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anatase interface: a dominant factor of photocatalytic activity
Chemical Physics Letters 390 (2004) 399–402 www.elsevier.com/locate/cplett
Charge separation at the rutile/anatase interface: a dominant factor of photocatalytic activity Takahira Miyagi a,b,*, Masayuki Kamei a,*, Takefumi Mitsuhashi a, Takamasa Ishigaki a, Atsushi Yamazaki a,b a
Advanced Materials Laboratory (AML), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan b Department of Resource and Environmental Engineering, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Sinjuku-ku, Tokyo 169-8555, Japan Received 25 November 2003; in final form 10 April 2004 Available online 4 May 2004
Abstract Epitaxial and polycrystalline anatase films were grown by pulse-powered magnetron sputtering. The photoreduction of Ag ions showed the difference in the distribution of the photocatalytic active sites in these films. The polycrystalline anatase film was covered with an Ag layer. In contrast, discrete Ag particles were interspatially deposited on the epitaxial anatase film. Evaluation of the epitaxial film by micro-Raman spectrometry revealed that the rutile coexisted at only the site where the Ag particle was precipitated. These results suggest that the rutile/anatase interface is the active site for photocatalysis and is one of the dominant factors of the photocatalytic activity. Ó 2004 Elsevier B.V. All rights reserved.
TiO2 has attracted much attention because of its photocatalytic capability. It has already been put to practical use in many applications, including anti-bacteria tile. Despite its use in multiple applications, since the overall photocatalytic performance has been suggested to be dependent upon various parameters such as the density of surface hydroxyl groups [1], light intensity, particle size and film thickness [2], further work must be done since it is still important to clarify what enables its high performance and to synthesize TiO2 with high photocatalytic activity. In addition to the above factors, it is well known that the crystalline quality is also a significant factor in the photocatalytic activity, and the high degree of crystalline brings the high photocatalytic activity. However, the polycrystalline anatase film showed higher photocatalytic activity than the epitaxial anatase film despite its inferior crystalline quality [3]. The cause of this reversal in photo*
Corresponding authors. Fax: +81-29-860-4701. E-mail addresses: [email protected] [email protected] (M. Kamei).
(T.
Miyagi),
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.04.042
catalytic activity has been of great interest. In order to clarify this point, we performed a spatially resolved evaluation of the photocatalytic activity by using process that combines the photoreduction of Ag ions with micro-Raman spectroscopy on epitaxial and polycrystalline films. The polycrystalline and epitaxial anatase thin films were prepared using a reactive pulse-powered magnetron sputtering system (MiniLabCoater 200: Frounhofer FEP) equipped with two rectangular targets [4] making a right angle with one other [5]. High purity Ti (99.9%) metal was used as a starting material. Epitaxial and polycrystalline anatase film were grown on LaAlO3 (1 0 0) and silica glass substrate respectively. The details of the deposition conditions for the growth of anatase films are listed in Table 1. The crystal structure was confirmed by the gracing incident and the h=2h X-ray diffraction (XRD). The gracing incident XRD pattern was obtained by a Rigaku Rint 2000 diffractometer using Cu-Ka1 radiation (40 kV, 450 mA) with an incident angle of 1.0°, and the high-resolution h=2h XRD pattern was obtained by a
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T. Miyagi et al. / Chemical Physics Letters 390 (2004) 399–402
Table 1 Detailed deposition conditions for TiO2 film growth Pressure during growth Target dimension Pulse frequency Duty cycle Substrate temperature Input power Oxygen flow rate Growth rate Film thickness
0.5 Pa 200 130 10 50 kHz 50% 973 K(epi), 473 K(poly) 500 W + 500 W 4 sccm 8 nm/min 240 nm
