Mesoporous Materials

DOI: 10.1002/ange.200504191

Chiral Mesostructured Silica Nanofibers ofMCM-41**

Bo Wang, Cheng Chi, Wei Shan, Yahong Zhang,Nan Ren, Wuli Yang, and Yi Tang*

Mesostructured silica MCM-41 has been one of the mostextensively studied mesostructured materials since its firstsynthesis by Mobil scientists in 1992.[1] Many importantapplications[2] of mesostructured silica MCM-41 in catalysis,separation, and nanoengineering are closely correlated to itsordered two-dimensional (2D) hexagonal mesostructure/mesopore. Besides the usual straight 2D hexagonal meso-structure,[3] various curved mesostructures of MCM-41 havealso been reported by several research groups in the lastdecade,[4] which has aroused great academic interest in their

enigmatic morphogenesis. Recently our research group hasinvestigated the topological transformation of a series ofvesicular MCM-41 compounds with different mesostructuresin an alkaline synthesis system[5] that was initialy developedby Rathousky and co-workers.[6] The self-assembly of sodiumsilicate (SS) and cetyltrimethylammonium bromide (CTAB)into a hexagonal mesostructure in such a method is driven bythe hydrolysis of ethyl acetate (EA). Herein we report thatchiral mesostructured silica nanofibers of MCM-41 can befabricated in this SS/CTAB/EA/H2O system by simply low-ering the SS and CTAB concentrations below 0.5 mol per1000 mol H2O. It is remarkable that two types of chiralmesostructures with different symmetries were synthesizedfrom the usual achiral materials in this study. Moreover, arelationship between the chiral and ordinary achiral meso-structures of MCM-41 was revealed through a systematicinvestigation of the synthesis system.

The first type of chiral nanofibers of MCM-41 (Fig-ure 1a,b) has a single twist axis. The XRD pattern of such a

single-axis nanofiber (Figure 1c) reveals a highly ordered 2Dhexagonal mesostructure with a lattice constant of 4.5 nm.The N2 sorption isotherms of the calcined product show asteep capillary condensation at a P/P0 ratio of 0.2:1–0.3:1,which corresponds to a BJH pore size of 2.4 nm (Figure 1d).The BET surface area andmesopore volume of the single-axisnanofiber are 960 m2g�1 and 0.63 cm3g�1, respectively. Anal-ysis of the chiral mesostructure of the single-axis nanofibersby electron microscopy showed: 1) The twisted crystal facetscould be distinguished from their field-emission SEM images(see Supporting Information); 2) periodic fringes along theaxis of the nanofiber in the TEM image (Figure 1b); and3) the observed fringes moved along the axis when the

Figure 1. a) SEM image, b) TEM image, c) XRD pattern, and d) N2

sorption isotherms of single-axis chiral nanofibers (SS/CTAB/EA/H2O,molar ratio 0.19:0.16:2.7:1000).

[*] B. Wang,[+] C. Chi,[+] W. Shan, Dr. Y. H. Zhang, N. Ren, Prof. Y. TangDepartment of Chemistry andShanghai Key Laboratory of Molecular Catalysis and InnovativeMaterialsFudan UniversityShanghai 200433 (P.R. China)Fax: (+86)21-6564-1740E-mail: yitang@fudan.edu.cn

Dr. W. L. YangDepartment of Macromolecular Science andKey Laboratory of Molecular Engineering of PolymersMinistry of EducationFudan UniversityShanghai 200433 (P.R. China)

[+] These authors contributed equally.

[**] We thank Prof. D. Y. Zhao for his very helpful discussions. This workwas supported by the NSFC (20233030, 20325313, 20303003,20421303, 20473022), the STCSM (05QMX1403, 05XD14002), andthe Major State Basic Research Development Program(2003CB615807).

