J O U R N A L O F C H E M I S T R Y Materials Thickness control and defects in oriented mesoporous silica films Hong Yang, Neil Coombs and GeoVrey A. Ozin* Materials Chemistry Research Group, L ash Miller Chemical L aboratories, University of T oronto, 80 St. George Street, T oronto, Ontario, Canada, M5S 3H6 By using a surfactant-based synthesis strategy, we have earlier demonstrated that the polymerization and growth of silicate micellar assemblies at the air–water interface, under quiescent and dilute acidic aqueous conditions, yields free-standing submicron thickness hexagonal mesoporous silica films in which the channels are oriented parallel to the film surface.TEM imaging studies of these thin films showed that microscopic defects pervade the channel structure with topologies resembling those found in lyotropic liquid crystals.This suggested that the mesoporous silica film evolved from silicification of a surface lyotropic silicate mesophase. Herein it is demonstrated that film growth, defect structure, extent of polymerization, and mesoporosity sensitively depend on the choice of synthesis acidity, temperature and mixing, and in the case of supported films, on the choice of substrate.In particular, a ten-fold increase in the thickness of the film can be obtained by simply lowering the acidity and moving to ambient temperature conditions whilst an alteration in mixing conditions can change the film from a discrete to a continuous morphology. Combined PXRD, TEM and nitrogen adsorption studies show that the silica films are hexagonal, oriented and mesoporous.Furthermore, the observation of a focal conic fan-type texture in the free-standing films shows that defect controlled director fields, that exist in a precursor hexagonal lyotropic silicate mesophase, are preserved in the channel structure of the mesoporous silica phase. Proof-of-existence of liquid crystalline texture in such free-standing mesoporous silica films, provides direct evidence that film growth evolves from the cooperative assembly and organization of silicate micellar species at the air–water interface.templating strategies have been successfully used to synthesize Introduction mesostructured materials, the formation pathways are not Liquid crystals represent a delicate phase of matter which has necessarily the same.4,5,8,16–22 In the concentrated surfactant lost long range positional order of ordinary crystals but retains regime, the formation of mesostructured materials likely proorientational order of anisotropic structural units.1–3 Defects ceeds through the mineralization of a pre-existing liquid crystal in liquid crystals, unlike their atomic scale counterparts in mesophase.21 By contrast, in the dilute regime, the mesostrucnormal crystals, are microscopic in size, their formation ture likely forms by mineralization of a co-assembly of silicate requires much less energy, and their topology determines the micellar templates.4,5,16–18 In this study, proof-of-existence of patterns of director fields.They are responsible for birefrin- hexagonal liquid crystalline texture in free-standing mesogence textures made visible between cross polarizers in an porous silica films that are grown at the boundary between optical microscope where distinct patterns are diagnostic of air and water under very dilute aqueous conditions, provides particular liquid crystal structure types.2,3 evidence that film growth occurs through the cooperative In this context, it has recently been demonstrated that the assembly pathway.polymerization of silicate micellar assemblies4,5 at the air– water interface, under dilute and acidic aqueous conditions, yields free-standing hexagonal mesoporous silica films with a Experimental thickness of ca. 0.5 mm, in which the channels are oriented parallel to the film surface.6–9 Free-standing hexagonal meso- Synthesis porous silica films with a diVerent space group, P63/mmc, were The film synthesis procedure involved a modified version of subsequently reported.10 Herring bone, U-shaped and S-shaped that reported earlier for the preparation of mesoporous silica channel designs have been observed by TEM in these hexagmorphologies. 7,15 Tetraethylorthosilicate (TEOS, 99.999+%, onal mesoporous thin film samples.7 Their channel architec- Aldrich), cetyltrimethylammonium chloride (CTACl, 29 wt.% ture6–8 displayed a close resemblance to the director field aqueous solution, Pflatz & Bauer) and hydrochloric acid patterns in hexagonal organic liquid crystals that are usually (36.5–38 wt.