Preparation of Aluminum-Containing Mesoporous Silica Films

Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, ... Laboratory for Materials Science and Technology, Waseda Universit...
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Langmuir 2002, 18, 744-749

Preparation of Aluminum-Containing Mesoporous Silica Films Makoto Ogawa,*,†,‡ Kazuyuki Kuroda,§,| and Jun-ichi Mori§ PRESTO, Japan Science and Technology Corporation, Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan, Department of Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan, and Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan Received September 11, 2001. In Final Form: November 5, 2001 Aluminum-containing mesoporous silica films were synthesized by the solvent evaporation method using tetraethoxysilane, aluminum tri(sec-butoxide), and octadecyltrimethylammonium chloride as the silica source, the alumina source, and a supramolecular template, respectively. The Al/Si ratios were 0.004-0.031 in the present synthesis. The resulting films with the Al/Si ratios of 0.004, 0.013, and 0.022 were highly transparent in the wavelength region of 200-800 nm. The porous structures of the aluminumcontaining films were characterized by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption. Although the hexagonal pore arrangements were disordered to some extent by the addition of aluminum, the pore size uniformity was confirmed by the nitrogen adsorption isotherms. The cation exchange ability was evaluated by the adsorption of a cationic azobenzene derivative, p-(ω-dimethylhydroxyethylammonioethoxy)-azobenzene bromide, which was not adsorbed effectively on a siliceous mesoporous film. The cation exchange ability of the aluminum-containing mesoporous silica films was semiquantitatively controlled by the amount of added aluminum.

Introduction Mesoporous materials prepared by the supramolecular templating approaches have been investigated extensively in this decade from both fundamental and scientific viewpoints owing to their unusually high surface area and porosity, ordered pore arrangements, pore size uniformity, and possible surface engineering.1 Efforts have been made to apply mesoporous silicas as adsorbents, as catalysts and their supports, and in host-guest chemistry. Besides the variation of microstructures and chemical compositions, the morphology of mesoporous materials is a key issue for their practical applications. Film is an ideal morphology for the applications of mesoporous materials to optical, electronic, and sensing devices as well as separation. We have successfully synthesized thin films of silicaalkyltrimethylammonium salt mesostructured materials by depositing a precursor solution containing surfactant and a soluble silica precursor derived from alkoxysilanes (solvent evaporation method).2 Due to the ease of synthetic operation and the homogeneity and quality of the resulting materials, the solvent evaporation method has become a versatile technique for the fabrication of silica-surfactant mesostructured materials. The controlled morphology of mesoporous silicas achieved by the solvent evaporation method includes thin coating films,3 self-standing films,4 fibers,5 and hollow spheres.6 Taking advantage of the transparency of the films, we have been interested in the immobilization of organic * To whom correspondence should be addressed. Tel: 81-3-52861511. Fax: 81-3-3207-4950. E-mail: [email protected]. † PRESTO, Japan Science and Technology Corporation. ‡ Department of Earth Sciences, Waseda University. § Department of Applied Chemistry, Waseda University. | Kagami Memorial Laboratory for Materials Science and Technology, Waseda University. (1) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. Ciesla, U.; Schu¨th, F. Microporous Mesoporous Mater. 1999, 27, 131. (2) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941.

