A facile and efficient approach to decoration of graphene nanosheets with gold nanoparticles

Applied Surface Science 258 (2012) 5348–5353

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

A facile and efficient approach to decoration of graphene nanosheets with gold nanoparticles Zhang-Gao Le a , Zhirong Liu a , Yong Qian b , Chunyan Wang a,∗ a b

Department of Applied Chemistry, East China Institute of Technology, Fuzhou 344000, China Department of Materials Science and Engineering, East China Institute of Technology, Fuzhou 344000, China

a r t i c l e

i n f o

Article history: Received 5 October 2011 Received in revised form 24 January 2012 Accepted 31 January 2012 Available online 8 February 2012 Keywords: Graphene nanosheets Poly(diallyldimethylammonium chloride) (PDDA) Gold nanoparticles

a b s t r a c t In this study, we present a simple and effective approach to the in situ decoration of gold nanoparticles on graphene nanosheets (AuNPs@GNs), employing graphene oxide (GO) as precursor and poly(diallyldimethylammonium chloride) (PDDA) as environmentally friendly reducing agent and stabilizer. The microstructures of as-prepared GO and AuNPs@GNs were characterized in detail. The results confirmed that the high dispersion of AuNPs with mean particle size around 4.1 nm on the surface of graphene nanosheets (GNs) could be easily obtained via using PDDA as reductant; moreover the AuNPs@GNs hybrids exhibited excellent electrochemical activity. This work presents a facile approach to synthesize GNs and opens up a new possibility for preparing graphene and graphene-based nanomaterials for large-scale applications. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of graphene by Geim et al. in 2004 [1], it has been touted as a “next generation material” because of its remarkable electronic, optical and thermal properties, chemical and mechanical stability, and large surface area for applications in various fields such as field-effect transistors [2], sensors [3], electrochemical devices [4], electromechanical resonators [5], polymer nanocomposites [6,7], batteries [8], capacitors [9,10]. At present, there are several methods reported to prepare graphene, such as chemical vapor deposition [11], micromechanical exfoliation of graphite using peel-off method with Scotch-tape [12], and epitaxial growth on electrically insulating surface [13]; these three methods yield graphene nanosheets (GNs) with good quality [14], which are more suitable for fabrication of graphene-based electronic devices. Other methods including bottom-up synthesis of graphene from organic molecules [15,16] and the reduction or deoxygenation of graphene oxide (GO) [17–20] are more realistic approaches to produce graphene in gram-level. Generally, the reduction of GO is carried out by chemical methods using different reductants, e.g. hydrazine, dimethylhydrazine, hydroquinone and NaBH4 [21–24]. The demerit of the reduction process is that it is very slow and the reductants used are rather toxic. Moreover graphene nanosheets (GNs) tend to form irreversible agglomerates through van der Waals attractive force,

∗ Corresponding author. Tel.: +86 794 8258320; fax: +86 794 8258320. E-mail address: [email protected] (C. Wang).

which leads to a great technical difficulty in the applications of graphene [25,26]. Recently, great efforts have been devoted to addressing this issue, among which attaching some molecules or polymers onto the surface of GNs is a promising approach to obtain well-dispersed graphene suspension [27]. As we all know, hybridization provides an effective strategy to enhance the functionality of nanomaterials [28]. Metal nanoparticles on carbon nanotubes (CNTs) composites have been applied in some field due to fascinating properties [29–31]; however, the high production cost of CNTs has placed an obstacle in their applications. Although CNTs have large surface areas, nanotube bundling and incomplete functionalization makes the accessible areas very limited. Moreover, depositing metal NPs on CNTs surface, functionalization needs to be done employing nitric and sulfuric acids. In comparison with CNTs, graphene possesses similar stable physical properties but larger surface areas, which can be considered as an unrolled CNT [24,32]. Furthermore, GO nanosheets can be easily obtained by chemical exfoliation of natural graphites. The production cost of GNs in large quantities is much lower than that of CNTs. Additionally using GO as a starting material for deposition of metal NPs has several advantages. Primarily, GO has large surface areas and both sides of nanosheets are accessible. Next, the abundance of functional groups (such as carboxyl, carbonyl, hydroxyl and epoxide) on the surfaces and edges of GO allows for favorable preparation of nanocomposites. Finally, GO can be easily converted by chemical reduction methods to graphene, which offers better electrical conductivity. Therefore it is very necessary to load nanoparticles on the surface of GNs for maximizing the availability of nanosized

