Scripta Materialia 49 (2003) 735–740

www.actamat-journals.com

Preparation of macroporous titania from nanoparticlebuilding blocks and polymer templates

Fengqiu Tang *, Hiroshi Fudouzi, Jianxin Zhang, Yoshio Sakka

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Received 12 February 2003; received in revised form 14 July 2003; accepted 14 July 2003

Abstract

A simple method based on hetero-coagulation for the preparation of ordered macroporous titania using commercial

titania nanoparticles as building blocks and polymer spheres as templates is reported. Surface modification plays a key

role in the microstructure of the porous titania.

� 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Oxide; Porous material; Hetero-coagulation; Powder consolidation

1. Introduction

There is currently a high interest in developing

macroporous materials with a wide range of pore

sizes over 50 nm as these open up new opportu-

nities in catalysis and separation technology.

Strategies for the preparation of macroporous

materials have been mainly focused on the efficient

path of template replication of opal structures

[1–5]. Other methods, such as the electrochemicaldeposition [6,7], the hydrodynamic infiltration of

nanoparticles [8] and the co-sedimentation of

microscale template particles and nanoparticles

[9,10], have also been developed for the purpose of

fabricating ordered porous structures. Titanium

dioxides have attracted much attention due to

their versatile applications in optical, electrical and

* Corresponding author. Tel.: +81-29-859-2463; fax: +81-29-

859-2401.

E-mail address: tang.fengqiu@nims.go.jp (F. Tang).

1359-6462/$ - see front matter � 2003 Acta Materialia Inc. Published

doi:10.1016/S1359-6462(03)00433-0

photocatalytic systems [11–13]. The potential

properties of the materials are highly dependent ontheir structures and morphologies. Consequently,

it is of significant importance in designing titania

materials with novel structures. Porous titania

materials have been the subject of many studies

over the past decades. To date, titania materials

with meso- and micropores have been widely in-

vestigated [14,15]. Macroporous titania has also

been produced using molecular precursors ornanocrystallites as building blocks [1,16]. How-

ever, a challenge remains in the fabrication of

porous materials with long-range order and a

robust framework. The economic and industrial

demands of low-cost processing for the large-scale

production of materials with the necessary per-

formance will most probably come from com-

mercially available nanopowders.In this paper, we demonstrate a simple hetero-

coagulation approach for the fabrication of mac-

roporous titania materials with a well-defined

structure on a micrometer scale. This method is

by Elsevier Ltd. All rights reserved.

736 F. Tang et al. / Scripta Materialia 49 (2003) 735–740

based on the self-assembly of a core-shell structure

via electrostatic adsorption using commercial

powders with opposite surface charges [17–19].

The preparation procedure is shown in Fig. 1.Monodispersed polymer spheres were used as the

template cores and nanosized titania particles were

used as the shell materials. The mixing of well-

dispersed suspensions of oppositely charged poly-

mers and titania particles resulted in flocculation

of the mixture. The hetero-coagulated suspension

containing the core-shell structure was filtered and

Fig. 1. Schematic procedure for the fabrication of macroporous

materials via core-shell flocculation of polymer spheres and

inorganic particles.

calcined to obtain the porous structure. This ap-

proach has several advantages. First, the utiliza-

tion of nanoceramic particles is expected to result

in less shrinkage than that of molecular precur-sors. Second, the core-shell structure can be uni-

formly and rapidly formed due to the electrostatic

adsorption of the two oppositely charged particles,

and as a result, the hetero-coagulation and filtra-

tion process is very time-efficient. Finally, a large

sample with enough mechanical strength can be

obtained using this simple processing, which may

be advantageous for manipulation and applica-tion. Consequently, this method may offer a gen-

eral route and is extendable to many other

materials.

2. Experimental procedure

Spherical TiO2 with an average particle diame-

ter of 30 nm and a BET surface area of 43 m2/g

(NanoTek, C.I. Kasei Co., Ltd., Tokyo, Japan)

was used as the inorganic building blocks; mo-nodispersed spherical polymethyl methacrylate

(PMMA) particles with an average diameter of

1300 nm (P1300, Soken Chemicals Co., Tokyo,

Japan) was employed as the template ‘‘core’’ ma-

terial. All the other chemicals used in this study

were reagent grade (Wako Pure Chemical Indus-

try, Ltd., Tokyo, Japan), such as polyethylenimine

(PEI) with an average molecular weight of 10,000was utilized to modify the inherent surface charges

of the ceramic particles, hydrochloric acid and

ammonium hydroxide were employed to adjust the

suspension pH. Distilled water was used in most of

the experiments while ultrapure water (less than

18.2 MX cm) from a Milli-Q water system (Ya-

mato Autopure WR600A, Yamato Scientific Co.,

Ltd., Tokyo, Japan) was used for the electrokineticmeasurements.

