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Electric field-induced mesostructure transformation of self-assembled
silica/copolymer nanocomposite thin films
Donghai Wang,a Xiangling Ji,a Jie-Bin Pang,a Qingyuan Hu,a Huifang Xub and Yunfeng Lu*a
a
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b
Department of Chemical Engineering, Tulane University, New Orleans, LA 70118, USA.
E-mail: [email protected]; Fax: +1 504 865 6744
Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences,
The University of New Mexico, Albuquerque, NM 87131, USA
Received 19th June 2003, Accepted 12th August 2003
First published as an Advance Article on the web 26th August 2003
Electric field-induced mesostructure transformation of silica/copolymer thin films has been investigated. Partial
transformation of the two-dimensional hexagonal mesostructure into irregular elliptical or rod shaped
mesostructure was observed using X-ray diffraction (XRD) and transmission electron microscopy (TEM)
techniques. The mesoscale pore structure of the thin films before and after the transformation was replicated
using metal nanostructures through an electrochemical templating growth technique. The comparison of the
replicates further confirms the mesostructure transformation of the thin films under an electric field.
1. Introduction
2. Experimental
Mesoporous materials with aligned channels are very useful
for separations, membranes, chemical sensors, templated
syntheses, and other applications. The surfactant templating
mesoporous materials possess unique pore structure and great
efforts have been made to study the transformation and alignment of the pore structure using electric fields,1 magnetic
fields,2 mechanical fields,3 the photoinduced method,4 and
the anisotropic surface-induced method.5 The electric field
alignment was typically conducted by applying two electrodes
crossing capillary tubes that contain silica–surfactant aqueous
precursor solution.1 Co-assembly of the surfactant and silica
within the capillary tubes under the applied electric field
aligned the hexagonal mesostructure parallel to the substrate.
However, the achieved alignment was limited within the small
capillary tubes and the applied electric field also caused undesired precursor solution electrolysis and inhomogeneous
morphology.
Recent research has developed rapid sol–gel assembly techniques to efficiently produce mesoporous silica thin films
through dip- or spin-coating silica/surfactant precursor solutions.6 Spontaneous mesostructural transformation within
these thin films has also been observed.7 The goal of this
research is to investigate electric field-induced mesostructure
transformation of such preformed silica/surfactant mesostructured thin films. Compared with the previous research, this
method directly transforms the mesostructure of the readily
made thin films without causing the undesired electrolysis
reactions, which may provide bases for the manufacture of
mesoporous thin films with aligned pore channels. The mesostructured thin films used for this study were prepared by a
sol-gel spin-coating process using block copolymer Pluoronic
P123 as the structure-directed reagent. Electric field was
immediately applied perpendicualrly cross the thin films after
the spin-coating process. Mesostructure of the thin films were
investigated using X-ray diffraction (XRD) and transmission
electron microscopy (TEM) techniques.
In a typical preparation, silica sols were prepared by mixing
tetraethoxylsilane (TEOS), H2O, Pluoronic P123 (EO20PO70EO20 , EO and PO represent ethylene oxide and propylene
oxide, respectively), HCl, and ethanol in a molar ratio of
1 : 5 : 0.0096 : 0.0089 : 22 at room temperature for 10 min.
Smooth, 400 nm thick films of silica/P123 nanocomposite
were prepared by spin-coating the precursor sols on conductive
Si wafer or ITO substrates which served as the bottom electrode. Aluminized Kapton thin films with a thickness of 50.8
mm were closely placed on silica/copolymer thin films with
the Kapton side facing the silica/copolymer thin films and
served as the upper electrode. The electrodes were then connected to a DC power supply at room temperature for 24 h.
The strength of the electric field between the electrodes was
estimated around 10 V mm1. The compared samples (samples
without electric field treatment) were prepared by aging the asmade silica/copolymer thin films at room temperature for 24 h.
All thin films were then calcined to obtain mesoporous silica at
400 C for 1 h in air with a heating rate of 1 C min1. Electrodeposition of the palladium nanostructures within the porous
thin films was conducted using the procedure described
before.8 X-ray diffraction patterns (XRD) were obtained with
a Philips Xpert X-ray diffractometer using Cu Ka radiation
(l ¼ 0.1542 nm). Transmission electron microscopy (TEM)
images were taken with a JEOL 2010 electron microscope with
an accelerating voltage of 200 kV.
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3. Results and discussion
Fig. 1 compares the XRD patterns of the uncalcined and calcined mesoporous silica thin films with (pattern a and c) and
without (pattern b and d) electric field treatment. The uncalcined thin film prior to electric field treatment (pattern b)
shows an intense (100) diffraction of the hexagonal mesostructure at the d-spacing of 8.9 nm; while electric field treatment
Phys. Chem. Chem. Phys., 2003, 5, 4070–4072
This journal is # The Owner Societies 2003
DOI: 10.1039/b306978h
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Fig. 1 XRD patterns of the uncalcined silica/copolymer thin films
with (a) and without (b) application of electric field and of the calcined
mesoporous silica with (c) and without (d) electric field treatment.
eliminates the intense (100) diffracition, resulting in a diffraction pattern with a very broad and weak diffraction centered
at the d-spacing of 5 nm (see pattern a). After calcination,
the mesoporous silica without electric field treatment (pattern
d) also shows intense (100) and (200) diffractions at the d-spacing of 6.4 and 3.3 nm repectively. The decreased d-spacing
after calcination is due to the shrinkage upon the removal of
surfactant and further silica condensation reaction. The
absence of (110) diffraction in both calcined and uncalcined
films without electric field treatment indicates the hexagonally
arranged surfactant or pore channels are oriented parallel to
the substrate. The calcined electric field treated thin film shows
a diffraction pattern with a very broad and weak diffraction
peak at the d-spacing of 4.6 nm (see pattern c). The comparison of the XRD patterns indicates that electric field can transform mesostructure of the preformed silica/surfactant
nanocomposited thin films and that the transformed mesostructure can be preserved after calcination.
