Electronic defects, i.e., shallow and deep donor, in uncharged and hydrogen-charged passivating $TiO_2$ films have been investigated by impedance measurement. Donor distribution in the crystalline and amorphous passivating $TiO_2$ films was detailed. In addition, an investigation was focussed on how hydrogen as a donor contributes to electronic conduction within the hydrogen-injected $TiO_2$ film. For this purpose, impedance of the $TiO_2$ film was measured as a function of applied potential and frequency. In an attempt to confirm the crystal structure of the film, the X-ray and TEM diffraction analyses were made.
The passive films on titanium were prepared galvanostatically in 1.0N $H_2SO_4$ solution with 5 and 10 mA $cm^{-2}$ at formation potentials ranging between 5 and 50 $V_{SCE}$. Hydrogen injection was performed into fresh films by scanning the applied potential range -1.7 to -1.5 $V_{SCE}$ and back at a rate of 2mV $S^{-1}$ in a 0.1N NaOH solution. The impedance measurement was conducted in 0.1N NaOH solution by superimposing an ac voltage of 5 mV amplitude over the frequency ranging from $10^2$ to $10^4$ Hz on a dc potential.
The crystal structure of the passivating $TiO_2$ film was experimentally substantiated by the TEM and X-ray diffraction analyses. The 5 $V_{SCE}$ -passive film was in an amorphous state, but the passive film formed more positive than 10 $V_{SCE}$ showed polycrystalline structure.
The Mott-Schottky plot proved to be nonlinear for the passive films irrespective of formation potential. From the analysis of the nonlinear Mott-Schottky plot obtained from the 30 $V_{SCE}$ -passive film(83nm in thickness) and 50 $V_{SCE}$ -passive film (106nm in thickness), it is concluded that the donor concentration remains constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases considerably with distance toward the film/metal interface. From the 5 $V_{SCE}$ -passive film (22nm in thickness), we deduced that the nonlinearity in the Mott-Schottky plot results due to the presence of multiple donor levels in the passive film.
Hydrogen injected into the film increases both donor concentration and electronic conductivity. The experimental results suggest that hydrogen donates electron to the conduction band inside the passive film. From the analysis of frequency dependence of the electronic conductivity, it is concluded that electrons which hydrogen donates in the conduction band flows in the interior of the grain and moves across the grain boundary by hopping. It is observed that flatband potential shifts to more positive potentials due to the adsorption of proton on the film surface.
