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Electrochemical detection of hydroquinone by graphene and Pt-graphene hybrid material synthesized through a microwave-assisted chemical reduction process
Electrochimica Acta 56 (2011) 2712–2716
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical detection of hydroquinone by graphene and Pt-graphene hybrid material synthesized through a microwave-assisted chemical reduction process Jing Li a,b , Chun-yan Liu a,∗ , Chao Cheng a,b a Key Laboratory of Photochemical Conversion and Optoelectronic Materials of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100806, PR China
a r t i c l e
i n f o
Article history: Received 17 June 2010 Received in revised form 17 September 2010 Accepted 14 December 2010 Available online 21 December 2010 Keywords: Hydroquinone Graphene Pt-graphene hybrid material Electrochemical detection Differential pulse voltammetry
a b s t r a c t We have synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application as electrode materials towards the electrochemical detection of hydroquinone. Graphene modified glass carbon electrode (GCE) showed a good performance for detecting hydroquinone due to the unique properties of graphene which increased the active surface area of the electrode and accelerated the electron transfer. The linear detection range of hydroquinone concentration was 20–115 M with a sensitivity of 1.38 A M−1 cm−2 ; the detection limit was estimated to be 12 M (S/N = 3). The electrocatalytic activity of the Pt-graphene modified GCE was further improved due to the enhanced electron transfer and the linear detection range was 20–145 M with the sensitivity of 3.56 A M−1 cm−2 , detection limit 6 M (S/N = 3). © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Hydroquinone is a phenolic compound which is important in a wide number of biological and industrial processes such as coal–tar production, paper manufacturing and photographic developers, and is considered as an important xenobiotic micropollutant [1]. Several analytical methods have been used to detect hydroquinone, including high performance liquid chromatography [2,3], flow injection analysis [4,5], spectrophotometry [6,7], and electrochemical methods [8,9], among which the electrochemical methods have attracted great attentions owing to the advantages such as efficiency, simplicity and quick response. In previous work, several carbon-based materials such as mesoporous carbon [10], borondoped diamond [11], conductive carbon cement [12], carbon fibre [13], and carbon nanotubes (CNTs) [14] have been explored for the electrochemical detection of hydroquinone. Graphene (G), a monolayer of carbon atoms packed into a dense, honeycomb crystal structure as well as being a fundamental building block for fullerenes, carbon nanotubes, and graphite [15], has shown fascinating properties and holds the promise for future carbon-based device architectures [16–19]. Recently, graphenebased materials have been explored for electrochemical sensor due to its large surface area, extraordinary electronic transport prop-
erties, strong mechanical strength, and its lower cost and easier preparation in mass quantities compared with carbon nanotubes, typical examples including detection of glucose by metal decorated graphene [20] and nitrogen-doped graphene [21] modified glass carbon electrode, electrochemical detection of paracetamol by graphene modified glass carbon electrode [22], and selective detection of dopamine by graphene modified glass carbon electrode [23]. However, to the best of our knowledgement, there is no report about the eletrochemical detection of hydroquinone by graphene-based materials. Up to now, numerous methods have been developed to synthesize graphene such as micromechanical cleavage of graphite [24], chemical vapor deposition technique [25], epitaxial growth on a single-crystal silicon carbide by vacuum graphitization [26], and chemical reduction of graphene oxide [27–31]. Among them, the chemical reduction of graphene oxide, involving graphite oxidation, exfoliation and reduction, is the most efficient approach to bulk production of graphene-based sheets at low cost. In this context, we synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application to electrochemical detection of hydroquinone. 2. Experimental 2.1. Preparation of graphene
∗ Corresponding author. Tel.: +86 010 82543573; fax: +86 010 62554670. E-mail address: [email protected] (C.-y. Liu). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.046
Graphene was prepared through a microwave-assisted chemical reduction of graphene oxide. In a typical procedure, graphite
J. Li et al. / Electrochimica Acta 56 (2011) 2712–2716