% aqueous solution, BDH) were used as received. associated with common line defects,11–13 such as +p, -p The reactant ratios used for making the film with a thickness disclinations, pairs of +p disclinations, edge dislocations, of ca. 5–10 mm were 100 H2O51.0 HCl50.11 CTACl50.13 bending and wall defects.14 TEOS. In a standard preparation, 2.9 g of a CTACl surfactant In this article we show that by simply employing less acidic aqueous solution (29 wt.%) was mixed well with 2.5 g of HCl synthesis conditions than those in our earlier study, it is solution (36.5–38%) and 40.8 g of deionized water by using a possible to realize at least a ten-fold increase in the thickness magnetic stirrer (Corning PC-351 hot plate stirrer) with a of the mesoporous silica films, while mixing conditions can 0.375 in×1.5 in stir bar in a polypropylene beaker followed by also aVect the morphology of the films.Polarizing optical adding 0.65 g of TEOS. The mixture was then stirred for 3 to microscopy shows that these thick films display a fan-type 10 min at room temperature. The final mixture was transferred texture diagnostic of a hexagonal lyotropic organic liquid into either a round or a square polypropylene (PP) or low crystal, HI phase.Significantly, the texture is maintained on density polyethylene (LDPE) bottle with diVerent diameters removing the surfactant and after thermally annealing the and length of edge (NalgeneA, LABCOR, Inc.), and allowed films. This implies that the optical birefringence emerges from to achieve a quiescent state. The mesoporous silica growth the polarization response of electron density circumscribing a process was typically allowed to proceed for a period of one hexagonal array of mesoscale glassy channels, that is, ‘mesosweek under static conditions at room temperature.Depending cale optical anisotropy in glass’.15 It is worth mentioning that while diVerent surfactant-based on the initial time period and the stirring rate for the mixing, J.Mater. Chem., 1998, 8(5), 1205–1211 1205diVerent optical birefringence textures were observed. The film on mica was grown by allowing a freshly cleaved muscovite mica substrate (J. B. EM. Services Inc., Dorval, Quebec) to float on the solution surface. The calcination of the film was achieved using two diVerent procedures. Direct calcination was done in air and in a furnace attached to an Omega CN- 2010 programmable temperature controller.The temperature ramp was less than 1 °C min-1 and typically the sample was held at 450 °C for 4–10 hours. The other calcination method involved two steps, the sample was first dehydrated at 150 °C for over 10 hours under 10-5 Torr dynamic vacuum and then heated in air at 450 °C for ca. 10 hours. Most of the characterization work for the as-synthesized and calcined film samples was done with those having a fan type texture and using the direct calcination procedure unless mentioned otherwise. Characterization Powder X-ray diVraction (PXRD) data were obtained on a Siemens D 5000 diVractometer using Ni-filtered Cu-Ka radiation with l=1.54178 A ° .Home made quartz low background sample holders and plexiglass sample holders were used for mounting the intact films or ground films, and for recording the PXRD patterns.Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4500 field emission microscope using a low acceleration voltage of 2 kV to minimize the charging of the mesoporous silica surfaces. Samples were uncoated and imaged directly. Transmission electron microscopy (TEM) images were recorded on a Philips 430 microscope operating at an accelerating voltage of 100 kV with a typical recording magnification in the range 80 000 to 160 000 times.The microscope has a working resolution of 3.5 A ° . In order to get ultrathin (ca. 400–600 A ° ) sections of the mesoporous silica morphologies, the samples were embedded in epoxy resin and sectioned using an RMC MT6000 ultrami- Fig. 1 Representative SEM images of an as-synthesized mesoporous crotome in combination with a Drukker diamond knife follow- silica film formed at the air–water interface showing (A) a typical surface with bending, and (B) cross section with a thickness of ing the standard procedure. Embedding in Spurr’s epoxy resin ca. 5–10 mm (TAAB laboratories equipment, Aldermaston, UK) was used for calcined film samples and a cyanoacrylate resin (SuperglueA) for as-synthesized film samples.Additional hardshows the thickness to be ca. 5–10 mm thick, Fig. 1(B). The ening of the embedding matrix was induced at 60 °C for 12 thickness of the film that grows at the air–water interface hours. Polarized optical microscopy (POM) images were depends on