0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.169

Z.-G. Le et al. / Applied Surface Science 258 (2012) 5348–5353

5349

Fig. 1. Scheme showing a proposed formation route to anchor Au nanoparticles (AuNPs) onto graphene nanosheets (GNs). (1) Oxidation of graphite (gray planes) to graphite oxide (yellow planes) with greater interlayer distance. (2) Exfoliation of graphite oxide by sonication in water solution. (3) Addition of Au ions to graphene oxide (GO) solution. (4) Formation of graphene-supported AuNPs hybrids (AuNPs@GNs) by reduction of the GOs and Au ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

electrocatalyst surface area for electron transfer and providing a better mass transport of reactants to the electrocatalyst. Deposition of metal oxide nanoparticles, such as MnO2 [33], TiO2 [34], Mn3 O4 [35], NiO [36] Co(OH)2 [37], ZnO [38], and decoration of noble metal nanoparticles, such as Pt [39–41], Ag [42,43], Au [44,45], onto GNs has been demonstrated to reveal special features and applications in new hybrids that can be widely utilized in sensors [46,47], catalysts [48–50], photocatalysts[51], Li-ion batteries, supercapacitors and electrocatalysts [52–54]. However, assembling Au nanoparticles in nanostructured materials with electronic and ionic conduction pathways for electrochemical applications still remains a challenge [55,56]. Herein, we report a facile and efficient route to disperse AuNPs on GNs using ordinary polyelectrolyte (PDDA) as reducing agent and stabilizer. We have successfully prepared AuNPs with the uniform size distribution on the surface of GNs without agglomerates through this approach; moreover the AuNPs@GNs hybrids exhibited high electrochemical activity. The microstructures and electrochemical activity of GO and AuNPs@GNs were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electrochemical measurements. Notably, the simple and straightforward synthesis of this procedure is expected to be a widely accepted approach with potential applications in future.

2.2. Synthesis of AuNPs@GNs In a typical synthesis, natural graphite powders were oxidized to GO using a modified Hummers method [57]. The AuNPs@GNs hybrid materials were prepared in one pot reaction. 100 mg GO powder in 100 mL H2 O was sonicated for 30 min to form a homogeneous GO suspension, and 5 mL of 50 wt% PDDA solution was mixed with it under vigorous stirring for 30 min. Next the solution was heated in refluxing conditions for 5 h. During this process, the color of the solution changed from yellow-brown to black. Then 4 mL HAuCl4 ·3H2 O (0.05 M) was added dropwise under vigorous stirring for 10 min. Subsequently 10 mL of 50 wt% PDDA solution was added dropwise to reduce HAuCl4 . After 24 h of stirring, the AuNPs@GNs suspension was filtered and washed with deionized water several times then dried at 60 ◦ C in vacuum for 3 h. 2.3. Preparation of GO, AuNPs@GNs modified electrode Prior to use, glassy carbon electrode (GCE, ˚ =3 mm) was polished with 0.05 ␮m gamma alumina powders, then rinsed thoroughly with ethanol and distilled water in an ultrasonic bath to remove any alumina residues, and finally dried with blowing N2 gas. Ten microliters of sonicated pure GO or AuNPs@GNs DMF suspension was dropped on the pretreated bare GCE using a micropipette tip and dried in air. After the film was dried, nafion solution (1.5% in ethanol) was pipetted on the catalyst surface for protection.

