MXenes represent a large and quickly growing family of two-dimensional (2D) early transition metal carbides and nitrides. Because of their 2D structure, tailorable thickness within the same atomic composition, and extraordinary performance, MXenes rise a significant interest for various (opto)electronic, sensing, as well as energy conversion and storage devices. However, their chemistry has been studied to a less extent. At the same time, knowledge of chemical properties of MXenes is important for their synthesis, storage, and application performance. It has long been known that MXene flakes in aqueous colloidal solutions are spontaneously converted into titanium oxide on the time scale from several hours to a few days, which limits their shelf life and complicates device manufacturing and operational stability. The conversion of Tin+1Cn MXenes into titania in aqueous colloidal solutions has been previously ascribed as oxidation and, correspondingly, oxygen was considered as the main reagent mediating the conversion, whereas water was playing a role of solvent. Here we analyze in more detail the chemical role of water as the main reagent mediating the conversion of Tin+1Cn into TiO2. We monitor chemical reactions of MXenes with water and other environments via direct detection of gaseous products of these reactions. Different environments, including water and organic solvents in equilibrium with oxygen and inert gas atmospheres were used to clarify the role of water. Our analysis leads to new important conclusions about the oxidation stability of MXenes and the choice of solvent for long-term storage of MXene sheets in a colloidal state, which may have important implications for the development of many applications of MXenes. In addition, we discuss effect of pH on chemical stability of MXenes in aqueous solutions and demonstrate how this knowledge can be used to significantly extend shelf life and quality of MXenes stored in aqueous colloidal state.

Structure, chemistry, and reactivity of 2D transition metal carbides/nitrides (MXenes)

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