Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
Vapor phase synthesis of mesoporous silica thin films with a 3D pore structure Takanori Maruo,a Kaori Nagata,a Norikazu Nishiyama,a Yasuyuki Egashira,a Korekazu Ueyama,a Christopher P. Muzzillo,b Michael P. Tate,b Hugh W. Hillhouseb a
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531, Japan b School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana, 47907, USA
Abstract Ordered mesoporous silica thin films were prepared using non-ionic alkyl poly(oxyethylene) surfactants (Brij56: C18EO10) by a vapor phase method. First, a Brij56/H2SO4 composite was deposited on a silicon substrate by spin-coating. The Brij56 film was treated with a tetraethoxysilane (TEOS) and hydrochloric acid (HCl) vapors in a closed vessel. The TEOS and HCl vapors were infiltrated into the film, resulted in a formation of the silica network. Results of a grazing angle of incidence small angle X-ray scattering (GISAXS) show that the films have an ordered structure with an Fmmm symmetry. From an Field emission scanning electron microscope (FESEM) observations, the film has a 3D pore structure. Keywords: mesoporous silica film, vapor phase synthesis, 3D pore structure.
1. Introduction Ordered mesoporous thin films have attracted much attention as promising materials for chemical sensors, low-k films, low-index films and separation. The ordered mesoporous films have been prepared mainly by dip-coating and spin-coating methods. However, the thermal stability of the films obtained by the so-called solvent evaporation method is insufficient although the procedures are very simple. Recently, we have developed a vapor phase synthesis method for a preparation of mesoporous silica thin films. In this synthesis, a silica/surfactant self-assembly is formed during the infiltration of a tetraethoxysilane (TEOS) vapor into the surfactant films. Silica films with 1D channel-like pores parallel to the film surface1 and with 2D cage-like pores2-3 have been reported so far. In this study, we prepared a mesoporous silica film with a 3D pore structure by the vapor phase method at low temperature. The structure of the films was investigated using a X-ray diffraction (XRD), an field emission scanning electron microscope (FESEM), a transmission electron microscope (TEM) and a 2D grazing angle of incidence small angle X-ray scattering (GISAXS).
2. Experimental 2.1. Synthesis A mixture of Brij56 surfactant, sulfuric acid, ethanol and water was deposited on a silicon substrate by spin-coating. The mole ratios of the solution were 0.16 Brij56: 0.9 H2SO4: 50 EtOH: 100 H2O. As shown in Fig. 1, the Brij56/H2SO4 composite film was placed vertically in a Teflon-lined stainless steel vessel (50 cm3) with small amount of
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TEOS and HCl (5N). The vessel was placed in an oven at 30°C for 10 min. The composite film was exposed with saturated TEOS and HCl vapors. Calcination was performed in air at 400°C for 5 h with a heating rate of 1°C /min to remove Brij56.
template film with H2SO4 HCl(5N) TEOS
Figure 1. A schematic illustration of the vapor phase synthesis of silica films.main text 2.2. Characterization The films were identified by XRD patterns recorded on a Rigaku Mini-flex using CuK radiation with k =1.5418 Å in the -2 Bragg–Brentano scan mode. FE-SEM images were recorded on a Hitachi S-5000L microscope at an acceleration voltage of 18 kV and TEM images were recorded on an FEI Tecnai 20 at 200 kV. Two dimensional GISAXS measurements were performed using a three-pinhole SAXS camera with a microfocus X-ray source, an Osmic MaxFlux confocal X-ray optic, and a gas-filled 2D multiwire detector at a camera length of 1424 mm. The scattering patterns were calibrated in q-space using an isotropic silver behenate powder standard. Due to the presence of an intense specular beam at grazing angles of incidence, aluminum strips were used to attenuate the scattering along the specular plane, blocking the region of -0.05 < qz < 0.2 Å–1 and -0.015 < qx < 0.015 Å–1. The measured scattering patterns were simulated using NANOCELL, a Mathematica-based program4. The predicted spot patterns from NANOCELL are overlaid on the GISAXS data to determine the structure, lattice constants, and orientation.
3. Results and discussion 3.1. Formation of Mesostructured Silica Films by vapor infiltration The XRD patterns of the mesostructured silica films by the vapor infiltration method were shown in Fig. 2. The spin-coated film of Brij56/H2SO4 already has a hexagonal structure before the vapor infiltration treatment. On the other hand, we did not find an ordered structure in Brij56 films without H2SO4 deposited on the Si substrate. We consider that a large amount of H3O+ and SO42– could adsorb on the hydrophilic head group of Brij56 molecules. The bulky head of surfactants caused an arrangement of the Brij56/H2SO4 composite into a hexagonal symmetry. A similar phase transformation of a composite of H2SO4 and cethyltrimethylammonium bromide (C16TAB) had been reported so far5. The reflection peaks of the mesostructured films treated with TEOS and HCl vapors were slightly shifted to lower angle. The d-spacing was increased from 5.8 nm to 6.3 nm. This result indicates that partially-hydrolyzed TEOS molecules penetrate into a hydrophilic H3O+ and SO42– parts around the hydrophilic head groups of Brij56, resulting in an expansion of the periodic distance. The hexagonal structure of the Brij56/H2SO4 composite was retained after the infiltration of TEOS vapor. The periodic distance shrunk from 6.3 nm to 5.0 nm in a direction perpendicular to the surface of the
Vapor phase synthesis of mesoporous silica thin films with a 3D pore structure
substrate after calcination. A complete removal of Brij56 and H2SO4 were confirmed by FTIR and EDX measurements. Fig. 3(a) shows the FE-SEM image of the Si surface where the film was peeled off. Mesopores can be observed in the direction perpendicular to the Si substrate. Mesopores can be also observed on the cross-section of the films, indicating that the mesopores run 3-dimensionally in the film. From the TEM image shown in Fig. 3 (b), the pore size of the film was about 2-3 nm in accord with the FE-SEM image.
Brij56/H2SO4 composite film
2 theta [degree] Figure 2. XRD patterns of mesostructured silica films. (b) (a)
100 nm Figure 3. (a) FE-SEM and (b) TEM images of the mesostructured silica films after calcination.
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3.2. Structure of the Mesoporous Silica Films Fig 3 shows the 2D GISAXS patterns of the mesoporous silica films after calcination. Some spots can be observed in the GISAXS patterns, indicating that the nanostructured films were highly oriented relative to the plane of the substrate. The patterns fit extremely well with an Fmmm space group oriented with the (010) direction parallel to the surface (a = 7.4 nm, b = 10.5 nm, c = 12.2 nm, and = = = 90°).
Figure 4. GISAXS pattern of the mesoporous silica film.
4. Conclusion Ordered mesoporous silica films were prepared by the vapor infiltration method. The mesoporous silica film had an Fmmm structure. The pores of this film run both parallel and perpendicular to the film surface. This film can be promising material as a separation membrane.
Acknowledgments This study was supported by Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan (No. 18360374). We acknowledge the GHAS laboratory and Mr. M. Kawashima at Osaka University for the FESEM measurements. T. Maruo acknowledges the global center of excellence (GCOE) program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University.
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