Characterization of the Structure of Mesoporous Thin Films Grown at

structure of surfactant-templated silica films grown at the air/water interface at different depths in the ... two methods, spontaneous growth at the ...
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Langmuir 2003, 19, 2639-2642

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Characterization of the Structure of Mesoporous Thin Films Grown at the Air/Water Interface Using X-ray Surface Techniques Tessa Brennan,† Stephen J. Roser,† Stephen Mann,‡ and Karen J. Edler*,† Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K., and School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. Received April 22, 2002. In Final Form: September 6, 2002 Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity have been used in situ to study the structure of surfactant-templated silica films grown at the air/water interface at different depths in the film. The results confirm that cylindrical silica-encased surfactant micelles are predominantly organized into a two-dimensional hexagonal structure, with the long axis parallel to the surface of the film. The arrangement of the micelles is well orientated near to the air/film interface but becomes disordered deeper into the sample. GIXD also reveals the existence of vertical channels extending down from the bottom of the film. This suggests a transition from in-plane to unconstrained growth.

Introduction Surfactant-silicate mesophase composites self-assemble from dilute solutions containing surfactant micelles and a silica precursor to form a variety of mesoscale and macroscale morphologies, dependent on solution conditions.1 These materials contain ordered arrays of surfactant micelles within the silica matrix, and because when calcined they have a uniform and adjustable pore size, they are of interest for applications such as catalysis, filtration, and encapsulation.2,3 The majority of structural investigations on surfactanttemplated materials have concentrated on the syntheses of bulk materials prepared using alkaline solutions or mesophase films on substrates from acid-catalyzed syntheses. Materials prepared from alkaline and acidic solutions have been shown to have different pore wall characteristics,4 which result from differences in the synthetic process. The current study is concerned with thin films grown from acidic solutions, as described below. Thin film synthesis from acidic solutions can occur via two methods, spontaneous growth at the solution/air or solution/substrate interface or evaporative self-assembly. The structure of thin film materials from acid catalyzed preparations is dependent on the surfactant-silica ratios, particularly for evaporation driven processes, but the most common is a two-dimensional hexagonal phase (p6mm)5-7 * Author for correspondence. E-mail: [email protected]. Phone: +44(0)1225 384192. Fax: +44(0)1225 386231. † University of Bath. ‡ University of Bristol. (1) Edler, K. J.; Roser, S. J. Int. Rev. Phys. Chem. 2001, 20 (3), 387. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; 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. (3) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (4) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (5) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (6) Ogawa, M.; Ishikawa, H.; Kikuchi, T. J. Mater. Chem. 1998, 8, 1783. (7) Donnay, J. D. H.; Donnay, G. International Tables for Crystallography, Vol. II, Section 3, Crystal Geometry; International Union of Crystallography: Dordrecht, London, 1985.

where the cylinder length of the micelle is many times that of the diameter. However, there are also a number of papers which have determined the structure of evaporative self-assembly thin films to have a three-dimensional pore network, be that cubic,8 hexagonal,9,10 or lamellar.11 Less work has been done on preparations such as the ones discussed here, where films grow spontaneously at the surface of acidic solutions, and the silica polymerization and the self-assembly process proceed more slowly. Under these conditions, free-standing thin films self-assemble at the air/water interface, on time scales between 3 and 8 h.12,13 Previous grazing incidence X-ray diffraction (GIXD) experiments on these films have been done by Holt et al.,14 who observed the time dependent structure development of the first two or three layers of film. In that study well defined diffraction peaks corresponding to an ordered 2D hexagonal structure were observed to develop out of very weak streaks which initially appeared parallel to the specular streak, separated from it by wave vector transfers of approximately (0.11 Å-1. The aim of these experiments is to use the in situ techniques of X-ray reflectivity and grazing incidence X-ray diffraction to determine the structure at different depths in a film of surfactant-templated silica grown under static conditions at the air/solution interface from acidic solution. X-ray reflectivity has been used to characterize the morphological properties of the thin film in the direction normal to the surface (Qz), and GIXD allows us to study the in-plane structure (Qxy). The GIXD measurements have been made at two different angles, so the structures can be investigated as a function of penetration depth. To our knowledge, this is the first example of using GIXD to probe the structure of these films at two different (8) Zhao, D.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Commun. 1998, 8, 1147. (9) Besson, S.; Ricolleau, C.; Gacoin, T.; Jacquiod, C.; Boilot, J.-P. J. Phys. Chem. B 2000, 104, 12095. (10) Besson, S.; Gacoin, T.; Jacquiod, C.; Ricolleau, C.; Babonneau, D.; Boilot, J.-P. J. Mater. Chem. 2000, 10, 1331. (11) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M.; Zink, J. I. Nature 1997, 389, 364. (12) Edler, K. J.; Goldar, A.; Hughes, A. V.; Roser, S. J.; Mann, S. Microporous Mesoporous Mater. 2001, 44-45, 661. (13) Edler, K. J.; Roser, S. J.; Mann, S. Chem. Commun. 2000, 773. (14) Holt, S. A.; Foran, G. J.; White, J. W. Langmuir 1999, 15, 2540.

