Self-assembly of designed oligomeric siloxanes with alkyl chains into silica-based hybrid mesostructures

Article abstract of DOI:10.1021/ja0541736

A novel self-assembly route to ordered silica-organic hybrids using well-defined siloxane oligomers with alkoxy functionality and covalently attached alkyl chains has been investigated. Various hybrid mesostructures were obtained by hydrolysis and polycondensation without the use of any structure-directing agents. The oligomers 1(Cn), having an alkylsilane core and three branched trimethoxysilyl groups, formed highly ordered lamellar phases when n = 14-18, while those with shorter alkyl chains formed cylindrical assemblies, slightly distorted two-dimensional (2D) hexagonal structures (n = 6-10), and a novel 2D monoclinic structure (n = 12). Furthermore, the mixtures of 1(Cn) with different chain lengths yielded well-ordered 2D hexagonal phases, possibly due to the better packing of the precursors. The hybrids consisting of cylindrical assemblies were converted to ordered porous silica with tunable pore sizes upon calcination to remove organic groups. The liquid-state ²⁹Si NMR analysis of the hydrolysis and polycondensation processes of 1(Cn) revealed a unique intramolecular reaction yielding primarily the oligomer with a tetrasiloxane ring which is a new class of amphiphilic molecule having both self-assembling ability and high cross-linking ability. We also found that the mesostructure (lamellar or 2D hexagonal) was strictly controlled by varying the number of siloxane units per alkyl chain. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of oligosiloxane precursors.

Full text of DOI:10.1021/ja0541736

Published on Web 09/21/2005
Self-Assembly of Designed Oligomeric Siloxanes with Alkyl
Chains into Silica-Based Hybrid Mesostructures
Atsushi Shimojima,*,|, Zheng Liu,‡,§ Tetsu Ohsuna,§ Osamu Terasaki,§ and
Kazuyuki Kuroda*,†,|,
Contribution from the Department of Applied Chemistry, Waseda UniVersity, Ohkubo-3,
Shinjuku-ku, Tokyo 169-8555, Japan, Research Center for AdVanced Carbon Materials,
National Institute of AdVanced Industrial Science and Technology (AIST),
Tsukuba, Ibaraki 305-8565, Japan, Structural Chemistry, Arrhenius Laboratory,
Stockholm UniVersity, S-10691 Stockholm, Sweden, Kagami Memorial Laboratory for Materials
Science and Technology, Waseda UniVersity, Nishiwaseda-2,
Shinjuku-ku, Tokyo 169-0051, Japan, and CREST,
Japan Science and Technology Agency (JST), Japan
Received June 24, 2005; E-mail: a-shimo@kurenai.waseda.jp; kuroda@waseda.jp
Abstract: A novel self-assembly route to ordered silica-organic hybrids using well-defined siloxane
oligomers with alkoxy functionality and covalently attached alkyl chains has been investigated. Various
hybrid mesostructures were obtained by hydrolysis and polycondensation without the use of any structure-
directing agents. The oligomers 1(Cn), having an alkylsilane core and three branched trimethoxysilyl groups,
formed highly ordered lamellar phases when n ) 14-18, while those with shorter alkyl chains formed
cylindrical assemblies, slightly distorted two-dimensional (2D) hexagonal structures (n ) 6-10), and a
novel 2D monoclinic structure (n ) 12). Furthermore, the mixtures of 1(Cn) with different chain lengths
yielded well-ordered 2D hexagonal phases, possibly due to the better packing of the precursors. The hybrids
consisting of cylindrical assemblies were converted to ordered porous silica with tunable pore sizes upon
calcination to remove organic groups. The liquid-state 29Si NMR analysis of the hydrolysis and polycon-
densation processes of 1(Cn) revealed a unique intramolecular reaction yielding primarily the oligomer
with a tetrasiloxane ring which is a new class of amphiphilic molecule having both self-assembling ability
and high cross-linking ability. We also found that the mesostructure (lamellar or 2D hexagonal) was strictly
controlled by varying the number of siloxane units per alkyl chain. These results provide a deeper
understanding of the present self-assembly process that is strongly governed by the molecular packing of
oligosiloxane precursors.
Introduction
relying on the weak interactions between surfactant and
inorganic species. The molecular design of inorganic-organic
The design of nanomaterials through self-assembly is one of
the most exciting areas of research in nanoscience and nano-
technology. Recently, self-assembly of amphiphilic molecules1
has been widely used for the production of inorganic-organic
nanocomposites with controlled structures and morphologies.
It is well-established that various mesostructured composites
can be obtained by using surfactants as structure-directing
agents.1d,2 However, their formation process is complicated,
hybrid precursors capable of self-assembly and subsequent
inorganic cross-linking would provide a direct and efficient
pathway to novel nanoarchitectures where each component is
precisely arranged at the molecular scale. Although there have
been a few reports on the use of precursors where surfactants
are covalently attached to inorganic species,3,4 the “surfactant-
free” synthesis of hybrid mesostructures still remains a signifi-
cant challenge for fundamental research.
Much attention is currently being paid to self-directed
assembly during hydrolysis and polycondensation of orga-
noalkoxysilane precursors with the general formula of R′[-Si-
(OR)3]m.5 Such a method is expected to provide a new class of
† Department of Applied Chemistry, Waseda University.
‡ AIST.
§ Stockholm University.
| Kagami Memorial Laboratory for Materials Science and Technology,
Waseda University.
CREST, JST.
