Tungsten-based mxenes and mxene precursors

Abstract

Methods for synthesizing tungsten-based MXenes without using a MAX phase precursor.

Description

Attorney Docket No. IUIC-170 TUNGSTEN-BASED MXENES AND MXENE PRECURSORS CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Application Serial No. 63 / 561,856, filedMarch 6, 2024, which is incorporated by reference herein in its entirety. STATEMENT ON FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under 2124478 awarded by theNational Science Foundation. The government has certain rights in the invention. TECHNICAL FIELD

[0003] Embodiments of the subject application are generally directed to methods forsynthesizing tungsten-based two-dimensional (2D) materials, also referred to as MXenes. Additionally, embodiments discussed herein further relate to the formation of novel compositions of MXenes using non-MAX phase precursors. BACKGROUND

[0004] With the increasing visibility of climate change impacts attributed to fossil fuelconsumption, there is a rising need for sustainable energy alternatives. The key to a sustainable energy future lies in developing materials that can be used to generate and store energy. A crucial aspect of this pertains to the progress of eco-friendly hydrogen fuel cells and the production of hydrogen fuel via water electrolysis. This paradigm shift necessitates the engineering of a proficient catalyst for the hydrogen evolution reaction (HER). Nevertheless, the sustainability of HER is hindered by the high cost of the most efficient noble metal catalysts, including platinum (Pt), iridium (Ir), and ruthenium (Ru). Therefore, to design cost-effective HER catalysts, it is imperative to focus on developing new catalysts that are scalable, effective, and reliable.

[0005] Tungsten (W) is relatively abundant in the earth’s crust, with an abundance ofapproximately 1.3 parts per million (ppm) by weight compared to platinum's 0.005 ppm and iridium's 0.001 ppm. The high HER catalytic activity of tungsten-based materials (such as tungsten carbide, tungsten nitride, and tungsten oxide) can be attributed to their ability to bind hydrogen atoms. Tungsten has a relatively high electronegativity (2.36) and a low work function (4.8 eV),Attorney Docket No. IUIC-170 which allows it to easily accept electrons from hydrogen molecules and form strong tungsten- hydrogen bonds. Additionally, the electronic structure of tungsten-based materials can be tuned by controlling their composition and morphology, which can further enhance their HER activity. These features, in combination with their durability and resistance to corrosion under harsh operating conditions, make tungsten-based materials a promising alternative for HER compared to precious metal catalysts. In recent years, layered W-based chalcogenides (WS2, WSe2, and WTe2) have been progressively applied in the field of HER electrocatalysis due to their crystal structures and favorable density of states at the Fermi level which promotes electronic conductivity and charge transfer during HER. As a result, these W-based materials have opened exciting possibilities for affordable alternatives in catalytic and sustainable energy technologies.

[0006] The growing palette of two-dimensional (2D) transition metal carbides, nitride andcarbonitrides, known as MXenes, are a promising class of electrocatalytic materials for clean energy applications because their 2D basal planes are electrochemically active, combined with their tunable chemistry, electrical conductivity (up to 24,000 S / cm for Ti3C2Tx), high mechanical strength (up to 480 GPa for Ti3C2Tx), and thermal stability in inert environments.

[0007] MXenes have the chemical formula of Mn+1XnTx (n = 1-4), where M stands for n+1layers of one or more transition metals (usually from groups 4-6 of the periodic table), X stands for n layers of carbon or nitrogen which interleave the M layers, and Tx represents a variety of surface functional groups (commonly -O, -OH, -F). MXenes are usually derived from their parent MAX phases, denoted by Mn+1AXn, with A being an A-group element, usually groups 13-16. The current synthesis approach of the majority of realized MXenes is through the selective etching of the A-group layers from their precursor MAX phase via different etching routes, including using hydrofluoric acid (HF), HF-containing etchants with hydrochloric acid (HCl), HF-forming etchants with fluoride salts (LiF, NH4HF2, FeF3, KF, and NaF) and HCl, molten salt etching (LiF, NaF, KF), alkali-assisted hydrothermal etching, and electrochemical etching. The use of top-down synthesis methods means processes to yield MXenes are scalable (up to 1 kg shown), making MXene a strong competitive 2D material for industrial-scale use.

[0008] MXenes’ entire basal planes are catalytically active for HER, unlike 2D transitionmetal dichalcogenides, where mainly the 2D edges are catalytically active in most cases. Thus, the basal plane provides more active sites in MXenes for HER. To date, Mo2CTxhas shown the best HER performance among all MXenes with the lowest overpotential of 189 mV at 10 mA / cm2.Attorney Docket No. IUIC-170 Beyond Mo2CTx, tungsten-based MXenes are useful for HER as they are anticipated to possess near-zero overpotentials with hydrogen adsorption-free energy ΔGad, approaching thermoneutral (ΔGad → 0), which makes them promising earth-abundant electrocatalysts for HER. However, as most MXenes are derived from their precursor MAX phases, synthesizing W-based MXenes has proven difficult due to the calculated instability of its hypothetical precursor MAX phases, such as W2AC and W3AC2. To date, the only W-based MAX phases are in-plane-ordered MAX (i- MAX) of (W2 / 3Sc1 / 3)2AlC and (W2 / 3Y1 / 3)2AlC. Wet-chemical selective etching of these MAX phases using HF led to the removal of the Al layer along with scandium (Sc) or yttrium (Y). The resulting MXene exhibited ordered divacancies in which ~ 33 % of the tungsten sites were vacant on each basal plane resulting in a W1.33CTxMXene stoichiometry rather than the prototypical M2CTx. The measured HER overpotential of W1.33CTxMXene was 320 mV at 10 mA / cm2. Ordered divacancies provide metastable adsorption sites for hydrogen that require extended exposure time or repeated cycling to improve the reaction kinetics and HER electrocatalytic properties of W1.33CTxMXene. As a result, there is a need for W-based MXenes in the form of M2CTx and M3C2Tx with a minimum number of M vacancies on the MXene surfaces to maximize HER performance.

[0009] Based on theoretical predictions, W-based MAX phases are inherently unstable innature. However, the combination of W with other transition metals (for example, W with Ti) on the M sites has been predicted to form stable double-metal M3AlC2 and M4AlC3 MAX phases. Despite this prediction, many attempts, including several from our group, have resulted in no confirmed synthesis of W-based M3AlC2and M4AlC3to date. While two solid solution Ti-W MXenes with 20 at.% of W in the M sites in M2C and M3C2 structures are reported, no W-based MXene (majority M being W) has been reported. In 2019, a nano-laminated non-MAX ternary carbide (W,Ti)4C4-ywas synthesized with an attempt to make a hypothetical out-of-plane orderedW2TiAlC2 MAX phase. In that work, a hexagonal and layered (W,Ti)4C4-y was synthesized bypressureless sintering of elemental powder mixtures of W, Ti, Al, C in the molar ratio 2:1:1.1:2, which showed a hexagonal structure (P63 / mmc) with twin plane layers of pure W and layers of mixed W and Ti all interwoven with layers of C as M6C octahedra. Even though aluminum is not present in the final crystal structure of (W,Ti)4C4-y, the presence of aluminum was found to be crucial for the formation of this nanolaminated phase. Further attempts were made to synthesizeAttorney Docket No. IUIC-170 a W-based MXene from this material, however, all the attempts until the development of the subject matter disclosed herein were unsuccessful.

[0010] Accordingly, there remains a need for further contributions in this area oftechnology. SUMMARY

[0011] In one embodiment, W2TiC2Tx is prepared as the first W-based ordered double-transition-metal (DTM) MXene, and it is one of few MXenes that have been derived from a non- MAX precursor. In embodiments, this phase was synthesized by etching covalently bonded layers of a transition metal carbide from its layered carbide precursor. To do so, a (W,Ti)4C4-y phase was modified using an excess amount of aluminum in the synthesis of the (W,Ti)4C4-yprecursor to improve the precursor quality, which enabled the successful synthesis of W2TiC2Tx MXene. The precursor and W2TiC2Tx MXene were characterized in detail for their structure and morphology. In addition, the electrical conductivity, linear optical properties, and nonlinear optical properties of this new MXene were measured for potential lasing and optoelectronic applications. In addition, HER reaction kinetics of the W2TiC2Tx MXene were studied via first-principle density functional theory (DFT) calculations, and its electrocatalytic performance was determined. The W2TiC2TxMXene exhibits highly efficient HER catalytic activity.

[0012] According to an embodiment, a MXene comprises W2MC2Tx wherein W istungsten; M is a transition metal; C is carbon; and Tx is a functional surface termination.

[0013] In embodiments, M is selected from the group consisting of Sc, Ti, V, Cr, Y, Zr,Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.

[0014] In embodiments, M is Ti.

[0015] In embodiments, a method of making a MXene comprises: synthesizing ananolaminated material having a general formula of (W,M)4C4-y wherein W is tungsten; M is a transition metal; C is carbon; and y is an integer from 1 to 3; chemically etching at least a portion of the nanolaminated material to form an etched material; washing the etched material to produce a multilayered MXene sediment; and isolating the MXene from the MXene sediment.

[0016] In embodiments, synthesizing the nanolaminated material comprises mixingpowders of each of W, M, aluminum, and calcined coke in a 2:1:1.1:2 or 2:1:2:2 molar ratio.Attorney Docket No. IUIC-170

[0017] In embodiments, synthesizing the nanolaminated material further comprisessintering the mixed powders at a temperature of approximately 1600 degrees Celsius for approximately four hours.

[0018] In embodiments, synthesizing the nanolaminated material further comprises, duringheating, exposing the mixed powders to a generally constant flow of an argon gas with a ramp rate of 3.5 degrees Celsius per minute.

[0019] In embodiments, sintering occurs under an inert atmosphere.

[0020] In embodiments, etching comprises mixing the nanolaminated material with ahydrofluoric acid (HF) solution.

[0021] In embodiments, etching further comprises stirring the mixture of thenanolaminated material and the HF solution at approximately 400 revolutions per minute (rpm) for around 96 hours at about 55 °C.

[0022] In embodiments, washing comprises washing the etched material with deionizedwater via repeated centrifugation.

[0023] In embodiments, the centrifugation is repeated for four or five cycles.

[0024] In embodiments, the centrifugation is repeated until a supernatant of themultilayered MXene sediment has a pH of approximately 6.

[0025] In embodiments, the method further comprises adding the multilayered MXene toa tetramethylammonium hydroxide (TMAOH) solution to a delaminated MXene suspension.

[0026] In embodiments of the method, M is selected from the group consisting of Sc, Ti,V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.

[0027] In embodiments of the method, M is Ti.