Supporting Information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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nanofiber was rotated around its twist axis, while the fringesappeared to be curved when the nanofiber was tiltedvertically against its original direction (see SupportingInformation). The above observations indicate that such ananofiber is composed of chiral nanochannels with a singletwist axis.[7b] Notably, the nanochannels at the end of somenanofibers become straight, which results in an additional setof fringes (Figure 1b). It was found by counting more than 400single-axis nanofibers that the number of left-handed nano-fibers is almost equal to the number of right-handed ones,thus demonstrating that the product is a racemic mixture.

The dimensions of the single-axis nanofibers vary signifi-cantly with the concentrations of the reactants in the SS/CTAB/EA/H2O system: The length of the nanofibersincreases with the concentration of CTAB (see SupportingInformation) and varies from tens of nanometers to tens ofmicrometers, and the diameter of the nanofibers increases asthe concentration of silicate increases (Figure 2). Interest-

ingly, nanofibers with a large diameter have a large twistperiod. The coexistence of a large diameter and a small twistperiod would result in nanochannels with high curvatures,which is not preferred from an energy viewpoint. The twistperiod of the fibers synthesized at higher silicate concen-trations becomes so large that the fibers are almost straightand chirality is no longer evident (Figure 2d). If the reactantconcentrations are further increased, the obtained productbecomes ordinary MCM-41 fibers (namely tubules[5]) withstraight hexagonal mesostructures. On the basis of theseresults, the ordinary straight MCM-41 mesostructure can beregarded as a special case of a chiral mesostructure with atwist period of infinite length.

Another type of chiral nanofibers appeared in the MCM-41 products when the EA concentration was decreased in thesynthesis system (Figure 3a). The nanofibers have two twistaxes: one lies outside the nanofiber, and the other one lies inthe center of the nanofiber. Some of these dual-axis nano-fibers do not wind very regularly around their primary twistaxes. The well-crystallized mesostructure of the dual-axisnanofiber enabled us to discover some distinct features. It wasfound that the primary and secondary twist not only share thesame chirality but they also have the same period (Figure 4a).

Surprisingly, every period of the dual-axis nanofiber has twoinstead of six[7c] sets of (100) fringes in the TEM image (seearrows in Figure 4c). This observation can be ascribed to the

primary twist operation that degrades the symmetry of thenanofiber from the D6h to the C2 point group (Figure 3b). Athree-dimensional model was built by performing two twistoperations on a straight hexagonal mesostructure, and it is inexcellent agreement with the SEM and TEM images of thedual-axis nanofiber (Figure 4).

Two factors are critical for the synthesis of chiral MCM-41nanofibers. One is the concentration ratio of CTAB to SS. Nosolid product could be obtained if the concentration ratio ofCTAB/SS was above a certain value (approximately 3:1–4:1at 25 8C). This result might be attributed to a shortage of the

Figure 2. Electron micrographs of single-axis nanofiber samplesobtained at different silicate concentrations (SS/CTAB/EA/H2O, molarratio x :0.16:2.7:1000), where a) x=0.17, b) x=0.20, c) x=0.23, andd) x=0.50. Both the diameter (D) and twist period (6L) of thenanofibers increase with the concentration of silicate. The chirality ofthe fiber obtained at a high silicate concentration becomes unobserv-able, and its mesostructure is close to the ordinary straight hexagonalmesostructure.

Figure 3. a) SEM image of dual-axis chiral nanofibers (SS/CTAB/EA/H2O, molar ratio 0.19:0.16:0.50:1000). b) Transformation from astraight hexagonal fiber to a dual-axis fiber through two twist oper-ations which is accompanied by a degradation of symmetry.

Figure 4. a) SEM image, b) simulated 3D model, c) TEM image, andd) simulated TEM image of a dual-axis nanofiber.

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silicate source, that is, the amount of the silicates in solution isnot sufficient for the cross-linking of silicate–CTA speciesinto an aggregated form. Another factor is the synthesistemperature. The length of the nanofibers gradually decreasesas the temperature increases, and is accompanied by a loss ofthe ordered mesostructure. No precipitate could be obtainedfrom the synthesis solution when the temperature exceededabout 50 8C, thus suggesting that the aggregation of thesilicate–CTA complexes was greatly hampered under suchconditions. The recommended temperature for the synthesisof chiral MCM-41 nanofibers is between 18 and 25 8C.

