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Preparation of Nitrogen-Doped Graphene Sheets by a Combined Chemical and Hydrothermal Reduction of Graphene Oxide Donghui Long,†,‡ Wei Li,† Licheng Ling,‡ Jin Miyawaki,† Isao Mochida,† and Seong-Ho Yoon*,† †

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan, and ‡ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Received June 15, 2010. Revised Manuscript Received August 25, 2010

Nitrogen-doped graphene sheets were prepared through a hydrothermal reduction of colloidal dispersions of graphite oxide in the presence of hydrazine and ammonia at pH of 10. The effect of hydrothermal temperature on the structure, morphology, and surface chemistry of as-prepared graphene sheets were investigated though XRD, N2 adsorption, solid-state 13C NMR, SEM, TEM, and XPS characterizations. Oxygen reduction and nitrogen doping were achieved simultaneously under the hydrothermal reaction. Up to 5% nitrogen-doped graphene sheets with slightly wrinkled and folded feature were obtained at the relative low hydrothermal temperature. With the increase of hydrothermal temperature, the nitrogen content decreased slightly and more pyridinic N incorporated into the graphene network. Meanwhile, a jellyfish-like graphene structure was formed by self-organization of graphene sheets at the hydrothermal temperature of 160 °C. Further increase of the temperature to 200 °C, graphene sheets could self-aggregate into agglomerate particles but still contained doping level of 4 wt % N. The unique hydrothermal environment should play an important role in the nitrogen doping and the jellyfish-like graphene formation. This simple hydrothermal method could provide the synthesis of nitrogen-doped graphene sheets in large scale for various practical applications.

1. Introduction Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, has attracted a great deal of interest due to its unique properties and potential applications.1-3 Currently, there are several major methods, including mechanical exfoliation,,5 epitaxial growth,6,7 bottom-up synthesis,8 and chemical reduction of graphene oxide (GO) suspension,9-13 used to prepare graphene sheets. Among them, the chemical reduction of GO is considered to be an efficient approach to produce graphene sheets in bulk quantity at relatively low cost. Generally, GO is synthesized by the Brodie,14 Staudenmaier,15 or Hummers method16 or some minor modifications of these *Corresponding author: Tel þ81 92 583 7959, Fax þ81 92 583 7897, e-mail [email protected]. (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (2) Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (3) Katsnelson, M. I. Mater. Today 2007, 10, 20. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, K. T.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Nature 2005, 438, 197–200. (5) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453. (6) 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. (7) Charrier, A.; Coati, A.; Argunova, T.; Garreau, Y.; Pinchaux, R.; Forbeaux, I.; Debever, J. M.; Sauvage-Simkin, M.; Themlin, J. M. J. Appl. Phys. 2002, 92, 2479–2484. (8) Zhang, W. X.; Cui, J. C.; Tao, C. A.; Wu, Y. G.; Li, Z. P.; Ma, L.; Wen, Y. Q.; Li, G. T. Angew. Chem., Int. Ed. 2009, 48, 5864–5868. (9) 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. (10) Szabo, T.; Tombacz, E.; Illes., E.; Dekany, I. Carbon 2006, 44, 537–545. (11) Park, S.; Lee, K.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. ACS Nano 2008, 2, 572–578. (12) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (13) Zhou, Y.; Bao, Q.; Tang, L. A. L.; Zhong, Y.; Loh, K. P. Chem. Mater. 2009, 21, 2950–2956. (14) Brodie, B. C. Philos. Trans. R. Soc. London 1859, 149, 249–259. (15) Staudenmaier, L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481–1487. (16) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.

