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• W1976684431 abstract "Orientation control: We demonstrate a facile method for the controlled orientation of 1D mesochannels by applying a novel rubbing method that utilizes lyotropic liquid crystals (LLC) made of highly concentrated surfactants. This method does not have any special requirements for the supports or the substrates. Here we applied this rubbing method for the uniaxial orientation control of mesochannels in silica films using two different surfactants: as P123 and Brij 56. The discovery of surfactant-templated mesoporous materials has attracted huge attention owing to their unique nanostructures and tailor-made properties.1–4 Ordered mesoporous materials are prepared by co-assembly of the surfactant and the inorganic species and subsequent removal of the surfactant. Although many works have been reported on different morphologies of mesoporous materials (e.g., nanoparticles,5–7 fibers,8, 9 monoliths,10–11 and films12–14), transparent mesoporous films have been paid huge attention owing to their attractive features and promising and potential applications in optical, gas sensors, low dielectric films, electronic, and electrochemical sensing devices.15–21 In particular, two-dimensional (2D) hexagonal mesoporous films have been widely prepared with different types of surfactants by various methods. Fine controls of 1D mesochannels in a certain direction are required in order to employ these materials as the recording media, nanofluidic, optoelectronic devices, and fabrication of the oriented nanowires. Thus, several efforts have been proposed to control the mesochannel orientation in mesoporous films. As well-known alignment strategies, external fields such as electric22 and magnetic fields23–25 and shear flows26–29 have been utilized for the alignment controls of the mesochannels. In some cases, however, the formation of noncontinuous and nontransparent films has been confirmed, which limits their application in several fields. The use of confined spaces30–34 and surface modification on the substrates35–39 are useful for orientation controls of the mesochannels. However, all these methods need some special requirements on supported substrates prepared with aluminum anodization, photolithography, electron beam lithography, and oriented polymer films. As another approach, the mesochannels can be spontaneously oriented on the anisotropic crystal surfaces such as mica, graphite, and silicon.40–42 This basic principle is the micelle assembly controlled by the solid–liquid interface, and the mesochannels can be aligned along the atomic facets. Recently, Kuroda and Miyata et al. reported the formation of mesoporous silica films with perfectly aligned mesochannels by using coating polyimides on the substrates.43–45 However, most of the works for the alignment of mesochannels were carried out using either ionic surfactants like CTAB or non-ionic surfactants like Brij 56. The pore size of uniaxially oriented mesochannels has been limited to be less than 3 nm. The limits of mesopore size seriously devalue the advantages of mesoporous films, because a small mesospace suppresses effective movement of guest species within the mesopores. Giant mesopores can incorporate large biological molecules and functional polymers. There have been only a very few reports using a block copolymer like Pluronic P123. Although mesoporous silica films with large mesochannels by using triblock copolymers are reported by using a dip-coating method, the definitive evidence of the alignments is not presented.46 Such a dip-coating method cannot achieve the precise control of alignment.47 Herein, we report the development of a facile process for uniaxial orientation control of mesochannels in silica films using two different kinds of surfactant such as P123 and Brij 56. Here we demonstrate a novel rubbing method utilizing lyotropic liquid crystals (LLC) made of highly concentrated surfactants. This method does not need any special requirement on the supports or the substrates. This method can provide a strict control of uniaxial orientation of the mesochannels, without any disorder or damage in the hexagonal arrangement. Furthermore, multilayered mesoporous silica thin films with different orientations of mesochannels can be achieved. Figure 1 shows a schematic presentation of mesoporous silica films on the substrates using the LLC-based precursor solution. The LLC-based precursor solutions were uniformly coated by a rubbing machine. After the coating, completely transparent films without any cracks were obtained (Figure 1 B). In the both cases of P123 and Brij 56 systems, the conventional θ–2θ XRD of the as-prepared films shows strong several diffraction peaks. Two peaks correspond to (10) and (20) diffractions of the 2D hexagonal structure. No peaks assignable to (11) diffraction can be observed, indicating that the (10) face is aligned parallel to the substrate.48 The same behavior has often been observed in mesoporous silica films reported previously.43–45 From these results, in both films, it was proved that an ordered 2D hexagonal mesostructure is indeed formed on the