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Graphite Oxide: Chemical Reduction to Graphite and Surface Modification with Primary Aliphatic Amines and Amino Acids Athanasios B. Bourlinos,† Dimitrios Gournis,‡ Dimitrios Petridis,*,† Tama´s Szabo´,§ Anna Szeri,§ and Imre De´ka´ny*,§ Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi Attikis, Athens 153 10, Greece, Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece, and Department of Colloid Chemistry and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, University of Szeged, H-6720 Szeged, Aradi v.t. 1, Hungary Received September 9, 2002. In Final Form: May 12, 2003 The chemical reduction of graphite oxide (GO) to graphite by either NaBH4 or hydroquinone and also its surface modification with neutral, primary aliphatic amines and amino acids are described. Treatment of GO with NaBH4 leads to turbostatic graphite that upon calcination under an inert atmosphere is transformed to highly ordered graphitic carbon, while the reduction with hydroquinone yields directly crystalline graphite under soft thermal conditions. On account of the surface-exposed epoxy groups present in the GO solid, its surface modification with neutral, primary aliphatic amines or amine-containing molecules (amino acids and aminosiloxanes) takes place easily through the corresponding nucleophilic substitution reactions. In this way, valuable GO derivatives can be obtained, like molecular pillared GO, organically modified GO affording in organic solvents stable organosols or hydrophilic GO affording in water stable hydrosols and possessing direct cation exchange sites. The potential combination of surface modification and chemical reduction of GO in producing novel graphite based materials is also presented.
Introduction Graphite oxide (GO) is a graphite derivative with covalently attached oxygen-containing groups to its layers. These groups are generated in the course of the GO synthesis by strong oxidation.1-3 In this sense, GO exhibits lamellar structure with randomly distributed unoxidized aromatic regions (sp2-carbon atoms), six-membered aliphatic regions (sp3-carbon atoms) as a result of oxidation, and a high concentration of exposed oxygen-containing functional groups, like hydroxyl, epoxy, and carboxyl, embedded in its carbon layers.1-3 Although not fully verified, it has been proposed that the epoxy and C-OH functional groups lie above and below each carbon layer, while the -COOH groups are located near the layers’ edges.2,3 Owing to the presence of such hydrophilic polar groups in the solid, GO is quite reminiscent of montmorillonites, which share common swelling and intercalation properties. As a result, GO is an excellent host matrix for the interlayer accommodation of long chain aliphatic hydrocarbons,1 transition metal ions,4 and hydrophilic molecules and polymers5 and is also promising for particle engineering processes, especially for the fabrication of thin films with smart properties.3,6,7 * To whom correspondence should be addressed. (D.P.) Fax: 003-210-6519430. tel: 003-210-6503343. E-mail: dpetrid@ims. demokritos.gr. † Institute of Materials Science. ‡ University of Ioannina. § University of Szeged. (1) De´ka´ny, I.; Kru¨ger-Grasser, R.; Weiss, A. Colloid Polym. Sci. 1998, 276, 570. (2) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477. (3) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (4) Kovtyukhova, N. I.; Karpenko, G. A.; Chuiko, A. A. Russ. J. Inorg. Chem. 1992, 37, 566. (5) Liu, P.; Gong, K.; Xiao, P. Carbon 1999, 37, 2073.
