Arrangement and Dynamics of Diamine, Etheric, and

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Arrangement and Dynamics of Diamine, Etheric, and Tetraalkylammonium Intercalates within Graphene or Graphite Oxide Galleries by 2H NMR Kazuma Gotoh,*,† Chika Sugimoto,† Ryohei Morita,† Tatsuya Miyatou,‡ Motohiro Mizuno,‡ Weekit Sirisaksoontorn,§,∥ Michael M. Lerner,§ and Hiroyuki Ishida† †

Graduate School of Natural Science & Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Department of Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Ishikawa 920-1192, Japan § Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331-4003, United States ‡

ABSTRACT: Ternary graphite intercalation compounds (GICs), which consist of graphite, alkali metal (Li, Na, K) cations, and organic cointercalates such as ethylenediamine (en) or tetrahydrofuran (thf), are useful precursors to graphene-based materials and tetraalkylammonium GICs. This study investigates the gallery arrangements and intercalate dynamics of the deuterated en(d4), thf(d8), piperazine(d10), or 1,4-diazabicyclo[2.2.2]octane(dabco)(d12) in ternary GICs containing Na+ or K+ cations using XRD and solid state 23Na and 2H NMR line-shape analyses. An en(d4)-graphite oxide (GO) intercalation compound and the trihexylmethyl(d3)ammonium (thma) GIC were also prepared and evaluated by XRD and NMR. The 2H NMR spectra exhibit a narrow peak ascribed to intercalates undergoing isotropic rotation and a broad powder pattern ascribed to intercalates in a rigid state or undergoing uniaxial rotations. The thma intercalates in thma(d3)GIC and the en(d4) intercalates in en(d4)-GO are relatively mobile and can diffuse; this may arise because there are no alkali metal cations in the galleries. The molecular dynamics as well as the synthetic challenges presented by some GICs are explained in terms of different affinities of alkali metal cations to the cointercalates.

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ion batteries.7,8 In the past few years, ternary GICs containing alkali metal cations and a wide range of amine/diamine molecules such as n-alkylamine,9,10 ethylenediamine (en),11 and 1,2-diaminopropane12 have been prepared and characterized by our group. Na-en-GIC (i.e., the GIC with Na+ and en cointercalates) can also serve as a precursor for the preparation of GICs containing tetraalkylammonium13−15 cations. In these reactions, Na and en are quantitatively displaced by the alkylammonium cations in a dimethyl sulfoxide (DMSO) solvent. These reactions can also result in surface alkylation of the GICs that affords remarkable chemical passivity in aqueous media, other protic solvents, and the ambient environment. Diaminoalkanes with the general molecular formula of H2N(R)NH2 are known to intercalate into a wide range of layered compounds including metal chalcogenides, metal oxychlorides, metal oxides, clays,16 graphite oxide (GO),17,18 as well as graphite. The product of the diamine-GO obtained by

INTRODUCTION Graphite intercalation compounds (GICs) form when graphite is reduced or oxidized, and cations or anions, respectively, intercalate between the graphene layers. This intercalation chemistry is important for electrochemical energy storage in lithium ion batteries, and GICs are also used in other important applications such as producing thermally exfoliated graphites. GICs present an interesting and challenging chemistry and can display potentially useful properties such as high electric conductivity, superconductivity, and thermoelectricity and have therefore attracted considerable attention from scientists in many disciplines.1 Several previous reports have described the formation of ternary GICs containing both alkali metal cations with an organic or ammonia cointercalate. Ethers, including tetrahydrofuran (thf),2 1,2-dimethoxyethane,3 and 1,2-diethoxyethane,4 can form ternary GICs with different alkali metal cations (Li, Na, K) via reaction in a suitable organic solvent. The resulting GIC structures have been investigated using powder X-ray diffraction (XRD), NMR,2,5,6 and other methods. It has been reported that some ternary GICs consisting of Na and ethers show long-term cycle stability as anode of sodium © XXXX American Chemical Society