In contrast to previous studies in which chiral surfactantswere used to fabricate chiral mesostructures,[7–9] our synthesisof chiral MCM-41 was carried out without any chiraladditives. Both the CTA+ and silicon-oxygen tetrahedronhave highly symmetric structures, thus the chirality of theMCM-41 nanofibers must originate from a chiral aggregationof these symmetric building blocks. Such chirality is analo-gous to that of many organic molecules which originates froman asymmetric combination of carbon, oxygen, and hydrogenatoms. Notably, the study by Che et al. showed that the use ofa chiral surfactant (namely, sodium N-acyl-l-alanine) led tomore left-handed fibers than right-handed ones,[7b] thussuggesting that the chiral surfactant may play a role in thebreaking of the racemization of the obtained mesostructuredsilica. Another important question arising from our work ishow the same chirality is maintained during the formation ofevery single MCM-41 nanofiber. We presume that twopossible assembly routes may lead to this result. Onepossibility is that some chiral silicate–CTA intermediatespecies are formed during the assembly process and thatspecies with the same chirality tend to assemble with eachother, thus forming nanofibers with specific chiralities.Another possibility is that nuclei with different chiralitiesare formed at the beginning of the reaction, and then theyinduce the silicate and CTA+ species to assemble intoaggregates with the same chirality on the seed surface. Asmentioned above, the chirality of the nanofibers becomesmore prominent as its diameter decreases (Figure 2), whichmay suggest that the chiral mesostructure is more thermody-namically stable than the achiral one at small dimensions. Therecent study of Stucky and co-workers[10] has shown that thephysical confinement of mesostructured silica could also leadto the formation of chiral architectures. Thus, the dimensionsmay possibly play an important role in the symmetry breakingof the mesostructured silica.

In conclusion, two types of chiral MCM-41 nanofiberswere synthesized from achiral surfactant and silicate reac-tants, and their chiral architectures were carefully character-ized. A transformation from a chiral mesostructure to astraight achiral mesostructure was observed, thus demon-strating an interesting relationship between these two seem-ingly different structures. Furthermore, the self-assembly ofnanosized chiral mesostructure in a diluted SS/CTAB/EA/H2O system is an important supplement to the formation ofmicrometer-sized vesicles observed at higher reactant con-centrations.[5] We expect that these results will contribute tothe understanding of the formation of mesostructured silicaMCM-41.[1,4d,11]

Experimental SectionIn a typical synthesis of MCM-41 single-axis nanofibers, sodiumsilicate (Na2SiO3·9H2O, 0.45 g) and CTAB (0.5 g) were dissolved inH2O (150 g) at 23 8C. Ethyl acetate (2.0 g) was added to the abovesolution under vigorous stirring. After stirring the mixture for about30 s, the solution was left to stand in a water bath at 23 8C overnight.The product was isolated by centrifugation and then washedrepeatedly with deionized water. The dried samples were calcinedin air at 600 8C for 6 h to remove the surfactants. The reactioncompositions for the synthesis of single-axis and dual-axis nanofiberswere SS/CTAB/EA/H2O 0.15–0.50:0.12–0.33:1.4–3.0:1000, and 0.15–0.30:0.16–0.50:0.3–0.5:1000 (in molar ratio), respectively.

Transmission electron microscopy (TEM) and scanning electronmicroscopy (SEM) images were obtained on JEOL JEM-2010 andPhilips XL 30 instruments, respectively. The X-ray diffraction (XRD)patterns were taken on a Rigaku D/MAX-IIA diffractometer withCuKa radiation at 30 kVand 20 mA. Nitrogen sorption isotherms weremeasured on a Micromeritics TriStar 3000 system. The samples weredegassed at 200 8C for about 3 h before the sorption measurement.

Received: November 24, 2005Published online: February 24, 2006

.Keywords: chirality · mesoporous materials · nanostructures ·silica

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