Microstructural modifications in macroporous oxides prepared via latex templating: Synthesis and thermal stability of porous microstructure

Article abstract of DOI:10.1111/j.1551-2916.2006.00996.x

Titanium and zirconium oxides with interconnected macropores presenting well-defined pore sizes and controlled three-dimensional ordering were prepared using a latex templating method. Polystyrene spheres were used as pore templates, whereas metal alkoxides were used as oxide precursors. The final thermal treatment at different temperatures showed macroporous microstructure stabilities up to 600°C. At higher temperatures, a collapse of the macroporous microstructure and sintering were observed, as well as phase transformations in the oxides structures during the calcination process, as shown by Raman spectroscopy and X-ray diffraction techniques.

Full text of DOI:10.1111/j.1551-2916.2006.00996.x

J. Am. Ceram. Soc., 89 [7] 2226–2231 (2006)
DOI: 10.1111/j.1551-2916.2006.00996.x
r 2006 The American Ceramic Society
ournal
J
Microstructural Modifications in Macroporous Oxides Prepared Via
Latex Templating: Synthesis and Thermal Stability of Porous
Microstructure
Carla Verissimow and Oswaldo L. Alves
LQES-Laboratorio de Quımica do Estado Solido, Instituto de Quımica, Universidade Estadual de Campinas,
´ ´ ´ ´
UNICAMP, 13084-971 Campinas, SP, Brazil
Titanium and zirconium oxides with interconnected macropores
presenting well-defined pore sizes and controlled three-dimen-
sional ordering were prepared using a latex templating method.
Polystyrene spheres were used as pore templates, whereas metal
alkoxides were used as oxide precursors. The final thermal
treatment at different temperatures showed macroporous micro-
structure stabilities up to 6001C. At higher temperatures, a col-
lapse of the macroporous microstructure and sintering were
observed, as well as phase transformations in the oxides struc-
tures during the calcination process, as shown by Raman spec-
troscopy and X-ray diffraction techniques.
has been an easier and more versatile method to obtain these
macroporous matrices.
The idea of this experimental approach is to use monodis-
perse particles such as self-assembled latex spheres to form an
ordered three-dimensional array. A precursor solution of the
desired final solid is added to the template and it infiltrates into
voids between the latex spheres. Wet chemistry techniques such
as the sol–gel process are then used to prepare the macroporous
solid material, so that it captures the self-organization from the
colloidal crystals via solidification of the continuous medium by
a sol–gel transition. The template can be removed by calcination
leading to a macroporous structure. In most cases, a final ther-
mal treatment is necessary not just to eliminate the organic tem-
plate but also to produce a solid having a desired crystalline
phase. This stage is very important as the macroporous micro-
structure associated with a specific crystalline phase can provide
the properties of the material leading to its final application.
In this paper, macroporous titanium oxide (TiO2) and zirco-
nium oxide (ZrO2) were prepared using latex spheres as pore
templates, and the influence of the thermal treatment tempera-
ture on the stability of the macroporous microstructure as well
as the phase transformations during the calcination process are
reported.
I. Introduction
ERAMIC macroporous materials with pore sizes between 50
C
nm and 10 mm, a narrow diameter distribution, and three-
dimensional ordering of pores can present very interesting prop-
erties, leading to several applications such as in separation proc-
esses, catalysis, adsorbents, insulators, lightweight structural
materials, and low dielectric constant materials.1–4 Moreover,
macroporous materials can also present unusual optical prop-
erties, permitting preparation of optical devices like photonic
crystals with optical bandgaps.1–8 However, until recently, there
has been no general method available for producing such ma-
terials.1,2 In the last few years, a new approach using colloidal
crystals as self-assembled templates for macropores has been
used to prepare macroporous materials with well-defined pore
sizes and controlled three-dimensional ordering.1–4,6–19
II. Experimental Procedures
(1) Materials
All chemicals were used without further purification. The chem-
icals used were titanium tetraisopropoxide (97%, Sigma-Aldrich,
St. Louis, MO), zirconium n-propoxide (23%–28% free alcohol,
Strem Chemical, Inc., Newburyport, MA), and 2,4-pentanedi-
one (499,5%, Merck, Whitehouse Station, NJ). A monodis-
perse polystyrene sphere suspension (380 nm diameter, 10 wt%)
was ordered from Bangs Laboratories Inc. (Fishers, IN).
