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Synthesis and Characterization of Wormlike Mesoporous Silica by Using Polyelectrolyte/Surfactant Complexes as Templates Cheng Tao and Junbai Li* International Joint Lab, Key Lab of Colloid and Interface Science, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Bei Yi Jie 2, Beijing 100080, China Received March 3, 2003. In Final Form: August 24, 2003 Wormlike mesoporous silica is prepared by using a complex template: mixed poly(styrenesulfonate sodium) and cetyltrimethylammonium bromide. Such a pore modification with polyelectrolyte under the control of a sol-gel process can be used as an efficient route in fabricating highly flexible and ordered mesoporous silica materials.
Introduction The synthesis of mesoporous material MCM-41 and its derivatives showed how the periodic crystal growth of inorganic material can be obtained by the self-assembly of surfactants as templates at the fluid/solid interface.1,2 The corresponding inorganic porous products with a three-dimensional structure have exhibited application superiority as light ceramics, catalyst supports, biomedical implants, and robust membranes for high-temperature separation technology.3,4 Therefore, they have been of great interest. To explore the diversity and complexity of the morphology and structure of inorganic porous materials, many different synthetic means have been attempted in the control of their size, shape, orientation, and polymorphic structure.5-8 The applications of various templates such as microemulsion,9 block copolymers,10,11 latex particles,12 colloidal crystals,13 spongelike polymer gels,14 and bacterial superstructures15 have been considered as very effective paths to organize the periodicity and regularity of the porous inorganic materials. The mixtures of polyelectrolytes and oppositely charged surfactants can * Corresponding author. Tel.: +86 10 82614087. Fax: +86 10 82612484. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 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.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Mann, S.; Burkett, S. L.; Davis, S. A.; Fowler, C. E.; Mendelson, N. H.; Sims, S. D.; Walsh, D.; Whilton, N. T. Chem. Mater. 1997, 9, 2300. (4) Ozin, G. A. Acc. Chem. Res. 1997, 30, 17. (5) Mann, S. J. Mater. Chem. 1995, 5, 935. (6) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098. (7) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (8) Mann, S. J. Chem. Soc., Dalton Trans. 1997, 3953. (9) Walsh, D.; Mann, S. Nature 1995, 377, 320. (10) Goltner, C. G.; Henke, S.; Weisenberger, M. C.; Antonietti, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 613. (11) Zhao, D.; Feng, J.; Huo, Q. S.; Melosh, N.; Fedrickson, G. H.; Chemelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (12) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477. (13) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lehnoff, A. M. Chem. Mater. 1998, 10, 3597. (14) Breulmann, M.; Co¨lfen, H.; Hentze, H. P.; Antonietti, M.; Walsh, D.; Mann, S. Adv. Mater. 1998, 10, 237. (15) Archibald, D. D.; Mann, S. Nature 1993, 364, 430.
form stoichiometric and stable complexes.16-20 Such a complex as templates with high flexibility may play an important role in controlling the growth of inorganic crystals in a well-defined way.21 Here, we report the synthesis of mesoporous silicate by the complex of a cationic surfactant and an anionic polyelectrolyte as the templates. The charge interaction between the surfactant and polyelectrolyte results in the formation of a threedimensional structure in the solution. With a controlled sol-gel process, the wormlike morphology of mesoporous silica with a longer length scale was obtained. Experimental Section Materials. Tetraethyl orthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) were purchased from Beijing Chemical Reagents Co. Poly(4-styrenesulfonate sodium) (PSS) (MW, 70 000), poly(ethylene glycol) (PEG) compounds, and poly(allylamine hydrochloride) (PAH) were from Sigma. All the reagents were used as received without further purification. Synthesis. In a typical synthesis, 0.2 g of PSS was added to a solution of 22.3 mL of HCl (37%) and 55.8 mL of H2O. After vigorously stirring at ambient temperature for 0.5 h, CTAB was added to the vessel. The obtained solution as a template was homogenized for 1 h under stirring, and then 5.6 mL of TEOS was dropped into the system to afford a reaction mixture with the molar ratio 1:0.13:1.14 × 10-4:10.4:130 TEOS/CTAB/PSS/ HCl/H2O. The mixture was stirred for 24 h at 85 °C. The resulting white precipitation was filtered, washed thoroughly with deionized water, and dried in air. Finally, calcination was carried out in air at 550 °C for 3 h to remove the organic templates. X-ray Powder Diffraction (XRD). XRD patterns were obtained on D/max rB using Cu KR radiation at 40.0 kV and 120.0 mA. Transmission Electron Microscopy (TEM). TEM images were achieved on a TECNAI 20 (Philips) at 120 kV. The powder specimens for the TEM measurements were preground and deposited on a grid with a holey film and then rapidly transferred to the TEM microscope with an accelerating voltage of 120 kV. (16) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (17) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (18) Antonietti, M.; Go¨ltner, C. G.; Hentze, H. P. Langmuir 1998, 14, 2670. (19) Antonietti, M.; Caruso, R. A.; Go¨ltner, C. Macromolecules 1999, 32, 1383. (20) Ganeva, D.; Antonietti, M.; Faul, C. F. J.; Sanderson, R. Langmuir 2003, 19, 6561. (21) Tao, C.; Zheng, S. P.; Mo¨hwald, H.; Li, J. B. Langmuir, in press.
