Structure Transition from Hexagonal Mesostructured Rodlike Silica to

Mar 19, 2008 - 0-1.4), (2) the subsequent HEX to multilamellar structure transition by ..... (43) Yu, C. Z.; Fan, J.; Tian, B. Z.; Zhao, D. Y.; Stucky...
0 downloads 0 Views 228KB Size
Download PDF
5038

Langmuir 2008, 24, 5038-5043

Structure Transition from Hexagonal Mesostructured Rodlike Silica to Multilamellar Vesicles Pei Yuan,†,‡ Sui Yang,† Hongning Wang,† Meihua Yu,† Xufeng Zhou,† Gaoqing Lu,§ Jin Zou,*,‡ and Chengzhong Yu*,†,§ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, P. R. China, School of Engineering and Centre for Microscopy and Microanalysis, The UniVersity of Queensland, St Lucia, QLD 4072, Australia, and ARC Centre of Excellence for Functional Nanomaterials, The UniVersity of Queensland, St Lucia, QLD 4072, Australia ReceiVed January 8, 2008. In Final Form: January 31, 2008 We studied the synthesis of siliceous structures by using a nonionic block copolymer (Pluronic P123) and perfluorooctanoic acid (PFOA) as cotemplates in an acid-catalyzed sol-gel process. Different siliceous structures were obtained through systematically varying the molar ratio (R) of PFOA/P123, and the resultant materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen sorption analysis, and Fourier-transform infrared spectroscopy. The results are consistent and reveal a structure transition from a highly ordered 2D hexagonal (HEX) mesostructure with a rodlike morphology to multilamellar vesicles (MLVs) with sharp edges when R is increased. The fact that the MLVs are initiated from the end of hexagonally mesostructured rods provides key evidence in such a novel structure transition. Our finding indicates that, at least in our observations, the MLVs are developed gradually from HEX structures, rather than by a direct cooperative self-assembly mechanism. It is suggested that PFOA molecules with rigid fluorocarbon chains closely interact with PEO. This interaction model may well explain (1) the “wall-thicken” effect in HEX mesostructures by enlarging the hydrophilic PEO moiety (R ) 0-1.4), (2) the subsequent HEX to multilamellar structure transition by modifying the hydrophilic/hydrophobic volume ratio (R ) 1.4-2.8), and (3) the formation of MLVs with sharp edges by increasing the bending energy. This model provides insight into the fabrication of novel porous materials by the use of block copolymers and fluorinated surfactant mixed templates.

Introduction Since the pioneering discovery of surfactant-templated mesoporous materials reported by Mobil researchers,1,2 increasing attention has been drawn to this new family of materials because of their emerging applications as catalysts, catalyst supports, adsorbents, molecular sieves, nanoreactors, media for the immobilization of biomolecules, and templates for nanoobjects and nanostructures.3-10 Because of their potential wide applications, the controlled assembly of ordered porous silica materials with different structures has always been a global focus in this field. Generally speaking, controlling the cooperative self-assembly of organic-inorganic composites is considered to be the key * To whom correspondence should be addressed. E-mail: czyu@fudan. edu.cn, [email protected]. † Fudan University. ‡ School of Engineering and Centre for Microscopy and Microanalysis, The University of Queensland. § ARC Centre of Excellence for Functional Nanomaterials, The University of Queensland. (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, 10834-10843. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (3) Cavallaro, G.; Pierro, P.; Palumbo, F. S.; Testa, F.; Pasqua, L.; Aiello, R. Drug DeliVery 2004, 11, 41-46. (4) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56-77. (5) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350-353. (6) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. ReV. 2002, 102, 3615-3640. (7) Davis, M. E. Nature 2002, 417, 813-821. (8) Yu, C. Z.; Tian, B. Z.; Zhao, D. Y. Curr. Opin. Solid State Mater. Sci. 2003, 7, 191-197. (9) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403-1419. (10) Linssen, T.; Cassiers, K.; Cool, P.; Vansant, E. F. AdV. Colloid Interface Sci. 2003, 103, 121-147.