[0028] This summary is not intended to identify key or essential features of the claimedsubject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Further embodiments, forms, features, and aspects of the present application shall become apparent from the description and figures provided herewith. BRIEF DESCRIPTION OF THE FIGURES

[0029] The concepts described herein are illustrative by way of example and not by wayof limitation in the accompanying figures. For simplicity and clarity of illustration, elements illustrated in the figures are not necessarily drawn to scale. Where considered appropriate,Attorney Docket No. IUIC-170 references labels have been repeated among the figures to indicate corresponding or analogous elements.

[0030] Figure (FIG.) 1 shows density functional theory (DFT) and experimental approachfor the synthesis of modified (W,Ti)4C4-yprecursor and its HF-etchability. In particular, FIG.1 is a crystal structure of (W,Ti)4C4-y precursor showing one pure M1 W layer with ~ 25 % vacancies, two M2 W-rich layers on either side of the M1 layer, and a M3 Ti-rich layer in between the M2 layers.

[0031] FIG. 2 shows a density functional theory analysis of the synthesizability ofW2TiC2Tx MXene structure with no W-Ti intermixing and no vacancies,

[0032] FIG. 3 shows a density functional theory analysis of the synthesizability ofW2TiC2TxMXene structure with W-Ti intermixing and no vacancies, and

[0033] FIG. 4 shows a density functional theory analysis of the synthesizability ofW2TiC2Tx MXene structure with W-Ti non-intermixed (left) and intermixed (right) structures with 25 at% W vacancies. Color code: Yellow-O, Turquois-W, Black-C, Pink-Ti.

[0034] FIG. 5 shows density functional theory (DFT) and experimental approach for thesynthesis of modified (W,Ti)4C4-y precursor and its HF-etchability. In particular, FIG.5 is a density functional theory (DFT) analysis of the synthesizability of W2TiC2TxMXene structure with intermixing between Ti and W without and with 25 at% W vacancies.

[0035] FIG. 6 shows density functional theory (DFT) and experimental approach for thesynthesis of modified (W,Ti)4C4-yprecursor and its HF-etchability. In particular, FIG. 6 is a schematic of synthesis procedure for 1.1Al-(W,Ti)4C4-yand 2Al-(W,Ti)4C4-y,

[0036] FIG. 7 shows density functional theory (DFT) and experimental approach for thesynthesis of modified (W,Ti)4C4-y precursor and its HF-etchability. In particular, FIG.7 is an XRD of as-synthesized and HF-etched 1.1Al-(W,Ti)4C4-y.

[0037] FIG. 8 shows density functional theory (DFT) and experimental approach for thesynthesis of modified (W,Ti)4C4-y precursor and its HF-etchability. In particular, FIG.8 is an XRD of as-synthesized and HF-etched 2Al-(W,Ti)4C4-y.

[0038] FIG. 9 shows Rietveld-derived occupancies of the zAl-(W,Ti)4C4-y (z = 1.1, 2)structures pre-HF treatment.

[0039] FIG. 10 shows density functional theory (DFT) and experimental approach for thesynthesis of modified (W,Ti)4C4-yprecursor and its HF-etchability. In particular, FIG.1 is a densityAttorney Docket No. IUIC-170 functional theory (DFT) analysis of the synthesizability of a W2TiC2TxMXene structure out of the zAl-(W,Ti)4C4-y precursor for z = 1.1, 2 based on Rietveld occupancies.

[0040] FIG. 11, panel a, shows an analysis of the byproducts formed after HF treatment ofzAl-(W,Ti)4C4-yusing XRD.

[0041] FIG. 11, panel b, shows an analysis of the byproducts formed after HF treatment ofzAl-(W,Ti)4C4-y using EDS.

[0042] FIG. 12 shows Rietveld-derived occupancies of the zAl-(W,Ti)4C4-y (z = 1.1, 2)structures after HF treatment.

[0043] FIG. 13 shows density functional theory calculations to evaluate etchingcapabilities as a function of W vacancies in the M1 layer.

[0044] FIG. 14 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG. 14 shows generalized etching and delamination reaction pathway.

[0045] FIG. 15 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG.15 shows SEM.

[0046] FIG. 16 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2TxMXene synthesis process. In particular, FIG.16 shows XRD of Al-(W2Ti)4C4-y.

[0047] FIG. 17 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG.17 SEM of HF etched W2TiC2Tx MXene.

[0048] FIG. 18 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2TxMXene synthesis process. In particular, FIG.18 shows XRD of HF etched W2TiC2TxMXene.

[0049] FIG. 19 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2TxMXene synthesis process. In particular, FIG.19 shows SEM of free-standing W2TiC2TxMXene film.

[0050] FIG. 20 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2TxMXene synthesis process. In particular, FIG.20 shows XRD of free-standing W2TiC2Txsingle-to-few layer film.

[0051] FIG. 21 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2TxMXene synthesis process. In particular, FIG.21 shows solution processable colloidal suspension of the delaminated W2TiC2TxMXene-like colloidal solution.Attorney Docket No. IUIC-170

[0052] FIG. 22 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG.22 shows the Tyndall effect showing the colloidal stability.

[0053] FIG. 23 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG.23 shows free-standing film prepared from the W2TiC2Tx MXene-like colloidal solution and its flexible nature (inset).

[0054] FIG.24 shows a cross-sectional HAADF-STEM image of W2TiC2Tx MXene layersin the horizontally aligned MXene pattern prepared by focused ion beam (FIB) lift out.

[0055] FIG. 25 shows atomic resolution HAADF-STEM image of W2TiC2TxMXenelayers. STEM images show that the horizontally aligned MXene patterns have well-stacked layers of W and Ti. The light elements of the surface termination groups (O, H, and F) cannot be seen between the layers, but the larger and non-uniform spacing seen in FIG. 34 indicates the weak interactions between the MXene layers after etching and the formation of a 2D structure.

[0056] FIG. 26 shows plan-view atomic resolution transmission electron microscopicimages of W2TiC2Tx MXene flakes.

[0057] FIG. 27 shows atomic resolution STEM micrographs including the line scans andEDX spectra.

[0058] FIG. 28 shows XPS of W2TiC2Tx MXene film for Ti2p.

[0059] FIG. 29 shows XPS of W4f.

[0060] FIG. 30 shows XPS of C1s.

[0061] FIG. 31 shows XPS of O1s.

[0062] FIG. 32 shows Full XPS spectra and deconvolutions of the precursor Al2-W2TiC2.

[0063] FIG.33 shows HER activity of W2TiC2Tx MXenes: The HER activity of W2TiC2TxMXene has been performed by well-defined density functional theory calculations. In particular, FIG.33 shows optimized structures of the adsorption site of H+on intermixed Ti and W with all - OH terminations, -OH vacancy, -F terminations, and –F vacancy.

[0064] FIG.34 shows HER activity of W2TiC2Tx MXenes: The HER activity of W2TiC2TxMXene has been performed by well-defined density functional theory calculations. In particular, FIG. 34 shows a free energy diagram of HER over MXene surface with Ti-W intermixing. The prepared MXenes electrode was examined in a three-electrode electrochemical cell using a rotating disk electrode apparatus at 1600 rpm in 0.5 M H2SO4.Attorney Docket No. IUIC-170

[0065] FIG. 35 shows density functional theory calculations of optimized structures of theadsorption site of H+on non-intermixed Ti and W with all -OH terminations, -OH vacancy, -F terminations, and –F vacancy.

[0066] FIG. 36 shows density functional theory calculated free energy diagram.

[0067] FIG. 37, panel a, shows density of states (DOS) of OH terminated surfaces withand without intermixing between Ti and W.

[0068] FIG. 37, panel b, shows density of states (DOS) of F terminated surfaces with andwithout intermixing between Ti and W.

[0069] FIG. 38 shows optimized structures of H adsorption over non-intermixed panel (a)stoichiometric surface, panel (b) -OH defect, panel (c) -F defect, and panel (d) free-energy diagram for the non-intermixed surface showing the HER limiting potential is -0.16 V, but the binding over OH and F vacancies are much stronger which results in -0.84 and -1.06 V limiting potential. Optimized structures of H adsorption over intermixed panel (e) stoichiometric surface, panel (f) - OH defect, panel (g) -F defect, and panel (h) free-energy diagram for the intermixed surface showing the HER limiting potential is -0.31 V, and the limiting potential over OH and F vacancies are also lower than the non-intermixed surface, -0.67 and -0.77 V, respectively.

[0070] FIG. 39 shows a FESEM of W2TiC2Tx MXene flake synthesized from HCl-LiFMILD etching route.

[0071] FIG. 40 shows linear sweep voltammetry (LSV) curves of studied W2TiC2TxMXenes. The HER activity of delaminated and etched multilayered W2TiC2TxMXene was compared. The LSV results show the lowest overpotential of 144 mV for delaminated W2TiC2TxMXene.

[0072] FIG. 41 shows Tafel analysis of studied delaminated W2TiC2Tx MXene for HER,which indicates a Tafel slope of 70 mV / dec. The HER activity of W2TiC2TxMXene may be linked to the highly active and ordered basal-plane vacancies of W.

[0073] FIG. 42 shows transport measurements revealing the temperature dependence ofresistivity and conductivity.

[0074] FIG. 43 shows VRH low-temperature fit of data for coeff. ½ (top). Middle andbottom – dependence of log conductivity for coeff.1 / 3 and ¼, showing non-linearity in the low- temperature part, implying fit with coeff. ½ describes the low-temperature part of the data best.Attorney Docket No. IUIC-170

[0075] FIG. 44 shows magnetoresistance of W2TiC2Tx MXene film at 1.8 K (top),corresponding to typical semiconductor and metal. The middle and bottom images show 2nd and 3rd harmonics.

[0076] FIG. 45 shows the absorption spectrum of a W2TiC2Tx film.

[0077] FIG. 46 shows a nonlinear optical transmission measurement of the W2TiC2TxMXene.

[0078] FIG. 47 shows Raman spectra of W2TiC2Tx MXene compared with Ti3C2TxMXene.

[0079] FIG. 48 shows fitted Raman spectra of Ti3C2Tx and W2TiC2Tx MXene.

[0080] FIG. 49 shows a phase evolution study to understand the formation of parent non-MAX precursor with varying amounts of aluminum (1.1Al and 2Al). The findings indicate that the formation of an intermetallic impurity (TiAl) at sintering temperatures (800 °C -1200 °C) leads to successful formation of the (W,Ti)4C4-y precursor with 1.1Al and 2Al.

[0081] FIG. 50 shows the effect of TiAl intermetallic impurity on the formation of(W,Ti)4C4-y precursor with 1.1Al (blue) and 2Al (orange) as shown by the XRD patterns.

[0082] FIG. 51 shows SIMS analysis of 1.1Al-(W,Ti)4C4-y.

[0083] FIG. 52 shows SIMS analysis of 2Al-(W,Ti)4C4-y.