2. Experimental 2.4. Characterization 2.1. Raw materials Graphite powder (99.9995% purity, 100 mesh, briquetting grade) was purchased from Alfa Aesar. HAuCl4 ·3H2 O, PDDA and KMnO4 were obtained from Shanghai Chemical Reagents Company. All chemicals were of analytical grade and all solutions used in the electrochemical experiments were freshly prepared with Millipore water having a resistivity of 18.2 M (Pure Lab Classic Corp., USA).

TEM images were obtained from JEM-2100 (Japan) with a 200 kV accelerating voltage. The TEM samples were prepared by drying a droplet of the GO or AuNPs@GNs suspensions on a Cu grid. AFM images were obtained on an Agilent 5500 AFM/SPM system with Picoscan v 5.3.3 software in tapping mode under ambient conditions. An X-ray powder diffractometer (XRD, Shimadzu, X-6000, Cu K␣ radiation) was used to determine the phase purity and crystallization degree. Raman spectra were recorded using a

5350

Z.-G. Le et al. / Applied Surface Science 258 (2012) 5348–5353

Fig. 2. (a and b) Typical AFM and TEM image of GO, (c) TEM image of AuNPs@GNs and (d) XPS spectrum of Au 4f of the as-prepared AuNPs@GNs composites deposited on a glass slide. The upright inset of (b) and (c) are digital photo of GO (yellow) and AuNPs@GNs (black) dispersed in water (1 mg/mL) respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Renishaw InVia micro-Raman system with an excitation wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS, K␣) analyses were carried out on a Thermo Fisher X-ray photoelectron spectrometer system equipped with Al radiation as a probe, with a chamber pressure of 5 × 10−9 Torr. The source power was set at 72 W, and pass energies of 200 eV for survey scans and 50 eV for high-resolution scans were used. The analysis spot size was 400 ␮m in diameter. All electrochemical measurements were performed on a CHI 660 electrochemical workstation (CH Instruments, USA). Cyclic voltammograms were collected in a three-electrode system (GCE or modified GCE as the working electrode, a Pt wire as counter electrode, and an Ag/AgCl (sat. KCl) as the reference) at room temperature. 3. Results and discussion Fig. 1 represents the schematic procedure of the preparation of AuNPs@GNs composites. Firstly, hydrophilic graphite oxide was prepared by chemical oxidation of graphite according to modified Hummers’ method described in the literature [57]. Then, individual graphene oxide (GO) nanosheets were obtained by exfoliating graphite oxide under ultrasonication in water. Thereafter, HAuCl4 was added to the GO nanosheets colloid suspension under stirring. Finally, the composites of graphene decorated by AuNPs were obtained via chemical reduction of GO and Au ion using PPDA as reductant and stabilizer. Atomic force microscopy (AFM) can directly characterize the morphologies and layers of GO. The samples were prepared by

dropping GO dispersed in water onto clean silicon wafers and dried at room temperature. Fig. 2a shows the AFM image of the exfoliated GO. Flattened GO nanosheets appeared with an average thickness of about 1.0 nm, it is somewhat thicker than the interlayer spacing of GO (0.78 nm). It could be ascribed to the presence of functional groups derived during the fabrication process. Morphologies of the obtained GO and AuNPs@GNs were also characterized by TEM. As shown in Fig. 2b, the GO nanosheets are randomly compact and stacked together, showing uniform laminar morphology like crumpled silk veil waves. The image of as-prepared AuNPs@GNs is shown in Fig. 2c. Highly dispersed AuNPs evenly distributed on GNs may provide large available surface and enhance the electrocatalytic activity. XPS pattern of the resulting AuNPs@GNs shows significant Au 4f signals corresponding to the binding energy of Au (Fig. 2d) and further supports that AuNPs have been effectively assembled on the surface of GNs. The dispersion of GO and AuNPs@GNs respectively in water (1 mg/mL) are shown in the upright inset of (b) and (c). The homogeneous colloidal suspensions of GO and AuNPs@GNs dispersed in H2 O can keep steadily for several months. It indicated clearly that the dispersion effect in water was very well. PDDA can adsorb on the surface of GNs through ␲–␲ interaction and electrostatic interaction, which results in electrostatic repulsion between GNs to prevent them from aggregating. AuCl4 − was confined in the positively charged PDDA via electrostatic interaction. The presence of PDDA prevents AuNPs from agglomeration. X-ray photoelectron spectroscopy (XPS) was performed to further confirm the removal of the oxygen-containing groups and formation of AuNPs. The C 1s XPS spectrum of GO as shown in

