From single to multiple TiO2 nanotubes layers: Analysis of the parameters which influence the growth

Highly-ordered vertically oriented TiO2 nanotube arrays (TiO2 NTs) are widely exploited in many different fields such as catalysis, electronics and biomedicine. TiO2 NTs can be synthetized by a number of methods; however, the synthesis via anodization in a fluoride-based electrolyte, proposed for the first time in 2001, has been proved to be the procedure which offers the best control over the nanotube dimensions. In literature, four generations of TiO2 NTs obtained with different types of anodization baths have been reported, each bath giving rise to TiO2 NTs with specific morphological features. In this work, we performed the growth of third generation TiO2 NTs by varying different parameters (i.e. voltage, temperature, anodization time, bath composition) and systematically analyzed their influence on NTs morphology. A deep knowledge of the effect of each parameter allowed their suitable combination in order to obtain double and triple NTs layers with different length and aspect ratio. The proposed method can be applied to synthetize multiple layers with predictable and well-defined features.