Resolving the Structure of Ti₃C₂T_x MXenes through Multilevel Structural Modeling of the Atomic Pair Distribution Function

Abstract

MXenes are a recently discovered family of two-dimensional (2D) early transition metal carbides and carbonitrides, which have already shown many attractive properties and great promise in energy storage and many other applications. However, a complex surface chemistry and small coherence length have been obstacles in some applications of MXenes, also limiting the accuracy of predictions of their properties. In this study, we describe and benchmark a novel way of modeling layered materials with real interfaces (diverse surface functional groups and stacking order between the adjacent monolayers) against experimental data. The structures of three kinds of Ti3C2T x MXenes (T stands for surface terminating species, including O, OH, and F) produced under different synthesis conditions were resolved for the first time using atomic pair distribution function obtained by high-quality neutron total scattering. The true nature of the material can be easily captured with the sensitivity of neutron scattering to the surface species of interest and the detailed “third-generation” structure model we present. The modeling approach leads to new understanding of MXene structural properties and can replace the currently used idealized models in predictions of a variety of physical, chemical, and functional properties of Ti3C2-based MXenes. The developed models can be employed to guide the design of new MXene materials with selected surface termination and controlled contact angle, catalytic, optical, electrochemical, and other properties. We suggest that the multilevel structural modeling should form the basis for a generalized methodology on modeling diffraction and pair distribution function data for 2D and layered materials.

Transmission Electron Microscopy of 2D Materials: Structure and Surface Properties

“…The experimental measurements and theoretical predictions are not always consistent, as the MXene surfaces are covered by a random distribution of surface groups and intercalants which makes prediction of properties difficult. This has been observed by the few atomic resolution experiments reported so far, such as Paper III, Paper IV, Wang et al [123], and Wang et al [127].…”

“…Following studies assumed a full coverage of a single type of surface group (fluorine, oxygen, or hydroxide) and defect free MXenes, which predicted many MXenes as semiconductors with varying direct or indirect band gaps [114,118,124,134,135,136,137,138]. Recently, a study assuming a random distribution of surface groups has been performed, and showed results in good correlation with measurements [123].…”

“…The sheets are held together through van der Waals interaction [114,123], which contribute to the stability of MXene [124]. Due to the low interaction between MXene sheets, the properties are independent of the number of sheets [111,125].…”

“…MXene is synthesized from the MAX phase by etching the A element, see Figure 3.4, which replaces the metallic M-A bonds with van der Waals interaction [16] or hydrogen bonds [123,127]. The M-X bonds are unaffected by the process [16].…”

“…After synthesis some MAX phase material might remain among the separated MXene sheets [111,135,151]. Different etchants and etchant concentrations have been employed in the synthesis of MXene, resulting in a variation of defects, crystallinity and stacking ordering of the sheets [33,109,128,123,129,135]. Depending on the choice of etchant, MXene can exhibit enhanced properties such as higher volumetric capacitance for lithium-based etchants [33,152].…”