X-ray Photoelectron Measurements. XPS spectra of UO2+x were recorded on a Kratos Axis Ultra DLD spectrometer using monochromatic Al-Kα radiation (hν=1486.6 eV) at 150 W X-ray gun power under 10–9 mbar at room temperature (Figure 2). The analyzed area was an ellipse with 300 and 700 m minor and major axes, respectively. Binding energy scale of the spectrometer was preliminarily calibrated by the position of the peaks of Au 4f7/2 (83.96 eV) and Cu 2p3/2 (932.62 eV) core levels for pure gold and copper metals. The spectra were acquired in the constant analyzer energy mode using a pass energy of 20 eV and a step size of 0.05 eV. The equipment resolution measured as the full width on the half-maximum (FWHM) of the Au 4f7/2 peak was less than 0.65 eV. The binding energies (BE) were measured relatively to the BE of the C 1s electrons from hydrocarbons adsorbed on the sample surface that was accepted to be equal to 285.0 eV. The FWHMs are given relatively to that of the C 1s XPS peak from hydrocarbon on the sample surface being 1.3 eV.50 The error in the determination of the BE and the peak width did not exceed ±0.05 eV, and the error of the relative peak intensity – ±5%. The inelastically scattered electrons-related background was subtracted with the Shirley method.75
Figure 2. A survey XPS scan from a UO2 film (sample AP3).
40Ar+ etching of 2×2 mm2 sample area was conducted at the accelerating voltage of 2 kV and beam current of 50 μA at 2×10-9 mbar and room temperature. The etching rate was 7.1 nm/min for SiO2 under these conditions. Thus, UO2 film on the YSZ substrate (AP7) was Ar+ etched at 2 keV and 2×10-9 mbar. The flux was maintained at about 3.1×1014 ions/(cm2 s).
The quantitative elemental analysis was performed for several nanometer-deep layers of the studied samples. It was based on the fact that the spectral intensity is proportional to the number of certain atoms in the studied sample. The following ratio was used: ni/nj=(Si/Sj)(kj/ki), where ni/nj is the relative concentration of the studied atoms, Si/Sj is the relative core-shell spectral intensity, kj/ki is the relative experimental sensitivity coefficient. The following coefficients relative to the C 1s were used: 1.00 (C 1s); 2.81 (O 1s); 10.51 (Nb3d); 36.0 (U 4f7/2; see ref 76).
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