Anionic surfactant induced mesophase transformation to synthesize highly ordered large-pore mesoporous silica structures

Abstract

Successive mesophase transformation induced by an anionic surfactant such as sodium dioctyl sulfosuccinate (AOT) has been demonstrated to fabricate four kinds of large pore mesoporous silica materials in a triblock copolymer F127 surfactant assembly system. The transformation of the highly ordered mesostructures from face-centered cubic (space group Fm3̄m) to body-centered Im3̄m then towards two-dimensional (2-D) hexagonal p6m and eventually to cubic bicontinuous Ia3̄d symmetries has been achieved by tuning the amount of AOT and 1,3,5-trimethylbenzene (TMB). Characterization by small-angle X-ray scattering (SAXS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ sorption isotherms reveals that all mesoporous silica structures have highly ordered regularity in large domains and possess high surface areas, large pore volumes and uniform pore sizes. The expansion of hydrophobic volume in the amphiphilic Pluronic F127 surfactant associated with AOT and TMB molecules in an acidic media is attributed to the observed mesophase transformation. A further swelling of the surfactant micelles can be achieved by adding TMB molecules into the mixed AOT and F127 surfactants system due to their synergistic solubility enhancement, which gives rise to a long-range ordered 2-D hexagonal mesoporous silica structure with very large cell parameter (a = 16.5 nm) and pore size (∼12 nm). The understanding of the blend–surfactant assembly mechanism will lead to a more rational approach for economical and large-scale production of mesoporous materials with controllable structures.

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Introduction

The proliferation of new mesostructures has drawn increasing interest in the properties and behaviors of surfactant assemblies for synthetic chemistry. Highly ordered large-pore materials with controllable structures and systematic tailoring of the pore architecture are very promising candidates for processes involving large molecules.^1–5 The combination of the above two objectives will inevitably result in a large range of mesoporous materials and meet the demand of the growing applications emerging in adsorption, separation, catalysis, drug delivery, etc.^6–11 Rational control of the synthetic approaches has been stimulated by the modulation of the surfactant. Nonionic triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers (PEO-PPO-PEO) with diverse structural characteristics, rich phase behavior, low cost and non-toxic degradation can direct the mesoscopic assembly and are becoming more and more popular and powerful in the synthesis of mesoporous silica materials.^12,13 Moreover, the resulting materials possess highly ordered mesostructures with larger pore sizes (usually larger than 5 nm), thicker pore walls and better thermal stability compared with their analogues derived from low-molecular-weight cationic surfactants.^12–15 Ample evidence is available that the pore structures, including channel connectivity and pore size, of such “cavity-crystals” can be remarkably controlled by adjusting the synthetic parameters, such as volume fraction of the polymer blocks, reaction temperature, acid concentration, composition of the solution, etc.^{13,14,16–18} For example, the 2-D hexagonal (p6m) silicate mesostructure, represented by the well-known SBA-15, can be acquired from triblock copolymers with relatively small EO block fractions, e.g. Pluronic P123 (EO₂₀PO₇₀EO₂₀), P104 (EO₂₇PO₆₁EO₂₇) and P85 (EO₂₆PO₃₉EO₂₆).^13,19 Body centered cubic (space group Im3̄m) mesoporous materials are usually obtained from copolymers with larger EO segments, such as Pluronic F127 (EO₁₀₆PO₇₀EO₁₀₆) and F108 (EO₁₃₂PO₇₀EO₁₃₂), in the presence of inorganic salts.¹⁷ Decreasing acidic concentration in the F127 triblock copolymer aqueous solution leads to face-centered cubic Fm3̄m mesoporous silica (KIT-5) through a thermodynamically controlled silica mesophase formation process.²⁰ Recent published reports indicate that the bicontinuous body-centered cubic Ia3̄d mesostructured silicates with large pores can be obtained by adding additives such as 3-mercaptopropyltrimethoxysilane, butanol, or inorganic salts into the dilute P123 solutions under relatively weak acid conditions.^21–24 In general, only individual silica mesostructures can be accomplished in a single triblock copolymer templated system even with significant variation in the reaction conditions. Although a three-mesostructure evolution process, from cubic Fm3̄m to Im3̄m and finally to 2-D hexagonal p6m, has been achieved recently by tuning the amount of butanol in a dilute F127 aqueous solution,²⁵ manufacture on a large scale is unmanageable due to the volatility of butanol and the familiar cubic Ia3̄d mesostructure is skipped. Can the phase transformation involving the above four well-known mesostructures occur in a simplified dilute surfactant solution? We attempted to find constituents in the surfactant architecture that can induce the continuous phase transformation so that rational fabrication of various mesostructures from a simple system becomes possible. This can, in turn, declare whether the cooperation of the surfactant components can endow a novel assembly property distinct from the component themselves. Anionic surfactants are a family of the surfactants which are extremely cheap and contribute to a large market in detergents. However, the repulsive interaction between anionic surfactant and silicate species prevents the organization of an ordered mesostructure. It is noted that synthetic anionic–nonionic surfactants have been widely used in industrial fields. Does the additive of anionic surfactant as co-template in the triblock copolymer system facilitate the silicate–surfactant cooperative assembling process? In fact, anionic surfactants have been successfully used as the co-templates to prepare bicontinuous cubic or vesicle-like mesoporous silica structures with cationic surfactants under basic conditions.^26–28 Previously, we reported that the cubic Ia3̄d mesostructure can be templated by using mixed structure-directing agents (SDAs) of commercially available nonionic block copolymer P123 and anionic sodium dodecyl sulfate.²⁹

