Pulsed particle formation

Ag2S04, all reactions are drastically accelerated. The sulfate anion complexes the silver intermediates, the result being a reduction of their mutual repulsion. 

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Pulsed particle formation has been used to study redox reactions on the surface of the growing particles as a ftmction of particle size. The standard redox potential of metallic silver. 

Microemulsion-mediated materials synthesis employs three basic methods, as illustrated in Fig. 3 [17]. The microemulsion-plus-trigger method (Fig. 3, method a) is based on a single microemulsion. The fluid system is activated in some way in order to initiate the reactions that eventually lead to particle formation. Pulse radiolysis and laser photolysis have served as triggers for the preparation of nanosize gold particles [41]. In the case of metal oxides, temperature elevation can provide the needed trigger action hydrated metal ions solubilized... 

In order to image the distribution of the solvent, the solution and CO2 and the location of detectable particle formation simultaneously, the setup from the experiments before has to be adapted to other requirements. For this scope, a three-camera system has to be applied as shown in Fig. 24.8. Two cameras are required to detect the inelastic Raman signals, which are scattered from the organic solvents and the CO2 molecules, and therefore indicate the distribution of the solvent and the antisolvent. The third camera is taken to detect the elastic light scattering, which comes from phase boundaries, and therefore indicates the presence of a multiphase flow. The light scattering processes are excited with a pulsed frequency-doubled... 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

The hydrated electrons then react according to e + Cd Cd, and the Cd ions which have a strong absorption at 300 nm react with the colloidal particles after the pulse. It was observed that the same bleaching took place during this reaction as in the reaction of e with CkiS particles, and it was concluded from this result that Cd" transfers an electron to a CdS particle Cd" + (CdS), - Cd + (CdS) . These observations also are of interest for our understanding of the formation of Cd atoms in the photocathodic dissolution of CdS (see Sect. 3.4). Cd" cannot be the intermediate of the overall reaction 2e + Cd - Cd° as already pointed out in discussing the mechanism of Eqs. (35) and (36)... 

In the pulse radiolysis studies on the reaction of MV with TiOj, the sol contained propanol-2 or formate and methyl viologen, MV Ionizing radiation produces reducing organic radicals, i.e. (CH3)2COH or C02 , respectively, and these radicals react rapidly with MV to form MV. The reaction of MV with the colloidal particles was then followed by recording the 600 nm absorption of MV . The rate of reaction was found to be slower than predicted for a diffusion controlled reaction. 

The reduction phase (phase 1) is slower than the re-oxidation one (phase 2). The C02 formation decreases regularly upon each CO pulse while the re-oxidation is achieved upon the first pulse of 02. This is a rather general phenomenon in catalysis. Oxides (like rare-earth oxides) reduced more slowly than their suboxides may be re-oxidized. It is interesting to note that the reverse phenomenon can be observed with the metals (Pt, Rh and Pd). Their oxides are reduced at a much lower temperature than the metal can be re-oxidized [19-21] even though the nature of support and the metal particle size may change the redox properties significantly [20,22,23],... 

Nickel oligomers prepared in the presence of PA (Amax = 540 nm) (Section 20.4.2) may also act as catalysts for the reduction of Ni by hypophosphite ions. This requires, as shown by pulse radiolysis, a critical nuclearity, while free Ni cannot be reduced directly by H2PO2. Very low radiation dose conditions, just initiating the formation of a few supercritical nuclei, will lead to large particles of nickel [96]. 

The time-resolved studies of the cluster formation achieved by pulse radiolysis techniques allow one to better understand the main kinetic factors which affect the final cluster size found, not only in the radiolytic method but also in other reduction (chemical or photochemical) techniques. Generally, reducing chemical agents are thermodynamically unable to reduce directly metal ions into atoms (Section 20.4) unless they are complexed or adsorbed on walls or dust particles. Therefore, we explain the higher sizes and the broad dispersity obtained in this case by in situ reduction on fewer sites. A classic... 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

Figure 5. An idealized mechanism of photoinduced electron transfer from CdS conduction band to methylviologen (MV +)( resulting in formation of methylviologen radical cation (MV,+). The colloidal CdS particle as represented, was generated at the inside surface of the DHP vesicle. Its exact location is based on fluorescence quenching experiments (Figure 5). Inserts oscilloscope trace showing the formation of MV by the absorbance change at 396 nm, after a laser pulse at 355 nm.

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