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oxide (25 mg) prepared from natural graphite powders (Beijing Chemical Factory of China) by Hummer’s method [32] was sonicated in water (50 mL) for 2 h using a JK-300 ultrasonic cleaner (300 W, 40 kHz) to achieve a clear, brown dispersion of graphene oxide (0.5 mg/mL). 5 mL of hydrazine hydrate (80%) was added into the graphene oxide and kept stirring for 3 h at 100 ◦ C in a microwave oven with program-control (Sineo MAS-II). The heating power was set to 300 W at the beginning of the reaction and automatically reduced to as low as about 10 W when a given temperature reached. The stirring rate was set to 700 rps. The black graphene was obtained after centrifugation, washed with water and ethanol, and naturally dried in air. 2.2. Preparation of Pt-graphene hybrid material The route to prepare Pt-graphene hybrid material was similar to graphene, except that 5 mL of hexachloroplatinic acid solution (0.0193 M) was added to the graphene oxide solution and kept stirring for 15 min at room temperature before the addition of hydrazine hydrate. 2.3. Preparation of graphene and Pt-graphene modified glass carbon electrode Prior to modification, the glassy carbon electrodes (GCE) of 3 mm diameter were polished with 0.5 m alumina powder, washed with deionized water and then dried in air. Graphene-based materials were dispersed in N,N-dimethylformamide (DMF) by sonication for 30 min to achieve a 2 mg/mL graphene-DMF suspension. Then, 5 L of the suspension was coated onto GCE and allowed to dry in air. 2.4. Characterization The AFM images were recorded using a Multimode Nanoscope IIIa AFM (Veeco Metrology LLC, Santa Barbara, CA). Tapping-mode imaging was applied to provide the largest amount of structural detail of the graphene oxide sheets as well as to prevent translocation of the sheets on the surface by the tip (Veeco MP-11100 silicon cantilevers, force constant k = 60 N/m, radius of curvature r = 10 nm, and resonance frequency f = 300 kHz). The sample was prepared by depositing graphene oxide dispersion in water (0.05 mg/mL) onto a new cleaved mica surface and dried under vacuum at room temperature. The TEM images were taken with a JEM 2100F transmission electron microscope, by using an accelerating voltage of 200 kV. The samples were dispersed in deionized water by sonication and dropped onto a conventional carbon-coated copper grid. The XRD pattern was obtained with a Bruker D8 Focus under Cu K␣ radiation at 1.54056 A˚ with a scanning speed of 4◦ min−1 . The XPS measurements were performed on a MICROLAB MK II spectrophotometer with Mg K␣ radiation. UV–vis absorption spectra were recorded with a Shimadzu UV-1601 PC spectrophotometer. The samples were dispersed in deionized water for optical measurements. Differential pulse voltammetry (DPV) was carried out at room temperature with a CHI 660C workstation (CH Instruments, Chenhua, Shang-hai, China) connected to a personal computer. A three-electrode configuration was employed, consisting of a modified glassy carbon electrode (3 mm in diameter) serving as the working electrode, saturated calomel electrode and platinum wire serving as the reference and counter electrodes, respectively. 3. Results and discussion AFM images of graphene oxide revealed the presence of sheets with thickness of 1.199 nm (Fig. 1), which was character-
Fig. 1. AFM images and height profile of graphene oxide.
istic of a fully exfoliated graphene oxide sheet [27,33,34]. Such thickness was larger than that of single-layer pristine graphene (0.34 nm), due to the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the displacement of the sp3 -hybridized carbon atoms slightly above and below the original graphene plane [27]. The TEM images of graphene sheets and Pt-graphene hybrid material were shown in Fig. 2, where the transparent graphene sheets with crumpled silk veil waves on the top of the carbon film were observed (Fig. 2a), and the rumples was intrinsic to graphene nanosheets [35]. A large number of Pt nanoparticles were supported on graphene sheets and few particles resided outside of the support (Fig. 2b), which indicated the strong interaction between the Pt nanoparticles and the graphene sheets. The size distribution of Pt nanoparticles was 8–45 nm (Fig. 2c). In Fig. 3, graphite showed a sharp diffraction peak at 26.2◦ corresponding to (0 0 2) plane with d-spacing of 0.34 nm (Fig. 3a). Compared with the graphite, the feature diffraction peak of graphite oxide appeared at 10.4◦ corresponding to d-spacing of 0.85 nm (Fig. 3b), which was larger than that of graphite due to the intercalated water molecules between layers [36]. After the exfoliation of graphite oxide by ultrasonic vibration and subsequent chemical reduction, the obtained graphene showed a broadened diffraction peak at 24.5◦ (Fig. 3c), meaning the layers of graphite along c-axis were exfoliated and the carbon sp2 bond were restored. The XRD pattern of Pt-graphene hybrid material is shown in Fig. 3d. Apart from the peak at 24.5◦ assigned to graphene, all other peaks can be indexed to Pt nanoparticles (face-centered cubic, JCPDS 040802).
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Fig. 3. XRD patterns of (a) graphite, (b) graphite oxide, (c) graphene, and (d) Ptgraphene hybrid material.
