hydrothermal synthesis of na2ti6o13 and tio2 whiskers

Journal of Crystal Growth 275 (2005) e2371–e2376 Hydrothermal synthesis of Na 2 Ti 6 O 13 and TiO 2 whiskers Dong-Seok Seo a , Hwan Kim a , Jong-Kook Lee b,Ã a School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea b Department of Advanced Materials Engineering, Development of Intelligent Materials, Chosun University, Gwangju 501-759, Korea Available online 21 December 2004 Abstract Na 2 Ti n O 2n+1 typed whiskers has been extensively used for frictional materials, reinforcement materials and high insulators, and TiO 2 whiskers can be applied for catalyst support and photocatalysts. Na 2 Ti 6 O 13 whiskers were easily synthesized by hydrothermal treatment of the mixed solution of spherical TiO 2 powder with anatase structure and NaOH solution at 250 1C for 4 h. The sodium titanate (Na 2 Ti 6 O 13 ) whiskers obtained had a smooth surface and high aspect ratio of 100 nm below in diameter and 100 mm above in length. TiO 2 whiskers were obtained by acid treatment of the Na 2 Ti 6 O 13 whiskers in 0.5 M HCl solution at 100 1C for 48 h. This suggests that Na ions in the Na 2 Ti 6 O 13 structure were extracted during acid treatment and the formed TiO 2 Á nH 2 O hydrate was turned to the TiO 2 whisker with anatase phase. r 2004 Elsevier B.V. All rights reserved. PACS: 61.82.Rx; 61.66.Fn Keywords: A1. Low-dimensional structures; A1. Nanostructures; A2. Hydrothermal crystal growth; B1. Nanomaterials 1. Introduction Nanostructured materials have received much attention because of their novel properties which differ from those of bulk materials. One-dimen- sional materials are an important category of nanostructured materials [1,2] and have been widely researched, yielding various special struc- tures such as nanowhiskers [3–5], nanowires [6] and nanobelts [7]. The crystal structures of alkali-metal titanates, A 2 Ti n O 2n+1 are well-known. All of them have a monoclinic structure with almost the same b value [4,8]. Alkali-metal titanates with a high alkali- metal content (n ¼ 2; 3; 4) are open-layered struc- tures having layers made of titanate groups held together by alkali-metal ions. They can be used as cation exchangers and catalysts because of their distinctive intercalation ability and catalytic activ- ity [9–12]. On the other hand, alkali-metal titanates with a low alkali-metal content ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.340 Ã Corresponding author. Tel.: +82 62 230 7202; fax: +82 62 232 2474. E-mail address: jklee@mail.chosun.ac.kr (J K. Lee). (n ¼ 6; 7; 8) are tunnel structures and exhibit high insolating, mechanical and chemical ability [13–15]. Specially, sodium titanate (Na 2 Ti 6 O 13 )(A 2 Ti- n O 2n+1 , n ¼ 6) whisker combined with Na 2 Ti 3 O 7 or TiO 2 has been applied for an oxygen electrode of CO 2 gas sensors and clarify the ion exchanges at the interface between the gas and electrolyte. For instance, Holzinger et al. [16] improved the long- term stability and selectivity of fast potentiometric CO 2 sensors using a reference electrode consisting of Na 2 Ti 3 O 7 /Na 2 Ti 6 O 13 or Na 2 Ti 6 O 13 /TiO 2 , which are chemically inert against CO 2 . Ramirez-Salgado et al. [9] also proved that those composites could be used as oxygen electrode materials in potentio- metric gas sensor devices. Furthermore, sodium titanate as an ion exchanger can be used for the removal of transition metals and anions from drinking water [17] and purification of heavy metals from industrial waste water [18]. There are several methods to synthesize alkali- metal titanates including sodium titanate such as calcination, melt reaction, flux growth and slow- cooling calcination. These methods usually need high reaction temperatures for a long period of time. For example, Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 were synthesized by heating mixtures of Na 2 CO 3 and TiO 2 or Na 2 O and TiO 2 , at 1000 1C for one day [16]. Meanwhile, the hydrothermal method has many advantages: (i) the crystallization tempera- ture