Synthesis of Atomic Gold Clusters with Strong Electrocatalytic

Oct 11, 2008 - Beatriz Santiago González , María J. Rodríguez , Carmen Blanco , José Rivas , M. Arturo López-Quintela , and José M. Gaspar Martinho...
1 downloads 0 Views 2MB Size
Download PDF
12690

Langmuir 2008, 24, 12690-12694

Synthesis of Atomic Gold Clusters with Strong Electrocatalytic Activities Marı´a J. Rodrı´guez-Va´zquez,*,† M. Carmen Blanco,† Ricardo Lourido,† Carlos Va´zquez-Va´zquez,† Elena Pastor,‡ Gabriel A. Planes,§ Jose´ Rivas,† and M. Arturo Lo´pez-Quintela*,† Laboratory of Magnetism and Nanotechnology, Technological Research Institute, UniVersity of Santiago de Compostela, E-15782 (Spain), Departamento de Química Física, Facultad de Química, UniVersidad de La Laguna, Astrofísico F. Sánchez s/n. E-38071 La Laguna, Tenerife (Spain), and Departamento de Química, Facultad de Ciencias Exactas, Físico-Químicas y Naturales. UniVersidad Nacional de Río Cuarto, Ruta Nacional 36 - Km. 601 C.P. X5804BYA - Río Cuarto, Córdoba (Argentina) ReceiVed June 12, 2008 Small atomic gold clusters in solution, Aun, stabilized by tetrabutyl ammonium bromide (TBABr), have been synthesized by a simple electrochemical technique, based on the anodic dissolution of a gold electrode in the presence of TBABr salt, and using acetronitrile as solvent. The presence of clusters in the range Au3-Au11 were detected by MALDI-TOF spectroscopy, and further characterized by UV-vis absorption spectroscopy, TEM, AFM, X-ray diffraction, and cyclic voltammetry. Clusters display a semiconductor behavior with a band edge of ∼2.5 eV. We report here their extraordinarily high electrocatalytic activity toward the O2 reduction reaction in acid solutions, which can explain Zhang’s results,5 showing that a four-electron mechanism seems to occur because of the facile reduction of H2O2 on gold clusters compared to bulk gold or larger gold nanoparticles.

1. Introduction Gold colloids have been used since ancient times to color glasses. Beautiful examples of these applications can be found in glass windows of many gothic European cathedrals (such as Chartres, Reims, etc), among which the Leon cathedral represents one of these unique masterpieces.1 The renewed interest in gold colloids is mainly due to Haruta’s discovery that small gold nanoparticles supported on TiO2 can catalyze a variety of reactions.2 The catalytic properties of gold nanoparticles depend very much on the support material, preparation method, and size of the gold nanoparticles.3 Although much activity has surrounded gold nanoparticle catalysis, questions about the fundamental catalytic mechanisms remain. As an example, a recent review about this subject found4 “direct evidence of the active site -referring to ionic or metallic gold- and reaction mechanism still need to be generated, and much better understanding of the energetic of the reaction pathway is needed to explain the unusually high activity of this catalyst”. In particular, last year, Zhang et al.5in an interesting paper demonstrated the use of gold clusters to prevent platinum dissolution in oxygen-reduction fuelcell electrocatalysis. Their surprising results found almost no change in the electrocatalytic activity, although they replaced as much as 1/3 of the platinum surface with gold clusters. This result is remarkable because, contrary to platinum, gold is not considered an active electrocatalyst for the oxygen reduction reaction (ORR) to H2O in acidic media. * To whom correspondence should be addressed. E-mail: malopez. [email protected] (M.A.L.-Q.), [email protected] (M.J.R.-V.). † University of Santiago de Compostela. ‡ Universidad de La Laguna. § Universidad Nacional de Rı´o Cuarto. (1) Located on the medieval French pilgrimage path to Santiago de Compostela, its impressive 2000 m2 color windows offer a unique view that warrants a visit. A virtual tour offers a flavor of these beautiful and relaxing atmospheres, see, for example http://www.magicspain.com/castilla-leon/leon/leoncat.html (2) Haruta, M. Catal. Today 1997, 36, 153. (3) Valden, M.; Lai, X.; Goodman, W. Science 1998, 281, 1647. (4) Kung, M. C.; Costello, C. K.; Kung, H. H. Catalysis 2004, 17, 152. (5) Zhang, J.; Sasaki, K; Sutter, E.; Adzic, R. R. Science 2007, 315, 220.

