Continuous Formation of Supported Unusual Mesostructured Silica

9570

Langmuir 2002, 18, 9570-9573

Continuous Formation of Supported Unusual Mesostructured Silica Films by Sol-Gel Dip Coating An-Wu Xu,*,†,‡ Jimmy C. Yu,§ Hua-Xin Zhang,† Li-Zhi Zhang,§ Dai-Bin Kuang,†,‡ and Yue-Ping Fang†,‡ School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, China, State Key Lab of Rare Earth Materials Chemistry and Applications, Beijing 100871, China, and Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, China Received June 5, 2002. In Final Form: September 16, 2002 Novel silicone surfactant was first used as the template to prepare mesostructured silica films with unusual lamellar structure. Complex mesostructured silica films can be obtained using the mixture of silicone surfactant and cetyltrimethylammonium bromide (C16TAB: CH3(CH2)15N+(CH3)3Br-) as the surfactant in conjunction with the sol-gel dip-coating process. The results show that silicone surfactants favor the formation of lamellar structure. Highly ordered long-range lamellar structure has the largest lattice constant known for lamellar materials to date reported. Moreover, the hexagonal mesophase can be easily created in layered silica walls just by adding C16TAB in the silicone surfactant templating system. The lamellar mesophase and the hexagonal mesoporosity in the walls were separately templated from silicone-based surfactant and C16TAB in the mixed surfactant templating systems, and thus hierarchically ordered silica formed. The examples presented supply strong evidence that oxide mesophases are governed by supramolecular chain configuration; i.e., unrestricted chain configuration (Si-O-Si chain in silicone surfactant) is responsible for the formation of the zero-curvature lamellar silica mesophase and restrictive chain configuration (C-C-C chain in C16TAB) for the high-curvature hexagonal mesophase, even their existing in a one-pot system.

Introduction Surfactant-templated syntheses based on the hydrolysis and cross-linking of inorganic precursors on the surfaces of supramolecular surfactant assemblies have been increasingly used to fabricate a wide range of mesoporous materials with hexagonal, cubic, or lamellar mesostructures.1-6 Thin films of surfactant-templated mesoporous materials may find applications in membrane-based separations, selective catalysis, and sensors. Several approaches are now available for the preparation of ordered structures at different length scales, such as micro-, meso-, and macroporous materials.7 However, the preparation of hierarchically ordered multistructures in a single body, such as seen in diatoms in nature, has remained an experimental challenge.8 So far, most * Corresponding author: e-mail [email protected]. † Zhongshan University. ‡ State Key Lab of Rare Earth Materials Chemistry and Applications. § The Chinese University of Hong Kong. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. Beck, J. S.; Vartuwli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (3) Antonelli, M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014. Go¨ltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 613. (4) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (5) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y.; Terasaki, O.; Park, S.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552. (6) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature (London) 1994, 368, 317. Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (7) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Science 1998, 282, 2244.

synthetic methods typically yield mesoporous materials in the form of fine powders, which hampers their wide applications. Above the critical micelle concentration of a bulk silica-surfactant solution, films of mesophases with hexagonally packed one-dimensional channels can be formed at solid-liquid and liquid-vapor interfaces through an interfacial silica/surfactant self-assembly process under acidic conditions.9-11 Lu et al.11 recently used C16TAB, under acidic conditions, to prepare well-ordered, uniform three-dimensional pore channel systems of cubic 3-dH (three-dimensional hexagonal) films by the sol-gel dip-coating process. Mesoporous silica with bimodal pore size distribution by simultaneous application of two different templates which do not interfere was first reported by Hentze and co-workers.12 However, the use of mixed surfactants to prepare transparent and continuous mesoporous silica films with hierarchically ordered multistructures has not been reported so far. Herein we report a generalized method for the synthesis of mesostructured silica films with hierarchically ordered multistructures using the mixture of silicone surfactant/ C16TAB as a structure-directing agent in conjunction with dip-coating process. The transparent films exhibit high thermal stability upon calcination at 500 °C and are crack-free. The silicone surfactant is composed of a poly(8) Sumper, M. Science 2002, 295, 2430. (9) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature (London) 1995, 378, 366. Feng, P.; Bu, X.; Stucky, G. D.; Pine, D. J. J. Am. Chem. Soc. 2000, 122, 994. McGrath, K. M.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Gruner, S. M. Science 1997, 277, 552. (10) Ogawa, M. Chem. Commun. 1996, 1149. Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature (London) 1996, 379, 703. Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1380. (11) 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 (London) 1997, 389, 364. (12) Antonietti, M.; Berton, B.; Go¨ltner, C.; Hentze, H. P. Adv. Mater. 1998, 10, 154.

