Capacitance study on anodically grown titanium dioxide films

Abstract

The capacitance technique was applied to the study on oxide film formed on titanium rod surface. The experiment was performed with potentiostat, using TiO$_2$ electrode as the working electrode and Pt electrode as the counter electrode in one mol sodium perchlorate solution. The properties of titanium dioxide film, which was anodically grown on titanium surface, were determined by measuring the film thickness and studying the effects of the oscillator frequency, the temperature of the film, and the pH value of the aqueous electrolyte solution. The dependence of the growth rate of the titanium dioxide film on the anodic current density was also examined and found to follow a parabolic oxidation rate. The anodically grown oxide films showed similar behavior to n-type semiconductors in which the oxygen vacancies in the oxide layer play the role of donors. The study of film thickness effect on capacitance showed that the donor density, which was determined from the Mott-Schottky slope, decreased exponentially as the film thickness increased. The conductivities of the oxide film, measured at various frequencies and temperatures, showed that the ionization energy of donor states in the titanium was 0.817 eV, and the donor density increased with the increment of the film temperature and decreased with the frequency increase. Form Mott-Schottky plots at various pH values of the solution, it was shown that the flat band potential of the TiO$_2$ film decreased with the pH increase by -90 mV/pH. From experimental results on the dependence of donor density on temperature, deep and shallow donors were suggested.