Passivation of nanocrystalline Al prepared by the gas phase condensation method: An x-ray photoelectron spectroscopy study

Nanocrystalline aluminum powders have been prepared by the gas phase condensation method. Samples have been synthesized in a conventional preparation chamber for gas phase condensation and also in the pretreatment chamber of an XPS (x-ray photoelectron spectroscopy) spectrometer so that in situ studies of the passivation process of nanocrystalline aluminum can be performed. For the range of particle sizes (12–41 nm) studied in the present work, we found a universal behavior during passivation with oxygen of the nanocrystalline Al⁰. An Al₂O₃ overlayer of 4 nm, which protects the material from further oxidation, was obtained for all samples independently of the route of oxygen dosage. A careful analysis of the photoelectron parameters (binding energy and Auger parameters) for Al and O shows that in the early stages of passivation the alumina overlayer is so thin (<2.5 nm thickness) that the Al₂O₃ –Al interface induces an increase in the relaxation energy of the photoholes as compared to that of bulk alumina. Conclusions have been drawn about the best way to proceed during passivation of Al ultrafine particles before exposure to an air atmosphere.