Direct Measurement of Surface Termination Groups and Their

May 26, 2015 - Nuclear-spin magnetization transferred from 1H nuclei in the hydroxide surface termination layer to 13C nuclei in the center of the MXe...
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Direct Measurement of Surface Termination Groups and Their Connectivity in the 2D MXene V2CTx Using NMR Spectroscopy

Kristopher J. Harris,† Matthieu Bugnet,‡ Michael Naguib,§ Michel W. Barsoum,§ and Gillian R. Goward*,† †

Department of Chemistry and ‡Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4M1, Canada § Department of Materials Science and Engineering and A.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: 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 V2CTx, 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 → 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 V2CTx 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.

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INTRODUCTION Two-dimensional materials are known to provide unique and promising characteristics, with graphene being perhaps the most recognizable member.1 The past few years have seen the development of an intriguing analogue of graphenes, the MXenes,2 generated via removal of the “A” layer of a MAX phase. The starting MAX phases are a conventional 3D solid formed from infinite 2D sheets of MX and A layers that are strongly bonded together, where M is a transition metal, X is C and/or N, and A is an element from groups 13 and 14.3 Thus far, approximately 10 MXene structures have been synthesized,4,5 and the potential for many more is hinted at by the availability of over 70 MAX phases formed by different combinations of the components and different numbers of MX layers between the A sheets. Changing the surface termination group chemistry, combined with techniques for doping with electrons or holes, has the potential to create an extremely large set of MXene structures. As found in studies of other classes of 2D and nanostructured materials, a variety of promising capabilities have been found in the MXenes that have been investigated so far. The physical, © 2015 American Chemical Society

electrical, and chemical properties of the MXenes have made them attractive candidates in a variety of roles, e.g., as Li-, Na-, K-ion battery anodes,6−8 electrochemical capacitors,9,10 photonics,11 electronics,12 and chemical/temperature sensors.13 Given the tunability of the diverse MXene class, the range of applications for these materials is likely to balloon in the near future. While there is a broad scope of potential applications for MXenes, there are still significant questions regarding their structure. Powder X-ray, electron microscopy, pair-distribution function, and selected-area electron-diffraction studies show clear evidence of exfoliation into thin sheets that maintain relatively regular atomic order.4,5,14 However, for materials lacking order between adjacent thin layers, complete structure solution via diffraction methods is very difficult. Detailed energy-dispersive X-ray spectroscopy (EDS),8,10,15 X-ray photoelectron spectroscopy (XPS),2,8 and electron energy loss Received: March 30, 2015 Revised: May 22, 2015 Published: May 26, 2015 13713

DOI: 10.1021/acs.jpcc.5b03038 J. Phys. Chem. C 2015, 119, 13713−13720

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The Journal of Physical Chemistry C spectroscopy (EELS)16 studies show the presence of significant amounts of oxygen and fluorine, with aluminum also detected in some samples. These have been attributed to surface termination with F and OH (or O) moieties, intercalated water, and, in some cases, unreacted Al-containing MAX starting material.8 With this in mind, the chemical formula for the MXenes is usually given as M2XTx, M3X2Tx, etc., where T is some mixture of OH and F surface termination groups. Accurate structures for the metal−carbide (or metal−nitride) sheets and details regarding the structure, fraction of surface coverage, and ratio of termination groups in their surface layers are still lacking. A recent plane-wave DFT computational study on possible structures for prototype MXenes with complete monolayers of two types of surface termination layers, Ti3 C2 F 2 and Ti3C2(OH)2, reported similar internal metal carbide structures with two distinct arrangements of the surface termination groups.17 Both fluoride and hydroxide groups were calculated to have accessible energies in two arrangements labeled Type I and Type II. In the Type II structure, the F atoms are positioned directly overtop of the C atoms in the center sheet of the stack, while in Type I, they are positioned above the interstices in the C layer (directly above the V atoms at the opposite side of the sheet), as shown in Figure 1. Of particular interest is the fact that different band gaps were calculated for the Type I and Type II structures. While these models were