photoactive species in the mesostructured materials with a view toward future photofunctional materials. Photochemistry in constrained media is a growing new field which yields a wide variety of useful applications such as sensitive optical media, reaction paths for controlled photochemical reactions, molecular devices for optics, and so forth.7 For such applications, mesoporous silica films possess attractive features such as large surface area and porosity, controllable pore sizes, reactive pore surfaces, and stability in a wide temperature range. Along these lines, the incorporation of organic dyes into thin films of silica-surfactant mesostructured materials8 and mesoporous silica films9 has been reported so far. The ways to immobilize organic functional groups on the mesopore surfaces can be classified into three: (1) (3) (a) Ogawa, M. Chem. Commun. 1996, 1149. (b) Anderson, M. T.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P.; Wilcoxon, J. P. Microporous Mater. 1997, 10, 13. (c) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. (4) (a) Ogawa, M.; Kikuchi, T. Adv. Mater. 1998, 10, 1077. (b) Ogawa, M.; Ikeue, K.; Anpo, M. Chem. Mater. 2001, 13, 2900. (c) Ryoo, R.; Ko, C. H.; Cho, S. J.; Kim, J. M. J. Phys. Chem. B 1997, 101, 10610. (d) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332. (e) Melosh, N. A.; Davidson, P.; Chmelka, B. F J. Am. Chem. Soc. 2000, 122, 823. (f) Melosh, N. A.; Davidson, P.; Feng, P.; Pine, D. J.; Chmelka, B. F. J. Am. Chem. Soc. 2001, 123, 1240. (g) Feng, P.; Bu, X.; Stucky, G. D.; Pine, D. J. J. Am. Chem. Soc. 2000, 122, 994. (5) Bruinshman, P. J.; Kim, J A.; Liu, Y.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (6) Ogawa, M.; Yamamoto, N. Langmuir 1999, 15, 2227. (7) Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991. (8) (a) Ogawa, M. Langmuir 1995, 11, 4639. (b) Ogawa, M.; Igarashi, T.; Kuroda, K. Chem. Mater. 1988, 10, 1382. (c) Ferrer, M.; Lianos, P. Langmuir 1996, 12, 5620. (d) Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I.; Langmuir 1998, 14, 7331. (e) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (f) Lebeau, B.; Fowler, C. E.; Hall, S. R.; Mann, S. J. Mater. Chem. 1999, 9, 2279. (g) Hata, H.; Kimura, T.; Ogawa, M.; Sugahara, Y.; Kuroda, K. J. Sol.Gel Sci. Technol. 2000, 19, 543. (h) Yang, P.; Wirnsberger, G.; Huang, H.; Cordero, S.; McGehee, M. D.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B.; Buratto, S. k.; Stucky, G. D. Science 2000, 287, 465.

10.1021/la011429m CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002

Aluminum-Containing Mesoporous Silica Films

adsorption of nonionic and cationic species, (2) grafting by the condensation reaction with surface silanol group, and (3) co-condensation of functionalized alkoxysilane with tetraalkoxysilanes during the mesostructure synthesis.10 For thin film samples, which are relatively fragile if compared with powder samples, the adsorption of cationic dyes into mesoporous silicas is attractive to construct functional silica-dye nanomaterials among the abovementioned approaches. Therefore, there is a demand for mesoporous silica films with controllable cation exchange capacity and acidic properties. The introduction of aluminum into mesoporous silicas is a possible solution for this purpose. The introduction of aluminum into mesoporous silica powders has been recognized as a way to impart acidity and cation exchange ability to the resulting materials.11 However, to our knowledge, there is no report on the preparation of aluminum-containing mesoporous silica films except our previous communication, where we reported the successful synthesis of aluminum-containing mesoporous silica films by a rapid solvent evaporation method.12 In this paper, we report the details of the synthesis and characterization of aluminum-containing mesoporous silica films. The resulting films have been utilized as immobilizing media of a cationic azobenzene derivative, p-(ω-dimethyl-hydroxyethylammonioethoxy)-azobenzene (hereafter abbreviated as AZ+) bromide. Because AZ+ cations are not adsorbed on purely siliceous mesoporous silica films effectively, the quantitative analysis of the adsorption of AZ+ is a way to evaluate the cation exchange ability and the amount of acidic sites of the mesoporous silica films. Note that conventional methods to evaluate those parameters, such as temperature-programmed desorption, which have been developed for bulk and powder samples, are difficult to apply for thin film samples. Experimental Section Materials. Tetraethoxysilane (abbreviated as TEOS), aluminum tri(sec-butoxide) (abbreviated as ATSB), and octadecyltrimethylmmonium chloride (abbreviated as C18TAC) were obtained from Tokyo Kasei Industries Co. and used without further purification. Ethanol and hydrochloric acid are reagent grade, obtained from Tokyo Kasei Industries Co. Phenylazophenol, dibromoethane, and dimethylaminoethanol were purchased from Tokyo Kasei Industries Co. A cationic azo dye, p-(ω-dimethyl-hydroxyethylammonioethoxy)-azobenzene bromide (AZ+Br-), was synthesized by the reaction between phenylazophenol and dibromoethane and by the subsequent reaction with dimethylaminoethanol. The dye was characterized by 1H NMR, CHN analysis, and mass spectrometry. The dye was used after recrystallization from ethanol. Sample Preparation. Aluminum-containing mesoporous silica films were synthesized by the rapid solvent evaporation method.2,3 ATSB was used as an aluminum source, and the cosolidification with silica was conducted by the reported solgel synthesis for the aluminosilica xerogels13 with a slight modification. A typical synthetic procedure for the transparent thin films of aluminum-containing silica-surfactant mesostructured materials is as follows: TEOS, ethanol, water, and HCl (at the molar ratio of 1:4:1.2:0.006) were mixed at 60 °C for 10 min with magnetic stirring. To this solution was added ASTB, and (9) (a) Ogawa, M.; Ishikawa, H.; Kikuchi, T. J. Mater. Chem. 1998, 8, 1783. (b) Nguen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (10) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. (11) (a) Stein, A.; Holland, B. J. Porous Mater. 1996, 3, 83. (b) Tuel, A. Microporous Mesoporous Mater. 1999, 27, 151. (12) Ogawa, M.; Kuroda, K.; Mori, J. Chem. Commun. 2000, 2441. (13) Pozamsky, G. A.; McCormick, A. V. J. Non-Cryst. Solids 1995, 190, 212.

