Preparation of Chiral Mesoporous Silica Nanotubes and Nanoribbons

Jan 23, 2013 - ABSTRACT: Single-handed helical silica nanotubes and nanoribbons have attracted much attention, due to their potential for applications...
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Preparation of Chiral Mesoporous Silica Nanotubes and Nanoribbons Using a Dual-Templating Approach Yi Li, Baozong Li, Zhuojun Yan, Zeli Xiao, Zhibin Huang, Kai Hu, Sibing Wang, and Yonggang Yang* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China S Supporting Information *

ABSTRACT: Single-handed helical silica nanotubes and nanoribbons have attracted much attention, due to their potential for applications in asymmetric catalysis and enantioseparation. However, their surface areas are usually lower than 200 m2/g. Herein, single-handed coiled silica nanoribbons with mesopores in their walls were prepared by changing the molar ratio of the chiral low-molecular-weight amphiphiles (LMWAs) and the F127 triblock copolymer, using a dual-templating approach. The obtained samples were characterized using field-emission scanning electron microscopy, transmission electron microscopy, N2 sorption measurements, and X-ray diffraction. The BET surface area reached 700 m2/g. The pore diameters did not change significantly at different LMWA/F127 triblock copolymer molar ratios. The formation of the coiled mesoporous silica nanoribbons was studied by taking TEM images at different times during the reaction. The results indicated that the morphology and the mesopores were controlled by the chiral LMWAs and the F127 triblock copolymer, respectively. Silica nanotubes with mesopores in their walls could also be prepared using this approach. KEYWORDS: mesoporous materials, chirality, silica, nanoribbons, nanotubes

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wormlike, and concentric pore channels were reported.4−9 Among these structures, silica nanotubes and nanoribbons with mesopores perpendicular to the surfaces should be more suitable for applications in catalysis, because of the rapid mass transfer. It has been reported that these structures can be prepared using a single-templating approach.9 The structure formation was proposed to follow a structural transition process. Single-handed coiled silica nanoribbons with mesopores perpendicular to the surfaces can be also prepared using this approach,10 but only a few LMWAs can be successfully applied in this synthesis. Neither the morphology nor the pore architecture can be tuned freely. Herein, a dual-templating pathway based on sol−gel transcription and the preparation of SBA-16 was developed. Self-assemblies of the chiral lowmolecular-weight amphiphiles acted as templates to control the morphology, and F127 triblock copolymers acted as templates for the mesopores in the walls.

INTRODUCTION Sol−gel transcription, a technique developed by Shinkai et al., is a powerful method to control the single-handedness of silica structures.1 Generally, self-assemblies of chiral low-molecularweight amphiphiles (LMWAs) serve as templates, which are formed through H-bonding, π−π interactions, and solvophobic associations. During the sol−gel transcription process, the silica oligomers adsorb and then polycondense on the surface of the helical organic self-assemblies. After calcination, helical silica nanostructures are obtained. Because the chiral organic selfassemblies tend to grow in one dimension, single-handed helical silica nanotubes, double helical silica nanotubes, and coiled silica nanoribbons are typically obtained. Their nitrogen BET surface areas are normally lower than 200 m2/g. Recent results indicated that single-handed helical mesoporous silica nanostructures could also be prepared using self-assembled LMWA structures as templates.2,3 TEM images taken after different reaction times indicated that the formation of the mesoporous structures followed a cooperation self-assembly process.3 Although the surface areas were larger than 700 m2/g, the pore channels in the silicas ran along the long axis, which is not favorable for mass transfer. It is, therefore, desirable to build mesopores within the walls of the nanotubes and nanoribbons, to increase the surface area and the mass transfer properties. Much effort has been dedicated to the preparation of silica nanotubes and nanoribbons with mesopores in their walls. Recently, silica nanotubes with coiled, radically oriented, © 2013 American Chemical Society

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MATERIALS AND METHODS

Methods. Field-emission scanning electron microscopy (FESEM) images were taken using a Hitachi 4800 instrument. Transmission electron microscopy (TEM) images were obtained using a TecnaiG220 instrument. The specific surface area and pore-size

