| Abstract
| - Highly ordered TiO2 nanotube arrays are of considerable interest for their use as gas sensors, materials forwater photoelectrolysis, and as photoanodes in dye sensitized solar cells. For example, under UV illuminationhighly ordered TiO2 nanotube arrays ≈35 μm in length achieve a light-to-chemical energy photoconversionefficiency of 16.25% [M. Paulose et al., J. Phys. Chem. B2005, 110, 16179−16184]. It is now well-establishedthat the properties of the nanotube arrays are dependent upon their specific architecture, including nanotubearray length, wall thickness, pore diameter, and tube-to-tube spacing. In this work we investigate the effectof five different cationic species on the formation of TiO2 nanotube arrays by potentiostatic anodization oftitanium in formamide−water mixtures containing fluoride ions. We find the cation choice to be a key parameterinfluencing both the nanotube growth rate and resulting nanotube length. The length and aspect ratio of thenanotubes increases with increasing cation size. Under similar conditions, electrolytes containing thetetrabutylammonium cation resulted in the longest nanotubes (∼94 μm), while the shortest nanotubes (∼3μm) were obtained when H+ ions were the sole cationic species in the anodization electrolyte. We attributethis difference in nanotube growth to the inhibitory effect of the quarternary ammonium ions that restrict thethickness of the interfacial (barrier) oxide layer; a thinner interfacial oxide layer facilitates ionic transportthus enhancing the nanotube growth. The aspect ratio of the resulting nanotubes is also voltage dependent,with the highest aspect ratio of ≈700 obtained at an anodizing voltage of 20 V in an electrolyte containingtetrabutylammonium ions. In a saturated solution of NaF in formamide, nanotubes with a pore diameter of 12nm were obtained, while a formamide Bu4NF solution resulted in nanotubes of 5 nm wall thickness; bothvalues are, to the best of our knowledge, the smallest reported values for anodically formed TiO2 nanotubearrays.
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