Defect mediated photocatalytic decomposition of 4-chlorophenol on epitaxial rutile thin films under visible and UV illumination

摘要

We show that pure rutile TiO_2 can be photo-responsive even under low energy visible light after annealing in vacuum where we envisage that the point defects, i.e.oxygen vacancies and titanium interstitials, serve an important role. In this study, single crystal rutile films were grown by the pulsed laser deposition technique and then vacuum annealed under different oxygen pressures to introduce defects into their lattices. 4-chlorophenol was selected as a model material and decomposed by the annealed TiO_2 films where the maximum photocatalytic reaction rate constants were determined as 0.0107 and 0.0072min ~1 under UV and visible illumination. Epitaxial growth along the [200] direction was confirmed by -scan and 2θ-scan XRD and the epitaxial relationship between the rutile film and the c-sapphire substrate was explained as. The formation of atomically sharp interfaces and the epitaxial growth were ascertained by annular dark-field STEM imaging. Based on the XPS, UV-vis and PL spectroscopy results, it was found that the defect concentration increased after annealing under lower pressures, e.g.5×10 ~6Torr. In contrast, more perfect crystals were obtained when the films were annealed under high oxygen pressures, namely 5×10 ~1Torr. The morphology of the films was also investigated by employing an AFM technique. It was observed that increase of the annealing pressure results in the formation of larger grains. It was also found that the electrical resistivity of the rutile films strongly increased by about three orders of magnitude when the annealing pressure increased from 5×10 ~4 to 5×10 ~1Torr.