Chem. Mater. 2002, 14, 83-88
83
Assembly of Highly Ordered Three-Dimensional Porous
Structure with Nanocrystalline TiO2 Semiconductors
Q.-B. Meng,*,†,‡ C.-H. Fu,† Y. Einaga,§ Z.-Z. Gu,† A. Fujishima,†,§ and O. Sato*,†
Special Research Laboratory for Optical Science, Kanagawa Academy of Science and
Technology, KSP Building East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi,
213-0012 Kanagawa, Japan, State Key Laboratory for Surface Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing 100080, China, and Department of Applied Chemistry,
School of Engineering, The University of Tokyo, 7-3-1, Hongo Bunkyo-ku,
Tokyo 113-8565, Japan
Received February 22, 2001. Revised Manuscript Received October 11, 2001
In this paper, we report on the assembly of a highly ordered three-dimensional porous
structure with nanosized crystalline TiO2 particles by a cooperative assembly method in
which the fabrication of the template and the infiltration of the voids of the template are
carried out at the same time and the related experimental parameters for the assembly,
including temperature, humidity, and concentrations and concentration ratio of the colloid
mixture. SEM (scanning electron microscope) images and transmission spectra of these
samples demonstrate that these films have a highly ordered three-dimensional structure.
The Bragg law was used to calculate the diameter of the spheres of air in the porous TiO2
structure. A good agreement between the calculated results for the diameter of the spheres
of air and those measured by SEM further confirms the high quality of the films fabricated
using this simple method. Additionally, based on these experimental results, a detailed
mechanism of the simple method is also discussed.
Introduction
Porous nanocrystalline TiO2 semiconductors have
recently attracted much attention because of their
various applications in electronic, electrochemical, and
photocatalytic systems, including photoelectrochemical
solar cells,1-3 electrocatalysts,4-6 sensors,7,8 and highperformance photocatlysts.9 In particular, highly ordered three-dimensional (3-D) porous TiO2 structures
with lattice spacings on the order of wavelengths of light
can be used as PBG (photonic band gap) materials
because TiO2 crystals have a large refractive index and
are transparent in the visible light region.10,11 Moreover,
catalysis, large-molecule separation processes, and other
* Corresponding authors. E-mail: [email protected] and
[email protected]. E-mail: [email protected].
† Kanagawa Academy of Science and Technology.
‡ Chinese Academy of Sciences.
§ The University of Tokyo.
(1) Barbe, C. J.; Arendse, F.; Comte, P. Jirousek, M.; Lenzmann,
F.; Shklover, V.; Gratzel, M. J. Am. Ceram. Soc. 1997, 80, 3157.
(2) ORegan, B.; Schwartz, D. T.; Zakeeruddin, S. M.; Gratzel, M.
Adv. Mater. 2000, 12, 1263.
(3) Park, N. G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem.
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(4) Hoyer, P. Langmuir 1996, 14, 1411.
(5) Moriguchi, I.; Maeda, H.; Teraoka, Y.; Kagawa, S. Chem. Mater.
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1995, 9, 857.
(7) Hoyer, P.; Masuda, H. J. Mater. Sci. Lett. 1996, 15, 1228.
(8) Fujishima, A.; Rao, N. T.; Tryk, D. A. Electrochim. Acta 2000,
45, 4683.
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1990, 94, 6435.
(10) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802.
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applications would also benefit from more uniform
porous supports that provide optimal flow and improve
efficiencies. The most common method used for assembling highly ordered porous structures is the template method.12-17 A major advantage of this method is
that the dimensions of the pores are set by the size of
the ordered template beads and the pore size can be
varied easily. In the generalized template method, 3-D
templates must first be fabricated, and then the voids
of the template must be infiltrated. Hereafter, we call
this generalized method the two-step method. It is wellknown that three challenging problems, namely, (1)
preparation of a high-quality template, (2) complete
infiltration of the voids of the template without damage
of the template itself when filled, and (3) minimization
of shrinkage after removal of the template, directly
influence the quality of the porous structure fabricated
by the two-step method. The biggest barrier for fabricating highly ordered porous structure using the twostep method is to resolve the three challenging problems
at the same time. Furthermore, one important limitation of the general two-step template method is that the
quality of the porous structure is strictly limited by the
quality of the template itself.
