Synthesis of ordered double transition metal (Mo2+αNb2-α)AlC3 MAX phases and their (Mo2+αNb2-α)C3Tx MXenes

MXenes are a growing family of two-dimensional transition metal carbides with an ever-widening variety of compositional possibilities with impressive material behaviors. One sub-group of MXenes that has grown in recent years in the search for more exotic material properties is double-transition metal (DTM) MXenes. Here, we present the synthesis of an out-of-plane partially ordered (Mo2+αNb2-α)AlC3Tx DTM MAX phase and their derivative out-of-plane ordered (Mo2+αNb2-α)C3Tx DTM MXenes. We investigated the synthesis of the full range of Mo-Nb compositions from Mo-rich to Nb-rich (Mon, Nb4-n)AlC3 MAX phases (0.3 ≤ n ≤ 3.7). Our x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy results indicated that only Mo-rich (n > 2) leads to the formation of a MAX phase which suggests the out-of-plane ordered nature of this phase. Using these methods, we further analyze changes within the a and c-lattice parameters as well as the atomic ratios across an as-mixed molar range of Mo and Nb. After verification of these partially ordered phases, we exfoliated the (Mo2+αNb2-α)C3Tx MXenes in an aqueous hydrofluoric acid solution and then delaminated the MXene to single-to-few layer flakes using tetramethylammonium hydroxide. These newly reported partially ordered (Mo2+αNb2-α)C3Tx MXenes represent further expansion of the possible compositions of DTM MXenes by adding Nb as an interior transition metal with Mo as an exterior transition metal in ordered MXene phases. While further characterization is still needed, we speculate that (Mo2+αNb2-α)C3Tx MXenes may provide a new composition to further tune their electrochemical, mechanical and electronic properties and provide another option toward application-driven design of MXene compositions.