Anodic Formation of High-Aspect-Ratio Titania Nanotubes

Nanotubular materials are of great interest because of their exceptional electronic and mechanical properties. Since the discovery of carbon nanotubes [1], extensive research has been carried out to explore nanotubular materials other than carbon, including the oxide materials SiO2, Al2O3, V2O5, MoO3, and TiO2. Titania nanotubes have attracted particularly wide attention because of their potential applications to photocatalysts, gas sensors, and photovoltaic cells. Titania nanotubes have been synthesized in different ways, such as liquid phase electrodeposition combined with a replication method [2], anodization of titanium in a fluoride-containing solution [3, 4], and alkaline hydrothermal treatment of titania nanoparticles [5, 6]. Recently, we found that anodization of titanium in a perchloric acid solution yields the formation of tubular titania, and we successfully obtained high-aspect-ratio titania nanotubes with diameters about 20 nm and lengths over 10 m. A titanium sheet (99.9% pure) was ultrasonically cleaned in distilled water and then in ethyl alcohol for 10 min. The titanium sheet was anodized in an aqueous perchloric acid solution (0.05 mol/l) by applying 30 V at 16 °C in a two-electrode cell with a Pt counter electrode. The white product thus formed on the sheet was rinsed with water and dried in air. The product was confirmed to be titania by XPS. The SEM image given in Fig. 1 shows that the titania is composed of bundles of nanofibers with length over 10 m. The detailed structure of a nanofiber was examined by HRTEM observation. As shown in Fig. 2(a) and 2(b), the as-prepared titania nanofiber was verified to have a nanoscale tubular structure. The diameter of a nanotube is around 18 – 23 nm and the thickness around 5 – 7 nm. The aspect ratio of a single titania nanotube reaches well over 500:1. The as-prepared titania nanotube was observed to be amorphous by selected area electron diffraction (SAED) as shown in Fig. 2(c). Annealing the titania at 250 C for 1 h in air induced a phase transition from an amorphous phase to a crystalline anatase phase. The morphology of titania nanotubes remained virtually unchanged after crystallization. The anodization of titanium in fluoridecontaining solution also yields formation of titania nanotubes, but their lengths are about 4 m at best [4]. Titania synthesized by alkaline hydrothermal treatment was once reported to form a nanotubular structure [5], but recently the titania “nanotube” was verified to be in fact a titania nanosheet curled up to form a tube-like structure. Moreover, the SAED pattern of this titania “nanotube” clearly differs from that obtained from our titania [5]. These facts indicate that the titania nanotubes given here can be classified as a new kind of titania materials. In summary, we successfully synthesized highaspect-ratio titania nanotubes by anodic oxidation of a titanium sheet in a perchloric acid solution. To the best our knowledge, the aspect ratio of 500:1 is the highest value so far reported. This new class of nanostructure titania is expected to find even wider applications in academic and industrial fields.