Herein, a facile process of anionic surfactant induced mesophase transformation to synthesize large pore mesoporous silica has been developed in a nonionic triblock copolymer assembly system. Highly ordered mesoporous silica materials with 2-D hexagonal p6m, cubic Im3̄m, Fm3̄m and Ia3̄d structures in high purity can be readily obtained by adjusting the amount of the anionic co-template, sodium dioctyl sulfosuccinate (AOT), or the volume of the swelling agent, 1,3,5-trimethylbenzene (TMB), in the triblock copolymer F127 system. Evident synergism of the anionic surfactant in the nonionic surfactant assembly renders it abundant phase behavior, which can guide the successive phase transformation in an individual system. Based on such organization, highly ordered hexagonal mesoporous silicates with very large pore sizes (up to 12 nm) and cell parameters (a = 16.5 nm) have been reproducibly obtained and proved to be stable in boiling water. Comprehension of the mechanism relating to the mixed-surfactant assembly will ultimately result in a rational approach to the preparation of mesoporous materials with diverse symmetries.

Experimental

Chemicals

All chemicals were used as received without purification. Sodium dioctyl sulfosuccinate (C₂₀H₃₇SO₇Na, AOT, 98%) and the triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer EO₁₀₆PO₇₀EO₁₀₆ denoted Pluronic F127 (M_w = 12 600) were purchased from Aldrich. Tetraethyl orthosilicate (TEOS) (99%) was obtained from Guanghua Chemical Company. Other chemicals were purchased from Shanghai Chemical Company. Millipore water was used in all experiments.

Synthesis

Highly ordered mesoporous silica materials with large pore sizes were hydrothermally prepared by using triblock copolymer Pluronic F127 as template and AOT as co-template in dilute H₂SO₄ aqueous solution at 45 °C. The batch compositions of F127 : AOT : H₂SO₄ : H₂O : TEOS were 3.4 × 10⁻³ : (0–0.034) : 0.97 : 93 : 1.0 (molar ratio). In view of the anion sequence (SO₄²⁻/HSO₄²⁻ > NO₃⁻ > Br⁻ > Cl⁻) in the phase transformation of silica mesostructures templated by triblock copolymers,³⁰ H₂SO₄ aqueous solution instead of HCl was used to catalyze the hydrolysis of TEOS molecules and it was found that high-quality samples were obtained with H₂SO₄ concentrations of 0.35–0.55 M in this process.

The face-centered cubic (space group Fm3̄m) mesostructured silica (S1)

A typical synthesis procedure was carried out as follows: 0.80 g F127 was dissolved in a mixture of 31 g water and 9.0 g H₂SO₄ (2.0 M) at 45 °C. To this solution, 3.8 g TEOS was added under vigorous stirring. The reaction mixture was stirred at the same temperature for 24 h, then sealed within a Teflon autoclave and heated at 100 °C for another 24 h. After being filtered, washed with water, and dried in air at room temperature, the solid products were collected, and then calcined at 550 °C for 5 h in air to remove the templates. Thus, the final mesoporous silica materials were obtained.

The body-centered cubic (Im3̄m) mesostructured silica (S2)

0.80 g F127 and 0.084 g AOT were dissolved in a mixture of 31.0 g water and 9.0 g H₂SO₄ (2.0 M) at 45 °C to obtain a clear solution. After 3.80 g TEOS was added to the solution under vigorous stirring, the mixture was continuously stirred at 45 °C for 1 day. The precipitate together with the solution was transferred into a Teflon autoclave and then heated at 100 °C for 24 h. Collection and calcination of the resultant mixture were performed according to the above procedure.

The two-dimensional (2-D) hexagonal (p6m) mesostructured silica (S3)

0.80 g F127 and 0.20 g AOT were dissolved in a solution of 31.0 g water and 9.0 g H₂SO₄ (2.0 M) at 45 °C. 3.80 g TEOS was then added as a batch. After continuous stirring at the same temperature for 24 h, the reaction mixture was sealed within a Teflon autoclave and subjected to a hydrothermal treatment at 100 °C for a period of 24 h. The products were collected and calcined by the same procedure as that adopted for sample S1. Large pore hexagonal (p6m) mesostructured silica materials (assigned as S4–S7) were prepared by adding TMB as a swelling agent during the synthesis process of sample S3. For example, 0.80 g F127 and 0.20 g AOT were dissolved in a mixture of 31.0 g water and 9.0 g H₂SO₄ solution (2.0 M) at 45 °C, then 0.10–0.40 g TMB was added with stirring to obtain a clear solution. After that, 3.80 g TEOS was poured into the solution and vigorously stirred at the same temperature for 1 day. The hydrothermal process and the calcination of the products were carried out as in the above procedure.

