Curly Graphene with Specious Interlayers Displaying Superior

Nov 21, 2013 - While almost all attempts for bulk synthesis of graphene result in agglomeration of flat graphene layers, a simple solvothermal route i...
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Curly Graphene with Specious Interlayers Displaying Superior Capacity for Hydrogen Storage Ali Eftekhari*,† and Parvaneh Jafarkhani‡,§ †

National Institute of Arts & Sciences, 411 Walnut Street, Green Cove Springs, Florida 32043-3443, United States NUSNNI-Nanocore, National University of Singapore, Singapore 117411, Singapore § Department of Materials Science and Engineering, National University of Singapore, 117576 Singapore ‡

ABSTRACT: While almost all attempts for bulk synthesis of graphene result in agglomeration of flat graphene layers, a simple solvothermal route is reported for the preparation of a graphene powder consisting of very flexible sheets. Curvature of these graphene sheets avoids vital agglomeration, which normally blocks the accessible surface of graphene sheets. The thickness of the graphene sample varies from one to a few layers, but the spaces among the graphene layers are sufficiently large enough to provide internal accessibility. Thus, this graphene sample shows the general (bulk) properties of graphene single sheets. For instance, it has a high specific surface area of ca. 1168 m2 g−1 and starts to burn at 350 °C because of a large interlayer spacing of graphene sheets (i.e., 5.1 Å). It should be considered that this specious interlayer is mostly due to the graphene curvature rather than to filling with functional groups (as in the case of graphite oxide). Owing to the high accessibility and active sites over a large surface area, this graphene sample shows a superior electrochemical behavior for hydrogen storage.

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perform thermogravimetric analysis (TGA) in air as single graphene sheet burns early at low temperatures around 400 °C. Unfortunately, it is popular in the literature to judge about the number of graphene sheets in a sample on the basis of selective techniques like various kinds of microscopy; however, it is necessary to utilize techniques which examine a considerable amount of the sample to be confident about a statistical conclusion. In the present research, we rely on methods analyzing the bulk properties of the graphene sample synthesized. The key point is that graphene layers in the graphite oxide structure are extremely flat and parallel to each other; then, upon removing the pillars, they attract each other like magnets (because of the van der Waals force). Instead, graphene sheets formed in other synthesis routes (utilizing organic carbon feedstocks) are curvy, which is the reason for stability of carbon nanotubes. Thus, the ultimate solution for effective reduction of graphite oxide (to keep the individual graphene sheets detached) is to reduce the contact surface area probably through a curvature of the graphene sheets. Otherwise, the graphite oxide reaches the condensed structure of graphite again. Hydrothermal route as an alternative method for the synthesis of carbon nanotubes is subject of recent studies, and various reactions have been proposed for this purpose.21−31 It has also been employed for the preparation of other carbon nanostructures.32−40 However, less attention has been paid to

raphene has recently attracted particular attention in different areas of research because of the unique properties of a single sheet out of graphite structure.1−5 However, most works are related to graphene-based nanomaterials as there is still no practical approach for bulk synthesis of graphene. The common method is to reduce graphite oxide,6−9 but the graphene sheets are closely attached together during the reduction process to reach a graphite-like structure; thus, the resulting product cannot be considered as an individual graphene sheet. A possible solution is to prepare curved graphene sheets as they cannot be attached together in the form of parallel layers. Upon formation of multilayers, the internal layers are still accessible. This is indeed the basis for the preparation of single-walled carbon nanotubes, which can be considered as an example of single layer graphene10 since the sheets are not flat to be attached (resembling the graphene structure). Here, a simple method is developed for the preparation of such a sample of graphene, showing the essential properties of individual graphene sheets. Although graphite oxidation is a successful approach for expanding the distance between graphene sheets in the graphite structure,11−20 the expansion is not permanent as the graphene sheets attach to each other during the reduction process. This process also results in partial exfoliation of the lateral layers (usually as a result of a thermal shock) but still is not a practical approach for large-scale production. On the other hand, this route does not lead to the formation of uniform nanostructures as the size distribution is extremely high (usually in the range of 3 orders of magnitude, from nano- to microsized particles, and graphene sheets are cut upon oxidation19). A reliable method to prove that a sample is actually single- or bilayer graphene is to © 2013 American Chemical Society

Received: October 9, 2013 Revised: November 8, 2013 Published: November 21, 2013 25845

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this method as the synthesis yield is very low in comparison with common methods (such as chemical vapor deposition, CVD), and thus, it is not capable of commercialization. However, this is not a reasonable obstacle to postpone studies of hydrothermal synthesis of carbon nanostructures as there are rational horizons for such studies; this may lead to the formation of new carbon nanostructures with different properties as is the case in the present study.

