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Jul 8, 2009 - Division of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan, The Institute of Scien...
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J. Phys. Chem. C 2009, 113, 14575–14581

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Efficient Solar Water Splitting with a Composite “n-Si/p-CuI/n-i-p a-Si/n-p GaP/RuO2” Semiconductor Electrode Satoshi Yamane,† Naoaki Kato,† Shinji Kojima,† Akihito Imanishi,*,†,‡ Shunsuke Ogawa,§ Norimitsu Yoshida,‡,§ Shuichi Nonomura,‡,§ and Yoshihiro Nakato‡,| DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, The Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, Ibaraki, Osaka, 567-0047, Japan, EnVironmental and Renewable Energy Systems DiVision, Graduate School of Engineering, Gifu UniVersity, 1-1 Yanagido, Gifu 501-1193, Japan, and CREST, JST, Kawaguchi, Saitama 332-0012, Japan ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: June 14, 2009

A composite semiconductor electrode with the structure “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2” was fabricated for the purpose of achieving efficient solar water splitting. The electrode showed a stable photoanodic current due to oxygen evolution with a large negative photoshift (Vp) of about 2.2 V from an anodic current at a RuO2 electrode. The photoshift was large enough for full water splitting. A photoelectrochemical (PEC) cell, composed of the composite electrode, a Pt counter electrode, and 0.10 M Na2SO4 (pH 6.3), generated a photocurrent density of 1.88 mA cm-2 under simulated solar illumination (AM 1.5 G, 100 mW cm-2), yielding a solar to chemical conversion efficiency of 2.3% as calculated from the photocurrent value. The result has shown that the combination of “crystalline Si/a-Si/GaP” is suitable for efficient solar water splitting. It is shown that the efficiency can be increased by use of GaP with a well-regulated p-n junction. Introduction The main target in recent studies on photovoltaic solar energy conversion is to realize a practically applicable low-cost conversion system with a sufficiently high efficiency and longterm durability. Of a number of approaches thus far studied, an interesting approach is direct solar to chemical conversion, such as solar water splitting, by use of a semiconductor/electrolyte junction.1-16 This approach has merits that (1) no current collection is necessary and a decrease in conversion efficiency by ohmic losses is minimized and (2) a storable fuel such as hydrogen is directly produced. Direct solar to chemical conversion can be divided into two types: a photocatalyst type17-34 and a photoelectrode type.1-16,35-40 The photocatalyst type has a strong merit in that the fabrication cost can be made extremely low but has a demerit in that it is quite difficult to obtain high conversion efficiencies owing to efficient carrier recombination and reverse reactions. In fact, the solar conversion efficiencies reported to date have remained very low values.21,27-29,31,34 On the other hand, in the photoelectrode type, carrier recombination and reverse reactions can in principle be avoided by the use of a Schottky or p-n junction. The photoelectrode type has another merit in that oxygen and hydrogen are produced separately. The main difficulty in this type is that it is not easy to find an efficient and stable semiconductor electrode. For example, titanium dioxide (TiO2) is stable and can photooxidize water into oxygen and H+ ions, as first reported by Fujishima and Honda,1 but it only absorbs UV light, thus resulting in a very low conversion efficiency of ca. 0.4%. Fairly recently, stacked high-quality p-n junction solar cells prepared by the * To whom correspondence should be addressed. E-mail: imanishi@ chem.es.osaka-u.ac.jp. † Graduate School of Engineering Science, Osaka University. ‡ JST. § Gifu University. | ISIR, Osaka University.

