Nano-scale oxidation of copper in aqueous solution

In the present paper, a method has been successfully developed to prepare the copper needles. These needles have sufficiently thin tip regions that allow the direct TEM observation of the oxide layers grown on the surface of copper subjected to the electrochemical oxidation in the aqueous solution at nano-scale. The results show that the duplex layers are formed on the copper surface in the alkaline solution during electrochemical polarization and the layer growth follows a sequential order. An impact inner layer consisting of Cu₂O is first formed at low anodic potentials, which will be then transformed into CuO when the anodic potentials increase before the occurrence of oxygen evolution. Subsequently, the production of Cu(OH)₂ on the top of CuO inner layer leads to the formation of the outer layer. The epitaxial orientation relationship between the inner oxide layer and its adjacent copper matrix is discovered. The outer oxide layer appears porous consisting of nano-crystals. Some nano-crystals are oriented well with the inner layer due to their direct growth from it. The rest randomly-oriented nano-crystals are produced by the dissolution of oxides into the solution forming copper hydroxide particles, which are immediately deposited back to the surface of the copper needle.