV were assigned to Inline graphic and Inline graphic (54), respectively. The O1s peak at 532.4 eV well agreed with that of Inline graphic in NH4NO3 (532.5 eV) (54). These results indicated that [BiO]+, Inline graphic, Inline graphic, and Inline graphic were contained in the orange film before calcination. TG-DTA measurement was carried out to examine the formation temperature of the BiVO4 thin film from these chemical species as shown in Fig. S2. Significant loss of weight was observed around 480 K accompanied with an exothermic process and a subsequent endothermic process. This exothermic process was probably due to the chemical reaction heat of [BiO]+ with Inline graphic to form BiVO4. The loss of weight with the endothermic process was mainly caused by the decomposition of NH4NO3 to release N2O and NH3 gases (55). These results led the formation process of the BiVO4 thin film as shown in Fig. S3. When Bi(NO3)3 of a starting material was dissolved in an aqueous nitric acid solution, [BiO]+ and Inline graphic species formed (56). The bright yellow color of the precursor solution suggested the existence of Inline graphic species that was formed from Inline graphic under a strongly acidic condition (57). Inline graphic species changed to Inline graphic after drying, and these chemical species turned into NH4NO3 and [BiO]+[VO3]-. Calcination above 480 K caused the decomposition of NH4NO3 and the reaction of [BiO]+[VO3]- to form BiVO4. Porous structure of the thin film observed by SEM (Fig. 2C) formed accompanied with running out of gaseous products.

Fig. 3.

Fig. 3.

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X-ray photoelectron spectra of BiVO4 thin film electrode. (A) Before and (B) after calcination at 673 K for 2 h.

Photoelectrochemical Property of BiVO4 Thin Film Electrode.

Effects of calcination temperature and film thickness on photoelectrochemical properties of BiVO4 thin films were examined as shown in Fig. 4. BiVO4 thin film electrodes calcined at different temperatures gave anodic photocurrents under visible light irradiation. The anodic photocurrent increased as the calcination temperature was raised. The photocatalytic activity of the BiVO4 (z-t) powder was lower than that of BiVO4 (s-m) for O2 evolution from an aqueous AgNO3 solution under visible light irradiation (35). The thin film containing BiVO4 (z-t) calcined at 573 and 623 K gave relatively small photocurrents corresponding to the photocatalytic property. The thin film electrodes with a single phase of BiVO4 (s-m) calcined at 673–723 K showed large photocurrents. The increase in the anodic photocurrent with the increase in the calcination temperature was due to the improvement of the denseness of the thin film as observed by SEM, and the purity and crystallinity of scheelite-monoclinic phase as confirmed by XRD. On the other hand, as the thickness of the BiVO4 film was increased, the anodic photocurrent became large. The BiVO4 thin film prepared using 200–300 mmol L-1 of precursor solutions gave an excellent photoelectrochemical property. Thicknesses of these thin films were sufficient for light absorption. These results concluded that the optimum calcination temperature and the concentration of the precursor solution were 673–723 K and 200–300 mmol L-1, respectively. Fig. 5 shows action spectra of the optimized BiVO4 thin film electrode. The onset of the IPCE agreed well with that of an absorption edge of BiVO4. This result indicated that the anodic photocurrent attributed to the band gap transition from the valence band of hybrid Bi6s-O2p orbitals to the conduction band of V3d (35). IPCE increased with an increase in the electrode potential. The BiVO4 thin film electrode gave an excellent anodic photocurrent with 73% of an IPCE at 420 nm at 1 V vs. Ag/AgCl. Modification with cobalt oxide on the BiVO4 electrode improved the photoelectrochemical property as previously reported (43, 45, 46). The onset potential of the anodic photocurrent for the cobalt oxide-modified BiVO4 electrode was more negative than that for the non-modified electrode as shown in Fig. S4. This result indicated that the loaded cobalt oxide improved the photoelectrochemical performance of the BiVO4 thin film electrode. XPS was carried out to examine the chemical state of the cobalt oxide loaded on the BiVO4 thin film electrode. Fig. S5 shows the Co2p and P2p spectra of cobalt oxide-modified BiVO4 electrode before and after photoelectrochemical water splitting. The XPS spectra of Co2p at 780.1 eV accompanied with the satellite were assigned to Co2+ in CoO. The peak of P2p was not detected, suggesting that the cobalt compound should be CoO. An average ratio of Co to Bi was estimated to be about 1∶1 by XPS.