10.1021/la0343728 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003
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Figure 1. SEM micrograph of the calcined samples templated by the complex of PSS/CTAB in a molar ratio of (a) 1.3:1.1 × 10-3 CTAB/PSS; (b) a calcined sample by pure CTAB as the template without PSS; (c) 1.3:5.7 × 10-4 CTAB/PSS; and (d) 1.3:2.3 × 10-3 CTAB/PSS.
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Figure 2. SEM images of the calcined samples synthesized with different aging times: (a) 2, (b) 4, (c) 8, and (d) 16 h.
Scanning Electron Microscopy (SEM). SEM was performed on a KYKY-2800 (made in China) at 20 kV by using the conventional sample preparation and imaging techniques. N2 Adsorption-Desorption Isotherms. This experiment was carried out at 77 K by using a Soptomatic 1800 instrument. Samples were degassed at 150 °C under a high vacuum for at least 4 h before the measurement. The data were analyzed according to the DS method. The specific surface area was determined from the linear part of the BET equation (P/P0 ) 0-1). The calculation of the pore size distribution was performed using the desorption branches of the N2 adsorption isotherms.
Results and Discussion Figure 1 displays the SEM images of calcined silica samples by using mixed PSS/CTAB as templates. A typical wormlike morphology is observed in Figure 1a. The samples have a very flexible shape and randomly aggregate together. The length and width of the wormlike particles are approximately 4.2 µm and in the range of 200-500 nm, respectively. This morphology is obviously different from that of the classical mesoporous molecular sieve MCM-41. Under the same experimental condition, by using pure CTAB as templates without PSS the particles show block morphology like that of pure MCM41, as seen in Figure 1b.1,2 However, as we varied the PSS amount in the mixture of PSS/CTAB the morphology change was also dramatically observed. If the PSS amount was reduced to 50%, a minority of wormlike shapes of the products have been observed. The particle morphology was principally dominated by the amorphous status, as shown in Figure 1c. The addition of an amount of PSS over twice that in the first case resulted in the total disappearance of the wormlike morphology, as presented in Figure 1d. These experimental results indicate that the regular wormlike morphology can be achieved only in a proper molar ratio of PSS to CTAB as complex templates. Experimentally, we have also realized that the wormlike morphology of the final particles can form in an aging time of approximately 2 h after calcination (Figure 2a). However, in the sol-gel process, the final products with
Figure 3. SEM images of the calcined samples (a) synthesized at a neutral condition; (b) synthesized by the template of the PEG/CTAB complex; and (c) synthesized by the template of the PAH/CTAB complex.
different aging times from 2 to 24 h under the same conditions have a similar stable wormlike morphology as that displayed in Figure 2b-d. This means that the initial state of the templates decides the final morphology of the yields. Also, the order change in adding polyelectrolyte or surfactant does not affect the morphology of the samples. This reflects that the formation of complex templates is based on the charge interaction. To prove this, we performed the experiments by changing the template condition. In a neutral stage, we used the same PSS/CTAB templates and let TEOS hydrolyze to undergo a sol-gel process. Consequently, we observed an amorphous morphology of the final particles, as shown in Figure 3a. This indicates that the pH value influences the formation of a wormlike morphology. On the other hand, the charge
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Figure 4. TEM micrographs of the sample before calcination at different magnifications.
