Using Molecular Level Modification To Tune the Conductivity of

Article pubs.acs.org/JPCC

Using Molecular Level Modification To Tune the Conductivity of Graphene Papers Jingquan Liu,*,† Rui Wang,† Liang Cui,† Jianguo Tang,† Zhen Liu,† Qingshan Kong,† Wenrong Yang,‡ and Justin Gooding*,§ †

College of Chemistry, Chemical and Environmental Engineering, Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China ‡ School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3217, Australia § School of Chemistry and the Australian Centre for Nanomedicine, The University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: Graphene's excellent electrical conductivity benefits from its highly conjugated structure. Therefore, the manipulation of graphene's electronic and mechanical properties should be realized by controlled destruction of its in-sheet conjugation. Here, we report the manipulation of the conductivity of graphene papers, at the molecular level, via either covalent bonding or π−π stacking interactions using either monofunctional or bifunctional molecules. The graphene papers can be tailored with controllable conductivity from around 100 to below 0.001 S/cm. The controlled destruction of the in-sheet graphene conjugation system using monoaryl diazonium salts (MDS) resulted in a tunable decrease in the graphene paper conductivity. However, when the graphene was modified with bifunctional aryl diazonium salts (BDS), a more subtle decrease in conductivity of the graphene papers was observed. It is suggested that the modification of the graphene with the bifunctional BDS linker showed more subtle changes in conductivity because of the between-sheet electron communication, thus boosting the collective graphene paper conductivity. Consequently, a bipyrene terminal molecular wire (BPMW) was also synthesized and used to modify the graphene sheets via π−π stacking interactions. The BPMW afforded graphene papers with better electrical conductivity than those modified with either MDS or BDS molecules.

1. INTRODUCTION

method usually retain more defects and can possess uncontrolled geometrical shapes. Single graphene sheets are highly conductive, where the mobility of charge carriers can reach 200000 cm2 V−1 s−1,13 and the measured electrical conductivity (EC) can reach up to 6000 S/m.14 However, the vast majority of investigations on graphene's EC have focused on the composite materials containing many graphene sheets mixed with other ingredients, for example, polymers via simple noncovalent mixing. In these cases, a broad range of EC values have been reported from 10−14 to 104 S/cm.15−18 However, these noncovalently mixing methods cannot be used to manipulate the EC of a single graphene sheet, and the so-prepared graphene composites are relatively unstable upon the outside stimuli: vibrations, temperature, and pressure. In a recent paper, Haddon and co-workers19 prepared a nine-layered ultrathin graphene film with the top layer

Graphene is the latest member of the nanocarbon family, in which fullerenes, carbon nanotubes, and now graphene are the most extensively studied. Mechanical exfoliation,1 thermal deposition,2,3 oxidation of graphite,4 and liquid-phase exfoliation of graphite5−9 are the mostly frequently used methods to synthesize graphene. Unzipping carbon nanotubes has also been employed to synthesize carbon nanoribbons.10,11 Templating of surfactants is another alternative to produce pure single-layer graphene sheets.12 Mechanical exfoliation and thermal deposition (like chemical vapor deposition) benefit for the preparation of high-quality, defect-free graphene sheets but at a high cost. Graphene sheets in large quantities can be produced using wet chemistry methods.8,9 Such methods usually involve an oxidation−reduction, in which the oxidation process assists in the formation of graphene oxide (GO) exfoliated from the expanded graphite, while the reduction process recovers the conductivity of the material by restoring the in-sheet conjugation.8 However, the graphene sheets made through this © 2012 American Chemical Society

Received: May 6, 2012 Revised: August 1, 2012 Published: August 2, 2012 17939

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

wavelength was 347 nm, and the emission wavelength was located at 379 nm for the pyrene fluorophore. The ECs of the graphene papers were measured using a standard four-probe analyzer (RTS-8, Probes Tech). The tensile strength (TS) was recorded on Instron 3300. The calculation of the most stable 3D structure of BPMW was performed using a B3LYP/3-21G method of density functional theory (DFT) in Gauss program (Gaussian 09). X-ray diffraction (XRD) patterns were obtained by using a D8 Advance (Bruker) X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å).