(12) Yin, J. S.; Wang, Z. L. Adv. Mater. 1999, 11, 469.
(13) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827.
(14) Vlasov, Y. A.; Yao, N.; Norris, D. J. Adv. Mater. 1999, 11,
165.
(15) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.;
Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630.
(16) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem.
Soc. 1999, 121, 7951.
(17) Velev, O. D.; Kaler E. Adv. Mater. 2000, 12, 531.
10.1021/cm0101576 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/01/2001
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Figure 1. Schematic of the assembly method. Sucking capillary pressure is directly used to drive the nanosized particles
to assemble themselves in the ordered template while the
template is being fabricated using the generalized vertical
deposition method.
Based on the advantages of several fabrication
methods,18-21 we have, more recently, developed a
simple method to solve all three problems discussed
above at the same time.22 By utilizing the local sucking
capillary pressure,23 ultrafine particles can be used
directly to assemble themselves in the voids of template
while the template is being assembled. Highly ordered
SiO2 porous structures with large areas and uniform
orientations have been fabricated successfully using this
method.22 In this paper, we report on the assembly of
highly ordered TiO2 porous structure using this simple
method and provide details on experimental parameters, such as temperature, humidity, and concentrations and concentration ratio of the colloid mixture. The
detailed mechanism of the simple method is also discussed in this paper. We believe that this work will
stimulate the wide application of highly ordered TiO2
porous structure films fabricated using nanosized TiO2
semiconductor particles.
Mechanism of the Simple Method
Our basic idea is to infill ultrafine particles (around
10 nm) into the voids of an ordered template directly
by means of capillary forces. Figure 1 shows the
schematic of the simple fabrication method used. The
substrate is dipped into a slurry that contains polystyrene spheres (several hundred nanometers) and the
ultrafine particles. In contrast to previously reported
methods,18-20 we employ a vertical deposition technique,21 not only because the technique itself can
fabricate high-quality opal, but also because this fabrication method can help realize our overall strategy.
Because of the small size of the TiO2 particles (around
10 nanometers) compared with that of the polystyrene
spheres (several hundred nanometers), the TiO2 particles are immersed in the liquid layer when the ordered
(18) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature
1999, 401, 548.
(19) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho,
K. M. Appl. Phys. Lett. 1999, 74, 3933.
(20) Subramania, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J.
Adv. Mater. 1999, 11, 1261.
(21) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem.
Mater. 1999, 11 2132.
(22) Meng, Q. B.; Gu, Z. Z.; Sato, O.; Fujishima, A. Appl. Phys. Lett.
2000, 77, 4313.
(23) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.;
Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183.
Meng et al.
template is assembling. With the evaporation of liquid
fluxes through the voids of the template, the nanosized
TiO2 particles can easily move and fill the voids of the
template as a result of a convective water flux that
carries along the particles toward the voids. A detail
discussion about this kind motion is provided by Denkov
et al.23 The distinctive feature of this method is that
the fabrication of the template and the infiltration of
the ultrafine particles are carried out at the same time.
Using this strategy, it is favorable for the ultrafine
particles to assemble themselves in the voids of the
template, as they need only move a short distance in
the voids before assembling. Therefore, the voids can
be completely filled by the ultrafine particles. Furthermore, in this simple fabrication process, the ultrafine
particles can immediately mend several kinds of defects
in the template when the template is assembling. It is
well-known that defects in fabricated opal are induced
by the random distribution of the diameters of the
spheres of the template. For example, when a larger
template sphere is introduced into the opal, sphere
vacancies must be created around it, and when a
smaller template sphere is introduced into the opal, a
large void is formed around it. These defects can easily
be mended by the ultrafine particles used in this simple
fabrication process. Therefore, we call this simple
method the cooperative assembly method. It is the
immediate mending in the cooperative assembly process
that greatly improves the fabrication of the template
itself. In addition, this mending can improve the mechanical stability of the composite film. It is thought
that the formation of cracks in opals is initiated at these
defects. The complete filling of the defects might therefore lead to a reduction in the generation of cracks in
fabricated opal, thus allowing for potential improvement
in the quality of the porous structures. Because of this
important advantage, the quality of the porous materials produced using this method is generally better than
that of the template itself, which is fabricated by a general vertical deposition method under the same experimental conditions, except for the absence of the ultrafine particles in the solution. Another advantage of this
method is that it is very simple and can be widely used.
