Mehran Mostafavi

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.

Absolute calibration for a broad range single shot electron spectrometer

This article gives a detailed description of a single shot electron spectrometer which was used to characterize electron beams produced by laser-plasma interaction. Contrary to conventional electron sources, electron beams from laser-plasma accelerators can produce a broad range of energies. Therefore, diagnosing these electron spectra requires specific attention and experimental development. Here, we provide an absolute calibration of the Lanex Kodak Fine screen on a laser-triggered radio frequency picosecond electron accelerator. The efficiency of scintillating screens irradiated by electron beams has never been investigated so far. This absolute calibration is then compared to charge measurements from an integrating current transformer for quasimonoenergetic electron spectra from laser-plasma interaction.

Bimetallic AgPt and AuPt aggregates synthesized by radiolysis

Abstract Irradiating aqueous solutions containing both Ag 2 SO 4 and K 2 PtCl 4 leads to intermetallic aggregates of various sizes according to the stabilizing agent: polyvinylalcohol, polyacrylic acid or polyacrylate. In the last case, the particle diameter is 1.5 nm. The bimetallic character is evidenced in all cases by the spectral changes of such sols compared to pure silver sols which display a characteristic surface plasmon absorption band. This plasmon band disappears when 10 to 20 at.% or more Pt is present. Observation by TEM gives an estimation of the particle sizes. Comparable results have been obtained for AuPt particles.

Radiation-induced copper aggregates and oligomers

Abstract γ radiolysis of CuSO 4 solutions containing surfactants (polyvinyl alcohol, polyacrylic acid) induces the formation of copper nanoaggregates which dissolve readily when brought into contact with oxygen. The pink copper clusters and the intermediate Cu I are studied spectrophotometrically under various conditions: pH, Cu 2+ concentration, radiolytic dose. The reactivity of the aggregates in redox reactions is used to determine, notwithstanding the presence of excess copper ions, the metal atom content and the radiolytic yield. The optical absorption band at 570 nm has thus been calibrated to be ϵ(Cu n ) 570 nm = 1950 ± 50 l mol −1 cm −1 at pH 6.2 and 2400 ± 100 at pH 2.6. Oligomers of lower nuclearity stabilized by polyacrylate ions are shown to absorb at 292, 350 and 455 nm.

Optical limitation induced by gold clusters: Mechanism and efficiency

The optical limiting effect induced by gold clusters was measured as a function of excitation wavelength. The limiting effect is most efficient below 530 nm, and decreases towards the red. Two different behaviors are seen in the time-resolved signals in the nanosecond and picosecond ranges. These behaviors have different time delays for the amplitude maxima and for the fluence thresholds where nonlinear effects are observed. This suggests that two types of scattering centers are responsible for the optical limitation. The fast mechanism, which reaches a maximum amplitude in less than 1 ns, occurs at relatively high fluence and for short pulses and is assigned to the vaporization of metal particles. It is more pronounced for large clusters where the absorbed light energy is primarily highly confined. The slow mechanism, which develops in a few nanoseconds, is assigned to the energy transfer from the gold particles to the surrounding solvent and to the formation of solvent bubbles. At lower fluence and for smaller size clusters, only the second mechanism is observed in the nanosecond range, because the efficient dissipation of energy from the small clusters to the solvent, which produces bubbles, precludes metal-particle vaporization.

Complexation of silver clusters of a few atoms by a polyanion in aqueous solution: pH effect correlated to structural changes

Abstract Gamma irradiation of aqueous solutions containing AgNO 3 and polyacrylic acid (PAA) in basic medium (pH=10 to 11) leads to the formation of a blue-coloured silver cluster (a-type) associated to the PAA polyanion, which displays a two-peak spectrum with one band (λ max =292 nm) characteristic for silver and the other (λ max =800 nm), not yet reported, characteristic for the polyanion-silver interaction. This a-type species is indefinitely stable at pH> 10 and can be dried as a composite film. Progressive acidification of a solution of this a-type species results in important changes in the spectrum: these changes indicate the successive formation of two other silver clusters b and c (λ max =325 and 375 nm, respectively) associated with the polymer ligand in a distinct way (ligand bands peaking at 640 and 465 nm) with an increasing aggregation number from a to c. The size of the c-type cluster is estimated to be close to eight. The whole process is discussed in terms of conformation changes in PAA induced by progressive protonation of the carboxylate groups. The shift of the ligand band shows that the metal-ligand interaction depends on the aggregation number and on the ligation power (higher for the anionic form -COO − than for the protonated form -COOH).

Study of the interaction between polyacrylate and silver oligomer clusters

Abstract γ-Radiolysis of aqueous solutions of silver sulfate in the presence of polyacrylic acid (PAH) or polyacrylate polyanion (PA - ) allows the synthesis of a series of silver oligomer clusters which are observed through their UV-visible spectra. According to the experimental conditions (pH, PA/Ag ratio, dose, ageing), the spectral band characteristic for distinct silver cores is located at ca 280 (Ag 2+ 4 cluster), 295 (a-type cluster), 335 (b-type), 370 (c-type) or 380 nm (subcolloidal particles). In part of the spectra, a second absorption band at higher wavelengths, mainly in the visible region, due to the polymer-silver interaction, has been studied and used to follow some reactions at the particle surface. A shift of this band upon ageing is understood in terms of slow reorganization of the polymer environment over days and months. At least two types of environment have been found. The competition of Cl - with PA - for the positive surface sites induces a weakening of the ligand bonding resulting in coalescence and/or increased corrodability.

Mechanism of trivalent gold reduction and reactivity of transient divalent and monovalent gold ions studied by gamma and pulse radiolysis.

The detailed kinetics of the multistep mechanism of the Au(III) ion reduction into gold clusters have been investigated by radiation chemistry methods in 2-propanol. In particular, a discussion on the steady state radiolysis dose-dependence of the yields concludes to a comproportionation reaction of nascent gold atoms Au(0) with excess Au(III) ions into Au(II) and Au(I). This reaction should be achieved through Au(III) consumption before the coalescence of atoms Au(0) into gold clusters may occur. Then gold clusters catalyze the reduction of Au(I) by 2-propanol. It was also found that a long-lived Au(II) dimer, (Au(II))(2), was transiently formed according to the quantitative analysis of time-resolved absorbance signals obtained by pulse radiolysis. Then the disproportionation of Au(II) is intramolecular in the dimer instead of intermolecular, as usually reported. The yields, reaction rate constants, time-resolved spectra, and molar extinction coefficients are reported for the successive one-electron reduction steps, involving especially the transient species, such as Au(II), (Au(II))(2), and Au(I). The processes are discussed in comparison with other solvents and other metal ions.

Bimetallic Au-Pt nanoparticles synthesized by radiolysis: Application in electro-catalysis

Abstract2 nm bimetallic alloyed Au-Pt nanoparticles were synthesized via electron beam irradiation. The nanoparticles were characterized using TEM and EDAX analysis, and tested as electrocatalysts for different fuel-cell related reactions. Our results show that these Au-Pt nanoparticles are promising for application in fuel cells.