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Ag nanocomposite with good dispersibility and electroconductibility via solvothermal method
Materials Chemistry and Physics 129 (2011) 270–274
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of graphene/Ag nanocomposite with good dispersibility and electroconductibility via solvothermal method Juan Yang ∗ , Chuanliang Zang, Lei Sun, Nan Zhao, Xiaonong Cheng School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 28 March 2011 Accepted 2 April 2011 Keywords: Composite materials Chemical synthesis Electron microscopy Electrical conductivity
a b s t r a c t In the present study, Ag nanoparticles were deposited onto graphene sheets to form graphene/Ag nanocomposites through solvothermal method using ethylene glycol or de-ionzed water/hydrazine as solvent and reducing agent. Ag particles were attached on the graphene sheets and well separated. The size and morphology of the particles were influenced by the reducing agent and solvothermal reaction. Because of the existence of Ag particles, graphene sheets were well separated both in the solution and in the film obtained via vacuum filtration method. The electroconductibility of graphene/Ag film was investigated. It was found that the film resistance were 0.85 cm and 0.60 cm for different reducing agents used, which were close to that of the pure graphene film with same carbon concentration (0.25 cm). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a monolayer of carbon atoms that are tightly packed into a two-dimensional, honeycomb crystal structure [1]. This extremely thin nanomaterial has shown fascinating properties and holds the promise for future carbon-based device architectures, owing to characteristics such as high mechanical stiffness [2], extraordinary electronic transport properties [3] and excellent antibacterial activity [4]. Promising approaches to graphene include mechanical exfoliation (“Scotch-tape” method) of bulk graphite [5], epitaxial chemical vapor deposition on substrates [6] and chemical vapor deposition starting from carbon precursors [7]. Although these routes might be preferred for precise device assembly, they can be less effective for large-scale manufacturing. Chemical efforts, involving exfoliation starting from the oxidation of graphite and post-reduction, have received the most attention recent years, with respect to large-scale production of graphene [8,9]. During the chemical process, the product of the chemical exfoliation of graphite oxide, that is, graphene oxide (GO), are strongly hydrophilic and can generate stable and homogeneous colloidal suspension in aqueous and various polar organic solvents due to the negatively charged GO sheets [3,8]. Subsequent deoxygenation via chemical reduction will remove most of the oxygen-containing groups and the obtained bulk graphene sheets-if left unprotectedwill restack to form graphite [8,10]. So, keeping the graphene sheets individually separated is the most important and challenging part
of graphene production. Aggregation can be reduced by the attachment of other molecules or polymers onto the sheets [10–13]. But the presence of foreign stabilizers is undesirable for some applications, especially in electronic device. For chemically converted graphene, only partial restoration of the graphitic structure can be accomplished by chemical reduction, which leads to the high sheet resistance [14]. The existence of insulate dispersant will further deteriorate the electroconductibility. Recently, the deposition of inorganic nanoparticles, such as metals or semiconductors, onto graphene sheets is supposed to be useful in preventing the restack of graphene sheets [15]. By selecting the appropriate nanoparticles, the deposition strategy not only leads to physical separation of the resultant graphene sheets, but also makes it possible to use these new hybrids in fields of chemical sensors [16], optical and electronic devices [17], energy storage [18], and so on. So we consider that if particles with high electric conductivity were deposited onto the graphene sheets, graphene sheets with good dispersity would be obtained and the good conductivity would also be maintained. In this paper, electrically conducting particle, Ag, was selected to form graphene/Ag nanocomposite via solvothermal method. Results showed that graphene/Ag nanocomposite with good dispersity and electroconductibility was obtained and the particle size of Ag was influenced by the solvothermal reaction. 2. Experimental 2.1. Sample preparation
∗ Corresponding author. Tel.: +86 511 88780195; fax: +86 511 88791947. E-mail address: [email protected] (J. Yang). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.04.002
Analytical-grade reagents, including silver acetate (AgAC), ethylene glycol, absolute ethanol and hydrazine, were purchased from Shanghai Chemical Reagent Corporation and used as received.
