0 Copyright 1995 American Chemical Society
MARCH 1995 VOLUME 11, NUMBER 3
Letters Effect of Colloidal Particles on the Formation of Ordered Mesoporous Materials Jun Liu,* Anthony Y. Kim, Jud W. Virden, and Bruce C . Bunker Pacific Northwest Laboratory,? Battelle Boulevard, Box 999, Richland, Washington 99352 Received August 31, 1994. In Final Form: November 11, 1994@ This research investigates the effect of colloidal particles on the preparation of ordered mesoporous materials (M41S). It demonstrates that colloidalsilica and colloidaltitania particles promote the formation ofthese materials. Under similar conditions no ordered structure is observedwithout the colloidal particles. A heterogeneous nucleation mechanism is proposed based on these observations and is supported by direct evidence from transmission electron microscopy (TEM).
Introduction The recent discovery of the M41S series of mesoporous materials has greatly expanded the prospect of designing open-structured inorganic materials.1,2 The new mesoporous materials are characterized by their ordered porosity, narrow pore size distribution, and adjustability of the pore size from 1.5 to 20 nm. Formation mechanisms for the M41S materials are of considerable interest in the literature. Currently the formation of these mesophase materials is attributed to the "templating" effect of the surfactant molecules used in the synthesis. Two possible routes were considered by Beck et a1.'g2 The first one assumes that the surfactant liquid crystal phase formed in the aqueous solution directs the growth of the inorganic materials. The second model suggests that the addition of the soluble silicate species assists the ordering of the surfactant micelles around which the silicate then forms. Monnier et al. later proposed ~~
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t Pacific Northwest Laboratory is operated by Battelle Memorial
Institute for the U.S.Department of Energy under Contract DEAC06-76RLO 1830. Abstract published in Advance ACS Abstracts, February 15, 1995. (1)Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz,M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker,J. L. J.Am. Chem. SOC.1992, 114, 10834. (2)Kresge, C.T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992,359,710. @
a cooperative mode for the synthesis of me so structure^.^ They suggested that three processes were involved in mesostructure formation: multidentate binding of silicate oligomers to the cationic surfactant, preferred silicate polymerization in the interfacial region, and charge density matching between the surfactant and the silicate. One issue that has not been addressed in the literature is the significance of a pre-existing solid phase in the solution. Similar to the synthesis of other zeolite materimost of the preparation methods of mesophase materials involve a heterogeneous mixture of several components, including colloidal silica particles. The question is whether these particles play a major role other than providing an alternative silicate source. It will be shown that colloidal silica particles do play a critical role: the presence of the colloidal silica, even a very small amount, will promote the formation of the ordered phase, but no ordering occurs without the colloidal particles under similar conditions. Further study shows that an inert material (titania particles) also has a similar effect as colloidal silica. These observations, together with direct evidence of the nucleation process from TEM, can be (3) Monnier, A.; Schuth, F.; Huo, Q.;Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A,; Janicke, M.; Chmelka, B. F. Science 1993,261,1299. (4)Barrer, R.M. In Zeolites, Synthesis, Structure, Technology and Application; Drhj, B., HoEevar, S., Pejovnik, S., Eds.; Elsevier: New York, 1985; p 1. (5)Kessler, H. In Recent Advances in Zeolite Science; Klinowski,J., Barrie, P. J., Eds.; Elsevier: New York, 1989; p 17.
0743-746319512411-0689$09.00/0 0 1995 American Chemical Society
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Figure 1. TEM images of the materials prepared with different amounts of colloidal silica: (a) disordered structure without colloidal silica: (b) ordered structure with one-tenth of the colloidal silica compared to the regular composition; (c) ordered structure with regular amount of colloidal silica.
explained by a heterogeneous nucleation mechanism. While this paper does not imply that ordered mesoporous materials cannot form without colloidal particles under all conditions, it proves that heterogeneous nucleation is a viable route to prepare such materials. This mechanism has been explored extensively in biomimetic processing of ceramic^^?^ and thin films.8 Therefore it is hoped that this study will open up new possibilities for studying growth kinetics and structural control.