the conditions of the solution phase. Representative obtained on a BH-2 Olympus optical microscope with a POM images obtained between crossed polarizers, for meso- Kodak Gold Ultra 400 film. Proton-decoupled 29Si solid state porous silica films formed under slightly diVerent conditions magic angle spinning nuclear magnetic resonance (MAS NMR) are shown in Fig. 2. In contrast to the previously reported spectra were recorded on a Bruker DSX 200 MHz spectrometer images for ca. 0.5 mm mesoporous silica thin films,6 the at 40 MHz using a 90° pulse with a delay time of 600 seconds. newly obtained thick films display classic liquid crystal textures. Computer simulation of the NMR spectra employed a Bruker The transformation of the films from ones displaying discrete deconvolution program.Thermal analysis (DSC and TGA) birefringent patterns, Fig. 2(A), to ones with fan-type textures, data were recorded on Perkin-Elmer Thermal Analysis Series Fig. 2(B), can be controlled by simply choosing the stirring 7 instrumentation under N2. The temperature ramp was at time or rate of the synthesis mixture before it is set into a 5 °C min-1. The surface area and mesoporosity of the film quiescent growth state.The films that have a fan-type texture were obtained on a McBain balance. Details of the set-up and were obtained for the mixture with a stirring time of about 10 measurement procedure have been published elsewhere.23 To minutes at low stirring rate, Fig. 2(B), or about 5 minutes at prepare a sample for the measurement of adsorption isotherms, high stirring rate, while the ones showing discrete birefringence a suYcient quantity of the as-synthesized films was first ground patterns emerged after a stirring period of 3 to 5 minutes at to a fine powder and then calcined following either one of the low stirring rate, Fig. 2(A). The observed fan-type texture is calcination procedures mentioned above. typical of a hexagonal lyotropic liquid crystal HI phase having the optical axis in the plane of the film.2,3 The fan-type texture Results is essentially invariant on removing the surfactant and after annealing the film at 450 °C, Fig. 2(C). This implies that the An optical microscopy image of a free-standing film obtained optical birefringence of the film does not require the surfactant in a synthesis that employed an agitation time of 10 minutes to be present in the channels and that strain anisotropy in the shows that the film is optically transparent.Scanning electron film is not the source of the fan-type texture. The study microscopy (SEM) images of the film are shown in Fig. 1. The herewith was conducted with samples that have fan-type film is continuous and the surface of the film that grows textures obtained for the mixture with a stirring time of about adjacent to the air interface has a smoother surface than that 10 minutes, at low stirring rate of the synthesis mixture before of the growing front emerging at the water interface, Fig. 1(A). it is set into a quiescent growth state, unless stated otherwise. The observed bending of the film presumably arises from a stress induced drying eVect.A cross-sectional view of the film Powder X-ray diVraction (PXRD) traces of these meso- 1206 J. Mater. Chem., 1998, 8(5), 1205–1211Fig. 3 PXRD traces of as synthesized mesoporous silica films, (a) without and (b) with grinding; calcined mesoporous silica films, (c) without and (d) with grinding Fig. 4 PXRD traces of (a) calcined ground film without dehydration and (b) calcined ground mesoporous silica films after dehydration treatment the oriented mesoporous films was investigated further by examining the eVect of grinding.The ground-up film samples are white powders. The PXRD trace for the ground sample, Fig. 3( b), showed the expected four peaks (100), (110), (200), (210) from diVraction planes that are typically seen in randomly oriented powder preparations of hexagonal mesoporous silica.4,5 The calcined and annealed ground-up samples, Fig. 3(d), also show a large contraction of the d100-spacing similar to that for the calcined oriented film, Fig. 3(c). It is observed that after the vacuum dehydration treatment at Fig. 2 POM images of mesoporous silica films under cross polarizers: 150 °C, the d100 diVraction peak contracts ca. 2A ° , and then (A) discrete texture of an as-synthesized film; fan-type texture of (B) an as-synthesized and (C) a calcined film (scale bar: 50 mm) further calcination of the dehydrated sample leads to a more well ordered mesoporous silica structure than that obtained without the vacuum pre-treatment. This is evidenced