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Fig. 1. XRD patterns of (a) GO, (b) GO–AgAC and (c) G–Ag(e) powders. GO was prepared according to the method described in the literature [19]. The synthesis of graphene/Ag nanocomposite involved two steps, according to the procedure in Ref. [20]. In the first step, 15 mg AgAC and 25 mg GO powders were dispersed into 50 mL absolute ethanol with sonication for 2 h. The mixture was magnetically stirred for 4 h, and then centrifuged and washed copiously with ethanol. The obtained sample was dried in a vacuum at 60 ◦ C for 8 h and labeled as GO–AgAc. In the second step, 25 mg of GO–AgAc composite was mixed with 25 mL ethylene glycol or 25 mL de-ionized water with 50 uL hydrazine, under sonication for 1 h, and the resulting mixture was transferred into two 25 mL stainless steel Teflon-lined autoclaves. The autoclaves were sealed, kept at 140 ◦ C, and then naturally cooled to room temperature. After the reaction, the solution was centrifuged, washed with ethanol and de-ionized water three times each, dried in a vacuum at 60 ◦ C for 6 h and the obtained products were labeled as G–Ag(e) and G–Ag(h) for different reducing agent, ethylene glycol and hydrazine, respectively. For film preparation, the G–Ag(h) and G–Ag(e) were-dispersed in water with sonication at a concentration of 0.1 mg mL−1 . The dilute suspension was vacuumfiltrated using a mixed cellulose ester membrane with 25 nm pores (Millipore). The film was allowed to dry in a vacuum at 30 ◦ C. To improve the conductivity, the products were further heat-treated in a tube furnace at 500 ◦ C under Ar/H2 atmosphere (volume ratio = 5:1) for 2 h and cooled naturally. 2.2. Characterization The crystal structure of the composites was characterized by a Philips 1730 pow˚ The morphology der X-ray diffractometer (XRD) with Cu K␣ radiation ( = 1.5406 A).
and microstructures were observed by a JEOL 6460 scanning electron microscope (SEM). High resolution transmission electron microscopy (HRTEM) was conducted on a JEOL 2011 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Infrared (IR) spectra of the samples were collected on a Nicolet Avatar 360 FTIR Fourier transform infrared (FTIR) spectrometer. The Raman spectra were recorded on a MXR Raman system with 532 nm (2.33 eV) excitation and with laser power at the sample below 0.5 mW to avoid laser-induced heating. The film resistance was tested on RTS-9 four point resistivity test system at room temperature.
3. Results and discussion Fig. 1 gives the XRD patterns of various samples. As shown in Fig. 1(a), the interlayer distance of the (0 0 2) peak for GO is 8.54 A˚ (2 = 10.35◦ ), larger than the origin graphite, 3.37 A˚ [20], indicating that the interlayer distance was remarkably expanded during the chemical oxidation. After reaction with AgAC in absolute ethanol, Ag+ was transformed to Ag by the sonication process. As shown in Fig. 1(b), characteristic peaks of Ag and GO coexisted in the XRD pattern of GO–AgAC. After the chemical reduction by solvothermal method, peak C(0 0 2) shifted to 24◦ , suggesting the reduction of GO. At the same time, the G–Ag(e) composite showed charac-
Fig. 2. SEM images of as-prepared (a) G–Ag(e); (b) Ag particles and TEM images of (c) G–Ag(e); (d) GO suspension.