Z.-G. Le et al. / Applied Surface Science 258 (2012) 5348–5353

C (002)

(a)

C1s

5351

Au (111)

C-OH

Au (200)

Intensity (a.u.)

Intensity (a.u.)

C=C

C-O-C O-C=O

Au(220)

C (002)

AuNPs@GNs C (100)

GO 280

282

284

286

288

290

292

294

10

20

30

Binding energy (eV)

40

50

60

70

2 theta (degree)

C1s

(b)

Fig. 4. XRD patterns of as-prepared GO and AuNPs@GNs. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

280

C-OH C-O-C

282

284

286

O-C=O

288

290

292

Binding energy (eV) Fig. 3. (a) The high resolution C 1s XPS spectrum of GO, and (b) C 1s spectra of AuNPs@GNs.

Fig. 3a clearly indicates oxidation with four types of carbon with peaks centered at 284.5, 286.2, 287.2, and 288.4 eV, corresponding to C C, C OH, C O C, and O C O groups, respectively. Fig. 3b shows clearly that the peak intensity of some oxygenated functional groups on graphene sheets after AuNPs deposition decreases, especially, the hydroxyl groups underwent considerable deoxygenation compared with GO (Fig. 3a), which could be assumed that hydroxyl groups act as anchoring sites for AuNPs. Meanwhile, the intensity of epoxy and carboxylic groups was also reduced, which may be owing to the reduction by PDDA and depositing Au nanoparticles. From these results, we believed that the GO could also be easily reduced by PDDA in our system. In general, the chemical reduction of GO always uses toxic reductants, such as, hydrazine, dimethylhydrazine, hydroquinone. Therefore, such findings may enable us to synthesize graphene using harmless chemical reagents in the future. The as-obtained GO and AuNPs@GNs are also characterized by XRD. As shown in Fig. 4, the XRD pattern of GO exhibits a diffraction peak centered at 2 = 11.4◦ , corresponding to the C (0 0 2) interlayer spacing of 0.78 nm, which indicates the complete oxidation of graphite to GO. After chemical reduction with PDDA, the diffraction peak at 2 = 11.4◦ disappears and a very broad peak around 23◦ which is assigned to the hexagonal graphite structures C (0 0 2) is observed in the AuNPs@GNs (green line), indicating that most oxygen functional groups were removed. The sharp peaks of the AuNPs-loaded graphene at 37.9◦ (1 1 1), 44.2◦ (2 0 0) and 65.6◦ (2 2 0) are characteristic of the crystalline nanodomains of gold, which are consistent with the standard card of cubic Au (JCPDS

