Nature of Terminating Hydroxyl Groups and Intercalating Water in Ti₃C₂T_x MXenes: A Study by ¹H Solid-State NMR and DFT Calculations

Since the discovery of two-dimensional transition metal carbides, referred to as MXenes, research efforts have targeted their applications in energy storage, as lithium-ion batteries and supercapacitors. This interest is attributable to MXenes’ large volumetric capacitance, high rate handling capability, and stable cycling performance, which largely rely on the surface chemistry provided by the terminating groups, such as −OH, −O, and −F. However, the atomic-scale characterization of these surface terminations is challenging for diffraction methods. Solid-state (SS)­NMR spectroscopy, especially ¹H SSNMR, is a promising approach for scrutinizing the surface terminations and the intercalated water on an atomistic-scale; yet, only a few SSNMR studies of MXenes have been reported to date, offering conflicting results and limited understanding of −OH terminations. Here, we used ¹H SSNMR experiments in concert with the DFT calculations of NMR parameters to identify multiple types of −OH groups residing on the external and internal surfaces in a commonly studied MXene, Ti₃C₂T_x. The study also identifies bulklike water trapped between the MXene flakes and interfacial water stranded on the surface. Lastly, two-dimensional ¹H–¹H correlation spectra elucidated the water–surface interactions and the mechanism of water deintercalation upon annealing.