Langmuir 1998, 14, 3041-3044
3041
Porous Silica of Self-Similar Morphology K. Aikawa and K. Kaneko* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan
M. Fujitsu, T. Tamura, and K. Ohbu Material Science Research Center, Lion Corporation, 7-13-12, Hirai, Edogawa-ku, Tokyo, 132, Japan Received September 5, 1997. In Final Form: February 9, 1998 New mesoporous silica was prepared using the template of the bicontinuous layered structure from the bicontinuous phase of poly(oxyethylene) dodecyl ether, H2O, and isooctane and the aqueous phase of 6 N HCl and teraethoxysilane. The synthesized silica was characterized by scanning electron microscopy (SEM) observation, X-ray diffraction, small angle X-ray scattering (SAXS), mercury porosimetry, and N2 adsorption at 77 K. The structure had a fractal nature of which cluster fractal dimensions are 1.7 and 1.83 from SEM observation and SAXS, respectively. The silica structure was noncrystalline like MCM-41. The prepared silica was highly porous and had a wide range pores from ultramicropores to macropores.
1. Introduction Recent chemistry on microporous and mesoporous materials has gathered much attention.1-4 Anomalous structures and properties of nanodimensional molecular assemblies of H2O,5 NO,6 SO2,7 and O28 were reported and thereby nanoporous materials have provided a good system for molecular science. Nanodimensional materials have been synthesized using micropores or mesopores.9 New mesoporous materials of the regular structure such as mesoporous silica (MCM4110,11 or FSM12) or carbon nanotubes13 have been applied to catalysis, molecular assembly, molecular recognition, separation, and mesoscopic physics. Although the pore walls of mesoporous silica prepared by the templating technique are noncrystalline, the long-range periodical pore structure gives sharp X-ray diffraction patterns. The regular porous silica is indispensable to develop the fundamental understanding of molecular processes in pores and chemistry of low dimensional systems. There are many important porous systems having the wide pore size distribution in nature. However, we cannot sufficiently control and describe quantitatively the pore (1) Kaneko, K. J. Membr. Sci. 1994, 96, 59. (2) Pinnavaia, T. J.; Thorpe, M. F. Access in Nanoporous Materials: Plenum Press: New York, 1995. (3) Tolbert, S. H.; Sieger, P.; Stucky, G. D.; Aubin, S. M. J.; Wu, Chi-Cheng; Hendrickson, D. N. J. Am. Chem. Soc. 1997, 119, 8652. (4) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. (5) Iiyama, T.; Nishikawa, K.; Otowa, T.; Kaneko, K. J. Phys. Chem. 1995, 99, 10075. (6) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J. Chem. Phys. 1987, 87, 776. (7) Wang, Zen M.; Kaneko, K. J. Phys. Chem. 1995, 99, 16714. (8) Kanoh, H.; Kaneko, K. J. Phys. Chem. 1996, 100, 755. (9) Yamamoto, T.; Shido, T.; Ichikawa, M. J. Am. Chem. Soc. 1996, 118, 5810. (10) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonoaicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Oison, D. H.; Sheppard, E. W.; McCulen, S. W.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (11) Gusev, V. Y.; Feng, X.; Bu, Z.; Haller, G. L.; O’Brien, J. A. J. Phys. Chem. 1996, 100, 1985. (12) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (13) Iijima, S. Nature 1991, 354, 56.
size distribution. If we can design the structure of porous materials using a mathematical rule, we will get a key to understand the irregular structure of important substances in nature. Fractal has offered one of the hopeful methods to describe the irregular structure of materials.14,15 On the other hand, chemistry on assemblies of surfactant molecules provides a variety of possibilities for templating for designing porous solids.2,4,16 Although a liquid crystalline structure provides the regular porous solids such as mesoporous silica, the bicontinuous layered structure could be available for design of other structural porous solids. A new porous silica having both of selfsimilarity and a wide-range pore size distribution, which was synthesized using the bicontinuous structure of surfactant molecules as the template,17-19 is described in this paper. 2. Experimental Section We used the nonionic surfactant, poly(oxyethylene) dodecyl ether, H2O, and isooctane to prepare the bicontinuous phase. We examined the stable bicontinuous phase region in the presence of tetraethoxysilane (TEOS). At 303 K the used composition of the aqueous phase was H2O/TEOS/6 N HCl ) 82.6:10.0:3.2 in wt ratio. The produced gel was dried by supercritical CO2 drying and it was heated at 773 K for 5 h in an N2 atmosphere, firing at 773 K for 4 h in air. The morphology of the prepared sample was examined by scanning electron microscopy (SEM) (S-520, Hitachi). The powdered X-ray diffraction was measured by an automatic diffractometer (MXP3, MAC Science) using monochromatic X-ray from Mo KR radiation at 50 kV and 35 mA. Small-angle X-ray scattering (SAXS) spectra were measured by use of a two-axial three-slit system (Mac Sci. Model No. 3310) (14) Pfeifer, P.; Avnir, D. J. Chem. Phys. 1983, 79, 3566. (15) Avnir, D., Ed. The Fractal Approach to Heterogeneous Chemistry; John Wiley & Sons: Chichester, 1989. (16) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature, 1994, 368, 317. (17) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (18) Sjoblom, J.; Lindberg, R.; Friberg, S. E. Adv. Colloid Interface Sci. 1996, 95, 125. (19) Olsson, U.; Wennerstro¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113.
