Hydration of Graphite Oxide in Electrolyte and Non-Electrolyte Solutions

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Hydration of Graphite Oxide in Electrolyte and Non-Electrolyte Solutions Serhiy M. Luzan and Alexandr V. Talyzin*  Department of Physics, Umea University, 90 187 Umea, Sweden

ABSTRACT: Pressure induced insertion of liquid media was studied for graphite oxide (GO) immersed in excess amounts of aqueous copper acetate and sucrose solutions and compared to previous experiments with GO immersed in solute-free water media. Compression of GO in copper acetate solution resulted in significant enhancement of high pressure anomaly compared to pure water: interlayer distance reached 17.4 Å at 2.3 GPa while for pure water the maximal observed layer separation was 13.08 Å. Compression of GO in sucrose solution was found to be very similar to compression in solute-free water. These results confirm that copper ions can be pressure-inserted into GO structure while the expansion of structure is attributed to osmotic swelling. Sucrose dissolves in water in molecular form (nonelectrolyte) which results in weaker absorption into the GO structure and the absence of osmotic swelling. Pressure induced insertion of various solutions into the GO structure could possibly be promising for synthesis of new graphite intercalation materials or graphenerelated composites.

’ INTRODUCTION GO is a unique material prepared using various oxidation routes from finely dispersed graphite powder.1 5 Recently, strong attention was drawn to this type of materials due to a possibility to use it as a precursor for synthesis of graphene and graphene-related materials.6 10 Graphite oxides with different degrees of oxidation can be prepared and properties of these materials depend on particular method used. GO has planar structure like graphite but with significantly increased interlayer distance (up to 6 Å).4,11 GO is a hydrophilic material due to the presence of various functional groups (e.g., epoxy, carbonyl, and OH) attached to the graphene skeleton,.4,8,11 14 In contrast to graphite, GO immersed in water and other polar solvents demonstrates rapid swelling, whereas 1 2 monolayers of solvent are inserted into interlayer space forming an expanded solvated structure (up to 12 Å upon the water inclusion).15 17 We showed also that water saturated GO exhibits very strong temperature dependent variations of interlayer spacing at ambient pressure.18 GO can be dispersed into separate layers in solution, deposited as a thin film on various substrates,10,19,20 and then converted into graphene by mild heat treatment, by reduction in solutions6,9,21 or even by light from a camera flash.22 Recently we found that GO immersed in excess of liquid media exhibits unusual (and reversible) anomalies of compressibility with a sharp maximum of unit cell volume at pressures below 2 GPa.23 26 Expansion of GO structure occurs when the aqueous media is liquid (due to pressure induced insertion of additional solvent between GO layers) while solidification of solvent results in downturn on compressibility dependence and partial desertion of solvent from the GO structure. The structure expansion of GO immersed in solute-free water results in of r 2011 American Chemical Society

28 30% expansion of (001) lattice at 1.3 1.5 GPa.23 Compression in acidic or basic solutions strongly modified the compressibility anomaly of GO. The interlayer of GO immersed into the water solution of NaOH increased by an enormous value of 85% reaching 22 Å separation at 1.6 GPa while HCl media suppressed the anomaly.26 High-pressure behavior of GO was studied in some other solvents including protic (e.g., ethanol, methanol)24 and aprotic (e.g., acetone)25 polar solvents. In this case a stepwise phase transformation with insertion of additional solvent monolayer was found at a certain pressure point, and no correlation with the solidification point of the liquid media was observed. The reasons behind the difference in the insertion of water and other polar solvents remain unclear. However, it can be noted that hydration (solvation) of GO is similar in many ways to swelling (at ambient pressure) of clay minerals, which typically exhibit water insertion by a certain number of monolayers (as in GO/ethanol system for example) but in some cases “osmotic swelling” is observed. Osmotic swelling may correspond to tens of water monolayers, and the separation of layers depends on osmotic pressure developed in the system when some ions are inserted between the layers of material.27 Therefore, experimental verification of an osmotic swelling mechanism in GO requires further experiments with various ions dissolved in water. The results of our previous study26 could possibly be interpreted directly as pH dependence: basic solution enhances pressure induced swelling and acidic suppresses it. On other hand, osmotic swelling would Received: August 31, 2011 Revised: November 10, 2011 Published: November 14, 2011 24611

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Figure 2. Pressure dependence of (001) d-spacing for GO compressed in solution of copper acetate (9) and sucrose (2) and compared to solute-free water (b). Inset shows pressure dependence of (010) d-spacing for GO in copper acetate solution.

