SCIENCE CHINA Technological Sciences Special Topic: Nano Materials
February 2014 Vol.57 No.2: 244–248
• Article •
doi: 10.1007/s11431-014-5454-z
Synthesis and characterization of amphiphilic graphene DU ZhuZhu1, AI Wei1, ZHAO JianFeng1, XIE LingHai1* & HUANG Wei1,2* 1
Centre of Molecular System and Organic Devices (CMSOD), Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing 210046, China; 2 Jiangsu-Singapore Joint Research Center for Organic/Bio- Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Technology, Nanjing 211816, China Received December 13, 2013; accepted December 27, 2013; published online January 17, 2014
A simple and effective method for the preparation of amphiphilic graphene (AG) is presented under an organic solvent-free synthetic condition. The synthetic route first involves a cyclization reaction between carboxylic groups on graphene oxide and the amino groups on 5,6-diaminopyrazine-2,3-dicarbonitrile, and subsequent reduction by hydrazine. Results of UV-vis spectroscopy, Fourier transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Raman spectroscopy have confirmed that the covalent functionalization of graphene can be achieved through the formation of imidazo[4,5-b]pyrazine on the graphene sheets. As a result, AG can be successfully dispersed in water and common organic solvents. This work successfully provides a facile and efficient way to fabricate AG and may extend the potential applications of graphene-based materials in nanoelectronic devices, polymer fillers and biological field. amphiphilic graphene, 5,6-diaminopyrazine-2,3-dicarbonitrile, cyclization reaction Citation:
Du Z Z, Ai W, Zhao J F, et al. Synthesis and characterization of amphiphilic graphene. Sci China Tech Sci, 2014, 57: 244248, doi: 10.1007/s11431014-5454-z
1 Introduction Graphene and its derivatives have been regarded as the next-generation material for flexible nanoelectronic devices and one of the next-generation nanofillers for polymer nanocomposites to improve the mechanical, electrical and thermal properties of polymer due to their excellent physical properties and the natural abundance of their precursor, graphite [1–3]. However, pristine graphene is hydrophobic and has very limited solubility in most of solvents. To impart solution processability of graphene, the most important way is to functionalize this two-dimensional insoluble material [4]. By functionalizing graphene with different organic moleculars, not only the solubility can be improved in different solvents, but also the electrical, mechanical, opti-
cal and other properties can be tuned [5–7]. Unfortunately, it is difficult to directly and chemically functionalize graphene because of the hydrophobic and chemically inert nature of pristine graphene. Thus, attempts to overcome this issue have mainly focused on functionalization of graphene oxide (GO) due to its more chemically reactive than grapheme [8]. GO is prepared from the oxidation of insoluble graphite, which is recognized as the major route to achieve the bulk quantities of graphene. In contrast to the pristine graphene, GO consists of sp2 hybridized carbon atoms and sp3 hybridized carbon atoms that are covalently bonded to oxygencontaining functional groups, such as hydroxyl and epoxy groups on the basal plane and carboxyl groups at the edges [9]. As a result, GO can be dispersed in water with desirable single layer due to the hydrophilic nature of the oxygen-containing functional groups, which also endows GO
*Corresponding authors (email:
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with perfect solution processability for devices [10]. However, GO is electrically insulating material due to its disrupted sp2 bonding networks. Moreover, the GO sheets tend to either form irreversible agglomerates or restack during the solution reduction process, consequently limiting the application of graphene in nanoelectronic devices and polymer fillers [11]. Therefore, structural functionalization of GO is needed to prevent the restacking of graphene sheets, as well as to improve its solubility in solvents. Currently, covalent and non-covalent functionalizations of graphene have been widely used to improve its dispersibility [12]. However, the resulting functionalized graphene can only be dispersed in certain solvents, due to its either lipophilic or hydrophilic properties, but not both [13,14]. Up to date, amphiphilic graphene (AG) that could be dispersed in water and organic solvents has been rarely reported. Herein, we present a simple and effective method to fabricate AG via an organic solvent-free synthetic method. This method first involves a cyclization reaction of carboxylic groups on GO with the amino groups on 5,6-diaminopyrazine-2,3-dicarbonitrile, and subsequent reduction by hydrazine. The abundant imidazo[4,5-b]pyrazine groups that are grafted on graphene sheet enable the functionalized graphene to function as amphiphilic. Thus, the as-obtained AG can be dispersed in water and common organic solvents four months without aggregation. The structure of AG has been verified by UV/Vis spectroscopy, Fourier transformed infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA).