by a higher diVraction intensity and a greater number of reflections porous silica films are shown in Fig. 3. The PXRD pattern of as-synthesized and calcined film confirm them to be hexagonal in the PXRD patterns of the pretreated samples, Fig. 4. Clearly the moisture content of an as-synthesized incompletely poly- mesoporous silica and not the silicate liquid crystal mesophase for which the d100 appears at much lower angles,17 Fig. 3. The merized mesoporous film can aVect the stability of the structure.24 presence of only (100, 200) low angle reflections, Fig. 3(a), implies that the channels preferentially run parallel to the The orientation of the channels for the as-synthesized and calcined hexagonal mesoporous films is confirmed by TEM surface of the film.6 This structure is retained after surfactant removal by calcination and is accompanied by contraction of images of microtomed thin sections, Fig. 5. The expected hexagonal honeycomb structure is seen in both as-synthesized the hexagonal mesostructure due to condensation–polymerization of residual hydroxyls in the silica channel walls.4 The and calcined mesoporous silica films. Careful inspection of the TEM images shows that the hexagonally packed pores are not relatively large contraction of the d100 spacing for the calcined film, Fig. 3(c), implies a much lower degree of polymerization as well organized as those in the thinner version of the film made at 80 °C. It is unlikely that such a diVerence is simply of the silica in the surfactant-containing precursor film relative to a higher acidity synthesis (see below).6–9 The structure of from the variation of imaging conditions. Note that a lower J.Mater. Chem., 1998, 8(5), 1205–1211 1207Fig. 5 Representative TEM images of mesoporous silica films: (a) as-synthesized, (b) calcined (no dehydration treatment), (c) calcined (with dehydration treatment) degree of condensation–polymerization of silica (see below) may lead to a less stable channel wall structure for the embedding and microtoming procedure.As far as we can judge from the TEM images, the order of the mesopores on the top (air interface) and bottom (water interface) faces of the films is about the same. Vacuum dehydration prior to calcination seems to be an essential step for stablizing a well ordered hexagonal mesoporous structure.Calcination without vacuum dehydration appears to cause a greater distortion of the hexagonal mesoporous structure, Fig. 5(b). Thermogravimetric analysis (TGA) of the as-synthesized film shows the expected weight changes corresponding to loss of imbibed water below 100 °C, surfactant template around 270 °C, and water from condensation of framework hydroxyls around 360 °C and 600 °C, Fig. 6. The total weight decrease is ca. 60–70% which represents a high-end loss for a hexagonal Fig. 6 A representative TGA trace of a mesoporous silica film mesoporous silica preparation. This is presumably because of the lower degree of polymerization and larger amount of surfactant template imbibed within the channels. combined results of the TGA and DSC show that the thermal properties of these mesoporous silica films closely resemble DiVerential scanning calorimetry (DSC) shows no evidence of an endotherm that can be ascribed to a liquid crystal or those of the powdered solid.Moreover, the imbibed surfactant within the channels is not behaving like a liquid crystal isotropic liquid melting transition of the encapsulated surfactant for a heating cycle between 20 °C and 250 °C, Fig. 7. The mesophase. 1208 J. Mater. Chem., 1998, 8(5), 1205–1211Fig. 7 A representative DSC trace of an as-synthesized mesoporous silica film with a heating cycle up to 250 °C Fig. 9 Nitrogen isotherms of free-standing oriented mesoporous silica films: (a) sample after dehydration at 150 °C, (b) sample calcined without dehydration pre-treatment, and (c) sample calcined with The proton-decoupled 29Si MAS NMR spectrum and comdehydration pre-treatment.All samples were ground into a powder puter deconvolution of the spectrum for the as-synthesized before the adsorption measurements mesoporous silica film are shown in Fig. 8. Three silicon sites are observed with a Q2[SiO2(OH)2]5Q3[SiO3(OH)]5 Q4(SiO4) ratio of 8542548. The observation of Q2 silicon shows both low and high pressure hysteresis.It is well docuspecies and the high value of Q2+Q3 relative to Q4 are mented that particular shapes of hysteresis loops can be consistent with the observed large contraction of the hexagonal associated with specific pore structures.25a Low pressure hystermesostructure on calcination (PXRD). This implies a lower esis in this case may be associated with distortion swelling of degree of silicate polymerization in the as-synthesized thick the walls of a partially condensed mesoporous silica films compared to the thin ones, as well as the usual meso- accompanying adsorption.25a The isotherm for the directly porous powders.calcined sample shows a lower uptake of nitrogen, has a much Quantification of the mesoporosity and surface area of the shallower inflection due to capillary condensation, and also film was conducted on a McBain balance.23 N2 isotherms at displays low and high pressure hysteresis.The BET surface liquid nitrogen temperature are shown in Fig. 9. The as- area is calculated to be ca. 750 m2 g-1. These observations synthesized silica films show negligible N2 adsorption after together with those from PXRD, TEM and NMR suggest that dehydration at 150 °C in vacuum implying that this treatment the vacuum-dehydration pre-treatment prior to calcination does not create void space for adsorbing N2.The isotherm for substantially improves the degree of order and mesoporosity the vacuum dehydrated–calcined sample is typical of that of the silica films.expected for Type IV demonstrating that the sample is meso- The thick mesoporous silica films can also be grown on a porous.25 The BET surface area is calculated to be ca. 1000 m2 variety of substrates, such as glass slides and tubes, and freshly g-1. The mean pore diameter estimated by the Dollimore–Heal cleaved muscovite mica. For the preparation of these films, the method from the adsorption branch of the isotherm is ca. 10 minute mixing period described above was used. Between 2.8 nm, which is consistent with the center-to-center distance crossed polarizers in the optical microscope, discrete birefrinof ca. 3.7 nm obtained from PXRD of calcined mesoporous gence patterns were observed for the films grown on planar film samples. Note that the desorption branch of the isotherm and curved glass substrates.The discrete patterns on glass appear to arise from the local nucleation and growth of silicatesurfactant assemblies with contributions from +2p disclination12 –14 concentric-type topological defects. Similar POM patterns have been observed for mesoporous silica discoid shapes that concurrently form in the bulk synthesis mixture.15 Interesting birefringence patterns were observed for the thick Fig. 8 The proton-decoupled 29Si MAS NMR spectrum and computer Fig. 10 A representative POM image of an as-synthesized mesoporous deconvolution of the spectrum for the as-synthesized mesoporous silica film silica film on mica (image size is 150×120 mm) J. Mater. Chem., 1998, 8(5), 1205–1211 1209films grown on freshly cleaved muscovite mica, Fig. 10. Light synthesized mesoporous silica films show no thermal events that can be associated with crystal–liquid crystal or isotropic coloured, grey and dark areas were observed for the thick films between crossed polarizers. melting transitions of an imbibed surfactant liquid crystal up to 250 °C, Fig. 6. Also, the fan-type texture is retained essen- Careful inspection of these images showed that striations on the patches with the same brightness align parallel on the film tially invariant on removing the surfactant and after annealing the film at 450 °C, Fig. 2(C). Therefore the optical birefringence surface and that the lines on patches with diVering brightness meet at 60° or 120° angles. Recall that oriented mesoporous of the film does not require the surfactant to be present in the channels.Furthermore, strain anisotropy in the film is not the silica films have been reported to grow on this surface.26 Thus the observed correlation of striations amongst similarly col- source of the fan-type texture. Finally, the PXRD and 29Si MAS NMR data show that the silica walls of the channels are oured domains, Fig. 10, presumably originates from preferential alignment of the channels in the mesoporous silica film glassy.One may safely conclude that the birefringence is associated with the optically uniaxial nature of the oriented along the hexagonal a,b-axes of the mica surface. hexagonal mesoporous silica film. The thick mesoporous silica films display two dominant Discussion morphologies which give rise to distinct POM birefringence patterns.The free-standing films synthesized with a stirring Quiescent, acidic and aqueous conditions are the key and essential prerequisites for our surfactant-templated synthesis period of 3 to 5 minutes at low stirring rate, Fig. 2(A), have a structure based upon discrete morphologies which appear