Formation of ordered mesostructured silica by vapor infiltration of tetraethoxysilane into hexagonally arranged surfactant-catalyst nanocomposites

Article abstract of DOI:10.1246/cl.2005.1148

Hexagonally arranged surfactant-catalyst composites were treated only with tetraethoxysilane vapor, resulting in the formation of the silica mesostructures without phase transition. Copyright

Full text of DOI:10.1246/cl.2005.1148

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148
Chemistry Letters Vol.34, No.8 (2005)
Formation of Ordered Mesostructured Silica by Vapor Infiltration
of Tetraethoxysilane into Hexagonally Arranged Surfactant–Catalyst Nanocomposites
ꢀ
Shunsuke Tanaka, Takanori Maruo, Norikazu Nishiyama, Yasuyuki Egashira, and Korekazu Ueyama
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received May 13, 2005; CL-050638)
Hexagonally arranged surfactant–catalyst composites were
treated only with tetraethoxysilane vapor, resulting in the forma-
tion of the silica mesostructures without phase transition.
2
H SO4
(a)
(b)
Syntheses of ordered mesoporous silicas require surfactant
templates which form lyotropic liquid crystal phases in the sol-
vent (usually water) and silicate species interacts with the surfac-
tant and polymerizes to form a silica network encapsulating the
surfactant micelles.1
(
c)
HCl
(d)
(e)
(f)
2
For a large range of applications such as low-k insulators,
3
4
antireflective coatings, and optical sensors, it would be desira-
ble to produce mesoporous silicas in the form of thin films. Fab-
rication of thin films made of mesoporous silicas has conven-
tionally been performed by liquid deposition methods such as
CH COOH
3
(
(
g)
h)
(i)
0
1
2
3
4
5
6
7
8
9
5
6,7
epitaxial growth or sol–gel method. Self-assembled surfac-
tant–silicate composites are deposited from a liquid phase under
acidic or basic conditions.
θ
α
2 CuK / degree
Figure 1. XRD patterns of the mesostructured films using
H2SO4, HCl, or CH3COOH as a catalyst. Traces (a), (d), and
g) show the spin-on surfactant films; Traces (b), (e), and (h)
show the surfactant films treated with TEOS vapor in a batch
reactor; Traces (c), (f), and (i) show the films after calcination.
On the other hand, we have reported a novel synthetic route
8
(
of mesoporous silica films from vapor phase. Spin-on surfactant
films were treated with tetraethoxysilane (TEOS) and hydro-
chloric acid (HCl) vapors in a closed vessel. Surfactant–silicate
composites were rearranged from lamer to hexagonal structure
structure with d ¼ 2:8 nm (Figures 1d and 1g). We consider that
8
a
þ
ꢁ
under vapor infiltration. A drawback of the previous method
is difficulty adjusting to a continuous vapor flow system because
it is unavoidable that the TEOS/HCl vapor reacts and produces
silicate particles not only on the film but also in gas phase.
Here, we report an advanced vapor phase synthesis of meso-
porous silica films using surfactant–catalyst composites. The ob-
jective of the present study is to prepare mesoporous silica films
using only TEOS in vapor phase. Thus, the catalyst should be
present in the surfactant film. Nonvolatile acids such as sulfuric
acid (H2SO4) and phosphoric acid (H3PO4) must be a candidate
as a catalyst instead of a volatile acid, HCl. Further, a novel phe-
nomenon of the formation of surfactant–catalyst nanocomposite
is presented here.
a large amount of H3O and SO4 could adsorb on the hydro-
philic head of CTAB molecules. The bulky head of surfactants
caused an arrangement of the H2SO4/CTAB composite into
hexagonal symmetry. After contacting the CTAB/H2SO4 com-
posite film with TEOS vapor using the batch reactor, the reflec-
tion peak was slightly shifted to lower angle (Figure 1b). The pe-
riodic structure swelled by 8%. It is confirmed by FTIR measure-
ment that silica network was formed on the substrate after
contacting with TEOS vapor. This result indicates that TEOS
molecules penetrate into the CTAB/H2SO4 composite film.
