Synthesis of Flower-like Gold Nanoparticles and Their Electrocatalytic

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Synthesis of Flower-like Gold Nanoparticles and Their Electrocatalytic Activity Towards the Oxidation of Methanol and the Reduction of Oxygen Bikash Kumar Jena and C. Retna Raj* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India ReceiVed NoVember 6, 2006. In Final Form: January 9, 2007 This article describes the synthesis of branched flower-like gold (Au) nanocrystals and their electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Gold nanoflowers (GNFs) were obtained by a one-pot synthesis using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) as a reducing/stabilizing agent. The GNFs have been characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electrochemical measurements. The UV-visible spectra show two bands corresponding to the transverse and longitudinal surface plasmon (SP) absorption at 532 and 720 nm, respectively, for the colloidal GNFs. The GNFs were self-assembled on a sol-gel-derived silicate network, which was preassembled on a polycrystalline Au electrode and used for electrocatalytic applications. The GNFs retain their morphology on the silicate network; the UV-visible diffuse reflectance spectra (DRS) of GNFs on the silicate network show longitudinal and transverse bands as in the case of colloidal GNFs. The GNFs show excellent electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Oxidation of methanol in alkaline solution was observed at ∼0.245 V, which is much less positive than that on an unmodified polycrystalline gold electrode. Reduction of oxygen to H2O2 and the further reduction of H2O2 to water in neutral pH were observed at less negative potentials on the GNFs electrode. The electrocatalytic activity of GNFs is significantly higher than that of the spherically shaped citrate-stabilized Au nanoparticles (SGNs).

Introduction Studies on the electrocatalytic oxidation of methanol and the reduction of oxygen using precious metal catalysts have received considerable attention mainly because of energy-related applications such as fuel cell technology.1 The development of an efficient catalyst for the direct methanol fuel cell (DMFC) is one of the key steps in achieving high efficiency. Precious metal platinum (Pt) has been extensively used as a catalyst in DMFC.1,2 However, the major problem associated with the Pt-based catalyst is the poisoning by adsorbed CO-like species generated during the oxidation of methanol.1,3 To overcome the problem of poisoning, bimetallic multicomponent catalysts such as Pt-Ru have been widely used.4 The development of a highly efficient non-Pt catalyst, which can overcome the problem of poisoning, is a challenging task. It has been shown recently that the Au electrode does not undergo poisoning during methanol oxidation by the electrogenerated CO.5,6 Assiongbon and Roy have demonstrated the use of time-resolved electrochemical impedance and surface plasmon resonance techniques in which the Au surface is free from site poisoning by chemisorbed CO during methanol oxidation.6 Although bulk Au is a poor catalyst, recent studies * Corresponding author. E-mail: [email protected]. Fax: +913222-282252. Tel: +91-3222-283348. (1) (a) Narayanan, S. R.; Valdez, T. I. In Handbook of Fuel Cells: Fundamentals, Technology, and Applications; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; John Wiley and Sons: Chichester, England, 2003; Vol. 4, pp 1133-1141. (b) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133-161. (c) Lamy, C.; Leger, J. M.; Srinivasen, S. In Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E, White, R. E., Eds.; Kluwer Academic/Plenum: New York, 2001; Vol. 34, pp 53-118. (2) (a) Love, J. G.; Brooksby, P. A.; McQuillan, A. J. J. Electroanal. Chem. 1999, 464, 93-100. (b) Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960-19966. (c) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792-5798. (d) Jambunathan, K.; Jayaraman, S.; Hillier, A. C. Langmuir 2004, 20, 1856-1863. (e) Luo, J.; Lou, Y.; Maye, M. M.; Zhong, C.-J.; Hepel, M. Electrochem. Commun. 2001, 3, 172-176. (3) (a) Paulus, U. A.; Endruschat, U.; Feldmeyer, G. J.; Schmidt, T. J.; Bonnemann, H.; Behm, R. J. J. Catal. 2000, 195, 383-393. (b) Seiler, T.; Savinova, E. R.; Friedrich, K. A.; Stimming, U. Electrochim. Acta 2004, 49, 3927-3936.