Received: March 30, 2015 Revised: April 24, 2015

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The KC8 (50 mg) was then reacted with either ppz(d10) (C/ D/N Isotopes) (23 mg) at 413 K or dabco(d12) (C/D/N Isotopes) (20 mg) at 443 K in a sealed glass tube at reduced pressure for 12 h. Caution: Sodium and potassium metals are highly f lammable when in contact with air or water and should be handled with care. The thma(d3)-GIC was prepared by stirring Na-en-GIC (80 mg), which contains 0.3 mmol of en, with (C6H13)3(CD3)NI (0.2 mmol) in DMSO (2 mL). The reaction was maintained at 333 K for 10 min under N2. The product was washed with acetonitrile, dried under vacuum at room temperature for several hours, and then rinsed with anhydrous methanol. Graphite oxide (GO) was synthesized using Brodie’s method 26,27 with 3 h of oxidation. The en(d4)-GO intercalation compound was prepared by mixing GO (50 mg) with en(d4) (0.05 mL), stirring for 2 h, and evacuating for 2 h to remove excess en(d4). All products were characterized by powder X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer with Cu Kα radiation. 2H NMR spectra (resonance frequency at 45.3 MHz) were obtained from 123 K to ambient temperature using a JEOL ECA-300 spectrometer and a quadrupole echo pulse sequence with pulse length of 2.8 μs. 23Na MAS NMR spectra (resonance frequency at 132.2 MHz) of Na-en-GIC were recorded using an Agilent Technologies DD2 spectrometer and applying a single pulse sequence with 2.0 μs pulse length. D2O and 1 M NaCl were used as references for the 2H and 23Na chemical shifts, respectively. Thermogravimetric analyses (TGA) were performed using a Shimadzu TGA-50 analyzer.

hydrazine reduction shows promise in energy storage, with measured capacitances of 45−130 Fg−1.19 The en-GO compound was found to generate reduced graphene oxide (rGO) sheets when heated to 300−400 °C,20 and rGO-metal nanoparticle (Pt, Pd, Ru, Au, Ag) composites can catalyze oxygen reduction21,22 or other organic reactions.23 The goal of this report is to help elucidate the gallery structures and dynamics in ternary GICs or related structures based on GO. We have reported previously on the use of 1H NMR to analyze Na-en-GIC;11 however, that methodology was limited by the strong background signal from the NMR probe. As described herein, this interference can be avoided by preparing ternary GICs containing deuterated intercalates and analyzing the molecular dynamics with solid-state 2H NMR. For example, Na-en(d4)-GIC and Na-thf(d8)-GIC can be simply prepared using en(d4) and thf(d8), respectively, in place of en and thf. In this study, two novel GICs, containing the deuterated forms of piperazine, ppz(d10), and 1,4diazabicyclo[2.2.2]octane, dabco(d12), were also characterized by the same methods. A GIC containing a deuterated alkylammonium intercalate, trihexylmethyl(d3)-ammonium (thma(d3))-GIC, and an intercalation compound of GO with en, en(d4)-GO were also prepared and characterized. The structures of all of the intercalates used in this study are presented in Figure 1.

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RESULTS X-ray Diffraction. XRD data for the deuterated GIC and GO products are presented in Figure 2. Although the smaller powder sample masses obtained result in lower XRD intensities for the products obtained, the XRD patterns clearly indicate the product phases present and provide accurate determination of the basal repeat distances and thus gallery heights for each phase (Table 1). The Na-en(d4)-GIC sample (Figure 2a) exhibits strong reflections from the stage 1 compound with a basal repeat distance, d = 0.693 nm, matching previous reports for [Na(en)1.0]C15.14 The derived gallery height of 0.358 nm for this product (for stage 1, gallery height = basal repeat distance−graphene sheet thickness) supports the cointercalation of en(d4) in the GIC.11 Na-thf(d8)-GIC yields a predominant stage 1 compound (d = 1.100 nm), in good agreement with previous reports,24,28 along with weaker reflections from unreacted graphite and a stage 2 GIC. Unreacted graphite is expected to be silent in the subsequent NMR analyses, and while the higher stage products contain less intercalate, the gallery dimensions and arrangement should be very similar to those in the predominant stage 1 products. The XRD pattern for K-ppz(d10)-GIC also shows a predominant stage 1 product (d = 1.210 nm) with a smaller stage 2 component. Three peaks, indicated by arrows in Figure 2a, did not correspond to either the ternary GIC or the ppz or KC8 precursors and were unassigned. The stage 1 gallery height (0.875 nm) indicates a ppz bilayer arrangement. The XRD pattern for K-dabco(d12)-GIC also indicates a predominant stage 1 product with a minor stage 2 component. The obtained gallery height (0.564 nm) indicates a monolayer gallery arrangement. The compositions of the latter two products were found by elemental analyses to have compositions in the ranges [K(ppz(d10))0.5−0.7]C8 and [K(dabco(d12))1.4−2.0]C8.

