Intercalation of Gas Molecules in Graphene Oxide Interlayer: The Role

Apr 29, 2014 - 2010-0029176); and (4) the WCU program (31-2008-000-10055-0), funded ...... Diamond , L. W.; Akinfiev , N. N. Solubility of CO2 in Wate...
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Intercalation of Gas Molecules in Graphene Oxide Interlayer: The Role of Water Daeok Kim,† Dae Woo Kim,† Hyung-Kyu Lim, Jiwon Jeon, Hyungjun Kim, Hee-Tae Jung,* and Huen Lee* Graduate School of EEWS, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, South Korea Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, South Korea S Supporting Information *

ABSTRACT: Intercalation phenomena of gas molecules in the interlayer of graphene oxide have been investigated using CO2, CH4, H2, and N2 gases. Intercalation of gas molecules is highly affected by the affinity between the hydrophilic surface of GO and target molecules. Among tested gases, CO2 can only be intercalated. Furthermore, the swelling of interlayer changed the intercalation phenomena. Amounts of intercalated gases are significantly enhanced, and all the gases can be intercalated by retarded dynamics of intercalated water.

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INTRODUCTION Intercalation phenomena in which guest molecules are contained in the interlayer of stacked two-dimensional materials such as graphite, dichalcogenide, and silicate clays, and so on, have been widely studied not only for scientific curiosity about the physical and chemical behavior of guest molecules in narrow interlayer but also for practical applications such as catalysts,1,2 electrodes,3,4 and batteries.5,6 Graphene oxide (GO) is a derivative of graphene containing abundant oxygen functional groups such as carboxyl, hydroxyl, and ether on the basal plane of carbon sheet, which makes the interlayer of GO to be hydrophilic and gives rise to peculiar intercalation phenomena as they are stacked into graphite oxide. Previous studies revealed that the hydrophilic surface of GO provides favorable environment for the intercalation of polar organic molecules such as polymer electrolytes,7−9 polypyrrole,10 organic ammonium ions,11 diaminoalkanes,12 oleylamines,13 alcohols and water.14 By using theses intercalation phenomena, GO has been applied in various area such as fabrication of intercalated GO composites8,9,15 and separation of mixture by selectively intercalating specific molecule.14 Especially previous studies that reported the successful separation of mixtures using GO-based membrane represent tremendous potential of GO in separation field.16−20 As we consider the theoretical surface area of graphene (2630 m2/g)21 and hydrophilic surface of GO, it is expected to have huge spacing to store polar and hydrophilic gas molecules, which can be used for storage and separation of hydrophilic gas. © 2014 American Chemical Society

However, to the best of our knowledge, the intercalation of gas molecules in interlayer of GO at pressurized condition has never been reported, which is fundamental information for further application of GO for gas technology. Here, we investigated the intercalation behavior of various gas molecules such as CO2, CH4, N2, and H2 in both dried and water swelled interlayer of GO. The result revealed that the pressure and types of gas molecule are important factors to determine the degree of intercalation, and the intercalation behavior totally changed with the presence of water molecules in GO interlayer. The dynamics of the intercalated water in the GO interlayer were retarded to an order of magnitude less than the bulk state by strong interaction with the GO surface, which leads to the enhanced storage of gas by the intercalated water. These phenomena were investigated by low-temperature X-ray diffraction (LT-XRD), synchrotron high-resolution powder diffraction, and measuring the amounts of various gases stored. Molecular simulation was also performed to understand the gas intercalation behaviors in swelled GO.

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EXPERIMENTAL SECTION Synthesis of GO. Graphene oxide was prepared using a modified Hummers’ method.22 Briefly, a graphite powder (1 g, Asbury Carbon) was added to sulfuric acid (98%, 150 mL), Received: March 18, 2014 Revised: April 29, 2014 Published: April 29, 2014 11142