show that the nanosized Au particles exhibit high catalytic activity.7,8 The electrocatalytic activity of Au nanoparticles has been investigated by different groups.8-13 Zhong et al. explored the possible utilization of monolayer-protected nanosized Au particles for the oxidation of methanol in alkaline solution.9 Ohsaka and co-workers have extensively studied the electrocatalytic behavior of electrochemically deposited Au particles on different conducting supports with respect to the reduction (4) (a) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020-12029. (b) Arico, A. S.; Poltarzewski, Z.; Kim, H.; Morana, A.; Giordano, N.; Antonucci, V. J. Power Source 1995, 55, 159-166. (c) Lu, Q.; Yang, B.; Zhuang, L.; Lu, J. J. Phys. Chem. B 2005, 109, 1715-1722. (d) Zhang, X.; Chan, K.-Y. Chem. Mater. 2003, 15, 451-459. (e) Giroir-Fendler, A.; Richard, D.; Gallezot, P. Faraday Discuss. 1991, 92, 69-77. (f) Park, K.-W.; Choi, J.-H.; Kwon, B.-K.; Lee, S.-A.; Ha, H.-Y.; Hong, S.-A.; Sung, Y.-E.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869-1877. (g) Gurau, B.; Viswanathan, R.; Lafrenz, T. J.; Liu, R.; Ley, K. L.; Smotkin, E. S.; Reddington, E.; Sapienza, A.; Chan, B. C.; Mallouk, T. E.; Sarangapani, S. J. Phys. Chem. B 1998, 102, 9997-10003. (h) Rajesh, B.; Ravindranathan Thampi, K.; Bonard, J.-M.; Xanthopoulos, N.; Mathieu, H. J.; Viswanathan, B. J. Phys. Chem. B 2003, 107, 2701-2708. (i) Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Vazquez-Alvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abruna, H. D. J. Am. Chem. Soc. 2004, 126, 4043-4049. (j) Lee, S.; Park, K.; Choi, J.; Kwon, B.; Sung, Y. J. Electrochem. Soc. 2002, 149, A1299-A1304. (5) (a) Borkowska, Z.; Tymosiak-Zielinska, A.; Shul, G. Electrochim. Acta 2004, 49, 1209-1220. (b) Borkowska, Z.; Tymosiak-Zielinska, A.; Nowakowski R. Electrochim. Acta 2004, 49, 2613-2621. (6) Assiongbon, K. A.; Roy, D. Surf. Sci. 2005, 594, 99-119. (7) Biswas, P. C.; Nodasaka, Y.; Enyo, M.; Haruta, M. J. Electroanal. Chem. 1995, 381, 167-177. (8) (a) Daniel, M. -C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (b) Raj, C. R.; Jena, B. K. Chem. Commun. 2005, 2005-2007. (c) Jena, B. K.; Raj, C. R. Chem.sEur. J. 2006, 12, 2702-2708. (9) (a) Lou, Y.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C.-J. Chem. Commun. 2001, 473-474. (b) Zhong, C.-J.; Maye, M. M. AdV. Mater. 2001, 13, 15071511. (c) Luo, J.; Maye, M. M.; Lou, Y.; Han, L.; Hepel, M.; Zhong, C. J. Catal. Today 2002, 77, 127-138. (10) (a) El-Deab, M. S.; Ohsaka, T. Electrochem. Commun. 2002, 4, 288292. (b) El-Deab, M. S.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2003, 150, A851-A857. (c) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochem. Commun. 2005, 7, 29-34. (11) Kumar, S.; Zou, S. J. Phys. Chem. B 2005, 109, 15707-15713. (12) Maye, M. M.; Lou, Y.; Zhong, C.-J. Langmuir 2000, 16, 7520-7523. (13) Cuenya, B. R.; Baeck, S.-H.; Jaramillo, T. F.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 12928-12934.

