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Photoinduced One-Electron Reduction of MV2+ in Titania Nanosheets Using Porphyrin in Mesoporous Silica Thin Films Tatsuto Yui,†,‡ Takako Tsuchino,† Toshio Itoh,† Makoto Ogawa,§ Yoshiaki Fukushima,| and Katsuhiko Takagi*,†,‡ Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603, Japan, Department of Earth Sciences, Waseda University, Nishiwaseda 1, Shinjuku-ku, Tokyo 169-8050, Japan, Toyota Central R & D Labs Inc., Yokomichi, Nagakute, Aichi 480-1192, Japan, and CREST (JST) Received October 23, 2004. In Final Form: January 31, 2005 Composite films of a meso-(tetramethylpyridinium)porphyrin (TMPyP) hybrid incorporated in mesoporous silica (MPS) and cast on a methyl viologen (MV2+)/titania nanosheet hybrid were synthesized and a lightinduced charge separation between the two could be observed. These composite thin films were able to initiate a one-electron reduction of the MV2+ ions accompanied by the simultaneous decomposition of the TMPyP organic dye within the mesoporous silica channels.
Introduction The laminated hybridization of organic dyes and electron acceptors spatially separated by mesoporous silica and inorganic layered materials has attracted much attention in recent years for its role in the suppression of the back electron transfer between the one-electronreduced and the one-electron-oxidized species in visible light responsive photocatalytic systems.1,2 In our previous work, we prepared a layered metal oxide semiconductor (LMOS, e.g., TiNbO5) including an organic dye (meso(tetramethylpyridinium) porphyrin, TMPyP) and an electron acceptor (methyl viologen, MV2+), and the obtained triad hybrid film of TMPyP/LMOS/MV2+ was confirmed to exhibit photofunctionality for visible-light-induced charge separations.3 Chart 1. Molecular Formula of TMPyP and
MV2+
Significantly, such charge separations could be achieved by the intervention of the LMOS sheets between the TMPyP as the sensitizer and MV2+ as the electron acceptor, * To whom correspondence may be addressed. E-mail: ktakagi@ apchem.nagoya-u.ac.jp. † Nagoya University. ‡ CREST (JST). § Waseda University. | Toyota Central R & D Labs Inc. (1) Ogawa, M. J. Photochem. Photobiol., C: Photochem. Rev. 2002, 3, 129. (2) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (b) Takagi, K.; Shichi, T. Solid State and Surface Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2000; Vol. 5, p 31. (c) Shichi, T.; Takagi, K. J. Photochem. Photobiol., C: Photochem. Rev. 2000, 1, 113. (d) Yamaguch, Y.; Yui, T.; Takagi, S.; Shimada, T.; Inoue, H. Chem. Lett. 2001, 644. (3) Tong, Z.; Shichi, T.; Oshika, K.; Takagi, K. Chem. Lett. 2002, 876. (b) Tong, Z.; Shichi, T.; Takagi, K. J. Phys. Chem. B 2002, 106, 13306. (c) Tong, Z.; Shichi, T.; Kasuga, Y.; Takagi, K. Chem. Lett. 2002, 1206.
Figure 1. Schematic structure of the composite TMPyP/MPS and MV2+/TN tetrad hybrid film.
