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Fabrication of Mesoporous Silica Films via a Novel Route Providing a Wide Processing Time Window Sajo P. Naik,†,‡ Masaru Ogura,† and Tatsuya Okubo*,†,‡ Department of Chemical System Engineering, The University of Tokyo, Tokyo, Japan, and PRESTO, Japan Science and Technology, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
We have developed an economical and environmentally benign process for the fabrication of mesoporous silica films. The approach involves redispersion of the as-synthesized silicasurfactant hybrid in an organic solvent to form the transparent coating sol. The sol provided a much wider processing time window for the films, and ordered mesoporous silica films could be fabricated even after storing the sol for a long period of time. The key feature of this work is that the uncontrolled silica condensation, proceeding during drying, in the as-synthesized silicasurfactant hybrid is arrested by redispersing it in an inert organic solvent. Introduction Ordered mesoporous silica,1 synthesized by the selfassembly of surfactant and silicate species in acidic or basic solutions, is a candidate material for various applications in the field of catalysis, adsorption, etc. The main reason for this interest is because the structure and pore dimensions of mesoporous silica are tunable and can be delicately controlled according to the requirements. Mesoporous silica exists as a powder, fiber, thin film, or monolith in macroscopic form and can have a hexagonal (p6mm), cubic (Ia3d, Pm3n, etc.), or lamellar structure. Currently, fabrication of a mesoporous silica thin film has gained much importance because thin films are the most compatible from the standpoint of direct industrial applications. The key issue for the application of mesoporous silica thin films is the orientation’s control of the mesophase and alignment of the pores. Oriented thin films of mesoporous silica have applications in adsorption, sensors, low-k dielectrics, membrane technology, and fabrication of sophisticated electronic and photonic devices. Thin films of mesoporous silica can be synthesized from a highly acidic surfactant-silicate sol via a solvent evaporation-induced self-assembly (EISA)2 route. Many studies have been performed so far to understand the mechanism underlying the formation of the films by EISA.3-7 From these studies, it is realized that control over self-assembly during the film formation is required to obtain films with preferentially oriented mesophases and pore openings. Several research groups8-11 have reported the synthesis of hexagonal mesoporous silica films oriented parallelly on supports such as cleaved mica or surface-modified glass. We, on the other hand, have synthesized mesoporous silica films with accessible pores from the top surface by using copolymer surfactant (F127) under acidic conditions.12-14 Recently, we have also studied the mechanism of drying-induced phase transformations in mesoporous silica powders.15 We found that the as-synthesized hexagonal p6mm mesophase, obtained under the employed experimental * To whom correspondence should be addressed. Tel.: +81-3-5841-7348. Fax: +81-3-5800-3806. E-mail: okubo@ chemsys.t.u-tokyo.ac.jp. † The University of Tokyo. ‡ Japan Science and Technology.
