Langmuir 1997, 13, 6363-6365
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Mesoporous Silicate Film Growth at the Air-Water InterfacesDirect Observation by X-ray Reflectivity A. S. Brown,† S. A. Holt,† Thien Dam,‡ M. Trau,‡ and J. W. White*,† Research School of Chemistry, The Australian National University, GPO Box 414, Canberra ACT 2601, Australia, and Department of Chemistry, The University of Queensland, St Lucia Qld 4067, Australia Received July 24, 1997. In Final Form: October 2, 1997X Templated silicate films with mesoporous structure on the scale of 40 Å grow at the air-water interface of concentrated surfactant solutions. X-ray reflectivity measurements show that the mechanism of this growth for our preparation involves two stagessthe formation of an organized surfactant layer at the interface in the first 10 h and the subsequent rapid growth of a structure which shows strong diffraction associated with the mesoporous repeat distance. The structure appears to grow when the interfacial layer is complete with a thickness of about 30 Å.
Introduction In previous work it has been shown that the mesoporous silicate MCM-41 can be produced at room temperature1 and that the surfactant solutions from which it comes contain highly ordered “tactoid” like structures.2 One of us has shown3 that thin films of such materials are produced at the air-water and other interfaces when the chemistry of a solution is appropriately adjusted so as to allow the production of silicate oligomers to be the ratelimiting step. In this letter we report the direct observation of the growth process at the air-water interface at room temperature using X-ray reflectometry. The reflectometer is that recently constructed at the Research School of Chemistry and uses the copper KR radiation from an Elliott GX-13 fine focus X-ray generator. The resolution function of the reflectometer is determined predominantly by the beam divergence, which is defined by the combination of slits used. In these experiments the reflectometer was operated with a beam divergence of ∆θ ) 0.017° in a reflectivity range of 1-10-8 and a scattering vector range of Qz ) 0.03-0.35 Å-1 (the resolution function ∆Q/Q was therefore in the range 1-8%, and Qz ) (4π/λ) sin θ, where θ is the angle incident to the surface). The experiments reported here used a solution of cetyltrimethylammonium bromide (CTAB) dissolved in a minimal amount of methanol. This solution was stirred into approximately 20 times its volume of pure distilled water acidified with hydrochloric acid. Into this solution was added approximately 0.4 g of tetraethoxysilane (TEOS), and the mixture was stirred at room temperature for 15 min before being placed on the reflectometer. The solution was observed at room temperature in a clean flat dish containing approximately 30 mL of solution showing a positive meniscus. The sample environment was kept at constant humidity (saturation vapor pressure of water at the measuring temperature) by a continuous stream of humidified nitrogen. The reflectometer was programmed * Author to whom correspondence should be addressed. † The Australian National University. ‡ The University of Queensland. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Edler, K. J.; White, J. W. J. Chem. Soc., Chem. Commun. 1995, 1155. (2) Edler, K. J.; Dougherty, J.; Durand, R.; Iton, L.; Kirton, G.; Lockhart, G.; Wang, Z.; Withers R.; White, J. W. Colloids Surf., A 1995, 102, 213. (3) 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.
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to take repeated scans over the Q range at approximately 1 h intervals for two days. Results Overview. Figure 1 illustrates the very marked changes occurring in the reflectivity profiles as the reaction proceeds. In the earliest stages the reflectivity falls as approximately Q4z and has superimposed Kiessig fringes which initially have a period ∆Q ∼ 0.2 Å-1 (not shown in Figure 1) and which develop with time to ∆Q ∼ 0.14 Å-1 (Figure 1a). This is immediately interpretable in qualitative terms as arising from a surface layer with an initial thickness ca. 31 Å which then increases to 45 Å with time. After about 10 h of “induction time”, pronounced bumps appear at Qz ) 0.142 and 0.284 Å-1. These continue to grow, becoming sharper and more pronounced, as the reaction proceeds. By comparison with the induction period, growth of the “crystalline” diffraction peaks is fast. These peaks index on a repeat distance, a, of 44 Å. The intensity of the first strong diffraction peak from the crystalline film has reached saturation within 3 h of its appearance. Because of the humidity control, evaporation from the film over the time of the experiment was minimized. Discussion Induction Period. The development of structure at the air-water interface in the “induction stage” has been modeled by fitting the reflectivity profile using standard optical matrix methods.4 The changes can be seen qualitatively by multiplying the reflectivity profiles by Q4z , to remove the component associated with the simple Fresnel reflectivity of a smooth surface. The data collected in this way, taken at 20, 120, 530, 608, and 687 min after mixing the TEOS into the solution and insertion into the instrument, are shown in Figure 2. At the earliest times, there is clear evidence of a fringe in the reflectivity profile, which differs markedly from the case for pure water and a surfactant only solution. The fringe has a long period in Qz, corresponding to a thickness of about 30 Å. The film shrinks a little to about 27 Å prior to the fast growth of the crystalline film. The first maximum in the Kiessig fringe is very close to the wave vector of the first strong maximum in the diffraction from the subsequently crystallized film. (4) Penfold, J. In Neutron, X-ray and Light Scattering; Lidner, P., Zemb, Th., Eds.; Elsevier: Amsterdam, 1991; p 223. (5) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304.