10.1021/la063243z CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

Synthesis of Flower-like Gold Nanoparticles

of oxygen.10 The electrooxidation of CO on Au nanoparticles has also been investigated by different research groups.11-13 The high electrocatalytic activities of these nanosized particles are attributed to the large surface-to-volume ratio and the existence of special binding sites on the surface of the particles. It is generally observed that the catalytic property of the nanostructured metal particles largely depends on the size and shape.14 For instance, Li and Shi observed that the electrocatalytic behavior of electrochemically grown Au particles depends on their shape. The electrochemically deposited flower-like Au nanostructure showed higher electrocatalytic activity than the pine cone and sheet structures.15 Kumar and Zou observed that the electrocatalytic behavior of Au nanoparticles depends on the particle coverage on the electrode surface.11 Because the catalytic activity of the nanostructured particle mainly depends on the size and shape, in the past few years various methodologies have been developed to synthesize the nanoparticles with desired shapes and sizes.16-26 It has been observed that small monodisperse nanoparticles can be obtained using mild reducing agents. The shape, size distribution, and stability of the nanoparticles depends on the reducing agents and stabilizers used.27-29 Yang et al. reported the preparation of 100300 nm gold platonic nanocrystals by a modified polyol process in the presence of surface-regulating polymers and polyvinylpyrrolidone (PVP).22 Chen at el. synthesized bipod, tripod, and tetrapod Au nanocrystals using cetyltrimethylammonium bromide (CTAB) surfactant, ascorbic acid, and NaOH.21b Murphy et al. synthesized anisotropic metal nanoparticles by the seedmediated growth approach in the presence of CTAB, ascorbic acid, and so forth.23 In all of these approaches, structure-directing reagents or structure-inducing techniques have been used to control the shape and size of the nanocrystals. Dong and coworkers synthesized hexagonal and truncated triangularly shaped single-crystalline Au nanoparticles using L amino acids without any templates.25 Sastry and co-workers recently reported the biological synthesis of triangular nanoprisms using lemongrass extract.18a However, in these approaches, the desired nanostructure was obtained after a very long time (6-12 h),18a,25 and the electrocatalytic property of these particles has not been investigated. (14) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 1266312676. (15) Li, Y.; Shi, G. J. Phys. Chem. B 2005, 109, 23787-23793. (16) (a) Jana, N. R. Angew. Chem., Int. Ed. 2004, 43, 1536-1540. (b) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633-3640. (17) Sun Y.; Xia, Y. Science 2002, 298, 2176-2179. (18) (a) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482-488. (b) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 5312-5313. (19) Simakin, A. V.; Voronov, V. V.; Shafeev, G. A.; Brayner, R.; BozonVerduraz, F. Chem. Phys. Lett. 2001, 348, 182-186. (20) (a) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2002, 18, 3694-3697. (b) Stoeva, S. I.; Prasad, B. L. V.; Uma, S.; Stoimenov, P. K.; Zaikovski, V.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2003, 107, 7441-7448. (c) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310-2312. (21) (a) Kuo, C. -H.; Huang, M. H. Langmuir 2005, 21, 2012-2016. (b) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186-16187. (c) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327-330. (22) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673-3677. (23) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648-8649. (24) Murphy, C. J. Science 2002, 298, 2139-2141. (25) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104-1105. (26) Zhou, M.; Chen, S.; Zhao, S. J. Phys. Chem. B 2006, 110, 4510-4513. (27) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065-4067. (28) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226-8230. (29) Duff, D. G.; Baiker, A.; Edwards, P. P. J. Chem. Soc., Chem. Commun. 1993, 96-98.

