Growth of titanium oxide or titanate nanostructured thin films on Ti substrates by anodic oxidation in alkali solutions

Anodic oxidation of Ti in aqueous NaOH solutions is employed to grow titanium oxide or titanate nanostructured thin films on Ti substrates. The effect of voltage (10-30 V), temperature (20-80 °C), NaOH concentration (0.5-4 M) and oxidation duration (0.5-5 h) on the chemical composition, morphology, and photoelectrochemical property of as-grown films has been investigated. It is found that the film composition is strongly related to the alkali concentration. Titanium oxide films are obtained in low-concentration NaOH solutions (≤ 2 M), whereas the films primarily consist of titanate if the NaOH concentration reaches 4 M. Our results have also shown that by changing the growth conditions the films with different morphology can be obtained. When anodizing Ti at 80 °C and 30 V for 3 h, for example, a nanowire film with net-like morphology and a nanoleaf film can be grown in 2 M and 4 M NaOH solutions, respectively. An analysis of the correlation of measured photocurrent with film thickness or surface roughness reveals that the photoelectrochemical property of the titanium oxide films is dependent on both their thickness and surface morphology. The nanowire film with net-like morphology is found to be more photoactive than other films due to its relatively large thickness (~ 510 nm), high surface roughness (~ 18.4 nm) and one-dimensional structural feature. Based on the experimental results, a possible mechanism for growth of titanium oxide or titanate thin films on Ti substrates in alkali aqueous solutions via anodic oxidation has been proposed.