10.1021/la0203786 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/27/2003

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Figure 1. Diagram showing the experimental geometry of the X-ray reflectivity (θi ) θf) and grazing incidence experiment (θi ) constant, and θg maps out an area with the linear detector as shown by the shaded area).

depths. It reveals that the structure at the interface is essentially an organized two-dimensional network orientated parallel to the surface; however, the deeper scan reveals features consistent with unconstrained threedimensional growth. Experimental Procedure The films were synthesized by mixing cetyltrimethylammonium bromide (CTABr), 0.2 M HCl solution, and tetramethoxysilane (TMOS) in the following molar ratio: 1.52 × 10-3 CTABr/1 water/3.63 × 10-3 HCl/0.011 TMOS. The solution was then poured into a Teflon trough (surface area 63 cm2 and 3 mm deep) until a positive meniscus was obtained. The trough was located in position on beam line ID10B (Tro¨ika II) at the ESRF, Grenoble. Tro¨ika II is a multipurpose undulator beamline used for both diffraction and reflectivity measurements. The film takes approximately 200 min to grow and was initially characterized by recording a profile of the specular reflectivity using the wavelength (λ) 1.556 Å-1. For the specular reflectivity measurements the incident beam and the detector are moved simultaneously so that the incident angle (θi) equals the reflected angle (θf), over the angular range 0° to 4° (Figure 1). The variation in intensity of the specular reflection with scattering vector Qz (where Q ) (4π sin θ)/λ) contains information on the thickness, electron density, and roughness of the various interfaces in a direction normal to the surface. At scattering vectors below a critical value Qc there is total external reflection and the X-rays do not penetrate into the sample. The position of the critical edge depends on the difference in scattering length density (F(z)) of the two phases. The scattering length density for X-rays is the product of the Thomson scattering amplitude ro (2.82 × 10-5 Å), which is a measure of the scattering strength of a single electron, and the electron density ne(z). Above the critical edge the reflected intensity decays approximately with a Q-4 dependence; however, this decay is also modified by variations in scattering length density in the interfacial region. The data have been analyzed using a formulation originally created for optics15 and adapted by Parratt in 1954.16 Parratt approximated the film to a series of discrete strata sitting on an infinitely thick substrate. Each layer can be characterized by a different density, thickness, and interfacial roughness. In the Parratt computer program produced by HMI Berlin,17 suggested model parameters are subsequently optimized by an iterative least-squares fitting routine. The experimental setup only needs a relatively simple modification to change the measurement from specular reflectivity to grazing incidence diffraction (GIXD), enabling these measurements to be made consecutively on the same film. By replacing the evacuated beam guide after the sample with a Bragg-Soller collimator and keeping the incident angle (θi) constant, the linear detector records scattering in Qxy space by sweeping the detector out of the plane of incidence (θg ) 2θ) between 2° and 20° (Figure 1). The technique of grazing incidence diffraction requires a well collimated intense X-ray beam, only available at synchrotron sources. Measurements have been made using the incident angles 0.13° (approximately 85% of the critical (15) Abele`s, F. Ann. D. Phys. 1948, 3, 504. (16) Parratt, L. G. Phys. Rev. 1954, 95, 359. (17) http://piotr.chem.tu-berlin.de/parratt.htm.

