The effect of the heating rate on anatase crystal orientation and its impact on the photoelectrocatalytic performance of TiO₂ nanotube arrays

Tuning the degree of anatase crystal orientation in TiO₂ nanotube arrays has emerged as a new strategy to control their photoelectrochemical performance, since electron transport through the nanotubular film is favored. However, the existing literature fails to provide an accurate approach to evaluate the impact of anatase crystal orientation, since it is normally studied using TiO₂ nanotube films synthesized in different anodizing baths, which in turn affects the properties of the films. In this work, anatase crystal orientation was controlled by changing the heating rate during sample heat-treatment (450^◦C for 30 min), which resulted in randomly oriented anatase crystals when the film was taken from room temperature to 450^◦C, to highly crystalline films with preferential anatase crystal orientation when the heating rate was decreased to 5 ^◦C min−¹ and 1 ^◦C min−¹. Even though the physicochemical properties of the films were almost unaltered, increasing anatase crystal orientation resulted in higher photocurrents and photoelectrocatalytic methyl orange discoloration, showing a maximum for the film treated at a heating rate of 5^◦C min−¹.