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. Copper and its alloys are unique in being the only nongray colored metals, other than gold and its alloys. Polished copper is a warm, reddish-brown color, and its surface color can change depending upon the exposure time and environment. The colored surface films (patina) that form on copper alloys are of particular interest to the fields of architecture and sculpture art. A copper office complex in Amsterdam, the Netherlands, was given a brown chocolate patina by progressive oxidation of the copper, while copper warehouses in Hamburg, Germany, became an almond green patina after many years in outdoor atmospheric exposure. 1 Copper has been widely studied, and there are many chemical techniques for coloring copper. Taguri et al. developed attractive colored finishes for copper and copper alloys by simply immersing the metal in a selenious acid solution of controlled concentration and pH at room temperature and they grew films with thicknesses between 10 and 70 nm, but these films were not stable in the atmosphere and had to be protected by a layer of clear lacquer. Taguri et al. showed that the colors were due to interference effects. 2 Miley oxidized copper by heating samples in an electric furnace at different temperatures. These colors varied from dark brown to silvery green with intermediate colors like red-brown, violet, or blue with an average thickness of tens of nanometers. Miley also showed that the cuprous‐cupric oxide layer contained an intimate mixture of the two oxides or possibly a solid solution with the greater concentration of the cupric oxide toward the outer surface and the cuprous oxide toward the base metal. Light could be reflected from the outer and inner surfaces of the cuprous‐cupric oxide film, and exhibit interference effects, unless a separate layer of cupric oxide grew over it. 3 Recently Allred described a method to build a rainbow by electrodepositing a cuprous oxide on glass at different times by deposition of thin films. 4 The nanoscale thickness of the film, which correlated to the color, was varied by changing the deposition time. The coloration is proposed to originate from iridescence, i.e., constructive interference of light reflected at the outer air/oxide and the inner oxide/ metal interfaces. 5,6 The electrochemical behavior of copper in alkaline media has been extensively studied, but mostly in dilute alkaline solutions. 7‐9 A few studies had been conducted in the concentrated alkaline solutions. 10‐12 Biton et al. studied the behavior of copper in 45% KOH solution between � 0.4 and 0.6 V/standard hydrogen electrode (SHE), but only at ambient temperature. In this potential range, they showed that the major surface oxide formed on copper at potential P1 (� 0.34 V/SHE) was Cu2O. Further polarization to the potential P2 (� 0.07 V/SHE) led to the precipitation of surface films compris