The bicontinuous cubic (Ia3̄d) mesostructured silica (S8)

S8 was prepared with the aid of co-template AOT and organic swelling agent TMB in the F127 template system: 0.80 g F127 and 0.28 g AOT were dissolved in a mixture of 31.0 g water and 9.0 g H₂SO₄ (2.0 M) at 45 °C to obtain a clear solution. To this solution, 0.45 g TMB was added with vigorous stirring. After about 1 h, a light blue solution was obtained, then 3.80 g TEOS was added under vigorous stirring. The reaction mixture was continuously stirred at 45 °C for 24 h, then transferred into a Teflon autoclave and hydrothermally treated at 100 °C for another 24 h. Subsequent collection and calcination of the products were performed according to the above procedure.

Characterization

The structural quality and the symmetry of the mesoporous materials were determined by a combination of powder X-ray diffraction (XRD) investigations, small-angle X-ray scattering (SAXS) analyses and transmission electron microscopy (TEM) characterization. The low-angle XRD patterns were recorded on a German Bruker D4 X-ray diffractometer with Ni-filtered Cu Kα radiation, while a NanoSTAR system (Bruker SAXS) with pinhole collimation and a 2-D detector (HiSTAR), mounted on a micro focus X-ray tube and equipped with crossed Göbel mirrors, was used for the SAXS measurements. TEM images were obtained with a JEOL 2011 microscope operated at 200 kV. Before TEM measurements, the powder samples were dispersed in ethanol, and then dipped and dried on Cu grids. Scanning electron microscopy (SEM) images were obtained on a Philips XL30 microscope operated at 20 kV. Nitrogen adsorption–desorption isotherms were measured at −196 °C by using a Micromeritics ASAP Tristar 3000 system. The samples were degassed at 180 °C overnight on a vacuum line. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas. The pore size distributions of the surfactant-free products were derived from the adsorption branches of the isotherms based on the BdB sphere or cylinder model.

Results and discussion

Mesoporous silica with cubic Fm3̄m structure

Highly ordered mesoporous silica materials can be hydrothermally synthesized in triblock copolymer F127 surfactant assembly systems with/without the aids of AOT as the co-template and/or TMB as the swelling agent. The mesophase transformation determination relies on the combination of small angle X-ray scattering (SAXS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The initial mesoporous silica structure (S1 sample) is derived from a pure F127 surfactant assembly at 45 °C. Nine resolved diffraction peaks can be detected for the as-synthesized S1 sample in the SAXS pattern (Fig. 1A). Calcination at 550 °C in air causes a structural shrinkage, as indicated by the above peaks in the SAXS pattern shifting to higher q values of 0.558, 0.641, 0.908, 1.07, 1.11, 1.28, 1.40, 1.44 and 1.58 nm⁻¹, respectively (Fig. 1A). The q-value ratios of these nine diffraction peaks are √3 : √4 : √8 : √11 : √12 : √16 : √19 : √20 : √24, which are in good agreement with the characteristic values of the face-centered cubic structure according to the principle of crystallography. These peaks can be indexed to 111, 200, 220, 311, 222, 400, 331, 420 and 422 Bragg reflections, respectively. The reflection conditions can be summarized as follows: {hkl: h + k, h + l, k + l = 2n}, {0kl: k, l = 2n}, {hhl: h + l = 2n} and {h00: h = 2n}. Consequently, F23, Fm3̄, F432, F3̄3m and Fm3̄m are possible space groups, and Fm3̄m is warily chosen as the highest space group. The cell parameters for as-synthesized and calcined samples S1 are calculated to be 21.3 and 19.6 nm, respectively. These values are similar to that of KIT-5 reported by Ryoo and co-workers.²⁰ Compared with another faced-centered cubic (Fm3̄m) mesoporous silica (FDU-12)² prepared by using triblock copolymer F127 as a template in the presence of inorganic salt KCl and organic additive TMB under a relatively high concentration, the resulting product S1 shows a quite different X-ray diffraction pattern. The former displays a strong 311 and unresolved 200 and 400 diffraction peaks, while the latter exhibits a strong 220 diffraction peak, together with a weak 311 reflection and a shoulder peak that can be indexed to the 200 reflection. The remarkable discrepancy between mesoporous silica FDU-12 and sample S1 with the same face-centered cubic Fm3̄m structure implies a difference of the pore configuration. The high-resolution TEM images of the calcined S1 sample taken along the [100] and [211] directions are shown in Fig. 1B and C, respectively. The highly ordered arrangements of mesopores further reveal that the product consists of a high-quality face-centered cubic Fm3̄m mesostructure. The cell parameter, a, obtained from TEM analysis is 19.5 nm, in good agreement with that calculated from the SAXS measurement.

Fig. 1 SAXS patterns (A) and TEM images (B, C) of the cubic (Fm3̄m) mesoporous silica sample (S1) taken along the [100] (B) and [211] (C) zone axes, respectively. Sample S1 was templated by triblock copolymer F127 surfactant in a dilute H₂SO₄ solution.

