Combined Emulsion and Solvent Evaporation (ESE) Synthesis

Langmuir 2007, 23, 1459-1464

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Combined Emulsion and Solvent Evaporation (ESE) Synthesis Route to Well-Ordered Mesoporous Materials Nina Andersson,† Bengt Kronberg,‡ Robert Corkery,§ and Peter Alberius* YKI, Institute for Surface Chemistry, Box 5607, Stockholm 11486, Sweden ReceiVed July 28, 2006. In Final Form: NoVember 2, 2006 Control over morphology and internal mesostructure of surfactant templated silicas remains a challenge, especially when considering scaling laboratory syntheses up to industrial volumes. Here we report a method combining emulsification with the evaporation-induced self-assembly (EISA) method for preparing spherical, mesoporous silica particles. This emulsion and solvent evaporation (ESE) method has several potential advantages over classic precipitation routes: it is easily scaled while providing superior control over stoichiometric homogeneity of templating surfactants and inorganic precursors, and particle sizes and distributions are determined by principles developed for manipulating droplet sizes within water-in-oil emulsions. To demonstrate the method, triblock copolymer P104 is used as a templating amphiphile, generating unusually well-ordered 2D hexagonal (P6mm) mesoporous silica, while particle sizes and morphologies were controlled by varying the type of emulsifier and the method for emulsification.

Introduction Surfactant-templated, porous inorganic materials have proven to be potentially advantageous in numerous applications spanning from sensors, catalysis, controlled release, chromatography, and separations.1-4 While control over pore structure and size, as well as particle size and morphology, strongly influences the efficiency of the material, independent tailoring of these properties is still challenging despite the enormous efforts over the past decade in optimizing different routes for mesoporous materials preparation.5-7 Crystallographically well-ordered mesostructured materials contain surfactant molecules and inorganic species self-assembled into composites with well-ordered organic and inorganic domains separated on a nanometer scale. Calcination of these leads to mesoporous materials via removal of their templates. These can be prepared either by solution precipitation routes8,9 or by solvent evaporation induced self-assembly (EISA).10,11 The EISA route combines sol-gel processing of inorganic oxides with surfactant self-assembly, resulting in lyotropic liquid crystalline structures incorporating the inorganic moieties. For the EISA route, a number of groups have used water-surfactant phase diagrams to guide * Corresponding author. Telephone: +4650106021. Fax: +468208998. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail: [email protected]. § E-mail: [email protected]. (1) Yamada, T.; Zhou, H. S.; Uchida, H.; Tomita, M.; Ueno, Y.; Honma, I.; Asai, K.; Katsube, T. Microporous Mesoporous Mater. 2002, 54 (3), 269-276. (2) Stein, A. AdV. Mater. (Weinheim, Germany) 2003, 15 (10), 763-775. (3) Taguchi, A.; Schueth, F. Microporous Mesoporous Mater. 2004, 77 (1), 1-45. (4) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science (Washington, D. C.) 1997, 276 (5314), 923-926. (5) Palmqvist, A. E. C. Curr. Opin. Colloid Interface Sci. 2003, 8 (2), 145155. (6) Cool, P.; Vansant, E. F.; Collart, O. Inorg. Chem. Focus II 2005, 319-346. (7) Yang, H.; Coombs, N.; Ozin, G. A. Nature (London) 1997, 386 (6626), 692-695. (8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359 (6397), 710-712. (9) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 8, 680-682. (10) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature (London) 1995, 378 (6555), 366-368. (11) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11 (7), 579-585.

