Effects of synthesis conditions on dimensions, structure, and oxygen content of photocatalytically active titania nanotu
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John Mansfield and Kai Sun Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109
Johannes Schwank Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109 (Received 27 May 2009; accepted 21 August 2009)
In this study, we report a method for the formation and characterization of aligned arrays of amorphous titania nanotubes by anodic oxidation in thin titanium films on SiO2 substrates using fluoride-containing electrolytes. Trends in titania nanotube geometries as a function of synthesis conditions were established. A titania nanotube array surface area of approximately 178 m2/g is reported. The titania nanotubes transitioned to the rutile crystal structure when heated in air at 530 C–705 C. The degradation of methylene blue under UV light showed that lower fluoride concentrations in the synthesis electrolyte result in higher photocatalytic activity of the titania nanotubes. These results indicate that the synthesis conditions affect the oxygen content of amorphous nanotubes, which determines their physical and chemical properties.
I. INTRODUCTION
Titania nanotubes are of particular interest because of their applications in photocatalytic water splitting. Titania is a photosensitive material and offers a wide band gap. In addition, a nanotube structure allows for an increased surface area per volume ratio. Titania nanotubes can be synthesized by anodic oxidation, which results in uniform arrays of nanotubes perpendicular to the substrate surface. Titania nanotube arrays synthesized from thin titanium films are widely studied to incorporate them into applications such as gas sensing, battery, and solar cell systems. Previous work indicates that the geometries and properties of titania nanotubes can be controlled by the synthesis conditions. Mor and coworkers1 studied the effect of electrolyte composition and applied electrical potential on titania nanotube dimensions. This group also showed that transparent titania nanotubes can be formed in sputter-deposited thin titanium films. Macak and Schmuki2 studied the formation of titania nanotubes in viscous electrolytes, in which the lower mobility of ions reduces local pH fluctuations to create smooth titania nanotubes. Mor and coworkers3 also suggest that the nanotube wall thickness determined the effectiveness of titania nanotubes formed on a thick titanium substrate to split water for hydrogen production. Enyashin and Ivanovskii4 constructed a density-functional theory based tight binda)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0011 J. Mater. Res., Vol. 25, No. 1, Jan 2010
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ing computational model of TiO nanostructures to predict that the “metallic-like” TiO nanotubes become increasingly stable with an increasing number of tube walls. According to Masahashi and coworkers,5 titania nanotubes in the rutile phase have a higher degree of photocatalytic activity than titania nanotubes in t
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