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Figure 1. X-ray diffraction patterns of the as-synthesized aluminum-containing films of silica-surfactant mesostructured materials with Al/Si ) 0 (a), 0.014 (b), and 0.031 (c). the mixture was allowed to react for another 10 min at 60 °C. Finally, an aqueous solution of C18TAC was added, and the mixture was allowed to react for 2 h at room temperature. The resulting solution was spin coated on a substrate and calcined in air at 300 °C for 12 h to prepare mesoporous silica films. The ASTB/TEOS ratio was varied at 0.05, 0.031, 0.025, 0.022, and 0.014. The adsorption of dye was conducted by immersing a calcined film into an ethanol solution of the dye at room temperature for 3 h. After the reaction, the films were washed with ethanol. Characterization. X-ray diffraction patterns were obtained on a Mac Science, M03XHF22 diffractometer using Mn filtered Fe KR radiation operated at 40 kV and 30 mA. Visible absorption spectra were recorded on a Shimadzu UV-3100PC spectrophotometer. Scanning electron micrographs were obtained on a Hitachi S-2840N scanning electron microscope. Transmission electron micrographs of the films were obtained on a Hitachi H8100 transmission electron microscope with an accelerating voltage of 200 kV. Nitrogen adsorption isotherms of the calcined films were measured at 77 K on a Bell Sorp TCV (Bell Japan Inc.). For the nitrogen adsorption measurements, films were prepared on a cover glass with submicrometer thickness. Prior to the measurement, the films were degassed under vacuum at 150 °C for 3 h. The adsorbed amounts of AZ+ were determined by the change in the concentration of AZ+ in the solution using visible absorption spectra. Photochemical Reaction. The photochemical reactions were carried out using a 500 W super high pressure Hg lamp (USHIO USH-500D). A band-pass filter, Toshiba UV-D35, the transmittance centered at 350 nm, was used for isolating the UV light. For the cis-to-trans backward reactions, a sharp cut filter, HOYA L42 (cutoff wavelength is 420 nm), was used to obtain visible light. The reactions were monitored by the change in the absorbance of the trans isomer of the azobenzene.

Results and Discussion By spin coating the precursor solution, transparent thin films with the thickness of a few hundreds of nanometers formed. The thickness can be controlled by changing the composition of the precursor solution and the spinning rate. To avoid the peeling of the films from the substrate and to obtain reproducible results, the thickness of the films was adjusted to a submicrometer range. The X-ray diffraction patterns of the as-synthesized films with different Al contents are shown in Figure 1. A single diffraction peak indicative of the formation of surfactanttemplated mesostructures was observed for all samples, though the diffraction peaks became broad with the increase in the loaded Al amount. The basal spacings were also consistent with those observed for the silica-

