Bimetallic Palladium−Gold Nanostructures: Application in Ethanol

Chem. Mater. 2009, 21, 3677–3683 3677 DOI:10.1021/cm901364w

Bimetallic Palladium-Gold Nanostructures: Application in Ethanol Oxidation Fayc-al Ksar,† Laurence Ramos,‡ Bineta Keita,† Louis Nadjo,† Patricia Beaunier,§ and Hynd Remita*,† †

‡

Laboratoire de Chimie Physique, UMR 8000-CNRS, Universit e Paris-Sud 11, 91405 Orsay, France, Laboratoire des Colloı¨des, Verres et Nanomat eriaux, UMR 5587-CNRS, Universit e Montpellier II, 34095 eactivit e de Surface, UMR 7197-CNRS, Universit e Montpellier Cedex 05, France, and §Laboratoire de R Paris-VI, 75252 Paris Cedex 05, France Received May 18, 2009. Revised Manuscript Received June 19, 2009

Bimetallic Pd-Au nanostructures were synthesized in the soft templates provided by surfactant hexagonal mesophases. The nanostructures are constituted by a core rich in gold and a Pd porous shell. The electrocatalytic activity of these nanostructures for ethanol oxidation in basic medium was compared with that of alloyed Pd-Au nanoparticles synthesized in solution. The Pd-Au alloy is active toward the oxidation of ethanol in an alkaline medium but is not durable in realizing this process. The Pdshell-Aucore nanostructures synthesized in mesophases are promising for application in direct ethanol fuel cells as they exhibit a very good electrocatalytic activity and a high stability. Introduction Bimetallic nanomaterials have attracted much attention because of their potential application in various technologically important fields due to their unique catalytic, electrocatalytic, electronic, and magnetic properties, which differ from their monometallic counterparts. Bimetallic nanoparticles often exhibit enhanced catalytic performances in terms of activity, selectivity, and stability, compared to the separate component.1,2 The catalytic and electrocatalytic activities could strongly depend on the size and shape of the metal nanoparticles.3-7 Therefore, synthesis of bimetallic nanoparticles that could exhibit well-controlled shapes, sizes, chemical composition, and structure has been explored in order to enhance their performances.1,2,8 Direct alcohol fuel cells (DAFCs) are one of the very promising power sources for portable electronic devices and on-board vehicles.9 Pt-based catalysts are extensively *Corresponding author. E-mail: [email protected].

(1) Redjala, T.; Remita, H.; Apostolescu, G.; Mostafavi, M.; Thomazeau, C.; Uzio, D. Gas Oil Sci. Technol. 2006, 61, 789. (2) Wang, D.; Villa, A.; Porta, F.; Prati, L.; Su, D. J. Phys. Chem. C 2008, 112, 8617. (3) Fukuoka, A.; Higashimoto, N.; Sakamoτo, Y.; Inagaki, S. Fukushima, Y.; Ichikawa, M. Microporous Mesoporous Mater. 2001, 48, 171. (4) Sasaki, M.; Osada, M.; Higashimoto, N.; Yamamoto, T.; Fukuoka, A.; Ichikawa, M. J. Mol. Catal. A: Chem. 1999, 141, 223. (5) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (6) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (7) Berhault, G.; Bisson, L.; Thomazeau, C.; Verdon, C.; Uzio, D. Appl. Catal., A 2007, 327, 32. (8) Teng, X.; Wang, Q.; Liu, P.; Han, W.; Frenkel, A. I.; Wen, W.; Marinkovic, N.; Hanson, J. C.; Rodriguez, J. A. J. Am. Chem. Soc. 2008, 130, 1093. (9) Lamy, C.; Belgsir, E. M.; Leger, J.-M. J. Appl. Electrochem. 2001, 31, 799. r 2009 American Chemical Society

