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Growth, photophysical and structural properties of Bi2InNbO7
Journal of Crystal Growth 229 (2001) 462–466
Growth, photophysical and structural properties of Bi2InNbO7 Zhigang Zoua,*, Jinhua Yeb, Hironori Arakawaa a
National Institute of Materials and Chemical Research (NIMC), 1-1Higashi, Tsukuba, Ibaraki 305-8565, Japan b National Research Institute for Metals (NRIM), 1-2-1 Sengen, Tsukuba, Ibaraki 305, Japan
Abstract A new phase of bismuth indium niobate, the Bi2InNbO7 compound, was grown by the sub-solidus reaction method. Rietveld refinement of the powder X-ray diffraction data revealed that the Bi2InNbO7 compound has the pyrochlore ( The optical crystal structure, cubic system with space group Fd3m and the lattice parameter is a ¼ 10:7793ð2Þ A. absorption and electrical properties of Bi2InNbO7 were investigated. It is found that the Bi2InNbO7 compound exhibits a direct gap semiconducting behavior. Conductivity measurement showed that the compound has an activation energy of 2.62(5) eV. UV-vis diffuse reflectance spectroscopy measurement revealed that the band gap of Bi2InNbO7 is about 2.7(4) eV. # 2001 Elsevier Science B.V. All rights reserved. Keywords: A2. Growth from solution; A2. Single crystal growth; B1. Oxides; B2. Magneto-optic materials; B2. Semiconducting materials
1. Introduction It is known that numerous compounds with the 4+ A3+ 2 B2 O7 pyrochlore structure exhibit antiferroelectric phases or dielectric anomalies, while only a few compounds exhibit a ferroelectric behavior [1,2]. Bi2InNbO7 belongs to the family of the A2B2O7 compounds, but the space group and lattice constants are not yet clear [1]. On the other hand, no photophysical property of the Bi2InNbO7 compound has been reported so far. We considered that In3+ and Nb5+ doping of B2 4+ sites in A3+ 2 B2 O7 might cause an increase in hole (carrier) concentration, and might provide a
*Corresponding author. Tel.: +81-298-61-4750; fax: +81298-61-4750. E-mail address: [email protected] (Z. Zou).
change in the magnetic, electrical transport and photophysical properties. Very recently, we have found that Bi2InNbO7 is paramagnetic and shows semiconducting behavior and the photocatalytic water splitting using Bi2InNbO7 was carried out under UV irradiation [3,4]. The H2 and O2 evolutions were observed from pure H2O under UV irradiation, without using other co-catalysts, as shown in Fig. 1. The process for photocatalysis of semiconductor oxides is explained generally by the fact that the photon is absorbed directly by the band gap of the conventional semiconductor, generating an electron and a hole in the conduction band and the valence band, respectively [5]. In order to gain a better understanding of the photocatalytic efficiency of the photocatalyst, it is important to investigate the photophysical and structural properties of the compound.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 2 0 7 - 6
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 1. Reaction time course of photocatalytic H2 evolution from pure water using Bi2InNbO7 compound under UV irradiation (400 W high pressure Hg lamp).
463
small single crystals of light yellow color were obtained. The chemical composition of the compound was determined by scanning electron microscope-X-ray energy dispersion spectroscopy (SEM-EDS) with an accelerating voltage of 25 kV. Powder X-ray diffraction was carried out on an X-ray diffractometer with Cu Ka radiation ( The optical absorption of Bi2InN(l ¼ 1:5418 A). bO7 was measured by using a UV-vis spectrometer (MPS-2000). The temperature dependence of the electrical resistivity was examined by the fourprobe technique. The resistivity measurement was made using direct current. Au wires and silver electrodes were used.
3. Results and discussion The synthesis of the sample is essential to investigate physical chemistry and structural properties of the Bi2InNbO7 compound. Herein, we report the synthesis and characterization of optical and structural properties of Bi2InNbO7.
