Palladium Nanoballs Synthesized in Hexagonal Mesophases - The

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Palladium Nanoballs Synthesized in Hexagonal Mesophases Geetarani Surendran,† Fayc¸al Ksar,† Laurence Ramos,‡ Bineta Keita,† Louis Nadjo,† Eric Prouzet,§,| Patricia Beaunier,⊥ Philippe Dieudonne´,‡ Fabrice Audonnet,† and Hynd Remita*,† Laboratoire de Chimie Physique, UMR 8000-CNRS, UniVersite´ Paris-Sud, 91405 Orsay, France, Laboratoire des Colloı¨des, Verres et Nanomate´riaux, UMR 5587-CNRS, UniVersite´ Montpellier II, 34095 Montpellier Cedex 05, France, Institut Europe´en des Membranes, UMR 5635-CNRS-ENSCM-UM2, CNRS, F-34293 Montpellier, France, Department of Chemistry, UniVersity of Waterloo, 220 UniVersity AV. West, Waterloo, ON, N2L 3G1 Canada, and Laboratoire de Re´actiVite´ de Surface, UMR 7609-CNRS, UniVersite´ Paris-VI, 75252 Paris Cedex 05, France ReceiVed: February 27, 2008; ReVised Manuscript ReceiVed: April 03, 2008

Three-dimensional connected Pd nanowires forming nanoballs have been synthesized by slow reduction of PdII in hexagonal mesophases made by a quaternary system (water/cyclohexane/surfactant/cosurfactant). Both confinement and slow reduction are necessary to obtain these new nanostructures. This material exhibits remarkable electrocatalytic behavior for ethanol oxidation in 1 M KOH and appears as a promising candidate for use in direct ethanol fuel cells. Introduction Mesoporous precious metals, especially platinum and palladium, with pore size in the range 2-50 nm, are of great interest because of their technological importance in different fields such as catalysis,1 electrocatalysis and sensing.2 Palladium, one of the most efficient metals in catalysis, is involved in many industrial applications, especially for the formation of C-C bonds in organic reactions such as Heck, Suzuki, and Stille coupling3–8 and for the hydrogenation of polyunsaturated hydrocarbons.9,10 Palladium is also very efficient in electrocatalytic oxidation of ethanol for fuel cell applications11,12 and displays as well a remarkable performance in H2 storage and sensing.13 Since it was demonstrated that the catalytic and electrocatalytic activities could strongly depend on the size and shape of the metal nanoparticles,14,15 synthesis of nanoparticles that could exhibit well-controlled shapes and sizes has been explored in order to enhance their performances.16,17 Different palladium nanostuctures have been reported, including nanoparticles,18,19 nanowires,20 mesoporous ball-shaped structures (with 1Dporosity), and others.21 In parallel, there is a continuous trend to reduce the total amount of palladium inserted in the catalyst while keeping the same catalytic or electrocatalytic performance. Two ways are explored to reach this goal: increasing the surface over volume ratio by a size reduction and improving the activity by creating specific shapes that will expose preferential crystallographic planes. Therefore, synthesis of small nanoparticles or porous nanostructures with interconnected structures can combine both assets in enhancing the catalytic or electrocatalytic activities as they can supply different adsorption sites for the reactions involving two or more reactants. Here we describe the soft template radiolytic synthesis of three-dimensional porous * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. † UMR 8000-CNRS. ‡ UMR 5587-CNRS. § UMR 5635-CNRS-ENSCM-UM2. | University of Waterloo. ⊥ UMR 7609-CNRS.