studied for their electrocatalytic applications. Indeed platinum is very efficient for different reactions involved in fuel cells such as methanol oxidation, hydrogen production, and oxygen reduction.10 Nevertheless, it has been shown recently that palladium is more active than platinum for ethanol oxidation in basic media.11-14 It has to be noted that ethanol is less toxic than methanol and can be produced in large quantities from biomass. As Pd is 50 times more abundant on earth and cheaper than Pt, Pd-based nanomaterials appear as alternative electrocatalysts for replacing Pt in DAFCs. Palladium plays also a crucial role in catalysis and is involved in various reactions, especially for the formation of C-C bonds in organic reactions such as Heck, Suzuki, and Stille coupling15-20 and for the hydrogenation of polyunsaturated hydrocarbons.1,7 Palladium displays also a remarkable performance in H2 storage and sensing.21 Palladiumbased bimetallic catalysts have been developed for many (10) Arico, A. S.; Srinavasan, S.; Antonucci, V. Fuel cells 2001, 1, 133. (11) Gupta, S. S.; Datta, J. J. Power Sources 2005, 145, 124. (12) Liu, J.; Ye, J.; Xu, C.; Jiang, S. P.; Tong, Y. Electrochem. Commun. 2007, 9, 2334. (13) Xu, C.; Wang, H.; Shen, P. K.; Jiang, S. P. Adv. Mater. 2007, 19, 4256. (14) Mackiewicz, N.; Surendran, G.; Remita, H.; Keita, B.; Zhang, G.; Nadjo, L.; Hagege, A.; Doris, E.; Mioskowski, C. J. Am. Chem. Soc. 2008, 130, 8110. (15) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (16) Astruc, D. Inorg. Chem. 2007, 46, 1884. (17) Franzen, R. Can. J. Chem. 2000, 78, 957. (18) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385. (19) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (20) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Lee, J.; Hyeon, T. J. Am. Chem. Soc. 2004, 126, 5026. (21) Tobiska, P.; Hugon, O.; Trouillet, A.; Gagnaire, H. Sens. Actuators, A 2001, 74, 168. Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. Langhammer, C.; Zoric, I.; Kasemo, B. Nano Lett. 2007, 7, 3122.

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years to increase the selectivity of the hydrogenation of polyunsaturated hydrocarbons, a reaction used to produce highly purified olefin cuts.1 The promotion effect of Au on Pd catalytic activity is a well-known phenomenon for different reactions,22,23 including the commercial production of vinyl acetate.24 Pd-Au and Pt-Au catalysts have been studied for polyol (sorbitol and glycerol) oxidation: the addition of Au to Pd or Pt catalysts does not only improve catalytic activity and selectivity for the polyol oxidation but also enhances the resistance to poisoning.2,25 On the other hand, it has been shown recently that gold-based nanoparticles are promising electrocatalysts for application in fuel cells.26,27 Surfactant mesophases have proved to be useful and versatile soft templates for the synthesis of nanostructured materials. Attard et al. demonstrated that standard direct hexagonal liquid crystals made by a ternary mixture (nonionic surfactant, metal salts, and water) can template the synthesis of bulk porous materials and porous metal films by electro-deposition.28 A recent review by Yamauchi and Kuroda reports on the use of mesophases for the synthesis of mesoporous metals.29 We have recently shown that giant direct hexagonal mesophases made by a quaternary system (water, surfactant, cosurfactant, and oil) can be used as nanoreactors to synthesize nanostructured materials (metals, polymers, oxides) both in the aqueous and in the oil phases.30-34 The swollen mesophases consist of surfactant-stabilized oil-swollen tubes that are arranged on a triangular lattice in an aqueous medium (see scheme insert of Figure 1).35 These mesophases are very stable in a large pH domain and can be doped with a large amount of metal salts of (22) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (23) Venezia, A. M.; La Parola, V.; Deganello, G.; Pawelec, B.; Fierro, J. L. G. J. Catal. 2003, 215, 317. (24) Neurock, M.; Provine, W. D.; Dixon, D. A.; Coulston, G. W.; Lerou, J. J.; van Santen, R. A. Chem. Eng. Sci. 1996, 51, 1691. (25) Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F. Catal. Today 2005, 102, 203. (26) Cameron, D.; Holliday, R.; Thompson, D. J. Power Sources 2003, 118, 298. (27) Mirdamadi-Esfahani, M.; Mostafavi, M.; Keita, B.; Nadjo, L.; Kooyman, P.; Etcheberry, A.; Imperor, M.; Remita, H. Gold Bull. 2008, 41, 98. :: (28) Attard, G. S.; Goltner, C. G.; Corker, J. M.; Henke, S.; Templer R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 1315. Attard, G. S.; Bartlett, P. N.; Colemen, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (29) Yamauchi, Y.; Kuroda, K. Chem.-Asian J. 2008, 3, 664. (30) Surendran, G.; Pena dos Santos, E.; Tokumoto, M. S.; Remita, H.; Ramos, L.; Kooyman, P. J.; Santilly, C. S.; Bourgaux, C.; Dieudonne, P.; Prouzet, E. Chem. Mater. 2005, 17, 1505. (31) Surendran, G.; Apostelescu, G.; Tokumoto, M.; Prouzet, E.; Ramos, L.; Beaunier, P.; Kooyman, P. J.; Etcheberry, A.; Remita, H. Small 2005, 1, 964. (32) Ksar, F.; Surendran, G.; Ramos, L.; Keita, B.; Nadjo, L.; Prouzet, E.; Beaunier, P.; Hagege, A.; Audonnet, F.; Remita, H. Chem. Mater. 2009, 21, 1612. (33) Surendran, G.; Ramos, L.; Pansu, B.; Prouzet, E.; Beaunier, P.; Audonnet, F.; Remita, H. Chem. Mater. 2007, 19, 5045. (34) Surendran, G.; Ksar, F.; Ramos, L.; Keita, B.; Nadjo, L.; Prouzet, E.; Beaunier, P.; Dieudonne, P.; Audonnet, F.; Remita, H. J. Phys. Chem. C 2008, 112, 10740. (35) Ramos, L.; Fabre, P. Langmuir 1997, 13, 682. (36) Pena dos Santos, E.; Tokumoto, M. S.; Surendran, G.; Remita, H.; Bourgaux, C.; Dieudonne, P.; Prouzet, E.; Ramos, L. Langmuir 2005, 21, 4362.