2. Experimental procedure The Bi2InNbO7 compound was grown by subsolidus reaction method using high purity Bi2O3(99.9%), In2O3(99.9%), and Nb2O5(99.9%) under ambient pressure. Fig. 2 shows a schematic procedure of growth of the compound in this work. First of all, Bi2O3, In2O3, and Nb2O5 were converted into Bi(NO3)3, In(NO3)3, and Nb(NO3)5, respectively, using HNO3(16 N) at 608C. The solutions were mixed and stirred well, then NH4OH(7.5 N) was added to the mixed solution. A precipitate was obtained from the mixed solution. The precipitate was dried at about 2008C in air, and calcined in a furnace at 9008C for 10 h. Then the calcined material was pressed into pellet, placed in a Pt crucible and inserted into the vertical furnace. The furnace was heated from room temperature to 11008C over 20 h, held at 11008C for 10 h, and slowly cooled to 6508C, then rapidly cooled to room temperature. The very
The crystalline grains of the Bi2InNbO7 compound synthesized by the subsolidus reaction method have a maximum size of 5.0 mm as the sample synthesized by solid state reaction. The chemical composition of the Bi2InNbO7 compound was determined using characteristic X-rays of In La, Bi Ma, and Nb La. The composition content was decided using the ZAF quantification method. The SEM-EDS analysis showed that the compound has a homogenous atomic distribution with no other additional elements. The atomic ratio of the compound was also confirmed by XRFS (X-ray Fluorescence Spectrometer) measurement, which showed similar result. Oxygen content was calculated from the EDS results [6]. The powder X-ray diffraction data of Bi2InNbO7 were collected at 295 K. The data were collected with a step scan procedure in the range of 2y ¼ 521008. The step interval was 0.0248 and scan speed, 18 min1. The power X-ray diffraction analysis showed that the Bi2InNbO7 compound was a single phase. This is consistent with the observation from SEM-EDS. Full-profile structure refinement of the collected powder diffraction data for Bi2InNbO7 was performed using the Rietveld program REITAN [7]. The result of refinements is shown in Fig. 3. Positional parameters and isotropic thermal parameters of all atoms of the Bi2InNbO7 compound were refined. The result is
464
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 2. Schematic diagram of synthetic procedure for the Bi2InNbO7 compound.
shown in Table 1. The result of refinements yielded R factors, Rp ¼ 10:5%, RWP ¼ 15:3% in space group Fd3m when the O atoms are included in the model. The obtained R factors seem to be larger. A possible source for the large R factors might be due to minor impurities in the compound. However, powder diffraction pattern and SEM-EDS analysis do not indicate the presence of any other phases. It is well-known that the disorder/order of a fraction of the atoms might lead to the large R factors. The structure of Bi2InNbO7 compound shown in Fig. 4, can be described as consisting of the three-dimensional network of NbO6, stacked along [1 1 0] and separated by a unit cell translation
( Considering the fact that the large (10.7793(2) A). R factors were obtained, there might be the disorder of a fraction in the Nb position, as a result of which there are the three-dimensional networks of (Bi, Nb,)O6 and/or (In, Nb,)O6. The outcome of the final refinement indicated that the Bi2InNbO7 compound is the pyrochlore type crystal structure, cubic system with space group Fd3m and the lattice parameter is a ¼ ( All the diffraction peaks for Bi2InN10:7793ð2Þ A. bO7 could be successfully indexed based on the lattice constant and the space group. Fig. 5 shows the transmission curve of Bi2InNbO7. An average transmission of more than 90% was obtained from 500 to 700 nm. This means that strong photoabsorption of Bi2InNbO7 occurs only at wavelengths shorter than 500 nm. It is interesting to notice that Bi2InNbO7 shows photoabsorption in the visible light region (l > 420 nm), but the photoabsorption is weak. This means that the Bi2InNbO7 compound has the ability to respond to wavelength of visible light region. It is known that the process for photocatalysis of semiconductors is the direct absorption of photon by band gap of the materials and generates electron–hole pairs in the semiconductor particles. The excitation of an electron from the valence band to the conduction band is initiated by light absorption with energy equal to or greater than the band gap of the semiconductor. Upon excitation of photon the separated electron and hole can follow surface of solid. If the conduction band potential level of semiconductor is more negative than that of hydrogen evolution, and the valence band potential level is more positive than that of oxygen evolution, decomposition of water can occur even without applying electric power [5]. The photoabsorption of Bi2InNbO7 suggests that Bi2InNbO7 has potential ability to generate H2 evolution from water under visible light. However, the photocatalyst does not work under visible light irradiation to directly decompose pure water, even Pt/ CH3OH/H2O solution as shown in Fig. 1 [4]. Alig et al. [8] found that direct absorption of photons by band gap oxides can generate electron–hole pairs in the solid and, generally, this is more than twice of the energy gap of the material and such excess energy requirement is common for many
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
465
Fig. 3. X-ray powder diffraction pattern of the Bi2InNbO7 compound.