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which served as a solvent. Potassium permanganate (99.0%, 2.5 g), employed as an oxidizing agent, was gradually added to the graphite solution, with vigorous stirring for about 10 min. After reaction at 35 °C for 2 h, the solution was cooled in an ice bath and diluted with 200 mL of deionized water. Following 4 h of stirring, 100 mL of hydrogen peroxide was added to the reaction solution. The mixture was then filtered with a glass filter and washed several times with hydrochloric acid (10%). After this washing process, the remaining solvent was evaporated under vacuum at room temperature for 12 h. Water-Swelled (Saturated) GO and Gas Storage. All of the water-swelled GO samples were prepared by placing 6 g of graphene oxide and 4 g of distilled water into a vacuum oven. The oven was rapidly evacuated then kept closed, where the relative humidity inside the vacuum oven was then increased to 100%. The fraction of water in GO was controlled by the exposure time of GO in the humid vacuum oven. The swelled GO with 0.35 water fraction was prepared by exposing 1 day (0.35 means weight fraction of water in swelled GO), and samples with water concentration lower than 0.35 were prepared by quitting the swelling procedure before saturation. The swelled graphene oxide (1.5 g) was loaded into a highpressure reactor with 20 mL of internal volume, pressurized at room temperature by injecting gas, and then placed in a −30 °C cooling bath for 24 h. The amount of stored gas was measured by the water displacement method as shown in Figure S1 of the Supporting Information. Diffraction Analysis. For the analysis, samples were collected by precooling the reactor with liquid nitrogen, degassing, collecting the samples, and storing them in liquid nitrogen. Then, each sample was finely ground to particles smaller than 200 um at 77 K and placed in a sample loader. Powder X-ray diffraction was conducted using a low-temperature XRD (D/MAX, Rigaku) with CuK radiation (λ = 1.5406 Å) at a generator voltage of 40 kV and a generator current of 300 mA. XRD measurement was performed with 0.05 steps at −180 °C. When investigating the change of samples in relation to temperature, temperature was increased stepwise at a rate of 5 °C/min, with a waiting time of 5 min to stabilize the temperature before initiating measurement. The synchrotron HRPD patterns were recorded at −180 °C using the synchrotron of the Pohang Accelerator Laboratory (λ = 1.54950 Å). The experiments were carried out in step mode with a fixed time of 4 s and at a step size of 0.01° for each hydrate sample. X-ray Photoelectron Spectroscopy Analysis. For the measurement, GO was dispersed in water and dropped on Si substrate then dried as a thin film and analyzed using a Sigma Probe (Thermo VG Scientific). Molecular Dynamic Simulation. A representative GO structure model, C10O2.9(OH)1.2, was generated using the Monte Carlo method with the Metropolis Algorithm. The chemical composition of the model was similar to that indicated by the experimental measurement of epoxy and hydroxyl contents (Figure 1). Considering the inhomogeneity and low crystallinity of GO and GO−water materials, 270−310 water molecules could be inserted into the GO unit layer, where the experimental water was 286 molecules per GO unit layer. “Unit layer of GO” indicates one layer of the three separated layers that compose our periodic GO model in the simulation part, and the number 286 was calculated based on the experimental weight fraction of water (0.35) in swelled GO and the molecular weight of GO unit layer (9569.88 g mol−1) by

Figure 1. X-ray photoelectron spectroscopy result of the synthesized graphene oxide. The numbers indicate the percentage of each bond.

converting weight-based water content into molar base (e.g., 0.35 g H2O → 0.01944 mol H2O; 0.65g GO → 6.8e-5 mol GO; molar ratio H2O/GO=285.88). We carried out NPT, NVT molecular dynamics simulations using the large-scale, atomic modeling, massively parallelized simulation (LAMMPS) code with the Nose−Hoover thermostat. The time-steps for dynamics simulation were 0.001 ps and trajectories with 0.1 ps intervals were used to perform the dipole moment analysis. The dielectric constants of water molecules were calculated from the fluctuation of the total dipole moment by eq 1, where M = ∑iμi.23 4π ϵτ = 1 + (⟨M2⟩ − ⟨M ⟩2 ) 3VkBT (1)