MOCVD technique were used as electrodes for high-efficiency solar water splitting,6-10 but unfortunately, these electrodes were expensive for practical application. Very recently, several studies have been made on low-cost oriented composite semiconductor electrodes.2,11-16 The conversion efficiencies for these electrodes are not high enough yet, but the electrodes of this type are of much interest because they have the potential ability to achieve both a high efficiency and low cost. Recently, we reported that solar cells with a structure of “ITO/ p-CuI/n-Si” generated a very high open-circuit photovoltage (Voc) of 0.617 V without surface texturing and a back surface field treatment.41 Investigations of current-voltage characteristics have shown that the p-CuI/n-Si contact forms an ideal minority-carrier controlled junction suitable for the generation of high Voc owing to a morphologically soft property of a p-CuI overlayer as well as a low density of surface states at H-terminated or CH3-terminated n-Si(111). Accordingly, we in the present work have investigated the performance of a composite electrode (including an n-Si/p-CuI contact) with the structure “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ ITO/RuO2” for the purpose of achieving efficient solar water splitting. Theoretical calculations have indicated that a tandemtype solar cell composed of Si (Eg ) 1.1 eV) and GaP (2.3 eV) can realize an ∼36% solar conversion efficiency.42 However, the combination of Si and GaP did not generate a sufficiently high photovoltage to cause full water splitting. Thus, we have here adopted the combination of crystalline Si, a-Si, and GaP for producing a photoelectrode for full water splitting. The latter type of electrode actually generated a high photovoltage of about 2.2 V enough for full water splitting. Note that the elements of Ga and P have relatively high natural reserves (the Clarke number of Ga is similar to that of Zn or Pb). Besides, transparent conductive oxide (TCO) films used in the composite electrode can be of an inexpensive low-conductivity type because a photocurrent flows only across the thin film.

10.1021/jp904297v CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Experimental Section The hetero-pn-junction of p-CuI/n-Si was obtained by the vacuum deposition of p-CuI on n-Si. Single crystal n-Si(111) wafers with a resistivity of 1-5 Ω cm and a thickness of 825 ( 25 µm were donated by Osaka Tokushu-Gokin, Co. Ltd. The n-Si surface was cleaned by the RCA cleaning method43 [i.e., successive immersion in a boiling mixture of 95% H2SO4 and 30% H2O2 (3:1 in volume) for 15 min, 5% HF for 5 min, and a boiling mixture of 25% aqueous NH3, 30% H2O2, and water (1:1:5 in volume) for 15 min]. The hydrogen (H)-terminated Si surface was obtained by immersion in 5% HF for 5 min, followed by immersion in 40% NH4F for 15 min.44-47 Atomic force microscopic inspection has shown that the Si surfaces prepared in the same way have a well-defined step and terrace structure, as reported in previous papers.48-51 The vacuum deposition of p-CuI was performed using a tungsten boat heated at about 600 °C under 1.0 × 10-3 Pa. The use of low-purity CuI as the source material did not give a CuI layer of good quality, and special-grade CuI (purity 99.99%, Aldrich) was used as the source material. The thickness of the CuI layer was monitored with a quartz oscillator, and the average thickness was about 300 nm. An ITO layer was deposited on p-CuI by use of the DC magnetron sputtering technique, under an Ar atmosphere of 1.0 × 10-4 Pa. The applied power was 20 W. A commercial ITO wafer was used as the target. Amorphous Si layers with an n-i-p junction (abbreviated as n-i-p a-Si) were prepared at Gifu University. Strictly speaking, the n-i-p a-Si layer had the structure of stacked n-type microscrystalline (µc) 3C-SiC:H (25 nm thick, Eg about 2.2 eV), i-type a-Si:H (400 nm thick, Eg about 1.7 eV), and p-type a-SiCx:H (25 nm thick) layers. First, an n-(µc) 3C-SiC:H layer was deposited on a TiO2 (40 nm thick)-covered F-doped SnO2 (FTO, Asahi type-U)/glass plate by the hot-wire CVD method. Hydrogen-diluted monomethylsilane (CH3SiH3) gas (2 Pa) including phosphine (PH3) was employed as the source gas. The filament temperature and the substrate temperature were 1700 and 300 °C, respectively. Next, an i-type a-Si:H layer was deposited on n-(µc) 3C-SiC:H by the RF plasma CVD method with hydrogen-diluted silane (SiH4) gas of 30 Pa used as the source gas. Finally, a p-type a-SiCx:H layer was deposited on the i layer by the hot-wire CVD method with hydrogen-diluted SiH3CH3 (1 Pa), SiH4 (2 Pa), and B2H6 (45 Pa) as the source gas. The temperature of the filament and substrate was kept at 1550 and 300 °C, respectively. A zinc oxide (ZnO) layer 100 nm thick was deposited on the p-a-SiCx:H layer by the DC magnetron sputtering method. Single crystal “GaP” wafers with n-p junctions (abbreviated as n-p GaP) were purchased from Iwaki semiconductor Co. The “GaP” wafers were prepared for use in LED devices and contained a certain amount of As in the region of the n-p junction. The wafers had n-p junctions suitable for LED but not suitable for solar cells, thus yielding relatively low photocurrents. However, the wafers were the only n-p GaP materials that were available at present. A piece of the wafer of 1 × 1 cm2 in area was washed with acetone under ultrasonic irradiation, followed by electrochemical etching in 0.1 M NaOH at an applied potential of 0 V vs Ag/AgCl/sat.KCl in order to increase photocurrents. The electricity passing across the electrode surface was about 900 mC/cm2. The wafer was then further etched in aqua regia to improve the light transmittance. Ohmic contact was obtained at the back side of the n-p GaP wafer by applying an indium-gallium alloy in the shape of cross stripes, and the wafers thus treated were heated at 500 °C for 60 min. On the front side of the n-p GaP wafer, an ITO layer