factor in the preparation of mesoporous materials. Until now, various routes have been explored and diverse mesostructures have been constructed.11-27 For instance, the structure transition can be induced by posttreatment methods, such as heat, water vapor, and so on.11-14 Alternatively, by tuning the hydrophilic/ hydrophobic volume ratio (VH/VL) of the mixed nonionic surfactants, ordered mesoporous materials with different symmetries can be rationally designed.24 Through adjusting the amount of an anionic cotemplate and the volume of a swelling agent 1,3,5-trimethylbenzene (TMB) in a triblock copolymer (11) Gross, A. F.; Ruiz, E. J.; Tolbert, S. H. J. Phys. Chem. B 2000, 104, 5448-5461. (12) Grosso, D.; Babonneau, F.; Soler-Illia, G.; Albouy, P. A.; Amenitsch, H. Chem. Commun. 2002, 748-749. (13) Landry, C. C.; Tolbert, S. H.; Gallis, K. W.; Monnier, A.; Stucky, G. D.; Norby, F.; Hanson, J. C. Chem. Mater. 2001, 13, 1600-1608. (14) Wu, C. W.; Pang, J. B.; Kuwabara, M. Chem. Lett. 2002, 974-975. (15) Stein, A. AdV. Mater. 2003, 15, 763-775. (16) Hampsey, J. E.; Arsenault, S.; Hu, Q. Y.; Lu, Y. F. Chem. Mater. 2005, 17, 2475-2480. (17) Bao, X. Y.; Zhao, X. S. J. Phys. Chem. B 2005, 109, 10727-10736. (18) Kim, T. W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601-7610. (19) Lin, H. P.; Mou, C. Y. Acc. Chem. Res. 2002, 35, 927-935. (20) Kosuge, K.; Sato, T.; Kikukawa, N.; Takemori, M. Chem. Mater. 2004, 16, 899-905. (21) Yao, B.; Fleming, D.; Morris, M. A.; Lawrence, S. E. Chem. Mater. 2004, 16, 4851-4855. (22) Hentze, H. P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069-1074. (23) Martines, M. A. U.; Yeong, E.; Larbot, A.; Prouzet, E. Microporous Mesoporous Mater. 2004, 74, 213-220. (24) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y. U.; Terasaki, O.; Park, S. E.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552-2558. (25) Chen, D. H.; Li, Z.; Yu, C. Z.; Shi, Y. F.; Zhang, Z. D.; Tu, B.; Zhao, D. Y. Chem. Mater. 2005, 17, 3228-3234. (26) Chen, D. H.; Li, Z.; Wan, Y.; Tu, X. J.; Shi, Y. F.; Chen, Z. X.; Shen, W.; Yu, C. Z.; Tu, B.; Zhao, D. Y. J. Mater. Chem. 2006, 16, 1511-1519. (27) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Langmuir 2006, 22, 6-9.

