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Postsynthesis Stabilization of Free-standing Mesoporous Silica Films R. Vogel,†,‡ C. Dobe,† A. Whittaker,§ G. Edwards,† J. D. Riches,| M. Harvey,‡ M. Trau,† and P. Meredith*,‡ Department of Chemistry & Centre for Nanotechnology and Biomaterials, Centre for Magnetic Resonance, and Centre for Microscopy and Microanalysis and Department of Physics & Centre for Biophotonics and Laser Science, The University of Queensland, Qld 4072, Australia Received September 25, 2003. In Final Form: January 21, 2004 Mixed ammonia-water vapor postsynthesis treatment provides a simple and convenient method for stabilizing mesostructured silica films. X-ray diffraction, transmission electron microscopy, nitrogen adsorption/desorption, and solid-state NMR (13C, 29Si) were applied to study the effects of mixed ammoniawater vapor at 90 °C on the mesostructure of the films. An increased cross-linking of the silica network was observed. Subsequent calcination of the silica films was seen to cause a bimodal pore-size distribution, with an accompanying increase in the volume and surface area ratios of the primary (d ) 3 nm) to secondary (d ) 5-30 nm) pores. Additionally, mixed ammonia-water treatment was observed to cause a narrowing of the primary pore-size distribution. These findings have implications for thin film based applications and devices, such as sensors, membranes, or surfaces for heterogeneous catalysis.
Introduction In 1992 researchers at Mobil discovered a new class of ordered porous adsorbents, M41S.1 These novel, silicabased materials possessed adjustable, uniform pore-sizes2 and large surface areas. Mesoporous silicates can be produced by either an alkaline route or an acidic route3 using surfactants as templates. Silicates produced under alkaline conditions exhibit good thermal stability. This is improved by postsynthesis hydrothermal treatment.4-6 Under acidic conditions, silicon alkoxide precursors are typically used, forming a more labile and soft network during the hydration process.7 Postsynthesis hydrothermal treatment in boiling ammonia-water improves the order and stability of these acid-synthesis mesoporous silica powders.8-10 An important feature of mesoporous silica materials is their ability to form thin films as well as bulk powders.11 * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry & Centre for Nanotechnology and Biomaterials. ‡ Department of Physics & Centre for Biophotonics and Laser Science. § Centre for Magnetic Resonance. | Centre for Microscopy and Microanalysis. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Sayari, A.; Yang, Y.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 1999, 103, 3651. (3) Huo, Q.; Margoleses, S. I.; Ciesla, U.; Feng, P.; Gier, D. E.; Sieger P.; Leon, B. F. R.; Petroff, P. M.; Schueth, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (5) Chen, L.; Horiuchi, T.; Mori, T.; Maeda, K. J. Phys. Chem. B 1999, 103, 1216. (6) Kruk, M.; Jaroniec, M.; Sayari, A. Micropor. Mesopor. Mater. 1999, 27, 217. (7) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Garces, J.; Olken, M. M.; Coombs, N. Adv. Mater. 1995, 7, 842. (8) Lin, H. P.; Mou, C. Y.; Liu, S. B. Chem. Lett. 1999, 1341. (9) Mou, C. Y.; Lin, H. P. Pure Appl. Chem. 2000, 72, 137. (10) Lin, H. P.; Mou, C. Y.; Liu, S. B.; Tang, C. Y.; Lin, C. Y. Micropor. Mesopor. Mater. 2001, 44-45, 129. (11) Pevzner, S.; Regev, O.; Yerushalmi-Rozen, R. Curr. Opin. Colloid Interface Sci. 2000, 4, 420.