[0084] FIG. 53 shows density functional theory analysis of the synthesizability ofW2TiC2Tx MXene with similar atomic compositions investigated by SIMS for 1.1Al-(W,Ti)4C4-y and 2Al-(W,Ti)4C4-y.

[0085] FIG. 54 shows a step-by-step characterization of the 2Al-(W2Ti)4C4-y precursor toW2TiC2Tx MXene synthesis process. In particular, FIG.54 shows SIMS analysis of free-standing W2TiC2Tx MXene film.

[0086] FIG. 55 shows full XPS spectra and deconvolutions of the precursor Al2-W2TiC2.

[0087] FIG. 56 shows HER activity over F terminated intermixed MXene surface, F andOH terminated surface with (red) and without (yellow) intermixing, and O terminated Mo2CTx (100) surface.

[0088] FIG.57 shows average turnover frequency (TOFavg) plots, normalized to all surfacesites, of multilayered and delaminated W2TiC2Tx MXene.

[0089] FIG. 58 shows electrochemical capacitance and active surface area measurementsfor etched multilayer and delaminated W2TiC2TxMXene flakes using cyclic voltammetry.Attorney Docket No. IUIC-170

[0090] FIG. 59 shows a stability study of the delaminated W2TiC2Tx MXene flakes in 0.5M H2SO4 electrolyte.

[0091] FIG. 60 shows the absorption spectrum of a thin film W2TiC2Tx sample across theultraviolet, visible, and short-wave infrared wavelengths. Focused wavelength spectra to show the key features. Spectrum of entire wavelength range with region-of-interest shaded (inset).

[0092] FIG. 61 shows intensity-dependent transmission at 800 nm wavelength for a thinfilm sample of W2TiC2Tx. Three distinct RSA trends labeled with Roman numerals (I-III) suggest nonlinear dynamics are controlled by an intensity-dependent equilibrium of a multilevel system. DETAILED DESCRIPTION

[0093] Although the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described herein in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives consistent with the present disclosure and the appended claims.

[0094] References in the specification to “one embodiment,” “an embodiment,” “anillustrative embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may or may not necessarily include that particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. It should be further appreciated that although reference to a “preferred” component or feature may indicate the desirability of a particular component or feature with respect to an embodiment, the disclosure is not so limiting with respect to other embodiments, which may omit such a component or feature. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. Additionally, it should be appreciated that items included in a list in the form of “at least one of A, B, and C” can mean (A); (B); (C); (A and B); (B and C); (A and C); or (A, B, and C). Similarly, items listed in the form of “at least one of A, B, or C” can mean (A); (B); (C); (A and B); (B and C); (A and C); or (A, B, and C). Further, with respect to the claims, the use of words and phrases such as “a,” “an,” “at leastAttorney Docket No. IUIC-170 one,” and / or “at least one portion” should not be interpreted so as to be limiting to only one such element unless specifically stated to the contrary, and the use of phrases such as “at least a portion” and / or “a portion” should be interpreted as encompassing both embodiments including only a portion of such element and embodiments including the entirety of such element unless specifically stated to the contrary.

[0095] In the drawings, some structural or method features may be shown in specificarrangements and / or orderings. However, it should be appreciated that such specific arrangements and / or orderings may not be required. Rather, in some embodiments, such features may be arranged in a different manner and / or order than shown in the illustrative figures unless indicated to the contrary. Additionally, the inclusion of a structural or method feature in a particular figure is not meant to imply that such feature is required in all embodiments and, in some embodiments, may not be included or may be combined with other features.

[0096] Unless defined otherwise, all technical and scientific terms have the same meaningas is commonly understood by one of ordinary skill in the art to which this disclosure belongs. All patents, applications, published applications and other publications are incorporated by reference in their entireties. If a definition set forth in this section is contrary to, or otherwise inconsistent with, a definition set forth in a patent, application, or other publication that is incorporated by reference, the definition set forth in this section prevails over the definition incorporated by reference.

[0097] As used in the description and the appended claims, the singular forms “a,” “an,”and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. The terms “including,” “containing,” and “comprising” are used in their open, non-limiting sense. Also as used herein, “and / or” refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative (“or”).

[0098] The term “about,” as used herein when referring to a measurable value such as anamount of polypeptide, dose, time, temperature, enzymatic activity or other biological activity and the like, is meant to encompass variations of ±20%, ±10%, ±5%, ±1%, ±0.5%, or even ±0.1% ofAttorney Docket No. IUIC-170 the specified amount. To provide a more concise description, some of the quantitative expressions are not qualified with the term “about.” It is understood that, whether the term “about” is used explicitly or not, every quantity is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximations due to the experimental and / or measurement conditions for such given value.

[0099] The terms “including,” “containing,” and “comprising” are used in their open, non-limiting sense. The transitional phrase “consisting essentially of” means that the scope of a claim is to be interpreted to encompass the specified materials or steps recited in the claim, “and those that do not materially affect the basic and novel characteristic(s)” of the claimed subject matter. See, In re Herz, 537 F.2d 549, 551-52, 190 USPQ 461, 463 (CCPA 1976) (emphasis in the original); see also MPEP §2111.03 (9thedition, 10threvision).

[0100] Embodiments disclosed herein include a tungsten-based MXene In embodiments,the MXene may have a general formula of W2MC2Tx, where W is Tungsten, M is a transition metal, such as, for example, a transition metal from the 3d to 5d blocks of the International Union of Pure and Applied Chemistry (IUPAC) groups 3-6 of the Periodic Table of Elements, C is carbon, and Txis a functional surface termination.

[0101] As noted above, M may be a transition metal from the 3d to 5d blocks of theInternational Union of Pure and Applied Chemistry (IUPAC) groups 3-6 of the Periodic Table of Elements. In embodiments, M is selected from Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, or a combination of two or more of these. In embodiments, M comprises Ti. In embodiments, M is Ti.

[0102] In embodiments, a method of making the MXene described above does not includesynthesizing the typical MAX phase of the MXene. Instead, the method includes synthesizing a nanolaminated material having a general formula of (W,M)4C4-y wherein W is tungsten; M is a transition metal; C is carbon; and y is an integer from 1 to 3. Then, at least a portion of the nanolaminated material may be chemically etched to form an etched material. The etched material may then be washed to produce a multilayered MXene sediment, from which the desired MXene may be isolated.

[0103] To synthesize the nanolaminated material, powders of each of W, M, aluminum,and calcined coke can be mixed in a 2:1:1.1:2 or 2:1:2:2 molar ratio. After mixing, the powderAttorney Docket No. IUIC-170 may be sintered at a temperature of approximately 1600 degrees Celsius for approximately four hours. Such sintering, which can occur in a pressureless and an inert atmosphere, can also include exposing the mixed powders to a constant flow of an argon gas with a ramp rate of 3.5 degrees Celsius per minute. The synthesis can provide one or more blocks of the nanolaminated, which can be milled, including, but not limited to, being crushed, so as to provide milled powders. The milled powders can be etched via, for example, mixing the milled powders with a hydrofluoric acid (HF) solution, and stirred at about 400 revolutions per minute (rpm) for around 96 hours at a temperature from about 40 °C to about 70 °C, such as about 55 °C.

[0104] The etched material can then be washed with, for example, deionized water, viarepeated centrifugation, such as, for example, centrifugation at about 4200 rpm for around approximately four or five cycles, and / or until the supernatant reaches a pH of approximately 6. Additionally, according to certain embodiments, for delamination, the etched multilayer MXene sediment can be added to 5% of a tetramethylammonium hydroxide (TMAOH) solution.

[0105] The MXene that are producible in accordance with the methods disclosed hereincan be utilized in a variety of different applications. For example, tungsten-based MXenes produced by the methods disclosed herein can be utilized as a topological insulator, as well as to provide catalytic activity useful for renewable energy and pollution reduction, among other applications. Additionally, the chemical synthesis methods disclosed herein may provide an optimal option for the scalable synthesis of two-dimensional materials, and thus provide pathways to synthesize tungsten-based MXenes using chemical synthesis methods.

[0106] In addition to the aspects and embodiments described and provided elsewhere inthe present disclosure, the following non-limiting list of embodiments are also contemplated.

[0107] 1. A MXene comprising:W2MC2Txwherein W is tungsten; M is a transition metal; C is carbon; and Tx is a functional surface termination.

[0108] 2. The MXene of clause 1, wherein M is selected from the group consisting ofSc, Ti, V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.Attorney Docket No. IUIC-170

[0109] 3. The MXene of clause 1, wherein M is Ti.

[0110] 4. A method of making an MXene, the method comprising:synthesizing a nanolaminated material having a general formula of (W,M)4C4-ywherein W is tungsten; M is a transition metal; C is carbon; and y is an integer from 1 to 3; chemically etching at least a portion of the nanolaminated material to form an etched material; washing the etched material to produce a multilayered MXene sediment; and isolating the MXene from the MXene sediment.

[0111] 5. The method of clause 4, wherein synthesizing the nanolaminated materialcomprises mixing powders of each of W, M, aluminum, and calcined coke in a 2:1:1.1:2 or 2:1:2:2 molar ratio.

[0112] 6. The method of clause 5, wherein synthesizing the nanolaminated materialfurther comprises sintering the mixed powders at a temperature of approximately 1600 degrees Celsius for approximately four hours.

[0113] 7. The method of clause 6, wherein synthesizing the nanolaminated materialfurther comprises, during heating, exposing the mixed powders to a generally constant flow of an argon gas with a ramp rate of 3.5 degrees Celsius per minute.

[0114] 8. The method of clause 5 or clause 6, wherein sintering occurs under an inertatmosphere.

[0115] 9. The method of any one of clauses 4 to 8, wherein etching comprises mixingthe nanolaminated material with a hydrofluoric acid (HF) solution.

[0116] 10. The method of clause 9, wherein etching further comprises stirring themixture of the nanolaminated material and the HF solution at approximately 400 revolutions per minute (rpm) for around 96 hours at about 55 °C.

[0117] 11. The method of any one of clauses 4-10, wherein washing compriseswashing the etched material with deionized water via repeated centrifugation.Attorney Docket No. IUIC-170

[0118] 12. The method of clause 11, wherein the centrifugation is repeated for four orfive cycles.

[0119] 13. The method of clause 11 or clause 12, wherein the centrifugation is repeateduntil a supernatant of the multilayered MXene sediment has a pH of approximately 6.

[0120] 14. The method of any one of clauses 4-13, further comprising adding themultilayered MXene to a tetramethylammonium hydroxide (TMAOH) solution to a delaminated MXene suspension.

[0121] 15. The method of any one of clauses 4-13, wherein M is selected from thegroup consisting of Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.

[0122] 16. The method of any one of clauses 4-14, wherein M is Ti.

[0123] Examples

[0124] Examples related to the present disclosure are described below. In most cases,alternative techniques can be used. The examples are intended to be illustrative and are not limiting or restrictive to the type, nature or composition of the embodiment of the material, or the scope of the invention as set forth in the claims.