Figure 1. (a) j vs U curves for “n-i-p a-Si/n-p GaP/ITO/RuO2”, “n-p GaP/ITO/RuO2”, and “ITO/RuO2” electrodes in 0.1 M Na2SO4, with simulated solar illumination for the former two electrodes. (b) The j vs U curve (in the dark) for a Pt electrode in the same 0.1 M Na2SO4.

was deposited by the DC magnetron-sputter technique. After the electrodeposition of RuO2 on the ITO layer in aq. 5 mM RuCl3,52 the GaP wafer was finally annealed at 350 °C for 1 h. A composite n-i-p a-Si/n-p GaP/ITO/RuO2 electrode was prepared by mechanically pressing n-i-p a-Si to n-p GaP/ITO/ RuO2 using a clamp. A composite n-Si/p-CuI/ITO/n-i-p a-Si/ n-p GaP/ITO/RuO2 electrode was obtained by electrically connecting the ITO layer on p-CuI and the F-doped SnO2 layer of n-i-p a-Si with a fine Cu wire. Photocurrent densities (j) vs potential (U) were measured with a commercial potentiostat (Hokuto-Denko HSV-100) and a potential programmer (Hokuto-Denko HSV-100), using a Pt plate as the counterelectrode and a Ag/AgCl/sat.KCl electrode as the reference electrode. A solar simulator (Kansai Kagaku Kikai XES-502S, AM 1.5 G, 100 mW cm-2) was used as the light source. The morphology of the electrode at the surface and the cross section were inspected with a Hitachi S-5000 highresolution scanning electron microscope (SEM). X-ray photoelectron spectroscopic (XPS) analysis was performed with a Shimadzu ESCA-1000 spectrometer using a Mg KR line. UV-vis spectra were measured with a JASCO V-570 spectrophotometer. Results Figure 1a shows typical j vs U curves for “n-i-p a-Si/n-p GaP/ ITO/RuO2”, “n-p GaP/ITO/RuO2”, and “ITO/RuO2” electrodes in 0.10 M Na2SO4 (pH 6.3) under simulated solar illumination. No dark current was observed for both electrodes in the potential region where an anodic photocurrent was measured. A schematic structure of an “n-i-p a-Si/n-p GaP/ITO/RuO2” electrode and a band diagram to explain the photoinduced water oxidation on it are shown in Figure 2. The anodic photocurrent, attributable to water oxidation, for “n-i-p a-Si/n-p GaP/ITO/RuO2” increases with the potential fairly steeply, in a similar manner to that for n-p GaP/ITO/RuO2, indicating that good electrical contact between a-Si and GaP is obtained, though a-Si and GaP were

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Figure 2. Schematic structure of the “n-i-p a-Si/n-p GaP/ITO/RuO2” electrode and an expected band diagram for the electrode under solar irradiation. λ denotes the wavelength of the light.

