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

Abstract

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