Understanding\nIn-Plane Sliding of Functionalized Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene by In Situ\nMicroscale Analysis of Electrochemical Actuation

Electrochemical actuation is based on the deformation\nof electrodes\nupon applying a mild potential (usually in the range of ± 5 V\nin a single cell). In this work, in situ local analysis of the electrochemical\nactuation of Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> (T = −O, −OH, −F) MXene at the microscale is\nachieved by Raman spectroscopy, atomic force microscopy (AFM), and\nscanning electrochemical microscopy (SECM) while applying potential\nin an electrochemical cell. First, Raman spectra recorded at a steady\nstate with different potentials confirm the intercalation/deintercalation\nof Li<sup>+</sup> ions, resulting in the change of out-of-plane vibrations\ndue to the interaction of Li<sup>+</sup> with the −O and −OH\nsurface group atoms of MXene. A methylcellulose functionalization\nof the MXene films results in more freedom for in-plane vibrations.\nSecond, SECM is applied to visualize the deformation of two-end-fixed\nMXene films both at a steady state (0 V vs Ag/AgCl QRE) and at scanning\npotential. The results show buckling-type actuation, which is more\npronounced for methylcellulose-functionalized samples. Electrochemical\nAFM provides evidence of a significant in-plane sliding of the MXene\nflakes. Out-of-plane changes appear to be negligible in the actuation\nmechanism, as supported by the AFM approach curve analysis. Overall,\nthe methylcellulose-functionalized MXene shows better actuation performance,\nespecially in the in-plane direction and at a high frequency. The\nmechanism is then discussed.