A simple method to synthesize nanowires titanium dioxid e from layered titanate particles Mingdeng Wei * , Yoshinari Konishi, Haoshen Zhou, Hideki Sugihara, Hironori Arakawa National Institute of Advanced Industrial Science and Technology (AIST), Energy Technology Research Institute, AIST Tsukuba Central #5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 11 September 2004; in final form 25 October 2004 Available online 11 November 2004 Abstract Nanowires TiO 2 were successfully synthesized from layered titanate Na 2 Ti 3 O 7 particles by a simple soft chemical process. Com- pared with other synthetic routes where some templates or reactants were introduced into reaction system, only raw material and dilute HCl were used in this simple method. The diameters of nanowires are ca. 20–100 nm and the length up to several hundred micrometers. Formation of brookite TiO 2 phase in the nanowires was confirmed by XRD and TEM measurement. Based on our experimental results, an exfoliating-splitting model was proposed for formation of nanowire structure. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Since carbon nanotubes were discovered by Ijima in 1991 [1], nanoscale materials have attracted immense interest due to structure, electronic and optical proper- ties and their potential applications in electronic-, pho- tonic-device [2], gas sensor, solar cell, lithium battery and fuel cell. The growing Ônano-toolboxÕ contains organic and inorganic components, which come in a variety of shapes and dimensionalities, includi ng nano- particles/quantum dots (zero-dimension, 0D), nano- tubes/nanorods/nanobelt/nanoribbons/nanowires (one dimension, 1D), and nanosheets/nanohorns (two dimen- sion, 2D). Materials in 1D form include TiO 2 [3],MnO 2 [4],V 2 O 5 [5], ZnO [6], CdSe [7] and MoS 2 [8]. Among them, nanoscale TiO 2 is particularly interesting because they have large surface area, leading to a higher poten- tial of application in environment purification, gas sen- sor, and photovoltaic cell [9]. Several methods have been reported to prepare 1D nanowires TiO 2 . Jiang et al. [10] reported the synthesis of nanowires TiO 2 using polyol-mediated process. How- ever, the products are required to remove organic com- pound, resulting in increase the cost of product. Using the hydrothermal process, nanowires TiO 2 were synthe- sized in high concentration KOH solution [11,12].As well known, alkali solution is a strong corrosiveness and very harmful to human health and environment. Nanowires TiO 2 were also formed in anodic alumina membranes by a sol–gel process [13–15]. However, the diameters of formed nanowires were decided by through-hole membrane. Xu et al. [16] prepared nano- wires TiO 2 via annealing TiS 2 precursor with or without the presence of molten-salt flux in ammonia gas atmos- phere. Among them, the templates or reactants were introduced into the reaction system. This means a much more complicated process, and might bring about an in- crease of impurity concentration in the final product. To our knowledge, however, synthesis of nanowires TiO 2 using a soft chemical process has not been concerned. In this Letter, we investigated the preparation of 1D structural nanowire TiO 2 from the layered material 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.114 * Corresponding author. Fax: +81 29 8616771. E-mail address: wei-mingdeng@aist.go.jp (M. Wei). www.elsevier.com/locate/cplett Chemical Physics Letters 400 (2004) 231–234 Na 2 Ti 3 O 7 particles using a soft chemical process, with no existence of reactants or templates, only the raw material and dilute HCl were used. Further, a formation mechanism of nanowires TiO 2 was proposed according to the results of SEM, TEM and XRD. 2. Experimental 2.1. Preparation of samples The layered material Na 2 Ti 3 O 7 was synthesized from Na 2 CO 3 (Wako) and TiO 2 (ST-01, Ishihara Sangyo Kaisha LTD.) in the stoichiometrical ratio 1:3. The pow- ders were mixed together and repeatedly ground in an agate mortar, and calcined at 1000 °C for 2 h in the air. Synthesis of nanowires TiO 2 was performed by a hydrothermal soft chemical process. In a typical synthe- sis, 0.15 g Na 2 Ti 3 O 7 was dispersed into a 15 ml 0.05–0.1 M HCl solution, then was transferred into a 30 ml auto- clave, and kept at 140–170 °C for 3–7 days. The as-prod- uct was filtered, washed with H 2 O, and finally dried at 60 °C for 4 h. 2.2. Characterization of samples X-ray powder diffraction (XRD) patterns were re- corded using a diffractometer (Mac Science). Scanning electron microscope (SEM) and transmission electron micrographs (TEM) were taken on a TOPCON DS instrument and a JEOL instrument, respectively. 