Toward a Better Control of Internal Structure and External Morphology

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Toward a Better Control of Internal Structure and External Morphology of Mesoporous Silicas Synthesized Using a Nonionic Surfactant A. Le´onard,†,‡ J. L. Blin,†,§ M. Robert,† P. A. Jacobs,⊥ A. K. Cheetham,|,# and B. L. Su*,† Laboratoire de Chimie des Mate´ riaux Inorganiques (CMI), I.S.I.S, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium, Equipe Physico-Chimie des Colloı¨des, Faculte´ des Sciences, UMR 7565, CNRS/Universite´ de Nancy-1, BP 239, F-54506 Vandoeuvre-les-Nancy Cedex, France, Centrum voor Oppervlaktechemie en Katalyse, Katholieke Universiteit Leuven (KUL), Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium, and Materials Research Laboratory, University of California, Santa Barbara, California 93106 Received October 18, 2002. In Final Form: March 12, 2003 Mesoporous silicas with controlled morphologies have been synthesized using decaoxyethylene cetyl ether [C16(EO)10] as a surfactant (10 wt % in aqueous solution). A large series of characterization results (SEM, TEM, XRD, and nitrogen adsorption-desorption analysis) have demonstrated how the surfactant/ silica molar ratio as well as hydrothermal treatment conditions can drastically influence not only the regular organization of the compounds on the nanometer scale but also the morphogenesis of the particles at the micrometer level. High loadings of a silica precursor yield highly structured mesoporous compounds with a morphology of ropes, gyroids and toroids whereas an increase of the surfactant/silica molar ratio results in spherical materials with a disordered channel array. A phase transition from hexagonal to disordered structure and then again to a regular hexagonal array with increasing gel heating temperature from 40 to 80 °C has been evidenced.

1. Introduction Ten years after the first synthesis by Mobil researchers, mesoporous molecular sieves are gaining increasing attention.1,2 Applications extend catalysis and fine chemicals production to the more elaborated nanotechnologies with integrated nanosystems. For example, the encapsulation of drugs into biocompatible silica nanovectors could lead to advanced systems that allow for a controlled release of the active substance.3-5 On the other hand, mesoporous materials containing immobilized enzymes could be very promising candidates for nanobiosensors for instance.6,7 The creation of semiconductor clusters that exhibit variable band gaps as a function of cluster size requires materials with well-defined opening sizes that can monitor the particle growth.8 Studies dealing with * Corresponding author. Telephone: +32-81 724531. Fax: +3281 725414. E-mail: [email protected]. † The University of Namur (FUNDP). ‡ FRIA fellow. § CNRS/Universite ´ de Nancy-1. ⊥ Katholieke Universiteit Leuven (KUL). | University of California, Santa Barbara. # International Francqui Chair of Belgium (2000-2002). (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. (2) 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.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Unger, K.; Rupprecht, H.; Valentin, B.; Kircher, W. Drug. Dev. Ind. Pharm. 1983, 9, 69. (4) Kortesuo, P.; Ahola, M.; Kangasniemi, I.; Yli-Urpo, A.; Kiesvaara, J. Biomaterials 2000, 21, 193. (5) Han, Y. J.; Stucky, G. D.; Butler, A.J. Am. Chem. Soc. 1999, 121, 9897. (6) Braun, S.; Rappoport, S.; Zusman, R.; Avnir, D.; Ottoglenghi, M. Mater. Lett. 1990, 10, 1. (7) Chung, K. E.; Lan, E. H.; Davidson, M. S.; Dunn, B. S.; Valentine, J. S.; Zink, J. I.Anal. Chem. 1995, 67, 1505.