10.1021/la8000569 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008

Structure Transition

Langmuir, Vol. 24, No. 9, 2008 5039

templating system (Pluronic F127), highly ordered mesostructured silica with 2-D hexagonal (HEX) p6m or cubic Im3m j , Fm3m j, and Ia3dh structures in high purity can be easily obtained.26 The successful synthesis of mesoporous silica materials with controlled pore geometries, pore sizes, and morphologies is important for their future applications.24,28,29 Recently, the synthesis of siliceous vesicular materials has attracted much attention. By combining the vesicle templating30 and liquid crystal templating,31-35 other novel materials are expected to be synthesized.36 In the case of a ternary templating system (cetyltrimethylammonium bromide, sodium dodecyl sulfate, and Pluronic P123), hollow silica spheres with mesostructured shells were prepared.27 In contrast, the structure transition from highly ordered mesostructures to uni- or multilamellar vesicles has received less attention. Such a structure transition is generally achieved by changing the packing parameter through the addition of organic additives such as swelling agents37-41 or by changing the mixing ratio of ionic surfactant mixtures as structure directing agents.27,42 In order to understand the detailed mechanism of the structure transition, study of structural variations at different stages is essential. In this paper, we systematically studied the synthesis of siliceous structures by using a nonionic block copolymer [Pluronic P123, EO20PO70EO20, where EO is poly(ethylene oxide) and PO is poly(propylene oxide)] and perfluorooctanoic acid (PFOA) as cotemplates through an acid-catalyzed silica sol-gel process. By increasing the molar ratio (R) of PFOA/P123 from 0 to 2.8, a structure transition from a highly ordered 2-D hexagonal (HEX) mesostructure with a rodlike morphology to multilamellar vesicles (MLVs) with sharp edges is observed through comprehensive characterization techniques. In addition, an interaction model between P123 and PFOA is proposed to reveal the structure transition. Experimental Section Synthesis. All chemicals were commercially available and used as received without purification. Both the triblock copolymers P123 and PFOA were purchased from Aldrich. Other chemicals were purchased from Shanghai Chemical Company. Deionized water was used in all experiments. In a typical synthesis, 2 g of P123 was dissolved in 15 g of deionized water. A 60 g amount of a 2 M HCl (28) Stevens, W. J. J.; Lebeau, K.; Mertens, M.; Van Tendeloo, G.; Cool, P.; Vansant, E. F. J. Phys. Chem. B 2006, 110, 9183-9187. (29) Zhang, H.; Sun, J. M.; Ma, D.; Weinberg, G.; Su, D. S.; Bao, X. H. J. Phys. Chem. B 2006, 110, 25908-25915. (30) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. AdV. Mater. 2000, 12, 1286-1290. (31) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366-368. (32) Templin, M.; Franck, A.; DuChesne, A.; Leist, H.; Zhang, Y. M.; Ulrich, R.; Schadler, V.; Wiesner, U. Science 1997, 278, 1795-1798. (33) 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, 1299-1303. (34) Huo, Q. S.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176-1191. (35) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (36) Djojoputro, H.; Zhou, X. F.; Qiao, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q. J. Am. Chem. Soc. 2006, 128, 6320-6321. (37) Sun, J. M.; Ma, D.; Zhang, H.; Wang, C. L.; Bao, X. H.; Su, D. S.; Klein-Hoffmann, A.; Weinberg, G.; Mann, S. J. Mater. Chem. 2006, 16, 15071510. (38) Singh, M.; Ford, C.; Agarwal, V.; Fritz, G.; Bose, A.; John, V. T.; McPherson, G. L. Langmuir 2004, 20, 9931-9937. (39) Yin, H. Q.; Lei, S.; Zhu, S. B.; Huang, J. B.; Ye, J. P. Chem.sEur. J. 2006, 12, 2825-2835. (40) Cui, K.; Cai, Q.; Chen, X. H.; Feng, Q. L.; Li, H. D. Microporous Mesoporous Mater. 2004, 68, 61-64. (41) Lind, A.; Spliethoff, B.; Linden, M. Chem. Mater. 2003, 15, 813-818. (42) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. D.; Lindman, B. J. Phys. Chem. B 1998, 102, 6746-6758.

Figure 1. XRD patterns of calcined silica samples S0-S4 prepared at different molar ratios of PFOA/P123 (R ) 0, 0.7, 1.4, 2.1, 2.8, respectively). *The structure of S4 cannot be determined from the XRD pattern, and the d-spacing is calculated from the first diffraction peak.

solution was added to the mixture under stirring at room temperature. A desired amount of PFOA (0, 0.1, 0.2, 0.3, and 0.4 g in five different syntheses) was then added into the solution. The R of PFOA/P123 is 0, 0.7, 1.4, 2.1, and 2.8, and the five samples are named as S0, S1, S2, S3, and S4, respectively. The temperature of each solution was raised to 35 °C and kept at that temperature before 4.25 g of tetraethyl orthosilicate (TEOS) was added. The reaction mixture was stirred at the same temperature for 24 h and then transferred to a Teflon autoclave and hydrothermally treated at 100 °C for another 24 h. The solid products were collected by filtration, washed with deionized water, and dried in air at room temperature. The as-synthesized samples were calcined at 550 °C for 5 h in air to remove the templates. Characterization. X-ray diffraction (XRD) was carried out using a German Bruker D4 X-ray diffractometer with Ni-filtered Cu KR radiation. Transmission electron microscopy (TEM) investigations were performed in a TECNAI 12 microscope operated at 120 kV. TEM specimens were prepared by dispersing the powderlike samples in ethanol by ultrasonic for 3 min and then depositing them on the holey carbon films. Scanning electron microscopy (SEM) was used on a JEOL 6400 microscope operated at 20 kV. Nitrogen adsorption/ desorption isotherms were measured at -196 °C by a Micromeritics ASAP Tristar 3000 system. The samples were degassed at 180 °C overnight on a vacuum line. The FTIR spectra were collected by using a Nicolet FTIR 360 spectroscope (KBr method).

