TiO2 Nanotube arrays: Influence of tube length on the photocatalytic degradation of Paraquat

Highly ordered TiO2 nanotubes have attracted a lot of attention for the photocatalytic removal of organic pollutants. A few studies demonstrated that the tube morphology plays a key role on the photocatalytic activity. However, no studies have been performed to explain the optimal photocatalytic activity observed for a precise tube length. Here we studied how the morphology of the tubes evolves during anodic growth and we evaluated the corresponding photocatalytic activity. In our conditions, the optimal activity was found for 7 μm long tubes. The SEM study reveals that short tubes have a small pore opening thus hindering pollutant diffusion into the tubes. On the other hand, tubes longer than 7 μm have thinner walls due to etching by the fluoride ions. Consequently the volume of TiO2 decreases strongly in the upper part of the tubes as the tube length increase and light should travel a longer pathway before absorption and production of electron-hole pairs. Hence the organic species have to diffuse further and the photocatalytic activity decreases.