Fig. 2(a) and 2(b) respectively show cross-section TEM
images of the uncalcined (2a) and calcined (2b) mesostructured
thin films without electric field treatment. Consistent with the
XRD result shown in Fig. 1, both images indicate the thin
films contain ordered hexagonal pore structure that are
oriented parallel to the substrate. Note the brick-like pore
structure shown in Fig. 2(b) is due to the one-dimensional
shrinkage of the thin film upon calcinations. Fig. 2(c) and
2(d) shows cross-section TEM images of the thin films after
electric field treatment, indicating the transformation of
ordered hexagonal mesostructure into homogenous mesostructure without the long-range order. As shown in the Fig. 2(c),
these pores are irregularly elliptical or rod-like with their long
axis parallel to the substrate. The average dimension of the
most rod-shaped pores along the direction vertical to the substrate is around 8–10 nm, which is similar to that of the hexagonal pore channels of the untreated sample. This implies that
the applied electric field breaks the long parallel hexagonal
tubes and results in the irregular, rod-like pores. Fig. 2(e)
and 2(f) respectively show the plain-view TEM images of
uncalcined thin films without and with electric field treatment.
The untreated thin films ,Fig. 2(e), show a swirling hexagonal
mesostructure similar to those reported previously8 while the
treated thin films ,Fig. 2(f), show a disordered mesostructure
connected through the thin film.
To further examine the pore transformation, we used the
calcined mesoporous films with and without electric field treatment as templates; electrochemically grew palladium within
the template.8 The crystalline structure defects connect the
swirling two-dimensional pore channels, which provide the
pathway for metal deposition from the bottom electrode
through the pore channels that swirl down to the substrate.
Fig. 2 Cross-section TEM images of uncalcined (a) and calcined (b)
mesostructured thin film without electric field treatment; High (c) and
low (d) magnification of cross-section TEM images of electric field
treated mesoporous silica thin film; Plan-view TEM images of the
uncalcined mesostructured thin films with (f) and without (e) electric
field treatment.
Removal of the silica template results in palladium nanostructure that replicates the pore structure. Consistent with the
TEM and XRD results, the use of thin films without the electric field treatment as templates results in long palladium nanowires (see Fig. 3a), indicating the untreated thin films contain
long two-dimensional hexagonal pore channels. While the use
of electric field treated templates results in nanoparticles and
nanorods (see Fig. 3b), confirming the transformation of the
hexagonal mesostructure into disordered irregular and rodshape mesostructure.
The co-assembly of silica/surfactant into mesophases is a
dynamic process involving the formation of intermediate
mesophases7 and the mesostructure can rearrange under certain conditions before the silica backbone has fully condensed
and solidified. It has been reported that mesostructure of the
Fig. 3 (a) A TEM image of palladium nanowires deposited using the
mesoporous silica template without electric field treatment. (b) A TEM
image of nanoparticles or nanorods deposited using the electric field
treated mesoporous silica thin films as template.
Phys. Chem. Chem. Phys., 2003, 5, 4070–4072
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block-copolymers can be rearranged under an electric field due
to the dielectric difference of individual blocks.9,10 Similarly,
due to the dielectrics difference of the meso-components in
the silica/copolymer nanocomposite silica, the applied electric
field may prefer to orient the dielectric interface parallel to
electric field direction to obtain its lowest energy conformation.9,10 Dense silica and block copolymer have dielectric
constants of around 4.4 and 3–5 respectively.11 However, the
presence of water and silanol groups enriched in the hydrophilic phase may further increase the dielectric difference (water
has a dielectric constant of around 80) to promote such mesostructure rearrangement. However, the low mobility of the
silica framework and copolymer in the current experimental
condition may limit the rearrangement. The partial rearrangement may contribute to the formation of the elliptical or rod
shape mesostructures observed in this research.
4. Conclusion
In a summary, we have demonstrated that electric field can
induce the mesostructure transformation of self-assembled
silica/copolymer thin films. Electric field partially induces the
transformation of hexagonal mesostructure into irregular elliptical or rod shaped mesostructure due to the poor mobility of
silica and block copolymer liquid crystalline phase. Silica/surfactant mesostructure with a better mobility should favor a
more complete rearrangement or potential alignment. These
results provide insight into electric field induced alignment of
the self-assembled silica/surfactant mesostructure. Research in
using different surfactant system, electric field strength, and
controlled condensation to reach alignment of mesoporous
silica under electric field is in progress.
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Phys. Chem. Chem. Phys., 2003, 5, 4070–4072
Acknowledgements
We thank Dr. Weilie Zhou and Dr. Jibao He for assistance in
TEM studies and Dr. Jinke Tang for assistance in X-ray diffraction studies. This work is supported by NASA under the
grant NAG-1-02070 and NCC-3-946, and by the Advanced
Materials Research Institute at the University of New Orleans
through DARPA Grant No. MDA972-97-1-0003 and through
the Louisiana Board of Regents Contract No. NSF/
LEQSF(2001-04)-RII-03.
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