4544
Langmuir 1999, 15, 4544-4550
Highly Oriented Mesostructured Thin Films: Shear-Induced Deposition of Optically Anisotropic Coatings of Tungsten Oxide/Surfactant Composites Hugh W. Hillhouse, Jan W. van Egmond, and Michael Tsapatsis* Department of Chemical Engineering, Goessmann Laboratory, University of Massachusetts, Amherst, Massachusetts 01003 Received August 10, 1998. In Final Form: December 15, 1998 A new method has been demonstrated to synthesize highly oriented mesostructured coatings of tungsten oxide/surfactant composites on amorphous substrates by a shear-flow-induced deposition mechanism. The applied flow field provides convective transport of film precursors and orients the domains of the mesostructure, resulting in its macroscopic alignment. Two-dimensional X-ray diffraction, scanning electron microscopy, and transmission electron microscopy suggest a hexagonal-close-packed channel structure with periodicity along the c-axis generated by a sequence of tungsten oxide polyanions. The mesostructure has been indexed with a hexagonal unit cell with lattice constants a ) b ) 46.45 Å, c ) 12.93 Å, R ) β ) 90°, and γ ) 120°. The channels have no preferred rotational orientation about the c-axis and are oriented in the direction of the applied flow field with an order parameter of 0.95. The high degree of orientation renders the films optically anisotropic and results in preferential scattering of light perpendicular to the flow direction. Also, the anisotropies of the real and imaginary parts of the refractive index tensor are measured as 0.005 and 0.002, respectively.
1. Introduction Mesostructured materials have generated a flood of interest since the introduction of mesoporous silica.1 Formation of mesostructured composites has since been shown to be a more general phenomenon2,3 of selfassembly, forming periodic arrays that may assemble from a variety of inorganic/amphiphilic systems at a wide range of synthesis conditions. In many systems, most notably silica, the inorganic oxides polymerize by condensation reactions, forming a cross-linked network. This allows for removal of the organic template, creating mesoporous powders that have promise as high-surface-area supports for catalysts, open catalytic structures, and novel media for new host-guest chemistry.4 Films of mesostructured composites also have been synthesized by several techniques: spin and dip coating methods relying on solvent evaporation,5-7 grafting of assembled Langmuir-Blodgett films8 onto substrates, growth of films at fluid interfaces,9,10 and finally growth from solution directly on substrates.11-13 (1) Kresge, C. T.; Leonnowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Huo, Q.; Margolese, D.; Cielsa, U.; Feng, P.; Gler, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317-321. (3) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1701. (4) Ozin, G. A.; Chomski, E.; Khushalani, D.; MacLachlan, M. J. Curr. Opin. Colloid Interface Sci. 1998, 3, 181-193. (5) Ogawa, M. Chem. Commun. 1996, 1149-1150. (6) Martin, J. E.; Anderson, M. T.; Odinek, J.; Newcomer, P. Langmuir 1997, 13, 4133-4141. (7) 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 1997, 389, 364-368. (8) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340-2347. (9) Schacht, S.; Huo, Q.; Voight-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768-771. (10) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589-592. (11) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892898.
Many of the most exciting applications of mesostructured and mesoporous materials will require the ability to orient the mesostructure in a chosen direction. However, since there are several characteristic length scales associated with these materials, there are several length scales over which we may speak of “order”. So, we first define the structural hierarchies over which we may observe order to better facilitate the analysis and discussion. We define the primary structure as the crystallographic unit cell and space group that describe the mesoscale structure (i.e. hexagonal P6m, cubic Ia3d, lamellar). Therefore, the primary order is defined over the length scale of the unit cell. The secondary structure refers to the curvature and persistence of directional alignment of successive unit cells within a domain. Secondary order then occurs over length scales of the domain size and may be from several unit cells to several hundred nanometers. Finally, the tertiary structure refers to the orientation of domains or groups of domains with respect to each other and the laboratory reference frame. Also, it is important to note two distinctions between these mesostructures and traditional crystals. In general, the walls of the mesostructure may be composed of amorphous material, and therefore the atomic positions in adjacent unit cells may be different. However, the unit cells do have the same spatial pattern of coarse-grained electron density. The amorphous character of the mesophase also provides an extra degree of freedom, allowing the structure to bend and form curved secondary structures. Using this terminology, typical MCM-41 mesoporous powders exhibit an ordered primary structure, a secondary structure that loses directional alignment in the range 10-100 nm, and a disordered tertiary structure. Other syntheses, such as of the nonionic surfactant templated materials with the “wormhole” morphology,14 show a less (12) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703-705. (13) Tolbert, S. H.; Schaffer, T. E.; Feng, J.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1997, 9, 1962-1967. (14) Pinnavaia, T. J.; Prouset, E. Angew. Chem. 1997, 36, 516.