Figure 1. (a) XRD patterns recorded from graphite oxide powder immersed in excess of aqueous sucrose solution upon pressure increase. Stars mark reflections from gasket. Pattern at 0.14 GPa shows indexing for GO and gold wire used as pressure standard. Some background subtracted. (b) XRD patterns recorded from graphite oxide powder immersed in excess of aqueous solution of copper acetate upon pressure increase. Some background subtracted.

occur in both basic and acidic solutions if certain ions are inserted between the layers of GO. In this study we performed experiments with compression of GO immersed in an excess of aqueous solutions of copper acetate (which easily dissociates to copper and acetate ions) and sucrose (which is not electrolyte) while both solutions are acidic.

’ EXPERIMENTAL SECTION The GO sample was prepared using Brodie’s method1 and showed a composition of CO0.38H0.12 according to elemental analysis. The XRD pattern recorded for the pristine sample of GO at ambient conditions was indexed by a turbostratic graphitelike hexagonal structure with the cell parameters a = 2.485 Å and c = 6.597 Å.23 High pressure experiments were performed using synchrotron radiation X-ray diffraction in a diamond anvil cell (DAC). High pressure experiments were performed using DAC with 0.4 0.8 mm flat culets. Water solutions of copper acetate and sucrose in concentrations of 0.4 mol/L and 2.3 mol/L correspondingly were used in the experiment. Real concentrations could possibly slightly exceed concentrations of pristine solutions due to evaporation of solvent from droplets in the process of SAC loading. The samples were loaded into a hole in

the steel gaskets together with a gold wire piece used for pressure calibration. Proportion of GO powder to the solution was approximately 1:1 by volume. The pressure was increased gradually, and XRD patterns were recorded on every step during compression and decompression. XRD patterns were recorded from graphite oxide samples using synchrotron radiation at ELETTRA X-ray diffraction beamline, the wavelength of λ = 0.688 81 Å, using a MAR345 image plate detector. The two-dimensional XRD patterns were integrated using Fit2D software. Two test heating experiments aimed at functionalization of GO were also performed. The DAC with GO immersed in the water solution of copper acetate was first pressurized up to 4.2 GPa and then was heated at 400 °C for ca. 30 min in the oven. DAC containing the water solution of sucrose was heated at 3.7 GPa to 200 °C for 5 min. Temperatures of heating were selected to be over the limit of thermal decomposition of copper acetate and sucrose.

’ RESULTS AND DISCUSSION Pressure induced insertion of solvent into the GO structure was found for both sucrose and copper acetate solutions; XRD patterns recorded at various pressure are shown in Figure 1. The GO structure consists of planar oxidized graphene sheets which are very rigid (similar to graphite) and compress very slightly in the studied interval of pressures below 4 GPa. For GO immersed in the copper acetate solution the (010) d-spacing decreased linearly from 2.152 to 2.137 Å. The oxidized (or more correctly functionalized) graphene planes are turbostratically packed with water inserted between the layers. Therefore, compressibility of graphite oxide is dominated by variations in interplanar distance which are rather strong due to relatively weak van der Waals bonding between the planes and pressure-dependent insertion/ desertion of solution into the structure. Therefore, the main information about pressure-induced structural breathing can be extracted from variation of (001) reflection which corresponds to the interplanar distance in the GO structure (Figure 2). As can be seen from this figure, pressure dependence of GO (001) d-spacing shows a sharp maximum for all three studied pressure media: copper acetate, sucrose, and solute-free water. The sharp downturn correlates with the pressure point of media solidification into the ice VI phase, while the ice VII phase was observed at higher pressures in a good agreement with a p T diagram of water (Figure 2). As expected, the solidification pressure is higher for 24612