the preparation of mesoporous materials have led to mesoporous materials with well-defined structural and textural features.2,9-16 In addition to these characteristics, it is also of importance for some applications to control the outer morphology.17-21 For instance, spherical particles are preferably used in chromatography for column packing as irregular particles tend to break down.22,23 Prouzet et al. tested spherical micrometric MSU-X particles in adsorption and separation processes.24 Mesoporous silica spheres also find applications in the chromatographic (8) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (9) Sayari, A.; Yang, Y.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B. 1999, 103, 3651. (10) (a) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (b) Yuan, Z. Y.; Blin, J. L.; Su, B. L. Chem. Commun. 2002, 504. (11) (a) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B. L. Langmuir 2000, 16, 4229. (b) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303. (12) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 56. (13) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (14) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Stud. Surf. Sci. Catal. 1994, 84, 53. (15) Bagshaw, S. A. Stud. Surf. Sci. Catal. 1998, 117, 381. (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 (London) 1994, 368, 317. (17) Yang, H.; Coombs, N.; Ozin, G. A. Nature (London) 1997, 386, 692. (18) Schultz-Ekloff, G.; Rathousky, J.; Zukal, A. Int. J. Inorg. Mater. 1999, 1, 97. (19) Gru¨n M.; Lauer, I.; Unger, K. K. Adv. Mater. 1997, 9, 254. (20) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 276, 768. (21) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D. Adv. Mater. 1997, 9, 974. (22) Chan, H. B. S.; Budd, P. M.; de V. Naylor, T. J. Mater. Chem. 2001, 11, 951. (23) Boissie`re, C.; Van der Lee, A.; El Mansouri, A.; Larbot, A.; Prouzet, E. Chem. Commun. 1999, 2047.

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purification of mixtures of biomolecules.25 We recently carried out a study with the aim of controlling morphology. This study dealt with the effect of the surfactant concentration on the properties of compounds prepared with a decaoxyethylene cetyl ether-TMOS system. In the study, we successfully synthesized, in mild acidic media, in a range of decaoxyethylene cetyl ether [C16(EO)10] weight percentages of 10-25, silica spheres of 1 µm in diameter with an ordered hexagonal network of uniformly sized mesoporous channels26 according to a LCT-type mechanism. Nontoxic and biodegradable nonionic polyoxyethylene alkyl ether type surfactants are of interest in the synthesis of ordered mesoporous materials. Since the assembly of the inorganics around the micelles occurs through Hbondings, the removal of most of the occluded surfactant molecules can be made by simple solvent extraction. Initial syntheses with these surfactants have led to materials with a disordered wormholelike framework.27-30 However, adjusting preparation parameters allows the formation of compounds that exhibit an ordered array of channels.26,31-33 Also, it is possible to tailor the pore diameter from 4.5 to 7.0 nm by adding some swelling agents to the micellar solution. It is now apparent that the morphology and texture of mesoporous materials are extremely important for industrial applications.34-36 A series of very recent papers have once again emphasized the importance of interaction between the hydrolyzed silicic species and the micelles during the gel treatment26,29,30,32 on the control of final structure. The surfactant/silica molar ratio has also been shown to be a key factor. The present paper deals with the effect of a template/silica molar ratio in a wide range of 0.25-3.50 on the external morphology and internal structure with the surfactant concentration (10 wt %) in the gel remaining constant. On the basis of this systematic study, we have tried to acquire more information on the formation of the internal geometry and the morphology of the silica particles. 2. Experimental Section 2.1. Synthesis. A 10 wt % micellar solution was prepared by dissolving decaoxyethylene cetyl ether [C16(EO)10, Brij 56] in twice distilled water. Sulfuric acid was then added to adjust the pH (24) Boissie`re, C.; Larbot, A.; Prouzet, E. Stud. Surf. Sci. Catal. 2000, 129, 31. (25) Yang, H.; Vovk, G.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1998, 8, 3, 743. (26) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542. (27) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature (London) 1995, 378, 366. (28) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (29) (a) Prouzet, E.; Cot, F.; Nabias, G.; Larbot, A.; Kooyman, P.; Pinnavaia, T. J. Chem. Mater. 1999, 11, 1498. (b) Boissie`re, C.; Larbot, A.; Bourgaux, C.; Prouzet, E.; Bunton, C. A. Chem. Mater. 2001, 13, 3580. (c) Boissie`re, C.; Martines, M. U. A.; Tokumoto, M.; Larbot, A.; Prouzet, E. Chem. Mater. 2003, 15, 509. (30) Blin, J. L.; Le´onard, A.; Su, B. L. J. Phys. Chem. B. 2001, 105, 26, 6070. (31) Coleman, N. R. B.; Attard, G. S. Microporous Mesoporous Mater. 2001, 44-45, 73. (32) (a) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (b) Bennadja, Y.; Beaunier, P.; Margolese, D.; Davidson, A. Microporous Mesoporous Mater. 2001, 4445, 147. (c) Davidson, A. Curr. Opin. Colloid Interface Sci. 2002, 7, 92. (d) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107. (e) Tolbert, S. H.; Landry, C. C.; Stucky, G. D.; Chmelka, B. F.; Norby, P.; Hanson, J. C.; Monnier, A. Chem. Mater. 2002, 13, 2247. (33) Zhang, W.; Glomski, B.; Pauly, T. R.; Pinnavaia, T. J. Chem. Commun. 1999, 1803. (34) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature (London) 1997, 390, 674. (35) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (36) Kim, S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 2244.