developed for the M3X2Tx system, they are also a good starting point for the thinner M2XTx MXenes. A list of potential structures should likely include those containing Type I and Type II arrangements of T groups on either side of the same sheet and intermixtures of OH and F groups. Important targets for future studies include confirming the identities of the surface groups, whether mixed geometries exist, and the possibility of surface vacancies. In an effort to address structural questions about the MXenes, particularly those regarding the surface termination layer, we present a nuclear magnetic resonance (NMR) study of a MAX-phase to MXene-phase conversion, using the example system V2AlC to V2CTx. A synthetic route for V2CTx has recently been published with a report of its promising capability as a lithium battery anode with a high rate capacity8 and results showing its use as a CO2 and temperature sensor.13 Here, 1H NMR spectra that show OH groups bonded to the MXene surface, along with a layer of hydrogen-bonded water molecules, are presented first. This is followed by 19F NMR experiments that show unusual effects assigned to bonding of surface fluoride ions to the metallic MXene backbone. Finally, 13C spectra that show an extremely high sensitivity to the MAX to MXene transformation are discussed, along with 1H−13C correlation, which further prove the connectivity at the surface. Taken together, these results provide deeper insight into the 2D MXene sheet with an approximately equal ratio of directly bonded hydroxide and fluoride surface groups, experimentally validating the hypothesized structural models.

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EXPERIMENTAL SECTION V2AlC and V2CTx were synthesized using literature procedures8 and characterized by powder X-ray diffraction. V2AlC was ball milled prior to conversion to V2CTx in order to reduce the particle size for better penetration of the pulsed radiofrequency (RF) electromagnetic radiation used during the NMR experiments. As might be expected, given that RF penetration into conductors decreases at higher frequency, 1H and 19F NMR experiments were only found to produce visible signal with ball milling as a preparatory step. Given that MAX phases are generally extremely tough, wear-resistant materials, it is expected8 that ball milling the precursor MAX phase rather than the more novel MXene phase avoids issues of sample alteration. The 1H and 19F NMR experiments were carried out under a 7 T applied magnetic field using a Bruker Avance III HD console and a 1.3 mm ultrafast MAS probe using spinning rates of 50−65 kHz. A series of data at lower spinning frequencies, and higher field, proved to be insufficiently resolved for detailed interpretation. The unique combination of ultrafast MAS together with low field (typically applied to paramagnetic systems) was essential to the success of this study. The only exception is the 1H T2-filtered experiment, which was acquired at an 11.75 T applied field with a Bruker Avance I spectrometer and a 2.5 mm probe under an MAS rate of 30 kHz. Pulsed RF fields of 150−170 kHz were applied in each case, and spectra were referenced to adamantane at 1.85 ppm (1H) or CFCl3 at 0 ppm (19F). RFDR homonuclear 1H dipolar recoupling during EXSY experiments were performed using a rotor-synchronized XY-8 pulse sequence.18 Background suppression was employed by using a pair of 90° RF pulses phased to alternately invert the initial magnetization (or leave its sign unchanged) on successive scans. For nuclei lying outside the coil, the effective

Figure 1. Sections of model structures for V2CT2 using T = F as an example. The 6-fold-coordinated C atoms are shown as gray polyhedra capped by V atoms in red; F capping atoms are shown in green. (a) Cross-sectional and perpendicular views of the infinite C, V, and F sheets of the Type II model structure. (b) The Type I model structure. 13714

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The Journal of Physical Chemistry C pulse tip angle is very small and the inversion half of this phase cycle is extremely ineffective, leading to selective cancelation of these “background” signals. The 13C NMR experiments were carried out under an 11.75 T applied magnetic field using a Bruker Avance I spectrometer with a 2.5 mm probe under MAS rates of 8−15 kHz. Pulsed RF fields of 80 kHz were employed, and the chemical shift scale was referenced to the high-frequency peak of adamantane at 38.5 ppm. Echoes in the Carr−Purcell/Meiboom−Gill (CPMG) experiment were rotor synchronized to align the Hahn echo and rotational echo pathways.19−22 Cross-polarization was performed using a 10 ms contact period with ∼60 kHz RF power (linearly ramped from 75 to 100% on the 1H channel during the contact period). The hexagonal structure of the MXene sheets was confirmed using high-resolution transmission electron microscopy. Electron energy loss spectroscopy performed in the electron microscope confirmed the presence of significant amounts of V, C, O, and F in V2CTx. Further details are given in the Supporting Information.