No. 04-784). Simultaneously it confirms that the HAuCl4 has been reduced to AuNPs by PDDA. The mean diameter of the Au nanocrystals was calculated to be around 4.1 nm, which was in agreement with estimation of TEM. Raman spectroscopy is the most direct and nondestructive technique to characterize the structure and quality of carbon materials, particularly to determine the defects, the ordered and disordered structures, and the layers of graphene. G band is usually assigned to the E2g phonon of C sp2 atoms, while D band is a breathing mode of ␬-point phonons of A1g symmetry [58,59]. Fig. 5 shows the Raman spectra of GO (red line) and AuNPs@GNs (green line), exhibiting two remarkable peaks at around 1350 and 1580 cm−1 corresponding to the well-defined D band and G band, respectively. The frequencies of AuNPs@GNs and GO are similar. But AuNPs@GNs has an increased D/G intensity (ID /IG ) ratio relative to GO. The ID /IG value increases from 0.95 to 1.13. This significant enhancement in the ratio of GNs indicates a decrease in the size of the in-plane sp2 domains and a partially ordered crystal structure of GNs. Fe(CN)6 3−/4− redox probe is selected to study the electrochemical properties of AuNPs@GNs. Cyclic voltammograms (CVs) curves of the bare GCE, GO/GCE and AuNPs@GNs/GCE are shown in Fig. 6a. The quasi-reversible one-electron redox behavior of ferricyanide ions is observed on the above three electrodes in 0.1 M KCl solution containing 5.0 mM K3 Fe(CN)6 . It is observed that the peak current of GCE modified by GO (red line), decreases relative to that

D

G

2D

Intensity (a.u.)

Intensity (a.u.)

C=C

D+G

AuNPs@GNs

GO 1000

1500

2000

2500

3000

Raman Shift (cm-1) Fig. 5. Raman spectra of as-prepared GO and AuNPs@GNs.

5352

Z.-G. Le et al. / Applied Surface Science 258 (2012) 5348–5353

0.12

(a) AuNPs@G/GCE

Current (mA)

0.09 0.06 0.03

bare GCE 0.00

GO/GCE

Simultaneously, a uniform distribution of AuNPs on the surface of GNs without agglomerates can be obtained through this route. Moreover, the as-prepared conductive nanocomposites could efficiently enhance the respective electrochemical response, suggesting that the AuNPs@GNs nanocomposites could afford a novel strategy to prepare the promising nanomaterial for the highly sensitive electrochemical sensors. Further study is underway in our lab. Acknowledgements

-0.03 -0.06 -0.09 0.0

0.1

0.2

0.3

0.4

0.5

Potential (V vs Ag/AgCl) 0.10

References

(b)

AuNPs@GNs/GCE

Current (mA)

0.08 0.06

bare GCE 0.04 0.02

GO/GCE 0.00

-0.2

This work was supported by the National Natural Science Foundation of China (No. 21004009), the Foundation of Jiangxi Educational Committee (No. GJJ11487), Natural Science Foundation of JiangXi Province (20114BAB213010) and the start-up funds of the East China Institute of Technology.

0.0

0.2

0.4

0.6

Potential (V vs SCE) Fig. 6. Cyclic voltammograms (CVs) obtained at a bare glass carbon electrode (GCE), GO/GCE, and AuNPs@GNs/GCE at the scan rate of 20 mV s−1 . (a) 0.1 M KCl solution containing 5.0 mM K3 Fe(CN)6 and (b) 0.1 M KCl containing 1 mM ascorbic acid (AA)/PBS solution (pH 7.40) in saturated nitrogen gas.

of bare GCE (blue line), suggesting that the GO acts as an insulating layer which makes the interfacial charge transfer difficult. AuNPs@GNs (green line) exhibits a higher electrochemical activity than the other two, indicating that the introduction of the GNs plays an important role in the increase of the electroactive surface area and providing the conductive bridges for the electron-transfer of Fe(CN)6 3−/4− . Moreover these effects are attributed to increase surface area and surface roughness due to the amount of AuNPs loaded on GNs. To further demonstrate the electrocatalytic activity of AuNPs@GNs, the electrochemical behaviors of ascorbic acid (AA), as the biomolecule which prevents scurvy and takes part in several biological reactions, was investigated. Fig. 6b shows CVs obtained from three electrodes in phosphate buffer solution (PBS, pH = 7.40) containing 1 mM AA. A larger peak current and lower overpotential of AA on the AuNPs@GNs-modified electrode appears as compared to the bare GCE (blue line) and GO/GCE (red line), which further reveal that AuNPs@GNs are excellent materials for electrochemical sensing and activity. This electrocatalytic activity toward AA could be due to the unique electronic properties of GNs and large surface area and high surface roughness of increased amount of AuNPs which accelerate electron transfer rate via improved conductivity and the good affinity of AuNPs@GNs to AA. 4. Conclusion In summary, we have demonstrated a simple and effective approach to the decoration of AuNPs on GNs by using PDDA as eco-friendly reducing agent and stabilizer for the first time.

