Effects of Applied Potential and Water Intercalation on the Surface Chemistry of Ti₂C and Mo₂C MXenes

Two-dimensional transition metal carbides and nitrides, also known as MXenes, represent an attractive class of materials for a multitude of electrochemical and other applications. While single sheets of MXenes have been widely studied theoretically, there have been much fewer studies on layered bulk MXenes, which are more representative of multi- or few-layer MXenes used in actual applications. Herein, we investigate the structural and electronic effects of water intercalation, multiple functional groups and applied potential on layered bulk Ti2C and Mo2C MXenes using density functional theory. The out-of plane lattice parameter, c, was found to vary significantly with the functional group, and is greatly increased upon intercalation of water. Experimental results confirm the change in lattice constant due to addition or removal of intercalated water. Under zero applied potential, both Ti2C and Mo2C were found to be functionalized by one monolayer of O; bare MXenes were never found to be stable, regardless of the applied potential. Applying a potential changed the adsorbate coverage, changing the systems from O covered to H covered at negative potentials and, in some cases, giving rise to a metal–insulator transition. Understanding of the effects of surface functionalization and water intercalation of MXenes provides a better insight of their use for catalytic and electronic applications.