The hexagonal structure of the CTAB/H2SO4 composite was re-
tained after the infiltration of TEOS vapor. The periodic distance
shrunk by 8% in a direction perpendicular to the surface of the
substrate after calcination (Figure 1c). FTIR measurements
showed that the surfactant molecules were removed from the
composite. Moreover, EDX measurements showed that the cal-
cined films were free of sulfur. On the other hand, in the case of
First, we have performed vapor phase synthesis in a batch
reactor using surfactant films containing different catalysts, such
as H2SO4, H3PO4, HCl, and acetic acid (CH3COOH). Cetyltri-
methylammonium bromide (CTAB) was used as a surfactant.
The detailed synthesis and characterization methods are shown
using volatile acids, HCl and CH COOH, hexagonal structure
3
did not formed (Figures 1e and 1h). There is no periodic struc-
9
in the references and notes section.
Figure 1a shows the XRD pattern of the spin-on film pre-
pared using CTAB and H2SO4. The XRD pattern has two reflec-
tion peaks, indicating that the (100) family of planes of the hex-
agonal unit cell (d ¼ 4:5 nm) is oriented parallel to the surface of
the substrate. The CTAB/H2SO4 nanocomposite already has a
hexagonal structure even before the TEOS vapor treatment.
For reference, we prepared spin-on films using CTAB and vola-
tile acid such as HCl or CH3COOH. The films have a lamellar
ture in the calcined samples indicating that TEOS did not infil-
trate into CTAB films prepared using volatile acid (Figures 1f
and 1i).
A similar hexagonal structure was obtained when CTAB/
H3PO4 composite films were used. Unlike in the case of H2SO4,
the framework included the phosphate compound even after cal-
cination. The molar ratio of P/Si in the calcined SiO2/P2O5 film
was determined to be about 0.4 by an EDX analysis. Figure 2
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1149
out phase transition. The periodic mesostructures of the films
were retained after calcination although the structural contrac-
tions were observed under heating. The film thickness was about
4
00 nm. On the other hand, the continuous flow synthesis using
the volatile acids such as HCl and CH3COOH was impossible.
The continuous vapor phase synthesis is expected to be a
useful technique for mass production. In addition, the formation
of mesostructured silica without phase transition is a simpler
way than the previous method for the control of the structure
orientation.
5
nm
Figure 2. FE-SEM image of a cross section of the mesoporous
film prepared using a spin-on CTAB/H3PO4 film.
shows the FE-SEM image of a cross section of the calcined silica
film. Periodic mesostructures were clearly observed around the
surface of the substrate, with uniform pores (3.0 nm), indicating
that diffusion of TEOS molecules is so fast that the TEOS mole-
cules penetrate to the H3PO4/CTAB film–substrate interface.
The distance from layer to layer is about 3.8 nm, which is consis-
tent with the result of the XRD measurements (see Supplemen-
tary Information Figure S1).
We proposed a surfactant–catalyst templating model for a
formation of the mesostructured silica film in Figure 3. A
spin-on surfactant–catalyst film has a hexagonal structure
already. TEOS molecules penetrate into a hydrophilic H2O/
catalyst part around the CTAB molecules without nano phase
transition.
We gratefully acknowledge the assistance of the GHAS
laboratory and Mr. M. Kawashima at Osaka University with
the FE-SEM measurements.
References and Notes
1
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and
J. S. Beck, Nature, 359, 710 (1992); J. S. Beck, J. C. Vartuli,
W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmidtt,
C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen,
J. B. Higgins, and J. L. Schlenker, J. Am. Chem. Soc., 114,
1
0834 (1992).
2
3
C. M. Yang, A. T. Cho, F. M. Pan, T. G. Tsai, and K. J. Chao,
Adv. Mater., 13, 1099 (2001).
