Evidence and analysis of parallel growth mechanisms in Cu₂O films prepared by Cu anodization

We have studied the preparation of Cu₂O films by copper anodization in a 0.1 M NaOH electrolyte. We identified the potential range at which Cu⁺ dissolution takes place then we prepared films with different times of exposure to this potential. The morphology, crystalline structure, band gap, Urbach energy and thickness of the films were studied. Films prepared with the electrode unexposed to the dissolution potential have a pyramidal growth typical of potential driven processes, while samples prepared at increasing exposure times to dissolution potential present continuous nucleation, growth and grain coalescence. We observed a discrepancy in the respective film thicknesses calculated by coulometry, atomic force microscopy and optical reflectance. We propose that anodic Cu₂O film formation involves three parallel mechanisms (i) Cu₂O nucleation at the surface, (ii) Cu⁺ dissolution followed by heterogeneous nucleation and (iii) Cu⁺ and OH^- diffusion through the forming oxide and subsequent reaction in the solid state.