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9
14108
J. AM. CHEM. SOC. 2005, 127, 14108-14116
10.1021/ja0541736 CCC: $30.25 © 2005 American Chemical Society

Self-Assembled Silica-Based Hybrid Mesostructures
A R T I C L E S
nanostructured silica-based hybrids that are not accessible by
using surfactants as structure-directing agents,6 or by post
modification of mesoporous silica or layered silicates by
silylation.6a,7 Long-chain alkyltrialkoxysilanes were found to
form hybrids with lamellar structures due to the amphiphilicity
of the hydrolyzed monomers.4a,8 Lamellar hybrids have also
been prepared from bistrialkoxysilane precursors, (RO)3Si-R′-
Si(OR)3, having rigid and/or self-associating organic groups.9
Furthermore, synthetic peptide lipids and amphiphilic block
copolymers bearing -Si(OR)3 groups have been used to form
vesicular assemblies.10,11 However, the chemical design of
organosilane precursors that allows for the control of meso-
structures has been very limited. In particular, higher-curvature
mesophases such as 2D hexagonal and cubic can be obtained
only when both a hydrophobic chain and a cationic head are
integrated in the organic group.4 Corriu et al. recently reported
the formation of a 2D hexagonal hybrid from bis(trimethoxy-
silyl)alkane with a long alkylene spacer; however, the structure
is not yet well characterized due to the instability of the
framework.12
surfactants.18 Thus, the self-assembly of the precursors contain-
ing an oligosiloxane part acting as both a hydrophilic head and
a cross-linking unit is very unique. However, until now, little
is known about their detailed phase behavior and hydrolysis
and polycondensation processes.
In this paper, we report the designed synthesis of mesostruc-
tured hybrids from a series of oligosiloxane precursors and
discuss the molecular factor affecting the self-assembly process.
In addition to the alkyl chain lengths (1(Cn), n ) 4, 6, 8, 10,
12, 14, 16, and 18), we also examined the variation of the
average number of siloxane units either by the addition of Si-
(OR)4 during the reaction or by the use of an oligomeric
precursor (2) containing only two -Si(OR)3 groups. The
structural characterization at molecular and mesoscopic scales
was performed using X-ray diffraction (XRD), transmission
electron microscopy (TEM), and solid-state NMR. Furthermore,
the hydrolysis and polycondensation processes of such oligo-
meric precursors were investigated by liquid-state 29Si NMR
to elucidate the structure of intermediate species involved in
the self-assembly.
To establish a new design concept, the incorporation of an
oligomeric siloxane unit instead of a single -Si(OR)3 group
should be important because of the potential control over the
molecular shape, hydrophobic-hydrophilic balance, and cross-
linking ability of the molecules. We recently succeeded in
synthesizing oligomeric precursors, tris(trimethoxysilyloxy)-
alkylsilanes (1(Cn), n ) 10 and 16), which can self-assemble
into either lamellar (n ) 16) or 2D hexagonal-like (n ) 10)
hybrids by hydrolysis and polycondensation.13 The well-defined
oligomeric siloxane unit as well as the alkyl chain length are
crucial for the variation of mesostructures, because alkyltri-
alkoxysilanes (CnH2n+1Si(OR)3, n ) 8-18) exclusively form
lamellar phases even by co-condensation with tetraalkoxysi-
lanes.14 Although siloxane- or carbosilane-based oligomers and
dendrimers have been used as building blocks to construct
nanomaterials with controlled microstructures,15-17their meso-
scale organization has been attained only in the presence of
Experimental Section
Precursor Synthesis. Tris(trimethoxysilyloxy)(alkyl)silane (Cn-
H2n+1Si(OSi(OMe)3)3, n ) 4, 6, 8, 10, 12, 14, 16, and 18) (1(Cn)).
The synthesis was accomplished by silylation of alkylsilanetriols
prepared by hydrolysis of alkyltrichlorosilanes (n ) 4-12) or alkyl-
triethoxysilanes (n ) 14-16) (see Supporting Information) with
tetrachlorosilane (SiCl4, Tokyo Kasei Kogyo Co., Ltd.), followed by
methanolysis of Si-Cl groups. All reactions were performed under
nitrogen atmosphere using standard Schlenk techniques. Typically, 12.0
g of alkylsilanetriols dissolved in dried tetrahydrofuran (THF, 600 mL)
was added to a mixture of SiCl4 (100 mL) and hexane (120 mL) with
vigorous stirring at room temperature. The solvents and unreacted SiCl4
were then removed in vacuo to afford slightly turbid liquids that
contained mainly RSi(OSiCl3)3 (R ) alkyl) and RSiCl(OSiCl3)2 (15-
20%, evaluated by 29Si NMR). From these mixtures, RSi(OSiCl3)3 was
roughly isolated by vacuum distillation. The methanolysis of RSi-
(OSiCl3)3 was performed by either (1) addition of an excess of dried
methanol with degassing of HCl under reduced pressure or (2) addition
of methanol (excess) in the presence of hexane and pyridine. The former
procedure was repeated several times to eliminate Si-Cl groups, while
the latter required a subsequent filtration step to remove pyridine
hydrochloride. The products were finally purified by vacuum distillation
to yield 1(Cn) as clear and colorless liquids. Spectroscopic data are
presented in the Supporting Information.
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Bis(trimethoxysilyloxy)(decyl)(methyl)silane (C10H21SiMe(OSi-
(OMe)3)2) (2). Decylmethyldichlorosilane (Tokyo Kasei Kogyo Co.,
Ltd., 24.3 g) in diethyl ether (250 mL) was added dropwise to a
vigorously stirred mixture of diethyl ether (250 mL), THF (400 mL),
H2O (3.8 mL), and aniline (19.1 mL) in an ice bath. After stirring for
1 h, the precipitates (aniline hydrochloride) were removed by filtration,
and solvents were then completely evaporated under reduced pressure.
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A R T I C L E S
Shimojima et al.
The solid residue, mainly consisting of decylmethylsilanediol, was all
dissolved in dried THF (500 mL) and added to the mixture of SiCl4
(75 mL) and hexane (200 mL) under nitrogen atmosphere. After the
removal of volatile components in vacuo, methanolysis of the siloxane
species was conducted in a similar manner as for 1(Cn). Vacuum
distillation of the oily residue gave 2 as a clear and colorless liquid.
1H NMR (500 MHz, CDCl3): δ (ppm) 0.20 (s, 3H; SiCH3), 0.64-
0.67 (m, 2H), 0.87-0.89 (t, 3H; CH3), 1.27 (m, 12H), 1.30-1.34 (m,
2H), 1.39-1.44 (m, 2H), 3.57 (s, 18H; OCH3); 13C NMR (125.7 MHz,
CDCl3): δ (ppm) -1.17, 14.14, 16.83, 22.67, 22.81, 29.46, 29.49,
29.69, 29.79, 32.06, 33.30, and 51.04; 29Si NMR (99.3 MHz, CDCl3):
δ (ppm) -85.51 (Q1), -18.04 (D2).
