Termination-Property Coupling via Reversible Oxygen Functionalization of MXenes

MXenes are a growing family of 2D transition-metal carbides and nitrides, which display excellent performance in myriad of applications. Theoretical calculations suggest that manipulation of the MXene surface termination (such as =O or -F) could strongly alter their functional properties; however, experimental control of the MXene surface termination is still in the developmental stage. Here, we demonstrate that annealing MXenes in an Ar + O₂ low-power plasma results in increased =O functionalization with minimal formation of secondary phases. We apply this method to two MXenes, Ti₂CT _<i>x</i> and Mo₂TiC₂T _<i>x</i> (T _<i>x</i> represents the mixed surface termination), and show that in both cases, the increased =O content increases the electrical resistance and decreases the surface transition-metal's electron count. For Mo₂TiC₂O _<i>x</i> , we show that the O content can be reversibly altered through successive vacuum and plasma annealing. This work provides an effective way to tune MXene surface functionalization, which may unlock exciting surface-dependent properties.