Table 1 Structural parameters of Bi2InNbO7 prepared by solid state reaction method Atom
x
y
z
Beq
Bi In O(1) O(2) Nb
0.0000 0.5000 0.1860 0.1250 0.5000
0.0000 0.5000 0.1250 0.1250 0.5000
0.0000 0.5000 0.1250 0.1250 0.5000
2.916 0.459 1.000 1.000 0.500
oxides. This means that to decompose water with the semiconductor under visible light irradiation, modification of the band gap of the semiconductor and/or increase of light energy may be effective. The value of band gap of Bi2InNbO7 was determined by extrapolations of the straight regions of the absorption coefficient, a2, versus photon energy hn. The absorption coefficient was determined by the equation a ¼ lnð1=TÞd, where T is the transmissivity and d is the thickness of sample [9]. The band gap of Bi2InNbO7 was estimated to be about 2.7(4) eV. The result is in good agreement with the value estimated from conductivity measurement. Fig. 6 shows the conductivity of the Bi2InNbO7 compound in temperature range 250–650 K. The conductivity decreases rapidly with temperature decrease, showing semiconducting behavior. The activation energy (Ea ) is 2.62(5) eV, according to the function
Fig. 4. The schematic structural diagram of the Bi2InNbO7 compound. Three-dimensional network of MO6, stacked along [1 1 0].
s ¼ s0 expðEa =KTÞ, where s is conductivity of the Bi2InNbO7 compound. This means the Bi2InNbO7 compound may be a direct gap semiconductor. The Bi2InNbO7 compound has pyrochlore crystal structure consisting of the network of MO6 as shown in Fig. 4. The structure of Bi2InNbO7 is built by forming infinite corner-sharing MO6 octahedra formed [MO3]a chains along [1 1 0]. This suggests that photogenerated electron–hole pairs in Bi2InNbO7 can move easily in this direction. The mobility of electron–hole pairs affects the
466
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 5. Diffused reflectance UV-vis spectrum of the Bi2InNbO7 compound.
photoabsorption as well as the conduction band level because it affects the probability of electrons and holes to reach reaction sites on the surface.
4. Conclusion In conclusion, we prepared a new phase of the bismuth indium niobate, the Bi2InNbO7 compound, and investigated its structural and photoabsorption properties. The powder X-ray diffraction showed that the Bi2InNbO7 compound has the pyrochlore crystal structure, cubic system with space group Fd3m and the lattice parameter ( The Bi2InNbO7 compound is a ¼ 10:7793ð2Þ A. shows photoabsorption in the visible light region (l > 420 nm), showing that Bi2InNbO7 has the ability to respond to wavelength of visible light region.
Fig. 6. Temperature dependence of the conductivity of the Bi2InNbO7 compound in the temperature range between 250 and 650 K.
References [1] D. Bernard, J. Pannetier, J. Lucas, Ferroelectrics 21 (1978) 429. [2] G.I. Golovshchikove, V.A. Isupov, A.G. Tutov, A.G. Nikove, I.E.Myl.P.A. Nikitina, O.I. Tulinova, Sov. Phys. Solid State 14 (1973) 2539. [3] Z. Zou, J. Ye, J. Alloys, Compounds 292 (1999) 72. [4] Z. Zou, J. Ye, R. Abe, H. Arakawa, Catal. Lett. 68 (2000) 235. [5] K. Honda, A. Fujishima, Nature 238 (1972) 37. [6] Z. Zou, J. Ye, K. Oka, Y. Nishihara, Phys. Rev. Lett. 80 (1998) 1074. [7] F. Izumi, J. Crystallogr. Assoc. Japan 27 (1985) 23. [8] R.C. Alig, S.W. Bloom, Phys. Rev. B 22 (1980) 5565. [9] S.B. Qadri, H. Kim, H.R. Khan, J. Mater. Res. 15 (2000) 21.
