Chemical bonding in carbide MXene nanosheets

Highlights: • The valence band of a new 2D material called MXene is probed by X-ray photoelectron spectroscopy. • Changes in the Ti-C bond strength of the carbide core is shown to depend on the atomic layer thickness. • The O 2p and the F 2p band positions of the termination groups depend both on the adsorption sites and bond lengths. • Due to a different bonding, the binding energy from the Fermi level of the F 2p band (∼9 eV) is larger than for the O 2p band (∼6 eV). • The samples must be freshly made to avoid surface oxidation and Ar+ sputtering should be avoided. - Abstract: The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Ti_n+1C_n-T_x (n = 1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO₂ that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s – Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar⁺ sputtering on the electronic structure is also discussed.