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The Journal of Physical Chemistry C solutions compared to pure water. The increase of interlayer spacing of GO is significantly stronger (up to 17.4 Å) for copper acetate media compared to solute- free water (13.08 Å). In contrast, compression in sucrose solution resulted in almost exactly the same pressure-induced expansion of the GO structure (up to 12.71 Å) as in previous experiments with solute-free water. These results confirm an osmotic swelling mechanism of pressureinduced insertion of solution into GO structure and demonstrate that enhancement of pressure-induced solvent insertion is not directly related to variation of pH (as could possibly be suggested from previous experiments with NaOH26). The copper acetate solution is acidic, but anyway, we observe stronger structure expansion of GO upon compression. The enhancement of pressure-induced insertion of solvent is related to the osmotic swelling mechanism. It suggests that copper ions are absorbed into the GO structure which causes an osmotic inflow of liquid into the GO interlayer space. The excess of copper ions in the GO structure compared to bulk solution is likely connected to interaction with functional groups attached to graphene oxide layers; e.g. copper ions can promote deprotonation of OH groups. Sucrose is dissolved in a molecular state without dissociation on ions and does not influence deprotonation of GO functional groups. This results in weaker absorption of sucrose into GO structure. The osmotic effect is related to the difference in concentrations of solute between bulk solution and solution inserted in GO structure which should be lower for sucrose, and as a result, the osmotic effect is not observed. In a much simplified image, the GO structure serves as a membrane for copper acetate but is not a membrane for sucrose in water solution which results in presence/ absence of osmotic swelling. It should be noted that sucrose is very likely inserted into GO structure together with water as can be expected from the analysis of Figure 2 looking at the first points of experiments. In absence of pressure the separation between GO layers is about 1 Å larger compared to GO in pure water. Experiments described above are not only interesting for understanding of the GO structural breathing mechanism but also open a road to possible synthesis of new graphite intercalation materials, graphite doped by metal nanoparticles, or graphene-containing composites using HPHT treatments. It is known that GO can be converted back to few layered graphite using heat treatment; it exfoliates explosively if heated rapidly around 350 °C (at ambient pressure). Our previous experiments proved that graphite oxide exfoliates and converts back to graphite also when heated at GPa pressures.28 However, no experiments were performed with exfoliation of hydrated GO. Such an experiment would be very difficult to perform at ambient pressure since the water boils at a temperature significantly below the point of GO exfoliation. High pressure, high temperature experiments in DAC would not allow water to escape form the system and increase the temperature limit for liquid state of water. On other hand, copper acetate is routinely used for synthesis of copper and copper oxide nanoparticles by thermal decomposition which occurs at temperatures (325 °C29) not far from the exfoliation point of GO. A simple test experiment confirmed that rapid heating of a GO/copper acetate solution sample at 4.2 GPa to 400 °C resulted in decomposition of GO to yield a graphite-like carbon phase and decomposition of copper acetate to yield Cu and copper oxides. The quality of data was not sufficient for detailed analysis of phase composition of the sample and to verify if some copper intercalated graphitic phase formed after exfoliation of GO. The possibility of simultaneous thermal decomposition of GO and copper acetate at GPa pressures gives

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a promise for synthesis of new carbon intercalation materials in the future.

’ CONCLUSIONS High pressure experiments with GO immersed in an excess of copper acetate and GO immersed in an excess of sucrose water solutions showed pressure induced insertion of solution. However, amplitude of high pressure anomaly for GO immersed in the copper acetate solution was stronger while for the sucrose solution smaller compared to pure water. The difference is explained by absorption of copper ions which causes osmotic inflow of water into interlayer space of the GO structure. Sucrose solution is not electrolyte and the osmotic effect is not observed. These experiments confirmed the previously proposed mechanism of osmotic pressure-induced swelling for GO immersed in aqueous media. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We would like to thank Giorgio Bais and Maurizio Polentarutti for support at Elettra Synchrotron Radiation facility. A.V.T. thanks Umea University for financial support provided by Young Investigator Award. ’ REFERENCES (1) Brodie, B. C. Ann. Chim. Phys. 1860, 59, 466–472. (2) Hofmann, U.; Frenzel, A. Ber. Dtsch. Chem. Ges. 1930, 63, 1248–1262. (3) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339–1339. (4) Scholz, W.; Boehm, H. P. Z. Anorg. Allg. Chem. 1969, 369, 327–340. (5) Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szabo, T.; Szeri, A.; Dekany, I. Langmuir 2003, 19, 6050–6055. (6) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282–286. (7) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (8) Gao, W.; Alemany, L. B.; Ci, L. J.; Ajayan, P. M. Nat. Chem. 2009, 1, 403–408. (9) Fan, X. B.; Peng, W. C.; Li, Y.; Li, X. Y.; Wang, S. L.; Zhang, G. L.; Zhang, F. B. Adv. Mater. 2008, 20, 4490–4493. (10) Cote, L. J.; Kim, F.; Huang, J. X. J. Am. Chem. Soc. 2009, 131, 1043–1049. (11) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dekany, I. Chem. Mater. 2006, 18, 2740–2749. (12) Lerf, A.; He, H. Y.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477–4482. (13) Casablanca, L. B.; Shaibat, M. A.; Cai, W. W. W.; Park, S.; Piner, R.; Ruoff, R. S.; Ishii, Y. J. Am. Chem. Soc. 2010, 132, 5672–5676. (14) Cai, W. W.; Piner, R. D.; Stadermann, F. J.; Park, S.; Shaibat, M. A.; Ishii, Y.; Yang, D. X.; Velamakanni, A.; An, S. J.; Stoller, M.; An, J. H.; et al. Science 2008, 321, 1815–1817. (15) Lerf, A.; Buchsteiner, A.; Pieper, J.; Schottl, S.; Dekany, I.; Szabo, T.; Boehm, H. P. J. Phys. Chem. Solids 2006, 67, 1106–1110. (16) Buchsteiner, A.; Lerf, A.; Pieper, J. J. Phys. Chem. B 2006, 110, 22328–22338. 24613

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