19954
J. Phys. Chem. 1996, 100, 19954-19958
Solid-State NMR Studies of the Structure of Graphite Oxide Heyong He,† Thomas Riedl,‡ Anton Lerf,‡ and Jacek Klinowski*,† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., and Walther-Meissner-Institut fu¨ r Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Walther-Meissner-Strasse 8, D-85748 Garching, Germany ReceiVed: May 29, 1996; In Final Form: September 10, 1996X
Graphite oxide (GO) and its derivatives have been studied using 13C and 1H NMR. The 13C NMR lines at 60, 70, and 130 ppm are assigned to C-OH, C-O-C, and >CdC< groups in the bulk of the material, respectively. The >CdC< double bonds are relatively stable, while C-OH groups may condense to form C-O-C (ether) linkages. There are at least two magnetically inequivalent C-OH sites, and the structure does not necessarily possess long-range order. Water molecules interact very strongly with the structure. The results reveal a number of new structural features.
Introduction Graphite oxide (GO) has first been prepared in 1860 by oxidizing graphite with KClO3/HNO3.1 The various structural models of GO that have been proposed2-8 differ with respect to the nature of oxygen-containing functional groups thought to be present. Different models were in favor over the last 60 years, depending on the characterization technique prevalent at the time. It is known that GO preserves the layered structure of graphite, but the conjugated bond structure is lost. Experiments using various techniques have established that GO contains different amounts of tertiary C-OH groups, ether C-O-C groups, >CdC< double bonds, as well as enol and ketone groups. Solid-state NMR rules out models that postulate the presence of carboxyl groups in the bulk of the sample.8 Although such groups may exist at the surface, their population is insufficient for NMR detection. However, as a result of the near-amorphous nature of GO, the precise structure of the material is still uncertain. 13C NMR gives three main resonances at ca. 60, 70, and 130 ppm. It is known that the first two peaks come from C-OH and C-O-C groups. However, the detailed assignments differ8-10 because of the closeness of the two chemical shifts and insufficient spectral information. We have examined GO and its derivatives (prepared by treating GO with deuterium oxide, maleic anhydride, sodium ethoxide (NaOEt), and MesO-(CH2)2-OMes) using magicangle spinning (MAS) NMR with conventional 1H-13C crosspolarization (CP), short-contact-time CP, dipolar-dephased CP, as well as 13C and 1H Bloch decay spectra. The 13C and 1H NMR spectra of GO have been measured in the 123-473 K temperature range. The derivatives of GO were prepared in order to distinguish different functional groups and to determine whether the double bonds are aromatic or nonaromatic. Spectra of chemically treated GO have provided additional information. As a result, we were able to reassign the 13C resonances at ca. 60 and 70 ppm and demonstrate a number of new structural features.
HCl the sample was dialyzed12,13 for 7 days in the dark (with several changes of the dialyzing water) until the solution was free of sulfate and chloride ions. After drying at 40 °C in 10-2 mbar vacuum to remove the adhering water, the sample was crushed to a powder and dried in 10-5 mbar vacuum for 2 days at room temperature. The interlayer distance in the product was 7.1 Å, corresponding to the water content of 8-15% if the results of Hofmann and Frenzel2 obtained for GO prepared by the method of Staudenmaier (who used H2SO4/HNO3/KClO3) can be applied to our samples. GO-DA was made, in order to test the hypothesis that GO may contain nonaromatic >CdC< double bonds, by treating 0.25 g of GO with 1 g maleic anhydride in 10 mL of toluene at 50 °C for 24 h. GO-D2O was prepared by exchanging GO with a mixture of 90% D2O and 10% H2O at room temperature. GO-base was obtained by treating 0.24 g of GO (corresponding to 1.44 mmol of -OH groups) with 0.68 g (10 mmol) of NaOEt in 10 mL ethanol for 2 h at room temperature with air being excluded. The supernatant solution was removed using a syringe to prevent contact with air. The sample was washed several times with ethanol until the washing solution in water was no longer basic and dried at 40 °C in 10-2 mbar vacuum for 16 h. GO-base(w) was made by washing GO-base three times with acetone and then water. It is generally assumed that NaOEt converts tertiary OH groups in GO to the corresponding sodium salt. GO-ether was prepared by treating the sample of GO-base with 0.314 g of 1,2-dimesylethane in 8 mL of dioxane at 45 °C for 16 h. The adhering solution was then extracted from the solid with chloroform in a Soxhlet extractor over 6 h. The product was washed with water and dried under vacuum in two steps (40 °C, 10-2 mbar, 2 days; 20 °C, 10-5 mbar, 2 days). The layer distance was 7.7 Å. The following reaction is expected to occur: (1) NaOEt/EtOH
