Cation effect on the electrochemical formation of very high aspect ratio TiO2 nanotube arrays in formamide -: Water mixtures

Highly ordered TiO2 nanotube arrays are of considerable interest for their use as gas sensors, materials for water photoelectrolysis, and as photoanodes in dye sensitized solar cells. For example, under UV illumination highly ordered TiO2 nanotube arrays approximate to 35 Am in length achieve a light-to-chemical energy photoconversion efficiency of 16.25% [M. Paulose et al., J. Phys. Chem. B 2005, 110, 16179- 16184]. It is now well-established that the properties of the nanotube arrays are dependent upon their specific architecture, including nanotube array length, wall thickness, pore diameter, and tube-to-tube spacing. In this work we investigate the effect of five different cationic species on the formation of TiO2 nanotube arrays by potentiostatic anodization of titanium in formamide-water mixtures containing fluoride ions. We find the cation choice to be a key parameter influencing both the nanotube growth rate and resulting nanotube length. The length and aspect ratio of the nanotubes increases with increasing cation size. Under similar conditions, electrolytes containing the tetrabutylammonium cation resulted in the longest nanotubes (similar to 94 mu m), while the shortest nanotubes (similar to 3 Am) were obtained when H+ ions were the sole cationic species in the anodization electrolyte. We attribute this difference in nanotube growth to the inhibitory effect of the quarternary ammonium ions that restrict the thickness of the interfacial (barrier) oxide layer; a thinner interfacial oxide layer facilitates ionic transport thus enhancing the nanotube growth. The aspect ratio of the resulting nanotubes is also voltage dependent, with the highest aspect ratio of approximate to 700 obtained at an anodizing voltage of 20 V in an electrolyte containing tetrabutylammonium ions. In a saturated solution of NaF in formamide, nanotubes with a pore diameter of 12 nm were obtained, while a formamide Bu4NF solution resulted in nanotubes of 5 nm wall thickness; both values are, to the best of our knowledge, the smallest reported values for anodically formed TiO2 nanotube arrays.