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RESULTS AND DISCUSSION H NMR. For clarity, we organize the results of the NMR by nucleus in this section, beginning with 1H NMR. Proton NMR provides extremely large signal intensity relative to other NMR nuclides, owing to the high natural abundance and large magnetic moment of the 1H nucleus. The 1H NMR spectrum of V2CTx (Figure 2a) can be accurately modeled using a sum of four sites (see Figure 2b,c and Table 1 for the model parameters). Over 90% of the spectrum is composed of two sites, at 6.5 and 85 ppm. The 6.5 ppm peak is at a typical chemical shift for water in porous materials and confirms the presence of significant amounts of water in the material. On the contrary, the 85 ppm peak is very unusual, lying well outside the typical 1H chemical shift range and having an extremely broad line shape. Both characteristics of the 85 ppm peak can only occur from magnetic interactions with unpaired electrons. In particular, the large isotropic chemical shift can only occur from unpaired electron density located at the 1H nucleus. Therefore, the moiety producing this peak is bonded to the MXene surface. As MXenes are likely metallic or low band gap semiconductors,17 the effect is most likely a Knight shift. Given the aqueous synthetic conditions, there is little doubt that the 85 ppm peak is from OH groups on the MXene surface. The spectrum also contains two minor contributors, which can be seen more clearly under the combination of a stronger applied magnetic field combined with a 1 ms T2 filter (that is found to selectively remove much of the water peak intensity) (Figure 2d). The 1.5 ppm peak reflects a site of less than 2% of the total sample and is assigned to a small amount of hydrocarbon (or hydrofluorocarbon) contamination. The final peak at 27 ppm, with ca. 6% integrated intensity, is obviously from a small portion of the sample. The most likely possibilities include sequestered HF, H2O/OH groups involved in extremely strong hydrogen bonds, or a small amount of MXene with a different structure such as a mixture of Type I and Type II MXenes. The longitudinal relaxation of each site was investigated for any relationships to structure, in particular, with respect to nearby conduction electrons. Inversion recovery experiments with background suppression (see Supporting Information for details) show that the sites are characterized by different relaxation time constants (T1). The 27 and 6.5 ppm peaks are 1

Figure 2. (a) Background-suppressed 1H NMR spectrum of V2CTx collected at 7 T under 60 kHz MAS. (b) Simulation of the full line shape shown in (a), using the components listed in Table 1 and simulated in (c). (d) 1H NMR spectrum of V2CTx collected at 11.75 T under 30 kHz MAS, using a background-suppressed Hahn echo with a total evolution time of 1 ms to attenuate components with faster transverse relaxation. Spinning sidebands are marked with an asterisk.

fully relaxed by 250 ms, reflecting T1 constants on the order of 50 ms. The hydroxyl protons on the MXene surface, on the other hand, are fully recovered from inversion by 5 ms, indicating a T1 value of ca. 1 ms. This extremely rapid relaxation undoubtedly has its source in the MXene conduction electrons, further corroborating the assignment of the 85 ppm peak to surface hydroxyl groups and suggesting a secondary means of assigning MXene surface groups in future studies. Considering that one would expect the intercalated water peaks to be strongly hydrogen bonded to the surface hydroxyls, it is interesting to probe the system for intersite correlations using 2D exchange spectroscopy (EXSY). In the EXSY experiment, the proton chemical shift is measured before and after a mixing period, τmix. The 2D EXSY spectrum of V2CTx collected with τmix = 993 μs is shown in Figure 3a. Any protons that have changed position, and therefore have changed their chemical shift, during τmix will appear off the main diagonal. Protons that are measured inside the water layer before the τmix period clearly produce observable magnetization in both water (at 6.5 ppm with spinning sidebands at ±200 and ± 400 ppm) and hydroxide sites (at 85 ppm) after the mixing time, as shown 13715

