Impact of Ionic Liquids on the Exfoliation of Graphite Oxide - The

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Impact of Ionic Liquids on the Exfoliation of Graphite Oxide Muge Acik,† Daniel R. Dreyer,‡ Christopher W. Bielawski,‡ and Yves J. Chabal*,† †

Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States

‡

S Supporting Information *

ABSTRACT: The fabrication of high performance, graphenebased electrochemical energy storage devices, such as ultracapacitors, depends on the reduction of graphite oxide (GO) and its interaction with ionic liquids (ILs), which may be used as the conductive electrolyte. To explore the physical and chemical interactions between ILs and thermally reduced GO (TRG) as a function of annealing temperature, three ILs with ammonium based structures were selected to differentiate the role of their anions and cations in the exfoliation process. Intercalation was accompanied by either covalent or noncovalent bonding, as supported by thermogravimetric analysis (TGA) and infrared (IR) absorption spectroscopy performed in situ during thermal annealing and by X-ray diffraction (XRD) analysis. Upon IL intercalation, covalent bonding between the IL and TRG prevented exfoliation, while noncovalently physisorbed ILs were readily removed and therefore facilitated exfoliation of the reduced GO. The anion and cation moieties of the ILs in GO−IL intercalation compounds investigated were found to affect the decomposition temperature as well as the degree of thermal stabilization. Indeed, the ammonium-based cations bearing long alkyl carbon chains did not functionalize the TRG and therefore promoted both sheet expansion and thermal exfoliation. The solventdependency of these properties was also investigated by forming GO−IL intercalation compounds from both deionized (DI) water and propylene carbonate (PC). In contrast to DI water, PC was found to decrease the thermal decomposition temperature of GO by about 100 °C in the presence of intercalated ILs, thus enabling highly efficient oxygen removal in GO−IL intercalation compounds.

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Graphite oxide (GO) is a common precursor to graphene and other similar carbon nanomaterials used in ultracapacitors and is accessible from graphite in one facile step.5 Although the intercalation chemistry of both graphite and GO, especially when coupled with chemical reduction of GO, has been studied,6−12 the intercalation mechanism and the role of the ILs’ structures in the intercalation process remain unclear in these systems. Aside from the aforementioned chemical reduction approaches, GO can also be subjected to thermal annealing to prepare thermally reduced graphene oxide (TRG)−IL composites suitable for use in ultracapacitors. One possible route is the direct covalent functionalization of TRG with an IL. For instance, reduced GO (prepared by heating GO at 150 °C for 45 min in vacuum) has been treated with 1-butyl-3methylimidazolium hexafluorophosphate,13 resulting in imidazolium-functionalized carbon electrode materials. Functionalization of TRG (prepared by annealing GO at 1000 °C, followed by the addition of 10% H2 in Ar) can also occur with a diazonium salt in 1-octyl-3-methylimidazolium tetrafluorobo-

igh surface area, conductive materials are necessary for high performance energy storage devices (e.g., Li-ion batteries or ultracapacitors).1 Supporting electrolytes are used in these devices to provide electrical communication between the high surface area, charge-storing electrodes. Ionic liquids (ILs) are attractive choices for use as electrolytes because of their valuable physical properties (e.g., low vapor pressure, high thermal stability, nonflammability, high ion conductivity, wide electrochemical window, and low melting temperature).2 In graphene-based ultracapacitors, electrodes are commonly prepared from modified graphite or graphene.3 Once the ILs are combined with the electrodes, the two components interact through Coulombic or charge polarization interactions, van der Waals, hydrogen bonding,and cation−π or π−π forces, which control their wetting of the electrode and internal order. The ILs’ structure and properties, such as dispersibility on the carbon and stabilization of charge buildup, determine the overall performance of the ultracapacitor. Beyond enhancing the energy storage capability of the carbon material, a proper match in surface tension of ILs and graphite has been shown to facilitate the interaction of these two components and promotes the intercalation and exfoliation of the individual graphene layers.4 © 2012 American Chemical Society

Received: January 24, 2012 Revised: March 10, 2012 Published: March 14, 2012 7867