Figure 1. Structural formulas of deuterated molecular or cationic intercalates employed in this study.

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EXPERIMENTAL SECTION GIC Syntheses. Na-en(d4)-GIC and Na-thf(d8)-GIC were prepared according to previously reported methods used to synthesize [Na(en)1.0]C1511,13 and [Na(thf)3.5]C32,1,24,25 by substituting en(d4) and thf(d8) for en and thf. In brief, 50 mg of graphite powder (TIMREX SLP50; Timcal) and 30 mg of Na metal (Wako Pure Chemical) were added to 0.5 mL of en(d4) (SI Science) or thf(d8) (SI Science) in a small test tube under a N2 atmosphere. 5−10 mg of naphthalene was also added to thf(d8) as a carrier transfer between sodium and graphite. The mixture was stirred for 1 to 2 days at ambient temperature using a glass-coated magnetic stirring bar in the test tube until the graphite powder color changed from black to dark blue. After the reaction, the solid product was separated by centrifugation and then dried at reduced pressure for 12 h. Kppz(d10)-GIC and K-dabco(d12)-GIC were obtained by direct solid-state reaction of KC8 and these diamine reagents. KC8 was first synthesized by combining graphite powder and K metal (Wako) at 543 K under reduced pressure in a sealed glass tube. B

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Figure 2. XRD patterns for GICs and GO compound products.

Table 1. Basal Repeat Distances, d, and Gallery Heights of Prepared Samples Obtained from XRD Na-en(d4)-GIC Na-thf(d8)-GIC K-ppz(d10)-GIC K-dabco(d12)-GIC thma(d3)-GIC GO en(d4)-GO

d/nm

height/nm

0.693 1.100 1.210 0.899 1.130 0.635 0.842

0.358 0.765 0.875 0.564 0.795 0.300 0.507

The XRD patterns of GO, en(d4)-GO, and thma(d3)-GIC are shown in Figure 2b. The GO basal repeat is observed at 2θ = 13.94° (d = 0.635 nm), and the gallery heights are 0.507 nm for en(d4)-GO and 0.795 nm for thma(d3)-GIC. TGA results (not shown) gave a thma(d3) mass content of 22%, corresponding to a product composition of [(C6H13)3(CD3)N]C85. Solid-State NMR. Figure 3 shows 2H NMR spectra obtained for the Na-en(d4)-GIC. The spectra include a broad powder pattern and a narrow peak at all temperatures studied, with a slight change in the intensity ratio of the two features. Therefore, two or more en molecular states should exist in this GIC. The quadrupole coupling constant (qcc) and the asymmetry parameter (η) of the powder pattern were fit as 167 kHz and 0.04, respectively, throughout the temperature range evaluated. This qcc value is consistent with previous reports for rigid deuterated methylene groups (−CD2−, 167 kHz).29 The narrow peak has a full width at half-maximum (fwhm) of 2 kHz and is ascribed to the en(d4) intercalates undergoing isotropic molecular motion. 23 Na MAS NMR spectra indicate the existence of multiple environments for Na in the Na-en-GIC product (Figure 4). The large observed peak at −10 ppm has a shoulder at +10 ppm, and a smaller peak at −60 ppm is also present. Complex 23 Na NMR spectra are not observed for the other Na-amineGICs prepared (not shown), which showed a single peak at about 5−10 ppm. In general, Na+ cations show shifts of +20 to

Figure 3. 2H NMR spectra of Na-en(d4)-GIC between 123 and 294 K. The red line on 123 K spectrum is a simulated powder pattern for I = 1 nuclei with qcc = 167 kHz and η = 0.04.

