Synthesis of highly ordered mesoporous alumina thin films and their framework crystallization to γ-alumina phase

Article abstract of DOI:10.1039/c1dt10166h

Here we report the preparation of highly ordered mesoporous alumina films existing both as P6₃/mmc and Fm-3m mesostructures by using triblock copolymer Pluronic P123 as the structure-directing agent. 2D grazing-incidence small-angle X-ray scatt

Full text of DOI:10.1039/c1dt10166h

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PAPER
Synthesis of highly ordered mesoporous alumina thin films and their
framework crystallization to c-alumina phase
Xiangfen Jiang,†a,b Hamid Oveisi,†a,c Yoshihiro Nemoto,a Norihiro Suzuki,a Kevin C.-W. Wu*d and
Yusuke Yamauchi*a,b,e
Received 30th January 2011, Accepted 8th April 2011
DOI: 10.1039/c1dt10166h
Here we report the preparation of highly ordered mesoporous alumina films existing both as P63/mmc
and Fm-3m mesostructures by using triblock copolymer Pluronic P123 as the structure-directing agent.
2D grazing-incidence small-angle X-ray scattering (GI-SAXS) completely proves the existence of two
different mesopore structures (i.e., [001]-oriented P63/mmc and [111]-oriented Fm-3m symmetries).
After calcination at 1000 ◦C, the amorphous alumina framework is successfully converted to g-alumina
crystals. During the crystallization process, large uniaxial shrinkage occurs along the direction
perpendicular to the substrate with the retention of horizontal mesoscale periodicity, thereby resulting
in formation of partially vertical mesoporosity in the film. Through detailed electron microscopic study,
we discuss the formation mechanism for the vertical mesoporosity upon calcination. The obtained
mesoporous g-alumina film shows high thermal stability up to 1000 ◦C, which is highly useful in wide
research areas such as catalyst supports and separators.
tures, and relative humidities to get highly ordered mesoporous
alumina.
1. Introduction
Since their discovery in the 1990s,1,2 ordered mesoporous metal
oxides have been widely investigated and developed, due to
There have been several successful reports on ordered meso-
porous alumina.8 Somorjai et al.8a demonstrated a sol–gel-based
their potential applications in various areas such as cataly-
self-assembly route to ordered mesoporous alumina with 2D
sis, separation, sensors, lithium batteries, optoelectronics, solar
hexagonal structure. The ordered mesostructure was formed after
cells, biotechnologies, and so on.3 Various mesoporous non-
strict control of hydrolysis–condensation rates during evaporation
siliceous metal oxides4,5 including TiO2 have been synthesized
6
of an ethanol-based solution containing block copolymer Pluronic
so far. Among these nonsiliceous oxides, g-alumina (Al2O3) is
highly attractive, because g-alumina can offer high mechanical
strength, corrosion resistance, thermal stability, and Lewis acidity.7
However, to control hydrolysis and condensation reactions of
aluminium alkoxides is more difficult than for silicon alkoxides.
In almost all previous cases, the mesostructural ordering of
mesoporous g-alumina is worse than that of mesoporous silica
materials. It is necessary to carefully adjust synthetic parame-
ters such as precursor compositions and pH values, tempera-
P123. However, the alumina in the framework existed as the
amorphous state, which devalues it for catalytic applications.
A sol–gel process with the presence of block copolymers was
developed systematically to generate crystalline g-alumina with
a highly ordered 2D hexagonal mesoporous structure and high
thermal stability to 1000 ◦C.8b Crystalline g-alumina spheres with
uniform mesopores stabilizing up to 900 ◦C were successfully
fabricated by aerosol-assisted self-assembly of block copolymers.8c
Very recently, Jaroniec et al. proposed a feasible approach to the
synthesis of mesoporous alumina by using Pluronic P123 block
copolymer without adding any acid. The mesostructural ordering
was tuned by changing the aluminium precursors.8d Meanwhile,
CMK-3 nanocasting was developed to synthesize ordered crystal-
lized mesoporous g-alumina by Zhang et al.8e Very recently, Zhao
et al. employed ordered mesoporous carbon obtained via organic–
organic self-assembly with tunable structures as a template for
ordered crystallized mesoporous g-alumina.8f Although hard-
templating is a powerful tool to prepare mesoporous material
with various compositions, a two-step procedure is complex and
time-consuming.
aWorld Premier International (WPI) Research Center for Materials
Nanoarchitectonics (MANA), National Institute for Materials Science
(NIMS), 1-1 Namiki, Tsukuba, 305-0044, Japan. E-mail: Yamauchi.