PANalytical X’pert MRD using monochromised CuKa1 radiation (45 kV, 40 mA). The photocatalytic activities of both films were evaluated by the photoreduction of Ag ions at the film surface. When the film is immersed in aqueous AgNO3 solution and exposed to UV light, the surface catalyzes the photoreduction of Agþ to Ag metal as described in the following reaction, 2H2 O + 4Agþ ! 4Ag + O2 + 4Hþ . The relative photoactivity of the film was assumed to be proportional to the amount of deposited Ag metal when illuminated by UV radiation for a fixed period of time. Moreover, this photochemical reduction of aqueous Agþ to Ag can be used as a local indicator of photochemical activity sites. In this study, the both polycrystalline and epitaxial anatase-TiO2 films were dipped in the aqueous AgNO3 solution (0.01 mol/l) and were irradiated with UV light (Hg–Xe lamp: 100 mW/ cm2 ) for 10 min. After exposure, the samples were rinsed with deionized H2 O and dried with forced N2 gas. Then the surface profiles of the samples were measured by a surface profilometer (Dektak3 Surface Profiler: Sloan). After the photoreduction of Ag, micro-Raman spectroscopy was carried out. The excitation light source was an Ar laser operating at 514.5 nm with an output power of 50 mW. The spectra were measured with a backscattering configuration at room temperature. Fig. 1a shows the gracing incident XRD pattern of the TiO2 film grown on a silica glass substrate. The diffraction peaks from the (1 0 1), (1 1 2), (0 0 4), and (2 0 0) planes of anatase TiO2 are observed at 2h ¼ 25:5 and 36.0°, 37.8°, 38.5° and 48.5°, and no diffraction peak from rutile TiO2 is observed. The standard h=2h XRD pattern for the TiO2 films grown on LaAlO3 (0 0 1) substrate is shown in Fig. 1b. Besides the diffraction peaks attributed to the LaAlO3 (0 0 1), only a peak assigned to the anatase (0 0 4) is observed. The full width at half maximum of the (0 0 4) peak rocking curve (not shown) was 0.1°, reflecting the high degree of crystalline quality. The photoreduction of Agþ was performed on these anatase films. Fig. 2 shows the surface profile of the polycrystalline (a) and epitaxial (b) anatase films after the photoreduction of Agþ . The regions of 0 nm in height are UV-unilluminated regions, that is, the sur-
Fig. 1. XRD patterns of TiO2 film grown on (a) silica glass and (b) LaAlO3 (1 0 0) substrate, respectively.
faces of anatase films. As shown in Fig. 2a, the polycrystalline anatase is covered by the Ag layer approximately 500 nm in thickness. In contrast, Ag is not uniformly deposited on the surface of epitaxial anatase film, as shown in Fig. 2b. Moreover, the height of the Ag (approximately 1000–9000 nm) is much higher than that of the Ag deposited on the polycrystalline film and there are some sites on the film where the Ag particles are not deposited. Therefore, there should be a difference in the distribution of the photocatalytic active sites in polycrystalline and epitaxial anatase films. The optical microscopy (OM) was carried out for these films. Fig. 3a shows the OM image of the polycrystalline anatase TiO2 film after the photoreduction of Agþ . The white spot in the center is the Ar laser spot used for the micro-Raman measurements. The upper left part and the lower right part of the image show the UV-illuminated and -unilluminated regions. It has been determined that the UV-illuminated part of the polycrystalline anatase film is completely covered with the Ag layer. In contrast, discrete Ag metal particles approximately 3 lm in diameter are deposited on the epitaxial thin film interspatially as shown in Fig. 3b. These OM images are in agreement with the profilometer profiles (Fig. 2). In order to examine the spatial inhomogeneity of the Ag deposited on the epitaxial anatase surface, MicroRaman spectrometry was carried out. Fig. 4a shows the
T. Miyagi et al. / Chemical Physics Letters 390 (2004) 399–402
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Fig. 2. The surface profiles of the Ag deposited on (a) the polycrystalline and (b) the epitaxial anatase films.
Fig. 3. (a) The optical microscopy images of the polycrystalline anatase TiO2 film after Ag photoreduction. The upper left part of image shows the deposited Ag and the lower right part of image show the unilluminated region. The white spot in the center is the spot of the Ar laser. (b) The optical microscopy images of the epitaxial anatase TiO2 film after Ag photoreduction. The Ag particles are sprinkled over the anatase raw surface. The circled region A indicates the large Ag particle.
