A study on the growth and structure of titania nanotubes
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Qinglei Wang and Elizabeth C. Dickey Department of Materials Science and Engineering, and Materials Research Institute, 221 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 8 July 2003; accepted 28 October 2003)
Titania nanotubes synthesized by a soft chemical process are described, having diameters of 8 nm to 10 nm and lengths ranging from approximately 0.1 m to 1 m. X-ray diffraction studies show the structure of the as-prepared nanotubes is the same as that of the starting anatase TiO2 nanoparticles. Energy-dispersive x-ray analysis and electron energy loss spectroscopy studies further indicate that the as-prepared nanotubes are composed of titania. Studies using transmission electron microscopy verified that the nanotubes are formed during alkali treatment, with subsequent acidic treatments having no effect on nanotube structure and shape.
Our interest lies in fabrication of nano-dimensional metal oxide materials and their application toward achieving gas sensors of exquisite sensitivity and performance due to their inherent nano-architectures.1,2 Earlier reports on fabrication of metal-oxide nanotubes include Al2O3,3,4 TiO2,5 VOx,6 and ZrO2.7 Titania is of particular interest for use in gas sensors, photocatalysis, and photovoltaics, hence considerable effort has focused on developing efficient routes of fabricating titania nanotubes of controllable dimensions.5,8–17 Among the different chemical methods available for titania nanotube fabrication, a method introduced by Kasuga et al.9,16 received much attention not only because the method is suitable for producing thin-walled nanotubes, but also due to the discrepancies in the conclusions they reached per nanotube formation. They used hydrothermal treatment of titania particles with NaOH solution at 110 °C, with the resulting product washed with water and hydrochloric acid to form titania nanotubes. Kasuga and coworkers16 concluded that this washing procedure was critical in the formation of titania nanotubes. However, Du and coworkers,15 using the same hydrothermal process at 130 °C, obtained nanotubes without washing with water and HCl. However, Du et al.15 concluded that the resulting nanotubes were not TiO 2 but rather H2Ti3O7. Sun and Li,18 following a similar process route, concluded that washing with water helps in nanotube formation and argued that the resulting nanotubes were not titania but of a titanate composition NaxH2-xTi3O7 (with x ≈ 0.75). Yao et al.19 recently contradicted the a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 2, Feb 2004
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claims of Sun and Li18 and tried to establish that the resulting nanotubes are TiO2. Consequently, as indicated by the historical record, there is yet considerable ambiguity regarding the underlying mechanism of nanotube growth using the process initially reported by Kasuga and coworkers9,16 and further questions on the chemical compos
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