3. Results and discussion Fig. 1 shows SEM images of the raw material Na 2- Ti 3 O 7 and the sample obtained at 170 °C for 7 days. It is very different between the raw material Na 2 Ti 3 O 7 and the sample. Only particles were observed for the raw material, as depicted in Fig. 1a. The SEM image of the sample shown in Fig. 1b has revealed the presence of numerous nanowires with typical lengths ranging from several micrometers to several tens of micrometers. It notice d that some brush-like sheets attached to some of the nanowires were observed (Fig. 1b, circle A). In addition, some particle aggregates and sheets with rolled edge were also found (Fig. 1b, circle B). These nano- wires lie close to each other. Further magnification clearly shows that the diameter of these nanowires is ca. 20–100 nm (see Fig. 1c). The formation of numerous nanowires was further confirmed by TEM measurement. Fig. 2 depicts TEM images of the sample containing lots of nanowires. The low magnification image, as shown in Fig. 2a, indi- cates that the size and length of nanowires are same as those observed in SEM experiment. One single nanowire was selected, as shown in Fig. 2b. It is clear that the end of nanowire is cone-shaped. Further magnification clearly shows the single-crystalline nature of sample (in- set in Fig. 2c, left upper), and the lattice fringes corre- spond to a d-spacing of 0.22 nm. XRD patterns of the raw material Na 2 Ti 3 O 7 and the product containing lots of nanowire were depicted in Fig. 3. The former shows a typical profile of layered compound, and could be indexed to a layered titanate Na 2 Ti 3 O 7 [17]. The latter is obviously different from the former. TiO 2 as the main phase was observed besides the phase of H 2 Ti 3 O 7 (JCPDS 47-0561), and it could be indexed to a brookite TiO 2 with an orthorhom bic struc- ture (JCPDS 72-1582). The d 202 -spacing is 0.22 nm. This is in agreement with the lattice fringe obtained from TEM image, indicating that the nanowires in product are TiO 2 with brookite structure. Formation of H 2 Ti 3 O 7 was contributed to ion-exchange reaction. In Fig. 1. SEM images of (a) the raw material Na 2 Ti 3 O 7 together with the product containing lots of nanowires (b) low magnification and (c) high magnification. 232 M. Wei et al. / Chemical Physics Letters 400 (2004) 231–234 the dilute HCl solution, Na + in the raw material Na 2- Ti 3 O 7 was replaced by H + to form H 2 Ti 3 O 7 phase. In this study, only the raw material Na 2 Ti 3 O 7 and di- lute HCl were used, no any templates or reactants were introduced into the reaction system. Based on the exper- imental results of XRD, SEM and TEM, an exfoliating- splitting model of nanowires formation from layered Na 2 Ti 3 O 7 was proposed, as illustrated in Scheme 1.It is well known that Na + cations reside between edge- shared [TiO 6 ] octahedral layers in Na 2 Ti 3 O 7 [17]. The strong static interaction between the Na + cations and [TiO 6 ] unit holds the layers together tightly. Under hydrothermal conditions, Na + cations will be replaced gradually with intercalated H þ 3 O molecule into the inter- layer space of [TiO 6 ] sheets. Because the size of H þ 3 Ois larger than that of Na + cation, the interlayer distance was enlarged, resulted in weakened static interaction be- tween neighboring [TiO 6 ] octahedral sheets. As a result, the layered compound Na 2 Ti 3 O 7 particles were gradu- ally exfoliated. In the meantime, Na + in Na 2 Ti 3 O 7 was exchanged by H + in the dilute HCl solution to form numerous H 2 Ti 3 O 7 sheet-shaped products. The nano- sheet does not has an inversion symmetry, i.e. the layer is asymmetric, an intrinsic tension exists which might gradually tends to roll from the edges of nanosheets [18]. In order to release the strong stress and lower the total energy, the nanosheets are splitted, resulting in for- mation of the nanowires. This formation model can be used to explain the formation of brush-like nanosheets observed in SEM images. There exist three procedures Scheme 1. An exfoliating-splitting model of nanowires formation from the layered Na 2 Ti 3 O 7 particles by a soft chemical process. Fig. 2. TEM images of the sample containing lots of nanowires (a) low magnification, (b, c) high magnification. Fig. 3. XRD patterns of (a) the raw material Na 2 Ti 3 O 7 and (b) product containing lots of nanowires. M. Wei et al. / Chemical Physics Letters 400 (2004) 231–234 233 in the reaction: (i) the exfoliation from layered Na 2 Ti 3 O 7 particles to form nanosheets; (ii) the split of nanosheets to form nanowires; (iii) the exfoliation accompanied by splitting to form brush-like nanosheets. Therefore, the formation of brush-like nanosheets can be contribu ted to fact that both exfoliating and splitting are incomplete. 4. Conclusions In this Letter, a simple method involving a soft chem- ical process started from the layered compound Na 2- Ti 3 O 7 particles has been used to synthesize single- crystal TiO 2 nanowires. According to the comparative experimental results, an exfoliating-splitting model was proposed for formation of nanowire structure. 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A simple method to synthesize nanowires titanium dioxid e from layered titanate particles Mingdeng Wei * , Yoshinari Konishi, Haoshen Zhou,. and MoS 2 [8]. Among them, nanoscale TiO 2 is particularly interesting because they have large surface area, leading to a higher poten- tial of application