MXene terminating groups O, –F or –OH, –F or O, –OH, –F, or O, –OH, –Cl?

MXenes are a novel family of two-dimensional (2D) materials that are fast gaining popularity due to their versatile characteristics. The surfaces of these materials are often functionalized by negatively charged terminal groups, such as O, OH, and F during their synthesis, and it has been hypothesized that regulating the surface terminators enables to control the material characteristics. However, there is still a large gap between computational and experimental investigations regarding comprehending the surface functional groups. Surfaces with mixed terminations are consistently synthesized in experiments, although pure terminated surfaces are predicted by computational research. Here we summarized the nature of chemical bonding in transition metal carbide materials (MXenes) by 1H and 19F nuclear magnetic resonance (NMR), Raman, X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS)/scanning transmission electron microscopy (STEM), and thermogravimetric analysis-mass spectrometry (TGA-MS) characterizations. Previous literature reveals that O, –OH, –F, and –Cl are typical MXene surface terminators. However, recent comparative investigations on the valence band intensity distribution in MXenes reveal that the –OH cannot be considered an intrinsic termination species in MXenes. The surface terminals (O, –OH, –F, and –Cl) of several MXenes, particularly V2CTx and Ti3C2Tx, will be identified and quantified here. We have also discussed different etching approaches for the synthesis of MXene, the dependence of MXene conductivity on MXene terminating groups, and the emission of various gaseous products that evolved during its chemical transformations. This paper provides significance, especially in the field of energy conversion and storage materials, where the intercalation process is crucial.