  A potential application of two-dimensional (2D) MXenes, such as Ti2CTx and
Ti3C2Tx, is energy storage devices, such as supercapacitors, batteries, and
hydride electrochemical cells, where intercalation of ions between the 2D
layers is considered as a charge carrier. Electrochemical cycling
investigations in combination with Ti 1s X-ray absorption spectroscopy (XAS)
have therefore been performed with the objective to study oxidation state
changes during potential variations. In some of these studies Ti3C2Tx has shown
main edge shifts in the Ti 1s X-ray absorption near-edge structure (XANES).
Here we show that these main edge shifts originate from the Ti 4p orbital
involvement in the bonding between the surface Ti and the termination species
at the fcc-sites. The study further shows that the t2g-eg crystal field
splitting (10Dq) observed in the pre-edge absorption region indicate weaker
Ti-C bonds in Ti2CTx and Ti3C2Tx compared to TiC and the corresponding MAX
phases. The results from this study provide information necessary for improved
electronic modeling and subsequently a better description of the materials
properties of the MXenes. In general, potential applications, where surface
interactions with intercalation elements are important processes, will benefit
from the new knowledge presented.