On the other hand, metallic atomic clusters, that is, metal (0) atomic aggregates with less than approximately 50-100 atoms (size < ∼1.5 nm) are of special interest because of the novel properties they can exhibit.6-9 In this article, we will report the electrochemical preparation and initial characterization of small and very stable gold clusters showing excellent electrocatalytic activity against ORR, which can explain Zhang’s results showing that a four-electron mechanism seems to occur because of the facile reduction of H2O2 on gold clusters compared to bulk gold or larger gold nanoparticles.10 These clusters display a semiconductor behavior with a band edge of ∼2.5 eV. The preparation of these Aun(TBABr)2.3n clusters, with n < 3-11, has been achieved using an extension of a previously developed electrochemical method11,12 for the production of metal nanoparticles on the basis of a technique first introduced by Reetz et al.13

2. Experimental Section Materials. All chemicals were purchased from Sigma-Aldrich and Fluka and used without further purification. Electrochemical Synthesis. Clusters were synthesized by an electrochemical method similar to that previously described for the synthesis of silver particles.12 Briefly, an Autolab PGSTAT 20 potentiostat was used in the synthesis and the temperature was kept at 25.0 ( 0.1 °C. All potentials were measured against an Ag/AgCl reference electrode. The experiments were carried out in a standard Metrohm electrolysis beaker containing a sacrificial gold sheet as (6) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345–352. (7) Joo, S. H.; Choi, S. J.; Oh, I.; Kawak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169–172. (8) Duan, H.; Nie, S. J. Am. Chem. Soc. 2007, 129, 2412. (9) Pyykko¨, P. A. Angew. Chem., Int. Ed. 2004, 43, 4412. (10) Adzic, R. Electrocatalysis. In Recent AdVances in the Kinetics of Oxygen Reduction; J. Lipkowski, P. N. Ross, Eds.; Wiley-VCH: New York, 1998; pp 197-242. (11) Lo´pez-Quintela, M. A.; Rivas, J. Procedure for the Synthesis of Atomic Quantum Clusters. Spanish Patent Application No. P200502041 (2005). PCT/ ES2006/070121 ( 2006). (12) Rodrı´guez-Sa´nchez, M. L.; Rodrı´guez, M. J.; Blanco, M. C.; Rivas, J.; Lo´pez-Quintela, M. A. J. Phys. Chem. B 2005, 109, 1183. (13) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401.

10.1021/la8018474 CCC: $40.75  2008 American Chemical Society Published on Web 10/11/2008

Synthesis of Atomic Gold Clusters

Figure 1. UV-vis gold cluster characterization. Absorption spectra at different times during the electrochemical synthesis of gold clusters.

the anode (counter electrode), and the same size platinum sheet was used as a cathode (working electrode). These two electrodes were placed vertically, face-to-face, inside the cell. The electrolyte solution, consisting of TBABr 0.1 M dissolved in acetonitrile, was deaerated by bubbling nitrogen for about 15 min, keeping an inert atmosphere during the whole process. Strong magnetic stirring was maintained during the galvanostatic electrolysis, which consists of the application of a current density of 5 mA cm-2 for 300s. ORR Catalysis. For this purpose, clusters were deposited on a glassy carbon (GC) electrode (d ) 3 mm, A) 0.707 cm2), allowing to evaporate 180 µL of a cluster solution with a concentration of 0.5 ppm (i.e., 9 × 10-8 g in Au), covering it with 8 µL of a 5% Nafion dispersion and drying it at room temperature. Equipment, Measurements, and Full Methods. They are available in the Supporting Information.

