J. Phys. Chem. C 2007, 111, 12827-12833
12827
Persistent Phenomena in Photocurrent of Niobate Nanosheets Kentaro Okamoto,†,‡ Hisako Sato,§,| Kazuko Saruwatari,§ Kenji Tamura,⊥ Jun Kameda,§ Toshihiro Kogure,‡,§ Yasushi Umemura,‡,# and Akihiko Yamagishi*,†,‡ Department of Chemistry, Faculty of Science, Ochanomizu UniVersity, Tokyo 112-8610, Japan, CREST, Japan Science and Technology Agency, Saitama, Japan, Department of Earth and Planetary Science, Graduate School of Science, The UniVersity of Tokyo, Tokyo 113-0033, Japan, PRESTO, Japan Science and Technology Agency, Saitama, Japan, Photocatalytic Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan ReceiVed: May 3, 2007; In Final Form: June 25, 2007
A thin film of trimethylammonium-exchanged perovskite-type niobate ((CH3)3NHSr2Nb3O10) was prepared by casting its exfoliated aqueous dispersion onto a glass substrate. The electric conductivity of a film (3-9 µm thick) was measured in vacuum under the illumination of light (280∼450 nm). Photocurrent raised and decayed slowly in the time range of 103 seconds in response to the on-and-off of an incident light. Atmospheric oxygen accelerated the decay rate of current in the dark. In the absence of oxygen, the decay curve obeyed the equation of stretched exponential relaxation (ip ) ip(0) exp(-(t/τ)β). From the dependence of the photocurrent on various parameters such as film thickness, light wavelength, and temperature, the observed persistent phenomena were interpreted according to the following mechanisms: (1) a free electron was photogenerated under the illumination of light most effectively when photon energy corresponded to the edge of a band gap transition; (2) the generated electrons were trapped at surface oxygen vacancies before they became conductive; and (3) a persistent current appeared through the multistep trapping-detrapping processes under the gradient of electric field. It was suggested that vacancies were produced by the elimination of lattice oxygen atoms as dianions during the acid treatment of original niobate (KSr2Nb3O10). As far as we know, the present finding is the first example of persistent photocurrents in inorganic thin films.
Introduction Layered perovskite-type niobates (e.g., KSr2Nb3O10; Scheme 1) are attracting wide interest in the field of material chemistry because of their unique properties such as photocatalytic activity and ion exchangeability.1-12 Photocatalysis is ascribed to the generation of an electron-hole pair due to the band gap transition. Probability of electron-hole recombination is thought to be reduced through their migration to the opposite sides of layer surfaces. As another characteristic, alkali ions in interlayer spaces (e.g., K+ in Scheme 1) are readily exchanged with protons or cationic compounds to form organic-inorganic nanocomposites.2,3 Furthermore, a protonated niobate is exfoliated by reacting with organic amines into single layers in an aqueous medium. The combination of photocatalysis with exfoliation properties leads to the preparation of a thin film with photoresponse. A prepared film is used as an element to fabricate nanostructured electronic devices in response to light.10 In constructing such systems, it is crucially important to make clear a role of surface states of a single layer in electronic and electric behaviors. Recently we have applied the Langmuir-Blodgett (LB) method to prepare a single layered film of exfoliated inorganic * Corresponding author. Phone: +81-3-5978-5575. Fax: +81-3-59785575. E-mail:
[email protected]. † Ochanomizu University. ‡ CREST. § The University of Tokyo. | PRESTO. ⊥ National Institute for Materials Science. # National Defense Academy.
