TiO₂ nanotubes: Interdependence of substrate grain orientation and growth characteristics

Highly ordered TiO₂ nanotube arrays have received great interest in the field of nanoscience and nanotechnology. In view of applications in devices, it is of paramount importance to control the growth over large areas. The results described in this paper show how the growth characteristics of anodic TiO₂ oxide films can be tailored by controlling the orientation of the underlying substrate. The correlation between the growth characteristics of anodic, self-organized TiO₂ nanotubes and the crystallographic orientation of the underlying titanium substrate grains has been studied using electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and micro-Raman spectrometry. The preferred formation site for self-organized, amorphous TiO₂ nanotubes is Ti(111), whereas on Ti(001) grains no nanotube growth is observed. Instead, a compact oxide film is formed, exhibiting a mixed anatase and rutile nanocrystalline character with a large degree of structural disorder. As an intermediate case, on Ti(010) and on Ti(110) nanotubes form but are capped by a strongly chemically etched overlayer. It has been observed that TiO₂ nanotubes exclusively form on grains with an orientation that allows for the formation of a thick valve metal oxide. This insight will enable controlled and effective growth over large areas of nanotubular films on titanium and presumably on other transition metals that did not show growth of nanotubular oxides before.