10.1021/la9810051 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/19/1999
Tungsten Oxide/Surfactant Composites
ordered primary structure with disordered secondary and tertiary structures. Also, mesostructured silica formed below the isoelectric point shows a vast array of morphologies and ordered secondary structures including spherical, toroidal, fibrous, and discoidal domains.15 Assembly and polymerization kinetics have been suggested to be a dominant factor in producing these secondary structures from hexagonal primary structures growing with a constrained radius of curvature. A morphology of particular interest is the mesostructured fiber; recently, Huo et al. demonstrated the use of such fibers as optical waveguides.16 However, fibers that are grown curve and lack long range directional alignment. Also, Bruinsma et al. have recently developed a spinning technique where mesostructured fibers, formed by solvent evaporation, are drawn and spooled.17 Developing long range order from the growth of mesostructured composites presents a challenge due to the generation of defects18 and may require the application of external fields19 to direct the growing structure. Mesostructured silica films have been grown on model substrates such as mica or graphite11,12,20 where the crystallographic planes of the substrate induced order during early stages of growth; however, defects rendered further growth disordered. Several techniques have been demonstrated to produce extended order in mesostructured silica. Tolbert et al. used an 11.7 T magnetic field to produce macroscopic orientation in mesostructured silica monoliths,21 resulting in a Gaussian distribution of directors with a full-width at half-maximum (fwhm) of 49°. Also, Trau et al. used confinement and electro-osmotic flow of a reaction solution through the microcapillaries of a preformed mould to form isolated silica bundles.22 In our previous work, it was demonstrated that an applied shear flow field during the growth of mesostructured silica thin films produced oriented structures;19 however, the resulting orientation is mainly of the tertiary structure with little order induced in the secondary structure. Since silica polymerizes and cross-links as the growth proceeds, the external field must affect the orientation on a faster time scale than that of polymerization or relaxation of the assembled aggregates in order to achieve significant orientation. Another inorganic system shown to self-assemble into periodic arrays is that of tungsten oxide and surfactants. Mesostructured tungsten oxide powders have been synthesized previously under mild pH conditions; mixtures of lamellar and hexagonal phases with possible crystallinity of the wall material23,24 and a pure hexagonal phase25 have been reported. However, Stein et al. proposed a (15) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692-695. (16) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schuth, F. Adv. Mater. 1997, 9, 974-978. (17) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507-2512. (18) Feng, J.; Huo, Q.; Petroff, P. M.; Stucky, G. D. Appl. Phys. Lett. 1997, 71, 620-622. (19) Hillhouse, H. W.; Okubo, T.; Egmond, J. W. v.; Tsapatsis, M. Chem. Mater. 1997, 9, 1505-1507. (20) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1997, 7, 1285-1290. (21) Tolbert, S.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264-268. (22) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature 1997, 390, 674-676. (23) Cielsa, U.; Demuth, D.; Leon, R.; Petroff, P.; Stucky, G.; Unger, K.; Schuth, F. J. Chem. Soc., Chem. Commun. 1994, 1387. (24) Huo, Q.; Margolese, D.; Cielsa, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176-1191. (25) Taguchi, A.; Abe, T.; Iwamoto, M. Adv. Mater. 1998, 10.