is obviously lower than that in the heat treatment process; (ii) hard agglomeration among particles can be prevented because crystallization proceeds under the high pressure; (iii) products without calcination or milling may guarantee a high quality of powder; (iv) it is easy to prepare nano-sized powder with controlled particle shape and size distribution, although the process shows slow reaction rate and is not appropriate for production on a large scale due to a volume limit of reaction vessel. In this work, we synthesized sodium titanate whiskers by the hydrothermal method, reacting between TiO 2 and NaOH. TiO 2 has attracted considerable interest due to its good characteristics of chemical stability, endurance, thin film transparency and lower production costs. Furthermore, TiO 2 photocata- lyst has been studied for applications in, for instance, water purification, decomposition of NO x and improvement of living conditions by removal of various pollutants, etc. [19–21]. In this paper, we also demonstrated the preparation of TiO 2 whisker by extracting Na species from the Na 2 Ti 6 O 13 whiskers using an acid treatment. 2. Experimental procedure For the preparation of Na 2 Ti 6 O 13 whiskers, TiO 2 nano-sized powder with anatase structure was used as a starting material. TiO 2 powder with anatase phase was obtained by precipitation reaction between TiOCl 2 and ammonium hydro- xide solutions, followed by heat treatment at ARTICLE IN PRESS Fig. 1. TEM micrograph and XRD pattern of TiO 2 powder with anatase structure. D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2372 450 1C for 1 h. In the hydrothermal process, the TiO 2 powder with 10 N NaOH solution was placed in a Teflon vessel and autoclaved at a temperature of 200–250 1C. Na 2 Ti 6 O 13 whiskers were obtained after filtering and washing. TiO 2 whiskers were prepared by extracting Na ions from the Na 2- Ti 6 O 13 whiskers using the acid treatment in 0.5 M HCl solution at 100 1C for 48 h. After washing repeatedly using distilled water until chloride ions were completely removed, the whiskers were dried at 80 1C. The crystallinity of the obtained powder was analyzed by means of an X-ray diffractometer (XRD) and transmission electron microscope (TEM) work was carried out to investigate the microstructures. 3. Results and discussion Fig. 1 shows XRD pattern and TEM micro- graph of TiO 2 powder prepared by precipitation and subsequent heat treatment at 450 1C. It has well-crystallized anatase structure and consists of spherical particles approximately 10 nm in size with narrow size distribution. The powder made of TiO 2 spherical particles was hydrothermally treated in an autoclave at 200 1C for various times. Fig. 2 demonstrates the microstructural evolution of TiO 2 particles with hydrothermal reaction time from 10 min to 4 h. The particles experienced a change of shape from spherical or spherulitic shapes to columnar crystals and the aspect ratio of the particles also increased during the hydrothermal reaction. The fiber-like particles were formed at the initial stage of the reaction (Fig. 2a). The fibers were actually produced from the needle-like particles, indicated by an arrow, which were generated from the spherical particles. As the reaction progresses, the fiber-like particles tend to grow into long and thin fibers of 300–400 nm length (Fig. 2b and c). On autoclaving for 4 h, the fibers grew in both ARTICLE IN PRESS Fig. 2. Microstructural evolution of TiO 2 powder from sphere to columnar particles with hydrothermal reaction times of (a) 10 min, (b) 30 min, (c) 1 h and (d) 4 h. D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376 e2373 diameter and length, producing the columnar crystallites rather than a whisker with 200 nm in diameter and 1–1.5 mm length. However, it seemed that there were still unreacted particles, resulting in a relatively rough surface and wide size distribu- tion. As shown in Fig. 3, hydrothermal treatment at a higher temperature of 250 1C gave rise to the long whiskers with