The UV–vis spectrum of GO exhibited two characteristic peaks, one was at 230 nm corresponding to → * transitions of aromatic C–C bonds, and a shoulder at 303 nm was attributed to n → * transitions of C O bonds (Fig. 5a) [40]. The peak at 230 nm was red shift to 268 nm after the chemical reduction treatment (Fig. 5b and c), which was an indication of the restoration of the electronic conjugation within the G sheets [41]. The activity of the graphene modified GCE and Pt-graphene modified GCE towards electrochemical detection of hydroquinone were investigated by differential pulse voltammetry (DPV). The GCE exhibited a weak and broad peak at 0.09 V corresponding to
Fig. 2. TEM images of (a) graphene sheets and (b) Pt-graphene hybrid material, (c) the size distribution of supported Pt nanoparticles.
Compared with graphite oxide, the C 1s XPS spectrum of graphene at 286–289 eV corresponding to oxygenated carbon showed a significant decrease (Fig. 4a), confirming that most of the epoxide, hydroxyl, and carboxyl functional groups were successfully removed through the reduction process. This observation was in agreement with that found in previous studies [27,31]. Fig. 4b represented the XPS signature of the Pt 4f doublet (4f7/2 and 4f5/2 ) for the Pt nanoparticles supported on graphene sheets. The Pt 4f7/2 and Pt 4f5/2 peaks appeared at 70.1 eV and 73.35 eV, respectively, which shifted remarkably to the lower binding energy compared with the standard binding energy of Pt 4f7/2 and Pt 4f5/2 for Pt0 state (70.83 eV and 74.23 eV) [37] due to the electron transfer from the graphene sheet to Pt nanoparticles. Because the work function of graphene (4.48 eV) [38] is smaller than that of Pt (5.65 eV) [39], electron transfer from the graphene sheets to Pt nanoparticles would occurred during the formation of the Pt-graphene hybrid structures.
Fig. 4. (a) C 1s XPS spectra of graphite oxide and graphene, and (b) Pt 4f spectra of Pt-graphene.
J. Li et al. / Electrochimica Acta 56 (2011) 2712–2716
Fig. 5. UV–vis absorption spectra of (a) graphene oxide, (b) graphene, and (c) Ptgraphene hybrid material.
the oxidation of hydroquinone (curve a in Fig. 6A). The graphenemodified GCE displayed a well-defined peak at 0.002 V with a much higher current intensity as compared with the GCE (the curve b in Fig. 6A), indicating the enhanced electrocatalytic activity towards hydroquinone, which could be due to the unique properties of graphene that increased the active surface area of the electrode and accelerated the electron transfer via improved conductivity and the good affinity of graphene to hydroquinone. When Pt nanoparticles were combined with graphene, the current density further increased, maybe due to the enhanced electron transfer in Pt-graphene hybrid system, which was attributed to the charge hopping through the metallic Pt nanoparticles and
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the effective charge migration through the graphene. The effective transport of the electrons to the electrode in the Pt-graphene matrix led to the efficient electrocatalytic oxidation of hydroquinone. We can observe in Fig. 6B and C that the peak current increased with the increase of the hydroquinone concentration (from the curve a to f). It could be seen from the curve a in Fig. 6D that the current at graphene modified GCE linearly increased with the increase of the concentration of hydroquinone over the 20–115 M range with sensitivity of 1.38 A M−1 cm−2 ; and the detection limit was estimated to be 12 M (S/N = 3). For Pt-graphene modified GCE (the curve b in Fig. 6D), a linear detection range was from 20 M to 145 M with sensitivity of 3.56 A M−1 cm−2 , the detection limit 6 M (S/N = 3). Additionally, the current intensity at the Pt-graphene modified GCE was higher than that at the graphene modified GCE in the whole concentration range. These results indicated that the Pt-graphene modified GCE showed higher current intensity, lower detection limit, and higher sensitivity towards electrochemical detection of hydroquinone compared with the pure graphene modified GCE, possibly due to the enhanced electron transfer in the Pt-graphene hybrid system. The applicability of the graphene modified GCE to the selective detection of hydroquinone in the presence of phenol was studied using DPV. Phenol and hydroquinone showed respectively an oxidation peak at 0.218 eV (Fig. 7a) and 0.002 eV (Fig. 7b). For a mixed solution of 0.15 mM hydroquinone and 10 mM phenol, there were two well-distinguished peaks at the potential of 0.002 eV and 0.218 eV, corresponding to the oxidation of hydroquinone and phenol, respectively (Fig. 7c), indicating that hydroquinone can be selectively detected in the presence of large concentration of phenol.