SCHEME 1: Schematic Representation of Potassium Perovskite-type Niobate (KSr2Nb3O10)
sheets by hybridizing with organic monolayers.13-15 As an example of such attempts, the electrical conductivity of a LB film of perovskite-type niobate was measured by alternating current (AC) and direct current (DC) analyses.13 It has been shown that the elimination of organic templates was possible by irradiating a film with a UV light. This treatment resulted in the appearance of electric photoconductivity. The nature of produced photoconductivity was, however, still to be clarified. In the present study, a thin film with a few micron thickness was prepared by casting an aqueous dispersion of exfoliated trimethylammonium-exchanged niobate. The DC conductivity of the film was measured under the illumination of light at 35∼100 °C. It was found that the current raised and decayed slowly in response to the on-and-off of an incident light. The relaxation curve of current decay was fitted by a stretched exponential relation over the time range of 103 seconds. Such persistent photoconductivity was previously reported for oxides
10.1021/jp073380k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
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with a large amount of oxygen vacancy or for amorphous semiconductors.16-26 As far as we know, the present finding is a first example of persistent electrical phenomena in exfoliated layered sheets. The observed photoconductivity of a nanosheet may open a possibility to fabricate photoresponsive electronic devices. Experimental Section Materials. KSr2Nb3O10 was synthesized according to the reported procedure.2,3,13 The material was transformed to the protonated form (HSr2Nb3O10) by being refluxed in an aqueous HCl (1.0 M) solution for 7 h. HSr2Nb3O10 was treated with an aqueous solution of 1.0 M trimethylamine to produce trimethylammonium-exchanged perovskite-type niobates ((CH3)3NHSr2Nb3O10). An aqueous dispersion of (CH3)3NHSr2Nb3O10 was supersonicated for 1 h at 40 °C. The dispersion was stirred at 70 °C for 3 weeks to complete exfoliation. Film Preparation. A thin film of exfoliated niobate nanosheets was prepared by casting 25-100 µL of a diluted dispersion of (CH3)3NHSr2Nb3O10 onto a hydrophilic quartz plate (12 mm × 12 mm). The film thickness was measured with an ID-C112B (Mitutoyo Corp, Japan). The average thickness ranged from 3 to 9 µm. Gold was sputtered to form electrodes of 50 nm thickness with an ion-sputtering instrument (JFC-1500, JEOL, Japan). The illuminated area of a sample was 2 mm (electrode length) × 12 mm (electrode width). A self-assembled single layered film was prepared by soaking a hydrophilic quartz plate (12 mm × 12 mm) in an aqueous solution of polyethyleneimine (12.5 mg mL-1) and thereafter in an aqueous dispersion of (CH3)3NHSr2Nb3O10 (12.6 mg mL-1).27 The deposition of niobate layers was ascertained by the increase of electronic absorption around 250-350 nm (see Figure 3). Gold was sputtered onto a glass substrate as electrodes to form a film sample (2 mm (electrode length) × 12 mm (electrode width)). A pellet of KSr2Nb3O10 was prepared by pressing approximately 0.2 g of the ground sample under 100 kg cm-2 for 5 min. Gold was sputtered onto the pellet to form electrodes. The size of the pellet was 2 mm (electrode length) × 20 mm (electrode width) × 2 mm (thickness). Film Characterization. Morphology of a niobate sample was observed by a JEOL 1010 (JEOL) transmission electron microscope (TEM) operated at 100 kV. A sample was prepared by soaking a gold mesh in an aqueous dispersion. XRD patterns were recorded with a RINT2100S X-ray diffractometer (Rigaku, Japan). A sample was prepared by casting an aqueous dispersion of exfoliated niobate onto a glass substrate. Thermogravimetric (TG) analyses of (CH3)3NHSr2Nb3O10 were performed under an air atmosphere using a Thermo plus TG 8120 (Rigaku, Japan) at a heating rate of 10 °C min-1. The surface of a self-assembled film was observed with an atomic force microscope (AFM; Nanoscope III scanning probe microscope (DI Instruments)). The images were recorded at room temperature in air. X-ray photoelectron spectroscopy (XPS) was performed with a VG Escalab 220 model using the Mg KR excitation source. A sample was prepared by casting a methanol dispersion of niobate onto a silicon wafer. Measurements ranged from 0 to 1000 eV in binding energy. The peak energy was calibrated by assuming that the peak due to Si(2p) was equal to 100.0 eV. Electrical Instruments. DC electrical conductivity was measured with a potentiostat (TOHO Technical Research 2020A, Japan) or a DC voltage current source/monitor TR6143 (Advantest). Data were transferred through a data-logger (Midi Logger GL450 (GRAPHTEC)) to a personal computer. A cell
Figure 1. TEM image of (CH3)3NHSr2Nb3O10 crystallites deposited on a gold mesh.