Yusuke@nims.go.jp
bFaculty of Science and Engineering, Waseda University, 3-4-1 Okubo,
Shinjuku, Tokyo, 169-8555, Japan
cDepartment of Metallurgy and Materials Engineering, Iran University of
Science and Technology (IUST), Narmak, Tehran, 16844, Iran
dDepartment of Chemical Engineering, National Taiwan University, Roo-
sevelt Road, Taipei, 10617, Taiwan. E-mail: kevinwu@ntu.edu.tw
ePrecursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama, 332-0012, Japan
Despite recent advances in mesoporous alumina materials, the
morphologies reported previously have been almost limited to
† These authors contributed equally to this work.
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powders. For specific applications such as nanofiltration, optics,
and electronics, morphology of film is highly demanded.7a,9 Only
a few reports presented ordered mesoporous alumina films.10
Sanchez et al.10a reported thermally stable ordered crystalline g-
alumina films with fcc mesoporosity by employing block copoly-
mer KLE22 (BH79-b-EO89) with relatively high thermal stability
to avoid collapse of ordered structure caused by early removal
of surfactants. In another report, Tian et al.10b also prepared
ordered mesoporous alumina films with 3D mesoporosity (Fm-
3m) by using aluminium inorganic salts as aluminium precursor
and commercial block copolymers such as Pluronic F127 and
P123 as templates. However, only poor mesostructural ordering
was obtained in their case without detailed structural analysis.
In this paper, we prepared highly ordered mesoporous alumina
film existing both as P63/mmc and Fm-3m mesostructures by
using triblock copolymer Pluronic P123 as a structure-directing
agent. After calcination at 1000 ◦C, we successfully converted the
amorphous alumina framework into g-alumina crystals. During
the crystallization process, large uniaxial shrinkage occurred along
the direction perpendicular to the substrate with the retention
of horizontal mesoscale periodicity, thereby forming vertically
oriented nanopillars on the film surface. The well-ordered arrange-
ment of the nanopillars can produce vertical mesoporosity in the
films. This porosity can be regarded as “inverse mesospace” of a 2D
hexagonal structure with “mesochannels” running perpendicular
to the substrate. Vertical mesoporosity in films can enhance pore
accessibility and then broaden potential applications.
Al2O3_“x”_“y” where “x” is the applied calcination temperature
and “y” is the added amount of P123. In the case of 1 g P123, only
Al2O3_“x” was used in short.
2.3. Characterization
2D GI-SAXS measurement was conducted on NANO VIEWER
(Rigaku, Japan) equipped with a Micro Max-007HF high intensity
micro focus rotating anode X-ray generator. The data were
˚
collected for 1 h using an X-ray wavelength (l) of 1.540 A on a
two-dimensional position sensitive vacuum chamber detector. The
surface morphology of the films was observed with a field-emission
scanning electron microscope (FE-SEM). The SEM images were
taken under 5 kV acceleration voltages by using a Hitachi SU8000
SEM. The cross-sectional TEM images were observed by a JEOL-
JEM 2100F. The accelerating voltage of the electron beam was
200 kV.
3. Results and discussion
To optimize the amounts of structure-directing agent (P123) for
highly ordered mesoporous alumina films, several alumina films
were prepared by using different amounts of P123. These as-
prepared films were calcined at 400 ◦C and carefully characterized
with SEM observation and 2D GI-SAXS measurement. The top-
surface SEM image of Al2O3_400_0.50 (Fig. 1a) showed striped
structures without specific orientation in long range, resulting
in no diffraction spots on the GI-SAXS patterns (Fig. 1d).
Various voids and cracks were also observed (Fig. 1a). The SEM
image of Al2O3_400_1.50 showed a partially ordered hexagonal
arrangement of mesopores (Fig. 1b). Its corresponding GI-SAXS
patterns (Fig. 1e) showed two in-plane diffraction spots and a
weak out-of-plane spot. With further increase of the amounts of
P123, the in-plane arrangement of mesopores was distorted, while
the out-of-plane mesoscale periodicity disappeared (Fig. 1c, 1f).
