The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo1.33C sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm−3, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo2C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.

The two-dimensional (2D) MXene Ti3C2Tx is functionalized by surface groups (Tx) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 °C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

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.

Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo4/3B2-xTz (where Tz is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2 in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.

charge storage, [7,8] electromagnetic interference shielding, [9] filtering, [10] and a range of additional applications. [7] MXenes constitute a large and growing family of 2D materials [11,12] that are obtained from the laminated M n+1 AX n (MAX) phases (M is a transition metal, A is a group A element-mostly groups 13 and 14-and X is C and/or N) [13] by chemical etching of the atomically thin A element layers that separate sheets of M n+1 X n . As the A element is removed, the MXene surfaces are immediately functionalized by surface terminating species, T x . [6,14] Hence, the proper MXene formula is M n+1 X n T x . Accordingly, the MXene properties can be tuned through structure, intrinsic composition, and surface terminations. The structure is inherited from the parent MAX phase (hexagonal, space group P6 3 /mmc), but compositional tuning displays an extraordinary toolbox for property tuning through MXenes based on single M and X elements, as well as alloys on both M and X. [12,15] In addition, there are reports on MXenes forming out-of-plane [16] and in-plane [17] double-M elemental ordering, as well as vacancy-ordered structures. [18,19] Manipulation of the surface terminations constitutes the final and most powerful variable for property tuning. [20] Despite several theoretical investigations, [21][22][23] noninherent terminations have remained experimentally unexplored. Currently, the MXene preparation dictates that T x is inherent to the etchant and predominantly a combination of O and F, where OH has also been considered as a minor [24] or even negligible contribution. [25] In the area of carbon capture (CC), MXenes are predicted to be highly efficient for capturing CO 2 , enabling capture of 2-8 mol CO 2 kg −1 . [20,21] However, the MXene surfaces were assumed to be termination free, an experimentally unrealistic starting point, given the current wet-chemical preparation routes for MXenes. To unlock the MXene potential for noninherent terminations or adsorption of other molecules, such as CO 2 , we have subjected the archetype Ti 3 C 2 T x MXene to a novel approach. Using in situ environmental transmission electron microscopy (ETEM), single Ti 3 C 2 T x sheets were subjected to an initial high-temperature treatment to desorb F, [25] and a subsequent H 2 exposure to remove the persistent O from the surfaces. The thereafter termination-depleted MXene was subsequently exposed to CO 2 gas, resulting in the first MXene to be terminated by a noninherent molecule. Additionally, termination-depleted MXene surfaces were exposed to N 2 gas after which no N adsorption was observed, Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the everincreasing CO 2 emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO 2 . However, challenges remain because of limited CO...

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo Y ) AlC, the resulting MXene can be either: i) (Mo Y ) C with in-plane elemental order through selective removal of Al atoms or ii) Mo C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo Y ) C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm is obtained, which is 40% higher than that of its Mo C counterpart. With H SO , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

they were stored in room-air after the annealing procedure and prior to the SEM/EDX study. 100The Si content of the A layers in the novel structure is negligible, based on EDX results, and and what is present in similar systems, see S3 and S6. These calculations showed that 115

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