being modified by 4-nitrophenyl diazonium salt molecule via a spontaneous grafting method. The reduced EC was observed with the modified graphene film. However, another group arrived at the completely opposite conclusion with the graphene sheet modified with the same 4-nitrophenyl diazonium salt molecule. They claimed that the modified graphene was more conductive than the pristine graphene as a result of charge transfer effect of nitrophenyl groups.20 Therefore, there is a demand to develop methodologies to subtly manipulate the EC of composites of graphene sheets. The excellent conductivity of graphene originates from its fully conjugated structure; therefore, controlled destruction of the conjugation system of graphene can be adopted to manipulate the graphene conductivity. In this article, we, for the first time, report the precise manipulation of the EC of graphene papers fabricated by graphene sheets modified using mono- or bifunctional molecules, via either covalent or π−π stacking functionalizations, that form links between the graphene sheets in a graphene paper.

3. RESULTS AND DISCUSSIONS 3.1. EC of Modified Graphene Papers. As shown in Scheme 1, the covalent modification is performed using either Scheme 1. Schematic Illustrating the Modification of Graphene Sheets with (A) MDS, (b) BDS, and (c) BPMW

2. EXPERIMENTAL DETAILS 2.1. Preparation of Graphene. Graphene was prepared by the reduction of GO, which was synthesized from natural graphite powder (KNGTM-150) by the method of Hummers and Offeman.21,22 The chemically converted graphene was obtained by the reduction of GO at 95 °C in a hydrazine solution at pH 10. 2.2. Preparation of Graphene Paper. The graphene and modified graphene papers were prepared by vacuum filtration through a membrane with a pore size of 220 nm. The membranes were selected as either water phase or organic phase depending on the media selected for graphene modification. In comparison, 8 mg of graphene was used for a single graphene film preparation throughout the whole experiment, and an ultrasonic cell disruptor was utilized to obtain a well-dispersed graphene or modified graphene. After filtration, the membrane was washed with the proper solvent, followed by air drying, and then dried in a vacuum oven prior to peeling off from the filtration membrane. 2.3. Synthesis of Monoaryl Diazonium Salt (MDS) and Biaryl Diazonium Salt (BDS). The synthesis of MDS was performed using a previously published procedure.23 The synthesis of BDS was developed in the early 1930s by Schoutissen.24 As it is very unstable, in the current research, we used the well-developed in situ preparation methods and directly used for modification of graphene.25 2.4. Synthesis of Bipyrene-Terminated Molecular Wire (BPMW). To a 50 mL round-bottom flask were added 1-amino pyrene (0.200 g, 0.92 mmol), terephthalaldehyde (0.056 g, 0.42 mmol), and ethanol (10 mL). The resulting mixture was heated up to 80 °C and was kept stirring for 12 h to a pink precipitate, which was then washed with CH2Cl2 to afford the expected product (0.201 g, 79%). 1H NMR (CDCl3, 298K, 300 MHz) δ (ppm from TMS): 7.82 (1H, Py), 7.83 (1H, Py), 8.02 (1H, Py), 8.03 (1H, Py), 8.04 (1H, Py), 8.05 (1H, Py), 8.07 (1H, Py), 8.08 (1H, Py), 8.10 (1H, Py), 8.15 (1H, Py), 8.17 (1H, Py), 8.19 (1H, Py), 8.20 (1H, Py), 8.21 (1H, Py), 8.22 (1H, Py), 8.23 (1H, Py), 8.3 (4H, Ph), 8.76,8.77, 8.86 (2H, NC−H) (Py represents the pyrene group, and Ph represents the phenyl group). 2.5. Analysis. 1H NMR spectra were obtained using a Bruker AC300F (300 MHz) spectrometer or a Bruker DPX300 (300 MHz) Spectrometer. The UV spectra were recorded on a UV− vis spectrophotometer (Cary 300). The fluorescence spectra of BPMW were obtained on a Perkin-Elmer LS50B scanning instrument employing a slit width at 5 nm. The excitation