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significant role as the RFDR sequence partially reintroduces dipolar couplings averaged by the ultrafast sample spinning. Furthermore, EXSY experiments (not shown) collected with the sample heated 45 °C above the room temperature (RT) measurements showed a less than 10% change in the intensity of the peak heights, demonstrating that chemical exchange is no more than a minor contributor. On the basis of the above findings, we conclude that spin diffusion is the principal exchange mechanism, mediated by the magnetic dipole−dipole coupling. Because the strength of this coupling is related directly to the internuclear distance, chemical information is often contained in the rate of exchange peak buildup. Measuring at RT with RFDR, the off-diagonal exchange peak is already measurable with a τmix of only 133 μs (see Figure 3c). With a τmix = 933 μs, the exchange peak has increased 3-fold, for a near equalization of the OH protons originating in water and OH sites before the τmix period (i.e., the off-diagonal peak is at ca. 80% of the diagonal peak). This relatively rapid buildup of the exchange peak strongly implies that a water layer is hydrogen bonded to the hydroxide surface. While EXSY experiments showed dipolar-coupling mediated magnetization exchange between sites, intrasite dipolar couplings cannot be observed with this method. Conversely, double-quantum coherences (DQCs) generated in proximal spin pairs can be used to measure correlations between nuclei at the same chemical shift. Using the back-to-back (BABA) method for DQC generation,23 a spectrum filtered through DQC states can be generated (Figure 4a). Given the unusual breadth of the spectrum, it is not surprising that the method is only effective over a small region centered at the transmitter frequency. The evolution frequency of the DQC state occurs at the sum of the two sites involved, so self-correlation is easy to identify in 2D spectra as such peaks occur on a diagonal line where the frequency in the indirect dimension is twice that of the direct one. The diagonal streak in the 2D spectrum of V2CTx (Figure 4b) shows this characteristic and provides definitive proof of OH sites near other OH sites. The surface cannot, therefore, be described as isolated hydroxyls separated by vacancies or fluorine moieties. 1 H NMR spectroscopy on a material such as the MXenes is also interesting from the viewpoint of NMR methodology. Most commonly, 1H NMR spectroscopy is applied to diamagnetic solids, where the broadening effects of homonuclear dipole−dipole couplings are usually reduced via multiple pulse sequences or, more recently, ultrafast magicangle spinning (MAS). In materials with magnetic effects from electrons, ultrafast MAS may be useful to reduce the associated peak broadening. For V2CTx, ultrafast MAS combined with a low applied magnetic field (7 T) is found necessary to produce a resolved 1H spectrum (Figure 2a). Using ball milling to reduce the particle size and thereby allow sufficient RF penetration of the electrically conductive material was also necessary. We also found that ball-milled samples and small rotor sizes combined to provide conditions where fast MAS is possible without undue increases in pneumatic pressure or the necessity for sample dilution with an electrically insulating filler; some increased sample heating from eddy currents is, however, likely. Furthermore, we found it critical to suppress the intense 1 H background signal from the protons in the stator of the NMR probe (see Experimental Section for more details). Previous studies of MXenes have had difficulties conclusively measuring hydroxides and have furthermore been unable to provide a measure of the OH to H2O populations. The

Figure 3. (a) 1H EXSY NMR spectrum of V2CTx collected at 7 T under 60 kHz MAS with τmix = 933 μs, during which RFDR dipolar recoupling pulses were applied. (b) A slice of the spectrum shown in (a), extracted at 6.7 ppm of the indirect dimension. (c) A slice of the spectrum shown in (a), extracted at 110 ppm of the direct dimension is shown in black. Overlays of equivalent slices extracted from EXSY experiments collected under identical conditions except for the removal of RFDR recoupling (in red) or a shorter recoupling time of 133 μs (in blue) are also displayed.