conditions, was transformed to the cubic Ia3d phase as drying of the mesophase proceeded, even at room temperature. The observed transformations did not follow the conventional rules of mesophase transformations that are based on the charge-density matching interpreted with the packing parameter (g) related to the effective shape of the surfactant.16 A comprehensive scheme for the understanding of the phase transformations and mesophase selection has been recently proposed by us.15 Through another study, we have demonstrated that, by controlling the degree of silicate condensation in silica sol, one could selectively obtain silica films with 3-D cubic (Pm3n) or 2-D hexagonal (p6mm) structures.6 The mesophase selection in these films was achieved by adjusting the time interval between the sol synthesis and actual coating. However, the fabrication of the ordered mesoporous films, especially Pm3n, was attained only for a short period immediately after the preparation of the coating sol because a longer synthesis/storage time resulted in the enhanced rigidity of the siloxane framework.6 To overcome the general difficulty of the utilization of only a limited part of the coating sol and to provide a much wider processing time window for the fabrication of films, important in many industrial processes wherein longevity of the coating sol is desired, we report herewith a novel approach for the fabrication of mesoporous silica thin films. Experimental Section Tetraethyl orthosilicate (TEOS), cetyl bromide (C16H33Br), and triethylamine were obtained from Tokyo Kasei Kogyo (TCI). HNO3 (35%) was obtained from Wako Chemicals. These chemicals were used without any further purification. Cetyltriethylammonium bromide (CTEAB) was synthesized as per the earlier described procedure.15 Synthesis of the SiO2-surfactant mesophase was also carried out by following the earlier procedure.15 Specifically, 0.25 g of the ionic surfactant was added to a solution of 0.83 g of HNO3 and 10.3 g of distilled water, and the resulting solution was stirred for 15 min at room temperature to form a clear solution, which was then cooled in an ice bath at 0 °C for 1 h. Separately, 1.0 g of TEOS was also precooled to 0 °C for 1 h in an ice bath and was subsequently added to
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Figure 1. (A) Clear and transparent coating sol produced by the redispersion of the as-isolated SiO2-surfactant hybrid in acetone. (B and C) FE-SEM images under different magnifications of the peeled calcined mesoporous silica film fabricated from the clear coating sol.
the surfactant solution under stirring. The emulsion formed after mixing was stirred at room temperature for 15 min and was then allowed to react under static conditions at 0 °C for 2 h. The as-formed SiO2surfactant hybrid mesophase was then collected by ultracentrifugation at 15 000 rpm for 5 min and immediately redispersed in 5 g of acetone under vigorous stirring for 2 h to form a clear coating sol. The redispersed coating sol is stored in glass vials at room temperature. Additional silica enrichment of the coating sol was performed by adding a small amount (0.3 g) of a silicate solution, prepared by hydrolysis of 1 g of TEOS in the presence of 1 g of 0.1 M HCl for 1 h, to 5 g of previously prepared SiO2-surfactant hybrid sol in acetone. The mesoporous silica thin films were coated onto polished thin glass (cover slide) substrates by spin or dip coating. The fabricated films were then dried at 100 °C for 30 min, followed by calcination at 500 °C for 5 h in air. The heating rate was ramped at 5 °C/min, and the furnace was maintained at 500 °C for 5 h. The X-ray diffraction (XRD) patterns of the fabricated films were recorded with an M03X-HF (Bruker AXS) using Cu KR radiation (40 kV and 40 mA) at a rate of 1 °C/min over a range of 1.5-6.0° (2θ). The scanning electron microscope (SEM) images were taken on a S-900 (Hitachi) instrument operated at an accelerating voltage of 6 or 8 kV. The samples were sputtered with Pt for 5 s in an ion-sputtering apparatus (Hitachi E-1030) in a 0.1 Pa argon atmosphere. A combined thermogravimetrymass spectroscopy (TG-MS) analysis was carried out by using a TG8120 (Rigaku) instrument equipped with M-QA200TS (ANELVA). The heating rate was 5 °C/min, and the measurement was performed in 10% O2/He. The N2 adsorption/desorption isotherms of the mesoporous silica peeled from the glass substrate were measured at 77 K using an Autosorb-1 instrument (Quantachrome Co.). The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) model from the adsorption branch of the N2 isotherm. Results and Discussion At first, we synthesized the SiO2-surfactant hybrid mesophase by the reaction of TEOS and CTEAB in the presence of nitric acid at 0 °C for 2 h. The as-formed SiO2-CTEAB hybrid was collected by centrifugation and immediately redispersed in acetone under stirring. Because of the short synthesis time at 0 °C, the silica condensation in the mesophase proceeded only to a limited extent, facilitating complete redispersion of the hybrid in acetone to form a clear silica coating sol. The image of the clear coating sol is shown in Figure 1A.
