Direct formation of mesostructured silica-based hybrids from novel siloxane oligomers with long alkyl chains

Article abstract of DOI:10.1002/anie.200351419

Lamellar and hexagonal structures of new silica-based hybrids were directly formed from newly designed alkylsiloxane oligomers 1. This approach based on the self-assembly of single precursors provides a novel pathway to ordered microporous silica as well as mesostructured silica-based hybrid materials (see scheme, 1) hydrolysis and polycondensation, 2) self-assembly).

Full text of DOI:10.1002/anie.200351419

Angewandte
Chemie
a wide variety of hybrids can be designed at a molecular scale
by the modification of the precursors, the control of their
[
1,2]
nanostructures is still an important subject of research.
Recently, intensive efforts have been made in the synthesis of
mesostructured (lamellar, hexagonal, or cubic) hybrids by
[
3–7]
using surfactant assemblies as structure directors.
How-
ever, their formations are governed by weak interactions
between surfactant molecules and silicate or organosiloxane
species. The control of the hybrid nanostructures without the
use of surfactants is a great challenge, and the preparation of a
novel building block with self-organizing ability should be
important. The design of such a single precursor would open
up the possibility of creating nanohybrid materials that are
not accessible through surfactant-directed processes.
Herein we report an innovative method for the formation
of ordered silica-based hybrids by using newly designed
siloxane-based oligomers 1, consisting of an alkylsilane core
and three branched trimethoxysilyl groups (C H2n+1Si-
n
(
OSi(OMe) ) , n = 10 or 16), as single precursors. The self-
3 3
assembly of 1 during the hydrolysis and polycondensation
leads to the formation of lamellar or hexagonal-like meso-
structures that are controlled by changing the length of the
alkyl chain (Scheme 1).
Mesostructured Silica-Based Hybrids
Direct Formation of Mesostructured Silica-Based
Hybrids from Novel Siloxane Oligomers with
Long Alkyl Chains**
Scheme 1. Formation of mesostructured hybrids from siloxane-based
oligomers 1 with two different chain lengths.
Atsushi Shimojima and Kazuyuki Kuroda*
The ability of organoalkoxysilanes to self-organize during
[
8–14]
Silica-based hybrids derived from organoalkoxysilanes have
been extensively studied because of their potential for novel
nanomaterials as well as their scientific significance. Although
hydrolysis and polycondensation was recently explored.
This is an attractive feature based on the weak interactions of
the organic groups and the hydrophilic nature of the silanol
groups. However, all attempts to prepare ordered hybrids
through self-assembly have resulted in layered structures,
except for the specific case in which a precursor bearing a
[
*] Prof. Dr. K. Kuroda, Dr. A. Shimojima
Department of Applied Chemistry
Waseda University
[
12]
surfactant molecule with a SiÀC bond was employed. We
have reported the formation of multilayered hybrids from
Ohkubo 3–4–1, Shinjuku-ku, Tokyo 169–8555 (Japan)
Fax: (+81)3-5286-3199
E-mail: kuroda@waseda.jp
[
13]
alkyltrialkoxysilanes. Although the morphological control
of the transparent films was attained by cocondensation of
alkyltrialkoxysilane with tetraalkoxysilane, oligomeric spe-
cies with well-defined structures have never been formed in
Prof. Dr. K. Kuroda
Kagami Memorial Laboratory for Materials Science and Technology
Waseda University
Nishiwaseda 2–8–26, Shinjuku-ku, Tokyo 169–0051 (Japan)
and
CREST, Japan Science and Technology Corporation (JST)
[
14]
precursor solutions,
which implies that highly organized
nanoarchitectures are formed with difficulty.
The precursors 1 (n = 10 and 16) were synthesized by the
reaction of corresponding n-alkylsilanetriols and tetrachlor-
osilane, followed by methanolysis of SiÀCl groups. Hydrolysis
[
**] The authors are grateful to Prof. Y. Sugahara (Waseda University) for
discussion. The work was partially supported by a Grant-in-Aid for
COE research, MEXT, Japan. A. S. thanks a financial support by a
Grant-in-Aid for JSPS Fellows from MEXT.
and polycondensation reactions of 1 were performed in the
solutions with a molar composition of 1/THF/H O/HCl =
2
1
:50:18:0.002. The mixtures were stirred at 258C for 12h,
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
and water (H O:1 = 32) was then added. The resulting
2
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DOI: 10.1002/anie.200351419
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4057