In contrast, when 1.0 g of P123 was dissolved in the precursor
solution, several intense spots clearly appeared in both in-plane
and out-of-plane directions of the 2D GI-SAXS patterns (Fig.
2a), indicating the formation of highly ordered 3-dimensional
(3D) mesostructure. The SEM observation also confirmed that
a highly ordered arrangement of the uniform mesopores spread
over the entire top-surface of the film (Fig. 3a). The single domain
sizes of mesoporous crystals were roughly calculated to be larger
2. Experimental
2.1. Materials
Triblock copolymer, Pluronic P123 (EO20PO70EO20), was pur-
chased from Sigma-Aldrich Co. and used as a structure-directing
agent for mesoporous alumina films. Hydrochloric acid (37 wt%),
ethanol, and aluminium tri-n-butoxide were purchased from Wako
Co. and used as catalyst, organic solvent, and aluminium source,
respectively.
2.2. Synthesis of ordered mesoporous alumina films
Aluminium tri-n-butoxide (2.46 g) was added to solution of
37 wt% hydrochloric acid (1.71 g) and absolute ethanol (4.74
g) under stirring. After vigorous stirring for 3 h, a solution
of different amounts of Pluronic P123 (0.50 g, 1.00 g, 1.50
g, and 2.0 g) dissolved in ethanol (9.48 g) was added into
the mixture. After further stirring for 5 h, the clear precursor
solutions were obtained. The mole ratios of P123 to aluminium
tri-n-butoxide were 0.0085, 0.017, 0.026, and 0.034, respectively.
When 1.00 g P123 was added in the precursor solution, the
final composition was 1.00 aluminium tri-n-butoxide : 1.70 ¥
10-2 P123 : 1.73 HCl : 3.10 ¥ 10 EtOH : 6.00 H2O in mole ratio.
The precursor solutions were spin-coated on clean silicon
substrates at 23 ◦C and 50% relative humidity. The spinning speed
and time were fixed at 3000 rpm and 30 s, respectively. After
that, as-prepared thin films were aged at -20 ◦C with 20% relative
humidity for 2 h. After the aging, the films were calcined in air
at 400 ◦C, 800 ◦C, 900 ◦C, 1000 ◦C, 1050 ◦C and 1100 ◦C for
4 h with a ramp rate of 1 ◦C/min. These films were labeled as
Fig. 1 (a–c) Highly magnified SEM images and (d–f) corresponding
GI-SAXS patterns of alumina films prepared using different amounts of
Pluronic P123. The calcination temperatures were fixed to be 400 ◦C. ((a,
d) Al2O3_400_0.50, (b, e) Al2O3_400_1.50, and (c, f) Al2O3_400_2.00).
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surface SEM image, it was confirmed that continuous alumina film
without any cracks/defects was successfully prepared (Fig. 3a).
The hexagonal arrangements of the uniform mesopores spread
over the entire area (Fig. 3a). Fast Fourier transform (FFT) clearly
showed perfect 6-fold symmetry (inset in Fig. 3a). The pore-to-
pore distance measured from the SEM image was around 17.7 nm.
The intense spots of the 2D GI-SAXS pattern (Fig. 5a) were
indexed to two different types of mesostructure: P63/mmc with
[001]-direction perpendicular to substrate (marked in red) and
Fm-3m with [111]-direction perpendicular to substrate (marked
in green). Schematic presentation for two different mesostruc-
tures is displayed in Fig. 6. To further investigate the internal
mesostructure in the films, the cross-sectional TEM image (Fig.
7a) of Al2O3_400 was taken. The observed mesopores were in the
shape of an ellipsoid with high aspect ratio, which resulted from
the vertical contraction. The ABABAB stacking of ellipsoidal
mesopores (derived from P63/mmc) was observable near the film
surface, while the ABCABC stacking of ellipsoidal mesopores
(derived from Fm-3m) was observable on the inner part of the film
(Fig. 7a).
Fig. 2 GI-SAXS patterns of alumina films calcined at various temper-
atures. ((a) Al2O3_400, (b) Al2O3_800, (c) Al2O3_900, (d) Al2O3_1000,
(e) Al2O3_1050, (f) Al2O3_1100.) For (a) Al2O3_400 GI-SAXS pat-
tern, the spots derived from [001]-orientated P63/mmc symmetry and
[111]-orientated Fm-3m symmetry are marked in red and green, respec-
tively. The detailed explanation is given in Fig. 5.
