Direct Measurement of Surface Termination Groups and Their Connectivity in the 2D MXene V2CTx Using NMR Spectroscopy

The MXenes are a class of 2D materials composed of transition-metal sheets alternating with carbide/nitride sheets, stacked just a few atoms thick. MXenes discovered thus far also have a surface termination layer that is likely a mixture of hydroxides and fluorides. While reasonable structural models based on X-ray diffraction and transmission electron microscopy data exist, the exact nature and distribution of the surface termination species are not well understood. Here, 1H, 19F, and 13C solid-state NMR spectroscopies are used to investigate the model MXene V2CTx, where T signifies the surface termination groups. 1H NMR experiments provide direct proof of hydroxide moieties in the surface layer by measuring interactions with the MXene surface. Furthermore, 1H NMR spectroscopy shows a significant amount of water hydrogen bonded to the surface hydroxide layer. 19F NMR experiments show fluoride moieties bonded to the MXene surface, with extremely unusual 19F spectra caused by strong interactions with the metallic/semiconducting MXene. 13C NMR observes the sample from the center of the MXene layer and shows that the 13C chemical shift is extremely sensitive to the MAX → MXene transformation. Nuclear-spin magnetization transferred from 1H nuclei in the hydroxide surface termination layer to 13C nuclei in the center of the MXene sheet yields further evidence of this connectivity. The multinuclear NMR experiments provide direct experimental verification of the structural models and depict the MXene V2CTx as infinite sheets of small-bandgap V2C sheets terminated by a mixed hydroxide/fluoride layer embedded in a matrix of strongly hydrogen-bonded water molecules.