Fig. 4.

Fig. 4.

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Current vs. potential curves of BiVO4 thin film electrodes under visible light irradiation. Concentration of precursor solution and calcination temperature, (A) 200 mmol L-1, 573 K, (B) 200 mmol L-1, 623 K, (C) 20 mmol L-1, 673 K, (D) 50 mmol L-1, 673 K, (E) 100 mmol L-1, 673 K, (F) 200 mmol L-1, 673 K, (G) 300 mmol L-1, 673 K, and (H) 200 mmol L-1, 723; calcination time, 2 h. Electrolyte, 0.1 mmol L-1 of an aqueous K2SO4 solution; sweep rate, 20 mV s-1; light source, 300 W Xe-lamp with a cut-off filter (λ > 420 nm).

Fig. 5.

Fig. 5.

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(A–D) Action spectra and (E) a diffuse reflection spectrum of BiVO4 thin film electrode. Preparation condition, calcined at 673 K for 2 h. Electrolyte, 0.1 mmol L-1 of an aqueous K2SO4 solution; potential, (A) 1 V, (B) 0.8 V, (C) 0.6 V, and (D) 0.4 V vs. Ag/AgCl.

Photoelectrochemical Overall Water Splitting.

It is important to see the stability of photocurrent and to determine the amounts of evolved H2 and O2 for photoelectrochemical water splitting, because we cannot always guarantee that obtained photocurrent is due to water splitting. Fig. 6 shows photoelectrochemical water splitting using a BiVO4 thin film (1.5 cm2) with 1.0 V of an applied potential vs. Ag/AgCl. Steady photocurrent was obtained after 30 minutes and the rate of H2 evolution agreed well with the photocurrent (Fig. 6B). H2 and O2 evolved in a stoichiometric ratio from Pt and BiVO4 electrodes, respectively. The amount of H2 evolved was similar to the half of the amount of electron passing through the outer circuit (Fig. 6A). This result indicated that the observed photocurrent was due to water splitting. Photoelectrochemical water splitting was also examined using a solar simulator (AM1.5) with applying an external bias smaller than 1.23 V that was a theoretical voltage required for electrolysis of water in order to see if the present photoelectrochemical cell consisting of BiVO4 thin film and Pt electrodes works for solar energy conversion as shown in Fig. 7A. The solar energy conversion efficiency was estimated to be 0.005% with applying an external bias at 1 V vs. C.E. Large photocurrent was obtained for the CoO-modified BiVO4 thin film even applying a smaller external bias (0.6 V vs. C.E.) as shown in Fig. 7B. The solar energy conversion efficiency was estimated to be 0.04% with 0.6 V vs. C.E.

Fig. 6.

Fig. 6.

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(A) Photoelectrochemical overall water splitting over BiVO4 thin film electrode and (B) stability of photocurrent using BiVO4 thin film electrode and Pt counter under visible light irradiation. Dashed line: the half of the amount of electron, open square: the rate of H2 evolution. Preparation condition, calcined at 673 K for 2 h. Electrolyte, 0.1 mmol L-1 of an aqueous K2SO4 solution; potential, 1 V vs. Ag/AgCl; electrode area, 1.5 cm2; light source, 300 W Xe-lamp with a cut-off filter (λ > 420 nm).

Fig. 7.

Fig. 7.

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Stability of photocurrents of (A) BiVO4 and (B) CoO-modified BiVO4 thin film electrodes under simulated sunlight irradiation (AM1.5). Preparation condition, calcined at 673 K for 2 h. Electrolyte, 0.1 mmol L-1 of an aqueous K2SO4 solution (phosphate buffer, pH = 6.9); applied voltage, (A) 1 V, (B) 0.6 V vs. C.E.