Growth, photophysical and structural properties of Bi2InNbO7 Zhigang Zoua,*, Jinhua Yeb, Hironori Arakawaa a
National Institute of Materials and Chemical Research (NIMC), 1-1Higashi, Tsukuba, Ibaraki 305-8565, Japan b National Research Institute for Metals (NRIM), 1-2-1 Sengen, Tsukuba, Ibaraki 305, Japan
Abstract A new phase of bismuth indium niobate, the Bi2InNbO7 compound, was grown by the sub-solidus reaction method. Rietveld refinement of the powder X-ray diffraction data revealed that the Bi2InNbO7 compound has the pyrochlore ( The optical crystal structure, cubic system with space group Fd3m and the lattice parameter is a ¼ 10:7793ð2Þ A. absorption and electrical properties of Bi2InNbO7 were investigated. It is found that the Bi2InNbO7 compound exhibits a direct gap semiconducting behavior. Conductivity measurement showed that the compound has an activation energy of 2.62(5) eV. UV-vis diffuse reflectance spectroscopy measurement revealed that the band gap of Bi2InNbO7 is about 2.7(4) eV. # 2001 Elsevier Science B.V. All rights reserved. Keywords: A2. Growth from solution; A2. Single crystal growth; B1. Oxides; B2. Magneto-optic materials; B2. Semiconducting materials
1. Introduction It is known that numerous compounds with the 4+ A3+ 2 B2 O7 pyrochlore structure exhibit antiferroelectric phases or dielectric anomalies, while only a few compounds exhibit a ferroelectric behavior [1,2]. Bi2InNbO7 belongs to the family of the A2B2O7 compounds, but the space group and lattice constants are not yet clear [1]. On the other hand, no photophysical property of the Bi2InNbO7 compound has been reported so far. We considered that In3+ and Nb5+ doping of B2 4+ sites in A3+ 2 B2 O7 might cause an increase in hole (carrier) concentration, and might provide a
*Corresponding author. Tel.: +81-298-61-4750; fax: +81298-61-4750. E-mail address: [email protected] (Z. Zou).
change in the magnetic, electrical transport and photophysical properties. Very recently, we have found that Bi2InNbO7 is paramagnetic and shows semiconducting behavior and the photocatalytic water splitting using Bi2InNbO7 was carried out under UV irradiation [3,4]. The H2 and O2 evolutions were observed from pure H2O under UV irradiation, without using other co-catalysts, as shown in Fig. 1. The process for photocatalysis of semiconductor oxides is explained generally by the fact that the photon is absorbed directly by the band gap of the conventional semiconductor, generating an electron and a hole in the conduction band and the valence band, respectively [5]. In order to gain a better understanding of the photocatalytic efficiency of the photocatalyst, it is important to investigate the photophysical and structural properties of the compound.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 2 0 7 - 6
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 1. Reaction time course of photocatalytic H2 evolution from pure water using Bi2InNbO7 compound under UV irradiation (400 W high pressure Hg lamp).
463
small single crystals of light yellow color were obtained. The chemical composition of the compound was determined by scanning electron microscope-X-ray energy dispersion spectroscopy (SEM-EDS) with an accelerating voltage of 25 kV. Powder X-ray diffraction was carried out on an X-ray diffractometer with Cu Ka radiation ( The optical absorption of Bi2InN(l ¼ 1:5418 A). bO7 was measured by using a UV-vis spectrometer (MPS-2000). The temperature dependence of the electrical resistivity was examined by the fourprobe technique. The resistivity measurement was made using direct current. Au wires and silver electrodes were used.
3. Results and discussion The synthesis of the sample is essential to investigate physical chemistry and structural properties of the Bi2InNbO7 compound. Herein, we report the synthesis and characterization of optical and structural properties of Bi2InNbO7.