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methods.17 All of them produce GO by the strong oxidation of graphite with acid. GO is characterized to be a lamellar solid with unoxidized aromatic regions and aliphatic regions containing many phenolic, carboxyl, and epoxide groups.18,19 These oxygen functionalities make GO highly hydrophilic, thereby stabilizing it to be easily exfoliated in aqueous media via strong stirring or ultrasonication.20 The exfoliated GO are then be subjected to chemical reduction to obtain graphene as individual sheets, typically using hydrazine,21 sodium borohydride,22 hydroquinone,23 or strongly alkaline solutions24 as reducing agents. The chemical reduction of GO usually results in significant restoration of sp2 carbon sites but is still unable to completely remove all the oxygen functionalities and often leaves a number of defects, such as topologic defects (e.g., pentagons, hepatgons), vacancies, adatoms, edges/cracks, and adsorbed impurities.25 Despite its importance, only a very limited effort has been undertaken to understand the chemistry of reduction reaction and discern the resultant surface chemistry of reduced graphene sheets. Graphene sheets occupy the interface of macromolecules and nanoscale objects. As such, they can be functionalized easily with (17) 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. (18) He, H. Y.; Riedl, T.; Lerf, A.; Klinowski, J. J. Phys. Chem. 1996, 100, 19954–19960. (19) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477–4482. (20) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560–10564. (21) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (22) Shin, H. J.; Sim, K. K.; Benayad, A.; Yoon, S. M.; Park, H. K.; Jung, I. S.; Jin, M. H.; Jeong, H. K.; Kim, J. M.; Choi, J. Y.; Lee, Y. H. Adv. Funct. Mater. 2009, 19, 1987–1992. (23) Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J. J. Phys. Chem. C 2008, 112, 8192–8195. (24) Fan, X.; Peng, W.; Li, Y.; Li, X.; Wang, S.; Zhang, G.; Zhang, F. Adv. Mater. 2008, 20, 4490–4493. (25) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prudhomme, R. K.; Aksay, I. A.; Car, R. Nano Lett. 2007, 8, 36–41.

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polymer or surfactants to improve their dispersion in aqueous or organic solvents.26 They are also doped with nitrogen atoms to tailor their band structure and the physicochemical properties, leading to widespread potential applications. Several approaches have been successfully demonstrated to dope nitrogen atoms into graphene sheets.27-29 Dai’s group developed a doping method through electrical joule heating in NH3,27 and recently they reported a chemical approach though thermal annealing of GO in ammonia.28 Liu et al. reported an in-situ doping method by adding NH3 gas during chemical vapor deposition growth of graphene.29 Currently, the chemical doping of graphene is an active area of research that should continue to rapidly grow. In this work, we developed a simple hydrothermal reaction to simultaneously dope nitrogen and reduce GO. Bulk quantities of graphene sheets doped with high level of nitrogen atoms were achieved via reduction of GO colloidal solution in the presence of ammonia and hydrazine under a hydrothermal environment. The effects of hydrothermal temperature on surface chemistry and structure of the reduced graphene sheets were also evaluated. Some self-organized graphene structures were observed at the elevated hydrothermal temperature. We expect our method could produce nitrogen-doped graphene sheets in large scale used for widely potential applications.

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Figure 1. UV-vis adsorption spectra of GO and reduced HR-80 sample. The inset is photos of GO, HR-80, and HR-160.