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RESULTS AND DISCUSSION First we investigated the gas intercalation behaviors in dried GO. Figure 2 represents the gas storage and desorption behaviors of dried GO. To analyze the degree of gas intercalation in the GO interlayer, we observed the interlayer spacing of GO after pressurization with various gases using lowtemperature XRD. All the GO pressurized with gas were recovered by rapid quenching with liquid nitrogen, and then analyzed at −180 °C in vacuum. As shown in Figure 2A, the interlayer distance of GO pressurized with nonhydrophilic gases such as N2, H2, and CH4 at 100 bar was 7 Å, which corresponds to that of dried GO itself. In contrast, the interlayer spacing of the graphene oxide gradually expanded from 7.5 to 8.7 Å when CO2 gas pressure was increased to 50 bar, as shown in Figure 2B. The amount of stored CO2 showed a tendency to increase as higher pressure was applied, indicating the higher degree of CO2 intercalation at higher pressure, as shown in Figure 2C. These results indicate that the hydrophilicity of the gas molecules and the strong interaction of the gas molecules with the GO surface are essential factors for the diffusion of gas into the GO interlayer. The similar gas storage behavior of N2O gas confirmed the conclusion in Figure S2.The desorption of intercalated CO2 from the GO interlayer was analyzed by LT-XRD with the GO pressurized at 15 bar, with increasing temperature from −160 to 0 °C, as shown in Figure 2D. The interlayer distance begins to contract at −80 °C and gradually decreases with increasing temperature and is completely removed at 0 °C. To investigate the gas interaction behaviors when the interlayer is swelled with water, we prepared water-swelled 11143

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Figure 2. Intercalation phenomena of gas molecules in the interlayer of dried graphene oxide. (A) XRD of graphene oxide after gas pressurization with N2, H2, CH4 at 100 bar. (B) XRD of graphene oxide after gas pressurization with CO2 at the range from 5 to 50 bar. (C) Storage amount of CO2 as a function of injecting pressure. (D) In-situ XRD analysis to observe the CO2 desorption behavior from GO interlayer as temperature increases, where the GO was pressurized at 15 bar.

function of pressure and shows 29.0 mL(NPT)/g at 20 bar, which is much higher than other gases, as well as being about 2 times higher than that of dried GO. And further, this amount of stored CO2 leads to 82.9 mL(NPT)/gwater based on the amount of interlayer water, which is about 5 times higher than the CO2 solubility of water (15.8 mL(NPT)/gwater) at 20 °C and 20 bar.24 To analyze how the gas is stored in swelled GO, LT-XRD investigations were conducted with swelled GO pressurized with CO2 (20 bar), CH4 (20 bar), N2 (100 bar), and H2 (100 bar). The LT-XRD showed peaks at 23.1, 24.6, 26.2, 34, and 40.4° corresponding to the (100), (002), (101), (102), and (110) planes of hexagonal ice with a GO (001) peak at low angle, as shown in Figure 4B. According to the structure analysis using Le-Bail fitting, the experimental diffraction peaks and calculated reference peaks exactly matched in Figure 5, which indicates that hexagonal ice exists in the noninterlamellar regions, rather than within the GO interlayers. This matched well with previous works on the dynamics of water in GO that showed two different types of water in GO such as intercalated water and excess water in noninterlamellar voids,25−28 and structural change of water swelled GO at low temperature.29 In addition, the interlayer spacing of GO changed with swelling and gas storage. Interlayer of GO expanded from 7.8 to 11.4 Å with water swelling and then slightly contracted with gas storing procedure: (10.2 Å, CO2), (8.8 Å, CH4, N2, H2). Considering that the gas storage capacity of hexagonal ice is insignificant and the larger expansion of interlayer spacing of CO2 sample than other gases, the only possible gas storing site is the interlayer. This conclusion was also confirmed by the different interlayer contraction behaviors observed when heating the

GO. For the entire experiment, GO containing 35 wt % water was used, because the interlayer of GO was saturated at 35 wt % (Figure 3). In Figure 4A, we show the amounts of various gas

Figure 3. Interlayer distance of GO in relation to weight percentage of water. The GO-interlayer is saturated with 35 wt % water. Excess water exists on the surface of the graphene oxide, and in voids between the graphene oxide stacks, at higher water content than 35 wt %.