Colloidal crystals can be considered as monodisperse particles
self-assembled to form an ordered three-dimensional array.
These artificial opals, as they are known, obtained from emul-
sion droplets or latex spheres, have been used as pore temp-
lates with appropriate dimensions for the preparation of
ceramic macroporous materials, as shown by Imhof and
Pine1,12 Velev et al.,2 Holland et al.,3,11 Wijnhoven and Voss4
and other authors.6–10,13–19
The emulsion templating method requires several efforts in
the preparation of monodisperse droplets, employing a tedious
fractionation procedure.20 Moreover, the latex spheres present
lower values of polydispersity when compared with emulsion
droplets and larger domain sizes of well-ordered spheres.21 In
general, the use of colloidal crystals prepared from latex spheres
(2) Preparation of Macroporous TiO2 and ZrO2
An aqueous suspension (0.3 mL) of 380 nm polystyrene spheres
(10 wt%) was centrifuged at 3000 rpm for 3 h and dried under
vacuum for 5 h to form latex macrocrystals, further used as a
macropore template.
TiO2 was prepared using titanium tetraisopropoxide treated
with two equivalents of chelating agent 2,4-pentanedione and
diluted in 1:1 v/v isopropanol. Later, 0.1 mL of this solution was
added dropwise to the latex macrocrystal previously placed over
an absorbent surface. TiO2 was also prepared by the direct ad-
dition of pure titanium tetraisopropoxide onto the latex macro-
crystal kept inside the centrifuge tube.
G. Franks—contributing editor
ZrO2 was prepared using zirconium n-propoxide diluted in
1:1 v/v isopropanol. This solution was added dropwise to the
latex macrocrystals previously placed over an absorbent surface.
After permeation of oxide precursor solutions into the voids
between the polystyrene spheres and solidification into an inor-
ganic network upon drying under atmospheric conditions for 24 h,
the samples were thermally treated at 6001, 8001, or 10001C in air.
Manuscript No. 20991. Received September 13, 2005; approved January 26, 2006.
This work was financially supported by the FAPESP, under Grant No. 99/06491-7, and
Millennium Institute for Complex Materials (PADCT/CNPq).
Presented at the Royal Society of Chemistry Materials Forum: Materials Discussion 5—
Porous Materials and Molecular Intercalation, Madrid, Spain, September 2002 (Winner of
the prize for Best Poster).
Based on the thesis submitted by C. Verissimo for the Ph.D. degree in chemistry, Campinas
State University, Campinas, SP, Brazil, 2003.
wAuthor to whom correspondence should be addressed. e-mail: carla@iqm.unicamp.br
2226

July 2006
Microstructural Modifications in Macroporous Oxides Prepared Via Latex Templating
2227
(3) Materials Characterization
Microstructure characterizations were performed by Raman
spectroscopy (model System 3000 Raman Imaging Microscope,
Renishaw, New Mills, UK) using the 632.8 nm line of a He–Ne
laser, and scanning electron microscopy (SEM; models JSM
6360LV and JSM 6340F, JEOL, Tokyo, Japan). SEM samples
were placed over an adhesive conductive carbon tape attached
to a metal stub and were coated with carbon and Au/Pd films,
respectively. Powder X-ray diffraction patterns (XRD) were
carried out at the XRD beamline of the Brazilian Synchrotron
Light Laboratory (LNLS, Campinas, Brazil) using a Si(111)
double crystal to monochromatize the incident X-ray beam
˚
(l 5 1.5405 A), at room temperature. Powder samples were
filled into a thin-walled glass capillary (7 mm) with 1 mm diam-
eter.
III. Results and Discussion
(1) Latex Macrocrystal
Different methods have been used to organize the latex spheres
into three-dimensional arrays, such as sedimentation, filtration,
and centrifugation. In this work, latex macrocrystals were pre-
pared by three-dimensional ordering of polystyrene spheres us-
ing centrifugation. This process led to iridescent sediments on
the bottom of the centrifuge tubes that were dried under vacuum
before use.
SEM images from latex macrocrystals showed high ordering
degree of polystyrene spheres after the centrifugation process.