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In the present investigation, we describe a simple environmentally benign route for the synthesis of nanostructured Au particles with flower-like morphology using HEPES as a reducing/ stabilizing agent and their electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Gold nanoflowers (GNFs) have been self-assembled on a thiolfunctionalized sol-gel silicate network derived from (3mercaptopropyl)trimethoxysilane (MPTS). The electrocatalytic activity of GNFs has been compared with that of spherical Au nanoparticles (SGN). Although the use of HEPES for the synthesis of Au nanoparticles has been reported30 during the course of the present investigation, the flower-like morphology has not been observed, and the electrocatalytic properties have not been investigated. Experimental Section Materials. HEPES, MPTS, and HAuCl4 were obtained from Sigma-Aldrich and used as received. Hydrogen peroxide (H2O2) was obtained from Merck Ltd. All other chemicals, unless otherwise mentioned in this investigation, were of analytical grade. All of the solutions were prepared with Millipore water. Gold grids (400 mesh) and carbon-coated copper grids were obtained from Pelco International. Instrumentation. TEM images of GNFs were obtained from a transmission electron microscope (JEOL JEM 2010 electron microscope) operating at 200 kV. The samples were obtained by dropping 2 µL of a colloidal solution onto a carbon-coated copper grids. The UV-visible spectra of the colloidal solutions were recorded on a Shimadzu UV-1601 spectrophotometer. The UV-visible diffuse reflectance spectra (DRS) of GNFs on gold-coated glass slides were measured with a Shimadzu UV-2401 PC spectrophotometer. X-ray diffraction analysis of self-assembled GNFs was carried out with a Phillips X’pert PRO X-ray diffraction unit using Ni-filtered Cu KR (λ ) 1.54 Å) radiation. The crystallite size of GNFs was determined using X-ray line broadening analysis by applying the Scherrer formula. Electrochemical measurements were performed using a two-compartment three-electrode cell with a polycrystalline Au working electrode, a Pt wire auxiliary electrode, and a Ag/AgCl (3 M NaCl) reference electrode. Cyclic voltammograms were recorded using a computer-controlled CHI643B electrochemical analyzer. All of the electrochemical experiments were conducted in an argon atmosphere. Synthesis of Colloidal GNFs. All glassware used in the preparation of colloidal nanoparticles was cleaned with freshly prepared aqua regia and rinsed thoroughly with water Caution! Aqua regia is a powerful oxidizing agent, and it should be handled with extreme care. In a typical synthesis, 10 mL of an aqueous solution of HAuCl4 (0.25 mM) was stirred for 2 min, and 0.1 mL of HEPES (5 mM) was then added and stirring was continued for another 15 min. The resulting colloidal nanoparticles were stored at 4 °C. Fabrication of Electrodes. The fabrication of electrodes for electrochemical measurements was carried out according to our previous report.8b,c Briefly, the MPTS sol was prepared by mixing MPTS, methanol, and water (as 0.1 M HCl) in a 1:3:3 molar ratio, respectively, and stirring vigorously for 30 min. The well-polished and electrochemically cleaned polycrystalline Au electrode (0.031 cm2) was soaked in MPTS sol for 10 min. MPTS sol self-assembles on the polycrystalline Au electrode and exists as a sol-gel 3-D silicate network.31 The MPTS-sol-modified electrode was then soaked in colloidal GNFs for 18 h. Hereafter, the MPTS-sol-modified electrode and GNFs self-assembled electrodes will be referred to as MPTS and GNF electrodes, respectively. The schematic illustration of the GNF electrode is shown in Scheme 1. These electrodes were (30) Habib, A.; Tabata, M.; Wu Y. G. Bull. Chem. Soc. Jpn. 2005, 78, 262269. (31) (a) Wang, J.; Pamidi, P. V. A.; Zanette, D. R. J. Am. Chem. Soc. 1998, 120, 5852-5853. (b) Bharathi, S.; Nogami, M.; Ikeda, S. Langmuir 2001, 17, 1-4.