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Fig. 3. SEM images of (a) graphene/Ag prepared with ethylene glycol at 160 ◦ C and (b) G–Ag(h); (c) TEM images of G–Ag(h); (d) HRTEM image of G–Ag(h) composite.
teristic peaks at 38.1◦ , 44.3◦ , 64.4◦ and 77.4◦ with high intensity (Fig. 1(c)), which can be assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the cubic Ag crystal (JCPDS No. 04-0783). Product G–Ag(h) reduced by hydrazine shows the same XRD pattern, indicating that graphene/Ag composite was successfully prepared during our experiments. Fig. 2(a) shows the typical SEM image of as-prepared G–Ag(e). It can be observed that graphene sheets are decorated by Ag particles with an average particle size of 50 nm. For comparison, we also prepared Ag with the same procedure but without GO and the SEM image is shown in Fig. 2(b). Generally, the aggregation of Ag particles leads to a significantly decreased surface area, which means, if un-protected, Ag particles would aggregate spontaneously. In our experiment, the oxygen-containing groups on GO sheets supply chemical active centers for Ag deposition. So Ag particle are well separated with each other and distributed randomly on the graphene sheet, which can be further confirmed by the typical TEM image of G–Ag(e) shown in Fig. 2(c). It can be clearly seen that the
morphology of Ag particles are irregular and particles are well separated. At the same time, we found that the dispersity of graphene sheets was remarkably improved by Ag deposition. Fig. 2(d) shows the TEM image of GO exfoliated by the ultrasonic treatment at concentration of 0.1544 mg mL−1 in water, which revealed that GO nanosheets tend to congregate together to form multilayer agglomerates. But in Fig. 2(c), graphene sheets are well exfoliated and the transparent, silk-like sheets are stable under the electron beam. If we increase the solvothermal temperature to 160 ◦ C, we found that the particle size and size distribution of Ag particles were changed. In Fig. 3(a), we can see that large particles are bigger than 1 m, but the small particles are still present. The influence of reducing agent on the particle size and size distribution of Ag was also investigated. Fig. 3(b) and (d) is the typical SEM and TEM images of graphene/Ag composite reduced by hydrazine. It can be seen that graphene sheets are perfectly decorated by large amounts of well-dispersed Ag spherical nanoparticles and the mean particle diameter is about 20 nm. Compared with those reduced with
Fig. 4. (a) FTIR spectra of GO and as-prepared G–Ag(e) nanocomposite; (b) Raman spectra of natural graphite, GO and G–Ag(e) nanocomposite.
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Fig. 5. (a) SEM image of the fracture surface of G–Ag(e) film obtained via vacuum filtration; (b) XRD patterns of graphene film and G–Ag(e) film.
ethylene glycol, Ag particles with relatively small particle size and narrow size distribution can be obtained with hydrazine as reducing agent. The result is similar with that reported in Ref. [21] and the multiple size-distribution might result from the slow reaction rate, which allows repetitive nucleation to occur, and hence crystal nucleation and growth occur together for a relatively long time. In our experiments, two reducing systems were used; the reducibility provided by ethylene glycol was weaker than that of hydrazine, which might further influence the reaction rate. Fig. 3(d) is the high-resolution image of Ag particles reduced by hydrazine. The fringe spacing shown in the image is about 0.205 nm, which agrees well with the (2 0 0) lattice plane reported in the JCPDS (No. 04-0783). The as-obtained GO and G–Ag(e) samples were preliminarily analyzed by means of FTIR spectroscopy and the results are shown in Fig. 4(a). The relatively broad peak at 3414 cm−1 and relatively sharp peak at 1620 cm−1 indicate that the samples contain adsorbed water. The peaks at 1397 cm−1 and 1081 cm−1 can be assigned to the deformation vibration of O–H and stretching vibration of C–O, respectively. Characteristic bands of C O carbonyl stretching and C–O–C vibration located at 1720 cm−1 and 1250 cm−1 are very weak, indicating the small amount of these two functional groups [22]. After the solvothermal