Synthesis of Silica-Based Hybrids. Hydrolysis and polycondensa-
tion reactions of the precursors (1(Cn) and 2) were performed in
solution with molar ratios of 1(Cn)/THF/H2O/HCl ) 1:50:18:0.002
and 2/THF/H2O/HCl ) 1:50:12:0.002, respectively. The mixtures were
stirred at 25 °C for 12 h, and water (H2O/1(Cn) ) 32, H2O/2 ) 38,
respectively) was then added. We note that the additional water ensures
the homogeneity of the films in the cases of n g 14 and appears to
have no effect on the nanostructures of the resulting hybrids. The
precursor solutions were cast on glass substrates (∼0.03 mL cm-2) and
air-dried at room temperature for 2 days to promote polycondensation.
The resulting thick films (15-20 µm in thickness) were scraped off
from the substrate and pulverized before characterization. Thin films
(450-600 nm in thickness) were also prepared by spin-coating (3000
rpm, 10 s) the precursor solutions on glass substrates (Corning #7059)
in air at a relative humidity of 35-45%. Furthermore, the incorporation
of an additional silica source was performed by adding TMOS and
H2O (TMOS/1(Cn) ) 1, H2O/TMOS ) 4) to the precursor solution of
1(Cn) (n ) 14, 16, and 18) after 12 h of reaction, and the mixtures
were stirred for another 2 h before casting on glass substrate.
Characterization. The XRD patterns of the products were obtained
on a Mac Science M03XHF22 diffractometer with Mn-filtered Fe KR
radiation. TEM studies were carried out on JEOL JEM-2010, JEM-
3010, and JEM-4000EX microscopes operated at 200, 300, and 400
kV, respectively. To prepare TEM samples, powders were ground with
mortar and pestle and dispersed in ethanol. A carbon-coated copper
grid was immersed in this dispersion and allowed to dry in air. Solid-
state 13C CP/MAS NMR spectra were recorded on a JEOL JNM-CMX-
400 spectrometer at a resonance frequency of 100.53 MHz with a
contact time of 2 ms and a recycle delay of 5 s. Samples were put into
7.5-mm (or 5-mm) zirconia rotors and spun at 5 kHz. Liquid-state 29Si
NMR spectra of the precursor solutions were recorded on a JEOL
Lambda-500 spectrometer at a resonance frequency of 99.05 MHz with
a pulse width of 6.5 µs, and 64 scans were acquired with a recycle
delay of 10 s. The solutions were put in a 5-mm glass tube, where
THF-d8 was used for obtaining lock signals, and a small amount of
chromium(III) acetylacetonate was added for the relaxation of 29Si
nuclei. Chemical shifts for both 29Si and 13C NMR were referenced to
tetramethylsilane at 0 ppm. Nitrogen adsorption measurements were
performed by an Autosorb-1 instrument (Quantachrome Instruments,
Inc) at 77 K. Samples were preheated at 120 °C for 3 h under 1 ×
10-2 Torr. The Brunauer-Emmett-Teller (BET) surface area was
calculated from the adsorption data in the relative pressure range from
0.02 to 0.05. The pore size distribution was evaluated using the nonlocal
density functional theory (NLDFT).19
Figure 1. Powder XRD patterns of the hybrids (as-synthesized) derived
from 1(Cn): (a) n ) 4, (b) n ) 6, (c) n ) 8, (d) n ) 10, (e) n )12, (f) n
) 14, (g) n ) 16, and (h) n ) 18. The inset shows the variation in the d
value as a function of the alkyl chain length n.
1. The patterns for n ) 14, 16, and 18 show the strongest
diffraction peaks at small scattering angle (d ) 3.05, 3.38, and
3.59 nm, respectively) and their higher-order reflections up to
the fifth order, indicating the lamellar structures of these
samples. On the other hand, the XRD patterns of the hybrids
with shorter alkyl chains (n ) 6, 8, 10, and 12) are quite
different, showing the strongest peaks at d ) 2.13, 2.52, 2.92,
and 3.35 nm, respectively. The relationship between the d value
and alkyl chain length is linear, but differs from that for the
lamellar hybrids with n g 14 (inset of Figure 1). The hybrids
with n ) 8 and 10 exhibit three main peaks, characteristic of a
2D hexagonal mesostructure (10, 11, and 20 reflections).
However, each (10) peak has a slight shoulder at smaller 2θ
angle (see Supporting Information, Figure S1, where the
intensities are plotted on a logarithmic scale), which might be
due to the distortion of the structure from an ideal 2D hexagonal
lattice. The pattern for n ) 6 also shows a sharp peak with a
small shoulder, though the peaks at higher angle are less-defined.
When the chain length decreased to n ) 4, only a weak and
broad peak (d ) ∼1.9 nm) was observed, suggesting the low
self-assembling ability of this precursor. Remarkably, the hybrid
with n ) 12 shows a well-defined pattern exhibiting a shoulder
peak and three additional peaks at d ) 2.40, 1.86, and 1.68
nm, and these peaks are not assigned to the 2D hexagonal
structure.
The nanostructures of these hybrids were further studied by
TEM. In contrast to the lamellar hybrids (n ) 14-18) showing
only striped patterns, the TEM images of the hybrids with n )
6-12 revealed that they consisted of arrays of cylindrical
assemblies. The hybrids with n ) 6, 8, and 10 show typical
images of 2D hexagonal mesostructures, where both hexagonally
ordered patterns (Figure 2) and striped patterns are observable,
depending on the direction of electron incidence either parallel
or perpendicular to the channel. However, it appears that the
structures are slightly distorted. As expected from the XRD data,
a unique structure was observed for n ) 12. The high-resolution
images (Figure 3) reveal an ordered arrangement of distorted
hexagonal rods, and the structure is far from being 2D
hexagonal.
Results and Discussion
Structures of Silica-Based Hybrids Derived from 1(Cn).
Thick films obtained by casting the precursor solution on glass
substrates showed birefringence due to the structural ordering
when n g 6. The XRD patterns of the powdered samples derived
from 1(Cn) with various alkyl chain lengths are shown in Figure
(19) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V.
