A practical technique for the fabrication of highly ordered macroporous structures of inorganic oxides

Article abstract of DOI:10.1016/j.materresbull.2005.08.021

Well-defined macroporous ceramics consisting of SiO₂, TiO ₂ and ZrO₂ have been fabricated via a template-assisted colloidal processing technique. Close-packed polymer spheres were first prepared as a template using centrifugation or gravitational sedimentation, followed by infiltration with alkoxide precursors. The centrifugation should be preferred because it is a less time-consuming process and the materials are better ordered. The removal of the template beads was achieved by calcination of the organic-inorganic hybrids at appropriate temperatures, yielding well-ordered macroporous ceramics. The arrangement of the porous structures could be changing the preparation of the packed polymer templates. Some novel arrangements of macropores were obtained in these macroporous ceramics: a simple square-packed arrangement for SiO₂, the coexistence of hexagonal close-packed and simple close-packed arrangements for TiO ₂, and face-centered cubic packed arrangement for ZrO₂. The resulting highly structured ceramics could have applications in areas ranging from quantum electronics to photocatalysis and battery materials.

Full text of DOI:10.1016/j.materresbull.2005.08.021

Materials Research Bulletin 41 (2006) 268–273
www.elsevier.com/locate/matresbu
A practical technique for the fabrication of highly ordered
macroporous structures of inorganic oxides
*
Fengqiu Tang, Tetsuo Uchikoshi, Yoshio Sakka
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Received 26 February 2005; received in revised form 9 August 2005; accepted 26 August 2005
Available online 19 September 2005
Abstract
Well-defined macroporous ceramics consisting of SiO
2
2 2
, TiO and ZrO have been fabricated via a template-assisted colloidal
processing technique. Close-packed polymer spheres were first prepared as a template using centrifugation or gravitational
sedimentation, followed by infiltration with alkoxide precursors. The centrifugation should be preferred because it is a less time-
consuming process and the materials are better ordered. The removal of the template beads was achieved by calcination of the
organic–inorganic hybrids at appropriate temperatures, yielding well-ordered macroporous ceramics. The arrangement of the
porous structures could be changing the preparation of the packed polymer templates. Some novel arrangements of macropores
were obtained in these macroporous ceramics: a simple square-packed arrangement for SiO , the coexistence of hexagonal close-
2
packed and simple close-packed arrangements for TiO
structured ceramics could have applications in areas ranging from quantum electronics to photocatalysis and battery materials.
2005 Elsevier Ltd. All rights reserved.
2 2
, and face-centered cubic packed arrangement for ZrO . The resulting highly
#
Keywords: A. Ceramics; A. Oxide; B. Sol–gel chemistry
1
. Introduction
The fabrication of periodic macroporous solids is currently a hot research topic due to the wide range of potential
applications of the materials including catalysis, separations, photonic crystals and nanoelectronics [1,2]. Various
processing techniques have been employed to form different porous structures in ceramic materials. The use of pore
formers or foaming agents that evolve gases during calcination is a common method [3,4]. Replacing form processes
can duplicate the macroporous microstructures of the foams that have interconnected pores [5,6]. Partial sintering has
also been used as an alternative approach to obtaining porous structures [7–9]. To date, a variety of materials that have
porous structures have been successfully fabricated, such as Al O , SiC, TiO and hydroxyapatite. However, these
2
3
2
processes generally produce macroporous structures with a wide distribution of pore sizes.
Recently, a promising new method for the preparation of ordered macroporous solids was developed using close-
packed arrays of monodispersed spheres (typically silica or polystyrene) as templates [1,10–13]. In this approach, the
interstices of these colloidal crystals are infiltrated with a reaction solution of the material that will eventually
comprise the replicate structure. The resulting inorganic–organic composites are then either calcined or chemically
etched, yielding the macroporous replicate solid with periodic voids. A variety of macroporous materials have been
*
Corresponding author. Tel.: +81 298592461; fax: +81 298592401.
E-mail address: SAKKA.Yoshio@nims.go.jp (Y. Sakka).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.08.021