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Scheme 1. Schematic Representation of the Self-Assembled GNFs on a Sol-Gel-Derived 3-D Silicate Network

rinsed repeatedly with water and kept in a phosphate buffer solution (PBS) before being subjected to electrochemical experiments. Sample Preparation for XRD, DRS, and TEM Measurements. For XRD measurements, a glass microscope slide or coverslip was cleaned well with acetone and water and soaked in a methanol solution of MPTS monomer for about 18 h. The MPTS-monomer-functionalized glass slide or coverslip was washed with a copious amount of water and then soaked in colloidal GNFs for 5 h. For DRS measurements, the gold-coated coverslip was first modified with the silicate network, and then the GNFs were self-assembled on the network as described earlier. To obtain the morphology and shape of the GNFs on the silicate network, gold grids (400 mesh) have been modified with the silicate network by soaking in MPTS sol for 10 min. The silicate-network-modified gold grids were subsequently soaked in colloidal GNFs for the self-assembly of nanoparticles.

Results and Discussion Progress of the formation of GNFs was followed by UVvisible spectral measurements. HAuCl4 (3 mL, 0.25 mM) and (15 µL, 10 mM) HEPES were mixed and introduced into the quartz cell, and the spectra were recorded at regular intervals. The color of the HAuCl4 solution changed from yellow to violetred within 15 min after mixing. Figure 1 is the time-dependent spectral response obtained during the growth of Au nanoparticles. The spectra recorded in the early stage (up to 3 to 4 min) show only one peak at ∼532 nm. The absorbance at 532 nm increases monotonically with time while a new shoulder on the longerwavelength side (650-750 nm) of the main peak appears after 4 min. With time, the shoulder shifts to longer wavelength and stabilizes at ∼720 nm after the completion of the reaction (15 min). These time-dependent features can be ascribed either to the aggregation of spherical nanoparticles or to the formation of anisotropic nanostructures. Because the absorbance of both bands increases monotonically with time and the position of the band at 532 nm remains the same, it is concluded that the observed feature is not due to the aggregation of nanoparticles. The intense band observed at 532 nm is ascribed to the existence of the transverse component of the SP absorption whereas the shoulder observed at 720 nm is assigned to the longitudinal component of SP absorption. The SP absorption of the coinage metal nanoparticles, in general, is more sensitive to the particle shape and surrounding medium than to the size.32 The presence of two well-separated absorption bands is a characteristic feature of anisotropic nanoparticles. Such features have been observed for nanostructured Au particles such as nanorods and nanowires.33 (32) (a) Mulvaney, P. Langmuir 1996, 12, 788-800. (b) Liz-Marzan, L. M. Langmuir 2006, 22, 32-41. (c) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269-6275. (33) (a) Link, S.; Mohamed. M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073-3077. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410-8426.

Figure 1. Time-dependent UV-visible absorption spectra of a solution containing HAuCl4 (0.25 mM) and HEPES (50 µM). The spectra were recorded at a time interval of 2 min. The inset shows photographs of corresponding colloidal nanoparticles and HAuCl4.