Recently, Lerf et al., by considering GO as a multifunctional organic network, applied a wide range of chemical transformations typical for the C-OH and epoxy groups.2 In particular, treatment of the C-OH groups of GO with Ac2O or hexamethylenediisocyanate led to the formation of ester or urethane derivatives, while treatment of the epoxy groups with nucleophilic reagents, for example n-C5H11NH2, gave access to the corresponding substitution reactions, as detailed 13C and 1H NMR spectra revealed. In the frame of enriching and consolidating these chemical concepts of GO, we describe (i) the chemical reduction of GO by sodium borohydride and hydroquinone to graphite, (ii) its surface modification with a series of neutral primary aliphatic amines and amino acids, and (iii) how the aforementioned treatments can lead to aqueous or organic colloidal dispersions of the GO platelets (hydrosols or organosols, respectively) and to novel carbonaceous layered structures (molecular pillared GO and graphite layered solids). Experimental Section Materials. GO (61% C, 35.7% O, 3.3% H) was synthesized from natural graphite (Kropfmu¨hl, Bayern, Germany) by oxidation with HNO3/NaClO3, according to the Brodie method.1,8 Sodium borohydride and hydroquinone were used for the reduction of GO. The following amines and amino acids were employed for the surface modification of GO: CnH2n+1NH2 (n ) 2, 4, 8, 12, 18), (CH3O)3SiCH2CH2CH2NH2, and H2N(CH2)nCOOH (n ) 1, 7). Deionized water and commercial ethanol and acetone were used as solvents. Chemical Reduction of GO to Graphite. NaBH4. GO (100 mg) was dispersed in 20 mL of water and to the dispersion was added 200 mg of NaBH4. The mixture was allowed to stir for 20 min and then heated in a steam bath (controlled reduction) for (6) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (7) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877. (8) Brodie, B. C. Liebigs Ann. Chem. 1860, 114, 6.
10.1021/la026525h CCC: $25.00 © 2003 American Chemical Society Published on Web 06/25/2003
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Figure 2. IR spectra in KBr of the samples GO (a) and reduced GO (b). Figure 1. XRD patterns of GO (a) and of GO after 1 h (b) and 3 h (c) reaction time with NaBH4. 3 h prior to the isolation of the as-made graphite solid (centrifugion, good washing with water and acetone, and drying). Hydroquinone. GO (100 mg) was dispersed in 20 mL of water and 300 mg of hydroquinone was added. The mixture was refluxed for 20 h and the as-made graphite solid was isolated by centrifugion, washed very well with water and acetone, and finally dried. Surface Modification with Amines and Amino Acids. CnH2n+1NH2 (n ) 2, 4, 8, 12) and (CH3O)3SiCH2CH2CH2NH2. In each case, a sample of 100 mg of GO was dispersed in 10 mL of water, followed by the addition of 300 mg of amine in 10 mL of ethanol. Upon amine addition the GO solid swelled instantly. Each mixture was allowed to stir for 20 h at room temperature before isolation of the resulting derivatives (centrifugion, good washing with 1:1 H2O/EtOH and acetone, and drying). C18H37NH2. GO (200 mg) was dispersed in 10 mL of water, and 600 mg of amine in 40 mL of hot ethanol was added. The mixture was refluxed for 90 h prior to centrifugion, good washing with hot ethanol, and drying of the modified solid. H2N(CH2)nCOONa (n ) 1, 7). GO (100 mg) was dispersed in 10 mL of water, and 300 mg of H2N(CH2)nCOOH (n ) 1 for glycine, 7 for ω-aminocaprylic acid) and an equimolar amount of NaOH in 10 mL of water were added. Each mixture was stirred for 24 h at room temperature. For the aminocaprylate derivative, the modified solid was centrifuged, washed well with water and acetone, and dried. In the glycinate case, due to the formation of a viscous, brown dispersion, the colloidal dispersion was treated with ethanol, and the resulting precipitate was centrifuged, washed well with a 1:1 H2O/EtOH mixture and acetone, and finally dried. Characterization. X-ray powder diffraction (XRD) patterns were taken on a D-500 Siemens diffractometer using Cu KR radiation. Infrared spectra were recorded on a Bruker FT-IR spectrometer (Equinox 55/S model). The samples were measured in the form of KBr pellets. 1H NMR spectra were recorded with a Bruker AC 250 NMR spectrometer operating at 250 MHz. The TG curves were obtained using a MOM Derivatograph Q-1500 D device. The measurements were made between 25 and 1000 °C with a heating rate of 10 °C min-1. Specific surface areas were determined in a Micromentic Gemini type 2735 automated adsorption apparatus with N2 gas at 77 K.