nitrogen gas flow rate of 20 cm3/min with a heating rate of 5°C/min. Raman spectra were measured by a WITEC CRM200 Raman system with 532 nm excitation laser. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) measurements were conducted on a JEOL JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV.
2 Experimental
The synthesis procedure of AG is shown in Figure 1(a). Firstly, the 5,6-diaminopyrazine-2,3-dicarbonitrile was grafted on the graphene sheets through the cyclization reaction between the carboxylic groups on GO and the amino groups on 5,6-diaminopyrazine-2,3-dicarbonitrile, which will simultaneously induce the hydrolysis of cyano groups [8,17]. Then, the functionalized GO was reduced by hydrazine to remove the oxygen-containing groups. UV-vis spectra of the samples confirm the transition from GO into AG. The absorption of GO dispersion has two typical peaks located at 228 and 280 nm, resulting from the -* and n- electronic transition on GO functional groups, respectively [18]. It was observed that the absorption of AG was red shifted to 270 nm, with the color of GO solution changing from brown to black after the functionalization (Figure 1(b)). It is very interesting to find that the as-obtained graphene has good dispersibility in water and common organic solvents (Figure 1(c)). The IR spectra and XPS spectra, shown in Figure 2, suggest the successful fabrication of AG through the formation of heterocyclic groups on the graphene sheets by using 5,6diaminopyrazine-2,3-dicarbonitrile as the functionalizing reagent. Figure 2(a) shows the typical FT-IR spectra of GO
2.1
Materials
Graphite powder (325 mesh) was purchased from Baichuan Graphite Co., Ltd (Qingdao, China). Sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), hydrazine hydrate (N2H4) purchased from Sinpharm Chemical Reagent Co., Ltd (Shanghai, China). Polyphosphoric acid (PPA), hydrochloric acid (HCl) and 5,6-diaminopyrazine-2,3-dicarbonitrile were purchased from Sigma-Aldrich Pte Ltd and used without further purification. All the reagents were used without further purification. 2.2
Characterization
The UV-vis spectra were obtained by using Shimadzu UV3600 spectrometer with deduction for the solvent background. FT-IR spectra were carried on IR-Prestige-21 FT-IR spectrophotometer. XPS analysis was performed on PHI5000 Versa Probe X-ray photoelectron spectrometer. A Shirley background was removed from the spectra. TGA curves were recorded on a DTG-60H instrument under the purified
2.3
Synthesis of GO
Graphite oxide was synthesized from graphite by a modified Hummers’ method following the procedures reported earlier [15,16]. 2.4
Synthesis of AG
800 mg GO was added in 400 mL deionized water by sonication for 50 min, then centrifugated at 1000 r/min for 10 min. 440 mg 5,6-diaminopyrazine-2,3-dicarbonitrile was added by sonication for another 5 min. 5 mL PPA was added under vigorous stirring. The solution was refluxed at 80°C in the darkness under the N2 for 24 h. The mixed solution was dispersed in 1:1 ethanol and water for stirring 30 min, then filtered and washed with 1:1 ethanol and water for several times. The obtained product was dispersed in 400 mL deionized water, and 3 mL N2H4 was added and refluxed at 80°C for 24 h. Then the dispersion was washed with water for several times and dried under vacuum at 100°C for 24 h.
