This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Copper roofs lose their reddish color over time because a pale green layer of copper oxide forms on their surface, protecting the roof from further corrosion for many years. But what exactly happens during the first steps of this process?
Might it even be possible to stop it? This question has become increasingly urgent since copper containers have come to be regarded as one of the best options for storing nuclear waste. Before pure copper oxide builds up, complex superstructures consisting of copper and oxygen atoms, such as '29'CuxO, are formed.
The oxidation state of copper in this complex compound has been highly controversial and could not be measured directly because conventional spectroscopic methods were barely able to detect differences between pure copper and the partially oxidized surface. Now, a team led by Professor Dr. Alexander Föhlisch has succeeded in doing so using Auger photoelectron coincidence spectroscopy (APECS) at BESSY II.
This extremely surface-sensitive method allowed for a clear distinction between the chemical states. The research is published in the Journal of the American Chemical Society. The result: Copper atoms in the well-known '29' surface oxide are very close to the metallic state; their oxidation state is only around 0.3.
This means that the surface resembles metallic copper much more closely than copper oxide (Cu₂O), even though it contains a great deal of oxygen. "This is surprising and truly remarkable, because the oxygen concentration on the surface is very high," says Swarnshikha Sinha, who carried out the measurements. "We have also identified the distribution of oxygen species that make up the protective surface oxide," she points out.
Their study demonstrates that the amount of oxygen on a surface does not automatically determine the extent to which the copper atoms are oxidized. These insights are helpful for two different fields of application. Copper is used as a catalyst, for example, in CO₂ reduction, methanol synthesis or oxidation reactions.
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