in Figure 3b. This correlation provides clear evidence of close contact between the water and hydroxide sites. A series of EXSY experiments under alternate conditions were explored next, in an effort to better quantify this contact. The exchange observed in an EXSY experiment can be caused by the movement of atoms between sites (chemical exchange) and/or by the transfer of magnetization to nearby nuclei via coupling of their magnetic dipole moments (spin diffusion). More intense exchange peaks were observed, as shown in Figure 3c, under the application of radio frequency driven dipolar recoupling, RFDR,18 during τmix. This experiment demonstrates that the spin diffusion mechanism plays a 13716

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conductivity on radio-frequency skin-depth effects, as well as the fact that sample aging effects have not yet been thoroughly investigated in MXenes, future NMR studies will no doubt report sample compositions with increased confidence as these quantities become known. 19 F NMR. Fluorine-19 NMR is employed in an attempt to characterize the fluorine capping sites that have been posited to exist on the MXene surface.5 The 19F spectrum collected at 65 kHz MAS (Figure 5a) is extremely broad despite the use of

Figure 4. 1H NMR spectra of V2CTx collected at 7 T under 60 kHz MAS at 7 T. (a) The unfiltered background-suppressed 1H spectrum (in black) is compared with the DQ-filtered spectrum (in red) with an arbitrary scaling factor. The DQ filter was achieved using the BABA pulse sequence, with a 1 rotor period for both DQ excitation and reconversion, and a 170 kHz rf field. (b) Two-dimensional 1H DQ-SQ correlation using the BABA DQ excitation and reconversion shown in (a).

Figure 5. (a) Background-suppressed 19F NMR spectra of V2CTx collected at 7 T under 65 kHz MAS. (b) Simulations of each component contributing to the total simulated line shape shown at top, generated using the parameters listed in Table 2. Comparison of background-suppressed 19F NMR spectra of V2CTx collected at 7 T at MAS speeds of 65 kHz (red) and 30 kHz (blue).

Table 1. 1H NMR Parameters Used in the Best-Fit Simulations Shown in Figure 2c isotropic chemical shift (ppm)

relative intensity (%)

line width at 1/2 height (kHz)

1.5(5) 6.5(5) 27(3) 85(10)

2.0(5) 43(5) 6(2) 49(6)

0.75 3 8 45

ultrafast MAS. The spectrum can be modeled with reasonable accuracy, as shown in Figure 5b and Table 2, using a simulation composed of one major and two minor sites. The longitudinal relaxation rates of each site was also analyzed (see Supporting Information for details), and it appears that the −265 and −158 ppm peaks are characterized by two (slightly different) T1 relaxation constants on the order of 20 ms, while the −122 ppm site has a much longer T1 constant. Given the short T1 constant and the extreme line broadening, the −265 ppm peak can be assigned with little doubt to fluorine atoms capping the surface of the conductive MXene sheet. The peak at −122 ppm is narrow, has a long T1 value, and contributes approximately 0.5% of the total spectral intensity; most likely, this peak is due to a small amount of hydrofluorocarbon impurity given it mirrors the chemical shift of the CF2 groups in Teflon. The final peak at −158 ppm comprises 9.5% of the total spectral intensity, reflecting a site that is a minor, but significant, component of the sample. This −158 ppm peak is not nearly so

integrated intensities of the sites in Table 1 show that the ratio is 46 OH moieties to 21.5 H2O molecules, with the 27 ppm peak assigned as contributing to this ratio an amount of either 6 OH moieties or 3 H2O molecules. The ratio of V:C:F:OH +H2O, measured by EDS as 2:1:1.2:1.5, therefore yields an NMR corrected value of V:C:F:OH = 2:1:1.2:(0.96−1.00), depending on the assignment of the 27 ppm peak as OH or H2O sites. Considering that there are likely very small amounts of residual fluorine from the HF treatment, it seems likely that the material differs very little from the idealized ratio of V2CF(OH). Given the importance of particle size and electrical 13717