Figure 4. 23Na MAS NMR spectra of Na-en-GIC at 213, 253, and 293 K.

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The Journal of Physical Chemistry C −20 ppm,30 whereas the Na− anion shift is at −63 ppm.31−33 The peak at lower frequency may arise from second-order quadrupolar interactions of the predominant species (qcc = 3.2 MHz). Alternately, a sodide salt can occur in the synthesis and that impurity may be present, but Na− anion is not a likely intercalate species in GICs containing negatively charged graphene sheets. The 2H NMR spectra of Na-thf(d8)-GIC presented in Figure 5 also includes two components at 294 K: a broad powder

Figure 6. 2H NMR spectra of K-ppz(d10)-GIC between 123 and 294 K. Red lines on 138 and 294 K spectra show powder simulations for I = 1 nuclei with qcc = 65 kHz, η = 0.3 and qcc = 55 kHz, η = 0.3.

Figure 5. 2H NMR spectra of Na-thf(d8)-GIC between 123 and 294 K.

pattern with qcc = 167 kHz and a narrow peak of fwhm = 2 kHz. The narrow peak intensity decreases at lower temperatures and is barely evident below 213 K. The 2H NMR spectra of K-ppz(d10)-GIC and K-dabco(d12)-GIC are shown in Figures 6 and 7. The spectra of Kppz(d10)-GIC exhibit a broad powder pattern between ±40 kHz and a narrow peak at ∼0 kHz at all temperatures, with only a small temperature effect on the spectral features. The estimated values of qcc and η for the broad pattern were 65 kHz and 0.3 at 138 K. This component width gradually narrows to qcc = 55 kHz with increasing temperature. The spectra of Kdabco(d12)-GIC also show a broad powder pattern between ±40 kHz along with a narrow peak. The estimated qcc value (50 kHz) for the broad pattern was temperature-independent. Spectra of thma(d3)-GIC (Figure 8) show two components. The narrow peak intensity increases with temperature from 253 to 296 K, as was observed in the Na-thf(d8)-GIC spectra. The estimated qcc value is 50 kHz, which is typical for CD3 groups rotating around a C−C(N) axis.29 The 2H NMR spectrum of en(d4)-GO at room temperature shows a very narrow peak with fwhm of 0.3 kHz (Figure 9a). The signal can be ascribed to en(d4) with significant overall rotation and diffusion; however, the signal width of liquid en(d4) obtained using the same solid-state NMR probe (Figure 9b) is much narrower than that in en(d4)-GO. This suggests that the en(d4) intercalates in en-GO can move rapidly and diffuse within the

Figure 7. 2H NMR spectra of K-dabco(d12)-GIC between 123 and 294 K. Red lines on the 294 K spectrum show a powder simulation for I = 1 nuclei with qcc = 50 kHz, η = 0.03.

gallery space but do interact with surface functional groups on the GO layers to a significant extent.

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DISCUSSION Arrangement and Dynamics of Intercalates. i. Naen(d4)-GIC and Na-thf(d8)-GIC. A comparison of the observed gallery height (0.358 nm) in Na-en(d4)-GIC with the van der Waals molecular dimensions for en(d4) (0.42 × 0.72 nm) suggests that the en(d4) intercalates are packed tightly between graphene layers (Figure 10a). Most of the en(d4) intercalates are rigid, corresponding to the broad powder feature in the 2H

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NMR spectra. These rigid en(d4) intercalates presumably chelate the Na+ ions in the galleries by coordination at both N atoms (Figure 10b), as has been observed in crystalline salts containing Na(en)+ cationic complexes.34 Some en(d4) intercalates may also coordinate to Na+ at a single N, allowing greater molecular motions such as fluctuation or turning over (Figure 10c), and these groups generate the narrow peak in the 2 H NMR spectra (Figure 3) and explain the presence of two similar Na chemical environments (−10 and 10 ppm peaks in Figure 4). The structure of Na-thf-GIC has been described previously24 with two known stage-1 phases, phase A with d = 1.12 nm and phase B with d = 0.72 nm. From XRD results, the product obtained in this study corresponds to phase A. In solution, Na+ can coordinate up to six thf ligands; the octahedral Na(thf)6+ complex is observed in crystal structures;35−37 however, the gallery height observed in Na-thf(d8)-GIC (0.765 nm) cannot accommodate this octahedral complex. Instead, each Na+ may coordinate an average of three or four thf molecules to form a distorted tetrahedral complex.22 For the phase A structure (Figure 11), 2H NMR results indicate that most thf(d8) is