Results and Discussion Chemical Reduction of GO to Graphite. Figure 1 shows the XRD patterns of GO and that of GO after treatment with NaBH4, a common reducing agent, for various reaction times. We discern a gradual change in the patterns to finally receive, after 3 h reaction time, a disordered carbonaceous layered solid, with basal spacing of 3.5 Å instead of 6.78 Å for the parent GO. Both the d002
value and broadness of this reflection in the final solid are typical for randomly ordered (turbostatic) graphitic platelets.9 Interestingly, considerable changes are also observed in the IR spectra of GO before and after its complete treatment with NaBH4. As shown in Figure 2a, GO exhibits the following characteristic IR features: a weak shoulder at 3550 cm-1 attributable to the hydroxyl stretching vibrations of the C-OH groups, a weak band at 1750 cm-1 assigned to the CdO stretching vibrations of the -COOH groups, a strong band at 1350 cm-1 assigned to the O-H deformations of the C-OH groups, and a strong band at 1100 cm-1 attributed to C-O stretching vibrations.3 The NaBH4 treatment eliminated all these bands, giving evidence for the total removal of the parent oxygen-containing groups and therefore for the reduction of the solid (Figure 2b). In fact, the reduced GO is IR inactive, like pure graphite. Definite evidence for the graphitic nature of the reduced GO comes from carbon elemental analysis of the final solid. Accordingly, while the parent GO contains 61% C with the remaining 39% being oxygen and hydrogen,1 the reduced solid is almost composed entirely of carbon (98.5% C). Thus, XRD, IR, and elemental analysis point to chemical reduction of GO to turbostatic graphite by NaBH4. On account of their different stoichiometry and structure, GO and its reduced form exhibit quite different thermal behavior (Figure 3). Thus, while heating the GO at 500 °C in an argon atmosphere affords a disordered, oxygen-containing graphitic carbon with d002 ) 3.5 Å,10 the disordered structure of reduced GO is transformed, after a similar treatment, to a highly ordered graphitic carbon with a basal spacing similar to that of pure graphite (d002 ) 3.39 Å).9 On the other hand, while the TG curve of GO in air exhibits two weight losses at 250 and 500 °C, attributable to the removal of its oxygen-containing groups as H2O and to carbon combustion, the reduced GO (oxygenfree) shows only a weight loss at 550 °C ascribed to carbon combustion (Figure 4). Of particular interest is the reduction of GO with hydroquinone, because of the direct transformation of GO to a crystalline and not to a turbostatic graphite under soft thermal conditions. Figure 5 presents the XRD patterns of GO after reaction with hydroquinone for 3, 5, (9) Dresselhaus, M. S. Supercarbon: Synthesis, Properties and Applications; Yoshimura, S., Chang, R. P. H., Eds.; Springer Series in Materials Science; Springer: New York, 1998; Vol. 33, p 9. (10) Hofmann, U.; Frenzel, A.; Csala´n. E. Liebigs Ann. Chem. 1934, 510, 1.
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Figure 3. XRD patterns after thermal treatment at 500 °C in an argon atmosphere of GO (a) and of reduced GO (b).
Figure 4. TG curves of GO (upper) and reduced GO (lower) in air (the weight loss at ∼100 °C observed for the hydrophilic GO is attributed to physically absorbed water).
and 20 h. Once again, we observe a gradual change in the patterns to a final, crystalline, IR inactive graphitic solid with d002 ) 3.39 Å (98% C), after 20 h reaction time. At this point, we must stress that the mechanisms of the above-mentioned reductions are not clear at present. Nevertheless, we believe that the formation of reactive radicals must play a key role in the observed transformations. It is worth noting that chemical reduction of GO to graphite can be also achieved by employing either metallic iron or zinc fine powder (turbostatic graphite, d002 ) 3.6
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Figure 5. XRD patterns of GO after 3 h (a), 5 h (b), and 20 h (c) reaction time with hydroquinone.