Fig. 3 Top surface SEM images of alumina films calcined at various tem-
peratures. ((a) Al2O3_400, (b) Al2O3_800, (c) Al2O3_900, (d) Al2O3_1000,
(e) Al2O3_1050, and (f) Al2O3_1100.) Inset images are obtained after Fast
Fourier transform (FFT) of the observed SEM images.
than 500 nm2. Based on the investigation above, the composition
of precursor solution was optimized to be 1.00 aluminium tri-n-
butoxide : 1.70 ¥ 10-2 P123 : 1.73 HCl : 3.10 ¥ 10 EtOH : 6.00 H2O
in mole ratio. In the following parts, a series of alumina films
prepared using this optimized precursor solution were studied.
Mesoporous alumina film before the crystallization (Al2O3_400)
was further characterized. The powder TEM image (Fig. 4a) was
a typical view of closely packed mesopores. The selected-area
electron diffraction (ED) (inset in Fig. 4a) showed no ring-like
patterns, indicating that the alumina in the framework existed
as the amorphous state. Based on the observation of the top-
Fig. 5 GI-SAXS patterns of Al2O3_400 with the simulation diffraction
spots of (a) [001]-oriented P63/mmc symmetry and (b) [111]-oriented
Fm-3m symmetry. In (c), the simulation diffraction spots for both symme-
tries are shown in the GI-SAXS patterns. Red-colored and green-colored
assignments are derived from [001]-oriented P63/mmc symmetry and
[001]-oriented Fm-3m symmetry, respectively.
The hexagonal arrangements observed on the top surface
(Fig. 3a) represent the view from [001]-direction of P63/mmc
mesostructure. From the 2D GI-SAXS data, the in-plane pore-
to-pore distance calculated from d100 of P63/mmc (15.2 nm) was
around 17.6 nm, which was consistent with the pore-to-pore
distance (around 17.7 nm) measured from the top-surface SEM
image. On the other hand, the distance between neighboring
mesopores along the out-of-plane direction was calculated to be
6.3 nm by using d002 spacing of P63/mmc. Therefore, as shown
in the cross-sectional TEM image (Fig. 7a), the mesopores were
stacked with neighboring mesopores in the vertical direction much
closer compared with the horizontal direction, which was caused
Fig. 4 TEM images of alumina films calcined at various temperatures. ((a)
Al2O3_400, (b) Al2O3_800, (c) Al2O3_900, (d) Al2O3_1000, (e) Al2O3_1050,
and (f) Al2O3_1100.) Inset images are ED patterns.
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Fig. 8 Changes of thicknesses of alumina films calcined at different
temperatures. As-prepared film dried at room temperature was chosen
as the initial reference.
Kuroda et al.11c reported a mesoporous titania film by using
titanium tetraisopropoxide (TTIP) as titanium source and P123
as surfactant. In these cases, almost all regions in the film showed
P63/mmc (i.e., ABABAB stacking of mesopores), but Fm-3m
(i.e., ABCABC stacking of mesopores) was also partially mixed
at the inner part of the film. Similarly, mesoporous silica films
prepared using P123 also showed coexistence of both P63/mmc
and Fm-3m.11d Because both of them are very stable close-packing
mesostructures, it is very difficult to selectively form the single
phase.11c,d The examples above indicated that a slight difference of
inorganic sources or kinds of surfactant greatly affected not only
the final mesostructures but also their orientation on substrates.
In the present study, the as-prepared alumina film possessed two
different mesopore structures (i.e., [001]-oriented P63/mmc and
[111]-oriented Fm-3m symmetries).
Fig. 6 Schematic presentation of (a, c) [001]-oriented P63/mmc symmetry
and (b, d) [111]-oriented Fm-3m symmetry. (c) and (d) are the correspond-
ing views along [001] direction for P63/mmc and [111] direction for Fm-3m,
respectively. The spheres represent the positions of mesopores.
The 2D GI-SAXS patterns of mesoporous alumina films
calcined at various temperatures are displayed in Fig. 2. With the
increase of the calcination temperatures, the out-of-plane spots
were obviously shifted into higher angle and finally disappeared,
while the in-plane spots were well maintained without obvious
shifts (up to 1000 ◦C). It was understood that such changes in in-
plane and out-of-plane spots in the temperature range from 400 ◦C
to 1000 ◦C were caused by uniaxial contraction along the vertical
direction (Fig. 8). The retention of in-plane hexagonal periodicity
was clearly shown in the top-surface SEM images (Fig. 3). For
the alumina films calcined below 1000 ◦C, no obvious collapse of
in-plane hexagonal periodicity was observed.
