Impedance behavior of TiO2 nanotubes formed by anodization in NaF electrolytes

The impedance behavior of nanotubular self-organized porous TiO2 (PTO) films with thickness of > 2 mu m, 100 nm pore diameter, and 150 nm average spacing formed on titanium by anodization in 1 M Na2SO4 solution +0.5 wt% NaF was investigated and compared to the behavior of compact TiO2 layers (CTO). At potentials close to the flatband potential (E-FB), PTO had an apparent interfacial capacitance at 30 Hz that was greater than the capacitance of CTO. This effect, however, was not only related to the larger effective area of PTO. Differentiation between both surface conditions vanished both at potentials significantly higher than E-FB and with increasing test frequency to 1 kHz. These findings together with observations of appreciable frequency dispersion suggest that the pore walls are rich in deep-lying localized states, which become evident only at lower test frequencies and at potentials negative enough that depletion zones do not merge at pore walls. PTO did not show a transmission line effect at frequencies as high as 100 kHz. Model calculations based on pore dimensions and electrolyte conductivity predicted that those effects could only take place at much higher test frequencies, but the slow response of deeper states might impair observation anyway. (c) 2008 The Electrochemical Society.