Identification of oxidation states in γ-graphyne by computational XPS and NEXAFS spectra

Oxidation is an inevitable process for γ-graphyne (γ-GY), a two-dimensional (2D) material of carbon, both in its natural state and during large-scale processing. Understanding the oxidation states within γ-GY is of fundamental importance for further applications. In this work, with density functional theory (DFT) simulations of seven representative oxygen-doped γ-GY structures, we demonstrate that the X-ray photoelectron (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra at the oxygen and carbon K-edges are sensitive for different local structures of oxygen dopants. The theoretical calculation results of γ-GY and seven oxygen-doped γ-GY configurations fit well with the experiment, and all characteristic experimental XPS peaks are assigned. We found that the C 1s ionization potential (IP) of sp2-hybridized carbons is higher than that of sp-hybridized carbons for the pristine γ-GY. By complete and elaborate analysis of the NEXAFS spectrum of each structure at the carbon and oxygen K edges, the seven oxygen-doped γ-GY configurations have been successfully identified. The results can be considered as a benchmark for the determination of oxygen-doped γ-GY configurations, and provide a deep insight into the structure–spectroscopy relationships.