Formation of anodic TiO2 nanotube arrays with bimodal pore size distribution

• Anodic TiO2 nanotubes grown on polycrystalline Ti foils actually have bimodally distributed pore sizes.
• Anodic nanotubes grown on the Ti (0001) plane are smaller, shorter and better crystallized than those grown on other planes.
• Anodic TiO2 nanotubes with monomodally distributed pore size can be grown on (0001) oriented Ti thin films.

It is revealed that TiO2 nanotube arrays fabricated by anodization of the widely used polycrystalline hexagonal Ti foils have a bimodal pore size distribution rather than the commonly believed monomodal distribution manner. As characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it is shown that nanotubes grown on the Ti (0001) plane have thinner barrier thickness, smaller pore size, shorter tube length and better crystallinity than those grown on other planes, due to the enhanced electron transfer reaction and lower oxide formation efficiency on the Ti (0001) plane. When (0001) oriented Ti thin films deposited by RF magnetron sputtering are anodized, the pore sizes of the grown anodic TiO2 nanotubes distribute monomodally and are close to the smaller pore size of the bimodally distributed TiO2 nanotubes grown on polycrystalline Ti foils under the same conditions.