J. Phys. Chem. B 1997, 101, 3671-3679.
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Self-Assembled Silica-Based Hybrid Mesostructures
A R T I C L E S
Figure 4. 13C CP/MAS NMR spectra (as-synthesized) derived from 1-
(Cn): (a) n ) 8, (b) n ) 10, (c) n ) 12, (d) n ) 14, (e) n )16, (f) n ) 18.
The inset shows the schematic illustrations of alkyl chains in the cylindrical
and lamellar assemblies.
different chain length, as described later. We therefore consider
that the structure for n ) 12 is associated with the molecular
packing of alkylsiloxane units and is possibly an intermediate
phase between lamellar and 2D hexagonal. Such a behavior is
inherent to the present system. The surfactant/silicate systems
generally afford highly symmetrical structures except for the
report by Zhao et al.,21 who discovered a 2D mesostructure with
a centered rectangular lattice (SBA-8) using rigid bolaform
surfactants.
Figure 2. Typical TEM images of the hybrids (as-synthesized) derived
from 1(Cn): (a) n ) 6, (b) n ) 8, (c) n ) 10. Scale bar, 20 nm. They were
taken by a JEM-2010 microscope.
The variation of the mesostructure (cylindrical or lamellar)
is accompanied by the conformational change of the alkyl
chains, as evidenced by the chemical shifts of the interior
methylene carbons in the 13C CP/MAS NMR spectra (Figure
4). The spectra for n ) 14-18 show the signals at 33.7 ppm
indicative of all-trans chains, whereas in the cases of n ) 8-12
the signals are shifted to 30.6-31.2 ppm due to those with mixed
trans-gauche conformations.22 The lamellar hybrids (n ) 14-
18) have highly ordered interlayer chains as compared to the
lamellar hybrids derived from alkyltriethoxysilane/tetraethoxy-
silane systems14 which contained the chains with the trans-
gauche conformation. The alkyl chains appear to be arranged
in an interdigitated monolayer fashion, because the d values
and the average increment of the d value per CH2 group (∆d/
CH2 ) ∼0.13 nm) are much smaller than those for the lamellar
alkylsiloxanes with bilayer arrangements of all-trans chains (d
) 4.28, 4.80, and 5.28 nm (∆d/CH2 ) 0.25)).8 The ∆d/CH2
value of ∼0.13 indicates that alkyl chains are aligned almost
perpendicularly to the siloxane layers.
Figure 3. TEM images of the hybrids (as-synthesized) derived from 1-
(C12). Views (a) perpendicular and (b) parallel to the long axis of the
cylindrical assembly. They were taken by a JEM-4000EX microscope.
A distortion of 2D hexagonal structure along the [10] direction
is often observed for thin films of mesostructured silica due to
the anisotropic shrinkage of siloxane networks.20 Some aniso-
tropic shrinkage could also occur in our system because the
hybrids were obtained as thick films on glass substrates.
However, the structure shown in Figure 3b is not simply derived
from the 2D hexagonal structure by distorting along the [10]
direction, and it was observed throughout the sample. It is also
noted that the degree of distortion is dependent on the alkyl
chain length (n ) 6-12), and well-ordered 2D hexagonal phases
can be obtained from the mixtures of two precursors with
The average thickness of the siloxane layer of the lamellar
hybrids is roughly estimated to be 1.2 nm by extrapolating the
plot (n ) 14-18 in the inset of Figure 1) to n ) 0. For the
(21) Zhao, D.; Huo, Q.; Feng, J.; Kim, J.; Han, Y.; Stucky, G. D. Chem. Mater.
1999, 11, 2668-2672.
(22) Wang, L.-Q.; Exarhos, G. J.; Liu, J. In Characterization of Nanophase
Materials; Wang, Z. L., Ed.; Wiley-VCH: Weinheim, 2000; pp 243-260.
(20) Klotz, M.; Albouy, P.-A.; Ayral, A.; Me´nager, C.; Grosso, D.; Van der
Lee, A.; Cabuil, V.; Babonneau, F.; Guizard, C. Chem. Mater. 2000, 12,
1721-1728.
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A R T I C L E S
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Figure 6. TEM image and ED pattern (inset) of the hybrid derived from
1(C12) after calcination at 500 °C for 8 h. They were taken by a JEM-
3010 microscope.
Figure 5. Powder XRD patterns of the hybrids derived from 1(Cn) after
calcination at 500 °C for 8 h: (a) n ) 6, (b) n ) 8, (c) n ) 10, (d) n ) 12.
hybrids with n ) 6-12, the thickness of the siloxane layers is
calculated to be ∼1.0 nm (0.9 nm (n ) 0) × 2/x3), when
assuming a 2D hexagonal structure. Even though there are some
errors due to the insufficient number of plots, these values
suggest that the siloxane frameworks consist of bilayers of
oligosiloxane units derived from 1(Cn).
Table 1. Structural Parameters of the Calcined Samples Derived
from 1(Cn) Evaluated from Nitrogen Adsorption Data
1
1
n
BET surface area (m2 g-
)
pore volume (cm3 g-
)
NLDFT pore diameter (nm)
6
510
840
950
800
0.22
0.34
0.43
0.46
1.1
1.7
2.2
2.7
8
10
12
It is noteworthy that the lamellar hybrids have the ability to
adsorb alkyl alcohols within their expandable interlayer spaces.
