2D Vacancy Confinement in Anatase TiO2 for Enhanced Photocatalytic Activities

Light-driven energy conversion devices call for the atomic-level manipulation of defects associated with electronic states in solids. However, previous approaches to produce oxygen vacancy (VO) as a source of sub-bandgap energy levels have hampered the precise control of the distribution and concentration of VO. Here, a new strategy to spatially confine VO at the homo-interfaces is demonstrated by exploiting the sequential growth of anatase TiO2 under dissimilar thermodynamic conditions. Remarkably, metallic behavior with high carrier density and electron mobility is observed after sequential growth of the TiO2 films under low pressure and temperature (L-TiO2) on top of high-quality anatase TiO2 epitaxial films (H-TiO2), despite the insulating properties of L-TiO2 and H-TiO2 single layers. Multiple characterizations elucidate that the VO layer is geometrically confined within 4 unit cells at the interface, along with low-temperature crystallization of upper L-TiO2 films; this 2D VO layer is responsible for the formation of in-gap states, promoting photocarrier lifetime (approximate to 300%) and light absorption. These results suggest a synthetic strategy to locally confine functional defects and emphasize how sub-bandgap energy levels in the confined imperfections influence the kinetics of light-driven catalytic reactions.