An XPS and UPS study on the electronic structure of ThOx (x ≤ 2) thin films

Abstract

Model systems are needed for surface corrosion studies of spent nuclear oxide fuels. For this purpose ThO2 films have been prepared in situ by adsorption of molecular and atomic oxygen on Th metal films and by sputter deposition of Th metal in an Ar/O2 gas mixture. Surface compositions and electronic structure were compared to the bulk oxide and oxygen substoichiometry effects investigated. X-ray and ultraviolet photoemission spectroscopy (XPS and UPS, respectively) were used to measure to Th-4f, O-1s core levels and the valence band region. The Th-4f line was analyzed in terms of the final-state screening model. The evolution of the binding energies with oxygen concentration has been studied. On Th metal, adsorption of molecular oxygen ceased after the formation of a ThO2 surface layer. In the presence of atomic oxygen, the oxidation proceeded into the underlying bulk. The formation of oxygen interstitials was shown by the broadening of the O-2p and O-1s lines and by the increase of the O-1s/Th-4f ratio. Once ThO2 is formed, all photoemission peaks from Th and O undergo a rigid shift to low binding energy.