Shigeyoshi Nakayama

Voltammetric Characterization of Oxide Films Formed on Copper in Air

Oxide layers thermally formed on copper have been studied using double sweep cyclic voltammetry in strongly alkaline electrolytes. It was found that the addition of I M LiOH in an electrolyte (6 M KOH) allowed perfect resolution of cathodic waves due to the reduction of Cu 2 O and CuO. Assignment of the two reduction waves has been achieved with the help of spectrophotometric techniques including X-ray photoelectron spectroscopy and X-ray diffraction; the cathodic wave appearing between -1.3 and -1.5 V (vs. Ag/AgCI) was attributed to the reduction of Cu 2 O, while that appearing at a less negative potential (-1.0 to -1.1 V) was attributed to the reduction of CuO. The electrochemical measurement of samples prepared under several conditions has revealed that CuO is reduced at once to Cu prior to the reduction of Cu 2 O, It was also confirmed that the formation of the oxide films was accelerated by elevating temperature, heightening humidity, and by preimmersion in electrolyte solutions. Water vapor was essential for the formation of CuO at a lower temperature (80°C).

Which Is Easier to Reduce, Cu2O or CuO ?

Chronopotentiometry (CP) using 0.1 M KCl as the electrolyte has been most frequently used for selective determination of cuprous and cupric oxides (Cu 2 O and CuO) formed on copper surfaces. However, there are conflicting views regarding the order of reduction of the oxides. This study was carried out to settle this problem. Differently prepared samples of Cu-duplex oxide films were partially reduced by means of CP with 0.1 M KCl and then submitted to X-ray diffractometry and also linear sweep voltammetry using a strongly alkaline electrolyte (6 M KOH + 1 M LiOH) in which reduction peaks of Cu 2 O and CuO could be obtained with good separation. The results clearly demonstrated that CuO was first reduced in 0.1 M KCl, followed by the reduction of Cu 2 O. However, the reductions of both oxides were found to occur simultaneously, to a greater or lesser extent, in 0.1 M KCl. It was also revealed that CuO was reduced to metal Cu in one step and that a partially reduced Cu-duplex oxide sample had an expected Cu|Cu 2 O|Cu sandwich structure.

A Mechanism for the Atmospheric Corrosion of Copper Determined by Voltammetry with a Strongly Alkaline Electrolyte

A recently developed voltammetric technique using a strongly alkaline electrolyte (6 M KOH + I M LiOH) was successfully applied to clarify a corrosion mechanism of copper under atmospheric conditions. In contrast to conventional potentiometric methods with neutral or weak alkaline electrolytes (e.g., 0.1 M KCl), the developed method could give qualitative and quantitative information about the corrosion products of copper, including oxides (i.e., Cu 2 O and CuO) and hydroxide [Cu(OH) 2 ]. In the presence of water under such atmospheric conditions, Cu(OH) 2 was the initial corrosion product formed on a copper surface. However, the surface Cu(OH) 2 layer did not grow much but dehydrated to become a layer of CuO. The thus-formed CuO layer grew until it became several molecules thick (∼2 nm). For further progress of corrosion, an inner Cu 2 O layer was generated by the proportionation reaction between the CuO layer and the base metal Cu. The inner Cu 2 O layer grew for the subsequent oxidation until the thickness reached a certain value (∼35 nm). For further oxidation, the top CuO layer grew again preferentially over the inner Cu 2 O laver.

Cathodic reduction of copper oxides

Abstract The determination of oxide thickness via cathodic reduction was developed in 1936. It is a simple laboratory practice that has the sensitivity and capability to analyze total oxide thickness of both cuprous and cupric oxides within the same run. Unfortunately, contradictory results and misidentification of oxide layers on the copper surface have caused ambiguity in the interpretation of many published results, leading to its incorrect use in industrial applications. This review surveys experimental parameters used by the authors, such as electrolyte composition and concentration, current density, current efficiency, sample history, interfacial impedance, crystalline orientation, and concentration of depolarizer on the cathodic reduction of copper oxides. Ultimately, the goal of the review is to propose solutions by which more precise measurement and correct identification of oxide layers could be derived from this type of experiment.