Nanoscale MXene Interlayer and Substrate Adhesion for Lubrication: A Density Functional Theory Study

Abstract

Understanding the interlayer interaction at the nanoscale in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer–substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. Previous computational studies considered MXenes’ interlayer coupling with oversimplified, homogeneous terminations while neglecting the interaction with underlying substrates. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An ad hoc modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole–dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet’s thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes’ interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS 2 as established references, we verify that MXenes’ tribological performance as solid lubricants can be significantly improved by avoiding −OH and −F terminations, which can be done by controlling terminations via post-synthesis processing.

Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes

“…It is known that as synthesized MXene surfaces are generally covered by a mixture of randomly distributed surface terminations [ 38 – 41 ]. Our group has already verified that the chemical behavior of mixed and homogeneous MXene surfaces can be quite different from each other [ 46 ]. To gain a better understanding of MXenes oxidation, we investigated the interaction of water with a Ti 2 CT x layer simultaneously passivated with –F, –O and –OH (in a stoichiometric ratio of about one-third each).…”

“…the preceding noble gas in the periodic table (hence the acronym NG). In a previous work, [ 46 ] we demonstrated that the use of PBE + D NG is more accurate in capturing long-range van der Waals interactions than other dispersion-corrected DFT functionals (e.g. vdW-DF2) when MXenes are involved.…”

“…We used the generalized gradient approximation (GGA) within the Perdew–Burke–Ernzerhof (PBE) parametrization to describe the exchange-correlation functional [ 45 ]. To properly consider the van der Waals interactions, we used an ad-hoc modification of the Grimme D2 dispersion correction scheme (i.e., D NG ) [ 46 , 47 ], which differs from the standard D2 only for the C 6 coefficient and the van der Waals radius R 0 of the Ti atoms, which are replaced with those of Ar, i.e. the preceding noble gas in the periodic table (hence the acronym NG).…”

“…vdW-DF2) when MXenes are involved. The electronic wave-function was expanded on a plane-wave basis, truncated with a cutoff of 50 Ry, while a cutoff of 400 Ry was applied for the charge density, in agreement with our previous study on Ti-based MXenes [ 46 ]. The ionic species were described by ultrasoft pseudopotentials.…”

A review of recent advances and applications of machine learning in tribology

“…The presence of higher amounts of –F and –O groups on MXenes demonstrates weaker adhesion to ferrous substrates which helps in lubricant deportation under sliding conditions. 375…”