Fig. 7 Cross-sectional observation of alumina films. (a) Cross-sectional
TEM image of Al2O3_400. (b, c) Cross-sectional SEM and TEM images
of Al2O3_1000. (d) Highly magnified TEM image of Al2O3_1000.
With the increase of the calcination temperatures, the film
thicknesses were decreased (Fig. 8). A similar phenomenon has
been reported in alumina film templated by KLE2210a and other
transition metal oxide films like titania.11c This large uniaxial
contraction of films during the calcination is different from the
contraction of powders. The preferential out-of-plane contraction
is caused by in-plane restriction on the substrate. As shown in
Fig. 8, the dramatic vertical contraction was carried out under
lower calcination temperature range, and the contraction under
higher temperatures proceeded with a slow speed (the thickness
of Al2O3_1000 contracted by only 18% from Al2O3_400). The
former contraction was attributed to decomposition of organic
components and densification of the inorganic walls, while the
latter contraction (800–1000 ◦C) was thought to be caused by
crystallization of the alumina framework. After 800 ◦C calci-
nation, the g-alumina crystal phase was developed (Figure S1).
by a large contraction (around 60%) under calcination at 400 ◦C
(Fig. 8).
In general, there are three possible symmetries in cage-type
mesostructures formed on a flat substrate, when triblock copoly-
mers such as F127 and P123 are used as structure-directing
agents. One is Im-3m mesostructure. Stucky and Chmelka
et al.11a reported synthesis of mesoporous titania thin films with
Im-3m mesostructure by using titanium tetraethoxide (TEOT;
Ti(OC2H5)4) as titanium source and triblock copolymer P123 as
surfactant. In their case, the [001]-direction of the mesostructure
was oriented perpendicular to the substrate. Grosso et al.11b used
TiCl4 as titanium source and F127 as surfactant to successfully
prepare mesoporous titania films with [110]-direction of Im-3m
mesostructure perpendicular to substrate. Another two possible
symmetries are Fm-3m and P63/mmc mesostructures. Wu and
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With further increase of calcination temperatures up to 1000 ◦C,
a-alumina phase was slightly generated, although a large part
of the frameworks were still g-alumina crystals. After 1050 ◦C
calcination, only a-alumina phase existed. To further investigate
the crystallization process of the alumina framework, selected-area
ED patterns were taken for all films (Inset image, Fig. 4). With
the increase of calcination temperatures, ring-like ED patterns
appeared with brighter intensity. The ED patterns for Al2O3_800
and Al2O3_900 can be assigned to (400) and (440) planes of
g-alumina crystals. From the results, it was revealed that the
main crystallization in the amorphous alumina framework was
progressed from 800 ◦C.
stacking of mesoporous layers) were merged along the c axis by the
large contraction in the perpendicular direction to the substrate.
Actually, from top-surface SEM images (Fig. 9), it was proved
that the interval of the mesopores before the structural transforma-
tion (ca. 18 nm) was almost the same as that of the Al nanopillars
after structural transformation (ca. 18 nm). Furthermore, the
in-plane periodicity detected by GI-SAXS measurements was
almost constant before and after the structural transformation
(Fig. 2a,d). These SEM and GI-SAXS results strongly supported
the formation mechanism above. Unfortunately, however, this
structural transformation from the [001]-oriented P63/mmc to the
nanopillars was confirmed only at the surface of the film. The
nanopillars were not formed at the inner parts of the film (Fig.
7b,c). As shown in 2D GI-SAXS patterns and the cross-sectional
TEM image (Fig. 5 and 7a), [111]-oriented Fm-3m mesoporous
structures were preferentially formed in the internal region of the
film. As seen in Fig. 6d, the [111]-oriented Fm-3m mesoporous
structure does not have continuous Al regions existing throughout
the film thickness. Therefore, vertically oriented nanopillars were
not developed well, but the disordered mesoporous structures were
formed at the inner parts of the film (Fig. 7b,c).