F. Tang et al. / Materials Research Bulletin 41 (2006) 268–273
269
prepared by this method [14,15]. However, modification of the synthetic parameters remains the main challenge for
the template approach, since this ultimately influences the microstructures of the resulting macroporous structures.
In this study, we report a practical technique for the fabrication of several ordered macroporous oxides including
SiO , ZrO and TiO using a template of polystyrene prepared by centrifugation or gravitation sedimentation.
2
2
2
Modification of the synthetic conditions resulted in various ordered macroporous structures in these materials. Some
of this feature a novel porous structure with a metastable square arrangement that has never been previously reported
for macroporous materials.
2
. Experimental
2
.1. Materials
Monodisperse polymethyl methacrylate (PMMA) spheres with an average diameter of 300 nm (P300) were
purchased from the Soken Chemical and Engineering Co., Japan, for use as template materials. Tetraethyl orthosilicate
TEOS), titanium tetraisopropoxide (TTIP) and zirconium butoxide of analytic purity were used as the inorganic
precursors.
(
2
.2. Macroporous material fabrication
In a typical process, 0.5 g of PMMA spheres were dispersed in 50 ml of distilled water adjusted at pH 9 using
NH OH solution, and then ultrasonicated for 10 min [16], followed by stirring for 1 h to ensure complete dispersal.
4
The resulting suspensions were then either centrifuged at 3000 rpm for 30 min or they underwent static gravitational
sedimentation for about 1 month in order to obtain close-packed polymer colloidal crystals. After drying, the templates
of the colloidal crystals were immersed in a mixed solution of alkoxide precursors and ethanol with a volume ratio of
1
:1 to 1:2 in a glove box with an argon atmosphere. By diluting the alkoxide in ethanol, it was possible to adjust the
viscosity and the hydrolysis/condensation rates of the alkoxide. The immersion procedure was as follows: (1) the
PMMA template was immersed in a mixed solution of alkoxide and ethanol and (2) the coated template was dried at
room temperature. This procedure was repeated several times to ensure the complete infiltration of the voids of the
template colloidal crystals with the inorganic precursors. The resulting coated PMMA spheres were dried in air, and
then the template was removed from the inorganic framework by calcination at 550 8C or higher for 4 h in air at the
heating rate of 1 8C/min [17,18].
2
.3. Characterization
The density and porosity of the calcined samples were measured by the Archimedes’ method using distilled water
as the immersion medium. The obtained macroprous ceramics have no closed pores and the pore structure and
morphology were observed by scanning electron microscopy (SEM: JSM 5400 JEOL, Tokyo, Japan). Phase
identification of the materials was achieved using X-ray diffraction (XRD: Model JDX-3500, Rigaku, Tokyo, Japan).
3
. Results and discussion
The microstructure of the macroporous inorganic materials is highly dependent on the structure of the polymer
crystal templates. Close-packed polymer spheres generally lead to well-ordered macroporous inorganic ceramics. In
order to fabricate macroporous materials with uniform structures, a key requirement is to prepare well-packed polymer
beads. Principally, this usually involves the slow sedimentation of monodispersed polymer spheres, which results in
hexagonal packed structures. In our experiments, we found that this form of sedimentation was time-consuming and
that the precipitate was only partially closed-stacked. On the other hand, centrifugation of the polymer spheres led to
the formation of an iridescent sediment on the bottom of the centrifuge tube, suggesting a well-ordered arrangement of
the polymer spheres. The supernatant solution was decanted and the polymer spheres were air-dried before the
infiltration process. Scanning electron microscopy images (Fig. 1) revealed that the centrifuged spheres were already
ordered into close-packed domains, although the sphere arrays contained stacking faults, point defects, line defects
and larger disordered regions. The square arrangement has been observed even in the absence of the alkoxide solution

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F. Tang et al. / Materials Research Bulletin 41 (2006) 268–273
Fig. 1. The arrangement of polymer spheres obtained using centrifugation.
using a careful sedimentation technique. Here, we developed a less time consuming and simple method for the square
arrangement using centrifugation.
The resulting macroporous ceramics obtained by the infiltration of templates with inorganic precursors followed by
calcination were first examined by X-ray diffraction. The XRD data revealed that the formation of an amorphous SiO2,
anatase phase of TiO or monoclinic structure of ZrO , which indicated the formation of inorganic ceramic structures.
2
2
The macroporous SiO that was obtained using gravitational sedimentation showed relatively poorly ordered
2
structures (Fig. 2a). On the other hand, the low-magnification SEM images of the macroporous SiO prepared using
2
centrifugation revealed uniformly distributed macropores of up to several hundred micrometers in size throughout the
Fig. 2. (a) SEM image of macroporous SiO
2
materials obtained using sedimentation of polymer templates; (b) low-magnification and (c) high-
magnification images of SiO
arrangement of macropores.
2
obtained by centrifugation of polymer templates; (d) SEM image of the SiO materials showing a unique square
2