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Results and discussion
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Figure 1 (a) Proposed chemical methods to prepare AG; (b) UV-vis spectra of GO and AG dissolved in water. Insert picture is the solution of GO and AG; (c) the dispersibility of AG in water, methanol and N,N-dimethylformamide four months after preparation.
before and after the functionalization. In the spectrum of GO, six typical peaks at 1725, 1610, 1420, 1220 and 1050 cm1 were observed, which could be attributed to the C=O stretching vibration, C=C stretching vibration, highly activated carbonyl C-O vibration and C-O stretching vibration, respectively [16]. In comparison to GO, most of the oxygen-containing functional groups disappeared in the FT-IR spectrum of AG. The sharp peak at 1650 cm1 can be assigned to the C=N stretching vibration, while the two peaks at 1345 and 1250 cm1 can be assigned to the C-N stretching vibration and C-H bending vibration, respectively [6]. Meanwhile, the structure of the as-synthesized AG was further investigated by XPS as shown in Figures 2(c) and (d). The C 1s spectrum of GO (Figure 2(b)) shows four peaks arising from C=C/C–C (284.6 eV), C–O (286.7 eV), C=O (288.0 eV) and O–C=O (289.1 eV) [2]. After the functionalization and reduction treatment, the peak intensity of oxygen functional groups was dramatically decreased. Moreover, the additional C–N peak at 285.7 eV was also observed (Figure 1(c)), indicating the existence of nitrogen-containing groups in AG. The bonding configurations of N atoms in AG were further investigated by high-resolution N 1s XPS spectrum (Figure 2(d)). The N 1s peak of AG could be fitted into two peaks, the higher energy peak at 401.2 eV is assigned to the pyridine-like N, and the lower energy peak at 399.1 eV is attributed to the pyrrole-like N [4]. These results suggest that GO has been successfully functionalized through the formation of imid-
azo[4,5-b]pyrazine on the graphene sheets. TGA and Raman spectra were also used to further confirm the structure of AG. In Figure 3(a), GO shows the weight loss before 100°C due to the removal of the adsorbed water molecules trapped between the sheets. However, a relatively large weight loss in the range of 150°C –250°C was observed due to the decomposition of the oxygen functional groups [15]. In contrast, AG exhibits a smaller weight loss about 10% in the range of 30°C–300°C due to the decomposition of imidazo[4,5-b]pyrazine on the graphene sheets. The Raman spectrum of GO displays two prominent peaks located at 1340 and 1580 cm1, which correspond to the well-documented D and G bands, respectively (Figure 3(b)) [16]. The Raman spectra of the obtained AG show the D and G bands at 1360 and 1620 cm1 with the increased D/G ratio of 1.09 to that of GO (0.99), which suggests the increased structural disorder caused by the functionalization and reduction [5]. The TEM image in Figure 3(c) reveals that AG has very thin and sheet-like structure with intrinsic wrinkles folding, indicating the good dispersion proprieties of AG [14]. The HRTEM image of AG shows the recovery of the sp2 network structure after the functionalization. (Figure 3(d))
4
Conclusion
In summary, we have developed an organic solvent-free
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(a) FT-IR spectra of GO and AG; (b),(c) the C 1s XPS spectra of GO and AG; (d) the N 1s XPS spectrum of AG.
Figure 3
(a) TGA curves of GO and AG; (b) Raman spectra of GO and AG; (c) TEM; (d) HRTEM images of AG.
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method for the synthesis of AG through the formation of imidazo[4,5-b]pyrazine on the graphene sheets. The structure of AG was investigated by FT-IR spectra, XPS analysis, Raman, TGA and TEM. The resulting AG could be dispersed in water and common organic solvents, suggesting its potential applications in nanoelectronic devices, polymer fillers and biological fields. This work was supported by the NSFC for Excellent Young Scholars (Grant No. 21322402), National Natural Science Foundation of China (Grant Nos. 21274064, 61204095, 51173081), the Program for New Century Excellent Talents in University (Grant No. NCET-11-0992), Natural Science Foundation of Jiangsu Province, China (Grant Nos. BK2011761, BK2012431, BK2009025), and NJUPT (Grant No. NY211022).
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