Prediction of Optimal Synthesis Conditions for the Formation of Ordered Double-Transition-Metal MXenes (o-MXenes)

o-MXenes are a family of layered double-transition-metal carbides and nitrides with a unique out-of-plane ordering of the two metal atoms. Their chemical versatility means they have novel applications in energy storage, catalysis, and sensing devices. o-MXenes are synthesized from their precursor o-MAX phase materials using a chemical etching process, although yields are typically low. The optimal conditions necessary for the etching of MAX phase materials can be predicted using Pourbaix diagrams. Here, Pourbaix diagrams produced from first-principles calculations are used to determine the conditions necessary to enhance the yields of o-MXenes. In agreement with experiment, we show that high yields of (Mo, Ti)2CTx are possible, as the o-MXene is stable relative to known competing ionic and molecular species in an aqueous solution across a wide range of pH and applied potentials. We show how the stability of the o-MXene depends on the nature of the terminating groups and the presence of metal vacancies on the o-MXene surface.