Langmuir, Vol. 19, No. 13, 2003 5485 to a value of 2. After homogenization at 70 °C, TMOS was added dropwise and, after further stirring for 1 h, the synthesis gel was poured into Teflon cartridges sealed in stainless steel autoclaves. The amount of TMOS was varied in order to obtain different surfactant/silica molar ratios (R) ranging from 0.25 to 3.50 while the surfactant concentration was kept constant. Hydrothermal treatment was performed at 40, 60, and 80 °C for 3 days, 2 days, and 1 day, respectively. Complementary syntheses were performed using the same scheme but with a hydrothermal treatment of 1 day at either 100 or 120 °C. The recovered gel was then extracted with ethanol using a Soxhlet apparatus, dried under vacuum at ca. 60 °C and calcined at 550 °C under nitrogen and oxygen to completely eliminate remaining surfactant molecules in pores. 2.2. Characterization. Information about structure was obtained by powder XRD and TEM techniques. A Siemens D-5000 diffractometer using Ni-filtered Cu KR radiation with λ ) 1.541 78 Å was employed. A transmission electron microscopy study was performed on a Philips Technaı¨ microscope with an acceleration voltage of 100 kV and a LaB6 filament. The powdery samples were embedded in an epoxy resin and sectioned with an ultramicrotome before being deposited on carbon-coated copper grids. The textural properties of our compounds were investigated by nitrogen adsorption-desorption measurements. After the samples were degassed overnight, the data were collected using either a Micromeritics ASAP 2010 or Tristar 3000. The pore diameter and the pore size distribution were determined by the BJH method37 applied to the adsorption branch of the isotherm. Although it is well-known that this method gives underestimated pore sizes38 and some interesting methods have been developed by Neimark and Jaroniec38,39 for mesopore measurements, this will not affect our systematic comparison, as the same method was used for all of the experimental results. Morphological features have been investigated intensively with a Philips XL20 scanning electron microscope. For conductivity purposes and in order to enhance the yield of secondary electrons, a thin layer of gold was deposited on powders by metallization.

3. Results and Discussion In this study, the surfactant weight percentage in the aqueous solution is maintained at 10 which, according to the phase diagram, yields isolated micelles.40 3.1. Structural Features of the Prepared Materials. Figure 1 depicts the XRD patterns of materials prepared at 80 (Figure 1A), 60 (Figure 1B), and 40 °C (Figure 1C) with different surfactant/silica molar ratios (R). Parts A and C of Figure 1 demonstrate that the samples prepared at 80 and 40 °C exhibit a threereflections pattern at a low 2θ angle range, with a strong feature and two weaker peaks. These peaks can be indexed on a hexagonal unit cell with a0 ) 2d100/(3)1/2. They arise from the (100), (110), and (200) reflections, resulting from the hexagonal array of the channels.41a The unit cell parameter ao is therefore the sum of pore diameter and pore wall thickness.41b The experimentally observed positions of the 100, 110, and 200 peaks are given in Table 1. Neither hydrothermal treatment conditions nor the R value has a drastic influence on the d spacings and unit cell parameters. In both cases (80 and 40 °C), the secondary reflections lose intensity as the R value increases, indicating that (37) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37. (38) Jaroniec, M.; Kruk, M.; Sayari, A. Stud. Surf. Sci. Catal. 2000, 129, 587. (39) Ravikovitch, P. I.; Wei, D.; Cheuh, W. T.; Haller, G. L.; Neimark, A. J. Phys. Chem. B 1997, 101, 3671; 2001, 105, 6817. Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 1550; Langmuir 2000, 16, 2419; Colloids Surf. A. 2001, 187, 11. (40) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (41) (a) Chen, C. Y.; Xiao, S. O.; Davis, M. E. Microporous Mater. 1995, 4, 20. (b) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B. L. Int. J. Inorg. Mater. 2001, 3, 75.

