A-Modified MXenes: Interlayer Incorporation of A-Elements into MXenes via Selective Etching of A' from M_n+1A'_1-x A''_xC_n MAX Phases

MXenes are a large family of two-dimensional materials with a general formula M_n+1X_nT<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_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 new MXene-based materials [1]. The general strategy for A-modified MXenes relies on the synthesis of a M_n+1A'_1-<i>x</i>A"<i>_x</i>X_n MAX phase, in which the higher reactivity of the A' element compared to A" enables its selective etching, resulting in A"-modified M_n+1X_nT<i>_z</i>. In general, the A" element could modify the interlayer spaces of MXene flakes in a form of metallic species or oxides, depending on its chemical identity and synthetic conditions. We demonstrate this strategy by synthesizing Sn-modified Ti₃C₂T<i>_z</i> MXene from 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 high quality and exhibited improved environmental stability, which we attribute to the effect of uniform Sn modification. We demonstrate a peculiar electrostatic expansion of Sn-modified Ti₃C₂T<i>_z</i> accordions, which may find applications in MXene-based nanoelectromechanical systems. Overall, these results demonstrate that in addition to different combinations of M and X elements in MAX phases, an A layer also provides exciting opportunities for the synthesis of new MXene-based materials. Synthetic approaches to MXenes modified with A-elements other than Sn, as well as applications of such materials, will also be discussed.<br/><br/>[1] S. Bagheri, A. Lipatov, N. S. Vorobeva, and A. Sinitskii. <i>ACS Nano</i> 2023, 17, 18747.