[0125] Example 1: (W,Ti)4C4-y precursor synthesis

[0126] (W,Ti)4C4- was synthesized using elemental powders of tungsten,titanium, aluminum, andcoke which were mixed in stoichiometric 2:1:1.1:2 and excess- metal 2:1:2:2 molar ratio. The powders were then jar-milled in 250 mL Nalgene high-density polyethylene bottles (~ 10 g batch) with yttria-stabilized zirconia balls for 18 h in 2:1 ball-to- powder mass ratio without any shear at 60 RPM. The precursor powder was not sieved after jar- milling and then packed into an alumina crucible. The top of the crucible was covered with graphite foil and placed in a high-temperature tube furnace (Carbolite Gero, 1700 °C model) for reactive pressureless sintering. Initially, the furnace was purged with argon (99.999% purity) for 20 minutes at room temperature. After purging, the ball-milled precursor powders were heated to 1600 °C at a 3.5 °C / min ramp rate and held for 4 hours under a constant argon flow. After 4 hours, the furnace was cooled to room temperature at a 10 °C / min. Then the alumina crucible containing sintered billets of stoichiometric 1.1Al – (W,Ti)4C4-y and excess-metal 2Al – (W,Ti)4C4-y precursor were removed and drilled to a fine powder using a TiN coated drill bit in a drill press setup.

[0127] Example 2: W2TiC2Tx MXene synthesisAttorney Docket No. IUIC-170

[0128] The synthesized precursor was sieved using a 40 µm sieve before MXene synthesis.For 1g of (W,Ti)4C4-y precursor, 10 mL of 28.4 M HF etchant was used. The etchant was taken in a 60 mL Nalgene high-density polyethylene bottle with a magnetic stir bar. Then, the (W,Ti)4C4-y presursor was added slowly at a rate of ~ 0.5 g / minute. The etching reaction was carried out at 55 °C for 96 hours in the hot plate at 400 RPM. The reaction vessel should be capped with a condenser column on the top to release evolved gases and entrap vapors during the reaction. The etched multilayered MXene powder was washed to neutral pH with ~ 250 mL deionized water by repeated centrifugation in an Eppendorf centrifuge at 3234 RCF for 5 minutes. Then the neutralized solution was vacuum-filtered using a 0.8 µm filter membrane and the wet masses were recorded directly after. For delamination, tetramethylammonium hydroxide (TMAOH) solution (25 wt% stock, Fisher Scientific) with 10 mL of 6 % TMAOH per gram of etched W2TiC2TxMXene was used, and the reaction was carried out 55 °C for 4 h with continuous stirring at 400 RPM. TMA+intercalated W2TiC2Tx MXene was washed with ~ 250 mL deionized water for every 1 g of the precursor by repeated centrifugation at 21913 RCF in the Thermo Scientific centrifuge for 5 minutes followed by decantation until neutral pH. Then the washed W2TiC2Tx MXene clay was delaminated by mechanical agitation using vortex mixer for 30 minutes. W2TiC2Tx MXene clay was re-suspended in deionized water and vortexed for 30 minutes to isolate single-to-few MXene layers. W2TiC2Tx MXene colloidal suspension was then processed by final centrifugation at 2380 RCF for 30 minutes. The supernatant containing single-to-few layered W2TiC2Tx MXene was collected, and vacuum filtered using 0.25 µm filter membrane to prepare MXene films.

[0129] For HER studies, a minimally intensive layer delamination (MILD) route wasperformed using HCl-LiF for the synthesis of W2TiC2Tx MXene. The synthesis involved 12 M LiF and 9 M HCl mixture for 1 g (W,Ti)4C4-x precursor at 55 °C for 96 h with continuous stirring at 400 RPM. The resulting powders were centrifuged at 21913 RCF in the Thermo Scientific centrifuge for 5 minutes followed by decantation until neutral pH. For delamination, the neutralized MXene sediment was bath-sonicated for 15 minutes and then centrifuged at 2380 RCF for 30 minutes to obtain single-to-few-layered delaminated flakes W2TiC2TxMXene.

[0130] Example 3: Characterization techniques

[0131] X-ray diffraction (XRD) patterns of the as-synthesized (W,Ti)4C4-y precursors andW2TiC2TxMXene were analyzed using a Bruker D8 X-ray diffractometer with a Cu Kα (λ = 1.5406 Å) emitter and a VANTEC 500 two-dimensional x-ray detector (XRD2). A corundumAttorney Docket No. IUIC-170 standard was used to ensure each detector was calibrated. The precursor samples were mountedon Kapton tapes and scanned from 5 ° to 75 ° with a step size of 5 ° and a dwell time of 30 secondsper step. The (W2Ti)C2Tx MXene films made from vacuum filtration were annealed in the vacuumat 200 °C for 18 hours to remove the water entrapped between the layers and then analyzed usingXRD. The traditional XRD plots were obtained by merging and integrating the XRD2data in DIFFRAC.SUITE EVA software. For Rietveld refinement, the XRD patterns were recorded usinga Lynxeye detector and scanned from 5 ° to 75 ° with a step size of 0.02 ° and dwell time of 1second per step.

[0132] Scanning electron microscopy (SEM) was performed on a JEOL JSM-7800F at anacceleration voltage of 15 kV to study the flake size and surface morphology. The solution concentration was maintained at < 0.1 mg / mL and loaded on an anodic disc followed by vacuum drying for 2 hours. The samples were gold sputtered to reduce the charging. Energy dispersive X- ray spectroscopy (EDS) measurements were conducted using an EDAX Octane Silicon Drift Detector in point scan mode with a 30s exposure time. The EDS data was subsequently analyzed using EDAX TEAM software.

[0133] Cross-sectional lamella for scanning transmission electron microscopycharacterization was prepared by conventional focused ion beam (FIB) lift-out on a Zeiss Nvision40. The thinning process was followed by low-energy final polishing in the FIB (2^kV and 60 pA) to minimize the ion-induced damage and to obtain a foil with a uniform thickness. During the preparation process, the site of interest was covered with a protective layer through FIB- assisted carbon deposition.

[0134] Scanning Transmission Electron Microscope (STEM) images were acquired in thehigh-angle annular dark-field (HAADF-STEM) condition using a probe-corrected Thermo Fisher Scientific Spectra 200 S / TEM operated at 200 kV with a beam current of 100 pA. This microscope is equipped with an ultra-high-brightness cold field emission gun (X-CFEG), a Super-X EDS system comprising four silicon drift detectors, and Velox acquisition software. Energy-dispersive X-ray spectroscopy (EDS) data were collected as spectrum images, in which a focused electron probe was scanned in raster across a region of interest in STEM mode.

[0135] X-ray photoelectron spectroscopy (XPS) spectra were collected for each sample ona Thermo K-Alpha XPS system with a spot size of 400 μm and a resolution of 0.1 eV. All spectraAttorney Docket No. IUIC-170 were processed using Thermo Avantage, which is a software package provided through ThermoScientific.

[0136] All SIMS measurements were conducted using the CAMECA IMS SC Ultrainstrument with a cesium ion source. To achieve atomic depth resolution, several procedural modifications were implemented as outlined previously: high-angle incidence bombardment (75°), ultra-low impact energy (100 eV), in-situ ion polishing, optimization of extraction settings, the use of a super cycle, and precise beam positioning. Furthermore, a deconvolution and calibration protocol was employed to quantify the data and accurately determine the composition of each atomic layer.

[0137] Example 4: Computational Studies

[0138] Density functional theory (DFT) calculations were carried out using the Vienna abinitio Simulation Package (VASP). The projector augmented (PAW) method was used to describe the wave function of the ionic cores; then the generalized gradients approximation (GGA) was used with Perdew-Burke-Ernzerhof (PBE) functional. The cutoff energy for bulk and surface geometry optimization was 520 eV and 400 eV, respectively. The Monkhost-pack mesh of 6 × 6 × 1 and 3 × 3 × 1 was used for bulk and surface geometry optimization, respectively. The break condition for ionic relaxation was set as 1E-6 eV. The geometry optimization was stopped when the forces on atoms were smaller than 0.02 eV / Å. To account for the dispersion forces, Grimme’s DFT-D2 method was employed, which has been proven sufficient for layered structures. The computed lattice for W containing MXene is a = b = 3.04 Å, and c = 20.22 Å which agrees well with reported values in the literature.

[0139] To study the surface chemistry and adsorption interaction between the reactionintermediates and the catalyst surface, periodic 3 × 3 surfaces that are passivated by 18 O atoms with and without intermixing between Ti and W were cleaved along the (001) direction from the optimized bulk structure. Furthermore, the reaction proposed by equation (4), where m and n represent numbers of Ti and W atoms in the etched layer, is used to mimic the selective etching process. Since the etching process takes place with HF, the formed products are likely WF6, TiF4, CF4, and H2. ^^^^^^^^ + (14^^ + 14^^)^^^^ + 36^^2^^ → 2^^^^^^^^^^^^^^ (18^^) + ^^^^^^6 + ^^^^^^^^6 + 2(^^ + ^^)^^^^4 +(7^^ + 7^^ + 18)^^2 (1)Attorney Docket No. IUIC-170The reaction energy, ∆^^, for etching were then calculated using equation (2) based on reactionproposed by equation (1).∆^^ =2∗^^^^^^^^^^^^^^^^(18^^)+^^∗^^^^^^6+^^∗^^^^^^^^4+2(^^+^^)^^^^^^4+(7^^+7^^+18)^^^^2−^^^^^^^^^^−(14^^+14^^)^^^^^^)−36^^^^2^^^^+^^ (2)− (3)where ∆^^ represents the total energy change upon reaction, which can be directly obtained from DFT calculations. ∆^^^^^^ and ^^∆^^ represent the changes in zero point-energy and entropic contribution, which can be calculated using statistical dynamics approximation using vibrational frequencies from DFT calculations as discussed in the literature.

[0140] For all the gas phase calculations, the molecules, i.e. H2 and H2O, were introducedinto a box with dimension of 25 × 25 × 25 Å for geometry optimization using single Γ k-point. Furthermore, the total energy of H, OH, and O was calculated using H2 and H2O as reference states as described by equation 1-3.^^^^ =1 2 ^^^^2(4)^^ =1 ^^^^ ^^^^2^^ −2 ^^^^2(5)(6)

[0141] Example 5: Electrochemical HER studies

[0142] W2TiC2Tx MXenes were drop-cast onto glassy carbon electrodes using Nafion as abinder. This process involved mixing 5 mg of W2TiC2Tx MXene with a 5% solution of 20 μL Nafion 117 in the water and ethanol mixture, followed by the drop-casting of 20 μL of this mixture on the glassy carbon electrode. W2TiC2TxMXene was then examined in a three-electrode electrochemical cell using a rotating disk electrode apparatus (AUTOLAB, Metrohm) at 1600 RPM in 0.5 M H2SO4. The W2TiC2TxMXenes on glassy carbon were used as the working electrode, Pt-sheet as the counter electrode and Ag / AgCl as a reference electrode. The electrochemical HER studies include linear sweep voltammetry (sweep rate of 1 mV / s).

