A photoelectrochemical and ac impedance study of anodic titanium oxide films

The electrochemical behavior of sulphuric acid anodised titanium prepared galvanostatically under high field conditions over a wide range of maximum potentials (V-max) was examined using photoelectrochemistry (PEG) and ac impedance spectroscopy (EIS). Film structure and morphology were examined by transmission electron microscopy (TEM). The variation in reciprocal capacitance against V-max obtained from EIS was evaluated using a constant phase element model. For V-max = 0-10 V a straight line relationship was observed, enabling evaluation in terms of a parallel plate capacitor. This was followed by a region of decreasing reciprocal capacitance, indicating a substantial change in the nature of the material. Finally a slow rise occurred beyond 30 V. Large changes in the band structure of the anodic oxide were observed by PEC near the optical band edge for 0-10 V and in the sub-band gap region for 10-20 V. A defective TiO2 structure with impurity bands located deeply inside the band gap was still observed for 60 V and 84 V, where microcrystals of anatase are present. Strong correlation between the reciprocal capacitance, optical band gap, sub-band gap photoresponse and changes in crystallinity and degree of order within the anodic films were established. (C) 1997 Elsevier Science Ltd.