Assessing the Surface Chemistry of 2D Transition Metal Carbides (MXenes): A Combined Experimental/Theoretical ¹³C Solid State NMR Approach

Abstract

The surface functionalization of 2D transition metal carbides or nitrides, so-called MXenes, is one of the fundamental levers allowing to deeply modify their physicochemical properties. Beyond new approaches to control this pivotal parameter, the ability to unambiguously assess their surface chemistry is thus key to expand the application fields of this large class of 2D materials. Using a combination of experiments and state of the art density functional theory calculations, we show that the NMR signal of the carbon�the element common to all MXene carbides and corresponding MAX phase precursors�is extremely sensitive to the MXene functionalization, although carbon atoms are not directly bonded to the surface groups. The simulations include the orbital part to the NMR shielding and the contribution from the Knight shift, which is crucial to achieve good correlation with the experimental data, as demonstrated on a set of reference MXene precursors. Starting with the Ti 3 C 2 T x MXene benchmark system, we confirm the high sensitivity of the 13 C NMR shift to the exfoliation process. Developing a theoretical protocol to straightforwardly simulate different surface chemistries, we show that the 13 C NMR shift variations can be quantitatively related to different surface compositions and number of surface chemistry variants induced by the different etching agents. In addition, we propose that the etching agent affects not only the nature of the surface groups but also their spatial distribution. The direct correlation between surface chemistry and 13 C NMR shift is further confirmed on the V 2 CT x , Mo 2 CT x , and Nb 2 CT x MXenes.

Galvanic Replacement and Etching of MAX-Related Phases in Molten Salts toward MXenes: An In Situ Study

“…Unit cell parameters were fixed to their experimental values while considering relaxed atomic positions. The structural models of Brette et al were used for Ti 3 AlC 2 , Ti 3 C 2 Cl 2 and Mo 2 Ga 2 C; Mo 2 GaC unit cell parameters were taken from ref and those of Ti 3 ZnC 2 were from ICSD #133140. Sufficient number of k- points in the full Brillouin zone were considered, using Monkhorst–Pack integration, so that a further increase would not noticeably change the results.…”

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure–Property Interplay for Next‐Generation Technologies

“…Reproduced with permission. [ 151 ] Copyright 2023, ACS. g) Depth profiles of SIMS measurements on Ti 3 AlC 2 MAX and multilayer Ti 3 C 2 T x MXene.…”

“…Figure 7f plots the sensitivity of the nuclear magnetic resonance (NMR) signal of C to the functionalization of Ti 3 C 2 T x MXene. [151] Using a combination of theoretical simulations and experimental NMR tests, Brette et al showed that the NMR signal of carbon, a common element of all MXene carbides as well as the corresponding MAX phase precursors, is extremely sensitive to MXene functionalization, even though C is not directly bound to the surface groups. Starting with Ti 3 C 2 T x MXene, the authors confirmed that the 13 C NMR shifts are extremely sensitive to the exfoliation process.…”

“…f) Schematic illustration over the sensitivity with respect to the NMR signal of C to Ti 3 C 2 T x MXene functionalization. Reproduced with permission [151]. Copyright 2023, ACS.…”

Characterization by NMR Spectroscopy of the SEI Layer Formed on Ti3C2 MXene Materials Prepared with Various Terminations

“…12,13 The enhanced performance is attributed to the variation in the surface terminations and is backed by various theoretical and experimental studies. 14 Surface termination characterization of the MXenes synthesized through various routes have been reported before: X-ray photoelectron spectroscopy (XPS), 15 Raman spectroscopy, 16,17 solid-state nuclear magnetic resonance (ssNMR), [18][19][20][21] Fourier transform infrared spectroscopy (FT-IR), 22,23 thermogravimetric analysis (TGA) 13,24 are few among the qualitative and quantitative techniques used for the characterizations. The characterization of the terminations has been difficult due to many reasons like variation in the probed volume, impurities present and the differences in intercalant species etc Only a few studies have been conducted with ssNMR and to the best of our knowledge all the studies were on wet chemically etched MXenes.…”