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Figure 1. SAXS spectrum of the Pd-Au doped hexagonal mesophase and scheme of the structure of the mesophase. The arrows point to the three main peaks.

various types.36 Pd nanowires and porous Pd nanoballs synthesized in these hexagonal mesophases display very high electrocatalytic activity for ethanol oxidation.32,34 Herein, we report the radiolytic synthesis of palladium-gold nanoparticles and nanostructures in solution or in hexagonal mesophases. We show that the bimetallic nanostructures synthesized in soft templates are promising electrocatalysts in fuel cell application as they exhibit both very good activity and stability for ethanol oxidation. Experimental Section We used tetraaminepalladium(II), Pd(NH3)4Cl2, and ethylene diamine gold(III), [Au(en)2]Cl3, as metallic salts. [Au(en)2]Cl3 was synthesized from the synthetic method reported in the literature,37 and Pd(NH3)4Cl2 was used as received. Bimetallic Pd-Au structures were synthesized respectively in solution and in hexagonal mesophases. They are respectively denoted Pd-Ausol and Pd-Aumeso. For the synthesis of Pd-Ausol, aqueous solutions, containing Pd(NH3)4Cl2 and [Au(en)2]Cl3 (total concentration 2
10-3 M and [PdII]/[AuIII]=4) in the presence of 0.1 M 2-propanol (as OH• scavenger to enhance the radiolytic yield)38 and 0.1 M poly(acrylic acid) (PA from Aldrich, molecular weight 2000 g/mol) as stabilizer for the nanoparticles, were exposed to γ-rays (Co60 panoramic source: dose rate 2.2 kGy 3 h-1). For the preparation of Pd-Au doped mesophases, 1.03 g of the surfactant cetyltrimethylammonium bromide (CTAB) is first dissolved in 2 mL of water with Pd(NH3)4Cl2 and [Au(en)2]Cl3 (total metal salts concentration = 0.1 M and [PdII]/[AuIII] = 4), to give a transparent and viscous micellar solution. The subsequent addition, under stirring, of 2.98 mL of oil (cyclohexane) into the micellar solution leads to a white unstable emulsion. The cosurfactant pentanol (240 μL) is then added to the mixture, which is strongly vortexed for a few minutes. This leads to a perfectly transparent and stable yellow gel: a doped hexagonal mesophase. These mesophases were exposed to γ-irradiation (dose rate 2.2 kGy 3 h-1) for radiolytic reduction to give Pd-Aumeso bimetallic structures. After reduction, the nanomaterials synthesized in the mesophase were extracted in 2-propanol, centrifuged, and washed (37) Block, B. P.; Bailar, J. C. J. Am. Chem. Soc. 1951, 73, 4722. (38) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M. O. New J. Chem. 1998, 22, 1239.

