- Email: [email protected]
Ag nanocomposites
Applied Surface Science 261 (2012) 753–758
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Green synthesis of graphene/Ag nanocomposites Wenhui Yuan a,∗ , Yejian Gu a , Li Li b a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangdong, Guangzhou 510640, PR China College of Environmental Science and Engineering, South China University of Technology, Guangdong, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2012 Received in revised form 23 August 2012 Accepted 23 August 2012 Available online 29 August 2012 Keywords: Graphene Graphene/Ag Composites Sodium citrate Environment-friendly
a b s t r a c t Graphene/Ag nanocomposites (GNS/AgNPs) were fabricated via a green and facile method, employing graphite oxide (GO) as a precursor of graphene, AgNO3 as a precursor of Ag nanoparticles, and sodium citrate as an environmentally friendly reducing and stabilizing agent. The synthesized GNS/AgNPs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectra (RS), respectively. The results indicated that graphite oxide was completely reduced to graphene, and the silver ion was reduced by sodium citrate simultaneously. Under a suitable dosage of silver ions, well-dispersed AgNPs on the graphene sheets mostly centralized at 20–25 nm. The surface plasmon resonance property of AgNPs on graphene showed that there was a interaction between AgNPs and graphene supports. In addition, antibacterial activity of silver nanoparticles was retained in the nanocomposites, suggesting that they can be potentially used as a graphene-based biomaterial. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a single-atom thick carbon sheet that is formed by two dimensional layers of sp2 bonded carbon, has attracted tremendous attention due to its unique nanostructure and fascinating properties [1–4]. Recently, graphene decorated with various inorganic nanoparticles, such as Pt, Au, Ag, CdS, TiO2 , Fe3 O4 , and ZnO [5–11], among which Ag nanocomposites (AgNPs) are good candidates for electronics, optics, electrochemistry and catalysis [12,13], GNS/AgNPs has been proved to be a promising material due to its potential applications in many fields. Pasricha et al. [14] prepared GNS/AgNPs in two steps, which GO/AgNPs were synthesized first, and hydrazine was then used to remove the oxygen functionalities. Shen et al. [15] presented a chemical synthesis approach that the chemical reduction of silver ions in chemically converted graphene (CCG) suspensions with mixed reducing agents, ethylene glycol and NaBH4 . Furthermore, it was shown that the antibacterial activity of free AgNPs was retained in the nanocomposites, which suggested that they could be used in the field of graphene-based biomaterials. Liu et al. [16] prepared GNS/AgNPs by microwave-assisted reduction of GO and AgNO3 with DMF as a reducing agent and solvent, and the resulting composites exhibited a good catalytic activity for the reduction
of hydrogen peroxide. However, all these methods reported are required complicated synthetic process, time-consuming or use of extra highly toxic reducing agents which pollute the environment, limiting their further practical applications. Hence, it is highly demand to find a new reducing agent with green and facile characteristic for producing high-quality of GNS/AgNPs. In this work, an environment-friendly and low-cost method was used to produce GNS/AgNPs using sodium citrate as the reducing agent (Fig. 1). The corresponding reduction reaction, the influence of silver nitrate dosage on the particle size and size range of the AgNPs, and the antibacterial activity of silver nanoparticles were investigated for explore its application in the field of disinfection. 2. Experimental 2.1. Raw materials Graphite powders were purchased from Alfa Aesar (Beijing, China) with an average particle size of 45 m. HCl (37%), H2 SO4 (98%), H2 O2 (30%), KMnO4 , KClO4 , NaNO3 , C6 H5 O7 Na3 ·2H2 O, AgNO3 and ethanol were supplied by Sinopharm Chemical Reagent Co., Led (Beijing, China). All chemicals were of analytical grade. 2.2. Preparation of GO
∗ Corresponding author. Present address: School of Chemistry and Chemical Engineering, South China University of Technology, Wushan, Tianhe, Guangzhou 510640, PR China. Tel.: +86 2087111887; fax: +86 2087111887. E-mail address: [email protected] (W. Yuan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.094
Graphite oxide was produced via a modified Hummers’ method [17], using graphite as raw material, KMnO4 , KClO4 , NaNO3 and 98% H2 SO4 as oxidants.
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W. Yuan et al. / Applied Surface Science 261 (2012) 753–758
Na+ O +O OO- OH OH Na OH - OHOH O O O O O OH OH OH OH OH + OH -OH O OH OH Na O O OH Na+ OH OH O + OHO OO Na OH O O OO + OH O AgNO3 O O O OH O Na OH OH + O Na+ O + Na O O OH HO - OHO O OH O O+ O O O O O Na O O + O 95ºC O OH OH OHNaO O OH + OH Na OH OH O OH HO O Agions OH O OH OH OH Agparticle ONa+OH OH OHOH
OH O
Graphite Oxide
Sodium Citrate
OH
Graphene/AgNPs
Fig. 1. Schematic illustration of the reduction process of GNS/AgNPs based on sodium citrate reduction.
