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 angstrom thick) and single-crystal TiO2 show a large bandgap region (approximately 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 approximately 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.