The presence of domains separated by boundaries caused by
packing failures was also observed, as well as the formation of
domains with different packing: hexagonal (H) and tetragonal
(T) (Fig. 1(A)). The presence of hexagonal packing—each
sphere surrounded by six other spheres in the same plane—sug-
gest two different structures for the latex crystallization since
hexagonal packing can be related to the face-centered cubic (fcc)
structure (111) plane and hcp structure (001) plane. The tetrago-
nal array also observed in the latex macrocrystal is related to the
fcc structure (100) plane, indicating that the latex sphere packing
resulted in a fcc structure (Fig. 1(B)).11,22,23 The fcc array has
been observed in several works where latex or silica spheres were
three dimensionally ordered, and computer simulations have
suggested that the fcc array is more stable than the hcp ar-
ray.11,22 The co-existence of both fcc and hcp arrays has also
been observed in latex macrocrystals, in addition to a bcc array
under specific conditions.11,23
Fig. 1. Scanning electron microscopy images of the latex macrocrystal:
(A) high ordering of polystyrene spheres: different domains with hex-
agonal (H) and tetragonal (T) packing; (B) a surface fracture presenting
(111) and (001) planes of the face centered cubic structure.
ation of small volumes of the precursor solution through the
latex macrocrystal, with the excess solution being absorbed by
the surface. Scanning electron micrographs obtained after the
infiltration of the alkoxide solution inside the latex macrocrystal
voids showed maintenance of polystyrene sphere ordering
(Fig. 2(A)). The sample was then thermally treated at 6001C
for 8 h, and the SEM image shows the hexagonal packing of the
latex spheres replicated in the pore arrangement of the inorganic
material (Fig. 2(B)). A high three-dimensional organization of
the macroporosity and a narrow distribution of pore diameters
(ranging from 150–170 nm, indicating a shrinkage of 55%–
60%) could be observed. The solid microstructure thus obtained
constituted interconnected macropores in a well-ordered hexag-
onal arrangement (Fig. 2(C)).
Latex macrocrystals were used as macropore templates to
prepare macroporous titanium and zirconium oxides, as ca.
74% of the volume was occupied by latex spheres and 26%
was available to be filled by inorganic precursor solutions.
(2) Macroporous Oxides
Different experimental approaches were used to prepare mac-
roporous oxides via the latex templating method, as shown in
the literature.2–4,6–11,13–19 In this work, latex macrocrystals were
embedded in alkoxide precursor solutions using different pro-
cedures, as described in the experimental section.
Initially, pure titanium tetraisopropoxide was added drop-
wise inside the centrifuge tube containing the ordered latex
spheres (latex macrocrystal). A porous oxide was not obtained
by this procedure after thermal treatment at 6001C for 8 h, as
observed by SEM images (figure not shown). The absence of
pores in this TiO2 sample is probably related to the high reac-
tivity of pure titanium tetraisopropoxide, which prevented its
permeation through the latex macrocrystal by capillary forces
before hydrolysis.
When a latex macrocrystal/oxide precursor nanocomposite
was formed, the drying process was started with solvent evap-
oration. Alkoxide hydrolysis was also taking place because of
the natural humidity of air and the precursor condenses, leading
to solidification of the material into an inorganic network
around the template. During the thermal treatment, the latex
spheres decomposed and a densification of the solid was ob-
served, caused by condensation of the hydroxyl groups, forming
an oxide network that resulted in a shrinkage of the pore diam-
eters when compared with the polystyrene sphere size. Usually,
shrinkages around 17%–35% have been reported in the litera-
ture,3,6,7,11,15,16 which indicates that the special conditions in our
experiments led to greater shrinkage. The chelation process al-
lowed alkoxide penetration inside the latex macrocrystal struc-
ture, so as to preserve the three-dimensional ordering of the
latex spheres before hydrolysis, leading to macroporous TiO2.
ZrO2 was also prepared by using the procedure described
above. Scanning electron micrographs of latex macrocrystal ob-
tained after the permeation of the alkoxide solution showed the
To decrease its reactivity, titanium alkoxide was treated with
the chelating agent 2,4-pentanedione. In this approach, a small
volume of the chelated titanium tetraisopropoxide solution was
added dropwise onto latex macrocrystal pieces previously placed
over an absorbent surface. The idea was to promote the perme-

2228
Journal of the American Ceramic Society—Verissimo and Alves
Vol. 89, No. 7
Fig. 3. Scanning electron microscopy images: (A) latex macrocrystal/
zirconium oxide (ZrO2) precursor nanocomposite before the thermal
treatment; (B) and (C) micrographs at different magnifications of the
macroporous ZrO2 obtained after the calcination at 6001C for 8 h in air.
