Formation of mesoporous oxide fibers in polycarbonate confined spaces

Article abstract of DOI:10.1039/b909251j

Mesoporous oxide fibers were prepared in one-pot by permeating surfactant-containing clear precursor solutions into the confined spaces of polycarbonate membranes and subsequent calcination.

Full text of DOI:10.1039/b909251j

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Chemical Communications
www.rsc.org/chemcomm
Number 38 | 14 October 2009 | Pages 5645–5768
ISSN 1359-7345
FEATURE ARTICLES
Shigeru Matsuoka and Masayuki Inoue
Application of REDOR NMR in natural
product chemistry
COMMUNICATION
YusukeYamauchi, Norihiro Suzuki
andTatsuo Kimura
Frederick G. West et al.
Interrupting the Nazarov reaction
Formation of mesoporous oxide fibers
in polycarbonate confined spaces
1359-7345(2009)38;1-T

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Formation of mesoporous oxide fibers in polycarbonate confined spacesw
Yusuke Yamauchi,*ab Norihiro Suzukia and Tatsuo Kimurac
Received (in Cambridge, UK) 11th May 2009, Accepted 30th June 2009
First published as an Advance Article on the web 27th July 2009
DOI: 10.1039/b909251j
Mesoporous oxide fibers were prepared in one-pot by permeating
surfactant-containing clear precursor solutions into the confined
spaces of polycarbonate membranes and subsequent calcination.
prepared using triblock copolymers in the confined spaces of
the PC membrane in a similar manner to mesoporous silica
fibers. We also found new insights to align and orient uniform
mesopores inside the fibers. For instance, 1-D mesopores were
oriented along the long axis of the fibers (columnar orientation)
on the basis of the shear flow effect by strong aspiration during
evaporation of solvents, though mesochannels generally prefer
to be stacked like doughnuts in cylinders (circular orientation).
Columnar orientation is quite important for use in optical
waveguides and light emitters.
Periodic mesoporous materials are prepared through the self-
assembly process of amphiphilic organic molecules attached to
soluble inorganic species.1–3 Morphological controls of the
mesoporous materials to films, fibers, tubes, monoliths, and
nanoparticles are of considerable interest for a variety of
practical applications.4 In particular, fibers with aligned one-
dimensional (1-D) mesopores have drawn scientific interests.
Mesoporous fibers have often been obtained with aggregated
morphology,5–9 affording interparticle macrospaces which are
helpful for the effective diffusion of reactants and products in
applications in catalysts, sensors, devices, batteries, and so on.
The synthesis of periodic mesoporous fibers composed of
silica,5,6 polymer,7 carbon,8 and metal9 has been reported so
far by using porous anodic alumina (PAA) membranes. Clear
precursor solutions are soaked into the regular cylinders of
PAA membranes, and then solvents are evaporated in order to
induce the formation of mesostructures. The PAA templates
are finally dissolved by chemical etching using acidic or basic
solutions to recover mesoporous fibers. However, this process
cannot be extended to the preparation of other inorganic
oxides such as alumina and titania because the oxides were
also dissolved during the chemical etching of the PAA
membranes. Accordingly, there have been few reports on
PAA-templated mesoporous oxide fibers (for instance, the
yield was very low for titania).10
A PC membrane filter with a pore diameter of ca. 0.2 mm
was used as a polymer template (see ESIw, Fig. S1). In the
preparation of
a precursor solution of 2-D hexagonal
mesoporous silica, TEOS (5.20 g), ethanol (6.00 g), and HCl
(pH = 2, 2.70 g) were stirred for 20 min and mixed with
Pluronic P123 (1.38 g) in ethanol (4.00 g). The stirring was
continued for another 3 h. According to the literature,12
a
precursor solution of 2-D hexagonal mesoporous alumina was
simply prepared as follows: Pluronic P123 (1.60 g), acetic acid
(2.29 ml), conc. HCl (1.00 ml), and aluminium tri-n-butoxide
(2.46 g) were mixed in ethanol (30.0 ml) for 5 h. In the case of
titania, a precursor solution was prepared to obtain titania
13
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with a different mesostructure (cubic Im3m). Conc. HCl
(35.5 wt%, 0.74 g) was slowly added to titanium tetraisopropoxide
(1.05 g) under vigorous stirring. After stirring for 10 min, the
solution was combined with an ethanolic solution (3.00 g) of
Pluronic F127 (0.2 g) and stirred for 15 min. The precursor
solutions were dropped onto the PC filter, leading to rapid
infiltration into the cylindrical spaces of the PC filter by
capillary force. The filters were air-dried (silica and alumina)
or aged at À5 1C for 1 day (titania), and then calcined at
500 1C for 8 h (silica) or at 400 1C for 5 h (alumina and titania)
to burn off both the surfactant and the PC filter.
Here we propose a new strategy to obtain mesoporous metal
oxides with replicated fibrous morphology using polymer
templates with confined spaces such as polycarbonate (PC)
membrane filter. The synthesis using PC membrane is more
advantageous than that using PAA membrane.11 Unexpected
reactions of inorganic precursors with the surfaces of polymer
membrane are suppressed because of the chemical resistance
of PC. The PC membrane and the surfactants can be removed
by calcination. Mesoporous alumina and titania fibers can be
The SEM images of the resultant solid products are shown
in Fig. 1. Fibrous morphologies were observed over the entire
areas for all the calcined products composed of silica, alumina,
and titania. The average sizes in diameter and length of the
fibers were carefully counted to be 170–210 nm and 5–9 mm,
respectively. The values are consistent with the shape of the
cylinders of the PC membrane filter, leading to replication of
inorganic oxides. The silica and titania fibers were oriented in
the same direction and packed closely, while the alumina fibers
were aggregated with random orientation.
a World Premier International (WPI) Research Center for Materials
Nanoarchitectonics (MANA), National Institute for Materials
Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
E-mail: Yamauchi.Yusuke@nims.go.jp; Fax: +81-29-860-4706
b Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST),
Kawaguchi-shi, Saitama 332-0012, Japan
Mesostructures of the replicated fibers were investigated by
small-angle X-ray scattering (SAXS) (see ESIw, Fig. S2). In the
silica fibers, several peaks were distinctly observed, which were
assigned as (100), (110), (200), (210), and (300) peaks of 2-D
hexagonal symmetry (p6m). The silica fibers contained highly
ordered 1-D mesopores. In the alumina fibers, a broad peak
was recorded at 0.91 with the higher order diffraction,
c Advanced Manufacturing Research Institute, National Institute of
Advanced Industrial Science and Technology (AIST), Shimoshidami,
Moriyama-ku, Nagoya 463-8560, Japan
w Electronic supplementary information (ESI) available: Detailed
characterization of mesoporous silica, alumina, and titania fibers.
See DOI: 10.1039/b909251j
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5689–5691 | 5689

