Nucleation and early growth of anodized TiO 2 film
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D. Regonini, C.R. Bowen, and R. Stevens Materials Research Centre, Department of Mechanical Engineering, University of Bath, Claverton Down BA2 7AY, United Kingdom (Received 2 October 2007; accepted 21 March 2008)
Anodized films of titanium were prepared under different controlled conditions in a water-based electrolyte containing fluorine ions, using either a constant potential or a potential gradually rising to 20 V. The films were then examined using transmission electron microscopy at different stages of growth, in particular, the very early stages of growth (30 s, 200 s, and 10 min) and when the ordered nano-tubular structure was finally established (2–4 h). The use of ramped voltage during the early stages of anodization allowed a well-interconnected porous network to develop and maintained active oxidation throughout anodization. The film, as formed, consisted mainly of amorphous oxide/hydroxides of titanium with small regions of nano-sized crystals. These were found more often in the denser regions of the amorphous network, particularly the arms of the coral-like structure that formed. As the anodized film grew in thickness, the pores tended to become aligned, leading to a surface layer of nanotubes on the electrode material. Electron optical characterization revealed that the nanotubes consist of a stack of rings where the passage of the current had been optimized.
I. INTRODUCTION
Titanium oxide is an important material largely used as a white pigment in paints and food as well as ultraviolet (UV) ray protection in cosmetic products. It is of great interest to scientists and technologists for its chemical and physical properties and its stability.1 It is suitable for many applications including biomedical,2–7 gassensors,8–14 photo-catalysis,15–23 protection of lightsensitive materials,24 and solar cell devices.25–32 In all these technologies, a very high surface area, in the form of nano-sized particles or nanotubes is required to maximize efficiencies and performance. Anodization, sol-gel, hydrothermal, and vapor deposition are some of the techniques normally used to generate different morphologies of the nanostructured titanium dioxide.33,34 A. Anodization
The generation of titania nanotubes via anodization of metallic titanium is a relatively new topic.35,36 Generally, a small addition of fluorine ions is required to initiate the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0276 2116
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 8, Aug 2008 Downloaded: 13 Mar 2015
formation of cavities in the anodic structure, enhancing conductivity and creating channels where current can flow,37 although aqueous electrolytes containing chloride ions have also been used.38,39 The tubes have been shown to consist of a remnant structure formed as a result of the collapse of the oxide/hydroxide gel formed by the electrochemical oxidation. This structure is made of interlocked rings produced by the collapse of the cavities formed in the gel layer. The
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