J. Khatouri

Radiation-induced and chemical formation of gold clusters

The kinetics of the γ-radiolytical or chemical reduction of AuIII Cl4-, or of the combination of both methods, is followed as a function of the experimental conditions through the time evolution of the surface plasmon spectrum of the gold nanoparticles formed or of their sizes as observed by AFM imaging. It appears from the discussion on the mechanism that even with the strongly reducing radiolytic radicals, the low valency AuI ions are somewhat protected by the more concentrated AuIII ions from reduction, up to a ratio of AuI/AuIII=1, and are stabilized for hours, unless clusters or 2-propanol (or PVA, but more slowly) catalyze their disproportionation. The cluster concentration increases correlatively with the dose.2-Propanol or PVA are mild reducing agents and are unable to reduce AuIII directly except at the surface of clusters, previously formed, for instance, by partial radiolytic reduction. In this case, the cluster concentration remains the same but the size obtained after reduction by the alcohol increases slowly with time up to 100–500 nm, as in a development process. In order to avoid the relative extent of this development, associated with chemical reduction and even with the direct γ-reduction of AuIII, in particular the AuI disproportionation and reduction steps, high dose rate radiolysis has been used up to total reduction of the same solutions. The mechanism of reduction and growth, step-by-step, is discussed.

Ionization potential of clusters in liquids

Time-resolved observations of the fast electron transfer from an electron donor to metal ions adsorbed on metal clusters in solution have shown that a critical size of the cluster is required to make it capable of accepting electrons. The threshold is attributed to a size dependent redox potential of the cluster, increasing with the nuclearity (in contrast with the ionization potential in the gas phase which decreases when n increases): it corresponds to the nuclearity for which the cluster redox potential becomes more positive than the potential of the electron donor acting as a monitor.New data of redox potentials (or IP) of Agn clusters (hydroquinone as monitor) and Cun cluster (sulfonatopropylviologen anion as monitor) are derived. The influence of n and of the solvation or the ligand is discussed.

Kinetics of cluster growth by aggregation

The classical problem of the coalescence of isolated species to produce growing clusters/colloids/polymers by successive statistical encounters having the same rate constant, is revisited using numerical simulation for a maximum nuclearity value of a few 103 units. The evolution with time of the abundance of clusters of a given nuclearity and of the total population, and the distribution of sizes at a given time are obtained and compared with models from the literature. A remarkable feature of these curves is that they exhibit parity effects for the nuclearity, even clusters being systematically more abundant than odd ones. For easier comparison with experiments, some simulated curves are presented in the form of an approximated analytical expression: kinetics of the total population, and of the monomer, dimer and higher oligomers populations, amplitudes at the maximum and delay for the maximum as functions of the nuclearity, size distribution at a given time. The validity of the approximations is discussed.

Kinetics of cluster aggregation in competition with a chemical growth reaction

The kinetics of abundances and the distribution in size of clusters at a given time are obtained by numerical simulation in the case where a chemical growth competes with the coalescence of clusters originating from isolated monomers. To take into account the size dependence of cluster reactivity, it is assumed that the chemical reaction occurs only beyond a critical valuenc. The kinetics of the total cluster concentration are unchanged by addition of the reactant but the size distribution is suddenly perturbed for sizes beyondnc. The decay of the reactant, the kinetics of a cluster of a given nuclearity and the distribution of sizes at a given time depend on the relative values of the rates of coalescence and of growth due to the reactant. It is shown how the initial conditions control the kinetics and the amplitude or the time of the maximum cluster abundance. The case of an attack by a chemical reactant on clusters, restricted to low values ofn, is also considered. The concentration kinetics (for all size clusters) display much lower amplitudes than for pure coalescence case. No parity effects are observed.

Influence of solvation on the ionization potential of metal clusters

Due to quantum size effects, the thermodynamic properties of transient metal clusters of very low nuclearity in solution vary with the number of atoms building up the aggregates. Since these transient species usually have very short lifetimes, their redox potential for a given size was evaluated by a kinetic method consisting of the pulse radiolytic generation of aggregates from an initial quasi instantaneous distribution of atoms in the presence of a redox probe of known potential. The aggregation of metal atoms and the time evolution of the redox probe were observed by time‐resolved optical absorption spectrophotometry. The size‐dependent values of the redox potentials are compared with the corresponding values of the ionization potentials for bare clusters of the same nuclearity in the gas phase. The influence of the solvation energy on the ionization potential is discussed in the light of literature data obtained by different methods and related to different kinds of environment.