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MXenes
In materials science, MXenes (pronounced "max-enes") are a class of two-dimensional inorganic compounds along with MBorenes, that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. The first MXene was reported in 2011 at Drexel University's College of Engineering, and was named by combining the prefix "MAX" or "MX" (for MAX phases), with "ene" by analogy to graphene.
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) for 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 close 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. Ordered double transition metal MXenes have the general formulas: M'2M"C2 or M'2M"2C3 where M' and M" are 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 control electrochemical properties.
Solid-solution MXenes 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.
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, 2D Mo1.33C sheets with ordered metal divacancies may be possible.
MXenes are typically synthesized by a top-down selective etching process. This synthetic route is scalable, with no loss or change in properties with batch size. Producing a MXene by etching a MAX phase occurs mainly by using 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 a Cl terminal can be realized. The Cl-terminated MXene is structurally stable up to 750 °C. A general Lewis acid molten salt approach can etch most of MAX phases members (such as MAX-phase precursors with A elements Si, Zn, and Ga) by some other melts (CdCl2, FeCl2, CoCl2, CuCl2, AgCl, and NiCl2).
The MXene Ti4N3 was the first nitride MXene reported, and is prepared by a different procedure than for carbide MXenes. To synthesize Ti4N3, the MAX phase Ti4AlN3 is mixed with a molten eutectic fluoride salt mixture of lithium fluoride, sodium fluoride, and potassium fluoride and treated at elevated temperatures. This procedure etches out Al, yielding multilayered Ti4N3, which can be delaminated into single and few layers by immersing the MXene in tetrabutylammonium hydroxide, followed by sonication.
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MXenes
In materials science, MXenes (pronounced "max-enes") are a class of two-dimensional inorganic compounds along with MBorenes, that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. The first MXene was reported in 2011 at Drexel University's College of Engineering, and was named by combining the prefix "MAX" or "MX" (for MAX phases), with "ene" by analogy to graphene.
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) for 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 close 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. Ordered double transition metal MXenes have the general formulas: M'2M"C2 or M'2M"2C3 where M' and M" are 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 control electrochemical properties.
Solid-solution MXenes 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.
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, 2D Mo1.33C sheets with ordered metal divacancies may be possible.
MXenes are typically synthesized by a top-down selective etching process. This synthetic route is scalable, with no loss or change in properties with batch size. Producing a MXene by etching a MAX phase occurs mainly by using 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 a Cl terminal can be realized. The Cl-terminated MXene is structurally stable up to 750 °C. A general Lewis acid molten salt approach can etch most of MAX phases members (such as MAX-phase precursors with A elements Si, Zn, and Ga) by some other melts (CdCl2, FeCl2, CoCl2, CuCl2, AgCl, and NiCl2).
The MXene Ti4N3 was the first nitride MXene reported, and is prepared by a different procedure than for carbide MXenes. To synthesize Ti4N3, the MAX phase Ti4AlN3 is mixed with a molten eutectic fluoride salt mixture of lithium fluoride, sodium fluoride, and potassium fluoride and treated at elevated temperatures. This procedure etches out Al, yielding multilayered Ti4N3, which can be delaminated into single and few layers by immersing the MXene in tetrabutylammonium hydroxide, followed by sonication.