MXenes

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In materials science, 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.[1][2]

Structure[edit]

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).[2]

MXenes adopt three structures, 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, 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, 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.[2] Therefore, Mn+1Xn layers are interleaved with the A element, which is metallically bonded to the M element.[3][4]

Double Transition Metal MXenes[edit]

MXene carbides have been synthesized that are composed of two transition metals. MXenes in this new family 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, and Cr2TiC2. In these particular MXenes, the Mo or Cr atoms are on outer edges of the MXene and these atoms control electrochemical properties of the MXenes.[5]

Preparation[edit]

MXenes are produced by selective etching of the "A" element from the MAX phase structure

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),[2] ammonium bifluoride (NH4HF2),[6] and a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF).[7] 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.[8][9]

The MXene Ti4N3, currently the only nitride MXene that has been reported to have been synthesized, is prepared by a different procedure than those used 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 further be delaminated into single and few layers by immersing the MXene in tetrabutylammonium hydroxide, followed by sonication.[10]

The following MXenes have been synthesized to date:

2-1 MXenes: Ti2C,[11] (Ti0.5,Nb0.5)2C,[11] V2C,[12] Nb2C,[12] Mo2C [13]

3-2 MXenes: Ti3C2 ,[1] Ti3CN,[11] Zr3C2[14] and Hf3C2[15]

4-3 MXenes: Ti4N3,[10] Nb4C3 ,[16] Ta4C3 [11]

Double transition metal MXenes:

2-1-2 MXenes: Mo2TiC2,[5] Cr2TiC2[5]

2-2-3 MXenes: Mo2Ti2C3[5]

Intercalation and delamination[edit]

Since MXenes are layered solids and the bonding between the layers is weak, intercalation of the guest molecules in MXenes is possible. Guest molecules include dimethyl sulfoxide (DMSO), hydrazine, and urea.[2] 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 (crystal structure parameter that is directly proportional to the distance between individual MXene layers), which weakens the bonding between MX layers.[2] Ions, including Li+, Pb2+, and Al3+, can also be intercalated into MXenes, either spontaneously or when a negative potential is applied to a MXene electrode.[17]

Delamination[edit]

Ti3C2 MXene produced by HF etching has accordion-like morphology with residual forces that keep MXene layers together preventing separation into individual layers. Although those forces are quite weak, ultrasound treatment results only in very low yields of single-layer flakes. For large scale delamination, DMSO is intercalated into ML-MXene powders under constant stirring to further weaken the interlayer bonding and then delaminated with ultrasound treatment. This results in large scale layer separation and formation of the colloidal solutions of the FL-MXene. These solutions can later be filtered to prepare MXene "paper" (similar to Graphene oxide paper).[18]

MXene clay[edit]

For the case of Ti3C2Tx and Ti2CTx, etching with concentrated hydrofluoric acid leads to open, accordion-like morphology with a compact distance between layers (this is common for other MXene compositions as well). To be dispersed in suspension, the material must be pre-intercalated with something like dimethylsulfoxide. However, when etching is conducted with hydrochloric acid and LiF as a fluoride source, morphology is more compact with a larger inter-layer spacing, presumably due to amounts of intercalated water.[7] The material has been found to be ‘clay-like’: as seen in clay materials (e.g. smectite clays and kaolinite), Ti3C2Tx demonstrates the ability to expand its interlayer distance hydration and can reversibly exchange charge-balancing Group I and Group II cations.[19] Further, when hydrated, the MXene clay becomes pliable and can be molded into desired shapes, becoming a hard solid upon drying. Unlike most clays, however, MXene clay shows high electrical conductivity upon drying and is hydrophilic, being easily dispersed into single layer two-dimensional sheets in water without surfactants. Further, due to these properties, it can be quickly rolled into free-standing, additive-free electrodes for energy storage applications.

Properties[edit]

With a high electron density at the Fermi level, MXene monolayers are predicted to be metallic.[20][21][22][23][24] In MAX phases, N(EF) is mostly M 3d orbitals, and the valence states below EF are composed of two sub-bands. One, sub-band A, made of hybridized Ti 3d-Al 3p orbitals, is near EF, and another, sub-band B, -10 to -3 eV below EF 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(EF) is 2.5-4.5 times higher for MXenes than MAX phases.[1] Experimentally, the predicted higher N(EF) for MXenes has not been shown to lead to higher resistivities than the corresponding MAX phases.

Only MXenes without surface terminations are predicted to be magnetic. Cr2C, Cr2N, and Ta3C2 are predicted to be ferromagnetic; Ti3C2 and Ti3N2 are predicted to be anti-ferromagnetic. None of these magnetic properties have not been demonstrated experimentally yet.[1]

Biological Properties[edit]