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Figure 2. Specular reflectivity profile from a CTABr-templated film. The line is the “best fit” arising from modeling the data. The inset displays the corresponding scattering length density profile. angle) and 1.27°, which allows us to probe deeper into the sample. At angles below the critical angle, an evanescent wave penetrates into the sample, to a depth of only 50-100 Å;14 hence, it investigates the near surface structure of the film and effectively eliminates scattering from the subphase.

Results and Discussion The specular reflectivity profile, R(Q), for the thin film grown at the air/water interface is shown in Figure 2. The specular profiles are similar to the X-ray and neutron reflectivity patterns for CTABr/TEOS and CTACl/TEOS films grown under different synthesis conditions measured by Brown et al.18 The intensity of the reflectivity R(Q) above the critical edge (Qc ) 0.02 Å-1) decays with the characteristic ∼Q-4 dependence. Three Bragg diffraction peaks are superimposed upon the decay, which correspond to a real space repeat distance of 46 ( 0.5 Å. This suggests relatively tight packing of the CTABr micelles which have a cylindrical diameter of ∼34 Å.19 The fits to the data are calculated using the slab model described in the previous section. The concomitant scattering length density plot is shown in the inset of Figure 2. The profile has been modeled using 21 layers, which can approximately be broken down into a series of two alternating layers with decreasing scattering length density amplitude. One of the layers has the thickness 30 ( 4 Å and the scattering length density F(z) ∼ 9.1 × 10-6 Å-2. This probably corresponds to the hydrocarbon tails in the surfactant interior of the micelle. The second layer is thinner, 15 ( 3 Å, has F(z) ∼ 10.5 × 10-6 Å-2, and can be associated with a low-density silica phase. These results are consistent with measurements made by Brown et al.18 on CTACl/TEOS films. They modeled their X-ray reflectivity profile (only up to 0.3 Å-1) using alternating layers of 33 Å with the scattering length density 8.0 × 10-6 Å-2 and d ) 12 Å when F(z) ) 10.0 × 10-6 Å-2. The sharpness of the diffraction peaks in our measurement indicates the film has a large coherence length perpendicular to the surface; hence, as we are fitting the profile with a limited number of layers, we have difficulty in modeling the exact shape of the peaks. The full-width at halfmaximum of the first diffraction peak (Qz ) 0.137 Å-1) is ∆Qz ) 0.007 Å-1; using the Scherrer equation,20 there must be at least 39 bilayer units present. Since this fwhm corresponds to the resolution function of the machine (equivalent to 186 nm), the value for the number of repeat (18) Brown, A. S.; Holt, S. A.; Reynolds, P. A.; Penfold, J.; White, J. W. Langmuir 1998, 14, 5532. (19) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (20) Warren, B. E. X-ray Diffraction; Dover Publication Inc.: New York, 1990.

Mesoporous Thin Films Grown at the Air/Water Interface

Figure 3. GIXD in the form of a two-dimensional intensity contour plot from a CTABr film after 12.5 h. Incident angle 0.13°. The horizontal line at Qz ) 0.22 Å-1 is a detector glitch.

Figure 4. GIXD in the form of a two-dimensional intensity contour plot from a CTABr film after 15 h. Incident angle 1.27°. The horizontal line at Qz ) 0.26 Å-1 is a detector glitch.

units is a lower limit. However, examining the peak shape closely, it appears that there is a very sharp peak superimposed on a broader, less intense peak, which could be due to different domain sizes of cylindrical micelles. The sharper peak is due to scattering from larger domains while the broader peak is from smaller regions of ordered micelles. If we assume that the film is formed from a two-dimensional hexagonal arrangement of cylindrical micelles parallel to the surface (p6mm6,7), the peaks can be labeled as the [100], [200], and [300] reflections. Although there is good agreement between the calculated and measured values, it is impossible to distinguish between a two-dimensional hexagonal structure and a lamellar bilayer arrangement using X-ray reflectivity if the c axis is in-plane. Grazing incidence X-ray diffraction has been measured at two different incident angles to examine the in-plane configuration and structural changes that occur between the surface and the bulk of the film. These patterns are from the same film as was used for the specular measurement. The patterns recorded at the incident angles 0.13° and 1.27° can be seen in Figures 3 and 4, respectively. The linear detector has a fault in channel 540, which produces an artificial line at constant Qz. The linear detector sweeps out an area up to 5°; beyond this any features are very weak and count times become prohibitively long. The features seen in Figure 3 have been indexed assuming a two-dimensional hexagonal structure. There is relatively good correlation between the predicted and measured positions; however, all