10.1021/la026035p CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002

Mesostructured Silica Films

Langmuir, Vol. 18, No. 24, 2002 9571

Scheme 1. Primary Structure of Silicone-Based Surfactant

(dimethylsiloxane) (PDMS) backbone and a side chain of poly(ethylene oxide) (PEO) (Scheme 1). Silicon-based copolymers were prepared by hydrosilylation addition reaction, as described elsewhere.13 The essential character of silicone surfactants is amphiphilic, low glass transition temperature, low cost, nontoxic, and biodegradability. Silicone surfactants show higher surface activity than conventional hydrocarbon surfactants and find wide applications in industries.13 Experimental Section Lamellar silica films (designated ZSU-L) were prepared following procedures analogous to those described by Lu et al.11 using silicone surfactant as the template (x ) 12, y ) 3, z ) 12; Mn ) 2980, D ) 1.10, purity >95%). In a typical synthesis, TEOS (tetraethyl orthosilicate), ethanol, water, and HCl (mole ratios: 1:5:8:7 × 10-5) were refluxed at 60 °C for 1 h. Then water and HCl were added, increasing the concentration of HCl to 8.88 mM. After stirring at 25 °C for 15 min, the sols were aged at 50 °C for 15 min and diluted 1:3 with ethanol. Finally, silicone surfactant was added in quantities corresponding to concentrations in the range 1.6-5.2 wt %. The final reaction mole ratios were 1 TEOS:25 C2H5OH:5 H2O:0.005 HCl:0.016 silicone surfactant. Silica films were deposited on polished (100) silicon wafers and glass slides by the dip-coating process using the sol solution. Complex mesostructured silica films (denoted ZSU-38) were prepared by the same method just using the mixture of silicone surfactant and C16TAB instead of pure silicone surfactant in above experiment. The final reaction mole ratios for ZSU-38 preparation were 1 TEOS:25 C2H5OH:5 H2O:0.005 HCl:0.008 silicone surfactant and 0.14 C16TAB. The template was removed by calcination at 500 °C for 5 h in air (heating rate: 1 °C min-1). TEM photographs were obtained with a JEOL 100CX operated at 100 kV. The samples were embedded in epoxy resin and ultramicrotomed for TEM measurements. Powder X-ray diffraction (XRD) analysis was performed on a Rigaku Rotaflex diffractometer equipped with a rotating anode and Cu KR radiation. N2 adsorption measurements were performed at 77 K using a Micromeritics ASAP 2010 system utilizing BarrettEmmett-Teller (BET) calculations for surface area and BJH calculations for pore size distribution for the adsorption branch of the isotherm.

Results and Discussion Silica films with highly ordered long-range lamellar structure are clearly seen from the TEM images of an ultrathin section of the as-synthesized ZSU-L product templated from pure silicone surfactant (Figure 1). The hybrid silica mesostructrue is constructed of bilayer aggregates of the surfactant being sandwiched by thick silica walls that are arranged parallel to each other. The interlayer distance measured from the micrograph is about 160 ( 10 nm, which shows to be much larger than that of all previously synthesized and natural layered materials.14-20 The silica walls do not bend to form curved (13) Randal, M. H. Silicone Surfactants; Marcel Dekker: New York, 1998. Kunieda, H.; Uddin, M. H.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem. B 2001, 105, 5419. Rheingans, O.; Hugenberg, N.; Harris, J. R.; Fischer, K.; Maskos, M. Macromolecules 2000, 33, 4780. (14) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature (London) 1995, 378, 47. (15) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 1941. (16) Dubois, M.; Gulik-Krzywichi, Th.; Cabane, B. Langmuir 1993, 9, 673. (17) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267.

