Current characterization and growth mechanism of anodic titania nanotube arrays
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Junlei Li, Xian Wang, Xueping Song, and Zhaoqi Suna) School of Physics and Material Science, Anhui University, Hefei 230039, China (Received 25 June 2010; accepted 22 September 2010)
TiO2 nanotube arrays were synthesized by anodic oxidation on a pure titanium substrate in solutions containing 0.175 M NH4F composed of mixtures with different volumetric ratios of DI water and glycerol. According to the results of the current curve recorded during anodization, the time of the first sharp current slope (corresponding to the initial oxide layer formation time) was found to vary from 8 to 171 s depending not only upon the water content in the electrolytes but also upon the voltage. The current curves exhibit oscillation with different amplitudes and periods. In combination with the scanning electron microscope (SEM) images, a growth mechanism, layer-by-layer model, of TiO2 nanotube arrays was presented. Based on this mechanism, many phenomena that appeared during anodization can be reasonably explained. Our results would be helpful for the design of nanoarchitectures in related material systems.
I. INTRODUCTION
In 2001, Gong and coworkers reported on anodization of Ti and synthesized highly ordered nanotube arrays.1 The precisely oriented structures were suitable for many applications such as photocatalytic applications,2–6 sensing,7–10 photoelectrolysis,11–15 polymer-based bulk heterojunction photovoltaics,16–18 dye-sensitized solar cells,19–25 biofluid filtration, drug delivery, and other biomedical applications.26–29 These promising prospects require a thorough understanding of the nanotube formation mechanism and controllable preparation of the arrays. Many reports briefly mentioned the growth mechanism of TiO2 nanotube arrays,30–34 in which the key processes of the nanotube growth were described as follows: (i) Oxide growth at the surface of the metal due to interaction of the metal with O2– or OH–. (ii) Metal ion (Ti4+) migration from metal foil at the metal–oxide interface; Ti4+ cations ejected from the metal–oxide interface under the application of an electric field move toward the oxide–electrolyte interface. (iii) Fieldassisted dissolution of the oxide at the oxide–electrolyte interface. (iv) Chemical dissolution of the metal, or oxide, by the electrolyte during anodization.35 Normally, the current curve recorded during anodization is a convenient way to characterize the electrochemical behavior and the tube growth process.7,15,17,18,21,31,36,37 Figure 1 shows a typical current–time transient for the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.33 J. Mater. Res., Vol. 26, No. 3, Feb 14, 2011
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anodization of Ti foil, in which three stages can be easily classified. After an initial exponential decay (stage I), the current increases for a short time (stage II), then the current reaches a quasi-steady state (stage III). In stage III, the current curve is usually found to exhibit oscillation. Typically, this beh
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