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Chiral Mesostructured Silica Nanofibers of MCM-41.

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enigmatic morphogenesis. Recently our research group has
investigated the topological transformation of a series of
vesicular MCM-41 compounds with different mesostructures
in an alkaline synthesis system[5] that was initialy developed
by Rathouský and co-workers.[6] The self-assembly of sodium
silicate (SS) and cetyltrimethylammonium bromide (CTAB)
into a hexagonal mesostructure in such a method is driven by
the hydrolysis of ethyl acetate (EA). Herein we report that
chiral mesostructured silica nanofibers of MCM-41 can be
fabricated in this SS/CTAB/EA/H2O system by simply lowering the SS and CTAB concentrations below 0.5 mol per
1000 mol H2O. It is remarkable that two types of chiral
mesostructures with different symmetries were synthesized
from the usual achiral materials in this study. Moreover, a
relationship between the chiral and ordinary achiral mesostructures of MCM-41 was revealed through a systematic
investigation of the synthesis system.
The first type of chiral nanofibers of MCM-41 (Figure 1 a, b) has a single twist axis. The XRD pattern of such a
Mesoporous Materials
DOI: 10.1002/ange.200504191
Chiral Mesostructured Silica Nanofibers of
MCM-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 most
extensively studied mesostructured materials since its first
synthesis by Mobil scientists in 1992.[1] Many important
applications[2] of mesostructured silica MCM-41 in catalysis,
separation, and nanoengineering are closely correlated to its
ordered two-dimensional (2D) hexagonal mesostructure/
mesopore. Besides the usual straight 2D hexagonal mesostructure,[3] various curved mesostructures of MCM-41 have
also been reported by several research groups in the last
decade,[4] which has aroused great academic interest in their
[*] B. Wang,[+] C. Chi,[+] W. Shan, Dr. Y. H. Zhang, N. Ren, Prof. Y. Tang
Department of Chemistry and
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials
Fudan University
Shanghai 200433 (P.R. China)
Fax: (+ 86) 21-6564-1740
E-mail: yitang@fudan.edu.cn
Dr. W. L. Yang
Department of Macromolecular Science and
Key Laboratory of Molecular Engineering of Polymers
Ministry of Education
Fudan University
Shanghai 200433 (P.R. China)
[+] These authors contributed equally.
[**] We thank Prof. D. Y. Zhao for his very helpful discussions. This work
was supported by the NSFC (20233030, 20325313, 20303003,
20421303, 20473022), the STCSM (05QMX1403, 05XD14002), and
the Major State Basic Research Development Program
(2003CB615807).
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2142
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).
single-axis nanofiber (Figure 1 c) reveals a highly ordered 2D
hexagonal mesostructure with a lattice constant of 4.5 nm.
The N2 sorption isotherms of the calcined product show a
steep 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 1 d).
The BET surface area and mesopore volume of the single-axis
nanofiber are 960 m2 g 1 and 0.63 cm3 g 1, respectively. Analysis of the chiral mesostructure of the single-axis nanofibers
by electron microscopy showed: 1) The twisted crystal facets
could be distinguished from their field-emission SEM images
(see Supporting Information); 2) periodic fringes along the
axis of the nanofiber in the TEM image (Figure 1 b); and
3) the observed fringes moved along the axis when the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2142 –2144
Angewandte
Chemie
nanofiber was rotated around its twist axis, while the fringes
appeared to be curved when the nanofiber was tilted
vertically against its original direction (see Supporting
Information). The above observations indicate that such a
nanofiber is composed of chiral nanochannels with a single
twist axis.[7b] Notably, the nanochannels at the end of some
nanofibers become straight, which results in an additional set
of fringes (Figure 1 b). It was found by counting more than 400
single-axis nanofibers that the number of left-handed nanofibers 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 significantly with the concentrations of the reactants in the SS/
CTAB/EA/H2O system: The length of the nanofibers
increases with the concentration of CTAB (see Supporting
Information) and varies from tens of nanometers to tens of
micrometers, and the diameter of the nanofibers increases as
the concentration of silicate increases (Figure 2). InterestFigure 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 a
straight hexagonal fiber to a dual-axis fiber through two twist operations which is accompanied by a degradation of symmetry.
Surprisingly, every period of the dual-axis nanofiber has two
instead of six[7c] sets of (100) fringes in the TEM image (see
arrows in Figure 4 c). This observation can be ascribed to the
Figure 2. Electron micrographs of single-axis nanofiber samples
obtained at different silicate concentrations (SS/CTAB/EA/H2O, molar
ratio x:0.16:2.7:1000), where a) x = 0.17, b) x = 0.20, c) x = 0.23, and
d) x = 0.50. Both the diameter (D) and twist period (6L) of the
nanofibers increase with the concentration of silicate. The chirality of
the fiber obtained at a high silicate concentration becomes unobservable, and its mesostructure is close to the ordinary straight hexagonal
mesostructure.
ingly, nanofibers with a large diameter have a large twist
period. The coexistence of a large diameter and a small twist
period would result in nanochannels with high curvatures,
which is not preferred from an energy viewpoint. The twist
period of the fibers synthesized at higher silicate concentrations becomes so large that the fibers are almost straight
and chirality is no longer evident (Figure 2 d). If the reactant
concentrations are further increased, the obtained product
becomes ordinary MCM-41 fibers (namely tubules[5]) with
straight hexagonal mesostructures. On the basis of these
results, the ordinary straight MCM-41 mesostructure can be
regarded as a special case of a chiral mesostructure with a
twist period of infinite length.