Large increases of the d-spacings (∆d ) ca. 1.3 nm) were
observed when the samples were immersed in liquid decyl
alcohol (Supporting Information, Figure S2). The hydrophobic
interaction with interlayer alkyl chains and the hydrogen bonding
with surface silanol groups should be essential for this adsorption
property, as reported for alkylsilylated derivatives of crystalline
layered silicates (e.g., kanemite and magadiite).7
parameters, including the BET surface areas, pore volumes, and
average pore diameters evaluated by the NLDFT method, are
listed in Table 1. Note that the pore diameters are larger than
those calculated by the Barrett-Joyner-Halenda (BJH) method23
(1.0, 1.4, and 1.7 nm for n ) 8, 10, and 12, respectively). This
is in agreement with the report that the BJH method underes-
timates the pore size of mesoporous silica (MCM-41).19
Although there are several reports on the use of alkyltri-
alkoxysilanes as templates for producing porous silica,24 this is
the first synthesis of well-ordered cylindrical pores. The BET
surface areas and pore volumes are comparable to those of
mesoporous silica prepared by using alkyltrimethylammonium
surfactants.2 Moreover, by varying the alkyl chain length, the
pore size can be controlled in the range of micropore to
mesopore, which is usually difficult to access by the surfactant-
directed approach.25 The self-assembling ability of 1(Cn) with
relatively short alkyl chains and the absence of organic
headgroups (i.e., the covalent bond between alkyl chains and
siloxane units) should contribute to the creation of well-
controlled small pores. It is of particular importance that an
ordered structure was formed even from 1(C6), because the
surfactants with C6 chains are generally not suitable for use as
structure-directing agents.26 Although the structural ordering
Conversion to Porous Silica by Calcination. The hybrids
with n ) 6-12 can be used to produce ordered porous silica
by calcination in air at 500 °C for 8 h to remove organic
components. The XRD patterns of the calcined samples are
shown in Figure 5. All the samples retain the ordered meso-
structures, though the d-spacings are smaller by about 0.3-0.5
nm than those before calcination. The patterns for n ) 6, 8,
and 10 (Figure 5, parts a-c) exhibit the strongest peaks at d )
1.78, 2.18, and 2.49 nm and ill-defined peaks at higher 2θ
angles. When n ) 12 (Figure 5d), several peaks are clearly
observed at both sides of the strongest peak (d ) 2.79 nm).
The TEM image and the corresponding electron diffraction (ED)
pattern (Figure 6) reveal a monoclinic lattice, similar to that
before calcination. The ED pattern was unchanged by varying
the tilt angle of the sample, confirming the absence of a three-
dimensional (3D) ordering. On the basis of these TEM results,
all of the XRD peaks are assigned to a 2D monoclinic structure
with a ) 3.05 nm, b ) 4.58 nm, and γ ) 66° (inset of Figure
5d).
(23) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73,
373-380.
(24) (a) Park, M.; Komarneni, S.; Lim, W. T.; Heo, N. H.; Choi, J. J. Mater.
Res. 2000, 15, 1437-1440. (b) Bu¨chel, G.; Unger, K. K.; Matsumoto, A.;
Tsutsumi, K. AdV. Mater. 1998, 10, 1036-1038.
The nitrogen adsorption isotherms of calcined samples
displayed type I (n ) 6 and 8) or type IV (n ) 10 and 12)
curves typical of microporous and mesoporous silica, respec-
tively (Supporting Information, Figure S3). The structural
(25) Ryoo, R.; Park, I.; Jun, S.; Lee, C. W.; Kruk, M.; Jaroniec, M. J. Am.
Chem. Soc. 2001, 123, 1650-1657, and references therein.
(26) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.;
Schramm, S. E. Chem. Mater. 1994, 6, 1816-1821.
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Self-Assembled Silica-Based Hybrid Mesostructures
A R T I C L E S
Figure 7. XRD patterns of the hybrids films (as-synthesized) derived from
1(Cn): (a) n ) 4, (b) n ) 6, (c) n ) 8, (d) n ) 10, (e) n )12, (f) n ) 14,
(g) n ) 16, and (h) n ) 18.
Figure 8. Powder XRD patterns of the as-synthesized hybrids derived from
the 1:1 mixtures of (a) 1(C8)/1(C12), (b) 1(C10)/1(C14), (c) 1(C10)/1(C16),
and (d) 1(C12)/1(C18).
decreased to some extent by calcination, to the best of our
knowledge, the calcined sample derived from 1(C6) possesses
one of the smallest cylindrical pores that has ever been reported.
Film Formation. The thin films of the hybrids obtained by
spin-coating were transparent and homogeneous, which is
attributable to the high cross-linking ability of 1(Cn) containing
three -Si(OR)3 groups. The XRD patterns of the films are
shown in Figure 7. The patterns for n ) 14-18 are similar to
those of the powdered samples, and are attributed to lamellar
structures. The scanning electron microscopy (SEM) images of
the cracked section of these films show undulating morphologies
(Supporting Information, Figure S4), suggesting the incomplete
orientation of the layers. On the other hand, the films for n )
6-12 have relatively flat and featureless cross-sections; they
show sharp and intense XRD peaks with second- and third-
order reflections. Such patterns are typical for a well-ordered
2D hexagonal structure with its long axis aligned parallel to
the substrate surface,27 as we confirmed for the 1(C10)-derived
film by cross-sectional TEM.13 Unfortunately, the structural
ordering of the films decreased significantly upon calcination
at 500 °C for 8 h. For example, the XRD pattern of the calcined
film derived from 1(C10) shows a small, broad peak at d )
1.6 nm (data not shown).
spectrum of this hybrid revealed the mixed trans-gauche
conformation of alkyl chains (Supporting Information, Figure
S6). These results indicate that both precursors are homoge-
neously distributed at the molecular level to form a new phase.
It appears that the periodicity of the mesostructure can be
adjusted in a certain range by varying the molar ratio of the
precursors. For example, increasing the 1(C10)/1(C16) ratio to
3:1 led to the formation of a 2D hexagonal structure with a
smaller d10 value of 3.38 nm.
Furthermore, various combinations of precursors formed 2D
hexagonal structures, except that the mixtures of long chain
precursors (14 e n) yielded lamellar phases. The 1:1 mixtures
of 1(C8)/1(C12) and 1(C10)/1(C14) formed 2D hexagonal
phases with d10 ) 3.11 and 3.47 nm, respectively (Figure 8a,b).
Importantly, their XRD patterns are well-resolved compared to
those obtained from the single precursors with intermediate
chain lengths, that is, n ) 10 and 12, respectively (Figure 1d,e),
suggesting the better packing of the mixed alkyl chains to form
circular cylinders. However, the 1(C12)/1(C18) mixture resulted
in a less-resolved XRD pattern where the (11) peak is relatively
weak (Figure 8d). This is attributed to the strong interaction
between C18 chains to induce inhomogeneity in the solid, as
suggested by the 13C CP/MAS NMR spectrum showing the
presence of both all-trans and mixed trans-gauche domains
(Supporting Information, Figure S6).
