Electroreduction as a viable strategy to obtain TiO₂ nanotube films with preferred anatase crystal orientation and its impact on photoelectrochemical performance

Preferential orientation of anatase crystals in TiO₂ nanotube (TNT) films was induced through an electroreduction pre-treatment prior to annealing at 450 °C in atmospheric air. This pre-treatment consisted of applying a cathodic potential between − 1 and −6 V vs. SCE, using either continuous or pulsed form. Relative to TNTs without pre-treatment, XRD characterization revealed that the ratio of intensities for anatase planes (004) and (101), I₍₀₀₄₎/I₍₁₀₁₎, increased around five times when pulsed electroreduction was carried out at − 3 V versus SCE in a 0.1 M Na2SO4 solution, compared to that obtained without the pre-treatment. Since additional ions were not included in the electrolyte, this result demonstrated that pulsed electroreduction could be used as a simple strategy to preferentially orient TiO₂ anatase crystals. However, it was also found that the induction of a preferred crystal orientation in the [001] direction negatively affected TNT photoelectrochemical performance. This response could be attributed to changes induced on the film during the electroreduction process, since electrochemical characterization showed that the electrical and semiconducting properties of the TNTs were drastically modified after pre-treatment. Therefore, these findings reveal that TNT preferential orientation in the [001] direction is not necessarily associated with improved photoelectrochemical behavior and that TNT photoresponse is strongly influenced by the methodology used to achieve such crystalline orientation.