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Figure 1. Structures of various ionic liquids studied to make GO−IL intercalation compounds investigate: (a) N-methyl-N,N,N-tris(2hydroxyethyl)ammonium iodide (1), (b) N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium methyl sulfate (2), and (c) N,N,N-tributyl-Noctylammonium methyl sulfate (3).

solutions in PC were tip-sonicated for 10 min using 20% amplitude with pulses of 2.1 s on and 1.6 s off. Thermal Annealing Method. Thermal annealing of GO samples on SiO2/Si substrates was achieved by direct resistive heating of the coated Si/SiO2 substrates. The annealing chamber (a closed system) was placed in the main compartment of a N2 purged spectrometer: a Thermo Scientific Nicolet 6700 FT-IR spectrometer with a KBr beam splitter, a deuterated triglycine sulfate (DTGS) detector, and constant nitrogen flow generated by a slight overpressure. The chamber was evacuated (10−3−10−4 Torr) to minimize the environmental effects. The sample was held by two tantalum clips to permit resistive heating while under vacuum (6010A DC Power Supply (0-−200 V/0−17A, 1000 W, Hewlett-Packard)). Once the sample was in place, the measurements were performed in situ (no sample transfer needed). All temperature readings were monitored by a Eurotherm unit using type K thermocouples spot-welded to a tantalum clip attached to the sample edge. Calibration with a pyrometer indicated that the thermocouple readings were systematically too low (by 20−50 °C) in the 500−900 °C range. However, this was a systematic error so that the relative measurements were reproducible. Materials Characterization. In Situ Infrared Transmission Spectroscopy Measurements. FT-IR measurements were performed in a transmission geometry (typically 500 scans per loop), using a deuterated triglycine sulfate (DTGS) detector with a mirror optical velocity of 0.6329 cm s−1 at 4 cm−1 resolution. The sample in the chamber was positioned so that the IR incident angle was close to the Brewster angle (70°) using direct transmission. Data collection per loop was performed at 60 °C after each annealing sequence. The annealing time during a sequence was 5 min at each temperature for a stepwise reduction, and the total annealing time for the overall experiment per sample was approximately 8 h. Differential spectra were referenced to either a reference temperature (60 °C) or to the spectrum collected at a previous annealing temperature. Each absorbance spectrum was referenced to the bare clean SiO2/Si substrate spectrum collected at 60 °C used as a reference room temperature. X-ray Diffraction (XRD) Analysis. XRD diffraction patterns were recorded for 2θ values ranging from 2° to 40° in order. Xray diffraction measurements were conducted with a Rigaku Ultima III diffractometer (Cu Kα radiation, X-ray wavelength of 1.54187 Å, operating at 40 keV with a cathode current of 44 mA). GIICs intercalated in DI water were directly deposited on SiO2/Si substrates for XRD analysis. Dispersions in PC were dried at 60 °C on a hot plate for 2−3 days.

rate and potassium carbonate without any covalent reaction with the ILs (i.e., physisorption).14 In these examples, the dispersibility of the TRG−IL hybrid material appears to depend on the nature of the anion of the IL. However, it has also been shown that ILs bearing long alkyl carbon chains promoted debundling of carbon nanotubes (CNTs) (i.e., separation of bundles of CNTs into individual nanotubes). Analogous15 interactions between ILs having long alkyl carbon chains and TRG have not yet been established. Therefore, in a broad sense, the nature of the chemical interactions at work during ILmediated exfoliation prepared in different solvent media and the ensuing functionalization (covalent or noncovalent) of reduced GO are not well understood. Herein, we examine factors that facilitate the intercalation of ILs into GO by varying the ILs’ anions and cations, as well as the solvent, in which GO is dispersed (either deionized water or propylene carbonate (PC)) to provide a guide for choosing ILs applicable to electrochemical energy storage devices. To this end, we combine thermal analyses and in situ infrared transmission spectroscopy techniques to determine how the ILs intercalate into GO and form reduced products, whether the ILs chemically interact with the carbon materials, and how the ILs affect the thermal reduction efficiency (i.e., the degree of oxygen removal from GO). These experimental findings are also supported by X-ray diffraction studies that investigate the efficiency of the intercalation of the ILs into the interlayers of GO at room temperature and upon thermal treatment.

