hydrothermal synthesis of na2ti6o13 and tio2 whiskers

Journal of Crystal Growth 275 (2005) e2371–e2376
Hydrothermal synthesis of Na
2
Ti
6
O
13
and TiO
2
whiskers
Dong-Seok Seo
a
, Hwan Kim
a
, Jong-Kook Lee
b,Ã
a
School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea
b
Department of Advanced Materials Engineering, Development of Intelligent Materials, Chosun University, Gwangju 501-759, Korea
Available online 21 December 2004
Abstract
Na
2
Ti
n
O
2n+1
typed whiskers has been extensively used for frictional materials, reinforcement materials and high
insulators, and TiO
2

whiskers can be applied for catalyst support and photocatalysts. Na
2
Ti
6
O
13
whiskers were easily
synthesized by hydrothermal treatment of the mixed solution of spherical TiO
2
powder with anatase structure and
NaOH solution at 250 1C for 4 h. The sodium titanate (Na
2
Ti
6
O
13
) whiskers obtained had a smooth surface and high
aspect ratio of 100 nm below in diameter and 100 mm above in length. TiO
2
whiskers were obtained by acid treatment of
the Na
2
Ti
6
O
13
whiskers in 0.5 M HCl solution at 100 1C for 48 h. This suggests that Na ions in the Na
2
Ti
6

O
13
structure
were extracted during acid treatment and the formed TiO
2
Á nH
2
O hydrate was turned to the TiO
2
whisker with anatase
phase.
r 2004 Elsevier B.V. All rights reserved.
PACS: 61.82.Rx; 61.66.Fn
Keywords: A1. Low-dimensional structures; A1. Nanostructures; A2. Hydrothermal crystal growth; B1. Nanomaterials
1. Introduction
Nanostructured materials have received much
attention because of their novel properties which
differ from those of bulk materials. One-dimen-
sional materials are an important category of
nanostructured materials [1,2] and have been
widely researched, yielding various special struc-
tures such as nanowhiskers [3–5], nanowires [6]
and nanobelts [7].
The crystal structures of alkali-metal titanates,
A
2
Ti
n
O
2n+1

are well-known. All of them have a
monoclinic structure with almost the same b value
[4,8]. Alkali-metal titanates with a high alkali-
metal content (n ¼ 2; 3; 4) are open-layered struc-
tures having layers made of titanate groups held
together by alkali-metal ions. They can be used as
cation exchangers and catalysts because of their
distinctive intercalation ability and catalytic activ-
ity [9–12]. On the other hand, alkali-metal
titanates with a low alkali-metal content
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doi:10.1016/j.jcrysgro.2004.11.340
Ã
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fax: +82 62 232 2474.
E-mail address: (J K. Lee).
(n ¼ 6; 7; 8) are tunnel structures and exhibit high
insolating, mechanical and chemical ability
[13–15].
Specially, sodium titanate (Na
2
Ti
6
O
13
)(A
2
Ti-

n
O
2n+1
, n ¼ 6) whisker combined with Na
2
Ti
3
O
7
or TiO
2
has been applied for an oxygen electrode
of CO
2
gas sensors and clarify the ion exchanges at
the interface between the gas and electrolyte. For
instance, Holzinger et al. [16] improved the long-
term stability and selectivity of fast potentiometric
CO
2
sensors using a reference electrode consisting
of Na
2
Ti
3
O
7
/Na
2
Ti

6
O
13
or Na
2
Ti
6
O
13
/TiO
2
, which
are chemically inert against CO
2
. Ramirez-Salgado
et al. [9] also proved that those composites could
be used as oxygen electrode materials in potentio-
metric gas sensor devices. Furthermore, sodium
titanate as an ion exchanger can be used for the
removal of transition metals and anions from
drinking water [17] and purification of heavy
metals from industrial waste water [18].
There are several methods to synthesize alkali-
metal titanates including sodium titanate such as
calcination, melt reaction, flux growth and slow-
cooling calcination. These methods usually need
high reaction temperatures for a long period of
time. For example, Na
2
Ti