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Figure 2. X-ray diffraction patterns of the calcined aluminumcontaining films of silica-surfactant mesostructured materials with Al/Si ) 0 (a), 0.014 (b), and 0.031 (c).

alkyltrimethylammonium mesostructured materials reported previously. The SEM images of the film surface (data not shown) indicate that the films were continuous and crack free when the ASTB/TEOS ratios were 0.031, 0.025, 0.022, and 0.014. When the ASTB/TEOS ratio was increased to 0.05, a homogeneous solution was not obtained under the experimental conditions and the resulting film was slightly turbid. All these observations indicate that the films prepared at the ASTB/TEOS ratios of 0.031, 0.025, 0.022, and 0.014 are homogeneous, while the film prepared at the ASTB/TEOS ratio of 0.05 contains isolated aluminum hydroxide particles. To remove surfactants from the samples to obtain porous silica films, the as-synthesized films were calcined in air at 300 °C. The transparency of the films was retained after the calcination. The X-ray diffraction patterns of the calcined films are shown in Figure 2a-c. The d values decreased upon calcination as shown in Figures 1 and 2, suggesting the shrinkage of the silica walls. This observation is in accordance with those reported previously for aluminum-free silica-alkyltrimethylammonium salt systems.3a,b Nitrogen adsorption isotherms of the calcined films (Al/ Si ) 0, 0.014, and 0.022) are shown in Figure 3. The isotherms are type IV irrespective of the Al contents, showing that the films are mesoporous. The average pore sizes were derived from the isotherms by the HorvathKawazoe method14 to be 3.2, 2.9, and 2.8 nm for the films with Al/Si ) 0, 0.014, and 0.031, respectively. Since the isotherms were obtained for the films supported on the substrate, it was very difficult to determine the exact weight of the porous film used in the measurements. Therefore, the surface area and the porosity of the film cannot be determined exactly at the present stage. Assuming that the film weight was ca. 1 µg/1 cm-2, the Brunauer-Emmett-Teller (BET) surface area was roughly estimated to be ca. 1000 m2 g-1 and the porosity was 0.3 mL/g. The detection limit of the conventional apparatus for the nitrogen adsorption isotherms is not so sensitive, so it has been very difficult to obtain nitrogen adsorption isotherms of thin films with the thickness of submicrometers. For such kind of samples, QCM equipped with SAW devices has been used to obtain isotherms.8e In the present study, a highly sensitive apparatus was employed and the isotherms were successfully obtained

for the thin coating films with the thickness of several hundreds of nanometers and the area of a few cm2. The TEM images of the films are shown in Figure 4. Hexagonally arranged mesopores are observed in the TEM image of purely siliceous product (Figure 4a). For the Alcontaining samples (Figure 4b,c), the pore arrangements are slightly distorted; however, pores with well-defined size distribution were clearly shown in the TEM images. With the increase in the Al contents, the more pronounced distortion of the pore arrangements was observed. These observations are consistent with those reported for the aluminum-containing mesoporous silica powders.15 Though it is possible to substitute Si with Al to some extent due to the similarity of ionic radius, the slight difference of the ionic radius causes the distortion of the microstructure especially when a larger amount of Al is incorporated. The maximum loading level of Al in the preset synthesis is around 1/50 of Al/Si, and a larger amount of Al is not effective to provide cation exchangeable (or acidic) sites on the mesopore surfaces. For the introduction of Al into mesoporous silica powders, various synthetic approaches have been em-

(14) Horva´th, G.; Kawazoe, K. J. J. Chem. Eng. Jpn. 1983, 16, 470.

(15) Ryoo, R.; Ko, C. H.; Howe, R. F. Chem. Mater. 1997, 9, 1607.

Figure 3. Nitrogen adsorption isotherms of mesoporous silica films with Al/Si ) 0 (a), 0.014 (b), and 0.031 (c).

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Figure 5. X-ray diffraction patterns of (a) as-synthesized and (b) calcined aluminum-containing films of silica-surfactant mesostructured materials with Al/Si ) 0.031. The trace in (c) shows (b) after the adsorption of AZ.

Figure 4. Transmission electron micrographs of the calcined aluminum-containing films of silica-surfactant mesostructured materials with Al/Si ) 0 (a), 0.014 (b), and 0.031 (c).