MDS (Scheme 1a) or BDS (Scheme 1b) molecules; both MDS and BDS will disturb the in-sheet conjugation of graphene. However, BDS can also create aryl linkages between the adjacent graphene sheets and, hence, enhance electronic coupling between sheets and thus conductivity between sheets. To increase between-sheet electronic coupling without any destruction of the within-sheet conjugation, graphene sheets were also modified with a BPMW that exploited π−π stacking interactions, without any destruction of the within-sheet conjugation (Scheme 1c). The BPMW linker should increase the intersheet electrical communication as it is a fully conjugated molecular wire; thus, the collective conductivity of the soprepared graphene paper is expected to be boosted as compared with the aryl diazonium salt modification strategies. GO and chemically converted graphene were prepared using the methods of Hummers and Li7,21 and employed to prepare yellowish GO and black graphene papers (Figure S1 in the Supporting Information), whose functionalities and ECs were analyzed. FTIR analysis of GO revealed a large number of oxygen-containing functionalitiesfor example, carboxylic acid, hydroxyl, ketone, and epoxide groups (Figure S2a in the Supporting Information)with these functional groups being almost completely removed after reduction (Figure S2b in the Supporting Information). Boukhvalov et al. have proposed a model structure of GO and used it to obtain the optimized structures of GO with the coverage of oxygen-containing functionalities.26 The FTIR spectrum of GO after reduction was hardly observed, demonstrating a complete reduction (Figure S2b in the Supporting Information). Mass analysis also revealed a ∼48% mass loss after reduction, evidence of these thermosensitive oxygen-containing functionalities. The GO 17940

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

paper exhibited almost insulating behavior with a resistance of more than 1900 kΩ (5.3 × 10−6 S/cm). The conductivity of the graphene paper, after annealing at 300 °C to reduce the GO to graphene, reached 50 S/cm, a value with more than seven orders of conductivity enhancement as compared with GO paper. However, this value is still incomparable with that of a single graphene sheet. This large disparity may result from the weak electron communications within the graphene paper due to the poor contacts between the adjacent graphene sheets. Furthermore, the fact that graphene sheets are often curved or corrugated morphology, making it more difficult to obtain closely packed graphene papers, may exacerbate this problem.27 Therefore, to control the electron communications between the graphene sheets within a graphene paper is still a crucial issue to manipulate the EC of graphene paper. It is noteworthy that the measured conductivities of the papers should be attributed to the whole paper rather than the paper surface as the four probes usually penetrate through the thin paper during the measurement. Aryl diazonium salt molecules with varied functionalities have been successfully employed to modify carbon [e.g., single-wall carbon nanotubes (SWCNTs), graphite, and glass carbon] and metal substrates via either C−C or C−metal covalent couplings.28−30 The attachment of aryl diazonium salts can be conducted under mild conditions by either simple agitation or electrochemical reduction adsorption. The modification is typically accompanied by the loss of N2 and the destruction of the double bond at the bonding point. To give us control over the extent of modification of the graphene with aryl diazonium salts, we add different amounts of aryl diazonium salt to a known amount of graphene, by comparing the number of diazonium moieties, (two for BDS) with the number of graphene double bonds (GDB), assuming all of the graphene nanosheets are defect-free and fully conjugated (Scheme 1a). The successful attachment was demonstrated by a FTIR spectrum, which shows the signals attributed to CC−H functionality (Figure S2b in the Supporting Information). The modified graphene was then used to prepare graphene papers whose conductivity was analyzed. As shown in Figure 1a, the EC was found to decrease with an increasing molar ratio of MDS molecules to GDB. Surprisingly, we discovered that a 80% feed ratio has switched off most conductivity of the graphene paper, indicating that conjugation pathways for charge transfer were destructed. We also observed that the subsequent formation of the graphene paper became increasingly more difficult with a higher content of MDS molecules. This is attributed to the destruction of the insheet aromatic conjugation of the graphene by the attached MDS molecules and, hence, a loss of the corresponding π−π interactions between graphene sheets. The insertion of the aryl linkers between the graphene sheets also expands the graphene papers, further compromising the intersheet electrical communication. However, the EC of the so-generated graphene papers can be recovered by thermal annealing at 500 °C (Figure 1a), a process that has been frequently utilized to boost the EC of graphene via the removal of the thermally sensitive functionalities or nonstrongly attached species as well as regulation of the orientation of the graphene sheets.7,18 Graphene sheets were also modified with a controlled amount of BDS molecules in a similar manner as with MDS and used to prepare the graphene papers whose ECs were measured. As shown in Figure 1b, the EC of the graphene papers dropped sharply at a lower feeding ratio of BDS and tended to keep at a stable value when the BDS feed was over 10%. When comparing