Fig. 1 SEM and TEM images of mesoporous fibers of (a, d) silica, (b, e) alumina with (e-1) columnar and (e-2) circular orientations,
and (c, f) titania.
revealing the presence of a less ordered 2-D hexagonal
structure. The N2 adsorption–desorption isotherms of the
silica and alumina fibers were type IV with hysteresis loops
(see ESIw, Fig. S3). The fibers have uniform mesopores with
pore diameters of 6.0 nm (silica) and 7.2 nm (alumina). The
BET surface areas were 325 m2 gÀ1 (silica) and 412 m2 gÀ1
(alumina), being similar to those observed for 2-D hexagonal
mesoporous silica and alumina powders prepared using
P123 by solvent evaporation.12
of the resultant silica fibers are shown in Fig. 2 with a
schematic model of the columnar orientation of 1-D mesopores.
The SAXS image showed a less-uniform distribution of the
scattering intensity corresponding to the (100) peak of a 2-D
hexagonal structure. The image reveals that the mesochannels
are uniaxially oriented in the fibers, as illustrated in the
The TEM images of the mesoporous oxide fibers are also
displayed in Fig. 1. Mesostructures inside the replicated fibers
were investigated directly by taking the images perpendicular
to the long axis of the fibers. Tubular (columnar) mesopores
were aligned along the long axis of the silica fibers, and
circular mesochannels were also present along the cylinders
of the PC filter. The orientation of 1-D mesopores with respect
to the long axis of the cylinders in the PAA membrane can be
tuned according to the silica to P123 ratio and salt content in
the precursor solution and relative humidity during solvent
evaporation.5d,g,h When a precursor solution was prepared
from TEOS in ethanolic P123 solution and fabricated in
PAA, most of the 1-D mesopores were oriented along the
PAA pore surfaces.5b,e,g In contrast, both columnar and
circular orientations of 1-D mesopores were observed for the
silica fibers prepared in the confined spaces of the PC membrane.
The mesoporous alumina fibers fabricated in the PC filter also
contained both orientations. The interaction of silica–P123
species with the PC surfaces is weaker than that with the PAA
surfaces, and any preferred orientations were not confirmed.
We also found an interesting insight to align 1-D mesopores
in PC-assisted fibers. Most of the 1-D mesopores were able to
orientate parallel to the long axis of the fibers by strong
aspiration. For instance, the PC membrane filter was set in
an ordinary membrane filtration apparatus. The precursor
solution for silica was dropped onto the PC filter, and then
aspirated strongly (flow speed of around 40 m sÀ1) until the PC
filter was dried completely. The 2-D SAXS and TEM images
Fig. 2 (a) 2-D SAXS image obtained by irradiation of an X-ray beam
perpendicular to the long axis of the fibers, (b) schematic presentation
of the columnar orientation of 1-D mesopores, and (c) TEM image of
mesoporous silica fibers prepared through an aspiration process.
ꢀc
This journal is The Royal Society of Chemistry 2009
5690 | Chem. Commun., 2009, 5689–5691

