Transpassive Dissolution of Copper and Rapid Formation of Brilliant Colored Copper Oxide Films

This paper describes an electrochemical technique for growing adhesive copper oxide films on copper with attractive colors ranging from gold-brown to pearl with intermediate colors from red violet to gold green. The technique consists of anodically dissolving copper at transpassive potentials in hot sodium hydroxide, and then depositing brilliant color films of Cu₂O onto the surface of copper after the anodic potential has been turned off. The color of the copper oxide film depends on the temperature, the anodic potential, the time t₁ of polarization, and the time t₂, which is the time of immersion after potential has been turned off. (At potentials lower than the transpassive region, that is, in the passive region, the oxide film is a dark gray or dark green matte with a blue powder hydroxide on the surface and it is not attractive.) The brilliant colored films were characterized using glancing angle x-ray diffraction, and the film was found to be primarily Cu₂O. Cyclic voltammetry, chronopotentiometry, scanning electron microscopy, and x-ray photoelectron spectroscopy were also used to characterize these films. These techniques show that the different colors were due to differences in porosity and thickness rather than changes in composition.