the synthesis of mesostructured materials in a systematic way.10,5-7,12-14 This enables rapid synthesis of mesostructured materials having different morphologies and select internal symmetries. Examples include mesostructured monoliths, thin films, fibers, and colloidal powders.15,16,17,18 A variety of strategies have been explored for tailoring the internal structure and pore sizes of mesoporous materials via classical precipitation and EISA pathways. An important development, in this respect, was the introduction of block copolymer as templating agents, rather than the smaller acylchain-based surfactants such as the ionic cetyltrimethylammonium bromide (CTAB). The non-ionic block-copolymer templates generated materials with larger (meso)crystallographic unit cells and pores with superior thermal and mechanical stabilities compared to materials templated by ionic surfactants. In many applications, mesostructured particles with specific morphologies can be advantageous. For example, when the material needs to be closely packed or dispersed in different medias, a spherical morphology is arguably superior. Both hollow and solid mesoporous spherical particles have been reported. These have been synthesized using various aerosol-based methods based on the EISA technique,17,18 by precipitation routes within emulsions19,20 and also via a modified Sto¨ber process.21,22 However, there is no report to our knowledge that combines the (12) Attard, G. S.; Edgar, M.; Goltner, C. G. Acta Mater. 1998, 46 (3), 751758. (13) Klotz, M.; Ayral, A.; Guizard, C.; Cot, L. J. Mater. Chem. 2000, 10 (3), 663-669. (14) Goltner, C. G.; Berton, B.; Kramer, E.; Antonietti, M. AdV. Mater. (Weinheim, Germany) 1999, 11 (5), 395-398. (15) Yang, P. Monolithic Materials. J. Chromatogr. Lib. 2003, 67, 301-322. (16) Nicole, L.; Boissiere, C.; Grosso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15 (35-36), 3598-3627. (17) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507-2512. (18) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature (London) 1999, 398 (6724), 223-226. (19) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science (Washington, D. C.) 1996, 273 (5276), 768-771. (20) Huo, Q.; Feng, J.; Schueth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17. (21) Buchel, G.; Grun, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Supramol. Sci. 1998, 5 (3-4), 253-259. (22) Schumacher, K.; Grun, M.; Unger, K. K. Microporous Mesoporous Mater. 1999, 27 (2-3), 201-206.

10.1021/la0622267 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

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versatility of the EISA route with an emulsion-based method for generating spherical mesostructured particles. In this paper we report the synthesis of well-ordered mesostructures, with controllable spherical morphologies (hollow or solid) using a new method that combines emulsion processing with the EISA route. We refer to this as the ESE method (emulsion and solvent evaporation). The ESE method makes it possible to control synthesis parameters such as emulsion droplet size, temperature, evaporation speeds, humidity, and the composition of the alkoxide-templating surfactant solution. These are critical variables in the synthesis of well-ordered mesostructures11,23, 24 with controlled particle size and shape. For these reasons, it is very promising for large-scale production of materials for industrial applications.25 To demonstrate the method, synthesis of well-ordered spherical mesoporous particles is reported here. The calcined, mesoporous materials produced via this new route were characterized and compared with analogous mesoporous materials prepared using the known aerosol-based and precipitation (from dilute solution) methods. Control of particle size was reproducibly demonstrated by the use of two different emulsifiers. Materials and Methods Spherical mesoporous silica particles were prepared via ESE synthesis. The ESE method consists of five distinct steps: (1) preparation of a precursor solution, (2) emulsification of the precursor solution in oil, (3) aging under vacuum, (4) separation of the mesostructured particles from the continuous phase, and (5) surfactant removal by calcination. In a typical synthesis the precursor solution was prepared by hydrolyzing 5.2 g of tetraethylorthosilicate (TEOS, Purum >98%) in 6 g of ethanol (99.7%) and 2.7 g of dilute hydrochloric acid (pH 2) under vigorous stirring at room temperature for 20 min.13,26 Next, 0.8 g (50 vol %) of the amphiphilic, triblock copolymer templating molecule (Pluronic, BASF: P104, EO27PO61EO27) was dissolved in 8 g of ethanol (99.7%) before being mixed with the hydrolyzed TEOS solution to complete the preparation of the precursor solution. Water in oil (w/o) emulsions, with the inorganic siliceous precursor solution as the water phase and hexadecane as the oil phase, were prepared with two different techniques, either by adding the precursor solution into hexadecane (1:3 mass ratio) during vigorous stirring or by adding hexadecane into the precursor solution (3:1 mass ratio) during vigorous stirring. All emulsions were prepared at room temperature, with either 0.1 wt % (in the oil) of the emulsifier Arlacel P135 (a polymeric PEG-30 dipoly(hydroxystearate); Uniqema), or 0.5 wt % (in the oil) of Solsperse 3000 (an anionic single-anchor polymeric dispersant; Avecia) dissolved in hexadecane (minimum purity, 99%; Sigma-Aldrich). The fresh emulsions were transferred to 1000 mL round-bottom flasks, tempered to 30 °C in a water bath, and held under a reduced pressure of approximately 3 mbar for 30 min using a vacuum pump. Finally, the particles were separated from the oil by centrifugation and calcined in air at 500 °C for 5 h to remove the internal template. Characterization. X-ray diffraction (XRD) experiments were performed on a PANalytical X’pert PRO diffractometer system, with a 3050/60 θ/θ goniometer and a PW3064 spinning stage. Cu KR radiation (λ ) 1.5418 Å) was used in all experiments, and the generator was operated at 45 kV and 40 mA. A beam knife was used to enhance the low-angle diffraction maxima. Multiple Bragg peaks from a well-characterized crystalline lamellar lanthanum palmitate (23) Svensson, B.; Alexandridis, P.; Olsson, U. J. Phys. Chem. B 1998, 102, 7541-7548. (24) Cagnol, F.; Grosso, D.; Soler-Illia, G. J. d. A. A.; Crepaldi, E. L.; Babonneau, F.; Amenitsch, H.; Sanchez, C. J. Mater. Chem. 2003, 13 (1), 61-66. (25) Nystroem, M.; Herrman, W.; Larsson, B. Eur. Pat. 88-85021.3, 298062, June 16, 1988. (26) Alberius, P. C. A.; Frindell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 2002, 14, 3284-3294.