The position and intensity of the longitudinal band depends on the size, surface morphology,and aspect ratio. For instance, in the case of nanorods and nanowires the longitudinal absorption band is dominant.33 However, branched nanocrystals have a single broad band, and nanocrystals having bipod and tripod structures have a less intense longitudinal band and a dominant transverse band.21a,b In the present investigation, the transverse band is more intense than the longitudinal band. The longitudinal band appears as a shoulder-like peak, indicating the presence of nanocrystals having a special shape. These spectral features are very similar to those observed for monopod and bipod nanocrystals.21b The time course of changes in absorbance of the transverse (532 nm) and longitudinal (720 nm) bands was measured (Supporting Information) to understand the kinetics of formation of nanoparticles. A rapid, steady increase in absorbance at 532 nm was observed in the initial stage. The absorbance of the transverse band tends to saturate at ∼7 min whereas the absorbance of the longitudinal band attained the saturation only after ∼15 min. High-resolution TEM measurements were made to investigate the surface morphology and shape of the nanoparticles. Figure 2 shows the representative TEM images obtained for the nanoparticles synthesized using HEPES. Interestingly, the TEM images show nanocrystals having flower-like morphology with multipode structure. The GNFs have an average size of 60 ( 5 nm. To the best of our knowledge, this is the first report that describe the rapid synthesis of flower-like nanocrystals without any template or structure-directing agent at room temperature. Figure 2b is the selected-area electron diffraction (SAED) pattern of the GNFs; SAED reveals that the GNFs have fcc structure corresponding to (111), (200), (220), and (311) gold crystalline facets, showing that GNFs include many small particles that have independent orientations. High-resolution TEM images of selected GNFs were taken to examine the morphology further (Figure 2c,d). The lattice fringe spacing of the GNFs was determined (Figure 2e). The interplanar spacing was measured to be 0.236 nm, revealing that the growth of the GNFs occurs preferentially on (111) planes. Further observation of the surface image reveals the presence of a twin boundary. The lattice planes are separated by a twin boundary as indicated by a line in the image (Figure 2f). Such twin boundaries have been observed for the branched nanocrystals synthesized by a seed-growth approach.21a Characterization of Self-Assembled GNFs on the Silicate Network. Figure 3 displays the diffuse reflectance spectrum obtained for GNFs self-assembled on the silicate-networkmodified gold-coated coverslip. As in the case of a colloidal solution, the GNFs on the silicate network show two bands at

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Figure 2. (a and c) TEM images and (b) the SEAD pattern obtained from GNFs. (d and e) HRTEM images of the GNF shown in c. (f) Selected square region of the image shown in e. A red line is drawn to indicate a twin boundary bisecting two adjacent facets. Figure 4. Representative TEM images obtained for the GNFs selfassembled on the silicate-network-modified gold grids.

Figure 3. UV-visible diffuse reflectance spectrum obtained for (a) GNFs on the silicate network and (b) a silicate network-modified gold-coated cover slip. The inset shows the UV-visible absorption spectrum obtained for the colloidal GNFs.

∼530 and ∼730 nm corresponding to transverse and longitudinal SP absorptions, respectively. These spectral features indicate that the GNFs retain their flower-like morphology on the silicate network. To further confirm the existence of flower-like morphology on the silicate network, TEM measurements were performed; GNFs were self-assembled on a gold grid modified with a silicate network. Interestingly, as shown in Figure 4, the nanoparticles on the silicate network show flower-like morphology as observed on the carbon-coated copper grids (Figure 2).

Figure 5. XRD patterns obtained for the self-assembled GNFs.

This further confirms that the flower-like nanostructure is retained on the surface of the silicate network. Figure 5 shows the XRD pattern obtained for the self-assembled GNFs. Four peaks corresponding to the (111), (200), (220), and (311) planes of a face-centered cubic lattice of Au have been observed. The peak corresponding to the (111) plane is more intense than peaks corresponding to the other planes. The ratio between the intensities of the (200) and (111) diffraction peaks is much lower (0.33) than the conventional value (0.52), demonstrating that the (111) plane is the predominant orientation. The size of the GNFs was also calculated from X-ray line

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Figure 6. Cyclic voltammograms of (a) MPTS and (b) GNF electrodes in 0.1 M KOH. Scan rate: 10 mV/s.

Figure 7. Cyclic voltammograms for the oxidation of methanol (0.25 mM) at (a) MPTS and (b) GNF electrodes in 0.1 M KOH. Scan rate: 10 mV/s.

broadening analysis applying Scherrer formula,34 and it is in close agreement (∼64 nm) with the TEM data. Figure 6 shows the cyclic voltammograms obtained for the MPTS and GNF electrodes in 0.1 M KOH. A broad oxidation wave in the potential range from +0.32 to 0.4 V and a reduction peak at ∼0.17 V, corresponding to the formation of surface oxides and their reduction, were observed.9c Such a voltammetric response was not observed for the MPTS electrode (Figure 6a), revealing that the voltammetric features of the GNF electrode are due to the presence of nanostructured Au particles on the silicate network. The charge consumed during the reduction of Au oxides has been estimated by integrating the area under the reduction wave. The surface area of GNFs on the silicate network was calculated to be 0.0253 cm2 from the charge consumed during the reduction of surface oxides using the reported value of 400 µC/cm2 for a clean Au electrode.35 The surface area was also calculated by chronoamperometry using K3Fe(CN)6 as a redox probe. The value (0.0249 cm2) calculated from chronoamperometry is in close agreement with those obtained by the voltammetric method.