For many applications and devices, such as membranes, sensors, or surfaces for heterogeneous catalysis, mesoporous thin films are required. Silica-based mesoporous thin films have been prepared by methods such as dipcoating,12 spin-coating,13 and growth on hydrophilic substrates14 as well as via unsupported growth at air/ liquid as well as liquid/liquid interfaces.15 Free-standing silica films produced under acidic conditions were reported to be labile and thermally unstable, as demonstrated by a partial collapse of the mesostructure during calcination.16 The use of a postsynthesis treatment involving boiling the film in ammonia-water would be wholly inappropriate for the stabilization of silica-based mesoporous thin films because their macrostructure, especially in the case of labile free-standing films, would be destroyed. For these reasons, Wu et al.17 and Grosso et al.18,19 recently reported the use of an ammonia atmosphere to treat silica films on glass substrates. Wu et al. applied a mixed ammonia-water atmosphere at approximately 400 °C to increase the scratch resistance of nanoporous silica films. These films were produced by a sol-gel process without using surfactants. Grosso et al. posttreated dipcoated thin silica films produced from a silica precursor (TEOS) and either cetyltrimethylammonium bromide (CTAB)18 or a pluronic triblock copolymer (F127)19 under an ammonia atmosphere at room temperature. This (12) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky G. D. Adv. Mater. 1998, 10, 1380. (13) Martin, J. E.; Anderson, M. T.; Odinek, J.; Newcomer, P. Langmuir 1997, 13, 4133. (14) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (15) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (16) Yang, H.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 1998, 8, 1205. (17) Wu, G.; Wang, J.; Shen, J.; Yang, T.; Zhang, Q.; Zhou, B.; Deng, Z.; Bin, F.; Zhou, D.; Zhang, F. J. Non-Cryst. Solids 2000, 275, 169. (18) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Lavergne, M.; Mazerolles, L.; Babonneau, F. J. Mater. Chem. 2000, 10, 2085. (19) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Ayral, A.; Amenitsch, H.; Babonneau, F. Chem. Mater. 2001, 13, 1848.
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treatment leads to a stiffening of the silica network and hence minimizes the network shrinkage upon surfactant removal. This paper demonstrates a simple way of stabilizing free-standing as opposed to supported mesostructured silica films via a soft mixed ammonia-water vapor postsynthesis treatment. The detailed effects of this treatment on the enhancement of structural stability, silica cross-linking, and changes in pore-size and pore structure are discussed. Experimental Section Material Synthesis. Free-standing mesostructured silica thin films were synthesized following the method presented in ref 14: 10 g of Milli-Q water (18 MΩ cm-1) and 0.72 g of HCl (32% w/w) were mixed, and then 0.96 g of cetyltrimethylammonium chloride (CTAC, 25% w/w, Aldrich) was added followed by 0.14 g of tetraethyl orthosilicate (TEOS, 99.999+%, Aldrich). This corresponds to reactant mole ratios of 925.7 H2O:9.4 HCl:1.12 CTAC:1 TEOS. After stirring for 5-10 min, the solution was poured into a clean, dry Petri dish of appropriate size. The solution was left covered for 7-14 days. The film which formed at the surface of the solution was lifted and dried (this is referred to as the as-synthesized film). Films to be subjected to mixed ammonia-water vapor were placed between two sintered glass disks, which were transferred into a 500 mL container filled with approximately 150-200 mL of aqueous ammonia solution (5%), preheated to 90 °C. The sintered glass disks with the film between them were suspended above the liquid surface by a hollow