Site-projected electronic structure of two-dimensional Ti3C2 MXene: the role of the surface functionalization groups

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti3C2Tx MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F, O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and the location of the T-groups on the MXene's surface and thereby allow for the characterization of the MXene's functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti d bands: these edges are thus relevant probes of the Ti d density of states close to the Fermi level which is of particular interest since it drives most of the Ti3C2Tx electronic properties. Finally, the crucial role in the MXene's functionalization of the etchant used for its synthesis is evidenced by locally determining the [O]/[F] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.