M.O. Delcourt

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.

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.

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.

STM identification of silver oligomer clusters prepared by radiolysis in aqueous solution

Abstract Silver oligomers prepared by radiolysis in aqueous solution in the presence of polyacrylate and absorbing at ≈ 290 nm have been imaged by STM in air after being deposited on a HOPG surface. Clear images with atomic resolution are observed, showing flat clusters of 0.7 nm. Each cluster contains seven (or possibly eight) atoms corresponding to the stoichiometry (Ag4+m)m+, m = 3 (or 4), for the clusters in aqueous solution. No interference due to the polyanion (present in the solution as a stabilizing agent) is found for adsorbate densities.

Ionizing radiation-liquid interactions: A comparative study of polar liquids

Abstract A general survey of the behavior under irradiation of liquids with a wide variety of properties is given here. It permits us to check the validity of models or radiation-liquid interaction. A main distinction first appears—the ability of the liquid molecules to attach electrons or to merely solvate them. In the last case, the fate of the common species es- is compared. Specific aspects of the partially diffusion-contrlled reactivity of es- and of a cooperative role for cations in electron-radical reactions are emphasized.

Pt and Pt/Cu carbonyl clusters synthesized by radiolysis

In the mixed solvent: 50/50% v/v water/2-propanol, (Pt3(CO)6)2n - clusters with n = 3 10 have been obtained by irradiating solutions containing K2PtCI 4 under 1 atm CO. n is deduced from the very typical UV-visible and IR absorption spectra. The reduction occurs by a combined effect of CO and of the radicals produced by radiolysis (radiocatalysis). The synthesis is selective: the nuclearity n can be chosen by adjusting the dose (high doses yield low n values). Increasing the Pt salt concentration leads to CO-stabilized subcolloid particles, Intermetallic cluster compounds are expected from solutions containing two metal salts: bimetallic Pt/Cu carbonyl clusters have been obtained. Two distinct compounds have been characterized by their UV-visible spectra. Attempts with Pt/Ru and Pt/Sn systems were unsuccessful.

Early steps of formation of silver/polyacrylate complexed oligomer clusters. A pulse radiolysis study

Abstract Pulse radiolysis has been carried out on aqueous solutions of silver ions complexed with polycrylate (PA). Over the time scale 10−8−10 s, only a few kinetic steps have been observed which have been identified through spectroscopic and kinetic analysis. The reaction rate of e-aq with Ag+ is lowered in the presence of PA (3.6×109lmol−1s−1 when [PA]/[Ag+]=100). The first products (Ag0 λmax≈360 nm, Ag+2 λmax=310 nm) are similar to those obtained in the absence of PA, except for an additional absorption band (350–450 nm) which indicates that Ag+2 is itself complexed. Then through dimerization (2k=3×109lmol−1s−1) and reaction with the alcoholic radical (in the presence of 2-PrOH, 1.5×109lmol−1s−1), Ag+2 leads to an “Ag2” species peaking at 300, 320 and 360 nm, noticeably different from the reported Ag2+4 species. Two further steps are observed which are assigned to a ligand reorganization of “Ag2” in the millisecond range (103s−1), followed by a coalescence to “Ag4” in the range of seconds (105lmol−1s−1). This last product is thought to be the stable Aga cluster known to be formed by γ-radiolysis. A kinetic simulation accounts for the whole mechanism which is much lower than in the absence of PA.

Pulse radiolysis of (RhI(CO)2Cl)2 and (RhII(CH3COO)2)2 solutions under CO or N2 atmosphere

Abstract Ethanolic solutions of (Rh I (CO) 2 Cl) 2 and aqueous solutions of (Rh II CH 3 COO) 2 ) 2 have been investigated by pulse radiolysis under CO or N 2 atmosphere. In the first case the reduction of the Rh I complex is shown to proceed via CO - formation. In the second case, several steps have been evidenced, one of them extremely fast, indicating an exceptional reactivity of such a binuclear rhodium structure towards the electron. Spectra of transient species at different times are presented. A species absorbing at 520 nm, already present at 10 ns, is assigned to a Rh I Rh II complex resulting itself from a reaction of the initial salt with pre-solvated electrons. A mechanism is proposed to account for the decay kinetics of e - aq and the spectral changes. The rate constants are evaluated for each of the five steps occuring within the first microsecond.

A new process using radiation for synthesising molecular metal clusters and complexes: First results concerning iron, ruthenium and osmium compounds

Abstract The radiolytic reduction of metal ions in alcohols has been achieved under CO atmosphere. Under such conditions, metal reduction, ligation and aggregation processes compete, leading to metal complexes and molecular clusters. These compounds are interesting for catalytic purposes. We report here the radiolytic synthesis of Fe(CO) 5 , Fe 3 (CO) 12 , [Ru(CO) 3 Cl 2 ] 2 , Ru 3 (CO) 12 , H 2 Ru 6 (CO) 18 and [Os(CO) 3 Cl 2 ] 2 and of some intermetallic compounds. These syntheses are carried out under very mild temperature and pressure conditions (i.e. room temperature and at most 1 atm of CO). It is shown that increasing the concentration of initial metal ions or decreasing the partial pressure of CO favors aggregation against ligation.