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Figure 1. Schematic of the emulsion solvent evaporation method (ESE), a synthesis route that combines emulsion processing with the EISA concept for production of well-ordered mesostructured materials. A pre-hydrolyzed alkoxide/templating surfactant solution is emulsified (left), and a vacuum is applied to evaporate off solvents and generate liquid crystalline phases within the emulsified droplets (middle); organic/inorganic mesostructured hybrid particles are formed (right). (lamellar spacing ) 44.73 ( 0.05 Å) were used as an internal and external small-angle standard. A JEOL 2000 FX transmission electron microscope (TEM), operated at an accelerating voltage of 200 kV and equipped with a CCD camera, was used for high-resolution imaging of milled as well as microtomed samples. Standard scanning electron microscope (SEM) images were obtained using a Philips XL-30 instrument operating under high vacuum conditions. High-resolution scanning electron microscope (HRSEM) images were obtained using a LEO Gemini 1550 FEG SEM. Specific surface areas and pore size distributions of the samples were determined from nitrogen sorption isotherms. A Micrometrics ASAP2010 (Norcross, GA) instrument was used for these measurements. The pore size distributions were calculated from the adsorption isotherms according to the BJH method.

Results and Discussion Highly ordered, spherical mesostructured particles with controlled size distributions have been produced here using a new combined emulsion and solvent evaporation method. This process is schematically described in Figure 1. In the first step, a w/o emulsion was prepared, with the pre-hydrolyzed alkoxide/ surfactant template solution as the water phase. In order to stabilize the emulsion, an oil-soluble emulsifier was required. The choice of emulsifier was dictated by two factors. In order to produce a w/o emulsion, the emulsifier should partition primarily to the organic phase, according to Bancroft’s rule. Hence the emulsifier surfactant should be relatively hydrophobic in nature. The second factor is that the emulsifier should not significantly interfere with the hydrophilic template dispersed in the aqueous precursor solution. If a significant interaction would occur between the emulsifier and templating amphiphile, the formation of the liquid crystalline intermediate confined to the aqueous system may become compromised in terms of the desired average surfactant packing parameter S ) V/al, where S is the surfactant packing parameter, V the molecular volume, a the head group area, and l the tail length of the molecule.27 For example, an original morphology being a hexagonal phase, with S ) 1/3 - 1/2, would tend toward forming a lamellar phase (S ) 1) or even reversed phases (S > 1). The effective S value would increase upon the (27) Hyde, S., Andersson, S., Larsson, K., Blum, Z., Landh, R., Lidin, S., Ninham, B. W., Ed. The Language of Shape: The Role of CurVature in Condensed Matter: Physics, Chemistry and Biology; Elsevier: Amsterdam, 1997; p 396.