2(GO)C-OH ssssssssssssf
(2) MesO-CH2-CH2-OMes/dioxane
(GO)C-O-CH2-CH2-O-C(GO) (1)
Experimental Section Sample Preparation. The parent sample of GO was prepared by oxidation with KMnO4/H2SO4 according to Hummers et al.11 After preparation and washing with 5% aqueous †
University of Cambridge. Bayerische Akademie der Wissenschaften. X Abstract published in AdVance ACS Abstracts, November 15, 1996. ‡
S0022-3654(96)01563-8 CCC: $12.00
MesO-CH2-CH2-OMes was prepared as follows. A sample of 16.3 mL (0.16 mol) of mesyl chloride (methanesulphonylchloride) was added dropwise at -20 °C under stirring to a solution of 1,2-ethanediol (0.1 mol) in 40 mL of a 1:1 CHCl3/ THF mixture. A sample of 14 mL (0.17 mol) of pyridine was then added and the solution warmed to 0 °C over 2 h. It was © 1996 American Chemical Society
Structure of Graphite Oxide
J. Phys. Chem., Vol. 100, No. 51, 1996 19955
then added to an ice-cooled mixture of 100 mL of water and 35 mL of concentrated HCl and extracted with 50 mL of CHCl3. The organic phase was washed with water and dried over sodium sulfate. The solvent was removed under vacuum at room temperature. The yellow solid was recrystallized from chloroform, giving colorless crystals (mp of 317 K) that were examined by 1H NMR. Solid-State NMR. 13C and 1H MAS NMR spectra were recorded using a Chemagnetics CMX-400 spectrometer operating at 100.6 and 399.9 MHz, respectively, and an MAS probehead with zirconia rotors 4 mm in diameter. 13C Bloch decay spectra with high-power (>60 kHz) 1H decoupling were recorded with spinning at 3.5-10 kHz, 45° pulses of 1 µs duration, and 20 s recycle delays. Conventional and shortcontact-time 1H-13C CP/MAS spectra were recorded with spinning at 3.5-6.5 kHz, 4 µs 1H 90° pulses, 4 and 0.05 ms contact times, and 4 s recycle delays. Dipolar-dephased 1H13C CP/MAS spectra were measured with spinning at 3.5-6.5 kHz, 4 µs 1H 90° pulses, 4 ms contact times, 50-400 µs delays, and 4 s recycle delays. Different spinning rates were used to distinguish central lines from sidebands. The Hartmann-Hahn condition was established with hexamethylbenzene. The 13C spectra were acquired using a spectral width of 40 kHz, acquisition length of 256 points, and exponential broadening in the time domain of 50-100 Hz. 1H Bloch delay measurements were recorded with MAS at ca. 10 kHz, 45° pulses of 2 µs duration, and 4 s recycle delays. 13C and 1H chemical shifts are given in ppm from external tetramethylsilane (TMS). Variable-temperature 1H and 13C Bloch delay and 1H-13C CP measurements were carried out in the 123-473 K temperature range. Results and Discussion GO and GO-DA. In agreement with the literature,8,9 the proton-decoupled 13C NMR spectrum of GO (Figure 1) consists of three lines at ca. 60, 70, and 130 ppm. The same applies to the spectrum of GO-DA. The first two peaks are known to originate from tertiary C-OH and C-O-C groups and the third from >CdC< double bonds. However, the specific assignments of the first two resonances differ.8,9 The intensity ratios of the lines at 60 and 70 ppm to the 130 ppm line in GO and GO-DA are 2.06:1 and 2.16:1, respectively. The efficiency of 1H-13C cross-polarization depends on the strength of the dipolar interaction, which is inversely proportional to the cube of the internuclear distance, r. Dipolar coupling is thus much stronger for carbon atoms bonded and/ or very close (r < 1.0-1.1 Å) to protons compared with carbons that are more distant from protons (r > 2.0 Å). Also, molecular motion can reduce the strength of the dipolar coupling. Thus, a spectrum of a rapidly rotating functional group (such as CH3) would exaggerate the distance between the central carbon atom and the surrounding hydrogens. The short-contact-time CP spectra contain resonances from carbons that are very close to relatively immobile, bonded and/or nonbonded, protons. The dipolar-dephased CP spectra show quaternary carbons and mobile CH3 groups. By contrast, conventional 13C CP/MAS spectra contain all resonances. The conventional, dipolardephased and short-contact-time CP spectra of GO and its derivatives are shown in Figures 2-4. All three experiments show very similar features for GO and GO-DA, indicating that the structure of both samples is very similar and that the reaction with maleic anhydride does not proceed. It is possible that toluene, the normally used solvent, prevents the uptake of maleic anhydride into the interlayer gap. However, the more likely explanation is that nonaromatic double bonds are present in GO
Figure 1. 13C Bloch decay spectra with high-power 1H decoupling of GO and its derivatives. “ssb” denotes spinning sidebands.