Pd nanostructures formed by interconnected nanowires. For sensing and catalysis, such materials with open porosity in three dimensions are favored because they allow unlimited transport of the molecules of the medium.1 These Pd nanostructures show a very good electrocatalytic activity for ethanol oxidation and appear as promising candidates for fuel cell applications. Very few examples have been reported for the preparation of three-dimensional porous Pd nanostructures. Pd nanowires have been previously synthesized in mesoporous silicas (FSM16 and HMM-1).20 In addition, arrays of palladium nanostructures have been prepared inside MCM-41, and these ball-shaped palladium nanocatalysts present a very good selectivity for the cleavage of benzyl ethers through cooperative properties.22 Nevertheless, a major drawback of the use of hard templates is that it requires a harsh chemical treatment such as hydrofluoric acid to extract the nanomaterials synthesized within the hardtemplate. Mesophases resulting from surfactant self-assembly provide a class of useful and versatile templates for generating one-, two-, or three-dimensional nanostructures in relatively large quantities and these nanostructures can be extracted easily by the dissolution of the mesophase. In 1997, Attard and co-workers successfully prepared mesoporous Pt from lyotropic liquidcrystal templates23 made by a ternary mixture of a nonionic surfactant octaethyleneglycol monohexadecyl ether (C16EO8), hexachloroplatinic acid (H2PtCl6), and water. Soft templating generally offers a high degree of control over the pore size as well as microstructure periodicity, but most reported techniques have resulted so far in materials with unidimensional porosity, such as arrays of tubes.22,23 Giant hexagonal mesophases made by a quaternary system (water, surfactant, cosurfactant, and oil) can be used as nanoreactors to synthesize nanostructured materials both in the aqueous and in the oil phases.24,25 These mesophases are composed of oil-swollen tubes, which are covered by a monolayer of surfactant and cosurfactant molecules, and are hexagonally packed in a continuous water domain. By adjusting the ionic strength of the aqueous medium, the diameter of the

10.1021/jp801703z CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

Palladium Nanoballs nonpolar tubes can be tuned continuously from 3 to 30 nm, while the distance between the adjacent tubes is nearly constant (∼3 nm).26 These swollen mesophases are very stable in an extended pH domain and can be doped with a large amount of metal salts of various types.27 Recently, porous platinum nanoballs have been synthesized in these mesophases by slow reduction provided by radiolysis.28 Radiolytic reduction is a powerful method to synthesize metal nanoparticles and nanostructures in solutions or in heterogeneous systems because it produces, from the solvent, radiolytic species of strongly reducing potential (hydrated electrons and the reducing radicals) homogeneously in the bulk.29 We note that, compared to chemical reduction processes that follow a diffusion front, radiolysis presents the advantage of inducing a homogeneous nucleation and growth in the whole volume. Experimental Section Preparation of the Mesophase. For the preparation of the swollen hexagonal mesophase, 1.03 g of cetyltrimethylammonium bromide (CTAB) is first dissolved in 2 mL of water with Pd(NH3)4Cl2 (0.1 M), 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 (240 µL of 1-pentanol) is then added to the mixture, which is strongly vortexed for a few minutes. This leads to a perfectly transparent and stable gel: a hexagonal mesophase. Then the mesophases were exposed to γ- or electron beam irradiation. Another hexagonal mesophase was also obtained with another surfactant cetylpyridinium chloride (CPCl; 1.02 g in 2 mL water; keeping the same concentration in PdII). Irradiation Facilities. The γ-irradiation source, located at Orsay, was a 60Co gamma-facility of 7000 Curies with a maximum dose rate of 2500 Gy h-1. The dose rate fixes the reduction kinetics. The solutions were irradiated in glass vessels with a rubber plastic septum. Before irradiation the mesophase was left under N2 bubbling for 15 min to eliminate any O2 traces. Electron beam irradiations were performed with a 20 kW and 10 MeV electron accelerator (CARIC-Ionisos Society located at Saclay) delivering trains of 14 µs pulses (10-350 Hz) through a scanning beam (1-10 Hz) of mean dose rate 2200 Gy s-1 (7.9 MGy h-1). All experiments were performed at room temperature. X-ray Experimental Setup. 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. Measurements of the hexagonal mesophase were performed at a sample-detector distance D ) 1 m, and measurements of the nanoballs were performed at D ) 1 m and D ) 180 mm. The mesophase was put in 1.5 mm thick capillary (Mark-Ro¨hrchen type from GLAS Co., Germany), and the nanoballs sample was sandwiched between two Kapton films. Transmission Electron Microscopy. Transmission electron microscopy (TEM) observations were performed with a JEOL JEM 100CXII transmission electron microscope at accelerating voltage of 100 kV and for high resolution with a JEOL JEM 2010 at 200kV. The sample drops were deposited and dried on carbon-coated copper grids.

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Figure 1. (A) SAXS spectrum of a Pd-doped hexagonal mesophase prior irradiation; (scheme) cross-section of an oil-swollen hexagonal phase. The center-center distance between adjacent tubes is 18.3 nm, and the water channel in between the tubes is ∼3 nm; (B) Small and midangle X-ray scattering spectrum of nanoballs. The two arrows mark the characteristic wave vectors of the scattering pattern and are associated with distances comparable to the pore size and to the thickness of the nanowire, respectively.