2. Experimental procedure The Bi2InNbO7 compound was grown by subsolidus reaction method using high purity Bi2O3(99.9%), In2O3(99.9%), and Nb2O5(99.9%) under ambient pressure. Fig. 2 shows a schematic procedure of growth of the compound in this work. First of all, Bi2O3, In2O3, and Nb2O5 were converted into Bi(NO3)3, In(NO3)3, and Nb(NO3)5, respectively, using HNO3(16 N) at 608C. The solutions were mixed and stirred well, then NH4OH(7.5 N) was added to the mixed solution. A precipitate was obtained from the mixed solution. The precipitate was dried at about 2008C in air, and calcined in a furnace at 9008C for 10 h. Then the calcined material was pressed into pellet, placed in a Pt crucible and inserted into the vertical furnace. The furnace was heated from room temperature to 11008C over 20 h, held at 11008C for 10 h, and slowly cooled to 6508C, then rapidly cooled to room temperature. The very
The crystalline grains of the Bi2InNbO7 compound synthesized by the subsolidus reaction method have a maximum size of 5.0 mm as the sample synthesized by solid state reaction. The chemical composition of the Bi2InNbO7 compound was determined using characteristic X-rays of In La, Bi Ma, and Nb La. The composition content was decided using the ZAF quantification method. The SEM-EDS analysis showed that the compound has a homogenous atomic distribution with no other additional elements. The atomic ratio of the compound was also confirmed by XRFS (X-ray Fluorescence Spectrometer) measurement, which showed similar result. Oxygen content was calculated from the EDS results [6]. The powder X-ray diffraction data of Bi2InNbO7 were collected at 295 K. The data were collected with a step scan procedure in the range of 2y ¼ 521008. The step interval was 0.0248 and scan speed, 18 min1. The power X-ray diffraction analysis showed that the Bi2InNbO7 compound was a single phase. This is consistent with the observation from SEM-EDS. Full-profile structure refinement of the collected powder diffraction data for Bi2InNbO7 was performed using the Rietveld program REITAN [7]. The result of refinements is shown in Fig. 3. Positional parameters and isotropic thermal parameters of all atoms of the Bi2InNbO7 compound were refined. The result is
464
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 2. Schematic diagram of synthetic procedure for the Bi2InNbO7 compound.
shown in Table 1. The result of refinements yielded R factors, Rp ¼ 10:5%, RWP ¼ 15:3% in space group Fd3m when the O atoms are included in the model. The obtained R factors seem to be larger. A possible source for the large R factors might be due to minor impurities in the compound. However, powder diffraction pattern and SEM-EDS analysis do not indicate the presence of any other phases. It is well-known that the disorder/order of a fraction of the atoms might lead to the large R factors. The structure of Bi2InNbO7 compound shown in Fig. 4, can be described as consisting of the three-dimensional network of NbO6, stacked along [1 1 0] and separated by a unit cell translation
( Considering the fact that the large (10.7793(2) A). R factors were obtained, there might be the disorder of a fraction in the Nb position, as a result of which there are the three-dimensional networks of (Bi, Nb,)O6 and/or (In, Nb,)O6. The outcome of the final refinement indicated that the Bi2InNbO7 compound is the pyrochlore type crystal structure, cubic system with space group Fd3m and the lattice parameter is a ¼ ( All the diffraction peaks for Bi2InN10:7793ð2Þ A. bO7 could be successfully indexed based on the lattice constant and the space group. Fig. 5 shows the transmission curve of Bi2InNbO7. An average transmission of more than 90% was obtained from 500 to 700 nm. This means that strong photoabsorption of Bi2InNbO7 occurs only at wavelengths shorter than 500 nm. It is interesting to notice that Bi2InNbO7 shows photoabsorption in the visible light region (l > 420 nm), but the photoabsorption is weak. This means that the Bi2InNbO7 compound has the ability to respond to wavelength of visible light region. It is known that the process for photocatalysis of semiconductors is the direct absorption of photon by band gap of the materials and generates electron–hole pairs in the semiconductor particles. The excitation of an electron from the valence band to the conduction band is initiated by light absorption with energy equal to or greater than the band gap of the semiconductor. Upon excitation of photon the separated electron and hole can follow surface of solid. If the conduction band potential level of semiconductor is more negative than that of hydrogen evolution, and the valence band potential level is more positive than that of oxygen evolution, decomposition of water can occur even without applying electric power [5]. The photoabsorption of Bi2InNbO7 suggests that Bi2InNbO7 has potential ability to generate H2 evolution from water under visible light. However, the photocatalyst does not work under visible light irradiation to directly decompose pure water, even Pt/ CH3OH/H2O solution as shown in Fig. 1 [4]. Alig et al. [8] found that direct absorption of photons by band gap oxides can generate electron–hole pairs in the solid and, generally, this is more than twice of the energy gap of the material and such excess energy requirement is common for many
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
465
Fig. 3. X-ray powder diffraction pattern of the Bi2InNbO7 compound.