[0143] Example 6: Transport properties

[0144] Thin film of W2TiC2Tx MXene was prepared by vacuum filtration of a suspensioncontaining delaminated single sheets. To remove the intercalated water between the stacked MXene flakes, the free-standing MXene films were annealed at 200 °C in vacuum. Subsequently, the sample was annealed at 500 °C in the vacuum, which is a known as the approach of removingAttorney Docket No. IUIC-170 the –OH functionalization, typically causing the drop of resistivity. The thickness of the film was estimated from SEM images ~ 5 μm. To measure transport properties, the sample was wired by four probe method, using copper wires and silver paint by TedPella, with distance between contacts app.1.5 mm.

[0145] Temperature dependence of resistance of thin film (R(T)) in the temperature rangeof 1.8-300 K and magnetoresistance (magnetic field -9 - 9 T) at 1.8 K were measured using Quantum Design PPMS DynaCool system. Excitation current was 0.1 mA with a frequency 18.3 Hz. Data were collected in the temperature sweep mode from 300 K down to 1.8 K. Conductivity was calculated using measured film thickness and distance between contacts. Data were fit by models of Variable Range Hopping and thermally activated Arrhenius conductivity, using custom- written Matlab script(s).

[0146] Example 7: Linear and nonlinear optical measurements

[0147] Linear and nonlinear optical absorption measurements were conducted on a thinfilm sample of W2TiC2TxMXene. The thin film samples were prepared via spin coating (Laurell WS 400BZ-6NPP / Lite spin coater) of ~ 2 mg / ml W2TiC2Tx MXene solution on a fused silica substrate. Before spin coating, the fused silica substrates were plasma etched (PE-50 Compact Benchtop Plasma Cleaning System) for 30 seconds. The thin film samples were prepared using single-step spin coating at 800 RPM for 60 seconds and were vacuum-dried before optical measurements. For optical absorption data, the first transmission of the sample (Tsample) and a bare substrate (Tsubstrate) was found using the RC2 ellipsometer system made by J.A. Woolam company. The absorbance was calculated using the formula A= Log (Tsample / Tsubstrate).

[0148] Nonlinear transmission data was found using an I-scan technique where a calibratedsilicon photodiode measured the transmitted power as the incident power increased. An 800 nm pulsed Q-switched femtosecond laser with a pulse width of 100 fs and a repetition rate of 1 kHz was used. Additionally, to mitigate thermal effects, a shutter opened for each measurement, only allowing for approximately 5 pulses to be incident on the sample. For each incident power, 80 measurements were averaged together. The I-scan was automated to minimize any external influence. Additionally, multiple reference measurements through the course of the I-scan at the lowest incident power determined no material damage occurred. For the calculation of fluence, the beam width was measured to be about 150 ^m using a knife-edge experiment.Attorney Docket No. IUIC-170

[0149] Example 8: Raman spectral measurements

[0150] Raman spectral measurements were performed using an alpha300R Witec Spectralimaging system with 1 mW of 532 nm laser light focused to an approximately diffraction-limited spot size of 360 nm with a 100X / 0.95 NA objective in a backscattering arrangement. Scattered light was dispersed with a Czerny-Turner spectrometer using a 300 l / mm grating resulting in a spectral accuracy of <1.5 cm-1. Average spectra were obtained that were representative of a given sample.

[0151] Results and Discussion

[0152] The only available precursor to make a tungsten-based M3C2 MXene is the non-MAX nanolaminated ternary carbide (W,Ti)4C4-y. This phase does not have any A-group layers and the corresponding metallic M-A layers, and there have been no known successful attempts for selective etching to remove atomic layers to form 2D carbide flakes of MXenes. To investigate whether selective etching of any atomic layer (W- or Ti-layers) is possible, computational methods using DFT were used to identify which atomic layer is most energetically favorable to be removed to yield a 2D carbide MXene, similar to the A-group layer removal in a MAX to MXene synthesis. The (W,Ti)4C4-y precursor was reported to consist of one pure W layer with ~ 25 at% vacancies (shown as M1 in FIG.1), two W-rich layers (labeled as M2) on either side of the M1 layer, and a Ti-rich layer (M3) in between the M2 layers (FIG. 1). A “perfect” (W,Ti)4C4-y structure with no W-Ti intermixing and no vacancies (M1 = 100 at% W, M2 = 100 at% W, M3 = 100 at% Ti) was first modeled. According to equations 1 and 2 (supra), and as shown in FIG.2, formation energies toward removal of any layer are endothermic, with Ti M3 layer removal at +9.09 eV / Ti and removal of the W M1 layer being +3.21 eV / W. While both reactions are endothermic, these values suggested a preference for selective removal of the W M1 layer over TiM3 layer from the precursor, which could yield an idealized ordered double transition metal (DTM) MXene W2TiC2Tx structure.

[0153] To investigate the effect of transition metal occupancy intermixing and the role ofdefects (vacancies) on the synthesizability of this potential 2D carbide W2TiC2Tx, intermixing between Ti and W in M2 and M3 layers were next considered in DFT models to match the previously reported (W,Ti)4C4-y phase composition. Although results for the reported structure with intermixing were still endothermic, they showed lower formation energies at +5.74 eV / Ti and +2.63 eV / W (FIG.3). Next, a structure with 25 at% W vacancies in the W M1 layer in both non-Attorney Docket No. IUIC-170 intermixed and intermixed structures was simulated. The inclusion of 25 at% W vacancies resulted in negative formation energies at -3.75 eV / W and -4.81 eV / W for the non-intermixed and intermixed structures, respectively (FIG.4). As a result, the DFT calculations suggest the presence of vacancies in the W M1 layer are keys to possible selective etching of the W M1 layer to yield a W2TiC2-like structure, and intermixing of W- and Ti in the M2 and M3 layers makes the selective etching even more energetically favorable, as shown in FIG.5.

[0154] Following the DFT prediction of the possibility of selective etching of the W-layersfrom (W,Ti)4C4-y to make MXene, (W,Ti)4C4-y precursor samples were prepared by mixing W:Ti:C in 2:1:2 molar ratio with 1.1 moles of Al and sintering the mixed elemental powders at 1600 °C for 4 h under flowing argon (FIG. 6). The X-ray diffraction (XRD) pattern of the synthesized precursor is shown in FIG.7, which shows (W,Ti)4C4-yas the main phase, and impurities such as WAl4 and TiAl3 in agreement with the previous study. Because a 1.1 mol Al was used in the synthesis and the main (W,Ti)4C4-y phase itself does not contain Al, this phase was denoted as 1.1Al–(W,Ti)4C4-y. After preparing the precursor 1.1Al–(W,Ti)4C4-ypowder, the powder was added to 28.4 M HF for 96 h at 55 °C, a similar etching condition for the synthesis of a Mo- containing M3C2Tx MXene. After etching, the XRD patterns of as-synthesized and HF-etched 1.1Al–(W,Ti)4C4-ysamples (FIG.7) were examined to identify any evidence of MXene formation. The first indication of 2D MXene formation from a precursor with 3D crystalline structure is usually the broadening and shift to lower 2θ of the (002) peak. However, the XRD pattern after HF-treatment shows (W,Ti)4C4-yremained unchanged with no discernable broad peaks lower thanthe precursor (002) peak at ~ 9.1° 2θ. The only change in the XRD patterns in FIG. 7 is thedisappearance of some peaks after HF treatment attributable to the dissolution of WAl4 and TiAl3, in agreement with the first report on (W,Ti)4C4-y, where no MXene was reported.

[0155] Recently, it was shown that the inclusion of excess content of Al beyond thestoichiometric one mole during the reactive sintering of Ti3AlC2 MAX phase resulted in larger grain sizes and fewer defects (fewer metal vacancies and oxygen in the carbon sublattice) than those of the typically stoichiometrically prepared Ti3AlC2. While the excess Al (~ 2 moles instead of the required one mole) did not go to the structure, it was reported that its presence enhanced the quality of the resulting Ti3AlC2 MAX phase and eliminated any oxygen in the carbon sublattice. Overall, these studies suggested that the presence of ~2 moles of Al during the MAX sintering,Attorney Docket No. IUIC-170 while not going into the MAX structure, controls the stoichiometry and oxygen substitution in the carbon sublattice of the resulting MAX phase.

[0156] Therefore, two moles of Al (instead of 1.1 moles) were used, keeping everythingelse unchanged. In other words, W:Ti:C was mixed with the molar ratio of 2:1:2 (the same as before) with two moles of Al (labeled as 2Al–(W,Ti)4C4-y) and the elemental mixture was sintered at the same conditions as 1.1Al–(W,Ti)4C4-y (see FIG.6). The XRD pattern of the 2Al–(W,Ti)4C4-ypowder after the 1600 ºC – 4 h showed similar peaks as the 1.1Al–(W,Ti)4C4-y,indicating the majority phase is (W,Ti)4C4-y with some intermetallic impurities (FIG. 8). To better understand (W,Ti)4C4-y phase formation from the starting precursors and the effect of Al, a systematic stepwise ex-situ XRD phase evolution study was conducted for the synthesis of 1.1Al-(W,Ti)4C4-yand 2Al- (W,Ti)4C4-yfrom 800 ºC to 1600 ºC with a 200 °C step and a dwell time of 5 minutes (FIG.49). Our results indicated that Al reacts with Ti and W to form TiAl3 and WAl4 intermetallic phases (FIG. 50), which is essential for the formation of (W,Ti)4C4-y (see Supporting Information discussion and Figure S3). Rietveld refinement was used to analyze the XRD results, as shown in FIG.9. Rietveld refinement methods reveal any global scale (non-localized) changes in transition metal occupancies on the (W,Ti)4C4-y precursor phase based on changing Al content in the powder mixture during the reactive sintering. As XRD methods are not accurate in analyzing carbon or oxygen occupancies, this analysis focused on the transition metal occupancies in the various layers. To communicate the changes in occupancies on the transition metal “layers” within the nanolamellar (W,Ti)4C4-yprecursor, “M1”, “M2”, and “M3” were labeled in the (W,Ti)4C4-yprecursor the same way as described above as to FIG.1, which correspond to the atomic sites (1 / 3, 2 / 3, 3 / 4), (1 / 3, 2 / 3, 0.1277), and (0, 0, 0), respectively. Comparing the occupancy of the transition metals in each site revealed that the use of two moles of Al in 2Al-(W,Ti)4C4-y precursor changed W (Ti) occupancy in the M1 site at +3 at% (0 at%), M2 site at +10 at% (+1 at%), and M3 site at +9 at% (-14 at%), compared to those of 1.1Al–(W,Ti)4C4-y (FIG.9). These changes indicate that more W appears to be in 2Al-(W,Ti)4C4-y precursor as compared to the 1.1Al-(W,Ti)4C4-y, which might play a role in the formation and stability of a MXene from this structure. Using our Rietveld analysis (χ2< 0.1), it was determined that the M1 layer was solely occupied by W layers with vacancies, which DFT calculations indicated as the most favorable atomic layer for selective etching to form a 2D structure.Attorney Docket No. IUIC-170

[0157] After confirming the successful synthesis of the precursor, the 2Al–(W,Ti)4C4-y wasnext placed in 28.4 M HF for 96 h at 55 °C with similar etching conditions as earlier. Similar to the HF-treated 1.1Al–(W,Ti)4C4-y the removal of Al4W and TiAl3 impurities were observed. However, unlike the results of the 1.1Al–(W,Ti)4C4-y, a new broad peak was observed at ~ 7° 2θ (FIG.8).