3. Results and Discussion In previous publications,12,14 we have postulated a mechanism, which includes the existence of different types of precursor clusters, to explain the kinetics of the synthesis of nanoparticles by the electrochemical technique. In this work, we have been able to show, for the first time, that stable clusters can be synthesized by stopping the reaction before the production of nanoparticles. These clusters can be isolated and display strong electrocatalytic activities when are deposited on a glassy carbon electrode. Figure 1 shows a typical time evolution of the absorption spectrum during the electrochemical synthesis of gold clusters. After just 50 s, four absorption bands clearly appear with peak positions at 265, 292, 394, and 470 nm. After 300 s (at the end of the synthesis), the band located at 292 nm is replaced by a new band with a peak position at 340 nm. Upon halting the production of clusters at 300 s, the resulting solution is stable for several months, although some evolution is observed due to an excess of Au+ ions present in the solution. It is to notice that the synthesized gold samples are not nanoparticles because of the complete absence of the plasmon band. The plasmon band appears only when the synthesis is extended until > ∼ 600-800s. For particles below ∼1.5 nm, the surface plasmon band disappears and the spectrum consists of a continuous increase in absorbance with decreasing wavelength.15 For very small clusters (Aun, where n < ∼10-20 atoms), discrete, molecular-like bands, like those (14) Rodrı´guez-Sa´nchez, L.; Lo´pez-Quintela, M. A.; Blanco, M. C. J. Phys. Chem. B 2000, 104, 9683. (15) See e.g. Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harries, J. E.; Vachet, R. W.; Clark, M. R.; Londo, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.

Langmuir, Vol. 24, No. 21, 2008 12691

Figure 2. Microscopy gold cluster characterization. A) Typical TEM picture and B) particle size histogram of the electrochemically synthesized gold clusters

observed in Figure 1, are expected.16 Taking the onset of the UV-vis absorption (∼ 520 nm, i.e., 2.4 eV) as the HOMO-LUMO bandgap,17 one can see that the electrochemically produced clusters have a larger bandgap than previously identified synthesized clusters Au11(PPh3)7Cl3, Au13(PPh3)4[S(CH2)11CH3]2Cl2 (1.7 eV),16,18 Au38C6H11S (1.2 eV),19 and Au147C6H11S (0.27 eV).19 If we apply the bandgap trend observed for these clusters, the electrochemically produced Aun clusters would consist of n < ∼11-13 atoms. X-ray fluorescence (XRF) analysis gives the following general formula for the clusters: Aun (TBABr)2.3n. The presence of TBA in the clusters was confirmed by NMR (Supporting Information). A transmission electron microscopy (TEM) image and the corresponding size distribution are shown in Figure 2. Notice the very poor contrast observed in the TEM pictures for the sizes obtained (2.3 ( 0.4 nm). As we pointed out before,20 TEM pictures are 2D projections, making it difficult to distinguish between nearly spherical single nanoparticles and aggregations of smaller subunits. In this particular case, the poor contrast observed in the TEM images points to the existence of subnm entities.21 Figure 3 shows a high resolution TEM (HRTEM) picture of one of these particles. It was observed that the gold crystal planes are detected only after few seconds of observation, indicating again that nanoparticles are formed from small clusters, which fuse under the intense irradiation beam.22 Figure 4 shows an atomic force microscopy (AFM) image of the gold cluster samples (also Supporting Information). 2D nanoislands with heights of about 0.5-0.6 nm are observed. It is interesting to note that the height of these nanoislands is a little smaller than the size of Au13 clusters recently identified by the Murray’s group21 (0.8 ( 0.1 nm), and supports that these nanoislands are formed by aggregation of Aun clusters with n < 13. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were performed to get information about the HOMO-LUMO energy gaps of the clusters, which can be (16) See e.g. Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. J. Phys. Chem. B 2002, 106, 9979. (17) Benfield, R. E.; Maydwell, A. P.; van Ruitenbeek, J. M.; van Leeuwen, D. A. Z. Phys. D 1993, 26(S1), 4. (18) Yang, Y.; Chen, S. Nano Lett. 2003, 3, 75. (19) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Konturi, K. J. Am. Chem. Soc. 2003, 125, 6644. (20) Guille´n-Villafuerte, O.; Garcı´a, G.; Anula, B.; Pastor, E.; Blanco, M. C.; Lo´pez-Quintela, M. A.; Herna´ndez-Creus, A.; Planes, G. A. Angew. Chem., Int. Ed. 2006, 45, 4266–4269. (21) Meard, L.D.; S; Gao, H; Xu, R. D.; Twesten, A. S.; Harper, Y; Song, G; Wang, A. D.; Douglas, J. C.; Yang, A. I.; Frenkel, R. G.; Nuzzo, R. W.; Murray, J. Phys. Chem.B 2006, 110, 12874. (22) Ijima, S.; Ichihashi, T. Phys. ReV. Lett. 1986, 56, 616.