to of hexagonal mesoporous silica films at the boundary between air and water.6,7 Under high acidity and 80 °C reaction con- have coalesced and show concentric birefringence patterns.Such morphologies have previously been observed for meso- ditions the films grow to a limiting thickness of about 0.5 mm. TEM images of the films show that while the channels are porous silica films grown on gold31 and glass32 surfaces. Similar morphologies can be found in bulk preparations that yield well organized in the plane of the film, they swirl and curl throughout the body of the film to create designs7,8 that discoid shape mesoporous silica where the channels run concentrically and coaxially around the main rotation axis of the resemble the patterns of director fields induced by topological defects such as +p, -p and +2p disclinations found in a discoid.33 These discrete types of POM patterns presumably arise from +2p disclinations with the rotation axes normal to discotic hexagonal liquid crystal mesophase.12,13 Studies of the early stages of film growth suggest that the the director field.Defect textures of this genre may reflect the seeds that spontaneously emerge in a lyotropic discotic hexag- process begins with the assembly of silicate liquid crystal seeds located at the air–water interface and templated by a surfactant onal phase and originate from vertical disclinations.13 Mesoporous silica films of this type tend to have domain overstructure.6,7,27–30 They silicify and expand in size through the accretion of silicate micelles and coalesce to form a structures and varied thickness.By contrast, free-standing films with a continuous and fan-type texture were obtained from a continuous film. Concurrent with film growth in two dimensions, mesoporous silica morphologies with well defined three synthesis mixture with a stirring time of about 10 minutes at low stirring rate, Fig. 2(B). The film shows good homogeneity dimensional shapes are evolving in the bulk aqueous phase.The reactant ratios, temperature and acidity of the synthesis and a smoother surface compared to those formed with less of a mixing period. Films with continuous and fan-type texture are important factors for controlling the curvature and size of these morphologies.15 In particular, it was found that less can also be obtained from a synthesis mixture with a stirring time of about 5 minutes at high stirring rate.The fan textures, acidic and room temperature conditions promoted the growth of well formed morphologies with dimensions as large as ca. however, did not transform into discrete textures on changing the vessel geometry, such as its shape and size. Thus, the 70 mm. With this knowledge of size and shape control of mesoporous silica morphologies, it has now proven possible diVerence in the POM birefingence patterns and surface morphologies might be best viewed as arising from a ‘switch’ in to gain command over the thickness and channel texture of mesoporous silica films grown at the air–water interface.the mode of formation of the free-standing film from one initiated by silicate liquid crystal seeds at the air–water The converging evidence from PXRD, TEM, and nitrogen isotherm measurements for thick mesoporous silica films shows interface, involving local growth and coalescence, to one involving the formation of a continuous silicate liquid crystal that they have oriented and hexagonally close-packed mesopores with the channel c-axis aligned parallel to the growth surface film.In both instances, silicification captures the defect structure and pattern of director fields in the precursor silicate interface. The as-synthesized and calcined films showed only (100) and (200) diVraction peaks, while additional (110) and mesophase, which is manifest in the channel design of the resulting mesoporous silica film. Boundary walls between the (210) diVraction peaks can be observed for the ground samples, Fig. 3. This establishes that the mesoporous silica films are defect domain structures are visible in the POM images. Notwithstanding this, the birefringence textures observed for hexagonal and oriented with the channels parallel to the growth surface. TEM images of these films also define the both kinds of free-standing mesoporous silica films show that the channel architecture is a silicified replica of the defect hexagonal mesopore arrangement and orientation of the channels for as-synthesized and calcined samples.The order of the induced pattern of director fields in a silicate liquid crystal precursor. It is also noteworthy that the texture traverses the mesopores