Synthesis of characterization of sodium titanate and potassium hollandite nanowires
C.Y. Xu a, Q. Zhang b, L. Zhen a, L.-C. Qin b, c *
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:
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 (3n8 and A=K, Na, Li), consist of a layer structure with 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 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 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] (Fig. 1b) and [1] (Fig. 1c). The relative orientational relationship between the three electron diffraction patterns is also given in the figure. For example, the [1] (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 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 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.
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)
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] and [1], 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] 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.
Fig. 1 C.Y. Xu et al.
Fig. 2 C.Y. Xu et al.
Fig. 3 C.Y. Xu et al.
Fig. 4 C.Y. Xu et al.