Figure 2. XRD pattern of a cast film of exfoliated (CH3)3NHSr2Nb3O10 sample on a glass plate.
mounting a quartz substrate was placed at constant temperature in an oven (AS-ONE ICV-300P, Japan). For photoconductivity measurements, the samples were illuminated by a 250 W Hg lamp (Ushio Optical Modulex, USH-250SC, Ushio, Japan). The intensity of light was measured by chemical actinography using potassium tris(oxalate)iron(III).28 Results Characterization of Layered Niobates. Figure 1 shows the TEM image of the crystallites of (CH3)3NHSr2Nb3O10 deposited on a gold mesh. Each particle took the shape of a thin plate. Figure 2 shows the XRD pattern of a film of the same sample cast on a glass plate. The peaks were assigned according to the previous works.3 Since all of the observed reflections were indexed as basal reflections (00l), the film consisted of layers deposited in parallel with the surface of a substrate. The UV absorption of a colloid suspension of the material is shown in Figure 3. The edge of the band gap transition was determined to be 340 nm (or 3.6 eV) by extrapolating the square root of absorbance (R) to zero as shown in the inset of the figure.29 The TG analyses were performed on a dried sample of (CH3)3NHSr2Nb3O10 (2.2 mg; not shown). The weight decreased monotonously until the weight loss attained 10.25% at 1000 °C. This might be caused by the decomposition of (CH3)3NH+ at higher temperature. No peak was observed in the differential thermogravimetric (DTG) curve, indicating that the
Photocurrent of Niobate Nanosheets
Figure 3. Electronic spectrum of an aqueous dispersion of exfoliated (CH3)3NHSr2Nb3O10 sample (6.8 mg L-1). The inset is a plot of the square root of absorption intensity against photon energy.
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12829
Figure 5. Time course of photocurrent for a cast film with various thickness of exfoliated (CH3)3NHSr2Nb3O10 sample under the irradiation of a 340 nm light at 90 °C. The intensity of an irradiated light was 8.2 × 10-8 Einstein cm-2 s-1.
Figure 6. Decay profiles of photocurrent for a cast film of exfoliated (CH3)3NHSr2Nb3O10 sample (ca. 3 µm thickness) at various temperatures. Figure 4. Comparison of the XPS profiles due to Nb(V)(3d) among three samples: KSr2Nb3O10, HSr2Nb3O10, and (CH3)3NHSr2Nb3O10. The dotted bald curve was for a 1:1 mixture of KSr2Nb3O10 and HSr2Nb3O10. Binding energy was calibrated by assuming that the peak due to Si(2p) was equal to 100.0 eV.
sample contained no water molecules in interlayer spaces. According to the XPS results, there was no appreciable difference in the peaks shape due to Nb(V)(3d3/2 and 3d5/2) among three kinds of niobate samples (Figure 4). The conclusion was further assisted by the measurement on a 1:1 mixture of KSr2Nb3O10 and HSr2Nb3O10. There was no broadening of the peak observed for the mixed sample. Thus, the valence of a niobium atom remained to be five during the acid treatment for exfoliation. Photocurrent Measurements of Cast Films. Figure 5 shows the time courses of photocurrent at constant voltage (1.0 V) when three kinds of cast films with different thickness (5, 6, and 9 µm) were exposed to a 340 nm light (8.2 × 10-8 Einstein cm-2 s-1.) at 90 °C under vacuum. Photocurrent appeared and increased gradually in the time range of 103 seconds. No abrupt increase of photocurrent was observed on the onset of light. When the incident light turned off, the current decayed nonexponentially in hours. As will be stated later, the curve was fitted by a stretched exponential. After introducing oxygen into the cell, the decay rate was accelerated, returning to zero value in minutes. Comparing the photocurrent among the films of different thickness, we find the current raised at a higher rate for a thinner film. Since the apparent absorbance of the present films at 340 nm was less than 0.01, the observed effects were not caused by the surface absorption of an incident light.
