Solid-state synthesis of sodium titanate nanowires at
low temperature
a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
150001, China
b
Department of Physics and Astronomy, University of North Carolina at Chapel Hill,
Chapel Hill, NC 27599-3255, USA
c
Curriculum of Applied and Materials Sciences, University of North Carolina at Chapel
Hill, Chapel Hill, NC 27599-3255, USA
Abstract
A simple method was developed for the solid-state synthesis of sodium titanate
nanowires from TiO2 and molten sodium chloride at relatively low temperature.
Single-crystalline sodium titanate nanowires with diameter of 15-100 nm and length up to
tens of microns were characterized with TEM and HREM. The facile technique offers an
alternative opportunity for the synthesis of other alkali titanates nanowires.
Keywords: Nanostructures; Nanofabrication; Transmission electron microscopy
PACS: 68.37.Lp; 81.07-b
1. Introduction
The alkali titanates A2O·nTiO2 (1n6 and A=K, Na, Li) have arrested increasing
attention due to their high photocatalytic activities and ion-exchange/intercalation
properties [1]. They can also serve as a reinforcing agent of metals and plastic [2], as
adiabatic materials [3] and as an oxygen electrode for potentiometric sensors [4]. Sodium
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titanates are usually prepared by solid-state reactions from the stoichiometric weights of
Na2CO3 and TiO2 or Na2O and TiO2. For example, Na2Ti6O13 in the form of colorless
needles can be made by heating the mixture of Na2CO3 and TiO2 (anatase) in the molar
ratio of 1:6, firstly at 1000 ºC to remove CO2 and then at 1300 ºC [5]. In the past decade,
one-dimensional nanostructures of most of the functional materials were extensively
studied due to the novel physical properties on nanoscale. Binary oxide nanowires and
nanobelts, such as TiO2, ZnO, SnO2, MnO2 and Ga2O3, were extensively studied during
the past several years [6, 7]. Ternary complex oxides are also expected to exhibit novel
properties on nanoscale, especially with one-dimensional nanostructures. The sodium
titanate nanowires were recently synthesized by hydrothermal treatment of TiO2 [8, 9] or
Ti powder and H2O2 [10] in NaOH aqueous solution at 160-220 ºC with typical reaction
time of more than one day.
In this Letter, a facile solid-state reaction at relatively low temperature was proposed for
the synthesis of sodium titanate nanowires in a NaCl medium with the presence of a
nonionic surfactant. Sodium titanate could be obtained from the reaction of TiO2
nanoparticles and molten NaCl. The morphology and structure of the single-crystalline
nanowires were characterized with transmission electron microscopy (TEM) and
high-resolution electron microscopy (HREM).
2. Experimental
In a typical procedure, 0.1 g TiO2 (anatase) nanoparticles (Aldrich) was mixed with 2 g
NaCl and 4 ml NP-9 (Aldrich), ground for 20 min and then sonicated for 5 min. The
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mixture was placed in a combustion boat and annealed in a tube furnace at 825 ºC for 3 h,
and subsequently cooled to room temperature. This method was also described elsewhere
[11] for large-scale synthesis of barium titanate nanowires. The resulting powders were
washed several times with distilled water and dried at room temperature.
The morphology, structure and size of the sodium titanate nanowires were characterized
with TEM and HREM. The nanowires were dispersed in ethanol by ultrasonic treatment.
One drop of the suspension was added to a holey carbon film supported on a copper grid.
TEM was performed on JEM-100CX operated at 100 kV and HREM on JEOL-2010F at
200 kV. Cerius 2 was employed to simulate electron diffraction patterns.
3. Results and discussion
The morphology of synthesized sodium titanate nanowires was characterized with a
transmission electron microscopy (TEM). Fig. 1(a) shows the typical morphology of
several curled sodium titanate nanowires accompanied with unreacted titania
nanoparticles. The diameter of nanowires is in the range of 15-100 nanometers, which is
much smaller than that of the fibres from conventional solid-state reaction. The reason is
that this method was carried out at relatively low temperature, typical 825 ºC, which is
about 20 ºC higher than the melting point of sodium chloride. The length of the titanate
nanowires can reach up to several tens of microns. The average length-diameter ratio is
larger than 100. Large quantities of needle-like nanowires with average length of about
one micron were also observed, as shown in Fig. 1(b).