2. Experimental Details 2.1. Preparation and Reduction of GO. GO were synthesized from flake graphite powder using a modified Hummer method.17 The as-synthesized GO was suspended in water to give a brown colloidal solution with a concentration of 2 mg/mL followed by 1 h of sonication (ultrasonic bath cleaner USK-4R). The obtained dispersion was then subjected to 5 min of centrifugation at 1000 rpm to remove some unexfoliated GO particles using a centrifuge (Hitachi, Himac CR-GII). Chemical reduction of GO solution was achieved using ammonia and hydrazine hydrate (N2H4) as reducing agents under a hydrothermal environment. Typically, 70 mL of above GO solution was adjusted the pH value to 10 using 30% ammonia. Then 2 mL of hydrazine hydrate was added with the magnetically stirring for 10 min. The solution was then transferred into a Teflon-lined autoclave and heated at 80-200 °C for 3 h. The reduced graphene sheets were collected with centrifugation, followed by washing with deionized water several times, and then purifying using dialysis membrane (Wako). Finally, the collected samples were dried using a freeze-drying equipment (EVELA, FD-1000). The obtained samples were denoted as HR-x, where x represents the hydrothermal temperature. 2.2. Characterization. X-ray diffraction was performed with Rigaku X-ray diffractometer with Cu KR target. The porosity was measured with nitrogen adsorption-desorption isotherm using a surface area analyzer (Sorptomatic 1990, Qunata Instruments). High-resolution solid-state 13C NMR experiments were carried out on a JEOL ECA400 spectrometer operated at 100.53 MHz using the single-pulse decoupling method. The structure of GO and reduced graphene was observed under a field-emission scanning electron microscope (SEM; JEOL-6300F, 3 kV) and a highresolution transmission electron microscope (TEM; JEM-2010F, 200 kV). The UV-vis adsorption spectra were recorded under a Shimadzu UV-3600 spectrophotometer. The elemental composition was obtained from the CHN elemental analysis (Yanako MT2 CHN Corder, Japan). X-ray (26) Liu, Z. B.; Xu, Y. F.; Zhang, X. Y.; Zhang, X. L.; Chen, Y. S.; Tian, J. G. J. Phys. Chem. B 2009, 113, 9681–9686. (27) Wang, X. R.; Li, X. L.; Zhang, L.; Yoon, Y. K.; Weber, P. K.; Wang, H. L.; Guo, J.; Dai, H. J. Science 2009, 324, 768–771. (28) Li, X. L.; Wang, X. R.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. J. J. Am. Chem. Soc. 2009, 131, 15939–15944. (29) Wei, D. C.; Liu, Y. Q.; Wang, Y.; Zhang, H. L.; Huang, L. P.; Yu, G. Nano Lett. 2009, 9, 1752–1758.

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Figure 2. XRD patterns of GO and reduced graphene sheets. photoelectron spectroscopy (XPS) spectra were obtained using a JPS-9000MC (JEOL) instrument equipped with an Mg KR X-ray source. Field emission (FE) tests were performed in a vacuum chamber at a pressure of 7  10-8 Pa. Prior to elemental analysis and XPS measurement, samples were dried for 2 h at 120 °C.

3. Results and Discussion 3.1. Hydrothermal Reduction of GO. The GO colloid solution was reduced using the N2H4 at pH of 10 under the hydrothermal environment. The reduction progress was monitored by ultraviolet absorption as shown in Figure 1. The spectrum of GO showed two peaks: a broad peak at 270 nm with a characteristic of π-π* electron transition in the polyaromatic system and a sharp peak at 220 nm which is related to π-π* electron transition in the polyene-type structure from the graphite oxide. After hydrothermal reduction at 80 °C, only one peak centered at the 260 nm would be observed, indicating the electronic conjugation of GO was restored. Depending on the hydrothermal temperature, the reduced graphene samples (HR-80 and HR-120) could be easily dispersed in the water without any precipitation for 10 days (photos were inset in the Figure 1). But for sample reduced at high temperature (160, 200 °C), a lot of small flocs were formed, giving very poor dispersion properties. 3.2. Structure of Hydrothermally Reduced Graphene Sheets. Figure 2 shows the XRD patterns of samples before and after the hydrothermal reduction. The GO has a sharp peak at the 11.7°, corresponding to an interlay distance of 0.75 nm (at a relative humidity of ca. 30%30). This peak completely disappears (30) Buchsteiner, A.; Lerf, A.; Pieper, J. J. Phys. Chem. B 2006, 110, 22328–22338.

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Figure 3. N2 adsorption-desorption isotherms of GO and reduced graphene sheets.

Figure 4. Solid-state 13C NMR spectra of GO and reduced graphene sheets.

