Kinetic mechanism of TiO 2 nanocarving via reaction with hydrogen gas
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Helene Rick and Kenneth H. Sandhage School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 21 January 2006; accepted 18 April 2006)
Dense polycrystalline titania (TiO2, rutile) was converted into oriented arrays of single-crystal titania nanofibers by reaction with a noncombustible, hydrogen-bearing gas mixture at only 680–780 °C. Such nanofiber formation resulted from anisotropic etching (“nanocarving”) of the titania grains. The fibers possessed diameters of 20–50 nm and lengths of up to several microns, with the long fiber axes oriented parallel to the [001] crystallographic direction of rutile. Mass spectroscopy and inductively coupled plasma spectroscopy indicated that oxygen, but not titanium, was removed from the specimen during the reaction with hydrogen. The removal of substantial oxygen and solid volume from the reacting surfaces, without an appreciable change in the Ti:O ratio at such surfaces, was consistent with the solid-state diffusion of titanium cations from the surface into the bulk of the specimen. The reaction-induced weight loss followed a parabolic rate law, which was also consistent with a solid-state diffusion-controlled process.
I. INTRODUCTION
A significant level of research is currently focused on the creation of nanostructured materials for a variety of applications, including for photonic, electronic, and chemical sensing devices.1,2 However, the widespread utilization of nanostructured materials has been complicated by the often-conflicting demands for precise control of fine features (down to the nanometer scale) and for large-scale mass production. Recently, Yoo et al. developed a novel technique, referred to as “nanocarving,” for converting bulk polycrystalline titania (TiO2) surfaces into arrays of titania nanofibers.3 The nanocarving process is a relatively simple and scalable approach that involves the reaction of bulk polycrystalline titania with a noncombustible hydrogen-bearing gas mixture at a modest temperature (e.g., reaction with a 5% H2/95% N2 mixture at 700 °C).3 This process yields oriented arrays of single-crystal nanofibers over the exposed TiO2 surfaces. Unlike other approaches that have relied upon deposition and growth (i.e., additive processes) to synthesize one-dimensional nanostructures,4–6 the titania a)
Present address: School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0225 1822 J. Mater. Res., Vol. 21, No. 7, Jul 2006 http://journals.cambridge.org Downloaded: 19 May 2014
nanofibers generated by the nanocarving technique result from the anisotropic etching of titania grains (i.e., a subtractive process). The objective of the present work was to evaluate the mechanism of nanocarving through the use of chemical analyses (mass spectrometry and inductively coupled mass spectroscopy), thermogravimetric analyses, and microstructural analyses. II. EXPERIMENTAL
The
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