Correlation of electron-transfer rates with the surface density of states of native and anodically grown oxide films on titanium

We report tunneling spectroscopy (TS) and surface density of states (SDOS) plots ((dI/dV)/(I/V) vs V) for native and anodically grown titanium dioxide (TiO₂) films on polycrystalline Ti. The results are compared to data obtained by using single-crystal TiO₂ ((001) and (110) surface orientations). SDOS plots for anodically grown TiO₂ films (160 Å thick) and single-crystal TiO₂ show a large bandgap region (∼2 eV) with a low state density separating the conduction and valence band edges. The similarity in the distribution of SDOS for anodically grown TiO₂ films and single-crystal TiO₂ indicates that the anodically grown film has a well-ordered rutile structure. In contrast, SDOS plots obtained on native TiO₂ films show a nearly constant density of states over a 5-eV range and a ∼20-50-fold increase in state density at energies corresponding to the bandgap region of anodically grown films and single-crystal TiO₂. Electron-transfer rates for the oxidation of ferrocene in acetonitrile at native and anodically grown TiO₂ films are reported and correlated with the measured SDOS of these materials.