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EXPERIMENTAL METHODS Synthesis of Graphite Oxide. To investigate the ILs’ ability to intercalate into GO, we first prepared GO using a modified Hummers’ method16 by pretreating graphite powder with K2S2O8 and P2O5, followed by oxidation with KMnO4 in H2SO4 (see Supporting Information for details). Intercalation Process. The three selected ILs (Figure 1) include two with a common cation, N-methyl-N,N,N-tris(2hydroxyethyl)ammonium (1 and 2), and two with a common anion, methyl sulfate (2 and 3), to better assess their respective contributions to intercalation into GO and thermal exfoliation. These ILs were synthesized as described in the Supporting Information. The as-prepared GO was then mixed with each of the ILs (ratio of 1:2 (w/w) of GO/IL in 4 mL of deionized water or PC. ILs 1, 2, and 3 were individually intercalated into GO by the addition of the IL to a dispersion of GO in deionized water or in PC keeping the weight ratio of GO to IL constant at 1:2 (in wt %). The aqueous GO−IL solutions were centrifuged for 30 min and then stirred overnight. GO−IL 7868

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Thermogravimetric (TGA) Analysis. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/ SDTA 851 at a heating rate of 10 °C min−1 under an atmosphere of nitrogen. The GO−IL intercalation compounds were prepared as previously described in the experimental methods section and then collected on a nylon membrane (0.2 μm) after vacuum filtration for TGA analysis.

Table 1. Summary of TGA and XRD Results for Graphite, GO, the Neat ILs, the GO−IL Intercalation Compounds, and the TRG−IL Intercalation Compounds (Prepared from Different Solvents: Deionized Water or Propylene Carbonate)a decomposition temp (°C)b

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IL 1 IL 2 IL 3 GO graphite

RESULTS AND DISCUSSION In a series of preliminary experiments, each of the three ILs (1−3) given in Figure 1 was mixed with GO (GO-to-IL ratio held constant at 1:2) in DI water. To deduce whether or not the ILs had intercalated into GO’s lamellar structure, X-ray diffraction (XRD) was performed on the recovered carbon materials. The diffraction patterns collected for these three samples are summarized in Figure 2 and in Table 1. The

GO−IL 1d GO−IL 1e GO−IL 2d GO−IL 2e GO−IL 3d GO−IL 3e TRG−IL 1d TRG−IL 2d TRG−IL 3d

227 304 266 200

189f, 290 (↑) 90f, 234 (↑) 173f, 227 (↓) 88f, 271 (↓) 173f, 251 (↓) 111f,g 290 (↑) 251 (↓)

d-spacing (Å)c

9.1 3.7 12.4, 6.0 21.6, 13.6, 6.3 20.3 3.8 3.7 4.4

a

Arrows indicate whether the observed decomposition temperature (Td) increased (↑) or decreased (↓) relative to the neat IL. b Determined via TGA by heating under nitrogen at 10 °C min−1. c Determined by X-ray diffraction. dPrepared in deionized water. e Prepared in propylene carbonate. fAttributed to the decomposition of GO. gSeparate IL decomposition not detectable by TGA (see Figure S1, Supporting Information).

Information).17 Figure 2d shows that, after the GO−IL intercalation compound was thermally reduced at 500 °C, graphitic spacing was not compressed to 3−4 Å as is typical of graphite powder (d = 3.7 Å). Although the initial interlayer spacings of TRGs prepared by annealing IL-intercalated GO with IL 1 (TRG-1) and TRG-2 differed after RT intercalation (12.4 Å and 13.6−21.4 Å, respectively), they both reached similar d-spacings (3.8 Å) after annealing at 500 °C (Figure 3). In contrast, the use of an IL with long alkyl chains, such as 3, increased the degree of exfoliation with further expansion upon annealing at 500 °C. Specifically, a d-spacing of 4.4 Å was measured after annealing GO intercalated with 3 at 300 °C

Figure 2. X-ray diffraction analyses of (a) 1, (b) 2, and (c) 3 intercalated into graphite oxide (d) at room temperature. Calculated dspacings are shown for each 2θ value.