Mesoporous silica films were then fabricated by spin or dip coating. By variation of the dip- or spin-coating rates, crack-free films of thickness 50 nm to 50 µm, as analyzed from field emission scanning electron microscopy (FE-SEM), could be produced. The FE-SEM images, under different magnifications, of the peeled, calcined film prepared by spin coating are shown in Figure 1B,C. Olson et al.17 reported the dip coating of a dispersed colloidal sol of MCM-41 onto a substrate, resulting in the formation of a noncontinuous and nonuniform coating. In our procedure, however, uniform, transparent films could be produced. Notice that the cracks appearing in mesoporous silica films, shown in Figure 1B, have formed during the peeling of the film from the substrate. Figure 2A shows the XRD patterns of the films coated at different time intervals after the sol preparation. The fact that well-ordered films could be synthesized from this sol, even after 6 weeks of sol preparation, confirms its high durability. Because of the redispersion of the mesophase in an organic solvent such as acetone, further condensation of silicate was prevented, resulting in the increased durability of the coating sol. We also did not observe gelation or precipitation of the sol on storage for a long period, providing a much wider processing time window for the films. Durability of the coating sol is one of the crucial considerations for the commercial realization of any coating-related process. As can be seen from Figure 2A, all of the XRD patterns of the as-coated films show a mixture of Ia3d and lamellar mesophases with varying percentages as indicated by the differences in the relative intensities of the first two peaks assigned to 211 (cubic) and 220 (cubic)/100 (lamellar) planes, respectively. We also observed slight variations in the positions of the XRD peaks of the films. This is ascribed to the flexible nature of the silicate mesophase, which has resulted from the higher content of the surfactant and partial condensation of silica in the framework. Upon drying at 100 °C for 30 min, all of the films were transformed to the lamellar mesophase, as can be seen from their XRD patterns shown in Figure 2B. The phase transformation in the as-coated film was further studied at room temperature. It was observed that the phase transformation proceeded very slowly at room temperature. The as-coated film, as discussed earlier, showed a mixture of two phases, cubic and lamellar; upon an increase of the drying time at room temperature, the percentage of the cubic phase started to decrease with a concomitant increase in the percentage of the lamellar phase. In Figure 3B, the XRD pattern of the film after drying for 36 h at room temperature is shown. The peaks assigned to 211 (C) and 210 (C)/100 (L) planes
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Figure 2. XRD patterns of (A) the films coated after different time intervals of sol preparation and (B) the same films after drying at 100 °C for 30 min. The peaks marked with asterisks are assigned to the surfactant CTEABr.
Figure 3. XRD patterns of (A) as-coated film, (B) film after drying for 36 h at room temperature, and (C) film B after heating it at 100 °C for 30 min.
have almost fused to form the 100 (L) plane of the lamellar phase. The intensity ratio for the 211 (C) and 220 (C) peaks was calculated as 1.154. None of the observed XRD patterns could be assigned to the hexagonal p6mm mesophase, hence ruling out the formation of this phase in the films. Upon further drying of the film at 100 °C for 30 min, the cubic phase has totally disappeared, leaving only the lamellar phase (Figure 3C). From these results, it is reasonable to assume that the Ia3d phase is first formed in the films immediately after coating, but as was already discussed, this phase is unstable and transforms to lamellar upon drying. The transformation from cubic to lamellar, observed in our films, proceeds toward a higher g value much against the conventional rules of mesophase transformation.16 In our previous study, by prolonging the synthesis time, we could obtain a well-ordered p6mm mesophase and the phase transformation was hardly observed. Drying at higher temperatures promoted the phase transformation of the mesophase not only from hexagonal to cubic but also from cubic to lamellar. We have explained the observed mesophase transformations by considering the degree of silicate condensation and the
Figure 4. TG-MS profile of the SiO2-CTEAB hybrid mesophase collected from the redispersed silicate sol at room temperature.