Results and Discussion XRD Results. Figure 1 shows the small angle XRD patterns of calcined samples S0-S4. For S0-S3, three characteristic diffraction peaks are generally observed in their XRD patterns, which can be indexed as (10), (11), and (20) diffractions a 2-D HEX plane group (p6m). The d10 can be determined and listed in Table 1. The lattice parameter a based on the p6m plane group for S0-S3 can be calculated as 10.9, 11.1, 12.1, 12.1 nm, respectively. A general trend in S0-S3 can be summarized as following: in the range of R ) 0-2.1, the use of PFOA/P123 cotemplates does not change the resultant HEX mesostructure. When R is increased from 0 to 1.4, the lattice parameter is increased from 10.9 to 12.1 nm, while further increasing R does not alter the lattice parameter (comparing S2 and S3). For S3 (R ) 2.1), the weak intensities of (11) and (20) diffraction peaks indicate that the order of the mesostructure is reduced. When R is further increased to 2.8 (S4), the XRD pattern shows only one broad diffraction peak with a very low intensity, indicating that this sample does not have long-range structural ordering. The average d-spacing of S4 calculated from the first diffraction peak is 10.4 nm, similar to that of S2 and S3 (10.5 nm).

5040 Langmuir, Vol. 24, No. 9, 2008

Yuan et al.

Table 1. Physicochemical Properties of Mesoporous Materials Templated by P123/PFOA samples

d(10), nm

D, nm

B, nm

Vp, cm3/g

SBET, m2/g

S0 S1 S2 S3 S4

9.4 9.6 10.5 10.5 10.4c

8.9 8.9 8.9 8.8 -

2.0 2.2 3.2 3.3 -

0.95 0.99 1.12 1.06 0.88

445 568 772 738 605

a Note: Samples S0, S1, S2, S3, S4 were synthesized at PFOA/P123. R ) 0, 0.7, 1.4, 2.1, 2.8, respectively. b Abbreviations: d-spacing (d (10)), pore diameter (D), pore wall thickness (B), pore volume (Vp), and BET surface area (SBET) of S0-S4. c The structure of S4 cannot be determined from the XRD pattern, and the d-spacing is calculated from the first diffraction peak.

Figure 3. TEM images (A-D) of calcined samples S1-S4 prepared at different molar ratios of PFOA/P123 (R ) 0.7, 1.4, 2.1, 2.8, respectively). The scale bar of the insert images is 200 nm.

Figure 2. SEM images (A-D) of calcined samples S1-S4 prepared at different molar ratios of PFOA/P123 (R ) 0.7, 1.4, 2.1, 2.8, respectively).

SEM and TEM Images. Figure 2 is SEM images and shows typical morphologies of S1-S4. For S0 (in the absence of PFOA), a similar morphology as reported35 has been observed, so that will not be discussed further. As can be seen from Figure 2, increasing R leads to different morphologies in the final materials. For R ) 0.7, a rodlike morphology with a diameter between 130-180 nm and a length of 1-4 µm is obtained (Figure 2A), similar to that of rodlike SBA-15 synthesized previously.43,44 It should be noted that the majority of these rods are straight. When R is increased to 1.4, a rodlike morphology with curved or even U-shaped is observed (Figure 2B). Further increasing R to 2.1 leads to curved rods mixed with some spherical morphology (Figure 2C). Interestingly, when R is equal to 2.8, flowerlike morphology is observed (Figure 2D). Each flower consists of several shrunken spheres, whose sizes are in the range of 100250 nm. Figure 2 indicates that R is a key parameter in manipulating the morphology. Figure 3 is large-area TEM images of calcined samples S1S4 showing the general information and the high-magnification TEM images (insets) demonstating the representative structures. (43) Yu, C. Z.; Fan, J.; Tian, B. Z.; Zhao, D. Y.; Stucky, G. D. AdV. Mater. 2002, 14, 1742-1745. (44) Boissiere, C.; Larbot, A.; van der Lee, A.; Kooyman, P. J.; Prouzet, E. Chem. Mater. 2000, 12, 2902-2913.

