Interlayer Incorporation of A-Elements into MXenes Via Selective Etching of A′ from M_<i>n</i>+1A′_1–<i>x</i>A″_<i>x</i>C_<i>n</i> MAX Phases

Abstract

MXenes are a large family of two-dimensional materials with a general formula M_<i>n</i>+1X_<i>n</i>T_<i>z</i>, where M is a transition metal, X = C and/or N, and T_<i>z</i> represents surface functional groups. MXenes are synthesized by etching A-elements from layered MAX phases with a composition of M_<i>n</i>+1AX_<i>n</i>. 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_<i>n</i>+1A'_1-<i>x</i>A″_<i>x</i>X_<i>n</i> 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_<i>n</i>+1X_<i>n</i>T_<i>z</i>. 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_<i>z</i> 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_<i>z</i> 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_<i>z</i> 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.