J. Belloni

Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids

This review is devoted to metal cluster synthesis in solution via radiolytic reduction of ionic precursors under the proper conditions. The size and structure of the final particles are described in relation to the nucleation and growth mechanism of the process with a special interest in oligomers and nanometric-sized particles. The influence of either a polymeric surfactant or a ligand or a support is described. The role of a chemical electron donor in the development of cluster size is explained. Particular attention is paid to the formation of bimetallic clusters and to the synthesis conditions required to obtain either a core-shell or an alloyed structure in relation to a possible intermetal electron transfer.

Ultra-slow aggregation process for silver clusters of a few atoms in solution

Abstract γ-irradiation of aqueous solutions containing Ag + and polyacrylate (PAA) ions generates extremely small metal clusters containing only a few silver atoms (oligomers) coordinated to polyacrylate anions. Three different types of oligomers have been characterized by their optical spectra peaking at 300, 335 and 380 nm and assigned to (Ag n 0 , Ag m + ) with approximate aggregation numbers respectively n ≲ 1–3, n ≲ 4–7 and n ≳ 4. The dose and time effects show that they undergo simultaneous aggregation and corrosion processes on an exceptionally long time scale: this indicates the highly protective effect of PAA anions with respect to aggregation as well as to corrosion, probably resulting from a partial electron transfer from the carboxylate to the metal. The lifetime of these solvated metal clusters, even in the presence of air, is now long enough (weeks) for widely expanding the field of fundamental studies of such oligomers and of their applications, e.g. catalysis.

Enhanced yield of photoinduced electrons in doped silver halide crystals

The conventional photographic process involves several steps: the photogeneration of electron–hole pairs in crystals of a silver halide; the reduction of silver cations to atoms by some fraction of these electrons; the subsequent build up of atoms to give clusters (the ‘latent image’); and the complete reduction by a developer of crystallites having more than a critical number of silver atoms per cluster. The effective quantum yield, Φeff, of photoinduced electron–hole pairs produced per photon absorbed is less than the theoretical limit (Φtheory = 1), because of the fast recombination of some fraction of the pairs. Here we describe an approach for enhancing the yield of useful photogenerated electrons, in which the silver halide is doped with formate ions, HCO-2. The dopant ions act as hole scavengers, thus enhancing the escape of electrons from pair recombination. Moreover, the resulting CO[supbolddot]-2 radical can itself transfer an electron to another silver cation, so raising the theoretical yield to two silver atoms per photon absorbed. This photoinduced bielectronic transfer mechanism is strictly proportional to the light quanta absorbed—the dopant ions do not induce spontaneous reduction of silver cations in the dark—and appears to be close to the theoretical limit of efficiency. The efficiency is constant at all illumination levels and applies to both dye-sensitized and unsensitized crystals. We suggest that this approach is a promising route for improving the performance of photographic emulsions.

Irradiation-induced Ag nanocluster nucleation in silicate glasses: Analogy with photography

The synthesis of Ag nanoclusters in soda lime silicate glasses and silica was studied by optical absorption and electron spin resonance experiments under both low (gamma ray) and high (MeV ion) deposited energy density irradiation conditions. Both types of irradiation create electrons and holes whose density and thermal evolution---notably via their interaction with defects---are shown to determine the clustering and growth rates of Ag nanocrystals. We thus establish the influence of redox interactions of defects and silver (poly)ions. The mechanisms are similar to the latent image formation in photography: Irradiation-induced photoelectrons are trapped within the glass matrix, notably on dissolved noble metal ions and defects, which are thus neutralized (reverse oxidation reactions are also shown to exist). Annealing promotes metal atom diffusion, which, in turn, leads to cluster nuclei formation. The cluster density depends not only on the irradiation fluence but also---and primarily---on the density of deposited energy and the redox properties of the glass. Ion irradiation (i.e., large deposited energy density) is far more effective in cluster formation, despite its lower neutralization efficiency (from

Dose rate effect on size of CdS clusters induced by irradiation

{\mathrm{Ag}}^{+}

Photography: enhancing sensitivity by silver-halide crystal doping

to

Radiation chemistry of nanocolloids and clusters

{\mathrm{Ag}}^{0}

Nucleation dynamics of silver aggregates simulation of photographic development processes

) as compared to gamma photon irradiation.

Size-dependent thermodynamic properties of silver aggregates. Simulation of the photographic development process

Abstract Size-controlled CdS clusters, with subnanometric radii, have been produced by γ-irradiation of aqueous solution containing Cd 2+ and thiol. Absorption and fluorescence spectra show that the mean CdS cluster size increases in the distribution with the dose or the post-irradiation time. On the contrary, at a given dose, the effect of the dose rate has been clearly demonstrated to lower the cluster size. The mechanism of CdS cluster growth is discussed via coalescence reactions. A numerical simulation taking into account the coalescence reactions between monomers produced by irradiation and then between clusters of any size is presented. Calculations show how the final nuclearity depends on the dose rate effect. The observation of isosbestic points during the slow coalescence (100 M −1 s −1 ) allowed us to derive the absorption spectra of oligomeric CdS clusters, successively formed with maxima at 270, 282 and 320 nm, respectively. At high dose rate and low dose, only the smallest clusters of CdS absorbing at 270 nm (and never observed previously) are formed.

Ionization potential of clusters in liquids

The physical chemistry of the silver photography processes, exposure, development and fixing, is briefly summarized. The mechanism of the autocatalytic development by the developer of the clusters produced in silver bromide crystals during the exposure which is controlled by the critical nuclearity of these clusters was understood from pulse radiolysis studies. The effective quantum yield Feff of photoinduced silver cluster formation in silver halide microcrystals is usually much lower than the photoionization theoretical limit Ftheor ¼ 1 electron–hole pair per photon absorbed, owing to a subsequent very fast intra-crystal recombination of a part of the electron–hole pairs. In order to inhibit this recombination and favor the silver reduction by photo-electrons, the AgX crystals were doped with the formate HCO2 as a specific hole scavenger. First, the dopant scavenges the photoinduced hole, thus enhancing the electron escape from the pair recombination. Second, the CO2� radical so formed transfers an electron to another silver cation, so that the Feff limit may be of 2Ag 0 per photon. This Photoinduced Bielectronic Transfer mechanism is strictly proportional to the light quanta absorbed and induces an exceptional efficiency for enhancing the radio- or photographic sensitivity insofar as it totally suppresses the electron–hole recombination. r 2003 Elsevier Science Ltd. All rights reserved.