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.<br>
Incorporation of 15N-TNT transformation products into humifying plant organic matter as revealed by one- and two-dimensional solid state NMR spectroscopy
