Photochemical mechanism of the formation of nanometer-sized copper by UV irradiation of ethanol bis(2,4-pentandionato)copper(II) solutions

The formation of colloidal metallic copper and of nanometer films on quartz and silicon substrates by irradiation of bis(2,4-pentanedionato)copper(II) ethanol solutions with UV light was investigated. The 254-nm-light absorption in the bulk of the solution causes formation of stable colloidal copper through two consecutive steps that could be distinguishable at low light intensity. In the first step, the starting complex releases one ligand and forms of copper(I) species that quickly release the second ligand and evolve to copper(I) alkoxides. In the second step, these latter undergo reduction to colloidal copper by the released acetylacetone with an electronic transfer process. Acetone sensitizes reduction to metallic copper with radical pathways. Features of the process depend on the acetone concentration so that either nanoparticles of colloidal copper or fine powder can be fabricated from the solution. In the presence of quartz or silicon substrate, placed in the suitable apparatus, the light absorption at the substrate-solution interface induces the formation of pure thin metallic copper films with nanometer characteristics. The photodeposition involves almost a heterogeneous process. The photoreaction has been followed by UV-vis and H-1 NMR spectroscopy while the copper(1) species have been detected by ESI-MS analysis and IR spectroscopy; copper powder and the films have been investigated by XRD, XPS, and SEM techniques. A mechanism of direct and acetone-sensitized formation of nanoparticles of copper is proposed, based on the role of the released ligand and on the photochemistry of the acetone.