reduction, removal of oxygen-containing groups is clearly indicated by the gradual disappearance of the most absorption bands as shown in Fig. 4(a). Raman spectroscopy was utilized as a direct and non-destructive technique for characterization of the changes between GO and asmade G–Ag(e) composite. Representative Raman spectra for the parent graphite, graphene oxide and our obtained graphene/Ag composite are shown in Fig. 4(b). In the Raman spectrum of GO, there are two prominent peaks, the D band (1355 cm−1 ) and the G band (1600 cm−1 ), which represent the amount of sp3 carbons in the surroundings and the E2g phonon of sp2 C atoms, respectively. Compared with parent graphite, the ratio of intensities of the D and G bands (ID /IG ) increases from 0.09 to 0.94, indicating a higher level of disorder and an increased number of defects in GO layers. After solvothermal reduction, the ID /IG ratio increases to 1.28 in graphene/Ag material. During the reduction process, most of the oxygen-containing groups were removed, and the conjugated G network (sp2 carbon) would be re-established. However, the size of the re-established G network is smaller than the original one, which would consequently lead to an increase in the ID /IG ratio [23]. In addition, the peak intensity of graphene/Ag is much higher than that of graphite and GO, which is due to surface-enhanced Raman scattering (SERS) from the intense local electromagnetic fields of Ag nanoparticles that accompanies plasmon resonance [24]. In order to test the electroconductibility of the products, graphene film and graphene/Ag composite film were prepared via vacuum filtration method followed by thermal annealing. Fig. 5(a) is the SEM image of the fracture surface of G–Ag(e) film. It can be
seen that Ag particles are intercalated in the interlayer of graphene sheets. Fig. 5(b) shows the XRD pattern of graphene film and G–Ag(e) film. Characteristic peaks of cubic Ag can be detected, together with the C (0 0 2) peak in the pattern of G–Ag(e) film. But compared with pure graphene film, the intensity of peak C (0 0 2) is quite low. The strong peak located at 26.2◦ in pattern of graphene film indicating that during the film preparation process, the graphene sheets were restack to form graphite-like structure. The deposition of Ag particles can effectively prevent the stacking of graphene sheets. In our study, the film resistance of G–Ag(e) and G–Ag(h) films are 0.85 and 0.60 cm, which are close to that of the pure graphene film with same carbon concentration, 0.25 cm. Although the density of the film is decreased by the intercalation of Ag particles, but the good conductivity of Ag will make contribution to the excellent conductivity of graphene/Ag film. 4. Conclusions Graphene/Ag nanocomposite with Ag particles randomly deposited on the graphene sheets was prepared by solvothermal method. The particle size and morphology of Ag were influenced by the reaction condition, especially the reducing agent. Compared with ethylene glycol, the reducibility provided by hydrazine was stronger, and Ag particles with relatively small particle size and narrow size distribution could be obtained with hydrazine as reducing agent. Graphene sheets were well separated by the deposition of Ag. The nanocomposite film of graphene deposited with Ag particles has excellent electroconductibility, which is possibly due to the peculiar conductivity of graphene sheets and the synergistic effect with the deposited Ag particles. Acknowledgment This work was financially supported by the National science foundation of China (50902061). References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [2] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Nano Lett. 8 (2008) 902. [3] D. Li, M.B. Muller, S. Gijie, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101. [4] W.B. Hu, C. Peng, W.J. Luo, M. Lv, X.M. Li, D. Li, Q. Huang, C.H. Fan, ACS Nano 4 (2010) 4317. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [6] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, Science 312 (2006) 1191. [7] C. Soldano, A. Mahmood, E. Dujardin, Carbon 48 (2010) 2127. [8] D. Li, R.B. Kaner, Science 320 (2008) 1170. [9] J.P. Zhao, S.F. Pei, W.C. Ren, L.B. Gao, H.M. Cheng, ACS Nano 4 (2010) 5245.