The rodlike morphology as well as the highly ordered HEX mesostructure of S1 can be clearly seen from Figure 3A. The straight 1-D channels viewed along the [11] directions are parallel to the axial direction of the rods (inset of Figure 1A). The distance between two adjacent channels is measured to be 9.5 nm, in good agreement with that calculated from the XRD pattern. The low-magnification TEM image of S2 is in accordance with the SEM results, where curved morphology is observed (Figure 3B). From the high-magnification TEM image (insert of Figure 3B), it is of interest to note that most of the rods have round ends, instead of “crew-cut” shapes in straight rodlike S1 samples. The mesostructure at the end is quite different from the HEX packing (indicated by black arrows). The details will be discussed below. Figure 3C shows that S3 is a mixture of MLVs and HEX mesostructures. The inset of Figure 3C is a reprensentive image showing a curved hexagonally ordered rod and a multilamellar vesicle. Figure 3D confirms the multilamellar structure of S4 at a low magnification, and the detailed feature of one multilamellar vesicle is shown in the inset of Figure 3D as an example. The average d-spacing is estimated to be 9.5 nm in S4. Taking all XRD, SEM, and TEM results into account, we can conclude that when the ratio of PFOA/P123 cotemplates is increased from 0.7 to 2.8, a structure transition from highly ordered HEX mesostructure with a straight rodlike morphology (R ) 0.7) to MLVs with sharp edges (R ) 2.8) occurs. S2 (R ) 1.4) is an intermediate step of this structure transition and has an interesting structure at the end of rods (Figure 3B). Figure 4 presents TEM images and shows detailed morphology of S2. As can be seen from Figure 4, within a rod, the body section has a HEX structure (also confirmed by XRD), while the multilamellar vesicular structure is grown at the end of rods. This is in great contrast to S3, in which the curved rodlike morphology with a HEX structure coexists with the MLVs particles (Figures 2C and 3C). The unusual structure of S2 can be viewed as an intermediate stage during the structure transition from HEX mesostructure to MLVs and should be critical for understanding the formation mechanism at both the mesoscale and the macroscale (morphology).

Structure Transition

Figure 4. TEM images of sample S2 showing details of the end of rods.

Figure 5. N2 sorption isotherms (A) and pore size distribution curves (B) of the calcined samples S0-S4 prepared at different molar ratios of PFOA/P123 (R ) 0, 0.7, 1.4, 2.1, 2.8, respectively).

N2 Adsorption/Desorption Isotherms. Figures 5A and 5B show the N2 adsorption/desorption isotherms and the pore size distribution curves calculated from the adsorption branch using the Barrett-Joyner-Halenda (BJH) model of calcined samples S0-S4, respectively. Based on the N2 adsorption/desorption and the XRD data, the values of the surface area, pore size, pore volume, and the wall thickness of siliceous samples with HEX mesostructures are determined and summarized in Table 1. S0S2 exhibit Type IV isotherms with Type H1 hysteresis loops, indicating that the calcined mesoporous materials have 1-D pore structures. The steep capillary condensation step occurs at the same position (P/P0 ) 0.70-0.85), and the pore diameter of S0-S2 can be calculated as 8.9 nm in three samples. This