cylindrical support. The container was covered and placed in an oven at 90 °C for 3, 12, 24, or 48 h. In a similar manner films were also subjected to air and plain water vapor. After this treatment samples were calcined at 500 °C for 4 h. Characterization. X-ray Diffraction (XRD). Low-angle (2°10°) XRD patterns were obtained using Cu KR radiation in both Bragg Brentano (BB) and asymmetric parallel beam configurations of a Philips X-Pert Multi Purpose Diffractometer (MPDPW 3050/10). The parallel beam was used in conjunction with the BB configuration in order to minimize errors associated with sample height displacement.2,20 These are especially problematic for a BB configuration at low angles.21 For the diffraction measurements the films were finely crushed and spread on a glass slide. Transmission Electron Microscopy (TEM). TEM studies on calcined mixed ammonia-water vapor treated films were carried out at an operating voltage of 200 kV on a FEI Tecnai 20 microscope. Samples were prepared by dispersing finely crushed films onto a holey carbon copper grid. Bright-field images were recorded parallel and perpendicular to the direction of the pore axes of the mesostructures. Nitrogen Adsorption-Desorption. N2 adsorption-desorption isotherms were recorded at a temperature of -196 °C using a Micromeritics Tristar 3000 instrument. The sample mass was typically 0.1 g. The measurements were performed to assess the total surface area and the pore-size distribution of the films. Prior to measurement, samples (crushed films) were thoroughly degassed for 3 h at 200 °C. The specific surface areas were calculated using a five-point BET analysis (Brunauer-EmmettTeller).22 The pore-size distribution curves were generated using the BJH (Barret-Joyner-Halenda) model of the adsorption branch.22 Nuclear Magnetic Resonance (NMR). Solid-state 13C cross polarization magic angle spinning (CPMAS) spectra were run on a Bruker MSL 300 at a 13C frequency of 75.482 MHz, using the standard cross polarization (CP) pulse sequence. The samples (typically 0.1 g) were spun at the magic angle (MAS) within a Bruker 4 mm MAS probe. The 1H and 13C 90° pulse times were (20) Gross, M.; Haaga, S.; Fietzek H.; Herrmann, M.; Engel, W. Epdic5, PTS 1 and 2 1998, 278-2, 242. (21) Jenkins, R.; Snyder, R. L. Introduction to X-ray powder diffractometry; J. Wiley & Sons: New York, 1996; pp 194-195. (22) Lowell, S.; Shields, J. E. Powder surface area and porosity; Chapman & Hall: New York, 1991; pp 14-34.
Figure 1. X-ray diffraction patterns of as-synthesized (s), 3 h mixed ammonia-water (- - -), and 48 h mixed ammoniawater treated (‚ ‚ ‚) films. For clarity, the spectra of the 3 and 48 h treated films were multiplied by a factor of 4 and 6, respectively. Inset shows expanded view of the (110) and (200) regions. Table 1. Mesostructure of Uncalcined Films time (h)
2Θ (deg)
d100 (nm)
FWHM (deg)
0 3 12 24 48
2.13 2.00 1.98 1.96 1.95
4.14 4.41 4.46 4.50 4.53
0.07 0.12 0.15 0.15 0.16
both 5.0 µs, and the CP contact time was 2 ms. The pulse repetition time was 3 s, and the MAS spinning speed was 7 kHz. The acquisition time was 51 ms. Chemical shifts were calculated relative to tetramethylsilane (TMS) via the higher chemical shift peak of adamantane, which lies at 38.23 ppm. Solid-state 29Si MAS spectra were run at a 29Si frequency of 59.627 MHz, using the standard pulse and collect sequence with high-power decoupling of the 1H nuclei. The samples (typically 0.1 g) were spun at the magic angle within a Bruker 4 mm MAS probe. The 29Si 90° pulse time was 6.0 µs, the pulse repetition time was 60 s, and the MAS spinning speed was 2.2 kHz. The acquisition time was 26 ms. Chemical shifts were determined relative to TMS via the higher chemical shift peak of kaolinite, which lies at -91.03 ppm.
