Impact of Cation Intercalation on the Electronic Structure of Ti₃C₂T_x MXenes in Sulfuric Acid

Intercalation in Ti₃C₂T_x MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti₃C₂T_x nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti₃C₂T_x MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li-, Na-, K-, and Mg-intercalated Ti₃C₂T_x MXenes dispersed initially in water and 10 mM sulfuric acid (H₂SO₄) 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 H₂SO₄. 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 H₂SO₄ 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 H₂SO₄ environment significantly alter the Ti₃C₂T_x surface chemistry.