TABLE 1: Temperature Dependence of Apparent Resistance for the Cast Films of Exfoliated (CH3)3NHSr2Nb3O10 T/°C
R/kΩ
thickness
75 48 35 100 100
0. 8 ( 0.1 1.4 ( 0.2 1.7 ( 0.2 1.4 ( 0.2 2.9 ( 0.2
3 µm 3 µm 3 µm 5 µm 9 µm
Reaching a stationary state, the photocurrent was measured by varying an applied voltage. As a result, the stationary photocurrent (Is) increased linearly with V or Is ) V/R. The apparent resistance (R) decreased with the increase of temperature as given in Table 1. The decay of a photocurrent was monitored at various temperatures on a thinner film as shown in Figure 6. The decay profile was expressed by the stretched exponential relaxation as eq 1: β
( (τt ) )
ip ) ip(0) exp -
(1)
where τ is the relaxation time and β is a constant parameter expressing the degree of stretch (Table 2). Figure 7A shows the dependence of the relaxation time (τ) on temperature. The relaxation time was expressed by eq 2:
τ ) τ0 exp
() E* kT
(2)
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Figure 8. Dependence of wavelength on photocurrent for a cast film of exfoliated (CH3)3NHSr2Nb3O10 sample under light irradiation with the intensity of 8.2 × 10-9 Einstein cm-2 s-1 at 95 °C.
Figure 7. (A) Dependence of a parameter (τ) on temperature (see eq 2). (B) Dependence of a parameter (β) on temperature (see eq 3).
TABLE 2: Parameters Describing Photocurrent Decay for Four Different Samples τ sec β
self-assembled film
5 µm
6 µm
9 µm
19 0.71
854 0.65
1400 0.66
597 0.73
Figure 9. Generation and decay of photocurrent for a cast film of exfoliated (CH3)3NHSr2Nb3O10 sample (ca. 3 µm thickness) at 90 °C. Half of a film surface was masked against irradiation. The wavelength and the intensity of an irradiated light were 340 nm and 8.2 × 10-8 Einstein cm-2 s-1., respectively.
SCHEME 2: Schematic Drawing of a Half-Masked Sample
The intrinsic relaxation time (τ0) and the activation energy (E*) were determined to be 4.68 × 10-8 sec and 0.68 eV, respectively. Figure 7B shows the dependence of β on temperature, leading to the following equation:
β)
T T0
(3)
From the slope of the straight line, the intrinsic temperature (T0) is obtained to be 499 K. Figure 8 shows the dependence of the stationary value of the photocurrent on the wavelength under the constant light intensity (8.2 × 10-9 Einstein cm-2 s-1). The current showed the maximum value at 340 nm. That wavelength was nearly equal to the edge of the band gap transition (Figure 3). At all wavelengths, the decay of the photocurrent followed eq 1, while the relaxation time was slower for the longer wavelength of the incident light. Direction of Photocurrent in a Partially Masked Film. In order to determine the sign of photogenerated carriers, photocurrent was measured on the cast sample in which half of a surface was masked to screen an incident light (Scheme 2). Figure 9 shows the time course of a current after irradiation. A current was defined as positive when it flowed from the masked portion to the irradiated portion. Initially a negative current flowed in a short time, which was followed by the growth of a positive current to a stationary value.
The results were consistent with a view that carriers were electrons as below. On the onset of light irradiation, carriers were generated in an irradiated portion (left half in the scheme) and flowed into a gold electrode. Since this corresponded to the initial negative current, the sign of the carriers was concluded to be negative. When the carriers continued to be generated in the irradiated portion, they diffused toward the masked portion (right half in the scheme) because of the concentration gradient. This corresponded to the slow build-up of the positive current in the figure, confirming that the sign of the carriers was negative. These facts indicated that the present material behaved as an n-type photosemiconductor. This presented a simple method of determining the sign of a photogenerated carrier on a thin film.30 Photocurrent Measurements of KSr2Nb3O10 Pellet. A pellet of KSr2Nb3O10 was prepared as described in the experimental section. Photocurrent was recorded under the illumination of a 340 nm light (8.2 × 10-8 Einstein cm-2 s-1.) at 100 °C. No current was observed within an error of 10 nA when imposed potential was raised up to 10 V. Thus, KSr2Nb3O10 was concluded to be an insulator in both dark and illuminated conditions.
Photocurrent of Niobate Nanosheets
Figure 10. A 2 µm × 2 µm AFM image of a self-assembled film of perovskite-type niobate ((CH3)3NHSr2Nb3O10). The height of deposited plates was estimated to be in the range of 1.5-10 nm.