Figure 1. EC of graphene papers fabricated with graphene sheets modified with (a) MDS at different ratios of MDS to GDB before (filled square) and after (filled circle) annealing and (b) with BDS at different molar ratios of BDS to GDB (filled square) and BPMW at different molar ratios of BPMW to a quarter of the phenyl rings (filled circle).

Figure 1a,b, it can be observed that BDS-modified graphene paper exhibited a higher EC than the MDS-modified one at the same feeding ratio of BDS or MDS. Importantly, the lowest conductivity measured with the BDS-modified graphene papers is almost 2 orders of magnitude higher than that observed with the MDS. We also noticed that the formation of the graphene papers was much easier than those with a MDS modifier, probably due to the cross-linking effect between the graphene sheets. The reason for the observed EC profile will be discussed in more detail in the next session, but the higher conductivities observed for the BDS-graphene papers relative to the MDSgraphene papers provides an indication that forming conductive links between sheets will give papers with higher conductivity. To further explore this, observation sort to synthesize conductive links between sheets that do not disrupt the insheet conjugation to see whether even higher conductivity papers could be achieved. Such a strategy exploits π−π stacking interactions. Pyrene-functional precursors have been successfully prepared31−33 and attached onto aromatic macromolecules such as carbon nanotubes,34−38 GO,39,40 and fullerene.41 A π−π 17941

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

stacking mechanism was also explored by us 42,43 and others18,44,45 to modify graphene using functional polymers or other precursors. As a consequence, we synthesized BPMW to modify graphene paper. The design of BPMW was inspired by the expectation that the π−π stacking interaction will not destruct the in-sheet conjugation46−48 and therefore maximally protect the in-sheet EC, while the fully conjugated MW should also help boost the intersheet electrical communication. Our results shown in Figure 1b provide good support for this hypothesis. The EC profile of the graphene paper modified with BPMW was found to exhibit only a minor drop in conductivity with increasing feed ratio before stabilizing at a much higher conductivity than either of the aryl diazonium salt modifiers. It is noteworthy that the feed ratio of BPMW was designed as the molar ratio of BPMW to a quarter of phenyl rings because theoretically each pyrene group can cover the area of four phenyl rings. It is well-known that pyrene is a fluorophore, and its fluorescence will be quenched when it is attached onto graphene due to photoinduced charge transfer.49,50 Therefore, the successful attachment of BPMW onto graphene can be demonstrated by monitoring the fluorescence of pyrene. As shown in Figure 2a, the fluorescence of BPMW was almost completely quenched, evidence for a successful pyrene− graphene interaction. In addition, we also observed the enhancement effect of pyrene on graphene absorption in the UV−visible absorption spectrum at ∼300 nm, a property that is often utilized for the determination of graphene concentration18,51 (Figure 2b). Raman scattering spectra have been employed to characterize the number of graphene layers52 or to verify the surface modification of graphene with electron donors and acceptors. The G-band in the Raman spectra has been shown to be shifted to lower frequencies by electron donors and to higher frequency by electron acceptors.13,18,53,54 As shown in Figure 2c, the modifications of graphene with MDS and BDS lead to slight 2 and 3 cm−1 G-band frequency increases, respectively. However, when the conjugated BPMW was utilized to modify the graphene sheets, a 5 cm−1 G-band upshift from 1594 to 1599 cm−1 was observed, revealing an electron-donating effect of the adsorbed pyrene derivative. In addition, a significant surface-enhanced Raman scattering (SERS) was also observed with the graphene modified with BPMW (Figure 2c). The intensity was found to be enhanced by more than 25 times after modification, evidence of the increase of graphene surface roughness. SERS of graphene can be aroused by the strong plasmon resonance generated by the surface-attached metallic nanoparticles.55 However, graphene itself can also enhance the Raman scattering for the adsorbed molecules on it via charge transfer.56,57 The observed SERS might imply that the graphene surface became rougher due to the attachment of twisted BPMW molecules. The influence of BPMW on the modification of graphene sheets and on the EC of the corresponding graphene paper will be discussed next. 3.2. Model for Illustrating the Conductivity of Modified Graphene Papers. To clarify the influence of the modification of graphene sheets on the so-generated graphene papers, we proposed a model to illustrate the in-sheet and intersheet electrical communications, which might occur within the modified graphene papers (Figure 3a). Generally speaking, there are probably two major paths for electron communication within the graphene papers: The first path should be the in-sheet charge transfer that should be dominated by the graphene conjugation system (1), and the second one should be the