Langmuir, Vol. 24, No. 9, 2008 5041

observation is in contrast to the XRD results (Table 1). For S0S2 with the same pore size, the lattice parameter increases from 10.9 to 12.1 nm. From the sorption isotherms of S3 and S4, different structures with respect to those in S0-S2 are expected. For convenience, we first analyze the sorption isotherm of S4. The adsorption isotherm of S4 (Type II) does not show a plateau at high relative pressure. The shape of the hysteresis loop (Type H4) is also different from that of S0-S2. This type of hysteresis loop has been observed in structures consisting of voids surrounded by a mesoporous matrix45 or hollow particles with mesoporous walls.46-48 In fact, TEM (Figure 3D) confirmed S4 having a multilamellar vesicular structure, which agrees with the N2 adsorption/desorption isotherms. Unlike S0-S2, there are two peaks in the pore size distribution of S4 and both peaks are very broad. A first broad peak centered at 9 nm (Figure 5B) is attributed to the spacing between two adjacent layers of the multilamellar vesicular structure, consistent with the XRD and TEM results. The majority of volume adsorbed in S4 is reflected in the second broad peak centered at ∼20-48 nm, which indicates that the insides of the MLVs are indeed hollow. The reduced surface area is another indication of the lamellar pore structure of S4. When the isotherm of S3 is carefully examined, it can be seen that the isotherm shows mixed features of S0-S2 and S4. From the adsorption branch of S3 (Figure 5A), a steep capillary condensation occurred at P/P0 ) 0.70-0.84 is similar to that observed in S0-S2, while the shape of isotherm after P/P0 > 0.80 shows a similar feature to that observed in the isotherm of S4. Moreover, from the desorption branch of S3, the capillary evaporation occurred at P/P0 ) 0.75-0.62 is also similar to that observed in S0-S2, while another evaporation process occurs at P/P0 ) 0.47-0.58. The desorption branch crosses the adsorption branch at P/P0 ≈ 0.47 for S3 and S4 only. The above observation is in accordance with the SEM and TEM results: S3 is a mixture of HEX and MLVs structures. The Structure Evolution Mechanism. The above XRD, SEM, TEM, and N2 sorption results are consistent and reveal that the structure evolution consists of two steps. In the first step when R is increased from 0 to 0.7 and 1.4, the HEX mesostructure is retained while a “wall-thicken” process is observed (Table 1). The second step occurs from R ) 1.4 to 2.8. Although the dominant structure is HEX mesophase at R ) 1.4, evidence of a structure transition from the HEX to multilamellar vesicular structure is observed. The HEX mesostructure and MLVs coexist in S3 (R ) 2.1), while MLVs are the main phase at R ) 2.8. In order to understand the structure evolution with increasing R, we first studied the FTIR spectrum of as-synthesized S0-S4 as well as PFOA in the 1100-2000 cm-1 region as shown in Figure 6. One typical peak centered at about 1766 cm-1 in the spectrum of pure PFOA is assigned to the carbonyl stretching vibration of the free carboxylic acid group. The asymmetric and symmetric CF2 stretches give rise to the strong bands located near 1205 and 1148 cm-1, respectively. These three peaks cannot be observed in as-synthesized S0 obtained in the absence of PFOA. In contrast, the as-synthesized materials S1-S4 obtained at different R exhibit weak broad peak near 1772 cm-1. The blue-shift of 6 cm-1 compared with pure PFOA for carboxyl groups suggests the existence of hydrogen bonding of COOH (45) Lin, H. P.; Wong, S. T.; Mou, C. Y.; Tang, C. Y. J. Phys. Chem. B 2000, 104, 8967-8975. (46) Prouzet, E.; Cot, F.; Boissiere, C.; Kooyman, P. J.; Larbot, A. J. Mater. Chem. 2002, 12, 1553-1556. (47) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267-1269. (48) Martines, M. U.; Yeong, E.; Persin, M.; Larbot, A.; Voorhout, W. F.; Kubel, C. K. U.; Kooyman, P.; Prouzet, E. C. R. Chim. 2005, 8, 627-634.

