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

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• 作者：Kristopher J. Harris ; Matthieu Bugnet ; Michael Naguib ; Michel W. Barsoum ; Gillian R. Goward
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：June 18, 2015
• 年：2015
• 卷：119
• 期：24
• 页码：13713-13720
• 全文大小：318K
• ISSN：1932-7455

文摘

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 V₂CT_x, 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 鈫?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 V₂CT_x 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.