Formation of cuprous oxide layers in Cu(II) solutions containing gluconic acid

In accordance with thermodynamic analysis, cuprous oxide layers are formed spontaneously in the Cu vertical bar Cu (II), gluconic acid system at pH>3.7 under open-circuit conditions. A current peak of Cu2O reduction is observed on cathodic voltammograms at ca -0.7 V, its height being dependent on the exposure time. The analysis of the charge transferred in this region yields the rate of Cu2O formation equal to 1.25 x 10(-10) mol cm(-2) s(-1). The light perturbation of Cu electrode under open-circuit conditions results in the generation of a negative photopotential, which is indicative of n-type conductivity. The threshold wavelength is equal to similar to 590 nm and is consistent with a band gap of similar to 2.1 eV. Anodic photocurrents, which are observed near the open-circuit potential, decrease with cathodic polarization and change their sign at similar to 0.05 V. Analysis of impedance data was performed, invoking the equivalent circuit that accounts for the two-step charge transfer. In the presence of Cu2O, some retardation of Cu(II) reduction was found to occur with a slight increase in the admittance of the double layer. The suggestion has been made that oxide layers formed in Cu(II) gluconate solutions cannot be compact and uniformly distributed over the entire electrode surface. Relevant investigations of surface morphology support this conclusion.