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

Kent J Griffith,Mark Anayee,Michael A Hope,Philip J Reeves,Yury Gogotsi,Clare P Grey

doi:10.1021/jacs.0c09044

Kent J Griffith, Mark Anayee + Show 4 more

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https://doi.org/10.1021/jacs.0c09044

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Abstract

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, Nb2AlC, Nb4AlC3, Nb2CTx, and Nb4C3Tx, with their unique electronic and mechanical properties, using solid-state NMR spectroscopy to examine a variety of nuclei (1H, 13C, 19F, 27Al, and 93Nb) with a range of one- and two-dimensional correlation, wide-line, 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.

Highlights

MAX phases (M = early transition metal; A= Al, Si, Ga; X = C, N) are a large class of MXenes are a diverse class of 2D compounds derived from MAX phases via etching of the A-site atoms.[5]
The quadrupolar parameters determined by high-resolution magic-angle spinning (MAS) and static wideline 93Nb and 27Al spectra give insights on the local structures of the MAX phases, while 93Nb and 13C spectra reveal the nature of the etching to the corresponding MXene phases, with the 93Nb nuclear magnetic resonance (NMR) further confirming that the latter are surface terminated. 1H, 19F and 93Nb two-dimensional NMR spectroscopy identifies the chemistry and connectivity of the MXene surface terminations
MAS rate and magnetic field are given for each spectrum

Summary

Introduction

MXenes are a diverse class of 2D compounds derived from MAX phases via etching of the A-site atoms.[5]. The quadrupolar parameters determined by high-resolution magic-angle spinning (MAS) and static wideline 93Nb and 27Al spectra give insights on the local structures of the MAX phases, while 93Nb and 13C spectra reveal the nature of the etching to the corresponding MXene phases, with the 93Nb NMR further confirming that the latter are surface terminated. The presence of diffraction-silent side-products is confirmed, including the observation of aluminum oxides via the 27Al NMR spectra Overall, these results have important implications for the synthesis, characterization, and functional properties of Nb MAX and MXene phases. To prepare Nb4AlC3, powders of niobium (99.9% metals basis, 325 mesh), aluminum (99.8% purity, 300 mesh), and carbon black (99% purity, 300 mesh) were mixed and the synthesis was performed as previously reported.[28] To prepare Nb4C3Tx, 0.4 g Nb4AlC3 powder was added to 30 mL HF solution (aqueous, 49%, Millipore-Sigma) and stirred at room temperature (20–25 °C) for 6 days. Computed EFG parameters were used as the starting point to fit the experimental 27Al and 93Nb spectra

Results and Discussion

T exhibits an unexpected resonance at 16

Conclusions