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, 1 H, 19 F, and 13 C solid-state NMR spectroscopies are used to investigate the model MXene V 2 CT x, where T signifies the surface termination groups. 1 H NMR experiments provide direct proof of hydroxide moieties in the surface layer by measuring interactions with the MXene surface. Furthermore, 1 H NMR spectroscopy shows a significant amount of water hydrogen bonded to the surface hydroxide layer. 19 F NMR experiments show fluoride moieties bonded to the MXene surface, with extremely unusual 19 F spectra caused by strong interactions with the metallic/semiconducting MXene. 13 C NMR observes the sample from the center of the MXene layer and shows that the 13 C chemical shift is extremely sensitive to the MAX → MXene transformation. Nuclear-spin magnetization transferred from 1 H nuclei in the hydroxide surface termination layer to 13 C 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 V 2 CT x as infinite sheets of small-bandgap V 2 C sheets terminated by a mixed hydroxide/fluoride layer embedded in a matrix of strongly hydrogen-bonded water molecules.