Fig. 6. DPV obtained at GCE (a), graphene modified GCE (b), and Pt-graphene modified GCE (c), using 115 M hydroquinone in 0.05 M (pH 7.4) phosphate buffer solution (A). DPV obtained at graphene modified GCE (B) and Pt-graphene modified GCE (C) with various concentration of hydroquinone: (a) 20 M, (b) 29 M, (c) 65 M, (d) 83 M, (e) 115 M and (f) 145 M in 0.05 M (pH 7.4) phosphate buffer. DPV conditions: pulse amplitude = 0.05 V, sample width = 0.0167 s, pulse width = 0.15 s, pulse period = 0.4 s, and quiet time = 2 s. (D) Calibration plots of background subtracted peak current at graphene modified GCE (a) and Pt-graphene modified GCE (b) versus the concentration of hydroquinone.
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J. Li et al. / Electrochimica Acta 56 (2011) 2712–2716
Fig. 7. DPV obtained at graphene modified GCE: (a) 10 mM phenol, (b) 0.15 mM hydroquinone, and (c) 10 mM phenol and 0.15 mM hydroquinone in 0.05 M (pH 7.4) phosphate buffer. DPV condition: pulse amplitude = 0.05 V, sample width = 0.0167 s, pulse width = 0.15 s, pulse period = 0.4 s, and quiet time = 2 s.
4. Conclusions In conclusion, we have synthesized graphene and Pt-graphene hybrid material through a microwave-assisted chemical reduction process. The resulting graphene and Pt-graphene hybrid materials have been used for the electrochemical detection of hydroquinone. Graphene modified GCE showed a good performance for detecting hydroquinone due to the unique properties of graphene, which increased the active surface area of electrode and accelerated the electron transfer. Compared with the pure graphene modified GCE, the electrocatalytic activity of the Pt-graphene hybrid material was further improved possibly due to the enhanced electron transfer in the Pt-graphene hybrid system. Acknowledgements The author thanks the support of Chinese Academy of Sciences and 973 Program. References [1] H. Cui, C. He, G.J. Zhao, J. Chromatogr. A 855 (1999) 171. [2] C.H. Lin, J.Y. Sheu, H.L. Wu, Y.L. Huang, J. Pharm. Biomed. Anal. 38 (2005) 414.
[3] N.A. Penner, P.N. Nesterenko, Analyst 125 (2000) 1249. [4] M.E. Rueda, L.A. Sarabia, A. Herrero, M.C. Ortiz, Anal. Chim. Acta 479 (2003) 173. [5] G.N. Chen, J.S. Liu, J.P. Duan, H.Q. Chen, Talanta 53 (2000) 651. [6] L.P. Jia, A.M. Zhang, Chin. J. Anal. Chem. 31 (2003) 1508. [7] T. Gajewska, A. Kaszuba, Polimery 44 (1999) 690. [8] I.C. Vieira, O. Fatibello-Filho, Talanta 52 (2000) 681. [9] L.H. Wang, Analyst 120 (1995) 2241. [10] Y. Hou, L.P. Guo, G. Wang, J. Electroanal. Chem. 617 (2008) 211. [11] D. Sopchak, B. Miller, Y. Avyigal, R. Kalish, J. Electroanal. Chem. 538 (2002) 39. [12] X.J. Huang, J.J. Pot, W.Th. Kok, Anal. Chim. Acta 300 (1995) 5. [13] R.M. Carvalho, C. Mello, L.T. Kubota, Anal. Chim. Acta 420 (2000) 109. [14] Y.P. Ding, W.L. Liu, Q.S. Wu, X.G. Wang, J. Electroanal. Chem. 575 (2005) 275. [15] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [16] T. Takamura, K. Endo, L. Fu, Y.P. Wu, K.J. Lee, T. Matsumoto, Eletrochim. Acta 53 (2007) 1055. [17] S. Gilje, S. Han, M. Wang, K.L. Wang, R.B. Kaner, Nano Lett. 7 (2007) 3394. [18] J.S. Bunch, A.M. van der Zande, S.S. Verbridge, I.W. Frank, D.M. Tanenbaum, J.M. Parpia, H.G. Craighead, P.L. McEuen, Science 315 (2007) 490. [19] A. Sakhaee-Pour, M.T. Ahmadian, A. Vafai, Solid State Commun. 147 (2008) 336. [20] T.T. Baby, S.S.J. Aravind, T. Arockiadoss, R.B. Rakhi, S. Ramaprabhu, Sens. Actuators B: Chem. 145 (2010) 71. [21] Y. Wang, Y.Y. Shao, D.W. Matson, J.H. Li, Y.H. Lin, ACS Nano 4 (2010) 1790. [22] X.H. Kang, J. Wang, H. Wu, J. Liu, I.A. Aksay, Y.H. Lin, Talanta 81 (2010) 754. [23] Y. Wang, Y.M. Li, L.H. Tang, J. Lu, J.H. Li, Electrochem. Commun. 11 (2009) 889. [24] 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. [25] N.G. Shang, P. Papakonstantinou, M. McMullan, M. Chu, A. Stamboulis, A. Potenza, S.S. Dhesi, H. Machetto, Adv. Funct. Mater. 18 (2008) 3506. [26] A. Charrier, A. Coati, T. Argunova, Y. Garreau, R. Pinchaux, I. Forbeaux, J.M. Debever, M. Sauvage-Simkin, J.M. Themlin, J. Appl. Phys. 92 (2002) 2479. [27] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y.Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558. [28] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, ACS Nano 3 (2009) 2653. [29] G.X. Wang, J. Yang, J. Park, X.L. Gou, B. Wang, H. Liu, J. Yao, J. Phys. Chem. C 112 (2008) 8192. [30] H.L. Wang, J.T. Robinson, X.L. Li, H.J. Dai, J. Am. Chem. Soc. 131 (2009) 9910. [31] Y. Zhou, Q.L. Bao, L.A.L. Tang, Y.L. Zhong, K.P. Loh, Chem. Mater. 