Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W M ) AC, where M = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two-with a monoclinic C2/c structure-are predicted to be stable: (W Sc ) AlC and (W Y ) AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.
Introducing point defects in two dimensional, 2D, materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms, results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases. 3 Two-dimensional (2D) materials have shown great promise for many applications. 1-6 The reduced dimension leads to an increase in the surface to volume ratio, and can fundamentally alter the chemical, optical and electronic properties of a material. The properties can be altered further, either chemically via surface functionalization, 7 intercalation 8 or structurally, by introducing defects. 8-9 About 7 years ago, a new class of 2D materials based on transition metal carbides and/or nitrides (MXenes) was discovered. 10-11 MXenes are mainly produced by etching the Mn+1AXn (MAX) phases or related ternary phases. 12 The MAX phases are a family of hexagonal, layered ternary transition metal carbides and/or nitrides where M stands for an early transition metal, A stands for group 13 and 14 elements, X stands for carbon and/or nitrogen and n = 1, 2, or 3. 13 Various acidic solutions, containing fluoride ions are used to selectively etch the A layers (either Al or Ga) and convert MAX to MXene. 10, 14-17 The A layers are replaced with oxygen, hydroxyl and/or fluoride surface terminating (T) groups. 18 MXenes show promise for a large host of applications including batteries, supercapacitors, transparent conducting electrodes, catalytic and photocatalytic applications, water treatment, electromagnetic shielding, gas sensors and biosensors. 19-25 MXene properties can be tuned in at least three ways that involve either altering their: i) composition, ii) surface terminations, Tx and/or, iii) structure/morphology. The composition can be changed by e.g. forming solid solutions though alloying on the M-, 26 and/or X-27 sites in the parent MAX phase. The quaternaries, (Nb0.8,Ti0.2)4C3Tx and (Nb0.8,Zr0.2)4C3Tx 28 are examples
Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5SiB2 (T2) phases, the finding of a family of laminated quaternary metal borides, M′4M″SiB2, with out‐of‐plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4MoSiB2, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4MoSiB2 and select removal of Si and MoB2 sub‐layers is validated by derivation of a 2D material, TiOxCly, of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of ≈4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.
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