© 1997 American Chemical Society
6364 Langmuir, Vol. 13, No. 24, 1997
Letters Table 2. Model Fits to the Reflectivity Profiles Recorded on the Dilute Solution during the Induction Perioda model parameters time after mixing (min) background (×107) scale film thickness (Å) Nb1 (×106 Å-2) interface roughness (Å) air/film roughness (Å) χ2
Figure 1. X-ray reflectivity from the air-water interface during the induction period showing the development of the reflectivity profile over time into diffraction peaks: (a) 530 min; (b) 608 min; (c) 687 min.
Figure 2. X-ray reflectivity multiplied by Q4z as a function of time after mixing. The data sets have been arbitrarily offset for clarity and the lines through a, b, and c are the fits to the data. (a) 25 min; (b) 120 min; (c) 530 min; (d) 608 min; (e) 687 min. Table 1. Fit to the Reflectivity Profiles from the Air-Water Interface during the Induction Period (0-10 h from Mixing)a model parameters time after mixing (min) background (×107) scale film thickness (Å) Nb1(×106 Å-2) air/film roughness (Å) χ2
25 7 (7) 1.1 (1) 31.1 (8) 9.9 (3) 4.5 (8) 12.60
120 2 (1) 1.1 (1) 25 (2) 10.0 (8) 5 (2) 14.28
530 6 (2) 0.73 (4) 27.7 (6) 13.1 (2) 6 (4) 53.41
a The subphase scattering length density (water) and interface roughness (4.0 Å) were fixed for all models, and the numbers in parentheses represent the uncertainty in the last figure.
As the thickness of a CTAB cylindrical micelle is about 34 Å,5 it is logical to model the film structure in the induction region in terms of a surfactant surface excess. This consists of a layer of CTAB organized at the airwater interface. However it is not possible using specular reflectivity data alone to distinguish between hexagonally close packed micellar rods with their long axes parallel to the interface and a bilayer lamella. Toward the end of the induction period, this layer has increased in thickness (Figure 2), consistent with a swelling of the CTAB or with the association of silicate species at the CTAB-water interface. Table 1 gives the fitted layer thicknesses at three times prior to the growth of the crystalline film. It should be noted that the modeling has assumed a single layer at the interface; this is obviously not adequate to describe the thickening film near the end of the induction period (e.g. 530 min) where a more complex description is required. Dilute Solutions. Part of the original solution made up in the manner described above was kept sealed in a plastic bottle during the course of the experiments so far described. This solution was diluted with about half as
20 2.3 (1) 1.1 (2) 28.6 (3) 11.0 (7) 3 (3) 5.5 (8) 3.493
150 2.3 (1) 1.1 (1) 28.6 (2) 10.48 (4) 0.9 (3) 5.4 (6) 2.407
210 4.4 (2) 0.8 (1) 28.1 (5) 12.5 (2) 7 (7) 6 (1) 2.086
290 6 (1) 0.8 (1) 28 (1) 12.2 (3) 6 (1) 6 (4) 4.676
a The subphase scattering length density (water) was fixed for all models and the numbers in parentheses represent the uncertainty in the last figure.
