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Anodic oxidation of titanium: Mechanism of non-stoichiometric oxide formation

Structurally sensitive in situ methods such as photopolarization and impedance were used to examine the passivation process and the properties of the protective oxide layers on titanium. The kinetics of anodic oxidation and the non-stoichiometry of the surface oxide were correlated. The composition of the anodic film on titanium changes with the relative potential from lower to higher oxidation stages according to TiH2+TiO→nTi2O3·nTiO2→Ti5O9orTi6O11→Ti3O5→TiO2. The characteristic behaviour of titanium as a member of the valve metal group can easily be seen at higher anodic potentials (approximately +1.5 V (SCE) in 5 mol H2SO4 dm-3) when the electrode is covered with a nearly stoichiometric TiO2 layer. The semiconducting properties of TiO2 were investigated using an anodic film stabilized at +2 V(SCE) and it was found that TiO2, like the lower titanium oxides, is an n-type semiconductor under anodic polarization. The flat-band potential for the stabilized TiO2 film was -0.2 V, as determined by the two methods in 5 mol H2SO4 dm-3. The donor concentration was 2×10^(20)cm-3 under the same conditions (a value of 60 was used for the dielectric constant).

titanium; anodic oxidation; photopolarization measurements; semiconducting properties

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