In all of the above systems, the mesostructures were retained
after calcination (Supporting Information, Figure S7). While the
XRD peaks became broad and weak in the case of the 1(C12)/
1(C18) system, other systems clearly show three peaks due to
the 10, 11, and 20 reflections. The mesoporosities of these
samples were confirmed by nitrogen adsorption analysis (Sup-
porting Information, Figure S8). The BET surface areas, the
average pore diameters (evaluated by the NLDFT method), and
pore volumes are listed in Table 2. The pore diameter increases
with increasing average chain length of two precursors. The
relatively low BET surface area and pore volume for the 1-
(C12)/1(C18) system suggest the partial collapse of the meso-
structure upon calcination.
Binary Systems of 1(Cn) with Different Alkyl Chain
Lengths. We have explored the possibility of controlling
mesostructures by mixing two precursors with different alkyl
chain lengths. Interestingly, a well-ordered 2D hexagonal phase
was obtained from the 1:1 molar ratio of 1(C10) and 1(C16).
The XRD pattern of the as-synthesized hybrid (Figure 8c)
displays five peaks indexed to the 10, 11, 20, 21, and 30
reflections associated with a 2D hexagonal symmetry (d10
)
3.70 nm). A well-ordered hexagonal structure was observed by
TEM (Supporting Information, Figure S5). The d10 spacing is
about 0.8 nm larger than that of the hybrids derived from 1-
(C10) and is even larger than the interlayer spacing of the
lamellar hybrid derived from 1(C16). The 13C CP/MAS NMR
(27) (a) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G.
A. Nature 1996, 379, 703-705. (b) Hillhouse, H. W.; van Egmond, J. W.;
Tsapatsis, M.; Hanson, J. C.; Larese, J. Z. Microporous Mesoporous Mater.
2001, 44-45, 639-643. (c) Alberius, P. C. A.; Frindell, K. L.; Hayward,
R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 2002,
14, 3284-3294.
Hydrolysis and Polycondensation Processes. To investigate
the initial formation process of these hybrids, the reaction of
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A R T I C L E S
Shimojima et al.
mass spectrometry. The 29Si NMR spectrum of the silylated
derivatives mainly showed the T3 (-66.9 ppm) and Q4 (-107.9
and -108.1 ppm) signals with the intensity ratio of 1:2:1, along
with the several M1 signals due to the trimethylsilyl groups
(Supporting Information, Figure S9), indicating that the original
siloxane structure was retained during silylation. The FAB-MS
spectrum of this sample exhibits fragment peaks ([M+-Me] and
[M+-SiMe3]) due to the cyclic tetrameric unit with seven
trimethylsilylated silanol groups (Supporting Information, Figure
S10). These results indicate that the cyclic oligomer composed
of three Q2 units and one T2 unit (inset of Figure 9d) is the
predominant species in the solution after 12 h of the reaction.
The quantitative analysis based on the internal standard (hex-
amethyldisilane) revealed that about 50-60% of 1(C10) trans-
formed into the cyclic tetrasiloxane species. Although other
species should coexist in the solution, the formation of well-
defined oligomeric species in such a high ratio is a very specific
phenomenon that cannot be attained when starting from the
mixture of alkyltrialkoxysilane and tetraalkoxysilane.28
Table 2. Structural Parameters of the Calcined Samples Derived
from the 1:1 Mixtures of 1(Cn) Evaluated from Nitrogen Adsorption
Data
BET surface
pore volume
NLDFT pore
diametera (nm)
1
1
samples
area (m2 g-
)
(cm3 g-
)
1(C8)/1(C12)
1(C10)/1(C14)
1(C10)/1(C16)
1(C12)/1(C18)
880
840
830
690
0.46
0.51
0.58
0.49
2.3
2.8
3.2
3.3
a For comparison: the BJH pore diameters are 1.4, 1.8, 2.1, and 2.2 nm
for 1(C8)/1(C12), 1(C10)/1(C14), 1(C10)/1(C16), and 1(C12)/1(C18)
systems, respectively.
The rearrangement of the branched structure of 1(Cn) into
the tetrasiloxane ring is triggered by the intramolecular con-
densation between two branched Q1 units to form a trisiloxane
ring. Such an intramolecular reaction is plausible because
cyclization of linear trimeric species was reported to be fast
and to compete with intermolecular condensation.29 Actually,
the presence of the intermediate species with a trisiloxane ring
is evidenced by the appearance of three 29Si signals (A-C) in
Figure 9c. The intensities of these signals decrease with increases
in the signals of the cyclic tetrasiloxane species, while keeping
a constant ratio of 1:1:2. Signals A and C can be assigned to
the T3 and Q2 units of a trisiloxane ring, and signal B represents
the branched Q1 unit attached to the T3 unit (inset of Figure
9c). These assignments are consistent with the general findings
that the T3 and Q2 signals due to cyclic trisiloxanes appear close
to the T2 and Q1 signals of branched (or linear) oligomers,
respectively.30,31 The presence of a trisiloxane ring was further
supported by the significant downfield shifts of the T3 and Q4
signals in the 29Si NMR spectrum of the trimethylsilylated
species (Supporting Information, Figure S9). The tetrasiloxane
ring can be formed by the cleavage of the trisiloxane ring to
form a linear tetrameric species (Q1-T2-Q2-Q1) and subse-
quent intramolecular condensation between terminal Q1 units.
In contrast to the lability of a strained trisiloxane ring, a
tetrasiloxane ring is known to be thermodynamically stable.31
In the present system, no change was observed in the 29Si NMR
spectrum shown in Figure 9d, even after the addition of water
(H2O/1(C10) ) 32), and the oligomer with a tetrasiloxane ring
was detectable over 1 day after the reaction. This indicates that
the tetrasiloxane ring is relatively stable against cleavage and
that the intermolecular condensation is very slow, probably due
to the steric repulsion between covalently attached long alkyl
chains.
Figure 9. Liquid-state 29Si NMR spectra of (a) 1(C10) in THF, and the
solutions after (b) 0.5 h, (c) 3 h, and (d) 12 h of reaction in the 1(C10)/
THF (10% THF-d8)/H2O/HCl ) 1:50:18:0.002 system.
