Preparation of transparent thin films of silica-surfactant mesostructured materials

PII:S0968-5677 (98) 00015-7

Supramolecular Science 5 (1998) 247—251  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

Preparation of transparent thin films of silica-surfactant mesostructured materials Markoto Ogawa PRESTO, Japan Science and Technology Corporation (JST), Institute of Earth Science, Waseda University, Nishi-waseda 1-6-1, Shinjuku-ku, Tokyo 169-50, Japan

Our recent studies on the simple synthetic route to the thin films of the periodic silica-surfactant mesostructured materials are summarized. By depositing the mixtures of the prehydrolyzed tetramethoxysilane and alkyltrimethylammonium salts under an acidic condition on solid supports, transparent thin films of the periodic silica-surfactant mesostructured materials formed. The resulting transparent films have been used as a precursor of the porous silica films and a support for organic photoactive species. Compared with other reported reactions for the silica-surfactant mesostructured materials, this method possesses advantages such as the ease of operation and the possibility to control microstructure and macroscopic morphology.  1998 Elsevier Science Limited. All rights reserved. (Keywords: supramolecular template; inorganic—organic nanocomposite; surfactant; porous film)

INTRODUCTION Self-organization of molecules into a highly ordered architecture has attracted increasing attention from a wide range of scientific interests. Polymerization of organic or inorganic monomers in an environment with an ordered structure is a method by which polymers or composites with unique structures and properties can be prepared. Recently, surfactant aggregates have been used as supramolecular templates for the preparation of mesostructured silicates\ and other metal oxides\. The preparation of mesostructured materials from a cooperative organization of inorganic species and surfactants is a current topic mainly because of the possible use of the mesoporous materials derived from the mesostructued composites in catalysis and host—guest chemistry. Beside the preparation of mesoporous solids, the as-synthesized materials may also find applications as novel states of surfactant aggregate. The synthesis of the inorganic-surfactant mesostructured materials in controlled morphology is an important step for the applications of the mesostructured materials\. Thin films of the inorganic-surfactant mesostructured materials might be applied to sensors, optical and electronic materials, etc., to which powder samples do not have an access. Moreover, the morphology of the mesostructured materials can provide information on the mechanism of their formation. Accordingly, supported and unsupported thin films of the silica-surfactant mesostructured materials have been prepared on solid substrates\ and at the air—water interface. This paper summarizes the recent studies on the preparation of the transparent thin films of the silica—alkyltrimethylammonium halides mesostructured

materials by depositing precursor solution from tetramethoxysilane and surfactant. Compared with other reported synthetic procedures for the thin films of the silica-surfactant mesostructured materials\, the present synthetic method possesses advantages such as the ease of operation and the possibility to control the microstructures as well as the macroscopic morphology.

EXPERIMENTAL Materials Tetramethoxysilane (abbreviated as TMOS) and alkyltrimethylammonium halides [(C H )(CH ) NBr and L L>  Cl; abbreviated as CnTAB, and CnTAC, where n denotes the carbon number in the alkyl chain] were obtained from Tokyo Kasei Industries Co., and used without further purification. Pyrene was used after recrystallization. Sample preparation The silica-surfactant mesostructured materials were prepared as follows: TMOS was partially hydrolyzed by a substoichiometric amount of deionized and distilled water (the molar ratio of TMOS : H O was 1 : 2) under an  acidic condition at room temperature. Initially the mixture was an emulsion but it became homogeneous as the hydrolysis proceeded. Then surfactant was added and the mixture was stirred at room temperature. The clear viscous solution was spin coated on Pyrex glass or fused silica substrates and dried in air at room temperature to remove solvent and to complete condensation of the

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Preparation of transparent thin films: Makoto Ogawa silica. Thus, transparent thin films formed on the substrates. In order to prepare porous films, the as-synthesized films were calcined in air to remove surfactants. Characterization The as-synthesized and calcined films have been characterized by X-ray diffraction, Si magic angle spinning nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurement. The thicknesses of the films were determined with a surface profilometer.

RESULTS AND DISCUSSION Preparation of the thin films of the silica—alkyltrimethylammonium salt mesostructured materials By the spin coating, transparent thin films formed on the substrate. The thickness of the films can be controlled to some extent by changing the spinning ratios. The thickness of the films has been adjusted to be ca. 1 lm, since thicker films can be peeled off from the substrate during the drying. Figure 1 shows the X-ray diffraction pattern of the thin films prepared from the TMOS and an aqueous solution of C14TAB (the molar ratio of TMOS : C14TAB for the starting solution was 2.5 : 1). A very sharp diffraction peak with the d value of 3.5 nm, which accompanied second order reflection (d value of ca. 1.8 nm), was observed in the XRD pattern. Transparent thin films were obtained for the composites with C8, C10, C12 and C16TABs. However, the diffraction peaks were very weak and broad when C8TAB was used. The d values changed linearly as a function of the alkyl chain length of CnTAB.

