The Growth and Reduction of Duplex Oxide Films on Copper

The formation and reduction of anodic oxides on copper was studied by potentiodynamic polarization curves, fast potentiostatic pulse experiments and capacity measurements in alkaline solutions. Four different oxide species could be distinguished by their kinetic and capacitive behaviour: the oxide CuOx, which represents few monomolecular layers of copper oxide, formed in short periods or at low potentials, the oxides Cu2O and CuO at intermediate potentials and for longer polarization times, and a non‐defined oxidation product CuOy·zH2O at high potentials. The first 3 oxides form semiconducting films, which grow according to a high field mechanism. At intermediate potentials and at longer polarization times, the passive film consists of an inner Cu2O‐ and an outer CuO‐film, the properties of which are limited by the electronic properties. The common analysis for a high field mechanism of oxide growth can be applied to both stable oxides, taking into account the local potential difference Δϕi < Δε. The potential drop Δϕi in each of these oxides does not exceed 0.5 V. The potential drop ΔϕH in the Helmholtz layer is constant at potentials up to 1.2 V, but it increases with ε at higher potentials. The capacitive behaviour cannot be explained only by capacitors in series but parallel with diodes short circuiting the layers at an upper value of Δϕi. The oxide formation of CuOx, Cu2O and CuO takes place homogeneously all over the surface with almost constant thickness. The reduction occurs by an island mechanism for the duplex film as well as for the CuOx, film. The results are explained on the basis of a tentatively band structure model of the passive film.