Langmuir, Vol. 15, No. 13, 1999 4545
layered structure with hexagonally undulated sheets26 which may show lamellar-like and hexagonal-like patterns in TEM images depending on which zone axis is aligned with the electron beam. All authors concur that the mesostructure formed collapses upon removal of the surfactant template. Commensurate with these results, Stein et al. also proposed that the Keggin clusters27 of tungsten oxide, used as precursors for the synthesis, do not react with one another to form covalent bonds. In this case, we have an opportunity to perform assembly and orientation of a mesostructure independent of polymerization kinetics similar to the silicate-surfactant liquid crystalline phases synthesized by Firouzi et al.28 In this report, highly oriented tungsten oxide/CTAB composite films have been formed on amorphous substrates by shear-induced deposition. The applied flow field transports the colloidal precursors to the substrate and aligns the domains of the deposited film. For these films chemical and colloidal effects govern the formation of the primary structure, assembly kinetics determine the secondary order, and the applied flow field directs the tertiary order and prevents defects from causing misalignment. The resulting thin deposits are anisotropic on the length scale of the substrate and may possibly be used as anisotropic optical coatings, static optical components in liquid crystalline display applications, or hosts for optical species at temperatures and in environments where traditional thermotropic liquid crystalline phases are not applicable. 2. Experimental Methods 2.1. Synthesis of Fibrous Tungsten Oxide/Surfactant Films. A precursor suspension was prepared at room temperature by dissolving ammonium metatungstate hydrate (Aldrich) in distilled water and adding ammonium hydroxide or HCl to adjust the pH. A 25 wt % cetyltrimethylammonium chloride solution (Aldrich) was then added while stirring, whereupon a gel formed within seconds. The precursor gel structure was then irreversibly destroyed by agitation, creating a white opaque suspension of film nutrients. The precursor suspension was maintained at room temperature and peristaltically pumped through Tygon tubing into a temperature-controlled bath. Approximately 1 m of tubing was reserved for preheating the suspension prior to contact with the substrate. The suspension was flowed over amorphous borosilicate glass substrates (VWR glass slides, or capillaries) immersed in the bath where the films were deposited for periods up to 24 h. Films were then dried at room temperature to form smooth continuous films or washed with distilled water and rapidly dried under a flow of air or dried at 90 °C to produce fibrous deposits. Films have been synthesized from room temperature to 90 °C; estimated shear rates at the substrate surface have been varied up to 500 s-1. The most ordered films were obtained from a precursor suspension composition of 0.35 (NH4)6W12O39/8 CTACl/1000 H2O with no additional acid or base (pH ) 3.8). These depositions were carried out for 1 h at 70 °C under a shear rate at the substrate surface greater than 50 s-1. 2.2. Characterization. Scanning electron micrographs were obtained by first coating the samples with gold using a Polaron E5100 sputter coater and analyzing with a JEOL 100 CX electron microscope operated at an accelerating voltage of 20 kV. Transmission electron micrographs were obtained by scraping the film off the substrate and suspending the fragments in ethanol. A small amount of the ethanol suspension was then transferred to a carbon grid and analyzed with a JEOL 2000 FX (26) Stein, A.; Fendorf, M.; Jarvie, T.; Mueller, K.; Benesi, A.; Mallouk, T. Chem. Mater. 1995, 7, 304-313. (27) Keggin, J. F. Proc. R. Soc. London, Ser. A 1934, 144, 75-100. (28) Firouzi, A.; Atef, F.; Oertli, A. G.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1997, 119, 3596-3610. (29) Siddiquee, S.; Egmond, J. W. v. Macromolecules 1998, 31, 26612669.
4546 Langmuir, Vol. 15, No. 13, 1999
Hillhouse et al.