Received: July 28, 2012 Revised: January 14, 2013 Published: January 23, 2013 307

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the organic self-assemblies.3c Left- and right-handed twisted or coiled nanoribbons were prepared using L- and D-18Val11PyBr, respectively. Figure 2 shows FESEM images of the mesoporous

distribution were determined by the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, using N2 adsorption isotherms measured using a Micromeritics ASAP 2020M +C instrument. Small-angle X-ray diffraction (SAXRD) patterns were taken on an X′ Pert-Pro MPD X-ray diffractometer. Materials. Tetraethyl orthosilicate (TEOS) was obtained from Aldrich. The characterization of L-18Val4PyBr, L-18Val6PyBr, L18Val11PyBr, and D-18Val11PyBr has been reported previously.3c,11 Preparation of Chiral Mesoporous Silica Nanoribbons. The typical synthetic procedure was as follows: LMWA L-18Val11PyBr (200 mg, 0.29 mmol), F127 (PEO106PPO70PEO106, 100 mg, 0.0079 mmol), and potassium chloride (250 mg, 3.35 mmol) were dissolved in 6.0 mL of an aqueous HCl solution (2.0 M). TEOS (0.505 mL, 2.25 mmol) was added to the solution under stirring at 38 °C. Stirring was stopped after 6 min. The reaction mixture was kept at 38 °C for 24 h. Subsequently, the mixture was placed in an autoclave at 100 °C, for hydrothermal treatment. Twenty-four hours later, the products were collected by filtration and extracted using a mixture of methanol and hydrochloric acid, for 24 h. Finally, the sample was calcined at 550 °C for 5 h, under aerobic conditions. The obtained silica is named here as SL11-37 according to the L-18Val11PyBr/F127 molar ratio, which is 37:1. The other samples were also named according to the LMWA/F127 molar ratio. The amounts of the other chemicals were kept constant, and the samples obtained using 50, 100, and 150 mg of 18Val11PyBr were named as SL11-9, SL11-18, and SL11-28, respectively. The samples prepared using 40 mg of L-18Val4PyBr (0.067 mmol), 40 mg of L-18Val6PyBr (0.056 mmol), and 250 mg of D-18Val11PyBr (0.36 mmol) were named as SL4-8, SL6-7, and SD11-46, respectively. Sample Preparation for Studying the Formation of Chiral Mesoporous Silica Nanoribbons. A certain time after the addition of TEOS, one drop of the reaction mixture was taken out and dropped on the supporting film of the TEM grid. The film was dried under nitrogen flow as quickly as possible.

Figure 2. FESEM images of the mesoporous silica nanoribbons: (a) SL11-9, (b) SL11-18, (c) SL11-28, and (d−f) SL11-37.

silicas prepared here, named as SL11-9, SL11-18, SL11-28, and SL11-37. The structures obtained most frequently were lefthanded coiled nanoribbons (Figure 2). The length of the ribbons was typically several micrometers. Some of the ribbons tended to coil into nanotubes (Figure 2c). Some nanoparticles were identified on the surfaces of SL11-9 and SL11-18 (Figure 2a,b). SL11-28 and SL11-37 were smoother (Figure 2c,d). For SL11-37, open mesopores were identified on the surfaces of the nanoribbons, and seemed to organize in a two-dimensional hexagonal structure (Figure 2e,f; Supporting Information, Figure S1). It was reported previously that silica nanotubes with mesopores on the surfaces could be prepared by changing the molar ratio of the silica source and gelators.3c The pores were formed by the disorderly accumulation of silica oligomers on the surfaces of organogel nanofibers. The pore diameters were not uniform, and the pores were not arranged in a periodic fashion. The approach shown here should, therefore, give a better pathway to control the diameters and arrangement of the mesopores. TEM images of the mesoporous silicas are shown in Figure 3 and Figure S2 (Supporting Information). For SL11-9 and SL11-18, double-layered mesoporous nanoribbons with fine, hollow nanospheres were identified (Figure 3a,b). For SL11-28 and SL11-37, double-layered mesoporous nanoribbons and nanotubes were identified (Figure 3c,d). No fine hollow nanospheres were identified on the surfaces. The mesopores were organized in a two-dimensional hexagonal-like structure. It seemed that the nanoribbons were constructed from hollow nanospheres. Each layer of the nanoribbons consisted of one layer of mesopores. When the amount of L-18Val11PyBr was low, some of the hollow silica nanospheres could not adsorb, and organized on the surfaces of the organic self-assemblies (Figure 3a,b). Hollow spheres were, therefore, identified on the surfaces. The organic self-assemblies should have existed within the voids of the double-layered mesoporous silica nanoribbons. These mesoporous silica nanoribbons were approximately