typical Raman spectrum from the raw surface of the epitaxial anatase film. There are three strong bands at 148, 397 and 517 cm1 and a weak band at 643 cm1 , which are assigned to the fundamental vibrational modes of anatase TiO2 [6]. In contrast, the Raman spectrum in the vicinity of Ag particles is quite different from that of spot A. As shown in Fig. 4b, three broad bands at approximately 220, 450 and 610 cm1 are observed as indicated by the arrows, in addition to the Raman bands assigned to the anatase phase. As for the band at approximately 220 cm1 , it originates from deposited Ag because the Raman spectrum of the large Ag particle, such as the spot of A in Fig. 3b, shows the same profile [shown in the inset of Fig. 4b]. The other two bands at approximately 450 and 610 cm1 correspond to the Raman band of rutile [7], suggesting that this site is composed of a mixture of anatase and rutile phases. Moreover, this Raman spectrum with the fundamental vibration of rutile could only be observed in the vicinity of the Ag particles, suggesting that the rutile/
anatase interface may be the active site for photocatalytic activity. The LaAlO3 (0 0 1) substrate used in this experiment has an excellent lattice matching with the anatase TiO2 (0 0 1) plane and is suitable for epitaxial growth of the anatase phase. This lattice matching between anatase (0 0 1) and LaAlO3 (0 0 1) is the main driving force for the epitaxial growth of the anatase phase. However, even the finest single crystalline LaAlO3 (0 0 1) single crystalline substrate possesses crystalline imperfections and/or surface micro-scratches due to the growth and polishing process. The anatase phase cannot grow on these imperfections epitaxially. Since the substrate temperature of 973 K is high enough for rutile formation, the rutile phase can grow on these imperfections with a small probability during the nucleation. As a result, anatase epitaxial film with a small precipitation of the rutile phase can grow. It is known that the conduction band edge of rutile is approximately 0.2 eV lower than that of anatase [8]. On
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rutile works as a reaction site of the silver reduction. Since no Ag is deposited on the surface of anatase, UV light can reach the surface of anatase and continue to generate the photoexcited electrons. The electrons continue to move to the rutile site and the continuous preferential deposition of Ag take place there. In summary, by means of two techniques with spatial resolution, i.e. the photoreduction of Ag ions and the micro-Raman spectroscopy, we confirmed that the dispersed rutile nano-particles in the anatase film play the role of the reaction site of photoreduction. How to fabricate the well-defined anatase-rutile interfaces is the key factor for producing the photocatalyst titania with high activity.
Acknowledgements The authors thank Dr. Kenji Watanabe and Dr. Hideki Tanaka of NIMS for performing the microRaman analysis.
Fig. 4. Micro-Raman spectra of epitaxial anatase film after Ag photoreduciton: (a) the typical Raman spectrum at the anatase raw surface; (b) the typical Raman spectrum from the site on which Ag particle was deposited. The inset shows the Raman spectrum from the spot A in the Fig. 3b.
this basis, the model that the photoexcited electrons are effectively transferred from the conduction band of anatase to that of rutile has been proposed [9]. Therefore, the preferential Ag deposition at the interface between anatase and rutile can be explained as follows, by considering this model and the results of micro-Raman spectra. When a small domain of rutile phase coexists with the anatase phase, the interface between anatase and rutile promotes the electron transfer and then the
References [1] Y. Oosawa, M. Gr€atzel, J. Chem. Soc., Faraday Trans. 1 84 (1988) 197. [2] H. Tada, M. Tanaka, Langmuir 13 (1997) 360. [3] M. Kamei, T. Miyagi, T. Ogawa, T. Mitsuhashi, A. Yamazaki, T. Sato, T. Ishigaki, Transaction of the Material Research Society of Japan (accepted). [4] P. Frach, K. Goedicke, C. Gottfried, H. Bartzsch, Surf. Coat. Technol. 142–144 (2001) 628. [5] T. Miyagi, M. Kamei, T. Ogawa, T. Mitsuhashi, A. Yamazaki, T. Sato, Thin Solid Films 442 (2003) 32. [6] T. Ohsaka, F. Izumi, Y. Fujiki, J. Raman Spectrosc. 7 (1978) 321. [7] S.P.S. Porto, P.A. Fleury, T.C. Damen, Phys. Rev. 154 (1967) 522. [8] L. Kavan, M. Gr€atzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc. 118 (1996) 6716. [9] T. Kawahara, Y. Konishi, H. Tada, N. Tohge, J. Nishii, S. Ito, Angew. Chem. Int. Ed. 41 (2002) 2811.
