Alumina-Supported Array of Co Nanoparticles: Size-Dependent Oxidation Kinetics?

The kinetics of oxidation of ordered arrays of Co nanoparticles (NPs) supported on the ultrathin aluminum oxide (AlOx) on a Ni3Al(111) template surface has been investigated at 300 and 470 K and an oxygen pressure of 1 × 10–6 mbar by high-resolution core level photoelectron spectroscopy as a function of the particles’ mean diameter, which was varied between 3 and 7 nm. For both temperatures, the oxidation of the Co NPs starts at their surface, which leads at intermediate oxygen exposures to core–shell NP morphologies and ends with the complete oxidation of the NPs to CoO under the employed oxidation conditions. At 300 K the Co NP oxidation rate is fast and does not depend significantly on the particles size. In contrast, at 470 K an apparent NP size dependence is observed, which is caused by the thermally activated penetration of oxygen through the alumina film and the preferential oxidation of Al atoms at the Ni3Al(111)/AlOx interface. The changes in the Co and Al oxidation rates with the particle size at 470 K have been attributed to the variation of the surface morphology with the Co coverage.