Oxidation of copper and reduction of Cu₂O in an environmental scanning electron microscope at 800°C

The present study reports on the in situ oxidation of copper to Cu₂O and subsequent reduction to metallic copper in an environmental scanning electron microscope. The oxidation was carried out at approximately 125 μm oxygen pressure while the reduction was done at the same pressure using H₂ gas. The first visible signs of oxidation occurred in about 2 min: submicron size nuclei of Cu₂O formed randomly on the metal surface. No preferred nucleation along grain boundaries could be observed. The surface was completely covered with Cu₂O in about 30 min of oxidation time, the final average grain size of Cu₂O being approximately 3 μm. The reduction kinetics of Cu₂O were slower than the oxidation kinetics, the first visible copper nuclei appearing only after about 6 min of reduction. A reduction mechanism is suggested where the diffusion of copper vacancies from the copper particle-Cu₂O interface to the Cu₂O-H₂ interface limits the overall kinetics. Based on this assumption, a copper supersaturation corresponding to a copper activity of 1.0005 in Cu₂O has been calculated.