crystallographic orientations of the intercalation compounds were found to vary significantly with the ILs’ structure and properties. While as-prepared GO exhibited a d-spacing of 9.1 Å (2θ ≈ 10°), the carbon recovered after mixing of GO with 1 exhibited an increase of 3.3 Å in its d-spacing. Similarly, the carbon recovered after stirring 2 with GO exhibited an increase in d-spacing of 4.5−12.5 Å, and the carbon recovered after stirring 3 with GO showed an increase of 11.2 Å. Each of the recovered carbons showed at least three different peaks in their diffraction patterns, as shown in Figure 2, and the c axis graphitic orientation at 2θ ≈ 21° exhibited an increase in its dspacing by ∼3 Å. Even though both 1 and 2 possessed the same cation, their room temperature (RT) intercalation varied based on the nature of the anion (iodide versus methyl sulfate). Collectively, these results indicate that the ILs can be successfully intercalated into GO, facilitating the formation of GO−IL intercalation compounds. XRD analysis following annealing of the TRG−IL intercalation compounds showed that although 2 and 3 bear methyl sulfate anions, their thermal reduction behavior in the presence of TRG was distinct. Indeed, the presence of the long alkyl chains in 3 increased the recovered carbon’s interlayer spacing, possibly due to the increased molecular volume of 3, compared to 1 and 2 (see calculations in the Supporting

Figure 3. X-ray diffraction analysis of GO intercalated with (a) 1, (b) 2 after annealing at 500 °C, and (c) 3 (shown for a 300 °C anneal). Calculated d-spacings are shown for each 2θ value. (d) Graphite powder spectrum. Spectra are offset on the y axis for clarification. 7869

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composite exhibited a decrease in Td of 15 °C (from 266 to 251 °C). Collectively, these results suggest that IL cations with long alkyl chains provide a greater thermal stability for ILs when intercalated into GO. The solvent used for intercalating GO with each IL was found to affect the decomposition temperature of the GO−IL intercalation compounds. In general, the Td of GO (determined from the first mass loss onset in the TGA of each GO−IL compound, as previously described) was lowered in the presence of PC (Figure S1, Supporting Information), as compared to DI water. The lack of a distinct second decomposition in the GO−IL intercalation compounds prepared from PC also confirmed that PC dramatically lowered the Td of the ILs in the presence of TRG. Perturbation of the Td resulted in GO’s exfoliation coupled with highly efficient expansion, as evidenced by the previously described XRD studies. As a result, all ILs reduced the Td of GO (first onset). For instance, 1 intercalated into GO from DI water lowered the Td of GO by about 10 °C (189 °C, as compared to 200 °C for as-prepared GO). Conversely, when prepared from PC, the Td (90 °C) decreased by approximately 100 °C. Even though intercalation in DI water increased the Td of 1 in the presence of GO, which bore an iodide anion, by about 63 °C (from 227 to 290 °C), intercalation performed in PC stabilized GO intercalated with 1 (GO-1) to a lesser extent (Td elevated by 7 to 234 °C). In contrast, the Tds of GO-2 and GO-3 were lowered. Having explored the physical structure and morphology of the GO−IL intercalation compounds, we next sought to characterize the chemical interactions occurring between the carbon materials and the ILs. To investigate whether the degree of thermal sheet expansion and IL intercalation was closely related to the chemical interactions occurring within the interlayer regions of GO, a series of IR studies was performed on each GO−IL intercalation compound. The degree of functionalization was determined from the differential spectra in Figures 5 and S2, Supporting Information, in which the initial GO−IL structures are shown for each corresponding IL intercalation (black spectra), where a positive peak is associated with the formation of a new functionality (i.e., covalent binding