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RESULTS AND DISCUSSION FESEM and TEM Images of the Mesoporous Silicas, SL11-9, SL11-18, SL11-28, and SL11-37. LMWAs derived from amino acids have attracted much attention, due to their potential for applications in drugs and cosmetics.12 The LMWAs shown in Figure 1 can gelate benzene, toluene,

Figure 1. Molecular structures of the LMWAs.

THF, chlorobenzene, and nitrobenzene at 25 °C (Figure 1).3c,11 However, they show high solubility in ethanol and methanol. The self-assembly behaviors of other LMWAs have been reported previously.3c,11 Their CD spectra indicated that all of them could self-assemble into chiral nanostructures in pure water. The synthetic procedure used to create the chiral mesoporous silica nanoribbons followed that used for the synthesis of SBA-16.13 We reported previously that chiral silica nanoribbons with rodlike pore channels could be prepared, using the selfassembled structures of L- and D-18Val11PyBr as templates. The handedness of the nanoribbons was controlled by that of 308

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Figure 3. TEM images of the mesoporous silica nanoribbons: (a) SL11-9, (b) SL11-18, (c) SL11-28, and (d) SL11-37.

200−400 nm wide, and 25 nm thick. The diameter of the mesopores was approximately 5.0−6.0 nm. It is interesting to note that some single-layer nanoribbons were constructed from two layers of mesopores. The formation of these structures was likely due to the shrinkage of the interlayer voids of the doublelayered ribbons.14 Nitrogen Sorptions of the Mesoporous Silicas SL11-9, SL11-18, SL11-28, and SL11-37. Figure 4a shows nitrogen adsorption−desorption isothermal plots for the mesoporous silicas. These four samples showed two hysteresis loops at relative pressures (P/P0) between 0.45 and 1.0. The two hysteresis loops were observed at relative pressures (P/P0) of 0.4−0.65 and 0.65−1.0, and originated from the mesopores within the walls and the voids among the nanoribbons, respectively. The BJH pore size distribution plots determined from the adsorption branches showed sharp peaks at 5.0−5.2 nm. Calcined SBA-16 had a pore size of 5.4 nm, similar to the pore diameters shown here.13 It is, therefore, likely that the F127 block copolymer acted as the template for the mesopores within the walls. Specifically, the phase separation between the F127 block copolymer and organogelator played an important role in forming this hierarchical porous structure, which was observed previously in mixtures of L-18Val6PyBr and cetyltrimethylammonium chloride.15 SL11-9, SL11-18, SL11-28, and SL11-37 exhibited nitrogen BET surface areas of 583, 477, 578, and 703 m2/g, respectively. Such high surface areas are significant for their potential applications. X-ray Diffraction Measurements for the Mesoporous Silicas SL11-9, SL11-18, SL11-28, and SL11-37. The SAXRD patterns of these samples are shown in Figure 5. The SAXRD pattern for SL11-37 showed one peak, at a 2θ of 0.79°. Because of the lack of large diffraction domains, no higher-order reflections were observed.16 The other samples did not show peaks at 0.5−7.0°. We believe that the coiled morphologies, single- or double-layered porous structures, and randomly adsorbed hollow nanospheres suppressed the formation of sharp diffraction peaks. FESEM and TEM Images of the Mesoporous Silicas SD11-46, SL4-8, and SL6-7. For SD11-46 prepared using D18Val11PyBr, right-handed coiled nanoribbons were observed. Open mesopores could be identified on the surfaces of the nanoribbons (Supporting Information, Figure S3). Doublelayered mesoporous nanoribbons were identified in the TEM image (Figure 6a). The left-handed twisted mesoporous silica