Figure 11. Time course of photocurrent for a self-assembled film of perovskite-type niobate ((CH3)3NHSr2Nb3O10) at 100 °C. The wavelength and the intensity of an irradiated light were 340 nm and 8.2 × 10-8 Einstein cm-2 s-1., respectively.
Photocurrent on a Single-Layered Self-Assembled Film. Photocurrent was measured on a single-layered self-assembled film of niobate. The deposition of niobate layers was confirmed by the absorption spectrum and the AFM observation (Figure 10). Figure 11 shows the rise and decay of the photocurrent under the illumination of a 340 nm light. Although these transient behaviors were much faster than the cast films, the time course of decay was expressed by the same equation as 1. Applying the equation to the decay profile, τ and β were obtained to be 19 s and 0.71, respectively. The results indicated that the persistent character appeared as the intrinsic properties of a single layer. Discussion It is demonstrated that a thin film of niobate ((CH3)3NHSr2Nb3O10) becomes photoconductive under the illumination of light. In contrast to the previous results on the hybrid LB films of niobate and amphiphilic ammonium,13 the present film contained no molecule with a long alkyl chain. This is a main reason that the film was conductive with no pretreatment by a UV light. The rise of a photocurrent took 103 seconds until the current reached a stationary value. The decay of a current in the absence of light was also a slow event in the time range of
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12831 a few hours. The observed transient behavior was quite different from known photosemiconductors such as titanium oxide.31 In anatase (TiO2), for example, the lifetimes associated with the decay kinetics of photogenerated electrons ranged between 10-7 and 10-1 s.31 The extended exponential decay of a photocurrent as expressed by eq 1 is a characteristic of persistent phenomena. The behavior has been reported for various kinds of inorganic materials such as layered titanium niobate, amorphous hydrogenated silicon, nanocrystalline titanium oxides, gallium nitrates, and CdS nanoribbons.16-26 The observed current is attributed to the multistep transport processes involving the trapping and detrapping of carriers. In analogy to the persistent characters of layered perovskite Nd2Ti3O9,16 the following mechanisms are proposed on the basis of the results as stated in the preceding section: (1) Electrons were generated as carriers on the irradiation of light (Figure 9). The wavelength dependence on photocurrent showed that the bound states of such electrons were located at the highest level of the valence bands (Figure 8). Electrons were trapped before they became a free carrier, since no instantaneous rise of a photocurrent was seen at the onset of the illumination of a light (Figure 5). (2) A current appeared as a result of the transport of trapped carriers through the random hopping from one trap site to another. The processes were activated thermally, since the apparent resistance decreased with the increase of temperature (Table 1). The build-up of a photocurrent under illumination corresponded to the increase of the population of trapped electrons. The stationary current was attained by the balance between the increase of the population of trapped electrons and their annihilation due to the recombination at positively charged sites. After turning off an incident light, the current decreased with the decrease of trapped carriers by recombination. (3) The dependence of photocurrent on film thickness (Figure 5) indicates that carrier generation took place on an external surface. Since the absorbance of films was lower than 0.05 at 340 nm, photogeneration processes occurred uniformly through the film. It was therefore suspected that the carriers generated in a bulk were annihilated before they were trapped. One possibility was that annihilation took place effectively at the parts in contact with neighboring layers. No neighboring layer existed on an external surface. An extended exponential decay equation (eq 1) is derived on the basis of the following assumptions: (i) the decay of carrier density is expressed by dn(t)/dt ) -k(t) n(t); (ii) k(t) depends on time as k(t) ) D0(ωt)-R/a2, in which D0 is a microscopic diffusion constant, ω hopping frequency, and a a hopping distance; (iii) the time-dependent diffusion constant or D0(ωt)-R has an origin in the waiting time distribution of carriers as Ψ(t) ) t-(1+β) with β ) 1 - R. A parameter, β, is related to the exponential energy distribution of traps or exp(-E/kT0) below the conduction edge by β ) T/T0. Under these assumptions, the relaxation time in eq 1 is expressed by τ int ) a2ωR τR/D0. The variation of τ int with temperature is expected to follow the activated behavior as τ int ) τ0 exp(-Ea/kT). By applying these analyses to the experimental results (Figure 7A,B), T0 and Ea were calculated to be 499 K and 0.68 eV, respectively. The above persistent character in the transient behavior of photocurrent was observed even on a single-layered self-assembled film. From the value of β for a single-layered self-assembled film, nearly the same value of T0 was obtained. The results indicated that the energy distribution of trapping sites was determined by the single layer structure.
