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Radiolysis of mixed Au III /Ag I solutions at different dose rates is examined. The progressive evolution with dose of the UV-visible absorption spectra of radiation-induced metal clusters is discussed and compared with those calculated by Mie theory. The clusters have been also observed by transmission electron microscopy and analyzed by X-ray microanalysis and diffraction. At low dose rate, reduced silver atoms transfer an electron to gold ions (either free or at the surface of aggregates). Then, when Au III ions are totally reduced, reduction of the silver ions occurs in a second step at the surface of gold clusters, and silver-coated gold aggregates are obtained. At high dose rate, the shape of the absorption spectrum does not change with an increase in the absorbed dose and X-ray microdiffraction confirms that bimetallic alloyed Ag/Au clusters are synthesized. These results imply the preponderant influence of kinetics in the competition between the reduction-coalescence processes and intermetallic electron transfer. The segregation or the alloying of the metals is controlled by the reduction rate; a fast total reduction of both types of metal ions prevents the redox equilibrium through electron transfer from being established. A perfectly ordered nanocrystal, as observed by electron microdiffraction, also implies an intimate association of metal atoms from the early steps of reduction and aggregation. 4310

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TiO 2 (commercial-P25 and synthesized by sol-gel method) was surface modified with platinum ions or [Pt 3 (CO) 6 ] 6 2-clusters to improve its photocatalytic activity. The physical properties of the synthesized photocatalysts were examined by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer, Emmett and Teller adsorption (BET) methods. To characterize the absorption ability of visible light, the diffusion reflectance spectra (DRS) were recorded. The charge-carrier lifetimes in TiO 2 after UV illumination were determined by microwave absorption experiments using the time-resolved microwave conductivity (TRMC) method. The photocatalyst activity was examined by degradation of exemplary aqueous phase pollutants, such as Rhodamine B and phenol. The impact of the adsorbates on the photocatalytic activity depends strongly on the titania precursor (commercial or synthesized), the irradiation wavelength (UV or visible), and the model compound (dye or phenol). The results show that it is possible to enhance the P25 photocatalytic activity under UV-visible light by doping it with Pt clusters. Also, P25 doping with Pt(II) or Pt clusters results in enhancement of the activity under visible light. Pt(IV)/TiO 2 synthesized by sol-gel method exhibits better photoactivity under UV-visible and visible light compared to the unmodified titania. In all mentioned systems, a positive effect of modification with platinum clusters and, in particular, an important enhancement in photocatalytic activity under visible light were obtained. These results are explained by enhancement in visible light absorption and inhibition of charge-carrier recombination.

The nonlinear optical response of gold particles (with 2.5, 9, or 15 nm radius) prepared by γ-radiolysis in water solution and stabilized by poly(vinyl alcohol) is size-dependent. The 2.5 nm clusters do not limit light transmission even at very high fluence of nanosecond laser pulses, while the larger clusters strongly limit the laser beam transmission at 530 nm. The threshold of limitation and the amplitude depend on the size of the particles. The rise of the optical limiting effect is measured by picosecond laser spectroscopy. For the largest particles, it lasts 1 ns. The limitation effect is attributed to large light-scattering centers induced by the pulse around the initial particles.

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