2.3. Preparation of GNS/Ag The GNS/AgNPs were prepared in one step reaction. In a typical procedure, 200 mg GO powder was dispersed in 100 mL of water by ultrasonication for 1 h, forming stable graphite oxide colloid, and 50, 230, 410 mg AgNO3 was added under stirring, respectively. Then 1 g sodium citrate was gradually added to the mixture with magnetic stirring for 30 min. Subsequently, the mixture was transferred to an oil bath and kept at 95 ◦ C for 16 h under constant stirring. Finally, the products were washed with ethanol and deionized water by centrifugation, and the resulting GNS/AgNPs were dried in a vacuum oven at 60 ◦ C for 24 h and marked GNS/AgNPs-1, GNS/AgNPs-2, GNS/AgNPs-3 respectively. Fig. 2. XRD patterns of (a) G, (b) GO, (c) GNS/AgNPs-1, (d) GNS/AgNPs-2 and (e) GNS/AgNPs-3.
2.4. Characterization X-ray diffraction (XRD) patterns were recorded by a Bruker-D8 Advance (Germany) powder diffractometer with Cu K␣ radiation ˚ operating at 40 kV and 40 mA. Scanning electron ( = 1.5406 A) microscope (SEM, LEO 1530 VP, Germany) and transmission electron microscope (TEM, JEOL-2100, Japan) were used to observe the morphology of the samples. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Vector 33 (Germany) FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) results were detected on an Axis Ultra DLD photoelectron spectrometer using Al K␣ (1486.6 eV) radiation. Raman spectra were recorded with Horiba Jobin Yvon LabRam Aramis Raman spectrometer, the excitation line at 632.8 nm provided by an Ar+ laser was used. UV–visible spectra were recorded in the range between 200 and 800 nm using Specord 2450 (Shimadzu, Japan). The GNS/AgNPs suspension was diluted 10 times to record the UV–visible spectra. Antibacterial activity tests were performed according to GB/T21510-2008. Colibacillus and Canidia albicans were grown in nutritional broth and stored at 0 ◦ C. The whole process of the control experiment was carried out in a biosafety cabinet and the concentration of GNS/AgNPs was 0.05 mg mL−1 . The disinfection rate R was calculated as follows: R=
A−B × 100% A
where A is the average bacterial counting of the reference sample and B is the average bacterial counting of the test piece.
3. Results and discussion 3.1. X-ray diffraction analysis The XRD patterns of the graphite (G), GO and GNS/AgNPs are shown in Fig. 2a shows that G exhibits a characteristic peak (0 0 2) of graphite at 26.4◦ , which indicates the high crystallinity of this material (JCPDS No. 41-1487). After oxidation (Fig. 2b), the native graphite powder position peak (0 0 2) of graphite at 26.4◦ disappeared, and the (0 0 1) diffraction peak of graphite oxide at 10.4◦ appeared (corresponding to interlayer spacing was 8.5 A˚ from 3.35 A˚ of graphite powder). In the XRD pattern of the GNS/AgNPs (Fig. 2c), there are four main peaks at 2 = 38.2◦ , 44.4◦ , 64.5◦ , 77.5◦ , which correspond 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), respectively, indicating that the metallic AgNPs are formed after reduction. The correspond˚ d2 0 0 = 2.04 A, ˚ ing d-spacing values of the AgNPs are d1 1 1 = 2.37 A, ˚ respectively. It is also observed that d2 2 0 = 1.45 A˚ and d3 1 1 = 1.24 A, the d-spacing value of the synthesized AgNPs using different dosage of the AgNO3 remains almost constant for the each crystallographic plane. 3.2. SEM and TEM analysis Fig. 3 shows the SEM images of GO and GNS/AgNPs with different silver doping dosage. As shown in Fig. 3a, a closely packed lamellar structure can be observed. After the GO was doped with Ag
W. Yuan et al. / Applied Surface Science 261 (2012) 753–758
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Fig. 3. SEM images of GO and GNS/AgNPs, (a) GO, (b) GNS/AgNPs-1, (c) GNS/AgNPs-2, and (d) GNS/AgNPs-3.
particles and reduced (Fig. 3b–d), the Ag crystallites are deposited on graphene surfaces as spacers to keep the neighboring sheets separate, and a curled and corrugated morphology is observed. The size and shape of the AgNPs are also affected by the dosage of AgNO3 . In low dosage of AgNO3 , it is difficult to find that AgNPs are deposited on graphene sheets. After the dosage of AgNO3 increases to 230 mg (Fig. 3c), the AgNPs are well separated from each other and distributed randomly on the graphene sheets as spacers to keep the neighboring sheets separate. When AgNO3 dosage increases to 410 mg (Fig. 3d), the size of AgNPs increased significantly and AgNPs tend to agglomerate. Fig. 4 is the typical TEM images of as-prepared GNS/AgNPs2 hybrids. As shown in Fig. 4a and b, well-dispersed AgNPs are deposited on GNS homogeneously, and the size range of AgNPs mostly distribute at 20–25 nm. On the surface of the GNS/AgNPs, some wrinkles are observed, which may be important for preventing aggregation of GNS and maintaining high surface area with a particular advantage of attaching AgNPs on the graphene sheets. HRTEM image (Fig. 4c) indicates that the lattice spacing of planes is 0.236 nm corresponding to the (1 1 1) crystal plane of Ag, which is consistent with the d-spacing values reported in the (JCPDS No. 04-0783).
