The Origin of Ti 1s XANES Main Edge Shifts and EXAFS Oscillations in the Energy Storage Materials Ti2CTx and Ti3C2Tx MXenes

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.