  MXenes are technologically interesting 2D materials that show potential in
numerous applications. The properties of the MXenes depend at large extent on
the selection of elements that build the 2D MX-layer. Another key parameter for
tuning the attractive material properties is the species that terminate the
surfaces of the MX-layers. Although being an important parameter, experimental
studies on the bonding between the MX-layers and the termination species are
few and thus an interesting subject of investigation. Here we show that the
termination species fluorine (F) bonds to the Ti3C2-surface mainly through Ti
3p - F 2p hybridization and that oxygen (O) bonds through Ti 3p - O 2p
hybridization with a significant contribution of Ti 3d and Ti 4p. The study
further shows that the Ti3C2-surface is not only terminated by F and O on the
threefold hollow face-centered-cubic (fcc) site. A significant amount of O sits
on a bridge site bonded to two Ti surface atoms on the Ti3C2-surface. In
addition, the results provide no support for hydroxide (OH) termination on the
Ti3C2-surface. On the contrary, the comparison of the valence band intensity
distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy
with computed spectra by density of states, weighed by matrix elements and
sensitivity factors, reveals that OH cannot be considered as an inherent
termination species in Ti3C2Tx. The results from this study have implications
for correct modeling of the structure of MXenes and the corresponding materials
properties. Especially in applications where surface composition and charge are
important, such as supercapacitors, Li-ion batteries, electrocatalysis, and
fuel- and solar cells, where intercalation processes are essential.
