Impact of Cation Intercalation on the Electronic Structure of TiCT MXenes in Sulfuric Acid.

Intercalation in TiCT MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the TiCT nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the TiCT MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li-, Na-, K-, and Mg-intercalated TiCT MXenes dispersed initially in water and 10 mM sulfuric acid (HSO) using X-ray absorption spectroscopy (XAS). The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted HSO. In situ XAS at the Ti L-edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in HSO aqueous electrolyte. By applying the sensitivity of the Ti L-edge to probe the oxidation state of Ti atoms, we demonstrate that cation-intercalation and HSO environment significantly alter the TiCT surface chemistry.