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 n +1 X n T 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 +1 AX 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 +1 A′ 1– x A″ x X 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 +1 X n T 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 Ti 3 C 2 T z MXene from the Ti 3 Al 0.75 Sn 0.25 C 2 MAX phase, which was used as a model system. Although the incorporation of Sn in the A-layer of Ti 3 AlC 2 decreases the MAX phase reactivity, we developed an etching procedure to completely remove Al and produce Sn-modified Ti 3 C 2 T 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 Ti 3 C 2 T 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.

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