indicating that such layered metal oxide semiconductors could function as both an electron-transporting mediator and a charge-separating species. In line with the pioneering work on titania nanosheets (TN) first carried out by Sasaki et al., we have conducted research on laminated LMOS hybrids using TN sheets by electrophoretic deposition, followed by soaking in a MV2+ solution to construct TN/MV2+ hybrid films in which a light-induced charge separation could be observed between the TN and MV2+.4,5 In the present communication, we have carried out investigations on a novel UV-light-induced charge transfer from the TMPyP organic dye within the mesoporous silica (MPS) channels to the MV2+ ions independently included within TN, as shown in Figure 1. Results and Discussion The laminated hybrid composite consecutively stacked with TMPyP/MPS and MV2+/MPS was synthesized by the following procedures, outlined in Scheme 1: An aqueous MPS precursor gel suspension of a cationic surfactant, i.e., CTAC (cetyltrimethylammonium chloride), was spin(4) Yui, T.; Mori, Y.; Tsuchino, T.; Itoh, T.; Hattori, T.; Fukushima, Y.; Takagi, K. Chem. Mater. 2005, 17, 206. (5) Sasaki, T.; Watanabe, M.; Hashizumi, H.; Yamada, H.; Hakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (b) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (c) Sasaki, T.; Ebina, Y.; Watanabe, M.; Decher, G. J. Chem. Soc., Chem. Commun. 2000, 2163. (d) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M.; Decher, G. Chem. Mater. 2001, 13, 4661.
10.1021/la047385+ CCC: $30.25 © 2005 American Chemical Society Published on Web 02/26/2005
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Figure 2. XRD profiles of the composite TMPyP/MPS hybrid coated with the TN thin film (a) before and (b) after soaking in a MV2+ aqueous solution. Scheme 1. Experimental Outline of the Composite TMPyP/MPS and MV2+/TN Tetrad Hybrid Film
coated on an ITO electrode and then calcinated at 500°C for 3 h.6 The resulting mesoporous silica thin films were observed to be cubic in structure by the presence of three characteristic X-ray diffraction (XRD) signals, indicating the regularity of the distance at 33.4 Å (i.e., d210) along the axis vertical to the (210) plane. The thin films were soaked for 1.0 h in an aqueous 9.68 × 10-5 M solution of TMPyP at ambient temperature, leading to the formation of brownish-colored films. The absorption spectra of the films showed a sharp Soret band at 430 nm, which is 5 nm red-shifted when compared with the band in aqueous solution. This spectral shift suggests the non-aggregated adsorption of TMPyP in the mesopores of the MPS, i.e., TMPyP/MPS, and was clearly different from the TMPyP adsorbed in the TN interlayers (λmax 480 nm).7 The TMPyP/ MPS hybrid films were hybridized with TN nanosheets by electrophoretic or casting deposition and were then soaked in an aqueous 2.06 × 10-4 M solution of MV2+ for 4 h at ambient temperature.4 The obtained TMPyP-MPS/ TN hybrid films were first examined by XRD analysis before and after soaking in a MV2+ aqueous solution, as shown in Figure 2. A sharp diffraction peak at d001 ) 17.4 Å was observed before soaking (Figure 2a); however, the corresponding d001 peak broadened and shifted to a higher angle at d001 ) 11.7 Å, implying the adsorption of MV2+ into the TN interlayers with a flat and parallel conformation against the TN surfaces.4 Weak XRD peaks of the MPS thin films still remained at around 2θ ) 2.8°, even after soaking in H2O. From these results, it could be confirmed that after the TMPyP/MPS hybrid films were deposited on the ITO electrode, the MV2+/TN lamella hybrid films could be separately and successively stacked without any cosolubilization of the dye and acceptors, as (6) Ogawa, M.; Masukawa, N. Microporous Mesoporous Mater. 2000, 38, 35. (7) Takagi, S.; Shimada, T.; Yui, T.; Inoue, H. Chem. Lett. 2001, 128. (b) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 2265. (c) Eguchi, M.; Takagi, S.; Tachibana, H.; Inoue, H. J. Phys. Chem. Solids 2004, 65, 403.
