Methods for the synthesis and alkyl-functionalization of mxenes

Abstract

Methods for the direct synthesis of carbide MXenes by reacting a transition metal with an alkyl halide compound are provided. The MXenes are useful in a wide range of applications and devices, including supercapacitors, batteries, and as electromagnetic interference (EMI) shielding. The direct synthesis methods can produce a variety of carbide MXenes with high purities, including MXenes that have not previously been synthesized, such as MXenes having mixed halide surface functionalization. Also provided are methods for the covalent surface substitution of MXenes with alkyl groups. These alkylated MXenes have applications in catalysis, including for the catalysis of acetylene and ethylene deletion and C-N coupling.

Description

Atty. Dkt. No. 05400-0081 -PCTMETHODS FOR THE SYNTHESIS AND ALKYL-FUNCTIONALIZATION OF MXENES CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority to U. S. Provisional Patent Application No.63 / 777,897 filed on March 26, 2025, the entire disclosure of which is incorporated herein by reference.REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under FA9550-22-1-0283 awarded by the Department of Defense Air Force Office of Scientific Research and under CHE-2318105 awarded by the National Science Foundation. The government has certain rights in the invention.BACKGROUND

[0003] Two-dimensional (2D) materials show unique properties that have led to their extensive study for diverse applications. 2D materials are mainly dielectrics, semiconductors, or semimetals. MXenes are an emerging family of 2D transition metal carbides and nitrides, most of which are metallic conductors. (B. Anasori et al., Nature Reviews Materials 2, 16098 (2017).) Thus, they are actively studied for their promising performance in energy storage, electromagnetic interference (EMI) shielding, and catalysis.

[0004] MXenes are represented by the general formula of Mn+1XnTx(n = 1-4), where M represents one or more early transition metals, X is carbon (C) or nitrogen (N), and Tv surface-terminating atoms or chemical groups. The MXene structure is characterized by n+1 hexagonally packed M layers interleaved with n layers of X atoms. The M layers are capped by the surface termination groups. Examples of MXenes include Ti2CCl2and Ti3C2Cl2.

[0005] The direct synthesis of Ti2CCl2MXene through a reaction between titanium (Ti) metal, graphite and TiCl4has been reported. (Wang, D. et al.. Science 2023, 379 (6638), 1242-1247.) The method bypasses etching of a precursor phase and produces stacked MXene layers with complete and uniform coverage of surface halides. However, the formation of titanium carbide, TiCxby-products in sizable amounts was reported. At lower reaction temperatures, the formation of MXene phases was more favorable than the formation of TiCx, but graphite is less reactive and remained as a by-product. Improvements in the efficiency ofAtty. Dkt. No. 05400-0081 -PCTthe direct synthesis of MXenes have been realized by reacting TiCl3with CH4in a fluidized bed reactor. (Xiang, M. Q. et al.. Innovation 2024, 5 (1).) However, the corrosive nature of titanium chlorides poses a significant problem for widespread adoption of MXenes synthesized by this method. (Kapias, T. et al., Journal of Hazardous Materials 2005, 119 (1-3), 41-52.) Thus, alternative synthesis schemes are needed to improve the purity of MXenes and to expand the choice of precursors available for MXene synthesis to accelerate the accessibility, growth, and production of MXene family members.

[0006] Chemical intercalation and functionalization of low-dimensional materials, including MXenes, can lead to profound changes in their properties and can stabilize them in organic solvents. Covalent attachment or replacement of functional groups typically has been carried out at elevated temperature in pressurized vessels or molten salts. For example, Lewis acidic molten salt (LAMS) etching has been used to form halogen terminated MXenes. (Kamysbayev, Vladislav, et al. Science 369.6506 (2020): 979-983; and Li, Youbing, et al. Nature Materials 19.8 (2020): 894-899.) Among these, MXenes with uniform halogen termination are gaining interest because their uniform surface termination enables precise surface engineering. It has been demonstrated that multiple MXenes with uniform halogen terminations can be obtained through direct synthetic routes. However, uniform surface termination without oxygen makes intercalation and delamination of MXenes very difficult. Therefore, additional methods for synthesizing and functionalizing MXenes are needed.SUMMARY

[0007] Two-dimensional transition metal carbide MXenes, alkylated two-dimensional transition metal MXenes, methods for synthesizing the two-dimensional transition metal carbide MXenes and the alkylated two-dimensional transition metal MXenes, and methods for using the alkylated two-dimensional transition metal MXenes to catalyze a deacetyl enative or deethyl enative C-N coupling of an alkene or alkyne molecule are provided.

[0008] One embodiment of a two-dimensional transition metal carbide MXene has a formula MH+IC» (T T”i-y)x, where 1 < n < 4, M represents one or more early transition metals, T’ and T” represent two different halogen atoms, and 0 <y < 1. An example of these two-dimensional transition metal carbide MXenes include Ti2C(ClyBr1-y)2, where 0 < y <1 and Nb2C(ClyBr1-y)2.Atty. Dkt. No. 05400-0081 -PCT

[0009] One embodiment of an alkylated two-dimensional transition metal MXene has a formula Mn+1XnTx, where 1 < n < 4, M is one or more transition metals, X is carbon or nitrogen, and Txare surface terminating atoms or groups, wherein at least some of the surface terminating atoms or groups are alkyl groups covalently bonded to M atoms of the MXene via C-M bonds.

[0010] One embodiment of a method for the synthesis of a two-dimensional transition metal carbide MXene having a formula Mn+1CnTx, where 1 ≤ n ≤ 4, M is an early transition metal, and Txare surface terminating halogen atoms includes the step of: reacting the early transition metal with one or more alkyl halide compounds at a temperature at which the reaction forms the two-dimensional transition metal carbide MXene.

[0011] One embodiment of a method of alkylating a two-dimensional transition metal MXene having a formula Mn+1XnTx, where 1 ≤ n ≤ 4, M is an early transition metal, X is carbon or nitrogen, and Txare surface terminating halogen atoms, the two-dimensional transition metal carbide MXene comprising a stack of monolayer MXene sheets, includes the steps of: electrochemically reducing the two-dimensional transition metal carbide MXene having the formula Mn+1XnTxin an electrolyte solution comprising an alkyl group-functionalized quaternary organic cation in a polar aprotic solvent, wherein the electrochemical reduction drives intercalation of the alkyl group-functionalized quaternary organic cation between the monolayer MXene sheets and the formation of an ionic quaternary organic cation-MXene compound; and replacing the polar aprotic solvent with a non-polar solvent to induce a carbocation transfer between alky l groups on the alkyl group-functionalized organic cation and early transition metal atoms of the two-dimensional transition metal carbide MXene to form the alkylated two-dimensional transition metal carbide MXene.

[0012] One embodiment of a method of catalyzing a deacetyl enative or deethylenative C-N coupling of an alkene or alkyne molecule includes the step of reacting the alkene or alkyne molecule with ammonia, a primary amine, or a secondary amine in the presence of the alkylated two-dimensional transition metal MXene of a type described herein under conditions that promote the C-N coupling of the alkene or alkyne molecule.

[0013] Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.Atty. Dkt. No. 05400-0081 -PCTBRIEF DESCRIPTION OF THE DRAWINGS

[0014] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0015] FIG. 1, panels (a)-(c): Electrochemical surface engineering of M2CCI2 MXenes. A schematic diagram of a mechanism involving a polarity inversion at MXene surfaces is shown.

[0016] FIG. 2: Deacetylenative C-N coupling catalysis using an Nb2CCl0.8Oct0.2MXenes. The reaction, examples of substrates, and a proposed mechanism are shown.

[0017] FIGS. 3A-3G: CH2Cl2and CH2Br2serve as efficient precursors for MXene synthesis. FIG. 3A shows X-ray diffraction (XRD) patterns of MXenes synthesized from CH2Cl2or CH2Br2. The arrows show the trend of in-plane diffraction peaks shifting to lower angles proceeding from Ti2C to Nb2C to Zr2C. FIGS. 3B-3G show a collection of SEM images of the products shown in (FIG. 3B) Ti2CCh, (FIG. 3C) Ti2CBr2, (FIG. 3D) Nb2CCh, (FIG. 3E) Nb2CBr2, (FIG. 3F) Zr2CCl2. (FIG. 3G) Zr2CBr2.

[0018] FIG. 4 shows XRD patterns of Ti2CCh products after reacting different Ti starting materials with CH2CI2 at 830 °C in a CVD synthesis.

[0019] FIGS. 5A-5E show SEM images of final Ti2CC12 MXenes after reacting with CH2CI2 at 830 °C from a Ti powder (FIG. 5A), Ti sponge (FIG. 5B), Ti foil (FIG. 5C), Ti felt (FIG. 5D), and Ti mesh (FIG. 5E).

[0020] FIG. 6A shows a scanning transmission electron microscopy (STEM) image of Ti2CBr2 flakes. FIG. 6B shows high resolution STEM imaging showing layered atomic columns in a Ti2CBr2particle. Br has a higher atomic number and interaction cross section with the electron beam than Ti and appears brighter in a dark field image.

