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MXenes are technologically interesting 2D materials that show potential innumerous applications. The properties of the MXenes depend at large extent onthe selection of elements that build the 2D MX-layer. Another key parameter fortuning the attractive material properties is the species that terminate thesurfaces of the MX-layers. Although being an important parameter, experimentalstudies on the bonding between the MX-layers and the termination species arefew and thus an interesting subject of investigation. Here we show that thetermination species fluorine (F) bonds to the Ti3C2-surface mainly through Ti3p - F 2p hybridization and that oxygen (O) bonds through Ti 3p - O 2phybridization with a significant contribution of Ti 3d and Ti 4p. The studyfurther shows that the Ti3C2-surface is not only terminated by F and O on thethreefold hollow face-centered-cubic (fcc) site. A significant amount of O sitson a bridge site bonded to two Ti surface atoms on the Ti3C2-surface. Inaddition, the results provide no support for hydroxide (OH) termination on theTi3C2-surface. On the contrary, the comparison of the valence band intensitydistribution obtained through ultraviolet- and x-ray photoelectron spectroscopywith computed spectra by density of states, weighed by matrix elements andsensitivity factors, reveals that OH cannot be considered as an inherenttermination species in Ti3C2Tx. The results from this study have implicationsfor correct modeling of the structure of MXenes and the corresponding materialsproperties. Especially in applications where surface composition and charge areimportant, such as supercapacitors, Li-ion batteries, electrocatalysis, andfuel- and solar cells, where intercalation processes are essential.

Chemical Bonding of Termination Species in 2D Carbides Investigated through Valence Band UPS/XPS of Ti3C2Tx MXene