Monodisperse metal cluster

While steady-state radiolysis of metal ions solutions is a powerful method to generate small and monodisperse metal clusters and to synthesize metal nano-objects with controlled size, shape and struc-ture, pulse radiolysis technique enables to follow, in particular by time-resolved spectroscopy, the nucleation steps and the growth kinetics of the nanoparticles. ... 

Radiation chemistry methods have been proven to be of high potentiality to induce small and size-monodispersed metal clusters and to study the dynamics of nucleation and growth of clusters. 

As a rule, short nucleation times are the prerequisite for monodisperse particle formation. A recent mechanistic study showed that when Pt(acac)2 is reduced by alkylalu-minium, virtually all the Pt cluster nuclei appear at the same time and have the same size [86]. The nucleation process quickly consumes enough of the metal atoms formed initially to decrease their concentration below the critical threshold. No new metal cluster nuclei are created in the subsequent diffusion-controlled growth stage. 

PAMAM dendrimers are large (G4 is 4.5 nm in diameter) and have a hydrophilic interior and exterior accordingly, they are soluble in many convenient solvents (water, alcohols, and some polar organic solvents). Importantly, the interior void spaces are large enough to accommodate nanoscopic guests, such as metal clusters, and are sufficiently monodispersed in size so as to ensure fairly uniform particle size and shape. As we will show later, the space between the ter-... 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

The size-selective precipitation (SPP) was predominantly developed by Pileni [50c]. One example (SPP) is monodisperse silver particles (2.3 nm, 0= 15%), which are precipitated from a polydisperse silver colloid solution in hexane by the addition of pyridine in three iterative steps. Recently, Schmid [52a] has reported the two-dimensional crystallization of truly monodisperse AU55 clusters. Chromatographic separation methods have thus far proven unsuccessful because the colloid decomposed after the colloidal protecting shell had been stripped off [42a]. The size-selective ultracentrifuge separation of Pt colloids has been developed by Colfen [52b]. Although this elegant separation method gives truly monodisperse metal... 

Metal clusters have been considered as models for monodispersed nanoscale metal particles in a dielectric matrix or as precursors for nanoscale particles (see Chapter 12.03 for organometallic-derived metals, colloids, and nanoparticles). The number of metal atoms in well-defined molecular compounds can be varied from 2 to 3 up to hundreds, and the physical properties change from localized molecular to nearly bulk metal. They offer the advantage that clusters can be studied by a wide variety of experimental techniques. 

A short laser pulse ( 200 fs) generates hot electrons in the metal clusters. A second laser pulse monitors the hot electron population. This technique enables the direct observation of electron relaxation as a result of electron-phonon coupling. Electron-phonon coupling is a decisive property enabling differentiation between bulk and molecular properties. Although this possibility has been exploited by different authors, the particles used have not been single-sized, which is necessary if valid results are to be obtained. The use of clusters of a distinct size such as Aun and Auss" and almost monodisperse 15-nm gold colloids and 3 nm and 35 nm Pt particles recently enabled more precise measurements to be made and better results to be obtained than in the past. It could be shown that the hot electron relaxation depends on the particle size in two different ways ... 

To generate small metal clusters (i.e., clusters of only a few nanometers), a continuous nucleation regime should be maintained during the phase precipitation process. The continuous nucleation requires a high nucleation rate, which can be achieved by the mercaptide decomposition at high temperature (300 to 400°C). Usually, in addition to the very small size, the generated nuclei form monodispersed, pseudo-spherical clusters. 

Heiz U, Sanchez A, Abbet S, Schneider W-D (1999) Catalytic oxidation of carbon monoxide on monodispersed platinum clusters each atom counts. J Am Chem Soc 121 3214—3217 Henglein A (1989) SmaU-particle research physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 89 1861-1873... 

Manufacturing composites that contain monodispersed nanodimensional clusters of metals would have potential use in heterogeneous catalysis, surface-enhanced Raman scattering, and other fields, where surface properties of materials and interfacial phenomena are important. 

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