[0021] FIG. 7A shows an XRD pattern of products from reacting Nb with a mixture of CH2CI2 and CH2Br2 (1:1 molar ratio) at 800 °C (denoted as DS-Nb2CClBr), in comparison to XRD patterns of DS-Nb2CCh and DS-Nb2CBr2 MXenes. FIG. 7B shows a zoomed in view of in-plane (lOlO) diffraction peaks showing the trend of increasing in-plane lattice parameters from DS-Nb2CCh to DS-Nb2CClBr to DS-Nb2CBr2 due to the increasing surface group sizes. FIG. 7C shows an energy dispersive x-ray spectroscopy (EDX) spectrum showing an elemental ratio of Nb: Cl: Br = 2: 1.1: 1.2 in DS-Nb2CClBr, which is close to the expected 2:1:1 ratio. FIG. 7D shows an SEM image of DS-Nb2CClBr. FIG. 7E shows theAtty. Dkt. No. 05400-0081 -PCTcorresponding EDX maps indicating an overall homogeneous distribution of Nb, Cl, Br and carbon. The above evidence demonstrates the successful preparation of Nb2C MXene with mixed (Cl and Br) terminal groups through a direct synthetic route.

[0022] FIGS. 8A-8F: Assessment of the thermodynamic accessibility of MXenes through the enthalpy of MXene direct synthesis and competing reactions calculated by density function theory (DFT). FIG. 8A shows MXenes form when the reactions forming MXene phases are more exothermic than the most competitive side reactions, ΔH* = ΔHMXene− ΔHBy< 0. FIG. 8B shows cubic transition metal carbide by-products are thermodynamically more favorable when ΔH* = ΔHMXene− ΔHBy> 0. Assessment of the feasibility of M2CX2 (M = Ti, Zr, Nb; X = Cl, Br) MXenes by plotting the ΔH* of reactions involving M = (FIG. 8C) Ti, (FIG. 8D) Zr, and (FIG. 8E) Nb and a series of organohalide precursors CH2Cl2(DCM), CHCl3, C2Cl4, C2Cl6, CCl4, CH2Br2(DBM), CBr4. FIG. 8F shows a theoretical comparison of ΔH* targeting Ti3C2Cl2and Ti2CCl2MXenes. All ΔH values have been normalized by the number of M participating reactions.

[0023] FIGS. 9A-9B: Gram-scale synthesis of M2CCI2 MXenes using reduced organohalides in a chemical vapor deposition (CVD) furnace. FIG. 9A shows an XRD pattern of Ti2CCl2synthesized from a titanium sponge. FIG. 9B shows an SEM image of the gram-scale Ti2CCl2product.

[0024] FIG. 10: EDX spectrum of a DS-Ti2CCl2sample for sample purity calculation.

[0025] FIG. 11A shows an SEM image of Ti2CCl2MXenes made from Ti-6Al-4V turnings. FIG. 11B shows an EDX map with 1 at.% V composition after reacting. This demonstrates early transition metal alloys used can retain mixed metal compositions in the MXenes after reacting to yield mixed metal MXenes.

[0026] FIG. 12 shows XRD patterns and elemental ratios of Nb2CCh-TBAC before and after an electrolysis.

[0027] FIGS. 13A-13C: Structural characterization of electrochemically engineered M2C MXenes. FIG. 13A shows XRD patterns and corresponding structure illustrations of an M2CCI2 MXene before and after and electrochemical treatment. The out-of-plane diffraction peaks and interlayer spacings are given. FIG. 13B and FIG. 13C are scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS) N Is spectra of different stages of a M2C MXene showing the morphology and composition differences.Atty. Dkt. No. 05400-0081 -PCT

[0028] FIG. 14 shows powder XRD patterns of alky l functionalized Nb2C MXenes.

[0029] FIGS. 15A-15D: Investigation of organometallic bonding in Nb2CClo.8Buo.2 MXene using solid-state nuclear magnetic resonance (ssNMR). FIG. 15A shows a1H{13C} HETCOR spectrum of Nb2CCl0.8Bu0.2MXene, showing the carbon signals are correlated with proton signals, indicating alkyl groups. The width of signal also suggests anisotropy of bound organic fragments. FIG. 15B shows signal and FIG. 15C shows curve fitting of {1H}95Nb DE-RESPDOR experiments. FIG. 15D shows direct excitation NMR spectra of93Nb under different magnetic fields.

[0030] FIG. 16 shows aNMR spectrum of C-N coupling product from aniline and 1 -octyne.

[0031] FIG. 17 shows a13C NMR spectrum of C-N coupling product from aniline and 1-octyne.DETAILED DESCRIPTION

[0032] Methods for the direct synthesis of carbide MXenes by reacting a transition metal with an alkyl halide compound are provided. The MXenes are useful in a wide range of applications and devices, including supercapacitors, batteries, and as electromagnetic interference (EMI) shielding. The direct synthesis methods can produce a variety of carbide MXenes with high purities, including MXenes that have not previously been synthesized, such as MXenes having mixed halide surface functionalization. The direct synthesis methods can be scaled up efficiently and can use a variety of different transition metal starting materials, including starting materials that include more than one type of transition metal. Also provided are methods for the covalent surface substitution of MXenes with alkyl groups. These alkylated MXenes have applications in catalysis, including for the catalysis of deacetylenative or deethylenative C-N coupling.

[0033] Direct Synthesis of MXenes.

[0034] The carbide MXenes are 2D transition metal carbides having the formula Mn+1CnTx, where 1 ≤ n ≤ 4; M represents one or more types of early transition metal elements, and Txare surface terminating atoms or chemical groups. While Txis generically used to represent surface terminating atoms or groups, the number of which may vary; in some embodiments x is < 2. The MXene structure can be described as n+1 hexagonally packed M layers interleaved with n layers of C atoms. The M layers are then capped by theAtty. Dkt. No. 05400-0081 -PCTsurface terminating groups. In the as-synthesized MXenes, T represents halogen atoms, such as bromine (Br), chloride (Cl), fluorine (F), iodine (I), or a combination of two or more thereof. However, post-synthesis, these halogen atoms can be eliminated or replaced by other surface terminating atoms or groups using, for example, nucleophilic substitution.

[0035] The MXenes include those having only single type of early transition metal element, M, and mixed metal MXenes comprising two or more types of early transition metal elements. In a mixed metal MXene (e.g., (Ti, V)2CCh), the Mn+1CnTxformula is interpreted as follows:wherein 1 ≤ n ≤ 4, and each Miis a different early transition metal either ordered or arranged as a solid solution;0 and = 1, and Txare surface terminating atoms or chemical groups. Mixed metal MXenes can be formed using mixed metal starting materials, as illustrated in Example 2.

[0036] The early transition metals are 3d - 5d block transition metals (Groups 3-7 of the periodic table), including scandium (Sc), yttrium (Y), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), and manganese (Mn). Any two or more of these elements can be combined in a mixed metal MXene. In some embodiments, one of the two or more early transition metal elements includes Ti or Nb.

[0037] The transition metal reactants are generally solid reactants and may be provided, for example, in the form of a metal, including metal alloy, powder, sponge, turning, foil, felt, or mesh on which the reaction occurs. The alkyl halide compound reactants can be represented by the general formula CH4-zXz, where X is the halogen atom that will become a surface terminating atom in the MXene and z is 1, 2 or 3. If a high-purity MXene product is required or desired, it is generally preferable to use lower-valence alkyl halides, such as CH2CI2, CHCh, CH2I2, or CH2Br2, in the reaction. The reaction mixture can include a single ty pe of alkyl halide or a mixture of two or more different alkyl halides, wherein the different alkyl halides contain different halogen atoms.

[0038] Without intending to be bound to any particular theories of the inventions disclosed herein, the ability to produce phase-pure or substantially phase-pure MXenes can be explained by the enthalpies of the reactions forming the MXene products, relative to the enthalpies of the reactions forming competing side reactions that produce non-MXene byAtty. Dkt. No. 05400-0081 -PCTproducts, such as cubic transition metal carbide phases, other carbides, or oxide and / or oxyhalide by-products; based on enthalpy, the synthesis of MXenes is energetically more favorable for the lower valence alkyl halides than for higher valence reactants. By way of illustration, when Ti metal is reacted with the high-valence reactant CBr4, non-MXene phase products are favored to the exclusion of the Ti2CBr2 MXene. However, replacing CBr4with a lower valence alkyl halide, such as CH2Br2, results in the successful synthesis of Ti2CBr2.

[0039] By way of illustration, MXenes can be synthesized with a purity of 85% or higher, 90% or higher. 95% or higher. 98% or higher. 99% or higher, and 99.9% or higher using the methods described herein. The purity of the MXenes can be measured via X-ray diffraction (XRD) full-pattern fitting (Reitveld refinement) and elemental analysis, as illustrated in Example 1.

[0040] The high-purity MXenes can provide benefits over their lower purity counterparts for various applications. For example, high-purity MXenes, including high-purity M2CT2MXenes (e.g., Ti2CCl2) having higher gravimetric capacitance relative to their less pure counterparts and can provide improved performance in supercapacitor and energy storage applications. Additionally, higher-purity' MXenes can be better catalysts in hydrogen evolution reactions relative to less-pure MXenes.