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several times before mounting on carbon-coated copper TEM grids for observations. A few drops of the irradiated solutions were also deposited on TEM grids. Transmission electron microscopy (TEM) observations were performed on a JEOL JEM 100 CXII transmission electron microscope at an accelerating voltage of 100 kV. For high resolution electron microscopy (HRTEM) images we used a JEOL JEM 2010 equipped with a LaB6 filament and operating at 200 kV. The images were collected with a 4008
2672 pixels CCD camera (Gatan Orius SC1000). The chemical analyses were obtained by selected energy-dispersive X-ray spectroscopy (EDS) microanalyzer (PGT-IMIX PC) mounted on the JEM 2010. X-ray scattering experiments were performed using an in-house setup with a rotating anode X-ray generator equipped with two parabolic mirrors giving a highly parallel beam of monochromatic Cu KR radiation (wavelength λ=0.154 nm). The scattered intensity is collected on a two-dimensional detector. The experimental data are corrected for the background scattering and the sample transmission. For electrochemical experiments, the source, mounting, and polishing of the glassy carbon (GC, Le Carbone Lorraine, France) electrodes39 and the fabrication technique of the modified electrodes40 have been described previously. Typically, an electrode is fabricated by depositing 3 μL of the bimetallic Pd-Au suspension in 2-propanol on the polished electrode surface, letting it dry in air, then covering with 3 μL of 5 wt % Nafion solution and letting it dry again in air. An alternative procedure consisting of adding the Nafion solution to the PdAu nanostructure suspension prior to its deposition on the electrode surface could also be used. The electrochemical setup was an EG & G 273 A driven by a PC with the M270 software. Potentials were measured against a Hg/HgO reference electrode. The counter electrode was a platinum gauze of large surface area. Pure water from a RiOs 8 unit followed by a Millipore-Q Academic purification set was used throughout. The solutions were deaerated thoroughly for at least 30 min with pure argon and kept under a positive pressure of this gas during the experiments. The supporting electrolyte was 1 M KOH.

Results and discussion We used a mesophase formed by cetyltrimethylammonium bromide (CTAB) as surfactant, pentanol as cosurfactant, tetraaminepalladium(II) Pd(NH3)4Cl2 and ethylene diamine gold(III) [Au(en)2]Cl3 as salts, and cyclohexane as swelling agent. The translucent Pd-Au doped phase (total metal concentration 0.1 M, [PdII]/[AuIII]=4) is birefringent revealing an anisotropic structure. Small angle X-ray scattering (SAXS) experiments show a p6mm hexagonal symmetry, with three Bragg peaks whose position are in the ratio 1:31/2:2. The measured lattice parameter dc is 24.5 nm (Figure 1). The Pd-Au-doped mesophase was used as a nanoreactor to synthesize bimetallic nanostructures. The samples were exposed to γ-ray irradiation. The hydrated electrons and the reducing radicals produced during the radiolysis of the solvent induce homogeneous reduction in the water phase.38 The pentanol, used as cosurfactant, (39) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 243, 87. (40) Keita, B.; Zhang, G.; Dolbecq, A.; Mialane, P.; Secheresse, F.; Miserque, F.; Nadjo, L. J. Phys. Chem. C 2007, 111, 8145.

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Figure 2. TEM images of palladium-gold nanostructures (a-c) formed in hexagonal mesophases and the corresponding SAED pattern of the core and the shell, dose 22 kGy; (d) synthesized in water solutions and their corresponding SAED pattern, dose 9 kGy.