Figure 1. TEM image of an ultrathin section of the assynthesized silica films (ZSU-L) with a highly ordered longrange lamellar structure. The ZSU-L material was prepared by using silicone surfactant as the template.

silica frameworks such as closed bilayer structures, i.e., vesicles populated with concentric parallel silica layers.14,20 The lamellar structures extend to the length of micrometer scale through layer propagation without curvature, as observed in Figure 1. Higher-magnification TEM observation of the ZSU-L sample under various tilting angles did not show evidence for any mesostructured framework other than lamellae, which is also demonstrated by powder X-ray diffraction (XRD) analysis. No peaks at low angles can be observed for as-made ZSU-L sample in Figure 3a. Examining many of the TEM micrographs, it was found that mesostructured lamellar silicas represent 100% of the solid materials. Even with substantial changes in the concentration of silicone surfactant and HCl, lamellar mesostructure is still preserved, suggesting that silicone surfactant favors the formation of the lamellar materials. It is unprecedented that lamellar silica has extremely large interlayer distance. There would exist a different templating mechanism between ZSU-L and SBA-15 or MCM-41. It is most likely that special properties of silicone copolymer would be responsible for the formation of lamellar structure with large lattice constant. Both silicone surfactant and Pluronics family (such as PEO20PPO70PEO20, P123) are amphiphilic supramolecules with the same hydrophilic headgroup PEO. The only difference is that silicone surfactant contains a hydrophobic PDMS tail but P123 has a hydrophobic PPO tail. PDMS chains are more flexible than PPO chain in Pluronics family copolymers or hydrocarbon chain in hydrocarbon chains in alkyltrimethylammonium salts (CTA+), because the bond angle (SiO-Si) is significantly wider (∼143°) and the bond length (Si-O) (0.165 nm) longer than comparable C-O-C (114°, 0.142 nm) and C-C-C (109°, 0.140 nm) bonds. Thus, the obstacle to rotation is very low (rotation barrier: 0.8 kJ/ mol), and the Si-O bond can freely rotate and tilt.13 That is the reason why even very long PDMS chain silicone surfactants are in the liquid state at room temperature. In contrast, the hydrocarbon surfactants tend to be in a solid state at room temperature since Krafft temperatures for long and linear hydrocarbon-chain surfactants are high.13 We suggest that silicone surfactants with more flexible chains (unrestricted chain configuration) than conventional hydrocarbon surfactants or copolymer (restrictive chain configuration) would be responsible for the formation of this unusual lamellar silica mesophase with unprecedented large lattice constants.21 In other words, we suggest that oxide mesophases are governed by supra(18) Tanev, P. T.; Liang, Y.; Pinnavaia, T. J. J. Am. Chem. Soc. 1997, 119, 8616. (19) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Science 1998, 282, 1302. (20) Sayari, A.; Karra, V. R.; Reddy, J. S.; Moudrakovski, I. L. J. Chem. Soc., Chem. Commun. 1996, 3, 411. Chenite, A.; Page, Y. Le.; Karra, V. R.; Sayari, A. J. Chem. Soc., Chem. Commun. 1996, 3, 413.

9572

Langmuir, Vol. 18, No. 24, 2002

Xu et al.

Figure 2. Low-magnification and higher-magnification (inset) TEM images of an untrathin section of the as-made complex mesostructured silica films (ZSU-38). The ZSU-38 sample was obtained by using the mixture of silicone surfactant and C16TAB as a structure-directing agent.

Figure 4. Nitrogen adsorption-desorption isotherms for the ZSU-38 silica films calcined at 500 °C in air for 5 h and corresponding BJH pore size distribution curve (inset) from the adsorption branch of the isotherm. The samples were outgassed overnight at 180 °C before the analyses.

Figure 3. Powder X-ray diffraction patterns of the assynthesized ZSU-L silica films (a, inset), the as-synthesized ZSU-38 sample (b), and the calcined ZSU-38 films at 500 °C in air for 5 h (c).