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EXPERIMENTAL METHODS In our experiment, 20 mL of dichloromethane was placed into a stainless-steel autoclave with 40 mL capacity, and then 1 g of cobalt chloride and 1 g of metallic sodium were added. The sealed autoclave was heated at 200 °C for 16 h and then was cooled to room temperature in the furnace slowly. The sample was sequentially washed with absolute methanol, dilute acid, and distilled water to remove residual impurities. Electron microscopic characterizations were performed using a Philips XL30 scanning electron microscope and a Philips CM200 transmission electron microscope with field emission gun (FEG). X-ray diffraction (XRD) pattern was recorded using a Philips PW 1371 diffractometer, and Fourier transform infrared (FT-IR) was recorded using a Bruker spectrophotometer. The TGA/differential thermal analysis (DTA) experiments were carried out using a PL-STA-1640 instrument with a heating rate of 5°/min. The electrical conductivity was measured by compressing the sample into pellets with thickness of ca. hundreds of micrometers and by measuring the conductivity by a conventional fixed film four-point probe setup.

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Figure 1. SEM images of the curly graphene formed hydrothermally. (a) Wrinkled structure of the graphene sheets and (b) parallel carbon nanotubes formed as a result of rolling both edges of the graphene sheets independently.

RESULTS AND DISCUSSION In the hydrothermal synthesis developed here, the carbon feedstock, namely, dichloromethane, will be reduced to hexagonal carbon clusters by metallic sodium. These hexagonal sheets are similar to graphene sheets. Thus, the Co catalyst generated via the same reduction process can transform these hexagonal sheets to form carbon nanotubes. This mechanism has also been proposed for the formation of multiwalled carbon nanotubes (MWCNTs).23,24 It is not still clear how an important process, that is, attachment of such graphene sheets, occurs. This is indeed the main process for the system under investigation as the graphene sheets are attached but with a specious interlayer. A possible but speculative mechanism for the formation of this quaint graphitic structure is that graphene sheets initially formed will be functionalized by −OH groups, which are trapped between attaching graphene sheets. Such functional groups may be partially removed in the course of hydrothermal treatment at higher temperatures.23 Intercalation of Na and formation of Na-graphite intercalation compounds (GIC) is not the case (though seems to be probable) as no Na was detected in elemental analysis of the sample, and the sample was completely burn in air with no inorganic remaining. In our experimental condition, there is a tendency toward the formation of nanosheets rather than nanotubes as can be judged from the typical scanning electron microscopy (SEM) image illustrated in Figure 1a. However, this novel structure is more interesting than nanotubes as the sheets are shrunken; this provides a higher specific surface area as neighbor nanosheets do not cover each other. In addition to such shrunken nanosheets, a kind of nanotube can also be detected in SEM (Figure 1b). In other words, this is not the only special feature of this novel nanostructure since such flexible

nanosheets can be individually rolled to form nanotubes under the hydrothermal treatment. Because the nanosheets are significantly wide, such rolling just occurs at edges. The latter phenomenon leads to the formation of two parallel nanotubes connected to each other via a nanosheet (Figure 1b). This linkage is highly strong as this connecting nanosheet is not a secondary material but is a part of the original carbon nanostructures. In a different context, Kim et al.41 have proposed a model for such collapse of coalescence chainlike single-walled carbon nanotube (SWCNT) bundle under thermal treatment at extremely high temperatures (e.g., higher than 2000 °C). Lopez et al.42 have also inspected the possible pathways for such phenomenon by molecular dynamic simulations. In this peculiar case, transmission electron microscopy (TEM) can assist to inspect the internal structure of these shrunken nanosheets. Figure 2a shows a wrapped nanosheet as detected in SEM (Figure 1a). The natural tendency of these thin nanosheets to be rolled at the edges is also observable in this TEM image. Further rolling results in the formation of nanotubes as shown in Figure 2b and c. Because of this feature, it is difficult to estimate the exact thickness of these nanosheets by means of TEM. However, our rough estimation via an extensive analysis of different samples suggested that the thicknesses of these nanosheets are mainly lower than 8 nm, while estimating the lower limit was quite difficult. In addition to the special morphology of the graphene, an interesting feature was observed in thermogravimetrical analysis 25846

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Figure 3. TGA (a, blue) and DTA (b, red) of the curly graphene sample.