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Figure 3. The absorption spectral changes in the TMPyP/ MPS incorporated MV2+/TN thin films upon UV light irradiation for 0-30 min.
shown in Figure 1. The TMPyP and MV2+ molecules could, thus, be observed to be spatially separated by the intervention of the mesoporous silica and titania nanosheets. The TMPyP/MPS incorporated MV2+/TN composite thin film was irradiated for 60 min by visible light of 390-550 nm which was absorbed only by the TMPyP; however, no detectable spectral changes in the absorption could be observed. This could be explained by the fact that TN has a more negative conduction band potential at -1.02 V (vs SCE)8 than the redox potential of TMPyP in the excited state at (E°(S+/S*) ) -0.73 V vs SCE),9 the difference of 0.29 V being endothermic for an electron transfer. In contrast, UV light irradiation at 270-380 nm turned the color of the hybrid films from brown to blue. Under these conditions, TN could be seen to function as a photocatalyst to form a charge-separated state since almost all of the incident light (>99%) could be absorbed by TN. Figure 3 shows the changes in the absorption spectra of the hybrid thin films upon UV light irradiation. With UV light irradiation, the Soret absorption band of TMPyP was seen to decrease with the appearance of new absorptions at ca. 410 nm and ca. 600 nm, while retaining the two isosbestic points at 420 and 500 nm. The absorption spectra centered at 410 and ∼600 nm could be assigned to the cation radicals of MV2+ (MV+•).4,10 These results can be explained by the bleaching of the color which is attributed to the oxidative decomposition of TMPyP by the holes in the valence band of the charge-separated TN. The decomposition of the TMPyP accommodated in the mesoporous silica is observed to proceed simultaneously with the reduction of MV2+ incorporated in the TN interlayers, resulting in the photoinduced charge separation between the TN and MPS. It could, therefore, be observed that the photochemically formed holes were able to migrate to the TMPyP through the cubic MPS nanocavities. The present reaction mechanisms can be expressed by the equations
TN + hν f e-cb + h+
(1)
e-cb + MV2+ f MV+•
(2)
h+ + TMPyP f decomposition
(3)
-
where, e cb indicates an electron in the conduction band of TN and h+ indicates the hole in the valence band of TN. Further investigation confirmed such hole trapping reductive agents as melcaptoethanol (RSH) and methylene blue (MB) to be incorporated together with TMPyP in the (8) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2004, 126, 5851.
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MPS mesopores.11 Upon UV light irradiation, MV+• could be formed, accompanied by suppression of the decomposition of TMPyP. These results clearly indicated the TMPyP molecules to be decomposed by the photochemically formed holes (h+). In conclusion, it could be seen that UV light irradiation of the laminated composite films of a nanostructured hybrid of TMPyP and MV2+ which were separately (9) Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163. (b) Worthinbton, P.; Hambright, R. F. X. Williams, J. Reid, C. Burnham, P. Shamin, J. Turray, D. M. Bell, R. Kinkland, P.; Little, R. G.; Datta-Gupta, N.; Eisner, U. J. Inorg. Biochem. 1980, 12, 281. (10) Kamogawa, H.; Mizuno, H.; Todo, Y.; Nanasawa, M. J. Polym. Sci. 1979, 17, 3149. (b) Lee, C.; Lee, Y. M.; Moon, M. S.; Park, S. H.; Park, J. W.; Kim, K. G.; Jeon, S-. J. J. Electroanal. Chem. 1996, 416, 139. (c) Monk, P. M. S.; Fairweather, R. D.; Ingram, M. D.; Duffy, J. A. J. Chem. Soc., Perkin Trans. 2 1992, 2039.
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adsorbed within MPS and TN was able to induce the simultaneous decomposition of TMPyP and the formation of MV+•, indicating a photoinduced charge separation through the mesoporous silica and TN. Such investigations on laminated stacked films are yet unprecedented and have great potential for the design and development of photofunctional devices and systems. Acknowledgment. This work was partly supported by a Grant-in Aid for Scientific Research on Priority Areas (417) of the Ministry of Education, Science, Culture, and Sports, Science and Technology (MEXT) of Japan. We express our thanks for their kind support. LA047385+ (11) Itoh, T.; Yano, K.; Inada, Y.; Fukushima, Y. J. Am. Chem. Soc. 2002, 124, 13437.
