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.
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 Mn+1XnTx 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 Mn+1AXn, 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, Mn+1Xn 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.
By designing a parent 3D atomic laminate, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, there is evidence for 2D Mo1.33C 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.