schematic model. On the basis of the brightness distribution of
the 2D-SAXS image, it is considered that 85% of the meso-
channels are oriented along the long axis of the fibers.
Actually, 1-D mesochannels are directed along the long axis
of the fibers in the TEM image.
polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock
copolymer. Spherical mesopores were formed, but were not
packed closely (See ESIw, Fig. S5). This is contrast to the
previous report on mesoporous silica powders with cubic
15
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(Fm3m) mesostructure using PS-b-PEO.
The orientation of 2-D hexagonal mesoporous silica in PAA
membrane was previously reported by Yamaguchi et al., who
applied aspiration of aged precursor solutions (silica).5h They
stated that 1-D mesopores were aligned according to the
‘‘effect of aging temperature and time’’, but not to the
‘‘aspiration effect’’.5h For comparison, we also prepared
mesoporous silica fibers in PAA membrane from the reported
P123 containing precursor solution without aging (see ESIw,
Fig. S4). After the precursor solution was dropped onto PAA
membrane and penetrated into the cylindrical spaces by
capillary force, mesoporous silica fibers were obtained with
or without strong aspiration and subsequent calcination.
Tubular mesochannels were oriented to be circular along
the PAA pore surfaces without aspiration, as reported
previously.5b,e,g,i On the basis of self-consistent field theory,
such a circular orientation was reported to be a stable
conformation.5b
Titania frameworks were converted by calcination at 400 1C
to nanocrystals of anatase with a crystal size of 3–5 nm,
leading to further distortion of the mesostructure (See ESIw,
Fig. S6). The selected area electron diffraction (SAED) pattern
showed several rings assigned as (101), (103), (200), (105), and
(204) diffractions of anatase, consistent with the lattice fringes
being observed as having random orientation.
To conclude, we demonstrated straightforward preparation
of mesoporous oxide (silica, alumina, and titania) fibers with
high surface areas by permeating clear precursor solutions in
the confined spaces of PC membrane filter. Alignment control
of tubular mesochannels on the basis of shear flow effect was
also presented as a convenient method. In combination with
the advantages of using polymer membranes, this approach
will be applicable to not only silica, alumina, and titania but
also a wide variety of mesoporous inorganic solids such as
oxides and phosphates including mixed oxides.
Mesochannel orientation was changed dramatically into a
columnar orientation by strong aspiration (see ESIw, Fig. S4).
The fibers (470%) contained 1-D mesopores oriented along
the long axis. Strong aspiration induces high-speed flow of the
precursor solution in the cylindrical spaces of PAA membrane,
leading to rapid motion of silica–P123 species in a uniaxial
direction to the long axis of the PAA channels. Circular
orientation was still partially observed even after strong
aspiration (see ESIw, Fig. S4), being related to strong inter-
action between silica–P123 species and the PAA surfaces. In
the case of using PC membrane filter, such an interaction is so
weak that 1-D mesochannels are smoothly aligned to the flow
direction in the PC filter. Under the strong shear force, tubular
micelles are anisotropically arranged along the flow direction.
The insight can be extended to the control in a mesoporous
silica film by hot air flow which contains mesopores that are
aligned along the flow direction.14
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Finally, further orientation control of surfactant-templated
mesopores in confined cylindrical spaces was investigated in
the case of cage-type mesopores combined with compositional
variation. As described above, fibrous particles were obtained
in the case of titania (Fig. 1c). The SAXS pattern of the titania
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(ESIw, Fig. S2), suggesting a lack of the mesostructural
ordering. The titania fibers contained wormhole-like meso-
pores over the entire particles, proved by TEM (Fig. 1f). The
N2 adsorption–desorption isotherm of the titania fibers
showed no obvious capillary condensation due to the presence
of uniform mesopores in the adsorption branch, and the
surface area was low (65 m2 gÀ1, 7.0 nm) (see ESIw, Fig. S3),
being related to the disordered arrangement of the mesopores.
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¨
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precursor solution for cubic Im3m mesostructured titania,
cannot be oriented in any direction in the cylinders. The
mesostructural ordering would be reduced according to the
difficulty to fill cage-type mesopores out into the tubular
spaces of the PC filter. A similar result was observed using
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5689–5691 | 5691