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