Fig. 2. Scanning electron microscopy images: (A) latex macrocrystal/
titanium oxide (TiO2) precursor nanocomposite before the thermal treat-
ment. (B) and (C) TiO2 sample prepared by adding the titanium alkoxide
solution onto latex macrocrystal pieces placed over an absorbent surface
after the calcination at 6001C for 8 h in air.
preservation of sphere ordering and a visible coating formed by
the initial hydrolysis of the ZrO2 precursor (Fig. 3(A)). After
thermal treatment at 6001C for 8 h, the SEM image showed
large domains with a high pore organization (Figs. 3(B) and
(C)). As observed previously for TiO2 samples, ZrO2 material
also presented interconnected macropores with diameters be-
tween 160 and 190 nm that demonstrated a narrow distribution
and shrinkage around 50%–60%.
SEM images allowed the visualization of the oxide micro-
structure constituted by interconnected macropores in a well-
ordered hexagonal arrangement–as shown by Figs. 2(C) and
3(C)–and the evaluation of the different experimental proce-
dures to obtain macroporous templated ceramic solids.
The thermal stability of the macroporous microstructures was
investigated by thermal treatment of the oxide samples at tem-
peratures higher than 6001C. For TiO2 treated at 8001C for 1 h,
SEM micrographs showed domains with different microstruc-
tures (Fig. 4). Figure 4(A) shows the loss of pore organization,
the beginning of macroporous microstructure disruption, with
the cracking of pore walls, as indicated by arrows, and TiO2
sphere growth. In the same sample, some regions were consti-
tuted only by TiO2 spheres, as shown in Fig. 4(B). A pattern in
the disposition of TiO2 spheres was observed, indicating a hex-
agonal distribution of these spheres. Taking into account the
slightly hexagonal shape of the pores, as observed in Fig. 2(C),
TiO2 spheres were formed in the vertices of the pores. Sintering
of the TiO2 spheres was also observed, as shown in Fig. 4(C),
which led to agglomeration and neck formation. In the case of
thermal treatment at 10001C for 1 h (Fig. 5), the macroporous
microstructure was completely damaged and microparticles pre-

July 2006
Microstructural Modifications in Macroporous Oxides Prepared Via Latex Templating
2229
Fig. 5. Scanning electron microscopy image of the titanium oxide ther-
mally treated at 10001C.
Fig. 4. Scanning electron microscopy images of the titanium oxide
(TiO2) thermally treated at 8001C showing a disruption of the macro-
porous microstructure: (A) crack of pore walls (arrows); (B) TiO2 sphere
growth; (C) sintering.
sented geometric shapes and sizes between 0.1 and 2.5 mm. ZrO2
samples treated at 8001 and 10001C exhibited the same behavior
as observed for the TiO2 (Fig. 6).
According to our results, thermal treatments at temperatures
above 6001C led to a disruption of the macroporous micro-
structure, to spherical particle formation, and to sintering, as
shown by SEM images. Possibly, temperatures higher than
6001C promoted a diffusion of the constituent material of the
macroporous walls, causing this material to reassemble into a
spherical shape due to energetic factors. The driving force for
this behavior could be related to the reduction in the surface
energy achieved by the new material microstructure formed by
spherical particles that led to a smaller surface area.
Fig. 6. Scanning electron microscopy images of the zirconium oxide
(ZrO2) thermally treated at 8001C showing a disruption of the macro-
porous microstructure: (A) crack of pore walls (arrows); (B) ZrO2 sphere
growth; (C) sintering.
Raman spectroscopy and XRD were used to investigate oxide
structures after thermal treatments at different temperatures.
TiO2 is known to exist in three distinct crystallographic forms:

2230
Journal of the American Ceramic Society—Verissimo and Alves
Vol. 89, No. 7
M
R
R
M
1000 °C
M
T
M
M
M
M
M
M
M
M
M
M
M
M
1000 °C
M
M
M
M
A
M
M
800 °C
M
T
A
A
M
M
A
A
T
T
M
M
A
800 °C
600 °C
600 °C
M
M
T
M
20
25
30
35
40
45
50
55
60
250 300 350 400 450 500 550 600 650 700 750 800
Wavenumber (cm–1
2θ (degree)
)
Fig. 9. X-ray diffraction pattern of the zirconium oxide samples ther-
mally treated at 6001, 8001, and 10001C (T, Tetragonal phase; M, Mono-
clinic phase).