Electrocatalysis Electrooxidation of Methanol. The main objective of the present investigation is to examine the electrocatalytic activity of GNFs toward the oxidation of methanol and the reduction of oxygen. Figure 7 illustrates the typical cyclic voltammograms obtained for methanol oxidation at the GNF electrode. A large (34) Cullity, B. D. Elements of X-ray Diffraction; Addision-Wesley: Reading, MA, 1978; p 102. (35) (a) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711-734. (b) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429-453.

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anodic wave at ∼0.235 V was observed for the catalytic oxidation of methanol at the GNF electrode. Such a characteristic voltammetric response was not observed at the MPTS electrode in the potential window used, indicating that the GNFs were responsible for the catalytic effect. In the absence of methanol, the GNF electrode shows a broad oxidation wave at ∼0.4 V and a sharp reduction wave at ∼0.17 V, and these voltammetric peaks are ascribed to the formation of Au oxides (vide supra). The voltammetric response observed in the presence of methanol is due to its oxidation by the surface-confined GNFs. The reduction wave observed for the Au oxide at ∼0.17 V (Figure 6b) disappears in the presence of methanol, confirming that the electrogenerated oxide species is involved in the electrocatalytic oxidation of methanol; the surface oxides function as an electron-transfer mediator in the oxidation process.36 The catalytic effect of GNFs can be rationalized by considering the incipient hydrous oxide/ adatom mediator model.36 The surface oxides generated by the process of premonolayer oxidation are responsible for the observed catalytic effect. The involvement of surface oxides in the catalytic oxidation of methanol is further confirmed by recording the voltammetric response at different concentrations of methanol (Supporting Information). The gradual decrease in the cathodic peak corresponding to the reduction of surface oxide on increasing the concentration of methanol in solution demonstrates the involvement of surface oxides in the catalytic reaction. The voltammetric response of the GNF electrode toward methanol at different scan rates (ν) reveals that the peak current is approximately linear with ν1/2 at lower scan rates (-0.6 V, and the voltammetric response was not well defined, demonstrating that GNFs efficiently catalyze the reduction of both O2 and H2O2. The catalytic peak current and the peak potential for the oxidation of methanol and the reduction of oxygen depend on the coverage of GNFs on the electrode surface. The coverage of GNFs (θ) was calculated from the surface area of GNFs and the geometrical surface area of the underlying electrode.37 The catalytic current for the oxidation of methanol and the reduction of oxygen increases with increasing coverage of GNFs on the electrode surface (Figure 9). It is interesting that the peak potential for the oxidation of methanol shifts to a less-positive potential and the potential for the reduction of oxygen shifts to less-negative potential while the coverage of GNFs on the electrode surface increases (Figure 9), showing that catalytic reactions are more favorable at high GNF coverage. However, we observed a decrease in the catalytic current density with particle coverage. These voltammetric features can be explained by considering the GNF electrode to be a nanoparticle ensemble electrode.11,38 The voltammetric response at an ensemble of micro/nanoelectrodes depends on the distance between the electrode elements and the scan rate.38 In the case of the total overlap regime, the diffusion layer at the individual elements of a nanoelectrode has overlapped to produce a diffusion layer that is linear with respect to the entire geometrical area of the nanoelectrode ensemble.38 The high current density observed at low GNF coverage is due to the efficient mass transport of the reactant (oxygen or methanol) to the electrode surface. The shift in the peak potential while changing the GNF coverage can be ascribed to the change in the diffusion pattern.11,38 The diffusion pattern is expected to change from mixed spherical and linear diffusion at low GNF coverage to linear diffusion at high GNF coverage. It was demonstrated earlier that such a change in the diffusion pattern results in a negative shift of the peak potential.11,38e (37) GNF coverage (θ) has been calculated from the ratio of the GNF area to the geometrical surface area of the underlying conducting substrate. (38) (a) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (b) Scharifker, B. R. J. Electroanal. Chem. 1988, 240, 61-76. (c) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762-766. (d) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51.