[0041] Another notable benefit of the absence or near-absence of non-MXene byproducts is that the alkyl halide reactants can produce MXenes with mixed halide surface terminations by using two different alkyl halide reactants. Such “mixed halide" MXenes have at least two different surface terminating halide atoms. MXenes having surface terminating groups that consist of only two different halides are represented by the formula Mn+iCn(T’yT”i-y)x, where M, n, and Txare as previously defined, but T’ and T"’ represent two different types of halide atoms, and 0 < y < 1. In some embodiments of the MXenes with mixed halide termination, T’ and T” represent a combination of two or more of Cl, 1, and Br atoms. Such MXenes can be formed using, for example, a reaction mixture containing a mixture of CH2CI2, CH2I2, and CH2Br2, including reaction mixtures of CH2CI2 and CH2Br2. The ability to produce MXenes, including mixed metal MXenes, with mixed halide termination can be attributed to the fact that halogen atoms do not get consumed by non-MXene by-product phases, such as oxyhalides, during the synthesis. Because the formation of halogen-containing non-MXene phases can preferentially remove one halide in a reaction system containing two or more different halides, the formation of non-MXene by-productAtty. Dkt. No. 05400-0081 -PCTphases can make the synthesis of MXenes with mixed halide terminations challenging and may even prevent the formation of MXenes with mixed halide terminations altogether. The ability to produce MXenes with mixed halogen terminations is advantageous because it expands the scope of possible post-synthetic modifications and enables precise control over the electronic structure of the MXenes. This can result in improved and / or tunable properties such as conductivity, work function, and catalytical performance.

[0042] MXene synthesis from reduced organohalides (e.g., reduced forms of CCl4, CI4, or CBr4) is also advantageous as it enables the use of relatively inexpensive, less pure early transition metal starting materials, such as metal sponges and metal alloys, to yield high purity single metal MXenes and mixed metal MXenes from cost-effective starting materials. The ability to form mixed metal MXenes is advantageous because it allows greater flexibility' in synthesizing MXenes with a broad range of physical and chemical properties and surface reactivities.

[0043] The direct reaction of the early transition metals and alkyl halide compounds can be carried out by combining the reactants in a reaction vessel where they react to form the MXene. The reactants can be heated to thermally induce the reaction, increase the reaction rate, and / or increase the yield of the MXene phase over other phases that are formed as products of the reaction. Because the alkyl halides are highly reactive, the reactions can be carried out at relatively low reaction temperatures (e.g., 1000 °C or lower), which facilitates the synthesis of high purity MXene products. The optimal reaction temperature will depend on the reactant being used. Generally, however, temperatures in the range from about 650 °C to 1000 °C, including from 700 °C to 900 °C, are suitable. In the synthesis of MXenes with mixed halide surface termination, the ratio of T’ to T” groups in the product MXenes can be adjusted by altering the molar ratio of T’ -containing and T”-containing alkyl halides in the reaction mixture and / or by adjusting the reaction temperature. By way of illustration only, T’ to T’" ratios in the range from 1: 10 to 10: 1, including in the range from 1:5 to 5: 1 can be achieved. Typically, a higher temperature favors smaller halide (e.g., chloride) incorporation over larger halide (e.g., bromide) incorporation.

[0044] Carrying out the direct synthesis of the MXenes with a hydrogenated transition metal starting material is advantageous, as it can improve the yield of the MXene phase. The transition metal starting material (e.g., powder or bulk metal substrate) can be hydrogenated by, for example, carrying out the reaction in a hydrogen-containing environment, reacting aAtty. Dkt. No. 05400-0081 -PCTtransition metal starting product with LiH, or purchasing a pre-hydrided metal. The hydrogenation of the transition metal reactant can improve the yield of the MXene phase by. for example, suppressing the formation of by-products, such as transition metal oxyhalides. This effect is less important when utilizing reduced alkyl halides as they can create a more reducing environment when heated to reaction temperatures.

[0045] The MXenes are formed as stacks of monolayer MXene sheets, and these stacks can be delaminated to obtain individual monolayer MXene sheets. Methods for delaminating stacks of MXene sheets are known. The monolayer MXene sheets have excellent pseudocapacitive energy storage properties originating from efficient ion intercalation between the 2D monolayers. Their excellent electrochemical energy storage characteristics stem from the combination of large surface-to- volume ratio and high electrical conductivity.

[0046] An alternative method for the direct synthesis of the carbide MXenes is direct synthesis via chemical vapor deposition (CVD). CVD growth is a bottom-up growth process in which the MXenes are formed epitaxially on the surface of an early transition metal growth substrate by the thermal decomposition of the alkyl halide compounds. CVD growth generally proceeds through a series of stages. First, the vapor-phase alkyl halides diffuse to the early transition metal surface of the substrate where they are adsorbed. Reactions betw een the transition metal of the surface and the vapor-phase reactants grow the MXenes from the surface, and gaseous by-products of the reactions are separated from the surface. In preparation for CVD growth, the growth substrate is loaded into a CVD reactor chamber and heated to a temperature at which the MXene growth w ill be carried out - typically a temperature in the range from 650 °C to 1000 °C. The alkyl halide reactants are introduced into the CVD reactor chamber as a vapor comprising the reactants and, typically, an inert gas carrier. The reactants are transported to the substrate surface via fluid transport and / or diffusion where they undergo decomposition and heterogeneous reactions with the transition metal growth surface to form the MXenes. In the CVD synthesis of MXenes with mixed halide surface termination, the ratio of T' to T” groups in the product MXenes can be adjusted by altering the molar ratio of T’ -containing and T”-containing alkyl halides in the reaction mixture and / or by adjusting the reaction temperature.

[0047] Because the reactions on the substrate surface may be thermally driven or controlled, the CVD reactor is equipped with a heater, or other energy source, in thermal communication with the substrate to heat the substrate to a temperature that promotes theAtty. Dkt. No. 05400-0081 -PCTreactions. The CVD reactor will also include a vacuum pump in fluid communication with the reactor chamber to remove the by-product gasses and maintain the chamber pressure at a desired sub-atmospheric pressure during film growth.

[0048] The CVD growth of the MXenes can be used to form a plurality of sheets of MXenes that are bound to the growth substrate along a sheet edge, referred to herein as “carpets.’’ Alignment of the surface-bound MXene sheets in the carpets allows for efficient ion intercalation and fast charging and discharging cycles for battery electrodes made from the CVD-grown MXenes.

[0049] A notable benefit of CVD growth is the ability to easily scale reaction mass compared to direct synthesis using sealed ampoules. This enables MXene synthesis limited only by the size of the CVD tube furnace rather than reagent pressure in a sealed reactor in direct synthesis. In addition, the flow of alkyd halide can be controlled with a mass flow controller (MFC) to increase or decrease the growth of MXenes. This allows for early transition metal substrates to have MXene carpets grown directly on their surface while maintaining the overall physical properties of the substrate. Alternatively, flow or reaction time could be increased to complete the conversion of early-transition metal substrate or reagent into pure MXene powders.

[0050] Covalent Surface Substitution of MXenes to Form Alkylated MXenes

[0051] The surface terminating halogen atoms of the carbide MXenes synthesized using the methods described above can be fully or partially substituted with a variety different surface groups using known chemistries. For example, surface terminating halogens can be substituted with amido- or imido-groups using the methods described in Zhou, Chenkun. et al. " Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido-and imido-terminated surfaces." Nature Chemistry 15.12 (2023): 1722-172.

[0052] Alternatively, the surface terminating halogen atoms of the carbide MXenes and other MXenes, including nitride MXenes, can be fully or partially substituted with alkyl groups using an electrochemical intercalation-based polarity’ inversion and covalent surface functionalization reaction. These reactions, which form X-M bonds between carbon or nitrogen atoms of an intercalated alkyl-functionalized quaternary organic cation and the metal atoms (M) of a MXene, can be used to covalently graft a variety of alkyl groups directly to the MXene.Atty. Dkt. No. 05400-0081 -PCT

[0053] The electrochemical alkyl grafting process is shown schematically in FIG. 1, panels (a)-(c). Initially, the MXene is characterized by an M-T bond polarity in which the metal atom has a partial positive charge and the surface halogen has a partial negative charge (FIG. 1, panel (a)). The partial positive charge on the metal atoms renders those atoms unreactive toward alkyl groups. In the electrochemical grafting methods, the MXene is electrochemically reduced in an electrolyte comprising an alkyl group-functionalized quaternary organic cation. As the result of the electrochemical reduction, the quaternary organic cations become intercalated between monolayer MXene sheets in the stacked MXene structure, the cathodic current inverts the polarity of the metal of the MXene from positive to negative, converting the electrophilic metal atoms to nucleophiles, and surface terminating halogens are eliminated from the MXene. The result is the formation of a quaternary organic cation-MXene ionic compound (FIG. 1, panel (b)). The electrolyte solution is then replaced with a non-polar solvent to promote a nucleophilic substitution reaction between the nucleophilic metal atoms of the MXene and an alkyl group on an alkyl group-functionalized quaternary organic cation, resulting in the elimination of the remainder of the now-neutralized organic cation and the covalent grafting of the alkyl group to the MXene via a carbocation transfer reaction (FIG. 1, panel (c)). The alkylation of the MXenes can be carried out until all surface terminating halogens or replaced by alkyl groups or until only a portion (for example, at least 50%, at least 60%, at least 80%. or at least 90%) of the halogens are substituted by the alkyl groups. As shown in the figure, intercalated MXenes are intermediates of alkylated MXenes. It is possible to stop the reaction at the intermediate intercalation step, isolate the intercalated MXenes, and then trigger the alkylation reaction. However, alkylation can be done as a direct one-pot reaction.