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Figure 2. TEM micrographs of materials synthesized with several surfactant/TMOS molar ratios (R) and at different treatment conditions: (A) 1 day at 80 °C; (B) 2 days at 60 °C; (C) 3 days at 40 °C.

Figure 1. XRD patterns of compounds prepared with several surfactant/TMOS molar ratios (R) and at different conditions of hydrothermal treatment: (A) 1 day at 80 °C; (B) 2 days at 60 °C; (C) 3 days at 40 °C. Table 1. Structural Parameters of Compounds Prepared at Different Hydrothermal Treatments and with Several R Values hydrothermal treatment 1 day at 80 °C

2 days at 60 °C

3 days at 40 °C

surfactant/ cell wall silica d100 d110 d200 param thickness molar ratio (nm) (nm) (nm) a0 (nm) (nm)a 0.25 0.50 1.00 1.50 2.00 2.50 0.25 0.50 1.00 1.50 2.00 2.50 0.25 0.50 1.00 1.50 2.50

5.5 5.4 5.3 5.7 5.6 5.6 5.3b 4.8b 5.1b 5.7b 5.8b 5.4b 5.3 5.3 5.9 5.3 6.0b

3.2 3.1 3.1 3.2 3.1 3.2 c c c c c c 3.1 3.1 2.9 3.1 c

2.8 2.7 2.7 2.9 2.8 2.8 c c c c c c 2.7 2.7 2.6 2.7 c

6.4 6.2 6.1 6.6 6.5 6.5 c c c c c c 6.1 6.1 6.8 6.1 c

1.8 2.1 2.3 2.5 2.1 2.2 2.3 2.8 3.2 2.3 3.1 c 2.6 2.3 3.3 2.5 2.2

a Wall thickness ) a - pore diameter from N adsorption. 0 2 Wormholelike structure, the peak represents the sum of the pore c diameter and the wall thickness 35. Not observered.

b

the materials become less well ordered when less silica is added to the micellar solution. TEM pictures confirm these observations (Figure 2, parts A and C). The honeycomblike hexagonal arrangement and the fast

Fourier transform images given as insets also show the 6-fold symmetry, confirming the observation from XRD measurements for the lower molar ratios. Although some zones showing the honeycomblike structure can still be found, most of the channels are rather disordered when the R value is increased from 1.50 to 2.50 (a decrease in the quantity of added inorganics). Evolution is different at 60 °C (Figure 1B). In addition to the main reflection peak at around 5.3-5.8 nm, the secondary reflections cannot be well distinguished, reflecting less ordering and homogeneity in the samples, even for the lower molar ratios. Reproducibility of these samples has been checked and TEM observations also show a more disordered channel arrangement (Figure 2B). The best organization of the channels is thus obtained with high amounts of added silica, especially at 80 and 40 °C. Such conditions typically yield hexagonally ordered materials. An increase in the molar ratio (i.e., lower quantities of silica) leads to the coexistence of ordered and wormholelike structures. This was previously reported by Bennadja et al.32b in the synthesis of SBA-15 using pluronic surfactants and TEOS. For the lower silica contents (higher R values), the system could not be rigidified enough to avoid gliding of the channels, thus giving a more disordered framework. Materials synthesized with a series of R values, at both 80 and 40 °C, present a hexagonal arrangement of channels; however, such structures could not be obtained at 60 °C. A phase transformation from hexagonal to disordered and then to hexagonal seems thus to occur with varying synthesis temperatures of 40, 60, and 80 °C, respectively. However, if the hydrothermal temperature increases further, for example to 100 or 120 °C, the synthesis leads only to the formation of amorphous materials. 3.2. Textural Characteristics Obtained by Nitrogen Adsorption)Desorption Measurements. 80 °C (Figure 3A). All the isotherms are type IV, characteristic of mesoporous compounds, with a type H1 hysteresis loop42,43 (not shown for the sake of clarity). The capillary (42) Gregg, S. I.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982.