Figure 1. Schematic of apparatus for simultaneous measurement of birefringence and dichroism. The relationships of the calculated angles c′ and c′′ to the flow direction are shown in the inset. Photodiode D1 registers dichroism while photodiode D2 registers both dichroism and birefringence. The true birefringence may then be calculated after correcting for dichroic effects. electron microscope operated at an accelerating voltage of 200 kV. X-ray characterization of powder samples was obtained in a reflection geometry with a Phillips X’Pert MPD PW3040 diffractometer using a Cu KR source and a curved graphite monochromator. Films were analyzed with transmission geometry X-ray diffraction in an evacuated chamber using a nonmonochromatic Cu KR source, a camera length of 194 mm, and exposure times of up to 4 days. Two-dimensional diffraction patterns were collected on Kodak direct exposure film. Relative intensity data were quantified from the X-ray film using an optical attenuation technique with a Optronic System’s C-4500 microdensitometer. 2.3. Optical Analysis. The birefringence and dichroism of the films were simultaneously measured using a 30 mW He-Ne laser (λ ) 632.8 nm) with an optical train as seen in Figure 1. A polarizer was placed immediately after the laser to control the intensity of the beam, and a half-wave plate rotating at 400 Hz was used to rotate the polarization state of the beam. The films were deposited on a flat borosilicate substrate to eliminate lensing effects seen with capillary substrates. After passing through the sample, the beam was split. For the dichroism measurement, one beam was passed through a neutral density filter and collected with a photodiode. The other beam was passed through a circular polarizer and collected with a photodiode, registering both birefringence and dichroism. The signals are then amplified and Fourier transformed. The retardance (δ′), angle of the fast axis (χ′), extinction (δ′′), and angle of extinction (χ′′) are then calculated.29 The birefringence (∆n′) and dichroism (∆n′′) are defined by the following expressions:
∆n′ )
δ′λ 2πd
(1)
∆n′′ )
δ′′λ 2πd
(2)
λ is the wavelength of light in the sample, and d is the film thickness estimated from SEM images of the film cross section. The film and substrate are placed normal to the incident beam,
Figure 2. Schematic of small angle light scattering apparatus. The scattering pattern is collected with a CCD camera from the reflected signal off a polarization sheet analyzer. The analyzer is then rotated 90° to collect either Vv or Hv scattering patterns. and the apparatus is calibrated such that the angles of orientation calculated are in the plane of the substrate with 90° corresponding to the direction of the applied shear flow. Small angle lightscattering patterns were obtained using an 8 mW He-Ne laser with an optical train consisting of a collimator, the sample, and a polarizing mat (Figure 2). The main beam is transmitted though a hole in the mat, and the scattering pattern is collected with a CCD camera. The image is then corrected for the off-axis orientation of the CCD camera. Vv or Hv scattering patterns were collected by aligning the second polarizer with the transmission axis either parallel or perpendicular to the first polarizer, respectively.
3. Results and Discussion 3.1. Structure and Orientation. The analytical techniques of SEM, TEM, and two-dimensional X-ray diffraction, each probing increasingly smaller length scales, were employed to reveal the orientation and structural hierarchies of the films. The tertiary structure of the films grown under shear is highly oriented in the direction of the applied flow field, as seen from the scanning electron micrographs in Figure 3, where the arrows indicate the flow direction. The three micrographs are from the same deposition conditions but with differing drying procedures. The samples shown in parts a-c of Figure 3 were dried under a flow of air, dried in an oven at 90 °C, and dried at room temperature, respectively. The slowly dried film (Figure 3c), experienced some structural rearrangement after the deposition, resulting in a transformation from a fibrous structure to a smooth morphology. However, the mesostructure and morphology of the fibrous films may be preserved by rapidly drying the washed samples (Figure 3a and b). Higher resolution SEM images of a fibrous film are seen in Figure 4, showing striations along the fiber axis, suggesting an oriented mesostructure. Also, note the uniformity and regularity of fractures perpendicular to the flow direction in Figure
Tungsten Oxide/Surfactant Composites
Langmuir, Vol. 15, No. 13, 1999 4547
Figure 3. Scanning electron micrographs at 1000× magnification of tungsten oxide deposits with differing drying procedures. The films were (a) washed with distilled water and dried under a flow of air, (b) washed and then dried at 90 °C, or (c) dried at room temperature. The films were grown for 1 h at 70 °C and pH ) 3.8 on a borosilicate substrate with an applied shear rate of approximately 450 s-1. The arrows indicate the direction of the applied flow field. The tertiary structure is seen to be highly oriented in the flow direction.
Figure 4. SEM images of the film seen in Figure 3b: (a) 20000× magnification of a fiber, showing striations along the fiber axis; (b) cross section of the deposit, showing a thickness of approximately 1 µm.