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[10] S. Stankovich, R.D. Piner, X.Q. Chen, N.Q. Wu, S.T. Nguyen, R.S. Ruoff, J. Mater. Chem. 16 (2006) 155. [11] C. Xu, X.D. Wu, J.W. Zhu, X. Wang, Carbon 46 (2008) 386. [12] J.L. Chen, X.P. Yan, J. Mater. Chem. 20 (2010) 4328. [13] G.X. Wang, B. Wang, J. Park, J. Yang, X.P. Shen, J. Yao, Carbon 47 (2009) 68. [14] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol. 3 (2008) 270. [15] J.F. Shen, M. Shi, N. Li, B. Yan, H.W. Ma, Y.Z. Hu, M.X. Ye, Nano Res. 3 (2010) 339. [16] S. Chen, J.W. Zhu, X. Wang, J. Phys. Chem. C 114 (2010) 11829. [17] W.B. Zou, J.W. Zhu, Y.X. Sun, X. Wang, Mater. Chem. Phys., doi:10.1016/j.Matchephys.2010.10.008.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of graphene/Ag nanocomposite with good dispersibility and electroconductibility via solvothermal method Juan Yang ∗ , Chuanliang Zang, Lei Sun, Nan Zhao, Xiaonong Cheng School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 28 March 2011 Accepted 2 April 2011 Keywords: Composite materials Chemical synthesis Electron microscopy Electrical conductivity
a b s t r a c t In the present study, Ag nanoparticles were deposited onto graphene sheets to form graphene/Ag nanocomposites through solvothermal method using ethylene glycol or de-ionzed water/hydrazine as solvent and reducing agent. Ag particles were attached on the graphene sheets and well separated. The size and morphology of the particles were influenced by the reducing agent and solvothermal reaction. Because of the existence of Ag particles, graphene sheets were well separated both in the solution and in the film obtained via vacuum filtration method. The electroconductibility of graphene/Ag film was investigated. It was found that the film resistance were 0.85 cm and 0.60 cm for different reducing agents used, which were close to that of the pure graphene film with same carbon concentration (0.25 cm). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Graphene is a monolayer of carbon atoms that are tightly packed into a two-dimensional, honeycomb crystal structure [1]. This extremely thin nanomaterial has shown fascinating properties and holds the promise for future carbon-based device architectures, owing to characteristics such as high mechanical stiffness [2], extraordinary electronic transport properties [3] and excellent antibacterial activity [4]. Promising approaches to graphene include mechanical exfoliation (“Scotch-tape” method) of bulk graphite [5], epitaxial chemical vapor deposition on substrates [6] and chemical vapor deposition starting from carbon precursors [7]. Although these routes might be preferred for precise device assembly, they can be less effective for large-scale manufacturing. Chemical efforts, involving exfoliation starting from the oxidation of graphite and post-reduction, have received the most attention recent years, with respect to large-scale production of graphene [8,9]. During the chemical process, the product of the chemical exfoliation of graphite oxide, that is, graphene oxide (GO), are strongly hydrophilic and can generate stable and homogeneous colloidal suspension in aqueous and various polar organic solvents due to the negatively charged GO sheets [3,8]. Subsequent deoxygenation via chemical reduction will remove most of the oxygen-containing groups and the obtained bulk graphene sheets-if left unprotectedwill restack to form graphite [8,10]. So, keeping the graphene sheets individually separated is the most important and challenging part
of graphene production. Aggregation can be reduced by the attachment of other molecules or polymers onto the sheets [10–13]. But the presence of foreign stabilizers is undesirable for some applications, especially in electronic device. For chemically converted graphene, only partial restoration of the graphitic structure can be accomplished by chemical reduction, which leads to the high sheet resistance [14]. The existence of insulate dispersant will further deteriorate the electroconductibility. Recently, the deposition of inorganic nanoparticles, such as metals or semiconductors, onto graphene sheets is supposed to be useful in preventing the restack of graphene sheets [15]. By selecting the appropriate nanoparticles, the deposition strategy not only leads to physical separation of the resultant graphene sheets, but also makes it possible to use these new hybrids in fields of chemical sensors [16], optical and electronic devices [17], energy storage [18], and so on. So we consider that if particles with high electric conductivity were deposited onto the graphene sheets, graphene sheets with good dispersity would be obtained and the good conductivity would also be maintained. In this paper, electrically conducting particle, Ag, was selected to form graphene/Ag nanocomposite via solvothermal method. Results showed that graphene/Ag nanocomposite with good dispersity and electroconductibility was obtained and the particle size of Ag was influenced by the solvothermal reaction. 2. Experimental 2.1. Sample preparation
∗ Corresponding author. Tel.: +86 511 88780195; fax: +86 511 88791947. E-mail address: [email protected] (J. Yang). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.04.002
Analytical-grade reagents, including silver acetate (AgAC), ethylene glycol, absolute ethanol and hydrazine, were purchased from Shanghai Chemical Reagent Corporation and used as received.