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, Science 306 (2004) 666–669. [2] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 319 (2008) 1229–1232. [3] C.H. Lu, H.H. Yang, C.L. Zhu, X. Chen, G.N. Chen, Angew. Chem. Int. Ed. 48 (2009) 4785–4787. [4] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Nat. Mater. 6 (2007) 652–655. [5] J.S. Bunch, A.M. Zande, S.S. Verbridge, I.W. Frank, D.M. Tanenbaum, J.M. Parpia, H.G. Craighead, P.L. McEuen, Science 315 (2007) 490–493. [6] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso, R.D. Piner, D.H. Adamson, H.C. Schniepp, X. Chen, R.S. Ruoff, et al., Nat. Nanotechnol. 3 (2008) 327–331. [7] C. Lv, Q. Xue, D. Xia, M. Ma, Appl. Surf. Sci. (2011), doi:10.1016/ j.apsusc.2011.04.056. [8] E. Yoo, J. Kim, E. Hosono, H.S. Zhou, T. Kudo, I. Honma, Nano Lett. 8 (2008) 2277–2282. [9] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498–3502. [10] Y. Qian, S.B. Lu, F.L. Gao, J. Mater. Sci 46 (2011) 3517–3522. [11] S. Park, R.S. Ruoff, Nat. Nanotechnol. 29 (2009) 217–224. [12] F.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, et al., Nature 457 (2009) 706–710. [13] X. Lu, M. Yu, H. Huang, R.S. Ruoff, Nanotechnology 10 (1999) 269–272. [14] S. Bae, H. Kim, Y. Lee, X.F. Xu, J.S. Park, Y. Zheng, et al., Nat. Nanotechnol. 5 (2010) 574–578. [15] J.S. Wu, W. Pisula, K. Müllen, Chem. Rev. 107 (2007) 718–747. [16] Q. Kuang, S.Y. Xie, Z.Y. Jiang, X.H. Zhang, Z.X. Xie, R.B. Huang, et al., Carbon 42 (2004) 1737–1741. [17] S. Stankovich, Carbon 45 (2007) 1558–1565. [18] S. Park, R.S. Ruoff, Nat. Nanotechnol. 4 (2009) 217–224. [19] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev. 39 (2010) 228–240. [20] S.Z. Zu, B.H. Han, J. Phys. Chem. C 113 (2009) 13651–13657. [21] V.C. Tung, M.J. Allen, Y. Yang, R.B. Kaner, Nat. Nanotechnol. 4 (2009) 25–29. [22] G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, et al., J. Phys. Chem. C 112 (2008) 8192–8295. [23] S. Stankovich, R.D. Piner, X.Q. Chen, N.Q. Wu, S.T. Nguyen, R.S. Ruoff, J. Mater. Chem. 16 (2006) 155–158. [24] S. Niyogi, E. Bekyarova, M.E. Itkis, J.L. McWilliams, M.A. Hamon, R.C. Haddon, J. Am. Chem. Soc. 128 (2006) 7720–7721. [25] Y.Y. Liang, D.Q. Wu, X.L. Feng, K. Mullen, Adv. Mater. 21 (2009) 1679–1683. [26] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101–105. [27] Y.X. Xu, H. Bai, G.W. Lu, C. Li, G.Q. Shi, J. Am. Chem. Soc. 130 (2008) 5856–5857. [28] K. Jasuja, V. Berry, ACS Nano 3 (2009) 2358–2366. [29] V. Georgakilas, D. Gournis, V. Tzitzios, L. Pasquato, D.M. Guldi, M. Prato, J. Mater. Chem. 17 (2007) 2679–2694. [30] Y.C. Xing, J. Phys. Chem. B 108 (2004) 19255–19259. [31] G. Girishkumar, T.D. Hall, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 110 (2006) 107–114. [32] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Nature 448 (2007) 457–460. [33] S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano 4 (2009) 2822–2830. [34] D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, D. Hu, C. Wang, L.V. Saraf, J. Zhang, I.A. Aksay, J. Liu, ACS Nano 3 (2009) 907–914. [35] H. Wang, L. Cui, Y. Yang, Y. Cui, H. Dai, J. Am. Chem. Soc. 132 (2010) 13978–13980. [36] Z.Y. Ji, J.L. Wu, X.P. Shen, H. Zhou, H.T. Xi, J. Mater. Sci. 46 (2011) 1190–1195. [37] S. Chen, J.W. Zhu, X. Wang, J. Phys. Chem. C 114 (2010) 11829–11834. [38] J. Wu, X. Shen, L. Jiang, K. Wang, K. Chen, Appl. Surf. Sci. 256 (2010) 2826–2830. [39] Y.J. Li, W. Gao, L.J. Ci, C.M. Wang, Carbon 48 (2010) 1124–1130. [40] Y. Qian, C. Wang, Z.G. Le, Appl. Surf. Sci. 257 (2011) 10758–10762. [41] C. Xu, X. Wang, J. Zhu, J. Phys. Chem. C 112 (2008) 19841–19845.

