2D Vacancy Confinement in Anatase TiO 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 (V) as a source of sub-bandgap energy levels have hampered the precise control of the distribution and concentration of V. Here, a new strategy to spatially confine V at the homo-interfaces is demonstrated by exploiting the sequential growth of anatase TiO under dissimilar thermodynamic conditions. Remarkably, metallic behavior with high carrier density and electron mobility is observed after sequential growth of the TiO films under low pressure and temperature (L-TiO) on top of high-quality anatase TiO epitaxial films (H-TiO), despite the insulating properties of L-TiO and H-TiO single layers. Multiple characterizations elucidate that the V layer is geometrically confined within 4 unit cells at the interface, along with low-temperature crystallization of upper L-TiO films; this 2D V layer is responsible for the formation of in-gap states, promoting photocarrier lifetime (≈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.