How Water Attacks MXene

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. However, the stability of MXene remains a concern, especially its quick degradation in aqueous solutions under ambient conditions. Here, we report on the water/Ti3C2O2-MXene interfacial chemistry from first-principles molecular dynamics simulations at room temperature. Surprisingly, we find that the water molecules can attack the basal plane of Ti3C2O2 and pull the surface Ti atoms out, thereby reconstructing the surface. By tracking close encounters of water molecules and surface Ti atoms on the basal plane of Ti3C2O2, we show that the attack is initiated by the chemisorption of a water molecule on a surface Ti atom, followed by the breaking of Ti-C bonds and deprotonation of the water molecule, leading to the formation of Ti-OH on the Ti3C2O2 surface and a hydronium ion in the aqueous phase. Our finding highlights the susceptibility of Ti3C2O2 MXene to water attack, supporting recent experimental observations. Furthermore, we demonstrate that preventing close encounters of water molecules and the surface Ti atoms is key to the stability of the basal plane and can be realized by negatively charging the surface (thereby reorienting the O atoms of water away from the surface) or converting the surface O to -OH groups (thereby shifting the water layer further away from the surface). Our insights and approach highlight the importance of the reactivity of water when interfacing with 2D materials such as MXenes.