U2O5 film preparation via UO2 deposition by direct current sputtering and successive oxidation and reduction with atomic oxygen and atomic hydrogen

Abstract

We describe a method to produce U2O5 film in-situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, essential part of the PAMEC (Properties of Actinides under Extreme Condition) laboratory allows preparing films and study samples surfaces using surface analytical techniques such as X-ray and Ultra-violet photoemission spectroscopy (XPS, UPS). All studies are made in-situ; the films, transferred under ultra-high vacuum from preparation to analyses chamber, are never in contact with the atmosphere. Initially a film of UO2 is prepared by direct current (DC) sputter deposition on an Au foil and then oxidised by atomic oxygen to produce UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are done after each step of oxidation and reduction, using high resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction time and the corresponding temperature of the substrate during the process have a severe effect on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging. First uranium-oxygen systems exist in numerous intermediate phases. Second the differentiation of the oxidation states of uranium is mainly based on satellites peaks whose intensity peak is weak. Also, one should be aware that the technique of x-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1 to 5%. Thus, it is important to obtain the complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s and the valence band (VB). The programmes used on the Labstation include the linear transfer programme developed by Specs Company, and the data acquisition programme and the sputter source programme, both developed in-house.