Exfoliation Energy as a Descriptor of MXenes Synthesizability and Surface Chemical Activity

MXenes are two-dimensional nanomaterials isolated from MAX phases by the selective extraction of the A component —a p-block element. The MAX phase exfoliation energy, Eexf, is regarded as a chemical descriptor of the MXene synthesizability. Here we show, by density functional theory estimations of the Eexf values for 486 different MAX phases, that Eexf decreases i) when MAX is a nitride, ii) when going along a d series of the metal M component, iii) when going down a group of the p-block A element, as well as iv) when having thicker MXene phases. Furthermore, Eexf is found to bias, even to govern, the surface chemical activity, as evaluated here on the CO2 adsorption strength, so that more unstable MXenes, displaying larger Eexf values, display a stronger attachment of species upon.

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Exfoliation Energy as a Descriptor of MXenes Synthesizability and Surface Chemical Activity

Nanomaterials ◽

10.3390/nano11010127 ◽

2021 ◽

Vol 11 (1) ◽

pp. 127

Author(s):

Daniel Dolz ◽

Ángel Morales-García ◽

Francesc Viñes ◽

Francesc Illas

Keyword(s):

Density Functional ◽

Co2 Adsorption ◽

Selective Extraction ◽

Chemical Activity ◽

Surface Chemical ◽

Block Element ◽

Max Phases ◽

Functional Theory ◽

Element Group ◽

Chemical Descriptor

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component—a p-block element. The MAX exfoliation energy, Eexf, is considered a chemical descriptor of the MXene synthesizability. Here, we show, by density functional theory (DFT) estimations of Eexf values for 486 different MAX phases, that Eexf decreases (i) when MAX is a nitride, (ii) when going along a metal M component d series, (iii) when going down a p-block A element group, and (iv) when having thicker MXenes. Furthermore, Eexf is found to bias, even to govern, the surface chemical activity, evaluated here on the CO2 adsorption strength, so that more unstable MXenes, displaying larger Eexf values, display a stronger attachment of species upon.

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DFT insights into the electronic structure, mechanical behaviour, lattice dynamics and defect processes in the first Sc-based MAX phase Sc2SnC

10.21203/rs.3.rs-870934/v1 ◽

2021 ◽

Author(s):

M. A. Hadi ◽

S.-R. G. Christopoulos ◽

A. Chroneos ◽

S. H. Naqib ◽

A. K.M.A. Islam

Keyword(s):

Density Functional ◽

Elastic Anisotropy ◽

Max Phase ◽

Ionic Character ◽

Max Phases ◽

Functional Theory ◽

Barrier Coating ◽

Important Addition ◽

Effective Valence ◽

Experimental Values

Abstract The ceramic and metallic properties of the MAX phases make them attractive for numerous technological applications. The very recent experimental synthesis of the first scandium (Sc) based MAX phase Sc2SnC is an important addition to the MAX phase family as it further expands the diversity of physical characteristics of this family. Here we employ density functional theory (DFT) calculations to investigate the structural, electronic, mechanical, lattice dynamical properties of Sc2SnC including defect processes to compare with those of existing M2SnC phases. The calculated structural properties are in good agreement with the experimental values. The new phase Sc2SnC is structurally, mechanically and dynamically stable. Sc2SnC is metallic with a mixture of covalent and ionic character. The covalency of Sc2SnC including M2SnC is mostly controlled by the effective valence. Sc2SnC in M2SnC family ranks second in the scale of deformability, softness and machinability. The elastic anisotropy level in Sc2SnC is moderate compared to the other M2SnC phases. Like other members of the M2SnC family, Sc2SnC has the potential to be etched into 2D MXenes and has the potential to be a thermal barrier coating (TBC) material. The hardness and melting point of Sc2SnC, including M2SnC, follows the trend of bulk modulus.

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Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo2Ga2C

Journal of Advanced Ceramics ◽

10.1007/s40145-021-0531-9 ◽

2022 ◽

Vol 11 (2) ◽

pp. 273-282

Author(s):

Xinxin Qi ◽

Weilong Yin ◽

Sen Jin ◽

Aiguo Zhou ◽

Xiaodong He ◽

...

Keyword(s):

First Principles ◽

Density Functional ◽

Harmonic Approximation ◽

Max Phase ◽

Max Phases ◽

Functional Theory ◽

High Temperature Heat Capacity ◽

Stiffness Model ◽

Temperature Heat ◽

Increasing Temperature

AbstractMo2Ga2C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties. To understand this unique MAX phase structure and promote possible future applications, the structure, chemical bonding, and mechanical and thermodynamic properties of Mo2Ga2C were investigated by first-principles. Using the “bond stiffness” model, the strongest covalent bonding (1162 GPa) was formed between Mo and C atoms in Mo2Ga2C, while the weakest Ga-Ga (389 GPa) bonding was formed between two Ga-atomic layers, different from other typical MAX phases. The ratio of the bond stiffness of the weakest bond to the strongest bond (0.33) was lower than 1/2, indicating the high damage tolerance and fracture toughness of Mo2Ga2C, which was confirmed by indentation without any cracks. The high-temperature heat capacity and thermal expansion of Mo2Ga2C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K. Because of the metal-like electronic structure, the electronic excitation contribution became more significant with increasing temperature above 300 K.

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Electrochemical Lithium Storage Performance of Molten Salt Derived V2SnC MAX Phase

Nano-Micro Letters ◽

10.1007/s40820-021-00684-6 ◽

2021 ◽

Vol 13 (1) ◽

Author(s):

Youbing Li ◽

Guoliang Ma ◽

Hui Shao ◽

Peng Xiao ◽

Jun Lu ◽

...

Keyword(s):

Molten Salt ◽

Redox Reaction ◽

Density Functional ◽

High Rate ◽

Max Phase ◽

Molten Salt Method ◽

Lithium Storage ◽

Max Phases ◽

Storage Mechanism ◽

A Charge

AbstractMAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage. Here, we report the preparation of V2SnC MAX phase by the molten salt method. V2SnC is investigated as a lithium storage anode, showing a high gravimetric capacity of 490 mAh g−1 and volumetric capacity of 570 mAh cm−3 as well as superior rate performance of 95 mAh g−1 (110 mAh cm−3) at 50 C, surpassing the ever-reported performance of MAX phase anodes. Supported by operando X-ray diffraction and density functional theory, a charge storage mechanism with dual redox reaction is proposed with a Sn–Li (de)alloying reaction that occurs at the edge sites of V2SnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at V2C layers with Li. This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.

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