The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxid
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Maggie Paulose Sentechbiomed Corporation, State College, Pennsylvania 16803
Oomman K. Varghese and Craig A. Grimesa) Department of Electrical Engineering, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 6 August 2004; accepted 18 October 2004)
We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose, N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 m, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures 艌280 °C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 °C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 °C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.
I. INTRODUCTION
Highly-ordered metal oxide nanostructures made by anodic oxidation1–11 are of considerable interest due to their unique electrical and electrochemical properties12,13 that are suitable for catalysis3,14 sensing3,4 and biomedical15,16 applications. Anodic oxidation is a relatively simple and well-established process that is known for its industrial viability. Our group made the first reports in literature on the fabrication of titania nanotubes through anodization of Ti foils using dilute hydrochloric acid electrolyte.10 A detailed study on crystallization and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0020 230
J. Mater. Res., Vol. 20, No. 1, Jan 2005
structural transformation of these nanotubes on thermal annealing was reported in this same journal.1 These nanotubes showed extremely high hydrogen sensitivity at room temperature11 as well as elevated temperatures4,5 and excellent photocatalytic activity.3 However, the length of these nanotubes was limited to a maximum of ≈500 nm,7 due to the restriction imposed by the high rate of chemical dissolution of titanium oxide by hydrofluoric acid electrolyte. An increase in le
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