The nitrogen adsorption–desorption isotherms (Fig. 2) of the calcined sample S1 prepared from Pluronic F127 without addition of anionic surfactant AOT in a dilute H₂SO₄ solution show a type IV isotherm with an H₂-type hysteresis loop, implying cage-like mesopores. It has a high BET surface area of 790 m² g⁻¹, a large pore volume of 0.567 cm³ g⁻¹, and a narrow pore size distribution at the mean value of 7.8 nm calculated from the adsorption branch of the isotherms based on the BdB sphere model (Table 1).

Fig. 2 Nitrogen sorption isotherms of the calcined mesoporous silica materials (samples S1, S2, S3 and S8). The isotherms of samples S2, S3 and S8 are shifted by 100, 250 and 450 cm³ g⁻¹ (STP), respectively.

Table 1 Physical properties of the calcined mesoporous silica materials

Sample	Mesostructure	AOT/F127 (molar ratio)	TMB/g	Unit cell a/nm	Pore size/nm	Pore volume/cm^3 g^−1	Surface area/m^2 g^−1^c
a Calculated from the adsorption branch of the isotherms based on the BdB sphere model. b Calculated from the adsorption branch of the isotherms based on the BdB cylinder model. c Calculated from the BET method.
S1	Fm3̄m	0	0	19.6	7.8^a	0.57	790
S2	Im3̄m	3	0	14.6	8.4^a	0.60	750
S3	p6m	7	0	12.5	8.5^b	0.61	720
S4	p6m	7	0.10	12.9	9.0^b	0.63	730
S5	p6m	7	0.20	13.0	9.3^b	0.63	720
S6	p6m	7	0.30	14.8	10.9^b	0.65	730
S7	p6m	7	0.40	16.5	12.1^b	0.68	700
S8	Ia3̄d	10	0.45	28.9	10.4^b	0.66	710

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Mesoporous silica with cubic Im3̄m structure

A mesophase transformation occurs as the anionic surfactant AOT participates in the F127 surfactant assembly. The SAXS pattern (Fig. 3A) of as-synthesized mesoporous silica (S2) prepared with an AOT/F127 molar ratio of 3 in a dilute H₂SO₄ solution shows five well-resolved diffraction peaks with q vectors of 0.518, 0.730, 0.903, 1.04 and 1.16 nm⁻¹, respectively. After calcination at 550 °C in air, these peaks shift to higher q vector direction as expected with the ratios remaining exactly 1 : √2 : √3 : √4 : √5. Combined with elaborate TEM observation, they can be indexed to 110, 200, 211, 220 and 310 reflections of the body-centered cubic structure (space group Im3̄m), which are analogous to those of SBA-16.¹³ The TEM images for the calcined S2 sample viewed along the [111] and [100] directions are shown in Fig. 3B, C. High purity of the body-centered cubic (Im3̄m) mesostructure can therefore be clarified. The unit cell parameters calculated from SAXS measurements for as-synthesized and calcined mesoporous silica S2 are 17.1 and 14.6 nm, respectively, in agreement with those from TEM images.

Fig. 3 SAXS patterns (A) and TEM images (B, C) of the cubic Im3̄m mesoporous silica (S2) viewed along the [111] (B) and [100] (C) directions, respectively. Sample S2 was prepared with an AOT/F127 molar ratio of 3 in a dilute H₂SO₄ solution.

Fig. 2 shows the nitrogen adsorption–desorption isotherms of the calcined mesoporous silica S2 structure. A cage-like mesopore model can also be applied in the calcined S2 sample due to the typical type IV isotherm with an H₂-type hysteresis loop. The BET surface area, pore volume and mean value of uniform pore size calculated from the BdB sphere model are 750 m² g⁻¹, 0.595 cm³ g⁻¹ and 8.4 nm, respectively (Table 1). The slightly larger pore volume and pore size of the S2 sample than those for the S1 sample prepared in the absence of AOT molecules imply that AOT molecules are associated with the hydrophobic segments (PPO) of the triblock copolymer F127.

Mesoporous silica with 2-D hexagonal p6m structure

It is found that body-centered cubic (Im3̄m) mesostructure can be formed with the AOT/F127 molar ratio varying in the range of 1–5. Further increasing the AOT amount brings about a second mesophase transformation from cubic Im3̄m symmetry to 2-D hexagonal p6m symmetry, which is suggested by the corresponding XRD, SAXS patterns and TEM images. An example of the mesoporous silica S3 sample with an initial AOT/F127 molar ratio of 7 is set out here. The SAXS pattern of the as-synthesized S3 mesostructure (Fig. 4A) displays the typical diffractions for 2-D hexagonal symmetry with a very narrow and intense diffraction peak with q values of 0.534 nm⁻¹, as well as three well-resolved diffraction peaks at 0.925, 1.07 and 1.41 nm⁻¹, which can be indexed as the 10, 11, 20 and 21 diffractions, respectively. The cell parameter (a) calculated is measured to be 13.6 nm. Heating the as-synthesized sample at 550 °C for 5 h does not alternate the d-spacing ratios of these four reflection peaks, but causes a slight shift to higher q values of the positions. A cell parameter (a) of 12.5 nm indicates a small structural shrinkage of 8.1%. Interestingly, the intensity of the 11 reflection is weaker than that for the 20 diffraction, and even the 21 diffraction, implying an unusual cylinder mesostructure. Based on previous reports,^25,31 this phenomenon could be explained with a microporous corona around the mesopores of the as-synthesized silica products. The typical stripe-like and hexagonally arranged TEM images, recorded along the [10] and [01] directions, respectively, confirm that S3 sample possesses a highly ordered 2-D hexagonal p6m mesostructure (Fig. 4B and C).