and broad peak centered at around 24° (d002 of ca. 0.37 nm) is observed for all reduced samples, confirming the recovery of graphitic crystal structure. No obvious differences are observed in broadness of the XRD peaks for reduced samples at various hydrothermal temperatures, suggesting the similar stacking thickness of graphene layers. The porosity structures of GO and reduced samples were measured using N2 adsorption. As shown in Figure 3, the N2 adsorption-desorption isotherm of GO has an II type isotherm with hysteresis type H3, which is indicative of the presence of mesoporosity and slit-shaped pores. This sample has a BET specific surface area of 59 m2/g and average mesopore size of 3.8 nm. After the hydrothermal reduction, the hysteresis loop shifts to very high relative pressure region, responding to the macropore characteristic. The BET specific surface areas of reduced samples are in the range of 100-200 m2/g. Even though we try to use the freeze-drying to suppress the negative effect of capillary force during the drying process, the obtained graphene sheets have much lower surface area than the theoretical value of fully singlelayer graphene (2600 m2/g), possibly due to the incompletely exfoliation or the self-aggregation in the reduction process. The solid-state 13C NMR spectra with the 1H decoupling of GO and reduced graphene sheets are shown in Figure 4. The GO contains four peaks at 60, 70, 110, and 130 ppm, which are assigned to C-O-C expoxide groups, C-OH groups, and conjugated double bonds in oxidized aliphatic rings and aromatic entities, respectively. These structures are in good agreement with the GO structural model proposed by Kinowski et al.18 Upon hydrothermal reduction at 80 °C, the peaks at 60 and 70 ppm 16098 DOI: 10.1021/la102425a

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disappeared and the dominate peak was in the range of 100-150 ppm. Further increase the hydrothermal temperature over 120 °C, only one peak centered at 120 ppm was observed, suggesting that the oxygen function groups have been removed and aromatic entities have been recovered greatly. 3.3. Morphology of Hydrothermally Reduced Graphene Sheets. Figure 5 shows the SEM images of hydrothermally reduced graphene sheets. These images demonstrate thin and wrinkled sheets transparent to electrons. Some differences should be noted for graphene sheets reduced at different temperatures. The low hydrothermal temperature (e120 °C) gives the leaflike graphene with relatively flat feature, while high temperature causes some aggregation of crinkly graphene sheets. Further morphology observation has been achieved though TEM. As shown in Figure 6, the transparent graphene sheets with wrinkled and folded feature are easily observed for the samples reduced at 80 or 120 °C. It seems that some ripples gather together, forming scattering centers as indicated by a white circle in Figure 6b. The ripples in graphene may originate from the heteroatoms or defects remained in the plane of graphene, and the scatting center of ripples should contain an abundant of heteroatoms or defects. Increasing the hydrothermal temperature to 160 °C, a very interesting intermediate structure is observed, as shown in Figure 7. The graphene sheet can self-organize into a jellyfish-like structure, consisting of a head inserting into a skirtlike graphene sheet. The head, generally in the size of 100 nm, can be separated from the graphene skirt due to the strong sonication. Further highresolution TEM observations reveal that the head is composed by the amorphous rather than the crystalline graphite structure. These heads are thermodynamically unstable, which are easily destroyed by the electron beam (see Figure 7d). We believe such amorphous head is formed by the close enfolding of graphene sheet around the scattering center of ripples, in where heteratoms or defects act as nucleation sites. This process should be energetically favorable, as it minimizes the interfacial energy of graphene sheet by adopting the self-enfolding graphene intermediate structure. The formation of this intermediate structure may be partially attributed to the unique hydrothermal environment, which is well suitable for the preparation of metastable phases,31 due to their unique features such as high self-generated pressure and containment of volatile in the closed reactor. However, the detailed formation process is still unclear, which needs further discussion. At high hydrothermal temperature of 200 °C, the graphene sheet can self-aggregate into agglomerate particles with size of 1-5 μm, according to the SEM (images are not shown) and TEM observations (Figure 8). The selected area electron diffraction (SAED) yields a ring-shaped pattern consisting of many diffraction spots for each order of diffraction. These spots make regular hexagons (e.g., the one marked in the inset of Figure 8a) with different rotational angles, indicating the essentially random overlapping of graphene sheets. A high-resolution TEM image (Figure 8b) shows that the edge of the graphene sheets is composed of a stacking of graphene with the number of layers ranging from 2 to more than 10. Therefore, a complicated reorganization of graphene sheets should take place at this temperature under the hydrothermal environment. 3.4. Surface Chemistry of Hydrothermally Reduced Graphene Sheets. Table 1 lists the elemental composition of GO (31) Byrappa, K.; Yoshimura, M. Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing; Noyes Publisher: Norwich, NY, 2001.

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Figure 5. SEM images of hydrothermally reduced graphene sheets: (a) HR-80, (b) HR-120, (c) HR-160, and (d) HR-200.

