Anodic Growth of Highly Ordered TiO2 Nanotube Arrays to 134 μm in Length

Described is the fabrication of self-aligned highly ordered TiO2 nanotube arrays by potentiostatic anodization of Ti foil having lengths up to 134 μm, 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 ≈835. The as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures. In initial measurements, 45 μm long nanotube-array samples, 550 °C annealed, under UV illumination show a remarkable water photoelectrolysis photoconversion efficiency of 16.25%.