Transient and Stable Silver Clusters Induced by Radiolysis in Methanol M. Mostafavi,* G. R. Dey, ² L. Franc ¸ ois, and J. Belloni Laboratoire de Chimie Physique, UMR CNRS-UPS 8000, Ba ˆ t. 349, UniVersite ´ Paris-Sud, 91405, Orsay, France ReceiVed: NoVember 19, 2001; In Final Form: July 2, 2002 The optical absorption spectra of Ag 0 , Ag 2 + , transient oligomers, and stable clusters were studied in deaerated methanol by pulse and γ-radiolysis methods and compared with data obtained from studies using other solvents. From the decay of e - solv at 620 nm, the formation rate constant of Ag 0 was determined to be k(e - solv + Ag + ) ) (2.6 ( 0.6) × 10 10 M -1 s -1 . The coalescence rate of transient oligomers is particularly slow and the growth process is not completed within 2 s after the pulse. The surface plasmon band of stable clusters obtained by γ-radiolysis in methanol is intense and sharp (ǫ 385 nm ) 1.8 × 10 4 M -1 cm -1 ). The clusters are readily oxidized by oxygen. By adding methyl viologen MV 2+ to the solution containing stable silver clusters, the blue color of MV +• is transitorily observed, and the small clusters are developed into larger ones. Similarly, by pulse radiolysis of a mixture of silver cations and MV 2+ , silver atoms and MV +• are formed immediately through solvated electron scavenging, but subsequently additional MV +• are produced by reduction of MV 2+ by Ag 0 and small oligomers. In a further step, MV +• radicals reduce Ag + cations adsorbed on large clusters and let them develop eventually to larger sizes. The general mechanism of the methanol radiolysis is revisited on the basis of electron and radical scavenging by Ag + and silver clusters, and values of the yields of methanol radiolysis species are discussed in detail. Introduction The preparation of small metal particles at nanometric scale has received increasing attention due to the unique properties of such particles which result from size effects. 1-3 The radiolytic method is particularly suitable for generating metal clusters in solution. 1 The radiolytic species, solvated electrons and second- ary radicals, exhibit strong reducing properties, such that metal ions are reduced at each encounter. Therefore, the atoms formed through the reduction reactions are distributed homogeneously throughout the solution. Metal atoms then tend to coalesce into oligomers which themselves progressively grow into larger clusters and eventually into precipitates. Moreover, the progres- sive extent of the reduction is accurately controlled by the dose absorbed and the reduction rate by the dose rate. At quite high dose rate as in pulse radiolysis, an instantaneous distribution of the reducing agent and then of atoms are obtained throughout the solution. For studying stable clusters generally or for specific applications, the coalescence is limited by adding a polymeric molecule acting as a cluster stabilizer. Functional groups with high affinity for the metal ensure the anchoring of the molecule at the cluster surface while the polymeric chain protects the cluster from coalescing at an early stage through electrostatic repulsion or steric hindrance. The final size of the clusters depends on the type of polymer or ligand, the metal/polymer ratio, and the dose rate. 3 Some changes in the stability and chemical reactivity of the particles are expected if organic solvents are used instead of water. It is already known that silver and gold clusters can be prepared in alcoholic solution by thermal reduction. 4,5 The rate constant of silver ion reduction by solvated electrons in methanol has been measured by γ-radiolysis by means of the competition method with the reaction of N 2 O. 6 Metal clusters have also been chemically prepared in aqueous solutions, and then have been transferred into other solvents of lower polarity. 7 Their absorption spectra are shifted to the red with a decrease of the solvent polarity, according to the Mie model. The formation dynamics, the size, and the optical properties of radiolytically induced silver clusters have also been recently studied in supercritical ethane/methanol mixtures. 8 In this work, we present the results concerning the formation of silver clusters in methanol. The size, the optical properties, the growth kinetics, the yields, and the size-dependent reactivity of these clusters with oxygen and with the methyl viologen redox couple are determined. 9 The silver cations in methanol, similar to silver cations in aqueous solution, could potentially scavenge the total of solvated electrons and of alcohol radicals issued from radiolysis. Thus, we demonstrate that the use of Ag + as a scavenger is helpful in establishing the mechanism of methanol radiolysis. 10-13 Experimental Section All the reagents were pure chemicals: AgClO 4 from Aldrich, and methanol, 2-propanol, ethylene glycol, formaldehyde, and methyl viologen chloride MV(Cl) 2 from Fluka. The poly(vinyl alcohol) (PVA, hydrolyzed at 80% with average M w 9000- 10000 g mol -l ) was from Aldrich. It was used as a stabilizing agent for clusters. For experiments on mixtures of 5 × 10 -4 M AgClO 4 and 10 -4 M MV(Cl) 2 , a supplementary concentration of 10 -4 M AgClO 4 was added in order to eliminate Cl - ions by precipitation of AgCl, then the solution was filtered. The irradiation source was a 60 Co γ-facility of 7000 Ci with a maximum dose rate of 8 kGy h -1 . PVA is not soluble in neat methanol. Therefore we used methanol with less than 1% water. The PVA (2 × 10 -1 M) and the silver salt (10 -1 M) were first dissolved in water, then diluted with methanol. Due to the extreme sensitivity of clusters in methanol to oxygen, the * Corresponding author. E-mail: mehran.mostafavi@lcp.u-psud.fr. ² Permanent address: EOL from Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. 10.1021/jp014257a CCC: $22.00 © xxxx American Chemical Society PAGE EST: 10.8 Published on Web 00/00/0000