in very low amounts, if at all. There are three resonances (at ca. 60, 70, and 130 ppm) in the conventional and dipolardephased spectra but only a weak resonance at ca. 60 ppm and a strong resonance at ca. 70 ppm in the short-contact-time spectra. The main source of protons in GO are tertiary C-OH groups and water molecules. 1H spectra indicate that water protons are relatively immobile (see below). It is known that all three kinds of carbons are quaternary,8 e.g., not directly bonded to protons. Therefore, all give lines in the conventional and dipolar-dephased CP experiments (but not in the shortcontact-time experiment), since the dipolar interaction with remote protons is weak. The lines in the short-contact-time experiment must come from interactions with indirectly bonded protons that are very close and fairly immobile. The short-contact-time CP spectra in Figure 4 indicate that the carbon atoms participating in double bonds are remote from protons, while some carbon atoms in C-OH and C-O-C groups are very close to protons. A possible explanation for the existence of such “nearby” protons is given in Figure 5. Since the layered structure of GO is warped, a C-OH proton in one layer may be very close to a C-OH or C-O-C carbon atom in the adjacent layer, giving rise to the short-contact-time CP resonance. However, in view of the steric restrictions, the likelihood of C-OH/C-O-C pairs is higher than that of C-OH/C-OH pairs. Another potential source of very close protons are fairly immobile water molecules (Figure 6). One close proton may affect two carbons in the C-O-C group but only one carbon in the C-OH group. For the above reasons, the short-contact-time lines from the C-O-C group are
19956 J. Phys. Chem., Vol. 100, No. 51, 1996
Figure 2. Conventional 1H-13C CP/MAS spectra of GO and its derivatives measured with a 4 ms contact time.
stronger than those from the C-OH group for both models. This would explain why the resonance at ca. 60 ppm in the short-contact-time spectra is only a low-intensity shoulder. In conclusion, short-contact-time spectra allow us to assign the peaks at ca. 60 and 70 ppm to the C-OH and C-O-C groups, respectively. GO and GO-DA each give two 1H NMR resonances at ca. 1.8 and 4.7 ppm (Figure 7), although in GO-DA the former resonance is a shoulder. Since the 1H chemical shift of the tertiary C-OH group is in the 1-2 ppm range, the lines at 1.8 and 4.7 ppm must be assigned to C-OH and H2O, respectively. The water resonance is markedly broad, indicating that the water molecules trapped between the layers are fairly immobile as a result of strong interactions with the structure. Such strong interaction also gives rise to short-contact-time lines. GO-D2O. Upon treatment with D2O, the H2O content of GO-D2O is reduced much more significantly than the content of tertiary C-OH groups (Figure 7), showing that water is exchanged much more easily than are the C-OH hydrogens. The decreased water content reduces the 1H-1H dipolar interaction and allows us to resolve two tertiary C-OH resonances (at 1.3 and 1.0 ppm), demonstrating the presence of at least two magnetically inequivalent C-OH sites. The peaks near 0 ppm could come from a small amount of impurities. Given the significant reduction of the H2O content, the intensity of the short-contact-time CP line corresponding to the model in Figure 6 should be considerably decreased, as is indeed the case. The spectrum is dominated by the interaction shown in Figure 5. For reasons discussed in connection with the
He et al.