3
O
7
and Na
2
Ti
6
O
13
were
synthesized by heating mixtures of Na
2
CO
3
and
TiO
2
or Na
2
O and TiO
2
, at 1000 1C for one day
[16]. Meanwhile, the hydrothermal method has
many advantages: (i) the crystallization tempera-
ture is obviously lower than that in the heat
treatment process; (ii) hard agglomeration among
particles can be prevented because crystallization
proceeds under the high pressure; (iii) products
without calcination or milling may guarantee a
high quality of powder; (iv) it is easy to prepare

nano-sized powder with controlled particle shape
and size distribution, although the process shows
slow reaction rate and is not appropriate for
production on a large scale due to a volume limit
of reaction vessel. In this work, we synthesized
sodium titanate whiskers by the hydrothermal
method, reacting between TiO
2
and NaOH.
TiO
2
has attracted considerable interest due to
its good characteristics of chemical stability,
endurance, thin film transparency and lower
production costs. Furthermore, TiO
2
photocata-
lyst has been studied for applications in, for
instance, water purification, decomposition of
NO
x
and improvement of living conditions by
removal of various pollutants, etc. [19–21]. In this
paper, we also demonstrated the preparation of
TiO
2
whisker by extracting Na species from the
Na
2
Ti

6
O
13
whiskers using an acid treatment.
2. Experimental procedure
For the preparation of Na
2
Ti
6
O
13
whiskers,
TiO
2
nano-sized powder with anatase structure
was used as a starting material. TiO
2
powder with
anatase phase was obtained by precipitation
reaction between TiOCl
2
and ammonium hydro-
xide solutions, followed by heat treatment at
ARTICLE IN PRESS
Fig. 1. TEM micrograph and XRD pattern of TiO
2
powder
with anatase structure.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2372
450 1C for 1 h. In the hydrothermal process, the

TiO
2
powder with 10 N NaOH solution was placed
in a Teflon vessel and autoclaved at a temperature
of 200–250 1C. Na
2
Ti
6
O
13
whiskers were obtained
after filtering and washing. TiO
2
whiskers were
prepared by extracting Na ions from the Na
2-
Ti
6
O
13
whiskers using the acid treatment in 0.5 M
HCl solution at 100 1C for 48 h. After washing
repeatedly using distilled water until chloride ions
were completely removed, the whiskers were dried
at 80 1C. The crystallinity of the obtained powder
was analyzed by means of an X-ray diffractometer
(XRD) and transmission electron microscope
(TEM) work was carried out to investigate the
microstructures.
3. Results and discussion

Fig. 1 shows XRD pattern and TEM micro-
graph of TiO
2
powder prepared by precipitation
and subsequent heat treatment at 450 1C. It has
well-crystallized anatase structure and consists of
spherical particles approximately 10 nm in size
with narrow size distribution.
The powder made of TiO
2
spherical particles
was hydrothermally treated in an autoclave at
200 1C for various times. Fig. 2 demonstrates the
microstructural evolution of TiO
2
particles with
hydrothermal reaction time from 10 min to 4 h.
The particles experienced a change of shape from
spherical or spherulitic shapes to columnar crystals
and the aspect ratio of the particles also increased
during the hydrothermal reaction. The fiber-like
particles were formed at the initial stage of the
reaction (Fig. 2a). The fibers were actually
produced from the needle-like particles, indicated
by an arrow, which were generated from the
spherical particles. As the reaction progresses, the
fiber-like particles tend to grow into long and thin
fibers of 300–400 nm length (Fig. 2b and c). On
autoclaving for 4 h, the fibers grew in both
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Fig. 2. Microstructural evolution of TiO
2
powder from sphere to columnar particles with hydrothermal reaction times of (a) 10 min,
(b) 30 min, (c) 1 h and (d) 4 h.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376 e2373
diameter and length, producing the columnar
crystallites rather than a whisker with 200 nm in
diameter and 1–1.5 mm length. However, it seemed
that there were still unreacted particles, resulting in
a relatively rough surface and wide size distribu-
tion.
As shown in Fig. 3, hydrothermal treatment at a
higher temperature of 250 1C gave rise to the long
whiskers with a considerable aspect ratio. TEM
micrographs showed that the whiskers had a clean
and smooth surface, which suggested no presence
of unreacted particles. The whiskers were uni-
formly distributed and had a size of less than
100 nm in diameter and a length exceeding 100 mm.
From the X-ray diffraction analysis, we found that
the whiskers were of Na
2
Ti
6
O
13
structure and grew
into almost single-crystalline structure and also
confirmed that the whiskers consisted of Na, Ti, O
atoms from EDS analysis (Fig. 4a).

