Fabrication of tapered, conical-shaped titania nanotubes
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Fabrication of tapered, conical-shaped titania nanotubes G.K. Mor and Oomman K. Varghese Department of Electrical Engineering, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802
Maggie Paulose and Niloy Mukherjee Sentechbiomed Corporation, State College, Pennsylvania 16803
Craig A. Grimesa) Departments of Electrical Engineering and Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 3 June 2003; accepted 28 July 2003)
Using anodic oxidation with a time-dependent linearly varying anodization voltage, we have made films of tapered, conical-shaped titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10–23 V at rates varying from 2.0–0.43 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the titanium substrate during the initial stages of the anodization process enabled us to propose a mechanism of nanotube formation. I. INTRODUCTION
Functional metal oxide nanostructures are of considerable interest due to their novel properties, as illustrated by the electron emission of zinc oxide nanowires,1 luminescence exhibited by europium-doped nanoporous titania,2 the high ethanol sensitivity of nanocrystalline tin oxide,3 the unusually high electrical conductivity of nanocrystalline thin films of CeO2,4 and the enhanced photocatalytic effect in nanometer-sized TiO2 powders.5 Material nanostructure has been shown to play a critical role in determining physical properties. For example TiO2 nanowires possess a significantly higher photocatalytic activity than the standard (DeGussa P25) titania nanoparticles.6 Films of TiO2 nanotubes made by anodization7 have been shown to have an ultrahigh sensitivity to hydrogen.8 Nanoporous alumina has been used to achieve wide-range humidity sensors 9 and highconcentration ammonia sensors,10 and are currently of considerable interest for use as biofilters and bioreactors.11 While a variety of techniques are available for metal oxide nanostructure fabrication, mechanistic studies on their formation are essential for the development of new structures and materials with advanced properties. Due to the novel and enhanced properties found in nano-architectured titania, there has been considerable
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J. Mater. Res., Vol. 18, No. 11, Nov 2003
interest in developing robust methods for the fabrication of titania nanotubes.7,12–16 We have already demonstrated that the anodization of titanium is a rapid, simple, inexpensive route for the fabrication of large arrays of titania nanotubes.7 In a 0.5% hydrofluoric (HF) solution (pH ≈3.0), we obtained nanotubes of well-define
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