contributes to PdII and AuIII reduction on the radiolysisinduced seeds. Radiolysis is a powerful method to synthesize bimetallic nanoparticles of controlled size, composition, and structure.38,41 Compared to chemical reducing processes that follow a diffusion front, radiolysis presents also the advantage to induce a homogeneous nucleation and growth in the whole volume. After a dose of 22 kGy, a homogeneous brown gel was obtained. Transmission electron microscopy (TEM) observations showed core-shell nanoparticles formed by a dense core and a porous shell (Figure 2a-c). The diameter of the core was about 60-100 nm, whereas the thickness of the shell was about 15 nm. The selected area electron diffraction (SAED) pattern recorded on the core presents rings giving the interplanar distances of 0.231, 0.202, 0.143, 0.124, and 0.118 nm. The core can be identified as a polycrystalline nanoparticle of gold. For the shell, the indexation of the SAED pattern indicates the d spacing of 0.227, 0.195, 0.145, and 0.120 nm. It can be assigned to cubic Pd (Figure 2c, inset). Figure 3 shows high resolution (HRTEM) micrographs and EDS analyses. The chemical analyses confirmed that the core was rich in gold (Figure 3, spectrum a2) while the porous shell was composed of palladium (Figure 3, spectrum a3). Moreover, small bromine signals, originating from the surfactant CTAB, were detected which indicated its presence even after several washings. The other peaks as C and Cu came from the carbon-coated copper grids. A very few nanoparticles were pure gold (Figure 3, spectrum a1). Figure 3b shows the polycrystallinity of the Pd rich shell. We observed small crystallized domains (5 nm in diameter). The fast Fourier transformation (FFT) revealed one main diffraction ring and two discrete symmetric points. The reciprocal distances (41) (a) Remita, H.; Khatouri, J.; Treguer, M.; Amblard, J.; Belloni, J. Z. Phys. D 1997, 40, 127. (b) Treguer, M.; de Cointet, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J.; De Keyser, R. J. Phys. Chem. B 1998, 102, 4310. (c) Remita, H.; Etcheberry, A.; Belloni, J. J. Phys. Chem. B 2003, 107, 31. (d) Remita, H.; Lampre, I.; Mostafavi, M.; Balanzat, E.; Bouffard, S. Radiat. Phys. Chem. 2005, 72, 575.

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Figure 3. High resolution TEM images (a and b) of palladium-gold nanostructures formed in hexagonal mesophases (dose 22 kGy) and the EDS analyses (a1-a3) showing a few pure gold particles (a1) and bimetallic nanostructures with a core rich in gold (a2) and a shell formed by Pd (a3) (* Au peak, b Pd peak, and 1 Br peak). Figure 3 b corresponds to a zoom of the selected area on Figure 3 a showing the Pd rich shell of a Pd-Au particle and its FFT pattern.

found, d1=0.223 nm and d2=0.196 nm, corresponded to the (111) and (200) planes lattice of cubic Pd. For comparison, nanoparticles from the same precursors were synthesized in aqueous solutions. The evolution of the UV-visible absorption spectrum of the aqueous solutions containing Pd(NH3)4Cl2 and [Au(en)2]Cl3 in presence of 0.1 M 2-propanol and 0.1 M poly(acrylic acid) with irradiation dose is shown in Figure 4. No plasmon band of gold nanoparticles at around 510 nm is observed, and the evolution of the spectrum (which conserves its shape) seems to indicate that the structure of the particle does not change with irradiation dose. This behavior is in agreement with an alloy structure.38,41 TEM and HRTEM observations showed small spherical particles with a diameter of 4-5 nm (Figure 2d and Figure 5). The SAED pattern presented several diffraction rings corresponding respectively to the interplanar distances of 0.226, 0.198, 0.138, 0.119, and 0.089 nm. Our measures are in agreement with the synthesis of a cubic Pd80-Au20 alloy, for which the interplanar distances (according to the Vegard’s law) are 0.227, 0.196, 0.138, 0.118, and 0.089 nm. Furthermore, the EDS spectrum confirmed the chemical homogeneity of the nanoparticles (Figure 5c). The reciprocal distances of 0.228 and 0.197 nm measured on the FFT (Figure 5b) are in agreement with the SAED data. Previous experiments have shown that radiolysis of a solution containing other metal precursors, PdCl2 and KAuCl4, leads to formation of Aucore-Pdshell nanoparticles (of typical diameter 3-4 nm) at low dose rate (by γ-irradiation) or to alloyed Au-Pd nanoparticles (of typical diameter 2-3 nm) at high dose rate (electron beam irradiation).41c This difference in the nanoparticles structure is due to electron transfer from PdI or nascent

Figure 4. Evolution with dose of the UV-visible absorption spectrum of a solution containing Pd(NH3)4Cl2 and [Au(en)2]Cl3 (total concentration 2
10-3 M and [PdII]/[AuIII]=4) in the presence of 0.1 M 2-propanol and 0.1 M poly(acrylic acid). Optical path length=2 mm.

