On the electrochemical reactivity of the redox couple Fe(CN)63−/Fe(CN)64− at the single crystal zinc oxide electrode

The system zinc oxide/hexacyanoferrate was used to test the applicability of the theoretical model for electrochemical reactions at semiconductor electrodes based upon direct electron transfer between levels of equal energy. This model appears to be valid for electron transfer involving the conduction band. From the pH-dependence of the cathodic reactivity, a value of 0.75 eV is found for the rearrangement energy of Fe(CN)₆^3?/Fe(CN)₆^4?. This result, combined with capacity and potential data, allows the construction of a general energy scheme for the system under consideration. It is shown that direct electron transfer from filled redox levels to empty levels in the valence band is highly improbable, so that the observed hole capture by Fe(CN)₆^4? from the UV-illuminated ZnO anode presumably occurs by a more complicated mechanism, e.g. involving surface states.