21 (2009) 2950. [32] W. Hummers, R. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [33] S. Stankovich, R.D. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff, J. Mater. Chem. 16 (2006) 155. [34] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc. 130 (2008) 856. [35] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth, Nature 446 (2007) 60. [36] A. Buchsteiner, A. Lerf, J. Pieper, J. Phys. Chem. B 110 (2006) 22328. [37] J.F. Moudler, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Prairie, MN, 1992. [38] X.J. Wu, X.C. Zeng, Nano Lett. 9 (2009) 250. [39] H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. [40] J.I. Paredes, S. Villar-Rodil, A. Martınez-Alonso, J.M.D. Tascón, Langmuir 24 (2008) 10560. [41] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical detection of hydroquinone by graphene and Pt-graphene hybrid material synthesized through a microwave-assisted chemical reduction process Jing Li a,b , Chun-yan Liu a,∗ , Chao Cheng a,b a Key Laboratory of Photochemical Conversion and Optoelectronic Materials of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100806, PR China
a r t i c l e
i n f o
Article history: Received 17 June 2010 Received in revised form 17 September 2010 Accepted 14 December 2010 Available online 21 December 2010 Keywords: Hydroquinone Graphene Pt-graphene hybrid material Electrochemical detection Differential pulse voltammetry
a b s t r a c t We have synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application as electrode materials towards the electrochemical detection of hydroquinone. Graphene modified glass carbon electrode (GCE) showed a good performance for detecting hydroquinone due to the unique properties of graphene which increased the active surface area of the electrode and accelerated the electron transfer. The linear detection range of hydroquinone concentration was 20–115 M with a sensitivity of 1.38 A M−1 cm−2 ; the detection limit was estimated to be 12 M (S/N = 3). The electrocatalytic activity of the Pt-graphene modified GCE was further improved due to the enhanced electron transfer and the linear detection range was 20–145 M with the sensitivity of 3.56 A M−1 cm−2 , detection limit 6 M (S/N = 3). © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Hydroquinone is a phenolic compound which is important in a wide number of biological and industrial processes such as coal–tar production, paper manufacturing and photographic developers, and is considered as an important xenobiotic micropollutant [1]. Several analytical methods have been used to detect hydroquinone, including high performance liquid chromatography [2,3], flow injection analysis [4,5], spectrophotometry [6,7], and electrochemical methods [8,9], among which the electrochemical methods have attracted great attentions owing to the advantages such as efficiency, simplicity and quick response. In previous work, several carbon-based materials such as mesoporous carbon [10], borondoped diamond [11], conductive carbon cement [12], carbon fibre [13], and carbon nanotubes (CNTs) [14] have been explored for the electrochemical detection of hydroquinone. Graphene (G), a monolayer of carbon atoms packed into a dense, honeycomb crystal structure as well as being a fundamental building block for fullerenes, carbon nanotubes, and graphite [15], has shown fascinating properties and holds the promise for future carbon-based device architectures [16–19]. Recently, graphenebased materials have been explored for electrochemical sensor due to its large surface area, extraordinary electronic transport prop-