molecular chain configuration. Unrestricted supramolecule chain configuration of silicone surfactants always leads to the formation of lamellar mesophase,21,22 in that silica polymerization leads to an increase in interfacial area that is achieved through tilting siloxane chains, while maintaining and stabilizing lamellar structure. The previously reported lamellar to hexagonal or to cubic mesophase transformation occurred through corrugation of the lamellar surfactant-silicate sheets for increasing interfacial area, because tilting hydrocarbon chains is entropically disfavored by the restrictive hydrocarbon chain configuration; thus, hexagonal or cubic mesophase formed in the final products.11,21 Complex mesostructured silica films (ZSU-38) were obtained using the mixture of silicone-based surfactant and C16TAB as a structure-directing agent instead of pure silicone surfactant. Figure 2 shows that lamellar mesostructure similar to that observed in the ZSU-L product is also preserved in the as-made ZSU-38 product templated from the mixed template of silicone surfactant and C16TAB, although ZSU-38 exhibits poorly ordered lamellar structure in comparison to highly ordered lamellar structure of ZSU-L. Interestingly, we find that hexagonal mesopores were formed in the thick silica walls of the ZSU-38 sample (inset in Figure 2). Both the lamellae and hexagonal mesophase are assembled in a single body, representing complex mesostructures (structure within (21) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (22) Xu, A. W.; Cai, Y. P.; Zhang, L. Z.; Yu, J. C. Adv. Mater. 2002, 14, 1064.

structure). No peaks are observed in XRD patterns for the as-made ZSU-L product, as indicated in Figure 3a. (The ZSU-L lamellar phase was not detected by XRD analysis because of its extremely large lattice constant.) However, at least one peak can be observed for the assynthesized and calcined ZSU-38 samples (Figure 3b,c). It is clearly shown that the lamellar mesophase and the hexagonal mesoporosity in the walls were separately templated from silicone-based surfactant and C16TAB in the mixed surfactant templating systems, and thus hierarchically ordered silica formed; even this was previously unexpected. The present examples further supply strong evidence that oxide mesophases are governed by supramolecular chain configuration; i.e., unrestricted chain configuration (Si-O-Si chain in silicone surfactant) is responsible for the formation of the zero-curvature lamellar silica mesophase, and restrictive chain configuration (C-C-C chain in C16TAB) for high-curvature hexagonal mesophase, even their existing in an one-pot system. The representative nitrogen adsorption-desorption isotherm and the corresponding BJH (Barret-JoynerHalenda) pore size distribution curve of the calcined ZSU38 product (500 °C) are shown in Figure 4. The N2 isotherm of the calcined ZSU-38 is a type Ib isotherm with a type H2 hysteresis loop,23 different from that of MCM-41-type silica films with no hysteresis loop.12 It can be seen that the hysteresis loop results from the lamellar structure (slitlike pores). Silica produced from silicone surfactant decomposition thickening and supporting walls prevents the lamellar structure from collapse during calcination. The results demonstrated that the lamellar structure was intact after calcination.22 The calcined ZSU-38 product prepared using the mixture of silicone surfactant and C16TAB as the template has a BET surface area of 863 m2 g-1. The BJH analyses show that calcined ZSU-38 exhibits pore sizes of 2.6 nm (Figure 4, inset). Conclusion In summary, highly ordered long-range lamellar hybrid silica films with large interlayer spacings were obtained using silicone surfactant as the template in a wide range (23) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscov, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.

Mesostructured Silica Films

of synthesis conditions. We found that silicone surfactant favors the formation of lamellar mesostructure. The addition of other conventional surfactant such as C16TAB can create hexagonal mesophase in the layered walls, which supply a novel simple route to synthesize and design new complex mesostructured materials with potential applications. Coassembly of inorganic and organic species together with special supramolecule chain configuration of silicone surfactants preferably maintaining the planar structure are responsible for the high regularity of the lamellar silica mesophase. We believe that this novel route is universal and offers significant overall benefits for the controlled design of a wide range of new materials with complex mesostructures. Oxide materials with hierarchically ordered mesostructures are of interest from the

Langmuir, Vol. 18, No. 24, 2002 9573

viewpoint of biominerization and may find wide applications in catalysis, adsorption and separation, etc. We believe that these findings have provided new insights into the mechanistic issues involved in the formation of this interesting class of mesostructured materials. Further investigation, however, will be required in order to adequately elucidate the actual formation mechanisms of complex mesophases formed in the mixed surfactanttemplating systems containing silicone surfactant. Acknowledgment. The support from the Guangdong Province “The Tenth Five-Year Plan” Key Projects (20010185C and A3040302) is gratefully acknowledged. LA026035P