Native Vacancy Defects in MXenes at Etching Conditions

Two-dimensional MXenes have recently received increased attention due to their facile synthesis process and extraordinary properties suitable for many different applications. During the wet etching synthesis of MXenes native defects such as metal and carbon or nitrogen vacancies are produced but the underlying defect formation processes are poorly understood. Here we employ first-principles calculations to evaluate formation energies of Ti C and N vacancies in Ti₃C₂ and Ti₂N MXenes under etching conditions. We carefully account for the mixed functionalization of the surfaces as well as the chemical environment in the solution (pH and electrode potential). We observe that the formation energies of the metal vacancies differ significantly for different types of surface functionalization as well as for different local and global environments. We attribute these differences to electrostatic interactions between vacancies and the surrounding functional groups. We predict that Ti vacancies will be prevalent on bare or OH-functionalized surfaces but not on O-functionalized ones. In contrast C and N vacancies are more prevalent in O-functionalized surfaces. In addition our results suggest that the pH value of the etching solution and the electrode potential strongly affect vacancy formation. In particular the predicted conditions at which abundant vacancy formation is expected are compared to experiments and found to coincide with conditions at which MXenes oxidize readily. This suggests that Ti vacancy formation is a crucial step in initiating the oxidation process.