It has been found that a TiO
2 coating under illumination can cathodically protect substrate metals, in a non-sacrificial way, by its photoelectrochemical characteristics. It has also been found that, when TiO
2 layer containing Fe is coated under a plain TiO
2 layer, the effect continues even after stopping illumination. In this work, the impedance of TiO
2 coatings on ITO glass substrate, plain TiO
2 (TiO
2/ITO), TiO
2 containing x
Fe% Fe (x
Fe Fe-TiO
2/ITO), and TiO
2 coating with 2 layers, plain TiO
2 for outer layer and x
Fe Fe-TiO
2 for inner layer (TiO
2/x
Fe Fe-TiO
2/ITO), were measured in a phosphate buffer solution, pH6, to study this slow decline of electrode potential after stopping illumination. A capacitance,
CIL, was detected in the x
Fe Fe-TiO
2/ITO and it was considered to be associated to the oxidation/reduction of Fe(II)/Fe(III) along with that of space charge layer,
CSP. The
CIL values reached a maximum around V=-400- -300mV vs. SCE. The maximum value increased with increasung x
Fe up to 6000μF/cm
2 for 20% Fe-TiO
2/ITO, being significantly larger than
CSP (10-100μF/cm
2). It was confirmed that the behavior of
CIL and the slow decline of electrode potential for x
Fe Fe-TiO
2/ITO could be explained by the model which assumed that the ratio of Fe(II)/Fe(III) depends on electrode potential. A significantly large capacitance was also detected in the TiO
2/x
Fe Fe-TiO
2/ITO. The flatband potential,
Efb, or onset potential,
Eon, for TiO
2/x
Fe Fe-TiO
2/ITO decreased down to the similar values to those of TiO
2/ITO, while those values of x
Fe Fe-TiO
2/ITO were relatively noble. Based on these results, it was confirmed that the TiO
2 coating with 2 layers (TiO
2/x
Fe Fe-TiO
2/ITO) combines both less noble photo potential for cathodic protection and good characteristic of slow decline of electrode potential after stopping illumination.
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