Nanostructural and optical properties of cobalt and nickel–oxide/silica nanocomposites

Cobalt, nickel and mixed cobalt–nickel nanostructured oxides have been obtained by thermal treatments in oxygen of Co, Ni and Co50Ni50/silica nanocomposites produced by ion implantation technique. The thermal treatment produces the total or partial oxidation of the original metallic species, which diffuse towards the surface and in the depth of the substrate. This process gives rise to the growth of different oxides in both regions depending on the kind of metal. In the Co sample, mainly Co3O4 is formed; in the Ni sample, Ni and NiO nanoparticles are present, while in the Co–Ni sample Co3O4, Ni2SiO4 and mainly (CoxNi1−x)O nanostructures are formed. The optical features of cobalt and nickel samples are determined by the optical properties of the Co3O4 and NiO p-type semiconductor phases, respectively, with energy gaps littler than the corresponding bulk ones. The Co–Ni mixed nanostructured oxide is characterized by a direct absorption gap at 3.4 eV that is smaller than the band gaps of CoO and NiO oxides.