Abstract
A density functional theory study is presented here aimed at investigating whether the atomic stacking on the new family of two-dimensional MXene materials has an influence on their adsorption properties and whether these properties can provide information about this structural feature. To this end, the Mo2C MXene, exhibiting two nearly degenerate crystal structures with either ABC or ABA atomic stacking, is chosen as a case study. The study of the adsorption of CO, CO2, and H2O on both polymorphs of Mo2C reveals substantial differences that could be used in experiments to provide information about the atomic stacking of a given sample. Particularly, we show that the asymmetric and symmetric stretching modes of the adsorbed CO2 and the CO stretching mode are clear features that allow one to identify the stacking of atomic layers of the Mo2C MXene. The present finding is likely to apply to other MXenes as well.
Highlights
The discovery of low-dimensional transition-metal carbides and nitrides, known as MXenes,[1,2] has generated great expectation because of the broad number of applications of these materials emerging from their unique electronic, optical, chemical, mechanical, catalytic, and sensing properties.[3−8] To some extent, these properties can be modulated by varying the MXene structure and composition
One has the conventional family of MXenes, whereas the second case leads to newer families that are referred to as i-MXenes or o-MXenes.[13−15] Recently, it has been theoretically suggested that the MXene synthesizability is related somehow to the exfoliating energy of the MAX precursor.[16]
The analysis of the results presented in this work provides compelling evidence that ABC and ABA stackings lead to different chemistries
Summary
The discovery of low-dimensional transition-metal carbides and nitrides, known as MXenes,[1,2] has generated great expectation because of the broad number of applications of these materials emerging from their unique electronic, optical, chemical, mechanical, catalytic, and sensing properties.[3−8] To some extent, these properties can be modulated by varying the MXene structure and composition. Apart from the MXene composition and, obviously, from surface functionalization, there are two additional features that, in principle, can influence the reactivity of a given MXene These are the number of atomic layers and the atomic layer stacking. It is often assumed that because of their good thermal stability,[18] MXenes feature the ABC stacking inherited from the MAX precursor In this stacking, each atomic layer is horizontally shifted with respect to the immediate predecessor layer, but a different ABA stacking is possible. The presence of adsorbates could change the relative stability order of the two phases This hypothesis has been confirmed in Mo2N and W2N MXenes, where the activation of the N2 molecule promotes somehow the mentioned structural distortion.[20] Another interesting case is V2N MXene, which after etching the MAX precursor initially exhibits the ABC stacking. We will show that the vibrational frequencies of the adsorbed species provide a simple and efficient way to identify the atomic stacking in the experiments
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