According to the hetero-coagulation strategy,

both the suspensions of the template polymers and

the nanosized coating particles should be electro-

statically stabilized whereas the surface charges of

these two particles should be opposite for the

purpose of forming uniform core-shell structures.

Because the PMMA spheres used in this study arenegatively charged, the PEI was selected to modify

Fig. 2. TEM photograph of the TiO2 nanoparticles.

F. Tang et al. / Scripta Materialia 49 (2003) 735–740 737

the surface of the TiO2 powder [20]. In a typical

synthetic procedure, 1.2 g of the TiO2 was dis-

persed into 30 ml of deionized water containing

0.075 g of PEI (concentration: 200 g/dm3) at pH 6,and 1 g of the P1300 was dispersed into 50 ml of

deionized water at the same pH. The suspensions

were ultrasonically treated at 160 W for 10 min

(USP-600, Shimadzu, Tokyo, Japan) to disperse

the powders, and further stirring was carried out

for another 1 h to ensure the saturated adsorption

of PEI on the surface of the TiO2 particles. The

modified TiO2 suspension was then dropped intothe suspension of P1300 at a speed of about 2 ml/

min. Immediately after the mixing, a flocculated

phenomenon was observed in the mixture due to

the electrostatic interaction of the oppositely

charged TiO2 and polymers. The resulting mixture

was subsequently vacuum filtered to pack the

flocculated particles together. After drying at

room temperature in air, the polymer spheres wereremoved by calcination at 500 �C for 4 h in air at

a heating rate of 1 �C/min in a muffle furnace

(Yamato FP100, Yaehashi, Japan). Further heat

treatment was continually conducted at 850 �Cfor 2 h in air to enhance the mechanical strength of

the framework.

The zeta potential (f) of the powders was char-acterized using a laser electrophoresis analyzer(LEZA-600, Otsuka Electronics Co., Osaka,

Japan) calculated by the Smoluchowski equation.

0.1 M NaOH and HCl analytical solution were

used for pH adjustment. Approximately 1 vol% of

TiO2 suspension was prepared by ultrasonication

for 10min, then several drops of the suspension was

diluted into a 150 ml of 10�2 M NaCl solution to

reach the necessary photoamount according to thecomputer, which was used for zeta potential mea-

surement after aging for 30 min. A TEM (JEOL

2000FXII, JEOL Ltd., Tokyo, Japan) was used to

observe the morphology of the TiO2/polymer core-

shell structures, which was collected immediately

after the mixing of the two suspensions. The multi-

point BET surface area (5 point) was characterized

on the Coulter SA 3100 equipment (Coulter corp.,Florida, USA) after outgasing at 150 �C for 12 h in

vacuum. A scanning electron microscope (JSM

5400, JEOL Ltd., Tokyo, Japan) was employed to

observe the microstructure and morphology of the

porous materials. The density, porosity and open

porosity of the calcined samples were measured by

the Archimedes method with distilled water as the

immersion medium using the following equations:

qdensity ¼ m1qw=ðm3 � m2Þ ð1Þ

Popen-porosity ¼ ðm3 � m1Þ=ðm3 � m2Þ ð2Þ

Ptotal-porosity ¼ 1� m1qw=ðm3 � m2ÞqTiO2ð3Þ

where qw is the density of distilled water, qTiO2is

the theoretical density of the TiO2, m1 is the dry

mass of the sample in air, m2 and m3 are the wetmass of the sample after saturated water absorp-

tion in water and in air, respectively.

3. Results and discussion

Fig. 2 shows a TEM photograph of the com-

mercial TiO2 particles used in this study. The

particles were spherical and the size varied from

several nanometers to 100 nm. The particle size

distribution of the powder provided by the com-

pany is shown in Fig. 3, which was calculated from

TEM images by the number of different particlesizes. It showed that 80% of the particle size was

below 30 nm, whereas some fraction of larger

particles of about 60 nm also existed. The utiliza-

tion of commercial titania powders in this study

with a wide ranging particle size distribution is

to prove the simplicity and validity of our strategy

for the preparation of a porous structure, which

Fig. 3. Particle size distributions of TiO2 powders.

Fig. 4. Zeta potential of TiO2, TiO2 modified with PEI and

P1300 particles.

