A comparative study of anodized titania nanotube architectures in aqueous and nonaqueous solutions

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The unique and highly utilized properties of TiO2 nanotubes are a direct result of nanotube architecture. To create different engineered architectures, the effects of electrolyte solution, time, and temperature on the anodization of titanium foil were studied along with the resultant anodized titanium oxide (ATO) nanotube architectures encompassing nanotube length, pore diameter, wall thickness, smoothness, and ordered array structure. Titanium foil was anodized in three different electrolyte solutions: one aqueous [consisting of NH4F and (NH4)2SO4] and two nonaqueous (glycerol or ethylene glycol, both containing NH4F) at varying temperatures and anodization times. Variation in anodization applied voltage, initial current, and effect of F ion concentration on ATO nanotube architecture was also studied. Anodization in the aqueous electrolyte produced short, rough nanotube arrays, whereas anodization in organic electrolytes produced long, smooth nanotube arrays greater than 10 lm in length. A position effect, relative to the solution–air interface, was observed in this work. Furthermore, it was found that anodization in glycerol at elevated temperatures for several hours could possibly produce freely dispersed individual nanotubes.

I. INTRODUCTION

As a result of the utilization of TiO2 nanotubes in a variety of solar applications, an extensive body of literature over the past decade has addressed the use of electrochemical anodization in TiO2 nanotube production. The highly ordered architecture of vertically aligned nanotubes creates a large surface-to-area ratio that is ideally suited for optical and catalytic performance. These TiO2 nanotubes have shown outstanding photoelectrochemical properties1–4 for applications including water photolysis for H2 production,1,4–10 as potential photoelectrodes in dye-sensitized solar cells,11–15 and photocatalysis12,16,17 (the most promising being the photocatalytic conversion of the greenhouse gas, CO2, to hydrocarbon fuels).18,19 To maximize these properties, long debris-free nanotubes are desired. A 45-lm-thick nanotube array yields photoconversion efficiencies as high as 16.25%,10 and 17.6-lm long nanotubes in dye-sensitized solar cells produce a power conversion efficiency of 6.9%.15 As is apparent from the literature, the most effective route to producing long TiO2 nanotubes is electrochemical anodization. The two main media in which titanium is anodized are aqueous electrolyte solutions20–31 and nonaqueous organic electrolyte solutions.10–12,32–40 The anodization takes place in two steps: electrochemical oxidation of the titanium surface followed by chemical dissolution of the oxides that form.21 The a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.243 2612

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chemical dissolution requires an acidic solution environment and a source of F anions. In their early work, Gong and coworkers20 used an aqueous electrolyte and hydrogen fluoride (HF) to achieve these anodization conditions. They found that the t