12692 Langmuir, Vol. 24, No. 21, 2008

Figure 3. HRTEM picture of gold NPs formed from fused clusters after sample irradiation by the intense electron beam of the microscope

determined from their first oxidation and reduction peaks.23 Figure 5 shows the electrochemical behavior of the sample freshly prepared and after aging for seven months (a cathodic current is clearly observed in the aged sample, which can be associated to the slow growth of some of the initial small clusters). In both cases, oscillations in the current occur at potentials less than about -2 V, indicating the system is sensitive to the atomiclevel structure of the clusters at the electrode surface.24 A flat voltammetric response is seen between +2 and -1.8 V (curves a and b) with no oxidation or reduction peaks occurring in this range (outside this potential range redox processes are related to the supporting electrolyte). In the DPV, displayed as an inset to figure 5, a large central gap delimited by the first oxidation peak of the clusters at +0.9 V in the anodic part is observed. No reduction peak for the clusters can be seen, indicating that the bandgap is larger than ∼ 2.7 eV. This value is larger than the HOMO-LUMO bandgap estimated by UV-vis spectroscopy. The difference, ∆ > ∼300 mV, is due to the electron-hole (e-h) Coulomb energy.25 The Coulomb energy affects the bandgap of very small nanoparticles or clusters, where confinement effects are important. In the simplest approximation, the difference ∆ between the optical and voltammetrically determined bandgaps can be defined as ∆ ) e/C, where e is the elementary charge and C is the capacitance of the cluster. From the estimated e-h Coulomb energy of the clusters, one can deduce (assuming C ) 4πεε0r; ε ) relative dielectric constant of the surrounding media: acetonitrile, 37.5; ε0 ) permittivity of vacuum; r ) particle radius) that their size is ∼0.3 nm, which is relatively close to the AFM measured particle height, in spite of the approximations involved in the calculations. Taken together, the evidence points to the existence of very stable and small clusters in the samples, with bandgaps larger than those previously reported for well-defined gold clusters such as Au11 and Au13. Figure 6 represents a typical mass spectrum (matrix assisted laser desorption ionization-time-of-flight, MALDI-TOF) of the sample (positive ions), showing the presence of naked Aun clusters with n e 11. The absence of ligands in the observed spectra points to a weak ligand-metal interaction. Fragmentation can be disregarded because the energy of the used laser (3.3 eV) damped by the matrix - should not be enough to break the (23) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193. (24) Nakanishi, S.; Mukouyama, Y.; Karasumi, K.; Imanishi, A.; Furuya, N.; Nakato, Y. J. Phys. Chem B. 2000, 104, 4181. (25) Franceschetti, A.; Zunger, A. Phys. ReV. Lett. 2000, 62, 2614.

Rodrı´guez-Va´zquez et al.