Figure 2. (a) Fluorescence of BPMW before and after attached to graphene at the excitation wavelength of 347 nm, (b) UV−vis spectra of graphene and graphene modified with BPMW in DMF medium, and (c) the G-bands in Raman scattering spectra of graphene and graphene modified with MDS, BDS, and BPMW, respectively.

intersheet charge transfer, which can be achieved through either simple electron hopping from one sheet to another when these sheets are close enough58−60 (2) or through the inserted molecular conduit (3) (Figure 3a). Raman scattering studies suggest that the electronic coupling between the graphitic layers 17942

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

collective EC of graphene papers. When graphene paper was modified with BPMW, one may naively expect the higher conductivity than that prepared using pristine graphene. This was not the case because the connection between a graphene sheet and the BPMW is the same as the π−π bonds between sheets, but a significant boost in EC of graphene paper when modified with BPMW is observed in comparison to BDS and even more so in comparison with MDS (Figure 1b). With the molecular structure of BPMW, the question arises as to how this molecule connects the graphene sheets. To answer this query, we investigated the energy-minimized structure of BPMW using DFT of Gauss program (Gaussian 09) and obtained the most stable 3D structure of BPMW. As shown in Figure 3b, the central aryl moiety and the coplanar pyrene groups are not in a same plane but with a plane-to-plane angle of 33.2°. Because the BPMW molecule is not a planar but a twisted 3D structure, when it is used to “weld” the graphene sheets via π−π stacking, an ordered graphene structure cannot be expected. The scanning electron microscope (SEM) analysis in the next session further confirmed this assumption. 3.3. Morphology and Mechanical Property of Graphene Papers. SEM was also used to analyze the morphology of the graphene papers fabricated with graphene sheets modified with different functional precursors. As shown in Figure 4, SEM analysis revealed that the graphene papers prepared by graphene and graphene modified with MDS are denser, and the graphene papers modified with BDS and BPMW are much thicker and netlike. The significant “film swelling” should be attributed to cross-linking effect by BDS or BPMW and the 3D twisted structure in the case of BPMW (Figure 3). As summarized in Figure 5, the graphene paper made of MDS modified graphene is 6.5 μm, which is slightly bigger than that made of pure graphene (4 μm). However, when the graphene papers modified by the bifunctional BDS and BPMW, the so-generated graphene papers were much thicker, with a thickness of 25 and 20 μm, respectively. XRD is an efficient tool to characterize the dspace of graphene materials.62−64 XRD was also utilized to analyze the graphene papers before and after modification with MDS, BDS, and BPMW. As shown in Figure S3 in the Supporting Information the XRD pattern of graphene exhibited a broad peak centered at 2θ = 23.4°, indicating a d-space of 3.74 Ǻ . When graphene was modified with MDS, BDS, and BPMW, respectively, the XRD pattern revealed a decreased degree of 2θ, and the d-space values were observed to be 4.06, 4.12, and 4.34 Ǻ . These results are consistent with those obtained by SEM. The TSs of the graphene papers fabricated from varied precursors were measured and compared with the respective thicknesses obtained from SEM analysis. As shown in Figure 5, pure graphene paper exhibited a TS of 29.4 MPa, while it has the smallest thickness of 4 μm. The TS of the MDS modified decreased a bit to 22.5 MPa, and the thickness increased a bit to 6.5 μm. However, when we looked into the graphene paper fabricated with BDS, even though the paper thickness was almost doubled the MDS-modified one, the TS still reached 35.7 MPa. One will not be surprised when considering the cross-linking effect by the bifunctional BDS. The BPMW-modified paper gave an even higher thickness of 20 μm, whereas the TS was only 17.5 MPa, indicating the swelling effect by the BPMW linkers. The decreased TS also implies that the π−π stacking interaction is weaker than the covalent attachment when comparing Figure 5c,d.