[0054] MXenes synthesized via direct synthesis can be used as the starting material for alkylation via electrochemical intercalation-based polarity inversion and covalent surface functionalization. However, MXenes made using other methods, including MXenes made by top-down and bottom-up methods, can also undergo alkylation via electrochemical intercalation-based polarity inversion and covalent surface functionalization. Thus, the methods can be applied to both carbide and nitride MXenes of the general formula Mn+1XnTx, where 1 < n < 4, where M and Txare as defined previously herein and X is a carbon or nitrogen atom. Other methods that can be used to form the starting MXene include, but are not limited to, those described in PCT application publication number WO 2024 / 044510, and MXenes made from MAX phases through selective etching of the A layers.Atty. Dkt. No. 05400-0081 -PCT

[0055] The alky l group-functionalized quaternary organic cations that act as carbocation transfer agents are bulky quaternary organic cations, such as quaternary ammonium cations and quaternary phosphonium cations. The quaternary ammonium cations have the formula R4N+and the quaternary phosphonium cations have the formula R4P+ where R represents alkyl groups, which may be the same alkyl group or different alky l groups. Suitable alkyl groups include C2 to C12 and C4 to C18 alkyl groups, such as butyl groups, hexyl groups, dodecyl groups, and octadecyl groups. Longer alkyl groups can be used provided their quaternary organic salts are sufficiently soluble in a polar p solvent. Illustrative examples of quaternary ammonium cations include tetrabutylammonium cations (TBA+), and tetraoctylammonium cations (TOA+). Additional illustrative examples include: C4 – C8 symmetric ammoniums, such as tetrapentyd ammonium, tetrahexylammonium, and tetraheptylammonium; and symmetric C4-C8 phosphoniums, such as tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraheptylphosphonium and tetraoctylphosphonium; and asymmetric ammoniums, such as cetyl(tributyl)ammonium, and didecyl(dioctyl)ammonium. Still other examples include ionic liquid phosphonium cations, such as trihexyl(tetradecyl)phosphonium, trihexyl(decyl)phosphonium, and trihexyl(octyl)phosphonium.

[0056] The electrochemical reduction of the metal atoms in the MXenes is carried out in an electrolyte comprising the alkyl group-functionalized organic cations in a polar aprotic solvent. The quaternary' organic cations are generally provided as salts, such as halide salts, and dissolved in the electrolyte solvent. Suitable solvents for the electrolyte solution include, but are not limited to, acetonitrile, propylene carbonate, dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl carbonate, diethyl carbonate, ethyl acetate, and mixtures of two or more thereof.

[0057] The electrochemical cell can be a two-electrode or three-electrode cell in which the MXene is used as a cathode in electronic communication with a counter electrode.Suitable electrode materials for the counter electrode and. if present, the reference electrode, include graphite and Ag / Ag+. However, other electrode materials can be used. When an electrochemical potential is applied, the organic quaternary cations are driven between the monolayer MXene sheets of the stacked, multilayered MXenes, and the electrophilic M atoms are converted into nucleophiles to produce an ionic intercalation compound. Cyclic voltammetry can be used to identify a suitable potential window for the electrochemical cells.Atty. Dkt. No. 05400-0081 -PCT

[0058] After the electrochemical intercalation, the polarity of the environment is changed from polar to non-polar by replacing the electrolyte solution with a non-polar solvent to induce the carbocation transfer of the alkyl chains on the alkyl-functionalized quaternary organic cations to the metal atoms of the MXene, resulting in the covalent functionalization of the MXene with alkyl groups. Suitable non-polar organic solvents include, but are not limited to, toluene, and hydrocarbons, such as pentane, hexane, and cyclohexane.

[0059] The alky lated MXenes can be used as starting reactants for further substitutions of the surface terminating alkyl groups or as catalysts. For example, the alkylated MXenes can serve as C-N coupling catalysts for the addition of ammonia or primary or secondary amines to non-activated alkenes and alkynes. The amines can be aliphatic or aromatic amines. These catalyzable addition reactions are an efficient route to the formation of higher substituted nitrogen-containing products. The use of the alkylated MXenes as C-N coupling catalysts is illustrated in Example 3, below, using an octyl functionalized Nb2C MXene as a representative alkylated MXene. During C-N coupling, the N-H bond of ammonium or amine is added across a double or triple bond. Non-limiting examples of amines and alkynes that can be reacted in a C-N coupling, along with a proposed catalytic reaction pathway are shown in FIG. 2. The specific alkylated MXene, solvents, and reaction temperatures and times shown in the figure are for illustrative purposes only. Other alkylated MXenes, solvents, and reaction temperatures and times can be used.EXAMPLES

[0060] EXAMPLE 1. This example illustrates the direct synthesis of carbide MXenes using alkyl halide reactants.

[0061] T12CCI2, Ti2CBr2, Zr2CCl2, Zr2CBr2, Nb2CCl2and Nb2CBr2MXenes (denoted as “DS-MXenes”) were directly synthesized through high-temperature reactions between hydrogenated transition metal powders and DCM or DBM. (Hydrogenated transition metals can be in the form of commercial hydrided metal, a regular metal with added LiH, or by running the reaction under hydrogen atmosphere.)

[0062] DCM and DBM, which are the reduced forms of " CCh” and ‘ CBr2” units, provided a spontaneous reducing atmosphere. The reaction of Ti, Zr, orNb powders, sponge, or other metal precursors with DCM or DBM gave a full collection of M2CX2MXenes, where M = Ti, Zr, or Nb and X = Cl or Br (FIGS. 3A-3G, FIG. 4, FIGS. 5A-5E). The DS-MXenes synthesized from DCM or DBM showed high phase pun tv. based on XRD (FIG.4).Atty. Dkt. No. 05400-0081 -PCTThe usage of DBM also enabled the direct synthesis of TizCB MXenes (FIGS. 6A-6B).Ti₂CBr₂ is a strained phase, showing a weak out-of-plane diffraction signal. The use of a combination of DCM and DBM produced MXenes with Cl and Br mixed terminal groups (FIGS. 7A-7E).

[0063] DFT calculations were performed to investigate the thermodynamics of the reactions involving a transition metal M = Ti, Zr, or Nb and DCM or DBM. The entropy contributions of these reactions were mostly minimal or cancelled each other out, thus the calculations focused on the standard enthalpy (ΔH) of reactions. A general trend emerged whereby, when a transition metal M was reacted with lower-valence organohalides DCM or DBM (FIGS. 8A-8F), the side reactions forming a transition metal carbide MC by-product phase were energetically less favorable than the formation of the M2CX2 MXenes (X = Cl, Br).

[0064] Materials and Methods

[0065] Chemicals and materials

[0066] Hydrided Ti powder (Ti(H)), containing 3.90 weight percent (wt.%) of H that corresponds to a nominal composition TiHi.94 (-325 mesh, 99%), was purchased from Thermo Fisher Scientific. Titanium sponge (28.4mm & down, 99.7%), titanium foil (0.127mm thick, annealed, 99.99+%), titanium powder (-325 mesh, 99.5%), and titanium gauze mesh (80 mesh woven from 0.13mm diameter wire) were purchased from Thermo Fisher Scientific. Titanium fiber felt (5 x 5 cm) was purchased from Fuel Cell Store.Titanium turnings and shavings (Ti-6A1-4V, nominal composition 6 wt.% Al and 4 wt.% V) were purchased from eBay and rinsed with hexanes and / or acetone to remove residual cooling fluid, then dried in a vacuum oven prior to use.

[0067] CH2CI2 (anhydrous, >99.8%, containing amylene as a stabilizer), CH₂Br₂ (stabilized, 99%), and N-methyl formamide (NMF, 99%) were purchased from Sigma. NMF was purified by distillation before handling in a glovebox.

[0068] Zirconium powder (-50 mesh, 99.5%), and niobium powder (-325 mesh, 99.8%) were purchased from Strem Chemicals. Fused quartz tubes (10 mm O. D., 8 mm I. D.) and rods (7 mm O. D.) for ampoule fabrication were purchased from Technical Glass Products, Inc. Ar gas (ultra-high purity grade), and Ar / Fh gas (5% hydrogen, ultra-high purity grade) were purchased from Airgas.Atty. Dkt. No. 05400-0081 -PCT

[0069] Direct synthesis of M2CX2 MXenes using CH2CI2 and CThBn (M = Ti, Zr, or Nb;X = Cl and / or Br).

[0070] M2CX2 MXenes were synthesized by the reaction of elemental M powder with CH2CI2 or CH2Br2. All the syntheses were conducted under Ar atmosphere in a dry glove box unless otherwise noted.

[0071] Ti₂CCl₂ and Ti₂CBr₂ commercial hydrided Ti powder was transferred into a quartz test tube using a glass funnel, followed by the addition of 0.650 molar equivalents of CH2CI2 or CH2Br2. The test tube w as chilled with liquid nitrogen to reduce loss of CH₂Cl₂ or CH₂Br₂ to vaporization and sealed under vacuum. To initiate the reaction, the ampoules were put inside a muffle furnace and heated to 800 °C (725 °C for Ti₂CBr₂) in 10 min. The temperature was maintained for 24 hours until the reaction was finished. The ampoules were then cooled to room temperature by switching off the furnace.

