In Situ TEM Approaches to Study Dynamic Transformations in MXenes

MXenes have become an important class of 2D materials with functional applications ranging from electrochemical energy storage, catalysis, nanoelectronics, optoelectronics and sensors. Their unique physical and chemical properties are derived from their surfaced functional group chemistry and transition metal carbide, nitride and carbonitride atomic structure. New insight into MXene synthesis and measurement of functional properties can be elucidated at the atomic and nanoscale using <i>in situ</i> scanning transmission electron microscopy (STEM) based characterization methods. Specialized heating and electrical biasing platforms have been used to investigate the thermally induce growth of new MXene stoichiometric structures and ionic transport mechanisms and kinetics. During thermal exposure of Ti₃C₂T_x MXenes at temperatures above 500C, it was found that through atomic resolution STEM imaging that new hexagonal TiC layers from on one or both sides of single layer Ti₃C₂ following a Frank-van der Merwe growth mechanisms. ReaxFF and DFT calculations reveals the step-edge barrier that needs to be overcome for Ti and C atoms to migrate and diffuse on the surface to form the new layers. Similar behavior is observed for other MXenes as will be discussed. To better understand ionic transport behavior, probe based <i>in situ</i> electrical biasing TEM holders were used to understand the dynamics response of ion intercalation and corresponding kinetics of lattice plane expansion at nanoscale spatial resolution. Ion transport kinetics measurements revealed a decrease in lattice expansion as a result of the formation of SEI like inorganic phases. The <i>in situ</i> S/TEM research presented here aide to better understand materials transformations barriers and pathways in 2D MXenes.