Nitrogen Porosimetry. After washing with 2-propanol and centrifugation, the Pd nanomaterials were dried in an oven under primary dynamic vacuum for a few hours at 60 °C to evaporate the solvent. The nitrogen gas adsorption isotherm was performed on a Belsorp-Mini (BelJapan Inc.), in a standard operating mode. The sample was preliminary degassed during 2 h at 35 °C under primary dynamic vacuum. Electrochemistry Tests. The source, mounting, and polishing of the glassy carbon (GC, Le Carbone Lorraine, France) electrodes has been described previously,30 as was also described the fabrication technique of the modified electrodes:31 the electrode is fabricated by depositing a few µL of Pd nanostructures suspension in 2-propanol on a polished electrode surface, covering with 6 µL of 5 wt % Nafion solution, and letting it dry in the air at room temperature. 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 swollen hexagonal mesophase as a soft template for synthesis of Pd nanostructures. This mesophase was made by cetyltrimethylammonium bromide (CTAB), as the surfactant, and the ammonium chloride of palladium Pd(NH3)4Cl2, as the salt (0.1 M). The resulting phase was translucent and birefringent, and exhibited the characteristic textures of a hexagonal liquid crystal phase when viewed between crossed polarizers. Figure 1A displays the small-angle X-ray scattering (SAXS) pattern of the Pd doped mesophase which exhibits the characteristic features of a hexagonal phase with three Bragg peaks whose positions were in the ratio 1:31/2:2. The measured lattice parameter is dc ) 18.3 nm and the diameter of the tubes is of the order 15 nm (see scheme inset Figure 1A). The doped CTAB-cyclohexane-water direct hexagonal liquid crystal was used as a nanoreactor to synthesize palladium nanostructures. The samples were exposed to γ-rays for a slow reduction. With radiolytic reduction Pd0 nuclei are produced homogeneously in the water phase. Along with radiolytic reduction, the pentanol, used as cosurfactant, also slowly reduces PdII on the radiolysis-

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Figure 2. TEM images of palladium nanostructures formed by γ-irradiation at different doses: (a) after 2 h irradiation, (b) after 6 h irradiation, and (c and d) after 20 h irradiation.

induced seeds. A homogeneous black gel was obtained after 20 h γ-irradiation. After reduction, the mesophase was destabilized by addition of 2-propanol. The Pd nanomaterials were extracted by centrifugation and washed several times with 2-propanol. TEM observations at different doses are shown in Figure 2. The TEM pictures after 20 h irradiation reveal that palladium nanostructures display ball-shaped domains of 80-100 nm (Figure 2c,d). These domains consist of a three-dimensionally interconnected network of Pd nanowires forming cells which show a hexagonal shape with an aperture of about 10-12 nm (Figures 2d and 3a), a dimension comparable to that of the tubes’ diameter in the hexagonal mesophase. The average diameter of the nanowires is about 3.5 nm. Note that this dimension is comparable to the thickness of the water channels inbetween the cylinders (3 nm). We show in Figure 1B the small and midangle X-ray intensity scattered by an assembly of Pd nanoballs (obtained after 20 h of irradiation). Given the range of wave vectors, q, accessible experimentally (0.2-20 nm-1), the internal structure of the nanoballs is probed. The intensity at small-angle displays a power law decrease with q, with a cross-over from a slope of ∼ -2 to a slope of ∼ -3, for q = 0.43 nm-1. This q vector is associated to a distance 2π/q = 14 nm, hence of the order of the pore size. Moreover, the escape for the -3 power law occurs for a wave vector q ) 1.4 nm-1, which is associated to a distance 2π/q = 4.5 nm, hence of the order of the thickness of the rods. Finally at higher q, several sharp crystalline peaks are detected. They do not correspond to the diffraction of pure Pd (this occurs at even higher q that could not be reached with our setup). Instead, by analogy with previous findings,32 we believe that these peaks originate from a crystalline phase of the remaining surfactant molecules. High-resolution (HRTEM) micrographs confirm that the Pd nanostructures are crystalline (Figure 3; see also the Supporting Information, Figure S1). The image shows that the nanowires are formed of interconnected monocrystalline domains which includes a series of parallel lattice fringes separated by 0.227 nm corresponding to the (111) lattice fringes. The fast Fourier transformation (FFT) of the image at higher magnification reveals two main diffraction rings. The reciprocal distances found d1 ) 0.225 nm and d2 ) 0.198 nm correspond to the (111) and (200) plane lattices of cubic Pd. To further understand the growth process for the porous 3Dnanostructures, samples at different doses were observed by TEM. After 2 h of irradiation (Figure 2a), two populations of individual particles of 2 and 8 nm were found on the TEM grid.