erties, strong mechanical strength, and its lower cost and easier preparation in mass quantities compared with carbon nanotubes, typical examples including detection of glucose by metal decorated graphene [20] and nitrogen-doped graphene [21] modified glass carbon electrode, electrochemical detection of paracetamol by graphene modified glass carbon electrode [22], and selective detection of dopamine by graphene modified glass carbon electrode [23]. However, to the best of our knowledgement, there is no report about the eletrochemical detection of hydroquinone by graphene-based materials. Up to now, numerous methods have been developed to synthesize graphene such as micromechanical cleavage of graphite [24], chemical vapor deposition technique [25], epitaxial growth on a single-crystal silicon carbide by vacuum graphitization [26], and chemical reduction of graphene oxide [27–31]. Among them, the chemical reduction of graphene oxide, involving graphite oxidation, exfoliation and reduction, is the most efficient approach to bulk production of graphene-based sheets at low cost. In this context, we synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application to electrochemical detection of hydroquinone. 2. Experimental 2.1. Preparation of graphene
∗ Corresponding author. Tel.: +86 010 82543573; fax: +86 010 62554670. E-mail address: [email protected] (C.-y. Liu). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.046
Graphene was prepared through a microwave-assisted chemical reduction of graphene oxide. In a typical procedure, graphite
J. Li et al. / Electrochimica Acta 56 (2011) 2712–2716
2713
oxide (25 mg) prepared from natural graphite powders (Beijing Chemical Factory of China) by Hummer’s method [32] was sonicated in water (50 mL) for 2 h using a JK-300 ultrasonic cleaner (300 W, 40 kHz) to achieve a clear, brown dispersion of graphene oxide (0.5 mg/mL). 5 mL of hydrazine hydrate (80%) was added into the graphene oxide and kept stirring for 3 h at 100 ◦ C in a microwave oven with program-control (Sineo MAS-II). The heating power was set to 300 W at the beginning of the reaction and automatically reduced to as low as about 10 W when a given temperature reached. The stirring rate was set to 700 rps. The black graphene was obtained after centrifugation, washed with water and ethanol, and naturally dried in air. 2.2. Preparation of Pt-graphene hybrid material The route to prepare Pt-graphene hybrid material was similar to graphene, except that 5 mL of hexachloroplatinic acid solution (0.0193 M) was added to the graphene oxide solution and kept stirring for 15 min at room temperature before the addition of hydrazine hydrate. 2.3. Preparation of graphene and Pt-graphene modified glass carbon electrode Prior to modification, the glassy carbon electrodes (GCE) of 3 mm diameter were polished with 0.5 m alumina powder, washed with deionized water and then dried in air. Graphene-based materials were dispersed in N,N-dimethylformamide (DMF) by sonication for 30 min to achieve a 2 mg/mL graphene-DMF suspension. Then, 5 L of the suspension was coated onto GCE and allowed to dry in air. 2.4. Characterization The AFM images were recorded using a Multimode Nanoscope IIIa AFM (Veeco Metrology LLC, Santa Barbara, CA). Tapping-mode imaging was applied to provide the largest amount of structural detail of the graphene oxide sheets as well as to prevent translocation of the sheets on the surface by the tip (Veeco MP-11100 silicon cantilevers, force constant k = 60 N/m, radius of curvature r = 10 nm, and resonance frequency f = 300 kHz). The sample was prepared by depositing graphene oxide dispersion in water (0.05 mg/mL) onto a new cleaved mica surface and dried under vacuum at room temperature. The TEM images were taken with a JEM 2100F transmission electron microscope, by using an accelerating voltage of 200 kV. The samples were dispersed in deionized water by sonication and dropped onto a conventional carbon-coated copper grid. The XRD pattern was obtained with a Bruker D8 Focus under Cu K␣ radiation at 1.54056 A˚ with a scanning speed of 4◦ min−1 . The XPS measurements were performed on a MICROLAB MK II spectrophotometer with Mg K␣ radiation. UV–vis absorption spectra were recorded with a Shimadzu UV-1601 PC spectrophotometer. The samples were dispersed in deionized water for optical measurements. Differential pulse voltammetry (DPV) was carried out at room temperature with a CHI 660C workstation (CH Instruments, Chenhua, Shang-hai, China) connected to a personal computer. A three-electrode configuration was employed, consisting of a modified glassy carbon electrode (3 mm in diameter) serving as the working electrode, saturated calomel electrode and platinum wire serving as the reference and counter electrodes, respectively. 3. Results and discussion AFM images of graphene oxide revealed the presence of sheets with thickness of 1.199 nm (Fig. 1), which was character-
Fig. 1. AFM images and height profile of graphene oxide.