A photoelectrochemical cell consisting of a SrTiO3∶Rh (7%) photocathode (25) and the present BiVO4 thin film photoanode was constructed for water splitting. Photoelectrochemical water splitting proceeded under visible light irradiation with applying non-bias, as shown in Fig. 8. Steady photocurrent was obtained and the rate of H2 evolution agreed well with the photocurrent (Fig. 8B). H2 and O2 evolved in a stoichiometric ratio on the SrTiO3∶Rh and BiVO4 electrodes, respectively. The amount of H2 evolved was similar to the half of the amount of electron passing through the outer circuit (Fig. 8A). Thus, we succeeded in constructing a photoelectrochemical cell consisting of metal oxide materials for water splitting without applying an external bias.

Fig. 8.

Fig. 8.

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(A) Photoelectrochemical overall water splitting and (B) stability of photocurrent using BiVO4 thin film electrode and SrTiO3∶Rh photocatalyst electrode under visible light irradiation. Dashed line: the half of the amount of electron, open square: the rate of H2 evolution. Electrolyte, 0.1 mmol L-1 of an aqueous K2SO4 solution; applied voltage, 0 V; electrode area, 4 cm2 each; light source, 300 W Xe-lamp with a cut-off filter (λ > 420 nm).

In summary, we have developed a facile fabrication of an efficient BiVO4 thin film electrode for overall water splitting from an aqueous nitric acid solution of Bi(NO3)3 and NH4VO3. It was revealed that the chemical species contained in the starting materials reacted with each other to form BiVO4 (s-m) via BiVO4 (z-t) by calcination. The BiVO4 (s-m) electrode showed a larger anodic photocurrent than BiVO4 (z-t). This was consistent with the order in an activity of powdered BiVO4 photocatalyst for O2 evolution from an aqueous AgNO3 solution under visible light irradiation. Anodic photocurrent of the BiVO4 thin film electrode depended on the concentration of a precursor solution and calcination temperature. The BiVO4 thin film electrode prepared at 673–723 K using 200–300 mmol L-1 of the precursor solution gave an excellent anodic photocurrent with 73% of an IPCE at 420 nm at 1 V vs. Ag/AgCl. Photoelectrochemical water splitting using the BiVO4 thin film proceeded with applying an external bias smaller than 1.23 V, which is a theoretical voltage required for electrolysis of water. The amount of H2 evolved was similar to the half of the amount of electron passing through the outer circuit, indicating that the photocurrent was due to water splitting. Modification with CoO on the BiVO4 electrode improved the photoelectrochemical property. The metal oxide photoelectrochemical system was constructed for water splitting under visible light irradiation with applying no external bias employing a SrTiO3∶Rh photocathode and the BiVO4 thin film photoanode.

Materials and Methods

Preparation of BiVO4 Thin Film and SrTiO3∶Rh Electrodes.

When 0.2–3 mmol of Bi(NO3)3 (Kanto Chemical; 99.9%) and NH4VO3 (Kanto Chemical; 99.0%) at an equal molar ratio was dissolved in 10 mL of an aqueous nitric acid solution (2 mol L-1), a yellow precursor solution was obtained. An FTO (F-doped SnO2) transparent substrate electrode (AGC Fabritech; A-110U80, sheet resistance: 20 Ω sq-1) was dipped in the precursor solution for 1–2 s. Then, the FTO substrate was covered with the yellow liquid of the precursor solution. After dryness at room temperature, an orange film was obtained on the FTO. Calcination of this film at 573–723 K for 2 h in air gave a yellow BiVO4 thin film. Cobalt oxide cocatalyst was loaded on the BiVO4 film with an area of 2.5 cm2 by an impregnation method with calcination at 673 K for 1 h in air using an aqueous solution (5 μL, 80 mmol L-1) of Co(NO3)2 (Wako Pure Chemical; 99.5%), if necessary (17, 18).

