Roughness of Ti Substrates for Control of the Preferred Orientation of TiO2 Nanotube Arrays as a New Orientation Factor

We report the surface roughness of a Ti substrate as a critical factor for controlling the degree of the preferred orientation of anatase TiO 2 nanotube arrays (NTAs) which are synthesized by anodization and a subsequent annealing process. The degree of the preferred orientation to the (004) plane of the anatase crystal structure has a strong dependency on the root-mean-square roughness ( S q ) of the initial Ti substrate when the roughness-controlled substrates are anodized in an ethylene glycol-based electrolyte containing ∼2 wt % of water. Highly preferred oriented NTAs were obtained from low- S q (<10 nm) substrates, which were accompanied by uniform pore distribution and low concentration of hydroxyl ions in as-anodized amorphous NTAs. The mechanism of the preferred oriented crystallization of nanometer-scaled tube walls is explained considering the microscopic geometrical uniformit…