Since diffraction did not reveal the Al-O, Al-F, or Al-(C,N,O) compounds identified here, these secondary phases must be amorphous, present as small nanoparticles, or hidden under the MAX/MXene diffraction peaks. These findings again highlight that the picture from diffraction alone can be misleading. Furthermore, NbCx was almost entirely hidden under the Nb4AlC3 XRD peaks. A method that is sensitive to amorphous phases such as solid-state NMR and/or quantitative diffraction with an internal standard should be routinely performed to characterize MAX and MXene phases. depending on the synthesis procedure . The free H2O signal can be reduced by drying the MXene at 200 °C in vacuo (Figure ), after which an additional resonance can be distinguished at 12.2 ppm, as well as sharper signals between -0.5 and 1.6 ppm; the former could be due to a further -OH termination environment, or bound H2O partaking in strong hydrogen bonding, while the latter are assigned to intercalated volatile contaminants, as was also observed for Ti3C2Tx. There is also a loss of intensity for the 26.0 ppm signal in particular, which may be due to loss of -OH terminations as H2O during the drying process. 61 The assignment of the 1 H NMR resonances at high chemical shift to surface species on the MXene layers can be confirmed by 1 H/ 93 Nb TRAPDOR experiments: 62 for these spectra, a 1 H spin echo is recorded with continuous irradiation of 93 Nb during only the first half of the echo, thus any dephasing due to recoupled 1 H-93 Nb interactions is only partially refocused, resulting in a loss of signal intensity for 1 H environments in the vicinity of 93 Nb; when the difference is taken between spin echoes recorded with and without 93 Nb irradiation, these 1 H environments (near 93 Nb) can be selectively observed. The signals at 12.2, 19.5 and 26.0 ppm can all be clearly observed in the TRAPDOR difference spectra (Figure and Figure ), confirming their vicinity to Nb atoms at the MXene surface; the signals at less than 5 ppm, on the other hand, are suppressed in the TRAPDOR difference spectra. A weaker TRAPDOR effect is observed for the free H2O resonance at 6.3 ppm, which suggests that only some of the free H2O is proximal to Nb and/or that it is mobile; this may be evidence of chemisorbed water, as proposed by Persson et al. The sideband-separated F NMR spectrum of Nb2CTx, recorded with a MATPASS pulse sequence, shows a number of different resonances (Figure ). The signal at -158 ppm is due to AlF3⋅nH2O, a by-product of the etching process also seen in the Al spectrum (Figure ); this resonance is suppressed in the 19 F/ 93 Nb TRAPDOR difference spectrum (Figure ), leaving the signals at -20, -132 and -330 ppm, which must be near Nb. 1 H/ F HETCOR experiments (Figure ) are completely dominated by the H2O↔AlF3 correlation, so cannot be used to aid assignment of the 19 F spectrum. After drying at 200 °C in vacuo the AlF3⋅nH2O resonance shifts to -164 ppm due to changes in hydration, so that the -132 ppm signal can be seen as a shoulder more clearly, and there is also a small, sharp peak at -121 ppm from a fluorinated organic impurity. The 19 F signal at -20 ppm is largely removed by drying, so may be tentatively ascribed to -F terminations with strong hydrogen bonding to H2O molecules, or adjacent -OH terminations (given that the intensity of the -OH signals in the 1 H NMR also decreases on drying). Conversely, the major 19 F signal at -132 ppm is assigned to -F terminations without strong H bonding. The minor signal at -330 ppm is strongly ionic; it may be a niobium fluoride species formed as a byproduct from excessive etching and could therefore be minimized by using milder etching conditions. The 1 H NMR spectrum of Nb4C3Tx (Figure ) is dominated by a peak at 1.0 ppm due to the organic impurity identified from the 1 H→ 13 C HETCOR experiment (Figure ). Further Both the organic contaminant and the H2O signals can be reduced by drying in vacuo at 200 °C overnight (Figure ), while the -OH terminations can be identified in the 1 H/ 93 Nb TRAPDOR difference spectrum (Figure and Figure ). In the TRAPDOR experiment, the 1.0 ppm peak is almost, but not entirely, suppressed; the residual signal and satellite peaks are due to the differential phase (Bloch-Siegert) shift introduced by Nb irradiation, which results in imperfect cancellation in the difference spectrum. Two other signals can also be distinguished, centered at ca. -190 and -80 ppm; these are assigned to -F terminations as they can clearly be observed in the F/ 93 Nb TRAPDOR difference spectrum (Figure ), although the sidebands of the terminal -F signals are unresolved. Further evidence for the assignment of the -OH and -F terminations can be seen in the 1 H/ 19 F HETCOR spectra: in the F→ 1 H spectrum (Figure ) a broad correlation is observed between both the H2O (6.0 ppm) and -OH (15 ppm) 1 H resonances and F intensity between ca. 50 and -300 ppm that corresponds to the signals from -F terminations at -190 and -80 ppm, with unresolved spinning sidebands. This supports the assignments of the 1 H and 19 F spectra, as well as showing that the terminations are intimately mixed, rather than being segregated into different regions, as also found for Ti3C2Tx. In contrast, the 1 H→ 19 F spectrum (Figure ) only shows correlation between the 1 H H2O (6.0 ppm) signal and the -80 ppm -F termination F signal. The lack of signal from the -OH terminations is ascribed to a short 1 H T1r for this resonance, so that the transverse magnetization decays before developing significant cross polarization. The fact that the H2O resonance correlates only with the -80 ppm F signal suggests that these -F termination environments are associated with interlayer water via hydrogen bonding, whereas those at -190 ppm are not. The higher frequency Table shows a comparison of the shifts of the observed surface species for the different MXenes studied by 1 H and 19 F NMR to date. For the -OH terminations, V2CTx is the exception with an extremely large shift of 85 ppm; this is due to a metallic Knight shift, as confirmed by the short T1 relaxation of this signal, Interestingly, the -F terminations of V2CTx do not appear to be significantly Knight shifted and are observed at a similar frequency to those of Ti3C2Tx. The -OH 1 H shifts obtained for Ti3C2Tx have been shown by both Hope et al. and Anayee et al. to depend on the synthesis procedure, but in both cases fall in the range 12.5-20 ppm. In contrast, Kobayashi et al. observed no 1 H signals at frequencies above 7 ppm, instead ascribing signals at 0.5-2.0 and 3.6 ppm to -OH terminations on the basis DFT calculations and the assignment of a Ti2CTx 1 H spectrum in a separate report by Sugahara 68 . These differences may be due to sample preparation, however, we note that the studies of The -OH 1 H shifts for the Nb MXenes are similar to those observed for Ti3C2Tx by Hope et al. and Anayee et al. , although as the 26 ppm signal for Nb2CTx is outside the diamagnetic range for 1 H NMR, there must also be at least a small Knight shift, as for V2CTx.