Search

MXenes are a relatively new and large family of two-dimensional early transition-metal carbides derived typically by etching the MAX phases in fluoride-containing solutions. Although numerous studies have investigated the role of cations in the interlayer distance (d0002) between MXene multilayers (MLs), little is known about the role of anions. Herein, using mainly X-ray diffraction, the effect of anions on d0002 of Ti3C2Tz MLs, where Tz represents various terminations, was systematically studied. The MLs were produced by etching Ti3AlC2 powders in hydrofluoric (HF) acid alone or in mixtures of HF and mineral acids with anions larger than F, viz., hydrochloric, hydrobromic, hydroiodic, sulfuric, or phosphoric acids. The nature of the cations present in the postetching washing solutions was also varied. The results show that the presence of the larger anions demonstrably facilitates water intercalation and deintercalation. The fact that, for the most part, d0002 in fully dried MLs is not a function of min...

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their two-dimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten-based nanolaminated ternary phase, (W,Ti)

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their twodimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten based nanolaminated ternary phase, (W,Ti)(4)C4-x, synthesized by an Al catalyzed reaction of W, Ti, and C powders at 1600 degrees C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure (P6(3)/mmc) with lattice parameters, a = 3.00880(7) angstrom, and c = 19.5633(6) angstrom and a nominal chemistry of (W,Ti)(4)C4-x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times. Funding Agencies|Swedish Foundation for Strategic Research (SSF) through a Synergy Grant; Knut and Alice Wallenberg (KAW) Foundation [KAW 2015.0043]; Swedish Research Council [642-2013-8020, 621-2014-4890]

In this study, a transition from metallic to semiconducting-like behavior has been demonstrated in two-dimensional (2D) transition metal carbides by replacing titanium with molybdenum in the outer transition metal (M) layers of M3C2 and M4C3 MXenes. The MXene structure consists of n + 1 layers of near-close packed M layers with C or N occupying the octahedral site between them in an [MX]nM arrangement. Recently, two new families of ordered 2D double transition metal carbides MXenes were discovered, M′2M′′C2 and M′2M′′2C3 – where M′ and M′′ are two different early transition metals, such as Mo, Cr, Ta, Nb, V, and Ti. The M′ atoms only occupy the outer layers and the M′′ atoms fill the middle layers. In other words, M′ atomic layers sandwich the middle M′′–C layers. Using X-ray atomic pair distribution function (PDF) analysis on Mo2TiC2 and Mo2Ti2C3 MXenes, we present the first quantitative analysis of structures of these novel materials and experimentally confirm that Mo atoms are in the outer layers of the [MC]nM structures. The electronic properties of these Mo-containing MXenes are compared with their Ti3C2 counterparts, and are found to be no longer metallic-like conductors; instead the resistance increases mildly with decreasing temperatures. Density functional theory (DFT) calculations suggest that OH terminated Mo–Ti MXenes are semiconductors with narrow band gaps. Measurements of the temperature dependencies of conductivities and magnetoresistances have confirmed that Mo2TiC2Tx exhibits semiconductor-like transport behavior, while Ti3C2Tx is a metal. This finding opens new avenues for the control of the electronic and optical applications of MXenes and for exploring new applications, in which semiconducting properties are required.

Pseudocapacitive electrodes based on electrochemically active organic materials are emerging for the energy storage applications.1 Organic materials are renewable, relatively inexpensive, lightweight, and their electrochemical properties can be tuned by changing their functional groups. Despite aforementioned advantages, they tend to degrade rapidly upon cycling mainly due to their poor conductivity. One option to improve their cycling performance is to graft/deposit them onto conductive carbon particles.2 Two-dimensional carbides and carbonitirides called MXenes, which generally offer metallic conductivity, may become a suitable option to replace carbon in pseudocapacitve electrodes to improve the electrochemical performance of the organic materials.3,4 Herein, we present simultaneous intercalation, alignment and oxidant-free polymerization of the pyrrole between the Ti3C2Tx MXene layers by mixing pyrrole monomer with the Ti3C2Tx colloidal solution. The optimized composition of the resulting composite yielded high volumetric and gravimetric capacitances of 1000 F/cm3and 416 F/g, respectively, at 5 mV/s. Furthermore, excellent capacitance retention of 92% was observed after 25,000 cycles at 100 mV/s. 1. M. Boota et al., Adv. Mater. (2015). DOI: 10.1002/adma.201504705 2. M. Boota et al., ChemSusChem, 8, 3576–3581 (2015) 3. M. R. Lukatskaya et al., Science., 341, 1502–1505 (2013) 4. M. Ghidiu, M. R. Lukatskaya, M.-Q. Zhao, Y. Gogotsi, and M. W. Barsoum, Nature, 516, 78–81 (2014)

Herein we report on the synthesis of a new layered ternary carbide, Mo2TiAlC2, that was synthesized by heating an elemental mixture at 1600 degrees C for 4 h under an Ar flow. Its hexagonal, a and c lattice parameters were calculated via Rietveld analysis of powder X-ray diffraction patterns to be, respectively, 2.997 angstrom and 18.661 angstrom. High-resolution scanning transmission electron microscopy showed that this phase is ordered, with Ti layers sandwiched between two Mo layers in a M(3)AX(2) type ternary carbide structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Funding Agencies|Army Research Office (United States) [W911NF-11-1-0525, W911NF-14-1-0568]; SSF synergy grant FUNCASE Functional Carbides and Advanced Surface Engineering Sweden; Knut and Alice Wallenberg Foundation