istic of a fully exfoliated graphene oxide sheet [27,33,34]. Such thickness was larger than that of single-layer pristine graphene (0.34 nm), due to the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the displacement of the sp3 -hybridized carbon atoms slightly above and below the original graphene plane [27]. The TEM images of graphene sheets and Pt-graphene hybrid material were shown in Fig. 2, where the transparent graphene sheets with crumpled silk veil waves on the top of the carbon film were observed (Fig. 2a), and the rumples was intrinsic to graphene nanosheets [35]. A large number of Pt nanoparticles were supported on graphene sheets and few particles resided outside of the support (Fig. 2b), which indicated the strong interaction between the Pt nanoparticles and the graphene sheets. The size distribution of Pt nanoparticles was 8–45 nm (Fig. 2c). In Fig. 3, graphite showed a sharp diffraction peak at 26.2◦ corresponding to (0 0 2) plane with d-spacing of 0.34 nm (Fig. 3a). Compared with the graphite, the feature diffraction peak of graphite oxide appeared at 10.4◦ corresponding to d-spacing of 0.85 nm (Fig. 3b), which was larger than that of graphite due to the intercalated water molecules between layers [36]. After the exfoliation of graphite oxide by ultrasonic vibration and subsequent chemical reduction, the obtained graphene showed a broadened diffraction peak at 24.5◦ (Fig. 3c), meaning the layers of graphite along c-axis were exfoliated and the carbon sp2 bond were restored. The XRD pattern of Pt-graphene hybrid material is shown in Fig. 3d. Apart from the peak at 24.5◦ assigned to graphene, all other peaks can be indexed to Pt nanoparticles (face-centered cubic, JCPDS 040802).
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Fig. 3. XRD patterns of (a) graphite, (b) graphite oxide, (c) graphene, and (d) Ptgraphene hybrid material.
The UV–vis spectrum of GO exhibited two characteristic peaks, one was at 230 nm corresponding to → * transitions of aromatic C–C bonds, and a shoulder at 303 nm was attributed to n → * transitions of C O bonds (Fig. 5a) [40]. The peak at 230 nm was red shift to 268 nm after the chemical reduction treatment (Fig. 5b and c), which was an indication of the restoration of the electronic conjugation within the G sheets [41]. The activity of the graphene modified GCE and Pt-graphene modified GCE towards electrochemical detection of hydroquinone were investigated by differential pulse voltammetry (DPV). The GCE exhibited a weak and broad peak at 0.09 V corresponding to
Fig. 2. TEM images of (a) graphene sheets and (b) Pt-graphene hybrid material, (c) the size distribution of supported Pt nanoparticles.
Compared with graphite oxide, the C 1s XPS spectrum of graphene at 286–289 eV corresponding to oxygenated carbon showed a significant decrease (Fig. 4a), confirming that most of the epoxide, hydroxyl, and carboxyl functional groups were successfully removed through the reduction process. This observation was in agreement with that found in previous studies [27,31]. Fig. 4b represented the XPS signature of the Pt 4f doublet (4f7/2 and 4f5/2 ) for the Pt nanoparticles supported on graphene sheets. The Pt 4f7/2 and Pt 4f5/2 peaks appeared at 70.1 eV and 73.35 eV, respectively, which shifted remarkably to the lower binding energy compared with the standard binding energy of Pt 4f7/2 and Pt 4f5/2 for Pt0 state (70.83 eV and 74.23 eV) [37] due to the electron transfer from the graphene sheet to Pt nanoparticles. Because the work function of graphene (4.48 eV) [38] is smaller than that of Pt (5.65 eV) [39], electron transfer from the graphene sheets to Pt nanoparticles would occurred during the formation of the Pt-graphene hybrid structures.
Fig. 4. (a) C 1s XPS spectra of graphite oxide and graphene, and (b) Pt 4f spectra of Pt-graphene.