3.3. FT-IR spectra analysis FTIR spectra of G, GO and GNS/AgNPs-2 are shown in Fig. 5. It can be seen from Fig. 5b that the peaks at 3414, 1725, 1626, 1371, 1245 and 1060 cm−1 of GO are assigned to the–OH stretching vibrations, C O stretching of COOH groups, skeletal vibrations of unoxidized graphitic domains, O H deformations of the C OH groups, epoxy symmetrical ring deformation vibrations and C O stretching vibrations, respectively [18]. In the FTIR spectrum of GNS/AgNPs-2 (Fig. 5c), the peaks at 1060, 1245, 1371, 1725 cm−1 are become relatively weak. Furthermore, the absorption peak around 1212 cm−1 is attributed to the C OH, and a new absorption band at 1570 cm−1 is attributed to the skeletal vibration of the graphene sheets [19]. The FTIR results demonstrate that the GO have been successfully exfoliated and reduced to GNS, and strong interactions may exist between AgNPs and the remaining surface hydroxyl groups [20]. 3.4. XPS spectra analysis The C1s XPS spectrum of GO (Fig. 6a) clearly indicates oxidation with five components that correspond to carbon atoms in
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Fig. 4. TEM and HRTEM images of GNS/AgNPs-2, (a) and (b) GNS/AgNPs-2 at different magnification, (c) HRTEM image of AgNPs.
removed. This observation is in agreement with those found in previous studies [6,21,22]. On the other hand, as shown in Fig. 6c, the Ag3d spectrum for GNS/AgNPs-2 can be detected and the binding energies of 3d5/2 and 3d3/2 electrons for Ag are identified to be 367.8 and 373.8 eV, respectively [23].
Transmittance(%)
(a) (b) 3429
1371 1725 1626
(c)
1060 1245
3414
3.5. Raman spectra analysis
3396 4000
3500
1570 3000
2500
2000
1212 1500
1000
500
Wavenumber cm-1 Fig. 5. FTIR spectra of samples (a) G, (b) GO and (c) GNS/AgNPs-2.
different functional groups: C C (284.6 eV), C OH (285.6 eV), C (epoxy) (286.7 eV), C O (287.8 eV) and O C O (289 eV). After GO reduction by sodium citrate (Fig. 6b), the peaks assigned to oxygencontaining functional groups are significantly decreased compared with the C1s spectrum of GO, which confirms that most of the hydroxyl, epoxyl and carboxyl functional groups are successfully
Raman spectroscopy is a powerful nondestructive tool to characterize carbonaceous materials, particularly for distinguishing ordered and disordered carbon structures. As seen in Fig. 7, the Raman spectrum of G displays two prominent peaks at 1332 cm−1 (D band) and at 1562 cm−1 (G band), which are usually assigned to the breathing mode of -point phonons of A1g symmetry and the E2g phonon of C sp2 atoms, respectively [24]. In the Raman spectra of GO, the G band is broadened and shifted to 1573 cm−1 and a broadened D band at 1324 cm−1 is also appeared. After chemical reduction of GO, the Raman spectrum of GNS/AgNPs-2 shows a G band at 1586 cm−1 and D band at 1330 cm−1 . The ratio of the intensities of the D and G bands (ID /IG ) increases from 1.135 to 1.255 (Table 1), indicating that there is the existence of reduction procedure of GO. In addition, the peak intensity of GNS/AgNPs-2 is much higher than those of G and GO, which is due to surface-enhanced
W. Yuan et al. / Applied Surface Science 261 (2012) 753–758
(a)
(b)
C-C/C=C
757
C-C/C=C
C-OH C epoxy C-OH
C=O
C epoxy C=O
O-C=O
280
285 290 Binding Energy(ev)
(c)
295
280
O-C=O
285 290 Binding Energy(ev)
295
367.8
373.8
364
368
372
376
380
Binding Energy(ev) Fig. 6. C1s XPS spectra of (a) GO and (b) GNS/AgNPs-2, Ag3d XPS spectrum of (c) GNS/AgNPs-2.
D
227 nm
G
Intensity (a.u.)
Intensity(a.u.)
263 nm
(c) (b)
302 nm
440 nm
(b) (a)
(a)
1000
1200
1400
1600
1800
2000
Raman shift/cm-1
200
300
400 500 Wavelength nm
600
700
Fig. 7. Raman spectra (632.8 nm excitation) of (a) G, (b) GO and (c) GNS/AgNPs-2.
Fig. 8. UV/vis absorption spectra of (a) GO and (b) GNS/AgNPs-2.
Raman scattering (SERS) from the intense local electromagnetic fields of AgNPs that accompanies plasmon resonance [25].
