Rutile TiO2, especially its (110) surface, has long been taken as the model system to understand mechanism of semi-conductor based photocatalysis. Revealing photocatalysis related electronic structure features and dynamics helps to identify the mechanism behind, which then provides great help for designing strategies. It is established that rutile itself has high symmetry. Rutile itself has a wide gap of ~3 eV, while intrinsic doping like Oxygen vacancy and Ti interstitials could create a gap state ~1 eV below conduction band minimum. Both the gap state and CB are mainly contributed by Ti 3d orbitals. This gap state could be excited into a state ~2.8 eV above fermi level via d-d transition. With 2PPE (2 Photon Photo-Emission Spectroscopy) we found that the d-d transition from gap state to conduction band is anisotropic, breaking the symmetry of bulk rutile, probably caused by surficial cutoff. With this anisotropy we could identify TiO6 octahedrons in different directions. One of this anisotropic intermediate state extends away from the surface into the vacuum side, making it a possible stepstone for charge transfer. Thereby, we propose that this upper state could be a possible channel for effective rutile based photocatalysis reactions with near-UV light.

Anistrophic d-d transition in rutile TiO2: Possible channel for interfacial electron transfer

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