Results and Discussion X-ray Diffraction (XRD). The X-ray diffraction patterns of the powdered as-synthesized films and the mixed ammonia-water treated films (3 and 48 h) before calcination are shown in Figure 1. The displayed patterns were recorded in the BB configuration and compared to the corresponding scans with parallel beam optics. The difference in 2Θ for the (100) peaks was less than 0.01°, which is of the order of the instrumental error. The diffraction patterns in Figure 1 are rescaled and offset for clarity. Spectra for the 3 and 48 h treated films were multiplied by a factor of 4 and 6, respectively. It can be seen that 2Θ of the (100) peak decreases with increasing duration of mixed ammonia-water vapor treatment. The attributed d-spacings are 4.14, 4.41, and 4.53 nm for assynthesized, 3 h, and 48 h gas treated films, respectively (Table 1). This swelling process is thought to be a temperatureinduced effect which is supported by evidence from 13C NMR spectra (discussed later) and/or a water-induced effect. As mentioned by Khushalani et al.,7 water can solvate a hydrogen bond to SiO/SiOH/SiOQ framework and ClQ anionic and CTAx cationic sites and so cause a pore expansion. Also listed in Table 1 are the values of the (100) peak full-width half-maxima (FWHM) of the as-
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Figure 2. X-ray diffraction patterns of calcined as-synthesized (s), 3 h mixed ammonia-water (- - -), and 48 h mixed ammoniawater treated (‚ ‚ ‚) films. The pattern for the 3 h treated film was rescaled for clarity (multiplied by a factor of 0.5). Inset shows expanded view of the (110) and (200) regions. Table 2. Mesostructure of Calcined Films time (h)
2Θ (deg)
d100 (nm)
FWHM (deg)
0 3 12 24 48
2.76 2.08 2.01 2.00 2.00
3.20 4.24 4.40 4.42 4.42
0.41 0.11 0.11 0.12 0.12
synthesized and posttreated films, which are inversely related to the associated crystallite sizes and the quality of the mesostructure.23 With increasing treatment duration the long-range order of the mesostructures seems to decrease; however, both as-synthesized and mixed ammonia-water treated films showed at least 3-4 diffraction peaks, indexed as (100), (110), (200), and (210), which are indicative of a two-dimensional hexagonal p6m phase with rodlike channels. The XRD patterns of the corresponding calcined samples are displayed in Figure 2. The pattern for the 3 h mixed ammonia-water treated film was rescaled for clarity (multiplied by a factor of 0.5). Calcination of as-synthesized films at 500 °C resulted in a partial collapse of the mesostructure, indicated by the significant decrease of the d-spacing, the broadening of the (100) diffraction peak (Table 2), and the loss of all higher-order diffraction peaks. Similar results were obtained by Yang et al.16 for calcined untreated films. The data suggest that partial collapse of the mesostructure upon calcination has been prevented by treating the silica films with mixed ammonia-water vapor at a temperature of 90 °C before calcination. Figure 2 shows that the treated films keep to some extent their longrange order mesostructure (at least three diffraction peaks) after calcination. It should be noted that ammonia treatment durations between 12 and 48 h resulted in equivalent d-spacings, while the peak intensities decreased with treatment duration (Table 2). This stabilization effect is thought to be due to the postsynthesis alkaline conditions (OH-) created by mixed ammonia-water vapor, which catalyzes the condensation of silica24 and changes the interaction forces between surfactant and silica species. Lin et al.8,10 proposed a reaction scheme for liquid ammonia treatment which assumes transformation of the (23) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures, J. Wiley & Sons: 1954, pp 491-494. (24) Iler, R. K. The colloid chemistry of silica and silicates; Cornell University Press: Ithaca, NY, 1955; pp 34-71.
Figure 3. X-ray diffraction patterns of calcined 48 h water vapor (s), 3 h water vapor (- - -), and 48 h air-treated (‚ ‚ ‚) films. The pattern for the 48 h water-treated film was rescaled for clarity (multiplied by a factor of 0.5).
