Dose Rate Effect on Bimetallic Gold-Palladium Cluster Structure Hynd Remita, Arnaud Etcheberry, † and Jacqueline Belloni* Laboratoire de Chimie Physique, Ba ˆ t. 349, UMR 8000-CNRS, UniVersite ´ Paris XI, 91400 Orsay Cedex, France, and IREM-Institut LaVoisier, 45 rue des Etats-Unis, UMR 8637-CNRS, UniVersite ´ Versailles Saint-Quentin-en-YVelines, 78035 Versailles Cedex, France ReceiVed: May 22, 2002; In Final Form: October 3, 2002 Radiolytic synthesis of mixed Au III /Pd II solutions at different dose rates is examined. The evolution of the plasmon spectra and the local X-ray micro diffraction has been systematically studied as a function of irradiation dose and dose rate. At low dose rate, a bilayered cluster of Au core-Pd shell is characterized. Due to inter- metal electron transfer from nascent Pd atoms to gold ions, the latter are reduced first and constitute mainly the core of the particle, Pd ions being finally reduced at the surface of gold clusters when all Au III ions have been reduced. In contrast, at high dose rate when the ion reduction is sudden and faster than a possible inter-metal electron transfer, it is shown by UV-vis spectroscopy, XPS, and EDAX results that genuine alloyed clusters are formed. Introduction Multimetallic clusters, particularly when intimately alloyed, are of high interest because of their optical and catalytic properties. 1-3 Bimetallic clusters were synthesized by numerous methods, but the synthesis of alloyed clusters is much more difficult than that of core-shell particles. Moreover, the characterization of the alloying is delicate, particularly for the smallest sizes. Bimetallic clusters often exhibit enhanced catalytic reaction rates and selectivity compared to the separate components. 4,5 For example, the turnover rate per surface Pd atom for the reaction between dihydrogen and dioxygen to form water was greater by a factor of 50 for the reaction over a supported AuPd catalyst than for Pd catalyst. 4 Radiation-induced reduction of metal ions in solution has been demonstrated to be a powerful method to synthesize bimetallic clusters. 2,6-8 However, by γ-radiolysis, a few systems as in chemical reduction were shown to yield intimately alloyed clusters. Indeed, an efficient competition is often occurring between the radiolytic reduction process of both types of ions and the electron-transfer reaction from the less noble metal atoms to the other metal ions. 8-10 Then, this preferential reduction of the more noble metal first results in a segregation between the metals and eventually in a core-shell structure of the cluster with the more noble metal in the core. A sudden and complete reduction of both types of metal ions by a train of irradiation pulses prevents this kind of redox process through electron transfer. It was shown that in the same mixed ion system the metal clusters obtained change with increasing dose rate from a bilayered core/shell structure to an alloyed structure or bimetallic solid solution. 10 In the literature, the combination of palladium with silver or gold are couples much studied. Bimetallic silver-palladium clusters have been prepared by various methods. 7,11-13 The morphologies of gold/palladium mixed particles have been studied since the early 1970s. 14 When prepared by mixing the vapor phases, the two metals are miscible at any ratio as it can be seen in their phase diagram. 15 A gold-palladium mixed phase has been also condensed from the vapor into various solvents by chemical liquid deposition. 16 The particle size was found to be solvent-dependent and lies between 2.5 and 3.8 nm. In contrast, reported studies on reduction of salts in mixed solution lead systematically to core-shell particles. For example, Au(core)/Pd(shell) clusters were synthesized by reduction of the mixed ion aqueous 17 or alcoholic solutions. 18 Gold/palladium bimetallic particles having a palladium-rich shell were synthe- sized by Liu et al. 19 Two-step alcoholic reduction gives “cluster- in-cluster” structured products of the mixture of monometallic particles. 20 The formation of core-thin-shell structures has been reported for large polymer-protected colloids 18,19 and highly dispersed core/shell Au/Pd clusters. 21 Schmid et al. 22 have demonstrated the controlled synthesis of core/shell Au/Pd colloidal particles which are efficient catalysts for coupling and cyclization of acetylene, even at room temperature. Lee et al. 23 synthesized a Pd-Au/SiO 2 catalyst that contains bimetallic clusters consisting of Au-rich core decorated by Pd when the ionic precursors were reduced at 300 °C under H 2 . In contrast, the Pd/Au catalyst that was reduced at 350 °C contained Pd- Au alloyed clusters. 24 Photoreduction was also used for the preparation of mixed Au/Pd core-shell particles. 25 Sonochemical preparation of Au core /Pd shell particles was reported recently. 26,27 Transient monovalent palladium ions and palladium atoms are oxidized by gold ions by inter-metal electron transfer under sonication, so inhibiting the alloying. 27 The same core-shell structure was also found by γ-irradiation of a mixed solution of gold(III) and palladium(II) ions. 27 As a matter of fact, it seems that alloyed clusters could be synthesized radiolytically from mixed solutions at high dose rate, when the ion reduction is sudden and faster than a possible inter-metal electron transfer. 10 Therefore the aim of this work is to use pulse electron beams to irradiate gold-palladium ion mixed solutions and to get alloyed Au-Pd clusters which were never obtained previously in solution. As the structure charac- terization of alloyed compared to core-shell nanoparticles is made difficult by the small size of these objects, we used for * Author to whom correspondence should be addressed. † IREM-Institut Lavoisier. BATCH: jp1a40 USER: rjb69 DIV: @xyv04/data1/CLS_pj/GRP_jp/JOB_i01/DIV_jp021277j DATE: December 4, 2002 10.1021/jp021277j CCC: $25.00 © xxxx American Chemical Society PAGE EST: 5.6 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89