RESULTS AND DISCUSSION
Determination of uranium oxidation state and UO2+x ionic composition, as mentioned above, employs both the traditional XPS parameters (BEs and peak intensities) and the structure parameters of the core- and valence spectra such as: U 5f relative intensity; OVMO (IVMO) – core level BE differences; spin-orbit splitting ∆Esl (eV) and multiplet splitting ∆Ems (eV); dynamic effect – related structure parameters; relative positions of core-level shake-up satellites ∆Esat (eV).50 These XPS parameters allow getting information on uranium physical and chemical properties in the studied samples.
The surface treatment with Ar+ ions was not used during the XPS study of the films in this work, as it is known that Ar+ etching can change ionic composition of the surface. Since the data from the surface etched by Ar+ can be useful in discussing the results of the study of the radiation damage of the uranium dioxide films, thus, for sample AP7 the effect of Ar+ etching on the surface composition was studied. It was found that after the 20 sec etching the C 1s intensity became ~10 times lower, and the O 1s peak became single with Γ(O1s)=1.2 eV. The U 4f spectrum was observed as a spin-orbit split (∆Esl=10.8 eV) of 1.8 eV wide doublet with symmetrical peaks. Intensity of the shake-up satellites (Isat=Is/Io), equal to the ratio of the satellite area (Isat) to the area of the basic peak (Io), was observed to have 30% intensity at ∆Esat1= 6.9 eV. This structure is typical for UO2 (ref 50)]. After the 60-120-180 sec etching and staying for a while in the spectrometer chamber (“annealing”) the XPS structure did not change significantly. After the 180 sec etching the U 4f7/2 XPS exhibited a weak shoulder at the lower BE side attributed to metallic U. For the 180 sec etching the oxygen coefficient kO was 1.98 based on the U 4f and O 1s intensities. The same coefficient found on the basis of the U 5f intensity was 2.11. The ionic composition of the sample was found to be: 42%(U4+), 47%(U5+) and 11(U6+) (Table 2). This unexpected result agrees with the fact that uranium oxides can self-organize and form a stable lattice UO2+x containing different phases.78,79 This must be taken into account in the studies of irradiation of UO2 films with xenon and uranium ions.
The survey XPS spectrum (Figure 2) provides important information on the studied sample. It consists of peaks of included elements and is typical for uranium oxide. This spectrum also contains the Auger peaks of carbon (C KLL), oxygen (O KLL) and the XPS peak at 207.1 eV identified as the Nb 3d5/2 peak of niobium in Nb2O5 (ref 80). Niobium impurity formed during the sample preparation. The Nb 3p3/2 peak of Nb2O5 was observed at Eb(Nb3p3/2)=362.7 eV. Despite the fact that the Nb 3p1/2 peak of Nb2O5 at Eb(Nb 3p1/2)=378.2 eV is superimposed with the U 4f7/2 peak at Eb(U 4f7/2)≈379.9 eV (Table 3), the Nb 3p1/2 intensity does not contribute significantly (within the error) to the U 4f7/2 intensity since the Nb 3p1/2 peak has many times lower intensity than the U 4f7/2 one because the U 4f7/2 photoionization cross-section is ~10 times higher than the Nb 3p1/2 one,81 and uranium content is many times higher than niobium content. The XPS spectra are shown in Figures 2,3,5,6 and 7, and the corresponding BE are given in Table 3.
Table 3. Electron Binding Energy Eba (eV), Line Width Γb (eV) in the Parentheses, Satellite Positions ΔEsatc (eV) of Unirradiated (AP1-4 on LSAT and AP5-7 on YSZ) and 129Xe23+ (OB1-4) and 238U31+ (OB5-7) Irradiated Thin Films of Uranium Dioxide.
No
|
Sample
(hkl)
|
U 5f
|
U 4f7/2
U4+
|
U 4f7/2
U5+
|
U 4f7/2
U6+
|
Γ
U 4f7/2
|
O 1s
|
1
|
AP1 (210)d
OB1
|
1.2(1.2)
1.3(1.2)
|
379.7(1.4)
6.8(3.0)
|
380.9(1.4)
7.9(2.1)
380.4(1.5)
|
382.2(1.4)
|
2.1
1.6
|
529.8(1.1)
530.1(1.4)
|
2
|
AP2 (001)
OB2
|
1.3(1.2)
1.2(1.2)
|
379.7(1.4)
6.8(2.0)
|
380.9(1.4)
7.9(2.0)
380.4(1.9)
|
382.2(1.4)
|
2.3
1.9
|
529.7(1.1)
530.1(1.5)
|
3
|
AP3 (001)
OB3
|
1.3(1.1)
1.0(1.2)
|
379.9(1.4)
6.8(1.9)
|
381.1(1.4)
7.9(2.1)
380.3(2.0)
|
382.4(1.4)
|
2.2
2.0
|
530.1(1.0)
529.8(1.4)
|
4
|
AP4 (111)d
OB4
|
1.2(1.2)
1.3(1.1)
|
379.7(1.4)
6.8(1.9)
|
380.8(1.4)
8.0(2.1)
380.5(1.8)
|
382.1(1.4)
|
2.3
1.8
|
529.7(1.1)
530.1(1.4)
|
5
|
AP5 (001)
OB5 (001)
|
1.6(1.0)
1.5(1.1)
|
380.3(1.8)
6.1 (2.1)
380.1(1.5)
6.2(2.1)
|
381.4(1.8)
7.5(2.1)
381.2(1.5)
7.7(2.1)
|
383.3(1.8)
382.7(1.5)
|
2.5
2.5
|
530.1(1.1)
530.1(1.1)
|
6
|
AP6 (110)
OB6 (110)
|
1.6(1.1)
1.5(1.0)
|
380.2(1.6)
6.0(2.5)
380.1(1.6)
6.1(2.1)
|
381.4(1.6)
7.6(2.3)
381.2(1.6)
7.7(2.1)
|
382.9(1.6)
382.8(1.6)
|
2.4
2.3
|
530.2(1.2)
530.1(1.1)
|
7
|
AP7 (111)
OB7 (111)
AP7(Ar+)e
|
1.5(1.1)
1.4(1.2)
1.2(1.6)
|
380.0(1.5)
6.1(2.1)
380.0(1.5)
6.6(2.3)
379.8(1.6)
|
381.2(1.5)
7.7(2.1)
381.1(1.5)
7.6(2.6)
381.5(1.7)
|
382.7(1.5)
382.6(1.5)
|
2.4
2.4
1.6
|
530.1(1.1)
530.0(1.1)
530.0(1.2)
|
a1-st peak – binding energy measured relative to the Eb (C1s)=285.0 eV of hydrocarbons on the sample surface.
bLine width reported relative to the Γ(C 1s) =1.3 eV in the parentheses.
c2-nd peak – satellite energy ΔEsat relative to the basic peak.
dPreferred crystallographic orientation.
eSample AP7 after the 180 sec Ar+ treatment.
|