(TRG-3), indicating that exfoliation with a highly efficient expansion of the intercalated GO was achieved at temperatures below 500 °C. These experimental findings indicate that long, flexible alkyl carbon chains incorporated into the ILs’ cations promote the thermal expansion and exfoliation of TRG to a greater extent than tetrasubstituted ammonium salts with shorter alkyl chains. The nature of the interactions between the TRG and the ILs (covalent versus noncovalent bonding) in the intercalation compounds was examined using thermogravimetric analysis (TGA), in situ IR spectroscopy, and XRD. TGA of the intercalation compounds revealed two decomposition onsets, rather than just one, as observed for pure GO. We attributed the first, lower temperature onset to the mass loss of GO. The second, higher temperature onset was attributed to the decomposition of the ILs. These TGA data were then correlated with in situ IR measurements that detected the functional groups and their evolution as a function of temperature. In addition, XRD measurements provided information on thermal exfoliation and expansion of the interlayer spacing of TRG upon intercalation with the IL. The anions of the IL were found to alter the decomposition temperature (Td) of both GO (∼200 °C) and the ILs. The TGA studies, summarized in Figure 4 and in Table 1, indicated

Figure 4. Thermogravimetric analysis of ILs after intercalation into the stacks of GO in DI water. Normalized mass (to the maximum mass of each sample and then to [0.1] scale of the y axis for comparison) is shown as a function of reference temperature (°C).

that while pure 1 exhibited a Td of 227 °C, an increase to 290 °C was observed upon intercalation of the IL into GO in DI water. Conversely, 2 exhibited a reduced Td (from 304 to 227 °C) when intercalated into GO. Thus, the iodide anion elevated the thermal stability of the TRG−IL composite, while methyl sulfate lowered it. More broadly, the presence of methyl sulfate anions also diminished the Td of GO, even more than the iodide anion did. For instance, the Td of GO intercalated with 1 was lowered from 200 to 190 °C and with 2 from 200 to 143 °C. Indeed, a moderate decrease in Td (173 °C) was observed for 3, which also promoted the removal of oxygen from GO. When 2 was intercalated into GO, the recovered TRG−IL

Figure 5. Transmission differential infrared absorbance spectra at 200 °C referenced to 60 °C (initial temperature, black) and at 250 °C referenced to 200 °C (red) for GO intercalated with (a) N-methylN,N,N-tris(2-hydroxyethyl) ammonium iodide (1), (b) N-methylN,N,N-tris(2-hydroxyethyl)ammonium methyl sulfate (2), and (c) N,N,N-tributyl-N-octyl ammonium methyl sulfate (3) in DI water. 7870

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we believe that the IL cation unit of 3 stimulates sheet expansion and thermal exfoliation without functionalizing the TRG (i.e., no new bond formation). Indeed, there was remaining functionality (C−O and N−H moieties) in TRG intercalated with 2, even after annealing at 500 °C. Although residual oxygen groups in TRG could not be observed in Figure S2a, Supporting Information, both the XRD and TGA results showed weak thermal expansion of the TRG layers intercalated with 1. These findings confirm that partial covalent functionalization at the intermediate stages of thermal reduction decreased the degree of exfoliation in these GO−IL intercalation compounds. These experimental findings are also summarized in Table 2. While the GO−IL intercalation compounds were found to be insulating (σ < 10−4 S m−1; determined using a powder conductivity method described in the Supporting Information), the TRG−IL intercalation compounds exhibited increased conductivity. Indeed, when GO-1 (prepared from PC) was annealed at 500 °C, a conductivity of 156.5 S m−1 was measured. Similarly, when GO-2 (also prepared from PC) was annealed at 500 °C, a conductivity of 562.2 S m−1 was measured. These results indicate that highly conductive TRG− IL compounds may be obtained via this approach, a property which is essential for the fabrication of high performance ultracapacitor energy storage devices. Additionally, we performed Raman spectroscopy (Raman intensity normalized to the intensity of G band) to distinguish whether GO-1 is likely to heal the defects in the form of TRG-1 after annealing at 500 °C. Figure S3, Supporting Information, compares the Raman intensity ratio of the D band (∼1350 cm−1) to the G band (∼1600 cm−1). The ID/IG ratio is extracted to be 1.1 from room temperature measurements and decreased to ∼0.92 after annealing at 500 °C. This result confirms that the defects were partially healed in the presence of IL (1), a process that may occur during thermal annealing. We have also studied exfoliation by dispersing each TRG−IL intercalation compound (initially GO-to-IL ratio = 1:2 wt % and annealed at 500 °C) in either 4 mL of DI water or in 4 mL of PC. The results revealed a Tyndall scattering effect (Figure 6). Figure 6a shows GO−IL 1 dispersed in DI water (room temperature). After annealing at 500 °C and redispersing in DI