Experimental Approach The materials were prepared using methods and chemicals similar to those reported by Beck et u Z . ~ . ~ The chemicals used include cetyltrimethylammonium chloride (CTAC, Kodak), sodium aluminate (54%Al2O3,41% NazO, EM Science),Hi-Si1233 as colloidalsilica (aggregates of 20 nm amorphous silica particles, PPG Industries), and tetramethylammonium silicate solution (TMA)(10%SiO2,0.5TMA+/Si,SACHEM, Inc.). CTAC solutions were prepared using deionized (DI) water. The chloride of a separate CTAC solution was exchanged with hydroxide by ion exchange (Dowex-SBR, Dow Chemical). The exchanged and unexchanged CTAC solutions were combined to give an effective chloride exchange. A typical composition is 6.416 g of CTAC, 2.786 g of CTAC/OH, 0.190 g of sodium aluminate, 1.127 g of Hi-Si1 233, and 4.593 g of TMA. The solution was mixed and placed in a small, sealed, Teflon-linedhydrothermal reactor and reacted at 105 "C. The reaction products were quenched with 25 "C DI water. Next, the whole suspension was run through a vacuum filter and washed with additional DI water to yield a whitish powder. To remove the organic materials, the sample was calcined at 540 "C first in flowing nitrogen for 1h and then in flowing air for 6 h. Similar experimentswere conducted using titania (Degussa P25) as colloidal particles in place of Hi-Si1 silica. X-ray photoelectron spectroscopy (XPS) confirmed that these titania powders were not previously contaminated with silica and other impurities. Samples for transmission electron microscopy (TEM) were prepared by ultramicrotomy. A small amount of powder was embedded into epoxy resin consisting of 5 parts (by weight) Buehler Epo-Kwick Resin (No. 20-8128-032)and 1part Buehler (6)Calvert, P.D.; Mann, S. J. Muter. Sci. 1988,23,3801. (7)Mann, S.;Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993,261,1286. (8)Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.;Virden, J. W.; McVay, G. L. Science 1994,264,48.
A.
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No Colloidal Silica
2-Theta
Figure 2. Diffraction patterns of the materials prepared with different amount of colloidal silica: (A) disordered structure without colloidal silica; (B) hexagonal structure with one-tenth of the colloidal silica compared with C; (C) hexagonal structure with the full recipe. Epo-Kwick Hardener (No. 20-8128-008). The epoxy resin was cured for 24 h at room temperature. Microtomy was conducted on an NT 6000 Ultramicrotome (Sorval)with a Microstar diamond knife. TEM characterization was performed on a Philips 400 or Jeol 1200 microscope at 120 kV. Low-angle X-ray diffraction (XRD) was performed on a Philips Type 150 100 00 wide range goniometer. The X-ray source was a fmed anode LFF Cu tube operated a t 40 kV/45 mA with a Philips XRG 3100 X-ray generator. The TEM and XRD results reported here were obtained from samples that had not been calcined a t 540 "C. Unlike some real zeolites, the ordered structures in the mesoporous materials are formed in the solution. The ordered porosity is derived from the ordered surfactant, but the ceramic materials themselves do not have long range ordering. TEM and XRD indicated that calcining a t 540 "C in air or nitrogen atmosphere did not cause further crystallization. In this paper the calcined samples are called mesoporous materials and the uncalcined samples are sometimes referred to as mesophase materials because they still contain the surfactant molecules.
Results and Discussion
TEM (Figure 1) and XRD (Figure 2) experiments illustrate that colloidal silica promotes the formation of
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Figure 3. TEM images of the microstructure at different times of reaction: (a) silica aggregates 10 min after the reaction is initiated, some mesophase grains begin to appear on the edge of the aggregate, as indicated by the arrows;(b)mesostructuresgrown around the colloidal aggregates 2 h after the reaction is started.
ordered structures. Figure l a shows the disordered structure resulting from a sample without colloidal silica. Similar random structures are observed when the colloidal silica is replaced with another precursor silicate, such as tetraethoxysilane (TEOS). In either case, the diffraction pattern reveals an amorphous structure (Figure 2a). Figure l b shows that only a small amount of colloidal silica, one-tenth of the Hi-Si1 of the full recipe, causes the ordered structure to form. Figure 2b shows the corresponding diffraction pattern, which has well-defined hexagonal reflections. The ordered structure in Figure l b is similar to that in Figure IC,which resulted from the materials using the complete composition developed by Beck et u Z . ~ * ~This full recipe gives a hexagonal structure (Figure 2C) similar to that reported in literature, with a (100) d spacing of 4 nm. Note that spectra B and C of Figure 2 have essentially the same structure, but Figure 2C is more ordered, indicatingthat a higher colloidal silica concentration promotes ordering. These results confirm that colloidal silica has a drastic effect on the structures that finally form. In order to further understand the role of colloidal silica, TEM images of samples at different reaction times at 105 “C were obtained. Although the reaction throughout the solution is not homogeneous, and some disordered structures persist to the end of the reaction, the TEM study nevertheless illustrates the general trend and provides insight into the early growth process. Figure 3 shows some typical structures during the reaction. Figure 3a shows an aggregate of colloidal silica 10 min after the reaction is initiated. Evident in Figure 3a are dark grain structures that appear on the edge of the aggregate, as indicated by the arrows. High-magnificationTEM reveals that these grains are the ordered hexagonal phase. Figure 3b clearly illustrates that the extended mesostructures form around colloidal silica particles 2 h after the reaction starts. At the center is the colloidal silica aggregate,which is surrounded by the smooth grains of the ordered hexagonal phase. Figure 4 shows a typical grain structure
d
Figure 4. Mesophase grain after 24 h of reaction with a few residual silica particles left, as indicated by the arrows.
