Unexpected oxidation behavior of Cu nanoparticles embedded in porous alumina films produced by molecular layer deposition

This work reports an unexpected oxidation behavior of Cu nanoparticles embedded in porous Al₂O₃ confinements that are produced by annealing alucone (an organic - inorganic hybrid material) deposited by molecular layer deposition. An oxidation of such encapsulated Cu nanoparticles by annealing in air produces Cu oxide nanoparticles attached to the outer surface of the hollow Al₂O₃ nanostructures, which is in strong contrast to bare or compact, nonporous Al₂O₃-coated Cu nanoparticles, which result in hollow oxide nanospheres or do not undergo oxidation, respectively. The conversion from encapsulated Cu to supported oxide nanoparticles is explained by a concerted pore-assisted diffusion and oxidation mechanism. The micropores in the films, having diameters of several angstroms, permit a selective out-diffusion of Cu atoms and prevent the inward diffusion of oxygen. The subsequent oxidation occurs at the pore entrances, which work as multiple nucleation sites for the formation of oxide nanoparticles with a small size and good dispersion.