[0072] Zr₂CCl₂ and Zr₂CBr₂. Zr powder was transferred into a quartz test tube using a glass funnel, followed by the addition of 0.650 molar equivalent of CH₂Cl₂ or CH₂Br₂. The test tube was chilled by liquid nitrogen to reduce loss of CH₂Cl₂ or CH₂Br₂ to vaporization and sealed under vacuum. To initiate the reaction, the ampoules were put inside a muffle furnace and heated to 800 °C (750 °C for Zr₂CBr₂) in 10 min. The temperature was maintained for 24 hours until the reaction w as finished. The ampoules were then naturally cooled to room temperature by switching off the furnace.

[0073] Nb₂CCl₂ and Nb₂CBr₂. Nb powder was transferred into a quartz test tube using a glass funnel, followed by addition of 0.650 molar equivalent of CH₂Cl₂ or CH₂Br₂. The test tube was chilled by liquid nitrogen to reduce loss of CH₂Cl₂ or CH₂Br₂ to vaporization and sealed under vacuum. To initiate the reaction, the ampoules w ere put inside a muffle furnace and heated to 830 °C (750 °C for Nb2CBr2) in 10 min. The temperature was maintained for 24 hours until the reaction was finished. The ampoules were then naturally cooled to room temperature by switching off the furnace.

[0074] Chemical vapor deposition synthesis of M2CX2 MXenes using CH2CI2 and CH2Br2 (M = Ti, Zr, or Nb: X= Cl and / or Br): M2CX2 MXenes were_synthesized using Ti powder, Ti sponge, a Ti foil, a Ti felt, and a Ti mesh as a starting material. The early-transition metal starting material was placed in an inert boat (e.g., alumina or boron nitride) and loaded into the center of the tube furnace. The system was sealed and placed under vacuum for 1 hour followed by refilling the system with an inert gas. e.g., Ar. This cycle was repeated threeAtty. Dkt. No. 05400-0081 -PCTtimes to ensure the reaction was done under inert atmosphere. To begin the reaction, the furnace was heated up to the reaction temperature while flowing 100 standard cubic centimeters per minute (SCCM) of pure inert gas for 20 minutes. Then, the inert gas was bubbled into the desired alkyl halide. This initiates the reaction between the metal precursor and the alkyl halide vapor. At low flow rates (e.g., 2 SCCM of CH2CI2), the reaction was slow enough to grow MXenes mainly on the surface. After prolonged flow at 2 SCCM, all of the metal precursor was consumed yielding MXene powder only. Alternatively, higher flow of 10 SCCM can be used if pure MXene powder is desired to decrease reaction time.Reaction times vary depending on the mass of the starting metal precursor. Often reactions using around 200 mg of metal powder require roughly 1 hour at 10 SCCM of DCM flow to complete.

[0075] FIG. 4 shows XRD patterns of the Ti2CCh products after reacting with the CH2CI2 at 830 °C. XRD patterns of Ti foil, felt, and mesh are run at 2 SCCM Ar into DCM yielding MXene carpets on the surface of each substrate. This is evidenced by the existence of Ti metal in the XRD patterns of each form of the Ti starting material. The Ti powder, sponge, and turnings (the turnings are discussed in more detail in Example 2, below) were synthesized with a higher flow rate of 10 SCCM Ar into DCM to yield pure MXene powders. These had no visible Ti metal in XRD as they were fully reacted to form MXenes.

[0076] FIGS. 5A-5E show SEM images of final T12CCI2 MXenes after reacting with CH2CI2 at 830 °C from starting titanium sources: Ti powder (FIG. 5A), Ti sponge (FIG.5B), Ti foil (FIG. 5C), Ti felt (FIG. 5D), and Ti mesh (FIG. 5E).

[0077] FIG. 9A shows an XRD pattern of Ti2CCh synthesized from 2 g of titanium sponge and excess CH2CI2 at 830 °C for 6 hours. FIG. 9B shows an SEM image of the gramscale Ti2CCh product.

[0078] Dispersion of DS-MXene in polar organic solvents

[0079] DS-MXenes could be easily dispersed in polar organic solvents such as DMF, NMF and propylene carbonate (PC) by hand shaking. The use of NMF gives the best colloidal stability of DS-MXene particles. As an example, 100 mg DS-Ti2CC12 was immersed in 5 mL NMF inside the N2 filled glovebox. After hand shaking or vortexing, the supernatant was collected after centrifuging at 240 g for 15 minutes to remove most insoluble byproducts. Finally, the supernatant containing the MXenes was centrifuged at 12100 g for 15 minutes to precipitate the MXenes, leaving small impurities in solution. The sediment wasAtty. Dkt. No. 05400-0081 -PCTredispersed in fresh NMF to form a stable colloidal solution (up to 70-g L'1) of DS-ThCCh MXene.

[0080] Characterization Methods

[0081] The structure and composition of MXenes were characterized using x-ray characterization, Raman spectroscopy, electron microscopy and a combination of energy dispersive x-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses.

[0082] XRD

[0083] The diffraction patterns in Bragg-Brentano geometry’ were obtained using a Rigaku benchtop X-ray diffractometer equipped with HyPix-400 MF 2D hybrid pixel array detector (HP AD) and a Cu Ka X-ray source (1.5406 A) operating at 40 kV and 15 mA.

[0084] XRD full-pattern fitting (Rietveld refinement) was performed using Bruker TOP AS Version 5 software. The Ti2CCh MXene samples were assumed to contain two major phases: Ti2CC12 MXene (space group P-3mY) and TiC% (space group Fm-3m). The Stephens model (trigonal symmetry) was used to account for the anisotropic peak broadening of the XRD patterns of the MXene phases.

[0085] Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX)

[0086] SEM imaging and EDX elemental mapping were performed in a Carl Zeiss Merlin Field-Emission Scanning Electron Microscope equipped with Oxford Ultim Max 100 Silicon Drift Detectors (SDD). The accelerating voltage was set to 10 kV and beam current was set to 1 nA.

[0087] Scanning transmission electron microscopy (STEM)

[0088] Atomic-resolution characterization of the MXene samples was conducted using aberration-corrected scanning transmission electron microscope (STEM) imaging and spectroscopy experiments at the University of Illinois Chicago. The JEOL JEM-ARM200CF microscope is equipped with a cold-field emission electron gun operated at 200kV. and a probe-forming aberration corrector. A probe convergence semi-angle of 28 mrad was used, resulting in a spatial resolution of 73 pm for imaging and spectrum imaging. The emission current was reduced to 14 pA to minimize damage from the electron beam. Under these conditions, an energy’ resolution of 350 meV can be achieved. The inner and outer acceptance angles for the annular detectors were set to 68 mrad and 280 mrad for high-angle annular dark-field (HAADF) imaging and to 40 mrad and 160 mrad for low-angle annular dark-fieldAtty. Dkt. No. 05400-0081 -PCT(LAADF) imaging. A post-column Gatan Continuum imaging filter was used for EELS and an Oxford XMAX100TLE X-ray detector for EDX analysis. The EDX spectra were collected with a pixel dwell time of 120 ps per pixel analysis. The EDX data were then processed using AZtec software to quantify the elemental composition with a binning factor of 2. The MXene powder was dispersed in NMF, a polar solvent, to form a colloidal solution. The particles were then drop-cast onto a 3 mm diameter TEM mesh grid covered with holey carbon. The grid was left to dry in a N2 glovebox for 2 hours. Subsequently, the samples were transferred from the glovebox into the microscope column using an air-free transfer workflow.

[0089] Computational details

[0090] Density function theory (DFT) total-energy calculations and geometry optimization of the MXene systems were performed using the Vienna ab initio Simulation Package (VASP) (Kresse, G. et al., Computational Materials Science 1996, 6 (1), 15-50.) The Perdew-Burke-Emzerhof (PBE) functional within the generalized gradient approximation (GGA) was used for electron exchange-correlation. (Perdew, J. P. et al., Physical Review Letters 1996, 77 (18), 3865-3868.) Projector- Augmented Wave (PAW) potential was used treat the electron-ion interaction. (Blochl, P. E., B 1994, 50 (24), 17953-17979.) The DFT-D3 method was employed to account for van der Waals interactions. (Grimme, S. et al., Journal of Computational Chemistry 2011, 32 (7), 1456-1465.) Spin-polarized calculations were performed, with a planewave energy cutoff of 500 eV. All atomic coordinates were relaxed until the total energy converged to within 10'6eV per atom and atomic force to within 0.01 eV A'1. A 9×9×1 Γ-centered k-point mesh was used to sample the Brillouin zones of the MXene layers w ith a slab model of sufficient vacuum thickness (~ 15 A).