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the peaks are situated at Q values (Q ) (Qz2 + Qxy2)1/2) slightly lower than calculated. The [100] reflection at Q ) 0.138 ( 0.005 Å-1 corresponds to the first Bragg diffraction peak seen in Figure 2. The diffraction spots are relatively well defined, which suggests that the surface structure is well ordered; the top layers (∼100 Å-1) of the film have a high degree of long-range order. As the footprint of the X-ray beam is ∼15 cm long (although only a few millimeters wide) at grazing incidence, the orientation of the channels within the film must be well defined over this length scale, or powder rings rather than diffraction spots would be observed. Holt et al.14 have reported on the growth of cetyltrimethylammonium chloride (CTACl)-templated films at the air/water interface using GIXD. They also observed well defined diffraction peaks in the surface layers, which were indexed using a two-dimensional hexagonal structure. At the incident angle 1.27° (Figure 4) the evanescent wave probes significantly deeper into the sample (several hundred angstroms), thus allowing in situ measurements to be made on lower sections of the film. Although the features exhibit excellent agreement with the p6mm structure (where Q100 is calculated to be 0.133 Å-1), instead of diffraction peaks, partial rings are observed. These are synonymous with materials having domains with multiple orientations, as in a powder. Hence, as we probe deeper into the sample, the film retains a high degree of hexagonal order, yet the domains of hexagonally packed micelles become orientationally disordered with respect to each other in the x-y plane. There is some discrepancy in the position of similar features between Figures 3 and 4 (up to 8%), but we believe that this can be attributed to the refractive index of the medium, as well as distortion of the hexagonal phase at the surface where drying and continued silica polymerization cause distortion in the lattice similar to that observed by Klotz et al.21 They recorded the structure of dip coated films after drying and thermal treatment, and observed unidirectional shrinkage of the layers in the direction normal to the film: the compression of a two-dimensional hexagonal arrangement to a centered rectangle (c2mm) during solvent evaporation. There will also be roughness effects from the surface, which could produce interference between diffuse surface scattering and the reflected signal. In addition to the powder rings, there are three vertical broad features centered at constant at Qz and extended along constant Qxy. These features, which are approximately 100 Å apart, could be caused by protrusions extending normal to the surface into the sample. Yao et al.22 and Aksay et al.23 grew CTACl-templated mesostructured films at the air/water interface and on an amorphous silica substrate, respectively, and found (using ex situ TEM, AFM, and X-ray diffraction techniques) that the tubules orientated away from the interface on the underside of the film. They propose that the ordering ability of the substrate or water interface is not strong enough to confine the cylinders parallel to the interface, and once the end of the micelle is nucleated, the long axes of the tube appear to wander over a slowly curving configuration in three dimensions. Their results show that the air/water interface acts to confine film growth to two dimensions during the initial stages, and once the film (21) Klotz, M.; Albouy, P.-A.; Ayral, A.; Me´nager, C.; Grosso, D.; Van de Lee, A.; Cabuil, V.; Babonneau, F.; Guizard, C. Chem. Mater. 2000, 12, 1721. (22) Yao, N.; Ku, A. Y.; Nakagawa, N.; Lee, T.; Saville, D. A.; Aksay, I. A. Chem. Mater. 2000, 12, 1548. (23) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Guner, S. M. Science 1996, 273, 892.