Figure 4. N2 adsorption−desorption measurements for the mesoporous silicas. Sorption isotherms (a) and BJH pore size distribution plots calculated from the adsorption branch (b).

Figure 5. SAXRD patterns for the mesoporous silicas.

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mixtures. Some ordered structures templated by the F127 block copolymer would then be formed (Supporting Information, Figure S8). Mechanism of the Formation of the Mesoporous Silica Nanoribbons. To obtain a better understanding of the formation of the hierarchical porous structures, TEM images were taken of the reaction mixture incorporating L 18Val4PyBr, at different reaction times. After 6 min, nanospheres were identified in the reaction mixtures.17 Some of the nanospheres adsorbed randomly on the surfaces of the nanoribbons (Figure 7a). The inner diameters of the core−shell

Figure 7. TEM images of the reaction mixture, taken after different reaction times: (a) 6 min and (b) 60 min.

Figure 6. TEM images of the mesoporous silicas: (a) SD11-46, (b) SL4-8, and (c−e) SL6-7.

spheres were in the range from 5.6 to 16 nm, larger than those of the final products (Supporting Information, Figure S9). It is likely that these core−shell silica spheres were constructed from less-condensed silicas.17 After removing the templates, hollow spheres could be identified (Figure 3b; see the surfaces of the coiled nanoribbons). After 60 min, twisted double-layered nanoribbons with hexagonally arranged mesopores were identified (Figure 7b). The pore diameters in these structures were uniform. It was reported previously that twisted silica nanoribbons with rodlike pore channels running along the long axis could be prepared using the chiral LMWAs under acidic conditions.15 For the samples shown here, no rodlike pore channels templated by single-stranded organic self-assemblies were identified in the TEM images (Figures 3 and 6). Under this acidic condition, silica oligomers with positive charges seem to be more feasible to assemble with the F127 block copolymer through H-bonding. The mesopores were then controlled by the F127 block copolymer. The morphologies of the mesoporous silicas should be controlled by the self-assemblies of the LMWAs. It was reported by us that coiled mesoporous nanoribbons and helical mesoporous nanotubes could be prepared using these LMWGs.3c,g,10,15 The morphologies are similar as these reported here. The proposed mechanism for the formation of the twisted mesoporous silica nanotubes and nanoribbons is shown in Figure 8. We believe that the phase separation between the F127 block copolymer and the low-molecular-weight amphiphile played an important role in the formation of this hierarchical porous structure. Before the addition of TEOS, the F127 block copolymer and the LMWAs self-assembled into micelles and twisted ribbons/helical nanotubes, respectively, in the reaction mixtures. A similar mechanism was proposed in the cetyltrimethylammonium bromide (CTAB)/sodium dodecylsulfate (SDS)/P123 block copolymer system for the preparation of the mesoporous silica nanoflakes.18 CTAB and SDS formed a bilayer structure through the electrostactic interaction. P123 acted as the template for the silica mesopores within the bilayer structure.18,19 Herein, the LMWAs shown here can selfassemble into single-handed twisted nanoribbons through the H-bondings and hydrophobic associations.3,8,11,15 However, the