1(C10) in the solution was monitored by liquid-state 29Si NMR.
As shown in Figure 9a, the spectrum for 1(C10) exhibits two
signals at -67.3 and -86.4 ppm with the intensity ratio of 1:3.
These signals are assigned to the T3 (Tx, CSi(OSi)x(OH)3-x) and
Q1 (Qx, Si(OSi)x(OH)4-x) units, originating from the alkylsi-
loxane unit and branched -Si(OMe)3 groups, respectively.
As the reaction proceeds (Figure 9b), several signals due to
the hydrolyzed -Si(OMe)3 groups appear downfield in the Q1
region, accompanying the slight shifts of the T3 signal.
Subsequently, three signals (labeled with A, B, and C) appear,
and the unreacted precursor almost disappears after ∼3 h (Figure
9c). No signals of Q0 species (from -71 to -79 ppm) were
observed during the reaction, suggesting the very slow rate of
hydrolysis of the Si-O-Si bonds in 1(C10). After 12 h (Figure
9d), a T2 signal at -57.1 ppm and two close Q2 signals at -89.9
and -90.0 ppm are dominant (labeled with D, E, and F,
respectively). The intensity ratio of these signals is 1:2:1, being
consistent with the T/Q ratio of 1:3 for the precursor 1(C10).
Similar results were also obtained using 1(C6) and 1(C16),
independent of the alkyl chain length.
The formation of cyclic oligomers from 1(Cn) is of great
interest because cyclic siloxane units play an important role in
the nucleation of zeolites and are present as building units in
(28) (a) Rodr´ıguez, S. A.; Colo´n, L. A. Chem. Mater. 1999, 11, 754-762. (b)
Shimojima, A.; Umeda, N.; Kuroda, K. Chem. Mater. 2001, 13, 3610-
3616.
(29) Sanchez, J.; McCormick, A. V. J. Non-Cryst. Solids 1994, 167, 289-294.
(30) Feher, F. J.; Newman, D. A.; Walzer, J. F. J. Am. Chem. Soc. 1989, 111,
1741-1748.
Trimethylsilylation of the alkylsiloxane species formed after
12 h of reaction was carried out to elucidate their structures by
(31) Damrau, U.; Marsmann, H. C. J. Non-Cryst. Solids 1994, 168, 42-48.
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Self-Assembled Silica-Based Hybrid Mesostructures
A R T I C L E S
their frameworks. Recently, several reports have suggested that
cyclic tetrasiloxane species are suitable building units for the
self-assembly of silica-surfactant composite mesophases.32 By
using IR spectroscopy, the presence of tetrasiloxane rings is
generally revealed by a weak and broad IR band at around 600
cm-1 32b
This band can be observed in the FT-IR spectrum of
.
the hybrid derived from 1(C10), confirming the presence of
residual tetrasiloxane rings in the siloxane networks (Supporting
Information, Figure S11). However, we have not obtained
evidence of structural ordering in the siloxane networks of the
hybrids.
We reported that the solid-state 29Si MAS NMR spectra of
the hybrids with n ) 10 and 16 showed signals corresponding
to the T1, T2, and T3 units and the Q2, Q3, and Q4 units.13 Despite
the difference in the mesostructures with alkyl chain length,
there was no large difference in the local structures of the
siloxane frameworks. The presence of the T2 units is consistent
with the above results that the hybrids are mainly assembled
from the cyclic tetrasiloxanes, though the small T1 signal
indicates further cleavage of siloxane bonds during the synthesis.
The degree of condensation for the T units (n ) 10) is calculated
to be 81% (Supporting Information, Figure S12 and Table S1).
For comparison, the value for the lamellar hybrid prepared from
the 1:3 mixture of decyltrimethoxysilane (C10TMS) and tet-
ramethoxysilane (TMOS) under the identical conditions is 64%.
Thus, the use of the oligomeric precursor (1(Cn)) overcame
the problem of low co-condensation rate of the T and Q units,
which should be responsible for the formation of the higher-
curvature mesophase.
Effect of Molecular Structure on Self-Assembly. The
hydrolyzed species of 1(Cn) are the polyhydroxy amphiphiles
containing a hydrophobic alkyl chain and hydrophilic silanol
groups. The oligomer with a tetrasiloxane ring is structurally
somewhat similar to sugar-based surfactants such as alkylgly-
cosides, which are reported to form liquid crystalline phases,33
in terms of having ring structures bearing OH groups. The
emergence of birefringence indicative of the transformation from
the isotropic phase to anisotropic solids during the drying
process was observed by cross-polarized microscopy.
The self-assembled structures of amphiphilic molecules are
generally explained by a geometrical packing parameter g )
V/aol, where V is the effective volume of the hydrophobic chain,
ao is the effective area of the headgroup, and l is the hydrophobic
chain length.6a,34 Self-assembly of hydrolyzed alkyltrialkoxysi-
lanes, having a single Si(OH)3 group, leads to lamellar phases
exclusively.8 Thus the formation of higher-curvature phases from
1(Cn) (n ) 6-12) is attributed to the larger headgroup area of
the oligosiloxane unit that reduces the g value. It is intriguing
that the structure of hybrids strictly changes to lamellar when
the alkyl chain length increases from n ) 12 (n ) 13)35 to n )
14, because the variation of n does not cause a substantial change
of the g value.4a We suppose that the stronger hydrophobic
Figure 10. Powder XRD patterns of the as-synthesized hybrids prepared
with the molar ratio of TMOS/1(Cn) ) 1: (a) n ) 14, (b) n ) 16, (c) n )
18.
interaction between longer alkyl chains favors the formation of
all-trans lamellar assembly,36 where interdigitated arrangements
of alkyl chains compensate for the intermolecular space resulted
from the large headgroup. The formation of 2D hexagonal
phases upon addition of short chain precursors is attributed to
the decrease in the effective volume of hydrophobic chains, in
combination with the difficulty in the close packing of mixed
long and short alkyl chains.
