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Langmuir 1999, 15, 2540-2542
Observation of Hexagonal Crystalline Diffraction from Growing Silicate Films Stephen A. Holt,† Garry J. Foran,‡,§ and John W. White*,† Research School of Chemistry, The Australian National University, GPO Box 414, Canberra, ACT 2601, Australia, and Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia Received October 6, 1998. In Final Form: January 14, 1999 Clear evidence for hexagonal crystallinity in surfactant-templated silicate films growing at the airwater interface is presented for the first time. Grazing incidence synchrotron radiation diffraction shows the development of diffraction spots just as the “induction phase”, identified previously, is completed. The observed hexagonal diffraction represents the in-plane ordering of the first two-three layers of film formed at the interface.
Introduction We have recently shown1,2 by X-ray and neutron reflectivity methods that silicate films,3 highly ordered in one dimension, may be grown at the air-water interface from concentrated surfactant solutions. This process has an induction period during which there is an accumulation and ordering of surfactant and silicate precursor at the interface, followed by the rapid growth of a layered structure showing strong Bragg-like diffraction in the specular reflecting plane. By the combination of X-ray and neutron reflectivity data from different isotopic combinations, the film growth processes and structures in one dimension have been elucidated.2 Further studies on similar films using powder diffraction and other methods show the potential value of these films as new materials.4 A limitation of our specular reflectivity studies performed to date has been the lack of in-plane information. It is therefore very difficult to unambiguously determine whether the multilayer film is hexagonal, lamellar, or perhaps some other structure during the growth process. It has been demonstrated in recent years that grazing incidence X-ray diffraction (GIXD) is a powerful method for the study of the in-plane structure of Langmuir films at the air/water interface.5 The diffraction signal from organic monolayer structures though is very weak, necessitating the use of high flux synchrotron radiation sources to enable reasonable data collection times over large enough areas of reciprocal space. More recently time-resolved GIXD studies of Langmuir-Blodgett multilayers using an image plate detection * To whom comments should be sent. E-mail: John.White@ anu.edu.au. Phone: +61 2 6249 3578. Fax: +61 2 6249 5995. † The Australian National University. ‡ Australian Nuclear Science and Technology Organisation. § Current address: Australian National Beamline Facility, KEKPF, Oho 1-1, Tsukuba, Ibaraki, 305, Japan. (1) Brown, A. S.; Holt, S. A.; Thien Dam; Trau, M.; White, J. W. Langmuir 1997, 32, 6363. (2) Brown, A. S.; Holt, S. A.; Reynolds, P. A.; Penfold, J.; White, J. W. Langmuir 1998, 14 (19), 5532. (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. (4) Yang, H.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 1998, 8 (5), 1205. (5) Als-Nielsen, J.; Mo¨hwald, H. In Handbook of Synchrotron Radiation; Ebashi, S., Rubenstein, E., Koch, M., Eds.; Elsevier: Amsterdam, 1991; Vol. 4.
method have been reported.6,7 With the reduced data collection times this method is amenable to the study of self-assembled systems, the structure of which evolve with time. This paper then reports on a GIXD study of the development of order at the air-liquid interface in a selfassembled silicated film. This is we believe the first report of the application of such methods with minute time resolution to processes at the air-liquid interface. Experimental Section In the present experiment, X-rays reflected and diffracted both in and away from the specular plane have been recorded for a highly collimated beam of 1.485 Å X-rays incident on the growing film surface at an angle of 0.12°. This angle is below the critical angle for the fluid and resulted in a beam footprint on the sample of 7 cm. The evanescent wave penetration was calculated8 as ca. 75 Å, which is the effective observation depth. The detector was an image plate (exposed for 180 s) situated 300 mm from the Teflon trough containing the growing film. The image plate was scanned and geometrical corrections made following Foran et al.6 The apparatus was set up at “Big Diff”9 on the Australian National Beamline (BL20B) at the Photon Factory, Tsukuba, Japan. The solutions used were prepared in the same manner as those studied previously.1,2 In parallel neutron reflectivity measurements films were grown from heavy water preparations and the clear deuterated “mother liquor” reserved and examined by small-angle neutron scattering to characterize the hydrogenous cetyltrimethylammonium chloride (CTAC) surfactant micelles. These were in strong contrast to the surrounding fluid and gave a scattering function which was closely fitted by the form factor for a sphere of radius 27 Å and neutron scattering length density difference between the interior of the sphere and the subphase of 6.58 × 10-6 Å-2. This corresponds to a hydrogenous micellar interior within a deuterated solution. The reaction mixture was prepared by stirring an aqueous solution of CTAC into pure distilled water acidified with hydrochloric acid, to which was then added tetraethoxysilane (TEOS). Final concentrations of surfactant and TEOS were ca. 2.0% w/w and 1.2% w/w, respectively, leading to a surfactant to silicate molar ratio of 1:1. The final surfactant concentration was ca. 70 times the critical micelle concentration. The resultant mixture was stirred at room temperature until completely clear (6) Foran, G. J.; Gentle, I. R.; Garrett, R. F.; Creagh, D. C.; Peng, J. B.; Barnes, G. T. J. Synchrotron Radiat. 1998, 5, 107. (7) Peng, J. B.; Foran, G. J.; Barnes, G. T.; Gentle, I. R. Langmuir 1997, 13 (6), 1602. (8) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (9) Barnea, Z.; Creagh, D. C.; Davis, T. J.; Garrett, R. F.; Janky, S.; Stevenson, A. W.; Wilkins, S. W. Rev. Sci. Instrum. 1992, 63 (1), 1069.