[0091] Purity of MXenes based on weight percentage

[0092] The purity of the MXene (e.g., DS-TizCCL) in the product of the direct synthesis, based on weight percent was determined based on combustion elemental analysis by Atlantic Microlab, Inc. The samples were carefully washed with hydrochloric acid, water, and ethanol to remove all soluble by-products, then vacuum dried before testing. Very low hydrogen content established that the samples had been dried properly. The weight percentage of the transition metal (e.g., Ti) was calculated based on the metal: halogen (e.g., Ti: Cl) ratio from SEM-EDX analysis of the MXenes (FIG. 10).Atty. Dkt. No. 05400-0081 -PCT

[0093] Using DS-Ti₂CCl₂ as an illustrative example: according to the Rietveld refinement results, only two crystalline phases were present: the MXene phase and a TiC phase. The Ti: Cl atomic ratio within MXene flakes was found to be close to 1: 1 by STEM-EDX analysis (FIG. 10). Based on this, the wt.% of MXene was calculated by based on the MXene phase the the only component that contained Cl and the stoichiometry of the MXene phase being Ti2CCh. (For this purity calculation, a DS-Ti₂CCl₂ MXene made using C₂Cl₄ as a reactant was used. However, this purity calculation methodology can also be applied to the MXenes made using alkyl halide reactants.)MXene phase Purity: wt % (MXene) = wt %(Cl) / (Cl)* [(Ti2CC12)i / 2]

[0094] Table 1. Weight percentage of Ti2CCh when Ti and C2CI4 were used as precursor.DS-Ti₂CCl₂ (MXene) elemental analysisElement: Cl. C.. H. Ti (based on an EDX ratio) others wt % J 34.93 7.36 0.24 51.94 5.53

[0095] The product of the direct synthesis had an MXene phase purity of Ti2CCh: 88.14 wt.%.

[0096] Calculation of enthalpy of reactions

[0097] The thermodynamics of reactions by reacting a transition metal M = Ti, Zr, or Nb and an alkyl halide precursor were screened by comparing the energy differences between forming MXenes phases with the most competitive side reactions producing cubic MC carbide phases. In most of these reactions, the numbers of gas molecules remain unchanged or can be cancelled in comparison, suggesting minimal entropy contributions in flipping the product selectivity. Therefore, the energy of involved chemical species (E) was calculated, and the enthalpy changes of reactions (ΔH) were derived.

[0098] Note: The E values of the listed chemical species were used to calculate the enthalpy changes. However, these values may differ from the standard enthalpy of formation, as they w ere directly derived from DFT calculation results, rather than referenced to the most stable or common elemental form as required by the definition of standard enthalpy ofAtty. Dkt. No. 05400-0081 -PCTformation. Nonetheless, this discrepancy is just a matter of baseline adjustment and should not affect the calculation of ΔH significantly.

[0099] The values of ΔH are also normalized to the number of M in the chemical reaction, in order compare the feasibility of different alkyl halides as precursors of MXene direct synthesis.

[0100] The initial states were fixed to the stoichiometries that are expected to provide the best yields given below:2M + CH₂Cl₂ → MXenes vs. MC2M + CHCl₃ → MXenes vs. MC2M + CH₂Br₂ → MXenes vs. MCNote: the 30% excess amounts of alkyl halides in the synthetic experiments were to compensate for their decomposition and evaporation during a flame sealing process.

[0101] Table 2. Energy of chemical species. Results are based on DFT calculations at 0 K. Energy of MXenes were derived based on single layers.Metals and carbides E’ / kJ mol'1TM halides E / kJ·mol⁻¹ Small molecules E / kJ·mol⁻¹ Ti (s) -815 TiCl4(g) -2262 CH2CI2 (g) -2020 Zr (s) -874 TiCh (g) -1797 CHCh (g) -1864 Nb (s) -1057 TiCh (s) -1607 C2Cl4(g) -2531 Ti2CCl2(s) -3573 TiBr4(g) -2015 C₂Cl₆ (g) -2990 Zr₂CCl₂ (s) -3755 TiBr3(g) -1613 CC14 (g) -1691 Nb2CCl2(s) -3865 TiBr2(s) -1471 CI2 (g) -348 Ti2CBr2(s) -3412 ZrCl4(g) -2404 HC1 (g) -589 Zr2CBr2(s) -3615 ZrCl3(g) -1875 CH2Br2(g) -1897 Nb2CBr2(s) -3730 ZrCh (s) -1717 CBr4(g) -1439 Ti₃C₂Cl₂ (s) -5466 ZrBr₄ (g) -2152 Br2(g) -293 Zr₃C₂Cl₂ (s) -5719 ZrBr3(g) -1686 HBr (g) -521 Nb₃C₂Cl₂ (s) -5885 ZrBr2(s) -1588 H₂ (g) -653 TiC (s) -1892 Nb₂Cl₁₀ (g) -5320 CH4(g) -2322 ZrC (s) -1956 Nb₂Cl₆ (g) -3855 C₂H₄Cl₂ (g) -3634 NbC (s) -2077 NbCl2(s) -1736Atty. Dkt. No. 05400-0081 -PCTC (a-graphite) -904 Nb₂Br₁₀ (g) -4702Nb₂Br₆ (g) -3503NbBr2(s) -1594

[0102] Table 3. Enthalpy of different reactions (AH) when transition metals Ti, Zr, or Nb react with CH₂Cl₂. In AS,indicates the total number of gas molecules remains unchanged during the reaction, suggesting a minimal entropy contribution.A77 per No. Reactions Aff / kJ-mol’1M / AS kJ- mol'11 2 Ti (s) + CH₂Cl₂ (g) = Ti₂CCl₂ (s) + H₂ (g) -575 -288 / 2 2 Zr (s) + CH₂Cl₂ (g) = Zr₂CCl₂ (s) + H₂ (g) -641 -320 / 3 2 Nb (s) + CH₂Cl₂ (g) = Nb₂CCl₂ (s) + H₂ (g) -383 -191 / 4 2 Ti (s) + CH₂Cl₂ (g) = TiC (s) + TiCl₂ (s) + H₂ (g) -502 -251 / 5 2 Zr (s) + CH₂Cl₂ (g) = ZrC (s) + ZrCl₂ (s) + H₂ (g) -558 -279 / 6 2 Nb (s) + CH₂Cl₂ (g) = NbC (s) + NbCl₂ (s) + H₂ (g) -331 -166 / 10 4 Ti (s) +2 CH₂Cl₂ (g) = Ti₃C₂Cl₂ (s) + TiCl₂ (s) + 2 H₂ (g) -1078 -269 / 11 4 Zr (s) +2 CH₂Cl₂ (g) = Zr₃C₂Cl₂ (s) + ZrCl₂ (g) + 2 H₂ (g) -1206 -302 / 12 4 Nb (s) +2 CH₂Cl₂ (g) = Nb₃C₂Cl₂ (s) + NbCl₂ (g) + 2 H₂ (g) -657 -164 /

[0103] Table 4. Enthalpy of different reactions (AH) when transition metals Ti, Zr, or Nb react with CHCh. In the AS column,indicates the total number of gas molecules remains unchanged during the reaction, suggesting a minimal entropy contribution. indicates the total number of gas molecules decreases during the reaction, suggesting a potential entropy factor to make the reaction less favorable at higher temperatures.AH per No. Reactions AH / kJ mol1M / AS kJ- mol’11 2 Ti (s) + CHCl₃ (g) = Ti₂CCl₂ (s) + HCl (g) -668 -334 / 2 2 Zr (s) + CHCl₃ (g) = Zr₂CCl₂ (s) + HCl (g) -732 -367 / 3 2 Nb (s) + CHCl₃ (g) = Nb₂CCl₂ (s) + HCl (g) -476 -238 / 4 2 Ti (s) + CHCl₃ (g) = TiC (s) + TiCl₂ (s) + HCl (g) -594 -297 / 5 2 Zr (s) + CHCl₃ (g) = ZrC (s) + ZrCl₂ (s) + HCl (g) -650 -325 / 6 2 Nb (s) + CHCl₃ (g) = NbC (s) + NbCl₂ (s) + HCl (g) -424 -212 / 7 4 Ti (s) +2 CHCl₃ (g) = Ti₃C₂Cl₂ (s) + TiCl₄ (s) + H₂ (g) -1392 -348 -1 8 4 Zr (s) +2 CHCl₃ (g) = Zr₃C₂Cl₂ (s) + ZrCl₄ (s) + H₂ (g) -1552 -388 -1 24 Nb (s) +12 CHCl₃ (g) = 6 Nb₃C₂Cl₂ (s) + 2 NbCl₂ (s) + 29 -5596 -233 -4Nb₂Cl₁₀ (g) + 6 H₂ (g)Atty. Dkt. No. 05400-0081 -PCT

[0104] Table 5. Enthalpy of different reactions (AH) when transition metals Ti, Zr, or Nb react with CH₂Br₂. In the AS column,indicates the total number of gas molecules remains unchanged during the reaction, suggesting a minimal entropy contribution.AH per M / No. Reactions A / f / kJ mol’1AS kJ mol’11 2 Ti (s) + CH2Br2(g) = Ti2CBr2(s) + H2(g) -538 -269 / 2 2 Zr (s) + CH₂Br₂ (g) = Zr₂CBr₂ (s) + H₂ (g) -623 -312 / 3 2 Nb (s) + CH₂Br₂ (g) = Nb₂CBr₂ (s) + H₂ (g) -371 -186 / 4 2 Ti (s) + CH2Br2(g) = TiC (s) + TiBr2(s) + H2(g) -489 -245 / 5 2 Zr (s) + CH2Br2(g) = ZrC (s) + ZrBr2(s) + H2(g) -552 -276 / 6 2 Nb (s) + CH2Br2(g) = NbC (s) + NbBr2(s) + H2(g) -313 -156 /

[0105] Example 2: This example illustrates the direct synthesis of a mixed metal MXenes using a metal alloy starting material and alkyl halide reactants.