Figure 6. TEM images of hydrothermally reduced graphene sheets: (a) HR-80 and (b) HR-120.

and reduced graphene sheets. The initial GO has very high oxygen content (48%). Upon hydrothermal reduction, the carbon content increases gradually to 86%, which is at the expense of the great decrease of oxygen content. This result indicates that the oxygen functionalities have been removed mostly. It should be noted that the nitrogen content, almost negligible in the initial GO, shows a great large amount in the reduced samples. With the increase of the hydrothermal temperature, the nitrogen content decrease slightly. These nitrogen atoms doped in the graphene sheets should come from the N2H4 and/or ammonia during the hydrothermal reaction. XPS analysis further reveals that the surface compositions of GO and reduced graphene sheets. Only carbon, oxygen, and nitrogen species are detected, and the resultant O/C and N/C atom ratios (Table 2) are slightly higher than these calculated by elemental analysis, except for the O/C ratio of GO. High-resolution C 1s, O 1s, and N 1s spectra of these samples are shown in Figure 9. The detailed deconvolution of the C 1s and O 1s spectra are given in the Supporting Information, and the deconvolution of the N 1s (32) Zalan, Z.; Lazar, L.; Fueloep, F. Curr. Org. Chem. 2005, 9, 357–376.

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Figure 7. TEM images of HR-160 graphene sheets with different magnifications. The inset is the resultant SAED pattern.

spectra yield five peaks:32,33 the peak at 395.7 ( 0.3 eV ascribed to nitride-like species or aromatic N-imines; the peak at 398.7 ( 0.3 eV attributed to pyridine-like structures; the peak at 400.3 ( 0.3 eV attributed to the nitrogen atoms in pyrrolic or amine moieties; the peak at 401.4 ( 0.3 eV contains a contribution from quaternary or protonated nitrogen; the peak at 402-405 eV corresponds to the nitrogen atoms in pyridine-N oxides. (33) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565.

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The GO has a strong peak at high binding energy in the C 1s spectrum, corresponding to large amounts of sp3 carbon with C-O bonds, carbonyls (CdO), and carboxylates (OdC-O) which resulted from harsh oxidation and destruction of the sp2 atomic structure of graphite. The hydrazine reduction under a hydrothermal environment results in significant restoration of sp2 carbon sites but is still unable to completely remove all the oxygen

Figure 8. TEM images of HR-160 graphene sheets with different magnifications. The inset is the resultant SAED pattern. Table 1. Elemental Composition of GO and Reduced Graphene Sheets samples

C (wt %)

GO 49.59 HR-80 80.43 HR-120 80.76 HR-160 84.15 HR-200 86.12 a By difference.

H (wt %)

N (wt %)

Oa (wt %)

N/C (at./at.)

O/C (at./at.)

2.28 1.23 1.15 1.15 1.23

0.03 5.21 4.57 4.09 4.01

48.05 13.13 13.52 10.61 8.64

0.0018 0.074 0.064 0.055 0.051

0.72 0.12 0.12 0.091 0.077

functional groups. After hydrothermal reduction, the O/C atom ratio decrease to ca. 0.1, and the oxygen species are still hydroxyl, carbonyl, and carboxyl, etc. However, a general tendency can be concluded that the content of oxygen atom in hydroxyls or ethers decreases and that in carbonyls increase after the hydrothermal reduction (see Supporting Information). In addition to oxygen reduction, the hydrothermal treatment of GO using N2H4 shows an obvious nitrogen-doping effect in the resulting graphene sheets. The N/C atom ratio decreases from 0.11 to 0.052 with the hydrothermal temperature increasing. This decreasing tendency is very similar to the result obtained from the elemental analysis, further confirming that the nitrogen doping is easily achieved even at relatively low hydrothermal temperature. The N functionalities and their distribution were identified by the curve-fitting procedures, and the results are summarized in Table 2. A nitride-like peak located at 395.7 eV is observed for the reduced samples at relatively low hydrothermal temperature, which should be ascribed to the adsorbed N2H4, even these samples experienced several-times washing and dialysis. This peak disappears gradually at high hydrothermal temperature, indicating the thermodynamic instability of this N functionality. In contrast, the pyridine structure, located at the 398.8 eV, shows an increasing tendency with the increase of hydrothermal temperature, while the N contents in pyrrolic, quaternary, and pyridine-N oxides show inconspicuous changes. These results suggest that higher temperature hydrothermal reduction of GO using N2H4 and ammonia affords more pyridine N incorporated into the carbon network of graphene sheets. The nitrogen and oxygen distribution in the plane of graphene is highly homogeneous, as can be proved by the TEM elemental mapping shown in Figure 10. The same C, N, and O mapping of