738 F. Tang et al. / Scripta Materialia 49 (2003) 735–740

is expected to be applied to various other kinds of

available inorganic particles.The structures of the macroporous materials are

highly dependent on the properties of the starting

materials and suspensions, such as the zeta po-

tential, particle size and volume ratio of the two

powders. In order to fabricate the core-shell

composite with a uniform structure via our strat-

egy, the key point is to prepare well-dispersed

suspensions of both the template and the nano-particles in a same pH range with opposite surface

charges.

The zeta potentials of the TiO2, PEI modified

TiO2 and P1300 are shown in Fig. 4. P1300 was

negatively charged in the measured pH range of 3–

12. A relatively high f value could be obtained

between pH 5 and 12, indicating that the P1300

suspension is well dispersed in this pH range. Onthe other hand, the isoelectric point (IEP) of the

TiO2 was located at pH 6.7, a highly positive

surface charge could be obtained only below pH 5.

For the purpose of preparing the TiO2 suspension

with a highly positive surface charge in a wider

range, PEI was used to modify the surface charge

of TiO2. The addition of PEI not only shifted the

IEP to the higher pH value of 10.8, but also re-sulted in a good dispersion of the suspension. The

zeta potential of the TiO2 modified with PEI was

much higher than that without the PEI modifica-

tion, moreover, the pH range with a highly posi-

tive zeta potential became wider, which makes the

coating of polymer spheres much easier in a wide

pH range, i.e., from pH 5 to pH 8, because the

highly opposite-charged polymers and TiO2 par-

ticles can be readily flocculated upon mixing due

to the driving force of the electrostatic interaction.

A TEM was employed to observe the morpho-logies of the resulting core-shell composites, as

shown in Fig. 5. The high magnification image

revealed that the surface of the P1300 polymer

became rough compared with the smooth curva-

ture of the bare P1300 (not shown here), which is

attributed to the coverage of the smaller TiO2

nanoparticles, indicating the formation of the

core-shell structure. Based on the low magnifica-tion image (Fig. 5b), it was observed that the

majority of the TiO2 particles have been adsorbed

on the surface of the P1300 particles, whereas few

TiO2 particles were dispersed freely from the

templates, suggesting the effectiveness of this

method to form the core-shell structure. The con-

necting of the core-shell composites through some

necks could be identified, which may result in theflocculation of the core-shell structure because of

the decreasing of net electric charge of the core-

shell structure due to the neutralization of the two

kinds of particles.

The images of the calcined sample (Fig. 6) show

the ordered porous structures. The pore was nearly

spherical with a uniform pore diameter of �1.0–

1.2 lm, being slightly smaller than that of theoriginal PMMA spheres due to the contraction

Fig. 5. TEM micrographs of the P1300 coated with TiO2

nanoparticles. (a) High magnification, (b) low magnification.Fig. 6. SEM micrographs of the typical macroporous structure

after calcination. (a) High magnification, (b) low magnification.

F. Tang et al. / Scripta Materialia 49 (2003) 735–740 739

and grain growth of the TiO2 nanoparticles upon

heat treatment. The framework of the materials

was relatively uniform with an average thickness

of �200 nm, even though the TiO2 particles with a

large size deviation was used as the starting ma-

terials, which is assignable to the slight sintering of

the nanoparticles. The BET measurement showed

that the surface area of the porous TiO2 was 6.8m2/g, corresponding to the titania grain size of

�208 nm, indicating the growth of the particles

upon calcination, which also enhanced the

strength of the titania framework. The grain size of

titania is the same as the wall thickness of the

material, suggesting that the titania particles

composed of the framework have grown up into

one layer of TiO2 grains. It should be noted thatthe slight sintering is essential for the formation of

porous materials with a uniform framework and

good mechanical strength. It can be observed from

the low magnification image (Fig. 6b) that the

monodispersed spherical pores are uniformly dis-

tributed on a large scale, indicating uniformity of

the high porosity in the sample. The total porosity

of the sample was determined to be �69.6% while

the open porosity was �68.7%, suggesting that

over 98% of the pores are opened pores, whichmay be advantageous for the transportation of

certain kinds of materials or use as a catalyst.

4. Conclusions

In this paper, a very simple hetero-coagulation

method was demonstrated for the preparation of

macroporous titania with a uniform three-dimen-

sional ordered pore structure, which is expected

to be a potential supporting material for catalytic

or adsorption applications. This kind of core-shell coagulation strategy may provide a general

740 F. Tang et al. / Scripta Materialia 49 (2003) 735–740

pathway for the preparation of macroporous

materials with various compositions.

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