Figure 3. Dipolar-dephased 1H-13C CP/MAS spectra of GO and its derivatives. The delay time before acquisition is 50 µs. “ssb” denotes spinning sidebands.
spectra of GO and GO-DA, the short-contact-time CP line from the C-OH groups almost disappears, and only the C-O-C line at ca. 70 ppm remains. However, the interaction with remote protons is relatively unaffected. The features of conventional and dipolar-dephased CP spectra of GO-D2O are still similar to those for GO. The 13C Bloch decay spectrum also shows that the structure of GO-D2O is similar to that of GO. GO-Base, GO-Base(w), and GO-Ether. Infrared spectroscopy has shown that the C-OH groups in GO may be neutralized by sodium ethoxide8 to form GO-Na and ethanol. This reaction is widely used to determine the amount of weakly acidic tertiary OH groups in GO.4,5 We applied it as a precursor step for connecting the adjacent GO layers via ether bridges. The NMR spectra of GO-base were collected to characterize further this sample and to make sure that both reaction steps of reaction 1 really do occur. The disappearance of the peak from water in the 1H NMR spectrum of GO-base (Figure 7) is due to the reaction of NaOEt with residual water in the interlayer space. Surprisingly, the peak at 1-2 ppm assigned to the GO(COH) groups does not vanish completely. However, an estimate of the amount of these remaining groups is difficult because the signals of the GO(C-OH) group are superimposed by proton signals originating from the CH3 and CH2 groups. The intensity of the line at ca. 60 ppm in the 13C spectrum of GO-base (Figure 1) decreases slightly with the appearance of a new peak at ca. 15 ppm, which is also found in all CP spectra. It is clear that the latter should be assigned to CH3 groups. The CH3 lines
Structure of Graphite Oxide
Figure 4. Short-contact-time 1H-13C CP/MAS spectra of GO and its derivatives measured with a 50 µs contact time.
Figure 5. Protons may be close to carbons in the C-OH and C-O-C groups.
Figure 6. Protons from intercalated water molecules may be close to carbons in the C-OH and C-O-C groups.
in both the short-contact-time and dipolar-dephased CP spectra show that CH3 groups rotate at slow and fast rates, respectively, e.g., the CH3 groups have different environments. The line from C-O-C units becomes a shoulder. However, these observations are insufficient to decide whether the additional lines may come from ethanol formed from neutralization of the GO(COH) groups and the reaction of sodium ethoxide with water or from the CH3-CH2-O- groups covalently bound to GO.
J. Phys. Chem., Vol. 100, No. 51, 1996 19957
Figure 7. 1H Bloch decay spectra of GO and its derivatives.
To resolve this problem, we have prepared GO-base(w) by washing GO-base three times with acetone and then water. All CP spectra of GO-base(w) (Figures 2-4) contain a peak at ca. 15 ppm (the chemical shift of the CH3 group in acetone is ca. 30 ppm). The results confirm that the C-OH groups in GO may indeed react with sodium ethoxide to form C-OCH2CH3. Since the lines in the 13C spectra with 1H decoupling (Figure 1) are very broad and the spectrum is quite noisy, the peak at ca. 15 ppm is not very prominent. The dipolar-dephased CP spectrum also shows the presence of C-OH groups. These observations indicate that only a small proportion of C-OH groups participates in the reaction. The peak at 60 ppm in the short-contact-time CP spectrum comes from CH2 groups. The CH3 lines in short-contact-time and dipolar-dephased CP spectra show that the methyl groups have different rotating rates corresponding to different environments. The water line dominates the 1H spectrum (Figure 7), since the sample is fully hydrated. From these results we conclude that some CH3CH2-O- groups are covalently attached to GO. It is difficult to evaluate the amount of these bound CH3-CH2-O- groups; further investigation of this new phenomenon is underway. From all that is known about the chemistry of GO,4,5 we assume that the main reaction pathway during treatment of GO with NaOEt is the formation of GO(C-ONa). GO-ether was obtained from reaction with sodium ethoxide and MesO-(CH2)2-OMes. A cyclic ether in the same layer and/or an ether bridge linking two adjacent layers is expected. The line from the CH3 group at ca. 15 ppm almost vanishes in