in the front and back surfaces of the film are comparable. Although the film mesostructure was maintained entire extent of the film implying that the mesoporosity is not confined to small domains.in all the preparations of this study, the lower degree of polymerization–condensation of the silica in the thick films It is pertinent to inquire into the relation between the optical birefringence patterns of mesoporous silica films comprised of that are formed at lower acidity, evidenced by the larger (Q2+Q3)/Q4 ratio, may be the cause of the lower thermal discrete ribbon and discoid morphologies and those with fantype textures.Recall that the channels that run down the stability compared to the thin films formed at higher acidity. Consistent with this proposal is the observed large contraction length of the ribbon are found to whirl around the unique rotation axis of the discoids.8,15 When these discrete morpho- of the diameter of the mesopores for the calcined film samples. Thermal vacuum dehydration prior to calcination helps stabil- logies are viewed orthogonal to the channel director, optical extinction is found to occur only when the channel axis ize the mesostructure, Fig. 4. Although the formation of mesoporous silica films involves coincides with the optic axes of the polarizer or analyzer. In the case of the discoids, a roughly symmetrical black cross silicification of a lyotropic liquid crystal, the observed POM texture of the film does not arise from organized surfactant (the isogyre) emerges from a +2p disclination defect, while for the ribbon-shaped morphologies, a +p disclination defect assemblies in the channels.To amplify, the DSC trace of as- 1210 J. Mater. Chem., 1998, 8(5), 1205–12113 N. H. Hartshorne, T he Microscopy of L iquid Crystals, Microscope leads to the observed textures. The observed birefringence Publications Ltd., London, 1974, pp. 104–138. patterns are therefore optical manifestations of the diVerences 4 (a) C.T. Kresge, M. Leonowicz, W. J. Roth, J. C. Vartuli and in the channel structures of the mesoporous silica films that J. C. Beck, Nature (L ondon), 1992, 359, 710; (b) J. S. Beck, are prepared by stirring the synthesis mixture for diVerent J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, times prior to film growth in the quiescent state. As the stirring K.D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. time or rate and presumably the homogeneity of the synthesis Soc., 1992, 114, 10 834. mixture are increased, the discrete birefringence pattern of the 5 Q. Huo, D. I. Margolese, U. Clesia, P. Feng, T. E. Gler, P. Sieger, films so formed gradually merge into ones displaying the R.Leon, P. M. PetroV, F. Schu� th and G. D. Stucky, Nature continuous fan-type texture. This behavior might be thought (L ondon), 1994, 368, 317. as a ‘switch’ in the mode of formation of the films at the air– 6 H. Yang, N. Coombs, I. Sokolov and G. A. Ozin, Nature (L ondon), water interface, from one primarily involving the polymeriz- 1996, 381, 589. 7 H. Yang, N.Coombs, O� . Dag, I. Sokolov and G. A. Ozin, J.Mater. ation, growth and coalescence of a population of silicate liquid Chem., 1997, 7, 1755. seeds to one based on the polymerization and thickening of a 8 N. Coombs, D. Khushalani, G. A. Ozin, S. Oliver, G. C. Shen, continuous silicate liquid crystal film. I. Sokolov and H. Yang, J. Chem. Soc., Dalton T rans., 1997, 3941. The liquid crystal POM patterns and surface morphologies 9 (a) I.A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, not only depend on the synthesis conditions, they also change P. Fenter, P. M. Eisenberger and S. M. Gruner, Science, 1996, 273, with the substrate properties. This can be seen from POM 892; (b) A. S. Brown, S. A. Holt, T. Dam, M. Trau and J. W. White, L angmuir, 1997, 13, 6363.images for films grown on amorphous surfaces, such as glass 10 S. H. Tolbert, T. E. Scha�Ver, J. Feng, P. K. Hansma and plates or tubes and the atomically flat mica surface. It is G. D. Stucky, Chem.Mater., 1997, 9, 1962. interesting to note that the POM patterns for the films grown 11 J. Feng, Q. Huo, P. M. PetroV and G. D. Stucky, Appl. Phys. L ett., on mica diVer from those on the other surfaces or subphases. 1997, 71, 620. Discrete and fan-type textures are replaced by extensive areas 12 Y. Bouligand, J. Phys., 1980, 41, 1297. of dark, grey and light patches, Fig. 10. The patterns of parallel 13 Y. Bouligand, J. Phys., 1980, 41, 1307. 