Preparation of cotton-like silica

Article abstract of DOI:10.1039/b402192d

Cotton-like silica composed of helical bundles was prepared using the sol-gel transcription method in strong acid conditions.

Full text of DOI:10.1039/b402192d

Preparation of cotton-like silica
Yonggang Yang, Masahiro Suzuki, Mutsumi Kimura, Hirofusa Shirai and Kenji Hanabusa*
Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University,
Ueda 386-8567, Japan. E-mail: hanaken@giptc.shinshu-u.ac.jp
Received (in Cambridge, UK) 12th February 2004, Accepted 8th April 2004
First published as an Advance Article on the web 3rd May 2004
Cotton-like silica composed of helical bundles was prepared
using the sol–gel transcription method in strong acid condi-
tions.
nanofibers is around 10–20 nm. When gelator 1 was used as the
template, cotton-like silica with elasticity was obtained (Fig. 2).
The cotton-like fibers (which should be bundles of nanofibers) were
identifiable even with polarized optical micrography and with the
naked eye when the fibers were suspended in methanol. In earlier
research,3,14 inorganic powders were often obtained through the
sol–gel transcription method, although the small particles were
composed of nanofibers.
Fig. 3 shows an FE-SEM image of the calcined silica with no
break up of the morphologies during the workup procedure. It was
found that the fibers shown in Fig. 2 were not single fibers, they
were bundles formed by intertwining nanofibers. Although the
helical pitches of the bundles varied greatly, all of the helical
bundles were configured in a right-handed helix. The length of the
bundles was around 40 mm. We can also find many ultra-fine
nanofibers linked between the bundles, which support this cotton-
like structure. According to the preparation procedure, the helical
bundles should be formed before the stirring has stopped, because
it was found that most of the bundles were aligned in nearly the
same direction on the surface of the reaction flask. The alignment
of the bundles brought about by stirring, oriented the gel fibers
during the preparation procedure. Then the oriented gel fibers
transfer this alignment information to the silica. As to the ultra-fine
nanofibers linked between the bundles, they should be formed after
stirring has stopped, because they are not aligned. Fig. 3 shows
enlarged images of the silica fibers. There are two kinds of
Nanostructured materials showed fascinating properties, and their
application was superior to their bulk counterparts. Consequently,
much research has been done on shaping inorganic materials at the
microscopic level.1 Up to now, many kinds of interesting structures
have been prepared, rods, belts, tubes, balls, and so on.2
Researchers have found that the helical structure in organogel fibers
can be transcribed into inorganic silica and TiO2, for example,
single-stranded helix,3,4 double helix5 and multiple helix.6 Because
the helical information could be transferred from inorganic
materials to organic compounds, they could be used as chiral
catalysts7 and chiral sensors.8 For applications, for example,
nanocapacitors or nanotransistors, the alignment and functionaliza-
tion of single particles is necessary.9 The self-assembly of nano
particles into more complex structures will also be necessary.10
Nanotubes are superior to other morphologies as both inner and
outer surfaces can be modified. Due to the one-dimensional
structure, nanotubes should be easily organized into ordered
structures. Sol–gel transcription procedure is a powerful method to
prepare nanotubes.11 By selecting organic reagents as the solvents,
stirring is not essential because tetraethylorthosilicate (TEOS) is
soluble in organic solvents. For economic reasons, however, we
selected water as solvent, HCl as the catalyst and chiral gelators
(which are shown in Scheme 1) as the templates. Because TEOS
cannot dissolve in water, stirring is essential during the sol–gel
transcription procedure.12 And some interesting phenomena were
found due to stirring.
Gelator 1 was synthesized by the reaction of 4-methylpyridine
and cyclo(
was prepared from cyclo(
L
-10-bromodecylasparaginyl-
L
-phenylalanine), which
L
L
-asparaginyl-
-phenylalanine)13 and
10-bromodecanol using the carbodiimide–DMPA method. Gelator
2 was synthesized by a similar procedure. Sol–gel polycondensa-
tion of TEOS was carried out according to the following procedure.
Gelator 1 (57 mg) was dissolved in 1 mL of 2.4 M HCl aq., 90 mg
of TEOS was dropped into the solution under strong stirring at
room temperature. After the mixture turned white, it was kept at
room temperature for 30 min. and 80 °C for 4 days under static
conditions. Finally, the gelator was removed by rinsing with
methanol and then calcination was performed at 250 °C for 2 h and
500 °C for 5 h under aerobic conditions.
Fig. 1 TEM image of the hydrogel formed by gelator 1.
Gelator 1 forms a transparent gel consisting of a three-
dimensional network in water (Fig. 1). The diameter of the
Fig. 2 SEM image of the silica obtained by sol–gel transcription of the
gelator 1 after calcination.
Scheme 1
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C h e m . C o m m u n . , 2 0 0 4 , 1 3 3 2 – 1 3 3 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

nanostructures, i.e., nanoribbons and nanotubes, in this material.
Fig. 4A shows a right-handed helical ribbon15 and Fig. 4B shows
the structure of a branched tube. The tunnel in the tube is easily
identified. Fig. 4C shows the TEM image of a fine nanotube, the
inner diameter of which is about 3–4 nm.
To elucidate the mode of formation of cotton-like silica,
experiments were carried out by gradually varying the reaction
conditions and replacing the gelators. The results indicate that a
concentration of gelators more than 3 wt% in the case of gelator 1
is essential. Furthermore, the concentration of the gelators should
be controlled until a stiff gel is formed in aqueous solution at room
temperature. When the concentration of the gelators is low, only
silica powder is obtained. This may be because a small amount of
silica fibers cannot support the cotton-like structures, resulting in
the cotton-like structures shrinking into small particles. From the
SEM image, it was found that the fibers also form bundles; these
bundles were composed of only two or three nanofibers. With
respect to the amount of TEOS, the use of an excess of TEOS will
bring out TEOS nanoballs instead of nanofibers. It was found that
the width of the fibers could be controlled by varying the reaction
temperature. High temperature brought out fine fibers. The
concentration of HCl can affect the morphologies: for instance, a
weaker acid condition will enable the fabrication of fine nanofibers
and stronger acid condition will allow the production of amorphous
silica.
Fig. 5 FE-SEM image of the silica obtained by sol–gel transcription in
gelator 2 after calcination.
and 2 is the cationic ion portion of the gelator, the straighter silica
fibers only twist slightly to form bundles. The properties of the
cationic ion part seem crucial in controlling the helicity of silica
bundles. This result is similar to our previous findings,3,14 in which
the helicity of TiO2 fibers could be controlled simply by changing
the ionic ion part of the gelator.
In conclusion, a kind of cotton-like silica has been fabricated
with a fiber alignment that could be controlled simply by stirring.
Because the orientation of fibers in some cases is very important for
future applications, further research will be carried out to study
fiber alignment.
This work was supported by Grant-in-Aid for 21st Century COE
Program and a grant (No. 15350132) by the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
Fig. 5 shows an FE-SEM image of the silica after calcination,
from a gel of gelator 2. Although the difference between gelator 1
Notes and references
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Fig. 4 (A) and (B) FE-SEM images of the silica obtained by sol–gel
transcription in gelator 1 after calcination. (C) TEM image of the silica tube
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C h e m . C o m m u n . , 2 0 0 4 , 1 3 3 2 – 1 3 3 3
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