[0106] To demonstrate the synthesis of a mixed metal MXene from an inexpensive early transition metal starting material, MXenes incorporating Ti and V was synthesized from a scrap metal Ti-6Al-4V turnings. The MXenes, which are designated '■(Ti. VhCCh” MXenes were synthesized using CVD with a CH2CI2 precursor, as described in Example 1.

[0107] FIG. 11A shows an SEM image of (Ti, V)2CCh MXenes made from the Ti-6A1-4V turnings. FIG. 11B is an EDX map showing that the (Ti,V)₂CCl₂ MXenes had a V content of 1 at.% V. This demonstrates early transition metal alloy starting materials can produce mixed metal MXenes using the methods disclosed herein.

[0108] EXAMPLE 3. This example illustrates the alkylation of MXenes by electrochemical processes and their use as C-N coupling catalysts.

[0109] By tuning the reductive potential applied to a Ti₂CCl₂ MXene electrode soaked in tetrabutylammonium (TBA) chloride acetonitrile electrolyte, MXenes were intercalated by TBA or got reduced to eliminate terminal Cl atoms. The most interesting observation was the substitution of surface Cl with butyl groups, forming buty l terminated MXenes.

[0110] Take the synthesis of butyl terminated M₂CCl₂ (M = Ti or Nb) MXenes as an example. First, MXenes were fully intercalated by tetrabutylammonium (TBA+). A three-electrode electrochemical cell was used to conduct the intercalation reaction. A copper foil pasted with Nb₂CCl₂ MXene powders, with polyvinylidene fluoride (PVDF) as a binder, andAtty. Dkt. No. 05400-0081 -PCTa graphite rod were placed as the anode and cathode, respectively. Non-aqueous Ag / Ag⁺ electrode (0.01 M AgNO₃ and 0.1 M tetrabutylammonium perchlorate in acetonitrile) was used as reference. Tetrabutylammonium chloride (TBAC) in acetonitrile (40 mL; 10 mg·mL⁻¹) served as the electrolyte. The intercalation reaction was performed at a potential of -7 to -10 V vs. Ag / Ag+for 15 to 30 min. The product powders were recovered by dissolving PVDF in N,N-dimethylformamide (DMF). In the meantime, a certain amount of surface Cl groups had been removed, depending on the liability of surface ligand. As an example, MS-Ti₂CCl₂ showed negligible surface removal after an electrolysis of 10 min at -10 V. However, more than half of the Cl groups of Nb₂CCl₂ were removed after full intercalation, leading to an estimated composition of Nb₂CCl₀.₈(TBA)₀.₂ (FIG. 12).

[0111] After the reaction, the center-to-center spacing expanded from the original 8.7 to 21.5 Å for Ti₂CCl₂, and 8.9 to 21.6 Å for Nb₂CCl₂. The ~ 12.8 Å expansion was the same as TBA+intercalated two-dimensional materials like MoS₂. (Z. Lin et al.. Nature 562, 254-258 (2018).) The TBA+intercalated MXenes themselves could also be delaminated and dispersed in DMF after ultrasonication.

[0112] After full intercalation, alkyl transfer could be finished by changing the polarity of the environment. The TBA+intercalated MXenes were precipitated from their DMF suspension by centrifugation at 12100 g for 30 min. The solids were dried, then immersed and stirred in toluene. The low polarity environment crushed the TBA-MXene ionic compound, facilitating the carbocation transfer from TBA+to MXene surfaces. FIG. 13A shows XRD patterns and cartoon structures of Nb₂CCl₂ MXene (starting material), TBA intercalated Nb₂C MXene, and butyl functionalized Nb₂C MXene. FIG. 13B shows SEM images of the corresponding structures. FIG. 13C is a X-ray photoelectron spectroscopy (XPS) N Is spectra. A characteristic N+ binding energy was observed in TBA intercalated MXene.

[0113] The alkyl incorporation in MXene flakes is supported by a series of analysis such as a change in the center-to-center spacing against a number of carbons in alky l chains (FIG.14)

[0114] Solid-state nuclear magnetic resonance (ssNMR) spectroscopy confirmed the structure of butyl groups bonded to Nb surfaces (FIGS. 15A-15D).

[0115] Sitting on the interface of molecular and solid compounds, the alkyl functionalized MXenes possessed a mixed character and was found capable to initiateAtty. Dkt. No. 05400-0081 -PCTunusual reactions (FIG. 2). The addition of ammonia or primary' or secondary' amine to nonactivated alkenes and alkynes, also known as C-N coupling, is an important class of catalyzable reactions because they are potentially the most efficient route to higher substituted nitrogen-containing products. (F. Pohlki, et al., Chemical Society Reviews 32, 104-114 (2003).)

[0116] The octyl functionalized Nb₂C MXene was found to be a C-N coupling catalyst with excellent yield and substrate scope. The addition reactions were performed with 5 mol% Nb₂CCl₀.₈(octyl)₀.₂ MXene, in toluene / DMF at 110 °C for 36 hours (FIG. 2). The product was isolated by gas chromatography (GC) and characterized with NMR (FIGS. 16 and 17) and mass spectroscopy (MS).

[0117] Materials and Methods

[0118] Synthetic Methods

[0119] Chemicals and materials

[0120] Tetraalkylammonium and tetraalkylphosphonium salts were dried under vacuum at 60°C for at least 24 hours before use. Tetrapentylammonium bromide (99+%) was purchased from Thermo Scientific. Tetraethylammonium bromide (99%), tetrapropylammonium chloride (98%), tetrabutylammonium chloride (TBAC, >97.0%), tetrahexylammonium bromide (98%), tetraheptylammonium bromide (99%), tetraoctylammonium bromide (TOAB, 98%), tetrabutylphosphonium bromide (98%), and tetraphenylphosphonium chloride (98%) were purchased from Sigma- Aldrich.

[0121] n-butyllithium (n-BuLi, 2.5 M in hexanes), N,N-dimethylformamide (DMF, 99.8%, anhydrous), toluene (99.8%, anhydrous), hexane (95%, anhydrous), MeOH (99.8%, anhydrous), MeCN (99.8%, anhydrous), and NMF(99%) were purchased from Sigma. NMF was purified by distillation before handling in a glovebox.

[0122] Electrochemical grafting of MXenes

[0123] A two-electrode electrochemical cell w as used to conduct the intercalation. A copper foil pasted with Ti₂CCl₂ MXene and graphite rod were placed as the cathode and anode, respectively. Tetrabutylammonium chloride solution in acetonitrile (30 ml; 5 mg·mL⁻¹) served as the electrolyte. The applied voltage was set to different values from -2 to -10 V. The intercalation process was allowed to proceed for 10 min.Atty. Dkt. No. 05400-0081 -PCT

[0124] A three-electrode electrochemical cell was used to conduct the intercalation reaction. A copper foil pasted with Ti₂CCl₂ MXene and a graphite rod were placed as the anode and cathode, respectively. Non-aqueous Ag / Ag⁺ electrode (AgNO₃ in acetonitrile) was used as reference. Tetrabutylammonium chloride (TBAC) in acetonitrile (40 ml; 5 mg·mL⁻¹) served as the electrolyte. The intercalation reaction was performed at a potential of -5 V vs. Ag / Ag+for 10 min. The product powders were recovered by dissolving PVDF binder in DMF.

[0125] Reaction of Nb2CClo.s(octyl)o.2 with 2-ethynylthiophene

[0126] The substitution reaction was performed in a N2-fdled glovebox. To mimic the catalytical conditions, 5 mg Nb₂CCl₀.₈(oct)₀.₂ MXene and 80 mg (20 eq vs. Nb) were added to 1 mL toluene. The mixture was allowed to be stirred at 80 °C for 16 hours. Then the precipitate was carefully washed with fresh DMF for 3 times and fresh toluene for 6 times before characterization.

[0127] Characterization Methods

[0128] The structure and composition of MXenes were characterized using X-ray characterization, Raman spectroscopy, electron microscopy and a combination of EDX and EELS analyses.

[0129] XRD

[0130] The diffraction patterns in Bragg-Brentano geometry were obtained using a Rigaku benchtop X-ray diffractometer equipped with HyPix-400 MF 2D HPAD and a Cu Kα X-ray source (1.5406 Å) operating at 40 kV and 15 mA.

[0131] SEM-EDX

[0132] SEM imaging and EDX elemental mapping were performed in a Carl Zeiss Merlin Field-Emission Scanning Electron Microscope equipped with Oxford Ultim Max 100 SDD. The accelerating voltage was set to 10 kV and beam current was set to 1 nA.