[0158] The appearance of the new low-angle broad peak at ~ 7° 2θ after selective etchingresembled lower angle (002) peaks of other MXenes, such as Ti3C2Txat ~ 6-7° 2θ as compared to its precursor ~ 9.5° 2θ (002) peak of Ti3AlC2 MAX. If this peak is assumed to be a shifted (002) peak from the (W,Ti)4C4-y precursor, the new etched phase has a c-lattice parameter (c-LP) of 27.77 Å, which is 8.89 Å larger than the c-LP of the (W,Ti)4C4-yphase, consistent with changes in c-LP of Ti3C2Txfrom Ti3AlC2. As a result, it is possible that the use of excess aluminum in the synthesis of the (W,Ti)4C4-y precursor may have affected the structural occupancies favorable for selective etching to yield a 2D carbide from the nanolamellar (W,Ti)4C4-y phase.

[0159] To investigate the differences in the as-synthesized 1.1Al–(W,Ti)4C4-y and 2Al–(W,Ti)4C4-y precursors, atomic layer-by-layer resolved secondary ion mass spectrometry (SIMS) methods were used to investigate the composition at each atomic layer in the layered precursor (W,Ti)4C4-yphase. SIMS results confirmed the ordering of transition metals (tungsten and titanium) in both precursors, as shown in FIG.51 and FIG.52, with M1 layer made of W, and M2 and M3 layers being W and Ti intermixed with M2 majority W (~60 to ~70 at% tungsten in 1.1Al– (W,Ti)4C4-yand 2Al–(W,Ti)4C4-y, respectively) and M3 majority Ti (~80 to 70 at.% titanium in 1.1Al–(W,Ti)4C4-yand 2Al–(W,Ti)4C4-y, respectively) as shown in Table 1. The major differences between the two precursors are in the M1 layer vacancy content and the oxygen occupancy in the M1 and C layers. SIMS results on 1.1Al–(W,Ti)4C4-y reveal M1 layer with 34 at% vacancy and W 53 at% and O 12 at% occupancy. Additionally, all the carbon layers in 1.1Al–(W,Ti)4C4-yshow O occupancy. In the carbon layers surrounding the M1 layers, ~ 26 at% O content in C sites was detected (FIG.51 and Table 1). However, when 2.0 moles of Al were used in the synthesis of the (W,Ti)4C4-yphase (that is, 2Al–(W,Ti)4C4-y), SIMS did not detect O in either the M or C sites (FIG. 52). The M1 layers showed 47 at% vacancy and the rest are occupied purely with W atoms (Table 1). Following the SIMS findings, another set of DFT calculations was conducted on structures with similar atomic compositions as FIG.51 and FIG.52, as shown in FIG.53). DFT results show etching of the W layer from 2Al–(W,Ti)4C4-y, with vacancy and intermixing ratios measured inAttorney Docket No. IUIC-170 SIMS, is more thermodynamically favorable (- 4.97 eV / W) than the original prediction (compare FIG. 53 with FIG. 5), which could be due to higher W vacancy concentration in the M1 layer. Additionally, DFT results show even less favorable etching for the 1.1Al-(W,Ti)4C4-y composition based on SIMS analysis, which could be due to the presence of oxygen in the M1 and X layers.Attorney Docket No. IUIC-170 Table 1 1.1Al–(W,Ti)4C4- Probed layer Layer W at% Ti at% C at% O at%Attorney Docket No. IUIC-170

[0160] The Rietveld results indicated substantial transition metal vacancies (~ 60 at%) inthe W M1 layer in the synthesized (W,Ti)4C4-y precursor phases (FIG. 9). To best compare the occupancies as derived by Rietveld, a system with the same W-Ti intermixing and occupancies in different layers as the experimental samples was modeled (FIG.10). The DFT calculations showed that a (W,Ti)4C4-y with occupancies similar to the synthesized 2Al–(W,Ti)4C4-y requires lower energy (-3.06 eV / W) for removal of the W M1 layers compared to the structure with the occupancies of the 1.1Al–(W,Ti)4C4-y(-2.33 eV / W). The lower predicted energy for the selective etching of the tungsten M1 layers in 2Al-(W,Ti)4C4-y agrees with the successful experimental selective etching in FIG.8, where only the HF-treated 2Al-(W,Ti)4C4-y demonstrated a clear, broad peak at ~ 7° 2θ. These findings suggest the slight change in occupancies in the (W,Ti)4C4-ystructure using 2 moles of Al in the starting powder forms a structure with an improved thermodynamic preference for selective removal of the W M1 layer to yield a 2D W2TiC2Tx carbide structure. It is noted that DFT results show that 2Al-(W,Ti)4C4-y is less favorable than the hypothetical (W,Ti)4C4-ywith intermixing and 25 at% W vacancies in the M1 layer (FIG.5), which suggests more control of transition metal occupancies and vacancies can further enhance the etching results.

[0161] To further validate the selective removal of the W layers, the byproducts formedafter HF treatment of 2Al-(W,Ti)4C4-ywere studied. The XRD and energy dispersive x-ray spectroscopy (EDS) in scanning electron microscope (SEM) (FIG. 11) showed WO3 in the supernatant after the first wash of the HF etching treatment, which then is transformed to WO3 due to hydrolysis. Therefore, a proposed reaction mechanism for etching of the precursor 2Al- (W,Ti)4C4-yphase is a two-step chemical reaction is as follows: ¾ (W,Ti)4C4-y + 6HF + H2O ^ (W, Ti)3C2F~0.5O~0.5 + 4 H2 + WF6(7)Followed by hydrolysis of WF6 in water as: WF6+ H2O ^ WOF4+ 2 HF (8) WOF4 + H2O ^ WO2F2 + 2 HF(9)WO2F2 + H2O ^ WO3 + 2 HF(10)

[0162] In order to experimentally analyze the post-HF treated (W,Ti)4C4-y precursors,Rietveld refinement was next conducted to analyze the difference in the non-etched, and non- delaminated crystalline (W,Ti)4C4-y precursor phase remained in each sample after HF treatment. A comparison of the pre-and post-HF treated (W,Ti)4C4-y powders was conducted to identify anyAttorney Docket No. IUIC-170 differences in W occupancy in the remaining non-etched (W,Ti)4C4-yphase compared with the original powder (FIG. 9) to confirm the importance of W occupancy for etchability of theprecursor. When comparing the pre-HF (FIG.9) to post-HF (FIG.12), the 1.1Al-(W,Ti)4C4-y phaseshows small changes in occupancy in the M1, M2, and M3 layers post-HF treatment (within 3 at%) of either W or Ti. However, the W M1 layers of the post-HF-treated 2Al-(W,Ti)4C4-y powder had an increased (~ 10 at%) content of W in that layer as compared to the pre-HF treatment 2Al- (W,Ti)4C4-ypowder (compare Figure S2 with S4). As Rietveld was used to analyze the remaining HF-treated (W,Ti)4C4-y powder, and not the etched with new broadened peaks at ~7° 2θ, these results may show the crystal structure and stoichiometry of the non-etchable precursor powder during the HF-treatment. The increased W content in the M1 layer of the non-etched portion of the 2Al-(W,Ti)4C4-yprecursor demonstrates that slight changes in W content in the layers affect the etchability of the powder at the same conditions and may not yield a 2D MXene. Furthermore, taking 1.1 Al composition as an example, the DFT calculations on models with different vacancy concentrations demonstrated that the stoichiometric M1 layer (no vacancy) results in etching energy as 2.44 eV / W; however, when the W vacancy concentration increases in the M1 layer, the etching reaction becomes gradually more thermodynamically favorable (FIG.13, 1.15 and -2.33 eV / W for 22 % and 60 % vacancy in the M1 layer, respectively). These results provide supportive evidence that W vacancies can promote the etching capability. Therefore, combining this data with the DFT and SEM / EDS by-products analyzed after HF treatment, the presence of ~ 60 % vacancy in the W M1 and the intermixing of W and Ti in the other two layers may be the reasons for the successful selective etching of the W M1 layers in the etching conditions. However, in-situ monitoring methods to directly observe the reaction mechanisms and by-products, such as in-situ gas chromatography or spectroscopic techniques, should be used to further clarify the reaction pathway to form this W2TiC2TxMXene structure.

[0163] After determining the potential reaction mechanism to form a W2TiC2Tx MXenestructure from the 2Al-(W,Ti)4C4-y precursor phase (FIG. 14), a morphological and structural characterization of the precursor, etched powder and delaminated flakes (FIGs. 14-23) was conducted using SEM, XRD, and SIMS. FIGs. 15 and 16 show the 2Al-(W,Ti)4C4-y grains that have layered morphology and plate-like clustered structures, and its XRD pattern with (W,Ti)4C4-yas the majority phase. FIG.17 highlights the effect of HF on the precursor’s morphology which causes the plate-like clusters of the 2Al-(W,Ti)4C4-ypowder (FIG. 15) to disassemble intoAttorney Docket No. IUIC-170 nanoplate-like structures. While the XRD of the powder after HF treatment (FIG.18) still has the precursor peaks, new broad peaks appear at ~ 7, 14, 21, and 28° 2θ, which suggest the formation of the (00L) peaks of MXenes.

