Tailoring morphology, structure and photoluminescence properties of anodic TiO₂ nanotubes

TiO₂ nanotube (TNT) structures were grown perpendicular to fluorine-doped tin-oxide-coated glass substrates by anodic oxidation of titanium films. The morphology, crystal structure and optical properties of the TNTs were shown to be dependent on the thickness of the titanium film, which acts as an electrode in electrochemical anodization. Field emission scanning electron microscopy measurements revealed that an increase in titanium thickness from 1.5 to 2.7 µm caused a considerable increase in both inner diameter and tube length, which in turn increases the porosity and the physical surface of the TNTs per unit area. Grazing-incidence small-angle scattering was used to infer the statistical lateral ordering of the TNTs over macroscopic length scales. X-ray diffraction data show an increase in the texture coefficient for the (004) plane as well as the I₀₀₄/I₁₀₁ intensity ratio with titanium film thickness. All these factors lead to a significant improvement in the photoluminescence intensity from titania nanotubes, which is about five times more than from titania nanoporous materials under similar circumstances.