Fig. 7. Raman spectra of the titanium oxide samples thermally treated
at 6001, 8001, and 10001C (A, Anatase peaks; R, Rutile peaks).
anatase (tetragonal), brookite (orthorhombic), and rutile (te-
tragonal). Both anatase and brookite phases can be converted
irreversibly into the rutile form when treated at high tempera-
tures. The temperature of the phase transition is sensitive to
several factors, such as the experimental procedure used to ob-
tain the TiO2, reagents, the size of particles, etc. As a general
trend, this phase transition begins at around 4001–6001C and it
is complete between 8001 and 10001C.24–28 As shown by the
Raman spectroscopy and XRD pattern of TiO2 samples treated
at different temperatures (6001, 8001, and 10001C), the phase
transition from an anatase to a rutile structure was promoted
when the temperature was increased from 6001 to 10001C (Figs.
7 and 8, respectively). Raman spectrum for TiO2 sample heated
at 6001C presented peaks near 396, 517, and 637 cmÀ1, which
were related to anatase structure vibrations (Fig. 7). XRD data
also corroborated the formation of the anatase phase up to
6001C (Fig. 8). Thermal treatment of the TiO2 at 8001C led to a
higher crystallization of the anatase structure, as can be ob-
served by the presence of well-defined peaks centered at 25.31,
37.01, 37.81, 38.61, 48.11, 54.01, and 55.11 (2y) in the XRD pat-
tern related to (101), (103), (004), (112), (200), (105), and (211)
planes of the anatase phase (Fig. 8).29 The initial formation of
rutile structure from XRD data was also observed at 8001C.
When the temperature was increased to 10001C, the Raman
spectrum presented only peaks correlated to the rutile structure
at 446 and 610 cmÀ1 (Fig. 7). XRD pattern obtained for this
sample presented well-defined peaks centered at 27.51, 36.21,
39.31, 41.31, 44.21, 54.51, and 56.81 (2y), corresponding to (110),
(101), (200), (111), (210), (211), and (220) planes of the rulite
structure, which corroborates the complete conversion of the
anatase to the rutile phase (Fig. 8).29
In the case of ZrO2, a similar behavior was observed. The
material initially formed at 6001C presented a tetragonal struc-
ture as the majority phase (Fig. 9). At higher temperatures,
a conversion of tetragonal into monoclinic phase took place,
resulting in grain growth and macroporous microstructure
disruption.15,16,30
R
R
Subramanian et al.31 observed that the grain growth of TiO2
prepared by latex templating at temperatures higher than 8001C
resulted in macropore collapse. These authors state that the
continuous growth of grains, responsible for macroporous
microstructure disruption, has not yet been understood com-
pletely. Nevertheless, it is known that, in systems where a phase
transition occurs through a process involving nuclei formation
and further growth, as in the case of TiO2, there is an increase in
the tendency for sintering. This phenomenon is known as the
Hedvall effect: during transformation, the breaking and forma-
tion of bonds increase the atomic motions, increasing mass dif-
fusion transport. In this way, the nucleation and growth of the
rutile phase are responsible for the increase in the diffusion
mass, according to the Hedvall effect, favoring macropore col-
lapse and sintering.32,33
R
1000 °C
R
R
R
R
A
800 °C
600 °C
A
A
A
A
A
A
R
R
A A
R
R
R
A
A
A
20
25
30
35
40
45
50
55
60
IV. Conclusions
2θ (degree)
Different experimental procedures can lead to the formation of
macroporous oxides presenting interconnected macropores with
a well-defined pore size and controlled three-dimensional order-
Fig. 8. X-ray diffraction pattern of the titanium oxide samples ther-
mally treated at 6001, 8001, and 10001C (A, Anatase peaks; R, Rutile
peaks).

July 2006
Microstructural Modifications in Macroporous Oxides Prepared Via Latex Templating
2231
13Y. Hotta, P. C. A. Alberius, and L. Bergstrom, ‘‘Coated Polystyrene Particles
¨
ing from latex templating method. Herein, TiO2 and ZrO2 ma-
terials were obtained from the latex templating method.
Unusual shrinkage of the pore volume was also observed for
the oxide materials after thermal treatment at 6001C. Such a
result suggests that different experimental procedures can lead
to different shrinkages, allowing the use of templates consider-
ably larger than the final pore size desired.
An investigation of the thermal stability of the oxide macro-
porous microstructures showed the disruption of macropores at
temperatures of 8001C associated with oxide phase transitions.
