Synthesis of characterization of sodium titanate and potassium hollandite nanowires
Chengyan Xu,a, b Qi Zhang,c Liang Zhen,b Lu-Chang Qin b,c *
a
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC
27599-3255, USA
b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
c
Curriculum of Applied and Materials Sciences, University of North Carolina at Chapel Hill, Chapel
Hill, NC 27599-3255, USA
* lcqin@physics.unc.edu
Abstract:
Single-crystalline sodium hexatitanate and potassium hollandite nanowires were prepared
from direct reaction of TiO2 nanoparticles and alkali chlorides with the presence of a
nonionic surfactant at relatively low temperature. The single-crystalline nanowires with
diameter around 100 nm and length up to a few tens of microns were characterized with
SEM, TEM and HREM. Crystal structure of the nanowires was determined with
selected-area electron diffraction (SAED) using series titling technique. The facile
technique offers an alternative opportunity for the synthesis of other ternary oxide
nanowires.
Keywords: Nanostructures, Nanofabrication, Transmission electron microscopy
1. Introduction
The alkali metal titanates with tunnel structure are of interest, both experimental and
theoretical, as model systems for fast ionic conductors and also for their high
photocatalytic activities [1-3]. A family of the alkali metal titanates, represented by the
general formula A2O·nTiO2 (3n8 and A=K, Na, Li), consist of a layer structure with
1
each layer consisting of zig-zag ribbons, in which the octahedra share edges at one level
in linear groups of three and are further joined together by corners. These ribbons share
the terminal corners with identical ribbons, resulting in an open octahedral framework
enclosing tunnels, or continuous rows of interstitial positions, in which the alkali ions are
situated [3]. In addition, a hollandite structure with general formula of Ax(ByTi8-y)O16,
where A=K, Cs or Ba and B is either a divalent a trivalent cations, such as Mg2+, Mn2+,
Al3+, or Ti3+, can be described as columns of four linear strings of edge-sharing octahedra
parallel to the c-axis. The chains of octahedra are linked together, forming a framework
of continuous parallel tunnels, which are only partial occupied by alkali ions [4].
In the past decade, various strategies have been developed to fabricate one-dimensional
nanostructures of most of the functional materials, due to their novel physical properties
on nanoscale. Binary oxide nanowires and nanobelts, such as TiO2, ZnO, SnO2, MnO2
and Ga2O3 etc, have been successfully synthesized [5, 6]. However, the approach to
one-dimensional nanostructure of ternary complex oxides remains a challenge except for
the template method. Recently, hydrothermal method was employed to the synthesis of
alkali metal titanate nanowires. Sodium titanate nanowires were synthesized by
hydrothermal treatment of TiO2 [7, 8] or Ti powder and H2O2 [9] in NaOH aqueous
solution at 160 - 220 ºC. K2Ti6O13 nanowires were successfully prepared by hydrothermal
reaction between TiO2 or Na2Ti3O7 with KOH solution [10, 11], and K2Ti8O17 was also
fabricated via the same hydrothermal route [12].
In this Letter, we described a simple method to fabricate sodium titanate and potassium
2
hollandite nanowires from the reaction of TiO2 nanoparticles with molten salt in the
presence of a nonionic surfactant. The morphology and structure of the single-crystalline
nanowires were characterized with SEM, transmission electron microscopy (TEM) and
high-resolution electron microscopy (HREM). Crystal structure of the nanowires was
determined with selected-area electron diffraction (SAED) using series titling technique.
2. Experimental
In a typical procedure, a mixture of barium oxalate, TiO2 (anatase) nanoparticles and
NaCl with molar ratio of 1:1:20 was ground with 5 ml NP-9 (Aldrich) for 20 min and
then sonicated for 5 min. The 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. The pristine
powders were washed with distilled water several times to remove NaCl and resulting
BaTiO3 nanoparticles, which were suspended on the surface during washing. For the
potassium hollandite, KCl was used instead of NaCl and no barium oxalate was
introduced into the reaction system. The process is almost the same except that reaction
temperature was set as 800 ºC. This method was also described elsewhere [13] for
large-scale synthesis of barium titanate nanowires. The resulting powders were washed
several times with distilled water and then dried at room temperature.
The morphology, structure and size of the synthesized nanowires were characterized with
SEM, 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
3
200 kV.
3. Results and discussion
Fig. 1a shows a SEM image of the synthesized sodium titanate nanowires. Large quantity
nanowires with uniform diameter around 100 nm were observed. The length of nanowires
ranges from several to a few tens of microns. TEM image shown in Fig. 1b further
revealed narrow diameter distribution of the synthesized nanowire. Selected area electron
diffraction (SAED) taken from different parts of the nanowires indicates that the
nanowires are single crystalline.
Series tilting with JEOL-2010F was performed on a selected nanowire and electron
diffraction patterns were obtained to determine the structure of synthesized sodium
titanate nanowires. Fig. 2a-c show three electron diffraction patterns taken from the same
nanowire, which was identified as monoclinic sodium hexatitanate 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). The three zone axes are [103] (Fig. 1a), [1 1 3 ] (Fig. 1b) and
__
[1 11 ] (Fig. 1c). The relative orientational relationship between the three electron
__
diffraction patterns is also given in the figure. For example, the [1 1 3 ] (Fig. 1b) zone axis
electron diffraction pattern was obtained by rotating the nanowire 6.5 º about its [602]
direction (illustrated in Figure 1a). The 2-D lattice image (Fig. 2d), corresponding to the
electron diffraction pattern in Fig. 2c, further illustrate that the nanowires are single
crystalline with no defects or dislocations. Interplanar spacings are 3.63 Å and 3.61 Å,
which are consistent with (110) and (202) interplanar spacing of the monoclinic
4
Na2Ti6O13, as shown in Fig. 2d. The growth direction of the nanowire was determined to
be its [010] crystal direction, consistent with other alkali titanates with the same
monoclinic structure [8, 11, 14].