Table 2. XPS Results on the N/C and O/C Atom Ratios and the N Distributions N distribution (at. %) samples

N/C (at./at.)

O/C (at./at.)

395.7 ( 0.3 eV

398.7 ( 0.3 eV

400.3 ( 0.3 eV

401.4 ( 0.3 eV

402-405 eV

GO HR-80 HR-120 HR-160 HR-200

0 0.11 0.095 0.071 0.052

0.45 0.16 0.13 0.13 0.098

16 11 0 0

18 27 39 42

37 24 25 28

18 21 17 16

11 17 19 14

Figure 9. C 1s, O 1s, and N 1s XPS spectra of GO and reduced graphene sheets. 16100 DOI: 10.1021/la102425a

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Figure 10. TEM mapping of nitrogen-doping graphene sheet (HR-80).

Figure 11. Schematic structure of GO and nitrogen-doping graphene produced by hydrothermal reduction.

graphene suggests that not only the edge but also the plane of graphene contain a large amount of N and O functionalities. The reduction and nitrogen doping of graphene are achieved simultaneously during the hydrothermal reaction. To gain further insight into the reaction pathway, a comparison study was performed to reduce the GO at 90 °C using the N2H4 and ammonia at pH 10 in the absence of hydrothermal environment. Although the reduction of GO is achieved very well, only few nitrogen atoms are incorporated into the graphene network (shown in the Supporting Information). This result confirms that the unique hydrothermal environment could play a dominate role in the nitrogen doping. A possible schematic structure evolution of GO during hydrothermal process is given in Figure 11. Multiple Langmuir 2010, 26(20), 16096–16102

reactions may occur simultaneously, such as hydrazine can react with epoxides to readily open the ring of epoxides and form hydrazino alcohols;32 amine groups can react with carboxylic acid groups through the amidation process;33 hydrazine can react with anhydrides and lactones to form hydrazides and with quinones to form hydrazones.34 Anyway, hydrazine can remove the oxygen from the oxygen functionalities such as expoxides, hydroxyls, carboxyls, etc., although the mechanism is still unclear. During deoxygenation, the reorganization of unsaturated carbon and N atoms incorporation spontaneously occur under the hydrothermal (34) Neidlein, R.; Dao, T. V.; Gieren, A.; Kokkinidis, M.; Wilckens, R.; Geserich, H. P. Chem. Ber. 1982, 115, 2898–2904.

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environment. These nitrogen atoms should inherit from the reducing reagent N2H4 and/or ammonia; however, how they incorporate into the carbon network is still an open question.

4. Conclusions Chemical reduction and nitrogen doping of GO were achieved simultaneously using N2H4 and ammonia as reducing reagents via a simple hydrothermal approach. The structure and surface chemistry of the resulting graphene sheets were strongly dependent on the hydrothermal temperature. At the relative low temperature, up to 5 wt % nitrogen-doped graphene sheets with slightly wrinkled and folded feature were obtained. With the increasing of hydrothermal temperature, the nitrogen content decreased gradually and more pyridine N incorporated into the graphene network. Meanwhile, a jellyfish-like graphene structure

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was formed by self-organization of graphene sheet. The unique hydrothermal environment, such as high self-generated pressure and containment of volatile products inside the close reactor, should play a key role in the nitrogen doping and the intermediate structure formation. We expect that N-doped graphene sheets may exhibit some unique electrical properties and potential applications including the electrode materials for supercapacitors. The self-organized jellyfish structure of graphene may help us understand some metastable structures of single-layer 2D carbon atoms. Supporting Information Available: Experimental details and supplementary experimental evidence. This material is available free of charge via the Internet at http://pubs. acs.org.

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