Nanostructured thin films of TiO₂ tailored by anodization

Abstract

Although nanostructured TiO2 layers have been widely prepared by anodization, thin films with thicknesses under 1 μm, over substrate other than Ti foils, with structures beyond the nanoporous, had remained a challenge. In this work, such nanostructured TiO2 thin films were synthesized by anodization of Ti films deposited by sputtering on FTO/glass substrates. Anodization was performed in an electrolyte based on 0.6 wt % of NH4F, a graphite cathode and the application of 30 V during lapses ranging from 3 to 14 min. The amorphous TiO2 structures acquired the crystal anatase phase after a post-annealing treatment at 450 °C /4 h. Porous morphologies were observed for anodizing times of 3 and 4 min, sponges were formed with 5 and 6 min and vertical tubular structures were achieved by using 7 up to 9 min; dissolution was observed for longer times. Pore diameters of the structures were in the range of 27 to 47 nm, lengths were within the 330 and 1000 nm interval, transmittance in the visible range of 70 ± 10%, the energy gap was 3.37 ± 0.02 eV, and the wet contact angle was between 20 to 27°. One major contribution of the findings herein developed, is that they can be extended to TiO2 thin films, with a specific nanostructure, grown on a wide gamma of substrates, relevant for particular applications.