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The Journal of Physical Chemistry C broad as the −265 ppm peak, though it does display a similarly short T1 value. The chemical shift is quite different from the main impurities that might be expected, as HF resonates at +40 ppm and the various polymorphs of AlF3 have shifts of −171.6 to −175.3 ppm. However, the −158 ppm shift does lie inside the range of simple ionic fluoride salts, so some sort of aluminum- and fluorine-containing byproduct of the formation reaction is one possible assignment. Another possibility is a second type of surface capping OH or F anion, such as the Type I and Type II structures discussed above. The dominant −265 ppm peak from the MXene fluorides is unusually broad. Notably, its width is of the same order of magnitude as the spinning speed available and is nearly unchanged in spectra collected at 65 and 30 kHz MAS (Figure 5c). It appears that the spinning speed is just becoming sufficient to resolve evidence of low-intensity spinning sidebands at the 65 kHz spinning rate. The −265 ppm peak is therefore subject to very large isotropic broadening factors, nearly equal to the anisotropic component. As observed in the 1 H spectroscopy, the large line width is not due to rapid T2 relaxation, but rather to a distribution of environments. The increased difficulty of acquiring high-quality 19F spectra as compared to 1H data is no doubt related to the fact that the fluorines lie closer to the MXene surface, while an oxygen atom separates the protons. It is worth noting that in the previous section the unusual 1H chemical shift of the MXene site was easily assigned to magnetic effects from the nearby conduction electrons because of the very small chemical shift range of 1H in diamagnetic systems. Conversely, the 19F chemical shifts that can be produced by the usual induced orbital-angular-momentum terms in diamagnetic solids are extremely large, making it difficult to quantify a specific Knight shift. In the 19F case, it is the extreme line broadening and relatively short T1 relaxation constant that demonstrate the connectivity in the MXene.

Figure 6. (a) 13C NMR spectrum of V2AlC collected at 11.75 T under 8 kHz MAS using a Bloch decay experiment. (b) 13C CPMG-MAS NMR spectra of V2CTx collected at 11.75 T under 15 kHz MAS. Experimental spectrum ii is displayed along with the two-component simulation i described in the text. (c) 1H → 13C cross-polarization enhanced 13C CPMG-MAS NMR spectrum of V2CTx collected at 11.75 T under 10 kHz MAS.

Table 2. 19F NMR Parameters Used in the Best-Fit Simulations Shown in Figure 5b isotropic chemical shift (ppm)

relative intensity (%)

line width at 1/2 height (kHz)

−265(50) −158(5) −122(0.5)

90(3) 9.5(3) ∼0.5

55 7 0.5

MAS enhancement will likely be useful for other MXenes and conductive carbides when line widths are broad (see Supporting Information for further discussion of the CPMGMAS method). The 13C CPMG-MAS spectrum of the V2CTx is modeled as a sum of two peaks in Figure 6b. Note that a side effect of the CPMG-MAS method is the signal collection into “spikelets” whose tops trace the line shape that would be recorded with a conventional method. The peak at 208 ppm evidences a significant amount of unreacted V2AlC remaining after the heterogeneous reaction, in agreement with diffraction studies.8 While NMR spectra are reliably quantitative in many cases, the CPMG-MAS enhancement depends on the value of T2 for each site, making the ratio of peak areas only approximate in this case. The single, very broad peak at 260(15) ppm is assigned to the MXene. The large change in 13C chemical shift upon etching of the Al layer that lies across the V layer shows the high sensitivity of NMR spectroscopy to structural and electronic changes. Given the extreme sensitivity of Knight shifts to small changes in the band gap of semiconductors, the frequency increase upon exfoliation may be due to a change in the band structure near the Fermi level.