Well-Ordered Mesoporous Materials by ESE Synthesis

Figure 2. Optical microscopy images of (a) emulsion droplets of alkoxide/templating surfactant solution in hexadecane when using emulsifier Arlacel P135 and (b) the same emulsion after solvent evaporation.

incorporation of a hydrophobic emulsifier, which is equivalent to stating that the spontaneous curvature of interfaces within the templating surfactant aggregates would decrease and then possibly increase. We hypothesized that the two conditions above could be met using hydrophobic, polymeric emulsifying surfactants soluble in the alkane phase, and thus a w/o emulsion would be expected to form with minimal effect on the self-assembly of templating amphiphiles. We further expected that a relatively high molecular weight hydrophobic emulsifier would be superior for our purposes, due to a reduced chance of partitioning to the aqueous phase, relative to lower molecular weight analogues. In testing this hypothesis, two polymeric emulsifiers were evaluated: Arlacel P135 and Solsperse 3000. The amount of emulsifier used in each experiment (less than 0.5 wt %) was set (by trial and error) as sufficient to give reasonably stable emulsions but low enough in concentration to avoid significant dissolution of emulsifiers into the dispersed emulsion droplets (Figure 2a). The oil’s ability to allow water transport out of the emulsion droplets during vacuum-driven liquid crystal formation and its lack of propensity for diffusing into the aqueous emulsion droplets are two important factors for consideration when choosing an oil for the continuous phase. Choosing a short-chain alkane (e.g., hexane) over a longer one (e.g., hexadecane) offers the advantage of relatively higher water solubility and thus a higher water flux through and out of, the organic phase. But a shorter chain alkane is more likely to negatively effect liquid crystal formation in the emulsion droplets, relative to longer chain alkanes. Both shortand long-chain alkanes will be solubilized to some extent in the surfactant aggregates of the aqueous phase. However, the locus of the shorter chain alkanes is expected to be concentrated close to the palisade layer of the templating surfactant aggregates, whereas for the longer alkanes it will be in the center of the