302 nm, which correspond respectively to –* transitions of aromatic C C bonds and n–* transitions of C O bonds (Fig. 8a) [26]. After GO was doped with Ag particles and reduced (Fig. 8b), the absorption peak of GO dispersion at 227 nm gradually red-shifted to 263 nm, and the shoulder absorption peak at 302 nm disappeared, which indicates that the extensive conjugated sp2 -carbon network is restored. Furthermore, it is also noteworthy that a new peak at 440 nm occurred, which can be assigned to the surface plasmon resonance (SPR) absorption band of Ag nanoparticles, suggesting the formation of AgNPs.
3.6. UV spectra analysis Fig. 8 shows the UV–vis spectra of the dispersions of GO and GNS/AgNPs-2, GO exhibits two characteristic peaks at 227 and Table 1 Raman data of G, GO and GNS/AgNPs-2 samples. Sample
D-band peak (Raman shift/cm−1 )
G-band peak (Raman shift/cm−1 )
ID /IG
G GO GNS/AgNPs-2
1332.36 1323.53 1329.69
1562.19 1572.63 1585.62
0.105 1.135 1.255
ID /IG is the integrated intensity ratio of D-band and G-band.
3.7. Antibacterial test The antibacterial tests are shown in Table 2. It is previously shown that graphene, GO and silver nanoparticles are biocompatible materials [27,28]. Thus, it is possible to use GNS/AgNPs as
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Table 2 Antibacterial activity tests of GNS/AgNPs-2. Bacterial culture
0 min colony counting
32 h colony counting
Disinfection rate (%)
Colibacillus Canidia albicans
5.5 × 106 5.5 × 106
1.3 × 104 1.1 × 104
99.76 99.80
an antibacterial material. As shown in Table 2, the bacterial cultures (Colibacillus and Canidia albicans) are almost killed when the concentration of GNS/AgNPs was 0.05 mg mL−1 , implying that the GNS/AgNPs are expected for potential use in the field of disinfection. 4. Conclusions A facile and green approach to synthesis of GNS/AgNPs is reported by using sodium citrate as the reducing agent. Graphite oxide is completely reduced to graphene, and the silver ion is reduced by sodium citrate simultaneously. Sodium citrate is a superior compound in biological branch owing to its mild reductive ability and nontoxic property. The well-dispersed AgNPs with an average diameter of 20–25 nm are deposited on GNS homogeneously under a suitable dosage of silver ions (230 mg). In addition, the surface plasmon resonance property and antibacterial activity of Ag nanoparticles on graphene provide the use of biocompounds for nontoxic and scalable production of GNS/AgNPs. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 20976057) and Joint Funds of NSFC-Guangdong (U0834004) and research fund of The Guangdong Provincial Engineering Research Center of Green Fine Chemicals, China. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183–191. [3] D. Li, R.B. Kaner, Materials science – graphene-based materials, Science 320 (2008) 1170–1171. [4] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Reviews of Modern Physics 81 (2009) 109–162. [5] Y. Qian, C.Y. Wang, Z.G. Le, Decorating graphene sheets with Pt nanoparticles using sodium citrate as reductant, Applied Surface Science 257 (2011) 10758–10762. [6] C. Xu, X. Wang, J.W. Zhu, Graphene-metal particle nanocomposites, Journal of Physical Chemistry C 112 (2008) 19841–19845.
[7] J. Yang, C.L. Zang, L. Sun, N. Zhao, X.N. Cheng, Synthesis of graphene/Ag nanocomposite with good dispersibility and electroconductibility via solvothermal method, Materials Chemistry and Physics 129 (2011) 270–274. [8] Z.Y. Gao, N. Liu, D.P. Wu, W.G. Tao, F. Xu, K. Jiang, Graphene-CdS composite, synthesis and enhanced photocatalytic activity, Applied Surface Science 258 (2012) 2473–2478. [9] J.C. Liu, L. Liu, H.W. Bai, Y.J. Wang, D.D. Sun, Gram-scale production of graphene oxide-TiO2 nanorod composites: towards high-activity photocatalytic materials, Applied Catalysis B: Environmental 106 (2011) 76–82. [10] G.M. Zhou, D.W. Wang, F. Li, L.L. Zhang, N. Li, Z.S. Wu, L. Wen, G.Q. Lu, H.M. Cheng, Graphene-wrapped Fe3 O4 anode material with improved reversible capacity and cyclic stability for lithium ion batteries, Chemistry of Materials 22 (2010) 5306–5313. [11] Y. Yang, T.X. Liu, Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid, Applied Surface Science 257 (2011) 8950–8954. [12] J. Hranisavljevic, N.M. Dimitrijevic, G.A. Wurtz, G.P. Wiederrecht, Photoinduced charge separation reactions of J-aggregates coated on silver nanoparticles, Journal of the American Chemical Society 124 (2002) 4536–4537. [13] X.P. Sun, S.J. Dong, E.K. Wang, One-step preparation and characterization of poly(propyleneimine) dendrimer-protected silver nanoclusters, Macromolecules 37 (2004) 7105–7108. [14] R. Pasricha, S. Gupta, A.K. Srivastava, A. Facile, Novel synthesis of Ag-graphenebased nanocomposites, Small 20 (2009) 2253–2259. [15] J.F. Shen, M. Shi, N. Li, B. Yan, H.W. Ma, Y.Z. Hu, M.X. Ye, Facile synthesis and application of Ag-chemically converted graphene nanocomposite, Nano Research 3 (2010) 339–349. [16] S. Liu, J.Q. Tian, L. Wang, X.P. Sun, Microwave-assisted rapid synthesis of Ag nanoparticles/graphene nanosheet composites and their application for hydrogen peroxide detection, Journal of Nanoparticle Research 13 (2011) 4539–4548. [17] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, Journal of the American Chemical Society 80 (1958) 1339. [18] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653–2659. [19] C. Nethravathi, M. Rajamathi, Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide, Carbon 46 (2008) 1994–1998. [20] R.V. Hull, L. Li, Y.C. Xing, C.C. Chusuei, Pt nanoparticle binding on functionalized multiwalled carbon nanotubes, Chemistry of Materials 18 (2006) 