Role of Surface Structure on the Charge Trapping in TiO2 Photocatalysts

Surface reconstruction of the rutile-TiO2(011)-2 x 1 surface places Ti ions in a distorted square pyramidal coordination environment. This variation of the crystal field compared to the common octahedral symmetry causes the binding energy for excess electrons occupying Ti 3d states to be 0.34 eV higher for the (011)-2 x 1 surface compared to that for the (110) surface. The role of adsorbates in the formation or annihilation of excess charges in TiO2 is studied on the example of acetic acid adsorption. We find that bridge bidentate adsorption on the (110) surface results in extraction of excess charges from the substrate, while monodentate adsorption on the (011)-2 x 1 surface causes net-charge donation to the substrate. Differences in adsorbate-induced charge donation and more importantly binding energy differences of excess charge for the two surfaces may explain the face-dependent photocatalytic activity of rutile TiO2.