after 24 h of reaction. Almost all the structure is ordered now, except for some residual silica particles, indicated by the arrows. From these observations it can be concluded that colloidal silica promotes the formation of mesophases by providing heterogeneous nucleation sites. The ordered structure first forms on the edge of silica aggregates and then grows toward the interior of the aggregates. The colloidal particles are partially consumed through a dissolution process, which provides additional soluble silicatespecies. At the end of the reaction a small amount of unconsumed silica particles are left and remain at the center of the aggregates, leaving a less dense region at the center of the mesograin due to the porosity in the aggregates. Such morphology is supported by scanning electron microscopyimages,2where an increased porosity is clearly visible at the center of most of the grains. This
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derived from Figure 3, which captures the early nucleation stage on the edge of colloidal silica aggregates. The results reported in this paper are not totally surprising. It has been long speculated that colloidal particles played an important role in the evolution of complex molecules and structures from simple ones,ll and new evidences have been reported.12 One possible mechanism will be through the adsorption and coadsorption of the organic molecule and the ceramic precursors. The adsorption and coadsorption process will introduce local structures, such as bilayers, micelles, and other higher order surfactant aggregates13J4 and change the local chemistry on the particle surface. It is not difficult to imagine that these regions will favor the nucleation of ordered mesophase structures.
Figure 5. TEM images of the ordered phase seeded by colloidal titania. The dark particles are titania.
process will give rise to an apparent inward growth pattern because the colloidal silica inside the aggregates plays a dual role, providing both nucleation sites and additional soluble silica that allows the hexagonal mesophase materials to form. Currently the exact mechanism through which the colloidal silica promotes the growth of mesophase materials is not fully understood. Because colloidal silica is soluble under the experimental conditions (pH = and silicate itself is an active ingredient in the reaction, the addition of colloidal silica could affect the silicate concentration, the ionic strength, chemical activity, and the phase behavior of the surfactant. To eliminate such complicating factors, colloidal titania particles were used to replace the colloidal silica. Since the solubility of titania is many orders of magnitude lower than that of silica,1° the addition of titania particles will not significantly affect the chemistry of the solution. Figure 5 shows that titania particles can be used to seed the growth of ordered structures. In the TEM micrograph the titania particles themselves appear to be intact, and electron energy disperse spectroscopy (EDS) indicates that titanium is not incorporated in the ordered region. Therefore the most likely role the titania particles played is to provide nucleation seeds. This is consistent with the conclusion (9) Baes, C. F.,Jr.; Mesmer, R. E. The Hydrolysis ofCutions; John Wiley & Sons: New York, 1976; Chapter 15. (10)Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis ofCutions;John Wiley & Sons: New York, 1976; Chapter 8.
Summary This study addresses one important aspect of the synthesis of mesoporous materials: the effect of a preexisting heterogeneous phase. On the basis of experimental observations and direct TEM evidence a heterogenous nucleation mechanism is proposed. Colloidal particles promote the formation of mesophases; the mesostructure does not nucleate without the colloidal particles under similar conditions. However,this research does not rule out any of the theories discussed in the introduction of this paper, nor does this paper conclude that seeding with colloidal particles is the only route to prepare such materials. Similar to growing regular crystalline materials, whether heterogeneous or homogeneous nucleation occurs depends upon which mechanism is energetically and kinetically favored under the experimental conditions. Heterogenous nucleation does have some advantage: it allows the crystals to form in a more controlled fashion. This is desirable under some conditions for both fundamental studies and practical applications. It can provide favorable conditions to study the interactions between the organic and inorganic components and can offer additional means to modify the morphology and the structure of the final product. In addition to the extensive literature in biomimetic processing of ceramic materials,6-8 this mechanism is also utilized to grow oriented zeolite ~rysta1s.l~ Therefore heterogenous nucleation of mesophase material is a subject worth further investigation.
Acknowledgment. The authors acknowledge the professional help from R. R. Adee for microtoming some of the samples for TEM study. This research is supported by the U.S. Department of Energy under Contract DEAC06-76RLO 1830. LA940685E ~~
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(11) Iler, R. K.The Chemistry of Silica; John Wiley & Sons: New York, 1979; Chapter 7. (12) Maddox, J. Nature 1994,371,101. (13) Bijsterbosch, B.H.J . Colloid Interface Sci. 1973,47,186. (14) Soderlind, E.;Stilbs, P. Langmuir 1993,9, 2024. (15) Feng, S.;Bein, T. Nature 1994,368,834.