Experimental Section
Materials and Substrates. The monodisperse polystyrene
particles used were obtained from Duke Scientific Corporation.
Titania dispersions were purchased from Catalysts and Chemical Industrial Co. Ltd. The mean size of the titania particles
used was about 13 nm. Ultrapure water (20.0 MΩ cm-1) was
used directly from a Milli-Q water system. The substrates used
in our experiments were glass slides, ITO glass, and quartz
glass. Glass vials (10 mL, Iuchi) were used as the experimental
cells.
Instrumentation. Scanning electron microscopy experiments were carried out on a JSM-5400 scanning microscope.
Transmission spectra were obtained using a Shimadzu UV3100PC spectrometer. An advanced AE-215 constant-temperature and -humidity chamber from the Toyo Seisakusho Co.
Ltd. was used to control the temperature and humidity in
these experiments.
Fabrication of the Highly Ordered Porous Structure
with Ultrafine TiO2 Semiconductor Particles. Prior to
use, all substrates and glass vials were soaked in a chromicsulfuric acid cleaning solution overnight, rinsed thoroughly
with ultrapure water, and dried in a stream of nitrogen. The
optimized concentrations of polystyrene and titania were about
0.08-0.2% by volume. The concentration can be varied from
Nanocrystalline TiO2 Semiconductors
Figure 2. Scanning electron micrographs at different magnifications illustrating the [111]-oriented regions of the TiO2
porous structure films on ITO glass. All of the scanning
electron micrographs in this work were taken with a JSM5400 scanning microscope.
0.05 to 0.5%, and the concentration ratio is about 1:1 for our
experimental conditions. After the full ultrasonic dispersion
of the mixture slurry of the polystyrene and titania colloids, a
clean substrate was then placed vertically into the mixture
slurry in a clean glass vial. We found that the concentrations
of the colloidal mixture, the concentration ratio between the
individual colloids, and monodispersity of the mixture colloid,
rather than the coating substrate, were the key parameters
in controlling the film deposition. Subsequently, the glass vial
was covered with a plastic film punched through with a few
small holes to keep out any external airflow. Then, the glass
vial was placed into a constant-temperature, constant-humidity chamber. The optimized temperature and humidity are 50
°C and 30%, respectively. The area of film fabricated under
these experimental conditions can reach about 0.5 cm2 in 24
h. The number of layer (or total thickness) of the films can be
controlled by the concentration of the slurry mixture or repeat
deposition times under the same experimental conditions.
After removal of the polystyrene spheres in the composite opal
by calcination (the composite opal was heated slowly to 450
°C in 7 h), a highly ordered three-dimensional TiO2 porous
structure was obtained.
Results and Discussion
Figure 2a and 2b shows the SEM images of TiO2
porous structures at different magnifications on ITO
glass. They show that a highly ordered hexagonal array
is produced. The hexagonal orientation indicates the
(111) plane of the fcc lattice. In Figure 2b, the holes
Chem. Mater., Vol. 14, No. 1, 2002 85
Figure 3. Scanning electron micrographs at different magnifications illustrating [100]-orientation regions of the TiO2
porous structure films on ITO glass.
connecting the pores are clearly visible. In particular,
a triangular pattern below each hole in the first layer
can be clearly observed. This is because, along the [111]
direction, each sphere of air rests on three neighboring
spheres below. The observation of a regular triangular
pattern over a large area by SEM strongly confirms the
perfect three-dimensional ordering of the structure.