molecules (CO2, CH4, H2, and N2) captured in swelled GO, by measuring the volume of stored gas normalized by 1 g of swelled GO with varying injection pressure. We note that all samples were isolated and cooled down to −30 °C after pressurizing the gas at room temperature. In contrast to the dried GO, which showed no intercalation and storage for N2, H2, and CH4 until 100 bar in Figure 4A, for the swelled GO it was 3−4 mL(NPT)/g for N2, H2, and CH4 at 20 bar. The exotic feature is that the amount of stored CO2 increases as a 11144

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Figure 4. Gas storage and desorption behavior of swelled GO. (A) The storage amount of various gas molecules about the injected gas pressure at 15 °C. After pressurizing the gases at room temperature, all samples were cooled to −30 °C. (B) The XRD analysis of swelled GO after gas pressurization and cooling. At −30 °C, water formed ice for the CO2 below 20 bar. (C,D) In situ XRD studies for (C) swelled GO without gas intercalation and (D) swelled GO with CO2 exposure at 20 bar at elevated temperature. The contraction of GO interlayer at 10 °C in C was due to the vacuum condition in the LT-XRD machine.

temperature (−50 °C) than that of gas-free swelled GO (−20 °C). At this stage, the exact explanation of how the desorbed gases from the GO interlayer make the ice melt at lower temperature is not known, and remains to be investigated in the future. Considering the results described above, it is clear that gas molecules were captured in the interlayers of the GO and not in other locations. Because direct observation of gas and water in the GO interlayer is difficult, we carried out molecular dynamics (MD) simulations to provide insights into the atomic structure of the CO2 in swelled GO interlayer. We used F3C, EPM2, and DREIDING forcefields for water, CO2, and graphene oxide, respectively. As shown in Figure 6, we optimized van der Waals parameters between water and CO2 to reproduce the quantum mechanical binding energies, where the quantum mechanics calculation was conducted with 6-31**G++ and M06-2X level of theory. Three GO surfaces in Figure 7A were included in our simulation box with periodic boundary conditions in Figure 7B. The number of water molecules and CO2 molecules were determined to coincide with the experimental interlayer spacing of the GO from the XRD peak and an experimental storage amount of 30 mL/g. Using the trajectories from the MD simulations at −30 °C and 10 bar, the locational preference of CO2 molecules was examined using atomic distributions along the normal direction to the GO basal plane (Figure 8A). We found that CO2 molecules were mainly located in the middle of the interlayer; mostly surrounded by water molecules in limited contact with

Figure 5. Le-Bail fittings of XRD patterns. Low-temperature HRPD data of swelled GO with pressurized under 20 bar, which forms hexagonal ice structure. Black and red lines indicate measured and calculated diffraction patterns, respectively.

swelled GO from −180 to 0 °C, depending on the presence of gas molecules. Figure 4C,D presents the LT-XRD results of gas free swelled GO and swelled GO including CO2, respectively, during the temperature change. While the interlayer distance and the structure of hexagonal ice in the gas free sample were maintained up to −20 °C, the gas incorporated sample showed both the decomposition of ice and contraction of the interlayer spacing simultaneously at −50 °C. Desorption of gas from the interlayer caused the extraction of intercalated water and made the ice formed in noninterlamellar voids melt at lower 11145

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Figure 6. Optimization of off-diagonal van der Waals parameters between H2O and CO2. The arithmatic mean was used for the off-diagonal van der Waals interactions.

Figure 7. Atomistic structure of GO models. (A) Final structure of the GO monolayer generated by the Monte Carlo process based on the experimental composition of functional groups. (B) Three-layer model for swelled GO and intercalated CO2 gas molecules.

dynamics of the intercalated water is dramatically retarded compared to that of bulk water, which matches well with previous study on the dynamics of nanoconfined water.25−27,30 Under ambient conditions, the diffusivity of intercalated water was calculated to be 5.4 × 10−6 cm2 s−1. This is 1 order of magnitude less than the bulk value of 4.5 × 10−5 cm2 s−1 from the MD simulation (cf. experimental value is 2.2 × 10−5 cm2/ s).31 More interestingly, from the calculation of dipole-moment autocorrelation function in Figure 9, we found that the decaying of rotational dynamics of intercalated water dipoles has the time scale of 17.2 ps, which is much slower than that of bulk water (7.8 ps). This retarded rotational dynamics leads to a dramatic decrease in the dielectric constant of water, to ∼10 (Figure 10). Therefore, the intercalated water in the GO-gap has characteristics very different from those of bulk water, which may cause different gas intercalation behavior from dried GO and enhanced CO2 storage.