This suggests that the spherical TiO
2
particles
take a dissolution and reprecipitation process.
During the reaction between spherical TiO
2
and
NaOH, the particles are dissolved and reprecipi-
tated, while NaOH may play a role accelerating
the continuous growth of the reprecipitated
particles to the whiskers with a specific direction.
In addition, the solubility of the spherical particles
for the NaOH can increase as the time and
temperature of hydrothermal reaction are longer
and higher; accordingly, whiskers with high aspect
ratio are formed in the case of the process
autoclaving at 250 1C for 4 h.
In order to obtain TiO
2
whiskers, sodium
titanate whiskers were placed in 0.5 M HCl
solution and refluxed at 100 1C for 48 h. From
the EDS analysis (Fig. 4b), it was observed
that there were no sodium atoms in the acid-
treated powder compared to the Na
2
Ti
6
O
13

whiskers. Furthermore, the XRD pattern shows
Fig. 4c that there is no peak corresponding to
Na
2
Ti
6
O
13
structure and all peaks are identical
to TiO
2
with anatase structure. There was no
ARTICLE IN PRESS
Fig. 3. TEM micrographs and XRD pattern of Na
2
Ti
6
O
13
whiskers autoclaved at 250 1C for 4 h.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2374
different crystalline peak between the spherical
particles in Fig. 1b and the acid-treated ones.
Sodium ions were almost completely extracted
from sodium titanate by acid treatment, leading
to the formation of TiO
2
Á nH
2
O hydrate. TiO

2
in
the hydrate contains different amounts of H
2
O
upon the degree of hydrolysis and TiO
2
Á nH
2
O
could be completely hydrolyzed and subsequently
crystallized to TiO
2
whisker because of an aging
effect that can happen during the acid treatment
process.
Fig. 5 presents the microstructure of the acid-
treated TiO
2
powder. TiO
2
whiskers, 100–200 nm
in diameter and 5–10 mm in length, could be
obtained from the Na
2
Ti
6
O
13
whiskers. TiO

2
whiskers had quite a smooth and clean surface
although the structure was partially disintegrated.
It was also confirmed the crystal planes in the
selected area diffraction (SAD) pattern were in
accordance with anatase structure.
4. Conclusions
This study has focused on the synthesis of
sodium titanate (Na
2
Ti
6
O
13
) whiskers by hydro-
thermal treatment using spherical anatase-typed
TiO
2
powder, and also on the preparation of TiO
2
whiskers by extracting Na ions from the sodium
titanate whiskers. The Na
2
Ti
6
O
13
whiskers ob-
tained had a clean surface and a considerable
aspect ratio with less than 100 nm diameter and a

length exceeding 100 mm. Dissolution and repreci-
pitation process for the TiO
2
spherical particles
possibly gave rise to a change in the shape of the
particles from needle-like, fiber, and eventually
to the long and thin whiskers with a smooth
surface. The morphology of the whiskers seemed
to be influenced by the reaction time and
temperature during the hydrothermal process.
After extraction of Na ions from the Na
2
Ti
6
O
13
structure, TiO
2
Á nH
2
O hydrate was formed and
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Fig. 4. EDS patterns of (a) Na
2
Ti
6
O
13
whiskers, (b) TiO
2

whiskers and (c) XRD pattern of TiO
2
whiskers.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376 e2375
readily turned to crystalline TiO
2
retaining the
whisker shape during the acid treatment.
Acknowledgments
This study was supported by research funds
from Chosun University, 2003.
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ARTICLE IN PRESS
Fig. 5. TEM micrographs and SAD pattern of TiO
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whiskers obtained by acid treatment.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2376