[0164] After etching 2Al-(W,Ti)4C4-y for 96 h, the wet powders were then admixed in 6wt% TMAOH at 55 °C for 4 h for TMA+intercalation followed by repeated centrifugation and subsequent vortex mixing for delamination. The resulting dispersed black solution was then processed by final centrifugation at 2380 RCF for 30 minutes to separate single-to-few layer 2D flakes. FIG. 19 displays the cross-sectional SEM of a free-standing film prepared by the vacuum filtration of the colloidal solution. This cross-section image shows the stacking of many 2D flakes and an electron-translucent single flake is shown in the inset of FIG.19. The XRD pattern of the free-standing film shows only broad (00L) peaks reminiscent of MXene (FIG.20). Visually, the aqueous solution color was dark grey and solution processible (FIG.21), and a Tyndall effect was observed (FIG. 22) in a dilute solution which is attributed to the formation of stable aqueous dispersions. The as-fabricated, additive-free, free-standing film (FIG. 23) was brittle in nature with metallic luster and did not require any post-treatment. The SIMS results of W2TiC2Tx MXene film further displayed the out-of-plane ordered structure (FIG.54), where the outer M layers are majority W (~ 70 at%) and the inner M layer is majority Ti. Comparing the SIMS results of theMXene (FIG. 54) with those of the precursor (FIG. 52), showed each M layer content remainsunchanged (within ± 2 at%), confirming the selective etching (removal of M1) with minimum to no change to the inner M layers (M2 and M3 of the precursor). Based on SIMS results, the surface terminations (Tx) comprise or consist of O ~ 36 at%, OH ~ 29 at%, and F ~ 44 at%, as shown in Table 2. SIMS results further confirm that the synthesized W2TiC2Tx MXene is a carbide MXene and not an oxide or oxy-carbide nanomaterial, and it is an out-of-plane ordered MXene.Attorney Docket No. IUIC-170 Table 2 Probed layer from the top Layer W at% Ti at% C at% O at% OH at% F at% ^^4444theformation of 2D MXene layers, scanning transmission electron microscopy (STEM) was conducted in the high-angle annular dark-field condition (HAADF). The cross-sectional atomic resolution images of the MXene layers acquired via aberration-corrected STEM show that the horizontally aligned 2D layers are made of three-atom-thick layers (FIG.24). Individual layers at higher magnifications (FIG.25) showed that the atoms in the outer layers had higher brightness than the atoms in the inner layer. These STEM results confirmed the 2D flakes are made of three- atom-thick layers of metals and the outer atoms are heavier than the middle atoms, which appears analogous to an ordered 2D double-metal M3C2 MXene with W-rich outer M layers and Ti-rich inner M layer. STEM results combined with SIMS agree with the DFT predictions (FIG.10) that pure W layers are etched from the (W,Ti)4C4-yprecursor structure and results in W2TiC2TxMXene. The plan-view STEM micrographs of W2TiC2Txflakes (FIG.26) further show the planar view of the 2D MXene sheets. The atomic resolution STEM micrographs include the EDX spectra and line scans which confirm the presence of W and Ti ordering in the layered structure (FIG.27).

[0166] After investigating the cross-section of the free-standing films and single-to-fewlayered flakes analogous to a M3C2 MXene structure, the Raman spectrum of W2TiC2Tx MXenefilm was measured and compared with the well-studied Ti3C2Tx MXene (FIG. 47), which has anAttorney Docket No. IUIC-170identical M3C2Tx structure. The spectral positions were determined by fitting the average Ramanresponse of W2TiC2Tx and Ti3C2Tx MXene to a sum of Lorentzian peak shapes consistent in methodology to the previous reports (FIG. 48). Similarities in the average Raman spectra were observed for both the MXenes having a characteristic response that is consistent with a “pseudo” P63 / mmc space group. For example, W2TiC2Tx MXene exhibits the expected spectral responses in three distinct energy regions termed flake (50-270 cm-1), Tx (230-500 cm-1), and carbon (470-800 cm-1) bands corresponding to vibrations of C-(W,Ti)-O, surface terminations, and carbon groups, respectively. The similarities in Raman response between the two structures and their consistency with previous reports further corroborate the conclusion from XRD and STEM on the W2TiC2Tx MXene structure.

[0167] Moreover, based on observations, analyzing the broadening and energy shifts in theTi3C2Tx and W2TiC2Tx MXene compositions provides insight into how the W incorporates into the lattice structure, influencing the bonding and scattering. Notably, the broadening in W2TiC2Tx MXene is larger in the flake and carbon spectral regions, which probe the bonds with the W-Ti species (FIG.47). In contrast, the intermediate energy in the Tx region has a spectral width that is relatively unaffected. The addition of heavier W atoms disrupts the periodicity of the crystal lattice causing the movement of these atoms or the bonds attached to them. This results in shortened phonon lifetime, which is observed as a broadening of the Raman response in both the flake- and carbon spectral regions. In addition to broadening in the spectra, substantial and quantified peak shifts are indicative of bond strength alterations that occur with the addition of W. The phonon energies increase with lighter atoms and stronger bonds. The low energy flake spectral feature is indicative of a group vibration that red-shifts in W2TiC2Tx MXene compared to Ti3C2Tx MXene. This is attributable to the introduction of substantially heavier W. On the other hand, phonon energies for both the Txand carbon regions blue-shift with the addition of heavier W suggesting a significant increase in bond strength for these vibrations. Quantitatively, the intensity in the carbon region is found to increase from 730 cm–1to 774 cm–1from the Ti3C2Tx to the W2TiC2Tx MXene, respectively, despite the increase in atomic mass (FIG.48). This large increase in energy indicates that the addition of heavier W in the W2TiC2Tx MXene induces a substantial change in the bonding environment portending implications for the chemical, optical, and mechanical properties.

[0168] X-ray photoelectron spectroscopy (XPS) of the W2TiC2Tx free-standing film werenext obtained (FIG.28 to FIG.31). Full XPS spectra and deconvolutions of the precursor powdersAttorney Docket No. IUIC-170 with the peak deconvolution tables are also shown in FIG.55 and Table 3, respectively. If W-Ti oxide or carbo-oxide complexes are formed, they should be reflected in XPS patterns with clear and dominant TiOx or WOx patterns. However, for the W 4f and Ti 2p pattern (FIG.28 and FIG. 29, respectively), the deconvoluted MOxpeaks were either lower in intensity or non-existent compared to the M-C peaks, which is different from oxides produced from carbide structures. This XPS data does suggest that the transition metals bond in M-C or Tx-M-C states, which is consistent with the coordination states of transition metals within previously reported MXenes. Further analysis of the C 1s or O 1s spectra (FIG. 30 and FIG. 31, respectively) also shows consistent behavior with -O or -OH terminated MXenes. By analyzing these peaks and the F-content, the surface group content can be roughly calculated to be that of about 37 at%, 28 at%, and 35 at% O, (OH), and F, respectively, which is consistent with a mixed surface termination W2TiC2(O0.74(OH)0.56F0.70) MXene, which is in agreement with the SIMS data shown in FIG.54. Therefore, based on the STEM, Raman, and XPS data shown in FIGs. 24, 25, 28-31, and 47, combined with the XRD, SEM, and SIMS data presented in FIGs.1, 5-8, 10, 14-23, 51 and 52, the HF and TMAOH treatment of the 2Al-(W,Ti)4C4-y precursor powder produced a W2TiC2Tx 2D MXene.Attorney Docket No. IUIC-170 Table 3 Area Ratio (relative to in[non-MAX (W,Ti)4C4-yphase, the HER behavior of this W2TiC2TxMXene was characterized. DFT methods were first employed to study the surface interactions and electrocatalytic activity during HER. The intermixed surfaces of Ti and W were simulated to observe the adsorption behavior of H with varying terminations (FIG.33). Because of the existence of the W0 peaks found in XPS (FIG.29), in other words, undercoordinated surface W atoms, some surface termination vacancies were considered in DFT to study their effects on H adsorption. Additionally, it is believed that, by interacting with an acidic environment and potential bias, the surface termination –O atoms are reduced to –OH groups. For this reason, only –OH and –F terminations, along with their corresponding vacancies, are considered for HER (FIG.33) and their free energy diagram (FED) indicating the adsorption energies is shown in FIG.34. Calculations suggest H adsorption energies are -1.55 and -0.37 eV over the OH- and F-terminated surfaces, respectively. Over the OH- terminated surface, H is adsorbed atop of the W atom, FIG.33, while on the F-terminated surface,Attorney Docket No. IUIC-170 it forms a 3-fold site with two W and one Ti, FIG. 33. At the –OH and –F vacancy sites, H adsorption is also at the 3-fold site with two W and one Ti (FIG.33), with adsorption energies of -0.57 and -0.71 eV, respectively. On the non-intermixed surface (pure W), the H adsorption is on top of W (FIG. 35) with the binding energy of -1.79 eV and -3.71 eV on OH- and F-terminated surfaces (FIG. 36). However, when considering the –OH and –F vacancy sites, the preferred binding sites for H become the vacancy sites (3-fold site) with the same H adsorption energy of - 0.63 eV (FIG.35 and FIG.36). By computing the overall DOS of OH- and F-terminated surfaces with and without Ti-W intermixing (FIG.37), the DOS of the intermixed surfaces shifts to lower energy compared with the non-intermixed surfaces. The d-band center of the intermixed OH- and F-terminated surfaces are lower in energy (both at -2.17 eV) compared with the non-intermixed cases (-1.96 and -2.12 eV for OH- and F-terminations). According to the d-band center theory, a lower d-band center results in weaker binding energy, which explains the reason for weaker H adsorption on the intermixed surfaces. Additionally, the mixed OH- and F-terminated MXene surfaces are considered to investigate the HER activity (FIG.38). Over the non-intermixed surface, the H adsorption over the periodic surface is 0.16 eV; however, considering the –OH and –F vacancies, the H adsorption becomes -0.84 and -1.06 eV, respectively (FIG.38 at panel D). On the intermixed surface, H adsorption over the periodic surface is -0.31 eV, and the adsorption over – OH and –F vacancies results in H adsorption energies of -0.67 and -0.77 eV (FIG.38 at panel H).

[0170] Further, the HER catalytic behavior of W2TiC2Tx MXene was experimentallystudied. It has been observed that HF-TMAOH MXenes possess TMA+cations on their surfaces, which hinder their adhesion to glassy carbon electrodes. Consequently, the minimally intensive layer delamination (MILD) route was employed using HCl-LiF to synthesize W2TiC2Tx MXene, which led to the successful etching with a lower yield (FIG.39) as discussed in the Methods section above. W2TiC2TxMXene was first drop-cast onto glassy carbon electrodes using Nafion 117 as a binder. The prepared W2TiC2Tx MXene electrode was then examined in a three-electrode electrochemical cell using a rotating disk electrode apparatus at 1600 RPM in 0.5 M H2SO4. In the resultant linear sweep voltammograms (LSVs), W2TiC2TxMXene was found to be a more active catalyst toward electrochemical water reduction reaction (FIG. 40). The HER activity of HF- etched W2TiC2Tx MXene multilayer powder and HCl-LiF delaminated W2TiC2Tx MXene were compared. The LSV results show an overpotential of 144 mV for delaminated W2TiC2TxMXene, which is about 1.55 times lower than that of etched multilayered W2TiC2TxMXene (252 mV).Attorney Docket No. IUIC-170 DFT calculations were also carried out to investigate HER activity over O-terminated Mo2CTxMXene surface. The HER activity of W2TiC2Tx MXene was compared with that of Mo2CTx MXene. According to DFT calculations (FIG.56), the hydrogen binding energy over the Mo2CTx MXene surface (shown in green) is -0.45 eV, with an associated overpotential of 0.45 V. For reference, literature reports 0.35 and 0.23 V overpotential. In comparison, the limiting potential for W2TiC2Tx MXene is 0.16 V (indicated in orange), suggesting that W2TiC2Tx MXene is more active toward HER than Mo2CTxMXene, consistent with the experimental results (FIG.40). The Tafel slopes derived from the potential vs. current density plots (FIG. 41) illustrate that delaminated W2TiC2Tx MXene continues to outperform as compared to other etched multilayered W2TiC2TxMXene with a slope of 70 mV / dec. This enhanced HER activity of W2TiC2TxMXene may be linked to the highly active and ordered basal-plane of W and the presence of vacancies.