Å) or sulfide ions from Na2S (turbostatic graphite, d002 ) 3.8 Å) as reducing agents.11 In all instances, the as-made graphite solids were IR inactive. Surface Modification with CnH2n+1NH2 and H2N(CH2)nCOONa and Perspectives. CnH2n+1NH2 (n ) 2, 4, 8, 12, 18). Figure 6 shows the XRD patterns of GO after treatment with a series of primary amines, possessing different aliphatic chain lengths (n ) 2, 4, 8, 12). In all cases, there is a systematic increment of the d001 value, as compared to the parent GO, indicating insertion of the amine molecules to the intergallery space of GO. The amine molecules can be inserted in to the interlayer zone of GO in the following ways: (i) hydrogen-bonding interactions between the amine molecules and the oxygen-containing functional groups of GO (C-OH‚‚‚H2N-R), (ii) protonation of the amine by the weakly acidic sites of the GO layers (-COO-+H3N-R), and (iii) chemical grafting of the amine to the GO surfaces via nucleophilic substitution reactions on the epoxy groups of GO.3 Concerning the first two options, it is important to note that treatment of the amino derivatives, e.g. the dodecylamine derivative, with either hot ethanol or NaOH alcoholic solution in order to induce deintercalation caused only a small decrease (1-2 Å) in the d001 values, which probably arises from overstaffing of the GO galleries with amine molecules via hydrogenbonding interactions or/and protonation. More importantly, we found that protonated amines, which lack nucleophilicity, do not intercalate to the GO solid. Of equal importance is the finding that tertiary amines, e.g. tributylamine, which cannot interact with the GO epoxy groups for chemical grafting, do not induce swelling of the GO, as was the case with primary amines. Finally, the pH of amine 1:1 H2O/EtOH solutions (between 11 and 12) was decreased by one unit after reaction with GO, suggesting that neutral amine molecules and not protonated (RNH2 + H2O T RNH3+ + OH-) were consumed during the reaction course. These observations, in conjunction with the observed chain length dependent d001 (11) In a typical preparation, 0.1 g of GO was dispersed in 20 mL of water and 0.5 g of either iron or zinc fine powder or Na2S was added. Each mixture was refluxed for 1 h under vigorous stirring. For the metal cases, the solid mixtures were isolated by rinsing, treated with a concentrated HNO3/HCl or warm HCl solution in order to remove any residual iron or zinc compounds from the solids, and finally, the asreceived graphite solids were well washed with water and acetone prior drying. For the Na2S case, the as-made graphite solid was centrifuged, washed well with water and acetone, and finally dried.
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Figure 6. XRD patterns of GO after treatment with ethylamine (a), butylamine (b), octylamine (c), and dodecylamine (d). The left part of the figure presents a simple model based on amination of the epoxide groups of GO as the main pathway for the insertion of the amine molecules in the interlayer zone of GO. The cited table presents the d001 values (experimentally versus calculated) as a function of n (number of carbon atoms).