Figure 8. 2H NMR spectrum of thma(d3)-GIC between 143 and 296 K. Red lines on 203 K spectrum show a powder simulation for I = 1 nuclei with qcc = 50 kHz, η = 0.00.

Figure 9. 2H NMR spectra of en(d4)-GO and liquid en(d4) at room temperature.

Figure 11. Arrangement of thf(d8) intercalates in Na-thf(d8)-GIC. Only two of the thf(d8) molecules coordinated to Na+ are shown.

rigidly coordinated to Na+ ion below 253 K, with a minor thf(d8) component rotating isotopically. With increasing temperature, the thf(d8) intercalate fraction undergoing isotropic rotation increases. ii. K-ppz(d10)-GIC and K-dabco(d12)-GIC. Some motional narrowing is observed in the spectra of K-ppz(d10)-GIC, as indicated in Figure 6 and previously described. In addition, the estimated qcc of 50 kHz for the spectra of K-dabco(d12)-GIC (Figure 7) indicates nonrigid intercalates. Spectral powder simulations were performed to help identify the specific molecular dynamics involved. The observed spectrum of Kppz(d10)-GIC at 138 K and simulated spectra for two different molecular motions are presented in Figure 12. One simulation is for molecular rotation around the N−N axis and the other is for a C3-like rotation parallel to the mean plane of the ring (Figure 13). The C3-like rotation simulation provides a good match to the observed broad powder pattern in the 2H NMR spectra. The large gallery height (0.875 nm) obtained from an XRD pattern is consistent with a ppz(d10) bilayer within the GIC galleries (Figure 14). Most ppz(d10) intercalates forming these bilayers undergo only the C3-like rotation, with a smaller intercalate fraction exhibiting overall isotropic rotation.

Figure 10. Arrangement and dynamics of en(d4) intercalates in Naen(d4)-GIC.

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Figure 15. Observed 2H NMR spectrum of K-dabco(d12)-GIC at 233 K and a simulated powder spectrum with C3 rotation around the N−N axis. Figure 12. Observed 2H NMR spectrum of K-ppz(d10)-GIC at 138 K and simulated powder spectra for rotation around the N−N axis and C3-like rotation parallel to the mean plane of the ppz(d10) ring.

Figure 13. Proposed motions of ppz(d10) intercalates: (a) rotation around the N−N axis and (b) C3-like rotation parallel to the mean plane of the ppz(d10) ring.

Figure 16. Arrangement (a) and dynamics (b) of the dabco(d12) intercalates in K-dabco(d12)-GIC.

the molecular dimension perpendicular to the N−N axis (0.657 nm), the intercalate orientation and dynamics are assigned as shown in Figure 16. Given the observed dimensions and steric requirements of dabco(d12), even with dabco(d12) oriented at minimum height requirement, K+ cannot effectively coordinate. The narrow peaks in the observed spectra (Figures 7 and 15) are ascribed to dabco(d12) intercalates with overall isotropic rotation. iii. thma(d3)-GIC and en(d4)-GO. As previously indicated, 2 H NMR shows mainly CD3 group rotation in the thma cation at low temperature (Figure 17a). The intensity of the narrow peak, ascribed to a restricted overall rotation in thma, increases gradually with increasing temperature. At ambient temperature, most thma can undergo this restricted overall rotation. Figure 17a,b presents the proposed thma bilayer gallery arrangement and intercalate dynamics for thma(d3)-GIC. Unlike the en(d4) intercalates in Na-en(d4)-GIC, in en-GO the en intercalates move rapidly and diffuse on the NMR experiment time scale. Because the expansion of GO to form en(d4)-GO (0.21 nm) is less than any en(d4) molecular dimension, the en(d4) intercalates must partially nestle into the surface functional groups present on the GO sheets, including hydroxyl (−OH), carboxyl (−COOH), and epoxide (−O−) groups (Figure 18). The en(d4) intercalates can hydrogen bond to these functional groups, but the strong coordination of

Figure 14. Intercalate arrangement in K-ppz(d10)-GIC.