Z.-G. Le et al. / Applied Surface Science 258 (2012) 5348–5353 [42] [43] [44] [45] [46] [47] [48] [49] [50]

C. Xu, X. Wang, Small 5 (2009) 2212–2217. W.B. Lu, G.H. Chang, Y.L. Luo, F. Liao, X.P. Sun, J. Mater. Sci. 46 (2011) 5260–5266. S.H. Yan, S.C. Zhang, Y. Lin, G.R. Liu, J. Phys. Chem. C 115 (2011) 6986–6993. Z.Q. Zhang, Y.H. Wu, Langmuir 26 (2010) 9214–9223. C.L. Nehl, H. Liao, J.H. Hafner, Nano Lett. 6 (2006) 683. S. Pierrat, I. Zins, A. Breivogel, C. Solnnichsen, Nano Lett. 7 (2007) 259. M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. M.S. Chen, D.W. Goodman, Science 306 (2004) 252. M.D. Hughes, Y.J. Xu, P. Jenkins, P. McMorn, P. Landon, D.I. Enache, A.F. Carley, G.A. Attard, G.J. Hutchings, F. King, E.H. Stitt, P. Johnston, K. Griffin, C.J. Kiely, Nature 437 (2005) 1132. [51] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Science 316 (2007) 732.

5353

[52] H.G. Liao, Y.X. Jiang, Z.Y. Zhou, S.P. Chen, S.G. Sun, Angew. Chem. Int. Ed. 47 (2008) 9100. [53] J. Zhang, K. Sasaki, E. Sutter, R.R. Adzic, Science 315 (2007) 220. [54] B. Lim, M. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X. Lu, Y. Zhu, Y. Xia, Science 324 (2009) 1302. [55] F.L. Leibowitz, W.X. Zheng, M.M. Maye, C. Zhong, J. Anal. Chem. 71 (1999) 5076–5083. [56] J.H. Fendler, Chem. Mater. 8 (1996) 1616–1624. [57] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, A.D. Gorchinskiy, Chem. Mater. 11 (1999) 771–778. [58] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126–1130. [59] S.J. Guo, S.J. Dong, E.K. Wang, ACS Nano 4 (2010) 547–555.