Morphological and Kinetic Insights into Cu2O–CuO Oxidation

Abstract

The conversion of Cu2O to CuO thin films is probed in situ with UV–vis transmission spectroscopy at various thermodynamic conditions. The obtained spectral series are simulated with different CuO/Cu2O growth models, using the transfer‐matrix method and Bruggeman's mean‐field approach to account for Cu residuals in the oxide matrix. The analysis reveals that Cu2O oxidation proceeds via a flat and homogeneous reaction front, in contrast to polycrystalline copper that preferentially oxidizes along the grain boundaries. The conclusion is supported by the measured activation energy for Cu2O oxidation, which is compatible with Cu diffusion via the bulk oxide and not via line defects. The kinetics of CuO growth is found to follow logarithmic and exponential rate laws at low and high temperature, respectively. While the logarithmic regime is governed by CuO nucleation within the Cu2O phase, the exponential dependence results from self‐heating of the sample in the exothermic reaction. In both cases, traces of unreacted copper are detected in the oxide matrix, giving rise to a characteristic tail in the transmission response.