14 M. Kle�man, Points, L ines and Walls: In L iquid Crystals, Magnetic striations within the diVerent patches of the mesoporous silica Systems and Various Ordered Media, John Wiley & Sons Ltd., t at angles of 60 or 120°.This observation suggests Chichester, 1983. that growth of the hexagonal mesoporous silica film occurs 15 H. Yang, N. Coombs and G. A. Ozin, Nature (L ondon), 1997, with the channel axis preferentially aligned along the hexagonal 386, 692. a,b-axes of the mica (001) surface. 16 A.Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, Finally, the observation of liquid crystal textures in meso- R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. PetroV, A. Firouzi, M. Janicke and B. F. Chmelka, Science, 1993, 261, 1299. porous silica films grown at the air–water interface under 17 (a) A. Firouzi, F. Atef, A. G. Oertli, G. D. Stucky and dilute aqueous and acidic conditions provides direct evidence B.F. Chmelka, J. Am. Chem. Soc., 1997, 119, 3596; (b) A. Firouzi, for a templating pathway based upon cooperative assembly D. Kumar, L. M. Bull, T. Sieger, Q. Huo, S. A. Walker, and organization of silicate micellar species4,5,16–18 rather than J. A. Zasadzinski, C. Glinka, J. Nicol, D. I. Margolese, a pre-formed silicate liquid crystal.21 G.D. Stucky and B. F. Chmelka, Science, 1995, 267, 1138. 18 (a) C.-Y. Chen, H.-X. Li and M. E. Davis, Microporous Mater., 1993, 2, 17; (b) C.-Y. Chen, S. L. Burkett, H.-X. Li and M. E. Davis, Conclusions Microporous Mater., 1993, 2, 27. 19 P. T. Tanev and T. J. Pinnavaia, Science, 1995, 267, 865. Optical birefringence patterns observed for free-standing meso- 20 D.M. Antonelli and J. Y. Ying, Angew. Chem., Int. Ed. Engl., 1995, porous silica films are shown to originate from mesoscale 34, 2014. optical anisotropy associated with the polarization response 21 (a) G. S. Attard, J. C. Glyde and C. G. Go� ltner, Nature (L ondon), of electron density circumscribing a hexagonal array of glassy 1995, 378, 366; (b) M. Antonietti and C. Goltner, Angew.Chem., silica channels. The existence of the birefringence shows that Int. Ed. Engl., 1997, 36, 910. 22 P. Behrens, Angew. Chem., Int. Ed. Engl., 1996, 35, 515. the channel architecture is a silicified replica of the defect 23 H. Yang, G. Vovk, N. Coombs, I. Sokolov and G. A. Ozin, induced pattern of director fields in a silicate liquid crystal J.Mater. Chem., 1998, 8, 743. precursor.Alteration of synthesis conditions, mixing and sub- 24 T. Tasumi, K. Koyano, Y. Tanaka and S. Nakata, Chem. L ett., strates enables control over the film texture, which reflects 1997, 469. changes in channel structure arising from the operation of 25 (a) S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and distinct film growth processes. The ability to synthesize Porosity, Academic Press, London, 2nd edn., 1982; (b) H. Naono, M.Hakuman and T. Shiono, J. Colloid Interface Sci., 1997, 186, 5–10 mm thick mesoporous silica films and tailor their channel 360; (c) M. J. Meziani, J. Zajac, D. J. Jones, J. Rozie`re and architecture can be considered to represent a significant step S. Partyka, L angmuir, 1997, 13, 5409; (d) M. Kruk, M. Jaroniec towards the practical realization of mesoporous silica thin and A. Sayari, L angmuir, 1997, 13, 6267. film-based devices and technologies. 26 H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara and G. A. Ozin, Nature (L ondon), 1996, 379, 703. Financial support from Mobil Technology Company is deeply 27 O. Regev, L angmuir, 1996, 12, 4940. 28 Y. S. Lee, D. Surjadi and J. F. Rathman, L angmuir, 1996, 12, 6202. appreciated. H.Y. is grateful for an Ontario Graduate 29 J. Bo� cker, M. Schlenkrich, P. Bopp and J. Brickmann, J. Phys. Scholarship held during this research period. We also thank Chem., 1992, 96, 9915. Mr. G. Vovk for setting up the McBain balance and for 30 J. R. Lu, Z. X. Li, J. Smallwood, R. K. Thomas and J. Penfold, valuable discussions on data analysis, and Dr. P. Aroca for J. Phys. Chem.. 1995, 99, 8233. assistance with the recording of solid state NMR spectra. 31 H. Yang, N. Coombs and G. A. Ozin, Adv.Mater., 1997, 8, 811. 32 (a) H. W. Hillhouse, T. Okubo, J. W. van Egmond and M. Tsapatsis, Chem. Mater., 1997, 9, 1505; (b) A. Kuperman, References S. Mamiche-Afara, G. A. Ozin and H. Yang, T echnical Report to Mobil T echnology Company, June 1995. 1 K. Hiltrop, L yotropic L iquid Crystals, in L iquid Crystals, ed. 33 G. A. Ozin, H. Yang, I. Sokolov and N. Coombs, Adv. Mater., H. Stegemeyer, SteinkopV Darmstadt, Springer, Germany, 1994, 1997, 8, 662. pp. 143–171. 2 D. Demus and L. Richter, T extures of L iquid Crystals, Verlag Chemie, Weinheim, Germany, 1978. Paper 8/00004B; Received 2nd January, 1998 J. Mater. Chem., 1998, 8(5), 1205–1211