Selected-area electron diffraction (SAED) was performed on several individual
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nanowires to determine the crystal structure with JEM-100CX at 100 kV. All the
diffraction patterns can be indexed as monoclinic Na2Ti6O13 with space group C2/m and
lattice parameters of a=15.131 Å, b=3.745 Å, c=9.159 Å and β=99.3 º (JCPDS 73-1398).
A typical TEM image of an individual nanowire with a rectangle tip was shown in Fig.
2(a). The diameter of this nanowire is only 30 nm. The inset of Fig. 2(a) is the
corresponding SAED pattern taken with JEM-100CX at 100 kV. The electron diffraction
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pattern can only be indexed as (40 1 ) and ( 11 0) reflections of monoclinic Na2Ti6O13.
_
Thus the zone axis is [1 1 5]. To further confirm the structure, Cerius 2 was used to
simulate the electron diffraction patterns. The atomic positions followed what first
proposed by Anderson et al. [5] and the electron energy adopted was 100 kV. The
simulated diffraction pattern is shown in Fig. 2(b). The intensities of each reflection
perfectly fit that of experimental one. To be noted, the disappearance of several
diffraction spots in simulated pattern was not due to extinction of the monoclinic
structure, but the weak scattering ability of these lattice planes. The missed diffraction
spots will appear again when the intensity is set higher during the simulation.
A layered structure was observed from the high-resolution electron microscopy (HREM)
image (Fig. 3(a)) of the nanowires. The growth direction of the titanate nanowires (dark
arrow in Fig. 3(a)) was determined as [010], consistent with other titanates with the same
monoclinic structure [8, 12, 13]. The corresponding SAED pattern shows tripling of
2.995 Å lattice spacing, as shown in Fig. 3(b). The similar phenomenon was first
observed in H-substituted Na2Ti3O7 heated to 800 ºC by Feist et al. [14]. A series of
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titling were also performed on the shown nanowire and SAED patterns were recorded on
JEOL-2010F equipped with a double tilting holder. Each diffraction pattern could be
indexed as Na2Ti6O13 and the experimental interplane angles also consist with calculated
one. The nanowires were found to be actual ribbon-like during tilting.
Although it is still not clear what role the surfactant plays in the reaction, the surfactant
was supposed to react with oxygen and release a lot of CO2, which made the molten
mixture of TiO2 and NCl tousy during the reaction process. Thus, one-dimension sodium
titanate nanowires could be formed in the channels or honeycombs along the direction of
their crystal easy axis. The unreacted TiO2 nanoparticles were indexed as rutile,
indicating that phase transformation was probably first completed before the formation of
sodium titanate nanowires. If the surfactant was removed from the reagent system, only
anatase titania nanoparticles were found without the formation of sodium titanate
nanowires or phase transformation of titania powders. The titania nanoparticles remain
spherical, the same as raw materials, which indicates there is no interaction of TiO2 and
NCl without the assistance of the nonionic surfactant.
4. Conclusions
Anatase TiO2 nanoparticles were found to react with molten sodium chloride and form
single-crystalline Na2Ti6O13 nanowires at 825 ºC with the presence of a nonionic
surfactant. The nanowires were found to grow along [010] direction with diameter of
15-100 nm and length up to tens of microns. The formation mechanism of nanowires was
also discussed.
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Acknowledgement:
C.Y. Xu would like to give thank to H. Zhang for help with experimental assistance.
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Figure captions:
Fig. 1 TEM images of sodium titanate nanowires. (a) Several curl nanowires with length
up to 10 microns. (b) Needle-like nanowires.
Fig. 2 (a) TEM image of an individual Na2Ti6O13 nanowire. Inset: Corresponding
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selected-area electron diffraction (SAED) pattern. (b) Simulated SAED pattern of
[1 1 5]
zone axis using Cerius 2.
Fig. 3 (a) HREM image of Na2Ti6O13 nanowire. (b) Corresponding SAED pattern
showing tripling of the 2.99 Å lattice spacing.
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b)
Fig. 1 C. Y. Xu et al.
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Fig. 2 C. Y. Xu et al.
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(010)
(001)
Fig. 3 C. Y. Xu et al.
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