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aggregates. In the former case the presence of the alkane will increase the surfactant packing parameter such that the templating surfactant aggregate morphology will change (e.g., hexagonal to lamellar), with the further complication of alkane volatilization. In the latter case, negligible change in surfactant aggregate morphology is expected, thus allowing more predictable behavior. The extent of this differential impact on liquid crystal selfassembly will be governed by the type of amphiphilic template but should be taken into account when choosing an oil type. The choice of oil also has an influence on the emulsion being formed. If we choose a short-chain alkane over a longer one, the templating surfactant is more easily partitioned to the continuous phase. Hence the partitioning of templating surfactant between the aqueous phase and the oil phase is more pronounced compared to a longer chain alkane. This implies that emulsion stability is a function of the oil type according to the “surfactant affinity difference” concept,28 so this needs be considered for a particular system. Nevertheless, we have chosen to work with a long chain alkane, hexadecane, for the reasons mentioned previously, i.e., a minimal interaction with the surfactant aggregate structure in the aqueous droplets and its relatively low vapor pressure. Saturated alkanes longer than hexadecane were not used on the basis of their significantly higher viscosities at the temperatures used here. Mixed alkanes and silicone-based oils may also be suitable but are not explored here. In a second step, the pressure in the emulsion chamber was reduced in order to increase the evaporation rate of the volatile components (e.g., ethanol, water, and HCl) from the emulsified precursor solution. This evaporation procedure was the driving force for generating a well-ordered intermediate lyotropic liquid crystalline (LLC) structure within the droplets (according to the well-established evaporation induced self-assembly technique11). The time and the magnitude of applied pressure can be carefully controlled, in combination with the emulsion temperature. The use of a relatively low emulsion temperature, (30 °C) and a controlled evaporation of solvents (3 mbar) has apparently provided ideal conditions for generating well-ordered mesostructured materials, with unusually large crystalline domains as compared with other block copolymer templated materials, including those generated by the EISA route.16,18,29 Particle Size and Morphology. A typical emulsion, stabilized with the emulsifier Arlacel P135, is shown in Figure 2. The diameter of the emulsion droplets is approximately twice the diameter of the mesostructured particles generated by the solvent evaporation step, which can be explained by the evaporation of solvents. Several critical parameters are known to influence the size of emulsion droplets. These include the following: stirring speed and duration, composition in terms of both oil and emulsifier, relative viscosities of the continuous phase and the emulsified liquid, and the temperature when the emulsification are performed.30 There are also a number of well-established techniques for fabrication of emulsions with distinct character, such as monodisperse emulsions, double emulsions, and microemulsions. It is anticipated that these methods will be applicable to variations on the ESE method reported here, for example in tuning specific particle sizes and distributions. The two different emulsifiers (Arlacel P135 and Solsperse 3000) were used to demonstrate the possibility of tuning the (28) Salager, J.-L. Pharmaceutical Emulsions and Suspensions. Drugs Pharm. Sci. 2000, 105, 19-72. (29) Andersson, N.; Alberius, P. C. A.; Skov Pedersen, J.; Bergstroem, L. Microporous Mesoporous Mater. 2004, 72 (1-3), 175-183. (30) Sjo¨blom, J. Encyclopedic Handbook of Emulsion Technology; Marcel Dekker, Inc.: New York, 2001.

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Figure 3. SEM images of spherical mesoporous silica particles synthesized using the ESE method. Different size distributions were achieved when using the same emulsification conditions but different emulsifiers (a) Solsperse 3000 centered around 40 µm and (c) Arlacel P135 centered around 10 µm. All particles were homogeneous, but some evaporation holes were achieved for a minority of the particles (b and d). (e) SEM and (f) optical micrographs of meso-/macroporous particles that were generated from a double emulsion (o/w/o).

particle size on the basis of the choice of emulsifier. Particle size distributions were centered on 10 and 40 µm for Arlacel P135 and Solsperse 3000 emulsifiers, respectively (Figure 3a,c), under a given set of conditions for generating the emulsions. All particles, independent of size, were homogeneous and did not contain any macropores, as has been observed for earlier attempts using emulsion-based preparations.19,31,32 For a minority of the particles, however, evaporation holes were observed (Figure 3b,d), but it was possible to minimize the abundance of such defects by reducing the rate used in the evaporation step. More monodisperse particles were obtained when the oil phase (containing Arlacel P135) was rapidly added to the precursor solution during magnetic stirring. We consider that under this rate and order of addition, inversion of the initial o/w emulsion to a w/o emulsion occurred. This procedure resulted in the (31) Yu, C.; Tian, B.; Fan, J.; Stucky, G. D.; Zhao, D. Chem. Lett. 2002, 1, 62-63. (32) Fornasieri, G.; Badaire, S.; Backov, R.; Mondain-Monval, O.; Zakri, C.; Poulin, P. AdV. Mater. (Weinheim, Germany) 2004, 16 (13), 1094-1097.

formation of a double emulsion that generated a final macro-/ mesoporous material after the calcination step (Figure 3e,f). The external morphology and surface character of selected particles was investigated using high-resolution SEM, Figure 4. All particles displayed a spherical morphology with a surface texture reflecting the internal hexagonal mesostructure as determined using TEM and XRD. The formation of hexagonal cylinders running along the surface on this type of SBA-15 material has been reported elsewhere.33 Internal Mesostructure. The ESE synthesis method reported here is designed to allow control of parameters that promotes the formation of well-ordered mesostructures. In previous reports, researchers have demonstrated that the internal mesostructure is critically determined by the ratio of hydrolyzed silica to amphiphilic templates and that the ratio can be related to stability fields of liquid crystals in their respective surfactant-water phase diagrams.23 Another important parameter concerning the prepa(33) Che, S.; Lund, K.; Tatsumi, T.; Iijima, S.; Joo, S. H.; Ryoo, R.; Terasaki, O. Angew. Chem. Int. Ed. 2003, 42 (19), 2182-2185.