values, point to an amine nucleophilic attack on the epoxy groups of GO as the main insertion pathway. In Figure 6, a simple model based on amination of the epoxy groups is presented. First of all, since the nitrogen atoms enter the polar “coordination sphere” of the carbon layers, the interlayer distance must be determined by the length of the hydrocarbon chain of the amine molecules and by their orientation relative to the layers. The latter can be parallel to the layers or tilted. However, due to the hydrophilic nature of the layers, the tilted orientation is more likely. In this case, the aptitude of the hydrocarbon chain is dictated by the sp3 hybridization of the nitrogen atom. Therefore, the inclination θ can be written as θ ) 90° φ, where φ is the angle defined by the nitrogen atom, the carbon atom of the GO layer, and the amine carbon atoms (Figure 6). Taking into consideration that the sp3 orbitals of nitrogen diverge by 108°, the φ angle in the isosceles triangle is 36° and therefore θ ) 54°. Accordingly, the d001 value is given by the following equation: d001 ) 6.1 + lc sin θ, where 6.1 is the thickness of the GO layers in Å,1 lc the length of the hydrocarbon chain of the amine molecule [lc(Å) ) 1.5 + 1.265(n - 1), n is the number of carbon atoms],12 and θ the hydrocarbon chain inclination. The inset table in Figure 6 depicts the experimentally found versus calculated (in parentheses) d001 values as a function of n. The good agreement (80-100%) between these two values favors the proposed model and therefore the nucleophilic insertion of the amine molecules. The 20% discrepancy observed in the d001 values for the dodecylamino-GO derivative probably arises from kinetic reasons and, in particular, from the large size of the inserted amine molecules, which, in turn, is expected to hinder the nucleophilic attack on the epoxy groups of GO. When longer reaction times were used (24-30 h), an increased basal spacing of 19 Å versus 18.5 Å (calcd) was found for this amine derivative. Surface modification of GO with longer aliphatic amines than dodecylamine requires more intense conditions than previously reported, probably due to kinetic reasons, but also secures GO derivatives that are readily soluble in polar organic solvents, affording clear, brown organosols (12) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682.
(0.1-0.5% w/v) after sonication or good stirring. Thus, modification of GO with C18H37NH2, as described in the Experimental Section, leads to an organophilic solid possessing d001 ) 28 Å versus 25Å (calcd) and increased dispersion in the following order: chloroform > tetrahydrofuran > toluene, dichloromethane. In addition, by spreading the organosols (∼0.1% w/v) over a glass support and evaporating the solvent, brownish transparent films are obtained. Strong evidence for the modification of GO with octadecylamine is provided by the IR spectrum (strong absorption bands below 3000 cm-1 associated with the presence of -CH3 and -CH2- groups in the solid) and its 1H NMR spectrum in CDCl3 as a colloidal dispersion [δ ) 0.9 (s) (-CH3, small intensity), 1.1 (s) (-CH2-, very intense)] (Figure 7). The facile insertion of primary amines to the interlayer zone of GO in conjunction with the ability, for instance, of sodium borohydride to transform GO into graphite, offers unique routes for obtaining molecular pillared GO and graphite layered structures. In this way, reduction of the hydrophobized dodecylamino-GO derivative with NaBH4 leads to an ordered pillared graphite solid with d002 ) 11.3 Å (Figure 8, left). The maintenance of the basal spacing in the reduced solid provides additional evidence for the chemical grafting of the inserted amine molecules onto GO surfaces. On the other hand, treatment of GO with (CH3O)3SiCH2CH2CH2NH2 leads to a organosilasesquioxane-pillared GO structure with d001 ) 13.7 Å (Figure 8, right) in which the -NH2 moieties of the aminosiloxane are bonded to the GO surfaces, leaving behind the alkoxy groups that through condensation with other neighboring alkoxy groups can build up the organosilasesquioxane pillars, possibly as organofunctionalized silicon cubanes.13,14 Similar increment in the interlayer distance (7.6Å) has been also observed for smectite clay mineral pillared with the particular aminosiloxane.13,14 The presence of silasesquioxane units in the solid was established by IR spectroscopy from a strong absorption band at ∼1100 cm-1, typical for the presence of SiO-Si bonds.15 Also, note that the protonated aminosi(13) Szabo A.; Gournis, D.; Karakassides, M. A.; Petridis, D. Chem. Mater. 1998, 10, 639. (14) Petridis, D.; Gournis, D.; Karakassides, M. A. Mol. Cryst. Liq. Cryst. 1998, 311, 345.
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Figure 7. 1H NMR spectrum in CDCl3 of the GO-octadecylamine derivative (the peak at ∼7 ppm is assigned to CHCl3). The inset photo depicts the IR spectrum near 3000 cm-1 of this derivative.