Another interesting feature of the graphene sample synthesized hydrothermally is high specific surface area. The Brunauer−Emmett−Teller (BET) measurement of the graphene sample under consideration suggested a value of 1168 m2 g−1. This is comparable with single-walled carbon nanotubes and, indeed, is significantly higher than multiwalled carbon nanotubes. In fact, there should be a single reason for both high specific surface area and low-burning temperature detected for the graphene sample. For carbon nanotubes, for instance, it is known that nanotubes with smaller diameter burn sooner.43 Similarly, the burning temperature of natural graphite decreases by increasing the d(002) spacing.46 Thus, the high specific surface area of this graphene sample guarantees the availability of carbon atoms on the surface to burn quickly at low temperature. However, it is still a vague point of why graphene nanosheets with thickness of a few nanometers should have such a large specific surface area as it is known that interlayer spaces of graphene sheets are not considered as active surface area. The specific surface area of conventional graphite with normal interlayer spacing of graphene sheets, that is, 3.35 Å, is very low, about 10 m2 g−1. XRD investigation clarified this vague point (Figure 4) as the common graphite peak of the 002 plane appearing at ca. 26° is absent, or in a better expression, it has been shifted toward lower values. This means that the interlayer spacing of graphene sheets detected by XRD is no longer in the size of the standard 002 plane of graphite (i.e., 3.35 Å). In the present Figure 2. TEM images of the curly graphene generated hydrothermally. (a) The graphene with curvature and (b, c) two parallel nanotubes closely attached to each other from a base sheet.

of it. According to the TGA illustrated in Figure 3, the graphene sample starts to burn at an extremely low temperature of 300 °C. The sample mostly burns at 350 °C, where a strong exothermic peak is observable in the DTA curve (Figure 3). This is indeed an unusual temperature for the burning of any type of carbonaceous materials except well-structured singlewalled carbon nanotubes like HiPCO,43−45 which are made of individual graphene sheets. It is noticeable that the burning peak has a shoulder at lower temperatures, which is attributed to the size distribution (and thickness) of the graphene nanosheets. In any case, 90% of the sample is burned before 500 °C, confirming that the entire sample consists of individual graphene layers spaciously separated from each other.

Figure 4. XRD pattern of the curly graphene sample. 25847

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graphene sample and GICs.53 The peaks of functional groups are strong here because of their dominant presence on the curly graphene surface, resulting in stronger exposure during FT-IR measurement. As proven by the TGA results, the percentage of functional groups (including water molecules) are less than 5% in the sample, which is far less than GICs. It is well-known that the existence of such functional groups extremely reduces the electrical conductivity as graphite oxide is almost an insulator. However, the graphene sample synthesized here has a good electrical conductivity of 1.6 × 102 S m−1, which is comparable with natural graphite and carbon nanotubes. Since the report of Nutzenadel et al.,54 considerable attention has been paid to electrochemical hydrogen storage in nanostructured carbonaceous materials. Among different types of carbonaceous nanomaterials, SWCNTs are the best candidates for hydrogen storage because of their high active surface area as the internal diameters of SWCNTs are larger than the interlayer in graphitic structure. The internal space of SWCNTs is comparable with the interlayer of the graphene sample synthesized here. Thus, we inspected the capacity of the curly graphene sample for hydrogen storage to detect how much the specific surface area is active in an electrochemical process. Figure 6 depicts a typical charge/discharge profile