Table 1 Structural parameters of Bi2InNbO7 prepared by solid state reaction method Atom
x
y
z
Beq
Bi In O(1) O(2) Nb
0.0000 0.5000 0.1860 0.1250 0.5000
0.0000 0.5000 0.1250 0.1250 0.5000
0.0000 0.5000 0.1250 0.1250 0.5000
2.916 0.459 1.000 1.000 0.500
oxides. This means that to decompose water with the semiconductor under visible light irradiation, modification of the band gap of the semiconductor and/or increase of light energy may be effective. The value of band gap of Bi2InNbO7 was determined by extrapolations of the straight regions of the absorption coefficient, a2, versus photon energy hn. The absorption coefficient was determined by the equation a ¼ lnð1=TÞd, where T is the transmissivity and d is the thickness of sample [9]. The band gap of Bi2InNbO7 was estimated to be about 2.7(4) eV. The result is in good agreement with the value estimated from conductivity measurement. Fig. 6 shows the conductivity of the Bi2InNbO7 compound in temperature range 250–650 K. The conductivity decreases rapidly with temperature decrease, showing semiconducting behavior. The activation energy (Ea ) is 2.62(5) eV, according to the function
Fig. 4. The schematic structural diagram of the Bi2InNbO7 compound. Three-dimensional network of MO6, stacked along [1 1 0].
s ¼ s0 expðEa =KTÞ, where s is conductivity of the Bi2InNbO7 compound. This means the Bi2InNbO7 compound may be a direct gap semiconductor. The Bi2InNbO7 compound has pyrochlore crystal structure consisting of the network of MO6 as shown in Fig. 4. The structure of Bi2InNbO7 is built by forming infinite corner-sharing MO6 octahedra formed [MO3]a chains along [1 1 0]. This suggests that photogenerated electron–hole pairs in Bi2InNbO7 can move easily in this direction. The mobility of electron–hole pairs affects the
466
Z. Zou et al. / Journal of Crystal Growth 229 (2001) 462–466
Fig. 5. Diffused reflectance UV-vis spectrum of the Bi2InNbO7 compound.
photoabsorption as well as the conduction band level because it affects the probability of electrons and holes to reach reaction sites on the surface.
4. Conclusion In conclusion, we prepared a new phase of the bismuth indium niobate, the Bi2InNbO7 compound, and investigated its structural and photoabsorption properties. The powder X-ray diffraction showed that the Bi2InNbO7 compound has the pyrochlore crystal structure, cubic system with space group Fd3m and the lattice parameter ( The Bi2InNbO7 compound is a ¼ 10:7793ð2Þ A. shows photoabsorption in the visible light region (l > 420 nm), showing that Bi2InNbO7 has the ability to respond to wavelength of visible light region.
Fig. 6. Temperature dependence of the conductivity of the Bi2InNbO7 compound in the temperature range between 250 and 650 K.
References [1] D. Bernard, J. Pannetier, J. Lucas, Ferroelectrics 21 (1978) 429. [2] G.I. Golovshchikove, V.A. Isupov, A.G. Tutov, A.G. Nikove, I.E.Myl.P.A. Nikitina, O.I. Tulinova, Sov. Phys. Solid State 14 (1973) 2539. [3] Z. Zou, J. Ye, J. Alloys, Compounds 292 (1999) 72. [4] Z. Zou, J. Ye, R. Abe, H. Arakawa, Catal. Lett. 68 (2000) 235. [5] K. Honda, A. Fujishima, Nature 238 (1972) 37. [6] Z. Zou, J. Ye, K. Oka, Y. Nishihara, Phys. Rev. Lett. 80 (1998) 1074. [7] F. Izumi, J. Crystallogr. Assoc. Japan 27 (1985) 23. [8] R.C. Alig, S.W. Bloom, Phys. Rev. B 22 (1980) 5565. [9] S.B. Qadri, H. Kim, H.R. Khan, J. Mater. Res. 15 (2000) 21.