Figure 3. (a) Model proposed to illustrate the in-sheet and intersheet charge transfer. (1) The in-sheet charge transfer, (2) the electron hopping from one sheet to another, and (3) the charge transfer through the intersheet molecular conduits. (b) The calculated 3D structure of BPMW (without hydrogen atoms) using Gauss program (upper) and orientated 3D BPMW structure showing the plane-to-plane angle between the coplanar pyrene and the phenyl groups.

is very weak;61 therefore, creation of charge transfer conduits between the graphene sheets should be critical to achieve boosted EC of chemically modified graphene papers. The EC of the graphene paper should be the joint effect of these three factors. As the in-sheet charge transfer is very fast, the collective charge transfer of the graphene paper should be dominated by the intersheet electrical communications. In the case of the graphene sheet modified with MDS, its in-sheet conjugation will be partially destroyed, resulting in the decreased EC. It is noteworthy that while the insertion of aryl linkers will increase the electrical communication between the graphene sheets, the concurrent increase of intersheet distance should compromise the charge transfer; therefore, the collective EC should be dependent on which process dominates. Similar to MDS, when BDS are employed to modify the graphene sheets, the diazonium salt functionalities will destruct the in-sheet conjugation and subsequently decrease the in-sheet conductivity. However, as one BDS molecule has two diazonium functionalities, each BDS can link the two adjacent sheets and thus create a molecular conduit in between, which should benefit the intersheet charge transfer. Even though the in-sheet EC will be significantly decreased due to the destruction of the graphene conjugation system, the molecular conduit that mediates the intersheet charge transfer should compensate the EC to some extent to the 17943

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

Figure 4. Micrographs of graphene papers. (a) The top view of graphene paper fabricated with pure graphene. Panels b−e are the side views of graphene papers fabricated with pure graphene and graphene modified with MDS, BDS, and BPMW, respectively.

4. CONCLUSIONS In summary, graphene sheets have been successfully modified by three different mono- and bifunctional precursors via either covalent or π−π stacking mechanisms. The ECs of graphene papers covalently modified by MDS can be tuned over 5 orders of magnitude dependent on the amount of MDS used. The graphene papers fabricated with bifunctional BDS revealed stronger, cross-linked structures, and those modified with

BPMW revealed further swelled, netlike morphologies but decreased TSs due to the twisted 3D structure of BPMW. Graphene also exhibited enhanced UV absorption at ∼300 nm because of BPMW modification. The modified graphene papers were also characterized by fluorescence spectroscopy, Raman scattering, and X-ray diffraction (XRD). These graphene papers with controlled EC and highly porous structures might envision potential application in electronics and material science. The 17944

