| Determination of the Oxygen Coefficient kO=2+x and Ionic Composition of Oxides UO2+x. Once uranium oxide UO2+x contains uranium ions of different oxidation states, the U 4f XPS spectrum is expected to consist of several peaks at different BEs. Usually, these peaks are superimposed because of low equipment resolution. This widens and distorts the shape of the main peak, which complicates its decomposition into components. The O 1s intensity in this case is usually higher due to oxygen-containing impurities on the surface. This increases the error in the oxygen coefficient kO=2+x determination, which is found from the U 4f/O 1s intensity ratio with sensitivity coefficients in mind. In this case we can only yield a qualitative interpretation of the results (Table 2, column 3).
Determination of uranium ions composition on the basis of intensities of U 4f-electron lines was carried out based on the known parameters of the spectra of uranium oxides. Both the primary and satellite peaks for U 4f were used in the fitting procedure. The binding energy of the U 4f7/2 electrons was taken as: ~380 eV (U4+); ~381 eV (U5+); ~382 eV (U6+) and shake-up satellites located from the basic peaks toward the higher BE by: ~7 eV (U4+); ~8 eV (U5+); ~4 eV and ~10 eV (U6+) were used, as was described in Section I. Although, it is known that the FWHM, Γ(U 4f7/2), of U 4f7/2-line decreases as: ~1.5 eV (U4+); ~1.4 eV (U5+) and ~1.2 eV (U6+), related to multiplet splitting and decrease of unpaired U 5f-electrons from 2 to 0, the FWHM value of 1.4 eV was taken for fitting the U4+, U5+ and U6+ curves to simplify the fitting. The curve shapes were approximated by a mixed Gaussian (~80%) and Lorentzian (~20%) function to get the best fit to the experimental curve. In the cases when some peaks were absent, the initial % composition was determined based on the method of U 5f/U 4f7/2-electron intensities (our suggested method), followed by an iterative fitting procedure to obtain a final fit. The results obtained by the spectra fitting procedure are shown in the parentheses in Table 2 in the fourth column.
Table 2. Surface Elemental Composition of the Epitaxial UO2+xa Thin Films, Peak Intensity of the U 5f Electronsb, Oxygen Coefficient kOc in UO2+x Oxide, Composition of Uranium Ions k(%)d of Unirradiated (AP1-7) and 129Xe23+ (OB1-4), and 238U31+ (OB5-7) Irradiated Thin Films of Uranium Dioxide.
No.
|
Sample
(hkl)
|
Elemental composition UO2+x
|
IU5f
(±0.001)
|
kO
in UO2+x
(± 0.01)
|
k
|
U4+
|
U5+
|
U6+
|
1
|
AP1 (210)e
OB1
|
UO2.8
UO47
|
0.025
0.013
|
2.11
2.31
|
44 (45)
0.3
|
45 (40)
68
|
11 (15)
31
|
2
|
AP2 (001)
OB2
|
UO3.5
UO21
|
0.029
0.025
|
2.07
2.11
|
58 (60)
44
|
35 (36)
45
|
7 (4)
11
|
3
|
AP3 (001)
OB3
|
UO3.6
UO15
|
0.028
0.023
|
2.08
2.14
|
52 (58)
33
|
39 (38)
52
|
9 (4)
14
|
4
|
AP4 (111)e
OB4
|
UO3.9
UO15
|
0.027
0.021
|
2.10
2.17
|
48 (54)
25
|
42 (41)
57
|
10 (5)
17
|
5
|
AP5 (001)
OB5 (001)
|
UO3.4
UO3.4
|
0.021
0.022
|
2.17
2.15
|
25 (47)
32 (45)
|
57 (48)
53 (47)
|
17 (5)
15 (8)
|
6
|
AP6 (110)
OB6 (110)
|
UO3.2
UO3.2
|
0.017
0.021
|
2.23
2.17
|
13 (37)
25 (44)
|
64 (55)
57 (48)
|
23 (8)
17 (8)
|
7
|
AP7 (111)
OB7 (111)
AP7(Ar+)f
|
UO3.6
UO3.4
UO1.98
|
0.019
0.020
0.025
|
2.20
2.18
2.11
|
18 (44)
23 (48)
42
|
61 (48)
59 (45)
47
|
21 (8)
18 (7)
11
|
aElemental composition obtained on the basis of the core line U 4f7/2, and O 1s intensities of uranium dioxide and atomic photoionization cross-sections σ: 2.81 (O 1s), 36.0 (U 4f7/2).
bPeak intensity of the U 5f electrons measured as a ratio: IU5f= I(U 5f)/I(U 4f7/2) without taking into account intensities of the shake-up satellites.
cOxygen coefficient kO = 2+x in UO2+x oxide found from Equation 1.
dComposition of uranium ions k (%) found by using Equations (4) – (6); the values obtained based on dividing the U 4f7/2 peak into components and intensities of the shake-up satellites are shown in the parentheses.
ePreferred crystallographic orientation.
fSample AP7 after the 180 sec Ar+ treatment.
The oxygen coefficient kO=2+x and ionic composition of UO2+x can be also determined on the basis of the intensity of the peak of the U 5f electrons not participating in the chemical bonding. The present work physically grounded this technique and considered it in more details than before.44,45,49 This technique considers oxygen ions directly bound with uranium ions. Adsorbed oxygen ions not participating in uranium-oxygen bonding do not affect the U 5f intensity; however, they affect strongly the uranium/oxygen ratio on the surface, which practically does not allow the traditional XPS quantitative analysis to be used. The U 5f electrons weakly participating in chemical bonding in uranium oxides are strongly localized and observed as a sharp peak at the lower BE side from the outer valence band (see Figure 3). The XPS from γ-UO3 not containing the U 5f electrons does not exhibit this peak.61 Since the U 5f BE is about ~3 eV lower than that of the low energy OVMO band edge (“quasi-gap”), one can suggest that the U 5f intensity must decrease discretely as uranium oxidation state grows from U4+ to U5+ and U6+, since the U 5f electrons transit from the localized state to the outer valence band. The U 5f intensity must first decrease twice and then vanish (Figure 3). Indeed, this was observed in the XPS of neptunium compounds, as neptunium oxidation state increased from Np(5f2)5+ to Np(5f1)6+ (ref 50). Therefore, the U 5f intensity can be used as a quantitative parameter of the number of U 5f electrons involved in chemical bonding in uranium oxides.
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