3c, again suggesting that the films are composed of building blocks oriented in a common direction. The secondary structure within the fibers is highly oriented, as seen from transmission electron micrographs in Figure 5. Fringes are observed along the edges of the fibers and show the persistence of directional alignment of the primary structure along the fiber axis. The spacing of the primary structure observed in the fringes is approximately 40 Å. Deposits synthesized at lower temperatures and lower nutrient concentrations showed a higher degree of curvature of the secondary structure, resulting in poorly defined fibers and smaller irregularly shaped domains. This suggests that the orientation of the successive unit cells within each fiber is controlled by assembly kinetics. The increased kinetic rates at higher temperatures and concentrations allow for extended growth in a given direction. Two-dimensional X-ray diffraction has been used to identify the primary structure of the film as well as to quantify the orientational order over macroscopic regions. The diffraction pattern from a fibrous deposit is seen in
Figure 5. Transmission electron micrographs of fibrous tungsten oxide deposits: (a) low-resolution image of a tungsten oxide fiber from the deposit shown in Figure 3b; (b) higher resolution image of the fringe of the fiber where the channel spacing may be seen. The secondary structure is seen to be highly oriented in the direction of the fiber axis. These fringes are ubiquitous in the sample, appearing on all the fibers and always aligning with the long axis of the fiber.
Figure 6 and shows a defined and highly oriented primary structure that may be indexed with a hexagonal unit cell with the lattice parameters a ) b ) 46.45 Å, c ) 12.93 Å, R ) β ) 90°, and γ ) 120°. The d spacings and azimuthal angles calculated from these parameters agree well with the observed d spacings seen in Table 1. Using this unit cell, all peaks observed may be indexed, and there are no additional peaks predicted that are not observed. Also, the spots from the hk0 planes along the east-west line of the diffraction pattern (φ ) 0°, 180°) indicate a high degree of orientation. If the director of the primary structure, the c-axis, were less oriented, the spots would appear as crescent-shaped lobes and gradually dissipate into concentric rings as the degree of orientation decreased. The fact that the h00 and hk0 planes are both observed
4548 Langmuir, Vol. 15, No. 13, 1999
Hillhouse et al.
〈cos
2
Figure 6. Two-dimensional X-ray diffraction from an oriented tungsten oxide deposit shown in Figure 3a. The pattern shows a highly oriented primary structure, which is indexed with a 3D hexagonal unit cell with the c-axis in the direction of flow. The arrow indicates the flow direction, which is at an azimuthal angle φ of 90°. The hkl planes are seen as spots at φ ) 0°, 180°. If the domains were randomly oriented, the high-intensity spots from these planes would appear as concentric rings with their origin at the center of the pattern. Also, note the azimuthal angle of the 00l is offset 90° from the hk0 planes, as would be expected for an oriented hexagonal lattice. The 101, 111, and 201 reflections appear in all four quadrants due to rotational sampling of the c-axis.
and are seen at the same azimuthal angle indicates a random rotational configuration of the channels about an oriented c-axis. Also, a high-intensity reflection is seen in the north and south regions of the image (φ ) 90°, 270°). From its orientation relative to the other reflections, this diffraction must come from periodicity down the c-axis and is indexed as the (001) plane. The 101,111, and 201 reflections are seen in all four quadrants of the pattern, again due to the rotational freedom of the channels. Also, the as-synthesized material formed in the bulk solution during film synthesis was analyzed by X-ray powder diffraction and compared well with an isotropic rendering of the diffraction data obtained from the fibrous films. We may quantify the directional order in the system by estimating an order parameter based on the ensemble average of the second Legendre polynomial.30 For a uniaxial system with a point source of X-rays, the order parameter S is
1 S ) (3〈cos2 φ〉 - 1) 2
(3)
where 〈cos2 φ〉 is the mean squared cosine of φ calculated from the angular distribution of diffracted intensity I(φ) by the following expression: (30) Finkelmann, H. Synthesis, Structure, and Properties of Liquid Crystalline Side Chain Polymers. In Polymer Liquid Crystals; Ciferri, A., Krigbaum, W. R., Meyer, R. B., Eds.; Academic Press: New York, 1982; p 55.