the GO surface. This indicates that the CO2 was mostly solvated by water molecules, and the hydrophilic functional groups on the GO surface (like ethers or hydroxyls) primarily interact with water rather than with CO2. In Figure 8B, we further quantified the population of functional groups of oxygen atoms surrounding the carbon of CO2 within the cutoff radius of 5 Å, which was determined by the distance to the second hydration shell. On average, a total of 18 oxygen atoms encapsulate one CO2 molecule, including 13 O atoms from water molecules, 3 O atoms from surface hydroxyl groups, and 2 O atoms from bridging epoxy groups. These ∼18 O atoms form a cage-like structure consisting of hydrogen bonding networks around CO2 as representatively shown in Figure 8C, which effectively confine the CO2. Also we observed that CO2 molecules in the swelled GO have a greater chance to form more stable binding structure because the dynamics of the water molecules are hindered by adsorption on the GO surface. We further show that the 11146

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Figure 8. Molecular dynamics simulation of CO2 in a swelled GO interlayer. (A) Atomic distribution along the normal direction to the GO basal plane of GO-water and GO-water-CO2 system at 240 K, 10 atm condition. CO2 is mainly located in the middle of water layer with limited contact with the GO surface. (B) Distribution probability of each oxygen component which constitutes hydrogen bonding network cage around CO2 within 5 Å range. (C) Representative local cage-like structure surrounding CO2. Hydrogen bonding network is shown with a dashed line.

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CONCLUSIONS In this paper, we investigated gas intercalation phenomena in a GO interlayer. The interlayer of dried GO exhibited discriminating affinity to hydrophilic gases; among CO2, CH4, N2, and H2, only the CO2 can be intercalated. The intercalation phenomena changed as the GO interlayer swelled with water. Contrary to dried GO, in water-swelled GO all the tested gases could be intercalated, but a noticeable difference in the amount of gas storage was observed for CO2, which increased as a function of pressure. The molecular dynamic simulation showed CO2 to be surrounded mainly by intercalated water molecules and partially with functional groups on the GO plane. Because the interaction between the intercalated water and the GO surface retards the dynamics of the water molecules, enhanced gas storage could be achieved within the intercalated water. We expect our finding will not only provide insight into the behaviors of gas and water molecules in a twodimensional hydrophilic nanogap and but also broaden knowledge about confined water science. Furthermore, the different gas storage behavior of CO2 relative to other gases may lead to the development of CO2-capturing technology via hybrid sorbents consisting of water and GO.

Figure 9. Calculated autocorrelation function of total dipole moment of water in bulk and water in GO.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic description of water displacement method to measure gas storage amount and the gas storage of N2O in both dried GO and swelled GO are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

Figure 10. Calculated dielectric constant and diffusion coefficient of water in bulk and GO.

*(H.-T.J.) Phone: +82-42-350-3931; fax: +82-42-350-3910; email: [email protected]. 11147

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*(H.L.) Phone: +82-42-350-3917; fax: +82-42-350-3910; email: [email protected].

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Author Contributions †

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the following grants: (1) National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning, Korea (MSIP, NRF-2012R1A2A1A01003537); (2) Grant funded by Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning, Korea (MSIP, NRF-2012M3A6A5055744); (3) The National Research Foundation Korea (NRF) funded by the Korean Government (MEST) (No. 2010-0029176); and (4) the WCU program (31-2008-000-10055-0), funded by the Ministry of Education and Science and Technology. We also gratefully acknowledge the Pohang Accelerator Laboratory (Beamline 9B-HRPD). H.-K.L., J. J., and H.K. acknowledge the support by Nano·Material Technology Development Program (2012M3A7B4049807), and also the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM), through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.

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