MXenes are a class of two-dimensional inorganic compounds. These materials consist of few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides. First described in 2011, MXenes combine metallic conductivity of transition metal carbides and hydrophilic nature because of their hydroxyl or oxygen terminated surfaces.

STRUCTURE

Scanning electron microscope image of the MXene produced by HF-etching of Ti₃AlC₂. As-synthesized MXenes prepared via HF etching have an accordion-like morphology, which can be referred to as a multi-layer MXene (ML-MXene), or a few-layer MXene (FL-MXene) when there are fewer than five layers. Because the surfaces of MXenes can be terminated by functional groups, the naming convention M_n+1X_nT_x can be used, where T is a functional group (e.g. O, F, OH, Cl).

Mono transition metal MXenes

MXenes adopt three structures with one metal in the M-site, as inherited from the parent MAX phases: M₂C, M₃C₂, and M₄C₃. They are produced by selectively etching out the A element from a MAX phase, or other layered precursor (e.g., Mo₂Ga₂C), which has the general formula M_n+1AX_n, where M is an early transition metal, A is an element from group 13 or 14 of the periodic table, X is C and/or N, and n = 1–4. 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.

Double transition metal MXenes

MXene carbides have been synthesized that are composed of two transition metals. MXenes in this new family have the general formulas M’₂M”C₂, M’₂M”₂C₃, or M’₄M”C₄, where M’ and M” are different transition metals. Double transition metal carbides that have been synthesized include Mo₂TiC₂, Mo₂Ti₂C₃, Cr₂TiC₂, and Mo₄VC₄. In some of these MXenes (such as Mo₂TiC₂, Mo₂Ti₂C₃, and Cr₂TiC₂), the Mo or Cr atoms are on outer edges of the MXene and these atoms control electrochemical properties of the MXenes. While, with others, such as Mo₄VC₄ or (Mo,V)₄C₃, the metals are randomly distributed throughout the structure in solid solutions.

Divacancy MXenes

By designing a parent 3D atomic laminate, (Mo_2/3Sc_1/3)₂AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, there is evidence for 2D Mo_1.33C sheets with ordered metal divacancies.

MXenes are typically synthesized by a top-down selective etching process. This synthetic route has been shown to be scalable, with no loss or change in properties as the batch size is increased. Producing a MXene by etching a MAX phase occurs mainly by using strong etching solutions that contain a fluoride ion (F⁻) such as hydrofluoric acid (HF), ammonium bifluoride (NH₄HF₂), and a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF). For example, etching of Ti₃AlC₂ in aqueous HF at room temperature causes the A (Al) atoms to be selectively removed, and the surface of the carbide layers becomes terminated by O, OH, and/or F atoms. MXene can also be obtained in Lewis acid molten salts, such as ZnCl₂, and Cl terminal can be realized. The Cl-terminated MXene is structurally stable up to 750 °C.