Interlayer Incorporation of A-Elements into MXenes Via Selective Etching of A' from M_n+1A'_1-xA″_xC_n MAX Phases

MXenes are a large family of two-dimensional materials with a general formula M_n+1X_nT_z, where M is a transition metal, X = C and/or N, and T_z represents surface functional groups. MXenes are synthesized by etching A-elements from layered MAX phases with a composition of M_n+1AX_n. As over 20 different chemical elements were shown to form A-layers in various MAX phases, we propose that they can provide an abundant source of very diverse MXene-based materials. The general strategy for A-modified MXenes relies on the synthesis of M_n+1A'_1-xA″_xX_n MAX phase, in which the higher reactivity of the A'-element compared to that of A″ enables its selective etching, resulting in A″-modified M_n+1X_nT_z. In general, the A″-element could modify the interlayer spaces of MXene flakes in a form of metallic or oxide species, depending on its chemical identity and synthetic conditions. We demonstrate this strategy by synthesizing Sn-modified Ti₃C₂T_z MXene from the Ti₃Al_0.75Sn_0.25C₂ MAX phase, which was used as a model system. Although the incorporation of Sn in the A-layer of Ti₃AlC₂ decreases the MAX phase reactivity, we developed an etching procedure to completely remove Al and produce Sn-modified Ti₃C₂T_z MXene. The resulting MXene sheets were of very high quality and exhibited improved environmental stability, which we attribute to the effect of a uniform Sn modification. Finally, we demonstrate a peculiar electrostatic expansion of Sn-modified Ti₃C₂T_z accordions, which may find interesting applications in MXene-based nano-electromechanical systems. Overall, these results demonstrate that in addition to different combinations of M and X elements in MAX phases, an A-layer also provides opportunities for the synthesis of MXene-based materials.