Fig. 4 SAXS patterns (A) and TEM images (B, C) of the 2-D hexagonal mesoporous silica (sample S3) prepared with an AOT/F127 molar ratio of 7, viewed along the [001] (B) and [110] (C) directions, respectively.

Although a type IV nitrogen sorption isotherm is measured on the calcined mesoporous silica S3 sample, a peculiar hysteresis loop between H₁ and H₂-type should be taken into account (Fig. 2). It can be attributed to an intermediate pore structure between cage-like and cylindrical shape.²⁵ The pore size calculated from the adsorption branch based on the BdB cylinder model is 8.5 nm, and the pore volume of S3 sample is 0.611 cm³ g⁻¹. Since the pore size and volume are a bit larger when more AOT molecules are involved in the synthesis, this is further evidence of the interaction of the anionic surfactant AOT with the hydrophobic segments of the copolymer.

Changing the molar ratio of AOT/F127 from 6 to 10 results in the highly ordered 2-D hexagonal silica mesostructure (p6m). Disordered mesostructured silica is produced under a higher concentration of AOT (AOT/F127 ratio >10). Another notable feature of the current synthesis system is that the unit cell size and pore size of the resulting hexagonal mesoporous silica can be expanded by adding TMB molecules as the swelling agent. On the basis of sample S3 which is prepared in the system with the AOT/F127 molar ratio equal to 7, different amounts of TMB are added. The XRD patterns (Fig. 5) of the resulting samples calcined at 550 °C exhibit well-resolved 2-D hexagonal diffraction peaks, suggesting highly ordered mesostructures with p6m symmetry. Increasing the TMB quantity results in an obvious shift of the 10 diffraction peaks to lower angles, indicating that the d-spacing and the cell parameter (a) are enlarged (Table 1). When the amount of TMB is in excess of 0.2 g in this system, a rapid increase in lattice constant of the product occurs. The largest cell parameter is 16.5 nm for the surfactant-free S7 sample prepared with 0.4 g TMB as swelling agent, representing an expansion of ∼50% compared to the conventional SBA-15 analog.^12,13 XRD and N₂ sorption measurements show that the mesostructure of these hexagonal silicates with large pores can be substantially maintained even after boiling in hot water for 14 days. It seems that the influence of the organic additive TMB materializes on at least two aspects. On one hand, the transfiguration of the cylindrical pore is apparently recovered by the introduction of TMB molecules to some extent based on the XRD patterns and N₂ sorption isotherms (Fig. 6). Both the growth in the intensities of the 11 diffraction peaks (Fig. 5) and the similarity to H₁-type hysteresis loops (Fig. 6) suggest the shift of the mesopore from a cucurbit-like configuration to an ideal cylindrical shape.^25,32 On the other hand, the swelling effect of the TMB molecules lies in the fact that the pore size and pore volume are enlarged from 9.0 to 12.1 nm and from 0.625 to 0.683 cm³ g⁻¹, respectively (Table 1).

Fig. 5 XRD patterns of the calcined mesoporous silica (samples S4–S7) with 2-D hexagonal p6m structure prepared by adding different amounts of TMB molecules into the F127–AOT mixed surfactant system.

Fig. 6 Nitrogen sorption isotherms of the calcined mesoporous silica materials (samples S4–S7) with 2-D hexagonal p6m structure prepared by adding different amounts of TMB molecules into the F127–AOT mixed surfactant system. The isotherms of S5, S6 and S7 are shifted by 150, 300 and 500 cm³ g⁻¹ (STP), respectively.

