Abstract
The prospects of the complementary use of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) have been demonstrated by the examples of highly oriented pyrolytic graphite, half-fluorinated graphite C2F, and half-fluorinated graphite C2F intercalated with Br C2FBr0.15. It has been shown that the photoelectron energy losses in XPS spectra conform well to valence band electron transitions resulted from the DFT calculations for relevant unit cells. This conformity justified the other results of joined XPS and DFT studies, which have revealed two arrangements of the Br2 embedded into the C2F framework. The first arrangement corresponds to separate Br pairs in which the Br state is similar to a free Br2 molecule, whereas the second one is an ultra-dense Br chain in which the Br state is between free Br2−1 and Br10 species. The specific energy losses in the XPS Br3d spectrum of C2FBr0.15 indicate a comparable content of both Br arrangements in a sample. Besides, a distinct structure in the difference F1s XPS spectrum is assigned to the expected strengthening of the C-F bond in a C2F matrix under the Br2 intercalation. The state and orientation of intercalated Br2 are juxtaposed with experimental studies by Near Edge and Extended X-ray Absorption Fine Structure spectroscopy and by Raman spectroscopy. A successful confluence of XPS and DFT can be useful in the field of material science, providing the local geometry, the state and bonding between atoms in a sample, and thereby revealing the wear performance of the material, regardless of its application.
Highlights
X-ray photoelectron spectroscopy (XPS) is a powerful analytical tool in the surface science
Energy losses in the X-ray photoelectron spectra of highly oriented pyrolytic graphite, pristine C2F, and the Br2-intercalated product C2FBr0.15 agree well with the valence-band electron transitions calculated by the density functional theory for the C24, C24F12, and C24F12Br2 unit cells, respectively
The combination of the XPS and density functional theory (DFT) data solves two important tasks. It rejects those DFT results that do not conform to the fine structure of XPS spectra
Summary
X-ray photoelectron spectroscopy (XPS) is a powerful analytical tool in the surface science. We attributed the XPS spectral structure above the energy of the main peak to the relaxation of photoelectrons via multiple conjugate electron excitations (CEE) of the valence band, accompanying the core level excitation.[8] It was found that the photoelectron energy losses in the XPS spectra of C2F agree well with the electron transitions of the valence band of a C24F12 unit cell, in which all C-F bonds were outside the bilayer cell This agreement was used as a benchmark in the determining the state of the embedded Br2 species, making a good choice from a set of DFT results, and interpreting the fine structures of XPS spectra. To verify the reliability of the XPS-DFT combination, the sample of Br2-embedded C2F was examined by X-ray absorption fine structure (XAFS) spectroscopy at the Br K edges and Raman spectroscopy
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