loss of occluded water upon drying as the main factors regulating the transformations. Several transformations tending toward higher g values are also reported in the literature.6,15,16,18-20 In some cases, the phase transformation is kinetically hindered by the reduced mobility of the surfactant micelles in the polymerized silicate network, and certain activation in the form of stimuli, such as heating, from the exterior to the interior of the system is required to overcome the kinetic barrier for the transformation. In the case of the present films, the occlusion of acetone or water within the hybrid mesophase is plausible, and the loss of these occluded molecules upon drying at 100 °C could have triggered the phase transformation in films. Phase transformation induced by the loss of occluded water molecules, upon drying, from the silica mesophase has already been reported by Ogura et al.15 and Liu et al.20 To verify the presence of occluded molecules, the as-coated film was subjected to a combined TG-MS analysis.21 The result of the TGMS analysis, shown in Figure 4, indicates that acetone and water molecules are indeed released from the hybrid mesophase upon drying. The loss of acetone starts at ∼40 °C and continues up to 100 °C, while the loss of H2O molecules, which was detected only after 90 °C, continues well above 150 °C.
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Figure 5. XRD patterns of the as-coated, 100 °C dried, and 500 °C calcined film, coated from the additional silica-enriched sol.
Judging from these results, it can be concluded that the loss of these occluded molecules, during drying, provides the driving force for the phase transformation in the film. From the earlier discussion, it is clear that the phase transformation to the lamellar phase, although complete below 100 °C, proceeds even at room temperature. The TG-MS results indicate that the loss of acetone from the mesophase is completed below 100 °C; therefore, the mesophase transformation must have primarily resulted as a consequence of the loss of acetone from the mesophase. In a recent study, Kleitz et al.22 have also observed phase transformation from face-centered cubic to 2-D hexagonal p6mm in silica mesostructures synthesized using triblock surfactants in the presence of butanol as a cosurfactant. The authors have proposed that butanol is located at the hydrophilic-hydrophobic interface, stabilizing the micellar aggregates and determining the surface curvature for the occurrence of phase transformation. In our previous studies,15 also we have observed mesophase transformations upon the loss of occluded water upon drying of the mesophase. Because the films were lamellar, they could not survive calcination at 500 °C in air. As such, we decided to enrich the coating sol with additional polymeric silica to reverse the direction of phase transformation in the films by enhancing the contribution of charge-density matching over drying-induced phase transformations with the intension of lowering the g value of the mesophase. To the as-prepared coating sol (5 g) was added a small quantity (0.3 g) of acid-hydrolyzed TEOS for further silica enrichment of the sol. The coating sol kept its clearness even after the addition of extra silicate, and gelation was not observed in it. The XRD patterns of the films fabricated from this sol are shown in Figure 5. The as-coated film showed a single Ia3d phase, which was transformed to p6mm upon drying at 100 °C. This is in contrast to the results obtained in the case of pure sol, wherein a mixture of lamellar and cubic phases was obtained in the as-coated films. The silica addition changes the silica-surfactant composition of the sol, effecting the micellar composition in the sol that changes the magnitude of the packing parameter (g) and resulting in the reversal of the phase transformation direction. The observed Ia3d to p6mm phase transformation indicates the retention of flexibility6,15 of the SiO2 hybrid mesophase. In any case, thermally stable films were produced by the additional silica enrichment of the sol
Figure 6. Nitrogen adsorption/desorption isotherm for a calcined mesoporous silica film.