1780–1788. [21] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. [22] Y. Zhou, Q.L. Bao, L.A.L. Tang, Y.L. Zhong, K.P. Loh, Hydrothermal dehydration for the “Green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties, Chemistry of Materials 21 (2009) 2950–2956. [23] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 3352–3357. [24] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman spectrum of graphene and graphene layers, Physical Review Letters 97 (2006) 187401. [25] J. Li, C.Y. Liu, Ag/graphene heterostructures: synthesis, characterization and optical properties, European Journal of Inorganic Chemistry 8 (2010) 1244–1248. [26] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Graphene oxide dispersions in organic solvents, Langmuir 24 (2008) 10560–10564. [27] O. Akhavan, E. Ghaderi, Photocatalytic reduction of graphene oxide nanosheets on TiO2 thin film for photoinactivation of bacteria in solar light irradiation, Journal of Physical Chemistry C 113 (2009) 20214–20220. [28] A. Panacek, L. Kvitek, R. Prucek, M. Kolar, R. Vecerova, N. Pizurova, V.K. Sharma, T. Nevecna, R. Zboril, Silver colloid nanoparticles: synthesis, characterization, and their antibacterial activity, Journal of Physical Chemistry B 110 (2006) 16248–16253.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Green synthesis of graphene/Ag nanocomposites Wenhui Yuan a,∗ , Yejian Gu a , Li Li b a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangdong, Guangzhou 510640, PR China College of Environmental Science and Engineering, South China University of Technology, Guangdong, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2012 Received in revised form 23 August 2012 Accepted 23 August 2012 Available online 29 August 2012 Keywords: Graphene Graphene/Ag Composites Sodium citrate Environment-friendly
a b s t r a c t Graphene/Ag nanocomposites (GNS/AgNPs) were fabricated via a green and facile method, employing graphite oxide (GO) as a precursor of graphene, AgNO3 as a precursor of Ag nanoparticles, and sodium citrate as an environmentally friendly reducing and stabilizing agent. The synthesized GNS/AgNPs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectra (RS), respectively. The results indicated that graphite oxide was completely reduced to graphene, and the silver ion was reduced by sodium citrate simultaneously. Under a suitable dosage of silver ions, well-dispersed AgNPs on the graphene sheets mostly centralized at 20–25 nm. The surface plasmon resonance property of AgNPs on graphene showed that there was a interaction between AgNPs and graphene supports. In addition, antibacterial activity of silver nanoparticles was retained in the nanocomposites, suggesting that they can be potentially used as a graphene-based biomaterial. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a single-atom thick carbon sheet that is formed by two dimensional layers of sp2 bonded carbon, has attracted tremendous attention due to its unique nanostructure and fascinating properties [1–4]. Recently, graphene decorated with various inorganic nanoparticles, such as Pt, Au, Ag, CdS, TiO2 , Fe3 O4 , and ZnO [5–11], among which Ag nanocomposites (AgNPs) are good candidates for electronics, optics, electrochemistry and catalysis [12,13], GNS/AgNPs has been proved to be a promising material due to its potential applications in many fields. Pasricha et al. [14] prepared GNS/AgNPs in two steps, which GO/AgNPs were synthesized first, and hydrazine was then used to remove the oxygen functionalities. Shen et al. [15] presented a chemical synthesis approach that the chemical reduction of silver ions in chemically converted graphene (CCG) suspensions with mixed reducing agents, ethylene glycol and NaBH4 . Furthermore, it was shown that the antibacterial activity of free AgNPs was retained in the nanocomposites, which suggested that they could be used in the field of graphene-based biomaterials. Liu et al. [16] prepared GNS/AgNPs by microwave-assisted reduction of GO and AgNO3 with DMF as a reducing agent and solvent, and the resulting composites exhibited a good catalytic activity for the reduction
of hydrogen peroxide. However, all these methods reported are required complicated synthetic process, time-consuming or use of extra highly toxic reducing agents which pollute the environment, limiting their further practical applications. Hence, it is highly demand to find a new reducing agent with green and facile characteristic for producing high-quality of GNS/AgNPs. In this work, an environment-friendly and low-cost method was used to produce GNS/AgNPs using sodium citrate as the reducing agent (Fig. 1). The corresponding reduction reaction, the influence of silver nitrate dosage on the particle size and size range of the AgNPs, and the antibacterial activity of silver nanoparticles were investigated for explore its application in the field of disinfection. 2. Experimental 2.1. Raw materials Graphite powders were purchased from Alfa Aesar (Beijing, China) with an average particle size of 45 m. HCl (37%), H2 SO4 (98%), H2 O2 (30%), KMnO4 , KClO4 , NaNO3 , C6 H5 O7 Na3 ·2H2 O, AgNO3 and ethanol were supplied by Sinopharm Chemical Reagent Co., Led (Beijing, China). All chemicals were of analytical grade. 2.2. Preparation of GO
∗ Corresponding author. Present address: School of Chemistry and Chemical Engineering, South China University of Technology, Wushan, Tianhe, Guangzhou 510640, PR China. Tel.: +86 2087111887; fax: +86 2087111887. E-mail address: [email protected] (W. Yuan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.094
Graphite oxide was produced via a modified Hummers’ method [17], using graphite as raw material, KMnO4 , KClO4 , NaNO3 and 98% H2 SO4 as oxidants.
