Two-Step Reduction Pathway of Copper(II) by Oleylamine for the Nucleation of Cu(0) Nanoparticles: A Joint Modelling and Experimental Study

Oleylamine is widely used in the synthesis of colloidal nanoparticles, as a solvent, as a stabilizing agent, and sometimes as a reducing agent. For example, metallic nanoparticles are obtained through reduction when Ni(II) and Pd(II) precursors are used or through disproportionation in the case of Ni(I) or Co(I). A similar dichotomy is observed for Cu precursors, with an additional complexity due to the nature of the precursor salt. In the present article, we report a combined DFT evaluation of possible reduction paths for Cu(II) and Cu(I) reduction by oleylamine, including the competition with Cu(I) disproportionation, and X-ray Absorption Spectroscopy monitoring of the oxidation state of copper(II) acetylacetonate in oleylamine. We show that the reduction of copper(II) acetylacetonate goes through a two-steps process, with the intermediate formation of Cu(I) complexes. The role of phosphine ligands is demonstrated as well as the relevance of these findings in case alternative copper sources such as copper halogen salts.