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C NMR. 13C NMR was employed to study the change from the MAX phase V2AlC to the MXene V2CTx, though the spectroscopy is somewhat more difficult than for 1H or 19F given the smaller magnetic moment of 13C and its very low isotopic abundance (1%). The single 208(4) ppm peak in the Bloch decay spectrum of V2AlC (Figure 6a) corresponds to the single unique site in the crystal structure. Bloch decay experiments attempted on V2CTx produced no visible signal, though 13C CPMG-MAS19−22 proved capable of enhancing the signal enough for it to be recorded, as shown in Figure 6b. CPMG-MAS enhancements are possible when the lifetime of the transverse magnetization is much longer than the inverse of the line width in Hz.22 The method has recently been shown to provide enhancements in 13C NMR spectroscopy of disordered graphitic carbons, where the line widths are broad and the T2 constants are very long; in V2CTx, the conductive nature of the sample leads to relatively short T2 values, but the line width is so large that enhancement still occurs. CPMG13718

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The Journal of Physical Chemistry C Given that the proposed reaction model for MXene formation involves forming a layer of hydroxyl groups on the MXene surface, it is interesting to probe the system for 1H−13C contacts using NMR. 1H−13C cross-polarization (CP) experiments involve polarizing 1H nuclei and transferring that magnetization to nearby 13C nuclei using the direct dipole− dipole coupling interaction, while filtering out any 13C polarization that does not originate from 1H nuclei. CP efficiency inherits the strong (rCH−3) distance dependence of the dipole−dipole coupling, with the C−V−OH topology lying at the longer end of the viable CP internuclear distance range. The 1H−13C CP-filtered, CPMG spectrum of V2CTx shown in Figure 6c demonstrates conclusively that the MXene formation brings protons into close contact with the carbon layer. This result provides further evidence that a significant fraction of the MXene surface is terminated by hydroxide groups.

ACKNOWLEDGMENTS

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CONCLUSIONS This report presents several new insights into the structure of the fascinating new MXene class of materials via a range of multinuclear NMR experiments. The broad peaks observed in all of the spectra are consistent with magnetic broadening and/ or a distribution of bond lengths and local structures. 1H NMR experiments carried out on V2CTx show hydroxide groups directly bonded to the MXene, with a layer of water hydrogen bonded to the hydroxide surface. 19F NMR experiments evidence fluoride moieties bonded to the MXene surface, showing extremely unusual 19F spectra from the strong interaction with the metallic MXene. 13C NMR provides a measurement at the center of the MXene layer, and the 13C chemical shift is extremely sensitive to the MAX → MXene transformation. Employing 13C NMR to a series of MAX to MXene transformations, perhaps in combination with quantum chemical calculations, looks like a highly promising means to obtain insight into the atomic and electronic structure of the MXenes. Polarization transferred from 1H nuclei in the hydroxide surface termination layer to 13C nuclei in the center of the MXene sheet further proves the connectivity of the OH groups to the surface. In summary, solid-state NMR yields a detailed view of the bonding within this structurally complex material and provides direct experimental evidence needed for the validation of structural models that have been proposed to date. Future studies of MXenes would clearly be aided by the NMR methods shown, and we note that an even greater amount of information will likely become available as empirical trends are tabulated and correlations with quantum chemical calculations are employed. ASSOCIATED CONTENT

S Supporting Information *

Inversion−recovery measurements of 1H and 19F longitudinal nuclear spin relaxation in V2CTx; comparison of Bloch decay and CPMG 13C NMR spectra for V2AlC and V2CTx; highresolution TEM and chemical analysis of V2CTx powder. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03038.

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The authors are grateful to Darren Brouwer and Carl Michal for useful suggestions on background suppression techniques. G.R.G. acknowledges support through the NSERC DG and the Automotive Partnerships of Canada programs. The authors thank Y. Gogotsi for helpful discussions on the structure of MXenes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.R.G.). Notes

The authors declare no competing financial interest. 13719

DOI: 10.1021/acs.jpcc.5b03038 J. Phys. Chem. C 2015, 119, 13713−13720

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DOI: 10.1021/acs.jpcc.5b03038 J. Phys. Chem. C 2015, 119, 13713−13720