The macroporous microstructure maintained at 6001C makes
these materials very interesting for applications in processes at
relatively high temperatures.
as Templates for Ordered Macroporous Silica Structures with Controlled Wall
Thickness,’’ J. Mater. Chem., 13 [3] 496–501 (2003).
14R. C. Schroden, M. Al-Daous, S. Sokolov, B. J. Melde, J. C. Lytle, A. Stein,
M. C. Carbajo, J. T. Fernandez, and E. Rodrıguez, ‘‘Hybrid Macroporous
´ ´
Materials for Heavy Metal Ion Adsorption,’’ J. Mater. Chem., 12 [11] 3261–7
(2002).
15W. Dong, H. J. Bongard, and F. Marlow, ‘‘New Type of Inverse Opals:
Titania with Skeleton Structure,’’ Chem. Mater., 15 [2] 568–74 (2003).
16M. A. Al-Daous and A. Stein, ‘‘Preparation and Catalytic Evaluation of
Macroporous Crystalline Sulfated Zirconium Dioxide Templated with Colloidal
Crystals,’’ Chem. Mater., 15 [13] 2638–45 (2003).
17L. Bechger and W. L. Vos, ‘‘Homogeneity of Oxide Air-Sphere Crystals from
Millimeter to 100-nm Length Scales: A Probe for Macroporous Photonic Crystal
Formation,’’ Chem. Mater., 16 [12] 2425–32 (2004).
18Y. N. Kim, S. J. Kim, E. K. Lee, E. O. Chi, N. H. Hur, and C. S. Hong,
‘‘Large Magnetoresistance in Three Dimensionally Ordered Macroporous Perovs-
kite Manganites Prepared by a Colloidal Templating Method,’’ J. Mater. Chem.,
14 [11] 1774–7 (2004).
19Z. Zhou, X. Bao, and X. S. Zhao, ‘‘Synthesis, Characterization and Optical
Properties of Ordered Macroporous Organosilicas,’’ Chem. Commum., [12] 1376–7
(2004).
Acknowledgments
CV acknowledges financial support from the Brazilian Agency FAPESP (99/
06491-7). The authors would like to thank LNLS—National Synchrotron Light
Laboratory, Brazil—for XRD measurements and LME/LNLS, Campinas, Brazil,
for the use of the scanning electron microscope. This is a contribution of
PRONEX and Millenium Institute of Complex Materials (IM2C) programs.
20J. Bibette, ‘‘Depletion Interactions and Fractionated Crystallization for Poly-
disperse Emulsion Purification,’’ J. Colloid Interface Sci., 147 [2] 474–8 (1991).
21V. N. Manoharan, A. Imhof, J. D. Thorne, and D. J. Pine, ‘‘Photonic Crystals
from Emulsion Templates,’’ Adv. Mater., 13 [6] 447–50 (2001).
22H. Mı
Mifsud, and V. Forne
´
guez, F. Meseguer, C. Lo
´
´
pez, A. Blanco, J. S. Moya, J. Requena, A.
s, ‘‘Control of the Photonic Crystal Properties of Fcc-Packed
Submicrometer SiO2 Spheres by Sintering,’’ Adv. Mater., 10 [6] 480 (1998).
23A. H. Cardoso, C. A. P. Leite, M. E. D. Zaniquelli, and F. Galembeck,
‘‘Easy Polymer Latex Self-Assembly and Colloidal Crystal Formation: The Case
of Poly[Styrene-co-(2-Hydroxyethyl Methacrylate)],’’ Colloids Surf. A, 144 [1–3]
207–17 (1998).
References
1A. Imhof and D. J. Pine, ‘‘Ordered Macroporous Materials by Emulsion
Templating,’’ Nature, 389 [6654] 948–51 (1997).
2O. D. Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, ‘‘Porous Silica Via
Colloidal Crystallization,’’ Nature, 389 [6650] 447–8 (1997).
24K. C. Song and S. E. Pratsinis, ‘‘Synthesis of Bimodally Porous Titania
Powders by Hydrolysis of Titanium Tetraisopropoxide,’’ J. Mater. Res., 15 [11]
2322–9 (2000).
3B. T. Holland, C. F. Blanford, and A. Stein, ‘‘Synthesis of Macroporous
Minerals with Highly Ordered Three-Dimensional Arrays of Spheroidal Voids,’’
Science, 281 [5376] 538–40 (1998).
