The sodium titanate Na2Ti6O13 whiskers obtained had a smooth surface and high aspect ratio of 100 nm below in diameter and 100 mm above in length.. Meanwhile, the hydrothermal method has
Trang 1Journal of Crystal Growth 275 (2005) e2371–e2376
Dong-Seok Seoa, Hwan Kima, Jong-Kook Leeb,
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
Na2TinO2n+1 typed whiskers has been extensively used for frictional materials, reinforcement materials and high insulators, and TiO2whiskers can be applied for catalyst support and photocatalysts Na2Ti6O13whiskers were easily synthesized by hydrothermal treatment of the mixed solution of spherical TiO2powder with anatase structure and NaOH solution at 250 1C for 4 h The sodium titanate (Na2Ti6O13) whiskers obtained had a smooth surface and high aspect ratio of 100 nm below in diameter and 100 mm above in length TiO2whiskers were obtained by acid treatment of the Na2Ti6O13whiskers in 0.5 M HCl solution at 100 1C for 48 h This suggests that Na ions in the Na2Ti6O13structure were extracted during acid treatment and the formed TiO2nH2O hydrate was turned to the TiO2whisker with anatase phase
r2004 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,
A2TinO2n+1 are well-known All of them have a monoclinic structure with almost the same b value
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
titanates with a low alkali-metal content
www.elsevier.com/locate/jcrysgro
0022-0248/$ - see front matter r 2004 Elsevier B.V All rights reserved.
doi:10.1016/j.jcrysgro.2004.11.340
Corresponding author Tel.: +82 62 230 7202;
fax: +82 62 232 2474.
E-mail address: jklee@mail.chosun.ac.kr (J.-K Lee).
Trang 2(n ¼ 6; 7; 8) are tunnel structures and exhibit high
insolating, mechanical and chemical ability
[13–15]
Specially, sodium titanate (Na2Ti6O13) (A2
Ti-nO2n+1, n ¼ 6) whisker combined with Na2Ti3O7
or TiO2has been applied for an oxygen electrode
of CO2gas 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
CO2sensors using a reference electrode consisting
of Na2Ti3O7/Na2Ti6O13or Na2Ti6O13/TiO2, which
are chemically inert against CO2 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, Na2Ti3O7and Na2Ti6O13were
synthesized by heating mixtures of Na2CO3 and
TiO2 or Na2O and TiO2, 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 TiO2and NaOH
TiO2 has attracted considerable interest due to
its good characteristics of chemical stability,
endurance, thin film transparency and lower
production costs Furthermore, TiO2
photocata-lyst has been studied for applications in, for
instance, water purification, decomposition of
NOx and improvement of living conditions by removal of various pollutants, etc.[19–21] In this paper, we also demonstrated the preparation of TiO2 whisker by extracting Na species from the
Na2Ti6O13whiskers using an acid treatment
2 Experimental procedure For the preparation of Na2Ti6O13 whiskers, TiO2 nano-sized powder with anatase structure was used as a starting material TiO2powder with anatase phase was obtained by precipitation reaction between TiOCl2 and ammonium hydro-xide solutions, followed by heat treatment at
Fig 1 TEM micrograph and XRD pattern of TiO 2 powder with anatase structure.
Trang 3450 1C for 1 h In the hydrothermal process, the
TiO2powder with 10 N NaOH solution was placed
in a Teflon vessel and autoclaved at a temperature
of 200–250 1C Na2Ti6O13whiskers were obtained
after filtering and washing TiO2 whiskers were
prepared by extracting Na ions from the Na
2-Ti6O13whiskers 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
micro-graph of TiO2 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 TiO2 spherical particles was hydrothermally treated in an autoclave at
200 1C for various times Fig 2 demonstrates the microstructural evolution of TiO2 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
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.
Trang 4diameter 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 inFig 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 Na2Ti6O13structure 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 TiO2 particles
take a dissolution and reprecipitation process
During the reaction between spherical TiO2 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 TiO2 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 Na2Ti6O13
whiskers Furthermore, the XRD pattern shows
Na2Ti6O13 structure and all peaks are identical
to TiO2 with anatase structure There was no
Fig 3 TEM micrographs and XRD pattern of Na Ti O whiskers autoclaved at 250 1C for 4 h.
Trang 5different 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 TiO2nH2O hydrate TiO2in
the hydrate contains different amounts of H2O
upon the degree of hydrolysis and TiO2nH2O
could be completely hydrolyzed and subsequently
crystallized to TiO2 whisker because of an aging
effect that can happen during the acid treatment
process
acid-treated TiO2powder TiO2whiskers, 100–200 nm
in diameter and 5–10 mm in length, could be
obtained from the Na2Ti6O13 whiskers TiO2
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 (Na2Ti6O13) whiskers by hydro-thermal treatment using spherical anatase-typed TiO2powder, and also on the preparation of TiO2
whiskers by extracting Na ions from the sodium titanate whiskers The Na2Ti6O13 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 TiO2 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 Na2Ti6O13
structure, TiO nH O hydrate was formed and
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.
Trang 6readily turned to crystalline TiO2 retaining the
whisker shape during the acid treatment
Acknowledgments
This study was supported by research funds
from Chosun University, 2003
References
[1] S Iijima, Nature 354 (1991) 56.
[2] A.M Morales, C.M Liber, Science 279 (1998) 208.
[3] J.K Lee, K.H Lee, H Kim, J Mat Sci 31 (1996) 5493.
[4] G.L Li, G.H Wang, J.M Hong, Mater Res Bull 34
(1999) 2341.
[5] Y.C Zhu, C.X Ding, Nanostruct Mater 11 (3) (1999)
427.
[6] C.N.R Rao, F.L Deepak, G Gundiah, A Govindaraj,
Prog Solid State Chem 31 (2003) 5.
[7] S.H Sun, G.W Meng, G.X Zhang, T Gao, B.Y Geng,
L.D Zhang, J Zuo, Chem Phys Lett 376 (1) (2003) 103.
[8] T.P Fiest, S.J Mocarski, P.K Davies, A.J Jacobson, J.T Lewandowski, Solid State Ionics 28–30 (1998) 1338 [9] J Ramirez-Salgado, E Djurado, P Fabry, J Eur Ceram Soc 24 (8) (2004) 2477.
[10] Y Fujiki, Y Komastsu, T Sasaki, Ceram Jpn 19 (1984) 126.
[11] M Dion, Y Piffard, M Tournoux, J Inorg Nucl Chem.
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 Fig 5 TEM micrographs and SAD pattern of TiO 2 whiskers obtained by acid treatment.