10.1021/la981390u CCC: $18.00 © 1999 American Chemical Society Published on Web 03/11/1999
Crystalline Diffraction from Silicate Films (ca. 10 min) and placed in a rectangular Teflon trough (length: width ratio approximately 3:1) longer than the beam footprint. The solution was added to the trough from a Pasteur pipet with drops placed randomly at various positions, first on the Teflon surface and then onto the liquid surface, as the volume increased. No shear force was applied to the solution. The solution was kept at about 25 °C in a large volume helium-filled enclosure throughout the experiment.
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a
Results Figure 1 shows a sequence of patterns recorded on the image plate. At short times (26 min) after mixing (Figure 1a) the pattern closely resembles that from pure water. The diffraction signal then develops on a time scale very similar to that observed with our specular reflectivity studies.2 As the latter stages of the “induction period" (270 min) are reached, very weak streaks (data not shown) develop parallel to the specular streak and separated by wavevector transfers of about (0.11 Å-1 from it. As the induction time increases well-formed diffraction spots develop out of these streaks. This process occurs over the next 30 min, a very similar time scale to our observations of specular reflectivity peak development on similar solutions.2 With the development of the in-plane diffraction spots (Figure 1b) it can also be observed that well-formed spots appear along the Qxy ) 0 direction. The equidistance of the three spots from one another (∆Q ) 0.139 Å-1) and from the origin indicates a hexagonal lattice. In previous X-ray and neutron specular reflectivity experiments from films grown using CTAB and CTAC there are Bragg diffraction peaks in the specular ridge at Qz ) 0.137 Å-1 and 0.274 Å-1. These are now interpreted as the first- and second-order reflections from a hexagonal lattice with a real space repeat distance of 46 Å. After 395 min of growth a well-developed second set of reflections (Figure 1c) indexable as the 2,0, 0,2, and 2,2 h reflections at precisely twice the spacing from the origin of the 1,0, 0,1, and 1,1h reflections are also identified. There is no sign of the 1,1 and 1,2 h reflections observed by Hamilton et al.10 in their study of a shear-induced hexagonal ordering from viscous micelles. In addition to the second set of reflections, the first-order reflections strengthen and some diffuse scattering appears off the specular ridge from the 0,1 point.
b
c
Discussion The most obvious fact in the present observations is that the residual micelles from the reserved substrate solution on which the film grew relate in their size to the crystallographic dimensions of the film. The hexagonal spacing (46 Å) is close to x3 times the micellar radius; this suggests close packing of the solution micelles to form the film structure. The induction time observed previously1,2 from specular reflectivity studies is also evident in the in-plane structure development. The appearance of the weak streaks which collapse into diffraction spots indicates that the air-water interface develops from an initially disorded state through to a hexagonally ordered crystalline structure. Within the depth probed by the evanescent wave it is obvious that the growing silicate film achieves a high degree of crystalline order. From the observed diffraction pattern there are a number of interface structures which can be conceived. The two most likely can be described as follows. The first involves the hexagonal arrangement of (10) Hamilton, W. A.; Butler, P. D.; Baker, S. M.; Smith, G. S.; Hayter, J. B.; Magid, L. J.; Pynn, R. Phys. Rev. Lett. 1994, 72, 2219.
d
Figure 1. Diffraction from a growing silicate film at the airwater interface: (a) 26 min after mixing; (b) 337 min from mixing (as the end of the “induction period” is approached); (c) firstand second-order spots at about 395 min as the induction process finishes; (d) hexagonal indexing of observed spots.
silicate-covered cylindrical surfactant micelles with the long axis is parallel to the liquid surface. The second, less likely possibility, involves the hexagonal ordering of spherical silicate-covered surfactant micelles; both models
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rely upon a strong association between the surfactant headgroup and the silicate species. It is unlikely that the film has grown as a single crystal with the micelles aligned along the beam direction. The likely surface structure consists of bundles or domain of hexagonally ordered cylindrical micelles. This is consistent with transmission electron microscope (TEM) results11 showing MCM-41 bulk phase synthesis gels contain tactoids or sausages of rods in which the spacing between the rods corresponds to the observed micellar spacing. Peng et al.7 have reported a simlar in-plane structure for 31 layer Cd stearate films deposited on silicon wafers and heated to 105 °C. Their film, by GIXD, was found to consist of rods with Cd2+ ions and headgroups in the center surrounded by hydrocarbon chains, a reverse cylindrical micellar structure. From measurement of the full width at half-maximum of the diffraction peak it should be possible to calculate the correlation lengths of the surface domains.7 The correlation length of the tactoids and indeed their width is of great interest. From the TEM results11 it is anticipated that the domains at the surface will be at least 200 nm accross with correlations lengths of the (11) Edler, K. J.; Dougherty, J.; Durand, R.; Iton, L.; Kirton, G.; Lockhart, G.; Wang, Z.; Withers, R.; White, J. W. Colloids and Surfaces, A 1995, 102, 213
Holt et al.
order of microns, presenting a self-assembly system with length scales in the angstrom, nanometer, and micrometer ranges. This still presents a technical challenge as the instrumental resolution (0.039 Å-1) dominates the peak widths. A final point which also supports the proposed structure is that our specular reflectivity studies of long-term film growth in addition to the strong peaks also display weak broad bumps between 0.06 and 0.07 Å-1 which would correspond to the projection of the 1,0 and 1,1 h peaks onto the specular direction. This would arise as a result of some small fraction of the surface domains being in the correct orientation and satisfy the diffraction condition for these spots, with the strong spots diffracting away from the instrumental specular condition. Acknowledgment. The authors wish to acknowledge access to the Australian National Beamline, Photon Factory, Tsukuba, Japan, through the Major Facilities Program and travel grants from the Australian Government ISTAC/ANSTO. S.A.H. gratefully acknowledges an Australian Synchrotron Research Program Fellowship. LA981390U
