Oxidation state and size effects in CoO nanoparticles

We have investigated the oxidation state and size effects on the electronic structure of 10 and 20 nm CoO nanoparticles by means of surface spectrosco pies. It has been concluded that the surface of the as-grown CoO nanopartic les is oxidized to Co3O4 After annealing of the nanoparticles in UHV at 600 degrees C, the surface is reduced to CoO. The results show that the electr onic structure of the as-grown 20 nm CoO nanoparticles is not significantly affected by surface effects and compares rather well with a CoO bulk sampl e. However, in the case of the 10 nm CoO nanoparticles, size effects are cl early observable since the relatively large surface-to-volume ratio of thes e nanoparticles produces a large amount of surface defects which act as ads orption centers.