Au-Au bonds.26 Moreover, the mild conditions used are similar to other reported studies with gold clusters without fragmentation.27,28 Additionally, an odd-even alternation, also seen for clusters observed in the gas phase,29 is clearly observed. The major abundance of clusters with an odd number of atoms would imply that they are positively charged (even number of electrons). On the other hand, the mass spectrum agrees with the previously mentioned finding that the synthesized Aun clusters have a number of atoms n e 11-13, with Au3 being the most abundant one. As far as we know, this is the first report of an odd-even alternation stability of clusters synthesized in solution. However, we are aware that more studies would be needed to confirm this aspect. The X-ray diffraction pattern of samples can be seen in part A of Figure 7. Instead of the typical peaks of bulk gold, several broad peaks resembling those previously reported for large gold clusters30 are observed. Considering that clusters with a small number of atoms have a preference to be planar,31 and assuming that the Au3 cluster is the most abundant one (as the massspectral analysis has shown), we have performed a tentative analysis of the X-ray diffraction pattern of the samples (for details refer to the Supporting Information). This analysis, shown in the lower curve of part A of Figure 7, was performed assuming that Au3 clusters self-assemble in a plane with a hexagonal arrangement, similar to the (111) plane of bulk gold (part B of Figure 7). A small percentage of other clusters is also included in the assembly (for simplicity, TBABr ligands linked to clusters are not included in the picture). It should be pointed out that increasing the percentage of large clusters (Aun, with n > 3) in the samples no good fittings are obtained confirming that Au3 is the most abundant one. It can be seen that the fitting qualitatively explains the main features of the observed XRD pattern. Moreover, the number of Au3 shells (around a central cluster) needed for the fitting (five shells) yields a size of these 2D cluster aggregates of 2.7 nm (part B of Figure 7), which nicely agrees with the sizes estimated by the TEM pictures. Then, this model of selforganization of planar clusters supports the TEM and AFM results. The cluster electronic structure was probed by X-ray photoelectron spectroscopy (XPS); see Supporting Information. The results show that the Au(4f) peak positions in the clusters are positively displaced with respect to bulk gold: Au 4f7/2 ) 84.70 eV (bulk ) 83.85 eV); Au 4f5/2 ) 88.37 eV (bulk ) 87.53 eV). A broadening of the peaks is also observed: fwhm (4f7/2) ) 0.99 eV (bulk ) 0.55 eV); FHWM (4f5/2) ) 0.92 eV (bulk ) 0.55 eV). These features are in agreement with those previously found in small gold nanoparticles and clusters.32-34 The observed positive displacement of the Au(4f) peak, ∆ ∼ 0.8 eV, can be associated with: 1) the Coulomb interaction between the photoelectron and the hole left behind in the cluster,35 which lowers the kinetic energy of the detected photoelectron by ∆1 ) e2/2C (∼ 0.3 eV, see above), and 2) the electron donation from (26) Wang, J.; Wang, G.; Zhao, J. Phys.ReV.B 2002, 66, 354181. (27) Balasubramanian, R.; Guo, R.; Mills, A. J.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 8126. (28) Dass, A.; Stevenso, A.; Dubay, G. R.; Tracy, J. B.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 5940. (29) See for example, Physics and Chemistry of Metal Cluster Compounds. Model Systems for Small Metal Particles; de Jongh, L. J.; Kluwer Academic: Dordrecht, The Netherlands, 1994. (30) (a) See for example, Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (b) Akola, J.; Walter, J.; Whetten, R. L.; Ha¨kkinen, H.; Gro¨nbeck, H. J. Am. Chem. Soc. 2008, 130, 3756. (31) Ha¨kkinen, H.; Moseler, M.; Landman, U. Phys. ReV. Lett. 2002, 89, 033401. (32) Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2003, 125, 4046. (33) Moriarty, P. Phys. ReV. Lett. 2004, 92, 109601. (34) Zhang, P.; Sham, T. K. Phys. ReV. Lett. 2004, 92, 109602. (35) Ohgi, T.; Sheng, H. Y.; Dong, Z. C.; Nejoh, H.; Fujita, D. Appl. Phys. Lett. 2001, 79, 2453.