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

(10) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Nature 2009, 458, 872−876. (11) Jiao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Nature 2009, 458, 877−880. (12) Zhang, W.; Cui, J.; Tao, C.-a.; Wu, Y.; Li, Z.; Ma, L.; Wen, Y.; Li, G. Angew. Chem., Int. Ed. 2009, 48, 5864−5868. (13) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351− 355. (14) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Nat. Nanotechnol. 2008, 3, 491−495. (15) 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. (16) Kim, H.; Miura, Y.; Macosko, C. W. Chem. Mater. 2010, 22, 3441−3450. (17) Liang, J.; Wang, Y.; Huang, Y.; Ma, Y.; Liu, Z.; Cai, J.; Zhang, C.; Gao, H.; Chen, Y. Carbon 2009, 47, 922−925. (18) Su, Q.; Pang, S. P.; Alijani, V.; Li, C.; Feng, X. L.; Mullen, K. Adv. Mater. 2009, 21, 3191−3195. (19) Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. J. Am. Chem. Soc. 2009, 131, 1336−1337. (20) Huang, P.; Zhu, H.; Jing, L.; Zhao, Y.; Gao, X. ACS Nano 2011, 5, 7945−7949. (21) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339−1339. (22) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771−778. (23) Saby, C.; Ortiz, B.; Champagne, G. Y.; Bélanger, D. Langmuir 1997, 13, 6805−6813. (24) Schoutissen, H. A. J. J. Am. Chem. Soc. 1933, 55, 4535−4541. (25) Liu, G.; Chockalingham, M.; Khor, S. M.; Gui, A. L.; Gooding, J. J. Electroanalysis 2010, 22, 918−926. (26) Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130, 10697−10701. (27) Qiu, L.; Zhang, X.; Yang, W.; Wang, Y.; Simon, G. P.; Li, D. Chem. Commun. 2011, 47, 5810−5812. (28) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491−501. (29) Liu, G. Z.; Liu, J. Q.; Davis, T. P.; Gooding, J. J. Biosens. Bioelectron. 2011, 26, 3660−3665. (30) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W. F.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201−16206. (31) Gacal, B. N.; Koz, B.; Gacal, B.; Kiskan, B.; Erdogan, M.; Yagci, Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 1317−1326. (32) Pei, X. W.; Xia, Y. Q.; Liu, W. M.; Yu, B.; Hao, J. C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7225−7237. (33) Yuan, W. Z.; Yuan, J. Y.; Zhou, M.; Pan, C. Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2788−2798. (34) Liu, Z.; Sun, X. M.; Nakayama-Ratchford, N.; Dai, H. J. ACS Nano 2007, 1, 50−56. (35) Marquis, R.; Greco, C.; Sadokierska, I.; Lebedkin, S.; Kappes, M. M.; Michel, T.; Alvarez, L.; Sauvajol, J. L.; Meunier, S.; Mioskowski, C. Nano Lett. 2008, 8, 1830−1835. (36) Zhao, H.; Yuan, W. Z.; Mei, J.; Tang, L.; Liu, X. Q.; Yan, M.; Shen, X. Y.; Sun, J. Z.; Qin, A. J.; Tang, Z. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4995−5005. (37) Mayo, J. D.; Behal, S.; Adronov, A. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 450−458. (38) Liu, J. Y.; Nie, Z. H.; Gao, Y.; Adronov, A.; Li, H. M. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7187−7199. (39) Liu, Z.; Robinson, J. T.; Sun, X. M.; Dai, H. J. J. Am. Chem. Soc. 2008, 130, 10876−10877. (40) Yang, X. Y.; Zhang, X. Y.; Liu, Z. F.; Ma, Y. F.; Huang, Y.; Chen, Y. J. Phys. Chem. C 2008, 112, 17554−17558. (41) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 3842−3843.

Figure 5. TS and film thickness of the graphene papers fabricated with graphene (a), graphene modified with MDS (b), BDS (c), and BPMW (d), respectively.

electronic properties of single graphene sheet with partially destructed conjugation are under investigation.