Surendran et al. After 6 h (Figure 2b) fewer 2 nm-seed particles were observed and more nanoparticles of 8-10 nm were seen. At this time, almost all the original seed particles have begun to grow into larger nanoparticles of 8 nm and a few of them already have a few branches (inset Figure 2b). These “embryonic” dendrites grow with irradiation time, and after 20 h of irradiation (Figure 2c,d), the 3D-dendrites have grown much larger forming porous nanoballs. The pore volume of the palladium nanoballs determined from N2 adsorption isotherm at a relative pressure of 0.98 is 0.12 cm3 g-1. The BET (Brunauer-Emmet-Teller)33 specific surface area is about 31 m2 g-1, and we determine a “hydraulic” pore diameter of 15.4 nm. It is in good agreement with TEM observations and SAXS measurements. The obtained specific surface area is close to that reported in literature for palladium black 17-23 m2g-1 (see the Supporting Information, showing, in Figure S2, a TEM picture of Pd black purchased from Aldrich and its nitrogen characterization in Figure S3). Compared to silica on a molar ratio basis (SiO2: MW ) 60 g mol-1 and Pd: MW ) 106.42 g mol-1), this value would correspond to a specific surface of 51 m2 g-1. If we apply the BJH (Barrett-Joyner-Halenda) model34 on the adsorption branch of the porous nanoballs, we obtain a large pore size distribution, with a broad maximum for pore sizes between 12.1 and 17.6 nm. On the desorption branch of the isotherm, the pore size is 14.2 nm. In both cases, there is still a good agreement with TEM observations. Details on the N2 adsorption and pore size distribution are shown in Figure S3 in the Supporting Information. In order to substantiate our proposed mechanism of uniform nucleation and subsequent growth leading to 3D nanostructures and to ascertain the role of the template and the particle growth kinetics, several control experiments were performed. Fast radiolytic reduction conducted with electron beams of PdII in hexagonal mesophases leads to small nanoparticles (3 nm) and short rods (3 nm diameter and 9-12 nm length; Figure 4a). In addition, when PdII salt is slowly reduced in micellar solutions containing Pd(NH3)4Cl2, CTAB and pentanol (with concentrations equivalent to those in the hexagonal phase but without cyclohexane), large particles heterogeneous in size are obtained (Figure 4b). Consistently with our previous findings with platinum,28 these experiments demonstrate that both confinement and slow reduction are necessary to obtain these 3D-Pd nanostructures forming nanoballs. Experiments were performed on hexagonal mesophases based on another cationic surfactant CPCl (cetylpyridinium chloride) containing Pd(NH3)4Cl2 and pentanol as cosurfactant. The radiolytic reduction led to spherical particles with diameters ranging from 3 to 8 nm. The nature of surfactant is also essential to obtain anisotropic growth. CTAB plays the role of structure-directing agent: it is known to favor formation of anisotropic nanostructures by preferential adsorption on (100) facets.35,10 The 3D-Pd nanostructures were tested both for their electrochemical behaviors and their electrocatalytic performances in the oxidation of ethanol in alkaline medium. As a matter of fact, it has been established that Pd-based electrocatalysts have no activity for alcohol electrooxidation in acidic media.12 In contrast, Pd has been demonstrated to be very active and even more efficient than Pt for ethanol oxidation in alkaline medium.12 Several advantages are attached to ethanol oxidation on Pd. First, this alcohol is not toxic and can be synthesized in large quantities from glucose-containing biomass, and second, Pd is 50 times more abundant than Pt in ores and is much cheaper. The voltammogram run in 1 M KOH with an electrode assembled by deposition of Pd nanoballs on a glassy carbon

Palladium Nanoballs

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Figure 3. HRTEM images of porous palladium nanoballs formed after complete reduction (20 h γ-irradiation) (a) showing cells, (b) a zoom of panel a where the (111) planes are clearly seen, and (c) the corresponding fast fourier transformation (FFT).

Figure 5. Representative cyclic voltammogram showing the electrocatalytic oxidation pattern of 1 M EtOH in 1 M KOH in the absence of dioxygen. The glassy carbon electrode was modified with a few microliters of Pd nanoballs and then covered with 3 µL of Nafion. The reference electrode was a Hg/HgO (1 M KOH) electrode; the scan rate was 50 mV s-1. See text for further details.