Charge separation at the rutile/anatase interface: a dominant factor of photocatalytic activity Takahira Miyagi a,b,*, Masayuki Kamei a,*, Takefumi Mitsuhashi a, Takamasa Ishigaki a, Atsushi Yamazaki a,b a
Advanced Materials Laboratory (AML), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan b Department of Resource and Environmental Engineering, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Sinjuku-ku, Tokyo 169-8555, Japan Received 25 November 2003; in final form 10 April 2004 Available online 4 May 2004
Abstract Epitaxial and polycrystalline anatase films were grown by pulse-powered magnetron sputtering. The photoreduction of Ag ions showed the difference in the distribution of the photocatalytic active sites in these films. The polycrystalline anatase film was covered with an Ag layer. In contrast, discrete Ag particles were interspatially deposited on the epitaxial anatase film. Evaluation of the epitaxial film by micro-Raman spectrometry revealed that the rutile coexisted at only the site where the Ag particle was precipitated. These results suggest that the rutile/anatase interface is the active site for photocatalysis and is one of the dominant factors of the photocatalytic activity. Ó 2004 Elsevier B.V. All rights reserved.
TiO2 has attracted much attention because of its photocatalytic capability. It has already been put to practical use in many applications, including anti-bacteria tile. Despite its use in multiple applications, since the overall photocatalytic performance has been suggested to be dependent upon various parameters such as the density of surface hydroxyl groups [1], light intensity, particle size and film thickness [2], further work must be done since it is still important to clarify what enables its high performance and to synthesize TiO2 with high photocatalytic activity. In addition to the above factors, it is well known that the crystalline quality is also a significant factor in the photocatalytic activity, and the high degree of crystalline brings the high photocatalytic activity. However, the polycrystalline anatase film showed higher photocatalytic activity than the epitaxial anatase film despite its inferior crystalline quality [3]. The cause of this reversal in photo*
Corresponding authors. Fax: +81-29-860-4701. E-mail addresses: [email protected] [email protected] (M. Kamei).
(T.
Miyagi),
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.04.042
catalytic activity has been of great interest. In order to clarify this point, we performed a spatially resolved evaluation of the photocatalytic activity by using process that combines the photoreduction of Ag ions with micro-Raman spectroscopy on epitaxial and polycrystalline films. The polycrystalline and epitaxial anatase thin films were prepared using a reactive pulse-powered magnetron sputtering system (MiniLabCoater 200: Frounhofer FEP) equipped with two rectangular targets [4] making a right angle with one other [5]. High purity Ti (99.9%) metal was used as a starting material. Epitaxial and polycrystalline anatase film were grown on LaAlO3 (1 0 0) and silica glass substrate respectively. The details of the deposition conditions for the growth of anatase films are listed in Table 1. The crystal structure was confirmed by the gracing incident and the h=2h X-ray diffraction (XRD). The gracing incident XRD pattern was obtained by a Rigaku Rint 2000 diffractometer using Cu-Ka1 radiation (40 kV, 450 mA) with an incident angle of 1.0°, and the high-resolution h=2h XRD pattern was obtained by a
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T. Miyagi et al. / Chemical Physics Letters 390 (2004) 399–402
Table 1 Detailed deposition conditions for TiO2 film growth Pressure during growth Target dimension Pulse frequency Duty cycle Substrate temperature Input power Oxygen flow rate Growth rate Film thickness
0.5 Pa 200 130 10 50 kHz 50% 973 K(epi), 473 K(poly) 500 W + 500 W 4 sccm 8 nm/min 240 nm