case, the XRD peak suggests an interlayer spacing of 5.1 Å. This value is very large for graphite structure and guarantees the availability of inner carbon atoms, resulting in a high specific surface area. Moreover, Raman spectroscopy confirmed the existence of graphite as G band. The Raman spectrum of the graphene sample was similar to other types of carbon nanostructures particularly carbon nanotubes (not shown). This simply contains two characteristic peaks of amorphous carbon and crystalline graphite as the ID/IG ratio was 0.23, which is an acceptable ratio of graphite to amorphous carbon for this class of carbonaceous materials. However, because of the specious structure confirmed by XRD, Raman spectrum is not reliable for judging the number of graphene layers.20 This large interlayer spacing reminds the specious structure of graphite intercalation compounds (GIC) and exfoliated graphite; though, in the latter one, the interlayer spacing is usually higher than 8 Å.47,48 In these cases, the functional groups intercalated between the graphene sheets increase the interlayer spacing. Somehow, this should also be the case for the present case. This phenomenon is not exactly what is detected for GICs as the existence of functional groups among the exfoliated structure of graphite avoids the generation of high specific surface area as it is normally about 30 m2 g−1 (and usually lower) for GICs.49,50 As a counterpart to Raman spectroscopy, FT-IR can provide useful information to inspect this quaint carbonaceous material. Because carbon bonds do not have intrinsic dipole moment, they are usually hidden in the presence of functional groups, which usually have larger IR cross sections.51−53 Thus, FT-IR is a reliable technique for the inspection of functional groups attached to the carbon atoms. FT-IR spectrum of the curly graphene sample (Figure 5) confirms the weakness of the strong C−O stretching band at 1091 cm−1, which normally appears for carbon nanotubes and GICs.

Figure 6. Electrochemical hydrogen storage in the graphene examined with an applied current density of 100 mA g−1 during (a, red) charge and (b, blue) discharge.

associated with hydrogen storage and uptake in/from the carbon nanosheets. According to these experimental results, the capacity of the graphene sample is significantly higher than other carbon nanostructures including MWCNTs.54−63 This extremely high capacity for electrochemical hydrogen storage is comparable with those of SWCNTs.64,65 Two factors may be responsible for this superior capability for electrochemical hydrogen storage: high specific surface area providing more accessible carbon atoms and the existence of the functional group C−OH participating in the hydrogen chemisorption. The former reason indeed guarantees excellent capability of high surface area carbon nanostructures such as SWCNTs for hydrogen storage. The latter one is accompanied by the formation of better pathways for the diffusion of the electroactive species. In fact, such functional groups change the hydrophobic nature of graphite to a hydrophilic one (as is the case for GICs). Thus, hydrogen ions and water molecules can easily pass among the curly graphene sheets. In a different context, it has been shown that graphene edges provide a

Figure 5. FT-IR spectrum of the curly graphene sample.

The FT-IR spectrum indicates that the functional groups attached to the curly graphene nanosheets are mostly carboxyl and carbonyl groups. The band appearing at 1710 cm−1 may be assigned to the stretching vibration of carboxyl groups on the edges of the layer planes or conjugated carbonyl groups. The deformation vibration of water molecules is also observed as the 1620 cm−1 band. However, the extremely strong and sharp band appearing at 1385 cm−1 can be assigned to the vibration of C−OH. This somehow indicates the similarity of this 25848

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the initial cycles. The decrease of the charging potentials indicates that the hydrogen saturation occurs at more negative potential. In other words, a strong deriving force is needed to store the hydrogen in the graphene sample. In general, these failures are negligible, and the excellent cyclability obtained is of practical interest.

different electron transfer capable of changing the pathway of electrochemical processes.66 The superiority of the present graphene sample for hydrogen storage is not restricted to a high specific capacity, and it shows an excellent cyclability too. Figure 7 shows 5000 charge/

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CONCLUSIONS A novel curly graphene was synthesized hydrothermally. Extensive analysis by different techniques revealed that the interlayer spacing of graphene sheets is 5.1 Å instead of 3.35 Å. This wideness of the graphene interlayers is responsible for its special properties. This is one of the few reports (if not the first) that show that the graphene synthesized sample has the material properties of single-walled carbon nanotubes, which are examples of individual graphene layers. Further research should be conducted in a manner to understand the synthesis mechanism and to control the graphene morphology in order to prepare desirable graphene precursors for various applications.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (904)297-8050. Fax: (904)297-5050. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 7. Excellent cyclability for hydrogen storage capability of the graphene. The charge/discharge profiles were recorded without cutoff limits under a high applied current density of 5 A g−1.

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dx.doi.org/10.1021/jp410044v | J. Phys. Chem. C 2013, 117, 25845−25851