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A MXene is a two-dimensional early transition metal carbide or carbonitride, produced by etching the A element from a MAX phase. This family of materials was discovered at Drexel University in 2011. MXenes combine the metallic conductivity of transition metal carbides and hydrophilic nature because of their hydroxyl or oxygen terminated surfaces. The following MXenes have been synthesized: Ti₃C₂, Ti₂C, V₂C, Nb₂C, (Ti_0.5,Nb_0.5)₂C, Ta₄C₃, (V_0.5,Cr_0.5)₃C₂, Ti₃CN, and many more are predicted and analyzed computationally.

Synthesis

MXenes are produced by selectively etching out the A element from a MAX phase, which has the general formula M_n+1AX_n, where M is an early transition metal, A is an element from group IIIA or IVA of the periodic table, X is C and/or N, and n = 1, 2, or 3. MAX phases have a layered hexagonal structure with P6₃/mmc symmetry, where M layers are nearly closed packed and X atoms fill octahedral sites. Therefore, M_n+1X_n layers are interleaved with the A element, which is metallically bonded to the M element. MAX phases are etched mainly by using strong etchants such as hydrofluoric acid (HF). Etching of Ti₃AlC₂ in aqueous HF at room temperature causes the A (Al) atoms to be removed, and the surface of the carbide layers to be terminated by O, OH, and/or F atoms. It has been shown how the higher the value of n in the formula, the more stable the MXene.

One possible method of synthesizing MXenes is etching a MAX phase in aqueous HF for a certain period of time that depends on the HF concentration and temperature at which the procedure is performed. After this, the mixture is centrifuged to separate the solid from the supernatant fluid, followed by washing the solid with deionized water until the pH of the suspension is between 4 and 6. This produces an accordion like structure, which can be referred to as a multilayer MXene (ML-MXene), or a few-layer MXene (FL-MXene) when there are fewer than five layers. Because surface terminations by functional groups in MXenes can occur, the naming convention M_n+1X_nT_x can be used, where T is a functional group.

Although theoretically possible to produce them, attempts to produce nitride-based MXenes have not yet been successful. The lower cohesion energy of Ti_n+1N_n compared to Ti_n+1C_n implies that the nitride structure is less stable, while the higher formation energy of Ti_n+1N_n from Ti_n+1AlN_n relative to the formation energy of Ti_n+1C_n from Ti_n+1AlC_n implies that the Al-N bond is stronger than the Al-C bond in their respective MAX phases. It is also possible that less-stable nitride MXenes had been produced but were dissolved in the etchant.

Structure

MXenes that have been produced have three structures inherited from MAX phases: M₂C, M₃C₂, and M₄C₃. According to DFT (density functional theory) studies, there are two energetically favorable orientations for the terminating group (T) in Ti₃C₂T₂: I and II. In configuration I, T is located above the hollow sites between three neighboring C atoms. In configuration II, T groups are located above C atoms on both sides of the MXene layers. A third stable configuration is possible, III, which occurs when one side of a MXene is terminated in configuration I, and the opposite in II. Relative DFT total energies suggest that the most stable configuration of these three is configuration I, and the least stable is configuration II.

Intercalation

Intercalation in MXenes is possible because of the weak bonds between layers. Different species can intercalate MXenes, whether organic, inorganic, or ionic. Among molecules that have been intercalated into Ti₃C₂T_x are dimethyl sulfoxide (DMSO), hydrazine, and urea. For example, N₂H₄ (hydrazine) can be intercalated into Ti₃C₂(OH)₂ with the molecules parallel to the MXene basal planes to form a monolayer. Intercalaction increases the MXene c lattice parameter, which weakens the bonds between MX layers. Ions, including Li, Pb, and Al, can also be intercalaction into MXenes, either spontaneously or when a negative potential is applied to a MXene electrode.

Properties

With a high electron density near the Fermi level, MXene monolayers are predicted to be metallic according to DFT studies. In MAX phases, N(E_F) is mostly M 3d orbitals, and the valence states below E_F are composed of two sub-bands. One, sub-band A, made of hybridized Ti 3d-Al 3p orbitals, is near E_F, and another, sub-band B, -10 to -3 eV below E_F which is due to hybridized Ti 3d-C 2p and Ti 3d-Al 3s orbitals. Said differently, sub-band A is the source of Ti-Al bonds, while sub-band B is the source of Ti-C bond. Removing A layers causes the Ti 3d states to be redistributed from missing Ti-Al bonds to delocalized Ti-Ti metallic bond states near the Fermi energy in Ti₂, therefore N(E_F) is 2.5-4.5 times higher for MXenes than MAX phases.

Only bare-surface MXenes exhibit magnetic properties; Cr₂C, Cr₂N, and Ta₃C₂ are ferromagnetic, Ti₃C₂ and Ti₃N₂ are antiferromagnetic. Electronic properties of MXenes are also dependent on surface groups. While Ti₃C₂ is a metallic conductor, addition of surface groups give rise to small band gaps, 0.05 eV for Ti₃C₂(OH)₂ and 0.1 eV for Ti₃C₂F₂.

Another interesting trait of MXenes is their mechanical properties. Their M-C and M-N bonds are some of the strongest known. High elastic moduli for MXenes have been predicted.

Applications

Because of spontaneous intercalation in aqueous salt solutions of cations between Ti₃C₂T_x layers, MXenes can be used to produce outstanding supercapictors. MXenes’ chemistries, morphologies, and electrical conductivities make them ideal candidates for applications including sensors, catalysts, conductive reinforcement additives to polymers, and energy storage. Although MXenes' gravimetric capacities may not be as high as that of Si anodes in lithium-ion batteries, their advantage is offering a high cycling rate with good capacity. Because MXenes exhibit high cycling rates, they are good candidates for hybrid cells, which combine high energy densities characteristic of lithium ion batteries and high power densities of electrical double layer capacitors. MXenes have also shown high volumetric capacitance on aqueous electrolytes.

Intercalation of MXenes by Na, Mg, and Al suggests that they can be used in multivalent ion batteries other than lithium ion batteries.

It is expected that MXenes used as additives in polymers can provide excellent mechanical properties and good electrical conductivity.

References

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External links

• http://max.materials.drexel.edu
• http://nano.materials.drexel.edu

• Pending AfC submissions
• Pending AfC submissions in article space
• AfC submissions by date/10 November 2014