Origin of Low Energy d-d Excitations Observed on Wet Chemically Prepared Cobalt Bearing Nanoparticles by 2p3d Resonant X-ray Emission Spectroscopy

The 2p3d resonant X-ray emission spectroscopic (RXES) measurements on 8.4 and 5.0 nm cobalt and 3.6 nm cobalt-nickel nanoparticles coated with oleate molecules are provided. The spectra reveal low energy resonant Raman features at 0.3 and 0.75 ev. In combination with time-dependent density functional theory (TD-DFT) and ligand field multiple (LFM) calculations, these are ascribed to d-d excitations of cobalt ions in a low symmetry ligand field. Two different chemical environments of the ion may cause the transitions. In the first model cobalt ions in the nanoparticle outer atomic layer, resulting from the adsorbate binding, cause the excitations. These are transitions from a mixture of B-4(1g) plus E-4(g) to E-4(g) at 0.3 eV. At 0.75 ev transitions to B-4(2g) and (4)A(1g) take place. In the alternative model the excitations occur in a cobalt molecular species that might coexist with the nanoparticles. Here the transitions are from (4)A(2g) to E-4(g) and to E-4(g) plus B-4(2g) at 0.3 and 0.75 eV, respectively. On the basis of two-dimensional 2p3d RXES planes of the models and the differences between the three different nanoparticle systems, we exclude the first model and conclude that the metallic particles indeed coexist with varying minor degrees of molecular species. We observe however a second type of cobalt species, which is possible related to surface-ligated cobalt ions.