The fabrication of highly-ordered TiO2 nanotube arrays up to 134 µm in length by anodization of Ti foil has recently been reported (Paulose et al 2006 J. Phys. Chem. B 110 16179). This work reports an extension of the fabrication technique to achieve TiO2 nanotube arrays up to 220 µm in length, with a length-to-outer diameter aspect ratio of ≈1400, as well as their initial application in dye-sensitized solar cells and hydrogen production by water photoelectrolysis. The highly-ordered TiO2 nanotube arrays are fabricated by potentiostatic anodization of Ti foil in fluoride ion containing baths in combination with non-aqueous organic polar electrolytes including N-methylformamide, dimethyl sulfoxide, formamide, or ethylene glycol. Depending upon the anodization voltage, the inner pore diameters of the resulting nanotube arrays range from 20 to 150 nm. As confirmed by glancing angle x-ray diffraction and HRTEM studies, the as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures.

Described is the fabrication of self-aligned highly ordered TiO(2) nanotube arrays by potentiostatic anodization of Ti foil having lengths up to 134 mum, representing well over an order of magnitude increase in length thus far reported. We have achieved the very long nanotube arrays in fluoride ion containing baths in combination with a variety of nonaqueous organic polar electrolytes including dimethyl sulfoxide, formamide, ethylene glycol, and N-methylformamide. Depending on the anodization voltage, pore diameters of the resulting nanotube arrays range from 20 to 150 nm. Our longest nanotube arrays yield a roughness factor of 4750 and length-to-width (outer diameter) aspect ratio of approximately 835. The as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures. In initial measurements, 45 mum long nanotube-array samples, 550 degrees C annealed, under UV illumination show a remarkable water photoelectrolysis photoconversion efficiency of 16.25%.

Highly ordered vertically oriented TiO(2) nanotube arrays fabricated by electrochemical anodization offer a large surface area architecture with precisely controllable nanoscale features. These nanotubes have shown remarkable properties in a variety of applications including, for example, their use as hydrogen sensors, in the photoelectrochemical generation of hydrogen, dye-sensitized and solid-state heterojunction solar cells, photocatalytic reduction of carbon dioxide into hydrocarbons, and as a novel drug delivery platform. Herein we consider the development of the various nanotube array synthesis techniques, different applications of the TiO(2) nanotube arrays, unresolved issues, and possible future research directions.