Figure 2b shows that spherical vacancy defects in the
template have been completely filled with nanosized
TiO2 semiconductor particles and that one small hole
(indicated by an arrow) is surrounded by more TiO2
semiconductor particles. These experimental results
directly demonstrate the existence of cooperative assembly effects in the simple fabrication method. Although the same kind of phenomena can also been found
in the samples fabricated by the two-step method, which
fabricate the template first and then fill the voids of
template, they are formed by different processes, and
the effects on the final porous structure are different.
It is the immediate mending of these defects by this
simple cooperative assembly process that can greatly
improve the fabrication of the template itself and,
therefore, the quality of the final porous structure.
Figure 3a and 3b shows SEM images of the [100]direction zone of the sample at different magnifications.
From Figure 3a, the low-magnification SEM image, we
can see a highly ordered square array over a larger area.
From Figure 3b, the high-magnification SEM image of
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Chem. Mater., Vol. 14, No. 1, 2002
Figure 4. Typical scanning electron micrograph of the
coexistence zone of the [111] and [100] orientations of the TiO2
porous films on ITO glass.
Figure 5. Scanning electron micrograph of the special zone
of the TiO2 porous films on ITO glass.
the sample, we can clearly see the holes connecting the
pores and, inparticular, a regular cross pattern below
each hole of the first layer. This is because, along the
[100] direction, each sphere of air rests on the four
neighboring spheres below. The observation of a regular
cross pattern strongly confirms the perfect threedimensional structure in this zone of the film.
Figure 4 shows the [111]- and [100]-direction coexistence zone. At the boundaries between the [111]- and
[100]-direction zones, we can clearly see that the zones
are very well connected together. There are no transition
zones between them. Furthermore, in the [111]-direction
zone, we can clearly see a regular triangular pattern
below each hole of the first layer, and in the [100]direction zone, we can also clearly see a regular cross
pattern below each hole of the first layer. These regular
patterns directly confirm the three-dimensional structure of the different orientation zones.
Figure 5 shows one special pattern in some zones of
the sample fabricated by this method. In the [100]direction zone, a regular [111]-direction zone is relayed.
The two zones form a beautiful pattern over a larger
area. By carefully examining the boundary between the
two kinds of zones, we can also see a perfect connection
between them. From the different regular patterns
below the holes of the first layer in different zones, we
Meng et al.
Figure 6. Scanning electron micrograph showing a side view
of the TiO2 porous structures.
can confirm the three-dimensional structures of the
different zones.
Figure 6 shows a side view of the porous structure.
The cross section shows that the closely packed structure of the spheres of air extends uniformly over 10
layers produced during a single deposition. This side
view SEM image also demonstrates the 3-D structure
of the film fabricated by this simple method.
By carefully examining most TiO2 samples fabricated
using this method, we find that the [111]-direction zones
comprise 35% of the total area and that such zones can
extend to several square millimeters in size. The [ 100]direction zones are also about 35% of the total area, and
such zones can extend to nearly 10 mm2 in area. The
special pattern zones are about 30% of the whole area,
and such zones can extend to several square millimeters
in size. Moreover, within one orientation zone or between zones, the whole sample is planar, and the thickness is uniform. By comparing the structure of porous
TiO2 with that of SiO2,22 both of which were fabricated
by using the same method, we find that the domain sizes
of SiO2 are generally larger than those of TiO2. The
dominant direction of the SiO2 porous structures is [111]
(the [111]-direction regions comprise more than 75% of
the whole sample area). In contrast, the dominant direction of TiO2 porous structure is the [100] direction (the
total percent of the [100] direction and special pattern
area is more than 65% of the whole area of the TiO2
sample). Although we cannot provide a detailed explanation as to why the dominant directions are different
in the SiO2 and TiO2 porous structures at present, we
believe that the main reason might be the large difference between the sizes of the ultrafine particles used.
The size of the SiO2 particles (around 7 nm) is much
smaller than that of the TiO2 particles (over 12 nm).
