The Capacity of Passivated Iron Electrodes and the Band Structure of the Passive Layer

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AbstractThe capacity of passivated iron electrodes was measured by the potentiostatic pulse method in dependence on the oxide layer thickness d and the electrode potential ϵ at ρH = 8.4. At constant d, the C(ϵ) dependence shows three characteristic ranges. With increasing potential, C decreases in the first range by a factor 2, in the second range it becomes constant, and finally C increases at high potentials. The results are discussed with respect to the band structure model of the semiconducting passive layer. In the first range a Schottky‐Mott type behaviour is found indicating donor concentrations N up to 1020 cm−3. The flat band potential ϵFB = 0.1 V is obtained. In the second range, C is proportional to 1/d. The capacity increase in the third range at high potentials is explained as a contribution from valence band charging at the oxide surface. The band gap Eg = 1.6 eV follows. The electronic properties of oxide covered metal electrodes are discussed in relation to bulk semiconductor electrodes.