Pd0 induced by radiolysis to gold complexes (AuIII or AuI) occurring only at low dose rate when the reduction kinetics is slow. The present experiments show that, in addition to the dose rate, the nature of the metal precursors has also an importance on the final structure of the bimetallic nanoparticles. The bimetallic nanostructures obtained in solutions and in mesophases are different probably because the reduction/nucleation kinetics are not similar in the two media. The origin of this difference can be attributed to the different metal precursor concentrations, to the confinement (which decreases the diffusion of the species), and to the interactions of the metallic precursors with the surfactant and the cosurfactant, for synthesis in mesophases. In solution, gold and palladium complexes are reduced simultaneously leading to alloys while in mesophase gold is presumably reduced first, leading to

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Figure 5. (a) High resolution transmission electron microscopy of Pd-Au nanoparticles synthesized in water solutions and stabilized by poly(acrylic acid) (the concentrations are the same as in Figure 4, dose = 6 kGy), (b) corresponding FFT, and (c) corresponding EDS analysis (*Au peak and b Pd peak).

particles with large Au cores. The templating effect provided by the mesophase is observed for Pd which is reduced on the Au core leading to a porous Pd shell formed by connected nanowires. It has to be noted that previous work has demonstrated the templating effect of the mesophase on Pd growth: Pd reduction in CTABbased hexagonal mesophases has shown that porous Pd nanoballs formed by connected nanowires were obtained by γ-irradiation.34 The structure of the Pd shell (Figure 2a-c) is remarkably similar to that of the Pd nanoballs. Pd-Aumeso nanostructures were used for ethanol electrocatalytic oxidation in an alkaline medium and the results compared with those obtained with Pd-Ausol bimetallic alloyed nanoparticles. In pure 1 M KOH, the voltammetric pattern associated with a bimetallic PdAumeso nanostructure-modified glassy carbon electrode (Figure S1, Supporting Information) combines characteristic features close to those of electrodes modified separately by Pd or Au- nanostructures. As Pd-modified electrodes were demonstrated to be very active for ethanol electrooxidation in the alkaline medium,12,32,34 this reaction was selected for electrocatalytic studies. Figure 6 (black solid line curve) shows the first voltammogram run in 1 M KOH containing 1 M EtOH at a scan rate of 50 mV s-1, with an electrode modified with the bimetallic PdAumeso nanostructures. This electrode was the same as that previously characterized in pure 1 M KOH (Figure S1, Supporting Information). The observed EtOH electrooxidation pattern is reminiscent of that of the methanol electrooxidation process on Pt-based electrocatalysts in acidic media.42 This voltammetric pattern is constituted by two well-defined current peaks, one on the forward and the other on the reverse potential scans. The forward scan peak current is related to the oxidation of freshly chemisorbed species issued from alcohol adsorption. The reverse scan peak represents the removal of carbonaceous species not completely oxidized in the forward scan. However, as a difference with methanol (42) Jiang, S. P.; Liu, Z.; Tang, H. L.; Pan, M. Electrochim. Acta 2006, 51, 5721.

Figure 6. Superposition of the first (black solid line curve) and the 200th (red solid line curve) cyclic voltammetric runs associated with the electrocatalytic oxidation of 1 M EtOH in 1 M KOH with continuous cycling of the electrode potential. The working electrode was a glassy carbon disk modified with the bimetallic Pd-Aumeso nanostructures synthesized in the mesophase as described in the text. The reference electrode was an Hg/HgO (1 M KOH) electrode. The scan rate was 50 mV s-1.

electrooxidation, the ratio of the forward anodic peak current density (If) to the backward anodic peak current density (Ib) cannot directly quantify the catalyst tolerance to carbonaceous species accumulation. As a matter of fact, even though the mechanism of ethanol oxidation in alkaline media is not exactly known, sufficient evidence from studies in acidic media would suggest here also a dual pathway mechanism.43,44 In that case, the (If/Ib) ratio remains a comparative measure of the overall electroactive pathways, provided the positive potential limit is strictly restricted to the domain of only one oxidation wave. The main features determined from the voltammogram (Figure 6) include Eonset, the onset potential of the faradaic current, as well as Ef and Eb, the potentials (43) Rao, V.; Hariyanto; Cremers, C.; Stimming, U. Fuel Cells 2007, 7, 417. (44) Rao, V.; Cremers, C.; Stimming, U.; Cao, L.; Sun, S.; Yan, S.; Sun, G.; Xin, Q. J. Electrochem. Soc. 2007, 154, B1138.