simply pressed mechanically with a clamp. The addition of n-i-p a-Si to n-p GaP/ITO/RuO2 causes a shift in the onset potential of the anodic photocurrent toward the negative by 0.9 V, which is the same as the photovoltage for n-i-p a-Si, again indicating that good electrical contact between a-Si and GaP is obtained. In addition, the light-intensity-limited photocurrent density, jll, for n-i-p a-Si/n-p GaP/ITO/RuO2, observed in potentials more positive than about -0.8 V vs Ag/AgCl/sat.KCl, is only slightly lower than that for n-p GaP/ITO/RuO2, observed in potentials more positive than about 0.3 V, indicating that a longerwavelength part of simulated solar light passes the GaP wafer and effectively reaches the a-Si layer. The comparison of the j vs U curve for water photooxidation on an “n-i-p a-Si/n-p GaP/ITO/RuO2” electrode (Figure 1a) with that for hydrogen evolution on a Pt electrode in the same electrolyte, shown in Figure 1b, indicates that full water splitting is possible in a PEC cell composed of an “n-i-p a-Si/n-p GaP/ ITO/RuO2” electrode, a Pt counter electrode, and 0.10 M Na2SO4 (pH 6.3) with no external bias. The solar to chemical conversion efficiency φchem can be calculated by an equation53

φchem )

(∆G/e) × j × 100(%) ∆Es

(1)

where ∆G is the Gibbs energy () 1.23 eV) for the decomposition of H2O into H2 and O2, e the elementary charge, j the observed photocurrent density (in units of mA cm-2), and ∆Es the input solar energy (in units of mW cm-2). ∆G expresses the stored chemical energy because it represents not only the decomposition potential of H2O but also the output voltage of a H2-O2 fuel cell. The stable performance of the composite electrode, explained later, indicates that the photocurrent is wholly attributed to water splitting without any electrode corrosion, demonstrating the validity of eq 1. A photocurrent of 1.2 mA/cm2 estimated from Figure 1 yielded a conversion efficiency of 1.5% for the PEC cell equipped with an “n-i-p a-Si/n-p GaP/ITO/RuO2” electrode. With an aim to improve the conversion efficiency, the combination of a p-n heterojunction, “n-Si/p-CuI/ITO”, and “ni-p a-Si/n-p GaP/ITO/RuO2” was investigated. The j vs U curve for an n-Si/p-CuI/n-i-p a-Si/n-p GaP/ITO/RuO2 electrode is shown in Figure 3a, together with those for the n-p GaP/ITO/

Figure 3. (a) j vs U curves for “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ ITO/RuO2”, “n-p GaP/ITO/RuO2”, and “ITO/RuO2” electrodes in 0.1 M Na2SO4, with simulated solar illumination for the former two electrodes. (b) The j vs U curve (in the dark) for a Pt electrode measured in the same 0.1 M Na2SO4.

RuO2 and ITO/RuO2 electrodes. A schematic structure of an “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2” electrode and a band diagram to explain photoinduced water oxidation on it are shown in Figure 4. We can see that the onset potential of the anodic photocurrent for “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ ITO/RuO2” is shifted toward the negative by 1.4 V compared with that for n-p GaP/ITO/RuO2. Although this shift is a little smaller than the value expected from the photovoltage of n-Si/ p-CuI/ITO/n-i-p a-Si/ITO (1.5 V), the steep increase of the anodic photocurrent for the former electrode near the onset potential indicates the formation of good electrical contact among semiconductors. In addition, the light-intensity-limited photocurrent density, jll, for the former electrode in potentials more positive than -1.1 V is nearly the same as that for the latter electrode in potentials more positive than about 0.3 V. This result indicates that incident solar light is divided properly into three semiconductors, crystalline n-Si, a-Si, and GaP.

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Figure 4. Schematic structure of the “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2” electrode and an expected band diagram for the electrode under solar irradiation. λ denotes the wavelength of the light.

bubbles were easy to accumulate at the surface. This problem can thus be easily solved by improving the electrode shape. The results of Figure 5 indicate that the photocurrent corrected for the effect of oxygen gas bubbles is kept constant for more than 180 min. Discussion

Figure 5. Photocurrent (j) vs irradiation time for the “n-Si/p-CuI/ITO/ n-i-p a-Si/n-p GaP/ITO/RuO2” electrode in 0.1 M Na2SO4 under simulated solar irradiation. The arrows in the figure indicate the times at which the electrode was shaken to remove oxygen gas bubbles accumulated at the surface.