J. Yang et al. / Materials Chemistry and Physics 129 (2011) 270–274
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Fig. 1. XRD patterns of (a) GO, (b) GO–AgAC and (c) G–Ag(e) powders. GO was prepared according to the method described in the literature [19]. The synthesis of graphene/Ag nanocomposite involved two steps, according to the procedure in Ref. [20]. In the first step, 15 mg AgAC and 25 mg GO powders were dispersed into 50 mL absolute ethanol with sonication for 2 h. The mixture was magnetically stirred for 4 h, and then centrifuged and washed copiously with ethanol. The obtained sample was dried in a vacuum at 60 ◦ C for 8 h and labeled as GO–AgAc. In the second step, 25 mg of GO–AgAc composite was mixed with 25 mL ethylene glycol or 25 mL de-ionized water with 50 uL hydrazine, under sonication for 1 h, and the resulting mixture was transferred into two 25 mL stainless steel Teflon-lined autoclaves. The autoclaves were sealed, kept at 140 ◦ C, and then naturally cooled to room temperature. After the reaction, the solution was centrifuged, washed with ethanol and de-ionized water three times each, dried in a vacuum at 60 ◦ C for 6 h and the obtained products were labeled as G–Ag(e) and G–Ag(h) for different reducing agent, ethylene glycol and hydrazine, respectively. For film preparation, the G–Ag(h) and G–Ag(e) were-dispersed in water with sonication at a concentration of 0.1 mg mL−1 . The dilute suspension was vacuumfiltrated using a mixed cellulose ester membrane with 25 nm pores (Millipore). The film was allowed to dry in a vacuum at 30 ◦ C. To improve the conductivity, the products were further heat-treated in a tube furnace at 500 ◦ C under Ar/H2 atmosphere (volume ratio = 5:1) for 2 h and cooled naturally. 2.2. Characterization The crystal structure of the composites was characterized by a Philips 1730 pow˚ The morphology der X-ray diffractometer (XRD) with Cu K␣ radiation ( = 1.5406 A).
and microstructures were observed by a JEOL 6460 scanning electron microscope (SEM). High resolution transmission electron microscopy (HRTEM) was conducted on a JEOL 2011 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Infrared (IR) spectra of the samples were collected on a Nicolet Avatar 360 FTIR Fourier transform infrared (FTIR) spectrometer. The Raman spectra were recorded on a MXR Raman system with 532 nm (2.33 eV) excitation and with laser power at the sample below 0.5 mW to avoid laser-induced heating. The film resistance was tested on RTS-9 four point resistivity test system at room temperature.
3. Results and discussion Fig. 1 gives the XRD patterns of various samples. As shown in Fig. 1(a), the interlayer distance of the (0 0 2) peak for GO is 8.54 A˚ (2 = 10.35◦ ), larger than the origin graphite, 3.37 A˚ [20], indicating that the interlayer distance was remarkably expanded during the chemical oxidation. After reaction with AgAC in absolute ethanol, Ag+ was transformed to Ag by the sonication process. As shown in Fig. 1(b), characteristic peaks of Ag and GO coexisted in the XRD pattern of GO–AgAC. After the chemical reduction by solvothermal method, peak C(0 0 2) shifted to 24◦ , suggesting the reduction of GO. At the same time, the G–Ag(e) composite showed charac-
Fig. 2. SEM images of as-prepared (a) G–Ag(e); (b) Ag particles and TEM images of (c) G–Ag(e); (d) GO suspension.
