Photocatalytic activity of anatase and rutile TiO2 epitaxial thin film grown by pulsed laser deposition

Epitaxial rutile-TiO2(011) and anatase-TiO2(001) films have been grown by pulsed laser deposition on Al2O3(1¯102) and LaAlO3(100), respectively. For low growth rates of ~ 0.0012 nm/s and growth temperatures of 600 °C high quality films are obtained. In particular the anatase films exhibit 100 nm wide flat terraces for 10-15 nm film thickness. Atomic steps of both ~ 0.25 and ~ 0.5 nm are measured, suggesting the existence of two different terminations, although only a 4 × 1 reconstruction is observed in reflection high-energy electron diffraction. The rutile films exhibit coherent twin domain boundaries, which is likely the reason for smaller terrace sizes compared to anatase films. Photocatalytic activity of the films is measured by decomposition of methyl orange solution under UV-irradiation. The anatase films are about twice as active as the rutile films. For both films the activity does not change significantly for films of 10 nm and thicker. For rutile films the activity is also compared to that of single crystal rutile TiO2(011) samples. Both rutile-films and single crystals exhibit very similar activity, indicating the good quality of the films. Post-growth annealing of the films in air at 600 °C decreases the photocatalytic activity of the films. On the other hand, reduction of thin films by vacuum annealing to 800 °C does not change the photocatalytic activity of the films significantly compared to the as grown films. This is in contrast to single crystal rutile samples whose photocatalytic activity for the decomposition of methyl orange almost doubles after reduction by high-temperature annealing in vacuum compared to the stoichiometric bulk-samples. The differences between films and bulk samples may hint differences in defect formation due to the substrate interface.