Communications
exhibits exclusively well-defined striped patterns whose
periodicity is close to the d spacing. In contrast, a hexago-
nal-like structure is observed for the hybrid with n = 10, in
agreement with the XRD data. The structure is slightly
distorted, possibly due to the anisotropic shrinkage of the
siloxane frameworks during the polycondensation process.
Although a future detailed structural study is needed, the
present results evidently show that the precursors have the
ability to form both lamellar and hexagonal mesophases
through self-assembly.
The formation of the siloxane networks was confirmed by
Si MAS NMR spectroscopy of the hybrids (Figure 3). The
2
9
Figure 1. Powder XRD patterns of the products derived from 1:
a) n=10 (as-synthesized), b) n=16 (as-synthesized), c) n=10 (cal-
cined), and d) n=16 (calcined). The patterns were recorded on a Mac
2
2
Science M03XHF diffractometer with Mn-filtered FeKa radiation.
precursor solutions were cast on glass substrates, and air-dried
at room temperature for 2days. The resulting platelike solids
were pulverized before characterization.
The powder X-ray diffraction (XRD) patterns of the
hybrids (n = 10 and 16) exhibit low-angle diffraction peaks
corresponding to the d spacings of 2.92 and 3.38 nm, respec-
tively (Figure 1a, b). In the case of n = 16, all the peaks
observed at higher angles (2q = 6–178) are attributed to
higher order diffractions, thus indicating the well-ordered
layered structure of the product. This ordering was revealed
by the increase of the d value to about 4.7 nm upon
adsorption of n-decyl alcohol. The interlayer chains appear
to take a monolayer arrangement, because the d value
2
9
Figure 3. Solid-state Si MAS NMR spectra of the products derived
from 1: a) n=10 (as-synthesized), b) n=16 (as-synthesized), and
c) n=10 (calcined). The spectra were recorded on a JEOL JNM-CMX-
400 spectrometer at a resonance frequency of 79.42 MHz with a 458
pulse and a recycle delay of 100 s. The relative intensity ratios of the
signals are listed in the Supporting Information.
spectra for n = 10 and 16 are similar and display signals
corresponding to the T , T , and T units (T , CSi(OSi) (OH)
), and also to the Q , Q , and Q units (Q , Si(OSi) (OH) ).
x x 4-x
The presence of the T and T units suggests the partial
1
2
3
x
x
3-
2
3
4
x
(
3.38 nm) is much smaller than that for lamellar alkylsilox-
[
13]
1
2
anes with a bilayer structure (d = 4.80 nm). On the other
hand, the pattern for n = 10 (Figure 1a) shows quite a
different profile with several small peaks at the higher
region, and the structure of the product can tentatively be
assigned as 2D hexagonal.
cleavage and the rearrangement of the original SiÀOÀSi
bonds in the precursor 1 during hydrolysis and polyconden-
0
sation. We confirmed that the signals arising from the Q
2
9
species did not appear in the Si NMR spectra of the solution
during the reaction, thus suggesting a very slow rate of the
hydrolysis of SiÀOÀSi bonds in 1 to form smaller species.
Figure 2shows the transmission electron microscopic
(
TEM) images of the products. The hybrid with n = 16
Further work to investigate the detailed reaction process is
underway and will be reported subsequently.
These results prove the formation of two types of nano-
structured hybrids as depicted in Scheme 1. The products are
structurally unique because organic regions are filled with
alkyl chains covalently attached to nanostructured-silica
frameworks. The formation of these products depends on
the self-assembly of the single precursor, and is quite different
from conventional methods, including cocondensation of
organotrialkoxysilane and tetraalkoxysilane in the presence
of surfactants, or postmodification of mesoporous silica or
Figure 2. Typical TEM images of the as-synthesized hybrids derived
from 1: a) n=10 and b) n=16. The images were obtained by a JEOL
JEM-100CX microscope at an accelerating voltage of 100 kV.
[
4,5]
layered silicates with organosilanes.
4
058
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Angewandte
Chemie
The difference in the nanostructures was further con-
firmed by the calcination of the hybrids at 5008C for 8 h to
2
9
remove organic components. The Si NMR spectra of the
3
4
calcined samples mainly showed the Q and Q signals
Figure 3c), which suggests the conversion of the Tunits to
(
Q units by thermal degradation of the organic moieties. While
the layered structure (n = 16) collapsed upon calcination
(
Figure 1d), the ordered structure was retained in the case of
n = 10 with a decrease in the d spacing (Figure 1c), which was
also supported by TEM (see Supporting Information). The
nitrogen adsorption measurement revealed that the calcined
hybrid (n = 10) was a microporous solid with a Brunauer–
2
À1
Emmett–Teller (BET) surface area of 1070 m g , the aver-
age Barrett–Joyner–Halenda (BJH) pore diameter of about