Figure 8. XRD patterns of dodecylamino-GO (left) and aminosiloxane-GO (right) derivatives before (a) and after (b) treatment with NaBH4.
loxane or the propyl chloride siloxane analogue do not intercalate to GO. Treatment of this GO derivative with NaBH4 leads to a silasesquioxane-pillared graphite with d002 ) 13 Å that retains the characteristic absorption band at ∼1100 cm-1. However, the pillared derivatives do not show thermal stability, as their calcination at 500 °C in an argon atmosphere led to products with collapsed structure. It is possible that at high temperatures the C-N bonds [CGO-NH-(CH2)3-Sit] are disrupted, and after the thermal decomposition of the side organic chains, the formed silica migrates to the external surfaces of the carbonaceous solid. Finally, it should be emphasized that primary amines do not interact with reduced GO samples, owing to the lack of oxygen-containing groups on its surface. Therefore, the reduction step must always follow the treatment of GO with the desirable modifier in order to receive reconstructed graphite solids. The specific surface area of pure GO (2.26 m2 g-1) and of reduced GO (2.91 m2 g-1) are small and have near the same value. This means that N2 can only adsorb on the external (15) Kamitsos, E. I.; Patsis, A. P.; Kordas, G. Phys. Rev. B 1993, 48, 12499.
Figure 9. XRD patterns of GO after modification with H2NCH2COONa (a) and H2N(CH2)7COONa (b) (the inset photo depicts the 1H NMR spectrum in D2O of the GO-glycinate derivative).
surfaces of GO and reduced GO. Nitrogen molecules are also inaccessible to get into the interlayer space of the dodecylamino-GO and its reduced form, as shown by the small specific surface areas (0.56 and 1.57 m2 g-1, respectively). The extremely low aSBET values indicate a “paraffin-like” behavior of the material, which is also evidenced visually and “by touch”. However, the siloxane derivative of GO had a specific surface area one magnitude higher (26.51 m2 g-1) than pure GO, supporting pillaring by the silosequioxane units. This result implies that nitrogen adsorbs not only on the external surface but also incorporates into the pores of the silicon cubanes. The reduced GO-siloxane derivative showed also increased BET surface area (12.1 m2 g-1) compared to reduced GO. H2N(CH2)nCOONa (n )1, 7). Figure 9 presents the XRD patterns of the particular amino acid-GO derivatives. In contrast to the amine molecules, the insertion of the amino acids induces only small changes in the basal spacing of GO, irrespective of their chain length (from 6.78 to 8.2 Å
Reduction of Graphite Oxide to Graphite
for both amino acids). Assuming that a nucleophilic attack of the -NH2 end group on the epoxide groups of GO is taking place, the results suggest that the amino acid molecules adopt a flat orientation in the interlayer zone of GO, probably as a result of hydrogen-bonding interactions between the carboxylate groups of the guest molecules and the oxygen-containing groups of GO’s layers. Nevertheless, it is quite possible that the incoming entities become locked to the GO layers, since for a flat orientation of the amino acid molecules (thickness 3.5 Å) a basal spacing of 9.6 Å instead of 8.2 Å would be expected.16 The choice of amino acids as surface modifiers for GO is of particular interest for two reasons: first, it introduces direct exchange sites to the GO solid through the carboxylate groups (-COO-Na+) of the grafted molecules, and second, it enables one to negatively charge the surface of GO layers by dissociation of the -COO-Na+ moieties in water and thus leads to a stabilized colloidal dispersion of the platelets. This is the case for the glycinate derivative.17 Dispersion of this derivative in water, followed by centrifugion and sonication of the supernatant liquid, yields clear brown sols (0.5% w/v) in which 80% of the modified solid has been dispersed [1H NMR in D2O: δ ) 3.4 (d) (-CH2-, intense), inset in Figure 9]. Although, the as-made sol “breaks down” within a few days, it can be readily redispersed in water by sonication. In addition, by spreading the aqueous sol over a glass support and evaporating the water, brownish, transparent films can be obtained. On account of the surface exposed -COOgroups in the modified layers, addition of C16H33N(CH3)3+Cl- to the sol leads to the precipitation of a gel-like solid as a result of the following charge-balancing reaction:
