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 (V_O) as a source of sub-bandgap energy levels have hampered the precise control of the distribution and concentration of V_O. Here, a new strategy to spatially confine V_O 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 V_O layer is geometrically confined within 4 unit cells at the interface, along with low-temperature crystallization of upper L-TiO₂ films; this 2D V_O 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.