much pure water and the reflectivity was measured. Again, appreciable differences from the reflectivity of a pure water surface were obtained. The profiles resemble those seen in the induction period of the more concentrated solution (above), but the induction period before diffraction peaks appeared in the reflectivity profiles was in excess of 23 h. Once again, the Keissig fringes could be fitted by a single layer model of thickness about 28 Å (see Table 2 for fitted results). Compared with the case for the more concentrated solution the fit to the data was much better in this case. This is partially due to the slower kinetics in the dilute solution, there being less change in film structure during the time required to collect a reflectivity profile. Transition Period. After approximately 10 h an interesting change occurred in the reflectivity. The pattern characteristic of a thin surface layer structure changed to that of a crystalline phase (Figure 1). This problem has been discussed in classical crystallography texts such as those of Born and Wolf6 and James.7 The peak shape in this region is a function of the number of layers and the angular difference away from the Bragg position, and so further details of the film growth in this region may be obtained by the appropriate peak shape analysis. The orientation of the momentum transfer, Qz, is perpendicular to the air-water interface in this experiment. The sharpness of the diffraction peaks a few hours after crystal growth indicates quite a large coherence length for the film perpendicular to the interface. Clearly this evolves very quickly, there being virtually no change in the shape or intensity after about 3 h. The peaks also have high intensity which suggests that there may be quite high crystalline orientation. This is also evident in Figure 3b and c, where the present film data are compared (Figure 3b) with data from calcined MCM-41 powder (Figure 3c) taken with the reflectometer. The powder intensities are much weaker and the x3a reflection (11) is absent in the film data. Growth Phase. Data taken subsequently to the last scans on Figure 1 index well on a repeat distance of 44.2 Å, which is very close to the distance seen in templated bulk MCM-41 preparations at room temperature.3 Two scans taken 1535 and 2600 min after saturation of the peak intensity are shown in Figure 3a and b, respectively. The principal peaks are well defined, but for Figure 3a (where the reflectivity has been corrected by Q4z because there is still a strong specular reflectivity component in the profile) there are subsidiary peaks at 0.07 Å-1 (88 Å) and 0.21 Å-1 (44 Å). These index approximately on odd (6) Born, M.; Wolf, E. Principles of Optics. Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 4th ed.; Springer: Berlin, 1970. (7) James, R. W. The Optical Principles of The Diffraction of X-Rays. In The Crystalline State; Bragg, L., Ed.; G. Bell and Sons LTD: London, 1965; Vol. 2, p 38.
Letters
Figure 3. Diffraction intensity from growing silicate films at the air-water interface. The data sets have been arbitrarily offset for clarity, and the lines between the points are a guide to the eye only. (a) 1535 min after mixing; (b) 2600 min after mixing; (c) diffraction scan of calcined MCM-41 performed on the reflectometer (note the logarithmic scale). The intensity scale on part a has been multiplied by Q4z to remove the Fresnel reflectivity, while the Lorentz correction, Q2z , has been applied to parts b and c for reasons described in the text.
multiples of 88 Å, i.e. twice the lattice parameter seen from the strong reflections. We believe that these peaks are too strong to be due to λ/2 contamination of the incident beam, and the variation of their intensity relative to those of the main peaks with time is not consistent with this hypothesis. Their appearance suggests some long period modulation of the basic 44 Å repeat. We provisionally index them as (1/2,0) and (3/2,0). That the (1/2,0) and (3/2,0) reflections are very weak indicates that the silicated micellar tubes are quite parallel to the liquid surface and that the modulation approximates to ABAB. After standing for almost 2 days in the reflectometer (under controlled humidity) a final diffraction pattern was recorded from the film. This is shown in Figure 3b. There is very little low-angle reflectivity from this film so the data have been corrected by the usual Lorentz factor (multiplication by Q2z ). The “(1/2,0), (1,0), (3/2,0), and (2,0)” reflections are all well developed, but the modulation
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period is different. The odd order peaks are also broader, and there may be a broad component under the (2,0) reflection. The increased intensity of (1/2,0) is notable and indicates the onset of a new texture in the film. The lattice parameter is 44 Å, in close agreement with that for room temperature synthesized MCM-41.3 It is to be expected that, in the growing time allowed, the silanol groups will not have had time to fully condense. The film structure, as for MCM-41 under these conditions, will be somewhat unstable and probably subject to a contraction of the order of 8 Å upon removal of the template by calcination. This could easily destroy the film. A test of the synthesis at higher temperatures shows that growth occurs, and so the possibilities of careful study of the kinetics of the induction and growth phases have been opened up. We presume that the solution, after 10 h of standing, has become saturated with oligomeric silicate anions. This might be the cause of the relatively fast, probably heterogeneous, nucleation of the crystal phase. With the reflectometer it is easy to measure the height of the liquid surface to within about 5 µm. That there was some evaporation is illustrated by the fact that approximately 8.4 mL (out of an initial volume of approximately 30 mL) was lost over the 2 days. This is compared with approximately 2.9 mL prior to the crystallization phase. We cannot at this stage rule out evaporation as part of the driving force for the effects seen. Conclusion The evidence presented strongly supports the hypothesis that film growth of mesoporous silicates at the air-water interface has two main stages in its mechanism. We have determined that the induction period is dependent upon the concentration of the solution and observed induction periods ranging from 6 h for a highly concentrated solution to in excess of 23 h for the diluted solution. The transition and growth phases follow quickly after the induction period at room temperature to produce a material which may be highly organized perpendicular to the interface with the long axis of the templated silicate cylinders parallel to the air-water interface or in a lamellar structure. Clearly, X-ray reflectivity is a powerful method for studying the mechanism of this and related processes. LA970832J