GO-NHCH2COO- + C16H33N(CH3)3+ f GO-NHCH2COO-C16H33N(CH3)3+ This insertion was evidenced by IR spectroscopy from the presence of strong absorptions below 3000 cm-1 due to the -CH3 and -CH2- aliphatic moieties of the entered surfactant cations. On the other side, an aqueous dispersion of unmodified GO does not uptake the C16H33N(CH3)3+ ions, as evidenced by IR and XRD measurements. Figure 10 shows the XRD patterns of GO (a) and the GOglycinate derivative (b) after their treatment with C16H33N(CH3)3+Cl-. Unlike GO, the XRD of the modified solid changes drastically after the treatment, giving a lowangle, sharp (001) reflection at d001 ) 37 Å, characteristic of a bilayer arrangement of the surfactant cations within the interlayer space of the modified solid.1 Similarly, treatment of the glycinate-GO sol with a commercially available cationic polyelectrolyte (Gafquat HS-100: vinylpyrrolidone/methacrylamidopropyltrimethylammonium chloride copolymer) leads to expanded interlayers with a d001 value of 27 Å (Figure 10c). Notice that all samples were well washed and dried prior measurements. To the best of our knowledge, these materials are the only well-characterized and readily obtainable organic derivatives of GO. Conclusions The present work describes the chemical reduction of GO to graphite using common reducing agents such as (16) Theng, B. K. G. The chemistry of clay-organic reactions; Adam Hilger Ltd: 1974; pp 158-186. (17) It is worth mentioning that treatment of GO with H2NCH2COOCH2CH3 and (CH3)3+NCH2COO- does not lead to water-soluble GO derivatives. Noticeably, the sodium salt of arginine can be employed as well in attaining a highly hydrophilic GO derivative.
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Figure 10. XRD patterns of GO after treatment with surfactant cations C16H33N(CH3)3+ (a) and of the glycinate-GO derivative after treatment with surfactant cations (b) and a polyelectrolyte (c).
NaBH4 and hydroquinone, the modification of its surface with a series of neutral, primary aliphatic amines and amino acids, and how the two processes can lead to reconstructed GO and graphite materials. Concerning the redox reactions, treatment of GO with NaBH4 affords turbostatic graphite with d002 ) 3.5 Å that upon thermal treatment under an inert atmosphere is transformed to crystalline graphite (d002 ) 3.39 Å), while treatment with hydroquinone yields directly crystalline graphite with d002 ) 3.39 Å under soft thermal conditions. Other effective reducing agents of GO include metallic iron or zinc fine powders and Na2S. Concerning the surface modification reactions of GO with amine or amine-containing molecules, these proceed easily through amine nucleophilic substitution reactions on the epoxy groups of GO as the main insertion pathway. Such reactions afford valuable GO derivatives, in which the GO layers inherit the physicochemical properties of the particular modifier. For instance, surface modification of GO with octadecylamine leads to an organophilic GO solid that can be readily dispersed in polar organic solvents to give organic-based colloidal dispersions of the hydrophobic platelets. Similarly, attachment of glycinate molecules (sodic form) to the GO surfaces endows the solid not only with direct cation exchange sites but also with negative charges through the -COO- groups of the modifier and thus secures aqueous colloidal dispersions of the platelets. Finally, the unique combination of redox reactions with the facile surface modification of GO with amine or aminecontaining molecules can lead to novel reconstructed GO and graphite derivatives, providing that the surface modification step always proceeds the redox reactions. Thus, an organically modified GO with dodecylamine can be easily reduced to the corresponding organophilic graphite derivative, while modification with aminosiloxanes leads to molecular pillared GO and subsequently, by reduction, to molecular pillared graphite. Acknowledgment. The financial support by a grant (PST, CLG 977550) from NATO is gratefully acknowledged. LA026525H
