Photoacoustic study on cathodic reduction of anodic oxide films formed on copper in borate solution

An in-situ photoacoustic (PAS) technique, using a piezoelectric detector with high sensitivity was applied to the study on duplex oxide films anodically formed on copper in pH 8.4 borate solution. The PAS signals from the copper electrode were produced by an irradiation of light beam with a wavelength of 514.5 nm. The PAS amplitude during cathodic reduction of the outer oxide layer to Cu₂O changed in the opposite direction, depending on the anodic potential of film formation and oxidation time. Assuming that the change in PAS amplitude is proportional to both optical absorption coefficient and film thickness, it was deduced from comparison of the estimated absorption coefficients for Cu (OH)₂, CuO and CuO_0.67 films that dehydration of the outer layer having an average composition of CuO_x (OH)_2?2x proceeded with increasing anodic potential of film formation and oxidation time during growth of the duplex oxide film. Moreover, it was found that the change in PAS amplitude during cathodic reduction of the total Cu₂O film involving the inner layer to metallic copper was proportional to the electric charge required for cathodic reduction, i.e., the film thickness, irrespective of anodic potential of film formation and oxidation time, which proved the validity of the above assumption.