SrTiO3 powder doped with 7 mol% of Rh to a Ti site was prepared by a solid state reaction in order to obtain a photocathode for the construction of a photoelectrochemical cell (25). Starting materials, SrCO3 (Kanto Chemical; 99.9%), TiO2 (Soekawa Chemical; 99.9%), and Rh2O3 (Wako Pure Chemical) were mixed with the ratio of Sr∶Ti∶Rh = 1.07∶0.93∶0.07. The mixture was calcined in air at 1,173 K for 1 h and then at 1,373 K for 10 h in an alumina crucible. SrTiO3∶Rh photocatalyst electrodes were prepared by coating the paste composed of 20 mg of SrTiO3∶Rh photocatalyst powders, 20 μL of acetylacetone (Kanto Chemical; 99.5%), and 40 μL of distilled water on an indium tin oxide (ITO) transparent electrode and then calcining at 573 K for 2 h in air (25).

Characterization.

The crystal form of obtained BiVO4 thin film was confirmed by X-ray diffraction (Rigaku; miniFlex). Diffuse reflectance spectra were obtained using a UV-vis-NIR spectrometer (Jasco; Ubest-570) and were converted from reflection to absorbance by the Kubelka-Munk method, if necessary. The formation process of the BiVO4 thin film from the precursor solution were examined using a thermobalance (Ulvac; TGD-9600) for thermogravimetric–differential thermal analyses (TG-DTA) and an X-ray photoelectron spectrometer (Jeol; JPS-9010MC) with a Mg Kα (1253.6 eV) target. Obtained XPS spectra were corrected by C1s of adventitious carbon (284.6 eV). The morphology of obtained BiVO4 thin films was observed by scanning electron microscopy (Jeol; JSM-6700F).

Photoelectrochemical Measurement.

Photoelectrochemical properties were evaluated using a potentiostat (Hokuto Denko; HZ-5000 or HSV-100) with a three-electrode H-type cell divided into working and counter electrode compartments by Nafion 117 (DuPont). The BiVO4 thin film electrode, a Pt electrode, and a saturated Ag/AgCl electrode (DKK TOA; HS-205C) were used as working, counter, and reference electrodes, respectively. A SrTiO3∶Rh electrode was also employed as a photocathode (25). 0.1 mol L-1 of an aqueous K2SO4 (Kanto Chemical; 99.0%) solution was used as an electrolyte. An aqueous phosphate buffer solution [0.025 mol L-1 of KH2PO4 (Kanto Chemical; 99.6%) +0.025 mol L-1 of Na2HPO4 (Kanto Chemical; 99.5%)] was employed to adjust the pH to 6.9, if necessary.

Photoelectrochemical water splitting was carried out in an Ar gas flow system at 10 mL min-1 of a flow rate. The amounts of evolved gases were determined with gas chromatography (Shimadzu; MS-5A column, Ar carrier, TCD). The light source was a 300 W Xe-lamp (PerkinElmer; Cermax- PE300BF). Wavelength of the incident light was controlled by cut-off filters (HOYA), an NIR-absorbing filter (Sigma Koki; CCF-50S-500C), and a Plano convex lens (Sigma Koki; SLSQ- 60-150P). The working electrode was irradiated with light from an FTO side. The electrolytes in both compartments were bubbled with N2 or Ar before measurements for removal of dissolved O2. A 300 W Xe-lamp (Ashahi Spectra; MAX-301) with band-pass filters was employed for IPCE measurement. The number of incident photons was measured using a photodiode head (OPHIRA; PD300-UV) and a power monitor (NOVA). IPCE was calculated according to Eq. 1.

graphic file with name pnas.1204623109eq15.jpg [1]

A solar simulator (Peccell Technologies; PEC-L11, 100 mW cm-2) was employed for solar energy conversion determined by Eq. 2.

graphic file with name pnas.1204623109eq16.jpg [2]

Eapply represents of the applied external bias between working and Pt counter electrodes.

Supplementary Material

Supporting Information
supp_109_29_11564__index.html (924B, html)

ACKNOWLEDGMENTS.

This work was supported by Toyota Motor Corporation.

Footnotes

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1204623109/-/DCSupplemental.

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