weak hydrogen-bonded interactions between surfactant and silicate under acidic conditions into strong electrostatic interactions under alkaline conditions. It can be assumed that a similar reaction scheme can be applied to the mixed ammonia-water vapor treatment discussed in this paper. XRD was also performed on films treated by mixed ammonia-water vapor at room temperature. The recorded XRD patterns revealed an almost total collapse of the mesostructure after treatment for 3-48 h, demonstrating the necessity of higher posttreatment temperatures for the stabilization of the mesostructure. Films were not only subjected to mixed ammonia-water vapor but also to water vapor (without any ammonia) and plain air at 90 °C. The X-ray diffraction patterns of these powdered calcined films are displayed in Figure 3. The pattern of the 48 h water-treated film was rescaled for clarity (multiplied by a factor of 0.5). The data show that a postsynthesis water vapor treatment at 90 °C has some stabilizing effect. It can be asssumed that this effect is due to the creation of weakly alkaline conditions (OH-).24 However, long exposure times (48 h) are required to observe this effect. No significant stabilization and hence a collapse of the mesostructure is seen at shorter exposure times (3 h). Ammonia vapor treatment fully stabilizes the mesostructure after 3 h. Hence, one may conclude that although the mechanism may be similar (creation of alkaline conditions), mixed ammonia-water vapor treatment is a significantly more efficient stabilizing agent than water vapor. Films which were postsynthesis treated by heating in air at 90 °C for 48 h partially collapsed (Figure 3). The corresponding XRD pattern is basically identical to the pattern of a calcined as-synthesized film, which shows that posttreatment in air has no stabilization effect whatsoever. Transmission Electron Microscopy (TEM). A bright-field TEM image for a typical calcined, assynthesized film is shown in Figure 4. Large crystallites with stripe-like pores25,26 are clearly visible. We term these “secondary pores” (5-15 nm) in line with Wang et al.25,26 These pores are thought to be associated with the collapse of pore walls because of incomplete condensation of the silica (we discuss this phenomenon in the Si NMR section). It has to be (25) Wang, X. W.; Dou, T.; Xiao, Y. Chem. Commun. 1998, 1035. (26) Wang, X.; Dou, T.; Wu, D.; Zhong, B. Studies in Surface Science and Catalysis 141; Elsevier Science B.V.: New York, 2002; pp 77-84.
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Figure 4. Bright-field TEM image for a typical calcined, assynthesized film.
Figure 5. Bright-field TEM images for a typical calcined, 3 h mixed ammonia-water treated film. Images were recorded parallel and perpendicular to the direction of the pore axes.
noted that the pore-center to pore-center distance is consistent with the shrinkage of the lattice after calcination, as measured by XRD (Figure 2). Figure 5a,b shows bright-field TEM images for a large crystallite within a calcined, 3 h mixed ammonia-water treated film. Images a and b were recorded parallel and perpendicular to the pore axes, respectively, and confirm that regions of well-ordered tubules, arranged in a hexagonal array, are present within these large crystallites. The repeat distance from pore center to pore center is typically 4.7-5.0 nm, i.e., close to the value obtained by XRD (4.9 nm). Bright-field images for a typical calcined, 48 h mixed ammonia-water treated film are shown in Figure 6a,b. Figure 6a shows a hexagonally arranged array of tubules. The inset shows a honeycomb of the array recorded along the [001] zone axis. Figure 6b shows a different region of the sample containing many small crystallites 10-50 nm in size with gaps on the same scale. We term these textural
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(interparticle) pores in line with Pauly et al.27,28 Intracrystallite defects are also apparent in these images; these also lead to porosity on the scale of nanometers to tens of nanometers. It should be noted that many small crystallites are also visible in a detailed TEM survey of the 3 h mixed ammonia-water treated films. In combination with the XRD results (Figure 2), these data confirm that the hexagonal mesostructure of the mixed ammoniawater treated films is largely retained after calcination. Nitrogen Adsorption-Desorption. Nitrogen adsorption/desorption isotherms of MCM-41 materials generally show a type IV behavior with a sharp inflection, characteristic of capillary condensation within uniform mesopores.22,29,30 This inflection occurs typically at P/Po ) 0.3-0.4 (referred to as inflection 1),1,10 corresponding to a pore-size of approximately 3 nm.31 The isotherms presented here (Figure 7) differ significantly from the general behavior of MCM-41 materials. The mesoporous silica films were synthesized under acidic conditions and have a less condensed silica network than the mesostructures of base-catalyzed MCM-41 materials.9 This makes the acid-catalyzed structures more unstable and leads to a partial collapse of the hexagonally ordered mesostructures during the calcination process. This collapse is reflected in the observed changes in the isotherms