Interaction and kinetics of H2, CO2, and H2O on Ti3C2Tx MXene probed by X-ray photoelectron spectroscopy

Abstract: One of the most explored MXenes is Ti3C2Tx, where Tx is designated to inherently form termination species. Among many applications, Ti3C2Tx is a promising material for energy storage, energy conversion, and CO2-capturing devices. However, active sites for adsorption and surface reactions on the Ti3C2Tx-surface are still open questions to explore, which have implications for preparation methods when to obtain correct and optimized surface requirements. Here we use X-ray photoelectron spectroscopy (XPS) to study the adsorption of common gas molecules such as H2, CO2, and H2O, which all may be present in energy storage, energy converting, and CO2-capturing devices based on Ti3C2Tx. The study shows that H2O, with a strong bonding to the Ti-Ti bridge-sites, can be considered as a termination species. An O and H2O terminated Ti3C2Tx-surface restricts the CO2 adsorption to the Ti on-top sites and may reduce the ability to store positive ions, such as Li+ and Na+. On the other hand, an O and H2O terminated Ti3C2Tx-surface shows the capability to split water. The results from this study have implications for the correct selection of MXene preparations and the environment around the MXene in different implementations, such as energy storage, CO2-capturing, energy conversion, gas sensing, and catalysts.