Another type of chiral nanofibers appeared in the MCM41 products when the EA concentration was decreased in the
synthesis system (Figure 3 a). The nanofibers have two twist
axes: one lies outside the nanofiber, and the other one lies in
the center of the nanofiber. Some of these dual-axis nanofibers do not wind very regularly around their primary twist
axes. The well-crystallized mesostructure of the dual-axis
nanofiber enabled us to discover some distinct features. It was
found that the primary and secondary twist not only share the
same chirality but they also have the same period (Figure 4 a).
Angew. Chem. 2006, 118, 2142 –2144
Figure 4. a) SEM image, b) simulated 3D model, c) TEM image, and
d) simulated TEM image of a dual-axis nanofiber.
primary twist operation that degrades the symmetry of the
nanofiber from the D6h to the C2 point group (Figure 3 b). A
three-dimensional model was built by performing two twist
operations on a straight hexagonal mesostructure, and it is in
excellent agreement with the SEM and TEM images of the
dual-axis nanofiber (Figure 4).
Two factors are critical for the synthesis of chiral MCM-41
nanofibers. One is the concentration ratio of CTAB to SS. No
solid product could be obtained if the concentration ratio of
CTAB/SS was above a certain value (approximately 3:1–4:1
at 25 8C). This result might be attributed to a shortage of the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
2143
Zuschriften
silicate source, that is, the amount of the silicates in solution is
not sufficient for the cross-linking of silicate–CTA species
into an aggregated form. Another factor is the synthesis
temperature. The length of the nanofibers gradually decreases
as the temperature increases, and is accompanied by a loss of
the ordered mesostructure. No precipitate could be obtained
from the synthesis solution when the temperature exceeded
about 50 8C, thus suggesting that the aggregation of the
silicate–CTA complexes was greatly hampered under such
conditions. The recommended temperature for the synthesis
of chiral MCM-41 nanofibers is between 18 and 25 8C.
In contrast to previous studies in which chiral surfactants
were used to fabricate chiral mesostructures,[7–9] our synthesis
of chiral MCM-41 was carried out without any chiral
additives. Both the CTA+ and silicon-oxygen tetrahedron
have highly symmetric structures, thus the chirality of the
MCM-41 nanofibers must originate from a chiral aggregation
of these symmetric building blocks. Such chirality is analogous to that of many organic molecules which originates from
an asymmetric combination of carbon, oxygen, and hydrogen
atoms. Notably, the study by Che et al. showed that the use of
a chiral surfactant (namely, sodium N-acyl-l-alanine) led to
more left-handed fibers than right-handed ones,[7b] thus
suggesting that the chiral surfactant may play a role in the
breaking of the racemization of the obtained mesostructured
silica. Another important question arising from our work is
how the same chirality is maintained during the formation of
every single MCM-41 nanofiber. We presume that two
possible assembly routes may lead to this result. One
possibility is that some chiral silicate–CTA intermediate
species are formed during the assembly process and that
species with the same chirality tend to assemble with each
other, thus forming nanofibers with specific chiralities.
Another possibility is that nuclei with different chiralities
are formed at the beginning of the reaction, and then they
induce the silicate and CTA+ species to assemble into
aggregates with the same chirality on the seed surface. As
mentioned above, the chirality of the nanofibers becomes
more prominent as its diameter decreases (Figure 2), which
may suggest that the chiral mesostructure is more thermodynamically stable than the achiral one at small dimensions. The
recent study of Stucky and co-workers[10] has shown that the
physical confinement of mesostructured silica could also lead
to the formation of chiral architectures. Thus, the dimensions
may possibly play an important role in the symmetry breaking
of the mesostructured silica.
In conclusion, two types of chiral MCM-41 nanofibers
were synthesized from achiral surfactant and silicate reactants, and their chiral architectures were carefully characterized. A transformation from a chiral mesostructure to a
straight achiral mesostructure was observed, thus demonstrating an interesting relationship between these two seemingly different structures. Furthermore, the self-assembly of
nanosized chiral mesostructure in a diluted SS/CTAB/EA/
H2O system is an important supplement to the formation of
micrometer-sized vesicles observed at higher reactant concentrations.[5] We expect that these results will contribute to
the understanding of the formation of mesostructured silica
MCM-41.[1, 4d, 11]
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Experimental Section
In a typical synthesis of MCM-41 single-axis nanofibers, sodium
silicate (Na2SiO3·9 H2O, 0.45 g) and CTAB (0.5 g) were dissolved in
H2O (150 g) at 23 8C. Ethyl acetate (2.0 g) was added to the above
solution under vigorous stirring. After stirring the mixture for about
30 s, the solution was left to stand in a water bath at 23 8C overnight.
The product was isolated by centrifugation and then washed
repeatedly with deionized water. The dried samples were calcined
in air at 600 8C for 6 h to remove the surfactants. The reaction
compositions for the synthesis of single-axis and dual-axis nanofibers
were 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 electron
microscopy (SEM) images were obtained on JEOL JEM-2010 and
Philips XL 30 instruments, respectively. The X-ray diffraction (XRD)
patterns were taken on a Rigaku D/MAX-IIA diffractometer with
CuKa radiation at 30 kV and 20 mA. Nitrogen sorption isotherms were
measured on a Micromeritics TriStar 3000 system. The samples were
degassed at 200 8C for about 3 h before the sorption measurement.
Received: November 24, 2005
Published online: February 24, 2006
.
Keywords: chirality · mesoporous materials · nanostructures ·
silica
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