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Fig. 3. SEM images of (a) graphene/Ag prepared with ethylene glycol at 160 ◦ C and (b) G–Ag(h); (c) TEM images of G–Ag(h); (d) HRTEM image of G–Ag(h) composite.
teristic peaks at 38.1◦ , 44.3◦ , 64.4◦ and 77.4◦ with high intensity (Fig. 1(c)), which can be assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the cubic Ag crystal (JCPDS No. 04-0783). Product G–Ag(h) reduced by hydrazine shows the same XRD pattern, indicating that graphene/Ag composite was successfully prepared during our experiments. Fig. 2(a) shows the typical SEM image of as-prepared G–Ag(e). It can be observed that graphene sheets are decorated by Ag particles with an average particle size of 50 nm. For comparison, we also prepared Ag with the same procedure but without GO and the SEM image is shown in Fig. 2(b). Generally, the aggregation of Ag particles leads to a significantly decreased surface area, which means, if un-protected, Ag particles would aggregate spontaneously. In our experiment, the oxygen-containing groups on GO sheets supply chemical active centers for Ag deposition. So Ag particle are well separated with each other and distributed randomly on the graphene sheet, which can be further confirmed by the typical TEM image of G–Ag(e) shown in Fig. 2(c). It can be clearly seen that the
morphology of Ag particles are irregular and particles are well separated. At the same time, we found that the dispersity of graphene sheets was remarkably improved by Ag deposition. Fig. 2(d) shows the TEM image of GO exfoliated by the ultrasonic treatment at concentration of 0.1544 mg mL−1 in water, which revealed that GO nanosheets tend to congregate together to form multilayer agglomerates. But in Fig. 2(c), graphene sheets are well exfoliated and the transparent, silk-like sheets are stable under the electron beam. If we increase the solvothermal temperature to 160 ◦ C, we found that the particle size and size distribution of Ag particles were changed. In Fig. 3(a), we can see that large particles are bigger than 1 m, but the small particles are still present. The influence of reducing agent on the particle size and size distribution of Ag was also investigated. Fig. 3(b) and (d) is the typical SEM and TEM images of graphene/Ag composite reduced by hydrazine. It can be seen that graphene sheets are perfectly decorated by large amounts of well-dispersed Ag spherical nanoparticles and the mean particle diameter is about 20 nm. Compared with those reduced with
Fig. 4. (a) FTIR spectra of GO and as-prepared G–Ag(e) nanocomposite; (b) Raman spectra of natural graphite, GO and G–Ag(e) nanocomposite.
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Fig. 5. (a) SEM image of the fracture surface of G–Ag(e) film obtained via vacuum filtration; (b) XRD patterns of graphene film and G–Ag(e) film.
ethylene glycol, Ag particles with relatively small particle size and narrow size distribution can be obtained with hydrazine as reducing agent. The result is similar with that reported in Ref. [21] and the multiple size-distribution might result from the slow reaction rate, which allows repetitive nucleation to occur, and hence crystal nucleation and growth occur together for a relatively long time. In our experiments, two reducing systems were used; the reducibility provided by ethylene glycol was weaker than that of hydrazine, which might further influence the reaction rate. Fig. 3(d) is the high-resolution image of Ag particles reduced by hydrazine. The fringe spacing shown in the image is about 0.205 nm, which agrees well with the (2 0 0) lattice plane reported in the JCPDS (No. 04-0783). The as-obtained GO and G–Ag(e) samples were preliminarily analyzed by means of FTIR spectroscopy and the results are shown in Fig. 4(a). The relatively broad peak at 3414 cm−1 and relatively sharp peak at 1620 cm−1 indicate that the samples contain adsorbed water. The peaks at 1397 cm−1 and 1081 cm−1 can be assigned to the deformation vibration of O–H and stretching vibration of C–O, respectively. Characteristic bands of C O carbonyl stretching and C–O–C vibration located at 1720 cm−1 and 1250 cm−1 are very weak, indicating the small amount of these two functional groups [22]. After the solvothermal reduction, removal of