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exceeds a critical thickness, there is a transition between two-dimensional (in-plane) growth and three-dimensional (unconstrained) growth. If similar effects are happening at the bottom of the film in our case, it would explain why the peaks are diffuse (as the signal is being averaged over a large number of different orientations) and broad (due to the small size of the tubules). Hillhouse et al.24,25 also found evidence for particles embedded in films grown onto substrates from similar solutions. Channels within the particles ran perpendicular to the film surface and so would provide different diffraction signatures to those channels running parallel to the interface, as found in the body of the film. The differences between these two patterns can be explained by the film formation mechanism which is operative at this CTABr/TMOS ratio. We have previously shown that the film grows via the formation of preformed mesostructured particles in the bulk solution and these particles rise to the surface to form the film.12 The particles therefore already consist of large domains of a well ordered hexagonal phase, although this structure is not frozen in, since the condensation rate of silica at this pH is slow. From the specular measurement the silica layers are low density and contain large amounts of solvent, assisting structural rearrangement, and so the domains are still able to reorient themselves. The particles themselves also may well be anisotropic, since they are composed of bundles of rods. The first particles reaching the solution/air interface are most affected by the energetic constraints of the interface and reorient to align the long axis of the micelles with the interface. Changes in the surface structures of the existing large domains are propagated through the particle, resulting in large domains of crystalline arrays which give rise to the sharp diffraction spots. The mix of larger and smaller domains observed in the specular scattering may be due to different sizes of particles layering at the surface or else infilling of gaps between particles with smaller domains. Particles arriving at the surface after the first monolayer of particles will be sufficiently far from the energetic constraints of the surface that addition of these particles in different orientations is possible. Since these particles are also incompletely condensed, it is probable that some, that reach the film in favorable orientations, are able to join to existing oriented particles; however, others will be included in the growing film at a range of angles producing the vertically oriented channels after the manner described by Hillhouse et al.24 These later arriving particles are responsible for the development of powder-like diffraction rings in the GIXD scan of the lower section of the film and (24) Hillhouse, H. W.; Okubo, T.; van Egmond, J. W.; Tsapatsis, M. Chem. Mater. 1997, 9, 1505. (25) Hillhouse, H. W.; van Egmond, J. W.; Tsapatsis, M.; Hanson, J. C.; Larese, J. Z. Chem. Mater. 2000, 12, 2888.

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the diffuse vertical features in the pattern at greater depth in the film. The experiments therefore suggest that the bulk film structure consists of large domains of micelles that arrange themselves at the surface to form highly ordered hexagonal structures, with the long axis parallel to the surface. As the film becomes thicker, it also becomes more disordered through the addition of new domains (particles) which are not influenced by the energetics of the interface or through the bending of some of the cylinders to form curved morphologies away from the film surface. Conclusion In situ reflectivity and grazing incidence measurements suggest that the film consists of domains of hexagonally ordered cylindrical micelles, with their long axis parallel to the surface. The micelles pack in such a way that the CTABr hydrocarbon tails form the interior of the micelle (diameter 30 Å), which is covered by a low-density silica layer. The structure appears well ordered at the air/film interface, becoming less oriented as we probe deeper into the sample. Orientational order is preserved within the domains, but on length scales greater than the domain size, the material appears macroscopically isotropic. As the film becomes thicker, the effect of the weakly interacting hydrophobic air/water surface loses its ability to align the micelles in the particles parallel to the surface. As the surface dries, the lattice becomes distorted at the interface; a deformation from two-dimensional hexagonal structure (p6mm) to centered rectangle (c2mm) has been suggested. Additional features consistent with channel orientation normal to the surface have also been observed growing in the lower sections of these films. The observed structures have been explained in terms of a film formation mechanism which involves the initial formation of mesostructured particles in the subphase which rise to the surface to form the film. Grazing incidence X-ray diffraction has proved a very useful tool in the in situ characterization of thin films. In the future we plan to make time-resolved GIXD measurements, to observe ordering and possible reorientation of the mesostructured particles at the surface. Acknowledgment. The beamline scientist on Troika II at the ESRF, Dr. Oleg Konovalov, is acknowledged for his assistance in the experiment. This work has been financially supported by both the EPSRC (Grant no. GR/ M15989) and the Royal Society (in the form of a Dorothy Hodgkin research fellowship for K.J.E.). The authors would also like to thank Dr. Roberto Felici for useful discussion. LA0203786