Well-Ordered Mesoporous Materials by ESE Synthesis

Figure 4. External surface texture of the 2D hexagonal (P6mm) mesoporous silica particles studied with high-resolution SEM. The mesoporous silica was synthesized via the ESE method using emulsifier Arlacel P135. (a) Morphology of a typical particle, and (b) the surface of the hexagonal mesostructure at higher magnification, showing rod-shaped cylinders curving in sheets along the surface, orthogonal to the a and b crystallographic directions. Lowmagnification views are inset.

ration of the precursor solution is the degree of hydrolysis of the inorganic alkoxide.10, 13,26,34 Other critical parameters include the temperature of the intermediate LLC material (particularly important when ethoxylated non-ionic surfactants are used as the templating molecules) and the water content in the intermediate LLC structure. The ESE method allows a high degree of control with respect to these critical parameters mentioned above. For materials produced here, synthesis conditions were tuned in order to achieve a 2D hexagonal (P6mm) mesostructure. In all experiments, the degree of hydrolysis of the alkoxides and the ratio of amphiphilic template molecules to hydrolyzed silica precursor (50 vol %, corresponding to the 2D hexagonal mesostructure in the corresponding water-block copolymer phase diagram23) were kept constant. The emulsions were tempered to 30 °C, and the water content in the intermediate structure was carefully controlled by using the same emulsion temperature and magnitude of vacuum and evaporation time for all the experiments. Structural changes were observed for large variations of these parameters, which is consistent with the work of Cagnol et al.24 These variations are the topic of a study under preparation. The TEM micrographs in Figure 5 show microtomed slices of well-ordered hexagonal mesoporous particles. All particles examined in TEM had large well-ordered domains, which were also confirmed for the central parts of the particles. The exceptionally well ordered structures, as compared with more poorly ordered materials, e.g., materials generated by the aerosol(34) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L.; Guo, Y. X.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature (London) 1997, 389 (6649), 364-368.

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Figure 5. TEM images of microtomed slices of 2D hexagonal (P6mm) mesoporous particles synthesized via the ESE method using emulsifier Arlacel P135. (a) The particles are well-ordered in the center. The view is approximately orthogonal the a-b crystallographic plane and (b) parallel to the a-b crystallographic plane. Fractures and defects are microtoming artefacts.

assisted EISA route,29 are attributed to the relatively slow evaporation rate of the solvents from the emulsion droplets through and out of the continuous phase. We postulate that this slow evaporation allows a high-degree of homogeneity of the components needed to form the liquid crystalline phase. In terms of synthesis parameters that influence the internal mesostructure, this is the most important distinction of the current ESE method from previously reported aerosol-based methods. The hexagonal structure was further characterized using XRD. Seven intense, well-resolved XRD peaks with d-spacings in the approximate ratio 1:(1/x3):(1/x4):(1/x7):(1/x9):(1/x12):(1/ x13) were observed in the small-angle region of the diffractogram. These are respectively indexed to the (hk) ) (10), (11), (20), (21), (30), (22), and (31) peaks of a 2D P6mm hexagonal structure (Figure 6). The positions of the peaks in the diffractogram (2θ), except for the poorly defined,21 were used in a least-squares refinement procedure to define the unit cell size a ) 105.4 ( 0.2 Å.35 We have been very careful to account for any sample displacement by use of an internal standard. Such a displacement would yield systematic shifts in multiple Bragg reflections from the lamellar standard, and none were observed. The nitrogen sorption isotherm for the hexagonally ordered material is shown in Figure 7. The isotherm displays the H1 characteristics and is of type IV according to the IUPAC classification36 as expected from materials with cylindrical pore geometry and high degree of pore size uniformity. The relatively large hysteresis indicates pore blocking, which we believe mostly (35) Holland, T. J. B.; Redfern, S. A. T. Mineral. Mag. 1997, 61 (404), 65-77. (36) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169-3183. (37) Miyazawa, K.; Inagaki, S. Chem. Commun. (Cambridge) 2000, 21, 21212122.