PANalytical X’pert MRD using monochromised CuKa1 radiation (45 kV, 40 mA). The photocatalytic activities of both films were evaluated by the photoreduction of Ag ions at the film surface. When the film is immersed in aqueous AgNO3 solution and exposed to UV light, the surface catalyzes the photoreduction of Agþ to Ag metal as described in the following reaction, 2H2 O + 4Agþ ! 4Ag + O2 + 4Hþ . The relative photoactivity of the film was assumed to be proportional to the amount of deposited Ag metal when illuminated by UV radiation for a fixed period of time. Moreover, this photochemical reduction of aqueous Agþ to Ag can be used as a local indicator of photochemical activity sites. In this study, the both polycrystalline and epitaxial anatase-TiO2 films were dipped in the aqueous AgNO3 solution (0.01 mol/l) and were irradiated with UV light (Hg–Xe lamp: 100 mW/ cm2 ) for 10 min. After exposure, the samples were rinsed with deionized H2 O and dried with forced N2 gas. Then the surface profiles of the samples were measured by a surface profilometer (Dektak3 Surface Profiler: Sloan). After the photoreduction of Ag, micro-Raman spectroscopy was carried out. The excitation light source was an Ar laser operating at 514.5 nm with an output power of 50 mW. The spectra were measured with a backscattering configuration at room temperature. Fig. 1a shows the gracing incident XRD pattern of the TiO2 film grown on a silica glass substrate. The diffraction peaks from the (1 0 1), (1 1 2), (0 0 4), and (2 0 0) planes of anatase TiO2 are observed at 2h ¼ 25:5 and 36.0°, 37.8°, 38.5° and 48.5°, and no diffraction peak from rutile TiO2 is observed. The standard h=2h XRD pattern for the TiO2 films grown on LaAlO3 (0 0 1) substrate is shown in Fig. 1b. Besides the diffraction peaks attributed to the LaAlO3 (0 0 1), only a peak assigned to the anatase (0 0 4) is observed. The full width at half maximum of the (0 0 4) peak rocking curve (not shown) was 0.1°, reflecting the high degree of crystalline quality. The photoreduction of Agþ was performed on these anatase films. Fig. 2 shows the surface profile of the polycrystalline (a) and epitaxial (b) anatase films after the photoreduction of Agþ . The regions of 0 nm in height are UV-unilluminated regions, that is, the sur-
Fig. 1. XRD patterns of TiO2 film grown on (a) silica glass and (b) LaAlO3 (1 0 0) substrate, respectively.
faces of anatase films. As shown in Fig. 2a, the polycrystalline anatase is covered by the Ag layer approximately 500 nm in thickness. In contrast, Ag is not uniformly deposited on the surface of epitaxial anatase film, as shown in Fig. 2b. Moreover, the height of the Ag (approximately 1000–9000 nm) is much higher than that of the Ag deposited on the polycrystalline film and there are some sites on the film where the Ag particles are not deposited. Therefore, there should be a difference in the distribution of the photocatalytic active sites in polycrystalline and epitaxial anatase films. The optical microscopy (OM) was carried out for these films. Fig. 3a shows the OM image of the polycrystalline anatase TiO2 film after the photoreduction of Agþ . The white spot in the center is the Ar laser spot used for the micro-Raman measurements. The upper left part and the lower right part of the image show the UV-illuminated and -unilluminated regions. It has been determined that the UV-illuminated part of the polycrystalline anatase film is completely covered with the Ag layer. In contrast, discrete Ag metal particles approximately 3 lm in diameter are deposited on the epitaxial thin film interspatially as shown in Fig. 3b. These OM images are in agreement with the profilometer profiles (Fig. 2). In order to examine the spatial inhomogeneity of the Ag deposited on the epitaxial anatase surface, MicroRaman spectrometry was carried out. Fig. 4a shows the
T. Miyagi et al. / Chemical Physics Letters 390 (2004) 399–402
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Fig. 2. The surface profiles of the Ag deposited on (a) the polycrystalline and (b) the epitaxial anatase films.
Fig. 3. (a) The optical microscopy images of the polycrystalline anatase TiO2 film after Ag photoreduction. The upper left part of image shows the deposited Ag and the lower right part of image show the unilluminated region. The white spot in the center is the spot of the Ar laser. (b) The optical microscopy images of the epitaxial anatase TiO2 film after Ag photoreduction. The Ag particles are sprinkled over the anatase raw surface. The circled region A indicates the large Ag particle.