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Figure 1. SEM micrographs of the porous silica. with Cu KR radiation and a linear-type position-sensitive proportional counter. The scattering parameter s ranging from
0.008 to 0.6 Å-1 was covered. The detailed description on the SAXS measurement was given in the previous artilce.20 The N2
Porous Silica of Self-Similar Morphology
Langmuir, Vol. 14, No. 11, 1998 3043
Figure 2. The N2 adsorption isotherm of fractal silica: O, adsorption; b, desorption. adsorption isotherm was measured gravimetrically at 77 K after preheating at 373 K and 10 mPa for 2 h. The macropore structure was determined by the mercury porosimetry (Poresiza 9320, Shimazu).
3. Results and Discussion Figure 1 shows SEM micrographs of the porous silica. The SEM micrographs of different magnifications are shown on Figure 1. Each figure shows the rolled layer structure of the stacked sheets with many pores of different scales. Three different figures show a similar structure regardless of the different magnifications. That is, the silica samples have widely distributed pores from 50 nm to 1 µm from the SEM observation. We applied the box counting method to determine the fractal dimension on the two-dimensional SEM picture. We divided the similar object into small cubes and the number (nc) of cubes covering the object was determined for each magnified picture. The logarithm of nc against the logarithm of the real length of the cube was plotted, giving the fractal dimension. The obtained fractal dimension (cluster fractal dimension Dc) of the projected picture was 1.7. However, this SEM observation cannot elucidate the presence of mesopores and micropores. The SAXS spectrum was analyzed by the logarithm of the scattered intensity I(s) vs s, which gave a linear plot in the s range of 8 × 10-3 to 1 × 10-1. Although the linear range was narrow, we determined the cluster fractal dimension from the slope of the I(s) vs s. The Dc was 1.83, being close the value obtained by the SEM observation. As the SAXS covers the scale of 1-10 nm, the frame structure of this porous silica must have a fractal nature in the range of 1 nm to 1 µm from the SEM and SAXS examinations. The XRD patterns had a broad peak at 10°, which is the reflection from 100 planes. The crystallite size from the 100 plane peak was only 1 nm. Therefore, the pore-wall structure is noncrystalline as well as MCM41. Hence the prepared porous silica is a noncrystalline material having the fractal nature. The above results indicate that the frame structure of the prepared silica has a fractal nature and great porosity. The N2 adsorption isotherm at 77 K can give the pore size distribution. Figure 2 shows the N2 adsorption isotherm (20) Ruike, M.; Kasu, T.; Setoyama, N.; Suzuki, T.; Kaneko, K. J. Phys. Chem. 1994, 98, 9594.
Figure 3. Rs-plots of fractal silica and MCM-41: b, fractal silica; O, MCM-41.
at 77 K. Here, the abscissa of Figure 2B is expressed in terms of log(P/P0), because the adsorption uptake at low pressure is substantial. The significant adsorption below P/P0 ) 0.1 indicates the presence of micropores, because the molecule-pore wall interactions from the opposite pore walls are overlapped to produce the deep molecular potential well for an adsorbate molecule. The N2 adsorption isotherm has the hysteresis of type H3, suggesting the presence of slit-shaped mesopores,21 which agrees well with the stacking sheet-structure from the SEM observation. The hysteresis commences near P/P0 ) 0.42, where the meniscus of liquid N2 becomes unstable. As P/P0 ) 0.40 corresponds to the pore width of 2 nm through the Kelvin equation, this sample should have micropores. The adsorption isotherm was analyzed by the high-resolution Rs-plot using the standard adsorption data on the nonporous silica in the literature.22 This Rs-plot was compared with that of MCM41,23 as shown in Figure 3. The Rs-plot (21) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 287. (22) Bhambhani, M. R.; Cutting, P. A.; Sing, K. S. W.; Turk, D. H. J. Colloid Interface Sci. 1972, 38, 109.
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Table 1. Pore Structure
macropores meso- and micropores
pore volume (mL g-1)
surface area (m2 g-1)
2.68 0.24
210 384
of MCM41 has a linear rise from the origin to the vertical jump at Rs ) 0.8 corresponding to the capillary condensation in the regular mesopores of 3.3 nm in width. The Rs-plot of the porous silica of the self-similarity has no linear region over the whole Rs-region, indicating a wide distribution of micropores and mesopores. Table 1 shows the pore volume and surface area. Here, the pore volume and surface area of macropores were determined by mercury porosimetry. The macropore volume is larger than the sum of micropore and mesopore volumes. On the other hand, the surface area of meso- and micropores is greater than that of macropores. The mesopore size distribution can be evaluated from the adsorption branch with the well-known Dollimore-Heal method for slitshaped pores.24 Figure 4 shows the mesopore size distribution. The pore size distribution is widely spread in the mesopore range. Avnir and Pfeifer showed that the pore size distribution is associated with the fractal dimension,14 as given by eq 1.
-
dVP ∝ r2-Ds dr
(1)
Here VP is the pore volume and r is the half pore width. Ds is the surface fractal dimension. The log(-dVP/dr) vs log r plot gave a linear relation over 2 orders of the magnitude. The obtained Ds was 3.2, indicating a highly porous structure; pores develop three-dimensionally in (23) Setoyama, N.; Inoue, S.; Hanzawa, Y.; Branton, P. J.; Kaneko, K.; Pekala, R. W.; Dresselhause, M. S.; Sing, K. S. W. Characterisation of Porous Solids 1997, 117. (24) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109.
Figure 4. Mesopore size distribution of fractal silica.
the prepared silica. Then the templating synthesis using the bicontinuous structure of a surfactant molecular assembly gives the highly porous silica of which structure has the fractal nature in the wide range of 1 nm to 1 µm at least. This artificial fractal pore structure should be helpful to understand the complex materials in nature. At the same time, the structure of silica indicates the bicontinuous structure in the liquid state, although we must be cautious to get an explicit conclusion. Acknowledgment. This work was funded in part by a Grant-in-Aid or Scientific Research on Priority Areas No. 288 from the Japanese Government. LA9710031