substrate, even though a strong rubbing process was applied during the synthetic process. A) Concept of synthetic procedure for mesoporous silica films. The precursor solutions with pre-organized lyotropic liquid crystals were spread out by a rubbing rotor. Then, mesoporous films were uniformly coated on the substrate. B) Photograph of the calcined mesoporous silica films prepared with P123 by the rubbing method. To investigate the anisotropy of the 1D mesochannels in the film, grazing-incidence small-angle X-ray scattering (GI-SAXS) measurements were carried out. The patterns in Figure 2 were measured with the geometry where the incident X-rays are perpendicular (A-1, C-1) and parallel (A-2, C-2) to the rubbing direction. The diffraction spots clearly showed that the mesostructured silica film had a single-crystal-like mesostructure and the 1D mesochannels are completely aligned parallel to the rubbing direction. A, C) 2D GI-SAXS patterns of as-prepared mesostructured silica films prepared with A) P123 and C) Brij 56. The 2D patterns were measured with the geometry where the incident X-rays are perpendicular (A-1, C-1) and parallel (A-2, C-2) to the rubbing direction. After alignment of the X-ray beam with the surface of the mesostructured films, the GISAXS measurements were carried out by using a Rigaku MicroMax-007HF diffractometer with CuKα radiation. The incident beam was shadowed with circular beam stops, and the signal was recorded on a CCD camera. B) Cross-sectional TEM image of as-prepared mesostructured silica film prepared with P123. The films were sliced perpendicular (B-1) and parallel (B-2) to the rubbing direction. Scale bars: 50 nm. The images were taken by TEM (JEOL-2000) operated at 200 kV. The cross-sectional TEM images of the as-prepared films are shown in Figure 2. The TEM samples were sliced perpendicular (B-1) and parallel (B-2) to the rubbing direction. The mesochannels are running parallel throughout to the substrate. Even at the interface between the films and the substrate surfaces, the 1D mesochannels of continuous hexagonal mesostructure were successfully oriented parallel along the rubbing direction (Figure S1 in the Supporting Information). From 2D GI-SAXS patterns (Figure 2), the values of d10 and d were calculated to be 9.9 nm and 6.0 nm (for the P123 system) and 4.3 nm and 2.8 nm (for Brij 56), respectively. The values (d10/d ratio) were 1.65 (for P123) and 1.53 (for Brij 56). Since the value (d10/d) of an ideal hexagonal structure is 1.73, the hexagonally ordered mesopores in the films are slightly distorted along the perpendicular direction to the substrate. The 2D GI-SAXS patterns of the calcined films show that the hexagonal mesostructures were completely retained after removal of the surfactants. The distortion of the hexagonal structure is amplified by the calcination, since the shrinkage of the mesostructure occurred only in the vertical direction. After the calcination, the values (d10/d) were reduced. The strong adhesion onto the glass substrate could prevent a horizontal structural shrinkage. Crystallographically, the formed mesostructure is not hexagonal but orthorhombic, in which the spacings (2d) of the horizontal distance between the neighboring mesopores are larger than the values calculated from the corresponding (10) spacings. Nitrogen adsorption–desorption isotherms of the calcined films were performed to characterize the mesopore sizes and surface areas (Figure S2 in the Supporting Information). Nitrogen adsorption–desorption isotherms of the calcined films prepared with P123-based solution showed type IV with a hysteresis loop, which is characteristic of mesoporous materials. The average mesopore size in the film was measured to be around 6.3 nm by using BJH analysis. The BET surface area showed a high value of 1100 m2 g−1, which discloses the accessible mesoporosity. The nitrogen adsorption–desorption isotherms of the calcined films prepared with Brij 56 based solution showed type IV isotherm without a hysteresis loop. The average mesopore size and the surface area was calculated to be around 2.0 nm and 970 m2 g−1, respectively. To investigate the distribution of the mesochannel alignment in the film, 2D patterns were obtained by irradiation of X-ray beam perpendicular to the film surface (Figure 3 A). In this measurement, since the X-ray beam passes through the whole thickness of the film, we can understand the distribution of mesochannels in the inner part of the film. The 2D obtained patterns showed less-uniform distribution of the scattering intensity corresponding to the (10) peak of a 2D hexagonal structure (Figure 3 B). On the basis of the brightness distribution of the 2D images, we measured the change of intensities during the rotation (α). As shown in Figure 3 C, two peaks were observed every 180°. It is considered that almost all mesochannels in the film are uniaxially oriented along the rubbing direction. The FWHM value (full width at half maximum) was less than 25°. In the case of the Brij 56 system (Figure 3 B-2), the FWHM value was less than 12°. A) Schematic representation of SAXS measurement with the perpendicular irradiation of