Electronic and structural peculiarities of Br2-embedded C2F: XPS and DFT study

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.