Mesoporous silica with cubic Ia3̄d structure

Another mesophase transformation from 2-D hexagonal p6m to cubic Ia3̄d structure occurs by simultaneously increasing the amount of AOT and TMB molecules in the F127 surfactant system. The SAXS patterns of as-synthesized and calcined mesoporous silica sample S8 prepared with an AOT/F127 molar ratio of 10 and a large amount of TMB (0.45 g) are shown in Fig. 7. Eleven resolved diffraction peaks for as-synthesized sample S8 are observed. After calcination at 550 °C in air, the SAXS pattern becomes more resolved, and the corresponding q vectors are 0.531, 0.608, 0.814, 1.02, 1.07, 1.19, 1.23, 1.35, 1.40, 1.47 and 1.53 nm⁻¹, respectively. Obviously, the q value ratios are √3 : √4 : √7 : √11 : √12 : √15 : √16 : √19 : √21 : √23 : √25. Together with the careful TEM observation, these peaks can be indexed as 211, 220, 321, 332, 422, 521, 440, 532/611, 541, 631 and 543 reflections of the bicontinuous cubic mesostructure (space group Ia3̄d), respectively. The cell parameters (a) calculated are 31.6 and 28.9 nm for as-synthesized and calcined samples, respectively, which are much larger than those of the Pluronic P123-templated bicontinuous cubic analogues.^21–24,29 This suggests that the block copolymer with greater molecular weight (e.g. F127) can yield a mesoporous silica structure with a larger cell parameter. The characteristic TEM projections along the [111] and [311] directions (Fig. 7B, C) manifest an ordered arrangement of mesopores with large domains, providing further evidence for the highly ordered bicontinuous cubic Ia3̄d mesostructure.

Fig. 7 SAXS patterns (A) and TEM images (B, C) of the cubic Ia3̄d mesoporous silica (sample S4) taken along the [111] (B) and [311] (C) zone axes, respectively.

Unlike samples S1, S2 and S3, calcined mesoporous silica S8 with the cubic Ia3̄d mesostructure reveals type IV nitrogen sorption isotherms with a sharp capillary condensation step at high relative pressures (P/P₀ = 0.7–0.8) and a perfect H₁-type hysteresis loop. It is in accordance with that from cubic Ia3̄d mesoporous silica templated by Pluronic P123,^21,24,29 indicative of a highly ordered mesoporous silica structure with typical bicontinuous channels. The pore size calculated from the BdB cylinder model is 10.4 nm, and the pore volume is 0.658 cm³ g⁻¹. Again, the expansion of the pores caused by adding TMB is evident in the respect that both the pore size and pore volume are larger than those obtained from the other three samples (S1, S2 and S3).

Morphologies of mesoporous silica with different structures

SEM images for the calcined mesoporous silica materials obtained from the AOT–F127–H₂SO₄ system are shown in Fig. S1 (ESI†). It is found that the morphologies of the resulting mesoporous silica materials are similar despite their diversified symmetries. All of the aforementioned four kinds of mesostructures (samples S1, S2, S3 and S8) consist of irregular particles about 2–10 μm in size without well-defined morphologies. This phenomenon is distinguishable from that in the Pluronic P123–KCl or F108–K₂SO₄ systems reported previously,^16,17 where clear-cut mesoporous single crystals are observed. The latter is related to the fast hydrolysis speed of TEOS promoted by the high concentration of acid, as well as the enhanced interaction between the triblock copolymer template and the inorganic silicic species facilitated by the high concentration of inorganic salt.

Effects of anionic AOT surfactant on the phase transformation

To elucidate the co-template effect of anionic surfactant AOT with double hydrophobic chains, another anionic surfactant with a single chain, sodium dodecyl benzene sulfonate (SDBS), was also applied in the triblock copolymer F127 templating system. As shown in Fig. 8, a mesophase transformation from cubic Fm3̄m to Im3̄m mesophase (curves a and b) can take place when the SDBS/F127 molar ratio varies from 1 to 6. However, successively increasing the SDBS amount generates a disordered mesoporous silica structure, validated by only one broad XRD peak (curve c). This indicates that anionic surfactant AOT is a co-template adapted to induce the multi-step mesophase transformation. Besides that, apolar TMB was added alone in the F127-templating synthesis system to investigate its effects on the mesophase transformation. An unresolved XRD pattern (Fig. 9) is detected as a small amount of TMB (no more than 0.1 g) is added, which is tentatively indexed to the body-centered cubic mesostructure (Im3̄m), suggesting a possible phase transformation from cubic Fm3̄m to Im3̄m mesostructure. A disordered mesoporous silica structure forms in a system with more than 0.2 g of TMB. These results indicate that TMB itself cannot effectively induce the phase transformation, whose influence is intently pertinent to the anionic surfactant AOT. Considering the synthetic temperature of 45 °C, it is higher than the critical micellization temperature of triblock copolymer F127, so that only a small number of TMB molecules may penetrate into the core of the micelles formed by Pluronic F127 surfactant, due to the countereffect of the large moiety of hydrophilic EO blocks located on the outer shell of the triblock copolymer F127 micelles.³³ This situation can be alternated by the participation of AOT molecules (see below).

Fig. 8 XRD patterns of the calcined mesoporous silica materials prepared with SDBS/F127 molar ratio of 1 (a), 6 (b) and 7 (c), respectively.

Fig. 9 XRD patterns of the calcined mesoporous silica samples synthesized in the presence of TMB molecules within a pure F127 surfactant system.