of the IL to TRG). For instance, a positive peak at 900−1300 cm−1 observed in TRG-1 corresponds to the formation of new C−O species in Figure 5a (highlighted in yellow). We attributed this to covalent functionalization (e.g., an ether linkage) between the TRG (having out-of-plane hydroxyl groups in GO) and the hydroxyl end groups of the cation in the IL. This additional formation of C−O species also confirmed the covalent functionalization of TRG with cation-bound hydroxyl groups of 1. The results of these studies are summarized in Table 2. After in situ annealing at 250 °C, the Table 2. Summary of the Data Collected for the Diagnostic Properties of the GO−IL Intercalation Compounds (GO−IL 1, GO−IL 2, and GO−IL 3) Formed When the ILs Were Blended and Annealed with TRG GO−IL intercalation compd GO−IL 1

functionalization ability

GO−IL 2

noncovalent (≤200 °C); partial covalent (250 °C) covalent

GO−IL 3

noncovalent

oxygen removal efficiency

exfoliation capability

expansion capability

high

yes

weak

weak (oxygen remains at 500 °C) high

yes

weak

yes

high

GO−IL intercalation compound prepared with 1 was found to covalently functionalize the TRG upon annealing; no chemical reaction took place at annealing temperatures of 200 °C or below (Figure 5a). However, 2 covalently functionalized TRG, even at temperatures below 200 °C (Figure 5b). These findings suggest that TRG−IL intercalation compounds prepared in DI water bearing methyl sulfate anions (TRG-2) promote interlayer functionalization of GO through covalent bonding, in contrast to the iodide anions (TRG-1). In addition to this new C−O linkage between TRG and 2 (highlighted in pink in Figure 5), new carbonyl (CO) species are also formed during annealing at 60−200 °C as shown in Figure 5b. This additional carbonyl formation is indicated by the emerging absorbance at approximately 1750−1850 cm−1 (also observed as a weak absorption from approximately 1500− 1600 cm−1; highlighted in green). The corresponding release of CO2 (appearing as a single peak at 2340 cm−1) suggests that defects are formed,18 presumably as a result of chemical oxidation taking place between trapped water in the interlayers of GO or oxygen groups and TRG’s defects during annealing. Conversely, the GO−IL intercalation compound formed from 3 is decomposed upon annealing, as evidenced by the loss of C−OH and C−H vibrational absorption at 3000−3600 cm−1 and at 2800 cm−1, respectively, corresponding to both water removal and the IL’s decomposition. Collectively, these spectroscopic studies support our analysis of the XRD data (Figure 4). While 1 and 2 are not able to expand the interlayer spacing of TRG even after annealing at 300 °C, the long alkyl chains of 3 facilitate this expansion and exfoliation with high efficiency due to a lack of covalent functionalization between TRG and 3 (i.e., efficient oxygen removal) as highlighted in cyan in Figure 5c. The replacement of the N-methyl-N,N,N-tris(2-hydroxyethyl) ammonium cation by the N,N,N-tributyl-N-octyl ammonium cation, keeping the same methyl sulfate anion (2 versus 3) clearly prevents the functionalization of TRG over the whole range of reduction temperatures explored (Figure S2c as compared to Figure S2b, Supporting Information). Therefore,

Figure 6. Tyndall scattering effect for GO−IL 1 (GO/IL, ratio = 1:2 wt %) (w/w)) dispersed in 4 mL of (a) DI water or (c) PC at room temperature. Both intercalation compounds were redispersed in 4 mL of DI water (b) or PC (d) after annealing at 500 °C showing an improvement in their colloidal properties. 7871

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would restrict flow of the electrolyte through the energy storage device. Conversely, ILs that do not possess reactive functional groups interact weakly with the carbon surface, and therefore generally do not form irreversible covalent linkages, may allow for their successful application in ultracapacitors. Ultimately, using the parameters described herein, appropriately designed IL electrolytes may facilitate access to high performance energy storage devices that are suitable for use in commercial applications.

water, the particle size was reduced, as shown with exposure to a laser beam in Figure 6b, which improved the colloidal properties. Dispersing the intercalation compound in PC (Figure 6c) also improved the colloidal properties of GO−IL 1, for which exfoliation is further improved after annealing at 500 °C and redispersing in PC (Figure 6d). Similar experiments were also performed for GO−IL 2 and GO−IL 3 leading to highly exfoliated material once annealed and redispersed in liquid media leading to a particle size reduction.