3
À1
1
.7 nm, and pore volume of 0.41 cm g , while the as-
synthesized hybrid was nonporous (see Supporting Informa-
tion). Although several researchers have proposed the idea of
using organoalkoxysilanes as organic templates to prepare
Figure 4. XRD patterns of the as-synthesized hybrid films derived from
[
15–18]
microporous silica,
this work is truly the first synthesis of
1: a) n=10 and b) n=16. Inset shows the cross-sectional TEM image
of the film with n=10 (scale bar; 20 nm). The image was obtained by
a JEOL JEM-2010 microscope at an accelerating voltage of 200 kV.
the hexagonally ordered porous silica. Such a method has the
advantage of producing relatively small pores compared to
those obtained by using surfactants with the corresponding
[
19]
chain length.
The siloxane oligomers formed by the hydrolysis and
polycondensation of 1 can be regarded as a polyhydroxy
amphiphile containing a hydrophobic alkyl chain and hydro-
In conclusion, we have demonstrated the successful
formation of silica-based hybrids with highly ordered lamellar
and 2D hexagonal nanostructures by using novel alkylsilox-
ane oligomers as single precursors. Such a method proves to
lead to the better control of composition, mesostucture, and
morphology of silica-based hybrids. It is of great interest that
a hexagonally ordered hybrid is prepared for the first time
without the use of surfactants and any other silica source, thus
providing a novel pathway to ordered porous silica. The
further design of precursors based on the organic group and/
or the siloxane part will allow functionalization as well as
variations of the architecture at molecular and nanometer
scales.
[
20]
philic silanol groups. It is plausible that the nanostructured
hybrids are formed by the self-assembly of the alkylsiloxane
species upon evaporation of the solvents, followed by siloxane
formation. The difference in the nanostructures of the hybrids
depending on the alkyl chain length can be explained in terms
[
21]
of the geometrical packing of alkylsiloxane oligomers. In
contrast to the several reports on the formation of lamellar
hybrids from organoalkoxysilanes with the general formula of
RSi(OR’)3 or (R’O) SiÀRÀSi(OR’)
(R, R’ = organic
this example is the first self-assembly of
3
3
[
9–11,13]
groups),
organosilane molecules into a hexagonal structure, which is
apparently due to the large occupying volume of the hydro-
philic headgroup in the precursor 1 compared to that of a
single Si(OH) group. It should be noted that such meso- Experimental Section
3
structures cannot be obtained by the random cocondensation
n-Alkylsilanetriols (n = 10 and 16) were synthesized by the hydrolysis
of decyltrichlorosilane (C H SiCl ) and hexadecyltriethoxysilane
[
14]
of alkylalkoxysilane-tetraalkoxysilane mixtures, thus sug-
gesting the importance of well-designed molecular structure
of the precursor.
Furthermore, the control of the macroscopic morphology
of the hybrids was achieved because of the high cross-linking
ability of 1. Transparent thin films with the thickness of about
10 21
3
(
C H Si(OEt) ). Detailed procedures are described in the Support-
16 33 3
ing Information. In a typical synthesis of 1, 12.0 g of alkylsilanetriol
dissolved in THF (600 mL) was added to a vigorously stirred mixture
of SiCl4 (100 mL) and hexane (240 mL) at room temperature.
Removal of the solvents and unreacted SiCl under reduced pressure
4
afforded a slightly turbid liquid which contained mainly RSi(OSiCl3)3
4
50–600 nm were obtained by spin coating (3000 rpm, 10 s)
(R = alkyl) and a small amount of RSiCl(OSiCl ) . RSi(OSiCl ) was
3 2 3 3
isolated by vacuum distillation, and the conversion of SiÀCl groups to
SiÀOMe groups was performed by the addition of methanol with
degassed HCl under reduced pressure. The precursor 1 was obtained
as a clear and colorless liquid. The spectroscopic data are contained in
the Supporting Information.
the precursor solutions on glass substrates. The XRD patterns
of the films (n = 10 and 16) are shown in Figure 4. Both
patterns show the smaller d spacings than those of powdered
samples, which may in part be ascribed to the difference in the
degree of polycondensation. It is noteworthy that the film for
n = 10 exhibits a very sharp and intense peak with only second
and third order diffractions, which is typical for the well-
ordered 2D hexagonal structures in which cylindrical assem-
Received: March 17, 2003
Revised: May 5, 2003 [Z51419]
[
22,23]
blies are oriented parallel to the substrate.
Evidence for
the hexagonal structure of the film was obtained from the
cross-sectional TEM image (Figure 4 inset).
Keywords: nanostructures · organic–inorganic hybrid
composites · self-assembly · sol–gel processes · thin films
.
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Communications
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