F. Tang et al. / Materials Research Bulletin 41 (2006) 268–273
271
Fig. 3. (a) Low-magnification SEM image of macroporous TiO
magnification SEM image of macroporous TiO with both hexagonal close-packed and simple close-packed porous titania; (c and d) high-
showing a hexagonal close-packed arrangement of macropores and simple close-packed macropores,
2
materials with an ordered arrangement of the porous structure; (b) medium
2
magnification SEM images of the TiO
respectively.
2
entire sample (Fig. 2b). This indicates that centrifugation is an effective method for the fabrication of macroporous
materials. Therefore, we used the centrifugation method in the following preparations. The enlarged image (Fig. 2c)
shows a highly ordered hexagonal arrangement of macroporous SiO with a pore size of appropriately 210 nm and a
2
wall thickness of about 80 nm. In comparison with the size of the original polymer, the shrinkage of the pore size is
about 38%. Interestingly, a unique square arrangement of macropores was also observed in some parts of the structures
(
Fig. 2d) when we used a small quantity of concentrated alkoxide solution, TEOS/ETOH = 1:0.8, to infiltrate the
polymer template crystal. The wall thickness is slightly thicker at about 90 nm and the pore size is slightly smaller at
about 190 nm. In principle, the free energy is lowest for a hexagonal closed-packed structure, whereas a square-packed
arrangement of the spheres is only a metastable structure. However, infiltration with the concentrated alkoxide solution
may result in this metastable arrangement of the templates, which could ultimately result in the formation of novel
inorganic structures. It is worth noting that very few reports have mentioned the formation of macroporous structures
with the square porous architecture of SiO as seen in Fig. 2d to date. The formation of this structure may also be
2
advantageous in terms of structural studies and could have possible applications in catalysis and separation. Others
have also reported that control of the wall thickness and the size of the window openings between the walls could be
achieved by adjusting the dilution of the precursors with alcohols, which might modify the hydrolysis and gelation
rates of the alkoxides [14,19,20].
Fig. 3a depicts an ordered hexagonal arrangement of macroporous titania over a large area. From this image, we can
find areas in which the hexagonal close-packed arrangements of pores and simple close-packed pores coexist (Fig. 3b).
Fig. 3c and d shows high-magnification SEM images of the hexagonal close-packed arrangement and simple close-
packed arrangement TiO macrospheres, respectively. Dark spots are pores (originally polymer templates) surrounded
2
by about 10–14 small particles of TiO with a size of about 60 nm, and that the wall thickness and the pores seem to be
2
of equal size (about 130 nm) for both the hexagonal-packed pore structure and the simple close-packed arrangement.
This phenomenon was not observed in the samples of the amorphous SiO macroporous materials, because the ready
2

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F. Tang et al. / Materials Research Bulletin 41 (2006) 268–273
2 2
Fig. 4. (a) Low-magnification and (b) high-magnification images of macroporous ZrO materials calcined at 550 8C; (c) macroporous ZrO
materials calcined at 850 8C.
condensation of titanium alkoxide (which is sensitive to humidity) will induce a thicker wall framework for the TiO2
porous materials. When this porous material was examined it was found to have a high porosity of up to 78%, which
makes it a promising candidate material for photocatalytic applications. This result suggests that the unique metastable
square structure can also be maintained with the appropriate external modification.
Fig. 4a and b shows another inorganic macroporous material consisting of ZrO . A highly closed-packed fcc porous
2
structure was obtained while no simple square packing was observed. The spherical pore size is about 200 nm and the
wall thickness is about 80 nm, indicating that the shrinkage of the framework was appropriately 33% during the
calcination process resulting in zirconia particles that were about 30–40 nm in size. The fracture surface in Fig. 4c also
clearly shows the close-packed structure of the pores. This macroporous structure almost remained intact even when
heated at temperatures as high as 850 8C (Fig. 4d), indicating the thermally stable nature of the ZrO ceramics. On the
2
contrary, the macroporous structures of SiO and TiO partially collapsed at this temperature.
2
2
4
. Conclusions
In summary, macroporous materials of SiO , TiO and ZrO with well-defined pore architectures were fabricated
2
2
2
through a template-assisted strategy. Modification of the template arrangement resulted in novel structures with
metastable square arrangements of the macropores. The data suggest that the PMMA spheres are not firmly fixed in the
dry template prepared by centrifugation and undergo rearrangement after infiltration of the alkoxide precursor
followed by heating. These macroporous materials with well-defined pore structures may find applications in areas,
such as photocatalysis for macroporous TiO and separation techniques for macroporous SiO and ZrO .
2
2
2
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