Enabling the transition to ductile MAX phases and the exfoliation to MXenes via tuning the A element
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Abstract Two‐dimensional MXenes, exfoliated from their parental precursors‐MAX phases, exhibit several outstanding properties and have achieved several accomplishments in a vast range of fields. Developing novel and high‐performance MXenes has become a vital task in materials science, so estimating the possibilities for exfoliation is a topic positioned at the research frontier. Here, the likelihood of exfoliating 36 M 2 AC MAX phases was explored by using density functional theory. For MAX phases, the composition‐dependent mechanical performances were investigated, highlighting evident trends, and, more essentially, improving MAX phases toughness, which can be achieved via modulating the A site. Two novel criteria were then introduced to assess the probability of exfoliating MXenes from MAX phases, having less complexity and lower computational cost than the prior studies. The excellent agreement provided by the new criteria with the reported results demonstrates that they are feasible, reliable as well as easily accessible. Furthermore, some key features that were previously suggested to be related to exfoliation are instead determined to be weakly correlated with it. We thus performed a detailed numerical analysis to locate representative and correlated features that are fundamental for the exfoliation. Our findings provide deep insight into the synthesis process and accelerate the discovery of new MXenes.Keywords:
MXenes
Exfoliation joint
MAX phases
The properties of MXenes, a new group of quasi-2D d-metal carbide or nitride nanomaterials derived from the MAX phases by chemical exfoliation, can be very sensitive to the presence of surface functional groups. Herein, the MXenes Ti2C and Ti3C2 functionalized by methoxy groups are considered by means of the density functional theory tight-binding method. Their structural and electronic properties and relative stability are discussed in comparison with related and experimentally fabricated hydroxy derivatives of MXenes.
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The rush to synthesize novel two-dimensional (2D) materials has excited the research community studying ternary-layered carbide and nitride compounds, known as MAX phases, for the past two decades in the quest to develop new 2D material precursors.
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The properties of MXenes, a new group of quasi-two-dimensional d-metal carbide or nitride nanomaterials derived by chemical exfoliation from the MAX phases, can be very sensitive to the presence of surface functional groups. Herein, the MXenes Ti2C and Ti3C2 functionalized by methoxy groups are considered by means of the density functional theory tight-binding method. Their structural, electronic properties, and relative stability are discussed in comparison with related and experimentally fabricated hydroxy derivatives of MXenes.
MXenes
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MAX phases
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Titanium carbide
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Abstract MXenes—2D carbides/nitrides derived from their bulk nanolamellar M n +1 AX n phase (MAX) counterparts—are, for the most part, obtained by chemical etching. Despite the fact that the MA bonds in the MAX phases are not weak, in this work it is demonstrated that relatively large MAX single crystals can be mechanically exfoliated using the adhesive tape method to produce flakes whose thickness can be reduced down to half a unit cell. The exfoliated flakes, transferred onto SiO 2 /Si substrates, are analyzed using electric force microscopy (EFM). No appreciable variation in EFM signal with flake thickness is found. EFM contrast between the flakes and SiO 2 not only depends on the contact surface potential, but also on the local capacitance. The contribution of the latter can be used to show the metallic character—confirmed by four‐contact resistivity measurements—of even the thinnest of flakes. Because the A‐layers are preserved, strictly speaking MXenes are not dealt with in this work, but rather MAXenes. This is important in the case where the “A” layers contain magnetic elements such as Mo 4 Ce 4 Al 7 C 3 , whose structure is a derivative of the MAX structure.
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MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti3C2 MXene proceeds via HF insertion through edges of Ti3AlC2 MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al-MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H2, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (ΔG) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. In the presence of water, mixed functionalized Ti3C2Fx(OH)1-x (x ranges from 0 to 1) MXene can be obtained. The ΔG values for the mixed functionalized MXenes are very close in energy, indicating the random and nonuniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application.
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Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.
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The properties of MXenes, a new group of quasi-two-dimensional d-metal carbide or nitride nanomaterials derived by chemical exfoliation from the MAX phases, can be very sensitive to the presence of surface functional groups. Herein, the MXenes Ti2C and Ti3C2 functionalized by methoxy groups are considered by means of the density functional theory tight-binding method. Their structural, electronic properties, and relative stability are discussed in comparison with related and experimentally fabricated hydroxy derivatives of MXenes.
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Exfoliation joint
MAX phases
Nanomaterials
Titanium carbide
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