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CONCLUSIONS In conclusion, both the anion and cation moieties of ammonium-based ILs intercalated into GO have been found to influence the degree of thermal expansion and exfoliation upon thermal annealing of the resulting intercalation compounds. The intercalated ILs are capable of modulating the Td of the TRG−IL intercalation compounds, which facilitates the stabilization of the IL in the TRG interlayer spacing. Cations bearing long, unfunctionalized alkyl chains stabilize the corresponding GO−IL intercalation compounds. The compounds are highly thermally reduced, exhibiting no chemical interlayer functionalization of the TRG by N-methylN,N,N-tris(2-hydroxyethyl)ammonium iodide, 1. The ILs survive annealing at high temperatures in the TRG interlayer, allowing the IL to heal defects through possible hydride transfer from the IL into the defects (absence of CO2 production), with no covalent functionalization until an annealing at 200 °C. The iodide-containing ILs also facilitate an efficient oxygen removal from GO, as determined by both infrared absorbance and differential spectra. Conversely, ILs bearing methyl sulfate anions exhibit enhanced chemical reactivity with the GO surface, leading to functionalization of TRG. While the anion structure influences the chemical reactions occurring within the GO interlayer region, the thermal stability of the GIICs is primarily influenced by the cation portion of the IL. For instance, cations bearing long alkyl chains do not interact with the TRG (i.e., no functionalization occurred during annealing), even when coupled with methyl sulfate anions. GO−IL intercalation compounds containing the heavily alkylated IL also exhibit an enhanced degree of thermal expansion. A significant alteration of the Td of both GO and its intercalation compounds also brings to attention the role of the solvent for the performance of the intercalation compound. A significant decrease in the Td of GO in the presence of PC for all three GO−IL intercalation compounds confirms that PC enhances the degree of oxygen removal from the GO surface. Overall, we envision that these experimental findings will help guide the scientific community toward the optimal choice of a suitable IL for graphene-based systems in energy storage applications. Importantly, we have shown that the carbon−IL interactions are often very specific to the chemical environments created by the two components, and thus a proper design and selection of an IL is imperative to maximizing ultracapacitor performance. Our results demonstrate that both the cation and anion moieties of the IL can play a role in the carbon−IL interactions. Highly functionalized ILs, such as those bearing pendant hydroxyl groups or reactive anions, interact strongly with carbon materials, such as GO, which would be anticipated to improve wetting of the carbon by the IL. However, the interactions can be covalent or otherwise irreversible in nature when the carbon and the IL are annealed together at high temperatures, which would likely be detrimental to ultracapacitor performance as covalent linkages

ASSOCIATED CONTENT

* Supporting Information S

Method descriptions for synthesis of GO and IL, intercalation process, furnace and thermal annealing, and specifications for in situ infrared spectroscopy, XRD, and TGA measurement. TGA analyses for TRG−IL in PC, differential infrared spectra of TRG intercalated with ILs 1, 2, and 3, followed by a thermal annealing and Raman spectra for GO−IL 1 (room temperature measurement and after annealing at 500 °C). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

M.A. performed and interpreted the IR and XRD measurements, as well as the intercalation and thermal annealing studies. D.R.D. synthesized all ionic liquids and performed the TGA measurements. M.A. wrote the manuscript with Y.J.C. C.W.B. and D.R.D. reviewed the manuscript. Y.J.C. and C.W.B. supervised the work. 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 We are grateful to Juan Juarez and Cheng Gong for their technical support in the process of data collection and to JeanFrançois Veyan for the vacuum chamber design. We acknowledge the full financial support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC001951.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp300772m | J. Phys. Chem. C 2012, 116, 7867−7873