P. Falcaro, D. Grosso, H. Amenitsch, and P. Innocenzi, J. Phys.
Chem. B, 108, 10942 (2004).
4
5
S. Subbiah and R. Mokaya, Chem. Commun., 2003, 860.
I. A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou,
P. Fenter, P. M. Eisenberger, and S. M. Gruner, Science, 273,
Vapor Infiltration
without
Nano-Phase Transition
8
M. Ogawa, J. Am. Chem. Soc., 116, 7941 (1994).
Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J.
Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang,
and J. I. Zink, Nature, 389, 364 (1997).
92 (1996).
6
7
−
H O
SO 2
4
2
8
9
a) N. Nishiyama, S. Tanaka, Y. Egashira, Y. Oku, and K.
Ueyama, Chem. Mater., 15, 1006 (2003). b) S. Tanaka, N.
Nishiyama, Y. Oku, Y. Egashira, and K. Ueyama, J. Am. Chem.
Soc., 126, 4854 (2004). c) S. Tanaka, N. Nishiyama, Y. Hayashi,
Y. Egashira, and K. Ueyama, Chem. Lett., 33, 1408 (2004).
A coating solution was prepared using surfactant, catalyst,
EtOH, and deionized water with the mole ratios of 0.5:0.9:
50:100 surfactant/catalyst/EtOH/H2O. The mixture was drop-
ped onto the silicon substrate while it was spinning at 50 rpm,
and then the substrate spin up to 4000 rpm for 60 s. The surfac-
tant films were treated only with TEOS in a batch reactor as
follows. The surfactant-coated silicon substrate was arranged
Si(OC H ) (OH)n
2
5 4−n
Figure 3. Proposed surfactant–catalyst templating model for a
formation of ordered mesostructured silica film.
Next, this technique was applied to a continuous vapor flow
system at atmospheric pressure. In a continuous flow synthesis,
surfactant films were exposed to a TEOS vapor using a tubular
reactor (see Supplementary Information Figure S2). The detailed
1
0
synthesis method is shown in the references and notes section.
Figure 4 shows XRD patterns of the mesostructured silica
films prepared under a continuous TEOS vapor flow. TEOS
molecules penetrate into the CTAB/H2SO4 composite film with-
3
to lie vertically in a closed vessel (50 cm ). Small amount of
TEOS was placed in the bottom of the vessel apart from the
ꢂ
substrate. The vessel was placed in an oven at 90 C for 2 h. Cal-
ꢂ
cination was performed at 400 C in air for 5 h with a heating
ꢂ
rate of 1 C/min. The films were identified by X-ray diffraction
(XRD) patterns recorded on a Rigaku Mini-flex. Fourier-trans-
H SO /CTAB film
form infrared (FTIR) spectra of the films were recorded using
a FTIR-8200PC spectrometer (Shimadzu Co.). Field emission
scanning electron microscope (FE-SEM) images were recorded
on a Hitachi S-5000L microscope.
0 A tubular reactor was a horizontal quartz tube of 60-cm length
and 2-cm inner diameter surrounded by an electrical heater
2
4
vapor treated film
calcined film
1
(
1
ISUZU EPKRO-11K). The temperature was controlled at 90–
ꢂ
50 C. The surfactant-coated substrate was arranged to lie
0
1
2
3
4
5
6
7
8
9
horizontally in the tubular reactor. TEOS was fed with a N2
carrier gas for 3 h. The feed rates of N2 and TEOS/N2 were
0–30 and 30 mL/min, respectively. The films were subsequently
2θ CuKα / degree
Figure 4. XRD patterns of the mesostructured films prepared
under continuous vapor flow of TEOS.
ꢂ
calcined in flowing air at 400 C for 5 h to remove the surfactant.
Published on the web (Advance View) July 16, 2005; DOI 10.1246/cl.2005.1148