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corresponding to the maxima of the forward peak current and of the backward peak current, respectively. We measure Eonset: -680 mV; Ef =-248 mV; and Eb = -308 mV. In short, the characteristics of the present bimetallic Pd-Aumeso nanostructure-modified electrode are more negative than those measured with other systems described in the literature. For example, the different potentials measured in the present work indicate a better electrocatalytic activity of the bimetallic Pd-Au nanostructures than the Pd nanoballs (Eonset: -550 mV; Ef = -151 mV; Eb = -296 mV) published recently34 as featuring one of the best literature achievements. It comes out that our results suggest the bimetallic Pd-Au nanostructures synthesized in soft hexagonal phases to constitute a new class of material, which is very promising for use in direct ethanol fuel cells, subject to durability. To test this point, a series of potential cycling experiments (200 cycles) and a one-hour chronoamperometry were carried out. The same modified electrode was used throughout for these two kinds of experiments. Figure 6 shows, in superposition to the first cyclic voltammogram (black solid line curve), the 200th cycle (red solid line curve) between -1020 mV and þ480 mV vs a Hg/HgO reference electrode. Table S1 (Supporting Information) gathers the main quantitative parameters measured from these two voltammograms. During these potential cycles, acquired at a scan rate of 50 mV s-1, no shift was observed for the onset potential, the forward peak potential, or the backward peak potential. Strikingly, the peak current densities increased for both the forward and the backward scans and eventually stabilized around 1 mA cm-2 (Figure 6). Meanwhile, the ratio of these currents remains constant (Figure 6 and Table S1, Supporting Information). This behavior stands in contrast with the classically expected decrease of alcohol oxidation current as a function of the number of cycles. As a consequence, the following lines of evidence were checked to confirm the actuality of our observation: (i) the ethanol oxidation was performed after prior stabilization of the modified electrodes in the 1 M KOH electrolyte, and (ii) replication of the same sequence of experiments with independently prepared electrodes results in the same observations. Even though the exact phenomenon behind this behavior is not known at present, a few hypotheses can be put forward. Such improvement in modified electrode characteristics might be attributed to some reorganization of the surface film. It is also possible that the remarkable electrocatalytic activity of Pd-Aumeso structures can be due to the high defect density of Pd in the particle shell.45 Finally, gradual cleaning of traces of the capping agents issued from the initial mesophase matrix cannot be discarded. The bimetallic Pd-Aumeso nanostructure-modified electrode appears to exhibit very good and stable characteristics after the potential cycling experiment. These characteristics serve as initial conditions at the beginning (45) Cherstiouk, O. V.; Gavrilov, A. N.; Plyasova, L. M.; Molina I. Yu.; Tsirlina, G. A.; Savinova, E. R. J. Solid State Electrochem. 2008, 12, 497.

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Figure 7. Chronoamperometric curves for ethanol electrooxidation at -0.3 V vs Hg/HgO on a glassy carbon electrode modified with bimetallic Pd-Aumeso nanostructures synthesized in a hexagonal mesophase (black curve) or Pd nanoballs (red curve). The solution was 1 M KOHþ1 M ethanol.