Figure 9. Particle-coverage-dependent electrocatalytic activity of the GNF electrode toward (A) the oxidation of methanol and (B) the reduction of oxygen. Particle coverages: (a) 0.1, (b) 0.22, (c) 0.39, (d) 0.75, and (e) 0.82. Scan rates: (A) 10 and (B) 50 mV/s.

Figure 10. Cyclic voltammograms for (A) the oxidation of methanol and (B) the reduction of oxygen at (a) GNF and (b) SGN electrodes. Scan rates: (A) 10 and (B) 50 mV/s.

It is worthwhile to compare the electrocatalytic activity of GNFs with spherical gold nanoparticles (SGNs). Figure 10 compares the electrocatalytic activity of GNFs and SGNs toward oxygen and methanol. The SGN electrode shows only one peak for the reduction of oxygen to H2O2 at (∼-0.21 V), which is 110 mV more negative than that on the GNF electrode. On the GNF electrode, the reduction starts at a much less negative

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potential than that on the SGN electrode (vide supra). In the case of methanol oxidation, the SGN electrode shows the oxidation peak at a more positive potential (0.315 V) than does the GNF electrode. The catalytic current for oxygen reduction and methanol oxidation on the SGN electrode is much lower than that on the GNF electrode. The high catalytic current obtained on the GNF electrode can be ascribed either to the high catalytic activity of GNFs or to the increase in the surface area of GNFs. To understand the possible effect of surface area on the catalytic current, the catalytic current density for the oxidation of methanol and the reduction of oxygen was calculated using the surface area of the nanoparticles on the silicate network. The surface area of SGN was determined to be 0.0298 cm2. It is interesting that the current density for the oxidation of methanol on the GNF electrode is ∼2 times higher than those on the SGN electrode, demonstrating that the high catalytic current observed at the GNF electrode is not due to the increase in surface area; the high catalytic activity of GNFs is ascribed to its flower-like morphology.

oxygen; the catalytic oxidation of methanol in alkaline solution occurs at ∼0.27 V, which is much less positive than that on a polycrystalline Au electrode. The GNF electrode shows two distinct voltammetric peaks for the two-step four-electron reduction of oxygen at a less-negative potential at neutral pH. The electrocatalytic activity of GNFs strongly depends on their coverage on the electrode surface, which is in accordance with the theory of micro/nanoelectrode ensembles. The GNFs have significantly higher electrocatalytic activity than the spherically shaped nanoparticle. The high electrocatalytic activity is ascribed to the flower-like morphology of the nanoparticles.

Conclusions

Supporting Information Available: Time-dependent changes in absorbance during the growth of GNFs, voltammograms obtained for the oxidation of methanol at different concentrations and at different scan rates, and voltammograms for the reduction of oxygen at different concentrations of hydrogen peroxide. This material is available free of charge via the Internet at http://pubs.acs.org.

Nanostructured Au particles having flower-like morphology are synthesized by a one-pot ecofriendly route. The GNFs have been self-assembled on the silicate network, and their spectral and electrocatalytic properties have been investigated. GNFs show excellent electrocatalytic activity toward methanol and

Acknowledgment. This work is supported by grants from CSIR (01/1895/03/EMR/-11) and DST (SR/S5/NM-80/2006). We are grateful to Dr. Asim Bhowmik, Indian Association for the Cultivation of Sciences, Kolkata, for DRS measuremenst. We thank Professor Chacko Jacob of the Materials Science Center, IIT Kharagpur, for helpful discussions.

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