Microstructure and formation mechanism of titanium dioxide nanotubes

Transcription

1 Chemical Physics Letters 365 (2002) Microstructure and formation mechanism of titanium dioxide nanotubes Y.Q. Wang a,b, *, G.Q. Hu a, X.F. Duan a, H.L. Sun c, Q.K. Xue c a Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing , PR China b H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK c National Laboratory of Surface Science, Institute of Physics, Beijing , PR China Received 14 June 2002; in final form 26 August 2002 Abstract Titanium dioxide nanotubes are synthesized using sol gel method. Electron energy-loss spectroscopy has been used to investigate the chemical composition of the nanotubes. The results show that the atomic ratio of O/Ti is very close to two. Electron diffraction studies indicate that the nanotubes have the anatase structure. High-resolution transmission electron microscopy studies show that the nanotubes possess a layered structure with layer spacing of about 7.1 A. The tube axis is determined to be along [0 1 0] direction of the anatase phase. The formation mechanism of the nanotubes can be explained as 3D! 2D! 1D. The two-dimensional lamellar TiO 2 is essential for the formation of the nanotubes. Ó 2002 Elsevier Science B.V. All rights reserved. Titanium dioxide, TiO 2, has attracted considerable attention because it has great potential for such applications as catalysis [1], photocatalysis [2] and dye-sensitized solar cells [3]. Extensive research has been conducted on TiO 2 nanoparticles [4] thin film [5] and mesoporous TiO 2 [6,7]. Recently, much effort has been directed at obtaining TiO 2 nanotubes with a large surface area and high photocatalytic activity. TiO 2 nanotubes with diameters of nm were produced using the sol gel method [8] or porous alumina as template [9]. Smaller TiO 2 nanotubes with diameters of about 8 nm were reported, whose structure was * Corresponding author. Fax: address: yiqian.wang@bristol.ac.uk (Y.Q. Wang). considered as anatase phase [10] or anatase rutile phase [11]. More recently, highly ordered TiO 2 nanowire arrays have been reported with the structure of anatase phase [12]. Up to now, the structure and the formation mechanism of TiO 2 nanotubes have not been clarified yet. In this Letter, parallel electron energy-loss spectroscopy (EELS) has been used to investigate the chemical composition of the nanotubes. The microstructure of the nanotubes has been studied using high-resolution transmission electron microscopy (HRTEM). The formation mechanism of the TiO 2 nanotubes is discussed. TiO 2 nanotubes were synthesized following the sol gel procedure reported by Kasuga et al. [9]. The anatase-phase granules (lattice parameter /02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 428 Y.Q. Wang et al. / Chemical Physics Letters 365 (2002) a ¼ b 3:78 A; c 9:52 AÞ with diameters of about nm were used as the raw material. Specimens for transmission electron microscopy (TEM) observation were prepared by dispersing the powder in alcohol by ultrasonic treatment for several minutes, dropping onto a holey carbon film supported by a copper grid. Selected-area electron diffraction (SAED), HRTEM and parallel EELS studies were carried out using a Philips CM200- Field Emission Gun (FEG) transmission electron microscope equipped with a Gatan imaging filter (GIF) system. The point-to-point resolution is 2.4 A. The EELS spectrum was acquired in image mode with half collection angle of 13 mrad. Low-magnification TEM observation indicates that the nanotubes with narrow size distribution. The diameters of nanotubes are around 8 10 nm and the lengths are from several tens to several hundreds of nanometers, similar to the results reported by Kasuga et al. [10,13]. The chemical composition of the nanotubes was studied using parallel EELS. Only the free-standing isolated nanotubes are selected for acquiring the EELS spectrum in order to avoid the influences of carbon film and other nanotubes. Fig. 1 is a typical EELS spectrum from a single nanotube. It can be found that the peak at 532 ev corresponds to O Kedge; the peak at ev corresponds to the L 2;3 edges of Ti. There is no obvious peak of any impurity in this EELS spectrum. The quantification of the EELS spectrum shows that atomic ratio of O/Ti is Fig. 1. A typical EELS spectrum acquired from a single nanotube. very close to two. So the chemical formula of the nanotubes could be determined as TiO 2. HRTEM experiments were carried out to investigate the microstructure of the nanotubes. Fig. 2a is the typical HRTEM image of a single nanotube. The end of this nanotube is open, which is typical for the TiO 2 nanotubes. The inner diameter of this tube is around 5 nm, and the outer diameter is about 9 nm. It shows a layered structure similar to a carbon nanotube with layer spacing of about 7.1 A, which is two times the {1 0 1} planar spacing of the anatase phase. At the middle part of the nanotube, the lattice fringes are of around 3.7 A, corresponding to the {0 1 0} planes of the anatase phase. To further clarify the crystal structure and crystal orientation of the nanotubes, SAED pattern (Fig. 2b) of the single nanotube (shown in Fig. 2a) was obtained. The diffraction spots in Fig. 2b can be indexed according to the anatase structure. Combining the electron diffraction result (Fig. 2b) with HRTEM image (Fig. 2a), the tube axis is determined to be along the [0 1 0] direction of the anatase phase. A cross-section HRTEM image of one nanotube is shown in Fig. 2c. The layer spacing is about 7.1 A, two times the {1 0 1} planar spacing of the anatase phase. From this image, it can be clearly seen that the outer shell is not closed and the inner shell is also not closed. Moreover, the outer layer is curved outside. From the TEM observation, it seems that the TiO 2 nanotube is formed through scrolling the TiO 2 layer sheet. It is confirmed that the number of the wall of some TiO 2 nanotubes on one side is one more than that on the other side (Fig. 2c). To understand the formation mechanism of the TiO 2 nanotubes, different processing time of hydrothermal reaction was adopted. With short reaction time, some lamellar titanium oxide products were produced. A low-magnification TEM image of the lamellar products is shown in Fig. 3a. The typical HRTEM image of the lamellar TiO 2 is shown in Fig. 3b. It has the same structure as the nanotubes. Seo et al. [14] discussed the formation mechanism of TiO 2 nanotubes. Considering the crystalline characteristics of anatase ða ¼ b 3:78 A; c 9:52 AÞ and rutile ða ¼ b 4:59 A;c 2:95 AÞ phase, they thought that anatase TiO 2 powder