Figure 1 X-ray diffraction pattern of the layered silica-C16TAB mesostructured material (the molar ratio of TMOS to C14TAB was 2.5 : 1)

Figure 2 Schematic diagram for the microstructure of the thin film of the layered silica—alkyltrimethylammonium bromide mesostructured materials

Considering the fact that the shorter alkyltrimethylammonium bromide (C8TAB) did not give a highly ordered structure, the aggregation of CnTABs seems to be essential for the periodic structures. The films are thought to be composed of lamellar aggregates of CnTAB being sandwiched by ultrathin silica layers. Since the silica polymers are hydrophilic after the hydrolysis, they tend to interact with the hydrophilic head groups of CnTABs. The d value (ca. 1.2$0.1 nm) extrapolated to n"0 in a plot of d spacings versus the alkyl chains lengths of the surfactants corresponds to the thickness of the layers of silica and head group (trimethylammonium bromide). Suppose that the CnTAB aggregate as bilayers with their alkyl chains fully extended, the alkyl chains of CnTAB inclined to the lamella at ca. 50°. The proposed schematic microstructure of the silica-CnTAB mesostructured materials is shown in Figure 2. (For simplicity, the alkyl chains are shown as fully extended.) The infrared spectrum of the layered silica-C14TAB composite showed the absorption bands characteristic to C14TAB (such as C—H stretching vibration of CH group  at 2925 and 2853 cm\, which can be assigned to aggregated C14TAB) and silica (such as Si—O—Si symmetric stretching vibration at around 1230 and 1080 cm\ and Si—O—Si bending vibation at around 460 cm\). This shows that the composites are composed of a siloxane polymer and aggregated C14TAB. Further study on the infrared absorption bands due to C—H bonds of alkyl chains is now underway to determine the nature of the surfactants in the mesostructured materials. Rapid evaporation of solvent before gelation is essential for the formation of the highly ordered nanocomposites. Thinner films gave more intense X-ray diffractions and the X-ray diffractions and the X-ray diffraction peaks of the powdered sample are broad compared with those of the films. Thus, the present system is apparently different from the so-called sol—gel

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Figure 3 The variation of the d values of the silica-C16TAB nanocomposite films as a function of the TMOS : C16TAB ratio

processes. The following reaction mechanism is proposed for the formation of the periodic silica-surfactant meso-structured materials. During the evaporation of solvents, CnTAB tends to aggregate and the hydrophilic silica oligomers interact with the hydrophilic head groups of the surfactants. Being surrounded by the silica layer, the aggregates solidified upon the evaporation of solvents to form the periodic silica-surfactant mesostructured materials without crystallization of CnTAB. It is worth noting as a merit of the present system that the composition of the products can be controlled by simply changing the relative ratios of TMOS to CnTAB in the starting mixture. The relative ratio of TMOS : C16TAB has been changed to vary the microstructures of the composites. Figure 3 represents the relationship between d values obtained from the X-ray diffraction patterns as a function of the relative ratios of TMOS : 316TAB. There is a general tendency toward the larger d values with the increase in the relative ratio of the TMOS to C16TAB. This change in the d values is assumed to be due to the change in the thickness of the silica layer. The silica layer in the composite became thicker with the increase in the relative ratios of TMOS to C16TAB. Since the silica layer is composed of an amorphous siloxane network where SiO tetrahedra are linked randomly as  shown by the Si—MAS—NMR result, the detailed microstrutures of the composites are difficult to elucidate. Supporting that the composites are fully lamellar where the thin silica layer and surfactant bilayer piled up alternatively, the change in the d values must be larger. Thus, the origin of the observed change in the d values is not so simple. The transition from the lamellar phase to the hexagonally packed cylindrical aggregate phase may be concerned for the observed change in the d values since a larger amount of silica is required to cover the cylindrical micelles than that required for the lamellar phase. Although the mesostructures need to be elucidated further, these results indicate that the microstructures of the silica-surfactant nanocomposites can be controlled by simply changing the composition of the starting mixture. This is a very important characteristic of the present synthetic approach because it is not so straightforward to control the microstructures of the products by other reported procedures. The thickness of the silica

Figure 4 Schematic diagram for the proposed microstructure of the porous silica films

layer must affect the thermal and mechanical properties that are very important for the applications of mesostructured materials. Preparation of porous silica films fom the silica-surfactant mesostructured materials The conversion of the silica-surfactant mesostructured materials to porous silica films has been carried out by calcining the as-coated films to remove surfactants. In order to obtain porous films with retaining the periodic structures of the as-coated films, synthetic conditions including the molar ratio of TMOS to surfactant must be optimized. The silica-C16TAC film prepared at the TMOS/C16TAC molar ratio of 7 was successfully converted to transparent porous silica film. Although the d value of the as-coated film decreased by the calcination, very sharp diffraction peaks were observed for the XRD patterns of the calcined films. In order to obtain porous silica films, the thickness of the films must be as thick as 1 lm. The thicker films may be peeled off from the substrates during the calcination. The proposed schematic diagram for the porous silica film is shown in Figure 4. For the bulk characterization, thicker films have been prepared on the substrate and the heat treatment and the characterization was conducted as powders. Figure 5 shows the XRD patterns of the silica-C16TAC composite before and after the heat treatment in air at 873 K for 1 h. No special precautions were made before the thermal treatment. The diffraction intensity does not change upon the calcination, showing that the ordered structure maintained even after the removal of the surfactant. The d  value of the calcined product (2.9 nm) was smaller than that of the as-synthesized product by ca. 0.8 nm. Different reaction conditions such as the drying rate and heating temperature led different d values of the calcined  products. The Brunauer—Emmett—Teller (BET) surface area of the calcined silica C16TAC mesostructured material was ca. 1000 m g\. From the Horva´th—Kawazoe pore size distribution curve for the calcined product, the average pore diameter was determined to be ca. 1.8 nm. These observations indicate that the periodic silica-surfactant nanocomposite synthesized in this study was successfully