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W. Yuan et al. / Applied Surface Science 261 (2012) 753–758
Na+ O +O OO- OH OH Na OH - OHOH O O O O O OH OH OH OH OH + OH -OH O OH OH Na O O OH Na+ OH OH O + OHO OO Na OH O O OO + OH O AgNO3 O O O OH O Na OH OH + O Na+ O + Na O O OH HO - OHO O OH O O+ O O O O O Na O O + O 95ºC O OH OH OHNaO O OH + OH Na OH OH O OH HO O Agions OH O OH OH OH Agparticle ONa+OH OH OHOH
OH O
Graphite Oxide
Sodium Citrate
OH
Graphene/AgNPs
Fig. 1. Schematic illustration of the reduction process of GNS/AgNPs based on sodium citrate reduction.
2.3. Preparation of GNS/Ag The GNS/AgNPs were prepared in one step reaction. In a typical procedure, 200 mg GO powder was dispersed in 100 mL of water by ultrasonication for 1 h, forming stable graphite oxide colloid, and 50, 230, 410 mg AgNO3 was added under stirring, respectively. Then 1 g sodium citrate was gradually added to the mixture with magnetic stirring for 30 min. Subsequently, the mixture was transferred to an oil bath and kept at 95 ◦ C for 16 h under constant stirring. Finally, the products were washed with ethanol and deionized water by centrifugation, and the resulting GNS/AgNPs were dried in a vacuum oven at 60 ◦ C for 24 h and marked GNS/AgNPs-1, GNS/AgNPs-2, GNS/AgNPs-3 respectively. Fig. 2. XRD patterns of (a) G, (b) GO, (c) GNS/AgNPs-1, (d) GNS/AgNPs-2 and (e) GNS/AgNPs-3.
2.4. Characterization X-ray diffraction (XRD) patterns were recorded by a Bruker-D8 Advance (Germany) powder diffractometer with Cu K␣ radiation ˚ operating at 40 kV and 40 mA. Scanning electron ( = 1.5406 A) microscope (SEM, LEO 1530 VP, Germany) and transmission electron microscope (TEM, JEOL-2100, Japan) were used to observe the morphology of the samples. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Vector 33 (Germany) FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) results were detected on an Axis Ultra DLD photoelectron spectrometer using Al K␣ (1486.6 eV) radiation. Raman spectra were recorded with Horiba Jobin Yvon LabRam Aramis Raman spectrometer, the excitation line at 632.8 nm provided by an Ar+ laser was used. UV–visible spectra were recorded in the range between 200 and 800 nm using Specord 2450 (Shimadzu, Japan). The GNS/AgNPs suspension was diluted 10 times to record the UV–visible spectra. Antibacterial activity tests were performed according to GB/T21510-2008. Colibacillus and Canidia albicans were grown in nutritional broth and stored at 0 ◦ C. The whole process of the control experiment was carried out in a biosafety cabinet and the concentration of GNS/AgNPs was 0.05 mg mL−1 . The disinfection rate R was calculated as follows: R=
A−B × 100% A
where A is the average bacterial counting of the reference sample and B is the average bacterial counting of the test piece.