It is demonstrated that the concentration of anionic AOT surfactant in a nonionic triblock copolymer F127 surfactant system switches on the mesophase transformation from cubic Fm3̄m to Im3̄m then towards 2-D p6m symmetry. Adjusting the amount of swelling agent TMB in this system with a high AOT concentration accomplishes the final transformation from p6m to Ia3̄d mesostructure. To the best of our knowledge, this is the first time that the above four well-known mesostructures have been derived in a single synthetic system. It is well known that AOT with a cone-like molecular structure, different from the single-chain anionic surfactant SDBS, exhibits a remarkably rich aqueous-phase behavior and has been widely used as a microemulsifier in industry.^34,35 In addition, anionic AOT surfactants prefer to bind with the more hydrophobic PPO blocks rather than with the hydrophilic PEO blocks, and thereby cause an increase in the hydrophobic volume.^36–38 The fact that anionic surfactant AOT can induce the multi-step mesophase transformation may be related to its hydrogen-bond interaction with hydrophobic segments (PPO) in triblock copolymer F127, as well as its specific inverted truncated cone-shaped structure. With increasing amounts of AOT in the triblock copolymer F127 system, continuous decreases occur both in the interface curvature and in the hydrophilic/hydrophobic volume ratio of the resulting mixed micelles, which gives rise to the Fm3̄m → Im3̄m → 2-D p6m mesophase transformation (Fig. 10). The fourth one from 2-D p6m to cubic Ia3̄d structure is believed to originate from the assistant of the swelling agent TMB molecules in the F127 surfactant system with a high AOT concentration. The large amount of AOT molecules in F127 solution results in a dramatic reduction in interface curvature of the resulting mixed micelles, and thus a synergistic solubility enhancement for aromatic hydrocarbons such as TMB molecules.^27,39 Therefore, the highly swollen micelles result in mesoporous silica with a cubic Ia3̄d structure. On the other hand, the aforementioned mechanism could be partly supported by the fact that there is an increase in the pore size (from 7.8 to 12.1 nm) and a slight decrease in the specific surface areas for calcined samples during the Fm3̄m → Im3̄m → p6m → Ia3̄d mesophase transformations (Table 1). Owing to the swelling of the micelles by AOT or TMB molecules, an increase in hydrophobic volume of the micelles accompanied by a decrease in hydration of the PEO groups might be induced by increasing the amount of AOT or TMB in the mixed system, which usually leads to a large mesopore size. It also results in less occlusion of the PEO chains in the silica matrix, and consequently a slight decrease in the total specific surface areas for the calcined samples.

Fig. 10 Schematic representation of the mesophase transformation induced by co-template AOT and swelling agent TMB in the amphiphilic triblock copolymer F127 assembly system. With the increase of anionic surfactant AOT and/or organic additive TMB concentration, the interface curvature of F127–AOT mixed micelles reduces, resulting in the mesophase transformation from cubic closed packing (face-centered structure) to loose packing (body-centered bicontinuous structure).

Conclusions

With the aid of anionic surfactant AOT as the co-template, a single amphiphilic block copolymer F127 template system can produce four different high-quality silica mesostructures with very large pore size (7.8–12.1 nm), including 2-D hexagonal p6m, 3-D cubic Im3̄m, Fm3̄m and Ia3̄d symmetries. Merely controlling the amount of AOT in the F127 aqueous solution results in the mesophase transformation from cubic Fm3̄m to Im3̄m and then to 2-D p6m symmetry. A large number of TMB molecules with a high concentration of AOT surfactant can assist with the formation of the cubic Ia3̄d mesostructure. The mesophase transformation induced by anionic surfactant AOT may be related to the enlargement of the hydrophobic volume in the nonionic Pluronic F127 template associated with the anionic AOT co-template via hydrogen bonds in acidic media. Moreover, the mixed micelles formed by AOT and F127 surfactants have a synergistic solubility enhancement for the swelling agent TMB. The resulting swollen micelles do, therefore, lead to a very large cell parameter (a = 16.5 nm) in the long-range ordered 2-D hexagonal mesoporous silica. Rational control of the high-purity mesostructure in one system becomes possible. These results may pave the way for a facile approach to highly ordered silicas with abundant mesostructure, which may find applications in catalysis, immobilization and controlled release of biomolecules, protein separation, etc.

Acknowledgements

This work was supported by National Natural Science Foundation of China (20233030, 20373013, 20421303 and 20521140450), State Key Basic Research Program of PRC (2001CB610506), Shanghai Science and Technology Committee (03527001 and 03QF14037), Shanghai Nanotechnology Center (0212nm043, 04JC14087), Shanghai Education Committee (02SG01), Program for New Century Excellent Talents in University, Shanghai HuaYi Chemical Group, Unilever research institute of China and Fudan Graduate Innovation Funds.