[0171] The HER catalytic behavior of W2TiC2Tx MXene was compared to other MXenes,such as W1.33CTx MXenes and Mo2CTx MXenes, and it was found that delaminated W2TiC2Tx MXene overpotential (144 mV) is about 1.75, 1.97, 2.22, and 1.36 times lower than that of etched multilayered W2TiC2Tx MXene (252 mV), W1.33CTx MXenes (320 mV), and Mo2CTx MXenes (196 mV), respectively. In comparison with other MXenes, W2TiC2Tx MXene outperformed electrochemical HER as shown in FIG.40, presenting a new and promising catalyst for HER with the highly HER active W-based basal planes. To investigate the intrinsic per-site activity of W2TiC2Tx MXene, average turnover frequencies (TOFavg) were calculated based on the theoretical surface coverages as active sites (FIG. 57). The TOFavgof delaminated W2TiC2TxMXene was estimated to be ∼0.01 H2s−1at a 144 mV overpotential. Additionally, to investigate the effect of exposed basal plane surface area offered by W2TiC2TxMXene, electrochemically active surface area was measured (FIG. 58). To assess the durability of the W2TiC2Tx MXene electrode, a stability analysis was conducted in an acidic electrolyte (0.5 M H2SO4) over 24 h, which confirmed its stable aqueous performance (FIG.59).

[0172] The electronic and optoelectronic behavior of W2TiC2Tx MXene were nextinvestigated. The resistivity R and conductivity c temperature dependence measurements (FIG. 42) indicated R increasing with increasing temperature, with RT conductivity equal to 427 S / cm, which is comparable with the V2C MXene (c =384.6 S / cm). The measured conductivity dependence of W2TiC2Tx MXene film can be described by two models: thermally activated Arrhenius type and low-temperature variable-range-hopping (VRH) model. The data were fittedAttorney Docket No. IUIC-170 using these two models, separately for the low and high-temperature parts. Low-temperature data of temperature dependence of conductivity are best described by the VRH model (FIG.43) with coeff. 1 / 2, which corresponds to one-dimensional conductivity or Coulombic effect. Low- temperature conductivity following the VRH model can be mostly explained by the inter-flake hopping due to the flake-like structure of thin films, as has been demonstrated for several MXenes. The high-temperature response, however, follows an Arrhenius model best. Magnetoresistance of W2TiC2TxMXene film is positive at 1.8 K (FIG. 44), which is typical for standard metals and semiconductors when the applied magnetic field in a direction perpendicular to the film causes drift of electrons, hence the increase of resistivity with increasing magnetic field.

[0173] Further, W2TiC2Tx MXene also boasts a unique range of optical properties incomparison to other members of the MXene family. The absorption spectrum of ~ 150-nm-thick spin-coated W2TiC2Tx film (FIG.60) indicates the material is highly absorptive in the ultraviolet spectra and is transmissive in the infrared. These characteristics are similar to that of Mo2TiC2Tx and Mo2CTxMXenes. Additionally, a distinct step-like pattern is seen on the blue side of the spectrum, indicating that the absorption is generated from multiple distinct transitions. While other MXenes (such as Mo2CTx, Mo2TiC2Tx, Nb2CTx, Ti3C2Tx, Ti2CTx, V2CTx) have local minimums or maximums in absorbance spectrum and W2TiC2Txhas a plateau in absorption that occurs near 400 nm. This could suggest the broadening of energy bands or more complex band interactions.

[0174] Next, the nonlinear optical transmission of the W2TiC2Tx MXene was measuredwith the I-scan technique using an 800-nm wavelength femtosecond pulsed laser, revealing reverse saturable absorption (RSA) behavior (i.e. increased input light intensity leads to a higher absorption). As shown in FIG.61, three distinct regions of varying slopes are evident. A common explanation for nonlinear dynamics in absorbing media is based on the absorption cross-sections of various energy levels. For a multi-level system, as is suggested for W2TiC2Txby the linear absorption spectra, plateaus, and other non-monatomic trends arise from changing equilibrium dynamics at different incident power. This unique behavior is not commonly reported in MXene literature. More generally, these results suggest the use of W2TiC2Txfor optical limiting applications or all-optical switching.

[0175] Lastly, the Raman spectrum of W2TiC2Tx MXene film was also measured andcompared with the traditional Ti3C2Tx MXene (FIG. 47). Spectral positions were determined byfitting the average Raman response to a sum of Lorentzian peak shapes consistent in methodologyAttorney Docket No. IUIC-170 to previous reports (FIG.48). Similarities are apparent in the average Raman spectra among both the MXenes with space group “pseudo” P63mmc. More specifically, W2TiC2Tx MXene exhibits the spectral responses in three distinct energy regions termed the flake (50-270 cm-1), Tx (230-500 cm-1), and carbon (470-800 cm-1) bands corresponding to vibrations of Ti-C-O, surface, and carbongroups, respectively. The broadening and energy shifts observed between Ti3C2Tx MXene andW2TiC2Tx MXene compositions are indicative of how the W incorporates into the lattice and impacts bonding and scattering. Notably, the broadening is much larger in the two spectral regions probing bonds with the W-Ti species, namely the lower frequency flake group vibration and the higher frequency carbon region. In contrast, the intermediate energy Tx region arising from surface groups has a spectral width relatively unaffected. The addition of ionically larger and more massive W atoms disrupts the periodicity of the crystal lattice, phonons inducing movement of these atoms or bonds anchored to them will be especially affected. This manifests here as a shortening of the phonon lifetime and the corresponding observed broadening of the Raman response for these flakes and C-modes. In addition to broadening in the spectra, substantial and quantified peak shifts are indicative of bond strength alterations that occur with the addition of W. Phonon energies increase with lighter atoms and stronger bonds. Expectedly, the low energy spectral feature in Ti3C2TxMXene, indicative of a group vibration, red shift in W2TiC2TxMXene, is attributable to the introduction of substantially heavier W. On the other hand, phonon energies for both the Tx and C regions blue shift with the addition of heavier W suggesting a significant increase in bond strength for these vibrations. Quantitatively, the peak of the signal in the C region is found to increase from 730 cm–1to 774 cm–1for the Ti-only, and Ti-W systems, respectively, despite the increase in atomic mass. This large increase in energy indicates that the addition of W induces a large change in the bonding environment portending implications for both optical and mechanical properties.

[0176] In summary, disclosed herein is the theory-guided synthesis of W-based MXeneW2TiC2Tx, by selective etching of the covalently bonded W-layers from a modified nanolaminated ternary carbide (W,Ti)4C4-yprecursor, was synthesized and characterized its structure and composition in-depth using a combination of XRD, XPS, TEM, EDS, Rietveld, SIMS, and DFT methods. W-rich basal plane endows W2TiC2Tx MXene with remarkable electrocatalytic HER performance in terms of lowest HER overpotential (~144 mV) under acidic conditions, which is supported by DFT studies. HER activity of W2TiC2TxMXene may be linked to W's highly activeAttorney Docket No. IUIC-170 and ordered basal planes. Further, the electronic and optoelectronic behavior of W2TiC2Txsuggest that this MXene could be used for optical limiting, all-optical switching, and other photonic and optoelectronic applications.

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[0290] While embodiments have been disclosed hereinabove, the present invention is notlimited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application isAttorney Docket No. IUIC-170 intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

Claims

Attorney Docket No. IUIC-170 WHAT IS CLAIMED IS:

1. A MXene comprising: W2MC2Txwherein W is tungsten; M is a transition metal; C is carbon; and Tx is a functional surface termination.

2. The MXene of claim 1, wherein M is selected from the group consisting of Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.

3. The MXene of claim 1, wherein M is Ti.

4. A method of making an MXene, the method comprising: synthesizing a nanolaminated material having a general formula of (W,M)4C4-y wherein W is tungsten; M is a transition metal; C is carbon; and y is an integer from 1 to 3; chemically etching at least a portion of the nanolaminated material to form an etched material; washing the etched material to produce a multilayered MXene sediment; and isolating the MXene from the MXene sediment.

5. The method of claim 4, wherein synthesizing the nanolaminated material comprises mixing powders of each of W, M, aluminum, and calcined coke in a 2:1:1.1:2 or 2:1:2:2 molar ratio.Attorney Docket No. IUIC-170 6. The method of claim 5, wherein synthesizing the nanolaminated material further comprises sintering the mixed powders at a temperature of approximately 1600 degrees Celsius for approximately four hours.

7. The method of claim 6, wherein synthesizing the nanolaminated material further comprises, during heating, exposing the mixed powders to a generally constant flow of an argon gas with a ramp rate of 3.5 degrees Celsius per minute.

8. The method of claim 5 or 6, wherein sintering occurs under an inert atmosphere.

9. The method of any one of claims 4 to 8, wherein etching comprises mixing the nanolaminated material with a hydrofluoric acid (HF) solution.

10. The method of claim 9, wherein etching further comprises stirring the mixture of the nanolaminated material and the HF solution at approximately 400 revolutions per minute (rpm) for around 96 hours at about 55 °C.

11. The method of any one of claims 4-10, wherein washing comprises washing the etched material with deionized water via repeated centrifugation.

12. The method of claim 11, wherein the centrifugation is repeated for four or five cycles.

13. The method of claim 11 or claim 12, wherein the centrifugation is repeated until a supernatant of the multilayered MXene sediment has a pH of approximately 6.

14. The method of any one of claims 4-13, further comprising adding the multilayered MXene to a tetramethylammonium hydroxide (TMAOH) solution to a delaminated MXene suspension.

15. The method of any one of claims 4-13, wherein M is selected from the group consisting of Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Tc, La, Hf, Ta, W, Re, and a combination of two or more of these.Attorney Docket No. IUIC-170 16. The method of any one of claims 4-14, wherein M is Ti.

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