CH3CH2OH + 3OH- f CH3CO (ads) + 3H2O + 3e- (2) OH-fOH (ads) + 1e-

(3)

CH3CO (ads) + OH (ads) f CH3COOH (rate limiting step)(4) -

CH3COOH + OH f CH3COO-

Figure 4. TEM images of palladium nanorods and nanoparticles obtained (a) by electron beam irradiation (48 kGy) of a Pd doped (0.1 M) hexagonal mesophase, (b) by γ-irradiation (20 h) of a micellar solution containing PdII (0.1M), CTAB, and pentanol, and (c) by γ-irradiation (20 h) of a hexagonal mesophase containing PdII (0.1M), CPCl, and pentanol.

electrode is shown in the SI section (Figure S4). It exhibits the characteristic pattern of bulk Pd electrode and thus confirms the presence of Pd in the zero oxidation state. Next, ethanol (EtOH) electrooxidation was selected as a test reaction for the present electrodes. Complete oxidation of EtOH reads:

CH3CH2OH + 3H2O f 2CO2 + 12H+ + 12e-

(1)

However, in alkaline medium, the generally accepted sequence is the following one:

(5)

As an illustration, Figure 5 shows the first voltammogram run with the Pd nanoball-modified electrode in 1 M KOH containing 1 M EtOH. This electrode was the same as that used for the characterization in pure 1 M KOH (Figure S4). During the subsequent several runs, a shift of the whole voltammetric pattern of less than fifteen millivolts in the positive potential direction was observed until stabilization. The main characteristics which can be measured from such voltammograms include: Eonset is the faradaic current onset potential; Ef is the forward peak potential; Eb is the backward peak potential. In the present case, with the voltammogram (Figure 5) obtained at a scan rate of 50 mV s-1, these values relative to ethanol oxidation are Eonset ) -550 mV; Ef ) -151 mV; Eb ) -296 mV. Related literature results were obtained recently at a scan rate of 5 mV s-1 either with Pd nanoparticles fixed on Vulcan XC-72 (denoted Pd/C) or with Pd nanoparticles fixed on carbon microspheres (Pd/CMS). For Pd/C the following values were given Eonset ) -520 mV; Ef ) -140 mV; Eb ) -251 mV (estimated from the curve); for Pd/CMS the values read Eonset ) -580 mV; Ef ) -140 mV; Eb ) -230 mV (estimated from the curve).12 This set of characteristics indicate that the oxidation

10744 J. Phys. Chem. C, Vol. 112, No. 29, 2008 of ethanol are more favorable with the present material, as the energy saving can be related to the negative potential shift of the whole voltammograms. The observed values qualify this new material for use in direct ethanol fuel cells. To fully establish the viability and economical aspect of this catalytic material in practical systems, future work will study its long-term stability and quantify the current intensities as a function of the amount of Pd deposited on the electrode surface through Pd nanoballs. Conclusion In summary, we have synthesized Pd nanostructures with 3Dporosity using soft templates made by a quaternary system as nanoreactors. The mesophases are doped by high concentrations of palladium (0.1 M) leading to relatively large quantities of Pd nanostructures. Slow reduction and confinement are both necessary to obtain these 3D-porous nanowires matrix. CTAB plays the role of structure-directing agent. These nanostructures exhibit a remarkable electrocatalytic activity for ethanol oxidation and show promise for use in direct ethanol fuel cells. These nanostructures might also find applications in catalysis, hydrogen storage and sensing. Acknowledgment. G.S. acknowledges the European Commission for a postdoctoral fellowship (MIF1-CT-2004-506469). The authors thank Sylvain Franger from LPCES (ICMMO, Universite´ Paris XI) for access to Belsorp-mini for N2 adsorption isotherms. Supporting Information Available: Figures S1-S4. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Rolison, D. R. Science 2003, 299, 1698. (2) Martin, C. R.; Kohli, P. Nat. ReV. Drug DiscoVery 2003, 2, 29. (3) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A Org. Lett. 2000, 2, 2385. (4) Reetz, M. T.; Westermann, E Angew. Chem., Int. Ed. 2000, 39, 165. (5) Astruc, D Inorg. Chem. 2007, 46, 1884. (6) Franze`n, R. Can. J. Chem. 2000, 78, 957. (7) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (8) 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.

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