Characterization and Oxidation of Fe Nanoparticles Deposited onto Highly Oriented Pyrolytic Graphite, Using X-ray Photoelectron Spectroscopy

The characterization of Fe-0 nanoparticles (NPs), both before and during oxidation, has been of concern for the last two decades. We have studied the 2p and 3p XPS core levels of Fe NPs evaporated onto highly oriented pyrolytic graphite (HOPG) under ultrahigh vacuum. Both components of the 2p spectrum of Fe-0 are found to be highly asymmetric to higher binding energy; each is composed of a major photoemission peak, plus several smaller peaks attributable to a vacancy cascade, a process known to occur in Fe. In contrast, the two Fe 3p spectral components are too close to be separated with precision, and were treated as one single component; as with the 2p components, it, too, is asymmetric, due to the vacancy cascade. The onset of oxidation affects both spectra somewhat differently, causing the introduction, and subsequent increase, of components on the high binding energy side of the 2p(3/2) spectrum, superimposed on the vacancy cascade; this is not as obvious for the 3p spectrum because of a larger probe depth, to which the surface contributes less. These new components represent the FeO, gamma-Fe2O3, and Fe3O4 formed on oxidation; their oxidation kinetics indicate that the initially formed FeO is rate controlling.