Figure 6. The variation of the absorbance due to trans-AZ at 340 nm as a function of the washing period.

ployed.11,15,16 However, not all synthetic procedures used for powder samples can be applied to the present film samples in order to retain the transparency and homogeneity of the supported films. By utilizing the synthetic conditions developed for alumino-silica xerogels,13 aluminum-containing mesoporous silica films were successfully synthesized. Assuming that all the added Al generates negative charges, the cation exchange capacity of the product (Al/Si ) 0.02) is estimated to be 70 mequiv/ 100 g. This value is in the range of those reported for the conventional inorganic cation exchangers such as smectite clays and zeolites. The state of Al in the mesoporous silica is a topic of interest. For Al-containing mesoporous silica powders, various techniques including temperature-programmed desorption and 27Al NMR spectroscopy have been employed. However, these techniques are difficult to apply for the present mesoporous silica films mainly due to the detection limit. Therefore, we propose the adsorption of cationic dyes for the quantitative evaluation of the acidic and cation exchange ability, since the sensitive determination of the amounts of the adsorbed dyes by UV-visible and luminescence spectra can be utilized for such purposes.

The adsorbed amount of AZ+ onto an Al free film was very small even when the reaction period was prolonged or a concentrated AZ+ solution was used.17 On the other hand, AZ+ was adsorbed effectively onto Al-containing mesoporous silica films to give yellow-orange colored films. It was thought that the electrostatic interactions between the negative charge and the cationic headgroup of the AZ+ led the adsorption of AZ+ into the mesopore. The mesostructure of the films was retained after the adsorption of the AZ+ dye. Figure 5 shows the X-ray diffraction pattern of the AZ+ adsorbed film (Al/Si ) 0.031) as a typical example. The UV-visible absorption spectrum of the film after the reaction with AZ+ showed an absorption band centered at 338 nm, which is ascribable to the π-π* transition of the trans isomer of AZ+. Since the thickness of the films is almost constant, the absorbance corresponds to the amount of adsorbed AZ+. The absorbance due to transAZ+ decreased by the washing with ethanol, indicating that the weakly bound AZ+ was removed from the films. The change in the absorbance of trans-AZ+ as a function of the washing period is shown in Figure 6. The absorbance decreased at the beginning while the values became almost constant after the washing for 50 h. The decrease in the

(16) (a) Luan, Z.; Cheng, C.-F.; He, H.; Klinowsky, J. J. Phys. Chem. 1995, 99, 10590. (b) Weglarski, J.; Dakta, J.; He, H.; Klinowsky, J. J. Chem. Soc., Faraday Trans. 1996, 92, 5161.

(17) Ogawa, M.; Kuroda, K.; Mori, J. Stud. Surf. Sci. Catal. 2000, 149, 865.

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Figure 7. The variation of the absorbance due to trans-AZ at 340 nm as a function of the aluminum content. Open and closed circles correspond to the data obtained for the aluminumcontaining mesoporous silica film calcined at 300 and 450 °C, respectively.