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Figure 3. Nitrogen adsorption isotherms (and pore size distributions given as insets) of compounds synthesized with different surfactant/TMOS molar ratios (R) and at different conditions of hydrothermal treatment: (A) 1 day at 80 °C; (B) 2 days at 60 °C; (C) 3 days at 40 °C.

condensation occurs at similar p/p0 values, suggestive of constant pore sizes. The pore size distributions are centered at between 3.8 and 4.6 nm (insets). The specific surface areas are very high (>900 m2/g), whatever the surfactant/silica molar ratio is. 60 °C (Figure 3B). For molar ratios inferior to 1.5, the isotherms are type IV; however, beyond this value, the isotherms tend to have a shape located between type IV and type I, describing supermicroporous structures.44 The pore sizes decrease from 3.4 nm to values below 2.0 nm as the C16(EO)10/TMOS molar ratio is increased. Also the specific surface areas decrease but remain higher than 800 m2/g. 40 °C (Figure 3C). All of the isotherms are type IV, and the specific surface areas are located between 900 and 1100 m2/g. If the surfactant/TMOS molar ratio is raised beyond 0.50, the adsorbed volume at saturation, instead of remaining constant, begins to increase indicating the presence of secondary interparticle meso- or macropores. This feature, which becomes more pronounced as less silica is added to the mixture, can be related to the change in external morphology due to unconsumed surfactant molecules, as will be shown below. It could also be useful in catalytic applications. The pore size (43) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (44) Dubinin, M. M. In Progress in Surface and Membrane Science 9; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; p 1.

distributions from internal structure remain very narrow and their maximum is centered between 3.5 and 3.8 nm. Mesoporous materials with high specific surface areas, narrow pore size distributions and well-defined type IV isotherms are thus obtained at 40 and 80 °C for high silica contents. From Table 1, the wall thickness of the obtained materials does not vary significantly with R or temperature. As the pore diameter remains nearly constant at 80 and 40 °C, this implies that increasing the amount of added silica (i.e., decreasing R value) allows more micelles to interact with silica, favoring the organization of the channels and leading to a greater quantity of recovered mesoporous material. This is also indirect proof of the proposed self-assembly mechanism that leads to ordered compounds, which will be discussed below. 100 and 120 °C. Materials subjected to a hydrothermal treatment at 100 °C show somewhat larger pore sizes, which is consistent with our previous observations.30 However, at 120 °C, the compounds show very broad pore size distributions and the nitrogen adsorption isotherms belong to type II (i.e., macroporous without well-defined mesostructure). 3.3. Discussion. The synthesis of mesoporous materials takes advantage of supramolecular assemblies formed by surfactant molecules as framework templates. In an aqueous solution, these molecules pack together and depending on preparation conditions (concentration, temperature, presence of other compounds, etc.), first form

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isolated spherical micelles, then cylindrical micelles, and finally highly ordered liquid crystal phases. Considering the phase diagram of C16(EO)10, established by Mitchell et al.40 for weight percentages superior or inferior to 30, a hexagonal (H1) liquid crystal phase, or isolated spherical and cylindrical micelles will be obtained, respectively. The formation mechanism of mesoporous materials is now well established.2,16,26,29,31-33 These compounds are obtained via a cooperative templating mechanism. In the initial step, the interactions between silica and isolated spherical or cylindrical micelles lead to the formation of an organic-inorganic mesophase. Then, the condensation of the inorganic precursor on the external surface of the micelles occurs leading to an inorganic layer covering the micelles. Finally, under the hydrothermal treatment at a higher temperature, the inorganic layers join together to complete the polymerization of the silica source, leading to the rigid mesostructured framework. It was clearly shown that micellar solutions of C16(EO)10 prepared at concentrations located in the H1 liquid crystal domain (>30 wt %) gave wormholelike channel structures. A hexagonal array of channels is obtained however when surfactant molecules are present in the form of the isolated spherical and cylindrical micelles in the aqueous solution (