[0133] STEM

[0134] Atomic-resolution characterization of the MXene samples was conducted using the aberration-corrected scanning transmission electron microscope JEOL ARM200CF at the University of Illinois Chicago, equipped with a cold field emission gun operated at 200 kV, a Gatan Continuum electron energy-loss spectrometer, and an Oxford XMAX100TLE X-rayAtty. Dkt. No. 05400-0081 -PCTdetector, providing a sub-A probe-size and 350 meV energy resolution. The emission current was reduced to 12 LIA to reduce damage from the electron beam. An electron probe convergence semi-angle of 24 mrad was used and the inner detector angle for HAADF imaging was chosen to be 75 mrad, while an inner angle for LAADF imaging was chosen at 30 mrad. The MXene samples were initially prepared for STEM analysis by drop casting particles suspended in methanol onto a 3 nm holey-carbon-covered TEM grid, which was allowed to dry over 2 hours in a N2 glovebox maintained at <0.1 ppm H2O and O2. The TEM grids were then loaded onto a plasma cleaned Fischione vacuum transfer holder and inserted into the microscope column without atmospheric exposure.

[0135] X-ray fluorescence (XRF)

[0136] XRF analysis was performed with a benchtop Energy Dispersive Rigaku NEX DE VS X-ray fluorimeter equipped with a Peltier cooled FAST SDD Silicon Drift Detector. All analyses were carried out under He atmosphere to increase sensitivity for lighter elements. Elemental ratios were determined using the standardless thin-film fundamental parameters method as programmed in QuantEZ software provided by Rigaku, using the standard Rigaku calibration protocols. The samples were prepared by drop casting powders dispersed in anhydrous methanol on a Si substrate of an approximate 1×1 cm² size. The analysis window during measurement was set to a diameter of 10 mm.

[0137] X-ray photoelectron spectroscopy (XPS)

[0138] XPS analysis was performed on a Kratos Axis Nova spectrometer using a monochromatic Al Kα source (1486.6 eV). Ti 2p, C Is, and N Is high-resolution spectra were collected using an analysis area of 0.3 × 0.7 mm² and a 20 eV pass energy with a step size of 100 meV. Peak fitting of high-resolution XPS spectra was performed with CasaXPS software. A Tougaard background was used for better quantification of transition metal-based compounds. (M. Repoux, Surface and Interface Analysis 18, 567-570 (1992).) The Ti 2p region consists of the two 2p₃ / ₂ and 2p₁ / ₂ components arising from spin-orbit splitting; the peak area ratio of 2p₃ / ₂ to 2p₁ / ₂ was fixed to 2:1. The Ti 2p region was fit using three pairs of 2p₃ / ₂ and 2p₁ / ₂ components for each sample: two for the MXene components and one for TiCh. The two pairs of MXene component peaks were fit using the asymmetric Lorentzian line shape. (M. C. Biesinger et al., Applied Surface Science 257, 887-898 (2010).) The T1O2 peaks were fit using a symmetric Lorentzian-Gaussian line shape. The C Is and N Is regions were fit using symmetric Lorentzian-Gaussian line shapes. Since the Fermi edges were notAtty. Dkt. No. 05400-0081 -PCTsharp enough to use for calibration, the binding energy of the C 1s peak for Ti-C-Ti was fixed at 282.0 eV to compensate for any shifts caused by sample charging as Barsoum et al. suggested for carbide MXenes. (V. Natu et al., Matter 4, 1224-1251 (2021).) For other MXenes, calibration was performed by setting the C Is adventitious carbon peak to 284.8 eV.

[0139] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0140] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

Atty. Dkt. No. 05400-0081 -PCTWHAT IS CLAIMED IS:

1. A method for the synthesis of a two-dimensional transition metal carbide MXene having a formula Mn+iCnT.v, where 1 < n < 4, M is an early transition metal, and Tx are surface terminating halogen atoms, the method comprising: reacting the early transition metal with one or more alkyl halide compounds at a temperature at which the reaction forms the two-dimensional transition metal carbide MXene.

2. The method of claim 1, wherein the one or more alkyl halide compounds comprise dichloromethane.

3. The method of claim 1, wherein the one or more alkyl halide compounds comprise dibromomethane.

4. The method of claim 1, wherein the one or more alkyl halide compounds comprise diiodomethane.

5. The method of claim 1. wherein the one or more alkyl halide compounds comprise a mixture of dichloromethane and dibromomethane.

6. The method of claim 1, wherein M is Ti, Zr, or Nb and T is Cl, Br, or a combination thereof.

7. The method of claim 6, wherein M is Ti and T is Br.

8. The method of claim 1. wherein the two-dimensional transition metal carbide MXene is a mixed metal two-dimensional transition metal carbide MXene comprising at least two different early transition metal elements.

9. The method of claim 1, w herein reacting the early transition metal with the one or more alkyl halide compounds comprises reacting the early transition metal with at least two different alkyl halide compounds, wherein the two different alkyl halide compounds comprise different halogen atoms, and the two-dimensional transition metal carbide MXene has a formula M«+iC» (T’yT”i-y)x, where T’ and T"’ represent the two different halogen atoms, and 0 <y < 1.Atty. Dkt. No. 05400-0081 -PCT10. The method of claim 1, wherein the synthesis of the two-dimensional transition metal carbide MXene is carried out using chemical vapor deposition growth on a surface of a substrate comprising the early transition metal by:exposing the surface to a vapor comprising the one or more alkyl halide compounds; andreacting the early transition metal surface with the one or more alkyl halide compounds at the temperature at which the reaction forms the two-dimensional transition metal carbide MXene.

11. A two-dimensional transition metal carbide MXene having a formula M„+iCn (T’yT”i-y)x. where 1 < n < 4, M is one or more early transition metal elements, T’ and T” represent two different halogen atoms, and 0 < y < 1.

12. The two-dimensional transition metal carbide of claim 11, having a formula Ti2C(ClyBri-y)2, where 0 < y <1.

13. The two-dimensional transition metal carbide of claim 11, having a formula Nb2C(ClyBri-y)2.

14. The two-dimensional transition metal carbide of claim 11, wherein 0.1 < y < 0.9.

15. A method of alkylating a two-dimensional transition metal MXene having a formula Mn+iX„Tr, where 1 < n < 4, M is one or more early transition metal elements, X is carbon or nitrogen, and Txare surface terminating halogen atoms, the two-dimensional transition metal MXene comprising a stack of monolayer MXene sheets, the method comprising:electrochemically reducing the two-dimensional transition metal MXene having the formula Mn+iX^Tx in an electrolyte solution comprising an alkyl group-functionalized quaternary organic cation in a polar aprotic solvent, wherein the electrochemical reduction drives intercalation of the alkyl group-functionalized quaternary organic cation between the monolayer MXene sheets and the formation of an ionic quaternary organic cation-MXene compound; andAtty. Dkt. No. 05400-0081 -PCTreplacing the polar aprotic solvent with a non-polar solvent to induce a carbocation transfer between alky l groups on the alkyl group-functionalized organic cation and early transition metal atoms of the two-dimensional transition metal MXene to form the alkylated two-dimensional transition metal MXene.

16. The method of claim 15, wherein the alkyl group-functionalized quaternary organic cation is a quaternary ammonium cation.

17. The method of claim 15, wherein the alkyl group-functionalized quaternary- organic cation is a quaternary phosphonium cation.

18. The method of claim 15, wherein the alkyl groups of the alkyl group-functionalized quaternary- organic cations comprise C'r to Cis alkyl groups.

19. The method of claim 15, wherein M is Ti, Zr, or Nb and T is Cl, Br, or a combination thereof.

20. The method of claim 15, wherein the two-dimensional transition metal MXene is a mixed metal two-dimensional transition metal MXene comprising at least two different early transition metal elements.

21. An alkylated two-dimensional transition metal MXene having a formula Mn+iXflTr, where 1 < n < 4, M is one or more early transition metal elements, X is carbon or nitrogen, and Txare surface terminating atoms or groups, wherein at least some of the surface terminating atoms or groups are alkyl groups covalently bonded in M atoms of the MXene via C-M bonds.

22. The alkylated two-dimensional transition metal MXene of claim 21, wherein the surface terminating atoms or groups comprise the alkyl groups and halogen atoms.

23. The alkylated two-dimensional transition metal MXene of claim 21 or claim 18, wherein the alkyl groups comprise C2 to C12 alkyl groups.

24. The alkylated two-dimensional transition metal MXene of claim 21, wherein M is Ti, Zr. or Nb.Atty. Dkt. No. 05400-0081 -PCT25. A method of catalyzing a deacetylenative or deethylenative C-N coupling of an alkene or alkyne molecule, the method comprising reacting the alkene or alkyne molecule with ammonia, a primary amine, or a secondary amine in the presence of the alkylated two-dimensional transition metal MXene of claim 21 under conditions that promote the C-N coupling of the alkene or alkyne molecule.

26. A composition comprising a two-dimensional transition metal carbide MXene phase having a formula M„ iC„T. where 1 < n < 4, M one or more early transition metal elements, and Txare surface terminating halogen atoms, wherein the two-dimensional transition metal carbide MXene phase makes up at least 90 weight percent of the composition.

27. The composition of claim 26, wherein M is Ti, Zr, or Nb and T is Cl, Br, or a combination thereof.

28. A device or device component comprising the two-dimensional transition metal carbide MXene of claim 11, the alkylated two-dimensional transition metal carbide MXene of claim 21, or the composition of claim 26.

29. The device or device component of claim 28, wherein the device or device component is a capacitor, a battery', an electrode, or an electronic display.

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