∫0π/2I(φ) sin(φ) cos2(φ) dφ φ〉 ) ∫0π/2I(φ) sin(φ) dφ
(4)
A value of the order parameter of one indicates orientation of all the directors in the flow direction, while a value of zero indicates an isotropic distribution of directors and -1/2 indicates an orientation perpendicular to the flow direction. Thus, the order parameter serves as a quantitative measure of the degree of orientation. The angular distribution of the intensity of the 100 peak is seen in Figure 7, showing a fwhm of 14°. I(φ) data for each peak were fit to Gaussian curves, showing excellent agreement. The baseline of the diffraction data seen Figure 7 is purely from air, substrate, and the beam stop and does not result from an isotropic distribution of nonaligned unit cells. This was determined by examining plots of intensity, integrated over an azimuthal slice from 150° to 200°, versus 2θ, where a monotonically decreasing background was seen at low angles with no detectable diffraction from the (100) plane. The order parameter calculated from the intensity data and eqs 3 and 4 was 0.95, indicating an extremely high degree of orientation. The fractures perpendicular to the flow direction and striations parallel to the fiber axis seen in SEM images of the tertiary structure, the fringes seen in TEM images of the secondary structure, and the hexagonal indexing of the primary structure suggest that the mesostructure of the film is a hexagonal-close-packed array of channels with periodicity of the wall structure along the c-axis. It appears that the Keggin structural unit of the tungsten oxide precursor is preserved upon film formation and acts as a building block in the wall material, creating the periodicity down the c-axis. In general, the Keggin structure consists of a central heteroatom or vacancy in a tetrahedral cavity surrounded by 12 MO6 octahedra (where M stands for either tungsten or molybdenum) linked with shared oxygens.27 This cluster is roughly spherical with a diameter of approximately 10-11 Å.31 Since these symmetric polyanions tend to pack in cubic arrangements with occluded water of crystallization, stacking of these units leads to the observed d001 spacing of 12.93 Å. 3.2. Optical Properties. For macroscopically isotropic materials such as air, water, and glass, the refractive index of the material is a real scalar quantity. However, for materials that possess anisotropy at macroscopic length scales such as oriented polymeric, crystalline, or liquid crystalline materials, the refractive index may be a tensor. The anisotropy may be quantified by evaluating the birefringence, which is defined as the difference between any two of the three components of the diagonalized refractive index tensor. The molecular origin for this macroscopic property is an anisotropic polarizability of the constituent molecules or an anisotropic network of molecules or clusters. The latter is referred to as form birefringence. Further, if the material attenuates light, the refractive index will be complex, and likewise the anisotropy of the imaginary part is quantified by the dichroism. This property results from absorption originating from an imaginary contribution to the molecular polarizability or from scattering if the material has anisotropic spatial fluctuations in the refractive index, on the order of the length scale of the incident light.32 (31) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: Berlin, 1983. (32) Fuller, G. G. Optical Rheometry of Complex Fluids; Oxford University Press: New York, 1995.
Tungsten Oxide/Surfactant Composites
Langmuir, Vol. 15, No. 13, 1999 4549
Table 1. Comparison of Observed and Calculated d Spacings from Two-Dimensional X-ray Diffractiona hkl 100 110 200 210 300 220 310 001 101 111 201
obsd calcd d spacing d spacing 40.22 23.57 20.15 15.49 13.29 11.84 10.92 12.93 12.13 11.56 10.57
40.23 23.22 20.11 15.20 13.41 11.61 11.16 12.93 12.31 11.30 10.88
obsd azimuthal angle
calcd azimuthal angle
0, 180 0, 180 0, 180 0, 180 0, 180 0, 180 0, 180 90.270 71, 161, 251, 341 60, 150, 240, 330 57, 147, 237, 327
0, 180 0, 180 0, 180 0, 180 0, 180 0, 180 0, 180 90, 270 74, 164, 254, 344 63, 153, 243, 333 61, 151, 241, 331
a Spacings and angles were calculated using a hexagonal unit cell with the lattice constants a ) b ) 46.45 Å, c ) 12.93 Å, R ) β ) 90°, and γ ) 120°.