3 Y.Q. Wang et al. / Chemical Physics Letters 365 (2002) Fig. 3. (a) A low-magnification TEM image of the lamellar titanium oxide; (b) a typical HRTEM image of the lamellar TiO 2. Fig. 2. (a) A typical HRTEM image of a single TiO 2 nanotube; (b) electron diffraction pattern of the single nanotube in Fig. 2a; (c) a cross-section HRTEM image of a single TiO 2 nanotube. (with longer c axis) rather than rutile one is preferred to form the nanotube. Theoretical studies [15,16] suggested that the surface energy of a anatase TiO 2 macroscopic crystal is smaller than that of rutile phase. This can also explain the fact that TiO 2 nanotubes are preferred to have the anatase structure. This is consistent with what we have observed from HRTEM. Based on the experimental results, a schematic model (Fig. 4) of 3D! 2D! 1D is proposed for the formation mechanism of TiO 2 nanotubes. Firstly, the raw anatase-phase TiO 2 (Fig. 4A), three-dimensional crystal, reacted with NaOH aqueous solution and formed lamellar products (Fig. 4B). The lamellar TiO 2 products possessed

4 430 Y.Q. Wang et al. / Chemical Physics Letters 365 (2002) A B two-dimensional layered structures (Fig. 4B). The edges of the lamellar products might have many atoms with dangling bonds, and they should possess enough energy to destabilize the two-dimensional structures. Subsequently, the lamellar TiO 2 bent to saturate these dangling bonds (Fig. 4B). Finally, the nanoscale tubular structure was achieved by rolling further and then one-dimensional nanotubes were formed (Fig. 4C). There are usually two rolling methods from 2D to 1D, so the nanotubes should have two possible structures: one is concentric and the other is non-concentric structure. Only the non-concentric structures (Fig. 2c) were observed in our experiments. In conclusion, EELS results show that the chemical formula for the nanotubes is TiO 2. SAED and HRTEM studies indicate that the tube axis is along [0 1 0] direction of the anatase phase. The ends of most nanotubes are open. The outer shells of some nanotubes are not closed and curved outside. The formation mechanism of the nanotubes can be explained as 3D! 2D! 1D, that is to say, the raw TiO 2 was first transformed into lamellar products and then bent and rolled to form the nanotubes. The two-dimensional lamellar TiO 2 is essential for the formation of the TiO 2 nanotubes. Acknowledgements The authors from BLEM would like to acknowledge the support of National Natural Science Foundation (No ) of China. Thanks for the helpful comments from Professor David Cherns. References C Fig. 4. A schematic model for the formation of TiO 2 nanotubes. [1] K.I. Hadjiivanov, D.G. Klissurski, Chem. Soc. Rev. 25 (1996) 61. [2] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [3] B. OÕRegan, M. Gratzel, Nature (London) 353 (1991) 737. [4] W. Chen-Chi, J.Y. Ying, Chem. Mater. 11 (1999) [5] N. Negishi, K. Takeuchi, T. Ibusuki, J. Mater. Sci. Lett. 18 (1999) 515. [6] N. Ulagappan, C.N.R. Rao, Chem. Commun. (Cambridge) 14 (1996) [7] J.S. Yin, Z.L. Wang, Adv. Mater. 11 (1999) 469. [8] P. Hoyer, Langmuir 12 (1996) 141. [9] H. Imai, Y. Takei, K. Shimizu, M. Matsuda, H. Hirashima, J. Mater. Chem. 9 (1999) 2971.

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