and the related pore-size distributions. Typical N2 adsorption-desorption isotherms for calcined as-synthesized and 3 h and 48 h mixed ammonia-water treated films are shown in parts a, b, and c of Figure 7, respectively. These isotherms all have a characteristic sharp step,22,29,30 occurring above the relative pressure P/Po ) 0.6 (referred to as inflection 2), corresponding to the filling of mesopores far larger than expected for typical MCM-41 materials. These are designated as secondary mesopores. Hysteresis of type H1 suggests strong capillary condensation into the secondary pores.29 Both a sharpening of inflection 2 and a shift of the inflection point to higher relative pressures with ammonia treatment were observed. The observed shift points to an enlargement in secondary pore-size with ammonia treatment, which is reflected in the calculated pore-size distribution (Figure 8). As mentioned in the TEM section, the existence of secondary porosity is due to textural (interparticle)27,28 and/or intracrystallite stripe-like porosity25,26 (Figures 4 and 6b). By confocal microscopy we measured the films to be typically 20 µm thick (before calcination). Additionally, a careful scanning electron microscopy survey failed
Figure 6. Bright-field TEM images for a typical calcined, 48 h mixed ammonia-water treated film. These images show that the mesophase consists of large crystallites with well-ordered channels arranged in a hexagonal array (a) but also of many smaller crystallites (b).
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Figure 8. Pore-size distributions of calcined as-synthesized (s) and 3 h (- - -) and 48 h mixed ammonia-water treated (‚ ‚ ‚) films. The pore-size distribution curves were generated using the BJH model of the adsorption branch. Table 3. Surface Areas, Pore Volumes, and Average Pore Diameters of Primary (Sp, Vp, Dp) and Secondary (Ss, Vs, Ds) Poresa time (h)
Sp (m2/g)
Vp (cm3/g)
Dp (nm)
Ss (m2/g)
Vs (cm3/g)
Ds (nm)
0 3 48
412 441 507
0.34 0.36 0.38
3.2 2.9 2.7
538 449 290
1.16 1.22 1.20
9.4 13.4 21.6
a It should be noted that the errors associated with pore diameters, volumes, and surface areas are approximately 10-20%.
Figure 7. Nitrogen adsorption-desorption isotherms for calcined films: as-synthesized (a), 3 h (b), and 48 h (c) mixed ammonia-water treated.
to reveal any extensive cracking. Hence, we believe that microscopic cracks are not the main reason for secondary porosity. In addition to inflection 2, a sharp increase in the adsorbed gas volume in the region P/Po ) 0.3-0.4 (inflection 1) was observed for the mixed ammonia-water vapor treated samples. This step corresponds to the inflection expected for typical MCM-41 materials and is due to the filling of primary pores with a diameter of approximately 3 nm (Figure 8, Table 3). With increasing duration of ammonia treatment a sharpening and an increase in height of inflection 1 were observed while the relative pressure of the inflection point remained unchanged. This indicates a narrowing of the primary poresize distribution but an unchanged average primary poresize. Using the Barrett-Joyner-Halenda (BJH) model, the pore-size distributions for calcined as-synthesized and mixed ammonia-water treated samples were calculated for the adsorption branch. For the sorption isotherms displayed in Figure 7, the BJH method which generally
underestimates the pore-size32-34 was applied to the adsorption as opposed to the desorption branch since it resulted in slightly higher values for the pore diameters.35 These results are shown in Figure 8 and indicate that all samples have a bimodal mesopore distribution. The primary and secondary pore-sizes are in general agreement with the TEM observations. The BJH adsorption cumulative surface areas (Sp, Ss), pore volumes (Vp, Vs), and mean pore diameters (Dp, Ds) of the primary (2-4 nm) and secondary pores (5-50 nm) are displayed in Table 3. The increase of Sp/Ss and the narrowing of the primary pore-size distribution with ammonia treatment suggest a stabilization of the primary pores. It should be noted that the overlap between the primary and secondary pore distributions makes the estimation of the primary poresize very difficult and prone to an error of 10-20%. This is particularly true for the calcined, as-synthesized films since they possess a greater overlap of primary and secondary pore-sizes. The same caution should be exercised when interpreting the pore volume and surface area of primary and secondary pores. 13C NMR. The solid-state 13C NMR spectra of the uncalcined, untreated (as-synthesized) and mixed am(27) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (28) Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 987. (29) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (30) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982; pp 111-132. (31) Galarneau, A.; Desplantier, D.; Dutartre, R.; Renzo, F. D. Micropor. Mesopor. Mater. 1999, 27, 297. (32) Ravikovitch, P. I.; Domhnaill, S. C. O.; Neimark, A. V.; Schu¨th, F.; Unger, K. K. Langmuir 1995, 11, 4765. (33) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. (34) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1997, 101, 583. (35) Mokaya, R. J. Phys. Chem. B 1999, 103, 10204.
