Comparison of the electronic structure of anatase and rutile TiO2 single-crystal surfaces using resonant photoemission and x-ray absorption spectroscopy

A comparison of the electronic structure of rutile (110), anatase (101), and anatase (001) single-crystal surfaces has been made using resonant photoemission and x-ray absorption spectroscopy. Under identical preparative conditions, the anatase (101) surface shows the lowest Ti 3d and 4sp hybridization in the states close to the valence-band maximum of the three surfaces. It also shows the highest concentration of surface-oxygen vacancies. The effect on the electronic structure of modifying the surface preparative route and thus the concentration of surface-oxygen vacancies is examined. The sigma-antibonding Ti 3d e(g)/O 2p hybridization (probed by XAS) is reduced by the removal of surface-oxygen. Photoemission shows that as the number of surface-defects is increased, the O 2p-Ti 3d t(2g) pi-bonding interaction is disrupted. For the anatase (101) surface it is found that as the number of surface-oxygen vacancies is increased, the Ti 3d and 4sp contributions at the valence-band maximum are reduced. We discuss the correlation between electronic structure and photocatalytic activity of the different polymorphs of TiO2.