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Concept
MXenes
Summary
In materials science, MXenes are a class of two-dimensional inorganic compounds , that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. MXenes were first reported in 2012.
As-synthesized MXenes prepared via HF etching have an accordion-like morphology, which can be referred to as multi-layer MXene (ML-MXene), or few-layer MXene (FL-MXene) given 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).
MXenes adopt three structures with one metal on 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 can take two forms, ordered double transition metal MXenes or solid solution MXenes. For ordered double transition metal MXenes, they have the general formulas: M’2M”C2 or M’2M”2C3 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.
For solid-solution MXenes, they have the general formulas: (M’2−yM”y)C, (M’3−yM”y)C2, (M’4−yM”y)C3, or (M’5−yM”y)C4, where the metals are randomly distributed throughout the structure in solid solutions leading to continuously tailorable properties.
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