The control of the mesostructure is also achieved by modify-
ing the effective area of siloxane headgroups. The increase of
the average number of SiO4 (or CSiO3) units per alkyl chain
was performed by adding TMOS (TMOS/1(Cn) ) 1) into the
precursor solutions of 1(Cn) with n ) 14, 16, and 18. The XRD
patterns of the resulting hybrids are shown in Figure 10. Most
importantly, the mesostructure obtained from 1(C14) changed
from lamellar to 2D hexagonal, which is ascribed to the decrease
of the g value. However, 1(C16) and 1(C18) formed lamellar
structures with thicker siloxane layers, as suggested by the
increased d-spacings from those of the hybrids prepared without
TMOS. It should be noted here that the TMOS-derived species
are rarely linked to the cyclic tetrasiloxane species in solution
under the conditions we employed. The liquid-state 29Si NMR
spectrum of the solution after 2 h of the reaction with TMOS
showed the signals of completely hydrolyzed monomer (-71.7
ppm) and dimer (-81.1 ppm) and other oligomeric species,
along with those of cyclic tetrasiloxane species derived from
1(Cn). Co-condensation between TMOS- and 1(Cn)-derived
species probably takes place upon evaporation of the solvent,
thereby increasing the headgroup area of alkylsiloxane species.
On the other hand, we examined the use of the oligomer (2)
having two branched -Si(OR)3 groups, which provide a smaller
headgroup than that of 1(C10). The liquid-state 29Si NMR
spectrum after 12 h of reaction (Supporting Information, Figure
S13) mainly shows three signals (-9.1, -12.8, and -20.7 ppm)
due to the decylmethylsiloxane unit (Dx, C2Si(OSi)x(OH)2-x),
together with four Q signals (-80.0, -80.9, -82.9, and -89.6
ppm). These signals are tentatively assigned to a hydrolyzed
oligomer (Q1-D2-Q1), a cyclic trisiloxane (Q2-D2-Q2) formed
by intramolecular condensation, and a linear trisiloxane (D1-
(32) (a) Doshi, D. A.; Gibaud, A.; Goletto, V.; Lu, M.; Gerung, H.; Ocko, B.;
Han, S. M.; Brinker, C. J. J. Am. Chem. Soc. 2003, 125, 11646-11655.
(b) Falcaro, P.; Grosso, D.; Amenitsch, H.; Innocenzi, P. J. Phys. Chem. B
2004, 108, 10942-10948.
(33) (a) Jeffrey, G. A. Acc. Chem. Res. 1986, 19, 168-173. (b) von Rybinski,
W.; Hill, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 1328-1345.
(34) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday
Trans. 1 1976, 72, 1525-1568.
(36) The lamellar structures were obtained from 1(C14) and 1(C16) even when
the temperature of the substrate was increased up to 60 °C during the self-
assembly, which suggests the strong tendency of these precursors to form
a lamellar assembly.
(35) Additional experiments carried out after the submission of this manuscript
confirmed that the precursor with n ) 13 formed a 2D monoclinic structure
with d10 ) 3.53 nm (as-synthesized) and 2.93 nm (after calcination).
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A R T I C L E S
Shimojima et al.
Q2-Q1) formed by cleavage of the trisiloxane ring. This
precursor solution produced a nanostructured solid that exhibits
the XRD peak with d ) 2.92 nm accompanying the second-
order reflection (Supporting Information, Figure S14). Although
this d value is very close to that of the 1(C10)-derived hybrid
(Figure 1c), a lamellar structure was confirmed by the collapse
of the structure upon calcination. It is likely that the alkyl chains
are arranged in a bilayer fashion because the d value is much
larger than that expected for an all-trans interdigitated monolayer
arrangement.
These results clearly show that the self-assembly of siloxane
oligomers with alky chains is governed by their packing
parameters, which can be controlled by the number of siloxane
units as well as the alkyl chain lengths. It is reasonable to expect
that other mesophases that were not observed in the present
study can be formed by adjusting the packing parameter of the
precursors. In particular, further increase of the headgroup area
should lead to the creation of higher curvature mesophases such
as 3D hexagonal (P63/mmc) and cubic (Pm3n) phases. The
molecular design of new oligomers having more than three -Si-
(OR)3 groups is underway in our laboratory.
mainly formed by the cross-linking of tetrasiloxane rings,
generated by the rearrangement of siloxane linkage of the
precursors during the reaction. This is in clear contrast to the
complicated hydrolysis and polycondensation processes when
monomeric alkoxysilanes are used as silica sources. We believe
that further modifications of the precursors will allow for the
emergence of various hybrid architectures. The use of more rigid
and stable siloxane units such as double-four-membered ring
(D4R) may lead to the creation of molecularly ordered
frameworks. The incorporation of either organic functional
groups or other metallic species in the precursors is also very
important for a wide range of applications.
Acknowledgment. We are grateful to Mr. D. Mochizuki, Mr.
N. Atsumi, and Mr. Y. Yamauchi (Waseda University) for their
help in nitrogen adsorption measurement and TEM observation
(JEM-2010). We also thank one of the reviewers for many
helpful corrections to the original manuscript. The work was
partially supported by a Grant-in-Aid for COE Research
“Molecular Nano-Engineering”, the 21st Century COE Program
“Practical Nano-Chemistry”, and Encouraging Development
Strategic Research Centers Program “Establishment of Con-
solidated Research Institute for Advanced Science and Medical
Care” from MEXT, Japan. A.S. is thankful for financial support
from a Grant-in-Aid for JSPS Fellows from MEXT.
Conclusions
We have demonstrated the formation of silica-based hybrids
with ordered mesostructures by self-assembly of designed
precursors consisting of oligosiloxane units and alkyl chains of
various lengths. The mesostructure of the hybrids, either lamellar
or cylindrical assemblies, was controlled by the alkyl chain
length and the number of SiO4 units per alkyl chain. The latter
phases could be used to produce porous silica with the pore
sizes tunable in the range of micropore (1.1 nm) to mesopore
(3.3 nm), which is of great practical interest. These hybrids were
Supporting Information Available: Experimental details,
spectroscopic data for 1(Cn), Figures S1-S14, and Table S1.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0541736
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14116 J. AM. CHEM. SOC. VOL. 127, NO. 40, 2005