interaction between PSS and CTAB should be considered as a main factor to induce the formation of wormlike morphology. To prove this, we selected the nonionic polymer PEG and cationic polyelectrolyte PAH instead of PSS to carry out similar experiments. As a consequence, it is found that both calcined samples have the similar amorphous morphology like that obtained by using the pure CTAB as templates (Figure 3b,c). This indicates that only negatively charged polyelectrolyte interacts with CTAB to form a complex template and induce the growth of a quite flexible wormlike morphology during the TEOS hydrolysis. To further confirm the formation of the wormlike morphology by using polyelectrolyte/surfactant complexes as templates, we performed the TEM measurements with TEOS before calcination. It is shown in Figure 4a that after the hydrolysis of TEOS in a sol-gel process without calcination the as-synthesized SiO2 particles already possess the wormlike morphology. The TEM image at a higher magnification selected from the Figure 4a indicates that no obvious mesopores are observed inside the wormlike particles (Figure 4b). This morphology is distinctly different from that obtained from the pure polyelectrolyte/surfactant complexes. Previous studies demonstrated that the structure of the complexes of poly(styrenesulfonate) with different alkyltrimethylammonium derivatives were essentially layered.16 However, the hydrolysis of TEOS was carried out in a three-dimensional way, which probably resulted in an appearance of a polymer structured-directing phase. In fact, we have observed the wormlike morphology of SiO2 particles after calcination, as shown in Figure 5a. The diameter of the “worm” is around 200 nm and similar to that obtained by SEM. In the sample preparation for TEM measurement, the sample powder was dispersed in alcohol. Thus, the TEM image in Figure 5a displays that the individual “worm” is separated. The shape of the samples is very flexible like the stretched status of PSS chains in a solution. From the inner structure of the sample presented in Figure 5b, it is seen that the single particle has an obvious pore structure. An enlarged TEM image in Figure 5c shows that the pore size of inside the mesoporous silica is within 2.0-4.0 nm and the wall thickness is approximately 2.0 nm. The pore direction in the middle of the particle is parallel to the long axis of the sample and at the edge is changed. This is much different from that in the classical mesoporous material MCM-41.1,2 This is most probably because in the complex template the polymer elongated the template growth along the molecular chain and led to the pores being enlarged toward the axis. Figure 6 shows the XRD patterns of the calcined sample with a wormlike morphology. One major peak at around 2° [denoted as (100)] was observed. The intensity of the
Figure 5. TEM micrographs of the calcined sample at different magnifications.
Figure 6. XRD pattern of the calcined sample synthesized at 85 °C by the complex template of CTAB/PSS in a molar ratio 1.3:1.1 × 10-4 CTAB/ PSS.
d100 peak, which is eventually considered as a sign of the crystallinity in the mesoporous MCM-41, is relatively weak.1,2 The peak in Figure 6 shows the existence of a mesoporous structure, but the pores are irregular in shape. In fact, the peak is the correct average result of two kinds of pores with different sizes. A similar conclusion is also drawn from the nitrogen adsorption isotherm. Figure 7 displays the N2 adsorption-desorption isotherm of the calcined samples from the PSS/CTAB complex as templates. Usually, nitrogen adsorption can be used to estimate the pore size distribution. It is seen that the N2 adsorption provides a type IV isotherm. From this it can be estimated that the surface area is 827 m2/g. N2 adsorption at a lower pressure (P/P0 < 0.30) is regarded as the monolayer adsorption of nitrogen on the walls of the mesopores. However, it does not directly reflect the presence of mesopores. As the pressure increases (P/P0 >
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Figure 7. N2 adsorption-desorption isotherm of the calcined sample.
0.30), the isotherm displays a sharp inflection in the range of 0.40-0.75, indicating the characterization of capillary condensation within mesopores. The P/P0 position of the reflection points is relevant to the diameter of the mesopore range. The broad hysteresis loop in the isotherm of the particles demonstrates the disorder of some mesopores in the shape, which limits the emptying and filling of the accessible volume. This is consistent with the results of the XRD and TEM measurements. Figure 8 shows the pore volume distribution curve of the sample. It is seen that the diameter of the mesopores is in the range of 2-5 nm. With such a size range, it can be assumed that the PSS molecules with longer chains dominate the template status. The cylindrical CTAB micelles are most likely tangled by polymer chains via charge interaction. Thus, the complex templates behave as the polymer in organic solvents. The appearance of a distinct phase structure is mainly controlled by the mutual geometric fit of the surfactant and polyelectrolyte.17 Antonietti et al. demonstrated that the polyelectrolyte/ surfactant complexes could form a well-defined solid-state structure, which was regarded as polyelectrolyte-stabilized phases.16 Thus, we deduced that the participation of a suitable ratio of polyelectrolyte during the formation of
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Figure 8. Pore size distribution curve of the calcined sample.
CTAB templates results in the wormlike morphology of TEOS hydrolysis in a long length scale. Conclusions Mesoporous silica of wormlike morphology with a high surface area and pore volume has been prepared by hydrolysis and cross-linking of the negative inorganic precursor (TEOS) on the surface of complex templates of PSS/CTAB in a proper ratio. The strong electrostatic interaction between the surfactant and polyelectrolyte results in the formation of the pore structure with two directions. The addition of soluble polyelectrolyte plays a role in forming the framework of a wormlike morphology of the TEOS hydrolysis. Such a complex template can be considered as an extension to the synthesis of MCM-41 derivatives for adsorption, ion exchange, and catalytic processes because of its simple preparation, exceptional thermal stability and high surface area and pore volume. Acknowledgment. We acknowledge financial support from the National Nature Science Foundation of China (NNSFC29925307 and NNSFC90206035) as well as the collaborated project of German Max Planck Society. LA0343728