Synthesis of Atomic Gold Clusters

Langmuir, Vol. 24, No. 21, 2008 12693

Figure 4. AFM pictures of electrochemically synthesized gold clusters.

Figure 5. Electrochemical gold cluster characterization. Cyclic voltammograms of the same gold cluster sample (a) as prepared and (b) seven months later, performed at 25 °C. The scan rate is 20 mV/s. Inset: differential pulse voltammetry at 25 °C (quasi-reference electrode: silver wire).

the gold cores to the associated ligands,32 which decreases the Fermi level energy (∆2 ) ∆ - ∆1 ∼ 0.5 eV). We could then deduce an approximate value for the binding energy of the clusters to the TBABr ligands ∼ 0.5 eV/cluster ) 0.08 kJ/cluster ) 48 kJ/mol of clusters ∼ 16 kJ/mol of gold atoms (assuming again that Au3 is the main cluster in the sample). This small value of the binding energy indicates that the TBABr ligands are weakly adsorbed onto the clusters rather than forming strong bonds, as in the case of thiol-capped synthesized clusters (Au-S bond energy ) 160 kJ/mol36). The week gold-ligand binding energy may explain that only naked clusters are observed in the MALDITOF mass spectra. Catalysis by noble metals is one of the most important fields at this moment because of its economical implications. As we mentioned earlier, gold has a poor electrocatalytic activity for ORR in acidic media because the reduction takes place in two (36) Ulman, A. Chem. ReV. 1996, 96, 1533.

Figure 6. MALDI-TOF mass spectrum (positive ions) of electrochemically synthesized gold clusters. Numbers 3 to 11 refer to the corresponding number of gold atoms per cluster.

2e- steps:37 the first step corresponds to the reduction of O2 to H2O2, and in the second step, which occurs only at high overpotentials, H2O2 is reduced to water. Figure 8 shows the electrocatalytic activity of gold clusters supported on glassy carbon for ORR in acidic media at room temperature. Contrary to what has been reported for gold nanoparticles or surfaces, we were surprised to see only one reduction peak for the ORR, located at -0.35V (curve b). For comparison, the reduction of H2O2 in the absence of O2 is also shown (curve c). In this case, two peaks are observed: one located at positive potential, 0.35 V, and the other one at -0.35 V. We observe that both peaks increase in proportion to the H2O2 concentration (Supporting Information). (37) Jena, B. K.; Raj, C. R. Langmuir 2007, 23, 4064.

12694 Langmuir, Vol. 24, No. 21, 2008

Rodrı´guez-Va´zquez et al.

found in previous catalytic studies.

4. Conclusions

Figure 7. X-ray diffraction gold cluster characterization. A) Typical XRD pattern of electrochemically synthesized gold clusters (black line). Red curve shows the simulation results. B) Schematic cartoon of the possible self-aggregation of clusters explaining the TEM pictures and XRD patterns (each circle represents one gold atom).

Figure 8. Electrochemical gold cluster activity. Voltammetric response of a GC electrode modified with gold clusters in H2SO4 0.1 M (a) saturated with N2, (b) saturated with O2, and (c) saturated with N2 in the presence of 18 mM hydrogen peroxide. The scan rate is 20 mV/s. Measurements carried out at room temperature.