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Andersson et al. Table 1. Comparison of Nitrogen Sorption Data for Block Copolymer P104 Templated SBA 15-Type Materials Produced with Three Different Synthesis Routes sample emulsion prep aerosol prep29 precipitated prep37

TEOS/P104 molar ratio

pore vol (cm3/g)

93 134a 90

0.56 0.52 not reported

pore diam BET surface BJH (nm) area (m2/g) 6.4 6.3 5.4

503 382 506

a A molar ratio of 134 was chosen for the aerosol-prepared particles because materials prepared with molar ratio 90 have a lamellar structure.29 For the emulsion-prepared material, emulsifier Arlacel P135 was used.

Figure 6. X-ray diffraction pattern from 2D hexagonal (P6mm) mesoporous silica particles formed with Pluronics P104 synthesized with the ESE method (the tale of the diffractogram is enlarged 50 times).

probably due to very short crystallization times. It is also wellknown from the literature that the formed mesostructure is affected by synthesis temperature especially for precipitated materials. However, the ESE method, while generating well-ordered materials more or less comparable with precipitated materials, combines the advantages of the EISA method with the simplicity and advantages emulsion processing can offer.

Conclusion

Figure 7. Nitrogen adsorption/desorption isotherms for 2D hexagonal (P6mm) mesoporous silica formed with block copolymer P104 and emulsifier Arlacel P135 using the ESE method.

occurs at the particle surfaces, where the pores are aligned along the surface (Figure 4 high-resolution SEM). The pore size is estimated from the adsorption branches of the isotherm. The result yields a pore size of 64 Å with a very narrow pore size distribution. The total BET surface area was measured as 503 m2‚g-1 and pore volume found to be 0.56 cm3‚g-1. These data suggest a well-ordered structure and are consistent with the findings from XRD and TEM. Material properties for the 2D hexagonal mesoporous material synthesized with the new ESE method are summarized in Table 1. These properties are roughly compared with the SBA 15-type materials templated with the same block copolymer (P104), but using different synthesis routes and conditions, i.e., an aerosol based method and a precipitation route. Differences in surface areas and pore sizes are observed between the three different materials, suggestive that aerosol-generated materials may form less well-ordered materials compared with the other routes, most

A new route for preparing surfactant-templated mesoporous oxides is described here. The new ESE method combines emulsion processing with evaporation-induced self-assembly. The proposed method allows for relatively independent and simultaneous tuning of emulsion droplet sizes and internal-phase stoichiometry and liquid crystal structure formation. This versatility has been illustrated here via synthesis of spherical particles containing unusually well-ordered hexagonal (P6mm) mesostructures. The combination of emulsification and the controlled rate of evaporation of solvents from the precursor solutions within droplets encourages stoichiometric homogeneity of the inorganics and amphiphilic templates, leading to development of wellordered mesophases, with symmetries predictable using the same principles governing formation of lyotropic liquid crystalline phases from surfactant-water mixtures. This ESE method is potentially useful for industrial materials production due to its inherent scalability and relatively low batchwise processing times (1-3 h), in addition to the combined materials advantages, e.g., for the controlled release or uptake of actives in pharmaceutical, agrochemical, or separations applications. These initial findings indicate the versatility of the new method and lay the foundation for new studies under way aimed at the synthesis of well-ordered mesoporous materials with threedimensionally interconnected pore systems and hierarchically ordered pore structures. Acknowledgment. Ernesto Coronel, Department of Engineering Sciences, Uppsala University, is acknowledged for providing HRSEM micrographs. LA0622267