Compared to graphene oxide, which has been widely reported as an antibacterial agent, Ti2C MXene shows lack of antibacterial properties [25] Investigations results revealed that Ti2C phase did not influenced negatively the gram-positive bacteria commonly occurring in the natural environment. Moreover, the slightly intensified growth of Bacillus sp. strain was observed in the vicinity of tested Ti2C. [25] The SEM investigations of the preferential sites for bacteria adsorption indicated the presence of minor apoptosis for only Bacillus sp. strain, especially when the cells were located between individual sheets of the expanded Ti2C. Adsorption of bacteria cells on Ti2C resulted in changing of its zeta potential to that of bacteria cells. [25] On the other hand, MXene of Ti3C2 MXene shows a higher antibacterial efficiency toward both Gram-negative E. coli and Gram-positive B. subtilis.[26] Colony forming unit and regrowth curves showed that more than 98% of both bacterial cells lost viability at 200 μg/mL Ti3C2 colloidal solution within 4 h of exposure.[26] Damage to the cell membrane was observed, which resulted in release of cytoplasmic materials from the bacterial cells and cell death.[26] The principal in vitro studies of cytotoxicity of 2D sheets of MXenes showed promise for applications in bioscience and biotechnology. [27] Presented studies of anticancer activity of the Ti3C2 MXene was determined on two normal (MRC-5 and HaCaT) and two cancerous (A549 and A375) cell lines. The cytotoxicity results indicated that the observed toxic effects were higher against cancerous cells compared to normal ones. [27] The mechanisms of potential toxicity were also elucidated. It was shown that Ti3C2 MXene may affect the occurrence of oxidative stress and, in consequence, the generation of reactive oxygen species (ROS). [27] Further studies on Ti3C2 MXene revealed potential of MXenes as a novel ceramic photothermal agent used for cancer therapy. [28]

Water Purification Properties[edit]

One-micron-thick Ti3C2 MXene membranes demonstrated ultrafast water flux (approximately 38 L/(Bar·h·m2) and differential sieving of salts depending on both the hydration radius and charge of the ions.[29] Cations larger than the interlayer spacing of MXene don’t permeate through Ti3C2 membranes.[29] As for smaller cations, the ones with a larger charge permeate an order of magnitude slower than single-charged cations.[29]

Applications[edit]

MXenes, as conductive layered materials with tunable surface terminations, have been shown to be promising for energy storage applications (Li-ion batteries and supercapacitors), composites, photocatalysis,[30] water purification [31] and gas sensors.[32]

Lithium-ion Batteries (LIBs)[edit]

Some MXenes have been investigated experimentally thus far in LIBs (e.g. V2CTx ,[33] Nb2CTx ,[33] Ti2CTx ,[34] and Ti3C2Tx[35]). V2CTx has demonstrated the highest reversible charge storage capacity among MXenes in multi-layer form (280 mAhg−1 at 1C rate and 125 mAhg−1 at 10C rate). Nb2CTx in multi-layer form showed a stable, reversible capacity of 170 mAhg−1 at 1C rate and 110 mAhg−1 at a 10C rate. Although Ti3C2Tx shows the lowest capacity among the four MXenes in multi-layer form, it can be easily delaminated via sonication of the multi-layer powder. By virtue of higher electrochemically active and accessible surface area, delaminated Ti3C2Tx paper demonstrates a reversible capacity of 410 mAhg−1 at 1C and 110 mAhg−1 at 36C rate. As a general trend, M2X MXenes can be expected to have greater capacity than their M3X2 or M4X3 counterparts at the same applied current, since M2X MXenes have the least number of atomic layers per sheet.

In addition to the high power capabilities of MXenes, each MXene has a different active voltage window, which could allow their use as cathodes or anodes in batteries. Moreover, the experimentally measured capacity for Ti3C2Tx paper is higher than predicted from computer simulations, indicating that further investigation is required to ascertain the charge storage mechanism on MXene surfaces.[36]

Sodium-ion Batteries[edit]

MXenes also exhibit promising performances for sodium-based energy storage devices. Na+ should diffuse rapidly on MXene surfaces, which is favorable for fast charging/discharging.[37][38] Two layers of Na+ can be intercalated in between MXene layers.[39][40] As a typical example, multilayered Ti2CTx MXene as a negative electrode material showed a capacity of 175 mA h g−1 and good rate capability for electrochemical sodium-ion storage.[41] It is possible to tune the Na-ion insertion potentials of MXenes by changing the transition metal and surface functional groups.[37][42] V2CTx MXene has been successfully applied as a positive electrode material for sodium-ion storage.[43] Porous MXene-based paper electrodes have also been reported, which exhibited high volumetric capacities and stable cycling performance, demonstrating that MXenes are promising for sodium-based energy storage devices where size matters.[44]

Supercapacitors[edit]

Supercapacitor electrodes based on Ti3C2 MXene paper in aqueous solutions demonstrate excellent cyclability and the ability to store 300-400 F/cm3, which translates to three times as much energy as for activated carbon and graphene-based capacitors.[45] Ti3C2 MXene clay shows a volumetric capacitance of 900 F/cm3, a higher capacitance per unit of volume than most other materials, and does not lose any of its capacitance through more than 10,000 charge/discharge cycles.[7]

Composites[edit]

FL-Ti3C2 (the most studied MXene) nanosheets can mix intimately with polymers such as polyvinyl alcohol (PVA), forming alternating MXene-PVA layered structures. The electrical conductivities of the composites can be controlled from 4*10−4 to 220 S/cm (MXene weight content from 40% to 90%). The composites have tensile strength up to 400% stronger than pure MXene films and show better capacitance up to 500 F/cm3.[46] A method of alternative filtration for forming MXene-carbon nanomaterials composite films is also devised. These composites show better rate performance at high scan rates in supercapacitors.[47] The insertion of polymers or carbon nanomaterials between the MXene layers enables electrolyte ions to diffuse more easily through the MXenes, which is the key for their applications in flexible energy storage devices.

Porous MXenes[edit]

Porous MXenes (Ti3C2, Nb2C and V2C) have been produced via a facile chemical etching method at room temperature.[48] Porous Ti3C2 has a larger specific surface area and more open structure, and can be filtered as flexible films with, or without, the addition of carbon nanotubes (CNTs).[48] The as-fabricated p-Ti3C2/CNT films showed significantly improved lithium ion storage capabilities, with a capacity as high as 1250 mA·h·g−1 at 0.1 C, excellent cycling stability, and good rate performance.[48]

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