25C. C. Wang and J. Y. Ying, ‘‘Sol–Gel Synthesis and Hydrothermal Processing
of Anatase and Rutile Titania Nanocrystals,’’ Chem. Mater., 11 [11] 3113–20
(1999).
4J. E. G. J. Wijnhoven and W. L. Vos, ‘‘Preparation of Photonic Crystals made
of Air Spheres in Titania,’’ Science, 281 [5378] 802–4 (1998).
5J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, ‘‘Photonic Crystals: Putting
a New Twist on Light,’’ Nature, 386 [6621] 143–9 (1997).
26L. J. Alemany, M. A. Banares, E. Pardo, F. Martin-Jimenez, and J. M. Blasco,
‘‘Morphological and Structural Characterization of a Titanium Dioxide System,’’
Mater. Charact., 44 [3] 271–5 (2000).
6G. Subramania, K. Constant, R. Biswas, M. M. Sigalas, and K. M. Ho,
‘‘Optical Photonic Crystals Fabricated from Colloidal Systems,’’ Appl. Phys. Lett.,
74 [26] 3933–5 (1999).
27D. Bersani, G. Antonioli, P. P. Lottici, and T. Lopez, ‘‘Raman Study of
Nanosized Titania Prepared by Sol–Gel Route,’’ J. Non-Cryst. Solids, 234, 175–81
(1998).
7A. Richel, N. P. Johnson, and D. W. McComb, ‘‘Observation of Bragg
Reflection in Photonic Crystals Synthesized from Air Spheres in a Titania
Matrix,’’ Appl. Phys. Lett., 76 [14] 1816–8 (2000).
28U. Balachandran and N. G. Eror, ‘‘Raman-Spectra of Titanium-Dioxide,’’
J. Solid State Chem., 42 [3] 276–82 (1982).
29Joint Committee on Powder Diffraction Standards (JCPDS), 21-1272
(anatase) and 21-1276 (rutile), 1997.
8H. Mı
guez, F. Meseguer, C. Lopez, F. Lopez-Tejeira, and J. Sanchez-Dehesa,
´ ´ ´ ´
‘‘Synthesis and Photonic Bandgap Characterization of Polymer Inverse Opals,’’
Adv. Mater., 13 [6] 393–6 (2001).
30B. Ksapabutr, E. Gulari, and S. Wongkasemjit, ‘‘Preparation of Zirconia
Powders by Sol–Gel Route of Sodium Glycozirconate Complex,’’ Powder Tech-
nol., 148, 11 (2004).
9Y. Xia, B. Gates, and Z.-Y. Li, ‘‘Self-Assembly Approaches to Three-
Dimensional Photonic Crystals,’’ Adv. Mater., 13 [6] 409–13 (2001).
10D. J. Norris and Y. A. Vlasov, ‘‘Chemical Approaches to Three-Dimensional
Semiconductor Photonic Crystals,’’ Adv. Mater., 13 [6] 371–6 (2001).
11B. T. Holland, C. F. Blanford, T. Do, and A. Stein, ‘‘Synthesis of Highly
Ordered, Three-Dimensional, Macroporous Structures of Amorphous or
Crystalline Inorganic Oxides, Phosphates, and Hybrid Composites,’’ Chem.
Mater., 11 [3] 795–805 (1999).
31G. Subramanian, V. N. Manoharan, J. D. Thorne, and D. J. Pine, ‘‘Ordered
Macroporous Materials by Colloidal Assembly: A Possible Route to Photonic
Bandgap Materials,’’ Adv. Mater., 11 [15] 1261 (1999).
32J. Nair, P. Nair, F. Mizukami, Y. Oosawa, and T. Okubo, ‘‘Microstructure
and Phase Transformation Behavior of Doped Nanostructured Titania,’’ Mater.
Res. Bull., 34 [8] 1275–90 (1999).
33K.-N. P. Kumar, ‘‘Porous Nanocomposites as Catalyst Supports. 1. 2nd-
Phase Stabilization, Thermal-Stability and Anatase-to-Rutile Transformation in
12A. Imhof and D. J. Pine, ‘‘Uniform Macroporous Ceramics and Plastics by
Emulsion Templating,’’ Adv. Mater., 10 [9] 697–700 (1998).
Titania-Alumina Nanocomposites,’’ Appl. Catal. A, 119 [1] 163–83.
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