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

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

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

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

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

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
ARTICLE IN PRESS

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

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

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
ARTICLE IN PRESS
Fig. 4. EDS patterns of (a) Na
2
Ti
6
O
13
whiskers, (b) TiO
2

40 (1978) 917.
[12] H. Izawa, S. Kikkawa, M. Koizumi, J. Solid State Chem.
69 (1987) 336.
[13] S. Andersson, A.D. Wadsley, Acta. Cryst. 15 (1962)
194.
[14] T. Sasaki, Y. Fujiki, J. Solid State Chem. 83 (1989) 45.
[15] T. Sasaki, M. Watanabe, Y. Fujiki, Y. Kitami, J. Int.
Biomed. Inform. Data 105 (1993) 481.
[16] M. Holzinger, J. Maier, W. Sitte, Solid State Ionics 86–88
(1996) 1055.
[17] K. Vaaramaa, J. Lehto, Desalination 155 (2003) 157.
[18] H. Leinonen, J. Lehto, A. Makela, React. Polym. 23 (1994)
221.
[19] K. Terabe, K. Kato, J. Mat. Sci. 29 (1994) 1617.
[20] C. Dominguez, J. Garcia, M.A. Perdraz, A. Torres,
M.A. Galan, Catal. Today 40 (1998) 85.
[21] J. Berry, M.R. Muller, Microchem. J. 50 (1994) 28.
ARTICLE IN PRESS
Fig. 5. TEM micrographs and SAD pattern of TiO
2
whiskers obtained by acid treatment.
D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2376