Crystallization and high-temperature structural stability of titanium oxide nanotube arrays
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Dawei Gong, Maggie Paulose, and Craig A. Grimes Department of Electrical Engineering & Materials Research Institute, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Elizabeth C. Dickeya) Department of Materials Science & Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 20 August 2002; accepted 17 October 2002)
The stability of titanium oxide nanotube arrays at elevated temperatures was studied in dry oxygen as well as dry and humid argon environments. The tubes crystallized in the anatase phase at a temperature of about 280 °C irrespective of the ambient. Anatase crystallites formed inside the tube walls and transformed completely to rutile at about 620 °C in dry environments and 570 °C in humid argon. No discernible changes in the dimensions of the tubes were found when the heat treatment was performed in oxygen. However, variations of 10% and 20% in average inner diameter and wall thickness, respectively, were observed when annealing in a dry argon atmosphere at 580 °C for 3 h. Pore shrinkage was even more pronounced in humid argon environments. In all cases the nanotube architecture was found to be stable up to approximately 580 °C, above which oxidation and grain growth in the titanium support disrupted the overlying nanotube array.
I. INTRODUCTION
The enhancement of surface area without an increase in the geometric area of ceramic materials has long been realized as a matter of utmost technological importance. Creating porous or tubular structures has been considered as the most suitable way to achieve this goal. Advancements in nanoscale technology facilitated the development of highly ordered nanoporous and nanotubular structured materials. The extensive literature on nanoporous oxide ceramics like titania, alumina, zirconia, and silica is a measure of their potential applications.1–10 Recent interest has focused on the creation of nanotube structures of these ceramics,11–15 especially that of titania which has a wide range of technologically relevant applications such as gas sensors, photovoltaics, photo and thermal catalysis, photoelectrochromic devices, and immobilization of biomolecules. Titania nanotubes of different geometrical shapes and microstructures have been fabricated by various research groups using techniques like sol-gel synthesis, a)
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J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 06 Jul 2014
anodization, electrodeposition, sonochemical deposition, and methods involving the chemical treatment of fine titania particles.15–24 While many of these fabrication processes are complicated due to the use of templates or the chemical processes involved, we have recently demonstrated that self-organized titanium oxide nanotube arrays can be fabricated using a simple anodization technique.15 This technique can yield nanotubes of inner diameters in the range a
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