References

1. Y. J. Han, G. D. Stucky and A. Butler, J. Am. Chem. Soc., 1999, 121, 9897.
2. J. Fan, C. Z. Yu, T. Gao, J. Lei, B. Z. Tian, L. M. Wang, Q. Luo, B. Tu, W. Z. Zhou and D. Y. Zhao, Angew. Chem., Int. Ed., 2003, 42, 3146.
3. A. Vinu, V. Murugesan, O. Tangermann and M. Hartmann, Chem. Mater., 2004, 16, 3056.
4. J. Fan, W. Q. Shui, P. Y. Yang, X. Y. Wang, Y. M. Xu, H. H. Wang, X. Chen and D. Y. Zhao, Chem.—Eur. J., 2005, 11, 5391.
5. M. Hartmann, Chem. Mater., 2005, 17, 4577.
6. J. Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem., Int. Ed., 1999, 38, 56.
7. M. E. Davis, Nature, 2002, 417, 813.
8. N. K. Mal, M. Fujiwara and Y. Tanaka, Nature, 2003, 421, 350.
9. A. Stein, Adv. Mater., 2003, 15, 763.
10. C. Z. Yu, B. Z. Tian and D. Y. Zhao, Curr. Opin. Solid State Mater. Sci., 2003, 7, 191.
11. G. Cavallaro, P. Pierro, F. S. Palumbo, F. Testa, L. Pasqua and R. Aiello, Drug Deliv., 2004, 11, 41.
12. D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.
13. D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 1998, 120, 6024.
14. D. Y. Zhao, J. Y. Sun, Q. Z. Li and G. D. Stucky, Chem. Mater., 2000, 12, 275.
15. F. Q. Zhang, Y. Yan, H. F. Yang, Y. Meng, C. Z. Yu, B. Tu and D. Y. Zhao, J. Phys. Chem. B, 2005, 109, 8723.
16. C. Z. Yu, J. Fan, B. Z. Tian, D. Y. Zhao and G. D. Stucky, Adv. Mater., 2002, 14, 1742.
17. C. Z. Yu, B. Z. Tian, J. Fan, G. D. Stucky and D. Y. Zhao, J. Am. Chem. Soc., 2002, 124, 4556.
18. P. Van der Voort, M. Benjelloun and E. F. Vansant, J. Phys. Chem. B, 2002, 106, 9027.
19. P. Kipkemboi, A. Fogden, V. Alfredsson and K. Flodstrom, Langmuir, 2001, 17, 5398.
20. F. Kleitz, D. N. Liu, G. M. Anilkumar, I. S. Park, L. A. Solovyov, A. N. Shmakov and R. Ryoo, J. Phys. Chem. B, 2003, 107, 14296.
21. F. Kleitz, S. H. Choi and R. Ryoo, Chem. Commun., 2003, 2136.
22. S. N. Che, A. E. Garcia-Bennett, X. Y. Liu, R. P. Hodgkins, P. A. Wright, D. Y. Zhao, O. Terasaki and T. Tatsumi, Angew. Chem., Int. Ed., 2003, 42, 3930.
23. K. Flodstrom, V. Alfredsson and N. Kallrot, J. Am. Chem. Soc., 2003, 125, 4402.
24. T. W. Kim, F. Kleitz, B. Paul and R. Ryoo, J. Am. Chem. Soc., 2005, 127, 7601.
25. F. Kleitz, L. A. Solvyov, G. M. Anilkumar, S. H. Choi and R. Ryoo, Chem. Commun., 2004, 1536.
26. F. X. Chen, L. M. Huang and Q. Z. Li, Chem. Mater., 1997, 9, 2685.
27. A. Lind, J. Andersson, S. Karlsson, P. Agren, P. Bussian, H. Amenitsch and M. Linden, Langmuir, 2002, 18, 1380.
28. A. Lind, B. Spliethoff and M. Linden, Chem. Mater., 2003, 15, 813.
29. D. H. Chen, Z. Li, C. Z. Yu, Y. F. Shi, Z. D. Zhang, B. Tu and D. Y. Zhao, Chem. Mater., 2005, 17, 3228.
30. J. W. Tang, C. Z. Yu, X. F. Zhou, X. X. Yan and D. Y. Zhao, Chem. Commun., 2004, 2240.
31. M. Imperor-Clerc, P. Davidson and A. Davidson, J. Am. Chem. Soc., 2000, 122, 11925.
32. P. Sakya, J. M. Seddon, R. H. Templer, R. J. Mirkin and G. J. T. Tiddy, Langmuir, 1997, 13, 3706.
33. J. Fan, C. Z. Yu, J. Lei, Q. Zhang, T. C. Li, B. Tu, W. Z. Zhou and D. Y. Zhao, J. Am. Chem. Soc., 2005, 127, 10794.
34. S. Nave, J. Eastoe and J. Penfold, Langmuir, 2000, 16, 8733.
35. S. Nave, J. Eastoe, R. K. Heenan, D. Steytler and I. Grillo, Langmuir, 2000, 16, 8741.
36. M. Almgren, J. Vanstam, C. Lindblad, P. Y. Li, P. Stilbs and P. Bahadur, J. Phys. Chem., 1991, 95, 5677.
37. E. Hecht and H. Hoffmann, Langmuir, 1994, 10, 86.
38. Y. Li, R. Xu, S. Couderc, D. M. Bloor, E. Wyn-Jones and J. F. Holzwarth, Langmuir, 2001, 17, 183.
39. L. Z. Zhu and S. L. Feng, Chemosphere, 2003, 53, 459.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1: SEM images for the calcined mesoporous silica materials. See DOI: 10.1039/b517975k

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