Nature of oxygen at rocksalt and spinel oxide surfaces

The chemical environment of oxygen in cobalt-containing metal oxides with compositions M xM′( x – 1) O and M xM′(3x – 1) O4 (M,M′ = Mn,Ni,Co) has been studied by Auger, x-ray and ultraviolet photoelectron, and high resolution electron energy loss spectroscopies. While there is a single type of lattice oxygen in the bulk structure of simple rocksalt and spinel oxides, the nature of oxygen at the surface of the spinel oxides is considerably more complex. Photoemission from core oxygen states in these materials often shows multiple peaks and satellite structure which have been attributed to a range of intrinsic and extrinsic oxygen states. All of these 3d transition metal oxides show a single, intense O 1s core photoemission peak at approximately 529.6 eV. In the spinel materials, a second state at 531.2 eV is also observed and is shown to be intrinsic to the spinel surface and not a result of hydroxylation or other surface contaminant. Similar photoemission features in Fe3O4 were previously attributed to final state effects; however, the nature of the multiple final states remains to be elucidated

Effect of the surface structure on the departure from stoichiometry of Co

The surface of an oxide may have a composition different from that of the bulk. A method of calculation of the composition of a powder from the surface structure is presented. It is applied to a Co3O4 sample for which data on composition are deduced from gravimetry, neutron scattering and magnetic susceptibility measurements. It is found that at most 20 % of the excess of oxygen above stoichimetry has to be attributed to cation vacancies, the rest being due to the surface. Charges are counted by the same method, showing that surface oxygen are almost exclusively O-