Disentangling the Contribution of Surface and Bulk Ti³⁺ Defects to the Band Gap States of Rutile TiO₂(011)

Band gap states (BGS) induced by Ti³⁺ defects play a pivotal role in the physical and chemical properties of TiO₂. However, there is no consensus on the relative contributions of surface and bulk Ti³⁺ defects to the BGS measured by ultraviolet photoelectron spectroscopy (UPS). This is mainly due to the lack of vertical spatial resolution of UPS and limitations in the preparation and quantitative characterization of bulk Ti³⁺ defects. In this study, we create surface and bulk Ti³⁺ defects in a controllable way by introducing surface and bulk hydroxyls into rutile TiO₂(011)-(2 × 1) via atomic deuterium exposure. Utilizing UPS combined with density functional theory (DFT) calculations, we successfully disentangled the contributions of surface and bulk Ti³⁺ defects to the BGS. The UPS data indicate that surface and bulk Ti³⁺ defects give rise to BGS at binding energies of approximately 0.85 and 1.57 eV, respectively. DFT calculations reveal that the separation of surface and bulk BGS originates from the distinct atomic environments of surface and bulk Ti³⁺ ions that induce characteristic 3d orbital splittings. Our finding that the surface and bulk Ti³⁺(OH) states are separated in energy could provide a fingerprint for the in situ monitoring of metal-support interactions and hydrogenation reactions in heterogeneous catalysis.