Natural rutile-derived titanate nanofibers prepared by direct hydrothermal processing
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Ryoji Kawahata Iwatani International Corporation, Osaka 541-0053, Japan (Received 28 July 2004; accepted 2 February 2005)
Long titanate nanofibers (typically 10–500 m in length and 20–50 nm in diameter) were successfully prepared in high yield by the direct hydrothermal processing using natural rutile as a starting material. Fourier transform infrared spectroscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron diffraction, and x-ray diffraction demonstrated that the as-synthesized nanofibers presumably consisted of sodium hydrogen trititanate [(Na,H)2Ti3O7, e.g., Na0.4H1.6Ti3O7] including some hexatitanate-type defects [(Na,H)2Ti6O13]. A partial topotactic condensation model explained their nanostructure well. Although the as-synthesized fibers are defective, they can be cured by a post-heat-treatment in air. The direct hydrothermal treatment for natural rutile will be a promising low-cost process for one-dimensional nanomaterials, which can act not only as a reaction step but also as a purification step.
I. INTRODUCTION
One-dimensional TiO2 and TiO2-based nanomaterials have received considerable attention for their unique microstructure and promising photo-electrochemical applications. A number of processing methods have been recently reported for one-dimensional (1D) TiO2-related nanomaterials. For example, electrospinning1 and hydrogen treatment2 have been proposed for TiO2-nanofiber synthesis, and anodic porous alumina templating,3,4 anodic oxidation of a titanium sheet,5 carbon nanotube inner templating,6 supramolecular assemblies templating,7 and hydrothermal NaOH (aq.) treatment8,9 have been investigated for TiO2-nanotube synthesis. Among these methods, the hydrothermal method to produce nanotubes, proposed first by Kasuga et al., has been widely investigated to clarify the nanotube-formation mechanism, crystal structure, and exact chemical compositions.10–18 Recent studies indicate that the obtained nanotubes are composed of a layered titanate10–18 (not titania, TiO2) and can be expressed as a general formula of NayH2−yTinO2n+1·xH2O.19 Nevertheless, this simple process has become more attractive for producing 1D TiO2-related nanomaterials because as-synthesized titanates can be further modified by post-treatments.17–19
Furthermore, this technique can be also applied for the synthesis of titanate nanowires (i.e., not hollow but solid) using KOH (aq.).20 The hydrothermal process is potentially cost effective; however, rather expensive synthetic TiO2 fine powders have been used as starting materials in previous work. To reduce the production cost, use of a well-identified natural source is one of the most promising strategies. Fujiki and Mitsuhashi,21 for example, prepared K2Ti6O13 fibers by a melting process at 1100 °C using natural rutile as a starting material. They successfully obtained bundles of micro fibers with the diameter of 1–50 m. However, for the 1D nanomaterials, the use of natural rutile has not been extensively reported. In this study, the present author
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