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Fig. 5. UV–vis absorption spectra of (a) graphene oxide, (b) graphene, and (c) Ptgraphene hybrid material.
the oxidation of hydroquinone (curve a in Fig. 6A). The graphenemodified GCE displayed a well-defined peak at 0.002 V with a much higher current intensity as compared with the GCE (the curve b in Fig. 6A), indicating the enhanced electrocatalytic activity towards hydroquinone, which could be due to the unique properties of graphene that increased the active surface area of the electrode and accelerated the electron transfer via improved conductivity and the good affinity of graphene to hydroquinone. When Pt nanoparticles were combined with graphene, the current density further increased, maybe due to the enhanced electron transfer in Pt-graphene hybrid system, which was attributed to the charge hopping through the metallic Pt nanoparticles and
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the effective charge migration through the graphene. The effective transport of the electrons to the electrode in the Pt-graphene matrix led to the efficient electrocatalytic oxidation of hydroquinone. We can observe in Fig. 6B and C that the peak current increased with the increase of the hydroquinone concentration (from the curve a to f). It could be seen from the curve a in Fig. 6D that the current at graphene modified GCE linearly increased with the increase of the concentration of hydroquinone over the 20–115 M range with sensitivity of 1.38 A M−1 cm−2 ; and the detection limit was estimated to be 12 M (S/N = 3). For Pt-graphene modified GCE (the curve b in Fig. 6D), a linear detection range was from 20 M to 145 M with sensitivity of 3.56 A M−1 cm−2 , the detection limit 6 M (S/N = 3). Additionally, the current intensity at the Pt-graphene modified GCE was higher than that at the graphene modified GCE in the whole concentration range. These results indicated that the Pt-graphene modified GCE showed higher current intensity, lower detection limit, and higher sensitivity towards electrochemical detection of hydroquinone compared with the pure graphene modified GCE, possibly due to the enhanced electron transfer in the Pt-graphene hybrid system. The applicability of the graphene modified GCE to the selective detection of hydroquinone in the presence of phenol was studied using DPV. Phenol and hydroquinone showed respectively an oxidation peak at 0.218 eV (Fig. 7a) and 0.002 eV (Fig. 7b). For a mixed solution of 0.15 mM hydroquinone and 10 mM phenol, there were two well-distinguished peaks at the potential of 0.002 eV and 0.218 eV, corresponding to the oxidation of hydroquinone and phenol, respectively (Fig. 7c), indicating that hydroquinone can be selectively detected in the presence of large concentration of phenol.
Fig. 6. DPV obtained at GCE (a), graphene modified GCE (b), and Pt-graphene modified GCE (c), using 115 M hydroquinone in 0.05 M (pH 7.4) phosphate buffer solution (A). DPV obtained at graphene modified GCE (B) and Pt-graphene modified GCE (C) with various concentration of hydroquinone: (a) 20 M, (b) 29 M, (c) 65 M, (d) 83 M, (e) 115 M and (f) 145 M in 0.05 M (pH 7.4) phosphate buffer. DPV conditions: pulse amplitude = 0.05 V, sample width = 0.0167 s, pulse width = 0.15 s, pulse period = 0.4 s, and quiet time = 2 s. (D) Calibration plots of background subtracted peak current at graphene modified GCE (a) and Pt-graphene modified GCE (b) versus the concentration of hydroquinone.
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Fig. 7. DPV obtained at graphene modified GCE: (a) 10 mM phenol, (b) 0.15 mM hydroquinone, and (c) 10 mM phenol and 0.15 mM hydroquinone in 0.05 M (pH 7.4) phosphate buffer. DPV condition: pulse amplitude = 0.05 V, sample width = 0.0167 s, pulse width = 0.15 s, pulse period = 0.4 s, and quiet time = 2 s.
4. Conclusions In conclusion, we have synthesized graphene and Pt-graphene hybrid material through a microwave-assisted chemical reduction process. The resulting graphene and Pt-graphene hybrid materials have been used for the electrochemical detection of hydroquinone. Graphene modified GCE showed a good performance for detecting hydroquinone due to the unique properties of graphene, which increased the active surface area of electrode and accelerated the electron transfer. Compared with the pure graphene modified GCE, the electrocatalytic activity of the Pt-graphene hybrid material was further improved possibly due to the enhanced electron transfer in the Pt-graphene hybrid system. Acknowledgements The author thanks the support of Chinese Academy of Sciences and 973 Program. References [1] H. Cui, C. He, G.J. Zhao, J. Chromatogr. A 855 (1999) 171. [2] C.H. Lin, J.Y. Sheu, H.L. Wu, Y.L. Huang, J. Pharm. Biomed. Anal. 38 (2005) 414.
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