X-ray beam to the film surface. The X-ray beam passed through the whole thickness of the film. The 2D patterns were taken on a Rigaku MicroMax-007HF diffractometer with CuKα radiation. B) 2D patterns of as-prepared mesostructured silica films prepared with B-1) P123 and B-2) Brij 56. C) Relationship between the angle of rotation (α) and the intensities. D) 2D patterns of multilayer film with the different orientations of mesochannels prepared with P123. We consider that the orientation of the 1D mesochannels was induced by the strong rubbing motion. The precursor solution before the rubbing shows liquid crystals with highly concentrated surfactants. The pre-organized mesophase had a 2D hexagonal mesophase (p6mm), as is confirmed by a conventional θ–2θ XRD experiment and polarized optical microscopy (see the Experimental Section.). The alignment of the anisotropic liquid-crystalline materials or molecules can be induced by the application of a variety of external fields such as shear fields.49 The rubbing and its direction itself leads to the rapid motion of the precursor solution in a uniaxial direction on the substrate. Then, the tubular-shaped micelles are anisotropically arranged along the rubbing direction. As the rotation speed of the rubbing rotor is gradually increased, the FWHM value can be reduced to around 10°. The present rubbing method is not applicable to the conventional precursor solutions (with low concentration of surfactants) which have been used for making mesoporous films by a solvent evaporation process. When the conventional precursor solution with low-concentrated surfactants was used, any uniaxial orientation of the mesochannels could not be confirmed. In such cases, the surfactants dissolved in the precursor solution were isolated as monomers, and were not well organized as anisotropic micelles (e.g., tubular-shaped), since the surfactant concentration is below CMC (critical micelle concentration). Therefore, the rubbing motion was not effective for inducing the orientation of the mesochannels. Furthermore, we can produce multilayer film with different orientations of mesochannels in different layers by simply controlling the rubbing direction. In order to prepare such a film, the precursor solutions were placed once again on top of the film and rubbed perpendicularly to the first layer film. The intersection angle (β) can be changed by controlling the direction for the second rubbing coating (Figure 3 D, and Figure S3 in the Supporting Information). Multilayer films consisting of more than two layers were also achieved (Figure S3 c in the Supporting Information). In conclusion, we developed the new strategy for orientation controls of mesochannels in films, in which a pre-organized lyotropic liquid crystal is used as a precursor solution. We can apply our concept to many types of nonionic surfactants which have been known to have lyotropic liquid crystal behavior at high surfactant concentrations.50–53 Also, not only mesoporous silica but also other mesoporous metal oxides and pure metals will be produced.54–60 Thus, this method will demonstrate wide applicability in tuning the mesopore size and the framework compositions over a wide range scale in the near future. Tetraethoxyorthosilicate (TEOS) as a silica source was purchased from Sigma–Aldrich. Ethanol was purchased from Nacalai Tesque and 0.1 M hydrochloric acid (HCl) was purchased from Wako Chemicals, Tokyo. In a typical preparation of the P123-based precursor solution, TEOS (21 g) was added with ethanol (50 g) and 0.1 M HCl (9 g). After stirring for about 2 h, a mixture of Pluronic P123 (8.5 g) and ethanol (30 g) was added to the resultant solution. The mixture was stirred over 12 h to form the liquid crystal phase with high viscosity. The highly viscous liquid solution with a pre-organized mesophase was obtained. The pre-organized mesophase had a 2D hexagonal mesophase (p6mm), as is confirmed by a conventional θ–2θ XRD experiment and polarized optical microscopy. In a typical preparation of Brij 56 based precursor solution, TEOS (5.4 g) was added with ethanol (6.0 g) and 0.5 M hydrochloric acid (3.0 g). After stirring for 1 h, a mixture of Brij 56 (3.0 g) and ethanol (7.5 g) was added. The resultant mixture was stirred over 7 h at room temperature to attain the liquid crystal phase with a 2D hexagonal structure. For making mesoporous films, the precursor solutions were coated on substrates by a rubbing machine at room temperature (see Figure 1). Before using the glass substrates, they were washed by acetone and ethanol. Any special pretreatment of the substrates was not needed at all. After drying, the as-prepared films were calcined for 3 h at 350 °C. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
• W1976684431 created "2016-06-24" @default.
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• W1976684431 date "2010-05-28" @default.
• W1976684431 modified "2023-10-18" @default.
• W1976684431 title "Precise Manipulation of One-Dimensional Mesochannel Alignments in Mesoporous Silica Films by Novel Rubbing Method Utilizing Lyotropic Liquid Crystals" @default.
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