Figure 7a shows transmission spectra of TiO2 films
with different pore sizes fabricated by this simple
method. The first-order diffraction peaks can be clearly
seen in the transmittance spectra. It should be noted
here that the spot size of the light beam in the
transmission experiments is about 12 × 2 mm2. The
results further confirm that the porous structures are
highly ordered over a large area. The diffraction peak
positions are at 576, 698, 735, and 986 nm for ordered
pore structures of different pore sizes. Figure 7b shows
Nanocrystalline TiO2 Semiconductors
Chem. Mater., Vol. 14, No. 1, 2002 87
Table 1. Calculated and SEM-Measured Values of the
Pore Size of the TiO2 Porous Structures Fabricated with
Polystyrene Spheres of Different Diameters
neffa
closest-packed
TiO2 particles
2.0804
a
pore diameter (nm)
whole
porous
structure
1.2809
peak
position
(nm)
calculated
by Bragg
law
measured
by SEM
579
698
735
986
319.63
385.32
405.75
544.31
326
388
410
555
neff means the effective refractive index.
light and the surface normal of the sample, and d is
the distance between parallel lattice planes. In this
calculation
d ) d100 ) (2.0)1/2R
Figure 7. (a) Transmission spectrum of TiO2 porous structures with different pore sizes. (b) Photonic band gap position
as a function of the pore size. The straight line is a linear fit
to the peak wavelengths. Very good scaling with the pore size
is observed.
the linear relationship of the PBG position to the pore
size. This indicates the intrinsic feature of the porous
structure. The results demonstrate that highly ordered
TiO2 films with different pore sizes have been successfully fabricated using this method.
Table 1 lists the effective refractive indexes of the
closest-packed films of ultrafine TiO2 particles and of
the ordered TiO2 porous structures, the calculated pore
diameters, and the diameters measured by SEM. In the
calculation of the pore diameters, we use the Bragg
equation
λmax ) 2dna sin θ
(1)
where na is the average refractive index of the photonic
crystalline assembly, θ is the angle between the incident
(2)
where R is the radius of spheres of air in the porous
structures, because the dominant direction of TiO2
porous structure fabricated under our experimental
conditions is the [100] direction. The value of the
refractive index for the TiO2 particles used in this
calculation is 2.46, because the TiO2 ultrafine particle
sample contains about 96% TiO2 and 4% SiO2 (the
average refractive index of TiO2 sample used in our
experiment is 2.46 ) 2.5 × 96% + 1.5 × 4%). We also
assumed that the TiO2 particles are in the closestpacked state (occupying 74 vol % of the total voids) in
the voids of the template in this cooperative assembly
process. A comparison of the values of the diameter of
the spheres of air with those measured by SEM indicates good agreement between them. This further
demonstrates that a highly ordered porous structure
exists in three dimensions and confirms the closestpacked state of the ultrafine particles in the voids of
the template. In other words, the ultrafine particles can
completely fill the voids of the template in the cooperative assembly process. Comparing the pore diameter
with that of the polystyrene measured by SEM, the
shrinkage is between 4.2 and 6.8% for all samples.
These values are slightly larger than those (shrinkage
is between 3.2 and 4.6%) of SiO2 porous films.22 The
main reason for this difference is the larger size of the
TiO2 used.
Conclusions
Using a very simple method, we have successfully
assembled large-area, highly ordered porous structures
using nanosized TiO2 semiconductor particles. At present,
we cannot strictly control the orientation of the porous
structure in the whole sample to obtain a single orientation. However, the areas exhibiting a single orientation
zone (on the order of several square millimeters) are
large enough for specialized applications in optical
science. Furthermore, our experimental results demonstrate that the smaller the ultrafine particle size, the
better the results obtained for controlling the orientation
of the porous structure fabricated and the easier the
fabrication of a larger single domain using this method.
The theoretical calculations confirm the existence of the
closest-packed state of the ultrafine particles in the
voids of the template. This special structure combines
the advantages of nanosized TiO2 particles with the
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Chem. Mater., Vol. 14, No. 1, 2002
special optical properties of the structure itself. We
believe that this kind of highly ordered TiO2 porous
structure, which were fabricated using nanosized TiO2
particles, can be widely applied in the areas of electrocatalysts, sensors, high-performance photocatalysts,
photonic crystals, and so on. As an additional note, we
Meng et al.
would like emphasis here that a high-quality 3-D porous
structure can easily be fabricated by using this method
with semiconductor quantum dots (whose size is around
3 nm).
CM0101576