The observed and simulated spectra of K-dabco(d12)-GIC at 233 K are presented in Figure 15. The simulation for C3 rotation of dabco(d12) about the N−N axis shows good agreement with the observed broad powder pattern in the 2H NMR. Because the gallery height of K-dabco(d12)-GIC (0.564 nm) is comparable to the dabco(d12) molecular dimension along the N−N axis (0.562 nm) but is significantly smaller than F

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produced. While a few examples of K+-en40 or Na+-en34,41 complexes have been reported, it is notable that K-en-GIC is much more difficult to prepare than Na-en-GIC.11 Similarly, the K-thf-GIC is far less stable than the Na analog.42 The analysis of Na-ternary GICs with dabco and ppz could serve as a useful test of this concept; however, the analogous solid reaction of NaCx with organic cointercalates is not feasible because no lowstage NaCx has been reported. We are planning to establish a new synthetic route to generate these ternary GICs.

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CONCLUSIONS The gallery structures and dynamic behavior of deuterated en(d4), thf(d8), ppz(d10), dabco(d12), and thma(d3) intercalates to form ternary GICs or a GO compound were investigated using XRD and 2H or 23Na solid-state NMR analyses. The 2H NMR spectra generally showed both a narrow peak and a broader powder pattern. The narrow peaks were ascribed to intercalate undergoing isotropic rotation, whereas the broad features were explained by rigid intercalate states or uniaxial rotations. The dynamics of the dabco(d12) and ppz(d10) intercalates and the relative instabilities of K-enGIC and K-thf-GIC are postulated to result from the lower affinities of K+ to amines and ethers. The thma(d3) cations in thma(d3)-GIC move and diffuse rapidly at ambient temperature, and a similar case occurs with the en(d4) intercalates in en(d4)-GO. These intercalates may show enhanced mobility due to the absence of alkali metal cations.

Figure 17. Intercalate arrangement and thma CD3 rotation in thma(d3)-GIC (a) and thma intercalate dynamics including restricted overall rotation (b).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 18. Gallery structure, intercalate dynamics (orange arrows), and diffusion (green arrows) in en(d4)-GO.

Present Address ∥

W.S.: Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand.

en to alkali metal ions such as in Na-en(d4)-GIC is absent in en(d4)-GO. Additionally, en(d4) diffusion may be enhanced because the Coulombic interactions between en(d4) and GO are weaker than those between the Na(en(d4))+ cationic complexes and the reduced graphene layers (Cx−) in Naen(d4)-GIC. Affinity of Alkali Metal Cations to Cointercalates. The alkali metal cations (Na+ and K+) coordinate to the diamine or thf cointercalates in ternary GICs produced. The coordination of organic ligands to cations depends on both the alkali metal ion and the ligand. Most of the en(d4) and thf(d8) cointercalates are rigid at low temperature, whereas most of the ppz(d10) and dabco(d12) cointercalates undergo at least axial rotations. The former GICs contain Na+ and the latter contain K+. The previously described results indicate that N atoms in dabco do not coordinate to K+, in contrast with the strong and coordination to Na+ observed for N in en and O in thf. These observations conform generally to the Lewis hard− soft bonding principle, where the softer K+ ion is expected to have lower affinity for the relatively hard Lewis bases present in diamines and ethers. In addition, the weak coordination between K+ and diamines helps explain the observed axial rotations of dabco and ppz intercalates at low temperature in those GICs. These differences also appear in other chemical examples; for example, no complex with dabco coordinating to K+ has been reported, whereas there are several known instances of Na+-dabco complexes.38,39 The affinity of different alkali metal cations and ligands can also help explain the overall stability of the ternary GICs

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Number 26870385.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b03016 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b03016 J. Phys. Chem. C XXXX, XXX, XXX−XXX