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Figure 9. Solid-state 13C NMR spectra of as-synthesized and 3 and 48 h mixed ammonia-water treated films (a). (b) Expanded view of C3, C16, and C17.
monia-water vapor treated (3 and 48 h) films are shown in Figure 9a. The assignment of the peaks is in accordance with Khimyak et al.36 The peaks at 14.5-15.2, 23.2-24.7, 26.9-27.4, and 30.6-34.3 ppm represent the terminal methyl group C17, C-R atom C16, the C3 methylene group, and the inner-chain methylene groups C4-C15, respectively. The weak peaks at 66.0-66.7 ppm are associated with the methylene group C2. Finally, the peaks at 53.353.5 ppm are assigned to the N-methyl group C1. According to ref 36, the peaks at 30.5-30.9 and 32.6-32.9 ppm correspond to short sequences of inner methylene units having gauche and all-trans conformations, respectively. A transition of gauche to all-trans, expressed by the increase in the ratio of peak intensities due to all-trans and gauge sequences, can be observed with increasing duration of the ammonia treatment. Figure 9b shows the C3, C16, and C17 peaks in more detail. A broadening of the C16 and C17 peaks with mixed ammonia-water vapor treatment was noticed. In addition to the broadening, which increased with treatment time, a second peak or shoulder (indicated by arrows) was observed. These downfield-shifted peaks again correspond to sequences adopting the all-trans as opposed to the gauche conformation. As further evidence for the transformation of gauche into all-trans conformation, the C3 peak is also downfield-shifted with ammonia treatment. Both the broadening and the additional peaks indicate an increase in the range of conformations adopted. This thermally conditioned disorder is suggested to be contributing to the expansive force that drives the swelling process of the mesostructure, shown in Figure 1. Under hydrothermal conditions, similar effects have been observed by Gross et al. and Tolbert et al.37-39 Additionally, the creation of basic conditions (mixed ammonia-water vapor and to a lesser extent plain water vapor) changes the charge balance at the surfactant headgroup-silica interface. This leads to a net surfactant removal.40,41 Note, additional evidence to support this hypothesis was gained from X-ray photoelectron spectroscopy (XPS) measure(36) Khimyak, Y. Z.; Klinowski, J. Phys. Chem. Chem. Phys. 2001, 3, 616. (37) Gross, A. F.; Le, V. H.; Kirsch, B. L.; Riley, A. E.; Tolbert, S. H. Chem. Mater. 2001, 13, 3571. (38) Tolbert, S. H.; Landry C. C.; Stucky, G. D.; Chmelka, B. F.; Norby, P.; Hanson, J. C.; Monnier, A. Chem. Mater. 2001, 13, 2247. (39) Gross, A. F.; Ruiz, E. J.; Tolbert, S. H. J. Phys. Chem. 2000, 104, 5448. (40) Lin, H. P.; Mou, C. Y. Micropor. Mesopor. Mater. 2002, 55, 69. (41) Øye, G.; Sjo¨blom, J.; Sto¨cker, M. Adv. Colloidal Interface Sci. 2001, 89, 439.