A photoelectrochemical (PEC) cell, composed of an n-Si/pCuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2 electrode, a Pt counter electrode, and 0.10 M Na2SO4 (pH 6.3), was really constructed. It generated a photocurrent density of 1.88 mA cm-2 under simulated solar illumination (AM 1.5 G, 100 mW cm-2). The solar to chemical conversion efficiency was 2.3%, as calculated from this photocurrent. Actually, we were able to observe continuous evolution of gas bubbles from both the composite semiconductor and a Pt counter electrode. The evolved gases can be assigned to oxygen and hydrogen because the electrolyte solution contained only Na2SO4 and water and the composite semiconductor electrode was stable under long-term irradiation, as described below. The stability of the semiconductor electrode is an important factor in practical application. Figure 5 shows the results of a stability test for an n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2 electrode in 0.1 M Na2SO4, in which the photocurrent density during continuous solar light irradiation is plotted against the irradiation time. The photocurrent decreased with the irradiation time but was quickly recovered by shaking the electrode and removing accumulated oxygen gas bubbles at the composite semiconductor electrode surface. The arrows in Figure 5 indicate the times when electrode shaking was added. The composite semiconductor electrode in the present work was placed in a Teflon holder and thus had a structure in which oxygen gas

The present work has shown that the composite electrode having the combination “crystalline Si/a-Si/GaP” yields a high water-oxidation photocurrent and is suitable for efficient solar water splitting. The maximum conversion efficiency of 2.3% was obtained for an n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/ RuO2 electrode in 0.1 M Na2SO4. The conversion efficiency can be much increased by improving the properties of the composite semiconductor electrode. First of all, n-p GaP wafers, used in the present work, were prepared for use in LED devices, as mentioned earlier, and were unsuitable for use in solar cells, yielding relatively low photocurrents. The n-p GaP wafers in the present work were, however, the only n-p GaP material that was available at present. It is thus expected that the use of Ga(As)P with well-regulated p-n junctions, prepared for use in solar cells, gives much higher photocurrent densities and conversion efficiencies. The inferior property of the n-p GaP wafers used in the present work was clearly shown by the observation of the incident photon to current efficiency (IPCE) vs illumination wavelength (λ). Figure 6 shows the effect of electrochemical etching on the IPCE vs λ (a) and on j vs U (b) for a (naked) n-p GaP electrode in 0.1 M NaOH. Note that the photocurrent in this case is due to the photoanodic dissolution of GaP. A notable point is that the IPCE vs λ before etching shows a photoresponse only in a λ region from 450 to 600 nm with a peak at 500 nm, indicating that this n-p GaP electrode loses photoresponses from 400 to 500 nm, which normal GaP should show. Figure 6 shows that electrochemical etching increases the IPCE value in a region from 400 to 500 nm, leading to a large increase in photocurrent. This is probably due to the presence of an inactive layer near the surface or in the p-type layer. However, too much electrochemical etching with electricity over 1200 mC/cm2 caused a positive shift in the onset potential of photocurrent, indicating the loss of the p-type layer (3-5 µm thick). Note that the data of Figures 1 and 3 were obtained with n-p GaP wafers with electrochemical etching (900 mC).

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Figure 6. (a) IPCE vs λ and (b) j vs U for an n-GaP electrode before and after electrochemical etching. Note that the photocurrent in this case is due to the photoanodic dissolution of n-GaP. See the Experimental Section for details of the electrochemical etching.

Figure 8. j vs U curves of ITO/RuO2 electrodes prepared by chemical deposition (a) and electrodeposition (b). The measurements were performed in the dark in 0.1 M NaSO4.

Figure 7. Transmittance spectra of a GaP wafer with (a) and without (b) chemical etching. Part c shows photos of a GaP wafer with (right) and without (left) chemical etching.