The vertical pores formed on the film surface can accelerate
diffusion of guest species from outside. Our results reported
here are the first demonstration of the formation of vertical
mesoporosity in alumina films. The formation of vertical meso-
porosity in mesoporous film is desirable for future applications.12
Another advantageous point is that the thermal stability of our
films is very high, compared to titania and other metal oxides
films. Actually, the mesostructural periodicity in the alumina films
could be retained up to 1000 ◦C (Fig. 3). Howev◦er, when the
calcination proceeded to higher temperature (1100 C), large a-
alumina crystal particles about 100 nm in diameter were formed
and destroyed the ordered pillar structures, as confirmed by SEM
observation (Fig. 3f) and wide-angle XRD measurement (Figure
S1). The GI-SAXS patterns also showed no spots (Fig. 2f).
After the calcination at 1000 ◦C, interestingly shape transfor-
mation of mesopores on the film surface occurred without serious
damage of in-plane mesoscale periodicity (Fig. 3d). From cross-
sectional SEM and TEM images (Fig. 7b,c), it could be seen that
the nanopillars were periodically distributed, leading to creation of
vertical porosity. The enlarged image of Fig. 7b is given in Figure
S2. This porosity can be regarded as “inverse mesospace” of a 2D
hexagonal structure with “mesochannels” running perpendicular
to the substrate. These nanopillars were crystallized to g-alumina
phase and partially interconnected with each other. From the high-
resolution cross-sectional TEM image (Fig. 7d), it was proved
that the alumina framework was composed of small g-alumina
nanocrystals (about 5 nm) without preferred orientations.
This unique structural transformation was induced by a large
contraction of [001]-oriented P63/mmc (i.e., ABAB stacking)
along the perpendicular direction to the substrate during the cal-
cination process, which has been already reported in mesoporous
titania film by Wu and Kuroda et al.11c As viewed from the [001]-
zone axis (top surface view, Fig. 9), the areas marked as “Al” are the
regions of amorphous Al existing throughout the film thickness.
Upon the calcination, the amorphous Al regions shrink along
the c axis (the direction normal to the paper) and crystallize into
the g-alumina phase. At the same time, as viewed from the [100]-
zone axis, the neighboring mesopores (well-organized ABABAB
4. Conclusion
We reported the preparation of ordered mesoporous alumina
films and their structural conversion to g-alumina nanopillars
by a calcination process. Before the crystallization, ordered
mesoporous alumina film prepared with triblock copolymer
Pluronic P123 as structure-directing agent possessed two stacking
ways. The ABABAB stacking of ellipsoidal mesopores (derived
from P63/mmc) was observable near the film surface, while the
ABCABC stacking of ellipsoidal mesopores (derived from Fm-3m)
was observable at the inner part of the film. Upon calcination at
1000 ◦C, the amorphous alumina framework was crystallized into
g-alumina phase. Interestingly, g-alumina nanopillars were formed
near the film surface, which was caused by the large contraction of
[001]-oriented P63/mmc along the perpendicular direction. This is
the first demonstration of the formation of vertical mesoporosity
in alumina films. Although the vertical mesoporosity was formed
only near the film surface, we consider that they can accelerate
diffusion of guest species from outside.12 The formation of vertical
mesoporosity in mesoporous film is very desirable for future
applications such as catalytic supports and molecular separation
films.
Fig. 9 Schematic presentation for formation mechanism of vertically
oriented nanopillars. Top-surface views from the [001]-zone axis for
mesoporous alumina films calcined at (a) 400 ◦C and (b) 1000 ◦C. The areas
marked as “Al” indicate the regions of amorphous Al existing throughout
the film thickness.
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Recently, anodic porous alumina films with various pore diam-
eters have been successfully fabricated by controlling anodization
conditions and by applying a retexturing process using molds.13
But, these systems, we think, cannot be extended to general
synthetic methodology as easily as surfactant-templated systems
can, because it is difficult to finely control pore diameters and
film thicknesses in a nanometre scale. In the surfactant-templated
system, the pore sizes are controllable by changing surfactant sizes,
while the film thicknesses are controllable by changing concentra-
tions of inorganic species in precursor solutions. The used methods
(e.g., spin-coating, dip-coating, and casting methods) also greatly
affect the film thicknesses. Such a bottom-up approach by using
self-assembled surfactants is a versatile route to a wide range of
pore sizes, pore spacings, and compositions of pore walls.
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10856 | Dalton Trans., 2011, 40, 10851–10856
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