Oxidation and reduction of copper oxide thin films

We have employed ¹⁶O(α,α)¹⁶O oxygen resonance measurement and transmission electron microscopy to study the oxidation and reduction of copper oxide thin films. The in- and out-diffusion of oxygen-induced oxygen concentration variations and microstructural changes in the films were monitored. The study of reduction was carried out by annealing CuO and Cu₄O₃ thin films in vacuum. Dark-field microscopic images show that isolated and large Cu₂O grains emerge from the small CuO and Cu₄O₃ grain matrices. The growth of Cu ₂O grains in both CuO and Cu₄O₃ matrices has been measured to be linear with time, and have activation energies 1.1 and 0.7 eV, respectively. The main controlling mechanism to the discontinuous morphology of the Cu₂O grain growth is the migration of the phase boundaries between the oxides induced by oxygen out-diffusion along the moving boundary. An oxygen in-diffusion study was performed by annealing Cu₂O and Cu₄O₃ in an oxygen ambient. The CuO phase nucleates randomly and rapidly in both Cu₂O and Cu₄O₃ matrices. The small grain growth rate of CuO suggests that nucleation rather than grain growth is the predominant event during oxidation. The kinetics study of the reduction and oxidation of copper oxides shows that the two processes are asymmetrical and the latter is faster.