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.

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. MXenes adopt three structures with one metal on the M site, as inherited from the parent MAX phases: M 2 C, M 3 C 2 , and M 4 C 3 . They are produced by selectively etching out the A element from a MAX phase or other layered precursor (e.g., Mo 2 Ga 2 C), which has the general formula M n+1 AX 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 P6 3 /mmc symmetry, where M layers are nearly close packed and X atoms fill octahedral sites. Therefore, M n+1 X n layers are interleaved with the A element, which is metallically bonded to the M element. ==Intercalation and delamination== Since MXenes are layered solids and interlayer bonding is weak, intercalation of guest molecules is possible. Guest molecules include dimethyl sulfoxide (DMSO), hydrazine, and urea. For example, N 2 H 4 (hydrazine) can be intercalated into Ti 3 C 2 (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. Ions, including Li + , Pb 2+ , and Al 3+ , can also be intercalated into MXenes, either spontaneously or when a negative potential is applied to a MXene electrode. ==MXene clay== For the case of Ti 3 C 2 T x and Ti 2 CT x , etching with concentrated hydrofluoric acid leads to an open, accordion-like morphology with a short distance between layers (this is common for other MXene compositions). To be dispersed in suspension, the material must be pre-intercalated with something like DMSO. 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 intercalated water. The material has been found to be 'clay-like': as seen in clay materials (e.g. smectite clays and kaolinite), Ti 3 C 2 T x demonstrates the ability to expand its interlayer distance hydration and can reversibly exchange charge-balancing Group I and Group II cations. 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. It disperses into single layer two-dimensional sheets in water without surfactants. Further, due to these properties, it can be rolled into free-standing, additive-free electrodes for energy storage applications. ==Properties== With a high electron density at the Fermi level, MXene monolayers are predicted to be metallic. In MAX phases, N(E F ) is mostly M 3d orbitals, and the valence states below E F are composed of two sub-bands. Sub-band A is made of hybridized Ti 3d-Al 3p orbitals, and is near E F , while sub-band B, −10 to −3 eV below E F , which is due to hybridized Ti 3d-C 2p and Ti 3d-Al 3s orbitals. 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 Ti 2 , therefore N(E F ) is 2.5–4.5 times higher for MXenes than MAX phases. Experimentally, the predicted higher N(E F ) for MXenes has not been shown to lead to higher resistivities than the corresponding MAX phases. The energy positions of the O 2p (~6 eV) and the F 2p (~9 eV) bands from the Fermi level of Ti 2 CT x and Ti 3 C 2 T x both depend on the adsorption sites and the bond lengths to the termination species. Significant changes in the Ti-O/F coordination are observed with increasing temperature in the heat treatment. The anisotropic electronic band structure of Ti 3 C 2 T x contains electron pockets, bulk band gaps, and a Dirac-like feature. Only MXenes without surface terminations are predicted to be magnetic. E.g., Cr 2 C, Cr 2 N, and Ta 3 C 2 are predicted to be ferromagnetic; Ti 3 C 2 and Ti 3 N 2 are predicted to be anti-ferromagnetic. No magnetic properties have been demonstrated experimentally. ===Biological properties=== Compared to graphene oxide, which has antibacterial properties, Ti 2 C MXene does not. However, MXene of Ti 3 C 2 MXene shows a higher antibacterial efficiency toward both Gram-negative E. coli and Gram-positive B. subtilis. More than 98% of both bacterial cells lost viability at 200 μg/mL Ti 3 C 2 colloidal solution within 4 h of exposure. Damage to the cell membrane was observed, which resulted in release of cytoplasmic materials from the cells and cell death. The principal in vitro studies of cytotoxicity of 2D MXene sheets showed promise for applications in bioscience and biotechnology. Studies of anticancer activity of Ti 3 C 2 was determined on two normal (MRC-5 and HaCaT) and two cancerous (A549 and A375) cell lines. Cytotoxicity results indicated that the observed toxic effects were higher against cancerous cells compared to normal ones. Potential toxicity mechanisms include, e.g., that Ti 3 C 2 MXene may affect oxidative stress and, in consequence, the generation of reactive oxygen species (ROS). Further studies on Ti 3 C 2 MXene revealed the potential of MXenes as a novel ceramic photothermal agent in cancer therapy. In neuronal biocompatibility studies, neurons cultured on Ti 3 C 2 are as viable as those in control cultures, and they can adhere, grow axonal processes, and form functional networks. A 2025 study extended this work for the first time to astrocytes, showing that physiologically relevant concentrations of Ti 3 C 2 T x MXene flakes are biocompatible, with preserved cell viability, morphology, and calcium signaling in astrocyte cultures. ===Sodium-ion batteries=== MXenes exhibit promising performances for sodium-ion batteries. Na + should diffuse rapidly on MXene surfaces, which is favorable for fast charging/discharging. Two layers of Na + can be intercalated in between MXene layers. As a typical example, multilayered Ti 2 CT x MXene as a negative electrode material showed a capacity of 175 mA h g −1 and good rate capability. It is possible to tune the Na-ion insertion potentials of MXenes by changing the transition metal and surface functional groups. V 2 CT x MXene has been successfully applied as a cathode material. Porous MXene-based paper electrodes were reported to exhibit high volumetric capacities and stable cycling performance, demonstrating promise for devices where size matters.

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