a considerable aspect ratio. TEM micrographs showed that the whiskers had a clean and smooth surface, which suggested no presence of unreacted particles. The whiskers were uni- formly distributed and had a size of less than 100 nm in diameter and a length exceeding 100 mm. From the X-ray diffraction analysis, we found that the whiskers were of Na 2 Ti 6 O 13 structure and grew into almost single-crystalline structure and also confirmed that the whiskers consisted of Na, Ti, O atoms from EDS analysis (Fig. 4a). This suggests that the spherical TiO 2 particles take a dissolution and reprecipitation process. During the reaction between spherical TiO 2 and NaOH, the particles are dissolved and reprecipi- tated, while NaOH may play a role accelerating the continuous growth of the reprecipitated particles to the whiskers with a specific direction. In addition, the solubility of the spherical particles for the NaOH can increase as the time and temperature of hydrothermal reaction are longer and higher; accordingly, whiskers with high aspect ratio are formed in the case of the process autoclaving at 250 1C for 4 h. In order to obtain TiO 2 whiskers, sodium titanate whiskers were placed in 0.5 M HCl solution and refluxed at 100 1C for 48 h. From the EDS analysis (Fig. 4b), it was observed that there were no sodium atoms in the acid- treated powder compared to the Na 2 Ti 6 O 13 whiskers. Furthermore, the XRD pattern shows Fig. 4c that there is no peak corresponding to Na 2 Ti 6 O 13 structure and all peaks are identical to TiO 2 with anatase structure. There was no ARTICLE IN PRESS Fig. 3. TEM micrographs and XRD pattern of Na 2 Ti 6 O 13 whiskers autoclaved at 250 1C for 4 h. D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376e2374 different crystalline peak between the spherical particles in Fig. 1b and the acid-treated ones. Sodium ions were almost completely extracted from sodium titanate by acid treatment, leading to the formation of TiO 2 Á nH 2 O hydrate. TiO 2 in the hydrate contains different amounts of H 2 O upon the degree of hydrolysis and TiO 2 Á nH 2 O could be completely hydrolyzed and subsequently crystallized to TiO 2 whisker because of an aging effect that can happen during the acid treatment process. Fig. 5 presents the microstructure of the acid- treated TiO 2 powder. TiO 2 whiskers, 100–200 nm in diameter and 5–10 mm in length, could be obtained from the Na 2 Ti 6 O 13 whiskers. TiO 2 whiskers had quite a smooth and clean surface although the structure was partially disintegrated. It was also confirmed the crystal planes in the selected area diffraction (SAD) pattern were in accordance with anatase structure. 4. Conclusions This study has focused on the synthesis of sodium titanate (Na 2 Ti 6 O 13 ) whiskers by hydro- thermal treatment using spherical anatase-typed TiO 2 powder, and also on the preparation of TiO 2 whiskers by extracting Na ions from the sodium titanate whiskers. The Na 2 Ti 6 O 13 whiskers ob- tained had a clean surface and a considerable aspect ratio with less than 100 nm diameter and a length exceeding 100 mm. Dissolution and repreci- pitation process for the TiO 2 spherical particles possibly gave rise to a change in the shape of the particles from needle-like, fiber, and eventually to the long and thin whiskers with a smooth surface. The morphology of the whiskers seemed to be influenced by the reaction time and temperature during the hydrothermal process. After extraction of Na ions from the Na 2 Ti 6 O 13 structure, TiO 2 Á nH 2 O hydrate was formed and ARTICLE IN PRESS Fig. 4. EDS patterns of (a) Na 2 Ti 6 O 13 whiskers, (b) TiO 2 whiskers and (c) XRD pattern of TiO 2 whiskers. D S. Seo et al. / Journal of Crystal Growth 275 (2005) e2371–e2376 e2375 readily turned to crystalline TiO 2 retaining the whisker shape during the acid treatment. Acknowledgments This study was supported by research funds from Chosun University, 2003. 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The whiskers were uni- formly distributed and had a size of