5042 Langmuir, Vol. 24, No. 9, 2008

Figure 6. FTIR spectra of as-made samples synthesized at different R. Because of the acid synthetic conditions, we use the spectrum of pure PFOA for comparison.

with the PEO segments.49 Furthermore, the relative intensity of the signal increases with increasing R, in accordance with the increasing percentage of PFOA molecules interacting with P123. In our study, all the materials S1-S4 were synthesized under exactly the same conditions except that different amounts of PFOA were employed. Thus, the structure transition should be induced directly by PFOA. The FTIR results clearly show that PFOA molecules are present within the as-synthesized samples. In our synthesis, the as-synthesized materials were washed with water, and PFOA has a solubility of 3.4 g/L in pure water. Therefore, for micelle-templated mesostructured materials, the structure transition observed in our experiments should be associated with PFOA molecules which are located in the composite micelles. The nature of PFOA molecules is important to interpret the structure evolution with increasing R. Fluorinated molecules and their derivatives represent a very interesting and stimulating class of chemicals in physical chemistry and polymer science because of their specific and unusual properties. They are more hydrophobic and more surface active compared to the hydrocarbon counterparts.50 Because of the higher rigidity of the fluorocarbon chain, there exists much stronger interchain van der Waals attraction between perfluorinated alkyl chains.50 Recently, fluorocarbon surfactants have been used together with hydrocarbon surfactants to synthesize ordered mesoporous silica-based materials.51-55 However, the influence of small perfluorocarboxyl molecules on the structure of block copolymer-templated mesostructures and the structure transition have not been observed previously. It has been reported that in the dilute aqueous solution, PFOtends to interact with poly(ethylene glycol) because of the extremely hydrophobic properties of perfluorinated surfactant.56,57 (49) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301-310. (50) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237-9246. (51) Han, Y.; Li, D. F.; Zhao, L.; Song, J. W.; Yang, X. Y.; Li, N.; Di, Y.; Li, C. J.; Wu, S.; Xu, X. Z.; Meng, X. J.; Lin, K. F.; Xiao, F. S. Angew. Chem., Int. Ed. 2003, 42, 3633-3637. (52) Zhang, Z. T.; Han, Y.; Zhu, L.; Wang, R. W.; Yu, Y.; Qiu, S. L.; Zhao, D. Y.; Xiao, F. S. Angew. Chem., Int. Ed. 2001, 40, 1258. (53) Han, Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2005, 44, 288-292. (54) Li, D. F.; Han, Y.; Song, H. W.; Zhao, L.; Xu, X. Z.; Di, Y.; Xiao, F. S. Chem.sEur. J. 2004, 10, 5911-5922. (55) Yang, S.; Zhao, L. Z.; Yu, C. Z.; Zhou, X. F.; Tang, J. W.; Yuan, P.; Chen, D. Y.; Zhao, D. Y. J. Am. Chem. Soc. 2006, 128, 10460-10466. (56) Gianni, P.; Barghini, A.; Bernazzani, L.; Mollica, V.; Pizzolla, P. J. Phys. Chem. B 2006, 110, 9112-9121. (57) Gianni, P.; Barghini, A.; Bernazzani, L.; Mollica, V. Langmuir 2006, 22, 8001-8009.

Yuan et al.