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On the basis of the above model, the total charge (Qt) integrated by the decay current corresponded to the lower limit of the number of trap sites in a film. The amount of Qt is calculated to be 0.31 C for the 5 µm film used in Figure 5. This amounted to 1.9 × 1018 electrons. Assuming that no water was included, we find that the number of unit lattice of ((CH3)3NHSr2Nb3O10; fw 674.07) in the film (3.0 mg) was calculated to be 2.7 × 1018. Thus, one unit lattice contained 0.70 trapping sites on an average. Since this estimation was minimum, the real value for the trap density was thought to be higher than this value. Thus, it was presumed that the trapping sites were distributed regularly as a structural origin. At present, it is unknown what kind of structure is responsible for a trapping site. It is not related to niobium atoms since the XPS measurements showed that the whole niobium atom took a form of Nb(V) under dark conditions. The hopping rate of trapping carriers was dependent on temperature remarkably (Figure 6). The results suggested that the trapping site was formed by the deficiency of surface oxygen atoms, if the process was coupled with the thermal diffusion of oxygen vacancies in a layer. Taking the crystal structure of a niobate layer into account, we proposed the following processes for the appearance of the persistent features. Electron-hole separation took place by ionization of an oxygen atom in a layer under the irradiation of light:
consumption. It is under investigation to develop such an electronic device by use of photoconductive inorganic nanosheets.
O2-(lattice) + hν f O-(lattice) + e (free)
Supporting Information Available: Results of the ohmic I-V characteristic, TG measurements, and XRD pattern of K-type and H-type powder samples. This material is available free of charge via the Internet at http://pubs.acs.org.
(2)
in which e (free) denotes a free electron.32 It was not certain which of the oxygen atoms in a triple structured layer of niobate (Scheme 1) was responsible for this reaction. A free electron migrated on a layer surface until it was trapped at the vacant oxygen site in the outer niobate sheet:
e (free) + 0 f e (0)
(3)
in which e (0) denotes a trapped electron and 0 denotes an oxygen vacancy. The electrons tapped at the oxygen vacant sites transported through a layer in accompany with the thermal diffusion of surface oxygen atoms, which represents a persistent current after the turning-off of incident light:
e (0) + 0 f 0 + e (0)
(4)
The termination of trapped electrons occurred with the recombination of trapped electrons with O- (lattice):
e (0) + O- (lattice) f O2- (lattice)
(5)
One possibility was that oxygen vacancy was produced under the acid treatment of an original niobate (KSr2Nb3O10). Oxygen atoms on a surface reacted with protons in an interlayer space to be eliminated as dianions in the form of water molecules, as:
O2- (lattice) + 2H+ f 0 + H2O
(6)
In spite of the uncertainty in the structural origin for trapping sites, the present work reports an initial example showing that the persistent behavior was intrinsic to the properties of a single inorganic nanosheet. The finding may open a possibility of the fabrication of constructing an electronic device by use of the present type of nanosheets. Such devices could be made on a scale of nanometer and operate under a very low level of energy
Conclusion This paper presents an initial example of persistent electrical phenomena for a sample of exfoliated layered materials. So far the persistent behavior has been thought to be a characteristic of bulk materials, in which the slow transport of carriers takes place through the trapping of de-trapping processes. In challenging this conventional view, we find the present work has demonstrated that the same transporting process is possible even in a thin layer of a few nanometer thickness. It may evoke an essential question on the mechanism of persistent phenomena and also may open a possibility to fabricate photoresponsive electronic devices on a basis of thin layered film. Acknowledgment. This work has been financially supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese Government. We are grateful to Professor Kazunari Domen, The University of Tokyo, for his advice in preparing the samples. We also thank Mr. Tomochica Idei, The University of Tokyo, for the XRD measurements of powder samples.
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