Figure 8. Optical analysis of film, showing high birefringence and dichroism. The birefringence and dichroism along with the angles of polarization and attenuation were measured simultaneously using a 632 nm He-Ne laser for (a) the baseline, (b) the glass substrate, (c) the thin film-coated substrate, and (d) again a noncoated area of the substrate. The birefringence and angle of polarization indicate that the refractive index of the sample is 0.005 smaller in the flow direction.
Figure 7. Integrated X-ray intensity as a function of the azimuthal angle from the 100 reflection, indicating a high degree of orientation over macroscopic length scales. The baseline is purely from air and substrate scattering. This was determined by examining plots of intensity versus 2q integrated over various azimuthal slices of the pattern whereupon no peak could be detected at the d100 spacing. A Gaussian curve was fit to each peak, showing an excellent fit. The standard deviation of the peak was 14°.
For mesostructured materials, fanlike mosaic patterns have been previously reported33 from polarized optical microscopy, indicating a birefringent phase. These patterns, observed frequently in liquid crystals, are typical of a hexagonal primary structure that is disordered over the thickness of the sample or length scale probed by the optical microscope. Also, silica fibers have been grown at low pH and exhibit birefringence15,17 with the fast axis oriented perpendicular to the axis of the fiber with a birefringence of 0.002. The tungsten oxide/surfactant composites in this report are expected to be birefringent due to the fact that the surfactant constituents of these films are birefringent and that there is a high degree of molecular orientation at macroscopic length scales. Also expected is form birefringence due to the oriented mesostructure and scattering dichroism due to the fibrous tertiary structure. However, the most interesting feature of these coatings is the persistence of the orientation over the entire substrate. Thus, the angle of the fast axis may be chosen in the lab (33) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366.
Figure 9. Small angle light-scattering images obtained from Vv scattering of an oriented tungsten oxide/surfactant film. The pattern on the left is from the substrate alone while the pattern on the right is of the substrate plus the film. The direction of the applied flow field is horizontal. The anisotropic scattered intensity perpendicular to the flow direction indicates scattering centers elongated in the direction of flow.
reference frame and is constant over the length scale of the substrate. The anisotropy in the real and imaginary parts of the refractive index was quantified by static birefringence and dichroism measurements, respectively. These measurements were carried out simultaneously to correct the birefringence for dichroism contributions (scattering). Shown in Figure 8, the birefringence of the film was 0.005 and the dichroism 0.002. The fast axis of the propagation of light through the sample was determined to be perpendicular to the flow direction, indicating a lower refractive index in this direction. Also, small angle light-scattering patterns were measured (Figure 9) and show an elongated Vv scattering pattern in the direction perpendicular to the flow field. This scattering comes from concentration fluctuations (spatial fluctuations of the magnitude of the refractive index) perpendicular to the flow field, typical of oriented fibrous or cylindrical scattering volumes. The characteristic distance between scattering centers was calculated to be 0.62 µm from a linearized plot of the intensity versus the magnitude of the scattering vector, 1/I(q) versus q2.