3. Results and discussion 3.1. X-ray diffraction analysis The XRD patterns of the graphite (G), GO and GNS/AgNPs are shown in Fig. 2a shows that G exhibits a characteristic peak (0 0 2) of graphite at 26.4◦ , which indicates the high crystallinity of this material (JCPDS No. 41-1487). After oxidation (Fig. 2b), the native graphite powder position peak (0 0 2) of graphite at 26.4◦ disappeared, and the (0 0 1) diffraction peak of graphite oxide at 10.4◦ appeared (corresponding to interlayer spacing was 8.5 A˚ from 3.35 A˚ of graphite powder). In the XRD pattern of the GNS/AgNPs (Fig. 2c), there are four main peaks at 2 = 38.2◦ , 44.4◦ , 64.5◦ , 77.5◦ , which correspond 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), respectively, indicating that the metallic AgNPs are formed after reduction. The correspond˚ d2 0 0 = 2.04 A, ˚ ing d-spacing values of the AgNPs are d1 1 1 = 2.37 A, ˚ respectively. It is also observed that d2 2 0 = 1.45 A˚ and d3 1 1 = 1.24 A, the d-spacing value of the synthesized AgNPs using different dosage of the AgNO3 remains almost constant for the each crystallographic plane. 3.2. SEM and TEM analysis Fig. 3 shows the SEM images of GO and GNS/AgNPs with different silver doping dosage. As shown in Fig. 3a, a closely packed lamellar structure can be observed. After the GO was doped with Ag
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Fig. 3. SEM images of GO and GNS/AgNPs, (a) GO, (b) GNS/AgNPs-1, (c) GNS/AgNPs-2, and (d) GNS/AgNPs-3.
particles and reduced (Fig. 3b–d), the Ag crystallites are deposited on graphene surfaces as spacers to keep the neighboring sheets separate, and a curled and corrugated morphology is observed. The size and shape of the AgNPs are also affected by the dosage of AgNO3 . In low dosage of AgNO3 , it is difficult to find that AgNPs are deposited on graphene sheets. After the dosage of AgNO3 increases to 230 mg (Fig. 3c), the AgNPs are well separated from each other and distributed randomly on the graphene sheets as spacers to keep the neighboring sheets separate. When AgNO3 dosage increases to 410 mg (Fig. 3d), the size of AgNPs increased significantly and AgNPs tend to agglomerate. Fig. 4 is the typical TEM images of as-prepared GNS/AgNPs2 hybrids. As shown in Fig. 4a and b, well-dispersed AgNPs are deposited on GNS homogeneously, and the size range of AgNPs mostly distribute at 20–25 nm. On the surface of the GNS/AgNPs, some wrinkles are observed, which may be important for preventing aggregation of GNS and maintaining high surface area with a particular advantage of attaching AgNPs on the graphene sheets. HRTEM image (Fig. 4c) indicates that the lattice spacing of planes is 0.236 nm corresponding to the (1 1 1) crystal plane of Ag, which is consistent with the d-spacing values reported in the (JCPDS No. 04-0783).
3.3. FT-IR spectra analysis FTIR spectra of G, GO and GNS/AgNPs-2 are shown in Fig. 5. It can be seen from Fig. 5b that the peaks at 3414, 1725, 1626, 1371, 1245 and 1060 cm−1 of GO are assigned to the–OH stretching vibrations, C O stretching of COOH groups, skeletal vibrations of unoxidized graphitic domains, O H deformations of the C OH groups, epoxy symmetrical ring deformation vibrations and C O stretching vibrations, respectively [18]. In the FTIR spectrum of GNS/AgNPs-2 (Fig. 5c), the peaks at 1060, 1245, 1371, 1725 cm−1 are become relatively weak. Furthermore, the absorption peak around 1212 cm−1 is attributed to the C OH, and a new absorption band at 1570 cm−1 is attributed to the skeletal vibration of the graphene sheets [19]. The FTIR results demonstrate that the GO have been successfully exfoliated and reduced to GNS, and strong interactions may exist between AgNPs and the remaining surface hydroxyl groups [20]. 3.4. XPS spectra analysis The C1s XPS spectrum of GO (Fig. 6a) clearly indicates oxidation with five components that correspond to carbon atoms in
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Fig. 4. TEM and HRTEM images of GNS/AgNPs-2, (a) and (b) GNS/AgNPs-2 at different magnification, (c) HRTEM image of AgNPs.
removed. This observation is in agreement with those found in previous studies [6,21,22]. On the other hand, as shown in Fig. 6c, the Ag3d spectrum for GNS/AgNPs-2 can be detected and the binding energies of 3d5/2 and 3d3/2 electrons for Ag are identified to be 367.8 and 373.8 eV, respectively [23].
Transmittance(%)
(a) (b) 3429
1371 1725 1626
(c)
1060 1245
3414
3.5. Raman spectra analysis
3396 4000
3500
1570 3000
2500
2000
1212 1500
1000
500
Wavenumber cm-1 Fig. 5. FTIR spectra of samples (a) G, (b) GO and (c) GNS/AgNPs-2.