■

ASSOCIATED CONTENT

S Supporting Information *

Photos of GO and graphene papers, FTIR spectra of GO and chemically converted graphene, and XRD spectra of graphene and modified graphene papers. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.) or [email protected] (J.G.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS J.L. thanks the NSF of China (51173087), NSF of Shandong (ZR2011EMM001), and Taishan Scholars Program for financial support.

■

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666− 669. (2) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C. C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; et al. Science 2006, 312, 1191−1196. (3) Wei, D.; Liu, Y.; Zhang, H.; Huang, L.; Wu, B.; Chen, J.; Yu, G. J. Am. Chem. Soc. 2009, 131, 11147−11154. (4) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457−460. (5) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Science 2008, 319, 1229−1232. (6) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538−542. (7) Chen, H.; Muller, M. B.; Gilmore, K. J.; Wallace, G. G.; Li, D. Adv. Mater. 2008, 20, 3557−3561. (8) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101−105. (9) Choucair, M.; Thordarson, P.; Stride, J. A. Nat. Nanotechnol. 2009, 4, 30−33. 17945

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946

The Journal of Physical Chemistry C

Article

(42) Liu, J.; Tao, L.; Yang, W.; Li, D.; Boyer, C.; Wuhrer, R.; Braet, F.; Davis, T. P. Langmuir 2010, 26, 10068−10075. (43) Liu, J.; Yang, W.; Tao, L.; Li, D.; Boyer, C.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 425−433. (44) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856−5857. (45) Liu, Z.; Robinson, J. T.; Sun, X.; Dai, H. J. Am. Chem. Soc. 2008, 130, 10876−10877. (46) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856−5857. (47) Zhang, Y.; Liu, C.; Shi, W.; Wang, Z.; Dai, L.; Zhang, X. Langmuir 2007, 23, 7911−7915. (48) Liu, L.; Wang, T.; Li, J.; Guo, Z.-X.; Dai, L.; Zhang, D.; Zhu, D. Chem. Phys. Lett. 2003, 367, 747−752. (49) Ramakrishna Matte, H. S. S.; Subrahmanyam, K. S.; Venkata Rao, K.; George, S. J.; Rao, C. N. R. Chem. Phys. Lett. 2011, 506, 260−264. (50) Chen, Q.; Wei, W.; Lin, J.-M. Biosens. Bioelectron. 2011, 26, 4497− 4502. (51) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856−5857. (52) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Phys. Rev. Lett. 2006, 97, 187401−187404. (53) Poenitzsch, V. Z.; Winters, D. C.; Xie, H.; Dieckmann, G. R.; Dalton, A. B.; Musselman, I. H. J. Am. Chem. Soc. 2007, 129, 14724− 14732. (54) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257−259. (55) Zhou, H.; Qiu, C.; Yu, F.; Yang, H.; Chen, M.; Hu, L.; Sun, L. J. Phys. Chem. C 2011, 115, 11348−11354. (56) Ling, X.; Zhang, J. Small 2010, 6, 2020−2025. (57) Ling, X.; Xie, L. M.; Fang, Y.; Xu, H.; Zhang, H. L.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z. F. Nano Lett. 2009, 10, 553−561. (58) Lee, E. J. H.; Zhi, L.; Burghard, M.; Müllen, K.; Kern, K. Adv. Mater. 2010, 22, 1854−1857. (59) Gómez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499−3503. (60) Eda, G.; Chhowalla, M. Nano Lett. 2009, 9, 814−818. (61) Faugeras, C.; Nerriere, A.; Potemski, M.; Mahmood, A.; Dujardin, E.; Berger, C.; Heer, W. A. d. Appl. Phys. Lett. 2008, 92, 011914. (62) Liu, A.; Li, C.; Bai, H.; Shi, G. J. Phys. Chem. C 2010, 114, 22783− 22789. (63) Murugan, A. V.; Muraliganth, T.; Manthiram, A. Chem. Mater. 2009, 21, 5004−5006. (64) Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J. J. Phys. Chem. C 2008, 112, 8192−8195.

17946

dx.doi.org/10.1021/jp304374r | J. Phys. Chem. C 2012, 116, 17939−17946