In our case, the synthesis was carried out at acidic conditions (∼2 M HCl); thus, PFOA molecules (pKa ) 2.5) are not deprotonated. The FTIR results indicate that in our acidic conditions, the PFOA molecules may also interact with the ether group of P123 through the COOH-COC hydrogen bond. Such an interaction model provides insight into the structure evolution process of our system. In the “wall-thicken” step (R ) 0-1.4), the lattice parameter of the HEX mesostructure increases from 10.9 to 12.1 nm while the pore size remains constant (8.9 nm, see Table 1) in all cases. This indicates that the wall is thickened by 60% (from 2.0 to 3.2 nm). Apparently, PFOA does not behave as a swelling agent, such as 1,3,5-trimethylbenzene (TMB),35 and it is difficult to dissolve in the PPO core of P123 micelles. It is suggested that the PFOA molecules are embedded in the PEO shell rather than in the PPO core of the P123 micelle. As a consequence, the use of PFOA does not increase the pore size. On the other hand, the inclusion of PFOA molecules with rigid fluorocarbon chains in the EO moiety may enlarge the hydrophilic part, leading to mesoporous materials with thicker walls. In the second HEX to multilamellar vesicular structure evolution step (R ) 1.4 to 2.8), the ratio of PFOA to P123 is further increased. The hydrogen bonding between PFOA and EO dehydrates the EO moiety. With increasing R, more hydrophobic PFOA molecules are present in the EO part and the volume of water excluded from EO is increased, resulting in the decrease of the hydrophilicity of EO units. The decreased VH/VL finally leads to the structure evolution from HEX to MLVs phase as observed in our experiments. Further Discussion of the Structure Evolution. As discussed above and in the literature, the structure transition is generally interpreted by the change of VH/VL and/or packing parameter,24,58 at a molecular or supramolecular level. It is not clear, however, how the structure transition takes place during HEX to multilamellar vesicular structure evolution, i.e., which is the favored pathway: MLVs are assembled directly from organic and inorganic precursors in solution or a gradual and smooth transition from HEX to multilamellar vesicular structure takes place? To answer this question, we note that as shown in Figure 3B and Figure 4, the multilamellar vesicular structure formed at the end of individual hexagonally mesostructured rods indicates a gradual structure evolution from HEX to multilamellar vesicular phase. The fact that MLVs are initiated at the end of rods can be helpful in understanding the driving force of this structure evolution. It is suggested that the interaction between PFOA and block copolymer is the driving force for a hexgonal to lamellar phase transition. However, it is the surface tension reduction that leads to the sheetlike lamella closure into the vesicular form.59 In this regard, it is well-known that at a constant volume, the specific external surface areas increase in the order of spheres, curveshaped rods, and straight rods. The tip region of a rod is the more unstable area because of the higher specific surface area compared to the body part, therefore the structure transition from HEX to multilamellar vesicular structure occurs most likely in the tip region. Normally, decreasing the VH/VL ratio should lead to a structure evolution from HEX to lamellar phase,60 an important initial step in the formation of vesicles.61,62 Mathematically,61,63 the (58) Huo, Q. S.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 11471160. (59) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323-1333. (60) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; Wiley: New York, 2003; pp 89-93. (61) Shioi, A.; Hatton, T. A. Langmuir 2002, 18, 7341-7348. (62) Xia, Y.; Goldmints, I.; Johnson, P. W.; Hatton, T. A.; Bose, A. Langmuir 2002, 18, 3822-3828.

Structure Transition

Langmuir, Vol. 24, No. 9, 2008 5043

total energy (E) of a bilayer membrane is the sum of unstable edge energy (Eedge) and bending energy (Ebend). Through the formation of vesicles, Eedge is minimized while Ebend is increased. Energy compromise between Eedge and Ebend may determine the size and shape of resultant vesicular structures. We propose that the formation of MLVs with sharp edges (S4) is also associated with the use of the PFOA cotemplate with rigid fluorocarbon chains. This rigidity may increase the Ebend, and the formation of a perfect spherical vesicular structure is not favored; thus, MLVs with many facets are obtained.

Conclusion A structure transition from a highly ordered 2-D HEX mesostructure with a rodlike morphology to MLVs with sharp edges is achieved by increasing the molar ratio of PFOA/P123 as cotemplates. Importantly, an intriguing intermediate structure transition state at the end of hexagonally mesostructured rods is first observed in our systematic study, which provides key evidence in understanding the structure transition mechanism. (63) Lipowsky, R. J. Phys. II 1992, 2, 1825-1840.

Our finding indicates that, at least in our observations, the MLVs are developed gradually from HEX structures, rather than by a direct cooperative self-assembly mechanism from organicinorganic precursors in solution. It is suggested that PFOA molecules with rigid fluorocarbon chains interact with PEO through hydrogen bonding. This interaction may well explain (1) the “wall-thicken” effect in HEX mesostructures by enlarging the hydrophilic PEO moiety (R ) 0-1.4), (2) the subsequent HEX to multilamellar structure transition by modifying the hydrophilic/hydrophobic volume ratio (R ) 1.4-2.8), and (3) the formation of MLVs with sharp edges by increasing the bending energy. It is anticipated that our understandings as well as the use of block copolymers and fluorinated surfactant mixed templates will lead to the fabrication of novel porous materials. Acknowledgment. The authors thank the State Key Research Program (2004CB217804), the NSF of China (20421303), Shanghai Science Committee (06JC14011), Shanghai Leading Academic Discipline Project (B113), NCET, and the Australian Research Council for their financial support. LA8000569