Photoactive titania nanostructured thin films: synthesis and characteristics of ordered helical nanocoil array

Titania nanostructured thin films have been prepared by anodization of titanium foils at different voltages in the 10–20 V range. They have been characterized by field emission scanning electron microscopy (FESEM), glancing angle X-ray diffraction (GAXRD), UV–vis diffuse reflectance and current–time transients during the anodization process. The observed nanostructure differs from that reported for titania nanotubes (hollow columns) and can be described as helical nanocoils which form an ordered array over the entire surface. The model of formation of these nanostructures (TNT) has been also proposed. The voltage applied during the anodization influences the mean dimension of these TNT, their wall thickness and also their degree of packing. A linear relationship between the wall thickness, in the 5–10 nm range, and the band gap is observed. The behavior of these samples in the photo-generation of current by irradiation with a low-power lamp simulating solar spectrum has been also studied. A main factor which determine with photo-behavior is the band gap which in turn depends on the characteristics of the TNT. Increasing the voltage during anodization increases the wall thickness and the band gap shifts toward visible region with an improvement of the photo-generated current density. It is also demonstrated that the formation of these helical nanocoils improve the photo-generated current with respect to the sample samples after short anodization where only a titania layer is formed.