Determination of energy distribution of donor levels in anodically passivating TiO2 film

Abstract

The present work is concerned with determination of energy distribution of donor levels by analysing a.c. impedance response from the anodically passivating TiO2 films, based upon a new numerical method, The fresh and halide ion-incorporated anodic TiO2 films were galvanostatically prepared on titanium substrate at 10 mA cm(-2) to a formation potential of 50 V in deaerated 0.5 M H2SO4 solution and deaerated 0.5 M H2SO4 solution containing 0.5 M of Cl- or Br- respectively. Both a.c. impedance and photocurrent spectra were measured from the fresh and halide ion-doped anodic TiO2 films to quantitatively determine the frequency dependence of donor concentration and energy distribution of deep donors formed by the halide ions. The new numerical method analysing the frequency dependence of donor concentration was proposed to determine the energy distribution of donor levels in the fresh and halide ion-doped anodic TiO, films. From the analysis of a.c. impedance response on the basis of the proposed new numerical method, it was suggested that donor levels are distributed continuously in the energy range of 0.55 to 0.67 eV below the conduction band edge, In addition, it was concluded that the donor concentration is reduced by the halide ion incorporation into the fresh anodic TiO2 films in the measuring frequency range of 10 to 10(3) Hz, suggesting that the doped halide ions occupy oxygen vacancy sites and simultaneously form deep donor levels in the band gap of the anodic TiO2 films.