Therefore, we attribute the peak at +0.35 V to the reduction of H2O2 and the peak at -0.35V to the reduction of the O2 generated by the decomposition of H2O2. Because the O2 reduction reaction takes place on gold clusters at much more negative potentials than the H2O2 reduction reaction (|∆E| g 0.7V), the H2O2 produced by the ORR is instantaneously reduced because of this large overpotential, explaining why only one peak appears in the ORR. Therefore, our results are in line with the recent findings of Zhang et al.,5 showing that the surprising catalytic properties of gold result from the presence of small gold clusters. As far as we know, this is the first clear evidence of ORR in only one 4e- step at the potential of -0.3 V RHE using Aun clusters with n < 11 atoms, and our results point to the idea that very small clusters with large bandgaps (approximately 2.5 eV) can play an important role in the catalytic properties observed for activated gold electrodes. Undetected gold clusters could be produced in small and uncontrollable quantities by the different procedures used for the preparation of gold particles for catalytic studies (as well as in gold electrode activation38), which can lead to large differences in the final catalytic properties.39 Therefore, our results may point to the underlying explanation for the large divergences (38) Burke, L. D.; O’Connell, M. A; O’Mullane, A. P. J. Appl. Electrochem. 2003, 33, 1125. (39) Luo, J.; Maye, M. M.; Lou, Y.; Han, L.; Hepel, M.; Zhong, C. J. Catal. Today 2002, 77, 127.

In conclusion, we have shown a simple electrochemical method for the production of small Aun clusters, with n e 11 atoms, displaying amazing electrocatalytic activities. The electrocatalytic effects observed here with atomic clusters are much higher than those reported before for gold nanoparticles because the complete ORR (4e- process) can be obtained at more positive potentials and with more efficiency due to the high reduction current obtained, which is one of the essential requirements for catalysts to find utilities in fuel-cell applications. For many years, metallic clusters were mainly prepared by physical methods, and these clusters were essentially assumed to be stable only in the gas phase40 or immersed in solid matrices.41 The preparation of clusters in solution was until now restricted to the use of some very specific templates or capping molecules.16,32,42-55 The method we report here has the advantage of being both simple and effective for the production of very small clusters, opening a fascinating method of producing multiple nanostructures with different properties in a bottom up approach by combining/ assembling subnm elemental pieces. Acknowledgment. We thank Donna Ardnt-Jovin and Biometa S.L. for the AFM measurements. We want to acknowledge the financial support of the MEC, Spain (projects No. MAT200507554-C02-01, NAN2004-09133-C03-02, NAN2004-09195C04-01 and NanoBioMed CONSOLIDER-INGENIO 2010) and the European Union (Contract No. 037465-FLUOROMAG, EUFP6 Framework Programme LIFESCIHEALTH-6). Maria Rodriguez thanks the MEC FPI grant. Supporting Information Available: Supplementary methods, equipment, supplementary figures with characterization data, and supplementary references. This material is available free of charge via the Internet at http://pubs.acs.org. LA8018474 (40) See for example, Elemental and Molecular Clusters; Benedek, G., Martin, T. P., Pacchioni, G., Eds.; Springer Verlag: Berlin, 1988. (41) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Verlag, Berlin, 1995. (42) Zheng, J.; Zhang, C.; Dickson, R. M. Phys. ReV. Lett. 2004, 93, 077402. (43) Schmid, G. Chem. ReV. 1992, 92, 1709. (44) Yonezawa, T.; Yasui, K.; Kimikuza, N. Langmuir 2001, 17, 271. (45) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (46) Esumi, K.; Hosoya, T.; Suzuki, A.; Torigoe, K. J. Colloid Interface Sci. 2000, 229, 303. (47) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (48) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2005, 127, 5261. (49) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322. (50) Jiang, D.; Tiago, M. L.; Luo, W.; Dai, S. J. Am. Chem. Soc. 2008, 130, 2777. (51) Akola, J.; Walter, M.; Whetten, R. L.; Ha¨kkinen, H.; Gro¨nbeck, H. J. Am. Chem. Soc. 2008, 130, 3756. (52) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883. (53) Pei, Y.; Gao, Y.; Zeng, X. C. J. Am. Chem. Soc. 2008, 130, 7830. (54) Negishi, Y.; Tsunoyama, T.; Suzuki, M.; Kawamura, N.; Matsushita, M. M.; Maruyama, K.; Sugawara, T.; Ykoyama, T.; Tsukuda, T. J. Am. Chem. Soc. 2006, 128, 12034. (55) Woehrle, G. H.; Warner, M. G.; Hutchinson, J. E. J. Phys. Chem. B 2002, 106, 9979.