XPS determination of uranium oxidation states

Abstract

This contribution is both a review of different aspects of X-ray photoelectron spectroscopy that can help one determine U oxidation states and a personal perspective on how to effectively model the X-ray photoelectron spectroscopy of complicated mixed-valence U phases. After a discussion of the valence band, the focus lingers on the U4f region, where the use of binding energies, satellite structures, and peak shapes is discussed in some detail. Binding energies were shown to be very dependent on composition/structure and consequently unreliable guides to oxidation state, particularly where assignment of composition is difficult. Likewise, the spin orbit split 4f 7/2 and 4f 5/2 peak shapes do not carry significant information on oxidation states. In contrast, both satellite-primary peak binding energy separations, as well as intensities to a lesser extent, are relatively insensitive to composition/structure within the oxide-hydroxide-hydrate system and can be used to both identify and help quantify U oxidation states in mixed valence phases. An example of the usefulness of the satellite structure in constraining the interpretation of a complex multivalence U compound is given. Copyright © 2011 John Wiley & Sons, Ltd.Keywords: XPS; uranium; oxidation states IntroductionOne of the great strengths of X-ray photoelectron spectroscopy ( XPS) is its ability to elucidate the valence state of a wide range of elements at the near-surface of materials. Although many features of interest concern bulk properties, it is the surface and the near-surface that has immediate contact with its surroundings and consequently mediates most reactions. With the renewed interest in nuclear energy, characterizing the surface composition and chemical state of actinide-containing materials is important for both industrial applications and understanding the fate and transport of actinides in the environment. This perspective article is focused on uranium as the data set is by far the most comprehensive of all the actinides where an appreciable literature on the XPS of U compounds dates back to the beginnings of the commercialization of XPS in the 1970s. For an early paper on the XPS of a broad range of actinide oxides we refer the reader to Veal et al.[1] A basic understanding of XPS principles is assumed throughout and will not be reviewed.On one level, determining oxidation states with XPS would appear to be a simple exercise of comparing the binding energies (BEs) of core levels to well characterized standards; so why bother? However, interpreting the oxidation states of samples with mixed valence, unknown structures, and complicated or different compositions can pose a challenge, as with any analytical technique. In fact, composition can strongly affect uranium core-level BEs and potentially confuse the determination of oxidation states. For example, perhaps one is working in an oxidizing system and only expects U(VI), yet the U4f 7/2 line has a peak at 382.0 eV and a shoulder at 380.5 eV. Does the shoulder represent a reduced species, po...

Temperature‐dependent Variations of the Interface between UO2 and Zr

“…Previous studies have revealed that the binding energy of U 4f 7/2 in UO 2 is 379.5-379.8 eV. [12][13][14][15] In addition, the separated binding energies for U(IV)-U(V) and U(V)-U(VI) states are 0.6 and 0.9 eV, respectively.…”

Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation

“…The binding energy (BE) of the U(VI) 4f 7/2 peak shifted from 381.6 eV to a higher value of 382.7 eV, greater than previously reported values for U(VI) solids. 36 The U(VI) species produced by noncrystalline U(IV) oxidation may not represent the known crystalline U(VI) phases but rather labile U(VI)-carbonate complexes, which would explain the fast release of soluble U in the presence of 10.8 mM DIC. Surprisingly, the U(VI) species quickly diminished, dropping to merely ∼24% by 13 τ when the dissolved U concentration reached its peak value (Figure 1a).…”

XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films

“…17,50,55,56,59,60,61,62 A special attention was paid to the study of mechanisms of structure formation, which leads to widening of main peaks and appearance of extra structure in the spectra. 50,59,63 The XPS spectra of some oxides exhibit typical shake-up satellites of about ~25% intensity of the basic peaks 41,51,60,64 The calculated spectroscopic factors f Anf reflecting the fractions of the basic peak XPS intensities with the deduction of shake-up satellite intensities for the U 4f and U 5f peaks are: f U4f =0.83 and f U5f =0.86. 65,66 Shake-up satellites are located from the basic peaks toward the higher BE by ∆E sat (U4f): ~7 eV (U 4+ ); ~8 eV (U 5+ ); ~4 eV and ~10 eV (U 6+ ).…”

“…65,66 Shake-up satellites are located from the basic peaks toward the higher BE by ∆E sat (U4f): ~7 eV (U 4+ ); ~8 eV (U 5+ ); ~4 eV and ~10 eV (U 6+ ). 60 Since the U 4f BE on the moving from UO 2 to UO 3 changes by ∆E b~2 eV, one can reliably determine uranium oxidation states for individual uranium oxides on the basis of the U 4f 7/2 BE and positions (∆E sat ) and relative intensities I sat (%) of the shake-up satellites. 56,60 The U 4f XPS structure is best resolved for the crystalline oxides.…”

“…60 Since the U 4f BE on the moving from UO 2 to UO 3 changes by ∆E b~2 eV, one can reliably determine uranium oxidation states for individual uranium oxides on the basis of the U 4f 7/2 BE and positions (∆E sat ) and relative intensities I sat (%) of the shake-up satellites. 56,60 The U 4f XPS structure is best resolved for the crystalline oxides. For complex amorphous oxides UO 2+x it is often difficult to segregate unambiguously the U 4f XPS peaks containing shake-up satellites into separate components for reliable quantitative information on uranium oxidation state and ionic composition of the studied oxide.…”