of the chronoamperometry experiment. The potential was stepped from -1020 mV to -300 mV vs Hg/HgO. With the potential set to -300 mV vs Hg/HgO, the current intensity was monitored for at least one hour (Figure 7, black line). It is worth pointing out that directly comparable literature chronoamperometry experiments were carried out for 17 min46 and for 30 min.13 However, in a recent work from our group, Pd nanoballs were shown to exhibit remarkable electrocatalytic behaviors for ethanol oxidation in 1 M KOH.34 Therefore, this material was selected for a comparison between the chronoamperogram recorded with the electrode modified with bimetallic Pd-Au nanostructures (Figure 7, black line) and that run, in identical experimental conditions, with an electrode modified with Pd nanoballs (Figure 7, red line). Qualitatively, the decrease of the current appears much steeper in the case of the Pd nanoball-modified electrode than for the bimetallic Pd-Aumeso nanostructures. Quantitative measurements at selected durations on the two chronoamperograms are collected hereafter and expressed as the ratio between the currents on the bimetallic Pd-Aumeso nanostructure modified electrode and the Pd nanoball-modified electrode. The ratio is approximately 1.1 at the starting of the experiments. Subsequent ratios were found to increase with time, and values of 1.9, 6.6, and 7.5 are obtained respectively after 200, 1000, and 3600 s monitoring of the currents. In short, Figure 7 shows a larger durability of the present bimetallic Pd-Aumeso nanostructure-modified electrode compared to Pd nanoball-modified electrode. Comparison between Pd-Aumeso and Pd-Ausol modified electrodes give strikingly contrasting results. While the current density increases and finally stabilizes during ethanol oxidation by the Pd-Aumeso modified electrode, the contrary is observed with Pd-Ausol modified electrode (Figure S2, Supporting Information). The characteristic parameters for this last electrode are gathered in (46) Xu, C.; Cheng, L.; Shen, P.; Liu, Y. Electrochem. Commun. 2007, 9, 997.

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Table S2 (Supporting Information). After 200 potential cycles in the same conditions as those used previously in Figure 6, the numerical values of the peak intensity, If and Ib, are only 25% of their initial numerical values. However, no shift in the peak potential locations was observed. Independent replications of these experiments confirm all these trends. The reasons for such radically contrasting behaviors might probably be paralleled with the difference in spatial composition observed in Figure 2 between Pd-Aumeso and Pd-Ausol and described in the text. Gold-based electrocatalysts have recently been recognized as very active for a series of reactions and are finding widespread use. For example, Pd-Au electrocatalysts are remarkably efficient for ethanol oxidation in alkaline media.47,48 Gold nanoparticles decorated with mono- or submonolayer Pd atoms (denoted as Pd@Au/C) show an enhanced activity compared to Au or Pd nanoparticles alone.47 In identical experimental conditions, the (If/Ib) ratio of 1.05 obtained with PdAumeso compares favorably with the 0.69 value for Pd@Au/C47 and highlights the better electrocatalytic activity of the former nanostructure. Also comparison with the Pd4Au alloy48 is clearly in favor of Pd-Aumeso. In conclusion, the results altogether suggest that the PdAumeso-modified electrode should be considered as very active and durable in the present experimental conditions. (47) Zhu, L. D.; Zhao, T. S.; Xu, J. B.; Liang, Z. X. J. Power Sources 2009, 187, 80. (48) He, Q.; Chain, W.; Mukerjee, S.; Chain, S.; Laufek, F. J. Power Sources 2009, 187, 298.

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Conclusion In summary, we have synthesized Pd-Au nanostructures. In solution synthesis, our measures indicate that alloyed Pd80-Au20 nanoparticles were obtained. The structures synthesized using hexagonal mesophases as a soft template are constituted by a core rich in gold and a Pd porous shell. As the mesophases are doped by high concentrations of palladium-gold (total concentration 0.1 M), relatively large quantities of Pd-Au nanostructures are obtained. The Pd80-Au20 alloy is active toward the oxidation of ethanol in an alkaline medium but is not durable in realizing this process. In contrast, the Pdshell-Aucore nanostructures synthesized in mesophases are promising for application in direct ethanol fuel cells as they exhibit a very good electrocatalytic activity for this process associated with a high stability. These nanostructures might also find applications in catalysis and sensing. Acknowledgment. Dr. Periasamy Selvakannan (LCP, Universite Paris-Sud 11) and Dr. Laurent Delannoy (LRS, Universite Paris-VI) are acknowledged for the synthesis of [Au(en)2]Cl3. We thank P. Dieudonne (LCVN) for assistance during the X-ray experiments. Supporting Information Available: Cyclic voltammogram of the bimetallic Pd-Aumeso and the main electrochemical characteristics of the Pd-Au nanostructures (Pd-Aumeso and Pd-Ausol) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.