19958 J. Phys. Chem., Vol. 100, No. 51, 1996 all 13C spectra in comparison with those of GO-base and GObase(w). However, the line from CH2 groups is still found in the short-contact-time spectrum. The possibility of trapped reactant surviving the washing may be ruled out. since no additional peaks are found in the 13C spectra of GO-DA and GO-base(w). The spectra of GO-base are also different from those of GO-ether. All these observations indicate that reaction 1 has occurred in part of the GO sample. The reaction forces part of the water molecules out of the GO structure. Therefore, the intensity of the line from aliphatic groups in the 1H spectrum of GO-ether is markedly increased. Variable-Temperature Measurements. 13C CP/MAS and 1H and 13C Bloch decay spectra of GO (not shown) were measured in the 123-473 K temperature range. Surprisingly, the spectra at all temperatures are very similar to those at room temperature. In particular, the full-width-at-half-maximum of the 1H water line remains constant at ca. 2.8 kHz between 123 and 423 K. The 13C CP/MAS line is still present at temperatures as high as 373 K. At 473 K the intensity of the 1H line from water suddenly decreases to 1/20 of the original intensity, showing that water molecules escape and the GO structure is damaged. This indicates an extraordinary state of water molecules in GO: water must interact strongly with the structure at high temperatures; otherwise, the 13C CP lines would not be observed. The motional rate of water molecules remains nearly constant: water does not freeze at 123 K or evaporate at 423 K. It could be that the layers of GO act as capillaries of molecular dimensions. Such microvessels would cause very strong capillary action and prevent water from freezing at low temperatures. Also, the water-GO interaction is stronger than the water-water interaction and much stronger than in liquid water. As a result, the boiling point of water in GO is greatly elevated. The strong interaction between water molecules and the GO layers may be the key factor in keeping the structure together and accounts for the presence of the short-contact-time 13C CP/MAS lines. Conclusions We have obtained the following new information on the structure of GO. (1) The 13C lines at ca. 60, 70, and 130 ppm
He et al. belong to C-OH, C-O-C, and >CdC< groups in the bulk of GO, respectively. (2) The >CdC< bonds are relatively stable, while the C-OH groups may condense to form C-O-C (ether) linkages. (3) GO has at least two inequivalent C-OH sites. (4) Relatively immobile water molecules interact very strongly with the GO structure in a wide range of temperatures. More work is required for a complete understanding of the structure of graphite oxide. Although it may seem premature to suggest a new structural scheme at this stage, it is clear that the previously proposed models2-8 require careful re-examination. The structure may not be necessarily regular or long-range ordered, and the distribution of functional groups could be inhomogeneous. 1H-13C CP/MAS spectra indicate that the strong interaction from protons in trapped water molecules makes the C-OH and C-O-C groups appear homogeneous. The models in Figures 5 and 6 may account for the appearance of lines in short-contact-time CP spectra. It is also possible that more thorough oxidation would produce C-O-OH and C-O-O-C groups with chemical shifts similar to those of C-OH and C-O-C groups, respectively. References and Notes (1) Brodie, B. C. Ann. Chim. Phys. 1860, 59, 466. (2) Hofmann, U.; Frenzel, A. Ber. Dtsch. Chem. Ges. 1930, 63, 1248. (3) Ruess, G. Monatsh. Chem. 1947, 76, 381. (4) Clauss, A.; Plass, R.; Boehm, H. P.; Hofmann, U. Z. Anorg. Allg. Chem. 1957, 291, 205. (5) Scholz, W.; Boehm, H. P. Z. Anorg. Allg. Chem. 1969, 369, 327. (6) Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357. (7) Nakayama, T.; Matsuo, Y. Carbon 1994, 32, 469. (8) Mermoux, M.; Chabre, Y.; Rousseau, A. Carbon 1991, 29, 469. (9) Blumenfeld, A. L.; Muradyan, V. E.; Shumilova, I. B.; Parnes, Z. N.; Novikov, Yu. N. Mater. Sci. Forum 1992, 91-93, 613. (10) Hamwi, A; Saleh, I. A. Mol. Cryst. Liq. Cryst. 1994, 244, 361. (11) Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (12) Boehm, H. P.; Hofmann, U. Z. Naturforscher 1962, B17, 150. (13) Melezhik, A.; Monakhova, I. Russ. J. Gen. Chem. 1992, 62, 961.
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