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Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy

preprint
submitted on 24.08.2020 and posted on 25.08.2020 by Kent Griffith, Michael Hope, Philip J. Reeves, Mark Anayee, Yury Gogotsi, Clare P. Grey
MXenes, derived from layered MAX phases, are a class of two-dimensional materials with emerging applications in energy storage, electronics, catalysis, and other fields due to their high surface areas, metallic conductivity, biocompatibility and attractive optoelectronic properties. MXene properties are heavily influenced by their surface chemistry, but a detailed understanding of the surface functionalization is still lacking. Solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to the interfacial chemistry, the phase purity including the presence of amorphous/nanocrystalline phases, and the electronic properties of the MXene and MAX phases. In this work, we systematically study the chemistry of Nb MAX and MXene phases, Nb2CTx and Nb4C3Tx, with their unique electronic and mechanical properties, using solid-state NMR spectroscopy and examine a variety of nuclei ( 1 H, 13C, 19F, 27Al and 93Nb) with a range of one- and two-dimensional correlation, wideline, high-sensitivity, high-resolution, and/or relaxation-filtered experiments. Hydroxide and fluoride terminations are identified, found to be intimately mixed, and their chemical shifts are compared with other MXenes. This multinuclear NMR study demonstrates that diffraction alone is insufficient to characterize the phase composition of MAX and MXene samples as numerous amorphous or nanocrystalline phases are identified including NbC, AlO6 species, aluminum nitride or oxycarbide, AlF3×nH2O, Nb metal, and unreacted MAX phase. To the best of our knowledge, this is the first study to examine the transition-metal resonances directly in MXene samples, and the first 93Nb NMR of any MAX phase. The insights from this work are employed to enable the previously-elusive assignment of the complex overlapping 47/49Ti NMR spectrum of Ti3AlC2. The results and methodology presented here provide fundamental insights on MAX and MXene phases and can be used to obtain a more complete picture of MAX and MXene chemistry, to prepare realistic structure models for computational screening, and to guide the analysis of property measurements.

Funding

Winston Churchill Foundation of the United States

Herchel Smith Scholarship

Oppenheimer Foundation

EPSRC EP/M009521/1

NSF DGE-1646737

NSF DMR-1740795

NSF DMR-0521267

NSF ECCS-1542205

History

Email Address of Submitting Author

kentjgriffith@gmail.com

Institution

University of Cambridge, Northwestern University, Drexel University

Country

United Kingdom

ORCID For Submitting Author

0000-0002-8096-906X

Declaration of Conflict of Interest

The authors declare no competing financial interest.

Version Notes

submitted v1

Licence

Exports