different functional groups: C C (284.6 eV), C OH (285.6 eV), C (epoxy) (286.7 eV), C O (287.8 eV) and O C O (289 eV). After GO reduction by sodium citrate (Fig. 6b), the peaks assigned to oxygencontaining functional groups are significantly decreased compared with the C1s spectrum of GO, which confirms that most of the hydroxyl, epoxyl and carboxyl functional groups are successfully
Raman spectroscopy is a powerful nondestructive tool to characterize carbonaceous materials, particularly for distinguishing ordered and disordered carbon structures. As seen in Fig. 7, the Raman spectrum of G displays two prominent peaks at 1332 cm−1 (D band) and at 1562 cm−1 (G band), which are usually assigned to the breathing mode of -point phonons of A1g symmetry and the E2g phonon of C sp2 atoms, respectively [24]. In the Raman spectra of GO, the G band is broadened and shifted to 1573 cm−1 and a broadened D band at 1324 cm−1 is also appeared. After chemical reduction of GO, the Raman spectrum of GNS/AgNPs-2 shows a G band at 1586 cm−1 and D band at 1330 cm−1 . The ratio of the intensities of the D and G bands (ID /IG ) increases from 1.135 to 1.255 (Table 1), indicating that there is the existence of reduction procedure of GO. In addition, the peak intensity of GNS/AgNPs-2 is much higher than those of G and GO, which is due to surface-enhanced
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(a)
(b)
C-C/C=C
757
C-C/C=C
C-OH C epoxy C-OH
C=O
C epoxy C=O
O-C=O
280
285 290 Binding Energy(ev)
(c)
295
280
O-C=O
285 290 Binding Energy(ev)
295
367.8
373.8
364
368
372
376
380
Binding Energy(ev) Fig. 6. C1s XPS spectra of (a) GO and (b) GNS/AgNPs-2, Ag3d XPS spectrum of (c) GNS/AgNPs-2.
D
227 nm
G
Intensity (a.u.)
Intensity(a.u.)
263 nm
(c) (b)
302 nm
440 nm
(b) (a)
(a)
1000
1200
1400
1600
1800
2000
Raman shift/cm-1
200
300
400 500 Wavelength nm
600
700
Fig. 7. Raman spectra (632.8 nm excitation) of (a) G, (b) GO and (c) GNS/AgNPs-2.
Fig. 8. UV/vis absorption spectra of (a) GO and (b) GNS/AgNPs-2.
Raman scattering (SERS) from the intense local electromagnetic fields of AgNPs that accompanies plasmon resonance [25].
302 nm, which correspond respectively to –* transitions of aromatic C C bonds and n–* transitions of C O bonds (Fig. 8a) [26]. After GO was doped with Ag particles and reduced (Fig. 8b), the absorption peak of GO dispersion at 227 nm gradually red-shifted to 263 nm, and the shoulder absorption peak at 302 nm disappeared, which indicates that the extensive conjugated sp2 -carbon network is restored. Furthermore, it is also noteworthy that a new peak at 440 nm occurred, which can be assigned to the surface plasmon resonance (SPR) absorption band of Ag nanoparticles, suggesting the formation of AgNPs.
3.6. UV spectra analysis Fig. 8 shows the UV–vis spectra of the dispersions of GO and GNS/AgNPs-2, GO exhibits two characteristic peaks at 227 and Table 1 Raman data of G, GO and GNS/AgNPs-2 samples. Sample
D-band peak (Raman shift/cm−1 )
G-band peak (Raman shift/cm−1 )
ID /IG
G GO GNS/AgNPs-2
1332.36 1323.53 1329.69
1562.19 1572.63 1585.62
0.105 1.135 1.255
ID /IG is the integrated intensity ratio of D-band and G-band.
3.7. Antibacterial test The antibacterial tests are shown in Table 2. It is previously shown that graphene, GO and silver nanoparticles are biocompatible materials [27,28]. Thus, it is possible to use GNS/AgNPs as
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Table 2 Antibacterial activity tests of GNS/AgNPs-2. Bacterial culture
0 min colony counting
32 h colony counting
Disinfection rate (%)
Colibacillus Canidia albicans
5.5 × 106 5.5 × 106
1.3 × 104 1.1 × 104
99.76 99.80
an antibacterial material. As shown in Table 2, the bacterial cultures (Colibacillus and Canidia albicans) are almost killed when the concentration of GNS/AgNPs was 0.05 mg mL−1 , implying that the GNS/AgNPs are expected for potential use in the field of disinfection. 4. Conclusions A facile and green approach to synthesis of GNS/AgNPs is reported by using sodium citrate as the reducing agent. Graphite oxide is completely reduced to graphene, and the silver ion is reduced by sodium citrate simultaneously. Sodium citrate is a superior compound in biological branch owing to its mild reductive ability and nontoxic property. The well-dispersed AgNPs with an average diameter of 20–25 nm are deposited on GNS homogeneously under a suitable dosage of silver ions (230 mg). In addition, the surface plasmon resonance property and antibacterial activity of Ag nanoparticles on graphene provide the use of biocompounds for nontoxic and scalable production of GNS/AgNPs. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 20976057) and Joint Funds of NSFC-Guangdong (U0834004) and research fund of The Guangdong Provincial Engineering Research Center of Green Fine Chemicals, China. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183–191. [3] D. Li, R.B. Kaner, Materials science – graphene-based materials, Science 320 (2008) 1170–1171. [4] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Reviews of Modern Physics 81 (2009) 109–162. [5] Y. Qian, C.Y. Wang, Z.G. Le, Decorating graphene sheets with Pt nanoparticles using sodium citrate as reductant, Applied Surface Science 257 (2011) 10758–10762. [6] C. Xu, X. Wang, J.W. Zhu, Graphene-metal particle nanocomposites, Journal of Physical Chemistry C 112 (2008) 19841–19845.
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