nanoribbons and nanotubes could be also prepared using L18Val4PyBr (Figure 6b, and Supporting Information, Figure S4). The single-layered nanoribbons were constructed from double layers of mesopores. It was shown previously that the minimum gel concentrations of this series of cationic amphiphiles are sensitive to pH.11 Although L-18Val4PyBr cannot gelate pure water at a concentration of 40 g/L at 20 °C, the growth of ribbons in the reaction mixtures was reasonable. It was found that the reaction mixture had a high viscosity, so it is possible that templating could occur and silica nanoribbons could be produced. The formation of the twisted mesoporous silica nanoribbons shown in Figure 6b was also likely due to the shrinkage of the interlayer voids in the double-layered ribbons.14 TEM images of the calcined SL6-7 are shown in Figure 6c−e. The FESEM images indicate that the nanotubes are left-handed twisted (Supporting Information, Figure S5). The mesoporous silica nanotubes, approximately 30−50 nm in diameter and 1.0−3.0 μm long, were open-ended. The walls of the nanotubes were constructed from a single layer of mesopores, which had inner diameters in the range from 5.0 to 6.0 nm. Both open and closed mesopores were identified in the walls (Figure 6e). The open mesopores might have formed during the calcination process. SL4-8 and SL6-7 exhibited similar hysteresis loops (Supporting Information, Figures S6a and S7a). For SL6-8, two hysteresis loops were observed, at relative pressures (P/P0) between 0.4 and 0.9. The hysteresis loop observed in the isotherms at relative pressures (P/P0) from 0.4 to 0.7 was close to the H2 type, suggesting that the mesopores within the walls were ink bottle-shaped. The larger mesopores were attributed to the voids within the nanotubes. The BJH pore size distribution plots determined from the adsorption branches of SL4-8 and SL6-7 showed sharp peaks at 5.3 and 5.5 nm, respectively (Supporting Information, Figures S6b and S7b). The SAXRD patterns for SL4-8 and SL6-7 showed peaks at 2θ values of 0.96° and 0.90°, respectively, which indicated a partially ordered structure (Supporting Information, Figures S6c and S7c). When the concentrations of the LMWAs are low, only few chiral organic self-assemblies exist in the reaction 310

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK2011354), the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD), and the National Natural Science Foundation of China (Nos. 21104053, 21071103, and 21074086).

Figure 8. Proposed mechanism for the formation of the twisted nanoribbons and nanotubes.

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width of the nanoribbons of L-18Val6PyBr was thinner than those of the others. The twisted silica nanotubes were then obtained after the sol−gel polycondensation and calcination. The hydrophobic associations among the alkylene chains were proposed to play an important role in the difference of the width. After TEOS was added in a dropwise fashion, TEOS-F127 micelles were formed through H-bonding.19 After polycondensation of the silica oligomers, core−shell nanospheres were formed.17 The formed nanospheres adsorbed and polycondensed on the surfaces of the nanoribbons or nanotubes. Under this acidic condition, both the organic self-assemblies of the LMWAs and the silica spheres have positive charges on the surfaces. Therefore, Cl− and Br− ions should play an important role in this adsorption.20 At the beginning, they adsorbed randomly. With extending the reaction time, a partially ordered structure was obtained. After the extraction of the mixture was performed using methanol, concentrated aqueous HCl solutions, and calcinations, mesoporous silica nanotubes and nanoribbons were obtained. The single-layered mesoporous silica nanoribbons were formed via the shrinking of the interlayer voids of the double-layered ribbons.

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CONCLUSION In conclusion, coiled mesoporous silica nanoribbons with mesopores in the walls were prepared using LMWAs and F127 block copolymers, via a dual-templating pathway. Their surface areas reached 700 m2/g. The morphologies of the silicas were controlled by the self-assembled structures of the LMWAs, and the mesopores within the nanoribbons were controlled by the F127 block copolymer. It was proposed that the phase separation of the F127 block copolymer and the LMWAs drove the formation of these structures. Mesoporous silica nanotubes with mesopores in their walls could also be prepared using this approach. The results shown here give us the opportunity to prepare chiral silicas with high surface areas and good mass transfer abilities. These nanotubes and nanoribbons have the potential to be applied in the fields of catalysis, sorbents, and controlled release.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

FESEM and TEM images, N2 sorption data, and SAXRD for the mesoporous silicas (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. 311

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