absorbance is almost the same irrespective of the Al/Si ratio. There is a linear relationship between the adsorbed amounts of AZ+ and the Al contents of the host mesoporous silica films, showing that the added Al cations substitute Si as a four-coordinated Al (framework) to generate the negative charge (or acidic site) for the adsorption of AZ+. The absorption maximum observed for the film essentially matched that of a dilute (3 × 10-5 M) ethanol solution of the dye. Considering the fact that the absorption spectra of aggregated AZ+ such as AZ+Br- crystals and the AZ+ adsorbed on a layered silicate18 are different from that of the present system, the adsorbed AZ+ cations were dispersed molecularly on the surface of the mesoporous silica irrespective of the loaded AZ+ amounts. Assuming that the molar absorption coefficient of the AZ+ adsorbed in the mesoporous silica is same as that of AZ+ in a dilute ethanol solution and the film thickness is 0.5 µm, the concentrations of the AZ+ in the films are determined to be 0.42, 0.37, 0.29, and 0.23 mol/L for the films with the Al/Si ratios of 0.031, 0.025, 0.022, and 0.014, respectively. Such high concentrations of AZ+ cannot be achieved in any solvents due to the poor solubility. Photochemistry of azobenzene derivatives in transparent solid matrixes such as silica gels and organic polymers has been reported so far.19-21 To obtain highly transparent host-guest systems, the concentration of the doped dye is usually very low and the hosts are amorphous. The very high concentration of AZ+ successfully achieved in the present system is worth noting as a merit of the aluminum-containing mesoporous silica films. The introduction of other cationic species into the present aluminum-containing mesoporous silica films is a promising way to construct photofunctional supramolecular materials. When the as-coated films were calcined at a higher temperature of 450 °C for 3 h, the capacity for the adsorption of AZ+ was dramatically decreased. Figure 7 shows the absorbance at 340 nm for AZ+ adsorbed on the mesoporous silica films calcined at 450 °C. The decrease in the AZ+ adsorption capacity is thought to reflect the decrease in the anionic (or acidic) sites of the mesopore surface. The condensation of hydroxyl groups on the mesopore surface and the diffusion of the loaded Al into (18) Ogawa, M.; Ishii, T.; Miyamoto, N.; Kuroda, K. Adv. Mater. 2001, 13, 1107. (19) Ueda, M.; Kim, H.-B.; Ichimura, K. Chem. Mater. 1994, 6, 1771. (20) Victor, J. G.; Torkelson, J. M. Macromolecules 1987, 20, 2241. (21) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558.

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Figure 8. The relationship between the absorbance due to trans-AZ at 340 nm and the yield of photochemical isomerization.

the silica wall from the pore surface upon heating at the higher temperature are possible reasons for the decreased AZ+ adsorption capacity. Similar phenomena have been observed for aluminosilica MCM-41.16 27Al MAS NMR studies showed that four-coordinated Al (framework; which yields a Brønsted site) is removed from the structure to give six-coordinate (extraframework) Al upon heating at a higher temperature. Since the films are thin (0.5 µm), the oxidative decomposition of the template is easy if compared with those occluded in the bulk samples. Thus, the mesoporous silica films with cation exchangeable sites on the pore surfaces became available by surfactant removal at relatively low temperatures (ca. 300 °C). The adsorbed AZ+ in the mesopore exhibits photochemical isomerization by UV and visible light irradiation as reported in our previous communication.12 The reactions were monitored by the change in the absorbance of the trans isomer of AZ+ at 340 nm. The ratio of the cis isomer formed by the UV irradiation at the photostationary state at room temperature was roughly estimated to be ca. 70% from the absorbance change. The value is almost the same irrespective of the adsorbed AZ+ amount, which is thought to be related to the proximity of the adjacent AZ+, as shown in Figure 8. It is known that the photoisomerization was affected by the free volume and the rigidity of the surroundings.19-21 It was reported that the van der Waals volume of azobenzene is 144 Å3 and its photoisomerization needs an extra 127 Å3 for isomerization.20 The pore size of the mesoporous silica films used in the present study is large enough for AZ+ to isomerize. The packing of the chromophore is another factor to affect the photoisomerization. The adsorbed AZ+ amount did not affect the efficiency of the photoisomerization in the present system, suggesting that the adsorbed AZ+ cations were separated molecularly on the mesopore surface. The spatial distribution of the negatively charged sites on the mesopore surface is thought to contribute the present effective photoisomerization. Conclusions Aluminum-containing mesoporous silica films were successfully synthesized, and the films were characterized by XRD, TEM, nitrogen adsorption, and the adsorption of a cationic azobenzene derivative. Although the slight distortion of the pore arrangements with the increase in the Al content was evidenced by the XRD and TEM results, the resulting aluminum-containing mesoporous silica films possess uniform pore size, very high surface area,

Aluminum-Containing Mesoporous Silica Films

and transparency from ultraviolet and visible wavelength regions. The adsorption capacity of the cationic azo dye was controlled semiquantitatively by the loaded aluminum amount in the films. From these characteristic features, the applications of mesoporous silica films as nanovessels for photochemical reaction of cationic dyes were promising.

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Acknowledgment. The present work was partially supported by Waseda University as a Special Project Research. LA011429M