Figure 10. Solid-state 29Si NMR spectra of as-synthesized (s) and 3 h (- - -) and 48 h mixed ammonia-water treated (‚ ‚ ‚) films.
ments which showed a net increase of the Si/N and Si/C atomic ratios with ammonia-water vapor treatment. This is also evident in the 13C NMR spectra, which show additional peaks and the broadening of the existing peaks upon mixed ammonia-water vapor treatment (increased disorder of the inner carbon atoms). The net decrease of surfactant may allow water to penetrate the headspace.7,40 It is noteworthy that the 13C NMR spectra of the mixed ammonia-water treated films show no thermally induced decomposition of CTAC into N,N-dimethylhexadecylamine (DMHA), which acts as a swelling agent.42 Sayari et al. showed that postsynthesis hydrothermal treatment of MCM-41 materials at approximately 150 °C resulted in a decomposition of CTAC into DMHA.42 29Si NMR. The 29Si NMR spectra of the uncalcined, as-synthesized and 3 and 48 h mixed ammonia-water treated films are depicted in Figure 10. Three resonances were seen, assigned by the “Qn notation”. Q stands for a silicon atom bonded to four oxygen atoms forming a SiO4 tetrahedron. The superscript n indicates the connectivity, i.e., the number of other Q units attached to the unit in question.43 Thus, Q4 stands for Si(OSi)4, Q3 for Si(OSi)3(OH), and Q2 for Si(OSi)2(OH)2. In other words, the progression from Q2 to Q4 corresponds to an increase in extent of polymerization of silica. (42) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. Adv. Mater. 1998, 10, 1376. (43) Romero, A. A.; Alba, M. D.; Zhou, W.; Klinowski, J. J. Phys. Chem. B 1997, 101, 5294.
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As seen in Figure 10, the mixed ammonia-water vapor treatment (3 and 48 h) increased the peak area ratio of Q4 (at -111 ppm) to Q3 (at -102 ppm) from 1.1 to 1.8 (3 h) and 2.9 (48 h) while the Q2 (at -92 ppm) disappeared almost completely. Thus, the mixed ammonia-water vapor treatment at 90 °C served to increase the condensation of silica in the channel walls, improving the thermal stability of the material. This hypothesis is supported by the related XRD spectra in Figure 2. Both, 3 and 48 h treatment resulted in a broadening of the 29Si NMR peaks. This can be explained by the loss of precise repeats of the Si atoms,44,45 meaning a widening of the range of Si-O-Si bond angles as well as a broadening of the distribution of Si-Si distances. The resulting strain in the silica network may be caused by two processes (as mentioned above in the 13C NMR section): temperature-related swelling through an enhanced mobility of the surfactant tails37-39 and/or water related swelling around the surfactant headgroup.7,40 The strain may be responsible for the loss of intensity of the XRD diffraction peaks with ammonia treatment (Figures (44) Romero, A. A.; Alba, M. D.; Klinowski, J. J. Phys. Chem. B 1998, 102, 123. (45) Ramdas, S.; Klinowski, J. Nature 1984, 308, 521.
Vogel et al.
1 and 2). However, the increased condensation of silica stabilizes the film mesostructure. Conclusion It has been shown that mixed ammonia-water vapor treatment at 90 °C of thin silica films, synthesized under acidic conditions at 90 °C, stabilizes the mesostructure. This can be understood in terms of base-catalyzed (ammonia) cross-linking of the silica network through the creation of alkaline conditions. The treatment increases the volume and surface area ratios of primary to secondary pores of the bimodal mesoporous structure. Ammonia vapor treatment at 90 °C is an effective and convenient method to produce stable mesoporous silica films with narrow primary pore-size distributions and long-range order. Our findings have implications for the use of freestanding films in applications such as catalytic membranes and sensors. Acknowledgment. We thank Mr. Tony Raftery for his kind assistance with the powder diffraction measurements. LA035788O