The chemical etching of a GaP wafer with aqua regia was also effective to improve the transmittance of light in a wavelength region longer than the λ value corresponding to the band gap. Figure 7 shows light transmittance spectra of a GaP wafer with (a) and without (b) chemical etching. We can see that transmittance in the region longer than ca. 530 nm was drastically enhanced by the chemical etching.

The formation of an effective RuO2 layer, which is a good catalyst for a decrease in overvoltage for oxygen evolution, on an ITO layer of an n-p GaP electrode is also a key factor in obtaining a high-performance electrode. Note that here is a dilemma because too small an amount of RuO2 is ineffective for lowering the oxygen-evolution overvoltage, while too much an amount of RuO2 hinders light transmittance and decreases the photocurrent. In the present work, we prepared a RuO2 layer by two methods, electrodepostion and chemical deposition, and compared the performance of the obtained RuO2 layers. The latter chemical process consisted of the following procedures.36,54 A commercial 20% HCl aqueous solution of 0.1 M RuCl3 was dried by the evaporation of water, and the obtained solid was again dissolved in a minimum volume of 2-propanol. The 2-propanol solution of RuCl3 was then applied on the surface of ITO by brush and heated at 350 °C in air for 15 min. This procedure was repeated five times and then finally heated at 350 °C in air for 1 h to fully convert RuCl3 to RuO2. Figure 8 shows j vs U curves for RuO2/ITO electrodes prepared by chemical deposition (a) and electrodeposition (b), both being measured in 0.1 M NaSO4. We can see that the photocurrent for the electrode prepared by chemical deposition was much smaller than that for the electrode prepared by electrodeposition. SEM images of surfaces of these electrodes are shown in Figure 9. A large amount of polygons were observed on both surfaces, indicating that RuO2 is formed as crystallites. However, RuO2 particles prepared by electrodeposition (b) are smaller in average size than those prepared by chemical deposition (a). In addition, RuO2 particles prepared

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Figure 9. SEM images of surfaces of the ITO/RuO2 electrodes prepared by chemical deposition (a) and electrodeposition (b).

by electrodeposition (b) exist as needle-like small crystallites. Thus, the high activity of the RuO2/ITO electrode prepared by electrodeposition can be attributed to a high surface area of RuO2. We should also note that a heat treatment after the electrodeposition of RuO2 is an indispensable process. Although a RuO2 layer gave almost the same photocurrent with and without the heat treatment, the RuO2 layer without the heat treatment was easily peeled off from the ITO surface during j vs U measurements. XPS Cl-2p spectra showed that Cl-containing species remained at the surface of the electrodeposited electrode probably in a form of Ru(OH)δCl3-δ · cH2O. In general, RuCl3 is transformed into Ru(OH)δCl3-δ · cH2O in an aqueous solution, which is easily dissolved in water.55,56 This reaction can be expressed as follows: H2O

RuCl3 · aH2O 98 RuCl3 · cH2O RuCl3 · cH2O + H2O f Ru(OH)δCl3-δ · cH2O + δH+ + δClAccordingly, when a RuO2/ITO electrode having an electrodeposited RuO2 layer with no heat treatment was immersed in an aqueous solution for j vs U measurements, Ru(OH)δCl3-δ · cH2O in the RuO2 layer was eluted into an aqueous solution, leading to the formation of apertures or cracks in the layer. Thus, it is expected that oxygen evolution occurred inside the RuO2 layer as well as at the surface, leading to the peeling off of the RuO2 layer from the ITO surface. On the other hand, the heat treatment at 200 °C or a higher temperature changes almost all Ru(OH)δCl3-δ · cH2O into RuO2, resulting in the formation of a stable RuO2 layer. Conclusion The composite semiconductor electrode composed of multiple films “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2” was fabricated for the purpose of efficient solar water splitting. The electrode showed a stable photoanodic current due to oxygen evolution with a large negative photoshift (Vp) of about 2.2 V from the corresponding anodic current at a RuO2 electrode. The photoshift was large enough for full water splitting. Actual water splitting experiments with a photoelectrochemical (PEC) cell composed of the composite electrode, a Pt counter electrode, and 0.10 M Na2SO4 (pH 6.3) under simulated solar irradiation (AM 1.5 G, 100 mW cm-2) showed a conversion efficiency of

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