(Figure 5). To further confirm the mesoporous structure of the film, N2 adsorption/desorption studies were conducted on the calcined thick film peeled from the glass substrate. The obtained N2 adsorption/desorption plot is shown in Figure 6, confirming the mesoporous structure. The pore size distribution was obtained using the BJH model for the adsorption isotherm. The Brunauer-Emmett-Teller surface area of the mesoporous silica film, calculated by assuming the covered area by nitrogen molecules as 0.162 nm2, was 800 m2/g. The pore diameter and the mesopore volume were found to be ∼22 Å and 0.340 cm3/g, respectively, demonstrating the pore accessibility in the film. Another major feature of this approach is that the mesophase can be redispersed not only in acetone but also in many other organic solvents such as ethanol, 2-propanol, and n-butanol. Furthermore, other ionic surfactants such as trimethylsterylammonium chloride, dodecetyltrimethylammonium chloride, etc., could also be used for synthesis of a redispersible mesophase, proving the versatility of this new redispersion approach of fabricating films. We have also found that the types of mesophases obtained and the directions of mesophase transformations were affected by the weight and nature of the surfactant or the organic solvent used. Details of these and other related studies will be published shortly. Conclusion In a departure from the conventional processes, we have adopted an economical and environmentally benign process for the fabrication of mesoporous silica films. An essential part of this approach is the redispersion of the as-collected silica-surfactant hybrid mesophase in an organic solvent to form a highly durable transparent coating sol. The sol provided a much wider processing time window for the film fabrication, and mesoporous films could be formed even after storing the sol for a long period (several weeks). The cubic to lamellar phase transformation observed during the film formation was found to be closely associated with the loss of occluded molecules during drying of the mesophase. Additional silica enrichment of the coating sol allowed us to fabricate thermally stable hexagonal mesoporous silica films. Acknowledgment We are grateful to Japan Science and Technology Agency for providing the financial support for this work.
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(13) Naik, S. P.; Yamakita, S.; Ogura, M.; Okubo, T. Studies on mesoporous silica films synthesized using F127, a triblock copolymer. Microporous Mesoporous Mater. 2004, 75 (1-2), 51. (14) Murakami, Y.; Yamakita, S.; Okubo, T.; Maruyama, S. Single-walled carbon nanotubes catalytically grown from mesoporous silica thin film. Chem. Phys. Lett. 2003, 375, 393. (15) Ogura, M.; Miyoshi, H.; Naik, S. P.; Okubo, T. Investigation on the Drying Induced Phase Transformation of Mesoporous Silica; A Comprehensive Understanding toward Mesophase Determination. J. Am. Chem. Soc. 2004, 126, 10937. (16) Huo, Q.; Margolese, D. I.; Stucky, G. D. Surfactant Control of phases in the Synthesis of Mesoporous Silica-based Materials. Chem. Mater. 1996, 8, 1147. (17) Olson, D. H.; Stucky, G. D.; Vartuli, J. C. Sensor Device Containing Mesoporous Silica. U.S. Patent 5,364,797, 1994. (18) Tolbert, S. H.; Landry, C. C.; Stucky, G. D.; Chmelka, B. F.; Norby, P.; Hanson, J. C.; Monnier, A. Phase Transitions in Mesostructured Silica/Surfactant Composites: Surfactant Packing and the Role of Charge Density Matching. Chem. Mater. 2001, 13, 2247. (19) Landry, C. C.; Tolbert, S. H.; Gallis, K. W.; Monnier, A.; Stucky, G. D.; Norby, P.; Hanson, J. C. Phase Transformations in Mesostructured Silica/Surfactant Composites: Mechanisms for Change and Applications to Materials Synthesis. Chem. Mater. 2001, 13, 1600. (20) Liu, M. C.; Sheu, H. S.; Cheng, S. Drying induced phase transformation of mesoporous silica. Chem. Commun. 2002, 2854. (21) Kleitz, F.; Schmidt, W.; Schuth, F. Calcination behavior of different surfactant-templated mesostructured silica materials. Microporous Mesoporous Mater. 2003, 65, 1. (22) Kleitz, F.; Solovyov, L. A.; Anilkumar, G. M.; Choi, S. H.; Ryoo, R. Transformation of Highly Ordered Large Pore Silica Mesophases (Fm3m, Im3m and p6mm) in Ternary Triblock Copolymer-Butanol-Water System. Chem. Commun. 2004, 1536.
Received for review January 5, 2005 Revised manuscript received March 28, 2005 Accepted April 16, 2005 IE0500218