4550 Langmuir, Vol. 15, No. 13, 1999
This length scale is of the same order as the observed average fiber diameter seen in Figure 3b. Scattering from orientation fluctuations (spatial fluctuations of the direction of the fast axis) was probed by collecting Hv scattering patterns with the sample oriented at 45° to the polarization axes. The Hv scattering was below the detection limit, indicating a high degree of alignment. 3.3. Formation Mechanism. A mechanism for the film formation is speculated to proceed through several steps: (1) formation of the primary hexagonal structure immediately after mixing the reagents; (2) formation of a gel network of particles composed of xCTA+[(NH4)6-xW12O39]-x species; (3) irreversible destruction of the gel by agitation to form a low-viscosity precursor suspension of gel fragments; (4) shearing the precursor suspension, resulting in aggregation, deposition, and morphological rearrangement of particles on the substrate surface; and (5) rapid drying of the film to prevent structural rearrangement. The synthesis conditions are above the isoelectric point of tungsten oxide (pH of 0.5).34 Therefore, the tungsten oxide species in solution are negatively charged, and hence the interaction between the inorganic oxide and the cationic surfactant is a direct electrostatic attraction between species of opposite charge. This interaction drives the formation of the hexagonal mesostructure, the presence of which is determined by X-ray powder diffraction of samples of the wet gel that forms upon mixing the surfactant and inorganic solutions. This X-ray diffraction pattern is an isotropic rendering of the 2D X-ray diffraction pattern obtained from the fibrous film and confirms that the mesostructure has already formed by step 2. The morphology of the particles in the suspension after breaking up the gel is not known at this point due to the fact that SEM images could not be obtained without drying the sample, and the drying process itself has been seen to induce structural rearrangement. However, since the X-ray patterns of the gel and the film concur and since needle-like particles have been observed in quiescent experiments,23,35 it is likely that the needles form concurrently with the mesostructure during gel formation. Also, these data and the observed loss of regularity of the needle shape from experiments at decreased temperature or concentration suggest that the secondary structure and hence the needle-like morphology are dictated by assembly kinetics. It is proposed that deposition and aggregation of the particles from the suspension is induced by the flow field, as films could not be synthesized from the suspension at (34) Rowell, R. L. Unpublished results from ζ potential measurements. (35) Janauer, G.; Dobley, A.; Guo, J.; Zavalij, P.; Whittingham, M. S. Chem. Mater. 1996, 8, 2096-2101. (36) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 4th ed.; Butterworth and Heinemann: Oxford, 1992. (37) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1987; Vol. 1. (38) Warren, L. J. J. Colloid Interface Sci. 1975, 50, 307-318. (39) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. A. Particle Deposition and Aggregation; Butterworth-Heinemann Ltd.: Oxford, 1995.
Hillhouse et al.
quiescent conditions. Either the films do not grow under quiescent conditions, or the growth rate is retarded to an extent that significant growth is not observed over a period of days. This fact suggests that collisions between the precursor nutrients play a role in the layer formation, as is often observed in colloidal systems.36,37 This phenomenon of shear-induced flocculation has been observed previously as a mechanism of particle formation in calcium tungstate suspensions in the presence of surfactants.38 Also supporting this mechanism, the deposition rate is observed to increase with decreasing pH. As the pH is reduced toward the isoelectric point, interparticle repulsion is reduced and particle aggregation and deposition increase. The applied flow field orients and drives the elongated particles or aggregates to the wall, where the substrate then acts as a collector as the suspension is flowed across the surface.39 After deposition, aggregates may fuse and undergo rearrangements. In agreement with previous reports,26 the tungsten oxide clusters do not condense with one another during the synthesis. This allows for morphological rearrangements after deposition and also during drying. 4. Conclusions A new mechanism for the formation of mesostructured thin films resulting in highly oriented mesostructures has been demonstrated. In this technique an applied shear flow field induces the deposition of colloidal precursors. Chemical and colloidal interactions govern the primary crystallographic structure, assembly kinetics control the orientation of the secondary structure within the domains, and the applied flow field orients domains, induces film deposition, and directs the tertiary structure. Also, the method of drying has been shown to cause morphological and structural changes where rapid drying “freezes” the tertiary structure to produce thin fibrous deposits with a hexagonal mesostructure while slow drying allowed structural rearrangement, producing smooth thin films. The length scales and degree of orientation observed in these films are unparalleled in similar inorganic/organic mesostructured systems. The primary, secondary, and tertiary structures are all highly oriented in the direction of the applied flow field, resulting in macroscopic orientation quantified by an order parameter of 0.95. The high degree of orientation renders these films both birefringent and dichroic with anisotropic light-scattering patterns, all with an angle of orientation that may be chosen in the lab reference frame. Directional alignment extends uniformly over the length scale of the substrate. This technique should be applicable to other chemical systems to synthesize ordered films from a suspension of a dispersed mesophase. Acknowledgment. Support for this work was provided by NETI, NSF, the David and Lucile Packard Foundation, and the Dreyfus Foundation. We thank Dr. Alan Waddon and MRSEC for the use of the MRSEC/ Polymer Science X-ray facilities. LA9810051