typical Raman spectrum from the raw surface of the epitaxial anatase film. There are three strong bands at 148, 397 and 517 cm1 and a weak band at 643 cm1 , which are assigned to the fundamental vibrational modes of anatase TiO2 [6]. In contrast, the Raman spectrum in the vicinity of Ag particles is quite different from that of spot A. As shown in Fig. 4b, three broad bands at approximately 220, 450 and 610 cm1 are observed as indicated by the arrows, in addition to the Raman bands assigned to the anatase phase. As for the band at approximately 220 cm1 , it originates from deposited Ag because the Raman spectrum of the large Ag particle, such as the spot of A in Fig. 3b, shows the same profile [shown in the inset of Fig. 4b]. The other two bands at approximately 450 and 610 cm1 correspond to the Raman band of rutile [7], suggesting that this site is composed of a mixture of anatase and rutile phases. Moreover, this Raman spectrum with the fundamental vibration of rutile could only be observed in the vicinity of the Ag particles, suggesting that the rutile/
anatase interface may be the active site for photocatalytic activity. The LaAlO3 (0 0 1) substrate used in this experiment has an excellent lattice matching with the anatase TiO2 (0 0 1) plane and is suitable for epitaxial growth of the anatase phase. This lattice matching between anatase (0 0 1) and LaAlO3 (0 0 1) is the main driving force for the epitaxial growth of the anatase phase. However, even the finest single crystalline LaAlO3 (0 0 1) single crystalline substrate possesses crystalline imperfections and/or surface micro-scratches due to the growth and polishing process. The anatase phase cannot grow on these imperfections epitaxially. Since the substrate temperature of 973 K is high enough for rutile formation, the rutile phase can grow on these imperfections with a small probability during the nucleation. As a result, anatase epitaxial film with a small precipitation of the rutile phase can grow. It is known that the conduction band edge of rutile is approximately 0.2 eV lower than that of anatase [8]. On
402
T. Miyagi et al. / Chemical Physics Letters 390 (2004) 399–402
rutile works as a reaction site of the silver reduction. Since no Ag is deposited on the surface of anatase, UV light can reach the surface of anatase and continue to generate the photoexcited electrons. The electrons continue to move to the rutile site and the continuous preferential deposition of Ag take place there. In summary, by means of two techniques with spatial resolution, i.e. the photoreduction of Ag ions and the micro-Raman spectroscopy, we confirmed that the dispersed rutile nano-particles in the anatase film play the role of the reaction site of photoreduction. How to fabricate the well-defined anatase-rutile interfaces is the key factor for producing the photocatalyst titania with high activity.
Acknowledgements The authors thank Dr. Kenji Watanabe and Dr. Hideki Tanaka of NIMS for performing the microRaman analysis.
Fig. 4. Micro-Raman spectra of epitaxial anatase film after Ag photoreduciton: (a) the typical Raman spectrum at the anatase raw surface; (b) the typical Raman spectrum from the site on which Ag particle was deposited. The inset shows the Raman spectrum from the spot A in the Fig. 3b.
this basis, the model that the photoexcited electrons are effectively transferred from the conduction band of anatase to that of rutile has been proposed [9]. Therefore, the preferential Ag deposition at the interface between anatase and rutile can be explained as follows, by considering this model and the results of micro-Raman spectra. When a small domain of rutile phase coexists with the anatase phase, the interface between anatase and rutile promotes the electron transfer and then the
References [1] Y. Oosawa, M. Gr€atzel, J. Chem. Soc., Faraday Trans. 1 84 (1988) 197. [2] H. Tada, M. Tanaka, Langmuir 13 (1997) 360. [3] M. Kamei, T. Miyagi, T. Ogawa, T. Mitsuhashi, A. Yamazaki, T. Sato, T. Ishigaki, Transaction of the Material Research Society of Japan (accepted). [4] P. Frach, K. Goedicke, C. Gottfried, H. Bartzsch, Surf. Coat. Technol. 142–144 (2001) 628. [5] T. Miyagi, M. Kamei, T. Ogawa, T. Mitsuhashi, A. Yamazaki, T. Sato, Thin Solid Films 442 (2003) 32. [6] T. Ohsaka, F. Izumi, Y. Fujiki, J. Raman Spectrosc. 7 (1978) 321. [7] S.P.S. Porto, P.A. Fleury, T.C. Damen, Phys. Rev. 154 (1967) 522. [8] L. Kavan, M. Gr€atzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc. 118 (1996) 6716. [9] T. Kawahara, Y. Konishi, H. Tada, N. Tohge, J. Nishii, S. Ito, Angew. Chem. Int. Ed. 41 (2002) 2811.