When barium oxalate was removed from the reaction system, only a few nanowires were
obtained and most of the titania nanoparticles remain unreacted. The nanowires were also
identified as the monoclinic sodium hexatitanate. It is expected that similar nanostructure
should be obtained if other alkali source is used. However, potassium hollandite in stead
of potassium hexatitanate was obtained when the titania nanoparticles were annealed in
the molten KCl with the presence of nonionic surfactant (NP-9) at 800 ºC. Fig. 3 shows
the typical morphology of the synthesized potassium hollandite nanowires. The diameter
of potassium hollandite nanowires ranges from 40 to 150 nm and the average diameter is
about 100 nm. The length of nanowires can reach up to a few tens of micrometers. An
individual nanowire (shown in Fig. 4a) was selected and tilted to obtain a series of SAED
patters. From the obtained SAED patterns the synthesized nanowires were identified to
be potassium hollandite K1.36Ti8O16 with tetragonal structure and lattice parameters of a =
10.16 Å and c =2.97 Å (JCPDS 73-1398). The tilting angle between two zone axes fits
well with the calculated one. The inset in Fig. 4a shows one electron diffraction pattern
from the studied nanowire, which indicates that the nanowire is single-crystalline with a
preferential growth direction along the [100] crystal direction. Fig. 4b is a representative
HREM image of a single-crystalline potassium hollandite nanowire, which shows the
clearly resolved interplanar distance d200 = 5.08 Å and further confirms that the
5
nanowires grow along the [100] direction.
4. Conclusions
Anatase TiO2 nanoparticles were found to react with molten alkali chloride and form
single-crystalline sodium hexatitanate and potassium hollandite nanowires with the
presence of a nonionic surfactant. The nanowires have a diameter around 100 nm and
length up to a few tens of micrometers. The synthesize sodium titanate and potassium
hollandite nanowires were found to grow along [010] and [100] crystal direction,
respectively. This method may also be used for the fabrication of other ternary oxide
nanostructures.
Acknowledgement
C.Y. Xu would like to thank H. Zhang for help with experiment setup and L. Yang and
J.T. Jiang for TEM assistance.
6
References:
[1] T. Sasaki, F. Kooli, M. Iida, Y. Michiue, S. Takenouchi, Y. Yajima, F. Izumi, B.C.
Chakoumakos, M. Watanabe, Chem. Mater. 10 (1998) 4123
[2] S. Andersson, A.D. Wadsley, Acta Cryst. 15 (1961) 194
[3] K.L. Berry, V.D. Aftandilian, W.W. Gilbert, E.P. Meibohm, H.S. Young, J. Inorg. Nucl.
Chem. 14 (1960) 231
[4] V. Vogt, E. Schweda, C. Wüstefeld, J. Strähle, A.K. Cheetham, J. Solid State Chem.
83 (1989) 61
[5] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q.
Yan, Adv. Mater. 15 (2003) 353
[6] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31
(2003) 5
[7] X. Sun, X. Chen, Y. Li, Inorg. Chem. 41 (2004) 4996
[8] X.D. Meng, D.Z. Wang, J.H. Liu, S.Y. Zhang, Mater. Res. Bull. 39 (2004) 2163
[9] Y.N. Zhao, U.H. Lee, M. Suh, Y.U. Kwon, Bull. Korean Chem. Soc. 25 (2004) 1341
[10] G.H. Du, Q. Chen, P.D. Han, Y. Yu, L.M. Peng, Phys. Rev. B 67 (2003) 035323
[11] B.L. Wang, Q. Chen, R.H. Wang, L.M. Peng, Chem. Phys. Lett. 376 (2003) 726
[12] Z.Y. Yuan, X.B. Zhang, B.L. Su, Appl. Phys. A 78 (2004) 1063
[13] Y.B. Mao, S. Banerjee, S.S. Wong, J. Am. Chem. Soc. 125 (2003) 15718
[14] H.Y. Zhu, X.P. Gao, Y. Lan, D.Y. Song, Y.X. Xi, J.C. Zhao, J. Am. Chem. Soc. 126
(2004)
7
Figure Captions:
Fig. 1. Morphology of the synthesized sodium titanate nanowires. (a) SEM; (b) TEM.
Fig. 2. (a-c) Electron diffraction patterns of the same sodium hexatitanate nanowire tilted
__
__
to zone axes [103], [1 1 3 ] and [1 11 ], respectively. Dash lines indicate the tilting axes
about which the nanowires were rotated to obtain the needed orientation. Circular arrows
give the tilting angles, and the straight arrows point to the tilting sequence. (d) Typical
__
HREM image of Na2Ti6O13 nanowire with zone axis of [1 11 ] corresponding to Fig. 2c.
Fig. 3. TEM images of the synthesized potassium hollandite nanowires.
Fig. 4. (a) Low magnification TEM image of an individual potassium hollandite nanowire.
The inset is its corresponding SAED pattern. (b) Representative HREM image of
potassium hollandite nanowire.
8
(a)
(b)
Fig. 1 C.Y. Xu et al.
9
(a)
(b)
25.8º
6.4º
(c)
(d)
(202)
(110)
Fig. 2 C.Y. Xu et al.
10
Fig. 3 C.Y. Xu et al.
11
(a)
(b)
Fig. 4 C.Y. Xu et al.
12