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Figure 5 X-ray diffraction patterns of the silica-C16TAC mesostructured material before (a) and after (b) the calcination at 873 K in air

converted to porous solid. By subtracting the Horva´th— Kawazoe pore size from the repeat distance (a) between pore center (3.3 nm, which is calculated from the XRD data with the formula a"2d /(3), the framework  wall thickness was estimated to be ca. 1.5 nm. A transmission electron micrograph of the calcined sample confirmed the porous structure analogous to MCM-41 and FSMs. Incorporation of pyrene into the silica-surfactant mesostructured materials Besides the conversion to porous silica films, the assynthesized silica-surfactant mesostructured materials are worth investigating as a novel state of surfactant aggregates immobilized by ultrathin inorganic layers. The transparency of the silica-surfactant mesostructured materials in a wavelength region from ultraviolet to nearinfrared opens a way to apply these novel inorganic— organic composites for the supports of photoactive species. Accordingly, the introduction of poorly water soluble organic species has been investigated so far. Fluorescence of pyrene has been used as a probe for the location and distribution in surfactant aggregates. For the incorporation of pyrene into the composites, pyrene was first solubilized into an aqueous solution of surfactants and allowed to react TMOS subsequently. Thus, pyrene molecules have been incorporated into the silica-surfactant mesostructured materials without any loss of transparency and structural regularity. Figure 6 shows the photoluminescence spectrum of the silicaC16TAB-pyrene composite film. (The molar ratio of pyrene to C16TAB was 1/9.) The vibronic structure of the fluorescence spectrum of pyrene is strongly dependent on the polarity of microenvironments. The change in the relative intensity of fluorescence peaks I (at 373 nm)

Figure 6 The luminescence spectrum of the silica—C16TAC—pyrene mesostructured material (the molar ratio of pyrene to C16TAC was 1/9)

and III (at 385 nm) is commonly used to monitor the polarity. For the present composite films, I/III intensity ratios are ca. 0.8 irrespective of the amounts of pyrene, and these values are similar to that of the pyrene in micellar C16TAB solution (pyrene : C16TAB"1 : 100). Taking the Py scale of solvent polarity, which was proposed by Dong and Winnik, into consideration, the incorporated pyrene molecules are thought to be surrounded by the alkyl chains of C16TAB with some interactions with the hydrophilic head group of C16TAB. When pyrene is forced into close proximity or in high concentration solution, an excited state dimer (excimer) forms and the emission from the excimer is observed at around 475 nm in the fluorescence spectrum. Although the amount of pyrene was high, the contribution of the excimer emission (at around 475 nm) was very small for the present composites. The excitation spectrum of the excimer emission (475 nm) was consistent with that of the monomer emission (monitored at 390 nm), suggesting that there was no significant ground state interactions between the incorporated pyrene molecules. These observations indicate that the added pyrene molecules are solubilized molecularity in the silica—C16TAB nanocomposite and that the mobility of the pyrene molecules is restricted. The relative intensity ratio of the excimer to monomer emission decreased with the decrease in the temperature, suggesting the change in the microviscosity of the probe microenvironment. It was thought that the states of the surfactant aggregate in the silica-surfactant mesostructured materials changed depending on the temperature. Details of the temperature dependent luminescence characteristics of the pyrene molecules in the silicasurfactant mesostructured materials are currently being reported.

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Preparation of transparent thin films: Makoto Ogawa CONCLUSION This paper shows a simple way for the preparation of silica-surfactant mesostructures and the conversion to porous materials. Transparent thin films of the periodic silica-surfactant mesostructured materials have been obtained by spin coating the mixtures of the prehydrolyzed tetramethoxysilane and alkyltrimethylammonium salts. The rapid evaporation of solvents is essential for the formation of the mesostructured materials. Possible uses as a precursor of the porous silica films and a support for organic photoactive species have been investigated. Compared with other reported reactions for the silicasurfactant mesostructured materials, this method possesses advantages such as the ease of operation and the possibility to control microstructure and macroscopic morphology.

ACKNOWLEDGEMENTS The author is grateful to Prof. K. Kuroda, Department of Applied Chemistry, Waseda University for valuable discussions. This work was partially supported by Waseda University as a Special Research Project.

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