Anisotropic d-d Transition in Rutile TiO2.

The band gap state of TiO2, which is dominated by Ti3+ 3d character, is of great relevance to light absorption, electron trapping, charge recombination, and conduction band structure. Despite the importance, the explanation of the excitation from this state is controversial. To this end, the electronic structures of TiO2(110) and TiO2(011)-(2 × 1) have been systematically measured with two-photon photoemission spectroscopy. The results reveal the anisotropic nature of the electronic structure in rutile TiO2 at seemingly equivalent directions of [110] and [11̅0], the long axes of the TiO6 blocking unit. Although the resonant energy of these two d-d transitions is identical, the energy levels are systematically shifted by 0.1 eV. We propose this anisotropy originates from the broken symmetry of the rutile TiO2 crystals caused by the surface. The proposed asymmetry-caused electronic structure anisotropy could be generalized to other similar materials and may affect associated catalytic properties. This work provides an important benchmark for related calculations.