oxygen-containing groups is clearly indicated by the gradual disappearance of the most absorption bands as shown in Fig. 4(a). Raman spectroscopy was utilized as a direct and non-destructive technique for characterization of the changes between GO and asmade G–Ag(e) composite. Representative Raman spectra for the parent graphite, graphene oxide and our obtained graphene/Ag composite are shown in Fig. 4(b). In the Raman spectrum of GO, there are two prominent peaks, the D band (1355 cm−1 ) and the G band (1600 cm−1 ), which represent the amount of sp3 carbons in the surroundings and the E2g phonon of sp2 C atoms, respectively. Compared with parent graphite, the ratio of intensities of the D and G bands (ID /IG ) increases from 0.09 to 0.94, indicating a higher level of disorder and an increased number of defects in GO layers. After solvothermal reduction, the ID /IG ratio increases to 1.28 in graphene/Ag material. During the reduction process, most of the oxygen-containing groups were removed, and the conjugated G network (sp2 carbon) would be re-established. However, the size of the re-established G network is smaller than the original one, which would consequently lead to an increase in the ID /IG ratio [23]. In addition, the peak intensity of graphene/Ag is much higher than that of graphite and GO, which is due to surface-enhanced Raman scattering (SERS) from the intense local electromagnetic fields of Ag nanoparticles that accompanies plasmon resonance [24]. In order to test the electroconductibility of the products, graphene film and graphene/Ag composite film were prepared via vacuum filtration method followed by thermal annealing. Fig. 5(a) is the SEM image of the fracture surface of G–Ag(e) film. It can be
seen that Ag particles are intercalated in the interlayer of graphene sheets. Fig. 5(b) shows the XRD pattern of graphene film and G–Ag(e) film. Characteristic peaks of cubic Ag can be detected, together with the C (0 0 2) peak in the pattern of G–Ag(e) film. But compared with pure graphene film, the intensity of peak C (0 0 2) is quite low. The strong peak located at 26.2◦ in pattern of graphene film indicating that during the film preparation process, the graphene sheets were restack to form graphite-like structure. The deposition of Ag particles can effectively prevent the stacking of graphene sheets. In our study, the film resistance of G–Ag(e) and G–Ag(h) films are 0.85 and 0.60 cm, which are close to that of the pure graphene film with same carbon concentration, 0.25 cm. Although the density of the film is decreased by the intercalation of Ag particles, but the good conductivity of Ag will make contribution to the excellent conductivity of graphene/Ag film. 4. Conclusions Graphene/Ag nanocomposite with Ag particles randomly deposited on the graphene sheets was prepared by solvothermal method. The particle size and morphology of Ag were influenced by the reaction condition, especially the reducing agent. Compared with ethylene glycol, the reducibility provided by hydrazine was stronger, and Ag particles with relatively small particle size and narrow size distribution could be obtained with hydrazine as reducing agent. Graphene sheets were well separated by the deposition of Ag. The nanocomposite film of graphene deposited with Ag particles has excellent electroconductibility, which is possibly due to the peculiar conductivity of graphene sheets and the synergistic effect with the deposited Ag particles. Acknowledgment This work was financially supported by the National science foundation of China (50902061). References [1] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [2] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Nano Lett. 8 (2008) 902. [3] D. Li, M.B. Muller, S. Gijie, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101. [4] W.B. Hu, C. Peng, W.J. Luo, M. Lv, X.M. Li, D. Li, Q. Huang, C.H. Fan, ACS Nano 4 (2010) 4317. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [6] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, Science 312 (2006) 1191. [7] C. Soldano, A. Mahmood, E. Dujardin, Carbon 48 (2010) 2127. [8] D. Li, R.B. Kaner, Science 320 (2008) 1170. [9] J.P. Zhao, S.F. Pei, W.C. Ren, L.B. Gao, H.M. Cheng, ACS Nano 4 (2010) 5245.
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